Nobutaka Osawa

THE ROLE OF RNA IN CELL‐MEDIATED IMMUNITY

In previous 13, l4 it was reported that mice immunized with a live vaccine of Salmonella enteritidis acquire a high antilethal resistance against infection with the same organism. Killed vaccines, however, were ineffective in protecting against infection despite the presence of high agglutinin titers against a virulent strain of S. enteritidis.9. Is, l4 Similarly, in experimental infections with other species of group D salmonellae, in particular, S. dublin and S. t yph imur i~rn ,~~ 28 killed vaccines increased the survival time of mice after challenge, but were largely ineffective in preventing death from a challenge infection. By contrast, suitable immunization with a live vaccine conferred a high antilethal protection against challenge with a virulent strain of the same organism. Thus, experimental salmonellosis may be considered as a model of infectious disease, similar to tuberculosis, brucellosis, and Listeria monocytogenes infection, in which the microorganisms persist mainly in the cells of infected animals and a live vaccine is effective in protecting against infection. Earlier reports have described the three characteristic features of infection with S . enteritidis: 1 ) effectiveness of the live vaccine,’3, 14923 2) the longlasting immunity conferred by the live vaccine even when very few living organisms were 27 and 3) the fact that the mononuclear phagocytes of immunized mice appeared to acquire cellular immunity and inhibited intracellular growth of bacteria in the absence of immune sera in a culture medium 13, 16, (FIGURES 1-3). Based on these findings, the following questions arose: what were 1 ) the chemical substance(s) responsible for the effectiveness of the live vaccine, 2) the chemical agent(s) that transmitted the cellular immunity from cell to cell throughout the whole body of the immunized animal, and 3) the chemical agent(s) responsible for the long-lasting immunity?