Essential roles for IL-3 in IL-4 production by basophils and in Th2 differentiation. (A-B) Cytokine production by CD4⁺ T cells differentiated in vitro in the presence of basophil-enriched (Bs[+]) or depleted (Bs[-]) spleen cells. Anti–IL-4 antibody was added in the indicated cultures. (C) IL-3 production on day 2 in the cultures established as in panel A with basophils from wild-type mice. (D) Indicated antibodies were added in the cultures established as in panel C, and IL-4 production on day 2 was examined. ND indicates not detectable (< 10 pg/mL). (E-F) Generation of Th1 and Th2 cells from wild-type CD4⁺ T cells induced by T cell-depleted spleen cells in the absence (-) or presence of the indicated antibodies. Examination of cytokine production was done by cytoplasmic staining (A,E) or by ELISA (B-D,F). In panels B and F, means and SDs of duplicate cultures with cells from control (□) and IRF-2^-/- (▪) mice, respectively, are shown. Representative of 2 (A-B) or 3 (C-F) independent experiments.