Interleukin (IL)-4 plays a crucial role in type 2 immune responses such as immunity against helminth infection and allergic responses through the induction of IgE class-switching in B cells and Th2 differentiation.Th2 cells and basophils/mast cells are known as IL-4-producing cells and express antigen recognition receptors TCR and IgE, respectively. IL-4 production is initiated by the engagement of these receptors with antigen, leading to antigen-specific immune responses.
Group 2 innate lymphoid cells (ILC2s), are a new type of innate lymphocyte that are known to regulate type 2 immune responses. ILC2s rapidly produce large amounts of IL-5 and IL-13 in response to IL-33 or IL-25, suggesting that ILC2s regulate the initiation of ‘antigen-nonspecific’ type2 immune responses. Although ILC2s highly express Il4 mRNA as well as Il5 and Il13, IL-4 production by ILC2s is difficult to detect at the protein level in vitro and in vivo. Recently, cystenyl leukotriene D4 (LTD4) was reported to induce ILC2 activation including IL-4 production, suggesting that IL-4 production in ILC2s is regulated by lipid metabolites. However, the molecular mechanisms that regulate IL-4 production in ILC2s and the physiological function of ILC2-derived IL-4 are unknown.
We found that blockage of calcium signaling completely inhibited LTD4-induced IL-4 production whereas Ionomycin induced IL-4 production, suggesting that the calcium signaling pathway is required for IL-4 production in ILC2s. Further, ILC2s from a protease allergen-induced asthma mouse model showed higher IL-4 production than ILC2s from naïve mice under LTD4 stimulation. ILC2s from IL33-deficient mice lacked LTD4-induced IL-4 production. These data indicate that ILC2s acquire the ability to produce IL-4 though IL-33-mediated activation in vivo. Taken together, LTD4-mediated calcium signaling and the IL-33 signaling pathway cooperatively regulate IL-4 production in ILC2s in an antigen-independent manner and ILC2s may regulate antigen nonspecific type 2 immune responses by alarmin and lipid mediators.