The protective and toxic properties of a ribosomal vaccine prepared from Salnonella typhimurium W118-2 were systematically compared with those of an acetone-killed whole cell vaccine, purified lipopolysaccharide, and living cells in CD-1 mice. Tests of graded immunizing doses of each vaccine against several challenge doses of live strain W118-2 showed that, although the protection given by ribosomes approached the levels of protection conferred by living organisms, acetone-killed cells administered in appropriate dosages provided levels of protection comparable to that of ribosomes. Lipopolysaccharide was found to be significantly less protective than the other vaccines. On a dry-weight basis, ribosomes were the least toxic with a 50% toxic dose (TD5o) of 5,000 jig; acetonekilled cells had an intermediate TD5o of 1,400 ,ug; and lipolysaccharide was the most toxic, with a TD50 of 320 ,ug. The dose of each vaccine that protected 50% of the mice against a challenge of 1,000 times the 50% lethal dose was determined and divided by the TD5o to give the therapeutic index. This ratio also indicated that the ribosomes and acetone-killed cells were equally effective, whereas lipopolysaccharide was markedly inferior.

Groups of mice were immunized with optimal doses of the following vaccines of Salmonella typhimurium W118-2: acetone-killed cells, lipopolysaccharide, ribosomes, and live cells. At 3 weeks, 1 month, 2 months, 4 months, or 6 months postimmunization, sera were collected from control and vaccinated animals, and the anti-lipopolysaccharide and whole-cell agglutination titers of the sera were determined. Other groups of similarly vaccinated mice were tested for resistance to infection by challenging with live W118-2 and scoring the number of survivors 30 days postinfection. It was found that only ribosomes and live cells afforded significant protection 6 months after immunization. Thus, in duration of protection ribosomes were superior to the other nonviable vaccines tested. At all time intervals tested, purified lipopolysaccharide was the least effective vaccine. Protection afforded by the acetone-killed cell and ribosomal vaccines correlated better with the whole-cell agglutination titers than with the anti-lipopolysaccharide titers. However, the longer duration of protection afforded by the ribosomal vaccine, as compared with the acetone-killed vaccine, could not be accounted for by differences in whole-cell agglutination titers. These studies show that ribosomal vaccines are equal in all parameters to acetone-killed cells and have the advantage of providing longer-lasting immunity.

No abstract

Phenol-water-extracted lipopolysaccharide of Salmonella typhimurium, strain W118-2, was tested for protective capacity and immunogenicity in C3H/HeJ and CD-1 mice 21 days post vaccination. It was found that in C3H/HeJ mice the LPS was not protective and gave no anti-O titer, as measured by passive hemagglutination, whereas in CD-1 mice it was protective and immunogenic. In contrast, acetone-killed cells and a ribosomal vaccine prepared from the same strain of Salmonella were protective and resulted in an anti-O titer in both the C3H/HeJ and the CD-1 mice. It was shown, therefore, that LPS co-extracted with ribosomes, or present in whole cells, is more immunogenic for C3H/HeJ mice than phenol-water purified LPS. These observations imply that ribosome-rich extracts and acetone-killed cells contain substances, in addition to LPS, that modulate the immune response in C3H/HeJ mice. It was also observed that ribosomal vaccine prepared from a rough strain of S. typhimurium, strain TA1659, that lacks O antigens and makes LPS of the Rc type, did not protect C3H/HeJ or CD-1 mice against Salmonella infection. Mixtures of LPS with ribosomal vaccine derived from this mutant were also tested for their protective capacity. The results are discussed with regard to the antigens involved in immunity to Salmonella infection.