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.

Mononuclear leukocytes preferentially accumulate in the central nervous system (CNS) during the course of experimental autoimmune encephalomyelitis (EAE). To address factors that govern leukocyte trafficking in EAE, we monitored expression of mRNAs encoding IP-10 and JE/MCP-1, which are members of a family of chemoattractant cytokines. A transient burst of IP-10 and JE/MCP-1 mRNA accumulation in the CNS occurred, in close relation to the onset of histologic and clinical disease. In situ hybridizations showed, unexpectedly, that astrocytes were the major source of mRNAs encoding IP-10 and JE/MCP-1. These observations implicate astrocyte-derived cytokines as potential chemoattractants for inflammatory cells during EAE.

Intrathymic expression of tissue-specific self antigens may be involved in immunological tolerance and protection from autoimmune disease. We have analyzed the role of T-cell tolerance to proteolipid protein (PLP), the main protein of the myelin sheath, in susceptibility to experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. Intrathymic expression of PLP was largely restricted to the shorter splice variant, DM20. Expression of DM20 by thymic epithelium was sufficient to confer T-cell tolerance to all epitopes of PLP in EAE-resistant C57BL/6 mice. In contrast, the major T-cell epitope in SJL/J mice was only encoded by the central nervous system-specific exon of PLP, but not by thymic DM20. Thus, lack of tolerance to this epitope offers an explanation for the exquisite susceptibility of SJL/J mice to EAE. As PLP expression in the human thymus is also restricted to the DM20 isoform, these findings have implications for selection of the autoimmune T-cell repertoire in multiple sclerosis.

SUMMARY Interleukin-17 (IL-17) secreted by T helper 17 (Th17) cells is essential in the development of experimental autoimmune encephalomyelitis (EAE). However, it remains unclear how IL-17-mediated signaling in different cellular compartments participates in the central nervous system (CNS) inflammatory process. We examined CNS inflammation in mice with specific deletion of Act1, a critical component required for IL-17 signaling, in endothelial cells, macrophages and microglia, and neuroectoderm (neurons, astrocytes, and oligodendrocytes). In Act1-deficient mice, Th17 cells showed normal infiltration into the CNS but failed to recruit lymphocytes, neutrophils, and macrophages. Act1 deficiency in endothelial cells or in macrophages and microglia did not substantially impact the development of EAE. However, targeted Act1 deficiency in neuroectoderm derived CNS resident cells resulted in markedly reduced severity in EAE. Specifically, Act1-deficient astrocytes showed impaired IL-17-mediated inflammatory gene induction. Thus, astroctyes are critical in IL-17-Act1 mediated leukocyte recruitment during autoimmune induced inflammation of the CNS.

Despite constant contact with the large population of commensal bacteria, the colonic mucosa is normally hyporesponsive to these potentially proinflammatory signals. Here we report that the single immunoglobulin IL-1 receptor-related molecule (SIGIRR), a negative regulator for Toll-IL-1R signaling, plays a critical role in gut homeostasis, intestinal inflammation, and colitis-associated tumorigenesis by maintaining the microbial tolerance of the colonic epithelium. SIGIRR-deficient (Sigirr(-/-)) colonic epithelial cells displayed commensal bacteria-dependent homeostatic defects, as shown by constitutive upregulation of inflammatory genes, increased inflammatory responses to dextran sulfate sodium (DSS) challenge, and increased Azoxymethane (AOM)+DSS-induced colitis-associated tumorigenesis. Gut epithelium-specific expression of the SIGIRR transgene in the SIGIRR-deficient background reduced the cell survival of the SIGIRR-deficient colon epithelium, abrogated the hypersensitivity of the Sigirr(-/-) mice to DSS-induced colitis, and reduced AOM+DSS-induced tumorigenesis. Taken together, our results indicate that epithelium-derived SIGIRR is critical in controlling the homeostasis and innate immune responses of the colon to enteric microflora.