Objective To report the clinicopathologic features and outcome of myositis in patients treated with immune checkpoint inhibitors (ICIs) (irMyositis). Methods We retrospectively analyzed patients diagnosed with irMyositis in tertiary centers in Paris, France, and Berlin, Germany, from January 2015 to July 2017. Main outcomes were clinical manifestations and muscle histology, which included major histocompatibility complex class I (MHC-I), C5b-9, CD3, CD4, CD8, CD20, CD68, programmed death-1 receptor (PD-1), programmed death ligand (PD-L) 1, and PD-L2 immunohistochemical stains. Results Ten patients with metastatic cancer were included; median age was 73 (range 56–87) years. Median follow-up duration was 48 (range 16–88) weeks. Six patients developed myositis during nivolumab therapy, 1 patient during pembrolizumab, 1 patient during durvalumab, and 2 patients during combined nivolumab and ipilimumab. Median delay between ICI initiation and myositis onset was 25 (range 5–87) days. Clinical manifestations were dominated by acute or subacute myalgia (8 patients) and limb-girdle (7), axial (7), and oculomotor (7) weakness. Four patients had evidence of myocarditis. In all patients, creatine kinase levels were elevated (median 2,668, range 1,059–16,620 U/L), while anti-acetylcholine receptor and myositis-associated antibodies were negative. Electrodiagnostic studies showed myopathic process without decrement in all patients. Muscle biopsy constantly showed multifocal necrotic myofibers, sarcolemmal MHC-I, and endomysial inflammation, consisting mainly of CD68+ cells expressing PD-L1 and CD8+ cells expressing PD-1. ICI treatment was withdrawn in all patients; 9 patients received immunosuppressive therapy, which consistently led to marked clinical improvement. Conclusions irMyositis presents with remarkably homogeneous and unique clinicopathologic features, expanding the nosologic spectrum of inflammatory myopathies in patients with cancer. ICI withdrawal and treatment with corticosteroids improve outcome.

In 2019, a new coronavirus variant with drastic global impact emerged; it currently sums up to 168 million confirmed cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and an estimated 3.5 million deaths from coronavirus disease 2019 (COVID-19).¹ Independent studies suggest that 30% to 60% of patients with SARS-CoV-2 infections experience myalgia.^2-4 Myalgia and increased levels of creatine kinase were found to be more pronounced in patients with critical illness needing intensive care support than individuals who were mildly affected.^5-7 Signs of skeletal muscle injury were associated with a more severe clinical course and higher mortality rates.^7,8 Several case reports describe rhabdomyolysis,^9-14 and magnetic resonance imaging findings have suggested myositis.^15-18 To our knowledge, 3 case reports included biopsies, 2 of which advocated a dermatomyositis-like phenotype.^18-21 Finally, in a large follow-up study, two-thirds of survivors of COVID-19 experienced fatigue or muscle weakness and 2% to 3% myalgia, even 6 months after an infection with SARS-CoV-2.²² Similar results have been found elsewhere.^23,24

Objective: To analyze antisynthetase syndrome–associated myositis by modern myopathologic methods and to define its place in the spectrum of idiopathic inflammatory myopathies (IIMs). Methods: Skeletal muscle biopsies from antisynthetase syndrome–associated myositis and other IIMs from different institutions worldwide were analyzed by histopathology, quantitative PCR, and electron microscopy. Results: Myonuclear actin filament inclusions were identified as a unique morphologic hallmark of antisynthetase syndrome–associated myositis. Nuclear actin inclusions were never found in dermatomyositis, polymyositis, sporadic inclusion body myositis, autoimmune necrotizing myopathy associated with signal recognition particle or 3-hydroxy-3-methylglutaryl-coenzyme A reductase autoantibodies, or nonspecific myositis associated with other systemic diseases, harboring myositis-associated autoantibodies, and presenting myofiber necrosis. We show that molecules involved in actin filament formation and actin shuttling mechanisms are altered in antisynthetase syndrome, and may thus be involved in pathologic myonuclear actin aggregation. In addition, we have identified a typical topographic distribution of necrotic myofibers predominantly located at the periphery of muscle fascicles accompanied by inflammation and destruction of the perimysial connective tissue. Conclusion: Antisynthetase syndrome–associated myositis is characterized by distinctive myonuclear actin filament inclusions, including rod formations and a typical necrotizing perimysial myositis. This supports the hypothesis that antisynthetase syndrome–associated myositis is unique and should not be grouped among dermatomyositis, polymyositis, sporadic inclusion body myositis, necrotizing autoimmune myositis, or nonspecific myositis. Classification of evidence: This study provides Class II evidence that for patients with IIMs, the presence of myonuclear actin filament inclusions accurately identifies patients with antisynthetase syndrome–associated myositis (sensitivity 81%, specificity 100%).