This project focuses on how antigens are processed in the intestine of mice, and how antigens presented by different populations of dendritic cells (DC) and macrophages influence immune responses in the intestine. While it is clear that the outcome of oral antigen exposure can be either positive, i.e., the development of mucosal IgA responses, and in some cases the induction of systemic immunity as well, or negative, i.e., the induction of oral tolerance, the details of why one or the other outcome occurs is complex and poorly understood. Furthermore, the normal intestinal immune response to symbiotic/commensal bacteria which allows for one to tolerate these organisms without the onset of inflammation is essential for immune homeostasis in the intestine, as an defect in this homeostasis results in inflammatory bowel disease. Furthermore, while it is known that the antigen formulation, the presence of adjuvants, and the antigen dose, as well as genetic factors, can affect mucosal immune responses, how these act to influence immunity has never been established. Therefore, this project focuses on how immune responses are regulated in the intestine with a focus on the roles of dendritic cells and macrophages in this regulation, and on factors that control inflammatory functions of these cells. In prior studies we defined different antigen-presenting cell populations in the Peyer's patch (PP) and lamina propria and have detailed the surface phenotype, function, and migration of DCs in the PP using in situ immunofluorecense microscopy and in situ hybridization, flow cytometry of purified cells, and in vitro assays of cytokine production (ELISA and quantitative RT-PCR) and T cell differentiation. PP DCs have the unique capacity to induce the differentiation of T cells that produce high levels of IL-10, a cytokine important for the IgA B cell differentiation. These studies thus were some of the first to directly demonstrate that DCs from different tissues may be unique in their ability to induce tissue specific immunity. We also demonstrated that DCs the subepithelial dome region of the PP process viral antigen from virally infected apoptotic epithelial cells following reovirus infection. Furthermore, we determined that clearance of lethal experimental infection with a model mucosal virus infection, type 1 reovirus, is dependent on type-1 interferon production in the PP, that type-1 interferon production by dendritic cells within the PP is a primary determinant of whether this mucosal pathogen survives and is disseminated to other tissues. In other studies, we localized and studied the function of plasmacytoid DCs (pDCs) from the Peyer's patch. PP pDCs were localized to the interfollicular and subepithelial dome region of the PP by immunohistochemistry, exressed typical surface markers for pDCs from other tissues, as examined by flow cytometry, and produced IL-12 but little type-1 interferon following stimulation with either influenza virus, or bacterial DNA. The lack of type-1 interferon production by PP pDCs is significant because this is different from pDCs from other tissues. Furthermore, factors present in the microenvironment of the PP, in particular PGE(2), TGFbeta, and IL-10 were able to suppress type-1 interferon production by pDCs from the spleen, suggesting that microenvironmental factors may influence type-1 interferon production by pDCs. Furthermore, we have performed studies in humans the ability of granulocyte-macrophage colony stimulating factor (GM-CSF), a growth factor that is commonly used to treat organ transplant patients to improve their production of neutrophil and monocyte after immunosuppression, to treat patients with inflammatory bowel disease. Treatment efficacy was correlated with changes in peripheral blood T cell production of IL-10, and sustained changes in the numbers of plasmacytoid DCs. As plasmacytoid DCs have been shown to drive non-pathogenic Th2 responses, these studies suggest that treatmetns that enhance the number of plasmacytoid DCs in the blood, may have therapeutic efficacy in Th1-mediated autoimmune disorders. In FY2011, we have focused our efforts on two areas. First, we have defined sub-populations of macrophages and DCs in the mouse colon and are exploring their role in maintaining immune homeostasis in steady-state conditions and during inflammation in murine models of inflammatory bowel disease. We demonstrated four populations of cells based on surface markers that correlate with either a macrophage or DC phenotype, and have begun to understand their function in vivo. More specifically, we found that F4/80hi CX3CR1hi (CD11b+CD103-) cells account for 80% of mouse colonic lamina propria (cLP) MHC-IIhi cells. Both CD11c+ and CD11c- cells within this population were identified as MPs based on multiple criteria, including a MP transcriptome revealed by microarray analysis. These MPs constitutively released high levels of IL-10 at least partially in response to the microbiota via an MyD88-independent mechanism. In contrast, cells expressing low to intermediate levels of F4/80 and CX3CR1 were identified as DCs, based on phenotypic and functional analysis and comprise three separate CD11chi cell populations: CD103+CX3CR1-CD11b- DCs, CD103+CX3CR1-CD11b+ DCs and CD103-CX3CR1intCD11b+ DCs. In non-inflammatory conditions, Ly6Chi monocytes differentiated primarily into CD11c+, but not CD11c- MPs. In contrast, during colitis, Ly6Chi monocytes massively invaded the colon and differentiated into pro-inflammatory CD103-CX3CR1intCD11b+ DCs, which produced high levels of IL-12, IL-23, iNOS and TNFα. These findings demonstrated the dual capacity of Ly6Chi blood monocytes to differentiate into either regulatory MP or inflammatory DCs in the colon, and that the balance of these immunologically antagonistic cell types is dictated by microenvironmental conditions. These studies are important in that they delineate for the first time the precise definitions of macrophages and dendritic cells in the colon based on the use of a comprehensive array of surface markers, gene expression analysis, and development from defined circulating precursors. They also demonstrate the primary role played by peripheral blood monocytes in contributing to intestinal inflammation, and thus indicate that blocking the trafficking of this cell type to the inflamed colon could provide a novel strategy for treatment of inflammatory bowel disease. Second, we have addressed the role of type-1 interferons in the regulation of intestinal immunity and found an essential role for type-1 interferons in preventing abnormal inflammation in mouse models of inflammatory bowel disease. Therefore, type-1 interferons have essential roles both in prevention of infection by some, but not all, intestinal viruses, as well as in controling abnormal intestinal inflammation. We are currently delineating the mechanisms by which type-1 interferons control intestinal immune homeostasis. Finally, in collaborative studies we have helped demonstrate that colitis and intestinal inflammation in IL10(-/-) mice results from IL-13Rα2-mediated attenuation of IL-13 activity. In the absence of IL-13Rα2, IL-13 suppresses pro-inflammatory Th1 and Th17 responses. This implies thaeagents that block the IL-13 decoy receptor IL-13Rα2 might be developed for inflammatory bowel disease associated with increased levels of IFN-γand IL-17. Furthermore, we have helped identify an essential role for CCR2 in the accumulation of monocytes in the brain of mice during experimental West Nile Virus infection, primarily by acting to enhance the trafficking of monocytes from the bone-marrow to the blood. We also helped show that CCR6 plays an important role in the trafficking of a subset of blood monocytes into atheroscerotic lesions in mice.
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