THE ROLE OF OSTEOMACS IN REGULATING STEM CELL FUNCTION AND THE HEMATOPOIETIC NICHE

Abstract

Networking between hematopoietic stem cells (HSC) and cells of the hematopoietic niche is not only critical for the maintenance of stem cell function, but also for the competence of the hematopoietic niche. Among the key cellular components of the niche are a group of specialized bone-resident macrophages known as osteomacs (OM). We previously documented that close interactions between OM, osteoblasts (OB) and megakaryocytes (MK) are critical for sustaining in vitro and in vivo stem and progenitor cell function. To understand the importance of OM-OB-MK interactions within the niche, we performed 3D tissue cytometry on fixed and stained unperturbed bone marrow sections. This approach identified the spatial relationships between OM, OB, MK and HSC within the niche and defined parameters, under which these cell types coexist in undamaged bone marrow. Next, we performed single cell mRNAseq and CyTOF to assess genetic and proteomic expression changes in OM following their interaction with MK. These studies revealed the upregulation of CD166 and embigin on OM via MK interactions. We then examined how these molecules impact hematopoietic function in clonogenic assays. When these assays were initiated with CD166 KO OM or treated with anti-embigin blocking antibodies, we established that loss of these surface molecules on OM caused a decline in the normal OM-mediated hematopoietic enhancing activity. Conversely, recombinant CD166 and embigin partially substituted for OM activity thus identifying potential mediators through which OM maintain hematopoietic function. Our data, for the first time, reveal intimate spatial interactions between OM, OB, MK and HSC in the hematopoietic niche. They also illustrate the importance of crosstalk between OM, OB and MK and reveal novel mediators such as CD166 and embigin that cooperate with other elements of the niche to support HSC function.