Background/Purpose As supported by recent studies, anti-histidyl tRNA synthetase (anti-Jo1) antibodies are specific for clinico-biological features and outcomes. Though, anti-Jo1 synthetase syndrome is a clinical distinct entity among idiopathic inflammatory myopathies and among other anti-synthetase syndromes, its pathological features are poorly described.

Methods This study was performed to define the histological characteristics of the associated myopathy, as well as to immunophenotype the inflammatory infiltrates, in a series of 53 anti-Jo1-positive patients with antisynthetase syndrome. 

Results Myopathic changes in myofibers were observed with predominant perifascicular topography: these changes included a characteristic peripheral necrosis (66%), or atrophy (45%).  A perimysial fragmentation, seen with a trichrome coloration or with histoenzymological stain for alkaline phosphatases was commonly noted (90%). C5b9 membrane-attack-complex staining showed a sarcolemmal perifascicular expression in 57% of the biopsies. Human-leucocyte antigen class-I (HLA I) expression was enhanced at the periphery of the fascicles in 60% of the cases. A perimysial inflammatory infiltrate was present in 90 %, extending to endomysium in 83 %. 

Conclusion This largest study to date is highlighting that anti-Jo1 synthetase syndrome is a distinct histological entity within the spectrum of myositis, with a particular perifascicular necrotizing involvement, associated with a perimysial fragmentation, and a striking membrane-attack-complex perifascicular sarcolemmal expression.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/distinctive-muscle-histopathological-features-of-anti-synthetase-syndrome/