Role of macrophages and HSC trogocytosis in the bone marrow niche

About This Grant

Project Summary The ability to migrate is a hallmark of hematopoietic stem cells (HSCs), which travel to different hematopoietic sites during development and continue to be released from the adult bone marrow throughout life. The migratory property of HSCs has facilitated their collection from blood and their transplantation for curative anticancer therapies. Our recent studies revealed that a large subset of HSCs display macrophage-associated markers, including F4/80 and CD169, on their surface. Remarkably, macrophage marker–presenting HSCs are selectively retained in the bone marrow (BM), whereas HSCs without detectable macrophage marker are exclusively mobilized into the peripheral blood upon forced mobilization. We discovered that HSC use trogocytosis – a rapid and highly effective transfer mechanisms enabling acquisition of membrane-bound proteins from adjacent cells – to license their BM residence and retention. Mobilized HSCs are intravenously injected into recipients previously conditioned with irradiation and/or chemotherapy to reestablish hematopoietic system. Homing of HSCs to the BM niche is the first and critical step that precedes successful transplantation. Preliminary data supporting this application provided evidence that BM-resident macrophages interact with HSCs and guide their homing to the BM niche post-transplantation. These results raise important new questions as to how macrophages interact with HSCs to initiate the transfer, what is transferred, and whether macrophages guide HSC homing via transfer-dependent or -independent mechanisms. In the Specific Aim 1, we propose to define the mechanisms of HSC trogocytosis. Our preliminary results suggest that macrophage markers along with retention machinery, including CXCR4, are transferred from macrophages to HSCs and that HSC trogocytic activity strongly correlates with c-Kit levels on HSCs. We will assess the role of CXCR4 transfer and c-Kit signaling in trogocytosis-mediated transfer between macrophages and HSCs using pharmacologic and genetic means. We will elucidate the molecular underpinnings of a putative stem cell synapse with macrophages. In the Specific Aim 2, we will investigate how macrophages interact with HSCs to regulate their homing and/or engraftment post-transplantation. We will use co-culture system in vitro to manipulate the interactions between macrophages and HSCs and test models to explain their functional consequences with respect to stem cell transplantation. We will further investigate the macrophage heterogeneity and their location and interactions with HSCs during homing and engraftment using single cel technologies and immunofluorescent imaging. These studies will provide foundational insights into the macrophage niches and their contributions to HSC homing, retention, and mobilization, all key to improving stem cell transplant therapy for patients with blood disorders.