Interferon-gamma and B-cell Activating Factor (BAFF) promote bovine B cell activation independent of TLR9 and T-cell signaling

We previously reported that CD21+ B cells purified from bovine blood do not respond to CpG-ODN stimulation unless either CD14+ monocytes or B-cell Activating Factor (BAFF), a cytokine produced by activated monocytes, are present. In this report, we present evidence that CD14+ monocytes are critical for CpG-specific lymphocyte proliferation within the peripheral blood mononuclear cell (PBMC) population but that this response is not mediated by soluble factors produced by CpG-activated monocytes. We further determine that bovine monocytes stimulated with IFN-γ induce expression of the BAFF gene and that recombinant IFN-γ and BAFF induced robust B cell activation when cultured in the absence of CpG ODN. These data suggest that CpG-stimulated monocytes may indirectly promote B cell activation by promoting release of cytokines and/or other soluble factors from accessory cells which in turn act on CpG-stimulated B cells to promote antigen-independent and T cell independent B cell activation. Understanding the T cell independent signals that induce B cell activation has important implications for understanding B cell development in locations where T cells are limited and in understanding polyclonal B cell activation that may contribute to autoimmune diseases.