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The composition of cellular infiltrates in anti‐HMG‐CoA reductase‐associated myopathy

Introduction: To characterize cellular infiltrates in muscle biopsies from patients with anti‐3‐hydroxy‐3‐methyl‐gulatryl‐CoA reductase (HMGCR)‐associated myopathy. Methods: Biopsies from 18 anti‐HMGCR myopathy and 7 control dermatomyositis patients were analyzed. Results: CD4+ and CD8+ T‐cells were scattered within the endomysium in 50% of anti‐HMGCR biopsies. All anti‐HMGCR biopsies included increased endomysial and/or perivascular CD163+ M2 macrophages; CD11c+ M1 macrophages were present in 18.8%. CD123+ plasmacytoid dendritic (PD) cells were observed within the endomysium and perivascular spaces in 62.5% of anti‐HMGCR biopsies. Membrane attack complex was deposited on endothelial cells in 50% and on the sarcolemma of nonnecrotic muscle fibers in 85.7% of anti‐HMGCR cases. Major histocompatibility complex class I antigen was up‐regulated in 87.5% of the anti‐HMGCR cases. Conclusions: In addition to necrosis, scattered CD4+, CD8+, and PD cells are characteristic of anti‐HMGCR myopathy. Predominant M2 polarization suggests infiltrating macrophages are more likely to be involved with tissue repair than destruction. Muscle Nerve, 2015. Muscle Nerve 52: 189–195, 2015