CM from expanded pitpα^−/−and pitpα^−/−/β^−/−MKs suppressed colony formation. (A) HPC colony assays were performed utilizing BM from WT C57Bl/6 mice in the presence of CM from pitpα WT littermate, pitpα^−/−, pitpα/β WT littermate, and pitpα^−/−/β^−/− MKs cultures and the indicated growth factors. CFU-GM, BFU-E, CFU-MK, and CFU-GEMM colony numbers were determined by morphology of the cells and colony formation. Each test was plated in triplicate. Data are the average of 2 different batches of CM ± SEM. *P < .05, **P < .005, and ***P < .0005 when compared with media alone control as determined by Student t tests. (B) TGF-β1 levels in pitpα WT, pitpα^−/−, pitpα/β WT, and pitpα^−/−/β^−/− MKs CM was examined by ELISA. Samples were plated in triplicate. The data are representative of 2 different batches of samples ± standard deviation. ND indicates that TGF-β1 was not detected in sample. **P < .005 when compared with either media alone or WT control as determined by Student t tests. (C) TGF-β1 levels in pitpα WT, pitpα^−/−, pitpα/β WT, and pitpα^−/−/β^−/− BM flush fluid was examined by ELISA. Samples were plated in triplicate. Data are the average levels of 5 mice per group. **P < .005 and ***P < .0005 when compared with WT BM flush as determined by Student t tests. EPO, erythropoietin; PWMSCM, pokeweed mitogen spleen conditioned media; SCF, stem cell factor.