Heterologous Prime-Boost HIV-1 Vaccination Regimens in Pre-Clinical and Clinical Trials

Abstract

Currently, there are more than 30 million people infected with HIV-1 and thousands more are infected each day. Vaccination is the single most effective mechanism for prevention of viral disease, and after more than 25 years of research, one vaccine has shown somewhat encouraging results in an advanced clinical efficacy trial. A modified intent-to-treat analysis of trial results showed that infection was approximately 30% lower in the vaccine group compared to the placebo group. The vaccine was administered using a heterologous prime-boost regimen in which both target antigens and delivery vehicles were changed during the course of inoculations. Here we examine the complexity of heterologous prime-boost immunizations. We show that the use of different delivery vehicles in prime and boost inoculations can help to avert the inhibitory effects caused by vector-specific immune responses. We also show that the introduction of new antigens into boost inoculations can be advantageous, demonstrating that the effect of `original antigenic sin' is not absolute. Pre-clinical and clinical studies are reviewed, including our own work with a three-vector vaccination regimen using recombinant DNA, virus (Sendai virus or vaccinia virus) and protein. Promising preliminary results suggest that the heterologous prime-boost strategy may possibly provide a foundation for the future prevention of HIV-1 infections in humans.

Figure 1.

Durable antibody responses elicited by successive HIV-1 envelope immunizations with three different recombinant delivery systems. Female C57BL/6 mice were purchased from the Jackson Laboratory (Bar Harbor, ME) and housed under specific pathogen-free conditions in a BL1 or BL2+ containment area at St. Jude Children’s Research Hospital animal facilities, with adherence to AAALAC Guidelines. Mice were grouped (n = 3) for immunizations with UG92005 (UG) envelope using recombinant vectors: (i) DNA (‘D’, 100 μg, carrying a gp140 envelope sequence) administered i.m. in the gastrocnemius muscles, (ii) CHO-derived recombinant UG envelope protein (‘P’, 2 μg, 100 μl gp140 envelope in PBS mixed with 100 μl CFA) administered i.p., and (iii) SeV (‘S’, 10⁴ plaque forming units, carrying a gp120 envelope sequence for expression in infected cells) administered i.n.. Groups of mice received one, two or three of the recombinant vectors (or no vaccine, ‘0’), administered with one month intervals. Eight months after the completion of inoculations, ELISAs were performed with serially diluted serum samples. Antibody binding titers were determined using Prism Software (Non-Linear Regression, GraphPad Prism®, San Diego, CA). Mean titers and standard errors are shown. Some data points represent fewer than 3 animals due to mouse death or non-conformance of data to the non-linear regression curve (O-S-D, O-D-P, O-P-D, O-S-P, S-P-D, P-S-D). To perform the ELISA, 96-well plates (BD Biosciences, Franklin Lakes, NJ) were coated overnight at 4 °C with 2 μg/ml of purified CHO-derived UG gp140 envelope protein in PBS. The plates were washed three times with 0.05% Tween 20 in PBS, blocked with 1% BSA/PBS at room temperature for 1 h, and washed an additional three times. Samples were serially diluted in PBS to a final volume of 50 μl and were incubated in wells for 2 h at room temperature. After three washes, alkaline phosphatase-conjugated anti-mouse IgG1 (50 μl/well; Southern Biotechnology Associates, Birmingham, AL) diluted 1/1000 in 1% BSA/0.05% Tween 20/PBS was added for 1 h at room temperature. Following five washes, the assay was developed with 75 μl/well of p-nitrophenyl phosphate (Sigma-Aldrich) substrate (1 mg/ml in diethanolamine buffer) and was read at OD_450nm after 1 hour at 37 °C.

Figure 2.

Cognate B-cell:T-cell interactions support T-cell help. This cartoon illustrates a situation in which a B cell and T cell recognize different determinants on the same particle (panel A). In this circumstance, the B-cell antibody may capture antigen via a lock-and-key type interaction (panel B). The antigen is then fragmented (panel C), after which the T-cell epitope is presented on the B-cell membrane as a complex with MHC. The peptide-MHC complex is bound by the T-cell receptor, after which ‘help’ may be relayed from the T cell to its B cell partner (panel D).

Figure 3.

Antibody responses elicited by heterologous prime-boost vaccination of humans. Three study participants received three different vaccination regimens. Study Participant 1 (black bars) received D (month 0), D (month 1), V (month 2), P (month 3), D (month 4) and P (month 5). Participant 2 (clear bars) received the first four inoculations and Participant 3 (grey bars) received the first three inoculations. Sera were collected and tests were conducted with the Abbott enzyme-linked immunosorbent assay (HIVAB HIV-1/HIV-2 (recombinant DNA) Abbott Laboratories, Abbott Park, IL, USA). Asterisks indicate an absence of sample.