T helper 9 cells induced by plasmacytoid dendritic cells regulate interleukin-17 in multiple sclerosis

Multiple sclerosis (MS) is a chronic disease of the central nervous system characterized by persistent inflammation orchestrated by CD4 T helper (Th) cells. In particular, Th1 and Th17 cells amplify, while T regulatory cells moderate inflammation. The role of other Th subsets in MS is not clear. In this study we investigated the generation of different Th responses by human dendritic cells (DCs) in MS. We compared the production of several Th cytokines by naive CD4 T cells polarized with myeloid and plasmacytoid dendritic cells (mDCs and pDCs) in healthy donors and relapsing-remitting (RR) MS patients. We found that resiquimod-stimulated mDCs were able to activate Th17 differentiation, while pDCs induced interleukin (IL)-10-producing Th cells. Surprisingly, resiquimod-stimulated pDCs from MS patients also significantly induced the differentiation of Th9 cells, which produce IL-9 and are known to be involved in allergic diseases. We investigated the potential role of IL-9 in MS. We found that IL-9 activated signal transducers and activators of transcription (STAT)1 and STAT5 phosphorylation, and interfered with IL-17 and interferon regulatory transcription factor (IRF)-4 expression in Th17-polarized cells. Moreover, in the cerebrospinal fluid (CSF) of 107 RR-MS patients, IL-9 inversely correlated with indexes of inflammatory activity, neurodegeneration, and disability progression of MS. High levels of IL-9 were associated with the absence of IL-17 in the CSF of RR-MS patients. Our results demonstrate a Th9-inducing potential of pDCs in MS, suggesting an immunoregulatory role leading to attenuation of the exaggerated Th17 inflammatory response.