Histopathological study of protective immunity against murine salmonellosis induced by killed vaccine.

Swiss-Webster mice were vaccinated with heat-killed salmonellae and then were infected with virulent Salmonella typhimurium. Only 1 of the 18 vaccinated mice died from a challenge of 10(4) X the 50% lethal dose, and about 70% of them survived a challenge of 10(5) X the 50% lethal dose. Histopathological examinations of the lesions developed in these vaccinated mice showed that they followed the characteristic features of a primary lesion in murine salmonellosis. There was an early necrosis with infiltration of polymorphonuclear leukocytes and abscess formation within the first 6 to 7 days after infection. However, these abscesses remained small and discrete. By days 7 to 10, the lesions began to transform into granulomas, first with the appearance of peripheral mononuclear cells and then by the replacement of polymorphs. By the third week of the infection, minute and discrete granulomas were seen scattered in the spleen, liver, and lymph nodes. Beyond this stage, healing and tissue regeneration followed. Thus, the characteristics of infectious lesions developed in mice vaccinated with heat-killed salmonellae are distinctly different from those developed in mice protected by the avirulent vaccine.