HIV/AIDS is a worldwide public health threat causing high morbidity and mortality. At the end of 2010, the total number of people living with HIV was estimated to be 34 million, up 17% from 2001. This reflects the continued large number of new HIV infections and a significant expansion of access to antiretroviral therapy, which has helped reduce AIDS-related deaths, especially in recent years . Despite promising but still fragile successes in prevention, care and treatment, the development of a safe and efficacious preventive HIV vaccine, as part of a comprehensive prevention program remains a global health priority, and the best tool for long-term control of the HIV epidemic , .
Although the nature of the immune response needed to confer protection against HIV infection is unknown, an effective immune response will likely comprise antibodies and T cells that neutralize free virus and/or recognize and eradicate cells infected with diverse strains of HIV before an infection becomes irreversibly established .
Monomeric gp120 envelope subunits failed to induce neutralizing antibodies against circulating isolates and to confer protection against HIV acquisition , . Generation of broadly neutralizing antibodies is still a challenge ,  despite the recent progress in isolating broad neutralizing monoclonal antibodies against HIV , , , , , , , . Recent efforts have focused on the development of HIV vaccines capable of inducing broad cell-mediated responses that could reduce viral replication after infection (“T-cell vaccines”) , . Although contradicted by some studies , control of viral replication could slow the rate of disease progression, as suggested by non-human primate (NHP) challenge studies , , , , , and/or reduce transmission of HIV from infected vaccine recipient to partner by reducing virus load in the infected person .
Replication-incompetent viral vectors, including adenoviruses and poxviruses are among current strategies for induction of cell-mediated immune (CMI) responses in humans. The Step (HVTN 502/Merck 023) and Phambili (HVTN 503) vaccine trials were the first human efficacy trials (phase IIb ‘test-of-concept’) to explore whether a vector-based HIV-1 prophylactic vaccine aimed at inducing CMI responses could prevent infection or reduce post-infection viremia. The Merck vaccine was composed of replication-incompetent adenovirus serotype 5 (MRKAd5 HIV-1) vectors expressing HIV-1 clade B non-envelope antigens. The Step study enrolled, predominantly high-risk populations including men who have sex with men (MSM) as well as heterosexual women in North and South America and Australia, and heterosexual women and men in the Caribbean , . The Phambili study enrolled heterosexual men and women in South Africa . HVTN 502/Merck 023 was unexpectedly halted for futility in achieving the study primary endpoints (follow-up continued for two years after interim analysis) with an HIV incidence greater in vaccine than in placebo recipients, mostly men having sex with men and subjects with pre-existing Ad5-specific neutralizing antibody titers. The biological basis for this observation remains unclear. Post-hoc multivariate analysis further suggested that the greatest increased risk was in men who had pre-existing Ad5-specific neutralizing antibodies and who were uncircumcised , . Although the MRKAd5 HIV-1 vaccine induced IFN-γ ELISPOT responses, and polyfunctional T cells by flow cytometry in the majority of recipients, it did not result in a decreased viral load in HIV-infected individuals . Moreover, the immune response was lower both in frequency and magnitude in individuals with pre-existing Ad5 antibody titer >18 , .
Recently, a phase IIb trial (RV144) of ALVAC-HIV and AIDSVAX® gp120 B/E prime-boost enrolling Thai volunteers at “community risk” for HIV infection showed that, by modified intent-to-treat analysis 3.5 years after initial vaccination, the vaccine regimen was 31.2% efficacious in preventing HIV infection. Six months after the last vaccination, HIV Env- and Gag-specific IFN-γ ELISPOT responses were detected in 19.7% of vaccine recipients, Env-specific intracellular cytokine staining in 34%, and lymphoproliferative responses and binding antibodies to Env in a majority of subjects. There was however, no effect on early post-infection HIV-1 RNA viral load or CD4+ T-cell count .
This paper describes a clinical study with an HIV vaccine based on adenovirus serotype 35 (Ad35) that was designed to overcome pre-existing humoral immunity, a hurdle faced by Ad5-based vaccines. Ad35 is a human adenovirus serotype with low seroprevalence. In addition, the prevalence and titers of Ad35 neutralizing antibodies are lower than those of Ad5 neutralizing antibodies in Africa, Europe, North America and Asia , . In adults, Ad35 seroprevalence was 10.6%–17.8% in South Africa, 14.8% in Kenya, 5.4% in Uganda, and 17.1% Thailand . Ad35 belongs to subtype B of adenoviruses, which use highly expressed CD46 as receptor, while Ad5 is a subtype C using the coxsackie-adenovirus receptor (CAR) .
Recombinant Ad35-based vectors have been studied alone or in prime-boost regimens with DNA or with another Ad vector, as vaccines against HIV (NCT00479999: HVTN 072 and NCT00801697: HVTN 077), tuberculosis (AERAS)  and malaria (NCT01018459, NCT01366534, NCT00371189). The vaccines are generally well tolerated and immunogenic. This report describes a phase I dose-escalation, placebo-controlled, randomized study of two Ad35 vectors, Ad35-GRIN containing HIV-1 subtype A gag, reverse transcriptase, integrase and nef genes and Ad35-ENV containing HIV-1 subtype A env gene tested in healthy HIV-uninfected adults at low-risk of HIV acquisition.
We conducted a phase I, randomized, double-blind, placebo-controlled trial to assess the safety and immunogenicity of escalating doses of two recombinant replication defective adenovirus serotype 35 (Ad35) vectors containing gag, reverse transcriptase, integrase and nef (Ad35-GRIN) and env (Ad35-ENV), both derived from HIV-1 subtype A isolates. The trial enrolled 56 healthy HIV-uninfected adults.
Ad35-GRIN/ENV (Ad35-GRIN and Ad35-ENV mixed in the same vial in equal proportions) or Ad35-GRIN was administered intramuscularly at 0 and 6 months. Participants were randomized to receive either vaccine or placebo (10/4 per group, respectively) within one of four dosage groups: Ad35-GRIN/ENV 2×109 (A), 2×1010 (B), 2×1011 (C), or Ad35-GRIN 1×1010 (D) viral particles.
No vaccine-related serious adverse event was reported. Reactogenicity events reported were dose-dependent, mostly mild or moderate, some severe in Group C volunteers, all transient and resolving spontaneously. IFN-γ ELISPOT responses to any vaccine antigen were detected in 50, 56, 70 and 90% after the first vaccination, and in 75, 100, 88 and 86% of Groups A–D vaccine recipients after the second vaccination, respectively. The median spot forming cells (SFC) per 106 PBMC to any antigen was 78–139 across Groups A–C and 158–174 in Group D, after each of the vaccinations with a maximum of 2991 SFC. Four to five HIV proteins were commonly recognized across all the groups and over multiple timepoints. CD4+ and CD8+ T-cell responses were polyfunctional. Env antibodies were detected in all Group A–C vaccinees and Gag antibodies in most vaccinees after the second immunization. Ad35 neutralizing titers remained low after the second vaccination.
Ad35-GRIN/ENV reactogenicity was dose-related. HIV-specific cellular and humoral responses were seen in the majority of volunteers immunized with Ad35-GRIN/ENV or Ad35-GRIN and increased after the second vaccination. T-cell responses were broad and polyfunctional.