Colitis-primed circulating T cells drive small intestinal barrier function through Nod2, microbiota and myosin light...

Inflammatory bowel diseases (IBD), including Crohn's disease (CD) and ulcerative colitis (UC), are characterized by intestinal inflammation and barrier dysfunction. While disruption of the intestinal barrier contributes to the pathogenesis of IBD, yet how colonic inflammation alters small intestinal homeostasis remains poorly defined. Here, we demonstrate that three models of colitis, including 2,4,6-Trinitrobenzenesulfonic acid (TNBS), Dextran Sulfate Sodium (DSS), and oxazolone, induce paracellular barrier dysfunction in the small intestine, with model-specific immune profiles. Th1/Th17-skewed responses (TNBS and DSS) were associated with microbiota-dependent upregulation of myosin light chain kinase (MLCK), RORγt⁺ CD4⁺ T cell expansion, and elevated expression of Nod2, which was not observed under Th2-dominant (oxazolone) conditions. Inhibition of MLCK restored barrier function and suppressed inflammation in a CD4⁺ T cell-dependent manner. Using Nod2-deficient and 2939insC mutant mice, we showed Nod2 as a critical regulator of small intestinal permeability during colitis. Bone marrow chimeras revealed compartment-specific roles of Nod2, with non-hematopoietic Nod2 sufficient to preserve epithelial integrity, while hematopoietic Nod2 expression is required to limit cytokine-mediated inflammation. Moreover, MLCK inhibition ameliorated intestinal lesions only in mice with Nod2 deficiency in the hematopoietic compartment. Finally, microbiota transfer experiments ruled out a causal role for dysbiosis in driving small intestinal permeability defects in Nod2-deficient mice. These findings uncover a Nod2-MLCK-CD4⁺ T cell axis linking colonic inflammation to small intestinal barrier dysfunction and highlight distinct immune-epithelial-microbial mechanisms shaping intestinal homeostasis during colitis.