Induction of T cell-mediated immunity by the recombinant L.inn::vgc strain A number of cell types are involved in host defense against Listeria. Antigen-specific T lymphocytes mediate recovery from primary listerial infections and protective immunity to subsequent infections [13, 19] . Both CD4 + (helper, MHC class II restricted) and CD8 + (cytotoxic, MHC class I restricted) T cell subpopulations have been implicated [20] . Experimental evidence indicates, however, that CD8 + T cells play the predominant role in mediating protective immunity [21] [22] [23] [24] . The ability of the recombinant L.inn::vgc to induce T-cell mediated immunity as a prerequisite for protective immunity was analyzed. Groups of BALB/c mice were infected with Lm (10 3 ), L.inn (10 7 ), or L.inn::vgc (10 7 ). Two months later, all mice were challenged with a lethal i.v. dose (10 5 ), corresponding to 206LD50, of the wild type Lm, and survival was monitored. As controls, a group of untreated BALB/c mice that received a similar lethal dose of the wild type Lm were included. A single pre-immunization with the L.inn::vgc strain led to a significant protection against subsequent lethal infection with Lm. As expected, all mice that were pre-immunized with sub-lethal doses of Lm were also protected against a lethal listerial infection and survived whereas all non-immunized mice as well as those preimmunized with L.inn died within 4 days after challenge (Fig. 4A) .