Immunity to Toxoplasma and Listeria induced by homologous and heterologous organisms.

Cross protection of animals against various organisms have been shown for many years. This type of resistance to phylogenetically unrelated organisms might be attributed to certain immunological phenomena such as non-specific macrophage activation. In this report the cross-protective effect of some organisms against Toxoplasma gondii RH strain and Listeria monocytogenes is described. Groups of mice were immunized with BCG, Toxoplasma lysate antigen, viable cysts of T. gondii Tehran strain and heat killed L. monocytogenes. Seventeen days after initial immunization, the animals were tested for delayed hypersensitivity by a skin test. The hypersensitive animals in each group were challenged with either lethal doses of T. gondii RH strain or 5 x 10(5) viable L. monocytogenes. Among the animals challenged with T. gondii, it was observed that complete protection was achieved only in those mice immunized with viable cysts of T. gondii Tehran strain. Although all other immunized mice eventually died after infection, they did show some degree of resistance as their deaths were delayed considerably as compared to non-immunized animals. In animals which were infected with 5 x 10(5) L. monocytogenes, complete resistance was observed only in BCG immunized mice. The other antigens including L. monocytogenes induced partial resistance as evidenced by their survival times and the multiplication of the bacteria in various internal organs.