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Keywords:

  • inflammatory bowel disease;
  • inflammation;
  • autoimmunity;
  • IL-23;
  • Th17 cells

Abstract

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

The IL-23/Th17 pathway has recently been identified to play a critical role in a number of chronic inflammatory diseases including inflammatory bowel disease (IBD). The identification in IBD patients of associations in IL23R and regions that include other genes in the IL-23/Th17 pathway has highlighted the importance of proper IL-23/Th17 pathway regulation in intestinal immune homeostasis. IL-23 plays a role in CD4+ Th17 lineage cells, characterized by IL-17 secretion and the expression of the transcription factor retinoic acid-related orphan receptor (ROR)γτ, and in other immune and nonimmune cells. The balance between effector T cell subsets, such as Th17 cells, and CD4+ T regulatory subsets is finely regulated; dysregulation of this balance can lead to inflammation and autoimmunity. As such, the IL-23/Th17 pathway contributes to immune responses that play a role in defenses to microbial infection, as well as in the intestinal inflammation observed in both animal models of colitis and human IBD.

(Inflamm Bowel Dis 2009)

The interleukin (IL)-23 pathway has recently been identified to play a critical role in a number of chronic inflammatory diseases, including inflammatory bowel disease (IBD), psoriasis, multiple sclerosis, and arthritis, through both murine and human studies.1–15 The identification in IBD patients of associations in IL23R and regions that include other genes in the IL-23/Th17 pathway (e.g., IL12p40, STAT3, JAK2, CCR6, TNFSF15) has further highlighted the importance of proper IL-23 pathway regulation in intestinal immune homeostasis.16–20 IL-23 and IL-12 share both cytokine and cytokine receptor components. As a result, interventions targeting these shared components lead to effects on both pathways. Numerous studies are dissecting the respective contributions of IL-12 and IL-23 to inflammatory processes. IL-23 activation of the IL-23 receptor complex activates the JAK (Janus-activated kinase)-STAT (signal transducer and activator of transcription) signaling pathway and ultimately leads to a variety of downstream immune responses. Many studies have focused on the role of IL-23 in CD4+ Th17 lineage cells, characterized by IL-17 secretion and the expression of the transcription factor retinoic acid-related orphan receptor (ROR)γτ, although IL-23 has effects on other immune and nonimmune cells. The balance between effector T cell subsets, such as Th17 cells, and CD4+ T regulatory subsets, is finely regulated; dysregulation of this balance can lead to inflammation and autoimmunity (Fig. 1). Here we review the contributions of IL-23 to immune responses that play a role in defenses to microbial infection, as well as in the intestinal inflammation observed in both animal models of colitis and human IBD.

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Figure 1. Dysregulation of intestinal immune homeostasis in CD. Under intestinal homeostatic conditions there is a balance between proinflammatory cytokines from innate cell populations (e.g., IL1β, IL-6, IL-12, IL-23) and effector T cell populations (e.g., Th1 and Th17 cells), relative to regulatory T cell populations that are influenced by factors such as IL-10, TGFβ, and retinoic acid (RA) in the environment. During CD there is dysregulation of this balance with an increased proportion of innate and effector T cell populations along with the cytokines they produce.

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IL-23 CYTOKINE AND RECEPTOR EXPRESSION

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

IL-23 consists of a heterodimeric protein, made up of a p19 and a p40 subunit; the p40 subunit is shared with IL-12.21 Splice variants of IL-23p19 have been identified, with differential production of these variants in different cells types.22 IL-23 is expressed by hematopoietic and nonhematopoietic cells, including activated macrophages and dendritic cells, and endothelial cells.21, 23 IL-23 expression is induced by stimulation of myeloid-derived cells with pathogen ligands,24, 25 as well as other stimuli such as prostaglandin E2 and proinflammatory cytokines.26, 27 Consistent with this, IL-23 p19 expression is regulated by NFκB,24, 28 as well as by SMAD-3 and ATF-2.29 Negative regulation of IL-23 occurs through various mechanisms, including cytokines such as IL-10,30 IL-12,31 IL-25,32 IL-27,33 and IFNγ.34 Of note is that IFNγ inhibits IL-23 production under many,34 but not all situations. In fact, in some circumstances IFNγ cooperates to increase the inflammation observed through IL-23-mediated pathways,35 such that interactions between these pathways can be context-specific and complex.

The IL-23 receptor complex consists of IL23R and IL12Rβ1; the latter subunit is common to the IL-12 receptor complex.36 IL23R contains an extracellular domain (comprised of a signal sequence, an N-terminal immunoglobulin-like domain, and 2 cytokine receptor domains), a single transmembrane domain, and a cytoplasmic domain.36 IL23R is expressed on hematopoietic cells, including activated and memory T cells, NKT cells, eosinophils, dendritic cells, macrophages, and microglia,8, 21, 36, 37 as well as nonhematopoietic cells such as keratinocytes.7, 38 Regulated expression of IL23R plays a key role in leukocyte subset differentiation and function. Factors increasing IL23R mRNA expression include IL-23 itself, IL-6, IL-21, T cell activation, and TGFβ; importantly TGFβ effects are dose-dependent.4, 36, 39–43 IL23R mRNA induction is dependent on STAT3 and RORγτ for a number of these stimuli.39, 41

Studies over the years that utilized blockade of either the shared cytokine subunit (p40) or receptor subunit (IL12Rβ1) in which effects were attributed to the IL-12 pathway are now being reanalyzed with a focus on the differential role of IL-23 and IL-12 pathways through deletion or blockade of the unique subunits of these cytokine pathways.

IL-23 RECEPTOR COMPLEX SIGNALING

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

Like many cytokines, IL-23 engagement of the IL-23 receptor complex results in activation of the JAK-STAT signaling pathway. JAK2 is normally associated with IL23R, while engagement of STAT3 is ligand-dependent.21, 36 Associations in regions that include JAK2 and STAT3 have been identified in patients with both Crohn's disease (CD) and ulcerative colitis (UC) (Fig. 2).17–20 In addition to what is predominantly STAT3 activation, IL-23 can activate STAT1, STAT4, and STAT5.36, 44 STAT activation results in STAT dimerization and translocation to the nucleus, where STATs function as transcription factors. SOCS (suppressor of cytokine signaling) proteins are potent cytokine inhibitors and can directly bind JAK kinases; IL-23-mediated signaling can be inhibited by SOCS3.9 Other signaling pathways contributing to IL-23 responses include PI3K/Akt, tyrosine kinase 2, and NFκB; the relative contributions of these pathways may vary depending on the responding cell.36, 38, 45

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Figure 2. IBD gene region associations include multiple proteins implicated in the IL-23/Th17 pathway. Activation of naïve CD4+ T cells in the context of TGF-β, IL-6, and IL-1β leads to induction of RORγτ and IL-23R expression, along with cytokines that can amplify Th17 differentiation, such as IL-21. Antigen presenting cells produce IL-23, which is likely involved at multiple stages of Th17 lineage fate, including differentiation, amplification, and stabilization. Key transcription factors leading to IL-17-producing cells include RORγτ, RORα, and STAT3. Surface expression of IL-23R and CCR6 represent phenotypic features of IL-17-producing cells. Upon engagement of IL-23 (composed of p19 and p40) to its IL-23 receptor complex (composed of IL23R and IL23Rβ1), activation of JAK2 and STAT3 occurs. The cytokines secreted by Th17 cells lead to a combination of inflammatory and regulatory functions. The asterisk represents proteins encoded by genes included in regions associated with CD. †Associations also observed with UC (e.g., IL23R, JAK2, STAT3, and p40).

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IL-23/TH17 PATHWAY FUNCTIONAL ROLES

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

The key role of the IL-23 pathway in mediating inflammation has been established through in vivo models targeting the IL-23p19 subunit by either increasing or blocking expression. IL-23p19 transgenic mice demonstrate severe systemic inflammation, including in the small and large intestine.46 In contrast, while IL-23p19-deficient mice demonstrate no overt abnormalities, they have defects in T cell-dependent antibody production, memory T cell function, and decreased IL-17 production.47 Importantly, murine models in which IL-23p19 expression is experimentally regulated have contributed to defining the role of this pathway in intestinal defense and inflammation and will be discussed in the relevant sections to follow.

IL-23 and Th17 Cell Function

Mucosal sites, such as the intestinal lamina propria, demonstrate the constitutive presence of IL-23 and of IL-17-producing cells.4, 48, 49 In addition to the IL-17 isoforms (e.g., IL-17A, IL-17F) that characterize Th17 cells, Th17 cells also produce IL-6, IL-21, IL-22, IL-26, and various chemokines, such as CCL20.7, 50–54 IL-17 ultimately drives microbial defenses, as well as chronic inflammation during autoimmunity, through various mechanisms.8, 55 For example, IL-17 can contribute to neutrophil migration, expansion, and function.56 Moreover, IL-17 enhances dendritic cell maturation, T cell priming, and cellular (e.g., fibroblasts, endothelial cells, macrophages, epithelial cells) production of inflammatory mediators (e.g., IL-1, IL-6, IL-8, TNF, GM-CSF, NOS-2, prostaglandin E2, metalloproteases, and chemokines),57 although IL-17 can also contribute to certain downregulatory outcomes.58 In addition to proinflammatory cytokines, Th17 cells produce cytokines that can downregulate inflammation (e.g., IL-10 and IL-22).

The role of IL-23 in Th17 cell function has been an area of active research and is not yet fully defined. The role of IL-23 in Th17 differentiation specifically has been controversial; however, there is evidence for IL-23 contributions to Th17 cell expansion, stabilization, and/or conditioning for a fully inflammatory cell phenotype.8, 31, 59–70 For some of the implicated IL-23 contributions, in vitro and in vivo studies do not fully corroborate these IL-23 roles and additional studies will be needed to resolve these differences.

In contrast to CD4+ and CD8+ naïve T cells, NKT cells constitutively express IL23R and RORγτ.71 Both murine and human NKT cells can rapidly produce IL-17 on IL-23 stimulation, in particular in cooperation with T cell receptor stimulation, thereby implicating a role for the innate immune system in early production of IL-17.71

Th17 Cell Differentiation and Regulation

There is a critical equilibrium between Th17 and Treg cells mediated in part by the common requirement for TGFβ during differentiation. Specifically, TGFβ in the context of murine naïve CD4+ T cell activation leads to induced regulatory FoxP3+ T cells, whereas addition to TGFβ of IL-6 or other Toll-like receptor (TLR)-induced inflammatory signals results in a distinct outcome of IL-17-producing T cells.11, 66, 72 IL-6, in turn, stimulates IL-21 that acts in an autocrine loop to induce additional IL-21, IL23R, RORγτ, and IL-17.4, 39, 51, 73 In vivo contributory roles to IL-17 production have been confirmed for TGFβ, IL-1, IL-6, and IL-21 in murine models.4, 39, 48, 66, 74–76

Signaling pathways contributing to establishment of IL-17-producing cells include STAT3, which is required for IL-17, IL-21, IL-22, and IL23R expression.39, 51, 77, 78 Requirements for PI3K, NFκB, and PKCθ have also been reported.75 Studies investigating the requirement for STAT4 in Th17 differentiation have yielded mixed results.64, 65, 78 On the other hand, SOCS3 negatively regulates IL-17 production and STAT3 phosphorylation in response to IL-23 stimulation.9, 79

Distinct Th17 cell lineage determination is accompanied by expression of specific transcription factors. The nuclear receptor RORγτ, requiring STAT3 and induced by IL-6, IL-23, and TGFβ, is necessary for IL-17 production.4, 39, 41, 52–54, 80 However, other transcription factors, including RORα, IRF4, STAT3, and Runx1, are also important in IL-17 production.77, 81–83 The balance and phenotype of IL-17-producing T cells is undergoing continuous cross-regulation with other cell populations, including Th1, Th2, and Treg cells. Th1 cytokines (e.g., IFNγ, IL-2, IL-12) and Th2 cytokines (e.g., IL-4) can downregulate IL-17 production.31, 48, 59, 64–66, 84–87 Of note is that IFNγ can also contribute to Th17 function,88 and Th1 and Th17 cells frequently coexist at sites of tissue inflammation, such that the interactions between Th lineage cytokines are complex. Interactions between FoxP3 and RORγτ also play a role in the Treg/Th17 cell equilibrium.82, 89–91 Moreover, CD25+ FoxP3+ Treg cells can differentiate into IL-17-producing cells.92 Therefore, there is an ongoing balance between differentiation toward regulatory T cell populations (tolerance) and specific effector T cells that enhance host defense against particular pathogens (defense) (Fig 1).

Specific Issues in Human IL-17-Producing T Cell Differentiation

Human IL-17-producing CD4+ T cells express RORγτ, CCR6 (chemokine receptor 6), and CD161 along with cytokines similar to those observed in murine Th17 cells (e.g., IL-6, IL-17, IL-21, IL-22, and IL-26).7, 52, 54, 80, 91, 93–95 CCR6 interacts with the chemokine CCL20 and the expression of this chemokine ligand:receptor pair is enriched at epithelial sites, including the intestine.96–98 Furthermore, association in a genetic region that includes CCR6 has been identified in patients with CD.17 Unexpectedly, CCR6 deletion in mice led to an increase in IL-17-producing cells,90 revealing a complex role for this receptor in regulating immune responses.

Certain human–murine Th17 cell differences have been noted to date, including: 1) conditions for in vitro Th17 differentiation and the precise role of IL-23 in this process, and 2) heterogeneity in the cytokines produced. In fact, some debate exists as to whether human Th17 cells can be classified as a distinct lineage; further studies will be required to resolve this controversy. Some reports demonstrated that conditions established for murine Th17 cell differentiation (e.g., TGFβ, IL-6, anti-IL-4, anti-IFNγ) did not enhance IL-17 production in human T cells, and that in fact, TGFβ decreased IL-17 production.7, 40, 72, 99 However, more recent studies demonstrated that TGFβ is necessary for optimal IL-17 production during differentiation, along with IL-1β with or without IL-6 and/or IL-21.52–54 In addition, these recent studies found that IL-23 enhanced IL-17 production during differentiation.34, 52–54 Differences between these human T cell studies have been attributed to the inability to fully recapitulate in vivo differentiation requirements, the composition of serum utilized in the media in particular with respect to TGFβ, and differences in the purity of the starting naïve T cell population. Consistent with this, IL-17 secretion is much higher in memory compared to naïve CD4+ T cells.40, 100 Importantly, levels of IL-17 secreted from both naïve in vitro differentiated Th17 cells and memory T cells can differ between individuals on the order of greater than 10-fold,7 likely reflecting a combination of genetic and environmental exposures. These interindividual differences in Th17 lineage cytokine secretion ultimately lead to differential outcomes at the level of both defenses and tissue inflammation.

Another distinction in human IL-17-producing T cells has been the observation that both ex vivo memory and in vitro differentiated IL-17-producing T cells contain populations of cells coproducing IFNγ.7, 40, 54, 80, 99, 101 Whether this represents transitional differentiating cells, a distinct population of Th17 cells, or functional plasticity of human Th17 cells is not yet clear. In fact, a number of secreted cytokines are shared between Th1 and Th17 cells and slight alterations of in vitro conditions can lead to heterogeneous T cell populations that secrete a spectrum of cytokines reflective of typically defined Th1, Th17, and Treg cells.7, 52–54 The degree of reversibility in various stages of human Th17 differentiation has yet to be fully determined. However, plasticity is likely important for flexibility in contending with varying environmental challenges.

Costimulation Requirements for Th17

Whether Th17 cells require a distinct constellation of costimulatory molecules is not yet known. CD28 and ICOS, in the presence of IL-23, can enhance IL-17 production, while OX40 studies are conflicting.65, 102, 103 Associations with regions that include ICOSLG and tumor necrosis factor superfamily (TNFSF)15 have been identified in CD patients.17 Interactions between TNFSF15, expression of which is increased colitis,104, 105 and DR3, enhance IL-17-producing T cell differentiation and proliferation.106 However, DR3 and TNFSF15 interactions can also enhance Th1104, 107–109 and Th2110 cytokine production, indicating some nonspecificity in these costimulation effects.

Microbial Defenses

The IL-23/Th17 pathway is important in antimicrobial defenses; the intestine is a critical site of ongoing host–microbial interactions. The IL-23/Th17 pathway contributes to defense against fungi (e.g., Cryptococcus neoformans,114Candida albicans,94, 115Aspergillus fumigatus,116), bacteria (e.g., Klebsiella pneumoniae,117Salmonella enteriditis,118Bordetella pertussis,119Citrobacter rodentium,67 and Mycoplasma pneumonia23) and mycobacteria.111–113 Moreover, vaccine-induced protective immunity can require an intact IL-23/Th17 pathway.70, 119 Studies have sought to identify specificity in the microbial antigens able to induce IL-23 relative to Th1- or Th2-related cytokines. IL-23 is induced in human dendritic cells stimulated with Nod2 and TLR2 ligands.34, 100, 115 These Nod2- and TLR2-activated human dendritic cells, in turn, induce IL-17-producing CD4+ T cells, particularly in memory T cell populations.100 However, other studies have demonstrated that this IL-23/Th17 differentiation is not maintained with the addition of other innate receptor ligands.120 Furthermore, studies have demonstrated IL-23 production by human dendritic cells or monocytes upon stimulation of various TLR, including TLR3, TLR4, TLR5, and TLR8.25, 72 Therefore, it is not yet clear if there is induction of a specific IL-23/Th17 pathway response in the context of distinct microbial ligands.

Th17-Independent IL-23 Functions

In addition to its role in IL-17-producing cells, IL-23 can mediate IL-17-independent functions.3, 7, 14, 38, 121 IL-23 can enhance antimicrobial peptide (e.g., β-defensin 2, DEFB4, S100A8, S100A9) upregulation from keratinocytes.7, 38 Moreover, IL-23 can result in tissue injury, including intestinal inflammation, in the absence of T cells and B cells,121, 122 suggesting a role for innate cells. Consistent with this, IL23R is expressed on innate cell populations (e.g., dendritic cells, macrophages, eosinophils),8, 36, 37 and IL-23 contributes to production of proinflammatory mediators from these cells.8, 37, 121 That innate cells both produce IL-23 and express IL23R, along with IL-23 functional roles on these cells,21, 36, 123 suggests autocrine amplification within innate cells.

IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

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.

IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

The identification of genetic variants that are associated with common, complex genetic disorders, such as IBD, has been greatly accelerated through the application of genome-wide association (GWA) studies. GWA studies type several hundred thousand single nucleotide polymorphisms (SNPs) throughout the genome. In most cases, the associations do not directly implicate causal alleles, but because of linkage disequilibrium patterns, likely represent genes driving the association signals in these regions. Recent GWA studies led to the identification of multiple SNPs in the IL23R gene region (chromosome 1p31) as having an association with both CD and UC, with the association now replicated in a number of studies.16–18, 20, 163–172 One of these polymorphisms, Arg381Gln, is in the proximal cytoplasmic tail of IL23R and is highly conserved between species. The Arg381Gln IL23R allele confers a 2–3-fold protection against developing CD.16 In addition to Arg381Gln, other independent associations were identified in intronic and intergenic SNPs contained within a haplotype block encompassing the C-terminus 7 exons (of 12) and extending into the intergenic region between IL23R and its close homolog, IL12B2.16 The mechanism through which these polymorphisms confer either risk or protection from IBD have yet to be reported. A number of the identified IL23R variants are intronic; they may affect regulation of expression or of splice variants. Studies have not yet found interactions between IL23R and Nod2 genotype status,165 and IL23R genotype–phenotype correlations have yet to be reported. IL23R polymorphisms have also been associated with a number of other inflammatory diseases including psoriasis173 and ankylosing spondylitis,174 highlighting shared pathways between certain inflammatory disorders. In addition to IL23R, associations in regions that include 3 other genes that play a direct role in IL-23/Th17 pathway signaling have been observed for both CD and UC patients, specifically p40 (IL12B), STAT3, and JAK2 (Fig. 2).17–20 Taken together, these data demonstrate genetic association of multiple IL-23 pathway genes to CD and UC susceptibility, highlighting the common role of the IL-23/Th17 pathway in both disorders.

Two additional CD-associated regions include genes that play a role in Th17 cells, namely, CCR6 and TNFSF1517 (Fig. 2). CCR6 is expressed by various immune cells, including IL-17-producing cells,52, 80, 94, 95, 175 and it participates in cell trafficking. TNFSF15 is of particular interest because in a Japanese GWA study176 it represented the most significant association, there by highlighting an association in CD observed in both European ancestry177, 178 and Asian populations.176 TNFSF15 expression is upregulated in CD,105, 179 and TNFSF15 enhances T cell IL-17-production,106 although multiple studies have shown roles for TNFSF15 in other Th subsets as well.105, 107–110 Finally, CD associations have been observed in the ICOSLG gene region,17 which may be important given the enhancing effect of ICOS stimulation on IL-17 production.65, 103 Although some of the implicated gene associations encode for proteins that are not unique to the IL-23/Th17 pathway, the combined associations of IL23R, p40(IL12B) STAT3, JAK2, CCR6, and TNFSF15 strongly implicate the IL-23 pathway and Th17 cells as central players in intestinal inflammation.

THERAPEUTIC IMPLICATIONS

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES

The IL-23 pathway has been targeted for therapeutic intervention in a number of diseases, generally through use of anti-p40 antibodies that target both IL-23 and IL-12. Improvement with anti-p40 therapy is observed in plaque psoriasis.180 Importantly, anti-p40 therapies have proved promising in CD,181, 182 with cytokine secretion from colonic lamina propria mononuclear cells decreased for IL-6, IL-12p70, IL-17, and IL-23 in patients after anti-p40 therapy.162 Because IL-23/Th17 pathway responses to microbes are important at mucosal surfaces in comparison to potentially more systemic effects of the IL-12 pathway,121 specific targeting of the IL-23p19/IL23R pathway may be particularly effective in blocking organ- and intestinal-specific inflammation. However, it remains to be determined if specific blockade of either IL-23 or IL-17 is safe and effective in human IBD. In light of the important role of the IL-23/Th17 pathway in microbial defenses, the complex cross-regulation of the various T cell lineage pathways, and the differential role these distinct lineage cytokines may play at different phases of a disease process, blockade of IL-23/Th17 pathways for therapeutic purposes will also require close monitoring for effects on these IL-23-mediated functions.

Blockade of costimulatory pathways such as TNFSF15-DR3 and ICOS-ICOS ligand interactions have been considered given their role in mediating T cell responses, including IL-17 production, although blockade effects would not be specific to IL-17 production. In addition, medications such as the statins, used as cholesterol-lowering agents, also demonstrate immunomodulatory activity with decreased IL-6 and IL-23 expression, thought to be due to increased SOCS3 and SOCS7 as well as cytokines that downregulate IL-23 expression (e.g., IFNγ, IL-4, and IL-27).183 Finally, the success of anti-TNF therapies highlights an important pathway mediating the inflammation observed in IBD. TNF can be secreted by both Th1 and Th17 lineage populations, with both populations present at sites of intestinal inflammation. TNF is secreted by various other cell populations (e.g., macrophages, dendritic cells) and has pleiotropic effects, such that insights into mechanisms of disease given the efficacy of anti-TNF therapies have yet to be fully understood and integrated into new insights in the IL-23/Th17 pathway. Importantly, as we develop a better understanding of the genetic, immune, and environmental mechanisms leading to IBD, we will hopefully be in a position to improve the specificity of inflammatory pathway targeting during therapy.

REFERENCES

  1. Top of page
  2. Abstract
  3. IL-23 CYTOKINE AND RECEPTOR EXPRESSION
  4. IL-23 RECEPTOR COMPLEX SIGNALING
  5. IL-23/TH17 PATHWAY FUNCTIONAL ROLES
  6. IL-23/TH17 PATHWAY AND INTESTINAL INFLAMMATION
  7. IBD GENE ASSOCIATIONS AND THE IL-23/TH17 PATHWAY
  8. THERAPEUTIC IMPLICATIONS
  9. REFERENCES
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