A fourth strategy to contain the threat of HIV and AIDS?A recent article by Dr.Philip Rosenburg in Science suggested that HIV (Human Immunodeficiency Virus), the retrovirus widely accepted to be the causative agent of AIDS (Acquired ImmunoDeficiency Syndrome), is endemic within the population of the United States. These data suggest that certain strains of HIV-1, the most prevalent form of HIV in the Western world, will continue to infect large numbers of individuals in both the heterosexual and homosexual populations for the foreseeable future. This excludes the possible impact of the introduction of more virulent strains of the virus such as HIV-1E, the form of the virus considered to be responsible for the recent epidemic in Thailand. HIV-1E, in contrast to the HIV-1B strain which was responsible for the epidemic in the U.S.A. and Europe, is believed to be transmitted predominantly via heterosexual intercourse.
Several approaches for the treatment of HIV and AIDS have been employed in clinical trials, or are currently under investigation and development. The first of these centered upon the presence of reverse transcriptase (RT) in retroviruses, an enzyme that produces a DNA copy of the virus' genetic code which is stored as RNA, the form of genetic code in which the cell communicates instructions for the synthesis of proteins. Reverse transcriptase is normally exclusively found in retroviruses, or in cells they have infected. Consequently Wellcome's azidothymidine (AZT), which prevents RT from making DNA copies from viral RNA in vitro, seemed an ideal way of preventing the virus from replicating itself. Unfortunately, in clinical trials AZT has shared a similar fate to other drugs which were designed to inhibit the action of reverse transcriptase and other viral proteins, including tat, which are responsible for the virus' capacity to self-replicate. The explanation for the virus's evasion of these 'smart' inhibitors, either alone or in combination, lies in nature's flawed design of reverse transcriptase. This protein, unlike our DNA copying proteins known as polymerases, does not have a built in editing function, and consequently it makes frequent mistakes when copying the virus's genetic information. Many of these errors result in transcripts that do not contain useful instructions for the production of viral protein. However, in the course of the millions of copies of the virus that are produced during an infection, some of these mistakes may result in structural changes of the virus' coat, structural and replicating proteins. Alterations in the form of reverse transcriptase and associated viral replication proteins make the 'smart' drugs ineffective in checking the advance of the virus, whilst others may mean that antibodies produced by the immune system no longer recognise and neutralize viral particles. To this extent, nature's stratagem of trial and error overwhelms the body's more reliable defense mechanisms, and as predicted by Darwin, these drugs effectively select for resistant forms of HIV.
A second strategy has been to develop vaccines directed against specific viral proteins, and in particular the coat glycoprotein gp120 of the virus, which through docking onto the CD4 receptor present upon specialized cells of the immune system, triggers entry of the virus into the cell1. Vaccination of primates against HIV or the related SIV with viral coat proteins or 'killed' whole virus aims to prevent infection by stimulating the B-cells of the immune system to generate antibodies that specifically recognise these viral docking proteins. Others have suggested injection of the purified CD4 receptor molecules to serve as decoy targets for the free viral particles, thereby preventing the virus from entering into the cell and allowing the body's defenses sufficient time to identify and destroy the intruder. However, this vaccination strategy does not necessarily overcome the capacity of the virus to evade the primed defense system through mutation, or eliminate the possibility of subsequent infection by more virulent forms of HIV.
A more recent strategy revolves around the observation that long-term survivors of HIV-1 infection have elevated numbers of so-called killer T-cells, which are characterized of the immune response that destroy cells infected with the virus and viral particles captured by antibodies. The proposal is to introduce agents or hormones into the body that stimulate CD8 killer T-cell activity, such as monoclonal antibodies against interleukin-4 or alpha-interferons. Further studies are attempting to elucidate which cellular defense mechanisms are responsible for the remarkable number of long-term survivors of HIV infection. As many as 20% of homosexuals infected with HIV-1 within a study group based in Amsterdam remained free from AIDS-related symptoms for up to eight years after being diagnosed as HIV-positive. Similar reports of long-term infection with HIV-2 without the appearance of AIDS-related symptoms have been reported in African prostitutes. These and other cohorts of long-term survivors are being extensively investigated in an attempt to try and determine the cellular basis for this acquired or innate resistance. If the cellular defense mechanisms that are responsible for preventing HIV from overwhelming the immune system can be elucidated, then a cure for AIDS may yet become a reality. However, a proportion of these infections may in fact result from less virulent forms of the virus, casting an interesting shadow over apparent cases of acquired or innate resistance to HIV.
One further strategy becomes apparent from our understanding of the genetic make-up of HIV and the now readily available tools of genetic engineering. This approach combines the idea of priming the immune system with a virus recognizable in its external form as HIV, and nature's principles of competition and survival of the fittest. The key to this approach is in our understanding of the biology of HIV; the principle is to engineer a 'good', or rather a benign virus, one which will not undergo mutation and assume a more threatening form after infection, or spread through the system with the speed and devastation of our more familiar adversary. To concept is to use this knowledge to engineer a virus which will not use this mechanism of replication and subsequent devastation of the immune system. Thanks to the pioneering work of Drs. Gallo and Montagnier isolated HIV can be grown and maintained in culture where its properties may be screened. This technique becomes important in screening the remarkable number of more malign and benign versions of HIV that have sprung from the inherent unreliability of HIV's self-replication through RT. The first step in engineering a 'good' virus is the construction or discovery of an existing mutant form of reverse transcriptase which does not make mistakes in the replication of the virus' genetic code. This strategy requires the screening of a large number of man-made and naturally occurring variants of reverse transcriptase in the test tube to quantify how accurately they replicate genetic information. The genetic sequence that defines this variant can be cloned and introduced into a new construct of HIV that does not mutate itself further after infection. Another feature of HIV that separates it from other retroviruses is its capacity to kill the CD4 receptor-bearing T-helper cells of the immune system that it infects. One of the mechanisms of this cytotoxity results from the fusion of the membranes of many CD4 positive T-helper cells to form a giant mass of cells known as a 'syncytium', through a mechanism that is similar in nature to the fusion of the virus' outer membrane and the cell membrane by the interaction of the viral gp120 coat glycoprotein and the CD4 receptors present upon T-helper cells. Certain variants of HIV are not as effective in producing these lethal syncytia. Again the cell culture system comes to our aid as it allows us to screen for such HIV variants which do not induce the formation of syncytia. The region of the genetic code of HIV responsible for this relatively benign form of the virus may be cloned and incorporated into our man-made viral construct. It may be the case that such a mutant gp120, as may well exist or be designed by structural biologists will also slow the rate of viral entry into the cell and consequently also the rate at which it spreads in the body, giving the immune response a more gradual time in which to organize its defenses against a man-made HIV variant. HIV in its familiar form multiplies rapidly resulting in high levels of free virus in the bloodstream, a condition known as a viremia, wherein the body's immune system is obviously overwhelmed by the rate of viral proliferation. Another unique feature of HIV and the related HTLV (Human T-cell Lymphotrophic Virus) family of retroviruses is the release of the viral protein tax from HTLV-infected cells. This viral protein tax, and its HIV-related version tat, stimulate nearby T-cells to express a receptor for a growth factor known as Interleukin-2 (IL-2). This results in an abnormal rate of T-cell growth stimulated by IL-2 in the vicinity of the virus, thus accelerating the rate of HIV proliferation. However, tax/tat is also important in the turning on of the virus' structural genes, an important step in viral proliferation. Hence, the strategy would be to isolate or construct a mutant of tat which was ineffective in switching on T-cell IL-2 receptors, but which does not completely prevent the virus from multiplying within the T-cell. A further strategy would be to select for HIV mutants which multiply at a slower rate in culture, possibly due to less potent growth promoting properties of the long terminal repeat regions of HIV's genetic code.
The construction of such a 'good' virus from all the less virulent variants of the genes that make up a fully functional virus will probably not cause the precipitous fall in the number of CD4 receptor-bearing helper T-cells that is a characteristic portent of the onset of 'full-blown' AIDS. This would be achieved through reducing the IL-2 stimulated proliferation of CD4 positive T-cells by a selected form of tat, slowing the rate of viral entry into cells and decreasing cell-to-cell fusion through a more benign form of gp120; by reducing the rate of viral replication and the number of associated errors by selecting for or constructing a slower and more accurate variant of reverse transcriptase, and by the creation of a more slowly assembling form of the virus through less active forms of tat and rex2. One could imagine that such a 'good' virus would prime the immune system without depleting the body's subset of CD4 -positive helper T-cells, the AWACS of the immune surveillance system, allowing the immune system to hold the virus at a persistent level of infection. Such a persistent level of infection with the benign form of the virus may slow the rate of expansion of subsequent infections with more virulent strains of HIV by competing for available sites of infection in T-cells whose defense mechanisms would already be primed. Through the activation of both antibody producing B-cell and killer and helper T-cell populations, any new infection with a more virulent strain of HIV might be quickly and successfully suppressed. The mechanisms of adaptation and survival found in the long-term survivors of HIV may have an opportunity to develop in the systems of those individuals who have been primed by chronic infection with the constructed virus, and such a long-term infection may stabilize without the dramatic decline in CD4 positive cell population that heralds the onset of AIDS. Who knows, perhaps the genetic code of HIV will be consigned, like those of many previous viral invaders, to the graveyard of the vast non-coding regions of the human genome.
Addendum (8th February 2003): Vaccine development has since become a major focus of scientific and economic attention with promising results. It might be noted that resistance genes have since been discovered, most notably the delta 32 mutation of the CCR5 HIV entry co-receptor which confers complete apparent resistance to HIV entry and infection within an estimated 1% of the U.S. population. These individuals appear to be homozygous for this delta 32 mutation, which may have originated within bubonic plague resistant survivors from Europe. However the incidence and frequency of this gene appear initially insufficient to explain alone the high (circa 20%) survival rates amongst infected HIV cohorts, in essence supporting the existence of other resistance mechanisms, such as those proposed above.
In addition to HIV infection via blood and exchanges of infected fluids through sexual contact, a third route of transmission may have been overlooked. Just as malaria, encephalitis, Yellow Fever and the West Nile Virus are propagated through mosquito bites and a mammalian/avian vector, so it may well be that the high incidence of HIV infection rates in South-East Asia, Equatorial Africa and Florida (which paradoxically has 18.5 percent of residents aged 65 or older, the highest in the U.S.) may correlate with mosquito distributions as shown below. An RT-PCR analysis of the presence of HIV markers within ingested blood from mosquitos in these areas is necessary to test this hypothesis.
 Global Adult prevalence of AIDS |
 United States Adult Incidence of AIDS Cases |
 Distribution of Dengue Fever and Carrier Mosquito |
