Stat5 deficiency reduces survival and proliferation of developing mast cells. (A) Wild-type and Stat5-deficient bone marrow cells were cultured for the indicated times in WEHI-3CM. Live cell number and percent apoptotic cells were determined by assessing subdiploid DNA content with PI-DNA staining, using timed flow cytometry counts. (B) Wild-type bone marrow cells were infected with MSCV-based bicistronic retrovirus expressing either wild-type Stat5A or Stat5A Δ757 and an IRES-translated GFP. GFP-positive cells were isolated by cell sorting and cultured in IL-3 (5 ng/mL) + SCF (50 ng/mL). Viable cell numbers were determined by trypan blue exclusion. *P < .05 when comparing cells expressing wild-type Stat5A and Stat5 Δ757 by Student t test. (C) Toluidine blue staining of wild-type bone marrow cells expressing either wild-type Stat5A or Stat5A Δ757, after 21 days of culture in IL-3 + SCF. Final magnification shown is × 1000. (D) Expression of mast cell surface antigens on wild-type and Stat5-deficient BMMC populations after culture in IL-3 (5 ng/mL) + SCF (50 ng/mL) for 21 days. (D) Survival and proliferation of wild-type and Stat5-deficient BMMCs in IL-3 (5 ng/mL) + SCF (50 ng/mL) or in IL-3 alone (5 ng/mL). Cells were cultured in the indicated conditions and fed on day 4 of culture. PI-DNA staining with timed flow cytometry counts was used to compare numbers of viable cells. *P < .05 when comparing wild-type and Stat5-deficient cells under identical conditions by Student t test.