The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial Infections

*Lixing Huang, Rongchao He, Youyu Zhang and Qingpi Yan*

## **Abstract**

Aryl hydrocarbon receptor (AhR), an important nuclear receptor, regulates the cellular response to environmental stressors. It is well known for its critical functions in toxicology, but is currently considered an essential regulator of diseases, with specific modulatory effects on immune, antimicrobial and inflammatory responses. The present chapter discusses AhR's function and mechanism in the immune response against microbial infections.

**Keywords:** aryl hydrocarbon receptor (AhR), functional mechanism, antimicrobial, immunity, gut immunity

## **1. Introduction**

The ligand-activated transcription factor aryl hydrocarbon receptor (AhR) is structurally similar to other members of Pern-Arnt-Sim (PAS) superfamily [1, 2], which consists of a conserved signaling network that regulates signal exchange between host and environment [3, 4]. It was originally found to play a role in regulating the reactions of exogenous chemicals such as 2,3,7,8-Tetrachlorodibenzop-dioxin (TCDD). However, AhR has been recently recognized as an essential regulator of host-pathogen interactions [5–9], especially affecting immunity, inflammatory response and antibacterial activity [5, 9–15]. The current chapter focuses on AhR's function in regulating immunity, inflammatory response and antibacterial activity.

## **2. Mechanism of AhR action**

As a highly conserved nuclear receptor [10], AhR can regulate gene expression after binding to a ligand. AhR binds to its co-chaperones and maintains cytoplasmic localization [16, 17]. Ligand binding by AhR results in its release by co-chaperones and translocation into the nucleus, where it forms a heterodimer with the aryl hydrocarbon receptor nuclear translocator (ARNT) [18, 19]. Via binding to the genomic DNA usually interacting with AhR response elements (AhREs, 5'-GCGTG-3′) [20, 21], also referred to as dioxin (DREs) or xenobiotic (XREs) response elements [9, 10],

the AhR-ARNT heterodimer regulates multiple target genes such as Cytochrome P450 Family 1 Subfamily A Member 1 (CYP1A1), CYP1A2, CYP1B1, TCDD Inducible Poly (ADP-Ribose) Polymerase (TIP ARP), and aryl hydrocarbon receptor repressor (AhRR), which can inhibit AhR via a negative feedback circuit [22]. Target gene regulation is considered to be ligand dependent [21].

As a highly heterogeneous nuclear receptor, AhR binds to many ligands, including exogenous synthetic aromatic hydrocarbons [10, 23], exogenous natural chemicals [5, 6, 10, 14, 24] and endogenous ligands [25–29]. Tryptophan, an essential amino acid in humans, constitutes the precursor of many important components in the human body. Interestingly, the tryptophan (TRP) pathway has a critical function in immune and inflammatory responses through providing many ligands for AhR. In addition, AhR controls the expression and activation of tryptophan 2,3-dioxygenase (TDO2), indoleamine 2,3-dioxygenase (IDO), kynureninase (KYNU) and kynurenine 3-monooxygenase (KMO). The aforementioned enzymes catalyze the synthesis of kynurenine (KYN), which is a product of TRP metabolism, thus enabling feedback inhibition because KYN and AhR are agonists [30, 31].

#### **3. AhR expression modulation**

The interactions of AhR and its ligands, including polycyclic aromatic hydrocarbons (PAHs), can be used as a cytoplasmic signal sensor. The conformation of AhR changes, and it is transferred from the cytoplasm to the nucleus. The high-affinity ligand TCDD can exert toxic effects by binding with and activating AhR [32, 33]. Structural analysis of AhR revealed three domains: 1) The amino-terminal DNA binding domain (DBD) comprises the basic helix–loop–helix (bHLH) region and the nuclear localization signal (NLS); 2) The central PAS region encompasses two degenerate repeats; 3) The carboxy-terminal region features the transactivation domain (TAD) [34]. In addition, phylogenetic data showed that AhR constitutes an ancient protein whose functional orthologues are found in reptiles, amphibians, birds and mammals. However, there are many structural differences between human and murine AhR genes. Sequence analysis revealed approximately 85% structural similarity in the amino-terminal sequence, while the C-terminal region shows a low homology. The TAD or N-terminal domain is the least conservative [34]. The C-terminal domain is a highly unstructured sequence containing a transcriptionally active region and contributes to receptor transformation [35, 36].

AhR, heat shock protein 90 and X-associated protein 2 form multiple protein complexes in the cytoplasm. In the presence of ligands or agonists, AhR complexes undergo nuclear translocation and form heterodimers with ARNT. With a core sequence of 5′-GCGTG-3′, the AhR/ARNT complex interacts with DREs in the proximal site of promoters of target genes. Both AhR and ARNT recruit additional transcription co-activators for gene regulation, e.g., CYP and AhRR. Once transferred into the nucleus, AhR undergoes proteasome-induced degradation [37]. AhR function is modulated and weakened by AhRR, another member of the PAS family. After AhR activation, the level of AhRR increases rapidly [38]. Meanwhile, AhRR has a transcriptional repressor domain and can dimerize with ARNT even without an agonist, to fulfill its function [39].

#### **4. AhR response to bacterial pathogens**

It is known that AhR has a critical function in controlling responses to a variety of microbial pathogens. For example, it is required to effectively clear the *The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

Gram-positive pathogenic bacteria *Listeria monocytogenes* (LM). In mice, AhR inhibits LM by inducing ROS production via upregulation of the anti-inflammatory cytokine IL-10 and macrophage apoptosis inhibitor, resulting in suppressed macrophage apoptosis, reduced amounts of pro-inflammatory cytokines (e.g., Interleukin 6 (IL-6) and Tumor Necrosis Factor alpha (TNF-α)), and decreased the nuclear factor kappaB (NF-κB) activation. In addition, AhR ligands can enhance the response of AhR WT mice to LM, but not of AhR−/−mice [27].

When inoculated with log-phase LM intravenously, AhR deficient C57BL/6 J mice (AhR−/−) showed higher susceptibility compared with AhR heterozygous (AhR+/−) littermates. In comparison with AhR+/− animals, AhR−/− counterparts showed more colony forming units (CFUs) of LM in the spleen and liver, and more pronounced alterations in liver histopathology. Serum monocyte chemotactic protein 1 (MCP-1), IL-6, TNF-α and Interferon γ (IFN-γ) amounts were similar in AhR−/−and AhR+/−mice infected with LM. Elevated IL-12 and IL-10 amounts were detected in AhR−/−mice infected with LM. In terms of capacity of uptake and inhibition of intracellular growth of LM, AhR+/−and AhR−/−macrophages were comparable *in vitro*. In addition, T cell-dependent response was similar in AhR−/−and AhR+/−mice, as determined by intracellularly labelling cluster of differentiation 4 and 8 (CD4+ and CD8+ ) splenocytes for IFN-γ and TNF-α. AhR−/−and AhR+/−mice with prior infection showed increased resistance to re-infection by LM. The above evidence suggests that AhR is necessary to build an effective resistance, but not required for adaptive immune reactions following LM infection [40].

*Streptococcus pneumonia*, a common respiratory pathogen, represents a major cause of morbidity and death in humans, especially the elderly and children. The immune response after *S. pneumoniae* infection begins quickly in the lung, and the innate immune response can contain bacterial colonization in the ideal situation. Death, and bacterial load, cytokine/chemokine amounts, and immune cell infiltration in the lung have been assessed at different times in TCDD treated mice after *S. pneumoniae* infection. The survival rate of mice administered TCDD was significantly increased, while bacterial load in the lung was reduced. However, intriguingly, no evidence suggested that the protective effect was caused by increased inflammatory response. In fact, neutrophil amounts and inflammatory chemokine/ cytokine levels in TCDD treated mice were lower than those of control animals. These findings suggest that AhR induction does not protect the animals by immune modulation, but likely by directly affecting lung cells upon infection [41].

*Pseudomonas plecoglossicida* represents the bacterial pathogen of fish visceral white spot disease with temperature dependent virulence [42]. AhR is also required for resistance to *P. plecoglossicida*. It was shown that *ahr1a*, *ahr1b*, *ahr2* and *cyp1a* amounts in various organs of *Danio rerio* and *Epinephelus coioides* infected with *P. plecoglossicida* have similar trends. It should be noted that the intestine, liver, heart and spleen are the most affected organs, while *ahr2* specifically shows a sharp increase in the spleen. After *P. plecoglossicida* infection, *ahr1a* amounts in macrophages are markedly reduced, while *ahr1b*, *ahr2* and *cyp1a* are overtly upregulated. The cell viability and immune escape rates of *P. plecoglossicida* were significantly increased in macrophages with *ahr1b* and *ahr2* knockdown. In conclusion, *ahr1a*, *ahr1b*, *ahr2* and *cyp1a* are involved in immune reactions to *P. plecoglossicida* in various fish organs, while *ahr1b* and *ahr2* might play a key role in splenic and macrophage immune reactions [43].

Huang et al. described the first pathogenic *Aeromonas salmonicida* (SRW-OG1) obtained from the warm water fish *E. coioides*, and studied AhR's role in the immune response to SRW-OG1 infection [44]. They found that AhR is induced by unknown ligands in the intestine, spleen and macrophages. At the same time, *ahr1a* and *ahr1b* amounts were markedly elevated in the intestine, spleen and macrophages,

while *ahr2* only showed an increase in the intestine, suggesting *ahr2* may contribute less to immune reactions compared with *ahr1a* and *ahr1b*. In SRW-OG1 infected *E. coioides*, major genes contributing to bacterial recognition, macrophage inflammatory response and gut immunity were overtly upregulated. However, decreased ROS amounts and the downregulation of other associated genes were equally detected, which indicated that SRW-OG1 could prevent ROS production by macrophages through its virulence mechanism. In addition, repression of AhR with an inhibitor or by gene silencing rescued the increases of *IL-1 β* and *IL-8* associated with SRW-OG1 infection, clearly demonstrating that induction of *E. coioides* macrophages by *IL-1 β* and *IL-8* is controlled by AhR. Nevertheless, AhR exerted no effects on bactericidal permeability-increasing protein/lipopolysaccharide-binding protein (BPI/LBP*)*, reactive oxygen species (ROS) biosynthesis and associated genes. Compared with wild-type macrophages, survival and immune escape rates after SRW-OG1 infection were significantly increased in *ahr1a*/*ahr1b*-knockdown and 3′, 4'-DMF treated macrophages. Taken together, *ahr1a* and *ahr1b* are necessary for the immune response to SRW-OG1 [44].

Lipopolysaccharide (LPS) stimulation is often utilized to model Gram-negative bacteria-induced sepsis for assessing AhR's functions in infection resistance and septic shock regulation. AhR and TDO2 are required for survival after the initial exposure to LPS [14, 20], while subsequent exposures are dependent on AhR and IDO1/2. LPS up-regulates TDO2 and IDO1/2, the rate-limiting enzymes of TRP transformation into KYN, and further induces AhR, thus downregulating proinflammatory cytokines and regulating long-term systemic inflammation [20]. In addition, compared with AhR wild type mice or immune cells, LPS challenged AhR−/−mice or immune cells produce higher concentrations of pro-inflammatory cytokines, including IL-1 β, IL-6, IL-18, IL-12, TNF-α and IFN-γ, as well as NLR Family Pyrin Domain Containing 3 (NLRP3) that regulates multiple pro-inflammatory cytokines. The AhR agonists 3-methylcholine (3-Mc), 6-Formylindolo[3,2-b] carbazole (FICZ), KYN and TCDD could protect AhR WT mice, but conferred no protection to AhR−/− animals, from extremely high amounts of pro-inflammatory cytokines and septic shock [45]. Thus, the immune response to bacterial pathogens requires AhR, and the underlying mechanisms are vital in identifying novel therapeutic agents to combat bacterial pathogens.

#### **5. AhR response to viral pathogens**

AhR is also associated with response to viral pathogens. For example, herpes simplex virus (HSV)-associated eye infection can lead to chronic immune-inflammatory response, causing blindness. However, in a mouse model, a single dose of TCDD could alleviate herpetic keratitis lesions, reduce viral load and decrease pro-inflammatory cytokine levels. However, similar effects were not obtained with FICZ, thus indicating a difference between both AhR ligands [46]. Therefore, response to viral pathogens requires AhR, and nontoxic AhR agonists could be used in the treatment of HSV-induced eye infections.

In influenza virus infection, activation of AhR doubles the number of neutrophils in the airway and interstitium of the lung, which reduces the survival rate from an otherwise sub-lethal infection [47, 48]. Interestingly, no increase in neutrophil inflammation or decreased survival was observed in AhR deficient mice treated with TCDD and influenza virus [37]. Innate immune reactions, including excessive pulmonary neutrophilia, can lead to severer pathological conditions and poor clinical outcomes after influenza virus infection [49–51]. Meanwhile, epidemiological reports have shown that exposure to environmental AhR ligands is associated with

*The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

elevated respiratory tract infection, pulmonary congestion and exacerbation of inflammatory lung disease [52–54]. Therefore, there is parallel evidence in rodent animal models and humans that AhR regulates neutrophil inflow during infection. Overall, these data suggest that AhR regulates a new pathway to regulate neutrophil migration during influenza virus infection. A possible new target gene of AhR is inducible nitric oxide synthase (iNOS). Meanwhile, activation of AhR can increase the expression of iNOS in the mouse lung upon infection with influenza virus [55].

#### **6. AhR response to parasitic pathogens**

The immune response to parasites also requires AhR. For example, immune response to *Toxoplasma gondii*, a pathogenic parasite causing toxoplasmosis, requires increased AhR-dependent production of IL-10. Indeed, AhR−/− mice have reduced response to *T. gondii* and a less pronounced IL-10 increase [56].

After intraperitoneal infection with *T. gondii*, the death rate of AhR−/− mice was significantly higher than that of WT mice. Moreover, AhR−/− mice showed greater liver injury, and higher levels of NO, IgE and TNF-α, but lower IL-10 secretion in the serum. Interestingly, fewer cysts were found in the brain. The increased mortality was related to reduced IL-10, 5-LOX and GATA-3 expression levels, but increased IFN-γ expression in the spleen. In addition, AhR−/− mice had increased IL-12 and IFN-γ amounts, but decreased TLR2 levels compared with wild-type mice in peritoneal exudate cells. These findings suggest that AhR is vital for limiting inflammation during toxoplasmosis [57].

Therefore, AhR is necessary for parasitic pathogen response. This provides information on a response pathway and can be used to design new treatments.

#### **7. AhR and the intestinal microbiota**

AhR is found at high levels in the epithelial barrier [58], and the intestinal barrier of AhR−/−mice is inadequate, suggesting AhR might be important in maintaining or generating a healthy intestinal barrier [19]. In addition, low levels of AhR and AhR's target genes are found in sterile mice [9], and AhR is needed for maintaining the ROR<sup>γ</sup>t+ innate lymphoblastoid cell (ILC) balance in the intestine [18]. In addition, the TRP metabolizing indole biosynthesized by select bacterial components of the intestinal microbiota is an AhR ligand [59, 60]. Diet without indole or antibiotic treatment can lead to the differentiation of mononuclear phagocytes, dependent on AhR, into dendritic cells (DCs) [48], which are more susceptible to gut pathogens in mice [17]. Overall, the above findings suggest AhR might be important in host gut-microbiota interactions.

AhR also plays a role in the reciprocal relationship among intestinal bacteria, bacterial metabolites and the intestinal immune system. AhR-deficient RORγt + ILCs (the main producers of gut IL-22) with lower IL-22 amounts make mice easily die upon *Citrobacter rodentium* infection. It was pointed out that treatment with FICZ markedly enhances RORγt + ILC accumulation in AhR+/− and AhR+/+ mice, but not in AhR−/− animals [61]. Lactobacillus species (nonpathogenic intestinal bacteria) are capable of producing AhR ligands, including indole-3-aldehydes, from tryptophan in the gut, thus enhancing the production of AhR dependent IL-22 [62]. Indole-3 aldehydes induces AhR-associated transcription, but exclusively at elevated concentrations, indicating its low affinity. However, indole-3-acetaldehyde (a product of indole-3-aldehydes) produces the high-affinity ligand FICZ [63], which may be related to the effect reported by Zelante et al. IL-22 affects epithelial cells and

causes them to produce antimicrobial peptides, such as type III Reg (regenerating gene product) gamma (RegIIIg), and to stimulate tissue regeneration. Meanwhile, symbiotic bacteria may outperform bacterial pathogens and inhibit *Candida albicans* colonization [51]. Similar to keratinocyte and skin immune cell levels, AhR amounts are high in IECs and intestinal immune system cells [64].

In AhR-null mice, the number of intraepithelial lymphocytes (IELs) in the small intestine is significantly reduced [6, 64, 65], which is related to lower IL-22 amounts, and therefore to downregulated ileal antimicrobial peptides, including RegIIIb and RegIIIg. The microbial loads of the small and large intestines are also elevated. Loss of IELs is cell-intrinsic since AhR-deficient bone marrow cells do not reconstruct the gut in Rag−/− mice [51]. Over time after birth, intestinal Group 3 Innate Lymphoid Cells (ILC3s) [66], ILC22 and CD32NKp46+ lymphoid tissue inducer cells are lost in AhR-deficient mice. Similarly, ILC3's inability to multiply in AhR-deficient mice constitutes an intrinsic function since AhR is required for the transcription of the cell-specific proliferator c-kit [67, 68]. As a result, secondary lymphoid structures, including cryptopatches and innate lymphoid follicles, are absent from the gut of AhR-deficient mice, which show susceptibility to *C. rodentium*. ILC3s feature the secretion of IL-17 and IL-22 [69]. AhR-deficient mice have elevated susceptibility to infection by *C. rodentium*, as well as dextran sulfate sodium (DSS)-associated colitis. DSS can damage the intestinal epithelium and induce inflammatory reactions and microbial dissemination. AhR-deficient mice containing wild-type IELs are resistant to DSS colitis, indicating IEL role in injury reduction.

AhR-deficient mice have lower amounts of skin and intestinal IELs and intestinal ILCs, thereby increasing susceptibility to *C. rodentium* infection. These cell types, and the generation of normal gut lymphoid follicles, are regulated by AhR ligands in the diet. In addition, activation of AhR by microbial products equally regulates the production of DP IELs, which constitute another critical group that controls intestinal immunity [70]. It may also be due to the lack of IL-22 that affects the commensal flora [71]. In fact, ID2, a transcription factor, regulates the expression of IL-22 in ILCs via AhR- and IL-23-dependent mechanisms, thereby modulating the intestinal colonization of *C. rodentium* [72]. In addition, AhR also controls the production of IL-22 by Th22 cells, which protect against intestinal pathogens [73, 74]. All these data suggest AhR has a critical function in controlling the interaction at environmental interfaces with microorganisms by regulating IL-22 and other cellular factors. Interestingly, *cyp1a1* overexpression leads to the exhaustion of physiological AhR ligands and also increases susceptibility to intestinal bacterial infections [75], highlighting that AhR ligand availability and metabolism are important in controlling AhR-dependent immune effects.

### **8. AhR and T cells**

AhR plays an important role in controlling adaptive immunity, and regulating T cell differentiation and direct or indirect functions by affecting antigen presenting cells. It was found that TCDD-activated AhR could inhibit the immune response [76], which is subsequently associated with CD4+ T cell induction [77–79]. In addition, the role of AhR in Th17 function and T cell-induced IL-22 biosynthesis have also been determined [74, 80–83].

#### **8.1 AhR and regulatory T cells (Tregs)**

AhR shows high expression in Th17 cells, undetectable amounts in Th1 and Th2 cells, and low expression in Tregs. Tregs constitute a T cell subgroup, which helps

#### *The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

maintain tolerance to autoantigens, preventing autoimmune pathologies. FoxP3<sup>+</sup> Tregs [84, 85] and IL-10-producing type 1 regulatory T cells (Tr1 cells) [86] are the most typical Treg entities. Foxp3<sup>+</sup> Tregs and Tr1 cells are associated with AhR.

TCDD, ITE, KYN and laquinimod derivatives activate AhR, thus increasing FoxP3+ Treg amounts via various mechanisms, e.g., by directly activating epigenetic modifications that regulate Foxp3 transcriptionally and via DC regulation [80, 87–92]. In the presence of TGF - β 1, activating AhR with TCDD can also upregulate SMAD1 in human Tregs, resulting in stable expression of FoxP3 [93]. It was shown in mice with AhR-deficient T cells that AhR could also inhibit the activation of STAT1, which in turn inhibits FoxP3+ Treg differentiation [94]. In addition, AhR regulates the epigenetic modifier Aiolos, which downregulates genes associated with T cell's effector function, such as IL-2 [87]. However, the effect of AhR on FoxP3+ Tregs may be affected by the applied experimental model, which may reflect the different effects of tissue-specific action and/or AhR agonist provided by the symbiotic flora [95].

Tr1 cells participate in controlling tissue inflammation via IL-10 secretion. IL-27 promotes the differentiation of Tr1 cells [96–98], while IL-21 plays an autocrine role in their stabilization [98, 99]. IL-27 upregulates AhR in Tr1 cells via STAT3. Then, AhR amounts are maintained by transactivation of the AhR promoter by AhR itself [100–102]. The important role of AhR in Tr1 cells *in vivo* is reflected by insufficient Tr1 cell differentiation induced by long-term anti-CD3 treatment of AhR-mutant mice.

AhR triggered CD39 equally affects Tr1 cell differentiation. After induction, T cells secrete eATP [103], which then interferes with the differentiation of Tr1 cells through hypoxia inducible factor-1α (HIF1-α). HIF1-α binding is superior to the interaction between AhR and ARNT, and promotes the degradation of AhR through the immune proteasome, thus inhibiting the differentiation of AhR dependent Tr1 cells [101]. The expression of CD39 driven by AhR can deplete eATP and promote the differentiation of Tr1 cells. Therefore, AhR regulates central genes in the Tr1 cell transcription program, while limiting the inhibitory effect of eATP-dependent HIF1-α induction on Tr1 cell differentiation. Overall, the above findings confirm AhR as a potential therapeutic target for immunomodulation.

#### **8.2 AhR and T helper 17 (Th17) cells**

Th17 cells, forming a unique CD4<sup>+</sup> T cell subgroup, can biosynthesize Th17 cytokines and play key roles in the pathogenetic mechanisms of multiple inflammatory ailments. Their differentiation is triggered by IL-6 and transforming growth factor-beta (TGF-β). AhR can modulate Th17 cells by binding to the DRE site in the IL-17 promoter. In addition, AhR and STAT3 can synergistically upregulate Aiolos (IKZF3), an Ikaros family member, which can decrease the expression of IL-2, thus increasing Th17 cell amounts [64].

Th17 cells, producing IL-17A and expressing ROR-γt, are involved in immune responses to extracellular bacterial and fungal pathogens, and participate in the pathological mechanisms of multiple autoimmune diseases [104, 105]. Their differentiation involves joint effects of TGF-β and IL-6 or IL-21 [106–108]. AhR shows high expression in Th17 cells and is activated by FICZ, which can enhance Th17 cell differentiation and promote IL-22 expression. On the contrary, AhR deficiency can cause Th17 cells to produce IL-22, which may reflect AhR's function in promoting RORγt recruitment to the IL-22 promoter.

#### **8.3 AhR and other T cells**

Th22 cells are a CD4+ T cell subpopulation. They produce IL-22 without IL-17's intervention and their differentiation is induced by IL-6, IL-21 or IL-23. AhR

controls the production of IL-22 in Th22 cells, and other cellular factors are essential for their mucosal immune functions [73, 109–111].

The AhR pathway also significantly affects CD8<sup>+</sup> T cells. Activation of AhR by TCDD indirectly inhibits the primary response of CD8+ T cells to influenza virus through the regulatory mechanism of DC function [112]. In addition, CD8<sup>+</sup> T cells of mouse models administered the AhR agonist TCDD in the developmental stage show a weak response to influenza virus infection later in life [113]. The above data indicate epigenetic alterations that can lead to prolonged functional defects in CD8<sup>+</sup> T cells detectable after viral attack. Compared with other CD8<sup>+</sup> T cell subsets, AhR expression is much higher in tissue resident CD8+ memory cells (TRMs). Taken together, these findings indicate that, similar to previously reported CD4+ T cell data, the AhR pathway plays a major role in regulating specific CD8<sup>+</sup> T cell subgroups, such as TRMs and DP IELs.

AhR equally regulates γδ T cells, which are tissue resident lymphocytes. It regulates first-line immune response at epithelial sites and controls tissue homeostasis [114]. Despite AhR expression in the totality of γδ T-cell subgroups, AhR-deficiency significantly reduces the amounts of skin intraepithelial lymphocytes, mostly composed of Vγ3 and Vγ5 γδ T cells in the intestine and CD8αα αβ T cells [115]. AhR also regulates IL-22 expression by γδ T cells that produce IL-17 [116, 117]. The above data indicate that AhR has a significant effect on T cells residing in tissues, which supports further investigation of AhR's function in non-CD4<sup>+</sup> T cells.

In conclusion, AhR controls T cell responses at many levels and regulates transcription factors, enzymes, epigenetic modifiers and effector molecules that modulate T cell stability and metabolism. Lineage-specific responses to AhR induction may lead to ligand-specific effects, which are combined with cytokine-driven activities on the genome, thereby regulating AhR-interacting chaperones and controlling the accessibility of AhR's direct and indirect transcription targets [118]. Comprehensive studies of these interactions should provide insights into the design of immune-modulators against AhR.

#### **9. AhR and B cells**

The B lymphocyte is an important part of humoral immunity, which has high specificity against a variety of pathogens. After stimulation via an antigen receptor, activation of immature B cells leads to clonal expansion, antibody isotype conversion and differentiation into antibody-secreting plasma cells, thus producing strong immune reactions [119]. In the process of infection, mature B cells in the lymph nodes and secondary lymphoid organs undergo somatic hypermutation and produce plasma cells featuring elevated antigen affinity and unique effector function [120].

It seems that all B cells produce AhR, but specific subsets, e.g., marginal B cell and B1 B cell subsets, have higher levels than the others. Li and collaborators demonstrated that AhR contributes to the development of B lymphocytes, based on cord blood CD34 and feeder cells, which promote B cell development. Meanwhile, AhR induction inhibits the formation of early B cells and pro-B cells. AhR controls B cell differentiation by transcriptionally suppressing the early B cell genes EBF1 and PAX5 [121].

AhR, overtly induced after activation of B cells, has a critical function in regulating the fate of activated cells. Vaidyanathan and colleagues revealed AhR suppresses switch-like recombination by changing the amounts of activated cytidine deaminase. These authors showed that AhR suppresses B cell transformation into plasmablasts and plasma cells that secrete antibodies [122]. In addition, Villa et al. *The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

provided evidence of a role for AhR in B cells, revealing that AhR expression is increased after administration of IL-4 as well as B cell receptor engagement. Nevertheless, the proliferation of AhR-deficient B cells is decreased, and cells could not progress to the S-phase. Furthermore, AhR-deficient B cells could not compete with the decreased AhR+/+ B cell capability of reconstructing the empty host, and could not induce antigen-dependent proliferation in mice. Gene expression profile analysis showed that AhR excision downregulates cyclin O, an important gene controlling the cell cycle [123].

#### **10. AhR and dendritic cells (DCs)**

DCs are essential in controlling T cell response and regulating immune tolerance [124]. AhR regulates DC differentiation and function, thereby profoundly affecting T cell-dependent immune reactions. AhR also affects antigen presentation by DCs. Bone marrow derived DCs (BMDCs) exposed to TCDD show decreased CD11c amounts, but increased production of MHC-II, CD86, IL6 and TNFα [125]. Similar findings were reported in TCDD treated splenic DCs [126]. However, different results were observed by using the AhR agonist ITE. The expression of MHC-II and co-stimulatory molecules and the production of Th1 and Th17 polarization cytokines in splenic DCs were decreased by ITE stimulation of AhR.

Recent experiments in ovalbumin-induced asthma models provide additional evidence for the physiological regulation of AhR in DCs, with AhR-deficient mice exhibiting enhanced inflammatory reactions, elevated Th2 differentiation and higher DC MHC-II and CD86 amounts [127]. In addition, AhR signaling has been reported to regulate the activity of CD103+ /CD11b+ DCs during influenza virus infection, thereby reducing induction in protective CD8+ T cells [128]. Overall, this evidence confirms that AhR is a potential therapeutic target for regulating T cell responses in DC.

Multiple mechanisms are involved in AhR-associated regulation of DC function. AhR upregulates IDO 1 and 2 [129, 130], which catalyze the production of KYN, thus promoting the differentiation of FoxP3<sup>+</sup> Tregs [131]. Indeed, AhR-deficient DCs could not induce Treg differentiation and Th17 cell proliferation in culture. It is consistent with the immunosuppressive effect of AhR in DCs. Recently, it was reported that IDO expression is maintained by an autocrine loop involving AhR and KYN in tumor infiltrating tolerogenic DCs [132]. Additionally, AhR induction in DCs induces a retinoic acid-dependent enzymatic mechanism, thus promoting FoxP3<sup>+</sup> Treg differentiation and inhibiting effector T cells [133–137].

#### **11. Conclusions**

Studies evaluating AhR's functions in immune cell development, immune response modulation and immune tolerance have aroused great interest. Originally, AhR was considered a protein sensing environmental substances and regulating drug metabolism. Recently, the role of AhR in regulating normal physiological processes has attracted increasing attention. The organism must perceive and mount substantial responses to environmental changes. Indeed, AhR senses biochemical, chemical and physical environments. Combined with a small amount of high-affinity physiological ligands, including FICZ and ICZ, AhR plays a role in cell proliferation, differentiation and function.

Current evidence indicates that AhR has a critical function in host response to bacterial pathogens. It also overtly influences resistance to infections by

extracellular and intracellular bacteria. AhR is considered the best resistance factor for LM. It may have a new function in the innate immunity of LM infection, and AhR-deficient mice have increased sensitivity to LM. Activation of AhR can protect mice from the deadly attack of *S. pneumoniae*, inhibit bacterial growth and fight infection. AhR can also react with viral pathogens and parasitic infections. After infection by viruses and parasites, lack of AhR aggravates the host's inflammatory response. AhR regulates host's immune cells, confirming that AhR is a regulatory molecule with essential functions in the activation and induction of immune cells, e.g., T cells and inflammatory factors. Barrier organs are critical in immunity; specifically, large amounts of *ahr* are expressed in the intestine, which has a high potential for preventive and treatment interventions. AhR has a critical function in controlling the degree of inflammation in response to symbiotic microbiota and tissue destruction. Progress is being made in determining the molecular mechanisms by which AhR affects different cell types. To understand the complex process of AhR in immunity and antibacterial, to mitigate risks, and to develop novel treatment and prevention tools, more research is needed.

## **Abbreviations**


*The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*


## **Author details**

Lixing Huang1 \*, Rongchao He1 , Youyu Zhang2 and Qingpi Yan1

1 Fisheries College, Key Laboratory of Healthy Mariculture for the East China Sea, Ministry of Agriculture, Jimei University, Xiamen, Fujian, P.R. China

2 Institute of Electromagnetics and Acoustics, Xiamen University, Xiamen, Fujian, P.R. China

\*Address all correspondence to: lixinghuang@outlook.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Y. Zhang, L. Huang, Z. Zuo, Y. Chen, C. Wang, Phenanthrene exposure causes cardiac arrhythmia in embryonic zebrafish via perturbing calcium handling, Aquatic toxicology 142 (2013) 26-32.

[2] L. Huang, Z. Xi, C. Wang, Y. Zhang, Z. Yang, S. Zhang, Y. Chen, Z. Zuo, Phenanthrene exposure induces cardiac hypertrophy via reducing miR-133a expression by DNA methylation, Scientific reports 6 (2016) 20105.

[3] L. Huang, Z. Zuo, Y. Zhang, C. Wang, Toxicogenomic analysis in the combined effect of tributyltin and benzo [a] pyrene on the development of zebrafish embryos, Aquatic Toxicology 158 (2015) 157-164.

[4] L. Huang, D. Gao, Y. Zhang, C. Wang, Z. Zuo, Exposure to low dose benzo [a] pyrene during early life stages causes symptoms similar to cardiac hypertrophy in adult zebrafish, Journal of hazardous materials 276 (2014) 377-382.

[5] T.V. Beischlag, J.L. Morales, B.D. Hollingshead, G.H. Perdew, The aryl hydrocarbon receptor complex and the control of gene expression, Critical Reviews™ in Eukaryotic Gene Expression 18(3) (2008).

[6] T. Nakahama, A. Kimura, N.T. Nguyen, I. Chinen, H. Hanieh, K. Nohara, Y. Fujii-Kuriyama, T. Kishimoto, Aryl hydrocarbon receptor deficiency in T cells suppresses the development of collagen-induced arthritis, Proceedings of the National Academy of Sciences 108(34) (2011) 14222-14227.

[7] H. Liu, I. Ramachandran, D.I. Gabrilovich, Regulation of plasmacytoid dendritic cell development in mice by aryl hydrocarbon receptor, Immunology and cell biology 92(2) (2014) 200-203.

[8] F.J. Quintana, LeA (H) Rning selfcontrol, Cell research 24(10) (2014) 1155-1156.

[9] B. Stockinger, P.D. Meglio, M. Gialitakis, J.H. Duarte, The aryl hydrocarbon receptor: multitasking in the immune system, Annual review of immunology 32 (2014) 403-432.

[10] P.B. Busbee, M. Rouse, M. Nagarkatti, P.S. Nagarkatti, Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders, Nutrition reviews 71(6) (2013) 353-369.

[11] M. Colonna, AHR: making the keratinocytes thick skinned, Immunity 40(6) (2014) 863-864.

[12] M.A. Wheeler, V. Rothhammer, F.J. Quintana, Control of immune-mediated pathology via the aryl hydrocarbon receptor, Journal of Biological Chemistry 292(30) (2017) 12383-12389.

[13] C.F. Vogel, E.M. Khan, P.S. Leung, M.E. Gershwin, W.W. Chang, D. Wu, T. Haarmann-Stemmann, A. Hoffmann, M.S. Denison, Cross-talk between Aryl hydrocarbon receptor and the inflammatory response a role for nuclear factor-κB, Journal of Biological Chemistry 289(3) (2014) 1866-1875.

[14] A. Korecka, A. Dona, S. Lahiri, A.J. Tett, M. Al-Asmakh, V. Braniste, R. D'Arienzo, A. Abbaspour, N. Reichardt, Y. Fujii-Kuriyama, Bidirectional communication between the Aryl hydrocarbon Receptor (AhR) and the microbiome tunes host metabolism, npj Biofilms and Microbiomes 2(1) (2016) 1-10.

[15] J. Fu, S.V. Nogueira, V. van Drongelen, P. Coit, S. Ling, E.F. Rosloniec, A.H. Sawalha, J. Holoshitz, Shared epitope–aryl hydrocarbon receptor crosstalk underlies the

*The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

mechanism of gene–environment interaction in autoimmune arthritis, Proceedings of the National Academy of Sciences 115(18) (2018) 4755-4760.

[16] L. Huang, C. Wang, Y. Zhang, J. Li, Y. Zhong, Y. Zhou, Y. Chen, Z. Zuo, Benzo [a] pyrene exposure influences the cardiac development and the expression of cardiovascular relative genes in zebrafish (*Danio rerio*) embryos, Chemosphere 87(4) (2012) 369-375.

[17] Y. Zhang, C. Wang, L. Huang, R. Chen, Y. Chen, Z. Zuo, Low-level pyrene exposure causes cardiac toxicity in zebrafish (*Danio rerio*) embryos, Aquatic toxicology 114 (2012) 119-124.

[18] L. Huang, C. Wang, Y. Zhang, M. Wu, Z. Zuo, Phenanthrene causes ocular developmental toxicity in zebrafish embryos and the possible mechanisms involved, Journal of hazardous materials 261 (2013) 172-180.

[19] Y. Zhang, L. Huang, C. Wang, D. Gao, Z. Zuo, Phenanthrene exposure produces cardiac defects during embryo development of zebrafish (*Danio rerio*) through activation of MMP-9, Chemosphere 93(6) (2013) 1168-1175.

[20] F.L. Casado, K.P. Singh, T.A. Gasiewicz, The aryl hydrocarbon receptor: regulation of hematopoiesis and involvement in the progression of blood diseases, Blood Cells, Molecules, and Diseases 44(4) (2010) 199-206.

[21] F.J. Quintana, Regulation of central nervous system autoimmunity by the aryl hydrocarbon receptor, Seminars in immunopathology, Springer, 2013, pp. 627-635.

[22] Y. Zhang, L. Huang, Y. Zhao, T. Hu, Musk xylene induces malignant transformation of human liver cell line L02 via repressing the TGF-β signaling pathway, Chemosphere 168 (2017) 1506-1514.

[23] H. Sekine, J. Mimura, M. Oshima, H. Okawa, J. Kanno, K. Igarashi, F.J. Gonzalez, T. Ikuta, K. Kawajiri, Y. Fujii-Kuriyama, Hypersensitivity of aryl hydrocarbon receptor-deficient mice to lipopolysaccharide-induced septic shock, Molecular and cellular biology 29(24) (2009) 6391-6400.

[24] S. Mohammadi, F.S. Seyedhosseini, N. Behnampour, Y. Yazdani, Indole-3 carbinol induces G1 cell cycle arrest and apoptosis through aryl hydrocarbon receptor in THP-1 monocytic cell line, Journal of receptors and signal transduction 37(5) (2017) 506-514.

[25] B. Stockinger, K. Hirota, J. Duarte, M. Veldhoen, External influences on the immune system via activation of the aryl hydrocarbon receptor, Seminars in immunology, Elsevier, 2011, pp. 99-105.

[26] H. Hanieh, Toward understanding the role of aryl hydrocarbon receptor in the immune system: current progress and future trends, BioMed research international 2014 (2014).

[27] A. Kimura, H. Abe, S. Tsuruta, S. Chiba, Y. Fujii-Kuriyama, T. Sekiya, R. Morita, A. Yoshimura, Aryl hydrocarbon receptor protects against bacterial infection by promoting macrophage survival and reactive oxygen species production, International immunology 26(4) (2014) 209-220.

[28] P. Di Meglio, J.H. Duarte, H. Ahlfors, N.D. Owens, Y. Li, F. Villanova, I. Tosi, K. Hirota, F.O. Nestle, U. Mrowietz, Activation of the aryl hydrocarbon receptor dampens the severity of inflammatory skin conditions, Immunity 40(6) (2014) 989-1001.

[29] A. Bessede, M. Gargaro, M.T. Pallotta, D. Matino, G. Servillo, C. Brunacci, S. Bicciato, E.M. Mazza, A. Macchiarulo, C. Vacca, Aryl hydrocarbon receptor control of a

disease tolerance defence pathway, Nature 511(7508) (2014) 184-190.

[30] T.D. Hubbard, I.A. Murray, G.H. Perdew, Indole and tryptophan metabolism: endogenous and dietary routes to Ah receptor activation, Drug Metabolism and Disposition 43(10) (2015) 1522-1535.

[31] D. Liu, B. Ray, D.R. Neavin, J. Zhang, A.P. Athreya, J.M. Biernacka, W.V. Bobo, D.K. Hall-Flavin, M.K. Skime, H. Zhu, Beta-defensin 1, aryl hydrocarbon receptor and plasma kynurenine in major depressive disorder: metabolomics-informed genomics, Translational psychiatry 8(1) (2018) 1-13.

[32] D.W. Nebert, J.R. Robinson, A. Niwa, K. Kumari, A.P. Poland, Genetic expression of aryl hydrocarbon hydroxylase activity in the mouse, Journal of cellular physiology 85(S1) (1975) 393-414.

[33] W.F. Greenlee, A. Poland, Nuclear uptake of 2, 3, 7, 8-tetrachlorodibenzop-dioxin in C57BL/6J and DBA/2J mice. Role of the hepatic cytosol receptor protein, Journal of Biological Chemistry 254(19) (1979) 9814-9821.

[34] M.B. Black, R.A. Budinsky, A. Dombkowski, D. Cukovic, E.L. LeCluyse, S.S. Ferguson, R.S. Thomas, J.C. Rowlands, Cross-species comparisons of transcriptomic alterations in human and rat primary hepatocytes exposed to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, Toxicological Sciences 127(1) (2012) 199-215.

[35] K.W. Schulte, E. Green, A. Wilz, M. Platten, O. Daumke, Structural basis for aryl hydrocarbon receptor-mediated gene activation, Structure 25(7) (2017) 1025-1033. e3.

[36] M.S. Denison, S.R. Nagy, Activation of the aryl hydrocarbon receptor by

structurally diverse exogenous and endogenous chemicals, Annual review of pharmacology and toxicology 43(1) (2003) 309-334.

[37] S. Luecke-Johansson, M. Gralla, H. Rundqvist, J.C. Ho, R.S. Johnson, K. Gradin, L. Poellinger, A molecular mechanism to switch the aryl hydrocarbon receptor from a transcription factor to an E3 ubiquitin ligase, Molecular and cellular biology 37(13) (2017).

[38] J. Mimura, M. Ema, K. Sogawa, Y. Fujii-Kuriyama, Identification of a novel mechanism of regulation of Ah (dioxin) receptor function, Genes & development 13(1) (1999) 20-25.

[39] T. Baba, J. Mimura, K. Gradin, A. Kuroiwa, T. Watanabe, Y. Matsuda, J. Inazawa, K. Sogawa, Y. Fujii-Kuriyama, Structure and expression of the Ah receptor repressor gene, Journal of Biological Chemistry 276(35) (2001) 33101-33110.

[40] L.Z. Shi, N.G. Faith, Y. Nakayama, M. Suresh, H. Steinberg, C.J. Czuprynski, The aryl hydrocarbon receptor is required for optimal resistance to *Listeria monocytogenes* infection in mice, The Journal of Immunology 179(10) (2007) 6952-6962.

[41] B.A. Vorderstrasse, B.P. Lawrence, Protection against lethal challenge with *Streptococcus pneumoniae* is conferred by aryl hydrocarbon receptor activation but is not associated with an enhanced inflammatory response, Infection and immunity 74(10) (2006) 5679-5686.

[42] L. Huang, Y. Zuo, Q. Jiang, Y. Su, Y. Qin, X. Xu, L. Zhao, Q. Yan, A metabolomic investigation into the temperature-dependent virulence of Pseudomonas plecoglossicida from large yellow croaker (*Pseudosciaena crocea*), Journal of fish diseases 42(3) (2019) 431-446.

*The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

[43] R. He, L. Zhao, X. Xu, W. Zheng, J. Zhang, J. Zhang, Q. Yan, L. Huang, Aryl hydrocarbon receptor is required for immune response in *Epinephelus coioides* and *Danio rerio* infected by Pseudomonas plecoglossicida, Fish & Shellfish Immunology 97 (2020) 564-570.

[44] L. Huang, W. Qi, Y. Zuo, S.A. Alias, W. Xu, The immune response of a warm water fish orange-spotted grouper (*Epinephelus coioides*) infected with a typical cold water bacterial pathogen *Aeromonas salmonicida* is AhR dependent, Developmental & Comparative Immunology 113 (2020) 103779.

[45] W. Huai, R. Zhao, H. Song, J. Zhao, L. Zhang, L. Zhang, C. Gao, L. Han, W. Zhao, Aryl hydrocarbon receptor negatively regulates NLRP3 inflammasome activity by inhibiting NLRP3 transcription, Nature communications 5(1) (2014) 1-9.

[46] T. Veiga-Parga, A. Suryawanshi, B.T. Rouse, Controlling viral immunoinflammatory lesions by modulating aryl hydrocarbon receptor signaling, PLoS Pathog 7(12) (2011) e1002427.

[47] T.K. Warren, K.A. Mitchell, B.P. Lawrence, Exposure to 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) suppresses the humoral and cellmediated immune responses to influenza A virus without affecting cytolytic activity in the lung, Toxicological Sciences 56(1) (2000) 114-123.

[48] S. Teske, A. Bohn, J. Regal, J. Neumiller, B. Lawrence, Exploring mechanisms that underlie aryl hydrocarbon receptor-mediated increases in pulmonary neutrophilia and diminished host resistance to influenza A virus, Am J Physiol Lung Cell Mol Physiol 289 (2005) 111-124.

[49] T. Mauad, L.A. Hajjar, G.D. Callegari, L.F. da Silva, D. Schout, F.R. Galas, V.A. Alves, D.M. Malheiros, J.O. Auler Jr, A.F. Ferreira, Lung pathology in fatal novel human influenza A (H1N1) infection, American journal of respiratory and critical care medicine 181(1) (2010) 72-79.

[50] L.A. Perrone, J.K. Plowden, A. García-Sastre, J.M. Katz, T.M. Tumpey, H5N1 and 1918 pandemic influenza virus infection results in early and excessive infiltration of macrophages and neutrophils in the lungs of mice, PLoS Pathog 4(8) (2008) e1000115.

[51] Zelante T, Iannitti RG, Cunha C, De Luca A, Giovannini G, Pieraccini G, Zecchi R, D'Angelo C, Massi-Benedetti C, Fallarino F, Carvalho A, Puccetti P, Romani L. Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22, Immunity 39(2) 372-85.

[52] F. Dallaire, É. Dewailly, C. Vézina, G. Muckle, J.-P. Weber, S. Bruneau, P. Ayotte, Effect of prenatal exposure to polychlorinated biphenyls on incidence of acute respiratory infections in preschool Inuit children, Environmental health perspectives 114(8) (2006) 1301-1305.

[53] S.B. Stølevik, U.C. Nygaard, E. Namork, M. Haugen, H.E. Kvalem, H.M. Meltzer, J. Alexander, J.H. van Delft, H. van Loveren, M. Løvik, Prenatal exposure to polychlorinated biphenyls and dioxins is associated with increased risk of wheeze and infections in infants, Food and chemical toxicology 49(8) (2011) 1843-1848.

[54] R.L. Van Den Heuvel, G. Koppen, J.A. Staessen, E.D. Hond, G. Verheyen, T.S. Nawrot, H.A. Roels, R. Vlietinck, G.E. Schoeters, Immunologic biomarkers in relation to exposure markers of PCBs and dioxins in Flemish adolescents (Belgium), Environmental Health Perspectives 110(6) (2002) 595-600.

[55] H. Neff-LaFord, S. Teske, T.P. Bushnell, B.P. Lawrence, Aryl hydrocarbon receptor activation during influenza virus infection unveils a novel pathway of IFN-γ production by phagocytic cells, The Journal of Immunology 179(1) (2007) 247-255.

[56] S. Wagage, B. John, B.L. Krock, A.O.H. Hall, L.M. Randall, C.L. Karp, M.C. Simon, C.A. Hunter, The aryl hydrocarbon receptor promotes IL-10 production by NK cells, The Journal of Immunology 192(4) (2014) 1661-1670.

[57] Y. Sanchez, J. de Dios Rosado, L. Vega, G. Elizondo, E. Estrada-Muñiz, R. Saavedra, I. Juárez, M. Rodríguez-Sosa, The unexpected role for the aryl hydrocarbon receptor on susceptibility to experimental toxoplasmosis, Journal of Biomedicine and Biotechnology 2010 (2010).

[58] J. Stange, M. Veldhoen, The aryl hydrocarbon receptor in innate T cell immunity, Seminars in immunopathology, Springer, 2013, pp. 645-655.

[59] H.U. Lee, Z.E. McPherson, B. Tan, A. Korecka, S. Pettersson, Hostmicrobiome interactions: the aryl hydrocarbon receptor and the central nervous system, Journal of molecular medicine 95(1) (2017) 29-39.

[60] C. Goudot, A. Coillard, A.-C. Villani, P. Gueguen, A. Cros, S. Sarkizova, T.-L. Tang-Huau, M. Bohec, S. Baulande, N. Hacohen, Aryl hydrocarbon receptor controls monocyte differentiation into dendritic cells versus macrophages, Immunity 47(3) (2017) 582-596. e6.

[61] J. Qiu, X. Guo, E.C. Zong-ming, L. He, G.F. Sonnenberg, D. Artis, Y.-X. Fu, L. Zhou, Group 3 innate lymphoid cells inhibit T-cell-mediated intestinal inflammation through aryl hydrocarbon receptor signaling and regulation of

microflora, Immunity 39(2) (2013) 386-399.

[62] T. Zelante, R.G. Iannitti, C. Cunha, A. De Luca, G. Giovannini, G. Pieraccini, R. Zecchi, C. D'Angelo, C. Massi-Benedetti, F. Fallarino, Tryptophan catabolites from microbiota engage aryl hydrocarbon receptor and balance mucosal reactivity via interleukin-22, Immunity 39(2) (2013) 372-385.

[63] A. Rannug, U. Rannug, H. Rosenkranz, L. Winqvist, R. Westerholm, E. Agurell, A. Grafström, Certain photooxidized derivatives of tryptophan bind with very high affinity to the Ah receptor and are likely to be endogenous signal substances, Journal of Biological Chemistry 262(32) (1987) 15422-15427.

[64] S. Chmill, S. Kadow, M. Winter, H. Weighardt, C. Esser, 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin impairs stable establishment of oral tolerance in mice, Toxicological Sciences 118(1) (2010) 98-107.

[65] Y. Li, S. Innocentin, D.R. Withers, N.A. Roberts, A.R. Gallagher, E.F. Grigorieva, C. Wilhelm, M. Veldhoen, Exogenous stimuli maintain intraepithelial lymphocytes via aryl hydrocarbon receptor activation, Cell 147(3) (2011) 629-640.

[66] H. Spits, D. Artis, M. Colonna, A. Diefenbach, J.P. Di Santo, G. Eberl, S. Koyasu, R.M. Locksley, A.N. McKenzie, R.E. Mebius, Innate lymphoid cells—a proposal for uniform nomenclature, Nature reviews immunology 13(2) (2013) 145-149.

[67] E.A. Kiss, C. Vonarbourg, S. Kopfmann, E. Hobeika, D. Finke, C. Esser, A. Diefenbach, Natural aryl hydrocarbon receptor ligands control organogenesis of intestinal lymphoid follicles, Science 334(6062) (2011) 1561-1565.

*The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

[68] J.S. Lee, M. Cella, K.G. McDonald, C. Garlanda, G.D. Kennedy, M. Nukaya, A. Mantovani, R. Kopan, C.A. Bradfield, R.D. Newberry, AHR drives the development of gut ILC22 cells and postnatal lymphoid tissues via pathways dependent on and independent of Notch, Nature immunology 13(2) (2012) 144-151.

[69] A. Diefenbach, Innate lymphoid cells in the defense against infections, European Journal of Microbiology and Immunology 3(3) (2013) 143-151.

[70] L. Cervantes-Barragan, J.N. Chai, M.D. Tianero, B. Di Luccia, P.P. Ahern, J. Merriman, V.S. Cortez, M.G. Caparon, M.S. Donia, S. Gilfillan, Lactobacillus reuteri induces gut intraepithelial CD4+ CD8αα+ T cells, Science 357(6353) (2017) 806-810.

[71] L.A. Zenewicz, X. Yin, G. Wang, E. Elinav, L. Hao, L. Zhao, R.A. Flavell, IL-22 deficiency alters colonic microbiota to be transmissible and colitogenic, The Journal of Immunology 190(10) (2013) 5306-5312.

[72] X. Guo, Y. Liang, Y. Zhang, A. Lasorella, B.L. Kee, Y.-X. Fu, Innate lymphoid cells control early colonization resistance against intestinal pathogens through ID2-dependent regulation of the microbiota, Immunity 42(4) (2015) 731-743.

[73] R. Basu, D.B. O'Quinn, D.J. Silberger, T.R. Schoeb, L. Fouser, W. Ouyang, R.D. Hatton, C.T. Weaver, Th22 cells are an important source of IL-22 for host protection against enteropathogenic bacteria, Immunity 37(6) (2012) 1061-1075.

[74] A. Yeste, I.D. Mascanfroni, M. Nadeau, E.J. Burns, A.-M. Tukpah, A. Santiago, C. Wu, B. Patel, D. Kumar, F.J. Quintana, IL-21 induces IL-22 production in CD4+ T cells, Nature communications 5(1) (2014) 1-13.

[75] C. Schiering, E. Wincent, A. Metidji, A. Iseppon, Y. Li, A.J. Potocnik, S. Omenetti, C.J. Henderson, C.R. Wolf, D.W. Nebert, Feedback control of AHR signalling regulates intestinal immunity, Nature 542(7640) (2017) 242-245.

[76] N.I. Kerkvliet, B. Smith, L.B. STEPPAN, J. Youngberg, M. Henderson, D. Buhler, Role of the Ah locus in suppression of cytotoxic T lymphocyte activity by halogenated aromatic hydrocarbons (PCBs and TCDD): structure-activity relationships and effects in C57BI/6 mice congenic at the Ah locus, Toxicological Sciences 14(3) (1990) 532-541.

[77] C.J. Funatake, N.B. Marshall, L.B. Steppan, D.V. Mourich, N.I. Kerkvliet, Cutting edge: activation of the aryl hydrocarbon receptor by 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin generates a population of CD4+ CD25+ cells with characteristics of regulatory T cells, The Journal of Immunology 175(7) (2005) 4184-4188.

[78] N.I. Kerkvliet, D.M. Shepherd, L. Baecher-Steppan, T lymphocytes are direct, aryl hydrocarbon receptor (AhR)-dependent targets of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD): AhR expression in both CD4+ and CD8+ T cells is necessary for full suppression of a cytotoxic T lymphocyte response by TCDD, Toxicology and applied pharmacology 185(2) (2002) 146-152.

[79] N.B. Marshall, W.R. Vorachek, L.B. Steppan, D.V. Mourich, N.I. Kerkvliet, Functional characterization and gene expression analysis of CD4+ CD25+ regulatory T cells generated in mice treated with 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin, The Journal of Immunology 181(4) (2008) 2382-2391.

[80] F.J. Quintana, A.S. Basso, A.H. Iglesias, T. Korn, M.F. Farez, E. Bettelli, M. Caccamo, M. Oukka, H.L. Weiner, Control of T reg and TH 17 cell differentiation by the aryl hydrocarbon receptor, Nature 453(7191) (2008) 65-71.

[81] S. Rutz, R. Noubade, C. Eidenschenk, N. Ota, W. Zeng, Y. Zheng, J. Hackney, J. Ding, H. Singh, W. Ouyang, Transcription factor c-Maf mediates the TGF-β-dependent suppression of IL-22 production in TH 17 cells, Nature immunology 12(12) (2011) 1238-1245.

[82] M. Veldhoen, K. Hirota, A.M. Westendorf, J. Buer, L. Dumoutier, J.-C. Renauld, B. Stockinger, The aryl hydrocarbon receptor links TH 17-cell-mediated autoimmunity to environmental toxins, Nature 453(7191) (2008) 106-109.

[83] M. Veldhoen, K. Hirota, J. Christensen, A. O'Garra, B. Stockinger, Natural agonists for aryl hydrocarbon receptor in culture medium are essential for optimal differentiation of Th17 T cells, Journal of Experimental Medicine 206(1) (2009) 43-49.

[84] J.D. Fontenot, M.A. Gavin, A.Y. Rudensky, Foxp3 programs the development and function of CD4+ CD25+ regulatory T cells, Nature immunology 4(4) (2003) 330-336.

[85] S. Hori, T. Nomura, S. Sakaguchi, Control of regulatory T cell development by the transcription factor Foxp3, Science 299(5609) (2003) 1057-1061.

[86] H. Groux, A. O'Garra, M. Bigler, M. Rouleau, S. Antonenko, J.E. De Vries, M.G. Roncarolo, A CD4+ T-cell subset inhibits antigen-specific T-cell responses and prevents colitis, Nature 389(6652) (1997) 737-742.

[87] J.A. Goettel, R. Gandhi, J.E. Kenison, A. Yeste, G. Murugaiyan, S. Sambanthamoorthy, A.E. Griffith, B. Patel, D.S. Shouval, H.L. Weiner, AHR activation is protective against colitis driven by T cells in humanized mice, Cell reports 17(5) (2016) 1318-1329.

[88] J. Kaye, V. Piryatinsky, T. Birnberg, T. Hingaly, E. Raymond, R. Kashi, E. Amit-Romach, I.S. Caballero, F. Towfic, M.A. Ator, Laquinimod arrests experimental autoimmune encephalomyelitis by activating the aryl hydrocarbon receptor, Proceedings of the National Academy of Sciences 113(41) (2016) E6145-E6152.

[89] N.I. Kerkvliet, L.B. Steppan, W. Vorachek, S. Oda, D. Farrer, C.P. Wong, D. Pham, D.V. Mourich, Activation of aryl hydrocarbon receptor by TCDD prevents diabetes in NOD mice and increases Foxp3+ T cells in pancreatic lymph nodes, Immunotherapy 1(4) (2009) 539-547.

[90] J.D. Mezrich, J.H. Fechner, X. Zhang, B.P. Johnson, W.J. Burlingham, C.A. Bradfield, An interaction between kynurenine and the aryl hydrocarbon receptor can generate regulatory T cells, The Journal of Immunology 185(6) (2010) 3190-3198.

[91] N.P. Singh, U.P. Singh, M. Rouse, J. Zhang, S. Chatterjee, P.S. Nagarkatti, M. Nagarkatti, Dietary indoles suppress delayed-type hypersensitivity by inducing a switch from proinflammatory Th17 cells to anti-inflammatory regulatory T cells through regulation of microRNA, The Journal of Immunology 196(3) (2016) 1108-1122.

[92] N.P. Singh, U.P. Singh, B. Singh, R.L. Price, M. Nagarkatti, P.S. Nagarkatti, Activation of aryl hydrocarbon receptor (AhR) leads to reciprocal epigenetic regulation of FoxP3 and IL-17 expression and amelioration of experimental colitis, PloS one 6(8) (2011) e23522.

[93] R. Gandhi, D. Kumar, E.J. Burns, M. Nadeau, B. Dake, A. Laroni, D. Kozoriz, *The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

H.L. Weiner, F.J. Quintana, Activation of the aryl hydrocarbon receptor induces human type 1 regulatory T cell–like and Foxp3+ regulatory T cells, Nature immunology 11(9) (2010) 846-853.

[94] A. Kimura, T. Naka, K. Nohara, Y. Fujii-Kuriyama, T. Kishimoto, Aryl hydrocarbon receptor regulates Stat1 activation and participates in the development of Th17 cells, Proceedings of the National Academy of Sciences 105(28) (2008) 9721-9726.

[95] T.A. Dant, K.L. Lin, D.W. Bruce, S.A. Montgomery, O.V. Kolupaev, H. Bommiasamy, L.M. Bixby, J.T. Woosley, K.P. McKinnon, F.J. Gonzalez, T-cell expression of AhR inhibits the maintenance of pTreg cells in the gastrointestinal tract in acute GVHD, Blood, The Journal of the American Society of Hematology 130(3) (2017) 348-359.

[96] A. Awasthi, Y. Carrier, J.P. Peron, E. Bettelli, M. Kamanaka, R.A. Flavell, V.K. Kuchroo, M. Oukka, H.L. Weiner, A dominant function for interleukin 27 in generating interleukin 10–producing anti-inflammatory T cells, Nature immunology 8(12) (2007) 1380-1389.

[97] D.C. Fitzgerald, B. Ciric, T. Touil, H. Harle, J. Grammatikopolou, J.D. Sarma, B. Gran, G.-X. Zhang, A. Rostami, Suppressive effect of IL-27 on encephalitogenic Th17 cells and the effector phase of experimental autoimmune encephalomyelitis, The Journal of Immunology 179(5) (2007) 3268-3275.

[98] J.S. Stumhofer, A. Laurence, E.H. Wilson, E. Huang, C.M. Tato, L.M. Johnson, A.V. Villarino, Q. Huang, A. Yoshimura, D. Sehy, Interleukin 27 negatively regulates the development of interleukin 17–producing T helper cells during chronic inflammation of the central nervous system, Nature immunology 7(9) (2006) 937-945.

[99] R. Spolski, H.-P. Kim, W. Zhu, D.E. Levy, W.J. Leonard, IL-21 mediates suppressive effects via its induction of IL-10, The Journal of Immunology 182(5) (2009) 2859-2867.

[100] L. Apetoh, F.J. Quintana, C. Pot, N. Joller, S. Xiao, D. Kumar, E.J. Burns, D.H. Sherr, H.L. Weiner, V.K. Kuchroo, The aryl hydrocarbon receptor interacts with c-Maf to promote the differentiation of type 1 regulatory T cells induced by IL-27, Nature immunology 11(9) (2010) 854-861.

[101] I.D. Mascanfroni, M.C. Takenaka, A. Yeste, B. Patel, Y. Wu, J.E. Kenison, S. Siddiqui, A.S. Basso, L.E. Otterbein, D.M. Pardoll, Metabolic control of type 1 regulatory T cell differentiation by AHR and HIF1-α, Nature medicine 21(6) (2015) 638-646.

[102] H.Y. Wu, F.J. Quintana, A.P. Da Cunha, B.T. Dake, T. Koeglsperger, S.C. Starossom, H.L. Weiner, In vivo induction of Tr1 cells via mucosal dendritic cells and AHR signaling, PloS one 6(8) (2011) e23618.

[103] M.C. Takenaka, S. Robson, F.J. Quintana, Regulation of the T cell response by CD39, Trends in immunology 37(7) (2016) 427-439.

[104] I.I. Ivanov, B.S. McKenzie, L. Zhou, C.E. Tadokoro, A. Lepelley, J.J. Lafaille, D.J. Cua, D.R. Littman, The orphan nuclear receptor RORγt directs the differentiation program of proinflammatory IL-17+ T helper cells, Cell 126(6) (2006) 1121-1133.

[105] T. Korn, E. Bettelli, M. Oukka, V.K. Kuchroo, IL-17 and Th17 Cells, Annual review of immunology 27 (2009) 485-517.

[106] T. Korn, J. Reddy, W. Gao, E. Bettelli, A. Awasthi, T.R. Petersen, B.T. Bäckström, R.A. Sobel, K.W. Wucherpfennig, T.B. Strom, Myelinspecific regulatory T cells accumulate in the CNS but fail to control autoimmune inflammation, Nature medicine 13(4) (2007) 423-431.

[107] R. Nurieva, X.O. Yang, G. Martinez, Y. Zhang, A.D. Panopoulos, L. Ma, K. Schluns, Q. Tian, S.S. Watowich, A.M. Jetten, Essential autocrine regulation by IL-21 in the generation of inflammatory T cells, Nature 448(7152) (2007) 480-483.

[108] L. Vikström Bergander, W. Cai, B. Klocke, M. Seifert, I. Pongratz, Tryptamine serves as a proligand of the AhR transcriptional pathway whose activation is dependent of monoamine oxidases, Molecular Endocrinology 26(9) (2012) 1542-1551.

[109] T. Duhen, R. Geiger, D. Jarrossay, A. Lanzavecchia, F. Sallusto, Production of interleukin 22 but not interleukin 17 by a subset of human skin-homing memory T cells, Nature immunology 10(8) (2009) 857-863.

[110] J.M. Ramirez, N.C. Brembilla, O. Sorg, R. Chicheportiche, T. Matthes, J.M. Dayer, J.H. Saurat, E. Roosnek, C. Chizzolini, Activation of the aryl hydrocarbon receptor reveals distinct requirements for IL-22 and IL-17 production by human T helper cells, European journal of immunology 40(9) (2010) 2450-2459.

[111] S. Trifari, C.D. Kaplan, E.H. Tran, N.K. Crellin, H. Spits, Identification of a human helper T cell population that has abundant production of interleukin 22 and is distinct from TH-17, TH 1 and TH 2 cells, Nature immunology 10(8) (2009) 864-871.

[112] B.P. Lawrence, A.D. Roberts, J.J. Neumiller, J.A. Cundiff, D.L. Woodland, Aryl hydrocarbon receptor activation impairs the priming but not the recall of influenza virus-specific CD8+ T cells in the lung, The Journal of Immunology 177(9) (2006) 5819-5828.

[113] B. Winans, A. Nagari, M. Chae, C.M. Post, C.-I. Ko, A. Puga, W.L. Kraus, B.P. Lawrence, Linking the aryl hydrocarbon receptor with altered DNA methylation patterns and developmentally induced aberrant antiviral CD8+ T cell responses, The Journal of Immunology 194(9) (2015) 4446-4457.

[114] A.C. Hayday, γδ T cells and the lymphoid stress-surveillance response, Immunity 31(2) (2009) 184-196.

[115] S. Kadow, B. Jux, S.P. Zahner, B. Wingerath, S. Chmill, B.E. Clausen, J. Hengstler, C. Esser, Aryl hydrocarbon receptor is critical for homeostasis of invariant γδ T cells in the murine epidermis, The Journal of Immunology 187(6) (2011) 3104-3110.

[116] D. Cibrian, M.L. Saiz, H. de la Fuente, R. Sánchez-Díaz, O. Moreno-Gonzalo, I. Jorge, A. Ferrarini, J. Vázquez, C. Punzón, M. Fresno, CD69 controls the uptake of L-tryptophan through LAT1-CD98 and AhRdependent secretion of IL-22 in psoriasis, Nature immunology 17(8) (2016) 985-996.

[117] B. Martin, K. Hirota, D.J. Cua, B. Stockinger, M. Veldhoen, Interleukin-17-producing γδ T cells selectively expand in response to pathogen products and environmental signals, Immunity 31(2) (2009) 321-330.

[118] N. Yosef, A. Regev, Writ large: genomic dissection of the effect of cellular environment on immune response, Science 354(6308) (2016) 64-68.

[119] J.J. Taylor, M.K. Jenkins, K.A. Pape, Heterogeneity in the differentiation and function of memory B cells, Trends in immunology 33(12) (2012) 590-597.

[120] C. Boboila, F.W. Alt, B. Schwer, Classical and alternative end-joining *The Role of the Aryl Hydrocarbon Receptor (AhR) in the Immune Response against Microbial… DOI: http://dx.doi.org/10.5772/intechopen.96526*

pathways for repair of lymphocytespecific and general DNA double-strand breaks, Advances in immunology, Elsevier2012, pp. 1-49.

[121] J. Li, S. Bhattacharya, J. Zhou, A.S. Phadnis-Moghe, R.B. Crawford, N.E. Kaminski, Aryl hydrocarbon receptor activation suppresses EBF1 and PAX5 and impairs human B lymphopoiesis, The Journal of Immunology 199(10) (2017) 3504-3515.

[122] B. Vaidyanathan, A. Chaudhry, W.T. Yewdell, D. Angeletti, W.-F. Yen, A.K. Wheatley, C.A. Bradfield, A.B. McDermott, J.W. Yewdell, A.Y. Rudensky, The aryl hydrocarbon receptor controls cell-fate decisions in B cells, Journal of Experimental Medicine 214(1) (2017) 197-208.

[123] M. Villa, M. Gialitakis, M. Tolaini, H. Ahlfors, C.J. Henderson, C.R. Wolf, R. Brink, B. Stockinger, Aryl hydrocarbon receptor is required for optimal B-cell proliferation, The EMBO journal 36(1) (2017) 116-128.

[124] P. Guermonprez, J. Valladeau, L. Zitvogel, C. Théry, S. Amigorena, Antigen presentation and T cell stimulation by dendritic cells, Annual review of immunology 20(1) (2002) 621-667.

[125] J. Bankoti, B. Rase, T. Simones, D.M. Shepherd, Functional and phenotypic effects of AhR activation in inflammatory dendritic cells, Toxicology and applied pharmacology 246(1-2) (2010) 18-28.

[126] J. Bankoti, A. Burnett, S. Navarro, A.K. Miller, B. Rase, D.M. Shepherd, Effects of TCDD on the fate of naive dendritic cells, Toxicological Sciences 115(2) (2010) 422-434.

[127] T.H. Thatcher, M.A. Williams, S.J. Pollock, C.E. McCarthy, S.H. Lacy, R.P. Phipps, P.J. Sime, Endogenous

ligands of the aryl hydrocarbon receptor regulate lung dendritic cell function, Immunology 147(1) (2016) 41-54.

[128] G.B. Jin, B. Winans, K.C. Martin, B.P. Lawrence, New insights into the role of the aryl hydrocarbon receptor in the function of CD11c+ cells during respiratory viral infection, European journal of immunology 44(6) (2014) 1685-1698.

[129] N.T. Nguyen, A. Kimura, T. Nakahama, I. Chinen, K. Masuda, K. Nohara, Y. Fujii-Kuriyama, T. Kishimoto, Aryl hydrocarbon receptor negatively regulates dendritic cell immunogenicity via a kynureninedependent mechanism, Proceedings of the National Academy of Sciences 107(46) (2010) 19961-19966.

[130] C.F. Vogel, S.R. Goth, B. Dong, I.N. Pessah, F. Matsumura, Aryl hydrocarbon receptor signaling mediates expression of indoleamine 2, 3-dioxygenase, Biochemical and biophysical research communications 375(3) (2008) 331-335.

[131] F. Fallarino, U. Grohmann, S. You, B.C. McGrath, D.R. Cavener, C. Vacca, C. Orabona, R. Bianchi, M.L. Belladonna, C. Volpi, The combined effects of tryptophan starvation and tryptophan catabolites down-regulate T cell receptor ζ-chain and induce a regulatory phenotype in naive T cells, The Journal of Immunology 176(11) (2006) 6752-6761.

[132] Q. Li, J.L. Harden, C.D. Anderson, N.K. Egilmez, Tolerogenic Phenotype of IFN-γ–Induced IDO+ Dendritic Cells Is Maintained via an Autocrine IDO–Kynurenine/AhR–IDO Loop, The Journal of Immunology 197(3) (2016) 962-970.

[133] J.L. Coombes, K.R. Siddiqui, C.V. Arancibia-Cárcamo, J. Hall, C.-M. Sun, Y. Belkaid, F. Powrie, A functionally

#### *Antimicrobial Immune Response*

specialized population of mucosal CD103+ DCs induces Foxp3+ regulatory T cells via a TGF-β–and retinoic acid–dependent mechanism, Journal of Experimental Medicine 204(8) (2007) 1757-1764.

[134] D. Mucida, Y. Park, G. Kim, O. Turovskaya, I. Scott, M. Kronenberg, H. Cheroutre, Reciprocal TH17 and regulatory T cell differentiation mediated by retinoic acid, science 317(5835) (2007) 256-260.

[135] J. Nolting, C. Daniel, S. Reuter, C. Stuelten, P. Li, H. Sucov, B.-G. Kim, J.J. Letterio, K. Kretschmer, H.-J. Kim, Retinoic acid can enhance conversion of naive into regulatory T cells independently of secreted cytokines, Journal of Experimental Medicine 206(10) (2009) 2131-2139.

[136] K. Pino-Lagos, Y. Guo, C. Brown, M.P. Alexander, R. Elgueta, K.A. Bennett, V. De Vries, E. Nowak, R. Blomhoff, S. Sockanathan, A retinoic acid–dependent checkpoint in the development of CD4+ T cell–mediated immunity, Journal of Experimental Medicine 208(9) (2011) 1767-1775.

[137] C.-M. Sun, J.A. Hall, R.B. Blank, N. Bouladoux, M. Oukka, J.R. Mora, Y. Belkaid, Small intestine lamina propria dendritic cells promote de novo generation of Foxp3 T reg cells via retinoic acid, The Journal of experimental medicine 204(8) (2007) 1775-1785.

## **Chapter 5**

## Metabotropic Receptors 4 and the Immune Responses

*Zhuoya Wan and Song Li*

### **Abstract**

Neurotransmitters (NTs) have recently received increasing appreciation as important immune modulators. The immune cells express receptors for many classes of NTs and the communication between NTs and their receptors establish neuro-immune interactions for regulating effective immune response in both central nervous system (CNS) and peripheral tissues. Metabotropic Glutamate Receptor 4 (mGluR4) is expressed at high level in CNS and plays a role in various physiological and pathophysiological processes in CNS. Recently, mGluR4 has been reported to be expressed on immune cells and have an impact on regulating the immune system. This chapter summarized the works associated with the immunogenic function of mGluR4 and its potential underlying mechanisms.

**Keywords:** metabotropic glutamate receptor (mGluR4), immune response, peripheral tissues, central nervous system (CNS), cancer, autoimmune diseases

### **1. Introduction**

Neurotransmitters (NTs) have recently received increasing appreciation as important immune modulators. The immune cells express receptors for many classes of NTs and the communication between NTs and their receptors establish neuro-immune interactions for regulating effective immune response in both CNS and peripheral tissues [1]. Interestingly, the role of NTs is very complicated and the same NTs can even exert opposing effects for promoting or inhibiting tissue immunity in different contexts [2–6].

Studies of the NTs and their receptors in modulating immunity are limited and therein are important areas of investigations. L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS [7]. It acts via two classes of receptors, ligand gated ion channels (ionotropic receptors (iGluRs))-regulating rapid responses upon activation, and G-protein coupled (metabotropic) receptorsmodulating signal transduction cascades. Eight different types of mGluRs, mGluR1 to mGluR8 are divided into groups I, II, and III on the basis of their intracellular signal transduction mechanisms, agonist pharmacology, and sequence homologies (see **Figure 1**) [8]. Group I includes mGluR1 and mGluR5, coupled to Gq protein; group II includes mGluR2 and mGluR3, coupled to Gi and Go proteins; group III includes mGluR4, mGluR6, mGluR7 and mGluR8, coupled to Gi and Go proteins in heterologous expression systems.

mGluR4 is expressed at high levels in CNS and plays a role in various physiological and pathophysiological processes in CNS [9, 10], such as learning, memory, and cognitive impairment. In addition, growing evidence indicates that mGluRs

**Figure 1.**

*The summary of mGluRs families. mGluRs are classified into three families: group I, group II, and group III. In the CNS, activation of mGluRs from group I leads to the induction of phosphoinositide hydrolysis with formation of inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG). The activation of groups II and III receptors induce a decrease on the intracellular levels of cyclic adenosine monophosphate (cAMP) [7].*

are expressed in the peripheral such as thymus and lymphocytes [11]. These results suggest a potential role of mGluR4 in immune regulation. In this chapter, we summarized the association of mGluR4 with immune responses and its role in different diseases. The potential of mGluR4 as a novel therapeutic target in immune-related diseases was also discussed.

## **2. Expression of mGluR4 on immune cells**

Clinical data indicated that elevated plasma concentrations of Glu are associated with immune deficiency [12, 13]. In addition, in vitro assays showed that high concentration of Glu (>100 uM) can inhibit mitogen-induced T-cell proliferation [12, 13]. Therefore, it is not surprising that immune cells express mGluRs. It has been proposed that mGluRs can mediate an emergency mechanism once high levels of Glu are reached.

Using immunostaining and Western blot analysis, Rezzani et al. observed the expression of mGluR4 in rat thymic cells [14]. The expression of mGluR4 was abundant in dendritic cells (DCs) and lymphocytes of the thymic medulla but was weak in lymphocytes of the cortex. It is interesting to note that a rapid inhibition on the expression of mGluR4 was induced in the rat thymus after treatment with cyclosporine (an immunosuppressant). The mGluR4 expression reached undetectable levels after a longer treatment regimen of cyclosporine.

Other evidence also pointed out that the expression of mGluRs is not exclusive to young immune cells because mature lymphocytes are activated by selective mGluRs ligands. In addition, rat peripheral lymphocytes responded by producing reactive oxygen species (ROS) when they were exposed to the group III mGluRs

*Metabotropic Receptors 4 and the Immune Responses DOI: http://dx.doi.org/10.5772/intechopen.100272*

agonist L-2-amino-4-phosphonobutyric acid (L-AP4) [15]. ROS play important roles in T-cell biology and participate in activation-induced T cell apoptosis and hence in the termination of the immune response [16]. Moreover, DCs are capable of secreting glutamate when interacting with T lymphocytes, a process that might be essential for the function of lymphocyte. This hypothesis is based on the fact that the absence of glutamate led to impaired Th1 (Interleukin-2 (IL-2) and interferon-γ) and proinflammatory (IL-6 and tumor necrosis factor-alpha) cytokine production. However, these changes were not correlated with a decrease in T-cell proliferation.

### **3. mGluR4 and Autoimmunity in CNS**

A role of mGluR4 in immune modulation was first described in an autoimmune disease model [17]. Fallarino et al. [17] reported that mGlu4 knockout mice (Grm4−/−) were highly susceptible to experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis. More specifically, Grm4−/− mice and their wildtype (WT) counterparts were immunized with myelin oligodendrocyte glycoprotein (MOG35–55), which can induce EAE in C57BL/6 mice. The EAE clinical scores were recorded periodically and a lack of mGluR4 was found to be associated with earlier onset, more severe, and ultimately fatal EAE in >40% of the hosts. Along with these changes, white matter demyelination and inflammatory infiltrates were more prevalent in the spinal cord of MOG-vaccinated Grm4−/− mice in comparison to their WT counterparts, according to the morphological changes. The phenotype has also been characterized in littermates as well (heterozygote breeding—with cohorts of mice being matched for gender and age) and the disease indications were also more severe in Grm4−/− and Grm4+/− mice than in WT mice. In contrast, treatment of N-Phenyl-7-(hydroxyimino) cyclopropa [b] chromen-1a-carboxamide (PHCCC), an Grm4-positive allosteric modulator led to increased resistance to EAE. This was in agreement with previous reports demonstrating that long-term treatment of L-AP4 can increase the recovery rate from EAE in Lewis rats [17, 18].

There was significant infiltration of CD4<sup>+</sup> T cells, CD8+ T cells and B220<sup>+</sup> B cells in both peripheral lymphoid organs and the CNS in both Grm4−/− and WT mice, but the percentages of CD4<sup>+</sup> and CD8+ T cells as well as CD11b<sup>+</sup> and CD11c<sup>+</sup> cells were significantly higher in the CNS of Grm4−/− mice at the peak of disease [17]. Extended studies using littermates from heterozygote breeding further showed that the disease course was more severe in Grm4−/− and Grm4+/− mice than in WT mice. The cytokine profiling of sorted CD4<sup>+</sup> T cells from brain-infiltrating leukocytes (BILs) and pooled lymph nodes demonstrated a significant increase in *Rorc* transcripts (encoding the TH17 specification factor), a reduction in *Foxp3* (Treg) transcripts, and no change in Tbx21 (coding for Tbet; a TH1 maker) in Grm4−/− mice during the neurologic signs. No changes were observed in *Gata3* (a TH2 marker) in both groups. These data suggested that Grm4−/− tipped the balance of transcriptional activation in favor of inflammatory genes in response to MOG vaccination. In particular, Grm4−/− favored the emergence of TH17 over Treg cells, which would sustain inflammation and exacerbate EAE [18].

Expression of mGluR4 was confirmed in several immune subpopulations, such as CD4+ T cells, CD8+ T cells, γδ T cells, B220+ B cells, CD11b<sup>+</sup> and CD11c<sup>+</sup> cells, particularly in CD4<sup>+</sup> T and CD11c<sup>+</sup> cells, suggesting those cells as potential targets for Grm4 mediated effects. The expression of mGluR4 was also confirmed in DC subsets in splenocytes, including conventional DCs (cDCs; CD11b+ CD11chigh) and plasmacytoid DCs (pDCs; mPDCA1<sup>+</sup> CD11clow). They have further shown that

treatment with toll-like receptor ligands such as lipopolysaccharide (LPS) and CpG-oligonucleotide (CpG-ODN) led to increased Grm4 expression. Modulation of mGluR4 expression in activated nTreg cells (CD4+ CD25+ ) and LPS-stimulated cDCs was also confirmed, further supporting that mGluR4 activation within an immunologic synapse contributes to the crosstalk and reciprocal influence between T and accessory cells [17].

IL-17-producing T helper (Th17) cells are considered mediators of autoimmunity in multiple sclerosis and EAE. The accumulation of Th17 cells in the CNS as well as in the periphery is also associated with the development of demyelinating plaques of multiple sclerosis [19]. Fallarino et al. [17] also pointed out that the absence of mGlu4 in dendritic cells is key to inducing a differentiation of T helper cells toward the Th17 phenotype. More specifically, possible regulatory function of mGluR4 in the interaction between CD4+ T cells and DCs has been examined. Both cDCs and pDCs from Grm4−/− mice produced higher amounts of IL-6 and IL-23, but less IL-12 and IL-27, compared to their WT counterparts in response to LPS or CpG-ODN, respectively [17].

The notable results of coculturing of WT CD4<sup>+</sup> T cell and Grm4−/− DCs demonstrated an increase of IL-17A+ CD4+ T cells, along with a significantly reduction of IFN-γ producing CD4+ T cells (a portion of which also expressed IL-17A). However, they failed to see this effect when the coculture consisted of WT DCs and Grm4−/− T cells, suggesting that the effect of mGluR4 depletion was largely dependent on DCs in this *in vitro* system. The cytokine production in culture supernatants has been examined and there are decreased amounts of TH1-associated IL-2 in coculture system involving Grm4−/− cDCs. IL-27 is known to counter the effect of IL-6 in directing TH17 cell development, which can limit the EAE progression. The decrease in IL-27 during activation of naïve CD4<sup>+</sup> T cells might be another reason for favoring the emergence of Th17 cells [17].

They also suggest that activation of mGlu4 (as a result of elevated levels of glutamate during the neuroinflammation) might exert a protective effect by preventing the unbalance in T helper cells. Such mechanism presents a clear therapeutic potential for treating autoimmune related disorders.

The underlying mechanism for Grm4-mediated immune regulation is not clear at present. However, there appears to be a cross-talk and reciprocal influences between Grm4 and indoleamine 2,3-dioxygenase 1 (IDO-1) pathways [20]. IDO1 has been well known to be involved in generating an immunosuppressive environment through catalyzing the metabolism of tryptophan, resulting in tryptophan depletion and accumulation of kynurenine [21]. A protective role of IDO-1 has been shown in mice with different forms of EAE including acute, relapsing–remitting, and adoptively transferred disease [22]. Interestingly, in addition to the direct immunosuppressive effect of kynurenine through inhibition of CD8<sup>+</sup> T cells and activation of Treg cells, kynurenine metabolites such as cinnabarinic acid (CA) act as selective, although weak, orthosteric agonists of mGluR4 [23]. The therapeutic effect of CA in acute EAE was attenuated in Grm4−/− mice [24]. On the other hand, activation of Grm4 could positively impact the IDO1 pathway. Treatment of DCs with ADX88178, a positive allosteric modulator (PAM) of Grm4, led to both increased expression levels of IDO-1 and phosphorylation of IDO-1 [20]. These effects require a Gi-independent, alternative signaling pathway that involves phosphatidylinositol-3-kinase (PI3K), Src kinase, and the signaling activity of IDO1. Moreover, the effect of ADX88178 on the expression of several cytokines was impaired in IDO1−/− DCs [20]. Therefore, Grm4 and IDO1 constitute a loop that provides a positive feedback mechanism to amplify the immune-protective effect in EAE and possibly other immune-related diseases [20].

## **4. mGluR4 and cancers**

Most studies on the role of glutamate receptor in cancers have been focused on iGluRs [25, 26]. Tumor cells originated from neuronal tissues express iGluRs subunits and iGluRs antagonists have shown inhibitory effect on the proliferation of the tumor cells. Similarly, iGluRs subunits have been shown to be expressed in several peripheral cancers and blockade of the N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) inotropic glutamate receptor subtypes leads to decreased cell proliferation and migration [26].

mGluRs are also expressed in several cell lines derived from human tumors, including neuroblastoma, thyroid carcinoma, rhabdomyosarcoma/medulloblastoma, lung carcinoma, multiple myeloma, glioma, colon adenocarcinoma, astrocytoma, T cell leukemia, and breast carcinoma [27]. In particular, mGluR1 has been shown to be expressed in subsets of human melanomas [28]. Ectopic expression of mGluR1 in melanocytes drives the development of melanoma in mouse models. Pharmacological inhibition of mGluR1 led to inhibition of tumor cell growth both in vitro and in vivo [28]. Riluzole, an antagonist of mGluR1 signaling has advanced to phase II clinical trial in patients with advanced melanoma [29, 30].

The studies on the roles of mGluR4 in cancers are very limited and controversial. Change et al. studied the expression pattern of mGluR4 in several healthy and diseases-derived human tissues [31]. mGluR4 receptor expression was identified in 68% of colorectal carcinomas, 50% of laryngeal carcinoma, and 46% breast carcinomas. In the case of colorectal carcinoma, overexpression of mGluR4 was correlated with poor prognosis, and cell lines derived from human colorectal carcinomas showed increased cell invasiveness when treated with L-AP4. In another study, comparative proteomics was used to characterize a human colon cancer cell line that was resistant to 5-fluororacil (5-FU, a common chemotherapy agent). Interestingly, 5-FU resistant cells were found to overexpress mGluR4 in comparison with parental cancer cells. It has been demonstrated that cell survival was increased by the group III mGlu receptor agonist L-AP4 in the nonresistant parent cancer cells; conversely, survival was synergically decreased by 5-FU and the group III receptor antagonist MAP4 in 5-FU-resistant cells. It is noteworthy to mention that 5-FU downregulated mGluR4 expression, and MAP4 has a dose dependent cytotoxic effect in both cell lines [32].

In contrast to the above reports, mGluR4 agonists are shown to inhibit the proliferation of human breast and bladder cancer cells in a GRM4-depenedt manner [33]. In the study by Lasek et al., the expression of mGlu4 was shown to be inversely correlated with the severity of human medulloblastoma [34]. After scoring the extent of immunoreactivity for mGlu4 in human biopsies of medulloblastoma, the absence of spinal metastases, cerebrospinal fluid spread, and tumor recurrence as well as the survival of patients were all shown to be associated with high levels of mGlu4 immunoreactivity. Treatment with PHCCC (which is considered a group I mGlu receptor antagonist but can also act as a positive allosteric modulator of mGlu4 receptor) reduced the proliferation of cultured medulloblastoma cells and inhibited the growth of medulloblastoma implants in mice. In addition, subcutaneous or intracranial injections of PHCCC during the first week of life reduced the incidence of medulloblastoma from 85 to 28% in a mutant mouse model known to develop the disease upon X-ray irradiation. This indicates that activation of mGlu4 receptors also affects early events in tumorigenesis [35].

The above studies focus on the role of tumor cell-derived Grm4. It has been reported that the plasma levels of Glu are generally elevated in patients with carcinoma and seem to correlate with an impairment in immune function [36]. However, the role of immune cell-derived mGluR including mGluR4 has hardly been studied. Kansara et al. reported that Grm4−/− mice showed accelerated radiation-induced tumor development in an irradiation-induced osteosarcoma model [37]. Outside the CNS, mGluR4 is highly expressed by DCs, as well as CD4<sup>+</sup> T cells [17]. In the mouse osteosarcomas, they found that mGluR4 is predominantly expressed by CD45+ CD11c+ MHC+ myeloid cells within the tumor microenvironment (TME) instead of tumor cells. Few CD4+ T cells were detectable to characterize mGluR4 expression. In consistent with the study by Fallarino et al. [17] in an EAE model, Grm4−/− DCs isolated from the tumors showed increased expression of IL-23. Interestingly, high expression of IL-23 has been observed in primary osteosarcomas and allografted cell lines relative to normal bone, while ex-vivo cultured osteosarcoma cell lines and primary tumor cells did not express IL23. A role of IL-23 in tumorigenesis has been well established from previous studies [38]. Indeed, IL23−/− mice were resistant to the irradiation-induced osteosarcoma. They hypothesized that knockout of Grm4 in DCs facilitates the oncogenesis of osteosarcoma through increased production of IL-23 [37].

We have recently shown in three murine syngeneic tumor models (B16, MC38, and 3LL) that either genetic knockout (Grm4−/−) or pharmacological inhibition led to significant delay in tumor growth (Wan et al., unpublished data). Mechanistically, perturbation of GRM4 resulted in a strong anti-tumor immunity by promoting nature killer (NK), CD4<sup>+</sup> and CD8+ T cells toward an activated, proliferative, and functional phenotype. We have further shown that the antitumor activity of Grm4 antagonists can be further improved through combination with anti-PD-1 antibody. The differing role of Grm4 in different tumor models may reflect the complex functions of Grm4 in different tumor environment. More studies are needed to further define the roles of immune cells-derived Grm4 and its potential as a novel therapeutic target for cancer immunotherapy.

## **5. Conclusions**

Although the neurological function of GRM4 in CNS has been well established, its role in modulating immune response just began to be appreciated. GRM4 is expressed in various immune cells and loss of GRM4 function in immune cells led to sensitization to EAE. GRM4 selective agonists may hold potential as a novel therapy for autoimmune disorders of CNS. GRM4 is also expressed in various cancer cells, however, conflicting results have been reported regarding whether GRM4 promotes or inhibits tumor cell proliferation. The role of immune cells-derived GRM4 in antitumor immunity is also controversial and may reflect the complex function of GRM4 in different tumor microenvironment. Further studies using more defined animal models and selective GRM4 modulators may not only advance our understanding of the complex immunobiology of GRM4 but also lead to the development of a new immunotherapy for the treatment of cancer.

## **Acknowledgements**

This work was supported, in part, by NIH grant RO1 CA219399.

### **Conflict of interest**

The authors declare no conflict of interest.

*Metabotropic Receptors 4 and the Immune Responses DOI: http://dx.doi.org/10.5772/intechopen.100272*

## **Author details**

Zhuoya Wan1,2 and Song Li1,2\*

1 Center for Pharmacogenetics, School of Pharmacy, University of Pittsburgh, Pittsburgh, PA, USA

2 Department of Pharmaceutical Sciences, School of Pharmacy, University of Pittsburgh, Pittsburgh, PA, USA

\*Address all correspondence to: sol4@pitt.edu

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] C. Chu, D. Artis, I.M. Chiu, Neuroimmune interactions in the tissues, Immunity 52(3) (2020) 464-474.

[2] C. Godinho-Silva, R.G. Domingues, M. Rendas, B. Raposo, H. Ribeiro, J.A. da Silva, A. Vieira, R.M. Costa, N.L. Barbosa-Morais, T. Carvalho, Lightentrained and brain-tuned circadian circuits regulate ILC3s and gut homeostasis, Nature 574(7777) (2019) 254-258.

[3] P. Baral, B.D. Umans, L. Li, A. Wallrapp, M. Bist, T. Kirschbaum, Y. Wei, Y. Zhou, V.K. Kuchroo, P.R. Burkett, Nociceptor sensory neurons suppress neutrophil and γδ T cell responses in bacterial lung infections and lethal pneumonia, Nature medicine 24(4) (2018) 417.

[4] J.R. Huh, H. Veiga-Fernandes, Neuroimmune circuits in inter-organ communication, Nature Reviews Immunology 20(4) (2020) 217-228.

[5] C.S. Klose, D. Artis, Neuronal regulation of innate lymphoid cells, Current opinion in immunology 56 (2019) 94-99.

[6] H. Veiga-Fernandes, D. Mucida, Neuro-immune interactions at barrier surfaces, Cell 165(4) (2016) 801-811.

[7] M. Julio-Pieper, P.J. Flor, T.G. Dinan, J.F. Cryan, Exciting times beyond the brain: metabotropic glutamate receptors in peripheral and non-neural tissues, Pharmacological reviews 63(1) (2011) 35-58.

[8] J.F. Cryan, K.K. Dev, The glutamatergic system as a potential therapeutic target for the treatment of anxiety disorders, Handbook of behavioral neuroscience 17 (2008) 269-301.

[9] P.J. Conn, Physiological roles and therapeutic potential of metabotropic glutamate receptors, Annals of the New York Academy of Sciences 1003(1) (2003) 12-21.

[10] R. Gerlai, J.C. Roder, D.R. Hampson, Altered spatial learning and memory in mice lacking the mGluR4 subtype of metabotropic glutamate receptor, Behavioral neuroscience 112(3) (1998) 525.

[11] R. Pacheco, H. Oliva, J.M. Martinez-Navío, N. Climent, F. Ciruela, J.M. Gatell, T. Gallart, J. Mallol, C. Lluis, R. Franco, Glutamate released by dendritic cells as a novel modulator of T cell activation, The Journal of Immunology 177(10) (2006) 6695-6704.

[12] C. Ferrarese, A. Aliprandi, L. Tremolizzo, L. Stanzani, A. De Micheli, A. Dolara, L. Frattola, Increased glutamate in CSF and plasma of patients with HIV dementia, Neurology 57(4) (2001) 671-675.

[13] G. Lombardi, C. Dianzani, G. Miglio, P.L. Canonico, R. Fantozzi, Characterization of ionotropic glutamate receptors in human lymphocytes, British journal of pharmacology 133(6) (2001) 936-944.

[14] R. Rezzani, G. Corsetti, L. Rodella, P. Angoscini, C. Lonati, R. Bianchi, Cyclosporine-A treatment inhibits the expression of metabotropic glutamate receptors in rat thymus, Acta histochemica 105(1) (2003) 81-87.

[15] A.A. Boldyrev, V.I. Kazey, T.A. Leinsoo, A.P. Mashkina, O.V. Tyulina, P. Johnson, J.O. Tuneva, S. Chittur, D.O. Carpenter, Rodent lymphocytes express functionally active glutamate receptors, Biochemical and biophysical research communications 324(1) (2004) 133-139.

[16] D.A. Hildeman, T. Mitchell, T.K. Teague, P. Henson, B.J. Day, J. Kappler, P.C. Marrack, Reactive oxygen species

*Metabotropic Receptors 4 and the Immune Responses DOI: http://dx.doi.org/10.5772/intechopen.100272*

regulate activation-induced T cell apoptosis, Immunity 10(6) (1999) 735-744.

[17] F. Fallarino, C. Volpi, F. Fazio, S. Notartomaso, C. Vacca, C. Busceti, S. Bicciato, G. Battaglia, V. Bruno, P. Puccetti, Metabotropic glutamate receptor-4 modulates adaptive immunity and restrains neuroinflammation, Nature medicine 16(8) (2010) 897-902.

[18] G. Besong, G. Battaglia, M. D'Onofrio, R. Di Marco, R.T. Ngomba, M. Storto, M. Castiglione, K. Mangano, C.L. Busceti, F.R. Nicoletti, Activation of group III metabotropic glutamate receptors inhibits the production of RANTES in glial cell cultures, Journal of Neuroscience 22(13) (2002) 5403-5411.

[19] J. Milovanovic, A. Arsenijevic, B. Stojanovic, T. Kanjevac, D. Arsenijevic, G. Radosavljevic, M. Milovanovic, N. Arsenijevic, Interleukin-17 in chronic inflammatory neurological diseases, Frontiers in Immunology 11 (2020) 947.

[20] C. Volpi, G. Mondanelli, M.T. Pallotta, C. Vacca, A. Iacono, M. Gargaro, *E. Albini*, R. Bianchi, M.L. Belladonna, S. Celanire, Allosteric modulation of metabotropic glutamate receptor 4 activates IDO1-dependent, immunoregulatory signaling in dendritic cells, Neuropharmacology 102 (2016) 59-71.

[21] Z. Wan, J. Sun, J. Xu, P. Moharil, J. Chen, J. Xu, J. Zhu, J. Li, Y. Huang, P. Xu, Dual functional immunostimulatory polymeric prodrug carrier with pendent indoximod for enhanced cancer immunochemotherapy, Acta biomaterialia 90 (2019) 300-313.

[22] Y. Yan, G.-X. Zhang, B. Gran, F. Fallarino, S. Yu, H. Li, M.L. Cullimore, A. Rostami, H. Xu, IDO upregulates regulatory T cells via tryptophan catabolite and suppresses encephalitogenic T cell responses in experimental autoimmune

encephalomyelitis, The Journal of Immunology 185(10) (2010) 5953-5961.

[23] F. Fazio, C. Zappulla, S. Notartomaso, C. Busceti, A. Bessede, P. Scarselli, C. Vacca, M. Gargaro, C. Volpi, M. Allegrucci, Cinnabarinic acid, an endogenous agonist of type-4 metabotropic glutamate receptor, suppresses experimental autoimmune encephalomyelitis in mice, Neuropharmacology 81 (2014) 237-243.

[24] F. Fazio, L. Lionetto, G. Molinaro, H.-O. Bertrand, F. Acher, R.T. Ngomba, S. Notartomaso, M. Curini, O. Rosati, P. Scarselli, Cinnabarinic acid, an endogenous metabolite of the kynurenine pathway, activates type 4 metabotropic glutamate receptors, Molecular pharmacology 81(5) (2012) 643-656.

[25] M.P. Ribeiro, J.B. Custódio, A.E. Santos, Ionotropic glutamate receptor antagonists and cancer therapy: time to think out of the box?, Cancer chemotherapy and pharmacology 79(2) (2017) 219-225.

[26] A. Stepulak, R. Rola, K. Polberg, C. Ikonomidou, Glutamate and its receptors in cancer, Journal of neural transmission 121(8) (2014) 933-944.

[27] A. Stepulak, H. Luksch, C. Gebhardt, O. Uckermann, J. Marzahn, M. Sifringer, W. Rzeski, C. Staufner, K.S. Brocke, L. Turski, Expression of glutamate receptor subunits in human cancers, Histochemistry and cell biology 132(4) (2009) 435-445.

[28] Y. Ohtani, T. Harada, Y. Funasaka, K. Nakao, C. Takahara, M. Abdel-Daim, N. Sakai, N. Saito, C. Nishigori, A. Aiba, Metabotropic glutamate receptor subtype-1 is essential for in vivo growth of melanoma, Oncogene 27(57) (2008) 7162-7170.

[29] C.L. Speyer, M.A. Nassar, A.H. Hachem, M.A. Bukhsh, W.S. Jafry, R.M. Khansa, D.H. Gorski, Riluzole mediates anti-tumor properties in breast cancer cells independent of metabotropic glutamate receptor-1, Breast cancer research and treatment 157(2) (2016) 217-228.

[30] J.M. Mehnert, A.W. Silk, J. Lee, L. Dudek, B.S. Jeong, J. Li, J.M. Schenkel, E. Sadimin, M. Kane, H. Lin, A phase II trial of riluzole, an antagonist of metabotropic glutamate receptor 1 (GRM 1) signaling, in patients with advanced melanoma, Pigment cell & melanoma research 31(4) (2018) 534-540.

[31] H.J. Chang, B.C. Yoo, S.-B. Lim, S.-Y. Jeong, W.H. Kim, J.-G. Park, Metabotropic glutamate receptor 4 expression in colorectal carcinoma and its prognostic significance, Clinical cancer research 11(9) (2005) 3288-3295.

[32] B.C. Yoo, E. Jeon, S.-H. Hong, Y.-K. Shin, H.J. Chang, J.-G. Park, Metabotropic glutamate receptor 4-mediated 5-Fluorouracil resistance in a human colon cancer cell line, Clinical cancer research 10(12) (2004) 4176-4184.

[33] Z. Zhang, Y. Liu, K. Wang, K. Zhu, X. Zheng, L. Wang, Y. Luan, X. Wang, H. Lu, K. Wu, Activation of type 4 metabotropic glutamate receptor promotes cell apoptosis and inhibits proliferation in bladder cancer, Journal of cellular physiology 234(3) (2019) 2741-2755.

[34] S.-Y. Park, S. Lee, I.-H. Han, B.-C. Yoo, S.-H. Lee, J.-Y. Park, I.-H. Cha, J. Kim, S.-W. Choi, Clinical significance of metabotropic glutamate receptor 5 expression in oral squamous cell carcinoma, Oncology reports 17(1) (2007) 81-87.

[35] G. Battaglia, C.L. Busceti, G. Molinaro, F. Biagioni, A. Traficante, F. Nicoletti, V. Bruno, Pharmacological activation of mGlu4 metabotropic

glutamate receptors reduces nigrostriatal degeneration in mice treated with 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine, Journal of Neuroscience 26(27) (2006) 7222-7229.

[36] W. Dröge, H.-P. Eck, M. Betzler, P. Schlag, P. Drings, W. Ebert, Plasma glutamate concentration and lymphocyte activity, Journal of cancer research and clinical oncology 114(2) (1988) 124-128.

[37] M. Kansara, K. Thomson, P. Pang, A. Dutour, L. Mirabello, F. Acher, J.-P. Pin, E.G. Demicco, J. Yan, M.W. Teng, Infiltrating myeloid cells drive osteosarcoma progression via GRM4 regulation of IL23, Cancer discovery 9(11) (2019) 1511-1519.

[38] I. Chan, R. Jain, M. Tessmer, D. Gorman, R. Mangadu, M. Sathe, F. Vives, C. Moon, E. Penaflor, S. Turner, Interleukin-23 is sufficient to induce rapid de novo gut tumorigenesis, independent of carcinogens, through activation of innate lymphoid cells, Mucosal immunology 7(4) (2014) 842-856.

## *Edited by Maria del Mar Ortega-Villaizan and Veronica Chico*

Infectious microbial agents such as viruses, bacteria, fungi, and parasites can cause pathological disorders and even death in organisms exposed to the environment. However, organisms have an immune system to control infection caused by pathogens. The immune system is divided into the innate and the adaptive immune systems. The innate immune system is the first mechanism to respond to infections, whereas the adaptive immune system is based on immune memory. This book provides an overview of antiviral and antibacterial immune responses in different immune-reactive organs and across different animal species, from higher to lower vertebrates.

Published in London, UK © 2021 IntechOpen © MikeMareen / iStock

Antimicrobial Immune Response

Antimicrobial Immune

Response

*Edited by Maria del Mar Ortega-Villaizan and Veronica Chico*