Using the intracellular bacterium Listeria monocytogenes as a model of pathogen crossing of host epithelial barriers, Gessain et al. provide a mechanistic explanation for the requirement of two internalins used by L. monocytogenes during infection at the placental barrier, showing that phosphoinositide 3-kinase (PI3-K) is a key factor regulating microbial pathogen permissiveness at epithelial barriers.
L. monocytogenes possesses two internalins (Inls), InlA and InlB, which bind to their respective receptors E-cadherin (Ecad) and c-Met. Both Inls–receptor interactions are species-specific with InlB–c-Met inducing membrane ruffling via PI3-K activation, a key step of L. monocytogenes invasion. L. monocytogenes is known to exhibit differential Inl requirements for crossing the intestinal and placental barriers. While both Inls are necessary for L. monocytogenes invasion at the placental barrier, only InlA is required for crossing the intestinal barrier. The mechanistic explanation for this difference was unknown.
In this study, Lecuit and colleagues used in vivo mouse models, human intestinal and placental cell lines and tissue samples, in vitro biochemical and microbial invasiveness assays, as well as state-of-the-art fluorescent microscopy analysis to test the hypothesis in vivo that PI3-K tissue levels affect L. monocytogenes invasion. They provide a thorough in situ correlation of the levels of PI3-K activity (as measured by Akt phosphorylation and Foxo1 cytosolic location) with L. monocytogenes invasiveness of the intestine and placenta. PI3-K activity, both in mouse and human intestinal tissues, was primarily detected in mucus-secreting goblet cells, extruding cells, and some enterocytes—which were all targeted by L. monocytogenes—yet it was barely detectable in placenta. Gain and loss of function experiments, knocking down or promoting overexpression of PI3-K activity in vitro, and comparative invasiveness assays with L. monocytogenes mutants for InlA, InlB, or both, together provide compelling support for a critical role of PI3-K in controlling host barrier permissiveness.
These results highlight how protective mechanisms may have been evolutionarily selected to restrict the invasiveness potential of mucosal pathogens. They also allow for an interesting reinterpretation of prior work that used an engineered L. monocytogenes with a murine version of InlA, which may not fully reflect the natural situation of intestinal infection in humans.
An important next step to follow up this study is to extend the proposed model to other organs and mucosal microbial infections, and to establish whether or not PI3-K, as proposed by the authors, is a general mechanism for other pathogens to breach epithelial barriers. In this regard, because many pathogens enter through epithelial barriers, the therapeutic potential of the reported finding may turn out to be very important.