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Two outbred mouse strains, ddY and CF1, were tested for their ability to be protected against infection with Salmonella typhimurium by several types of salmonella vaccines. These strains have the same levels of innate susceptibility to S. typhimurium, and also have the same capacity to develop delayed-type hypersensitivity (DTH) to salmonella antigens. Both the crude ribosomal fraction (CRF) and live-cell vaccines conferred acquired resistance on both strains, characterized by greater responses of T cells to salmonella antigens. Mice of the ddY strain were also protected by the purified transfer RNA (tRNA) vaccine, which was free of O antigens, but CF1 mice were not, despite the presence of T-cell reactivity with salmonella antigens. Neither strain was protected by the phenol-water-extracted lipopolysaccharide (LPS). The tRNA-immunized CF1 mice were protected by transfer of antiserum to CRF, but not by transfer of anti-LPS antibody. This antiserum to CRF, however, did not transfer acquired resistance into non-immune mice of either strain. These observations suggest that CF1 mice may require an antibody to another non-O antigen existing in CRF to develop acquired resistance, and that stimulation of the defence system by tRNA may be essential to the development of acquired resistance in CF1 mice.

The active component in crude ribosomal fraction (CRF) of Salmonella typhimurium, capable of inducing protective antibody, was partially purified by two series of chromatography (Sephadex G-150 and DEAE-Sepharose CL6B) after sodium dodecyl sulphate (SDS)-treated CRF was precipitated with ammonium sulphate. The major active component was eluted by 0.4-0.45 M NaCl from DEAE-Sepharose CL6B, and its molecular weight was 43,000 as determined by SDS-polyacrylamide gel electrophoresis. Immunization with the fraction containing 43,000 component alone did not always confer protection on CF1 mice, but its administration together with either the purified transfer RNA (tRNA) or Freund's complete adjuvant (FCA) was much more effective against infection with S. typhimurium. Antibody to the fraction containing 43,000 component was not only free in serum but also associated with peritoneal cells. Macrophages that had been exposed to the antibody had enhanced anti-bacterial activity. Western blot analysis showed that 43,000 component did not react to antiserum to lipopolysaccharide (LPS), but to antiserum to CRF. The antibody elicited by non-O antigenic component and the cell-mediated resistance stimulated by the adjuvant effect of RNA together confer effective protection on CF1 mice.

In order to compare the potential of salmonella vaccines prepared from Salmonella typhimurium to provide the longer-lasting protection from the aspects of cell-mediated immunity, groups of mice were immunized with optimal doses of the following preparations: live cells, ribosome-rich extract, acetone-killed cells, and heat-killed cells. At various intervals post-immunization, mouse peritoneal macrophages and splenic T cells were tested for biological activities. The capacity of each vaccine to confer mouse protection against a lethal challenge with S. typhimurium correlated with the degree of macrophage activation engendered by each of them in the early stage of immunization. In the late stage of immunization, the level of mouse protection conferred by each vaccine was found to be based on the capacity of T cells to respond to salmonella antigens, which correlated with the degree of adoptive immunity by T cells. The live-cell and ribosomal vaccines were superior to killed-cell vaccines in inducing the cell-mediated protection. Thus, the longer-lasting immunity provided by the live-cell and ribosomal vaccines can be accounted for by the fact that T cells of mice immunized with both vaccines have the persistent reactivity to salmonella antigens.