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.