Age-Dependent Effects of Macrophage Derived Cathelicidin Antimicrobial Peptide on Bone Fracture Repair

The pace of fracture repair declines with age, increasing the risk of delayed union and nonunion. Delayed union and nonunion are associated with significant morbidity and even mortality with few effective non-surgical interventions. Here, we identify cathelicidin antimicrobial peptide (CAMP) as an age-dependent regulator of bone repair. Single-cell RNA sequencing and immunohistochemistry demonstrated higher CAMP expression during fracture healing in young compared with aged mice. Global or myeloid lineage-depletion of Camp delayed fracture repair in young mice, whereas Camp depletion in aged mice increased callus bone formation. Heterochronic bone marrow transplantation using the conditional genetic knockout further implicated hematopoietic-lineage derived cells in these effects and demonstrated that myeloid-specific Camp expression is important in the rejuvenation of the pace of fracture healing. CAMP promotes osteogenic differentiation through P2X7 receptor (P2X7R)-dependent signaling, influencing the downstream regulation of NLRP3 expression. NLRP3 expression is also correlated with osteogenesis following Camp modulation in both young and old mice, supporting CAMP-P2X7R-NLRP3 signaling as a regulator of osteogenesis. Together, these findings identify CAMP as a hematopoietic-derived regulator of bone repair whose function is age-dependent, promoting fracture healing in young animals while constraining it in aged animals.