BAFF promotes Th17 cells and aggravates experimental autoimmune encephalomyelitis

Background: BAFF, in addition to promoting B cell survival and differentiation, may affect T cells. The objective of this study was to determine the effect of BAFF on Th17 cell generation and its ramifications for the Th17 cell-driven disease, EAE. Methodology/Principal Findings: Th17 cells were increased in BAFF-Tg B6 (B6.BTg) mice and decreased in B6.Baff ^-/- mice. Th17 cells in B6.Baff ^-/- mice bearing a BAFF Tg (B6.Baff ^-/-.BTg mice) were identical to those in B6.BTg mice, indicating that membrane BAFF is dispensable for Th17 cell generation as long as soluble BAFF is plentiful. In T + non-T cell criss-cross co-cultures, Th17 cell generation was greatest in cultures containing B6.BTg T cells and lowest in cultures containing B6.Baff ^-/- T cells, regardless of the source of non-T cells. In cultures containing only T cells, Th17 cell generation followed an identical pattern. CD4 ⁺ cell expression of CD126 (IL-6R α chain) was increased in B6.BTg mice and decreased in B6.Baff ^-/- mice, and activation of STAT3 following stimulation with IL-6 + TGF-β was also greatest in B6.BTg cells and lowest in B6.Baff ^-/- cells. EAE was clinically and pathologically most severe in B6.BTg mice and least severe in B6.Baff ^-/- mice and correlated with MOG _35-55 peptide-induced Th17 cell responses. Conclusions/Significance: Collectively, these findings document a contribution of BAFF to pathogenic Th17 cell responses and suggest that BAFF antagonism may be efficacious in Th17 cell-driven diseases.