Myxovirus Resistance A Protein Expression in Idiopathic Inflammatory Myopathies and Hereditary Muscle Diseases with Inflammatory Cell Infiltration: A North African Study

Muscle biopsy (MB) is an important tool to help differentiate idiopathic inflammatory myopathies (IIMs) from hereditary muscular diseases (HMDs). The usefulness of immunohistochemical stains of the major histocompatibility complex class I and the membrane attack complex are controversial, as both may be identified in some HMDs. More sensitive markers of IIMs have recently been used, such as myxovirus resistance A (MxA), a type I interferon-inducible protein. We selected skeletal MB samples from 81 patients diagnosed with IIM and HMD harbouring overt inflammatory infiltrates on their MBs in the period between March 2022 and September 2024. Two groups were identified: the IIM group (46 cases) and the HMD group (35 cases). We characterized and compared the patterns of MxA protein expression among the two groups. In the IIM group, positive sarcoplasmic MxA expression was detected on the myofibres of 10 patients (24%), among whom were eight dermatomyositis patients. In the HMD group, we did not identify any sarcoplasmic positivity. However, five patients (14%) showed positive labelling restricted to the sarcolemmal membrane, including non-necrotic or regenerating fibres. Our study demonstrates the value of MxA for increasing dermatomyositis diagnostic accuracy and suggests the potential role of interferon type I in the pathophysiology of HMD.