Analysis of Flagellin-Specific Adaptive Immunity Reveals Links to Dysbiosis in Patients With Inflammatory Bowel Disease.

Bacterial flagellin is an important antigen in inflammatory bowel disease, but the role of flagellin-specific CD4+ T cells in disease pathogenesis remains unclear. Also unknown is how changes in intestinal microbiome intersect with those in microbiota-specific CD4+ T cells. We aimed to quantify and characterize flagellin-specific CD4+ T cells in Crohn's disease (CD) and ulcerative colitis (UC) patients and study their relationship with intestinal microbiome diversity. Blood was collected from 3 cohorts that included CD patients, UC patients, and healthy controls. Flow cytometry analyzed CD4+ T cells specific for Lachnospiraceae-derived A4-Fla2 and Escherichia coli H18 FliC flagellins, or control vaccine antigens. Serum antiflagellin IgG and IgA antibodies were detected by enzyme-linked immunosorbent assay and stool samples were collected and subjected to 16S ribosomal DNA sequencing. Compared with healthy controls, CD and UC patients had lower frequencies of vaccine-antigen-specific CD4+ T cells and, as a proportion of vaccine-specific cells, higher frequencies of flagellin-specific CD4+ T cells. The proportion of flagellin-specific CD4+ T cells that were CXCR3negCCR4+CCR6+ Th17 cells was reduced in CD and UC patients, with increased proportions of CD39+, PD-1+, and integrin β7+ cells. Microbiome analysis showed differentially abundant bacterial species in patient groups that correlated with immune responses to flagellin. Both CD and UC patients have relative increases in the proportion of circulating Fla2-specific CD4+ T cells, which may be associated with changes in the intestinal microbiome. Evidence that the phenotype of these cells strongly correlate with disease severity provides insight into the potential roles of flagellin-specific CD4+ T cells in inflammatory bowel disease.