Background: Mast cell localization within the airway smooth muscle (ASM)‐bundle plays an important role in the development of airway hyper‐responsiveness (AHR). Genomewide association studies implicate the ‘alarmin’ IL‐33 in asthma, but its role in mast cell–ASM interactions is unknown.Objectives: We examined the expression and functional role of IL‐33 in bronchial biopsies of patients with and without asthma, ex vivo ASM, mast cells, cocultured cells and in a mouse model system.Methods: IL‐33 protein expression was assessed in human bronchial tissue from 9 healthy controls, and 18 mild‐to‐moderate and 12 severe asthmatic patients by immunohistochemistry. IL‐33 and ST2 mRNA and protein expression in human‐derived ASM, epithelial and mast cells were assessed by qPCR, immunofluorescence and/or flow cytometry and ELISA. Functional assays were used to assess calcium signalling, wound repair, proliferation, apoptosis and contraction. AHR and inflammation were assessed in a mouse model.Results: Bronchial epithelium and ASM expressed IL‐33 with the latter in asthma correlating with AHR. ASM and mast cells expressed intracellular IL‐33 and ST2. IL‐33 stimulated mast cell IL‐13 and histamine secretion independent of FcεR1 cross‐linking and directly promoted ASM wound repair. Coculture of mast cells with ASM activated by IL‐33 increased agonist‐induced ASM contraction, and in vivo IL‐33 induced AHR in a mouse cytokine installation model; both effects were IL‐13 dependent.Conclusion: IL‐33 directly promotes mast cell activation and ASM wound repair but indirectly promotes ASM contraction via upregulation of mast cell‐derived IL‐13. This suggests that IL‐33 may present an important target to modulate mast cell–ASM crosstalk in asthma.

CD38/cyclic adenosine diphosphate ribose (cADPR) signaling plays an important role in the regulation of intracellular calcium responses to agonists in a variety of cells, including airway smooth muscle (ASM) cells. The present study was aimed at determining the effect of interleukin (IL)-13, a cytokine implicated in the pathogenesis of asthma, on CD38/cADPR signaling and to ascertain the contribution of CD38/cADPR signaling to IL-13–induced airway hyperresponsiveness. Human ASM cells maintained in culture were exposed to 50 ng/ml IL-13 for 22 h and levels of CD38 expression and intracellular calcium responses to agonists were measured. Treatment of human ASM cells with IL-13 resulted in increased CD38 expression as determined by real-time polymerase chain reaction, Western blot analysis, and indirect immunofluorescence. Increased CD38 expression was reflected as increased ADP-ribosyl cyclase activity in the ASM cell membranes. The net intracellular calcium responses to bradykinin, thrombin, and histamine were significantly (P ≤ 0.05) higher in cells treated with IL-13 compared with controls. Furthermore, 8-bromo-cADPR, a cADPR antagonist, attenuated IL-13–induced augmented intracellular calcium responses to agonists in human ASM cells. These findings indicate that the CD38/cADPR–dependent pathway has a major role in IL-13–induced modulation of calcium signaling in human ASM.