Effective DNA vaccination against listeriosis by prime/boost inoculation with the gene gun

Protective immunity against Listeria monocytogenes strongly depends on CD8( +) T lymphocytes, and both IFN-gamma secretion and target cell killing are considered relevant to protection. We analyzed whether se could induce a pr otective type 1 immune response by DNA vaccination with the gene gun using plasmids encoding for two immunodominant listerial Ags, listeriolysin and p 60. To induce a Th1 response, we 1) coprecipitated a plasmid encoding for G M-CSP, 2) employed a prime/boost vaccination schedule with a 45-day interva l, and 3) coinjected oligodeoxynucleotides (ODN) containing immunostimulato ry CpG motifs. DNA immunization of BALB/c mice with plasmids encoding for l isteriolysin (pChly) and p60 (pCiap) efficiently induced MHC class I-restri cted, Ag-specific CD8(+) T cells that produced IFN-gamma. Coinjection of Cp G-ODN significantly increased the frequency of specific IFN-gamma-secreting T cells. Although pChly induced specific CD8(+) T cells expressing CTL act ivity, it failed to stimulate CD4(+) T cells. Only pCiap induced significan t CD4+ T cell and humoral responses, which were predominantly of Th2 type. Vaccination with either plasmid induced protective immunity against listeri al challenge, and coinjection of CpG ODN improved vaccine efficacy in some situations. This study demonstrates the feasibility of gene gun administrat ion of plasmid DNA for inducing immunity against an intracellular pathogen for which protection primarily depends on type 1 CD8(+) T cells.