Despite the importance of Salmonella infections in human and animal health, the target antigens of Salmonella -specific immunity remain poorly defined. We have previously shown evidence for antibody-mediating protection against invasive Salmonellosis in mice and African children. To generate an overview of antibody targeting in systemic Salmonellosis, a Salmonella proteomic array containing over 2,700 proteins was constructed and probed with immune sera from Salmonella -infected mice and humans. Analysis of multiple inbred mouse strains identified 117 antigens recognized by systemic antibody responses in murine Salmonellosis. Importantly, many of these antigens were independently identified as target antigens using sera from Malawian children with Salmonella bacteremia, validating the study of the murine model. Furthermore, vaccination with SseB, the most prominent antigenic target in Malawian children, provided mice with significant protection against Salmonella infection. Together, these data uncover an overlapping immune signature of disseminated Salmonellosis in mice and humans and provide a foundation for the generation of a protective subunit vaccine.

Systemic Salmonella infection commonly induces prolonged splenomegaly in murine or human hosts. Although this increase in splenic cellularity is often assumed to be due to the recruitment and expansion of leukocytes, the actual cause of splenomegaly remains unclear. We monitored spleen cell populations during Salmonella infection and found that the most prominent increase is found in the erythroid compartment. At the peak of infection, the majority of spleen cells are immature CD71−Ter119+ reticulocytes, indicating that massive erythropoiesis occurs in response to Salmonella infection. Indeed, this increase in RBC precursors corresponded with marked elevation of serum erythropoietin (EPO). Furthermore, the increase in RBC precursors and EPO production required innate immune signaling mediated by Myd88/TRIF. Neutralization of EPO substantially reduced the immature RBC population in the spleen and allowed a modest increase in host control of infection. These data indicate that early innate immunity to Salmonella initiates marked splenic erythropoiesis and may hinder bacterial clearance.

Typhoid fever and non-typhoidal bacteremia caused by Salmonella remain critical human health problems. B cells are required for protective immunity to Salmonella but the mechanism of protection remains unclear. Here, we immunized WildType, B cell deficient, antibody-deficient and class-switched antibody-deficient mice with attenuated Salmonella and examined protection against secondary infection. As expected, WildType mice were protected and B cell deficient mice succumbed to secondary infection. Interestingly, mice with B cells but lacking secreted antibody or class-switched antibody had little deficiency in resistance to Salmonella infection. The susceptibility of B cell deficient mice correlated with marked reductions in CD4 T cell IFN-γ production after secondary infection. Together, these data suggest that the primary role of B cells in acquired immunity to Salmonella is via the development of protective T cell immunity.

Pathogen-specific CD4 T cells are activated within a few hours of oral Salmonella infection and are essential for protective immunity. However, CD4 T cells do not participate in bacterial clearance until several weeks after infection, suggesting that Salmonella can inhibit or evade CD4 T cells that are activated at early time points. Here, we describe the progressive culling of initially activated CD4 T cells in Salmonella-infected mice. Loss of activated CD4 T cells was independent of early instructional programming, T cell precursor frequency, and Ag availability. In contrast, apoptosis of Ag-specific CD4 T cells was actively induced by live bacteria in a process that required Salmonella pathogenicity island-2 and correlated with increased expression of PD-L1. These data demonstrate efficient culling of initially activated Ag-specific CD4 cells by a microbial pathogen and document a novel strategy for bacterial immune evasion.

Summary Typhoid fever is a persistent infection caused by host‐adapted Salmonella strains adept at circumventing immune‐mediated host defences. Given the importance of T cells in protection, the culling of activated CD4+ T cells after primary infection has been proposed as a potential immune evasion strategy used by this pathogen. We demonstrate that the purging of activated antigen‐specific CD4+ T cells after virulent Salmonella infection requires SPI‐2 encoded virulence determinants, and is not restricted only to cells with specificity to Salmonella‐expressed antigens, but extends to CD4+ T cells primed to expand by co‐infection with recombinant Listeria monocytogenes. Unexpectedly, however, the loss of activated CD4+ T cells during Salmonella infection demonstrated using a monoclonal population of adoptively transferred CD4+ T cells was not reproduced among the endogenous repertoire of antigen‐specific CD4+ T cells identified with MHC class II tetramer. Analysis of T‐cell receptor variable segment usage revealed the selective loss and reciprocal enrichment of defined CD4+ T‐cell subsets after Salmonella co‐infection that is associated with the purging of antigen‐specific cells with the highest intensity of tetramer staining. Hence, virulent Salmonella triggers the selective culling of high avidity activated CD4+ T‐cell subsets, which re‐shapes the repertoire of antigen‐specific T cells that persist later after infection.