PRMT6 is required for initiating and amplifying macrophage-induced inflammation in heterotopic ossification by increasing CCL2 expression

Heterotopic ossification (HO) is a debilitating disorder marked by ectopic bone formation in soft tissues, frequently triggered by inflammation after trauma. While macrophage-driven inflammation plays a critical role in HO pathogenesis, the molecular mechanisms governing its initiation, amplification and resolution remain elusive. Using a trauma/burn injury (TBI)-induced mouse model of HO, we identified rapid and sustained macrophage accumulation at the injury site during the early inflammatory phase, and macrophage depletion markedly suppressed HO formation. Transcriptomic profiling identified a pronounced upregulation of protein arginine methyltransferase 6 (PRMT6) in macrophages following injury. Genetic deletion or macrophage-targeted knockdown of Prmt6 reduced macrophage accumulation and significantly attenuated HO, without impairing tendon repair. Consistently, pharmacological inhibition of PRMT6 suppressed HO only when administered during the early inflammatory phase, indicating a restricted therapeutic window. Mechanistically, PRMT6 amplified macrophage chemotactic signaling by transcriptionally and epigenetically upregulating CCL2. Genetic disruption of macrophage-derived CCL2 phenocopied Prmt6 deficiency, whereas CCL2 supplementation rescued macrophage recruitment and partially restored HO in Prmt6-deficient mice. At the molecular level, PRMT6 formed a coactivation complex with NF-κB and catalyzed H3R17 asymmetric dimethylation at the Ccl2 promoter, thereby promoting sustained chemokine expression. Collectively, our findings identify PRMT6 as a central epigenetic amplifier of macrophage-driven inflammation that links early injury responses to ectopic bone formation. Targeting PRMT6 during the early inflammatory phase represents a promising strategy to prevent HO while preserving physiological tissue repair.