IRF-2 acted as a negative regulator of IL-3–induced proliferation on basophils. (A) IRF-2 messages in highly purified (> 95% pure) basophils from wild-type mice were examined by reverse transcription-polymerase chain reactions on serially diluted RNA preparations. Reverse transcriptase was not added in control amplifications (RT(-)). (B) BM cells from control (□) and IRF-2^-/- (▪) mice were cultured in vitro in the presence of IL-3. The numbers of c-kit^-FcϵRIα⁺ cells recovered on day 13 were enumerated. Data represent the means and SD of duplicated cultures. (C) BM cells prepared from B6-Ly5.1 and IRF-2^-/- (Ly5.2) mice were mixed 1:1 and cultured as in panel B. On day 13, Ly5.2 expression was examined in c-kit^-FcϵRI⁺ cells recovered from the cultures. Representative of 3 independent cultures. (D) Highly purified basophils (> 95%) prepared from control (□) or IRF-2^-/- mice (▪) were stimulated with IL-3 for 24 hours, and the production of IL-4 and IL-6 was examined. Cumulative data were obtained from 2 independent trials, showing the means of 4 independent cultures and SD. (E) Basophils enriched from the BM were stimulated with IL-3 and basophils (Dx5⁺NK1.1^-) were stained for phospho-Stat5 and phospho-Stat1. Open and filled histograms represent unstimulated and stimulated basophils, respectively.