Abstract Commensal microbiota critically regulates osteoimmune processes mediating post-pubertal skeletal development. Our studies have shown that the commensal gut microbiota suppresses osteoblastic bone- forming cells and enhances osteoclastic bone-resorbing cells, which impairs bone mass accrual. However, mechanisms discerning commensal microbiota effects on bone across the lifespan are unclear. The study of osteoimmunology has shown that immune cells in the bone marrow regulate bone modeling/remodeling. Despite knowledge that the commensal gut microbiota directs crosstalk with host immunity, immune mechanistic studies elucidating the commensal gut microbiota immunomodulatory effects on skeletal maturation are unclear. Preliminary findings from the investigator’s postdoctoral research work suggest that commensal microbiota-host interactions stimulate complement signaling to have system catabolic effects in the maturing skeleton in health and disease. Complement signaling protects the host from infection and modulates the immune response, highlighting the role of complement in maintaining a homeostatic relationship with the commensal microbiota. Our preliminary data demonstrated that the commensal microbiota upregulates circulating complement anaphylatoxin C3a, which has also been implicated in inflammatory bowel pathogenesis. Complement receptor C3aR is expressed on both osteoblasts and osteoclasts, implying that C3a may be a critical regulator of commensal microbiota effects on the maturing skeleton in health and disease. Three specific aims will address critical in vivo and in vitro studies utilizing transgenic mice deleting C3aR in both osteoblasts and osteoclasts in health and under dextran sodium sulfate (DSS)-induced colitis. These three aims will investigate the overall hypothesis that the commensal gut microbiota upregulation of C3a regulates C3aR derived skeletal maturation in health and disease throughout the lifespan. Aim 1 will elucidate the role of commensal gut microbiota on C3aR-mediated osteoblastogenesis in skeletal maturation and deterioration under normal and inflammatory bowel conditions. Aim 2 will examine commensal gut microbiota actions on C3aR-osteoclastogenic signaling in the maturing and aging skeleton in health and disease. Aim 3 will determine whether probiotic administration regulates C3a/C3aR signaling during post-pubertal skeletal development. Elucidating the relationship between C3/C3aR signaling, the gut microbiota, and bone will provide opportunities for therapeutic interventions to optimize bone mass accrual in young service members and protect against skeletal deterioration in aging Veterans. This research seeks to define osteoimmunological processes regulating peak bone mass accrual to withstand either age-related and/or disease-related skeletal deterioration. These studies will innovatively use bone cells specific C3aR knockdown models to determine microbiota derived complement signaling effects on ske...