Maturation of Human Intestinal Epithelial Cell Layers Fortifies the Apical Surface against Salmonella Attack

The active invasion of intestinal epithelial cells (IECs) represents a key event in the infection cycle of many gut pathogens. Studies of how Salmonella enterica Typhimurium (S.Tm) bacteria enter transformed cell lines have shaped the paradigm for swift type-three-secretion-system-1 (TTSS-1)-driven IEC invasion, fueled by expansive membrane ruffles. However, comparative studies suggest that non-transformed IECs in the intact gut context comprise a much more challenging target for the attack. The molecular and cellular features that explain these discrepancies remain undefined. By live-cell imaging in human enteroid-and colonoid-derived IEC layers, we demonstrate that the maturation state of gut epithelia dramatically impacts permissiveness to S.Tm invasion. IEC layers kept under immature progenitor-cell-promoting conditions are permissive to the bacterial invasion, whereas maturation towards an enterocyte/colonocyte fate reduces the frequency of S.Tm-induced epithelial entry structures, and lowers the invasion efficiency by up to tenfold. This phenotypic shift during IEC maturation couples to an altered expression of actin regulatory proteins implicated in the invasion process, and an increased dependence on the S.Tm TTSS-1 effector SipA for successful entry. In addition, IEC maturation involves upregulation of cell surface mucins, e.g. MUC13, and shifts in glycocalyx composition, as revealed by multiple lectin stainings. Enzymatic treatment of the apical surface with the StcE mucinase converts maturing IEC layers back to the S.Tm-invasion-permissive state of their immature counterparts. Taken together, these results showcase how the maturation state of human IECs dictates the susceptibility to invasion by a prototype enterobacterium.