Macrophage-mediated PDGF Activation Correlates With Regenerative Outcomes Following Musculoskeletal Trauma

Objective: Our objective was to identify macrophage subpopulations and gene signatures associated with regenerative or fibrotic healing across different musculoskeletal injury types. Background: Subpopulations of macrophages are hypothesized to fine tune the immune response after damage, promoting either normal regenerative, or aberrant fibrotic healing. Methods: Mouse single-cell RNA sequencing data before and after injury were assembled from models of musculoskeletal injury, including regenerative and fibrotic mouse volumetric muscle loss (VML), regenerative digit tip amputation, and fibrotic heterotopic ossification. R packages Harmony, MacSpectrum, and Seurat were used for data integration, analysis, and visualizations. Results: There was a substantial overlap between macrophages from the regenerative VML (2 mm injury) and regenerative bone models, as well as a separate overlap between the fibrotic VML (3 mm injury) and fibrotic bone (heterotopic ossification) models. We identified 2 fibrotic-like (FL 1 and FL 2) along with 3 regenerative-like (RL 1, RL 2, and RL 3) subpopulations of macrophages, each of which was transcriptionally distinct. We found that regenerative and fibrotic conditions had similar compositions of proinflammatory and anti-inflammatory macrophages, suggesting that macrophage polarization state did not correlate with healing outcomes. Receptor/ligand analysis of macrophage-to-mesenchymal progenitor cell crosstalk showed enhanced transforming growth factor β in fibrotic conditions and enhanced platelet-derived growth factor signaling in regenerative conditions. Conclusion: Characterization of macrophage subtypes could be used to predict fibrotic responses following injury and provide a therapeutic target to tune the healing microenvironment towards more regenerative conditions.