IL-33, but not IL-18, activates MAP-kinase and NF-kB signaling pathways in blood basophils. (A) IL-33 rapidly activates signaling pathways of the TLR/IL-1R family in freshly isolated blood basophils. Cells were stimulated with IL-33 or IL-18 (50 ng/mL each) for the indicated time periods. Western blot analysis of cell extracts shows the rapid activation of the MAP-kinases, Erk, JNK, and p38, and the phosphorylation and degradation of IκB-α in response to IL-33, but not to IL-18. The lack of responsiveness of human basophils to IL-18 was confirmed with different preparations of IL-18 from 2 different suppliers. (B) Anti-ST2 antibodies block IL-33–induced basophil activation. Basophils were left untreated, or pre-incubated with anti-ST2 antibodies (a mixture of 3 mAbs, clones 2A5, FB9, and HB12; 3 μg/mL each) or anti-CD28 (control mAb; at 10 μg/mL) for 15 minutes at 37°C. Cells were stimulated with 2 ng/mL IL-33 for 15 minutes as indicated. Erk and p38 activation and the degradation of IκB-α were assessed by Western blotting.