IL-33 is known to promote type 2 immune responses through ST2, a component of the IL-33R complex, expressed primarily on mast cells, Th2 cells, group 2 innate lymphoid cells and regulatory T cells, and to a lesser extent, on NK cells and Th1 cells. Consistent with previous studies, we found that IL-33 polarized alternatively activated macrophages (AAMΦ) in vivo.
However, in vitro stimulation of murine bone marrow–derived or peritoneal macrophages with IL-33 failed to promote arginase activity or expression of YM-1 or Retnla, markers of AAMΦ. Furthermore, macrophages have low/no basal expression of ST2. This suggested that alternative activation of macrophages may involve an IL-33–responsive third-party cell.
Because mast cells have the highest expression of ST2 relative to other leukocytes, we focused on this cell type. Coculture experiments showed that IL-33–stimulated mast cells polarized AAMΦ through production of soluble factors. IL-33–stimulated mast cells produced a range of cytokines, including IL-6 and IL-13. Mast cell–derived IL-13 was required for induction of AAMΦ, whereas mast cell–derived IL-6 enhanced macrophage responsiveness to IL-13 via upregulation of the IL-4Rα receptor. Furthermore, we found that AAMΦ polarized by IL-33–stimulated mast cells could suppress proliferation and IL-17 and IFN-γ production by T cells.
Finally, we show that AAMΦ polarized by IL-33–stimulated mast cells attenuated the encephalitogenic function of T cells in the experimental autoimmune encephalomyelitis model. Our findings reveal that IL-33 can promote immunosuppressive responses by polarizing AAMΦ via mast cell–derived IL-6 and IL-13.
