IL-23 plays a key role in Helicobacter hepaticus-induced T cell-dependent colitis

Distinct functional motifs within the IL-17 receptor regulate signal transduction and target gene expression

IL-17 is the founding member of a novel family of proinflammatory cytokines that defines a new class of CD4+ effector T cells, termed "Th17." Mounting evidence suggests that IL-17 and Th17 cells cause pathology in autoimmunity, but little is known about mechanisms of IL-17RA signaling. IL-17 through its receptor (IL-17RA) activates genes typical of innate immune cytokines, such as TNFalpha and IL-1beta, despite minimal sequence similarity in their respective receptors. A previous bioinformatics study predicted a subdomain in IL-17-family receptors with homology to a Toll/IL-1R (TIR) domain, termed the "SEFIR domain." However, the SEFIR domain lacks motifs critical for bona fide TIR domains, and its functionality was never verified. Here, we used a reconstitution system in IL-17RA-null fibroblasts to map functional domains within IL-17RA. We demonstrate that the SEFIR domain mediates IL-17RA signaling independently of classic TIR adaptors, such as MyD88 and TRIF. Moreover, we identified a previously undescribed"TIR-like loop" (TILL) required for activation of NF-kappaB, MAPK, and up-regulation of C/EBPbeta and C/EBPdelta. Mutagenesis of the TILL domain revealed a site analogous to the LPS(d) mutation in TLR4, which renders mice insensitive to LPS. However, a putative salt bridge typically found in TIR domains appears to be dispensable. We further identified a C-terminal domain required for activation of C/EBPbeta and induction of a subset IL-17 target genes. This structure-function analysis of a IL-17 superfamily receptor reveals important differences in IL-17RA compared with IL-1/TLR receptors.

Maturation of the mucosal immune system underlies colitis susceptibility in interleukin-10-deficient (IL-10−/−) mice

Elevated mucosal IL-12/23p40 and IFN-gamma accompany early inflammation in IL-10-deficient (IL-10(-/-)) mice and then later decline while inflammation persists. This report addresses whether this cytokine profile reflects disease progression or inherent, age-related changes in mucosal immunity. IL-10(-/-) and wild-type (WT) mice were maintained in an ultrabarrier facility or transferred to conventional housing at 3, 12, or 30 weeks of age. Weight, stool changes, and histologic features were followed. Lamina propria mononuclear cells were cultured for cytokine analysis by ELISA. Ultrabarrier-housed IL-10(-/-) mice are statistically indistinguishable from WT mice by weight, disease activity index, and histologic inflammation. IL-10(-/-) mice but not WT, transferred at 3 weeks, develop colitis gradually, reaching a significant, sustained maximum by 15 weeks of age. Transfer at 12 weeks induces rapid disease onset in both strains, maximal at 15 weeks of age. Inflammation persists in IL-10(-/-), and WT recover. IL-10(-/-) and WT mice transferred at 30 weeks demonstrate transient diarrhea and weight loss but no chronic inflammation. Probiotics delay symptom onset only in the 12-week-old group. IFN-gamma production from ultrabarrier-housed IL-10(-/-) mice is elevated at 12 weeks of age, and older animals have decreased IFN-gamma and increased IL-4. IL-10 is important for suppressing inflammation after transfer at 3 weeks of age and limiting inflammation after transfer at 12 weeks but has little influence at 30 weeks of age. Colitis onset, progression, and response to probiotic therapy vary with immune system age, suggesting that a distinct, Th1-driven, age-dependent cytokine profile may contribute to increased colitis susceptibility in otherwise healthy mice.

The straw that stirs the drink: insight into the pathogenesis of inflammatory bowel disease revealed through the study of microflora-induced inflammation in genetically modified mice

Abnormal response to enteric microflora is a critical factor driving bowel inflammation in patients with inflammatory bowel disease (IBD). Mice with genetically engineered mutations have played a central role in both formulating this hypothesis and elucidating the mechanism that normally protect the host from excessive inflammation within the bowel. One emerging theme is the importance of regulation within the innate immune system in protecting from microflora-driven pathology. In this review, I describe how genetically engineered mice have played a crucial role in shaping our conceptual understanding of pathways that regulate the development of chronic bowel inflammation, and furthermore, explore data derived from the study of genetically engineered mice that implicates the fundamental importance of regulation within the innate immune system in the control of this process.

Inhibitory effect of enterohepatic Helicobacter hepaticus on innate immune responses of mouse intestinal epithelial cells

Enterohepatic Helicobacter species infect the intestinal tracts and biliary trees of various mammals, including mice and humans, and are associated with chronic inflammatory diseases of the intestine, gallstone formation, and malignant transformation. The recent analysis of the whole genome sequence of the mouse enterohepatic species Helicobacter hepaticus allowed us to perform a functional analysis of bacterial factors that may play a role in these diseases. We tested the hypothesis that H. hepaticus suppresses or evades innate immune responses of mouse intestinal epithelial cells, which allows this pathogen to induce or contribute to chronic inflammatory disease. We demonstrated in the present study that the innate immune responses of intestinal epithelial cells to lipopolysaccharide (LPS) via Toll-like receptor 4 (TLR4) and to flagellin-mediated activation via TLR5 are reduced by H. hepaticus infection through soluble bacterial factors. In particular, H. hepaticus lysate and the soluble component LPS antagonized TLR4- and TLR5-mediated immune responses of intestinal epithelial cells. H. hepaticus lysate and LPS inhibited development of endotoxin tolerance to Escherichia coli LPS. Suppression of innate immune responses by H. hepaticus LPS thus may affect intestinal responses to the resident microbial flora, epithelial homeostasis, and intestinal inflammatory conditions.

The adaptor Act1 is required for interleukin 17-dependent signaling associated with autoimmune and inflammatory disease

T helper cells that produce interleukin 17 (IL-17) are associated with inflammation and the control of certain bacteria. We report here the essential involvement of the adaptor protein Act1 in IL-17 receptor (IL-17R) signaling and IL-17-dependent immune responses. After stimulation with IL-17, recruitment of Act1 to IL-17R required the IL-17R conserved cytoplasmic 'SEFIR' domain, followed by recruitment of the kinase TAK1 and E3 ubiquitin ligase TRAF6, which mediate 'downstream' activation of transcription factor NF-kappaB. IL-17-induced expression of inflammation-related genes was abolished in Act1-deficient primary astroglial and gut epithelial cells. This reduction was associated with much less inflammatory disease in vivo in both autoimmune encephalomyelitis and dextran sodium sulfate-induced colitis. Our data show that Act1 is essential in IL-17-dependent signaling in autoimmune and inflammatory disease.

Dependence of intestinal granuloma formation on unique myeloid DC-like cells

Granulomas represent a localized inflammatory reaction that is characteristically observed in many inflammatory conditions. However, the mechanisms of granuloma formation have not been fully defined. Herein we demonstrate, by using experimental models of intestinal inflammation, that a unique CD11c+ DC-like cell subset that exhibits phenotypic and functional features of immature myeloid DCs and is characterized by the expression of a macrophage marker (F4/80) produces large amounts of IL-23 and directly induces the development of granulomas under a Th1-predominant intestinal inflammatory condition. Importantly, both IL-4 and IgG contribute to the suppression of F4/80+ DC-like cell-mediated granuloma formation by regulating the function and differentiation of this cell subset. In addition, enteric flora is required for the F4/80+ DC-like cell-mediated granuloma formation. Collectively, our data provide what we believe are novel insights into the involvement of F4/80+ DC-like cells in intestinal granuloma formation and demonstrate the role of host (IL-4 and IgG) and environmental (enteric flora) factors that regulate this function.

Cytokine regulation of immunopathology in toxoplasmosis

Toxoplasma gondii infection is an important cause of central nervous system and ocular disease, both in immunocompromised and in certain immunocompetent populations. Although parasite-mediated host cell lysis is probably the principal cause of tissue destruction in immunodeficiency states, hypersensitivity and inflammatory responses may underlie severe disease in otherwise immuno-sufficient individuals. In this review, we have critically evaluated the body of experimental evidence indicating a role of CD4 T cells in systemic and local immunopathology associated with T. gondii infection. We also discuss the pathogenic roles of cytokines produced by T helper (Th) 1 and Th17 cells and the protective and homeostatic roles of interleukin (IL)-10, transforming growth factor-beta and IL-27 in modulating hypersensitivity responses induced by T. gondii.

Molecular pathogenesis of inflammatory bowel disease: genotypes, phenotypes and personalized medicine

Crohn's disease (CD) and ulcerative colitis (UC), also known as inflammatory bowel diseases (IBD), are characterized by chronic inflammation of the gastrointestinal tract. IBD is among the few complex diseases for which several genomic regions and specific genes have been identified and confirmed in multiple replication studies. We will review the different loci implicated in disease risk in the context of three proposed mechanisms leading to chronic inflammation of the gut mucosa: 1) deregulation of the innate immune response to enteric microflora or pathogens; 2) increased permeability across the epithelial barrier; and 3) defective regulation of the adaptive immune system. As our knowledge of genetic variation, analytical approaches and technology improves, additional genetic risk factors are expected to be identified. With the identification of novel risk variants, additional pathophysiological mechanisms are likely to emerge. The resulting discoveries will further our molecular understanding of IBD, potentially leading to improved disease classification and rational drug design. Moreover, these approaches and tools can be applied in the context of variable drug response with the goal of providing more personalized clinical management of patients with IBD.

Interleukin-23 drives innate and T cell-mediated intestinal inflammation

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract involving aberrant activation of innate and adaptive immune responses. We have used two complementary models of IBD to examine the roles of interleukin (IL)-12 family cytokines in bacterially induced intestinal inflammation. Our results clearly show that IL-23, but not IL-12, is essential for the induction of chronic intestinal inflammation mediated by innate or adaptive immune mechanisms. Depletion of IL-23 was associated with decreased proinflammatory responses in the intestine but had little impact on systemic T cell inflammatory responses. These results newly identify IL-23 as a driver of innate immune pathology in the intestine and suggest that selective targeting of IL-23 represents an attractive therapeutic approach in human IBD.