Background:  A current promising strategy for AIDS vaccine development is to elicit cytotoxic T lymphocyte (CTL) responses that broadly recognize highly diversified HIV. However, it has remained unclear how broadly vaccine-induced CTL can recognize heterologous viruses in vivo. In our previous vaccine trial eliciting simian immunodeficiency virus (SIV)_mac239 Gag-specific CTL responses, we have found a group of Burmese rhesus macaques possessing an MHC-I haplotype 90-120-Ia which consistently show vaccine-based viral control against a homologous SIV_mac239 challenge and Gag_206-216 epitope-specific CTL responses essential for this control. Here, we have evaluated in vivo efficacy of the vaccine-induced CTL responses against a heterologous SIV_smE543-3 challenge.

Methods: Of 5 Burmese rhesus macaques, 2 were 90-120-Ia-positive and 3 90-120-Ia-negative. All received a prophylactic DNA-prime/SIV_mac239 Gag-expressing Sendai virus vector-boost vaccination and were intravenously challenged with SIV_smE543-3 that has the same Gag_206-216 amino acid sequence with SIV_mac239.

Results:  At the set point, 3 90-120-Ia-negative vaccinees showed viral control, but unexpectedly both of the 2 90-120-Ia-positive vaccinees failed to control heterologous SIV_smE543-3 replication despite efficient elicitation of Gag_206-216-specific CTL responses by vaccination. Neither of them showed any mutation in the Gag_206-216 epitope region, although a Gag_206-216-specific CTL-escape mutation was selected in all the 90-120-Ia-positive macaques infected with SIV_mac239. We then focused on a single amino acid difference, Gag205D (SIV_mac239) and Gag205E (SIV_smE543-3), in the epitope-flanking region between the 2 virus strains. Flow-cytometric analysis of interferon-g induction revealed that Gag_206-216-specific CTL recognized SIV_mac239 Gag_202-216 peptide-expressing cells, but not SIV_smE543-3 Gag_202-216 peptide-expressing cells, although this CTL recognized SIV_smE543-3 Gag_202-216 peptide-pulsed cells as well as SIV_mac239 Gag_202-216 peptide-pulsed cells. These results indicate failure in Gag_206-216 epitope presentation in SIV_smE543-3-infected cells due to the single amino acid difference in the epitope flanking region.

Conclusions:  Our results show that a single amino acid change in CTL epitope flanking region can abrogate in vivo efficacy of vaccine-induced CTL responses, underlining the influence of epitope flanking regions on vaccine-induced CTL efficacy in vivo.