Injection of T3SS effectors not resulting in invasion is the main targeting mechanism of Shigella toward human lymphocytes

The enteroinvasive bacterium Shigella is a facultative intracellular bacterium known, in vitro, to invade a large diversity of cells through the delivery of virulence effectors into the cell cytoplasm via a type III secretion system (T3SS). Here, we provide evidence that the injection of T3SS effectors does not necessarily result in cell invasion. Indeed, we demonstrate through optimization of a T3SS injection reporter that effector injection without subsequent cell invasion, termed the injection-only mechanism, is the main strategy used by Shigella to target human immune cells. We show that in vitro-activated human peripheral blood B, CD4^{+} T, and CD8^{+} T lymphocytes as well as switched memory B cells are mostly targeted by the injection-only mechanism. B and T lymphocytes residing in the human colonic lamina propria, encountered by Shigella upon its crossing of the mucosal barrier, are also mainly targeted by injection-only. These findings reveal that cells refractory to invasion can still be injected, thus extending the panel of host cells manipulated to the benefit of the pathogen. Future analysis of the functional consequences of the injection-only mechanism toward immune cells will contribute to the understanding of the priming of adaptive immunity, which is known to be altered during the course of natural Shigella infection.