A Multi-Vector, Multi-Envelope HIV-1 Vaccine

Abstract

The St. Jude Children's Research Hospital (St. Jude) HIV-1 vaccine program is based on the observation that multiple antigenically distinct HIV-1 envelope protein structures are capable of mediating HIV-1 infection. A cocktail vaccine comprising representatives of these diverse structures (immunotypes) is therefore considered necessary to elicit lymphocyte populations that prevent HIV-1 infection. This strategy is reminiscent of that used to design a currently licensed and successful 23-valent pneumococcus vaccine. Three recombinant vector systems are used for the delivery of envelope cocktails (DNA, vaccinia virus, and purified protein), and each of these has been tested individually in phase I safety trials. A fourth FDA-approved clinical trial, in which diverse envelopes and vectors are combined in a prime-boost vaccination regimen, has recently begun. This trial will continue to test the hypothesis that a multi-vector, multi-envelope vaccine can elicit diverse B- and T-cell populations that can prevent HIV-1 infections in humans.

Keywords: HIV-1 vaccine; Immunology; clinical trial; envelope; multi-vector.

Figure 1.

Rationale for the design of a multi-envelope HIV-1 vaccine. (A) B-cells have evolved to bear unique surface antibodies that bind and destroy pathogens with a lock-and-key interaction. (B) Vaccines can be designed to mimic pathogens and thereby induce specific B-cells to divide and secrete antibodies that target the pathogen. (C) The activation of B-cells (or T-cells) with a single-component vaccine harnesses only a fraction of the human immune potential. (D) A multi-component vaccine will activate a diverse population of immune cells that can counteract a broad array of target viruses.

Figure 2.

(□) Control. () B24 only. () 114-9C10 only. (▪) B24 + 114-9C10. A combination of antibodies mediates synergistic virus neutralization. Mice were vaccinated with a single HIV-1_IIIB–derived envelope or with a combination of envelopes including envelope from HIV-1_IIIB. Delivery vehicles included recombinant DNA, vaccinia virus, and protein. Splenic cells from vaccinated mice were then harvested for B-cell hybridoma production. Once stable hybridomas (B24 and 114-9C10) were derived and cloned, respective antibodies were harvested from hybridoma culture media and purified by affinity chromatography with protein G sepharose. The neutralization assay was initiated by incubating HIV-1_IIIB (approximately 10 TCID-50 per well in a 96-well microtiter plate) with or without antibodies in R10 medium (RPMI 1640 plus 10% heat-treated fetal bovine serum, penicillin, streptomycin and 4mM glutamine). Antibodies were used at concentrations selected to yield fractional virus inhibition, in order to measure additive or synergistic effects (final concentrations of B24 and 114-9C10 were 0.05 μg/mL and 10 μg/mL, respectively). After overnight incubation, the contents of wells were transferred to confluent GHOST-CXCR4 cells in 96-well plates and incubated overnight. Cells were washed with R10 medium and incubated an additional 3 days, after which supernatants were analyzed for virus growth with a Coulter HIV-1 p24 antigen assay (Beckman-Coulter, Miami, FL). Percent inhibition was determined by comparing test wells with wells containing virus and no antibody. Control wells were with virus and HIV-1 negative human serum.

Figure 3.

Antibodies can recognize conformationally similar antigenic determinants among proteins with diverse amino acid sequences. Each of these six hypothetical envelope proteins is represented as a string of beads, with different colors representing different amino acid residues. The successful capture of HIV-1 envelope as a crystal structure has been difficult and the prediction of three-dimensional protein structure cannot easily be predicted based solely on amino acid sequences. Antibodies may thus be used to assist in the characterization and categorization of envelopes. For example, the binding and neutralization of the first three envelopes (among six) by a specific monoclonal antibody may reveal a shared conformational structure that may be pertinent to a specific mechanism of infectivity. Envelopes with such similar conformational structures might be represented by a single component in a vaccine cocktail, and envelopes with other common structures (e.g., envelopes 4–6 in this series) might require a different single vaccine component as a representative. Information from antibody-antigen analyses has assisted the design of successful cocktail vaccines in other research fields.