Gut microbiota and intestinal permeability in rheumatoid arthritis: pathogenic mechanisms.

Rheumatoid arthritis is a chronic systemic autoimmune disease in which the earliest breaks in immune tolerance may arise at mucosal surfaces before the clinical onset of synovitis. Among these sites, the gut has emerged as a particularly compelling candidate because it integrates microbial, epithelial, metabolic, and immune pathways with the potential to shape systemic inflammation. In this review, we examine the biological foundations of the gut-joint axis in rheumatoid arthritis, focusing on intestinal barrier structure, microbiome alterations, mucosal immune crosstalk, and mechanisms of barrier dysfunction. Current human evidence links rheumatoid arthritis to heterogeneous shifts in gut microbial composition, depletion of beneficial metabolite-producing commensals, altered immune-metabolic signaling, and biomarker patterns consistent with epithelial injury and microbial-product translocation. At the same time, available data do not support the existence of a single, universal microbial or permeability signature that defines the disease across populations. Recent longitudinal studies further challenge the concept of stable, long-standing dysbiosis and instead suggest a late, transient phase of ecological instability arising close to symptom onset. Experimental models provide stronger mechanistic support, showing that dysbiotic microbial communities, impaired barrier integrity, and strain-specific host-microbe interactions can promote T helper 17-skewed immunity and aggravate arthritis. Collectively, these findings support a context-dependent contribution of the gut to rheumatoid arthritis pathogenesis while underscoring the need for longitudinal, strain-resolved, and multi-omic human studies to clarify causality, refine disease models, and identify clinically meaningful windows for intervention.