Skeletal Muscle Resident Macrophages Undergo Expansion and Pathogenic Activation in mdx5cv/Ccr2-/- mice

Infiltrating macrophages contribute to muscle dystrophic changes in Duchenne muscular dystrophy (DMD). In a DMD mouse model, mdx5cv mice, CC chemokine receptor type 2 (CCR2) deficiency diminishes Ly6Chi macrophage infiltration by blocking blood Ly6Chi inflammatory monocyte recruitment. This is accompanied by transient improvement of muscle damage, fibrosis, and regeneration. The benefit is lost, however, after expansion of intramuscular Ly6Clo macrophages. To address mechanisms underlying the Ly6Clo macrophages expansion, we compared mdx5cv/Nur77-/- and mdx5cv/Ccr2-/-/Nur7-/- mice with mdx5cv and mdx5cv/Ccr2-/- mice, respectively, and found no evidence to suggest Ly6Clo monocyte recruitment by dystrophic muscles. Single-cell RNA sequencing analysis and Flt3cre/Rosa26LSL-YFP-based lineage tracing of macrophage origins demonstrated expansion and pathogenic activation of muscle resident macrophages in CCR2-deficient mdx5cv mice. The expansion was associated with increased cell proliferation, which appeared induced by colony stimulating factor 1 derived from fibro/adipogenic progenitors. Our study establishes a pathogenic role for skeletal muscle resident macrophages in muscular dystrophy.