Spectrum of immune-mediated necrotizing myopathies and their treatments

Purpose of review This review aims to describe the spectrum of clinical, histological, and serological features in patients with immune-mediated necrotizing myopathies (IMNMs). Recent findings Autoantibodies recognizing the signal recognition particle (SRP) or 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) define two unique subtypes of necrotizing myositis patient with distinct clinical features. For example, the major histocompatibility class II human leukocyte antigen allele DRB1∗11:01 is a strong immunogenetic risk factor for developing anti-HMGCR autoantibodies whereas B∗5001 and DQA1∗0104 are over-represented in patients with anti-SRP autoantibodies. Furthermore, statin exposure is a risk factor only for anti-HMGCR autoantibodies. And while skeletal muscle involvement is predominant in most patients with both autoantibodies, lung involvement appears in ∼20% of anti-SRP-positive patients but is more rare in anti-HMGCR-positive patients. Of note, ∼20% of anti-SRP and anti-HMGCR positive patients have significant lymphocytic infiltrates on muscle biopsy and thus would not be formally categorized as having IMNM; aside from this, these patients are clinically indistinguishable from other patients with the same autoantibody profile. Summary Anti-SRP and anti-HMGCR autoantibodies define unique populations of IMNM patients. It may be more appropriate to subtype myositis patients based on these autoantibodies than on their muscle biopsy features.