New dog and new tricks: evolving roles for IL-33 in type 2 immunity.

IL-33 is a more recently identified member of the IL-1 cytokine family, expressed in the nucleus of epithelial cells and released into the extracellular space following tissue damage. The impact of IL-33 as a regulator of the adaptive immune response has been studied extensively, with an understood role for IL-33 in the effector functions of CD4(+) Th2 cells. IL-33, however, is now being shown to initiate the Th2-polarizing function of DCs, and stimulate the secretion of the type 2-associated cytokines, IL-4, IL-5, and IL-13, from tissue-resident innate-immune cells, especially ILCs and MCs. IL-33 also initiates and perpetuates local inflammatory responses through the recruitment and activation of type 2- and inflammatory-associated effectors, such as eosinophils, basophils, and neutrophils. As such, IL-33 drives and amplifies type 2-dependent immunity, as well as type 2-dependent tissue destruction and inflammation. It is also becoming apparent that IL-33 supports the reparative capacity of macrophage and ILCs, but these functions may also contribute to chronic fibrotic diseases. Herein, we review new developments in the understanding of IL-33 as it functions in Th2 cells and type 2 immunity. This includes a discussion of our evolving understanding of how IL-33 directly and indirectly promotes type 2 immune responses through action on innate cells in immunity and the pathogenesis of atopic and fibrotic diseases.