**8. Discussions**

*Immune Response Activation and Immunomodulation*

**6. Effects of I3C and indirubin on immunoregulation**

mediators in splenocytes and microglial cells [49].

DCs treated with AhR-ligands, the increased production of CD4+

**7. Effects of curcumin on Treg/Th17 balance and autoimmunity**

The protective effect of curcumin was evaluated using ovalbumin (OVA) induced allergic inflammation in mouse model of allergic asthma. This mouse

ment of autoimmune and inflammatory diseases [50].

Indole-3-carbinol (I3C) (AhR-ligand) is found in cruciferous vegetables. Indirubin (IO) is another AhR-ligand, and is one of the components of the traditional Chinese medicine Danggui Longhui Wan. Although both of them are AhRligands, neither of these compounds bind the AhR as potently as TCDD. I3C and IO have anticancer properties, because they could inhibit cyclin dependent kinases that leads to cell cycle arrest in various cell lines. Moreover, both AhR-ligands were used to treat cancer. I3C has been used for the treatment of both breast and prostate cancer [45], while IO has been traditionally used for the treatment of chronic myelocytic leukemia [46]. I3C could downregulate the production of pro-inflammatory cytokines in macrophages [47, 48], whereas IO was reported to suppress these

A study was conducted to evaluate the effects of I3C and IO on specific immune cell populations, such as murine bone marrow-derived DCs, and the effect of these AhR-ligands was tested *in vivo*. The results showed that I3C and IO have immunosuppressive effects on DCs, which could promote a regulatory environment, thus could be useful to suppress chronic inflammatory diseases and/or autoimmunity *in vivo*. In addition, activating DC with lipopolysaccharide (LPS), after treating the cells with both AhR-ligands, suppresses the production of pro-inflammatory mediators including tumor necrosis factor-α (TNF-α), IL-1β, IL-6, IL-12, and nitric oxide but increased IL-10 levels. The DC treated with AhR-ligands was reported to upregulate some immune-regulating genes such as ALDH1A, IDO, and TGFB [50]. Both AhR-ligands were reported to suppress the levels of nuclear factor-kappa B (NF-кB), but only I3C suppressed the LPS-induced activity of RelB transcription factor encoded by the RELB gene. Finally, when naïve T cells were cultured with

The above observations suggest that I3C and IO have immunosuppressive and anti-inflammatory effects on DCs. Since these ligands are significantly less toxic than TCDD, these natural products may become useful therapeutics for the treat-

Foxp3+

(Treg

environment.

is activated with TCDD. In addition, suppression in the severity of EAE disease by a TGF-β1-dependent mechanism [44] was seen. Moreover, C57Bl/6J mice carrying the *d* allele of the Ahr gene (Ahr*d* mice) were characterized by a reduced affinity of about 10–100-fold for AhR-ligands due to a mutation in its ligand-binding site, and treating Ahr*d* mice with (1 μg/mouse) TCDD, had no significant effect on the severity of EAE and the development of Treg cells [44]. In contrast, when AhR binds to FICZ, the activation of the receptor will interfere with the differentiation of Treg development, and cause a significant induction of the Th17 subset and worsen EAE disease which suggests that AhR regulates Treg/Th17 subset differentiation in a ligand-specific manner [44]. These data suggested that different AhR-ligands have different effects on the production of pro- or anti-inflammatory T helper cell subsets, by controlling the production of different cytokines in the surrounding

**156**

cells) [50] was seen.

How might different AhR-ligands, all with the ability to stimulate AhRdependant gene transcription and promote Th17 cell development, promote either concomitant increases in Treg cells and lessen autoimmunity, or suppress Treg cell development and increase autoimmune activation? The presumed main function of AhR-induced transcriptional responses is to induce cytochrome P450 (e.g., CYP1A1) for detoxification of the detected aryl hydrocarbon. Indeed, FICZ is rapidly metabolized in a CYP-mediated reaction, within 1–3 hours [53] with a corresponding drop in AhR activation [54]. Thus, a transient AhR activation, even though promoting Th17 development and expansion, may ultimately terminate and allow Treg populations to emerge and dominate. In contrast, sustained AhR signaling might promote Foxp3 suppression and conversion of Treg to Th17 and Th1 cells.

Dietary AhR-ligands have also been suggested to act in an antagonistic manner to TCDD-induced AhR activation [55]. Additionally, although curcumin is able to act as a substrate for CYP1A1-mediated catabolism, it could partially decrease the accumulation of CYP1A1 mRNA [55] and antagonize CYP1A1 activity [56]. Therefore, interference with full AhR function, or metabolism of the inducing AhR-ligand or other endogenous ligands may be important in determining whether AhR-ligands result in regulatory and/or effector T cell development. Alternatively, certain AhRligands may induce distinct gene expression profiles [57], some of them promoting Th17 at the expense of Treg and others allowing the emergence of Treg.

The activation of AhR in DCs by some ligands may increase tolerogenic mediators, such as IDO, which promote Treg development. In support of this mechanism, IDO expression was found to be increased in DCs by TCDD or FICZ [38]. The conversion of Treg to Th17 and Th17 to Th1 profiles has been reported and reprogramming of subsets might be possible by additional cytokine provision, such as IL-23, IL-6, or removal of reinforcement factors, such as IL-23 or AhR-ligands [38]. The reported ability of IDO products (i.e., tryptophan metabolites) to suppress ROR-γt and induce Foxp3+ Treg cells [58] may indicate Th17 to Treg conversion, or shift to an IL-10-producing subset might result during exposure to some AhR-ligands. Since some AhR-ligand treatments lead to Th17 responses in the absence of Treg responses, allowing enhanced autoimmunity, this suggests that these ligands may be useful to promote antitumor immunity. It also raises the possibility that the anticancer effects of curcumin and quercetin may be due to their ability to promote potent effector T cell subsets in addition to suppressing some chronic inflammatory states. Another potentially beneficial use of AhR-ligands that have the ability to increase Treg populations is for the prevention or treatment of autoimmune diseases.

Experimental evidence has shown that flavonoids could be used to treat many diseases including cancer [59, 60]. The administration of curcumin was found to block the formation of lesions and tumors in C57Bl/6J mice after implanting murine melanoma B16F10 cells in their neck and brain. Furthermore, curcumin treatment was observed to significantly inhibit the proliferation of PC-3 prostate tumor cells.

The proposed mechanism for this effect of curcumin was its ability to significantly suppress NF-ĸB and AP-1 signaling pathways in tumor cells [61, 62]. Curcumin was given orally at concentrations in the micro-molar range; however, results showed that the concentration of curcumin was in the nano-molar range in the plasma [63, 64], due to the extensive metabolism of curcumin in the intestine and liver, which prevents the maintenance of high concentration of curcumin in the plasma and tissues after taking it orally [65, 66]. The curcumin is effective on the cancer cells at high concentration which is difficult to be maintained for several hours even in the gastrointestinal tract [63]. This suggests that the potential of using curcumin for cancer treatment is limited when given orally and the intraperitoneal injection may be more effective.

In contrast, other studies have shown that high concentrations of curcumin were found to enhance chromosome malformation in different cell lines. The curcumin could cause DNA damage both *in vivo* and *in vitro* and increase the incidence of thyroid gland follicular cell hyperplasia and carcinogenic activity in the small intestine [67–69]. This was proposed mainly due to its ability to increase the production of reactive oxygen species (ROS) [70]. Other studies have shown that curcumin has the ability to suppress cytochrome P450 enzyme, glutathione, S-transferase, and UDPglucuronosyltransferase, causing toxicity due to the increased level of drugs in the plasma [71]. Although lower concentrations of curcumin could enhance antioxidant activity, high concentrations of curcumin have shown pro-oxidant effects [63, 72].

Similarly, quercetin is known as an antioxidant, anti-inflammatory, and antimicrobial compound at low doses [73, 74]. In contrast, quercetin can enhance the production of ROS at higher concentrations [75]. ROS production by quercetin was found to kill some cancer cells, and quercetin complexes with bioactive compounds and metal ions such as lanthanum was reported to have powerful cytotoxic and antitumor properties at a concentration in the range of 100–1000 mM and the exposure time of tumor cells was around 3 hours. A quercetin/lanthanum complex was found to have a genotoxic effect on human cervical carcinoma cells due to ROS production [76].

**159**

**Author details**

Hana'a Burezq

provided the original work is properly cited.

Kuwait Institute for Scientific Research, Shuwaikh, Kuwait

\*Address all correspondence to: haborizq@kisr.edu.kw

*Role of Aryl Hydrocarbon-Ligands in the Regulation of Autoimmunity*

*DOI: http://dx.doi.org/10.5772/intechopen.80840*

© 2018 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,
