IL-10 and ICOS differentially regulate T cell responses in the brain during chronic Toxoplasma gondii infection
Control of chronic CNS infection with the parasite Toxoplasma gondii requires an ongoing T cell response in the brain. Immunosuppressive cytokines are also important for preventing lethal immunopathology during chronic infection. To explore the loss of suppressive cytokine exclusively during the chronic phase of infection we blocked IL-10 receptor (IL-10R). Blockade was associated with widespread changes in the inflammatory response, including increased antigen presenting cell (APC) activation, expansion of CD4+ T cells, and increased neutrophil recruitment to the brain, consistent with previous reports. We then sought to identify regulatory mechanisms contributing to IL-10 production, focusing on ICOS (inducible T cell costimulator), a molecule that promotes IL-10 production in many systems. Unexpectedly, ICOS-ligand (ICOSL) blockade led to a local expansion of effector T cells in the inflamed brain without affecting IL-10 production or APC activation. Instead, we found that ICOSL blockade led to changes in T cells associated with their proliferation and survival. Specifically, we observed increased expression of IL-2 associated signaling molecules, including CD25, STAT5 phosphorylation, Ki67, and Bcl-2 in T cells in the brain. Interestingly, increases in CD25 and Bcl-2 were not observed following IL-10R blockade. Also unlike IL-10R blockade, ICOSL blockade led to an expansion of both CD8+ and CD4+ T cells in the brain, with no expansion of peripheral T cell populations or neutrophil recruitment to the brain Overall, these results suggest that IL-10 and ICOS differentially regulate T cell responses in the brain during chronic T. gondii infection.