In vivo CTL activity induced by prime-boost vaccination using recombinant vaccinia virus and DNA vaccine expressing epitope specific for CD8⁺ T cells

DNA vaccine approaches have been applied to generate the protective immunity against various pathogens. However, the strength of immune responses induced by DNA vaccine is weak compared with conventional vaccines. The prime-boost vaccination using DNA vaccine and other viral vector has been suggested as one way to circumvent this limitation. In the present study, we used in vivo CTL activity assay to determine CD8⁺ T cell-mediated immunity induced by prime-boost vaccination with a DNA vaccine (gB _498-505 DNA) and recombinant vaccinia virus (VVg_B498-505) expressing gB_498-505 epitope peptide (SSIEFARL) of herpes simplex virus type 1 (HSV-1) glycoprotein B (gB). The most potent in vivo CTL activity was induced in mice received VVgB_498-505 when both gB _498-505 and VVgB_498-505 were used at priming step and boosted with the alternative vaccine vector expressing whole antigen protein (gBw). Priming with vaccine vector expressing gBw followed by the use of VVgB_498-505 at boosting step also induced strong in vivo CTL activity. We also examined in vivo CTL activity after immunization of mice with epitope-expressing vaccine vector at both priming and boosting step. Curiously, in vivo CTL activity mediated by CD8⁺ T cells was strongly elicited at memory stage when animals were primed with VVgB_498-505 and subsequently boosted with gB_498-505 DNA. Because the use of VVgB _498-505 at priming followed by boosting with gB_498-505 DNA induced most optimal immunity, these results suggest that the order of vaccine type should be carefully considered when used vaccine type expressing only epitope for prime-boost vaccination.