Temporal Macrophage Activation Is Reduced by CCR2 Knockout in a Murine Subacromial Impingement Model.

BACKGROUND: Rotator cuff (RC) tendinopathy is a prevalent cause of shoulder pain and disability, yet its underlying molecular drivers remain poorly defined. Increasing evidence highlights the role of early inflammation and macrophage activation in symptom development and disease progression. C-C chemokine receptor 2 (CCR2) regulates monocyte recruitment and pro-inflammatory macrophage signaling in tendinopathy. This study characterizes the temporal inflammatory profile of RC tendinopathy and evaluates the impact of CCR2 knockout in a murine subacromial impingement (SAI) model.

METHODS: Sixty mice (40 C57BL/6J wild type [WT SAI] and 20 CCR2 knockout [CCR2KO]) underwent bilateral placement of a subacromial surgical clip to induce RC impingement. Four additional C57BL/6J mice underwent sham surgery (WT sham controls). Animals were assessed through 42 days. Macrophage activation was quantified using microPET/CT (micro-positron emission tomography/computed tomography) and gallium-68-labeled folate receptor ligand. Selected WT SAI mice were killed at early and intermediate time points for temporal gene expression profiling. The remaining mice were killed on day 42 for histological, biomechanical, or gene expression analyses.

RESULTS: WT SAI mice demonstrated a reproducible pattern of early macrophage activation, peaking on postoperative days 3 and 7. CCR2KO mice exhibited significantly reduced macrophage activation at these time points. Gene expression analysis of WT SAI supraspinatus tendons revealed early upregulation of inflammatory and M1 macrophage-related genes, followed by later increases in anti-inflammatory mediators and M2 macrophage-related markers. CCR2KO tendons demonstrated fewer tendinopathic changes on histology but did not exhibit superior biomechanical properties compared with WT SAI tendons.

CONCLUSIONS: RC tendinopathy exhibits a distinct early inflammatory phase characterized by macrophage activation and M1 macrophage-dominant signaling. CCR2KO reduces early inflammatory activity and may attenuate tendon degeneration.

CLINICAL RELEVANCE: Defining the inflammatory progression of RC disease may help to refine treatment strategies-particularly the use and timing of nonsteroidal anti-inflammatory drugs, corticosteroid injections, and emerging biologic therapies. CCR2 represents a potential therapeutic target for modulating early inflammation and improving tendon healing.