Current HIV Research - Volume 1, Issue 2, 2003

Human immunodeficiency virus type 1 (HIV-1) can infect quiescent cells, however, viral production is restricted to actively proliferating cells. Recent evidence has indicated that HIV-1 viral proteins, Vpr and Tat, perturb the cell cycle to optimize HIV-1 replication. Vpr arrests the cell cycle at G2 by inactivating the cyclin B / cdk1 complex. Tat regulates the cell cycle by altering factors involved in proliferation and differentiation (i.e. the cdk inhibitor p21 / waf1) and associating with cyclin / cdk complexes (i.e. cyclin E / cdk2, cyclin H / cdk7, and cyclin T / cdk9). These studies indicate the importance of host cellular factors, such as cyclin / cdk complexes, in regulating HIV-1 replication and therefore represent novel targets for antiviral therapeutics.Recently, the efficacy of pharmalogical cdk inhibitors (PCIs) in abrogating viral replication has been under development. To date there are 25-30 PCIs that have been synthesized against known cdks, several of which have been shown to inhibit HIV-1 and other AIDS-associated viruses in vitro and in vivo. Targeting these critical cyclin / cdk complexes needed for viral propagation may solve the problems inherent in current HAART therapy, including the emergence of drug-resistant viruses. Thus, PCIs have the potential to become novel therapeutic antiviral drugs that can inhibit HIV-1 transcription and opens the possibility of new avenues of treatment.

One of the major obstacles to effective prolonged CD8 T cell control over HIV and other latent infections may be the intrinsic, genetically programmed barrier to unlimited proliferation that is characteristic of all normal human somatic cells. Replicative senescence, characterized extensively in cell culture for a variety of cell types, comprises both irreversible cell cycle arrest and striking changes in function. CD8 T cells with features similar to senescent CD8 T cell cultures (i.e., absence of CD28, inability to proliferate, telomeres in the 5-7 kb range, resistance to apoptosis) increase progressively during aging and in chronic HIV infection, suggesting that replicative senescence may be occurring in vivo, and, in fact, may constitute the final stage in the normal differentiation of human T cells. CD8 T cells with characteristics suggestive of senescence have also been implicated in modulating immune function and altering bone homeostasis. Further characterization of the underlying mechanism leading to the generation of senescent memory CD8 T cells and analysis of their functional attributes will help elucidate their role in HIV disease pathogenesis.

Upon binding to the CD4 receptor the HIV envelope protein undergoes conformational changes that culminate in the fusion of the viral and cellular membranes. A few hours later, a sophisticated set of processes is initiated to ensure the down-modulation of the viral receptor. Three viral proteins participate in this process: Nef, Env, and Vpu, suggesting that this function is critical for virus replication. The mechanisms of action of these proteins have been extensively characterized. However, the physiological relevance of the virus-induced CD4 down-modulation remains a focus of controversy, and the impact of this function on the viral life cycle has been underestimated. This review summarizes current hypotheses explaining why HIV needs to reduce expression of its own receptor, and discusses the experimental evidence supporting them. Recent findings indicate that efficient CD4 down-modulation is essential for the production of infectious particles, and highlight the importance of this function in HIV pathogenesis in vivo. Progression to disease correlates with enhanced viral induced CD4 down-modulation, and a subset of long-term nonprogressors carry viruses defective in this function. To date, the HIV-induced CD4 down-modulation has not been targeted for therapeutic intervention. Addressing the reasons why this function is so critical and understanding the interplay between viral and host factors governing surface expression of CD4 may provide clues for the development of new antiviral strategies.

Human allelic variants influence the susceptibility to HIV-1 infection and/or the subsequent rates of disease progression towards AIDS that average ten years, although they vary greatly among infected subjects. In this respect, studies involving multiply exposed persons who remain uninfected, long-term nonprogressors (who remain asymptomatic for fifteen years or more) or, in contrast, rapid progressors (who develop AIDS within two to three years post-infection) as well as seroincident cohorts of patients with defined seroconversion dates have contributed to our comprehension of the effects of different natural human polymorphisms on HIV-1 disease. The current article aims at providing an up-to-date review on these polymorphisms that may be broadly classified into three general categories: (1) those that control viral entry into susceptible cells (namely, chemokine and chemokine receptor polymorphisms), (2) mutational variants of genes involved in immune regulation, such as interleukin-10 (IL-10), interleukin-4 (IL-4), tumor necrosis factor-alpha (TNF-alpha), and mannose-binding lectin (MBL), and (3) polymorphisms in genes involved in the adaptive immune recognition by T cells, [human leukocyte antigen (HLA) type]. Particular emphasis has been placed on the state-of-the-art biotechnological methodologies, such as “spectral genotyping” that utilizes molecular beacons in conjunction with polymerase chain reaction in real-time (real-time-PCR), which were developed to assist with the characterization of some of these determinants. Elucidating the functional role of these factors via the application of such biotechnological assays is expected to further enhance our understanding of the pathogenesis of HIV-1 infection, and, eventually, to enrich our therapeutic arsenal with novel antiviral agents or strategic approaches.

Treatment of human immunodeficiency virus (HIV)-1 infection with potent antiretroviral medications has provided considerable clinical benefit. However because of the limitations of current therapy, innovative approaches are needed to better control HIV-1 infection. Several studies have suggested that robust CD4+ T helper and CD8+ T cell responses may contribute to the immunologic control of HIV-1 infection in certain individuals. Most chronically infected patients, however, cannot control the infection and may benefit from stimulation of cellular immunity with immunotherapy. Dendritic cells (DCs) are potent professional antigen-presenting cells (APCs) and have a central role in directing the adaptive immune response to pathogens. The ability of DCs to stimulate naïve T cells has long been thought to be crucial in initiating an effective immune response. As DCs are uniquely situated at the interface between the innate and adaptive immune systems, they are currently under intense scrutiny as potential adjuvants for vaccines in many clinical settings. Studies in healthy volunteers and patients with cancer have shown that antigen-pulsed DCs can boost both CD8+ and CD4+ T cell responses in vivo. Based on these promising findings, ex vivo antigen-pulsed DCs are being actively investigated in studies aimed at developing a therapeutic vaccine for individuals with HIV-1 infection.

HIV-1 coreceptor usage is believed to play a critical role in pathogenesis. To initiate infection, HIV-1 interacts with two cell surface receptors, CD4 is the primary receptor and the β-chemokine receptors CCR5 and CXCR4 usually serve as secondary receptors. HIV-1 strains transmitted in vivo generally use CCR5. Viruses that use CCR5 (R5 viruses) appear to be associated with relatively stable infection. Years after chronic infection is established, CXCR4 utilizing strains emerge in ;50% of infected individuals. Viruses that use the coreceptor CXCR4 (X4 viruses) are associated with rapid CD4+ cell decline and disease progression. However, the mechanism by which X4 viruses are associated with accelerated disease progression has never been properly elucidated. For example, the association between X4 virus and acceleration of HIV-1 disease progression has been ascribed to the expanded spectrum of CXCR4+ precursor cells susceptible to infection by X4 strains. It has also been postulated that the decline of the host immune system associated with clinical AIDS may allow X4 viruses to evolve and replicate freely in late-stage infection. Discriminating between these and other alternatives is central to increasing our understanding of the fundamental pathogenic processes involved in HIV-1 infection. In this article, we critically review those studies published over the last few years that purport to examine the relationship between HIV-1 coreceptor usage, transmission, CD4+ T-cell depletion, and disease progression.

The failure to develop vaccines to protect against important infectious diseases such as human immunodeficiency virus type I (HIV-1) or Hepatitis C virus (HCV) has increased the interest in new vaccine strategies. One of these methods is immunization with an attenuated recombinant viral vector expressing a foreign antigen, which could protect individuals from later exposure to the respective pathogen. A new method to recover a non-segmented negative-stranded RNA virus (NNSV) from cDNA was described for the first time for rabies virus (RV), a member of the rhabdovirus family. The same approach was successfully used for another rhabdovirus, vesicular stomatitis virus (VSV), and opened the possibility to use rhabdoviruses as vaccine vehicles and biomedical tools. Further research showed that the genomes of rhabdoviruses are highly flexible, easy to manipulate, and able to express large and even multiple foreign genes, and therefore are excellent vaccine candidates. In addition, it has been shown for both RV and VSV that their single surface glycoprotein G, which is responsible for attachment and fusion to the host cell, can functionally be replaced by other viral or cellular glycoproteins. This review gives an overview of the use of RV and VSV as promising new candidates in the fight against HIV-1 and other human diseases.

Foremost amongst human pathogens, the human immunodeficiency virus (HIV) exhibits a great genetic variability. The resultant fluidity of HIV enzymatic proteins allows them to remain functional whilst simultaneously evading immune surrveillance and antiretroviral therapy. This very variability, however, has been turned to powerful advantage in the study of the movement and evolution of HIV strains within and between human populations. Molecular analyses that estimate the relatedness between viral isolates, conducted in tandem with epidemiological studies, provide a new clarity of insight into the modes and routes of HIV transmission and epidemic spread. In this paper the principles underlying the molecular study of HIV and the achievements of this new field of epidemiology in southern and eastern Asia are reviewed.

In recent years, CD4 and CD8 T cell responses to HIV and SIV infection have been increasingly measured with the use of single-cell assays such as ELISPOT, MHC-peptide oligomers, and cytokine flow cytometry. The results of these assays have been compared to those obtained with traditional bulk assays such as lymphoproliferation (by 3H-thymidine incorporation) and cytotoxicity (by 51Cr release). Such comparisons have led to some general understanding of the T cell responses that characterize progressive disease, long-term non-progressors, and individuals with viral suppression achieved by anti-retroviral therapy. In addition, prophylactic and therapeutic vaccine trials have also begun to use these assays of T cell immunity to gauge the immunogenicity of the vaccines. Whether such analyses will allow us to pick the best vaccine constructs, and whether they will provide us with an improved understanding of what constitutes protective cellular immunity to HIV, are major questions for the field. These questions will be examined in this review from the standpoint of current data and comparisons to other viral diseases. It is hypothesized that sophisticated multiparametric assays will be required to sort out the factors relevant for protective immunity in this complex disease. These parameters may include functional avidity, epitope breadth and specificity, proliferative capacity, cytokine repertoire, degree of anergy, and differentiation phenotype, as well as magnitude, of HIV-specific CD4 and CD8 T cells.