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IL-17–producing CD4⁺ T cells (Th-17) contribute to the pathogenesis of experimental autoimmune encephalomyelitis and are associated with active disease in multiple sclerosis (MS). In addition to IL-17, Th-17 cells can also express IL-21, IL-22, and IL-6 under Th-17–polarizing conditions (IL-6 and transforming growth factor-β). In this study we investigated IL-21 and IL-21 receptor (IL-21R) expression in MS lesions by in situ hybridization and immunohistochemistry. We detected strongly IL-21⁺ infiltrating cells predominantly in acute but also in chronic active white matter MS lesions in which IL-21 expression was restricted to CD4⁺ cells. In contrast, IL-21R was much more broadly distributed on CD4⁺, CD19⁺, and CD8⁺ lymphocytes but not major histocompatibility complex class-II⁺ macrophages/microglia. Interestingly, in cortical areas we detected both IL-21 and IL-21R expression by neurons. These findings …