The L-DBF vaccine cross protects mice against different Shigella serotypes after prior exposure to the pathogen.

Shigellosis (bacillary dysentery) is a severe diarrheal disease caused by members of the genus Shigella , which results in 90 million cases annually around the world. The Shigella type III secretion system (T3SS) is a specialized secretion system that is the primary virulence factor it uses to infect the colonic mucosa. The type III secretion apparatus (T3SA) proteins IpaB and IpaD, as well as the genetic fusion, DBF, have been demonstrated to protect mice from Shigella spp. infection in a lethal pulmonary model. In a previous study, we fused LTA1, the active moiety of lethal toxin from enterotoxigenic Escherichia coli to DBF to produce a self-adjuvanting vaccine candidate L-DBF, which cross-protected mice against four serotypes of Shigella flexneri and Shigella sonnei . Here, we exposed mice with one or two sublethal doses of S. flexneri 2a to identify whether the immune response induced by L-DBF in the host would be affected by prior infection by homologous or heterologous Shigella serotypes. We demonstrate that pre-infection with two sublethal doses of S. flexneri 2a did not elicit cross-protection against S. sonnei , while vaccination with L-DBF did. Our results indicate that L-DBF is a feasible vaccine candidate that offers cross-protection against Shigella 's different serotypes even after prior exposure to the pathogen. This work provides a proof of concept that a novel subunit vaccine can not only protect a naïve host from Shigella challenge, but also can protect against challenge after prior infection by the same or different Shigella serotypes. IMPORTANCE Shigellosis is endemic to low- and middle-income regions of the world where children are especially vulnerable. In many cases, there are pre-existing antibodies in the local population and the effect of prior exposure should be considered in the development and testing of vaccines against Shigella infection. Our study shows that L-DBF-induced immune responses are not adversely affected by prior exposure to this pathogen. Moreover, somewhat different cytokine profiles were observed in the lungs of vaccinated mice not having been exposed to Shigella , suggesting that the immune responses elicited by Shigella infection and L-DBF vaccination follow different pathways.