Microbial threads in the tapestry of rheumatoid arthritis.

Rheumatoid arthritis (RA) has a preclinical period of 5-10 years preceding the appearance of joint pain and swelling characteristic of clinical RA. Preclinical RA has been characterized by circulating IgA and IgG classes of autoantibodies targeting citrullinated protein antigens (ACPAs) that are highly specific for future clinical RA, circulating IgA plasmablasts, and autoantibody production at mucosal sites, all of which point toward mucosal tissues as the origin of immune dysregulation. In individuals at risk for developing and with established RA, oral and gut microbial shifts correlate with immune activation. Specific bacterial taxa such as Segatella copri, Subdoligranulum didolesgii, Eggerthella lenta, and Streptococcal species have been shown to contribute to the development and/or perpetuation of RA through mechanisms that include molecular mimicry, antigen citrullination, and disruption of mucosal immunity. Furthermore, microbial metabolites, including short-chain fatty acids, bile acids, and tryptophan derivatives, regulate immune homeostasis and offer potential therapeutic avenues. The gut microbiome also influences therapeutic responses by modulating conventional disease-modifying antirheumatic drugs. This Review synthesizes current knowledge on the bacterial microbiome's role in RA pathogenesis and treatment responses, highlighting microbiome-targeted interventions as potential strategies for disease prevention and management.