An intrinsic BM hematopoietic niche occupancy defect of HSC in scid mice facilitates exogenous HSC engraftment.

Although scid mice have been widely used for human HSC engraftment studies, the function of HSCs of scid mice has not been characterized. We hypothesized that the DNA repair defect of scid mice results in a stem cell defect that facilitates HSC engraftment. scid BM cells showed severely impaired repopulation potentials in the competitive repopulation assay. To assess the BM hematopoietic niche occupancy ability of scid HSC, WT BM cells were transplanted into scid mice without any conditioning and observed to achieve long-term engraftment. Furthermore, the defects of scid HSCs are independent of their inability to perform lymphopoiesis because a similar defect in hematopoietic niche occupancy was not observed with Rag1(-/-) recipients. These results demonstrate that scid HSCs are impaired in maintenance within the niche, which may explain the nature of the conducive marrow niche environment of scid mice for xenotransplantation.