Osteomacs support osteoclast-mediated resorption and contribute to bone pathology in a postmenopausal osteoporosis mouse model

Osteal macrophages (osteomacs) support osteoblast function and promote bone anabolism, but their contribution to osteoporosis has not been explored. While mouse ovariectomy models have been repeatedly used, variation in strain, experimental design and assessment modalities, have contributed to no single model being confirmed as comprehensively replicating the full gamut of osteoporosis pathological manifestations. We validated an ovariectomy model in adult C3H/HeJ mice and demonstrated that it presents with human post-menopausal osteoporosis features, including reduced bone volume in axial and appendicular bone and bone loss in both trabecular and cortical bone including increased cortical porosity. Bone loss was associated with increased osteoclasts on trabecular and endocortical bone and decreased osteoblasts on trabecular bone. Importantly, this OVX model was characterised by delayed fracture healing. Using this validated model, we demonstrated that osteomacs are increased post-ovariectomy on both trabecular and endocortical bone. Dual F4/80 (pan-macrophage marker) and TRAP staining revealed osteomacs frequently located near TRAP+ osteoclasts and containing TRAP+ intracellular vesicles. Using an in vivo inducible macrophage depletion model that does not simultaneously deplete osteoclasts, we observed that osteomac loss was associated with elevated extracellular TRAP in bone marrow interstitium and increased serum TRAP. Using in vitro high-resolution confocal imaging of mixed osteoclast-macrophage cultures on bone substrate, we observed macrophages juxtaposed to osteoclast basolateral functional secretory domains scavenging degraded bone by-products. These data demonstrate a role for osteomacs in supporting osteoclastic bone resorption through phagocytosis and sequestration of resorption by-products. Finally, using Siglec1 knockout mice, we demonstrated that loss of the macrophage-restricted molecule Siglec-1/CD169 is sufficient to cause age-associated low bone mass, emphasizing the macrophages, independent of osteoclasts, contribute to optimal skeletal health. Overall, our data expose a novel role for osteomacs in supporting osteoclast function and provide the first evidence of their involvement in osteoporosis pathogenesis.