Host STAT5 dosage in adult mice controls HSC engraftment during nonablative transplantation. Recipient mice (CD45.2) were transplanted with either 5 × 10⁶ GFP-transgenic or CD45.1 BM cells under nonablative conditions. (A) Percentage of donor chimerism in each recipient mouse 16 weeks after transplantation. From 2 separate injection dates, wild-type (n = 10), STAT5ab^+/ΔN (n = 8), STAT5ab^+/null (n = 6), and from 10 separate injection dates (29 mice total), STAT5ab^ΔN/ΔN (n = 9 surviving mice). (B) Percentage of donor-derived overall and Gr-1⁺, B220⁺, or CD4⁺ cells in each recipient mouse 16 to 24 weeks after transplantation. From 4 separate injection dates, wild-type (n = 4), STAT5ab^ΔN/null (n = 4). (C) Peripheral blood hematology of each STAT5ab^ΔN/null mouse before and 16 to 24 weeks after injection with donor BM cells. (D) Percentage of donor-derived Gr-1, B220, Ter119, or CD4 cells obtained in lethally irradiated secondary recipients. Two wild-type and 2 STAT5ab^ΔN/null engrafted mice from panel B were used as donors for the secondary transplantation. Each donor was transplanted into 5 recipients. The representative dot plot from one secondary recipient is shown. Mean plus or minus SD values for all recipients are indicated above each plot. (E) E14.5 fetal liver cells from wild-type, STAT5ab^ΔN/ΔN, or STAT5ab^null/null CD45.2⁺ donors were transplanted into lethally irradiated CD45.1 recipients. Sixteen weeks later, the transplanted BM chimeras were challenged with 5 × 10⁶ GFP-transgenic BM cells. The percentage of donor-derived (GFP⁺) overall and Gr-1, B220, and CD4 cells in each mouse was determined by flow cytometry 16 weeks later. The number of chimeras challenged from 2 separate injection dates were wild-type (n = 9), STAT5ab^ΔN/ΔN (n = 7), and STAT5ab^null/null (n = 5). For t tests relative to wild-type, P < .001.