Poor survival but high immunogenicity of IL-2-expressing Salmonella typhimurium in inherently resistant mice

An IL-2-expressing, attenuated strain of Salmonella typhimurium (strain GIDIL2) was previously shown to survive poorly and to have lower immunogenicity in susceptible mice than its parental, non-cytokine-expressing strain (designated BRD509). In the present study, we compared the immune responses induced by both bacterial strains in inherently Salmonella-resistant C3H/HeN mice. Analysis of the bacterial loads in the peritoneum and spleen revealed that colony-forming units (CFUs) of GIDIL2 were consistently lower than the corresponding BRD509 CFUs. As early as 48 h after inoculation, there were 60-fold lower CFUs of GIDIL2 than BRD509 organisms in the peritoneal cavity. Similarly, the differences in splenic CFUs of GIDIL2 were 20- to 50-fold lower than those of BRD509 over a period of 3-21 days post-injection. This rapid rate of clearance of the GIDIL2 organisms correlated with significantly decreased infection-induced splenomegaly and nitric oxide production by spleen cells. However, despite the poor survival of GIDIL2 organisms in vivo, they could activate peritoneal NK cells efficiently. As early as 48 h after immunization, equivalent levels of NK-mediated cellular cytotoxicity were induced by BRD509 and GIDIL2 strains. Direct evidence for priming of the immune response was shown by demonstrating increased production of IFN-γ in a recall response by spleen memory T cells obtained 3 weeks after immunization. Finally, mice inoculated with a single dose of either BRD509 or GIDIL2 organisms were fully protected against a challenge of >100-fold the LD₅₀ dose of virulent Salmonella. Taken together, our data demonstrate that despite their rapid clearance from the reticuloendothelial system, IL-2-expressing Salmonella are immunogenic and fully capable of affording excellent protection against virulent challenge in Salmonella-resistant C3H/HeN mice.