Defining Parameters Attributing to the Role of Osteomacs in Regulating Stem Cell Function and the Hematopoietic Niche

Networking between hematopoietic stem cells (HSC) and cells of the hematopoietic niche is critical for the maintenance of stem cell renewal and function. HSC maintenance in the hematopoietic niche is considered to be the product of intimate interactions between cellular and soluble elements of the niche and stem cells. Among the cellular components of the niche participating in this function are a group of specialized bone-resident macrophages known as osteomacs (OM). Previously, we established the importance of osteoblasts (OB) in hematopoiesis and quite recently, we described the importance of OM and their interactions with OB and megakaryocytes (MK) in sustaining HSC function. We have also illustrated that CD166 is a critical functional marker of stem cell function and competence of the hematopoietic niche. Interestingly, immature OB which are CD166+ mediate the highest level of hematopoietic enhancing activity. We report here the importance of CD166 on calvarie-resident OM (identified as CD45+F4/80+ cells) and outline how these cells require cooperation from MK to increase CD166 expression and sustain HSC function.

Bone resident-osteomacs, which are phenotypically similar but functionally different from bone marrow-derived macrophages, were collected by the enzymatic digestion of neonatal calvarial cells (NCC) or long bones of adult mice. Transplantation assays indicated that OM are relatively radioresistant and survive several weeks post lethal radiation. However, they eventually deplete and are replenished by progeny of donor HSC. To understand the importance of OM-OB-MK interactions in maintaining HSC function in the niche, we performed 3D cytometry on fixed and stained bone marrow sections that revealed intimate spatial interactions between OM, OB, MK and HSC. To assess changes in gene expression observed due to these interactions, we cultured NCC for 16hr in the absence or presence of MK prepared from fetal liver followed by sorting out OM from each group. These cells were then captured as single cells and sequenced to identify potential targets through which OM enhanced hematopoietic activity. Strikingly, several genes involved in the hematopoietic stem cell differentiation pathway including lmo2, fli1 and ikzf1 were upregulated in OM cultured in the presence of MK. Other genes that were upregulated were embigin and PF-4, both of which have been implicated in the maintenance of HSC function. Interestingly, OM express embigin, angiogenin and IL-18 mRNA similar to proximal osteolineage cells which we previously described as HSC regulators. To investigate changes at the translational level, we performed single cell proteomics using CyTOF. NCC were cultured for 2 days in the absence and presence of MK followed by staining for a panel of 29 surface and intracellular markers. Expression of markers such as CD166, embigin, mac-2 and STAT3 amongst others was elevated on OM cultured with MK compared to OM cultured without. These data informed our decision to focus our future investigations on CD166 and embigin.

Next CD166+OM and CD166-OM were isolated by cell sorting and used in co-culture assays with OB to support the production of clonogenic cells in vitro. Only the CD166+ fraction of OM maintained hematopoietic activity similar to unsorted OM, implicating CD166 as one of the mediators of OM function. These results were validated using recombinant CD166 protein to substitute for OM function. Under these conditions, recombinant CD166 supported the hematopoietic enhancing activity of OB in the absence of OM. Recombinant Angiogenin and IL-18 were unable to augment the CD166-mediated support of hematopoiesis. Interestingly, CD166 knockout OM were unable to mediate the same hematopoietic enhancing activity observed with WT OM regardless of the presence or absence of MK in culture. In vivo transplantation studies to corroborate these findings have been initiated and are very early to yield meaningful conclusions.

These data demonstrate that CD166 is one of the important mediators through which OM maintain HSC function. However, CD166-OM mediated HSC function is only maintained in conjunction with OB-MK interactions. Our data indicate the importance of crosstalk between OM, OB and MK which leads to the expression of novel mediators such as CD166 to support HSC function.