O-Antigen Inhibits Internalization, Vacuole Escape, and Inflammasome Activation.

Two species, and , cause approximately 90% of bacterial dysentery worldwide. While is the dominant species in low-income countries, causes the majority of infections in middle- and high-income countries. is a prototypic cytosolic bacterium; once intracellular, it rapidly escapes the phagocytic vacuole and causes pyroptosis of macrophages, which is important for pathogenesis and bacterial spread. In contrast, little is known about the invasion, vacuole escape, and induction of pyroptosis during infection of macrophages. We demonstrate here that causes substantially less pyroptosis in human primary monocyte-derived macrophages and THP1 cells. This is due to reduced bacterial uptake and lower relative vacuole escape, which results in fewer cytosolic and hence reduced activation of caspase-1 inflammasomes. Mechanistically, the O-antigen (O-Ag), which in is contained in both the lipopolysaccharide and the capsule, was responsible for reduced uptake and the type 3 secretion system (T3SS) was required for vacuole escape. Our findings suggest that has adapted to an extracellular lifestyle by incorporating multiple layers of O-Ag onto its surface compared to other species. Diarrheal disease remains the second leading cause of death in children under five. remains a significant cause of diarrheal disease with two species, and , causing the majority of infections. are well known to cause cell death in macrophages, which contributes to the inflammatory nature of diarrhea. Here, we demonstrate that causes less cell death than due to a reduced number of bacteria present in the cell cytosol. We identify the O-Ag polysaccharide which, uniquely among spp., is present in two forms on the bacterial cell surface as the bacterial factor responsible. Our data indicate that differs from in key aspects of infection and that more attention should be given to characterization of infection.