Pathogens use numerous methods to subvert host immune responses, including the modulation of host IL-10 production by diverse cell types. However, the B cell sources of IL-10 and their overall influence on innate and cellular immune responses have not been well characterized during infections. Using Listeria as a model pathogen, infection drove the acute expansion of a small subset of regulatory B cells (B10 cells) that potently suppress inflammation and autoimmunity through the production of IL-10. Unexpectedly, spleen bacteria loads were 92–97% lower in B10 cell-deficient CD19−/− mice, in mice depleted of mature B cells, and in mice treated with CD22 mAb to preferentially deplete B10 cells before infection. By contrast, the adoptive transfer of wild type B10 cells reduced bacterial clearance by 38-fold in CD19−/− mice through IL-10-dependent pathways. B10 cell depletion using CD22 mAb significantly enhanced macrophage phagocytosis of Listeria and their production of IFN-γ, TNF-α, and nitric oxide ex vivo. Accelerated bacteria clearance following B10 cell depletion significantly reduced Ag-specific CD4+ T cell proliferation and cytokine production, but did not alter CD8+ T cell responses. B10 cell regulatory function during innate immune responses was nonetheless dependent on cognate interactions with CD4+ T cells since B10 cells deficient in IL-10, MHC-II or IL-21 receptor expression did not influence Listeria clearance. Thus, Listeria manipulates immune responses through a strategy of immune evasion that involves the preferential expansion of endogenous B10 cells that regulate the magnitude and duration of both innate and cellular immune responses
