IDENTIFICATION OF ADHERENT-INVASIVE ESCHERICHIA COLI (AIEC) VIRULENCE DETERMINANTS LEADING TO ACTIVATION OF PATHOGENIC TH17 CELLS IN CROHN¿S DISEASE.

Crohn’s disease (CD) is a chronic inflammatory bowel disease characterized by a relapsing-remitting clinical behaviour resulting from inappropriate and persistent activation of the gut mucosal immune system in genetically predisposed individuals. So far, the accepted dogma is that CD arises from the overly activation of tissue-resident T-cells in response to a dysbiotic composition of the gut microbiota. In particular, the pathogenesis of CD has been linked to a significant enrichment of pathogenic IFNγ-producing Th17 cells with a concomitant selective and significant enrichment of adherent-invasive E. coli (AIEC) pathotype. Notably, in a recent yet unpublished work, we described a new subset of pathogenic Th17 cells (pTh17), selectively enriched in the gut of CD patients that are specifically activated by AIEC strain. However, the molecular mechanism by which AIEC induces pTh17 trans-differentiation and activation is still totally unknown. To this aim, a library of 10,000 AIEC-LF82 mutants was generated and screened in order to identify AIEC-virulence determinants specifically involved in pTh17 generation, looking for mutants with an impaired ability to trigger the secretion of the polarizing cytokines IL-23/IL-1β, by human dendritic cells (DCs), linked to pTh17 differentiation. Our data demonstrated that AIEC-LF82 is able to persist and replicate within CD-derived DCs, promoting a significantly higher release of IL-23, but not IL-1β, compared to DCs isolated from healthy donors (HD). Among 10,000 mutants tested, 22 strains significantly reduced IL-23 secretion in HD-derived DCs, while only 13 mutants out of these 22 strains maintained their ability to reduce IL-23 secretion also in CD-derived DCs, thus confirming the genetic diversity and predisposition of CD-derived immune cells to a higher inflammatory response. Moreover, these 22 mutants differently persisted within DCs and displayed very different levels of IL-1β secretion, thus indicating that distinct AIEC-antigens are involved in promoting its intracellular survival and in triggering IL-23/IL-1β secretion. Interestingly, only 6 mutants out of 13 with a compromised capacity to stimulate IL-23 secretion in CD-derived DCs significantly reduce the generation of pTh17 cells, demonstrating that a strong decrease in IL-23 secretion is not enough to totally prevent the differentiation of pTh17 cells. Finally, sequencing of LF82-mutants showed that only a restricted number of AIEC-pathways are involved in the activation of IL-23/pTh17 axis. The identification of these AIEC-determinants, directly linked to IL-23 hypersecretion and to the activation of pTh17 could pave the way for the development of new and more efficient therapeutic strategies able to prevent the activation of pathogenic T cells leaving protective ones unaffected in CD.