Immune Boosters
Can we jump-start the immune system to control HIV?
by Brenda Lein

Recent progress in the field of anti-HIV therapies has led to increased attention on the immune system, the body's defense against infections and disease. In many people, the new triple-drug therapy regimens (called highly active anti-retroviral therapy, or HAART) can potently suppress HIV replication and prevent further ravaging of the immune system by the virus. But to improve immune health in the long term, we may also need complementary approaches to boost the immune system or help repair damage already done by the virus. These approaches are called immune-based therapies.

In general, immune-based therapies can be thought of in terms of how they affect the immune environment-the number of immune cells and their function, or their ability to protect cells from infection by HIV. These therapies are designed to reverse the progressive immune deficiency caused by HIV and restore specific immune responses that are typically lost in people with chronic infection. By restoring these immune responses, the immune system might then be strong enough to control HIV. At least that's the dream (see "Slouching Toward a Cure")

Now, encouraging news has come from researchers around the world regarding the immune system's response to anti-HIV therapy. In a nutshell, when HIV is potently suppressed, the immune system appears to heal and restore itself. After initiating potent anti-HIV therapy regimens, many people see almost immediate increases in the levels of their immune T-cells (both CD4 and CD8 cells). However, a group in the Netherlands suggests that this initial, rapid increase in CD4 counts is more likely due to immune cell redistribution-a reshuffling of T-cells out of lymphoid tissue and into the blood-and doesn't reflect true immune restoration.

CD8 T-cells are important cells in seeking out and destroying virally infected cells. Their function is highly dependent on help from healthy and functioning CD4 cells. While the number of CD8 cells initially increases after starting a potent anti-HIV therapy regimen, it subsequently begin to decline, mirroring the CD4-to-CD8 cell ratios observed in healthy, uninfected people. To date, studies suggest that among people who initiate anti-HIV therapy early, when their immune system is intact, evidence of immune restoration can be seen in a matter of weeks to months. But for those who initiate therapy later, after signs of abnormalities and damage are evident, immune restoration appears to take longer, possibly months to years.

In almost all instances, however, despite dramatic improvements in immune health following the initiation of a successful anti-HIV regimen, researchers aren't seeing a level of immune restoration equivalent to what is observed in healthy HIV-negative individuals. In an October Lancet report involving 159 people with HIV but no symptoms of disease, a Spanish team found that even after a year on HAART, people still lacked HIV-specific immune T-cell responses (as did people taking double therapy and those not taking any drugs). They concluded that "HAART alone will most likely not be enough to restore HIV-1 specific T-cell helper responses in people with chronic HIV-1 infection." To do that, other immune-based strategies may be needed, they added.

How much damage does HIV do to the immune system of people who have been infected for many years? That question is important, say researchers, in determining whether it's possible to fully regenerate the immune arsenal. If HIV has destroyed the architecture of the immune system-its lymphoid organs and tissue-can it restore itself? Today, frontline research is centered on two important aspects of our immune system: bone marrow and the thymus, the primary immune lymphoid organs that house and generate immune cells (see Box). If bone marrow and thymus functions have been damaged by HIV, efforts to restore them might be critical, especially in people with low CD4+ cell counts.

Ongoing research by Mike McCune of the Gladstone Institute in San Francisco, a leader in the HIV immunology field, suggests that the thymus is not only present in people with HIV but more likely to be detectable in an HIV-infected adult than it is in an HIV-negative one. Moreover, adults with higher CD4 T-cell counts appear more likely to have a detectable thymus than adults with less than 300 T-cells.

It may be that when people have HIV and their CD4 T-cell counts fall below normal ranges and new cells need to be reproduced, the thymus regenerates. But when HIV overpowers the immune system, including the thymus, the organ becomes exhausted. In a small study presented at the 12th World AIDS Conference in Geneva, an Italian group of investigators looked at the size of the thymus in children before and after administering anti-HIV therapy. They cited the case of a 7-year-old child with advanced AIDS whose strong response to HAART therapy included a rise in CD4 T-cell counts and a decrease in viral load (amount of virus in the blood) that corresponded with a 100-fold increase in the size of the child's thymus. What all this means remains is somewhat unclear. But these early studies are encouraging, suggesting the thymus may play a role in post-HAART immune recovery.

Other approaches to restoring the thymus include thymus transplantation, which uses fetal thymus tissue. As of October, two studies of thymus transplantation had been completed, one in people with high CD4 T-cell counts (more than 400) and another in people with low counts (less than 200). In the first group, the thymic transplant was apparently rejected by the body. But in people with lower cell counts, the procedure appeared safe and, in some cases, may be contributing to improved immune status. However, these studies are small, and no conclusions should be drawn about the effectiveness or failure of this approach.

A variety of strategies to either boost or decrease thymic activity is being investigated in conjunction with potent anti-HIV therapies. McCune says research is aimed at gaining a better understanding of whether there are signals in the body that are used to turn the thymus on-or off. Therapies that may increase thymus function include thymosin-alpha 1, thymopentin, inflammatory molecules like interleukin IL-2 (also called Proleukin), human growth hormone, and insulin-like growth factors. Those that may decrease thymus function include interferon-alpha, androgens, testosterone, and glucocorticoids.

Today, intensive research is also focused on interleukins (cytokines), natural inflammatory molecules produced by immune cells that, like words in a language, allows cells to communicate. There are about 16 different interleukins being studied, and all of them evoke different immune responses. Perhaps the most extensively studied is IL-2. At higher doses, it can dramatically boost T-cell levels in people by inducing CD4 cells to become active. At lower doses, IL-2 causes cytotoxic T-cells (or CD8) cells to reproduce.

In one of the largest IL-2 studies, people with average CD4 cell counts of 400 were divided into two groups receiving anti-HIV therapy (AZT and/or ddI)-with only one group also taking IL-2. After a year, those receiving IL-2 had an average CD4 T-cell count of 950, whereas those receiving only the anti-HIV drugs saw their T-cell counts fall to 350. At the Geneva conference, a French team examined the impact of IL-2 on a number of measures of immune function. In their study, people on HAART who added IL-2 to their regimens showed better immune responses, including higher CD4 T-cell counts and the improved ability of immune cells to perform tasks.

Since IL-2 stimulates T-cells to become active, researchers have been worried that it might also increase HIV activity by presenting more target T-cells for the virus to infect. In fact, in test tubes IL-2 can and does stimulate HIV replication, but in humans this doesn't appear to be a major concern. So far, for unexplained reasons, people receiving IL-2 and HAART appear to have a greater drop in viral levels then those receiving only HIV combination therapy, based on ultrasensitive HIV RNA viral-load tests. In the French study mentioned above, a greater percentage of people receiving IL-2 with HAART saw viral levels drop to below the limit of detection. Taking it a step further, researchers hunting for signs of the virus in more than 300 million cells of a few people on the IL-2/HAART regimen couldn't find any. But the virus was found universally when just 10 million cells were examined in those receiving only anti-HIV therapy. Researchers think that IL-2 may be "turning on" dormant HIV-infected cells, which express HIV as they become active; the emerging virus is then killed by potent anti-HIV therapies. At least that's the theory. A large study of IL-2, is expected to begin shortly.

Another promising avenue for enhancing immune function involves granulocyte macrophage colony stimulating factor, or GM-CSF (also called Leukine), which stimulates the growth of macrophage cells that also play an important role in fighting diseases. Macrophages are important for killing mycobacteria (such as those that cause HIV-related infections like MAC and tuberculosis), fungi, parasites, and other infections that commonly cause diseases associated with AIDS. In test-tube studies, GM-CSF enhances the ability of macrophages to kill these various organisms. GM-CSF is proven to prevent serious life-threatening infections in bone-marrow transplant patients, whose immune systems are severely compromised. A study is under way in people with low CD4 T-cell counts to see whether adding GM-CSF to their existing anti-HIV and other regimens will decrease their risk of infections and prolong survival.

Others are studying vaccines to stimulate HIV-specific CD4 and CD8 T-cell responses (also called lymphoproliferative responses, or LPR). Dr. Bruce Walker of Massachusetts General Hospital in Boston, among others, has observed that in long-term non-progressors these HIV-specific LPR responses are preserved, while they are lost in people with chronic infection. Walker has also looked at people who were treated with anti-HIV therapy shortly after acute infection (that is, within days to weeks after exposure to HIV). In this group, early treatment appears to preserve HIV-specific immune responses.

A leading candidate being studied is the HIV-1 Immunogen, also called Remune, the renamed Salk vaccine, used in conjunction with HAART. Based on a traditional vaccine approach, Remune is a deactivated form of HIV that can't reproduce. Based on recent studies, there's hope that Remune might be able to generate HIV-specific CD4 and CD8 responses seen in long-term non-progressors. The vaccine is currently being evaluated in studies involving thousands of people.

Results from previous, smaller Remune studies were not overwhelmingly encouraging. Those receiving the vaccine didn't see appreciable increases in CD4 T-cell counts or decreases in viral levels.

Recently New York immunologist Fred Valentine presented early data showing that Remune can elicit a broad range of HIV-specific immune responses in test-tube studies. But no one knows whether these will be sufficient to protect against all strains of the virus.

Other approaches that aim to protect immune cells from infection by HIV primarily involve cell- and gene-therapy approaches. By and large this field is in its infancy. Researchers are trying to genetically modify immune cells and render them resistant to HIV infection. This involves taking cells from the body, inserting a protective gene, and re-infusing the cells. Results from studies conducted at the University of Michigan Health Systems using a Rev M10 gene to modify cells show some success. The cells with Rev M10 appear to live longer than cells modified with a "placebo" gene, suggesting that the gene is indeed protecting the cells from infection and destruction by HIV.

Another study, by Stan Deresinski at the AIDS Clinical Research Consor-tium in Redwood City, California, involves inserting protective genes into bone-marrow stem cells. The goal here is to protect offspring CD4 T-cells and other cells derived from these stem cells.

Another approach includes inserting ribozymes into cells. Ribozymes can be likened to molecular scissors, which literally cut genetic pieces of HIV into bits, inside a cell. Some of these are being developed under the leadership of federal researcher Flossie Wong-Staal of the University of California at San Diego, in conjunction with Immusol, a Southern California biotechnology company.

The frontier of HIV immunology includes using immune-suppressive therapies to calm an overactivated immune system. That's based on studies that suggest immune activation increases during HIV infection and may fall, along with viral load levels, in people on therapy (see "Charting the (Troubled) Waters,"). Several such therapies have moved into the clinic, including cyclosporine and total lymphoid eradication. A novel cell-therapy approach is also being developed. As with HIV drugs, we may find that for some people, combining immune therapies is what's needed to support full immune recovery.

What's in an Interleukin? Interleukins are natural inflammatory molecules produced by immune cells that can be viewed as parts of the immune system's language; they're how immune cells communicate with one another. There are about 16 different interleukins currently being studied. They are identified by number, ranging from interleukin-1 (IL-1) to interleukin-16 (IL-16), and they evoke and promote very different types of immune responses. IL-2, administered at high doses, for example, causes CD4 T-cells to reproduce. But when administered at lower doses, it causes cytotoxic T-cells (natural killer cells) to reproduce. IL-2 also appears to promote macrophage function. Macrophages, sometimes called antigen-presenting cells, serve as important scouts of the immune system. They ingest organisms and either destroy them directly or carry them to CD4 T-cells, which then signal either CD8 cells or B cells to clear the organisms from the body. IL-7 may be useful in promoting B-cell responses, which are important in the development of antibodies, and is being considered as an adjunctive therapy for preventative vaccines. IL-15 is one of the newest interleukins moving toward human study. Apparently, according to test-tube studies, IL-15, like IL-2, is able to induce CD4 cells to reproduce. -Brenda Lein