With the sobering results of the STEP and HVTN 505 HIV vaccine clinical trials, novel approaches to HIV vaccine development must be explored. The HIV protease is a 99-amino acid aspartic enzyme that mediates the cleavage of Gag, Gag-Pol and Nef precursor polyproteins. This process is a highly specific, temporally regulated and essential for the production of infectious viral particles. A total of 12 proteolytic reactions are required to generate an infectious virion, and a single impaired cleavage reaction can render the virus noninfectious. Thus, HIV vaccine-elicited responses targeting the protease cleavage sites (PCS) could be highly efficacious.
We assess vaccine immunogenicity elicited against PCS immunogens using a modified Vesicular stomatitis virus (VSV) vector combined with a nanodelivery system. We then evaluate protective efficacy to disrupt SIV acquisition and disease progression using a Cynomolgus macaque (Macaca facicularis, Philippines) and SIVMAC239 intrarectal challenge model. To examine whether the immune driven viral mutations surrounding the PCS were detrimental to the virus, we amplified and sequenced the plasma viruses by 454-pyrosequencing and correlated the amino acid mutations surrounding PCS with alterations in viral load and CD4 count.
PCS peptides expressed by rVSVs and packaged in nanoparticles are able to generate both antibody and T cell responses in macaques. The ability of macaques to withstand high dose SIVMAC239 challenge was significantly correlated with the antibody and antibody/T cell responses to the number of PCS peptides(p=0.0005, R=0.8005). This combination imparted resistance to all but the higher SIV doses that were required to infect vaccinnees (p=0.01). the vaccine group maintains significantly higher CD4 counts (p=0.0002). Amino acid mutations (SN or NS) surrounding PCS correlated significantly with reduced viral RNA levels (p<0.0001).
We show here in a Cynomolgus macaque/SIV model that a candidate HIV vaccine focusing immunologic responses to the regions surrounding the SIV protease cleavage sites can force viral mutation resulting in impaired SIV fitness. Focused immune response to the PCS region enables the macaques to withstand multiple high dose pathogenic SIVMAC239 intrarectal challenges. Virus recovered from vaccinnees harbored mutations surrounding the protease cleavage region that correlated significantly with reduced viral load, and the maintenance of CD4+ T cells in vivo.