In vivo pathogenicity of IgG from patients with anti-SRP or anti-HMGCR autoantibodies in immune-mediated necrotising myopathy.

Objectives In autoimmunity, autoantibodies (aAb) may be simple biomarkers of disease or true pathogenic effectors. A form of idiopathic inflammatory myopathy associated with anti-signal recognition particle (SRP) or anti-3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) aAb has been individualised and is referred to as immune-mediated necrotising myopathy (IMNM). The level of aAb correlates with IMNM activity and disease may respond to immunosuppression, suggesting that they are pathogenic. We aimed to evaluate the pathogenicity of IgG from patients with anti-SRP or anti-HMGCR aAb in vivo by developing the first mouse model of IMNM. Methods I gG from patients suffering from anti-SRP or anti-HMGCR associated IMNM were passively transferred to wild-type, Rag2(-/-) or complement C3(-/-) mice. Muscle deficiency was evaluated by muscle strength on electrostimulation and grip test. Histological analyses were performed after haematoxylin/eosin staining or by immunofluorescence or immunohistochemistry analysis. Antibody levels were quantified by addressable laser bead assay (ALBIA). Results Passive transfer of IgG from patients suffering from IMNM to C57BL/6 or Rag2(-/-) mice provoked muscle deficiency. Pathogenicity of aAb was reduced in C3(-/-) mice while increased by supplementation with human complement. Breakage of tolerance by active immunisation with SRP or HMGCR provoked disease. Conclusion This study demonstrates that patient-derived anti-SRP+ and anti-HMGCR(+) IgG are pathogenic towards muscle in vivo through a complement-mediated mechanism, definitively establishing the autoimmune character of IMNM. These data support the use of plasma exchanges and argue for evaluating complement-targeting therapies in IMNM.