ADMINISTRATION OF KILLED BACTERIA TOGETHER WITH LISTERIOLYSIN-O INDUCES PROTECTIVE IMMUNITY AGAINST LISTERIA-MONOCYTOGENES IN MICE

It is known that only listeriolysin O (LLO)-producing Listeria monocyt ogenes strains are able to induce protective immunity, but the underli ning relationship between LLO produced by virulent strains and generat ion of protective immunity in the infected host remains poorly underst ood. In the present study, it was found that LLO gene expression was o nly detected in the mice infected with virulent strain which was able to induce protective immunity, while non-virulent strains or killed ba cteria were not able to generate protective immunity, When mice were i mmunized with LLO plus killed bacteria in the presence of incomplete F reund's adjuvant, the protective immunity was partially generated, and adoptive transfer experiment confirmed that this protection was antig en specific. Reverse-transcription polymerase chain reaction revealed that LLO plus killed bacteria induced the expression of interferon-gam ma (IFN-gamma) and interleukin-12 (IL-12). Our results also showed CD4 (+) T cells were the principal cells constituting protective immunity. Taken together, it may be concluded that LLO produced from virulent s trains of L. monocytogenes was essential for the generation of protect ive immunity, and that LLO plus killed bacteria induced IFN-gamma and IL-12 expression which resulted in the generation of protective immuni ty.