Basic studies on prevention of experimental salmonellosis(3). Protective effect induced by S. enteritidis SPA in mice (author's transl)]

Mice immunized by a single injection of SPA from cultures of S. enteritidis strain 2547 (high SPAproducing strain) tolerated a challenge with 100 LD50 of the same strain of oraganisms. But, with non-SPAproducing strain (strain 2822), no such effect was observed. Normal mice injected with mouse immune serum to SPA of strain 2547 proved capable of tolerating a challenge dose of 10 LD50, thus being protected from fatal infection. After absorption of the immune serum with SPA, the injection was no longer effective against infection. The and-SPA immune serum practically did not contain antibodies to 0 or H antigen of the organism, and that the protective effect of immune serm varied among strains though sharing the same structural patterns of 0 and H antigens. It appears to follow that the protective antibody may be distinct from anti-0 and anti-H antibodies.Experiments have shown that the protective effect correlates well with the SPA production even when assessed using heat-killed organisms (HK) or formol vaccine (FV).The protective effect of SPA became manifest at 3 days after injection for immunization and was still demonstrable at 180 days, whereas that of the passive transfer of immune serum disappeared by 90 days. The finding indicates that not only humoral antibodies but also other factors as well are involved in the protection against salmonella infection by immunization with SPA.Suppression of macrophage migration was evident on the migration inhibition test with peritoneal cells from mice immunized with SPA, thereby suggesting a possible involvement of cell-mediated immunity.Groups of mice immunized with SPA were examined for enumeration of bacteria in the blood and viscera following a challenge by a highly virulent strain. The inoculant organism showed a slight increase in number and then gradually diminished to become completely eradicated. With non-SPA-producing strains, no such effect was observed; the bacterial populations in the blood and organs increased progressively, with eventual death of the animals.