Oral Presentation 30th Australian and New Zealand Bone and Mineral Society Annual Scientific Meeting 2020

Osteal macrophage contributions to post-menopausal osteoporosis bone pathology (#9)

Lena Batoon 1 , Susan Millard 1 , Wenhao Sun 1 , Andy Wu 2 , Simranpreet Kaur 3 , Cheyenne Sandrock 1 , Martin Wullschleger 4 5 6 , Liza Raggatt 1 5 , Nathan Pavlos 7 , Allison Pettit 1 5
  1. Mater Research Institute - The University of Queensland, Translational Research Institute, Woolloongabba, QUEENSLAND, Australia
  2. Translational Research Institute, Woolloongabba, Queensland, Australia
  3. The University of Queensland, Diamantina Institute, Translational Research Institute, Woolloongabba, Queensland, Australia
  4. Gold Coast University Hospital, Southport, Queensland, Australia
  5. The University of Queensland, Faculty of Medicine, Herston, Queensland, Australia
  6. Griffith University, School of Medicine, Southport, Queensland, Australia
  7. The University of Western Australia, Crawley, Western Australia, Australia

Osteal macrophages (osteomacs) support osteoblast function and promote bone anabolism, but their contributions to osteoporosis have not been explored. While mouse ovariectomy models have been repeatedly used, spontaneous osteoporosis in female C57Bl/6 mice limits utility of this strain and there are no comprehensive studies confirming ovariectomy recapitulated all pathological features of osteoporosis in other mouse strains. We characterised ovariectomy model of post-menopausal osteoporosis in C3H/HeJ mice. Ovariectomy caused reduced trabecular bone volume (-46%), thickness (-21%) and number (-23%) as well as reduced cortical thickness (-8%) with increased cortical porosity (17.5%) at 4-weeks post-surgery. High resolution micro-CT revealed enlargement of cortical vascular canals post-ovariectomy. Bone loss was associated with increased osteoclasts on trabecular (p<0.0001) and endocortical bone (p<0.0016), and decreased osteoblasts on trabecular bone (p<0.0001). While there was no impact on overall osteocyte number, the TRAP-expressing osteocyte frequency was increased in cortical bone (p=0.0005) which is suggestive of osteocytic osteolysis, especially when considered in the context of the increased cortical porosity. Unexpectedly, osteomac frequency was increased on both trabecular (p<0.0001) and endocortical bone (p<0.0001) post-ovariectomy. Dual F4/80 (pan-macrophage marker) and TRAP staining revealed osteomacs frequently located near TRAP+ osteoclasts and containing TRAP+ intracellular vesicles, suggesting osteomac-mediated phagocytosis of extracellular TRAP at resorption sites. Using an in vivo inducible macrophage depletion model (CD169-DTR mouse), that does not simultaneously deplete osteoclasts, we observed that osteomac loss was associated with elevated serum TRAP (p=0.0017). 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 osteoclast by-products. These data demonstrate a role for osteomacs in supporting bone resorption through sequestering resorption byproducts. Overall, our data expose a novel role for osteomacs in supporting osteoclast function and provide the first evidence of their involvement in osteoporosis bone pathology.