Bacillus-derived poly-γ-glutamic acid reciprocally regulates the differentiation of T helper 17 and regulatory T cells and attenuates experimental autoimmune encephalomyelitis

Forkhead box protein 3 (FoxP3⁺) regulatory T (T_reg) cells and interleukin (IL)-17-producing T helper 17 (Th17) cells have opposing effects on autoimmunity, as the former are crucial for maintaining self-tolerance while the latter play a key role in precipitating inflammatory autoimmune diseases. Here we report that Bacillus-derived poly-γ-glutamic acid (γ-PGA) signals naive CD4⁺ T cells to promote the selective differentiation of T_reg cells and to suppress the differentiation of Th17 cells. The γ-PGA inducibility of FoxP3 expression was due partially to transforming growth factor (TGF)-β induction through a Toll-like receptor (TLR)-4/myeloid differentiating factor 88 (MyD88)-dependent pathway. However, this pathway was dispensable for γ-PGA suppression of Th17 differentiation. γ-PGA inhibited IL-6-driven induction of Th17-specific factors including signal transducer and activator of transcription-3 (STAT-3) and retinoic acid-related orphan receptor γt (RORγt) while up-regulating the STAT-3 inhibitor suppressor of cytokine signalling 3 (SOCS3). Importantly, in vivo administration of γ-PGA attenuated the symptoms of experimental autoimmune encephalomyelitis and at the same time reduced Th17 cell infiltrates in the central nervous system. Thus, we have identified the microbe-associated molecular pattern, γ-PGA, as a novel regulator of autoimmune responses, capable of promoting the differentiation of anti-inflammatory T_reg cells and suppressing the differentiation of proinflammatory Th17 cells. These findings draw attention to the potential of γ-PGA for treating Th17 cell-mediated autoimmune diseases.