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. In WT mice, sIgM is produced prenatally and limits the expansion of IL-10+ B cells. Lack of the high affinity receptor for sIgM, Fc mu R, in B cells translates into an intermediate IL-10+ B cell phenotype relative to WT or sIgM-/- mice. Our study thus shows that sIgM regulates IL-10 programming in B cells in part via B cell-expressed Fc mu R, thereby revealing a function of sIgM in regulating immune homeostasis. Il-10-expressing B cells play a pivotal role in immune homeostasis, but little is known about the factors and pathways that affect the development of this heterologous population of regulatory B cells. Here authors show in a mouse model that in embryonic life, soluble IgM restrains the expansion of Il-10-positive B cells, via utilizing Fc mu R and other putative receptors.