1004 – REGULATION OF THE HEMATOPOIETIC STEM CELL POOL BY C-KIT-ASSOCIATED TROGOCYTOSIS

Hematopoietic stem cells (HSCs) are routinely mobilized from the bone marrow (BM) to the blood circulation for clinical transplantation. However, the precise mechanisms by which individual stem cells exit the marrow are not understood. This study identified cell-extrinsic and molecular determinants of a mobilizable pool of blood-forming stem cells. We found that a subset of HSCs displays macrophage-associated markers on their cell surface. While fully functional, these HSC are selectively niche-retained as opposed to stem cells lacking macrophage markers which exit the BM upon forced mobilization. Macrophage markers on HSCs could be acquired through direct transfer via trogocytosis, regulated by cKIT, from BM-resident macrophages in mouse and human settings. Our study provides proof-of-concept that adult stem cells utilize trogocytosis to rapidly establish and activate function-modulating molecular mechanisms.