Reduced IL-7–dependent proliferation of BM cells from STAT3KO mice. BM cells of control (▪) or STAT3KO (□) mice were stimulated with or without IL-7 (A) or IL-3 (B) at the indicated doses for 48 hours, and cell proliferation was measured by [³H]-thymidine incorporation. (C) BM cells of control (left panels) or STAT3KO (right panels) mice were stimulated without (top panels) or with (50 ng/mL; bottom panels) IL-7 for 38 hours. Proliferation of different cells was measured by incorporation of BrdU for 3 hours, followed by surface staining of anti-B220 and intracellular staining of anti-BrdU antibodies and FACS analysis. B220⁺BrdU⁺ indicated proliferating B cells, and B220^–BrdU⁺ indicated proliferating non-B cells. An identical procedure was performed as described in panel C except that cells were treated without or with IL-3 (5 ng/mL) and IL-7 (10 and 50 ng/mL), respectively. Samples were subjected to FACS analysis, and stimulation indexes for B220⁺ (D) and B220^– (E) were shown. Stimulation index = % cytokine stimulated/% medium alone. *P < .05; Student t test; n = 3. Proliferation results are presented as mean ± SE of replicate samples.