IL-33 broadens its repertoire to affect DC.

IL-33 is a novel multi-functional IL-1 family member that, in contrast to other family members, is associated with Th2 responses. IL-33 signals via a heterodimer composed of its receptor, IL-1 receptor-like-1 (IL-1RL1), more commonly known as ST2L, and the IL-1R accessory protein. ST2L is expressed by endothelial cells, mast cells, basophils, Th2 cells, and DC. IL-33 has been associated with several immune-mediated disorders, including asthma, arthritis, and inflammatory bowel disease. In contrast, there is evidence that IL-33 can inhibit atherosclerosis development. A report in this issue of the European Journal of Immunology reveals a novel function of IL-33: the ability to promote myeloid DC generation in murine BM cell cultures, by triggering GM-CSF production by other BM cells, likely basophils. DC generated in the presence of IL-33 are maturation resistant, with only minimal T-cell stimulatory ability, associated with comparatively high levels of programmed death receptor ligand expression. This commentary discusses several questions raised by these findings, and provides a basis for further evaluation of IL-33 and ST2L in regulation of APC generation and function in both innate and adaptive immunity.