Expansion and pathogenic activation of skeletal muscle-resident macrophages in mdx^5cv/Ccr2^-/- mice

Infiltrating macrophages contribute to muscle dystrophic changes in Duchenne muscular dystrophy (DMD). In a DMD mouse model, mdx^5cv mice, CC chemokine receptor type 2 (CCR2) deficiency diminishes Ly6C^hi macrophage infiltration by blocking blood Ly6C^hi inflammatory monocyte recruitment. This is accompanied by transient improvement of muscle damage, fibrosis, and regeneration. The benefit, however, is lost after the expansion of intramuscular Ly6C^lo macrophages. To address the mechanisms underlying the Ly6C^lo macrophage expansion, we compared mdx^5cv/Nur77^-/- and mdx^5cv/Ccr2^-/-/Nur7^-/- mice with mdx^5cv and mdx^5cv/Ccr2^-/- mice, respectively, and found no evidence to suggest Ly6C^lo monocyte recruitment by dystrophic muscles. Single-cell RNA sequencing analysis and Flt3^cre/Rosa26^LSL-YFP-based lineage tracing of macrophage origins demonstrated the expansion and pathogenic activation of muscle resident macrophages in CCR2-deficient mdx^5cv mice. The expansion was associated with increased cell proliferation, which appeared induced by colony-stimulating factor-1 (CSF-1) derived from fibro/adipogenic progenitors (FAPs). Our study establishes a pathogenic role for skeletal muscle resident macrophages and supports a regulatory role of FAPs in stimulating the expansion of resident macrophages in the DMD mouse model when the inflammatory macrophage infiltration is inhibited.