Mechanism of long-term robust human HSC engraftment NSGW41 mice

Xenotransplantation models enable the in-depth analysis of human hematopoietic stem cell (HSC) function in vivo.However, human HSC engraftment and self-renewal are limited in xenotransplantation settings restricting the study of mechanisms regulating human HSC function. By introducing a loss-of-function KIT receptor into NOD/SCID Il2rg-/- (NSG) mice we created a novel mouse strain, NOD/SCID Il2rg-/- KitW41/W41 (NSGW41), which combines an impaired endogenous HSC compartment with immunodeficiency. This mouse strain efficiently supports stable long-term engraftment of human HSCs without the need for conditioning and shows highly improved multilineage engraftment, including the myeloid and megakaryocyte-erythroid lineages. Thus, we postulate that stable human HSC engraftment is a major prerequisite for the continuous differentiation of all hematopoietic lineages. To investigate whether self-renewal und functionality of engrafted human HSCs is conserved in NSGW41 mice we transplanted titrated numbers of human HSCs in primary and secondary recipients and could show enhanced pick-up, maintenance and expansion of human HSCs in vivo compared to conventional KIT-proficient NSG mice. We hypothesized that endogenous HSCs with a defective KIT receptor have a competitive disadvantage compared to human KIT-proficient HSCs and thus allow for their stable engraftment. In fact, in NSGW41 mice the endogenous HSC pool is significantly decreased after engraftment of human HSPCs, supporting our hypothesis that endogenous HSCs are actively replaced from their niche. Analysis of the murine bone marrow niche revealed a significant expansion of mesenchymal stromal cells (MSCs) after transplantation of human cells. The increase in MSCs strictly correlated with the number of engrafted human HSCs, suggesting that human donor HSCs modulate their niche cells. The transcriptional profile of murine MSCs is also strongly altered after humanization, favoring the expression of genes involved in blood formation and differentiation. We suggest that MSCs provide a niche for human HSCs in mice and that human HSCs actively modify the murine bone marrow niche after xenotransplantation.