IL-33–induced signaling and cytokine release in mast cells. (A) HMC-1.1 cells were stimulated with IL-33 or SCF as indicated. Cell lysates were separated by SDS-PAGE, blotted, and probed with anti–pY721-c-Kit. Because of the high basal phosphorylation level, the exposure time was less than 5 seconds. (B-C) HMC-1.1 cells were preincubated with imatinib before stimulation with IL-33. Cell extracts were analyzed with the indicated antibodies (B). Phosphorylation was quantified in 3 independent experiments and results are shown as increase (mean ± SEM) relative to nonstimulated cells. Supernatants were analyzed for IL-4, IL-6, and IL-8 (C).