Apelin Endothelial Niche Cells Control Hematopoiesis and Mediate Vascular Regeneration after Myeloablative Injury.

Radiotherapy and chemotherapy disrupt bone vasculature, but the underlying causes and mechanisms enabling vessel regeneration after bone marrow (BM) transplantation remain poorly understood. Here, we show that loss of hematopoietic cells per se, in response to irradiation and other treatments, triggers vessel dilation, permeability, and endothelial cell (EC) proliferation. We further identify a small subpopulation of Apelin-expressing (Apln) ECs, representing 0.003% of BM cells, that is critical for physiological homeostasis and transplant-induced BM regeneration. Genetic ablation of Apln ECs or Apln-CreER-mediated deletion of Kitl and Vegfr2 disrupt hematopoietic stem cell (HSC) maintenance and contributions to regeneration. Consistently, the fraction of Apln ECs increases substantially after irradiation and promotes normalization of the bone vasculature in response to VEGF-A, which is provided by transplanted hematopoietic stem and progenitor cells (HSPCs). Together, these findings reveal critical functional roles for HSPCs in maintaining vascular integrity and for Apln ECs in hematopoiesis, suggesting potential targets for improving BM transplantation.