The BM niche of Mx1^CreSbds^Exc mice exhibits impaired capacity to engraft donor HSC after BMT. (A) Schematic of studies. Mx1^CreSbds^Exc mice and Sbds^l/l controls received BMT of 10⁶ whole BM from GFP⁺ donor mice at 24 hours after receiving 1100 cGy of TBI. Donor engraftment in primary BMT recipients was assessed 1 week after primary BMT using histologic analysis and competitive secondary transplantation assays, in which defined volumes (25% of BM volume collected from bilateral hindleg bones) of whole BM from Mx1^CreSbds^Exc or Sbds^l/l control primary recipients were transplanted with 2 × 10⁵ WT competitor (GFP^neg) whole BM cells into irradiated WT secondary recipients. (B) Cumulative survival curves showed increased mortality among Mx1^CreSbds^Exc recipients (n = 10) after BMT compared with controls (n = 8). ***P < .001; log-rank test. (C) Impaired donor engraftment was seen in Mx1^CreSbds^Exc primary recipients by hematoxylin and eosin staining at 1 week after BMT. Scale bar: 200 µm. (D) In competitive secondary BMT studies, donor engraftment efficiency in Mx1^CreSbds^Exc or control (Sbds^l/l) primary recipients (n = 4 mice per group) was assessed by competitive secondary BMT assay. HSC and hematopoietic progenitor engraftment in primary recipient Mx1^CreSbds^Exc mice was significantly impaired, as indicated by lower GFP⁺ reconstitution of secondary recipients (n = 14 mice per group) receiving Mx1^CreSbds^Exc vs control primary recipient BM in all blood lineages, including RBCs, platelets, Gr1⁺myeloid cells, B220⁺ B cells, CD3⁺ T cells, and total white blood cells (WBC). *P < .05; **P < .01; ***P < .001; χ-squared test.