Purified Vi antigens as possible typhoid vaccines

Immunity to Salmonella infection

The foregoing literature review and data presentation have been set forth in the hope of clarifying some complex and confusing issues in regard to Salmonella infection. From a practical point of view, the information presented has implications for the direction to take with regard to improving the current typhoid vaccine, as the presently used acetone-killed cell preparation has considerable toxicity. The issues are important from a theoretical standpoint, because they have bearing on the nature of the concepts researchers and clinicians carry as working hypothesis with regard to the mechanisms of immunity to Salmonella infection. An incomplete appreciation of the literature seems to have led many scientists to believe that only cellular immunity can protect a mouse, and by analogy a human, against Salmonella. The logical deduction from such a premise is that only live vaccines will be effective in humans againsT S. typhi. Such a conclusion would appear unfounded, as documented in this review, for killed vaccines have been shown to be highly effective in vaccinating many mouse strains, as well as humans, against enteric fever.

Evaluation of two Salmonella typhimurium hybrids as challenge organisms in a system for the assay of typhoid vaccines

A mouse-virulent Salmonella typhimurium hybrid (H42), which expresses the Salmonella typhi Vi antigen in addition to S. typhi O antigens 9 and 12, and a mouse-virulent S. typhimurium hybrid (H1), which expresses only the 9 and 12 antigens of S. typhi, were compared in their behavior as challenge organisms in a system developed to assay the protective capabilities of typhoid vaccines. Swiss-Webster white mice, vaccinated intraperitoneally with live Escherichia coli hybrids expressing the S. typhi O antigens 9 and 12, were significantly protected against death from intraperitoneal challenge with each of the S. typhimurium hybrid strains. Vaccination with an E. coli hybrid expressing the S. typhi Vi antigen in addition to O antigens 9 and 12 was seen to confer no advantage in protection against either S. typhimurium hybrid challenge organism over that obtained by vaccination with an E. coli hybrid expressing only the O antigens of S. typhi. However, a notable difference in the behavior of the two S. typhimurium hybrids was seen in mice vaccinated with the parent of the E. coli hybrid vaccinating strains, E. coli F464, which expresses no surface antigens common to either of these S. typhimurium hybrid challenge organisms. A nonspecific (with respect to the vaccinating strain) protective effect, believed to be associated with Vi antigen expression by the challenge organism, was seen against the challenge with S. typhimurium hybrid H42 after F464 vaccination, whereas no protection was conferred by F464 vaccination against the challenge with Vi-nonexpressing S. typhimurium hybrid H1. Inasmuch as neither S. typhimurium hybrid discriminates between the expression or nonexpression of the Vi antigen in a vaccinating strain, it is concluded that the Vi-nonexpressing S. typhimurium hybrid H1, which more clearly indicates the vaccine-specific protective role of the S. typhi O antigens and does not exhibit the nonspecific protection response of hybrid H42, is the better choice as challenge organism for this vaccine assay system.

Development of a localized hemolysis-in-gel assay for Vi antigen: characterization of the Vi-specific PFC response of nude and normal mice

An assay to detect specific plaque-forming cells (PFC) to Vi antigen (Vi) was developed and the optimal conditions for sensitization of sheep erythrocytes (SE) and plaque development were determined. Using PFC and passive hemagglutination (PHA) assays, Vi-specific immune responses of athymic (nude) and normal mice were characterized. Vi was found to elicit only IgM PFC. No discernable secondary response was detected following a second injection of antigen. Nude and normal mice responded in a quantitatively similar manner to all doses of Vi tested and responded similarly on varying days following immunization. Also, both nude and normal mice produced the greatest number of Vi-specific PFC 4 days following immunization with an optimally immunogenic dose of Vi (1.0 microng/mouse). These results indicate that functional thymus-derived cells are not necessary to elicit an immune response against Vi antigen.

Ineffectiveness of Vi and chemically treated endotoxins as typhoid vaccines in mice challenged with a Salmonella typhosa-Salmonella typhimurium hybrid

Purified Vi antigen, acetic anthydride-treated Salmonella typhosa endotoxin, and potassium methylate-treated S. typhosa endotoxin employed as vaccines in Swiss white mice failed to protect these animals against challenge with a virulent S. typhimurium hybrid expressing S. typhosa antigens.