Interleukin-33 Triggers B1 Cell Expansion and Its Release of Monocyte/Macrophage Chemoattractants and Growth Factors.

B1 B lymphocytes are natural IgM-producing cells primarily found in peritoneum and mucosal sites. They perform vital functions during the early defence against viral and bacterial infections. Murine B1 cells express IL-33 receptor complex on activation. IL-33 is a new addition to the IL-1 family with a strong role in Th2 immunity. B1 cells have been recognized to exacerbate contact sensitivity by producing IgM and IL-5 in response to interleukin-33. However, the exact response of IL-33/ST2 signalling in B1 cells is not completely understood. In this study, we report that murine B1 cells respond directly to IL-33 in a ST2-dependent manner. This interaction instigates B1b cell proliferation in a time-dependent manner in vivo. Furthermore, it also mediates monocyte/macrophage and granulocyte recruitment via B1 cell release of chemokines (MCP-1 and MIP-1 alpha). It was noted that upon stimulation, B1b cells additionally release an angiogenic inducer vascular endothelial growth factor and granulocyte-monocyte colony-stimulating factor (GM-CSF). Our findings suggest that these IL-33-mediated B1 cells might be able to play a vital role in the recruitment and growth of monocytes and granulocytes.