The last several years have seen considerable developments in the area of HIV biomedical prevention, and the field has been reinvigorated by the results of several randomized controlled clinical trials in the area of antiretrovirals for prevention, and both systemic and topical antiretrovirals as pre-exposure prophylaxis.1–4 Yet, it seems axiomatic that the key to the ultimate control and eradication of the HIV epidemic is development of a safe, effective, durable, and universal HIV vaccine. Despite advances in elucidating the structure of broadly neutralizing antibodies (BnAbs), approaches to circumvent the great diversity of HIV, this goal is still some years away. Yet, insights from recently completed efficacy trials have unlocked new avenues of investigation that may inform design and implementation of future HIV vaccine studies.
EFFICACY TRIALS COMPLETED TO DATE
Since 1987, more than 200 vaccine products have been tested but only 4 have advanced to efficacy trials.5,6 These studies represent diverse approaches to inducing protective immunity, for example, by eliciting neutralizing antibodies, cell-mediated immune responses, or combined humoral and cellular responses (Table 1). The first 2 efficacy studies, VAX004 and VAX003, evaluated a bivalent gp120 subunit (AIDSVAX) designed to elicit antibodies specific to the viral envelope (Env). The products failed to prevent HIV-1 acquisition, delay progression of clinical disease, or reduce HIV viral load among those who seroconverted.7–10 Although the vaccine was immunogenic, antibodies elicited were not capable of neutralizing genetically diverse circulating HIV strains.11
The next immunogen, recombinant adenovirus serotype 5 (MRKAd5) vector vaccine, evaluated in the Step and Phambili trials, represented a shift in focus to eliciting the production of HIV-specific cytotoxic T lymphocytes, which in infected individuals contribute to control of viral replication to varying degrees.12 In non-human primates, depletion of CD8+ lymphocytes has been shown to correlate with rapid increase in viremia, and, conversely, vaccine-induced potent cytotoxic T lymphocytes responses have resulted in control of viral replication and prevention of disease progression.13–17 This evidence lent support to exploration of a vaccine strategy that may reduce HIV viral load and potentially prolong disease-free survival rather than prevent acquisition. The Step Study was halted in 2007 after the first interim analysis because it failed to achieve its primary end points of preventing HIV-1 infection and/or lowering viral load set point.18–20 Furthermore, the vaccine demonstrated an enhanced risk of infection in uncircumcised men with preexisting immunity to adenovirus serotype 5. Extended post-unblinding follow-up data from the Step cohort revealed that the risk of HIV acquisition peaked shortly after vaccination and waned after 18 months for uncircumcised and adenovirus serotype 5 seropositive men who received the vaccine.21
No efficacy was seen in the Phambili study but, in contrast to the Step trial, adenovirus serotype 5 seropositivity and uncircumcised status among men was not significantly associated with increased HIV acquisition risk; a nonsignificant trend toward lower early viral load and slower decline in CD4 count among female vaccine vs. placebo recipients was observed.19
The results of the MRKAd5 vector T-cell vaccine were both a disappointment and time for reflection for the field. Despite the lack of efficacy, however, viral sequencing of earliest breakthrough isolates among HIV-infected Step vaccinees (sieve analysis) demonstrated that the vaccine induced T-cell–mediated immune pressure on the viruses, particularly in individuals with protective HLA class 1 alleles.22,23 This demonstration of an immune selective pressure, although much weaker than what had been hoped for, provided clues about potential strategies to improve on the T-cell–based vaccine concept and highlighted the importance of collecting samples at key timepoints (eg, earliest post-seroconversion) to allow assessment of whether or not vaccine-induced immune responses have the potential to block certain viruses.
After the negative results of 3 efficacy trials, the RV144 vaccine trial in Thailand was an important milestone for the vaccine field, and its results infused investigators with a renewed sense of enthusiasm. The study evaluated a heterologous prime–boost vaccination strategy consisting of a recombinant canarypox vector vaccine expressing gag, pol, and env followed by a bivalent gp120 subunit vaccine boost; the gp120 protein was identical to the immunogen used in VAX003/VAX004.24 The RV144 vaccine did not elicit BnAbs nor did it elicit measurable CD8+ T-cell responses to reduce viral replication. However, it induced antibody-dependent cellular cytotoxicity (ADCC) responses and neutralizing antibodies only to easy-to-neutralize viruses.25 The vaccine's modest efficacy of 31% in preventing heterosexually acquired HIV infection approached nearly 60% through 6 months after immunization and appeared lower in higher-risk vaccinees in post hoc analyses. These results highlighted the importance of the viral challenge dose and inducing sustained antibody responses over time.
Although longer follow-up of HIV-infected individuals (roll-over study RV152) revealed that the vaccine had no effect on the plasma HIV-1 RNA levels and CD4 cell count after seroconversion, reduced HIV viral load was noted in the seminal fluid in male vaccinees (but not in cervicovaginal lavage samples in women). These results suggest that the vaccine was capable of eliciting immune responses in the mucosal compartment that were not apparent in the peripheral blood,26 thus underscoring the critical importance of evaluating responses at both sites. Unfortunately, limited sampling in RV144 did not permit assessment of vaccine-induced cellular or humoral immune responses at the mucosa. Thus, in the wake of these findings, concerted efforts have been made to optimize sampling techniques to assess humoral and cellular responses in the genital compartments and incorporate mucosal sample collections into vaccine trials to extend the analyses of correlates of protection or risk.
LESSONS LEARNED FROM COMPLETED CLINICAL TRIALS TO DATE TO HELP US CONFRONT MAJOR CHALLENGES IN THE FIELD
AIDSVAX failed, when administered alone in VAX004 and VAX003, but led to modest success, in RV144, in combination with a viral vector prime. The reasons for the success of RV144 remain to be fully elucidated, but these results have refocused the efforts on eliciting potent and durable humoral responses, have emphasized the desire to include an envelope containing protein boost in the regimen, and have given further stimulus to developing an immunogen that will induce broadly acting and potent neutralizing antibodies.
Intense laboratory and biostatistical analyses were launched to identify correlates of protection in a case–control study of 41 infected and 205 uninfected vaccine recipients in RV144.27 A range of immune parameters was assessed and 6 [five different antibody responses: HIV-1 neutralizing antibodies, binding of plasma IgA antibodies to Env, IgG antibodies to variable regions 1 and 2 of gp120, IgG avidity for Env, level of Env-specific CD4+ T cells, and ADCC; one cellular response: CD4+ T-cell cytokine production] were chosen to evaluate their relationship with HIV-1 infection risk. Two strong correlates of risk of infection were found: (1) level of plasma IgG antibodies binding to the V1V2 loop region of gp120 was associated with decreased risk of HIV and an estimated 71% reduction in the risk of infection (odds ratio = 0.29, P = 0.02) was noted in vaccinees with high, compared with low, antibody responses to V1V2; and (2) high plasma level of IgA antibodies to Env was associated with increased infection risk. Further analysis of binding antibody levels revealed that in vaccinees with low, but not high, levels of IgA antibodies, the other immune parameters (IgG avidity, ADCC, nAb, and Env-specific CD4+ T cells) were inversely correlated with risk of infection, although the correlations were of borderline significance.27 It remains to be seen what the significance of these binding antibodies is, but the proposed hypotheses are that the protective effect of high concentrations of IgG antibodies to scaffolded V1V2 region and its effector functions is diminished by high plasma levels of IgA to the HIV-1 envelope.23,28 These non-neutralizing, or binding antibodies, can recruit innate immune cells via their Fc fragments and trigger killing of infected cells via ADCC, thus underscoring the importance of exploring these Fc-related antibody activities, in addition to classic neutralization, in future vaccine strategies. The role of Fc receptor polymorphisms and other genetic factors that may play a role in modulating the immune responses to the vaccine is under evaluation.29,30
In parallel analyses, Rolland et al31 compared the viruses isolated from infected vaccine and placebo recipients and found evidence that the vaccine induced selective pressure on the virus either by blocking certain viruses from establishing infection or driving escape mutations after infection. Specifically, in an analysis restricted to V1V2, 2 amino acid sites were identified in the V2 region (at positions 169 and 181) that were associated with protective vaccine-induced immune responses, suggesting that the vaccine “blocked” or “sieved” viruses with specific signatures in the V2 region of the envelope. Recent post hoc analyses that focused on a wider range of antibody responses and epitope mapping to the V2 region confirmed a preferential targeting of regions in gp120 identified in the sieve analysis and the correlation with a lower rate of infection in the vaccinees.32,33
These data taken together with the correlates analyses and ongoing work pointing to the critical nature of the V2 loop in early viral transmission, mediating ADCC, and neutralization,32,34–36 further support the hypotheses that antibodies to V2 had a role in the partial protection conferred by the RV144 regimen.31
These results have influenced our approach to the next generation of vaccine strategies. For example, vaccine candidates are being screened for their ability to induce IgG antibodies to scaffolded V1V2 of gp120. It is worth noting, however, that given the complex steps in the viral entry, interaction with multiple receptors and an interplay of host and viral factors, it will be important to investigate vaccine strategies that elicit antibodies against other parts of the viral envelope and stimulate effective cellular responses.37
CIRCUMVENTING NEUTRALIZING ANTIBODY AND VIRAL DIVERSITY
The RV144 correlates findings dovetail with recent advances in isolating BnAbs from humans. It has been shown that 2–4 years after infection, up to 25% of HIV-1–infected individuals develop BnAbs, creating optimism that a vaccine inducing the “right” antibody could be successful.38–42 These BnAbs and the epitopes they recognize have been studied extensively to better define targets on the HIV envelope that could be used to design active immunogens with the hope of eliciting antibodies with strong neutralizing potential. Importantly, these antibodies can also be evaluated as passive immunoprophylaxis agents, perhaps in combination with other monoclonal products or with vaccines.37,43 Other approaches under investigation include using vector-mediated delivery of genes expressing the desired BnAb, an approach that has recently been evaluated in animal models44 and has the potential advantage of circumventing the need for repeated injections of antibodies.
The enormous diversity of the virus is emblematic of the challenges to HIV vaccine development. Very high number of replication cycles, the error-prone reverse transcription due to lack of proofreading activity, and high rate of recombination between variants within an infected person45–49 all lead to rapid creation of a large pool of HIV-1 variants in each infected individual. Pressure from host immune cellular and humoral responses leads to even more viral diversity.22,31,50–58 As a result, the amount of diversity within an individual can exceed the variability generated over the course of a global influenza epidemic, the latter of which results in the need for modification of the vaccine inserts each year.59 Most heterosexually infected subjects are infected with a single HIV-1 transmission/founder variant and very few mutations occur in the first 2 months after infection. Focusing on the very early events before establishment of HIV-1 infection and understanding the complex host and viral factors leading to one or few founder viruses getting through are thus critical to circumventing the diversity challenge.60,61
Equally critical is understanding of the interplay between early viral evolution from the time of transmission and the development and maturation of BnAbs. It is known that very high level of mutations (somatic mutations) over time are necessary for the evolution of broad and potent anti-HIV antibodies that pose a considerable challenge for vaccine design.62 Recent investigation into the coevolution of the virus and the BnAb shortly after seroconversion presented an opportunity to map out the pathways that lead to generation of these antibodies.63 Evidence that certain envelope proteins of the founder virus are more likely to stimulate evolution of BnAbs may present an opportunity to vaccinate with naturally derived viral envelopes that could drive the desired B-cell responses and induce the development of broad and potent antibodies.64
The results of RV144 and correlates analyses that followed were an important milestone for the vaccine field by opening new avenues of research and investigation.28,37,65 There is, for example, considerable interest in extending the RV144 findings to other populations, HIV-1 subtypes, and risk groups. Plans are underway for phase 2 and 3 studies to explore whether the addition of booster dose of protein or other adjuvants would result in more potent and durable antibody responses over time.
It is more than likely that an efficacious and durable vaccine will need to elicit a balance of responses,23,66–68 and current prime–boost vaccine strategies aim to elicit a combination of B-cell, CD4+, and CD8+ T-cell responses. For example, vaccine regimens that use DNA and viral vectors (eg, NYVAC and MVA) are under investigation alone or in combination with protein boosts in an attempt to induce durable cellular and humoral responses.
Although the HIV-1 vaccine field has experienced its share of disappointments and challenges with a succession of negative efficacy trials, translational research results from completed and fully analyzed studies generated new critical questions that have advanced the HIV vaccine field in pertinent ways (Table 1). Experience from the past 5 years, and in particular lessons learned from the immune correlates work in RV144, highlight the critical importance of conducting efficacy studies that continue to drive us closer toward a safe and effective preventive vaccine.69 Moreover, as the biomedical prevention landscape evolves and prevention technologies intersect, opportunities may emerge to evaluate combination strategies to achieve incremental but important reduction in HIV incidence.
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