Autoantibody internalization in myositis skeletal muscle: Emerging evidence, mechanistic insights, and therapeutic relevance.

The inflammatory myopathies-including dermatomyositis, immune-mediated necrotizing myopathy, anti-synthetase syndrome, and overlap myositis-are systemic autoimmune diseases characterized by myositis-specific and myositis-associated autoantibodies targeting intracellular antigens. These diseases can be subclassified by autoantibody seropositivity, based on the understanding that each myositis autoantibody is associated with distinct clinical features. Traditionally, given the intracellular nature of their targets, myositis autoantibodies were thought to be non-pathogenic. However, this idea is now being challenged based on data from recent and older studies showing that autoantibodies reach their intracellular targets in vivo and exert functional pathogenic effects. In this review, we summarize experimental evidence supporting a model of pathogenic autoantibody internalization in the skeletal muscle of different inflammatory myopathies. We also address gaps in the evidence for this model, including the lack of a proven mechanism of autoantibody entry, while offering suggestions for future studies to fill these gaps. We discuss possible mechanisms of autoantibody entry, as well as the diagnostic, prognostic, and therapeutic implications of this model. Finally, we propose that autoantibodies targeting intracellular antigens in other autoimmune diseases-including certain autoimmune neurologic disorders, systemic sclerosis, systemic lupus erythematosus, and vasculitis-could potentially play a role in these conditions.