ITAM-Syk signaling mediates the rebound phenomenon after anti-RANKL antibody discontinuation

Rebound bone loss following discontinuation of antiresorptive therapy, especially denosumab, represents a major clinical issue due to rapid osteoclast activation and increased fracture risk. However, the molecular basis of this phenomenon remains poorly understood. In this study, we investigated the role of the DAP12/TREM2-Syk signaling pathway in rebound-associated bone resorption using a murine model treated with and withdrawn from a RANKL-neutralizing antibody. Wild-type (WT) mice exhibited marked rebound bone loss, with elevated expression of DAP12/TREM2-related genes (Tyrobp and Trem2), increased infiltration of CD206⁺ M2 macrophages, and enhanced osteoclast formation. In contrast, DAP12/DAP10 double-knockout (DKO) mice showed diminished osteoclast activity, reduced M2 macrophage presence, and preserved bone mass, indicating the essential role of this signaling axis. Furthermore, pharmacological inhibition of Syk in WT mice effectively prevented rebound bone loss without disturbing normal bone turnover. Histological and ultrastructural analyses revealed hyperactive osteoclasts with enlarged ruffled borders in WT but not in DKO mice. These results identify the DAP12/TREM2-Syk pathway as a key regulator of pathological osteoclast activation after anti-RANKL therapy withdrawal, suggesting that its inhibition may provide a novel therapeutic approach to prevent rebound bone loss while maintaining physiological bone remodeling.