Contribution of macrophages in the HSC niche

PROJECT SUMMARY: Previous studies from our laboratory have suggested that CD169+ macrophages of the bone marrow (BM) contribute to the hematopoietic stem cell (HSC) niche activity by regulating CXCL12 synthesis in stromal cells and their retention in the BM. Preliminary results reveal two novel functions of macrophages in directly regulating the HSC function. First, we provide evidence that macrophages are critical for HSC regeneration after genotoxic challenge via the regulation of iron availability mediated by signals from the gut microbiota. Second, we have found that BM macrophages can transfer to HSCs/progenitors key retention signals that confer residence in BM. Indeed, HSCs that have received the transfer from macrophages are retained in the BM, whereas only those that have not are mobilized from the BM microenvironment following G-CSF administration. These results raise important new questions as to whether the various functions of macrophages in regulating HSCs, RBC production, or clearance are achieved by the same cells or whether the BM macrophages have specialized functions. In this proposal, we will explore the hypothesis that macrophage can directly contribute to niche activities by regulating HSCs’ ability to regenerate and to egress from the BM. In Specific Aim 1, we will investigate how macrophages interact with the microbiota to promote HSC regeneration. We will use genetic models to manipulate iron delivery pathways in HSCs and macrophages to dissect the mechanism by which iron is supplied to HSCs/progenitors during hematopoietic regeneration. We will also evaluate how BM macrophages can sense signals from the microbiota. In Specific Aim 2, we will investigate the mechanisms by which macrophages assign bone marrow residence. We will assess the role of connexins in the cell-cell communication using CRISPR/Cas9-mediated targeting and determine the role of trogocytosis as transfer mechanism. In Specific Aim 3, we will further define the bone marrow-resident macrophage population. We will evaluate the spatial relationship of these macrophages with vascular structures, HSCs and erythroblasts using immunofluorescence imaging. We will investigate the origin (embryonic or hematopoietic) of BM- resident macrophages using genetic tracing methods. These studies will shed light into the critical functions of an under-appreciated component of the HSC niche and uncover new therapeutic approaches for blood disorders.