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 life cycle in the cytosol of the infected eucaryotic cells. Within this cellular compartment originates the endogenous pathway of antigen processing and presentation. We thus assumed that recombinant L. monocytogenes expressing an heterologous protein, the nucleoprotein of the lymphocytic choriomeningitis virus (LCMV), should be able to induce antigen-specific CD8+ T cells in vivo. The LCMV nucleoprotein gene was inserted in phase with the sequence coding for the putative signal sequence of the hemolysin of L. monocytogenes in order to target its secretion into the cytosol of the infected cell. The ability of this recombinant bacterium to induce LCMV-reactive CD8+ T cells was then monitored in BALB/c mice. The immune status of the immunized BALB/c mice was studied on the seventh day after a single i.v. injection of a sublethal dose of the recombinant bacteria: (i) cytotoxic CD8+ T cells were detected in liver; (ii) using in vitro re-stimulation with PMA and ionomycin, secondary cytotoxic CD8+ T cells were detected in spleen; (iii) an early inflammatory reaction dependent on the presence of CD8+ T cells occurred in the footpad after intraplantar inoculation of live LCMV; and (iv) mice were protected against an otherwise lethal intracerebral LCMV challenge; the protection was accompanied by elimination of the virus. When the immune status of the immunized hosts was monitored for a longer period post-immunization, the balance between immune protection and immunopathology described for the anti-LCMV immune responses was observed; two phases of protection were detected, flanking a transitory phase of exacerbation of the lymphocytic choriomeningitis disease (weeks 2-5).(ABSTRACT TRUNCATED AT 250 WORDS)