Secreted IgM modulates IL-10 expression in B cells.

IL-10 B cells are critical for immune homeostasis and restraining immune responses in infection, cancer, and inflammation; however, the signals that govern IL-10 B cell differentiation are ill-defined. Here we find that IL-10 B cells expand in mice lacking secreted IgM ((s)IgM) up to 10-fold relative to wildtype (WT) among all major B cell and regulatory B cell subsets. The IL-10 B cell increase is polyclonal and presents within 24 hours of birth.