Human chorionic gonadotropin and IL-35 contribute to the maintenance of peripheral immune tolerance during pregnancy through mediating the generation of IL-10+ or IL-35+ Breg cells.

Regulatory B cells (Breg cells) play critical roles in modulating immune responses during autoimmune diseases and infection. Here we explored the participation of two main Breg subsets, including IL-10+ Breg (B10) and IL-35+ Breg cells, in maintaining successful pregnancy. We first observed an elevated percentage of B10 cells in peripheral blood from first-trimester pregnant women compared with non-pregnant controls. Serum from pregnancy induced the augmentation of B10  in peripheral blood mononuclear cells from non-pregnant women. In animal models, we demonstrated that there were significant augmentation of B10 cells and obvious increase of IL-10 level in splenic B cells from normal pregnant mice compared to that in abortion-prone pregnant mice and virgin mice. Further analysis showed that both hCG and IL-35 suppressed the proliferation of mouse splenic B cells. Moreover, IL-35 induced the expansion of both mouse splenic B10 and IL-35+ Breg cells while hCG only mediated the generation of B10 cells. Subsequent study in mice demonstrated that the activation of STAT1 and STAT3 in B cells caused by IL-35 and the activation of STAT3 caused by hCG were the predominant mechanism of IL-35+ Breg and B10 cells augmentation. These findings suggested that hCG and IL-35 induced the amplification of B10 and IL-35+ Breg cells which played a vital peripheral regulatory role during pregnancy.