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Th17 Cells Are the Dominant T Cell Subtype Primed by Shigella flexneri Mediating Protective Immunity

The T cell response to Shigella, the causative agent of bacillary dysentery, remains poorly understood. Using a murine model of infection, we report that Shigella flexneri primes predominately IL-17A– and IL-22–producing Th17 cells. Shigella-specific Th1 cells are only significantly induced on secondary infection, whereas specific Th2 and CD8+ T cells are undetectable. Apart from Th17 cells that are primed in a MHC class II- and IL-6–dependent, but IL12/23p40-independent manner, we identified γδ T cells as an additional but minor source of IL-17A. Priming of IL-17A+ γδ T cells is dependent on IL12/23p40, but independent of MHC-class II and IL-6. Th17 cells have emerged as important players in inflammatory, autoimmune, and infectious diseases. Among the yet unresolved questions is their role in long-term immunity to pathogens. In this study, we show that the elicited S. flexneri-specific Th17 pool gives rise to an enhanced recall response up to 12 mo after priming, suggesting the presence of a long-term memory state. The clearance of primary infection is impaired in the absence of T cells, but independently of IL-17A. However, after reinfection, IL-17A produced by S. flexneri-specific Th17 cells becomes important to ultimately restrict bacterial growth. These findings bring new insights into the adaptive immune response to Shigella infection and highlight the importance of pathogen-specific Th17 cell immunity for secondary immune protection.