Mechanical Competition Triggered by Innate Immune Signaling Drives The Collective Extrusion of Bacterially-Infected Epithelial Cells

Abstract

Several intracellular bacterial pathogens alter the mechanics of their mammalian host cells to promote their dissemination through tissues. Conversely, host cells may respond to the presence of intracellular pathogens by altering their own mechanical behavior to limit infection spread. Here, we monitored epithelial cell monolayers sparsely infected with the intracellular bacterial pathogens Listeria monocytogenes or Rickettsia parkeri over the course of several days. We found that, under conditions where these pathogens trigger innate immune signaling through the NF-κB pathway and use actin-based motility to spread non-lytically from cell to cell, domains of infected cells form enormous three-dimensional mounds. We show that these infection mounds result from uninfected cells moving toward the site of infection, collectively squeezing the softer and less contractile infected cells upward and ejecting them from the epithelial monolayer. Bacteria in infection mounds are less able to spread laterally in the monolayer, limiting the growth of the focus, while extruded infected cells eventually undergo cell death. This cellular competition between infected and uninfected cells leading to infected cell extrusion en masse is driven by active directional migration in uninfected bystander cells, intact cell-cell junctions and innate immune signaling. in addition, cells in infected monolayers exhibit behavioral and molecular signatures of the epithelial to mesenchymal transition (EMT). in summary our findings showcase that coordinated forceful action by uninfected bystander cells actively eliminates large domains of infected cells, consistent with the hypothesis that this collective cell response represents an innate immunity-driven process that could help to limit the local spread of infection.