Intestinal dysbiosis exacerbates skin inflammation via microbial metabolite-driven Th2 cell differentiation

The interplay between gut microbiota and the mucosal immune system critically regulates systemic immunity and disease susceptibility. Here, we demonstrate that intestinal epithelial Toll-like receptor (TLR)4 deficiency reshaped the gut microbiome and subsequently exacerbated atopic dermatitis (AD) in mice. Mechanistically, TLR4 deficiency reduced Akkermansia muciniphila abundance and enriched choline trimethylamine-lyase (CutC)-expressing bacteria. This enhanced microbial choline-to-trimethylamine conversion and elevated circulating trimethylamine oxide (TMAO) levels. Clinically, AD patients exhibited increased plasma TMAO levels that positively correlated with disease severity and immunoglobulin E (IgE) levels. UK Biobank data also showed that higher dietary choline intake was associated with increased AD risk. TMAO promoted T helper (Th)2 differentiation by directly interacting with protein phosphatase 5 (PPP5) and enhancing PPP5-mediated dephosphorylation of PPARγ. CD4⁺ T cell-specific PPARγ deletion abolished TMAO-driven skin pathology in AD mice. Our results reveal intestinal dysbiosis, as a result of innate immune deficiency, as a driver of inflammatory Th2 cells and AD pathology, highlighting a link among the gut immune environment, microbial metabolites, and skin disease.