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Background & Aims: The regulatory roles of interleukin-10 (IL10)-producing B cells in colitis are not fully understood, so we explored the molecular mechanisms by which these cells modulate mucosal homeostasis. Methods: CD4+ T cells from wild-type (WT), Il10â/â, or Il27raâ/â mice were cotransferred with B cells from specific pathogen-free (SPF) or germ-free (GF) WT or Il10â/â mice into Rag2â/âIl10â/â(double-knockout) mice, and the severity of colitis and intestinal regulatory T-cell populations were characterized. In vitro, WT or Il10â/â B cells were cocultured with unfractionated, naïve or regulatory T cells plus Il10â/â antigen-presenting cells and stimulated with cecal bacterial lysate (CBL) with or without IL27 or anti-IL10R blockade. Gene expressions, cytokines in the supernatant and cell populations were assessed. Results: WT but not Il10â/â B cells attenuated T helper cell TH1/TH17-mediated colitis in double-knockout mice that also received WT but not Il10â/â T cells. In vitro, CBL-stimulated WT B cells secrete abundant IL10 and suppress interferon-γ (IFNγ) and IL17a-production by T cells without requiring cell contact. Although both WT and Il10â/â B cells induced Foxp3+CD4+ T-regulatory cells, only WT B cells induced IL10-producing (Foxp3-negative) T regulatory-1 (Tr-1) cells both in vivo and in vitro. However, IL10-producing B cells did not attenuate colitis or induce Tr-1 cells in the absence of T cell IL27 signaling in vivo. WT B cell-dependent Tr-1 induction and concomitant decreased IFNγ-secretion were also mediated by T-cell IL27-signaling in vitro. Conclusions: IL10-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 IL27-signaling on T cells. Keywords: Experimental Colitis, IBD, Immunoregulation, Regulatory B Cells