O157:H7 suppresses host autophagy and promotes epithelial adhesion via Tir-mediated and cAMP-independent activation of protein kinase A.

Autophagy is a pivotal innate immune response that not only degrades cytosolic components, but also serves as one of the critical antimicrobial mechanisms eliminating intracellular pathogens. However, its role in host defense against extracellular pathogens is largely unknown. Here we showed that O157:H7 altered autophagy to evade host defense and facilitate adhesion. Enhancing host cell autophagy with tumor necrosis factor (TNF), host starvation or rapamycin reduced the adherence of O157:H7 to HT-29 cells. As a key regulator of autophagy, protein kinase A (PKA) was activated by O157:H7 infection. PKA inhibition by H89 abrogated O157:H7 inhibition of autophagy and prevented bacterial epithelial adhesion. Thus, PKA had a mediatory role in blocking autophagy and O157:H7 epithelial adhesion. Furthermore, deletion of translocated intimin receptor () prevented PKA activation, whereas ectopic expression in a Δ mutant strain restored its ability to activate PKA and inhibited autophagy in host cells. This indicated that Tir and PKA played pivotal roles in manipulating host autophagy during infection. Consistent with autophagy inhibition, O157:H7 infection inhibited endoplasmic reticulum (ER) stress in HT-29 cells, which was reversed by TNF, starvation, or H89 treatment. Additionally, O157:H7-induced PKA activation suppressed extracellular signal-regulated kinase 1/2 (ERK1/2) activation and enhanced phosphatidylinositol 3-kinase/Akt (PI3K/Akt) signaling, thereby repressing autophagic signaling. Conversely, PKA inhibition prevented downregulation of ERK1/2 signaling due to O157:H7 infection. In summary, O157:H7 inhibited host autophagy via Tir-mediated PKA activation that favored bacterial persistence on intestinal epithelial cell surfaces.