HIV-1 vaccine development : tackling virus diversity with a multi-envelope cocktail

A major obstacle to the design of a global HIV-1 vaccine is viral diversity. At present, data suggest that a vaccine comprising a single antigen will fail to generate broadly reactive B-cell and T-cell responses able to confer protection against the diverse isolates of HIV-1. While some B-cell and T-cell epitopes lie within the more conserved regions of HIV-1 proteins, many are localized to variable regions and differ from one virus to the next. Neutralizing B-cell responses may vary toward viruses with different i) antibody contact residues and/or ii) protein conformations while T-cell responses may vary toward viruses with different (i) T-cell receptor contact residues and/or (ii) amino acid sequences pertinent to antigen processing. Here we review previous and current strategies for HIV-1 vaccine development. We focus on studies at St. Jude Children's Research Hospital (SJCRH) dedicated to the development of an HIV-1 vaccine cocktail strategy. The SJCRH multi-vectored, multi-envelope vaccine has now been shown to elicit HIV-1-specific B- and T-cell functions with a diversity and durability that may be required to prevent HIV-1 infections in humans.

Clade, country and region-specific HIV-1 vaccines : are they necessary?

Today, scientists are often encouraged to custom-design vaccines based on a particular country or clade. Here, we review the scientific literature and then suggest that the overwhelming endeavor to produce a unique vaccine for every world region or virus subtype may not be necessary.

Multi-envelope HIV-1 vaccine devoid of SIV components controls disease in macaques challenged with heterologous pathogenic SHIV

A central obstacle to the design of a global HIV-1 vaccine is virus diversity. Pathogen diversity is not unique to HIV-1, and has been successfully conquered in other fields by the creation of vaccine cocktails. Here we describe the testing of an HIV-1 envelope cocktail vaccine. Six macaques received the vaccine, delivered by successive immunizations with recombinant DNA, recombinant vaccinia virus and recombinant envelope proteins. Following vaccination, animals developed a diversity of anti-envelope antibody binding and neutralizing activities toward proteins and viruses that were not represented by sequence in the vaccine. T-cells were also elicited, as measured by gamma-interferon production assays with envelope-derived peptide pools. Vaccinated and control animals were then challenged with the heterologous pathogenic SHIV, 89.6P. Vaccinated monkeys experienced significantly lower virus titers and better maintenance of CD4+ T-cells than unvaccinated controls. The B- and T-cell immune responses were far superior post-challenge in the vaccinated group. Four of six vaccinated animals and only one of six control animals survived a 44-week observation period post-challenge. The present report is the first to describe pathogenic SHIV disease control mediated by a heterologous HIV-1 vaccine, devoid of 89.6 or SIV derivatives.

A multi-vector, multi-envelope HIV-1 vaccine

The St. Jude Children's Research Hospital (St. Jude) HIV-1 vaccine program is based on the observation that multiple, antigenically distinct HIV-1 envelope protein structures are capable of mediating HIV-1 infection. A cocktail vaccine comprising representatives of these diverse structures (immunotypes) is therefore considered necessary to elicit lymphocyte populations that prevent HIV-1 infection. This strategy is reminiscent of that used to design a currently licensed and successful 23-valent pneumococcus vaccine. Three recombinant vector systems are used for the delivery of envelope cocktails (DNA, vaccinia virus, and purified protein) and each of these has been tested individually in phase I safety trials. A fourth clinical trial, in which diverse envelopes and vectors are combined in a prime-boost vaccination regimen, has been FDA-approved and is expected to commence in 2007. This trial will continue to test the hypothesis that a multivector, multi-envelope vaccine can elicit diverse 8- and T-cell populations that can prevent HIV-1 infections in humans.

Alteration of the proline at position 7 of the HIV-1 spacer peptide p1 suppresses viral infectivity in a strain dependent manner

The HIV-1 spacer peptide p1 is located in the C-terminus of the Gag polyprotein and separates the nucleocapsid (NC) and p6(Gag). Research centered on p1 has been limited and as yet no function has been ascribed to this spacer peptide. We have previously found that the conserved p1 proline residues (position 7 and 13) are critical for replication in the HIV-1 strain HXB2-BH10. In this study we have focused on the proline rich p1-p6(Gag) C-terminus of HIV-1. We individually examined the role of p1 proline's in multiple strains of HIV-1 and investigated the role of three proline residues in p6(Gag) (P24, P25 and P30). Assessment of the HXB2-BH10 based mutants revealed that Gag-Pol incorporation relative to Gag decreased in the p1 mutant virions, with the double proline mutant the most impaired. Mutating both p1 proline residues was found to abolish infectivity in multiple strains of HIV-1. Independent mutation of the p1 proline at position 7 resulted in a strain-dependent suppression of viral infectivity. This defect correlates with the presence of a tyrosine residue at position 9 of p1 and occurs in the early phase of the HIV-1 replication cycle. The p1 proline residues were found to be functionally distinct from P24, P25 and P30 in p6(Gag). This work affords novel insights into our understanding of the role of p1 in HIV-1 replication.

The packaging and maturation of the HIV-1 Pol proteins

The Pol protein of human immunodeficiency virus type 1 (HIV-1) harbours the viral enzymes critical for viral replication; protease (PR), reverse transcriptase (RT), and integrase (IN). PR, RT and IN are not functional in their monomeric forms and must come together as either dimers (PR), heterodimers (RT) or tetramers (IN) to be catalytically active. Our knowledge of the tertiary structures of the functional enzymes is well advanced, and substantial progress has recently been made towards understanding the precise steps leading from Pol protein synthesis through viral assembly to the release of active viral enzymes. This review will summarise our current understanding of how the Pol proteins, which are initially expressed as a Gag-Pol fusion product, are packaged into the assembling virion and discuss the maturation process that results in the release of the viral enzymes in their active forms. Our discussion will focus on the relationship between structure and function for each of the viral enzymes. This review will also provide an overview of the current status of inhibitors against the HIV-1 Pol proteins. Effective inhibitors of PR and RT are well established and we will discuss the next generation inhibitors of these enzymes as well recent investigations that have highlighted the potential of IN and RNase H as antiretroviral targets.

Innate immunity and intracellular trafficking : insights for novel anti-HIV-1 therapeutics

It is now evident that host cells have evolved a remarkable variety of antiretroviral activities to defend themselves against viral invaders and in return viruses have developed ingenious ways to circumvent these defences and, in some cases, actually hijack cellular proteins in order to facilitate their replication. Study of this cat and mouse interplay between viruses and their host cells throughout evolution has lead to the identification of some of the most sophisticated antiviral strategies that mammals have developed to prevent viral infection. Recently, a wave of publications has significantly enhanced our understanding of the relationship between human immunodeficiency virus type 1 (HIV-1) and its host, including: 1) the HIV-1 protein Vif and its interaction with host cell nucleic acid editing enzymes; 2) the host cell restrictive factors that provide protection against retroviral infection, such as TRIM5; and 3) the late domains of retroviruses and their relationship with the host cell vacuolar protein sorting pathway. The focus of this review is to provide an up-to-date account of these important areas of HIV-1 research and highlight how some of these new discoveries can potentially be exploited for the development of novel anti-retroviral therapeutics.

X4 and R5 HIV-1 have distinct post-entry requirements for uracil DNA glycosylase during infection of primary cells

It has been assumed that R5 and X4 HIV utilize similar strategies to support viral cDNA synthesis post viral entry. In this study, we provide evidence to show that R5 and X4 HIV have distinct requirements for host cell uracil DNA glycosylase (UNG2) during the early stage of infection. UNG2 has been previously implicated in HIV infection, but its precise role remains controversial. In this study we show that, although UNG2 is highly expressed in different cell lines, UNG2 levels are low in the natural host cells of HIV. Short interfering RNA knockdown of endogenous UNG2 in primary cells showed that UNG2 is required for R5 but not X4 HIV infection and that this requirement is bypassed when HIV enters the target cell via vesicular stomatitis virus envelope-glycoprotein-mediated endocytosis. We also show that short interfering RNA knockdown of UNG2 in virus-producing primary cells leads to defective R5 HIV virions that are unable to complete viral cDNA synthesis. Quantitative PCR analysis revealed that endogenous UNG2 levels are transiently up-regulated post HIV infection, and this increase in UNG2 mRNA is ∼10–20 times higher in R5 versus X4 HIV-infected cells. Our data show that both virion-associated UNG2 and HIV infection-induced UNG2 expression are critical for reverse transcription during R5 but not X4 HIV infection. More importantly, we have made the novel observation that R5 and X4 HIV have distinct host cell factor requirements and differential capacities to induce gene expression during the early stages of infection. These differences may result from activation of distinct signaling cascades and/or infection of divergent T-lymphocyte subpopulations.