Letvin: rAdenoviral & rMycobacterial Vector Immunogens
T-cell vaccines for AIDS often employ viral vectors that carry HIV-1 and other genes into the body and into cells. However, some of the vectors are constructed using natural viruses that infect humans. As a result, some individuals have preexisting immunity to the vectors. Data from the Merck-STEP trial suggest that preexisting anti-viral vector immunity may impact the performance of an HIV-1 vaccine, and some researchers suspect that it could perhaps enhance an individual’s risk of acquiring HIV-1. However, data from comprehensive research projects performed by several laboratories do not substantiate this hypothesis. Moreover, the modest success of the RV 144 Thai trial suggests that a vector prime/protein boost vaccination regimen may elicit sufficient antibody responses at genital mucosal surfaces to block HIV-1 infection. There is reason to believe that more potent vaccine regimens based on prime/boost immunizations might provide more robust and durable protection against HIV-1 acquisition.
Researchers in the Letvin research team are studying the biology of anti-vector immunity and exploring ways to make better vectors from recombinant adenoviruses (rAd) and mycobacteria (rMyco). The rAd team has generated novel, replicating and non-replicating Ad vectors from adenoviral strains that are not commonly found in the human population. The team is also pursuing optimal vaccine delivery strategies and improving regimens that target mucosal immune responses. The rMyco team has generated novel mycobacteria strains with enhanced immunogenicity due to gene deletions as potential candidates for clinical development. The goal is to create vector-based vaccines to be used in regimens that are optimized for eliciting durable systemic and mucosal cellular immune responses and that will not be suppressed by preexisting anti-vector immunity.
- Develop novel, rare serotype rAd vectors that evade preexisting anti-Ad5 immunity, and study the biology of anti-Ad vector immunity;
- Develop and improve prime-boost combinations of vaccines targeting the mucosa and mucosal immune responses;
- Develop novel rMyco vectors with enhanced immunogenicity; and
- Establish an administrative core to facilitate the proposed studies.
Priorities of the Letvin consortium were modified in light of the failed Merck STEP trial and of the modest success of the RV 144 Thai trial. To help clarify issues raised by the STEP results, and to study the biology of preexisting anti-vector immunity and how it might affect vaccine performance, VDC researchers have studied in detail the Ad vector-specific cellular and humoral immunity following rAd5-Gag immunization in human samples from the Merck phase I studies. No correlation was found between baseline Ad5-specific T-cell responses and neutralizing antibody titers, challenging the hypothesis that anamnestic Ad5-specific CD4+ T cell responses following rAd5 vaccination were responsible for the enhancement of HIV-1 acquisition in the Ad5 sero-positive subjects in the STEP study. The research team has also found that natural infection by Ad5 and vaccination with rAd5 vectors have different immunogenicity profiles resulting in the generation of neutralizing antibodies (Nabs) that target different proteins of the viral capsid. Studies have been completed in mice and with human samples that confirmed and delineated the differences in CD4 and CD8 epitopes between Ad5 and other rare Ad serotypes, and the extent of cross-reactive Ad immunity. These results provide reassurance to the vaccine community and will facilitate clinical development of rare serotype replication-incompetent and competent Ad vectors for HIV-1.
The rAd team has also made substantial progress towards the original project goals of developing vectors for clinical production. They have generated and tested a comprehensive portfolio of novel, primate rare serotype, non-Ad5 vectors that evade preexisting anti-Ad5 immunity. The portfolio includes the replication incompetent recombinant Ad26 (rAd26) and rAd28 vectors, the replication competent Ad26 (rcAd26) vectors, HVR-chimeric rAd5 vectors, prototype chimeric rAd35/41 vectors, and a panel of simian Ad-based vectors. Preclinical studies of these vectors have been performed with the rcAd26 vectors showing the most promising results. The rAd program has maintained a two-pronged approach: pursuing work aimed at advancing prototype rcAd26 constructs for clinical production, and using the existing portfolio of rAd vectors to continue to pursue optimal delivery strategies to induce and assess mucosal immune responses in nonhuman primate models.
The rMyco team is developing second generation Bacillus Calmette-Guerin (BCG) vectors with improved immunogenicity and safety. In a pilot nonhuman primate study the first generation rBCG-SIV vectors provided effective priming for a rAd5-SIV boost. To augment vector immunogenicity, genome-wide screens were performed to identify BCG genes that are responsible for immune evasion, and molecular biology to improve transgene expression and secretion. Using different approaches, the team has identified and characterized over 100 rMyco mutants that are associated with increased antigen presentation in the MHC class I and/or class II pathways. When benchmarked against industry-standard vaccine vectors, some of the mutants demonstrated superior immunogenicity. The rMyco team has designed a streamlined roadmap to select candidate mycobacteria vectors for clinical production. Following this roadmap, the team has completed the comparative immunogenicity studies in mice and the evaluation of the first group of lead-candidate rBCG mutants as priming immunogens in a heterologous prime/boost vaccine regimen in nonhuman primates. Preliminary data indicate that at as early as week 8 after priming, monkeys immunized with the two lead candidate rBCG, J13R-SIV Gag and AF25R-SIV Gag have enhanced levels of Gag-specific CD8+ and polyfunctional CD4+ T cells expressing mucosal homing markers. The team has also begun to evaluate and optimize immunogen formulation and delivery; a nonhuman primate pilot study has been initiated to test the efficacy of glycolipid a-C-GalCer adjuvanted live rBCG-SIV Gag. The rMyco team is also evaluating the potential of recombinant M. Smegmatis, and auxotrophic and non-replicating mycobacteria strains as vaccines against HIV-1. Based on the success of the RV 144 trial, selected rMyco constructs will also be evaluated as priming immunogens for use with adjuvanted Env protein or viral vector boosts for eliciting cellular and humoral immune responses to HIV-1 Env. The most immunogenic combination regimen will be advanced for clinical development.