The classical approach to vaccine development against HIV has shown only limited efficacy in dealing with viral antigenic diversity. The present study was initiated to explore the efficacy of a novel HIV vaccine candidate targeting the highly conserved protease cleavage sites (PCS) in a cynomolgus macaque-SIV-challenge model. In that study, PCS specific antibody and T cell responses correlated with reduced acquisition and disease progression after SIVmac239 challenge.
We sequenced SIV populations isolated after challenge from vaccinees (n=12) immunized with a VSV vector expressing a 20-amino acid peptides overlapping each of the SIVmac239 12 PCS regions, then boosted intra-nasally with nanopackaged peptides, or from control animals that received sham inoculations (n=5). Unique mutations were identified in vaccinees and associations were analyzed between the immune driven viral mutations surrounding the PCS and alterations in viral load and CD4 count. To evaluate whether the vaccine-elicited mutations were detrimental to the virus, we produced 11 transfection-derived viral stocks harboring each PCS mutation alone or in combination. Each of these full-length mutant viral stocks was evaluated for viral p27-CA content, SIV RNA levels, replication rate and proteolytic processing of Pr55Gag.
Vaccinees showed significantly higher frequencies of mutations in both PCS2 (p27/p2) and PCS12 (Nef) regions that correlated with reduced viremia post-challenge. All recombinant mutant clones containing only PCS2 preparations were impaired. Virus preparations of these mutants contained significantly reduced viral RNA levels and p27-CA content. These mutants also displayed impaired proteolyitic Gag processing. All PCS2 mutant clones were significantly impaired in their ability to replicate in CEMx174 cells. Interestingly, recombinant virus harboring only PCS12 mutations replicated at comparable rates to wild type SIV. Importantly, we observed compensation of the defects incurred by PCS2 by the addition of the PCS12 mutation. This compensating PCS12 mutation restored viral replication to PCS2 mutants, compared to those mutants harboring mutation in PCS2 region alone.
These results show that focused immune responses targeting the PCS region result in mutations surrounding the protease cleavage region, impair the replication ability of virus and correlate significantly with reduction in viral load, and the maintenance of CD4+ T cells in vivo.