Screening of HIV-1 Env glycoproteins for the ability to raise neutralizing antibody using DNA immunization and recombinant vaccinia virus boosting

HIV-1 envelopes from two series of primary isolates (from Swedish patients 5 and 6), from JR-FL and BaL (prototypic monocyte/macrophage tropic viruses) and from HXB-2 (a prototypic T-cell-line-adapted virus), have been screened for their ability to elicit neutralizing antibody to HIV-1. Rabbits were primed by gene gun inoculation with plasmids expressing secreted monomeric (gp120) and oligomeric (gp140) forms of each Env. After four to six DNA immunizations administered over a 1-year period, rabbits were boosted with 10⁸ plaque-forming units of a mixture of seven recombinant vaccinia viruses which express chimeric gp140 Envs (primary clade B sequences in a IIIb-related BH10 backbone). Neutralizing antibodies were assayed against two T-cell-line-adapted viruses (MN and IIIb), two non-syncytium-inducing (NSI) and two syncytium-inducing (SI) primary isolates, and two HIV-1-NL4-3-recombinants with patient 5 or 6 Envs (NL4-3/5A, NL4-3/6C). The DNA priming and recombinant vaccinia virus boosting raised low titers of neutralizing antibody in 10 of 19 rabbits. The highest titers of neutralizing activity (~ 1:150 for MN) were raised in rabbits DNA primed with Envs from Swedish patient 5. These sera cross-neutralized IIIb and MN but did not neutralize the primary isolates or the NL4-3 recombinant with the homologous 5A Env. Sera from rabbits primed with the HXB-2 Env DNA were, for the most part, type-specific for neutralization of IIIb. In one of three assays, sera from rabbits primed with plasmids expressing the JR-FL and BaL Envs had possible low titer neutralizing activity for two NSI, but not two SI, primary isolates. Our results highlight the low immunogenic potential of the HIV-1 Env and demonstrate that different Envs have different potentials to raise low titer neutralizing antibody.