A close relative of HIV called SIV (simian immunodeficiency virus) causes an AIDS-like disease in susceptible monkeys. SIV-infected monkeys are often used to assess potential medicines and vaccines prior to testing in HIV-positive people.
For at least the past six years a team of researchers in the U.S. has been conducting experiments with SIV-infected monkeys, giving them intravenous infusions of an antibody designed to reduce the spread of virus-infected cells within their immune system. The team's most recent experiment has found that SIV-infected monkeys that were given a combination of SIV treatment and the specialized antibody several weeks after the onset of SIV infection subsequently developed very low levels of SIV in their blood and very few SIV-infected cells. Furthermore, this outcome continued after scientists withdrew SIV treatment from the monkeys. Also, once treatment was completed, the monkeys' immune systems appeared to be largely undamaged by this infection, as they had normal levels of CD4+ cells.
This result was unusual and unexpected. To be clear, the monkeys were not cured of SIV. However, taken as a whole, this experiment raises the possibility that a clinical trial in HIV-positive people might have a similar result. Using an antibody -- vedolizumab (Entyvio) -- that is approved for use in people with inflammatory intestinal conditions, researchers in the U.S. have launched such a trial in HIV-positive people. Before we discuss the details about the experiment with monkeys mentioned above, we first provide some information about the immune system and its interaction with HIV.
The immune system is largely distributed in cells and small pockets of tissue throughout most of the body. There are major organs that are part of the immune system -- including the bone marrow, the spleen and the thymus gland. However, a large part of the immune system is in the many lymph nodes and lymphatic tissues that are around the gastrointestinal tract. As a result, most (98%) of the key cells of the immune system -- CD4+ cells -- are also found in these lymph nodes and lymphatic tissues. Only about 2% of the body's CD4+ cells are in the blood. Therefore, research that focuses on what is happening in the lymphatic tissues and lymph nodes is important.
After entering the body, HIV encounters cells of the immune system. These cells capture HIV and take it to lymph nodes and lymphatic tissues scattered around the gastrointestinal tract. Cells of the immune system are drawn to the lymphatic tissues and lymph nodes in the GI tract because of specialized receptors. Once in these pockets of the immune system, the cells that have captured HIV help educate other cells of the immune system about this invading virus so that they can also attack it. Unfortunately, through mechanisms not fully understood, HIV quickly subverts the immune system's defences and infects many more cells. These infected cells release chemical signals that further weaken the immune system and the virus spreads throughout the body.
Some researchers think that by interfering with the receptors used by cells to hone in on the gut lymphoid tissue it may be possible to greatly reduce the spread of HIV within the body and, therefore, reduce the injury to the immune system. Researchers in the U.S., Canada and Western Europe have been studying ways to interfere with a specific receptor found on T-cells and other cells of the immune system. The technical name for this receptor is called alpha4beta7, but we will shorten it to a4b7.
Cells of the immune system, particularly CD4+ cells, use a4b7 to help them migrate to pockets of the immune system that are distributed around the gut. Some research also suggests that SIV (and HIV) can use this receptor to assist the virus in infecting cells. By interfering with a4b7 -- using antibodies and small molecules -- some researchers think that it may be possible to protect the majority of the body's CD4+ cells from HIV. However, before embarking on clinical trials with this antibody in HIV-positive people, experiments in SIV-infected monkeys were first necessary.
Previous research in the U.S. has led to the development of an antibody that can be safely used in SIV-infected monkeys. This antibody binds to a4b7, blocking access to it by cells and SIV. Recent experiments have found that when given prior to or during SIV infection, antibodies to a4b7 are able to protect some monkeys from developing SIV infection. In those monkeys that did develop SIV infection and also received infusions of the antibody, levels of CD4+ cells were near normal and the number of SIV-infected cells was very low. These results motivated researchers to perform a study in SIV-infected monkeys using SIV treatment combined with the antibody against a4b7.
Key points from the latest experiment with monkeys are as follows:
The viral loads of the eight monkeys were initially high -- about 3 million copies/mL. However, after three consecutive weeks of therapy, their viral loads fell below the level of detection.
Researchers were somewhat surprised by the results they obtained after they stopped administering SIV treatment.
Among the eight monkeys that received the a4b7 antibody there were different responses, as follows:
In contrast, monkeys that were not given the a4b7 antibody had their viral loads surge to about 1 million copies/mL and remain high for the duration of the experiment.
After the cessation of SIV treatment in monkeys that were given a4b7 antibodies, the level of SIV-infected cells became undetectable. This suggests that the antibody had a major impact on SIV's ability to infect cells. In contrast, among monkeys given the non-specific antibody, the level of SIV-infected cells remained high.
During the early stage of untreated SIV infection, levels of CD4+ cells in the blood declined. Subsequently, only monkeys that received the a4b7 antibody had significant increases in their CD4+ cell counts. Furthermore, after cessation of a4b7 antibody infusions, CD4+ cell counts remained elevated, approaching normal levels.
Analysis of the lymphatic tissue around the gut of the monkeys also showed high levels of CD4+ cells only in those that had received the a4b7 antibodies. The researchers think that the antibody may have protected CD4+ cells in the gut and elsewhere from SIV infection.
An important group of the immune system's cells is called natural killer (NK) cells. These cells can destroy virus-infected cells and tumours. Researchers found that levels of NK cells were initially similar in both groups of monkeys. However, levels of NK cells increased only in monkeys given a4b7 antibodies. Researchers think that this increased level of NK cells may have helped the monkeys control SIV levels.
Viruses such as HIV and SIV cause a significant increase in the general level of inflammation and injury to many different organ-systems in the body. Elevated inflammation was found in all monkeys in the study, at least initially. However, inflammation decreased only in monkeys given a4b7 antibodies.
The active form of vitamin A in the body is called retinoic acid. At the start of the study, during early SIV infection, researchers found that all monkeys had lower-than-normal levels of retinoic acid. However, in monkeys subsequently exposed to a4b7 antibodies, levels of retinoic acid rose to near-normal levels. Researchers are not sure exactly what role, if any, retinoic acid might have played in the overall study results because retinoic acid levels went down as a result of SIV infection and only rose because of infusion of the a4b7 antibody. It may be that the elevated levels of vitamin A are a consequence, and not a cause, of the effect of the a4b7 antibodies. Other experiments have found that different levels of vitamin A are found in different species of monkeys whose cells express a4b7, further confounding any clear conclusions about the role vitamin A might have played in this experiment.
Analysis of blood samples from the monkeys suggested that only those animals given the a4b7 antibodies subsequently developed antibodies that specifically attacked SIV. It is plausible that these antibodies may have played a role in helping the immune systems of the monkeys control SIV.
The latest experiment with monkeys treated with a4b7 antibodies yielded unexpected and promising results. They need to be reproduced by at least another team of researchers.
An antibody designed for human use, vedolizumab (Entyvio), also binds to the a4b7 receptor of CD4+ cells in people. This antibody is licensed for the treatment of certain inflammatory conditions of the intestine -- Crohn's disease and ulcerative colitis. The monkey researchers think that vedolizumab might have beneficial effects in humans with HIV.
The U.S. National Institutes of Health (NIH) has launched a clinical trial of vedolizumab in HIV-positive people who are taking potent combination anti-HIV therapy (ART), have more than 450 CD4+ cells/mm3 and are otherwise healthy. As with the monkey study described earlier, eventually participants in the NIH trial will take a supervised treatment interruption to assess the effect of the antibody on the immune system. Researchers are uncertain of the outcome of this study in people for at least the following reasons:
Despite these caveats, the NIH trial is very important and exciting, as it holds much potential and is studying a drug that has already been approved for human use (for other conditions).
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