Towards the development of a therapeutic vaccine for the treatment of HIV-1 infection: are we closer than ever?

Since 1983, when it was first appreciated that AIDS resulted from infection with HIV-1, an enormous amount of effort has gone into the research and development of drugs with antiviral activity. Almost 25 years later, over 24 antiviral medications capable of blocking HIV-1 replication are currently licensed for use in the USA. In countries where there is early diagnosis of HIV infection and access to these antiviral medications in the form of highly active antiretroviral therapy (HAART), HIV-1 infection is considered a chronic, manageable disease. Despite these medical advances in the treatment of HIV-1 infection, access to HAART in developing countries is often limited or nonexistent, although slow progress is being made [1].

The WHO estimates that in 2006, 40 million people were living with HIV-1 or suffering from AIDS and 2.9 million died as a result of HIV infection. Despite social programs intended to stem the spread of HIV-1, 4.3 million people were newly infected with HIV-1 in 2006 [2]. It is unlikely, in the current global environment, that the development and licensure of additional antiviral medications for the treatment of HIV infection will change these numbers substantially. However, an affordable and easy to administer therapeutic HIV-1 vaccine, which is made available worldwide, does have the potential to change these statistics.

A large body of published evidence generated in both animal models and clinical studies suggests that HIV-specific CD8⁺ cytotoxic T cells play a principal role in controlling virus replication and disease progression [3,4]. As a result of this increasing body of evidence, the therapeutic HIV-1 vaccine field is reaching a consensus with regards to what sort of HIV-specific immune responses should be augmented by therapeutic immunization. Thus, the question becomes, can therapeutic immunization in the context of HIV-1 infection improve or augment the virus-specific immune response and, as a result, alter disease progression in a meaningful way?

Mellors and colleagues followed disease progression and death from AIDS in a cohort of 180 HIV-1-seropositive men over a period of 10 years [5]. The results of this analysis revealed that the risk of developing AIDS-like symptoms and/or death in study subjects was related directly to their set point plasma viral load at study entry. HIV-1 infected men with set point viral loads that were relatively low tended to survive the longest and as set point viral load levels increased, survival potential decreased. Collectively, these data suggest that if virus-specific CD8⁺ T-cell responses early in HIV-1 infection can be improved through therapeutic immunization, this may impact viral load set point or improve control of virus replication and translate into a measurable clinical improvement.

The concept of therapeutic immunization as a treatment for HIV-1 infection was first proposed by Jonas Salk in 1987 [6]. Since then, a wide variety of candidate therapeutic HIV-1 vaccines have been tested in Phase I clinical studies in HIV-1-infected individuals [7]. However, until recently, a therapeutic immunization regimen capable of providing clear clinical benefit in HIV-1-infected individuals had not been described. In December 2004 the results of a clinical study were published suggesting that a therapeutic dendritic cell vaccine could, in fact, affect a clinical benefit in people with chronic HIV-1 infection [8,9]. Lu and colleagues, using a cohort of 18 subjects with untreated chronic HIV-1 infection, prepared autologous monocyte-derived dendritic cells and pulsed them with whole, chemically inactivated autologous HIV-1. These HIV-1-pulsed dendritic cells were then injected into the patients at three different timepoints and immunological and virological parameters were assessed for 1 year following vaccination. The investigators found that there was a substantial decrease in mean plasma viral loads after immunization and this effect was maintained for at least 1 year after the initial immunization.

Lu and colleagues also described the HIV-1-specific immune responses that were correlated with, and most likely responsible for, the significant reduction in plasma viral loads following therapeutic immunization [9]. They found that patients with the greatest reduction in plasma viral load also tended to demonstrate higher frequency HIV-1-specific CD4⁺ and CD8⁺ T cells. Importantly, this clinical study serves as a ‘proof-of-principle’, demonstrating that therapeutic immunization could be a promising strategy for the treatment of HIV-1 infection.

So where does the therapeutic HIV-1 vaccine field go from here? The next step for the field should be to translate this early success into a more feasible vaccine approach. A feasible therapeutic vaccine approach would, as Lu and colleagues suggest, induce robust HIV-1-specific cellular immune responses in both the CD4⁺ and CD8⁺ T-cell compartments and simultaneously target multiple HIV-1 structural and regulatory components to achieve adequate breadth [9]. However, unlike the autologous dendritic cell approach, a feasible therapeutic vaccine approach will need to be easily manipulated, easily administered and compatible with regular booster administrations. These characteristics are perhaps best met by a multi-antigen plasmid DNA (pDNA)-based vaccine.

To this end, a number of Phase I clinical trials have been conducted evaluating pDNA-based vaccine approaches for the treatment of HIV-1 infection [10–14]. Similar to many prophylactic pDNA vaccine trials conducted to date, these therapeutic vaccines appeared safe and well tolerated. Unfortunately, these early therapeutic pDNA vaccines were only moderately immunogenic and unable to affect a measurable clinical benefit. While great strides have been made with regards to improving the in vivo performance of current generation pDNA vaccines, perhaps the single best way to further improve pDNA vaccine performance in the clinic will be in the area of pDNA vaccine delivery.

In vivo electroporation (EP) has emerged as a delivery technology capable of dramatically improving the intracellular delivery of gene-based vaccine and small molecules [15]. For pDNA vaccine delivery, one typically injects the pDNA vaccine into the target muscle using a standard needle and syringe. This is then followed by the application of brief electrical impulses. The electrical impulses result in the creation of pores in the target cell membrane, which facilitate the intracellular delivery of the vaccine resulting in increased gene expression and increased vaccine-specific immune responses.

Recently, Luckay and colleagues clearly demonstrated the extent to which in vivo EP can improve the induction of pDNA vaccine antigen-specific cell-mediated and humoral immune responses [16]. In this study, rhesus macaques were immunized with three pDNA vaccine vectors encoding multiple HIV-1 antigens in combination with a fourth plasmid encoding the immunostimulatory cytokine interleukin (IL)-12. For macaques receiving the pDNA vaccine by standard intramuscular injection, a relatively high 10 mg total plasmid DNA dose (8.5 mg vaccine pDNA in combination with 1.5 mg IL-12 pDNA) was used. For macaques receiving the pDNA vaccine in combination with in vivo EP, one-fifth of the total vaccine dose (1.7 mg vaccine pDNA in combination with 0.3 mg IL-12 pDNA) was used and EP was performed using a two-needle array and mild electrical conditions (voltages of less than 70 V/cm). The results of this study demonstrated that a multiantigen pDNA vaccine, when delivered in combination with in vivo EP, led to a faster onset and stronger cellular responses, as measured by IFN-γ enzyme-linked immunosorbent spot (ELISPOT) assay. For example, at 8 and 22 weeks after the final pDNA immunization, there was a ten- and 40-fold increase, respectively, in HIV-1-specific ELISPOT responses in macaques receiving the pDNA vaccine in combination with in vivo EP, compared with macaques receiving the same pDNA vaccine without in vivo EP. Given the fact that the macaques immunized in combination with in vivo EP received one-fifth of the total pDNA dose, this translates into an apparent 50- or 200-fold increase in pDNA vaccine potency, respectively. Importantly, in vivo EP was especially efficient at enhancing the cell-mediated immune response against the less immunogenic antigens contained within the vaccine (nef-tat-vif), resulting in a more balanced immune response overall. Macaques receiving the pDNA vaccine in combination with in vivo EP also showed a significant increase in total HIV-1 env gp120-specific serum immunoglobulin G antibody titers relative to macaques receiving a fivefold higher dose of pDNA in the absence of in vivo EP.

For pDNA vaccine delivery into humans in the context of in vivo EP, issues related to safety and tolerability are paramount. Luckay and colleagues have demonstrated that a dramatic increase in pDNA vaccine immunogenicity can be achieved in nonhuman primates with in vivo EP. Importantly, in this nonhuman primate study, in vivo EP was performed using a minimalistic electrode configuration and using in vivo EP conditions shown to be tolerable in unanesthetized human volunteers [17].

With the demonstration that a highly experimental dendritic cell-based vaccine can affect a clear and measurable clinical benefit in untreated, HIV-1-infected patients [9], the notion of using therapeutic immunization to treat and limit the spread of HIV-1 infection has taken a quantum leap forward and now appears to be an achievable goal. In addition, in vivo EP has emerged recently as a delivery technology with the potential to greatly improve the clinical performance of pDNA vaccines. Given the ease of manufacturing, stability and potential for unlimited booster immunization, the research and development of pDNA vaccines for use as therapeutic vaccines for treatment of HIV-1 infection appears to have the greatest potential for worldwide success.