Froebel, J.; Rahmig, S.; Metz, J.; Kucinski, I.; Svensson, C.-M.; Reinhardt, S.; Salbach-Hirsch, J.; Coppin, E.; Mende, N.; Henning, N.; Percin, G. I.; Weschenfelder, F.; Koehler, A.; Platz, A.; Gottgens, B.; Rauner, M.; Figge, M. T.; Hoefer, T.; Dahl, A.; Waskow, C.

The regulation of human hematopoietic stem cell (HSC) function through its native environment is virtually unknown. Cross-species chimeras, particularly humanized mice, are essential tools for investigating human HSC function in vivo. However, conventional models often require toxic conditioning that impairs niche and donor cell function, or simplify niche complexity. We utilize NSGW41 mice, which harbor a KIT receptor mutation, to achieve robust human leukocyte engraftment without prior treatment. We demonstrate that KIT-proficient human HSCs possess a clear advantage, effectively outcompeting endogenous murine stem cells and progenitors to establish stable, multilineage human hematopoiesis. Crucially, the murine niche undergoes significant plastic adaptation in response to humanization. We identify that mesenchymal stromal cells (MSCs) expand and undergo a transcriptional shift, transitioning from a mixed adipo- or osteo-primed state toward predominantly Lepr+ adipo-primed HSC-supporting cells. This adaptation is vital; the depletion of Lepr+ MSCs or the targeted deletion of Stem Cell Factor (SCF) from these cells leads to the mobilization or loss of human HSC engraftment, respectively. These findings provide compelling evidence for functional cross-species niche-HSC communication, identifying Lepr+ MSCs as primary regulators of human HSC maintenance in xenotransplantation models. By mapping this molecular dialogue, our work establishes a physiological in vivo platform to study human HSC biology and evaluate niche-targeted therapeutic interventions to improve transplantation outcomes.