Immunosuppression and nitric oxide production induced by parenteral live Salmonella vaccines do not correlate with protective capacity: a phoP::Tn10 mutant does not suppress but does protect.

Previous work from our laboratory showed that an aroA mutant strain of S. typhimurium, SL3235, induces profound immunosuppression 7 days post-parenteral inoculation, and that the suppression is mediated by nitric oxide. Suppression was measured by the capacity of spleen cells to mount a primary in vitro plaque-forming cell response to sheep red blood cells in Mishell-Dutton cultures. In the present studies, the capacity of a panel of strains of attenuated Salmonella with various genetic lesions was tested. Most of the strains were S. typhimurium, but several were S. dublin. It was found that a variety of Salmonella strains induced suppression, demonstrating that suppressive capacity is not unique to SL3235 or to S. typhimurium. A strong correlation was obtained between the log10 of the microbial burden (cfu spleen-1) on the seventh day post-vaccine inoculation and the degree of immunosuppression. Strains that gave high spleen counts gave greater suppression. Microbial burden also correlated with the size of the spleen and the amount of nitrite produced by spleen-cell cultures, a measure of nitric oxide. Finally, the degree of immunosuppression was found to be linearly related to the log10 of the amount of nitrite produced. The capacity of the various strains of Salmonella to protect against challenge with virulent S. typhimurium, strain W118-2, was also tested. No correlation was found between suppressive and protective capacities of the various strains. Two strains suppressed, but did not protect. While most strains that protected grew or persisted in vivo, a phoP::Tn10 mutant of S. typhimurium did not grow or persist; this phoP mutant did not cause immunosuppression, but gave 100% protection against challenge with wild type S. typhimurium, suggesting that such mutants have advantageous properties as live vaccines.