Macrophages and Bone Marrow Stem Cell Niches: The Roles of PGE2 and Coagulation

Blood-forming hematopoietic stem cells (HSC) functionally express the chemokine receptor CXCR4, which is required to control their directional motility, cell cycle, adhesion, and bone marrow (BM) repopulation. HSC are mostly BM-retained in a quiescent, non-motile mode via CXCR4-induced adhesion interactions with BM niche cells, which functionally express surface CXCL12. Surface CXCL12-mediated adhesion interactions with BM stromal cells protect quiescent CXCR4⁺ stem cells from DNA damaging agents while preserving their developmental potential. On the other hand, CXCL12 secretion by BM stromal cells and its release to the blood increase CXCR4 expression and signaling in hematopoietic stem and progenitor cells (HSPC), inducing their egress and clinical mobilization. Finally, CXCR4⁺ HSC also follow CXCL12 gradients to the BM when they home back from the blood. Intriguingly, dynamic CXCR4/CXCL12 signaling cascades control not only HSC retention in the BM, but also their homing to the BM and their release and mobilization to the blood. In addition to their central role in host defense, myeloid cells participate in organ homeostasis, including HSC localization. Monocyte-derived bone resorbing osteoclasts cleave and release endosteal membrane-bound CXCL12, stem cell factor, and osteopontin, factors needed for HSC adhesion and BM retention, leading to CXCL12/CXCR4^-mediated HSPC mobilization. Recently, we identified a rare population of BM αSMA⁺ macrophages that highly express, which protects HSC from inflammatory insults via COX-2-mediated PGE2 secretion and reactive oxygen species inhibition. In vitro PGE2 upregulates CXCR4 on enriched human cord blood CD34⁺ and murine HSC via cAMP activation, leading to enhanced CXCL12-induced migration, homing and BM repopulation. However, preliminary results reveal that in vivo PGE2 treatment reduced CXCR4 expression on BM HSC by increasing surface CXCL12 expression by stromal cells. Surface CXCL12 upregulation is due to PGE2-mediated secretion and autocrine signaling of lactate, which is followed by cAMP inhibition in BM stromal cells. Indeed, antagonizing COX-2 or activating cAMP induced HSPC mobilization via BM CXCL12 secretion and increased CXCR4 expression and signaling in HSPC. Coagulation cascades also navigate HSC localization. We revealed that anticoagulant microenvironments in the BM mediate HSC adhesion and retention via inhibition of nitric oxide (NO) production and HSC migration. Physiologic stress induced extensive production of the pro-coagulant factor thrombin, which activates NO generation, CXCL12 secretion and enhanced CXCR4⁺ stem cell motility and mobilization. We found multinucleated pre-osteoclast cell clusters in femoral metaphysis expressing Tissue Factor (TF) a potent initiator of coagulation leading to thrombin generation. Osteoclast maturation and stress signals also activate TF induced pro-coagulation cascades, mediating HSPC egress and recruitment to the circulation. Daily light and darkness cues regulate many physiological processes, including osteoclast/osteoblast bone remodeling, BM CXCL12 production and secretion and CXCR4⁺ HSC egress. In addition to the previously identified morning peak of HSC egress to the blood, we identified two peaks of BM HSC proliferation: a morning peak in conjunction with stem cell egress and an evening peak of HSC expansion without egress. Higher BM levels of the HSC-protecting aSMA⁺ monocyte/macrophages documented in the evening and their associate “low-CXCR4/high CXCL12” BM microenvironment provide the mechanism for these fluctuating patterns. Finally, we found higher PGD2 levels and reduced PGE2 levels in monocyte/macrophages in the morning, whereas at night we documented the opposite patterns. PGD2 induced CXCL12 secretion and HSC egress, while PGE2 induced their retention. Our studies attribute central roles for BM myeloid cells in HSC regulation.