Safety and Immunogenicity of the MRKAd5 gag HIV Type 1 Vaccine in a Worldwide Phase 1 Study of Healthy Adults

Review of preventative HIV vaccine clinical trials in South Africa

New HIV infections continue relentlessly in southern Africa, demonstrating the need for a vaccine to prevent HIV subtype C. In South Africa, the country with the highest number of new infections annually, HIV vaccine research has been ongoing since 2003 with collaborative public-private-philanthropic partnerships. So far, 21 clinical trials have been conducted in South Africa, investigating seven viral vectors, three DNA plasmids, four envelope proteins, five adjuvants and three monoclonal antibodies. Active vaccine candidates have spanned subtypes A, B, C, E and multi-subtype mosaic sequences. All were well tolerated. Four concepts were investigated for efficacy: rAd5-gag/pol/nef showed increased HIV acquisition in males, subtype C ALVAC/gp120/MF59 showed no preventative efficacy, and the trials for the VRC01 monoclonal antibody and Ad26.Mos4.HIV/subtype C gp140/ aluminum phosphate are ongoing. Future trials are planned with DNA/viral vector plus protein combinations in concert with pre-exposure prophylaxis, and sequential immunization studies with transmitted/founder HIV envelope to induce broadly neutralizing antibodies. Finally, passive immunization trials are underway to build on the experience with VRC01, including single and combination antibody trials with an antibody derived from a subtype-C-infected South African donor. Future consideration should be given to the evaluation of novel strategies, for example, inactivated-whole-virus vaccines.

The role of follicular helper CD4 T cells in the development of HIV-1 specific broadly neutralizing antibody responses

The induction of HIV-1-specific antibodies that can neutralize a broad number of isolates is a major goal of HIV-1 vaccination strategies. However, to date no candidate HIV-1 vaccine has successfully elicited broadly neutralizing antibodies of sufficient quality and breadth for protection. In this review, we focus on the role of follicular helper CD4 T-cells (Tfh) in the development of such cross-reactive protective antibodies. We discuss germinal center (GC) formation and the dynamics of Tfh and GC B cells during HIV-1/SIV infection and vaccination. Finally, we consider future directions for the study of Tfh and offer perspective on factors that could be modulated to enhance Tfh function in the context of prophylactic vaccination.

Biodistribution and residence time of adenovector serotype 5 in normal and immunodeficient mice and rats detected with bioluminescent imaging

As concerns increase about adenovirus type 5 (Ad5) being a safe gene transfer vector, it is important to evaluate its distribution, residence time, and possible toxicity in immunodeficient populations. To characterize the potential risk associated with different Ad5 vector delivery modes, we used immunocompetent and immunodeficient Rag2^−/− animals to establish mouse and rat models that could be monitored with bioluminescent imaging following intramuscular or intravascular infection with an engineered replication-incompetent Ad5 virus carrying the firefly luciferase gene (Ad5-Fluc). The Ad5 vector was less well-tolerated by Rag2^−/− animals than by wildtype ones, with delayed residence time, wider virus dissemination, less weight gain, and relatively severe pathological changes. In intravascularly Ad5-Fluc-infected Rag2^−/− mice, systemic virus dissemination extended from the abdomen to the limbs and head on day 9 post-infection. Additionally, significant increases in plasma TNF-α and IFN-γ, which may be important factors in the heightened immunopathology in the liver and brain, were detected in the Rag2^−/− mice 30 days after intravascular delivery. The Ad5 vector was better tolerated after intramuscular delivery than after intravascular delivery. Ad5-Fluc/Rag2^−/− mice and rats can be used as reliable models of an immunodeficient population in which to evaluate the safety of Ad5-vectored vaccines or gene therapy products.

A Little Help From the Follicles: Understanding the Germinal Center Response to Human Immunodeficiency Virus 1 Infection and Prophylactic Vaccines

Human immunodeficiency virus 1 (HIV-1) is the causative agent of AIDS. There are currently more than 35 million people living with HIV infection worldwide, and more than 2 million new infections occur each year. The global pandemic caused by HIV-1 is the subject of numerous research projects, with the development of a prophylactic vaccine and a therapeutic cure being the ultimate goals. The classic paradigms of vaccinology have proven incapable of producing a viable vaccine due to the complexity of the virus' replication cycle, its genetic diversity, and a lack of understanding of the immune correlates of protection. Here, we briefly discuss recent vaccine approaches and the immune correlates of protection from HIV-1 infection with a focus on the role of the germinal center as a reservoir of replication-competent virus and its role in the development of broadly neutralizing antibodies in response to vaccination.

Role of CD8+ T Cells in the Selection of HIV-1 Immune Escape Mutations

Human immunodeficiency virus type-1 (HIV-1) infection represents one of the biggest public health problems worldwide. The immune response, mainly the effector mechanisms mediated by CD8⁺ T cells, induces the selection of mutations that allows the virus to escape the immune control. These mutations are generally selected within CD8⁺ T cell epitopes restricted to human leukocyte antigen class I (HLA-I), leading to a decrease in the presentation and recognition of the epitope, decreasing the activation of CD8⁺ T cells. However, these mutations may also affect cellular processing of the peptide or recognition by the T cell receptor. Escape mutations often carry a negative impact in viral fitness that is partially or totally compensated by the selection of compensatory mutations. The selection of either escape mutations or compensatory mutations may negatively affect the course of the infection. In addition, these mutations are a major barrier for the development of new therapeutic strategies focused on the induction of specific CD8⁺ T cell responses.

Assessment of the Safety and Immunogenicity of 2 Novel Vaccine Platforms for HIV-1 Prevention: A Randomized Trial

BACKGROUND

A prophylactic HIV-1 vaccine is a global health priority.

OBJECTIVE

To assess a novel vaccine platform as a prophylactic HIV-1 regimen.

DESIGN

Randomized, double-blind, placebo-controlled trial. Both participants and study personnel were blinded to treatment allocation. (ClinicalTrials.gov: NCT01215149).

SETTING

United States, East Africa, and South Africa.

PATIENTS

Healthy adults without HIV infection.

INTERVENTION

2 HIV-1 vaccines (adenovirus serotype 26 with an HIV-1 envelope A insert [Ad26.EnvA] and adenovirus serotype 35 with an HIV-1 envelope A insert [Ad35.Env], both administered at a dose of 5 × 1010 viral particles) in homologous and heterologous combinations.

MEASUREMENTS

Safety and immunogenicity and the effect of baseline vector immunity.

RESULTS

217 participants received at least 1 vaccination, and 210 (>96%) completed follow-up. No vaccine-associated serious adverse events occurred. All regimens were generally well-tolerated. All regimens elicited humoral and cellular immune responses in nearly all participants. Preexisting Ad26- or Ad35-neutralizing antibody titers had no effect on vaccine safety and little effect on immunogenicity. In both homologous and heterologous regimens, the second vaccination significantly increased EnvA antibody titers (approximately 20-fold from the median enzyme-linked immunosorbent assay titers of 30-300 to 3000). The heterologous regimen of Ad26-Ad35 elicited significantly higher EnvA antibody titers than Ad35-Ad26. T-cell responses were modest and lower in East Africa than in South Africa and the United States.

LIMITATIONS

Because the 2 envelope inserts were not identical, the boosting responses were complex to interpret. Durability of the immune responses elicited beyond 1 year is unknown.

CONCLUSION

Both vaccines elicited significant immune responses in all populations. Baseline vector immunity did not significantly affect responses. Second vaccinations in all regimens significantly boosted EnvA antibody titers, although vaccine order in the heterologous regimen had a modest effect on the immune response.

PRIMARY FUNDING SOURCE

International AIDS Vaccine Initiative, National Institutes of Health, Ragon Institute, Crucell Holland.

Adenoviral Vector Vaccines Antigen Transgene

In the past decade adenovirus-based vaccines have progressed from preclinical studies, which universally showed the vectors’ high immunogenicity, to testing in humans. Clinical trials showed that adenovirus vectors are well tolerated by humans. They induce robust immune responses that can be expanded by booster immunization. The effect of preexisting neutralizing antibodies on vectors’ immunogenicity appears to be less severe than was observed in experimental animals and can readily be circumvented by using vectors to which most humans lack neutralizing antibodies. Additional clinical studies are needed to firmly establish the efficacy of adenoviral vector vaccines.

Continued Follow-Up of Phambili Phase 2b Randomized HIV-1 Vaccine Trial Participants Supports Increased HIV-1 Acquisition among Vaccinated Men

Background

The Phase 2b double-blinded, randomized Phambili/HVTN 503 trial evaluated safety and efficacy of the MRK Ad5 gag/pol/nef subtype B HIV-1 preventive vaccine vs placebo in sexually active HIV-1 seronegative participants in South Africa. Enrollment and vaccinations stopped and participants were unblinded but continued follow-up when the Step study evaluating the same vaccine in the Americas, Caribbean, and Australia was unblinded for non-efficacy. Final Phambili analyses found more HIV-1 infections amongst vaccine than placebo recipients, impelling the HVTN 503-S recall study.

Methods

HVTN 503-S sought to enroll all 695 HIV-1 uninfected Phambili participants, provide HIV testing, risk reduction counseling, physical examination, risk behavior assessment and treatment assignment recall. After adding HVTN 503-S data, HIV-1 infection hazard ratios (HR vaccine vs. placebo) were estimated by Cox models.

Results

Of the 695 eligible, 465 (67%) enrolled with 230 from the vaccine group and 235 from the placebo group. 38% of the 184 Phambili dropouts were enrolled. Enrollment did not differ by treatment group, gender, or baseline HSV-2. With the additional 1286 person years of 503-S follow-up, the estimated HR over Phambili and HVTN 503-S follow-up was 1.52 (95% CI 1.08–2.15, p = 0.02, 82 vaccine/54 placebo infections). The HR was significant for men (HR = 2.75, 95% CI 1.49, 5.06, p = 0.001) but not for women (HR = 1.12, 95% CI 0.73, 1.72, p = 0.62).

Conclusion

The additional follow-up from HVTN 503-S supported the Phambili finding of increased HIV-1 acquisition among vaccinated men and strengthened the evidence of lack of vaccine effect among women.

Trial Registration

clinicaltrials.gov NCT00413725

SA National Health Research Database DOH-27-0207-1539

Effect of rAd5-Vector HIV-1 Preventive Vaccines on HIV-1 Acquisition: A Participant-Level Meta-Analysis of Randomized Trials

Background

Three phase 2b, double-blind, placebo-controlled, randomized efficacy trials have tested recombinant Adenovirus serotype-5 (rAd5)-vector preventive HIV-1 vaccines: MRKAd5 HIV-1 gag/pol/nef in Step and Phambili, and DNA/rAd5 HIV-1 env/gag/pol in HVTN505. Due to efficacy futility observed at the first interim analysis in Step and HVTN505, participants of all three studies were unblinded to their vaccination assignments during the study but continued follow–up. Rigorous meta-analysis can provide crucial information to advise the future utility of rAd5-vector vaccines.

Methods

We included participant-level data from all three efficacy trials, and three Phase 1–2 trials evaluating the HVTN505 vaccine regimen. We predefined two co-primary analysis cohorts for assessing the vaccine effect on HIV-1 acquisition. The modified-intention-to-treat (MITT) cohort included all randomly assigned participants HIV-1 uninfected at study entry, who received at least the first vaccine/placebo, and the Ad5 cohort included MITT participants who received at least one dose of rAd5-HIV vaccine or rAd5-placebo. Multivariable Cox regression models were used to estimate hazard ratios (HRs) of HIV-1 infection (vaccine vs. placebo) and evaluate HR variation across vaccine regimens, time since vaccination, and subgroups using interaction tests.

Findings

Results are similar for the MITT and Ad5 cohorts; we summarize MITT cohort results. Pooled across the efficacy trials, over all follow-up time 403 (n = 224 vaccine; n = 179 placebo) of 6266 MITT participants acquired HIV-1, with a non-significantly higher incidence in vaccine recipients (HR 1.21, 95% CI 0.99–1.48, P = 0.06). The HRs significantly differed by vaccine regimen (interaction P = 0.03; MRKAd5 HR 1.41, 95% CI 1.11–1.78, P = 0.005 vs. DNA/rAd5 HR 0.88, 95% CI 0.61–1.26, P = 0.48). Results were similar when including the Phase 1–2 trials. Exploratory analyses based on the efficacy trials supported that the MRKAd5 vaccine-increased risk was concentrated in Ad5-positive or uncircumcised men early in follow-up, and in Ad5-negative or circumcised men later. Overall, MRKAd5 vaccine-increased risk was evident across subgroups except in circumcised Ad5-negative men (HR 0.97, 95% CI 0.58−1.63, P = 0.91); there was little evidence that the DNA/rAd5 vaccine, that was tested in this subgroup, increased risk (HR 0.88, 95% CI 0.61–1.26, P = 0.48). When restricting the analysis of Step and Phambili to follow-up time before unblinding, 114 (n = 65 vaccine; n = 49 placebo) of 3770 MITT participants acquired HIV-1, with a non-significantly higher incidence in MRKAd5 vaccine recipients (HR 1.30, 95% CI 0.89–1.14, P = 0.18).

Interpretation and Significance

The data support increased risk of HIV-1 infection by MRKAd5 over all follow-up time, but do not support increased risk of HIV-1 infection by DNA/rAd5. This study provides a rationale for including monitoring plans enabling detection of increased susceptibility to infection in HIV-1 at-risk populations.

Recombinant adenovirus type 5 HIV gag/pol/nef vaccine in South Africa: unblinded, long-term follow-up of the phase 2b HVTN 503/Phambili study

BACKGROUND

The HVTN 503/Phambili study, which assessed the efficacy of the Merck Ad5 gag/pol/nef subtype B HIV-1 preventive vaccine in South Africa, was stopped when futility criteria in the Step study (assessing the same vaccine in the Americas, Caribbean, and Australia) were met. Here we report long-term follow-up data.

METHODS

HVTN 503/Phambili was a double-blind, placebo-controlled, randomised trial that recruited HIV-1 uninfected, sexually active adults aged 18-35 years from five sites in South Africa. Eligible participants were randomly assigned (1:1) by computer-generated random numbers to either vaccine or placebo, stratified by site and sex. Cox proportional hazards models were used to estimate HIV-1 infection in the modified intention-to-treat cohort, all of whom were unmasked early in follow-up. The trial is registered with ClinicalTrials.gov, number NCT00413725 and the South African National Health Research Database, number DOH-27-0207-1539.

FINDINGS

Between Jan 24, 2007, and Sept 19, 2007, 801 participants (26·7%) of a planned 3000 were randomly assigned (400 to vaccine, 401 to placebo); 216 (27%) received only one injection, 529 (66%) received only two injections, and 56 (7%) received three injections. At a median follow-up of 42 months (IQR 31-42), 63 vaccine recipients (16%) had HIV-1 infection compared with 37 placebo recipients (9%; adjusted HR 1·70, 95% CI 1·13-2·55; p=0·01). Risk for HIV-1 infection did not differ according to the number of vaccinations received, sex, circumcision, or adenovirus type 5 (Ad5) serostatus. Differences in risk behaviour at baseline or during the study, or annualised dropout rate (7·7% [95% CI 6·2-9·5] for vaccine recipients vs 8·8% [7·1-10·7] for placebo recipients; p=0·40) are unlikely explanations for the increased rate of HIV-1 infections seen in vaccine recipients.

INTERPRETATION

The increased risk of HIV-1 acquisition in vaccine recipients, irrespective of number of doses received, warrants further investigation to understand the biological mechanism. We caution against further use of the Ad5 vector for HIV vaccines.

FUNDING

National Institute of Allergy and Infectious Diseases, Merck, and South African Medical Research Council.

Mucosal immunology of HIV infection

Recent advances in the immunology, pathogenesis, and prevention of human immunodeficiency virus (HIV) infection continue to reveal clues to the mechanisms involved in the progressive immunodeficiency attributed to infection, but more importantly have shed light on the correlates of immunity to infection and disease progression. HIV selectively infects, eliminates, and/or dysregulates several key cells of the human immune system, thwarting multiple arms of the host immune response, and inflicting severe damage to mucosal barriers, resulting in tissue infiltration of 'symbiotic' intestinal bacteria and viruses that essentially become opportunistic infections promoting systemic immune activation. This leads to activation and recruitment or more target cells for perpetuating HIV infection, resulting in persistent, high-level viral replication in lymphoid tissues, rapid evolution of resistant strains, and continued evasion of immune responses. However, vaccine studies and studies of spontaneous controllers are finally providing correlates of immunity from protection and disease progression, including virus-specific CD4(+) T-cell responses, binding anti-bodies, innate immune responses, and generation of antibodies with potent antibody-dependent cell-mediated cytotoxicity activity. Emerging correlates of immunity indicate that prevention of HIV infection may be possible through effective vaccine strategies that protect and stimulate key regulatory cells and immune responses in susceptible hosts. Furthermore, immune therapies specifically directed toward boosting specific aspects of the immune system may eventually lead to a cure for HIV-infected patients.

Development of prophylactic vaccines against HIV-1

The focus of most current HIV-1 vaccine development is on antibody-based approaches. This is because certain antibody responses correlated with protection from HIV-1 acquisition in the RV144 phase III trial, and because a series of potent and broad spectrum neutralizing antibodies have been isolated from infected individuals. Taken together, these two findings suggest ways forward to develop a neutralizing antibody-based vaccine. However, understanding of the correlates of protection from disease in HIV-1 and other infections strongly suggests that we should not ignore CTL-based research. Here we review recent progress in the field and highlight the challenges implicit in HIV-1 vaccine design and some potential solutions.

HIV epidemic in Asia: optimizing and expanding vaccine development

The recent evidence in Thailand for protection from acquisition of HIV through vaccination in a mostly heterosexual population has generated considerable hope. Building upon these results and the analysis of the correlates of risk remains among the highest priorities. Improved vaccine concepts including heterologous prime-boost regimens, improved proteins with potent adjuvants and new vectors expressing mosaic antigens may soon enter clinical development to assess vaccine efficacy in men who have sex with men. Identifying heterosexual populations with sufficient HIV incidence for the conduct of efficacy trials represents perhaps the main challenge in Asia. Fostering translational research efforts in Asian countries may benefit from the development of master strategic plans and program management processes.

NDV-3, a recombinant alum-adjuvanted vaccine for Candida and Staphylococcus aureus, is safe and immunogenic in healthy adults

The investigational vaccine, NDV-3, contains the N-terminal portion of the Candida albicans agglutinin-like sequence 3 protein (Als3p) formulated with an aluminum hydroxide adjuvant in phosphate-buffered saline. Preclinical studies demonstrated that the Als3p vaccine antigen protects mice from oropharyngeal, vaginal and intravenous challenge with C. albicans and other selected species of Candida as well as both intravenous challenge and skin and soft tissue infection with Staphylococcus aureus. The objectives of this first-in-human Phase I clinical trial were to evaluate the safety, tolerability and immunogenicity of NDV-3 at two different antigen levels compared to a saline placebo. Forty healthy, adult subjects were randomized to receive one dose of NDV-3 containing either 30 or 300 μg of Als3p, or placebo. NDV-3 at both dose levels was safe and generally well-tolerated. Anti-Als3p total IgG and IgA1 levels for both doses reached peak levels by day 14 post vaccination, with 100% seroconversion of all vaccinated subjects. On average, NDV-3 stimulated peripheral blood mononuclear cell (PBMC) production of both IFN-γ and IL-17A, which peaked at day 7 for subjects receiving the 300 μg dose and at day 28 for those receiving the 30 μg dose. Six months after receiving the first dose of NDV-3, nineteen subjects received a second dose of NDV-3 identical to their first dose to evaluate memory B- and T-cell immune responses. The second dose resulted in a significant boost of IgG and IgA1 titers in >70% of subjects, with the biggest impact in those receiving the 30 μg dose. A memory T-cell response was also noted for IFN-γ in almost all subjects and for IL-17A in the majority of subjects. These data support the continued investigation of NDV-3 as a vaccine candidate against Candida and S. aureus infections.

Vaccine-induced HIV-specific CD8+ T cells utilize preferential HLA alleles and target-specific regions of HIV-1

Most T cell-based HIV-1 vaccine candidates induce responses of limited breadth for reasons that are unclear. We evaluated vaccine-induced T-cell responses in individuals receiving an HIV-1 recombinant adenoviral vaccine. Certain HLA alleles (B27, B57, B35, and B14) are preferentially utilized to mount HIV-specific responses, whereas other alleles (A02 and B07) are rarely utilized (P < 0.001). This preference seems due to 4 following factors individually or in combination: higher affinity of specific peptides to specific HLA alleles; higher avidity of T-cell receptor; HLA and peptide interaction; and/or higher surface expression of certain HLA. Thus, HLA immunodominance plays a substantial role in vaccine-induced T-cell responses.

Pre-existing adenovirus immunity modifies a complex mixed Th1 and Th2 cytokine response to an Ad5/HIV-1 vaccine candidate in humans

The results of the recent Step Study highlight a need to clarify the effects of pre-existing natural immunity to a vaccine vector on vaccine-induced T-cell responses. To investigate this interaction, we examined the relationship between pre-existing Ad5 immunity and T-cell cytokine response profiles in healthy, HIV-uninfected recipients of MRKAd5 HIV-1 gag vaccine (HVTN 050, ClinicalTrials.gov #NCT00849732). Participants were grouped by baseline Ad5 neutralizing antibody titer as either Ad5-seronegative (titer ≤18; n = 36) or Ad5-seropositive (titer >200; n = 34). Samples from vaccine recipients were analyzed for immune responses to either HIV-1 Gag peptide pools or Ad5 empty vector using an ex vivo assay that measures thirty cytokines in the absence of long-term culture. The overall profiles of cytokine responses to Gag and Ad5 had similar combinations of induced Th1- and Th2-type cytokines, including IFN-γ, IL-2, TNF-α, IP-10, IL-13, and IL-10, although the Ad5-specific responses were uniformly higher than the Gag-specific responses (p<0.0001 for 9 out of 11 significantly expressed analytes). At the peak response time point, PBMC from Ad5-seronegative vaccinees secreted significantly more IP-10 in response to Gag (p = 0.008), and significantly more IP-10 (p = 0.0009), IL-2 (p = 0.006) and IL-10 (p = 0.05) in response to Ad5 empty vector than PBMC from Ad5-seropositive vaccinees. Additionally, similar responses to the Ad5 vector prior to vaccination were observed in almost all subjects, regardless of Ad5 neutralizing antibody status, and the levels of secreted IFN-γ, IL-10, IL-1Ra and GM-CSF were blunted following vaccination. The cytokine response profile of Gag-specific T cells mirrored the Ad5-specific response present in all subjects before vaccination, and included a number of Th1- and Th2-associated cytokines not routinely assessed in current vaccine trials, such as IP-10, IL-10, IL-13, and GM-CSF. Together, these results suggest that vector-specific humoral responses may reduce vaccine-induced T-cell responses by previously undetected mechanisms.

Nonhuman primate models and the failure of the Merck HIV-1 vaccine in humans

The adenovirus type 5 (Ad5)-based vaccine developed by Merck failed to either prevent HIV-1 infection or suppress viral load in subsequently infected subjects in the STEP human Phase 2b efficacy trial. Analogous vaccines had previously also failed in the simian immunodeficiency virus (SIV) challenge-rhesus macaque model. In contrast, vaccine protection studies that used challenge with a chimeric simian-human immunodeficiency virus (SHIV89.6P) in macaques did not predict the human trial results. Ad5 vector-based vaccines did not protect macaques from infection after SHIV89.6P challenge but did cause a substantial reduction in viral load and a preservation of CD4+ T cell counts after infection, findings that were not reproduced in the human trials. Although the SIV challenge model is incompletely validated, we propose that its expanded use can help facilitate the prioritization of candidate HIV-1 vaccines, ensuring that resources are focused on the most promising candidates. Vaccine designers must now develop T cell vaccine strategies that reduce viral load after heterologous challenge.