IL-23/Th17 Pathway and Intestinal Immune System Homeostasis
A central challenge of the intestinal immune system is balancing responses to pathogens and excessive entry of luminal bacteria, while simultaneously coexisting with resident luminal microbiota. A number of mechanisms contribute to defenses, including cytokines, against a range of microbes. Under physiological conditions, IL-23 is constitutively expressed in ileal mucosa124 and IL-17-producing cells are highly enriched in mucosal tissues and lymphoid structures.4, 48, 49, 125 Consistent with this, RORγτ is expressed in small intestinal and colonic TCRαβ and TCRγδ T cells.4, 90 The enriched IL-17-producing T cells in the intestine may be due to a number of factors, and are modulated by intestinal microbiota,48, 49, 125, 126 with microbial secreted factors (e.g., ATP) contributing to this modulation.49
Multiple mechanisms downregulate IL-23/Th17 responses in the intestine. For example, specific intestinal macrophage populations secrete low levels of IL-23 and IL-12 and, in fact, serve to downregulate dendritic cell-instructed T cell IL-17 secretion.30, 127 In addition to regulation at the level of antigen-presenting cells, FoxP3+ Treg cells are critical for regulating intestinal IL-17-producing T cells. Retinoic acid, which is important in induction of intestinal specific trafficking molecules on T cells, is also capable of inhibiting TGFβ+IL-6-driven induction of Th17 cells and promoting antiinflammatory Treg cell differentiation.128–132 In fact, retinoic acid contributes to a more stable regulatory T cell phenotype.129 Deficient IL-23, in turn, can lead to increased FoxP3+ T cells in the mesenteric lymph nodes and intestinal lamina propria,122 thereby highlighting the critical cross-regulation between the IL-23/Th17 pathway and FoxP3+ Tregs in the intestine (Fig. 1).
IL-23/Th17 Pathway in Colitis
IL-23 is constitutively expressed in the terminal ileum and IL-23 production is increased during intestinal inflammation.30, 123, 124 Colitis models demonstrating increased intestinal IL-23 and Th17-lineage cytokines and transcription factors (e.g., RORγτ) include the CD4+CD45RBhi transfer model, IL-10−/− colitis, dextran sodium sulfate (DSS) colitis, and trinitrobenzenesulfonic acid (TNBS) colitis.60, 76, 133–135 Transgenic expression of IL-23p19 results in severe systemic inflammation, including in the small and large intestine,46 and addition of rIL-23 exacerbates IL-10−/− colitis.60 Th17 lines derived from C3Bir-induced colitic mice resulted in severe colitis upon transfer into SCID recipients. Treatment with anti-IL-23p19 could both prevent and treat this colitis.136 Similarly, either blocking IL-23p19 or utilizing p19−/− mice results in less severe IL-10−/− colitis,60Helicobacter hepaticus-anti-IL-10R-induced colitis,35 and CD4+CD45RBhi transfer colitis.35 Deficiency in other Th17 differentiating and lineage cytokines can also lead to improvement in colitis, as observed with either deficiency or blockade of IL-21 in DSS- and TNBS-induced colitis.76 On the other hand, the intestinal tissue injury mediated by IL-23 is also observed in the absence of IL-17 in some of these same models122 and in the absence of T cells, demonstrating both IL-17- and T cell-independent effects of IL-23.121 Finally, despite the current focus of the IL-23/Th17 pathway on mediating intestinal inflammation and the evidence for negative cross-regulation between Th1 and Th17 pathways, there is significant evidence for Th1-mediated inflammation, as well as for the combined effects of Th1 and IL-23/Th17 pathways in intestinal inflammation.35
In addition to contributions to tissue-mediated inflammation, there is also evidence for IL-23/Th17 pathway contributions in downregulating inflammation.85, 137–139 For example, IL-23p19-deficient mice developed more severe TNBS-induced colitis than did their wildtype littermate controls.140 This was attributed to a role for IL-23 in the downregulation of IL-12.140, 141 Further, direct neutralization of IL-17 increases severity of DSS-induced colitis,142 although in a report of TNBS-induced colitis, IL-17R−/− mice were protected from disease.143 Finally, myeloid- and hematopoietic-specific STAT3 deficient mice develop spontaneous colitis.144, 145 The colitis was reversed in the absence of p40, implicating IL-12/IL-23 in mediating the inflammation.144, 145 Of note is that STAT3 plays a role in a number of cytokine pathways, including regulatory cytokines (e.g., IL-10) and proinflammatory cytokines (e.g., IL-6, IL-17, IL-21, IL-22, IL-23, and IL-27).41, 145–148 Therefore, increased STAT3 phosphorylation in inflammatory diseases likely reflects the effects of a complex inflammatory milieu, involving both primary and secondary mechanisms. Overall, these studies highlight the complexity in cross-regulation of cytokine pathways required for maintaining intestinal immune homeostasis.
The complexity of IL-23/Th17 pathway roles is also reflected in that fact that in addition to the proinflammatory cytokines secreted by Th17 lineage cells, cytokines that can downregulate inflammation are also secreted, such as IL-10 and IL-22. For example, IL-22, which is highly inducible by IL-23, can downregulate both liver149 and intestinal150 inflammation. IL-22 can contribute to STAT3-mediated restitution of mucus-producing goblet cells,150 colonic subepithelial myofibroblasts effects,151 and induction of lipopolysaccharide (LPS)-binding protein, which may serve to downregulate the inflammation mediated by LPS.134 Evidence for similar IL-22-mediated epithelial cell restitution is observed at other mucosal sites, including the lungs.152 Therefore, Th17 lineage cytokines and STAT3 activation can act in defenses against microbes at mucosal surfaces while simultaneously providing mechanisms for downregulating inflammation.
In human CD and UC, elevated IL-17, IL-21, IL-22, IL-23, RORγτ, and IL23R expression is observed in colonic mucosa.76, 80, 108, 151, 153–156 Increased IL-21, STAT3, phospho-STAT3, STAT4, and SOCS3 are also observed in lamina propria cells from CD individuals.157–160 Some of these cytokines can be observed in plasma or serum of patients.80, 134 Furthermore, a unique macrophage subset in the human intestine that expresses IL-23, TNF, and IL-6 is expanded in number and cytokine production in CD patients.161 IL-23 stimulation of lamina propria mononuclear cells from CD individuals leads to increased production of IFNγ and TNF compared to healthy controls or compared to increased IL-17 from UC individuals.30, 156 Elevated IL-17 and IL-23 expression decreases after treatment with steroids and anti-p40 antibodies.154, 162 Therefore, there is evidence for dysregulation of multiple T cell pathways in IBD, as well as for differential dysregulation between UC and CD individuals.