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

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 mediators in splenocytes and microglial cells [49].

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 DCs treated with AhR-ligands, the increased production of CD4+ Foxp3+ (Treg cells) [50] was seen.

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 treatment of autoimmune and inflammatory diseases [50].

## **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

**157**

**8. Discussions**

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

cumin significantly enhanced the production of CD4<sup>+</sup>

model was established by ovalbumin. Mice were treated with different doses of curcumin (50, 100, and 200 mg/kg), and then the level of Treg/Th17-secreted cytokines was measured by enzyme-linked immunosorbent assay (ELISA). In addition, the percentages of Treg and Th17 were measured using flow cytometry assay. Results showed that curcumin caused a significant suppression in the production of Th17 subsets, and the secretion of IL-17 cytokines. In contrast, the AhR-ligand cur-

findings suggest that curcumin could be used as therapeutic agent for patients with allergic asthma, because of its ability to significantly affect Treg/Th17 balance [51]. Curcumin plays an important role in multiple sclerosis (MS) autoimmune disease. It is characterized by some pathophysiological features such as breaching of bloodbrain barrier (BBB) and injury to axons and myelin sheaths. Th17 cells play an important role in the pathophysiological process of MS. Curcumin is well known as active anti-inflammatory and neuroprotective agent if used prophylactically. Curcumin could inhibit neuroinflammation through multiple mechanisms in MS. First, CNS antigens will be captured by DC, and then the antigen will be presented to T cells, which will help in initiating inflammatory response [52]. This action will be followed by the secretion of different pro-inflammatory cytokines and enhancement of production of Th17 cells in circulation. The blood-brain barrier (BBB) usually expresses IL-17R and IL-22R receptors and the expression of these receptors will bridge the gap between Th17 and BBB tight junction that results in the disruption of tight junctions. This action will enhance the transmigration of Th17 across the BBB followed by the enhanced secretion of granzyme-B which in turn is found to initiate the killing of neurons. In contrast, curcumin treatment was found to inhibit the production and expansion of Th17 subsets in circulation. In addition, curcumin was reported to increase the expression of ZO-1 protein, an important tight junction protein, suggesting that curcumin can reduce neuroinflammation in MS autoimmune disease [52].

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,

CD25+

T cell subsets. These

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

#### *Role of Aryl Hydrocarbon-Ligands in the Regulation of Autoimmunity DOI: http://dx.doi.org/10.5772/intechopen.80840*

model was established by ovalbumin. Mice were treated with different doses of curcumin (50, 100, and 200 mg/kg), and then the level of Treg/Th17-secreted cytokines was measured by enzyme-linked immunosorbent assay (ELISA). In addition, the percentages of Treg and Th17 were measured using flow cytometry assay. Results showed that curcumin caused a significant suppression in the production of Th17 subsets, and the secretion of IL-17 cytokines. In contrast, the AhR-ligand curcumin significantly enhanced the production of CD4<sup>+</sup> CD25+ T cell subsets. These findings suggest that curcumin could be used as therapeutic agent for patients with allergic asthma, because of its ability to significantly affect Treg/Th17 balance [51].

Curcumin plays an important role in multiple sclerosis (MS) autoimmune disease. It is characterized by some pathophysiological features such as breaching of bloodbrain barrier (BBB) and injury to axons and myelin sheaths. Th17 cells play an important role in the pathophysiological process of MS. Curcumin is well known as active anti-inflammatory and neuroprotective agent if used prophylactically. Curcumin could inhibit neuroinflammation through multiple mechanisms in MS. First, CNS antigens will be captured by DC, and then the antigen will be presented to T cells, which will help in initiating inflammatory response [52]. This action will be followed by the secretion of different pro-inflammatory cytokines and enhancement of production of Th17 cells in circulation. The blood-brain barrier (BBB) usually expresses IL-17R and IL-22R receptors and the expression of these receptors will bridge the gap between Th17 and BBB tight junction that results in the disruption of tight junctions. This action will enhance the transmigration of Th17 across the BBB followed by the enhanced secretion of granzyme-B which in turn is found to initiate the killing of neurons. In contrast, curcumin treatment was found to inhibit the production and expansion of Th17 subsets in circulation. In addition, curcumin was reported to increase the expression of ZO-1 protein, an important tight junction protein, suggesting that curcumin can reduce neuroinflammation in MS autoimmune disease [52].
