Proinflammatory macrophage ablation in CD11b‐diphtheria toxin receptor (DTR) transgenic mice impairs skeletal muscle regeneration

Abstract

Macrophages can be beneficial or detrimental to regeneration following muscle injury and macrophage polarization states are not well‐defined during this process. We previously demonstrated that macrophage ablation with diphtheria toxin (DT) given at day 1 after cardiotoxin injury increased residual necrotic myofibers and resulted in smaller regenerated myofibers in DTR mice compared to control mice that received mutant DT while mice receiving DT at day 4 were similar to controls. We used flow cytometry to evaluate the kinetics of macrophage surface markers to investigate the role of macrophage subtypes in muscle regeneration. In control mice, the majority of macrophages (CD11b+/F480+) were Ly6C+7/4+proinflammatory macrophages at day 1–2 after injury and switched to Ly6C−7/4−macrophages at day 4 after injury. M2 markers CD206+ or CD301+ were present on a small percentage of cells. Interestingly, the predominate macrophage population present from day 3–6 was negative for all 4 markers (Ly6C, 7/4, CD206 and CD301). Administration of DT at day 1 in DTR mice resulted in macrophage ablation at day 2–3 after injury with an increase in macrophages at day 4. Thus, the early recruitment of proinflammatory macrophages was critical for muscle regeneration; further studies are needed to define markers present on macrophages in the later stages of the regenerative process.Funding: Veterans Administration and NIH HL07423