Attenuated Listeria monocytogenes as a live vector for induction of CD8+ T cells in vivo: a study with the nucleoprotein of the lymphocytic choriomeningitis virus

Listeria monocytogenes spends most of its intracellular lifecycle in the cytosol of the infected eucaryotic cells. Withinthis cellular compartment originates the endogenous pathwayof antigen processing and presentation. We thus assumed thatrecombinant L. monocytogenes expressing an heterologous protein,the nucleoprotein of the lymphocytic choriomeningltis virus(LCMV), should be able to induce antigen-specific CD8⁺ T cellsin vivo. The LCMV nucleoprotein gene was inserted in phase withthe sequence coding for the putative signal sequence of thehemolysin of L. monocytogenes in order to target its secretioninto the cytosol of the infected cell. The ability of this recombinantbacterium to induce LCMV-reactive CD8⁺ T cells was then monitoredin BALB/c mice. The immune status of the immunized BALB/c micewas studied on the seventh day after a single i.v.injectionof a sublethal dose of the recombinant bacteria: (i) cytotoxicCD8⁺ T cells were detected in liver; (ii) using in vitro re-stimulationwith PMA and ionomycin, secondary cytotoxic CD8⁺ T cells weredetected in spleen; (iii) an early inflammatory reaction dependenton the presence of CD8⁺ T cells occurred in the footpad afterintraplantar inoculation of live LCMV; and (iv) mice were protectedagainst an otherwise lethal intracerebral LCMV challenge; theprotection was accompanied by elimination of the virus. Whenthe immune status of the immunized hosts was monitored for alonger period post-immunization, the balance between immuneprotectiosn and immunopathology described for the anti-LCMVimmune responses was observed; two phases of protection weredetected, flanking a transitory phase of exacerbation of thelymphocytic choriomeningitis disease (weeks 2–5). Takentogether, these results indicate the feasibility of using attenuatedL. monocytogenes as a model of a live vector to induce in vivoCD8-ependent immune responses against intracellular pathogens.