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IL-17–producing CD8 + T (Tc17) cells are detectible in multiple sclerosis (MS) lesions; however, their contribution to the disease is unknown. To identify functions of Tc17 cells, we induced EAE, a murine model of MS, in mice lacking IFN regulatory factor 4 (IRF4). IRF4-deficient mice failed to generate Tc17 and Th17 cells and were resistant to EAE. After adoptive transfer of WT CD8 + T cells and subsequent immunization for EAE induction in these mice, the CD8 + T cells developed a Tc17 phenotype in the periphery but could not infiltrate the CNS. Similarly, transfer of small numbers of WT CD4 + T cells alone did not evoke EAE, but when transferred together with CD8 + T cells, IL-17–producing CD4 + (Th17) T cells accumulated in the CNS and mice developed severe disease. Th17 accumulation and development of EAE required IL-17A production by CD8 + T cells, suggesting that Tc17 cells are required to promote CD4 + T cell–mediated induction of EAE. Accordingly, patients with early-stage MS harbored a greater number of Tc17 cells in the cerebrospinal fluid than in peripheral blood. Our results reveal that Tc17 cells contribute to the initiation of CNS autoimmunity in mice and humans by supporting Th17 cell pathogenicity.

Central nervous system (CNS) autoimmunity is regulated by the balance of pro-inflammatory cytokines and IL-10. Here we identify the transcriptional regulator Blimp1 as crucial to induce IL-10 in inflammatory T helper cells. Pre-committed Th17 cells respond to IL-27 and IL-12 by upregulating Blimp1 and adopt a Tr-1-like phenotype characterized by IL-10 and IFN-γ production. Accordingly, Blimp1-deficient effector T cells fail to produce IL-10, and deficiency in Tr-1 cell function leads to uncontrolled Th17 cell-driven CNS pathology without the need to stabilize the Th17 phenotype with IL-23. IL-23 counteracts IL-27 and IL-12-mediated effects on Tr-1-development reinforcing the pro-inflammatory phenotype of Th17 cells. Thus, the balance of IL-23 vs IL-12/IL-27 signals into CD4(+) effector T cells determines whether tissue inflammation is perpetuated or resolves.

PI3Ks control signal transduction triggered by growth factors and G-protein-coupled receptors and regulate an array of biological processes, including cellular proliferation, differentiation, survival and migration. Herein, we investigated the role of PI3Kγ in the pathogenesis of EAE. We show that, in the absence of PI3Kγ expression, clinical signs of EAE were delayed and mitigated. PI3Kγ-deficient myelin oligodendrocyte glycoprotein (MOG)(35-55) -specific CD4(+) T cells appeared later in the secondary lymphoid organs and in the CNS than their WT counterparts. Transfer of WT CD4(+) cells into PI3Kγ(-/-) mice prior to MOG(35-55) immunisation restored EAE severity to WT levels, supporting the relevance of PI3Kγ expression in Th cells for the pathogenesis of EAE; however, PI3Kγ was dispensable for Th1 and Th17 differentiation, thus excluding an altered expression of these pathogenetically relevant cytokines as the cause for ameliorated EAE in PI3Kγ(-/-) mice. These findings demonstrate that PI3Kγ contributes to the development of autoimmune CNS inflammation.