Background: HIV-1 has proven to be an extremely difficult target for vaccine development, in part because of its capacity to mutate to escape immunologic recognition. Furthermore, circulating strains are becoming more and more divergent with time. Thus, one conundrum facing vaccine developers is the choice of viral strains from which to formulate immunogens capable of eliciting broad immunity.

Methods: We have designed candidate vaccines that encompass the common features of present day viruses through reconstruction of their ancestral states. The ancestral state of the HIV-1 clade B protein (An1-EnvB) was deduced using maximum likelihood methods, and the gene was synthesized following human codon usage optimization. gp160 and gp140 versions of the gene were tested for functionality in transfected COS-7 and GHOST cells. An1-EnvB DNA was used to immunize rabbits via Gene Gun and tested for generation of neutralizing antibodies against a variety of primary and culture-adapted HIV strains in a cMAGI assay.

Results: The human-codon optimized synthetic ancestral An1-EnvB gene was shown to produce a glycoprotein that caused fusion of cells expressing the HIV-1 co-receptors CD4 and CCR5, but not CD4 plus CXCR4. The gp140 form bound to CD4 in vitro. As DNA vaccines, An1-EnvB gp140 and gp160 elicited antibody titers in rabbits up to 1:100,000. We measured neutralizing antibody against 2 clade B HIV-1 isolates, SF162 and 92US657, which are heterologous to the immunogen. After 4 immunizations, 50% neutralization titers against the SF162 strain ranged from 1:5 to 1:11 in 6 of 8 rabbits. Three (3) of those animals had titers of 1:4 at the 75% neutralization level. Neutralization against the primary viral isolate 92US657 was also seen in 2 animals at 50% and 90%.

Conclusions: Presentation of ancestral state features of variable immunogens provides a promising new approach to strain choice for the generation of HIV vaccines.