Antiviral cytotoxic T-cell memory by vaccination with recombinant Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular bacterium that is able to escape phagocytic vesicles and replicate in the cytoplasm of infected cells, As with viral vectors, this intracytoplasmic life cycle provides a means for introducing foreign proteins into the major histocompatibility complex class I pathway of antigen presentation, Using recombinant L. monocytogenes (rLM) strains expressing the full length nucleoprotein (NP) or a single cytotoxic T-lymphocyte (CTL) epitope from lymphocytic choriomeningitis virus (LCMV), we analyzed antiviral CTL responses induced by rLM vaccination, After vaccination, rLM was cleared from the host within 7 days while inducing an LCMV-specific ex vivo CD8(+) effector CTL response, Virus-specific CTL memory was maintained for 6 months postvaccination, as demonstrated by vigorous secondary CTL responses after in vitro stimulation, A single immunization with rLM that expressed either the full-length NP gene or the CTL epitope alone resulted in LCMV NP-specific CTL precursor frequencies of approximately 1/10(4) CD8(+) T cells. A second rLM vaccination resulted in enhanced virus-specific CTL activity and in vitro proliferation. rLM-vaccinated mice were protected against chronic viral infection by an accelerated virus-specific memory CTL response, These mice cleared infectious virus as well as viral antigen, suggesting that sterilizing immunity was achieved, In contrast to mice that received mild-type LM, rLM-vaccinated mice were protected from virally induced immunosuppression and splenic atrophy associated with chronic LCMV infection. The ability to elicit long-term cell-mediated immunity is fundamental in designing vaccines against intracellular pathogens, and these results demonstrate the efficacy of recombinant LM vaccination for inducing protective antiviral CTL memory.