P069 Elevated adaptive immune responses to multiple Lachnospiraceae bacterial flagellins in therapy-naïve pediatric CD patients are associated with distinct immune pathology.

Abstract Background In inflammatory bowel disease (IBD), including Crohn’s disease (CD) and ulcerative colitis (UC), CD4+ T cell-responses to unknown microbial antigens drive intestinal inflammation. Both hypo- or hyperactive anti-microbial innate immunity may underlie these pathogenic T-cell responses. Previously, IgG and T-cell responses to Lachnospiraceae flagellins were detected in spontaneously colitic mice with an innate immune defect and adult CD patients with complicated disease. However, whether these IgG responses occur in therapy-naïve pediatric patients and whether they relate to hypo- or hyperresponsiveness is unknown. We hypothesize that, at diagnosis, a subgroup of pediatric CD patients has elevated anti-Lachnospiraceae flagellin IgG and concomitant inflammatory flagellin-specific T-cell responses associating with insufficient anti-microbial innate immunity. Methods Plasma IgG reactivity to 20 flagellins from Lachnospiraceae that colonize mouse and human gut was measured in therapy-naïve pediatric IBD patients (CD n=49; UC n=16), non-IBD (n=12) and age-matched healthy controls (HC n=17). CD patients were divided in two groups: ‘flagellin-hi’ (elevated IgG to 11-20 flagellins) and ‘flagellin-lo’ (elevated IgG to 0-10 flagellins), whose immunological and clinical profiles were compared. Results The proportion of flagellin-hi patients was higher in CD (41%) compared to UC (6%), non-IBD (8%) and HC (6%). In CD patients, clinical disease activity score, CRP and ESR did not relate to flagellin reactivity. Compared to flagellin-lo patients, flagellin-hi patients had increased frequencies of inflammatory gut-homing (CD38+CD62Lneg) circulating CD4+ T cells. Crucially, flagellin-hi patients had high frequencies of circulating flagellin-specific CD4+ T cells. Although high anti-flagellin IgG reactivity associated with increased flagellin-specific- and gut-homing- T-cell frequencies in the circulation, it associated with very limited immune cell infiltration, epithelial damage and phosphorylated NFκB in colonic biopsies on histology. Moreover, the plasma concentration of EN-RAGE, a neutrophil-derived protein, was decreased in flagellin-hi compared -lo patients. Conclusion In sum, a subgroup of pediatric therapy-naïve CD patients has high IgG responses to Lachnospiraceae-derived flagellins. While these flagellin-hi patients have higher frequencies of circulating gut-homing and flagellin-specific CD4+ T cells, they have lower colonic histological disease activity and NFκB activation, possibly suggesting insufficient innate immune cell activation and/or infiltration. Together, our data raise the question whether the enhanced anti-Lachnospiraceae adaptive immune response in a subgroup of CD patients may be driven by insufficient local innate immunity.