Epitope interactions of monoclonal antibodies targeting CD20 and their relationship to functional properties.

OBJECTIVES: Anti-signal recognition particle antibody immune-mediated necrotising myopathy (anti-SRP-IMNM) is characterised by prominent muscle weakness and poor neurological outcomes. This study aimed to evaluate the relationship between endomysial fibrosis and the clinical, muscle magnetic resonance imaging (MRI) and myo-pathological features of patients with anti-SRP-IMNM.

METHODS: We collected the clinical, imaging, and myo-pathological data of patients diagnosed with anti-SRP-IMNM. Differences between patients with and without increased endomysial fibrosis on muscle biopsy were compared.

RESULTS: Ninety-four patients were included in the study, comprising 12 paediatric and 82 adult patients. The mean age at onset was 41.2±17.3 years. The mean serum creatinine kinase concentration was 6885.8±6300.7 IU/L. MRI revealed muscle oedema in 87.3% of patients and fatty infiltration in 77.5%, which was particularly severe in the muscles of the posterior thigh. Endomysial fibrosis was found in 50% of patients and was significantly associated with early onset (p=0.004), paediatric age (p=0.044), muscle fatty infiltration on MRI of the thigh (p=0.045), and inflammatory cell infiltration on pathology (p<0.05).

CONCLUSIONS: In anti-SRP-IMNM, endomysial fibrosis may be the pathological basis of fatty infiltration observed on MRI and may be associated with resistance to immunotherapy.