AIDS & HIV
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HIV (3D-rendered image) |
The world’s response to HIV has been described as the “fastest in medical history”, and now, 25 years of research into the HIV retrovirus and AIDS have given hope and quality of life to people diagnosed with HIV. Once HIV had been isolated, the discovery that infection with the closely related retrovirus SIV could cause a fatal immunodeficiency syndrome in monkeys was a key piece of evidence that helped persuade many in the medical community that HIV was the cause of AIDS.
The transmission of HIV to chimpanzees in the lab gave the first animal model of HIV. This was crucial to the development of blood tests allowing both diagnosis and the screening of blood donations, leading to an ELISA test for HIV, in which diluted blood serum is applied to a plate with HIV antigens attached. The test detects whether the blood contains HIV antibodies which bind to the antigens, and quickly became accepted to screen for HIV. A negative ELISA test is sufficient to show that HIV is not present in a sample, but a second test is used to confirm a positive result from the ELISA test.
> Control of HIV through anti-retroviral medication
> Current priorities for HIV research
> Approaches to vaccine development
> STEP and Phambili vaccine trials
> Why did the MRK-Ad5 vaccine fail?
> The next vaccine trials - PAVE 100
Control of HIV through anti-retroviral medication
1986 saw the first clinical trial of an anti-retroviral medicine. AZT had been studied in retroviral research in mice before the AIDS epidemic began, and experiments had shown it to have activity against retroviruses at doses which were non-toxic in animals1. The activity of AZT against HIV was soon confirmed in animals, and in 1987 it became the first anti-retroviral to be licensed by the FDA as a treatment for AIDS and HIV, offering hope to thousands.Soon other anti-retroviral drugs followed, which were more effective and less toxic than AZT, but the virus quickly developed resistance to all the compounds and treatment with combinations of drugs became the norm. A major advance came with the development of drugs that inhibit the activity of HIV-1 protease, an enzyme that is necessary for the formation of virus particles. Saquinavir entered clinical trials in 1991, and in 1995 became the first protease inhibitor to be approved by the FDA. During pre-clinical testing in vitro studies had shown it to have potent anti-HIV activity, while animal tests indicated that it had an acceptable toxicity profile and that clinically relavent doses could be achieved by oral dosing18 Other protease inhibitors soon followed, in particular the publication of the 3-D structure of the HIV-protease, determined by X-ray crystallography in 1989, guided the development of the more potent drug Indinavir. Over 150 lead and intermediate compounds were tested during the develiopment of Indinavir, and once again studies of oral bioavailability and toxicity in animals played a key role in identifying the compound that was taken into clinical trials19 Indinavir quickly bacame a standard componant of highly active antiretroviral therapy (HAART) that has greatly reduced AIDS related mortality in developed countries.
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Most patients now take a combination of around 3 anti-retrovirals each day to control HIV. © istockphoto/Angel Rodriguez |
By 1996 combination therapy, consisting of cocktails of anti-virals taken together, had increased life expectancy enormously and with early diagnosis and treatment life-expectancy was longer than the 10 years for which treatment had been available. While these drugs are not a cure, and need to be taken for life, the development of around 20 anti-virals which are effective against HIV is considered one of the great medical successes of the late 20th Century.
Anti-retroviral treatment has now turned HIV into what many consider to be a chronic, but manageable disease. In 2006 Stefano Vella, director of drug research and evaluation at the Institute Superiore di Sanita in Rome and former president of the International AIDS Society said: "Today, I can tell my patients with HIV that they can have a normal life expectancy." In February 2007, however, a study of more than 26,000 people who took anti-retroviral drugs showed that ‘normal’ life-expectancy was over-optimistic. Although mortality rates had progressively fallen since 1996, the life expectancy of a 20-year-old starting treatment in 2005 was 58 years2. In a more recent study life-expectancy was found to vary between social groups, but on average remained at 2/3 the normal life-expectancy of the general population for each country.17 While anti-retroviral drugs can allow HIV-positive people to lead full and active lives, it remains unlikely that they will live past middle age with current treatments.
HIV can infect chimpanzees, but the virus does not usually cause AIDS – a discovery which initially dashed hopes of a monkey model for this disease. Although there have been instances of chimpanzees developing AIDS3, these are very rare. However, it was soon discovered that macaques are susceptible to the retrovirus SIV, which causes a fatal immunodeficiency syndrome, and is closely related to HIV4. SIV is sensitive to similar drugs to HIV and these similarities have been used to develop and test many antiviral medications.
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| Electronmicrograph of HIV-1 virions (green spheres) budding from a lymphocyte. © CDC/C. Goldsmith, P. Feorino, E. L. Palmer, W. R. McManus |
HIV and SIV are from the genus lentivirus, named for the slow course of disease progression, often remaining unnoticed for years before causing disease. The AIDS-like condition caused by SIV in macaques affects the nervous system and behaviour in addition to infecting and damaging T-cells of the immune system.
There were initial concerns that SIV in the macaque model would not behave like HIV in humans. Part of the reason for this speculation was that studies showed an immediate and rapid decline in the levels of T cells in the gut mucosa of infected macaques. After only a week the T cell count in the gut mucosal tissue appeared to be only half that of uninfected macaques. This was not thought to happen in humans, suggesting that SIV was significantly different to HIV5. However, when scientists examined mucosal tissue in human studies and it was realised that T cells in the guts of many patients all but disappear after HIV infection6, 7. This was a very important discovery as for the first time it became clear that SIV and HIV could quickly destroy large populations of CD4+ T-cells, and subsequently they showed that this is because the virus targets CD4+ cells that are in an activated state, cells that are particularly common in the mucosa where they form part of a defensive barrier against infection. This knowledge has greatly improved scientists understanding of how HIV infection eventually leads to the collapse of the immune system20. The observation that sexually-transmitted HIV and SIV initially target T-cells of the mucosal tissue has significant implications for vaccine research, and vacines are now under development that are designed to block this initial onslaught21.
It is now known that there are two strains of HIV, HIV-1 and HIV-2. There are over 11 strains of SIV, each affecting different species of monkeys. SIVs do not usually cause disease in their natural hosts, but most strains of SIV cause an AIDS-like illness in macaques. HIV-1 is most similar to the chimpanzee strain of SIV, which it is believed to have mutated from, jumping species to infect humans. The virus is particularly prone to mutation, so even a single individual will have variations in the RNA structure of the HIV they carry over their lifetime. The amino acid structure of an individual HIV-1 protein has around 95% similarity with the same viral protein isolated from another individual. The similarity with the corresponding SIV protein isolated from a chimpanzee will be about 85%.
HIV-2 is a less common form of the virus, and has far fewer similarities with the chimpanzee strain of SIV. HIV-2 is like the SIV strain found in sooty mangabeys, and its structure has greater similarities to this SIV virus than to HIV-1.
Concerns include halting the spread of the epidemic, and access to anti-retroviral treatments, which are still very expensive, making them difficult to obtain in poorer countries. In developed nations the greatest threat to people with HIV is TB, which is becoming increasingly common and resistant to treatment. The WHO has major campaigns to provide worldwide access to anti-retrovirals by 2010 and to support the fight against TB for people living with HIV.
The need for a vaccine to protect populations is clear, and difficulties controlling the spread of HIV, particularly in sub-Saharan Africa, combined with the high cost of anti-retroviral drugs, have driven the need for an HIV vaccine.
Another important area of research is prophylaxis. This is the use of drugs to reduce the risk of infection following exposure to HIV. Information on HIV prophylaxis is available here.
There are currently over 30 potential HIV vaccines in development around the world. Despite the optimism about vaccination in the early days of HIV,8 it was soon realised that developing a vaccine would pose more technical difficulties than anticipated. While a vaccine based on the viral envelope protein was developed quickly using genetic engineering techniques, it failed to prevent infection.
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| ©istockphoto/José Carlos Pires Pereira |
No virus studied before behaves like HIV, which is very prone to mutation. This characteristic helps it to evade the body’s immune response, as the antibodies generated by vaccines are only able to recognise specific regions of the viral surface. The viral envelope of HIV is also very flexible, and changes shape as it attaches to T-cells and prepares to infect them, making it difficult for antibodies to bind to the virus and stop the process. However, the past 25 years of research have given several leads to an effective HIV vaccine. Regions of the virus which are very stable and do not tend to mutate have been identified as potential targets for vaccines. Despite the difficulties, antibodies are able to target particular regions of the viral envelope protein.
Research showing that: human antibodies can neutralise HIV in animal models9 ; some people have immune responses which are able to control the virus for years without developing AIDS; and vaccines are effective against SIV in macaques10; all indicate that developing an effective vaccine is possible.
Approaches to vaccine development
Many approaches have been investigated in the search for a successful HIV vaccine. The classical approach to vaccine development is to use an inactivated form of the virus or part of the virus, such as the viral envelope protein, to induce an immune response, generating antibodies which will recognise the virus and prevent an infection from establishing itself in the body. The outer envelope protien of the virus, gp120, was an obvious target and a purified form was developed to be used as a vaccine. Unfortunately this type of vaccine failed to protect immunised chimpanzees from the virus,11, 12 and other strategies were needed.
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X-ray crystallographic image showing the broadly neutralizing antibody b12 (green ribbon) in contact with a critical target (yellow) for vaccine developers on HIV-1 gp120 (red). © NIH |
A newer method of creating a vaccine is DNA immunisation which uses viral DNA, either alone or with a chemical carrier, to generate an immune response. The response to this type of vaccine in primates was much lower than predicted in small animal studies, and had limited success in humans.13 To be effective, antibodies must fit the specific, vulnerable, regions of the viral envelope, and it is rare that people make a sufficient quantity of these antibodies to protect against HIV. This has led to a mortality rate near to 100% if HIV is untreated, and the failure of vaccines to protect against infection. A new approach may be to find ways of stimulating these particular antibodies.
Live-attenuated vaccines are forms of the live virus which have been altered, either genetically, by mutating specific points on the RNA, or by other means, rendering them unable to cause disease. This type of vaccine gives a closer simulation of natural HIVinfection than inactivated viruses, and generates good immune responses. Live-virus vaccines can infect macaques without causing disease.14 Attenuated SIV vaccines have had mixed success in protecting macaques against the disease in challenge studies, where they are exposed to SIV after inoculation. In some studies animals were successfully protected15, but not in others.16
Live recombinant vaccines have also been developed, in which another virus is used to transfer genes from HIV or SIV into cells. The infection in these cells can then be ‘recognised’ by the immune system, leading to a full immune response. This has proved a relatively successful strategy, and a number of vaccines of this type have been developed and studied in human trials.
Live-attenuated and live-recombinant vaccines are able to fully activate the immune system in a way that is similar to natural HIV exposure. In addition to generating antibodies they are able to stimulate cytotoxic T- cells, a type of lymphocyte which lacks the CD4 receptor HIV uses to enter the cell, and so cannot be infected by HIV. The normal role of these cells in the body is to control a viral infection and kill virus-infected cells before they divide. There is good evidence that these cells control the HIV during the first stages of infection, preventing the development of AIDS, often for many years. Animal studies in macaques have shown that the ability of vaccines to stimulate cytotoxic T-cells has an impact on the spread of the infection, reducing viral load. Cytotoxic T-cells only attack cells which contain the virus, so they do not prevent infection, but they do reduce the impact of the virus on the immune system, and a low viral load can prevent transmission, reducing the spread of disease.
STEP and Phambili vaccine trials
The search for a vaccine against HIV recently faced setbacks when a vaccine developed by Merck, MRK-Ad5, was withdrawn from phase II b clinical trials in October 2007. This vaccine had already undergone a full development process, using non-animal and animal-based tests, and the first stages of trials in humans. It had stimulated cytotoxic T-cells strongly in animal studies, and it was hoped that it would prevent infection, and reduce viral load in people who became infected with HIV. The second set of phase II trials were designed to test how effective the vaccine would be. Two separate trials were run, one, known as STEP, took place in America and Australia, while a separate trial, ‘Phambili’, took place in South Africa.
Over 90% of the volunteers in the STEP trial were male, while volunteers in the Phambili trial were more evenly split between men and women. These trials were smaller than a usual phase III clinical trial, with analysis taking place after the first infections with HIV were seen in people who had received the candidate vaccine. This precaution was designed to reduce the risk to those taking part, and it was this analysis that raised concerns over safety.
An independent Data and Safety Monitoring Board (DSMB) studied the trial and concluded that the vaccine did not either prevent HIV infection, or reduce the amount of the virus in those who became infected. The DSMB recommended that no further immunisations were made in either trial.
By November both trials had been ‘unblinded’, so that the volunteers who took part were told whether they had received the trial vaccine or a placebo. Initial analysis of the STEP study showed that of 741 people who received a single dose of the vaccine 24 became infected with HIV. This was compared with 762 people who received the placebo, of whom 21 became infected with HIV. Although this result was not statistically significant, further analysis was carried out, and an increased likelihood of infection was found in relation to a particular group of male volunteers.
The MRK-Ad5 vaccine consisted of 3 HIV genes, carried into the body using a common cold virus, adenovirus-5, as a vector. The key HIV genes carried by the virus were hoped to be sufficient to give immunity to HIV, allowing the body’s T-cells to identify and kill cells containing the virus, without either the HIV genes or the adenovirus which carried them being able to cause disease.
However, male volunteers who already had strong immunity to adenovirus-5 prior to vaccination seemed more likely to become infected with HIV once they had received the vaccine (21 out of 392 men who had high levels of antibody to adenovirus, 9 out of 386 in the control group). The reason for the increased risk may have been unrelated to the vaccine, but since HIV infects immune cells, it was possible that the immune response generated by the adenovirus when they were vaccinated may have been responsible. New vaccines which do not use adenovirus-5 are now being developed.
For now, trials of vaccines using this vector will have strict selection criteria for volunteers, requiring a low level of immunity to adenovirus-5. Adenovirus-5 immunity is most common in Africa, where around 80% of people have a high level of antibody to the virus in their blood.
Why did the MRK-Ad5 vaccine fail?
In December 2007 a subcommittee of the NIH AIDS Research Advisory Committee met to discuss the STEP data and the future of HIV vaccine trials. They hoped to understand why the vaccine did not work as intended, since this will tell us more about HIV. The analysis of the pre-clinical data, from in-vitro and animal studies with MRK-Ad5, will help to re-design future tests and animal models used for assessing HIV vaccines.
The MRK-Ad5 vaccine was fully developed, and extensively studied in non-human primates before human trials began. Macaques were given a vaccine similar to MRK-Ad5, and were then infected with SHIV 89.6p, a hybrid SIV, genetically engineered to contain genes from HIV. Monkeys which received the vaccine and were then infected with SHIV had lower viral loads than those which did not receive the vaccine. A small group of the STEP volunteers who became infected did have a smaller than usual viral load in their blood, but the sample size was very small and since the trial ended early it is not known whether this is a significant finding.
A criticism of the primate studies is that MRK-Ad5 failed to prevent infection or have an impact on the SIV virus in macaques. The SHIV hybrid-virus retains key properties of HIV, but can cause disease in macaques like SIV. However, this model has limitations, and is frequently no more reliable than the SIV model at predicting how HIV will behave in humans. It is likely that future vaccines will be tested in challenge studies involving both SIV and SHIV.
Following the failure of the STEP discussion about future research has focused on the continued need for research and funding into the development of an HIV vaccine. The report into the withdrawal of the vaccine recommended that research should focus on the basic discovery which makes vaccine development possible, and the development of better animal models. This research includes the creation of antibodies which can neutralise HIV, and identifying what constitutes a protective immune response to this virus. As well as developing new animal models, researchers need to rate how predictive they are of how HIV behaves in humans and understand the limitations of each model better.
The next vaccine trials - PAVE 100
Another HIV vaccine candidate has yet to enter phase IIb clinical trials following the unblinding of the MRK-Ad5 trials. The trial design was halted in 2007 so that recommendations could be made following the STEP trial. The new vaccine also uses adenovirus-5 as a vector and following the analysis of the previous trial data, it was decided that a smaller, more focused study with strict criteria for volunteers would take place.
NIAID announced on the 17 July that the PAVE 100 trial would not go ahead, but that the smaller study designed would become a normal clinical trial phase in the development of vaccines for HIV.
There is still an urgent need for new treatments to control HIV, and it is clear that more basic research needs to be done. It is likely that research into HIV will return to academic laboratories and research institutes as new strategies are developed, and the problems highlighted by the recent trials are studied in depth.
July 2008
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Research Fields: Infection and Immunity, Disease characteristics, Drugs & toxins(yes - 3 items)Animals Used: Mouse, Primates(required - 2 items)
