The microbiome in autoimmune rheumatic disease.

Microbial contributions to the immunopathogenesis of autoimmune rheumatic diseases have been studied since the advent of germ theory in the 19th century. With the exception of Group A Streptococcus in rheumatic fever, early studies failed to establish causal relationships between specific pathobionts and rheumatic disease. Today, systemic autoimmune diseases are thought to result from a complex interplay of environmental factors, individual genetic risk, and stochastic events. Interactions of microbiota and the immune system have been shown to promote and sustain chronic inflammation and autoimmunity. In mechanistic studies, microbe-immune cell interactions have been implicated in the initiation of autoimmune rheumatic diseases, e.g., through the posttranslational modification of autoantigens in rheumatoid arthritis or through neutrophil cell death and cross-reactivity with commensal orthologs in systemic lupus erythematosus. In parallel, modern molecular techniques have catalyzed the study of the microbiome in systemic autoimmune diseases. Here, I review current insights gained into the skin, oral, gut, lung, and vascular microbiome in connective tissue diseases and vasculitis. Mechanism relevant to the development and propagation of autoimmunity will be discussed whenever explored. While studies on autoimmune rheumatic disease have almost invariably shown abnormal microbiome structure (dysbiosis), substantial variability in microbial composition between studies makes generalization difficult. Moreover, an etiopathogenic role of specific pathobionts cannot be inferred by association alone. Integrating descriptive studies of microbial communities with hypothesis-driven research informed by immunopathogenesis will be important in elucidating targetable mechanisms in preclinical and established rheumatic disease.