Antiflagellin antibodies recognize the autoantigens Toll‐Like Receptor 5 and Pals 1‐associated tight junction protein and induce monocytes activation and increased intestinal permeability in Crohn’s disease

. Background and objectives. Bacterial flagellin is considered an important antigen in Crohn’s disease (CD) as it activates innate immunity through Toll‐Like Receptor 5 (TLR5) engagement and induces an elevated adaptive immune response. Little is known about the presence of an autoimmune process in CD. We aimed to identify pathogenically relevant autoantigen targets in CD. Methods. We screened a random peptide library with pooled sera of patients with active CD. Transepithelial flux of [3H] mannitol in T84 human intestinal epithelial cell line was used to study the epithelial barrier function. Monocyte activation was evaluated by surface expression of activation markers and by production of pro‐inflammatory cytokines. Gene modulation of T84 cells exposed to antipeptide antibodies was analysed by gene array. Results. We identified a peptide that shares homology with Salmonella typhimurium flagellin and with self‐antigens such as TLR5 and cell junction protein, Pals 1‐associated tight junction protein. The affinity‐purified antipeptide antibodies recognized the self‐antigens and induced increased intestinal epithelial cell permeability. Moreover, the antibodies induced monocyte activation upon binding TLR5. Finally, in cultured intestinal cells (T84) the purified antibodies induced the modulation of clusters of proinflammatory genes similar to the one induced by the engagment of TLR5 by its natural ligand flagellin. Conclusions. Antibodies directed against an immunodominant peptide of flagellin recognize self‐antigens and are functionally active suggesting the presence of an autoimmune process that can both facilitate loss of tolerance to intestinal microflora by increasing cell permeability and amplify the innate immunity involvement through a novel mechanism of TLR5 activation.