IL-33 activates blood eosinophils. (A) Differential activation of eosinophils and neutrophils by IL-1 family members. Neutrophil and eosinophil granulocytes isolated from the same donors were stimulated with IL-1β, IL-18, IL-33 (all at 50 ng/mL), or TNF-α (100 ng/mL) for 10 minutes. Activation of ERK and p38 and the phosphorylation and degradation of IκB-α was analyzed by Western blotting. (B) IL-33 promotes IL-8 secretion in human eosinophils. Cells were incubated with the cytokines (all at 50 ng/mL) alone or in combination as indicated. After 24 hours, IL-8 was measured in the cell supernatants. Western blots of a representative experiment (of 4) are shown in A, and the mean plus or minus SEM of 4 independent experiments with cells from different donors is shown in panel B. The direct induction of IL-8 by IL-33 alone (control vs IL-33) as well as the synergistic induction by IL-33 in combination with cytokines of the GM-CSF family (IL-33 alone or 1 of the GM-CSF family members alone vs IL-33 combined with IL-3 or IL-5 or GM-CSF) were all statistically significant (P < .01 or P < .001) as assessed by the Student t test.