BACKGROUND: Regulatory B cells that exhibit the cell-surface CD1d(hi)CD5(+) phenotype and produce IL-10 are termed B10 cells. Although B10 cells exert potent suppressive functions in patients with various allergic and autoimmunity disorders, the precise signaling mechanisms required for B10 cell functions remain unknown. B-cell linker protein (BLNK) is an essential component of the B-cell antigen receptor signaling pathway and is required for optimal B-cell development. OBJECTIVE: We sought to elucidate the signaling pathways that are responsible for IL-10 production in B10 cells and in vivo mechanisms of how impaired B10 cell functions influence allergic and autoimmune responses. METHOD: For in vitro assays, splenic CD1d(hi)CD5(+) B cells from BLNK-deficient (BLNK(-/-)) mice were analyzed for intracellular signaling pathways and cytokine production. Contact hypersensitivity (CHS) and experimental autoimmune encephalomyelitis were examined by using BLNK(-/-) mice. RESULTS: Although the CD1d(hi)CD5(+) B-cell population was present in BLNK(-/-) mice, IL-10 production was impaired both in vitro and in vivo. BLNK(-/-) mice had exaggerated CHS and experimental autoimmune encephalomyelitis responses, which were normalized by adoptive transfer of splenic CD1d(hi)CD5(+) B cells from wild-type mice. In mice with CHS, BLNK(-/-) mice exhibited decreased B-cell and regulatory T-cell percentages and increased CD8(+) T-cell percentages in the skin and lymph nodes. In vitro BLNK was required for LPS-induced signal transducer and activator of transcription 3 phosphorylation in CD1d(hi)CD5(+) B cells. Finally, secreted IL-10 leads to autocrine expansion of IL-10-producing B cells. CONCLUSION: BLNK serves as a critical signaling component for B10 cell function by mediating IL-10 production.