Macrophage-mediated PDGF Activation Correlates with Regenerative Outcomes Following Musculoskeletal Trauma.

Abstract

Our objective was to identify macrophage subpopulations and gene signatures associated with regenerative or fibrotic healing across different musculoskeletal injury types. Subpopulations of macrophages are hypothesized to fine tune the immune response after damage, promoting either normal regenerative, or aberrant fibrotic healing. 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 (DTA), and fibrotic heterotopic ossification (HO). R packages Harmony , MacSpectrum and Seurat were used for data integration, analysis and visualizations. There was substantial overlap between macrophages from the regenerative VML (2mm injury) and regenerative bone (DTA) models, as well as a separate overlap between the fibrotic VML (3mm injury) and fibrotic bone (HO) 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 pro-inflammatory and anti-inflammatory macrophages, suggesting that macrophage polarization state did not correlate with healing outcomes. Receptor/ligand analysis of macrophage-to-mesenchymal progenitor cell (MPC) crosstalk showed enhanced transforming growth factor beta (TGF-β) in fibrotic conditions and enhanced platelet derived growth factor (PDGF) signaling in regenerative conditions. 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.