The V3 loop is a major neutralizing determinant of the HIV-1 virus. Cholera toxin subunit B (CTB) is a highly immunogenic protein and it has been used in fusion constructs to enhance immunogenicity of target proteins. We hypothesized that a rationally designed immunogen, based on V3 and CTB, could induce high titers of Abs with binding mode and epitope specificity similar to known broadly neutralizing anti-V3 mAbs.
We used molecular modeling and available crystallographic structures of the V3 loop in the HIV-1 gp120 context, the V3 loop fragment bound to broadly neutralizing mAbs, as well as the structure of the CTB to design two novel V3-scaffold immunogen constructs: a full-length V3-CTB presenting the complete 35 amino-acid residue V3 loop in a structural context mimicking the V3 in gp120, and a short V3-CTB presenting a smaller segment of V3 in the conformation recognized by mAb 447-52D.
Antigenic properties were evaluated on a panel of 24 anti-V3 human mAbs. The full V3 construct was recognized with high affinity by the large majority of mAbs, with some preference by mAbs derived from clade B-infected individuals. Short V3 construct exhibited high affinity binding to mAb 447-52D and only a few additional mAbs. Immunogenicity of the constructs was evaluated in rabbits using a DNA prime/protein boost protocol. Boosting with the full-length V3-CTB resulted in serum anti-V3 titers that neutralized multiple HIV virus strains from various HIV-1 clades. Short V3-CTB construct was ineffective in boosting the Ab
The results suggest that while (1) a scaffold immunogen that presents a single epitope (designed to fit a single mAb) may result in a poor Ab response, (2) focusing the immune response on a specific immunogenic region, such as the V3 loop, can elicit a robust Ab response, and (3) the CTB scaffold can efficiently present V3