Correlates of relative resistance against low-dose rectal simian immunodeficiency virus challenges in peripheral blood mononuclear cells of vaccinated rhesus macaques

Current advances in HIV vaccine preclinical studies using Macaque models

The macaque simian or simian/human immunodeficiency virus (SIV/SHIV) challenge model has been widely used to inform and guide human vaccine trials. Substantial advances have been made recently in the application of repeated-low-dose challenge (RLD) approach to assess SIV/SHIV vaccine efficacies (VE). Some candidate HIV vaccines have shown protective effects in preclinical studies using the macaque SIV/SHIV model but the model's true predictive value for screening potential HIV vaccine candidates needs to be evaluated further. Here, we review key parameters used in the RLD approach and discuss their relevance for evaluating VE to improve preclinical studies of candidate HIV vaccines.

Antibody responses to prime-boost vaccination with an HIV-1 gp145 envelope protein and chimpanzee adenovirus vectors expressing HIV-1 gp140

OBJECTIVES

Over 2 million individuals are infected with HIV type 1 (HIV-1) each year, yet an effective vaccine remains elusive. The most successful HIV-1 vaccine to date demonstrated 31% efficacy. Immune correlate analyses associated HIV-1 envelope (Env)-specific antibodies with protection, thus providing a path toward a more effective vaccine. We sought to test the antibody response from novel prime-boost vaccination with a chimpanzee-derived adenovirus (AdC) vector expressing a subtype C Env glycoprotein (gp)140 combined with either a serologically distinct AdC vector expressing gp140 of a different subtype C isolate or an alum-adjuvanted, partially trimeric gp145 from yet another subtype C isolate.

DESIGN

Three different prime-boost regimens were tested in mice: AdC prime-protein boost, protein prime-AdC boost, and AdC prime-AdC boost. Each regimen was tested at two different doses of AdC vector in a total of six experimental groups.

METHODS

Sera were collected at various time points and evaluated by ELISA for Env-specific antibody binding, isotype, and avidity. Antibody functionality was assessed by pseudovirus neutralization assay.

RESULTS

Priming with AdC followed by a protein boost or sequential immunizations with two AdC vectors induced HIV-1 Env-specific binding antibodies, including those to the variable region 2, whereas priming with protein followed by an AdC boost was relatively ineffective. Antibodies that cross-neutralized tier 1 HIV-1 from different subtypes were elicited with vaccine regimens that included immunizations with protein.

CONCLUSION

Our study warrants further investigation of AdC vector and gp145 protein prime-boost vaccines and their ability to protect against acquisition in animal challenge studies.

Systems biology and the quest for correlates of protection to guide the development of an HIV vaccine

Over the last three decades, a myriad of data has been generated regarding HIV/SIV evolution, immune evasion, immune response, and pathogenesis. Much of this data can be integrated and potentially used to generate a successful vaccine. Although individual approaches have begun to shed light on mechanisms involved in vaccine-conferred protection from infection, true correlates of protection have not yet been identified. The systems biology approach helps unify datasets generated using different techniques and broaden our understanding of HIV immunopathogenesis. Moreover, systems biology is a tool that can provide correlates of protection, which can be targeted for the production of a successful HIV vaccine.

Signatures in Simian Immunodeficiency Virus SIVsmE660 Envelope gp120 Are Associated with Mucosal Transmission but Not Vaccination Breakthrough in Rhesus Macaques

Mucosal surfaces are vulnerable to human immunodeficiency virus (HIV)/simian immunodeficiency virus (SIV) infection and thus are key sites for eliciting vaccine-mediated protection. Vaccine protocols carried out at the Yerkes Primate Research Center utilized SIVmac239-based immunization strategies with intrarectal and intravaginal SIVsmE660 challenge of rhesus macaques. We investigated whether there were genetic signatures associated with SIVsmE660 intrarectal and intravaginal transmissions in vaccinated and unvaccinated monkeys. When transmitted/founder (T/F) envelope (Env) sequences from 49 vaccinated and 15 unvaccinated macaques were compared to each other, we were unable to identify any vaccine breakthrough signatures. In contrast, when the vaccinated and control T/F Envs were combined and compared to the challenge stock, residues at gp120 positions 23, 45, 47, and 70 (Ile-Ala-Lys-Asn [I-A-K-N]) emerged as signatures of mucosal transmission. However, T/F Envs derived from intrarectal and intravaginal infections were not different. Our data suggest that the vaginal and rectal mucosal environments both imposed a strong selection bias for SIVsmE660 variants carrying I-A-K-N that was not further enhanced by immunization. These findings, combined with the strong conservation of A-K-N in most HIV-2/SIVsmm isolates and the analogous residues in HIV-1/SIVcpz isolates, suggest that these residues confer increased transmission fitness to SIVsmE660.

IMPORTANCE

Most HIV-1 infections occur across a mucosal barrier, and it is therefore important to understand why these sites are vulnerable and how to protect them with a vaccine. To gain insight into these questions, we studied rhesus macaques that were vaccinated with SIVmac239 and unvaccinated controls to determine whether the SIVsmE660 viral variants that infected these two groups were different. We did not find differences between viral variants in the absence versus presence of vaccination-induced immunity, but we did find that the SIVsmE660 viral variants that infected the monkeys, regardless of vaccination, were different from the dominant population found in the viral challenge inoculum. Our data suggest that the mucosal environments of the vagina and rectum both impose a strong selection for the SIVsmE660 variants in the challenge inoculum that are most like SIV and HIVs that circulate in nature.

Enhancement of recombinant adenovirus vaccine-induced primary but not secondary systemic and mucosal immune responses by all-trans retinoic acid

Vaccination is an important tool for enhancing immune responses against mucosal pathogens. Intramuscularly administered adenovirus (Ad) vectors have been demonstrated to be strong inducers of both systemic and mucosal immune responses. Further enhancement of immune responses following Ad vaccination is highly desirable. All-trans retinoic acid (ATRA), a biologically active vitamin A metabolite, has been explored as an adjuvant for primary immune responses following vaccination. In this study, we investigated the effect of ATRA on a heterologous Ad prime boost regimen. ATRA co-administration during priming increased mucosal and systemic antibody responses as well as mucosal but not systemic CD8(+) T cell responses. However, this effect was no longer apparent after boosting regardless of whether ATRA was administered at the time of priming, at the time of boosting, or at both immunizations. Our findings confirm ATRA as an adjuvant for primary immune responses and suggest that the adjuvant effect does not extend to secondary immune responses.

Using nonhuman primates to model HIV transmission

PURPOSE OF REVIEW

One of the major obstacles in fully understanding HIV transmission comes from the impracticality of studying transmission in humans. Because of this encumbrance, the early phases of HIV transmission and systemic dissemination are poorly understood. In order to fully comprehend these critical steps in HIV infection, animal models must be devised to accurately reflect HIV's mode of action. This review seeks to highlight the essential nature of modelling HIV transmission in nonhuman primates (NHPs).

RECENT FINDINGS

Recently, it was discovered that HIV infection is established in newly infected recipients by a single or few transmitted/founder variants. This has reshaped how animal modelling is conducted with researchers currently recapitulating a physiologically relevant, low-titre infection. Pertinent animal models have been established for the most common routes of infection, including rectal, vaginal and penile transmission; models for intravenous and oral transmission are still in developmental stages.

SUMMARY

These limited dose models now accurately reflect HIV transmission in humans and provide a realistic experimental platform for vaccine development and other intervention strategies that can be used to inform vaccine development in humans. Using information obtained in NHP and human trials, it is conceivable to envision effective prevention modalities in the near future.