The good news is that we are now witnessing the first decrease in HIV/AIDS mortality since the inception of the epidemic seventeen years ago.(1, 2) The chief cause of this turnaround is the ability of powerful new multidrug antiretroviral regimens to reduce viral load to undetectable levels in HIV-positive individuals. When triple-drug therapy was offered to the participants in one large clinical study, mortality dropped by about two-thirds, even in patients with CD4 counts below 100 cells/mm3.(3)
The bad news is that these potent new regimens pose some real problems. They are expensive and toxic, they demand strict adherence to complex dosing schedules, and they can actually promote the development of viral resistance, especially when compliance with therapy is less than optimal.(4) The biggest shortcoming of these combination therapies, however, is that they are incapable of clearing all the virus from an infected individual's body. As a result, these multidrug regimens must be taken daily -- perhaps for the rest of the patient's life -- to ward off the ravages of AIDS.
Because no antiretroviral treatment yet developed completely eradicates HIV, researchers are still looking for a treatment regimen that will deliver a knockout punch to the virus -- which remains sequestered in lymph nodes and in nondividing cells, including macrophages and lymphocytes, even after months of highly suppressive therapy (see "HIV found in lymph tissue of patients with 'undetectable' viral levels," Vol. 3, No. 6). Some recent developments in the study of chemokines and HIV co-receptors suggest a new avenue of investigation, one that offers significant hope for novel anti-HIV therapies.(5-8)
Over the past two years investigators have made a series of provocative discoveries about the process by which HIV infects cells (see "Long-term non-progressors may hold clue to vaccine," Vol. 2, No. 2; "Genetic mutation appears to confer immunity to HIV," Vol. 2, No. 5; and "How HIV penetrates CD4 cells," Vol. 2, No. 6). We now know that when HIV enters the bloodstream, it usually seeks out macrophages. Primary isolates of HIV are typically macrophage-tropic -- meaning that they will infect isolated macrophages in the laboratory but will not infect cultured T-lymphocytes.
Macrophage-tropic viruses enter cells using a complex of two cell-surface receptors, CD4 and CCR5. The former is familiar to all of us; the latter, an acronym for chemokine receptor 5, is a huge protein that traverses lymphocyte cell membranes and, when bound to certain peptides, facilitates the repair of inflamed tissue. Several additional chemokine- receptor proteins, all related to CCR5, also serve as co-receptors for macrophage-tropic HIV isolates.
Once HIV has entered a macrophage, it can remain sequestered there for years, protected from immune surveillance. Then, late in the course of infection, HIV cellular tropism broadens -- and in most patients a T-cell-tropic form of HIV emerges as a result of genetic changes in the virus population inside each patient. T-tropic HIV, which preferentially infects CD4 and other T-cells, replicates far faster and is considerably more virulent than macrophage-tropic HIV.
|"Several large epidemiological studies of people with clinically-defined AIDS have revealed that the CCR5-Delta32 allele, when inherited from both parents, confers nearly complete genetic resistance to HIV infection, even in individuals who are at extremely high risk."|
The T-tropic form of HIV enters and infects lymphocytes by means of a different combination of cell-surface receptors: CD4 and CXCR4. The appearance of these "hot" T-tropic viruses usually coincides with rapid T-cell depletion and is thought to contribute directly to the progression from asymptomatic HIV infection to C.D.C.-defined AIDS.
The crucial role played by these various chemokine co-receptors as unsuspecting collaborators in disease progression became evident when a common mutational variant of the CCR5 coding gene, CCR5-Delta32, was discovered in 1996.(9-11) Since then, several large epidemiological studies of people with clinically-defined AIDS have revealed that the CCR5-Delta32 allele, when inherited from both parents, confers nearly complete genetic resistance to HIV infection, even in individuals who are at extremely high risk of infection.(5, 11-14) A few CCR5-Delta32 homozygotes have become infected -- usually with the T-tropic, late-stage form of HIV -- but this occurrence is extremely rare. Individuals who inherit the CCR5-Delta32 allele from only one parent do become infected with HIV, but in such individuals the onset of AIDS-defining illness is postponed for two to four years longer than it is in individuals without the mutational variant.
A study published only last month indicates that this variant allele, CCR5-Delta32, also slows disease progression in children born to HIV-infected mothers. Because this mutation is a rare one, investigators were unable to determine if homozygosity for CCR5-Delta32 prevents HIV infection -- as it seems to do in adults -- but they did establish that heterozygosity substantially reduces disease progression (see "More evidence that genes confer protection against AIDS progression" in the Newsline section of this issue).
Additional genetic protection against disease progression has also been attributed to mutational variants in the genes for CCR2, a minor chemokine receptor that some strains of HIV employ to enter macrophages, and in the genes for SDF-1, a chemokine ligand that effectively blocks the entry of T-tropic viruses into cultured T-lymphocytes. (SDF-1 is an acronym for stromal-derived factor, a gene-encoding substance that normally binds to the T-tropic HIV co-receptor CXCR4 and prevents it from entering cells.)
|"Roughly 35% of the fortunate few who are known as long-term survivors carry one or more of the genes that confer resistance to AIDS progression. Conversely, about 40% of so-called rapid progressors, the unfortunate few who develop AIDS within four years of infection, do so because they lack genetic protection by any of these genes."|
The importance of SDF-1 in restricting disease progression in HIV-infected individuals has only recently been recognized (see the Newsline section of this issue). Genetic studies of thousands of HIV-exposed subjects show that roughly 35% of the fortunate few who are known as long-term survivors -- because, a decade or more after their initial exposure to the virus, they have yet to develop AIDS -- carry one or more of these three genes, CCR5, CCR2, and SDF-1. Conversely, genetic studies show that about 40% of so-called rapid progressors, the unfortunate few who develop AIDS within four years of infection, do so because they lack genetic protection by any of these genes.(15, 16)
Once it became clear that chemokine receptors actively mediate HIV infection -- and that both chemokines and certain mutations in cell-surface receptors block HIV infection -- efforts began to develop ways to mimic the genetic protection that these mutations afford.(17, 18) Interfering with the interaction between HIV and the chemokine receptors was an appealing approach to controlling the virus for several reasons. First, unlike HIV itself, which mutates at an astronomical rate and soon develops resistance to standard therapies, the cellular receptors do not mutate. Hence, compounds that prevent HIV from binding with cell-surface receptors should retain their effectiveness over time -- because they are insensitive to viral genetic adaptations that lead to drug resistance.
Second, CCR5-Delta32 homozygosity is not associated with particular pathology or with immune-system problems,(11) probably because the receptors are redundant in the genome, with several alternative versions recognizing the same chemokines.(8) These findings predict that therapies that block or abrogate chemokine-receptor availability will be safe to administer and will produce few adverse effects.
Third, the so-called 7-transmembrane receptors, the group to which chemokine receptors belong, are already familiar to pharmaceutical companies; a number of extant drugs, including several highly profitable ulcer medications, are inhibitors of 7-transmembrane spanners.(17) At the time CCR5-Delta32 was discovered, several major pharmaceutical companies were in the process of screening large stores of chemokine-receptor inhibitors, on the theory that these stores might yield new treatments for inflammatory diseases such as asthma, arthritis, and psoriasis.
|"At present more than a dozen biotech and pharmaceutical companies are actively investigating the HIV-chemokine receptor nexus, searching for drugs that will stem or even eradicate HIV infection."|
At present more than a dozen biotech and pharmaceutical companies are actively investigating the HIV-chemokine receptor nexus, searching for drugs that will stem or even eradicate HIV infection. Two derivatives of the CCR5 ligand are currently being tested, to see if they will block viral replication and spread in vivo.(19, 20) Others, which target the CCR5 and CXCR4 receptors, are also under development.
In addition, efforts have been directed at vaccinating people against specific motifs of these cell-surface receptors, in the hopes that vaccinated patients will produce antibodies to the receptors -- and these antibodies will then prevent HIV from gaining access to the receptors. Monoclonal antibodies that recognize the receptor are now being synthesized for use as potential treatments; they too would be used to block receptor access.
Another approach may be to use gene therapy to disarm the cell-surface receptors' availability to circulating HIV. This could be done with a gene construct that would bind to co-receptors inside the cell, preventing HIV from using the same receptors, or with an anti-sense gene structure that would bind to the receptor gene and prevent transcription in much the way the CCR5-Delta32 mutation does.
A provocative recent study hooked an SDF-1 ligand to a molecular tag that then attached the ligand to the endoplasmic reticulum.(21) When CXCR4, the T-tropic HIV co-receptor, was assembled, it was captured by this SDF-1 variant before it could be expressed on the cell surface. The cells were thereby protected, at least in vitro, from being infected by virulent T-tropic HIV.
In patients, gene-based treatments are not without uncertainties, particularly concerning toxicities and the half-life of treatment molecules in the body.(22) Nevertheless, the epidemiologic data -- which show that natural genetic mutations effectively postpone the onset of AIDS for many years by preventing the emergence of late-stage, T-tropic viral mutants -- are reassuring. The clinical effectiveness of these therapies has, in a sense, already been tested in the patient population at risk for AIDS, by Mother Nature herself.
In sum, dazzling recent advances in our understanding of the crucial roles played by chemokines, their receptors, and their genes in the pathogenesis of AIDS offer a panoply of new therapeutic possibilities. Perhaps these amazing developments prefigure the discovery of a whole new class of anti-HIV drugs in the near future. We shall see.
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Stephen J. O'Brien, Ph.D., is Director of Laboratory of Genomic Diversity, National Cancer Institute, Frederick, MD.