Resident bacteria-stimulated IL-10-secreting B cells ameliorate T cell-mediated colitis by inducing Tr-1 cells that require IL-27-signaling.

Objective: Regulatory roles of IL-10-producing B cells in colitis are not fully understood. The aim of this study is to explore the molecular mechanisms by which these cells modulate mucosal homeostasis.

Methods: CD4+ T cells from WT, Il10-/- or Il27ra-/- mice were co-transferred with B cells from specific pathogen-free (SPF) or germ-free (GF) WT or Il10-/- mice into Rag2-/-Il10-/- (DKO) mice and the severity of colitis and intestinal regulatory T cell populations were characterized. In vitro, WT or Il10-/- B cells were co-cultured with unfractionated, naïve or regulatory T cells plus Il10-/- antigen-presenting cells and stimulated with cecal bacterial lysate (CBL) with or without IL-27 or anti-IL-10R blockade. Gene expressions, cytokines in the supernatant and cell populations were assessed.

Results: WT but not Il10-/- B cells attenuate Th1/Th17-mediated colitis in DKO mice that also received WT but not Il10-/- T cells. In vitro, CBL-stimulated WT B cells secrete abundant IL-10 and suppress IFNγ and IL-17a-production by T cells without requiring cell contact. Although both WT and Il10-/- B cells induce Foxp3+CD4+ Tregs, only WT B cells induce IL-10-producing (Foxp3-negative) T regulatory-1 (Tr-1) cells both in vivo and in vitro. However, IL-10-producing B cells did not attenuate colitis or induce Tr-1 cells in the absence of T cell IL-27-signaling in vivo. WT B cell-dependent Tr-1 induction and concomitant decreased IFNγ-secretion were also mediated by T-cell IL-27-signaling in vitro.

Conclusions: IL-10-secreting B cells activated by physiologically-relevant bacteria ameliorate T cell-mediated colitis and contribute to intestinal homeostasis by suppressing effector T cells and inducing Tr-1 cells via IL-27-signaling on T cells.