Immune Restoration: Repairing the Damage
The ability of Highly Active AntiRetroviral Therapy (HAART) to suppress HIV replication, increase CD4 cell counts in the blood, and prevent or delay opportunistic infections is now well documented. Individual responses can vary, toxicities remain a problem and the best time to start HAART continues to be debated, but the overall trend of restored immunity and prevention of illness has come as a welcome surprise. Many researchers feared that the damage to the immune system caused by HIV would be irreversible, but HAART studies have contradicted this assumption. These studies paint a picture of immune restoration occurring in multiple phases -- some fast and others slow and variable -- ultimately leading to near-normal immune system function in many individuals. Research into immune restoration also provides a new opportunity to understand the mechanisms by which HIV damages the immune system, a necessary step for designing therapies that might speed immune recovery or help people whose immunity remains impaired despite HAART.
HIV infection progressively impairs the immune system in several important ways. The pool of naïve T-cells (both CD4 and CD8) needed to mount responses to new infections slowly declines, making it harder for people with advanced disease to respond to vaccinations or new infectious challenges. At the same time, the teams of memory CD4 cells specific for common infections -- such as pneumocystis carinii pneumonia (PCP), toxoplasmosis, thrush, mycobacterium avium complex (MAC), and cytomegalovirus (CMV) -- are reduced in number. Eventually, the ability of these memory cells to police their respective pathogens is lost, leading to the illnesses known as opportunistic infections (OIs).
One marker for this process is the CD4 cell count. This routine test gives you the number of CD4 cells in a milliliter of blood. Declining counts are linked to an increasing threat of illness -- especially when specific thresholds are crossed. Studies have found that a drop below 200 CD4 cells is the most significant risk factor for the development of OIs.
Along with the fall in numbers, there are profound changes in CD4 cell function, first revealed in the late 1980s by immunologist Gene Shearer and colleagues at the National Institutes of Health. Shearer demonstrated what can best be described as a spreading dysfunction among CD4 cells over the course of HIV infection. First to be impacted are memory CD4 cell responses to specific common antigens (pieces of infectious agent, from infections like influenza virus), which decline to below-normal levels early on. But eventually the CD4 cell population as a whole is affected. Due to the central coordinating role played by CD4 cells, this loss of function is almost inevitably accompanied by defects in B-cell and CD8 cell responses.
The pace of immune system impairment is known to be linked to the level of HIV replication (the viral load) and the degree of abnormal immune activation. One of the great mysteries of HIV infection is that it both suppresses immune function and also hyper-activates some immune system cells. The late Janis Giorgi from the University of California at Los Angeles pioneered this area of research, showing that certain markers of T-cell activation (particularly surface molecules called HLA-DR and CD38) actually increase as HIV infection progresses.
One of the first detailed investigations of HAART's effect on the immune system was published by Parisian immunologist Brigitte Autran and colleagues in 1997. Autran described a two-phase process of immune reconstitution that many other researchers have since confirmed. The first phase involves a rapid rise in CD4 cell counts of 100 and more during the first two months of therapy as HIV viral load is reduced. Autran showed that these are almost all memory CD4 cells (which can be distinguished from naïve cells by specific markers on their surface). Autran's theory -- subsequently confirmed in a detailed analysis conducted by Pat Bucy from the University of Alabama -- was that this initial gain reflected redistribution of memory CD4 cells that had been trapped in the lymphoid organs. The speed and extent of this early CD4 cell recovery is linked to the rate of CD4 cell loss during the year before HAART is started -- the more rapid the loss, the more rapid the recovery. Factors such as age or how quickly viral load drops do not seem to have much influence on the first phase of immune restoration. Autran also noted that the improvement in absolute CD4 cell counts is accompanied by an increase in the CD4 percentage and normalization of the CD4:CD8 ratio, because the number and percent of memory CD8 cells drops as CD4 cell numbers rise.
The second phase of immune restoration described by Autran comprised a much slower but steady increase in naïve T-cells (both CD4 and CD8) that became detectable about four months after HAART was initiated. This increase continued during a year of follow-up. Longer-term studies have since shown that this slow gain of naïve T-cells can continue for a period of years, until normal or near-normal levels are attained. The source of these naïve T-cells has since been shown to be the thymus, an organ that was once thought to be inactive in adulthood. A young English researcher based in Texas, Danny Douek, overturned this assumption using a test that can identify naïve T-cells that have recently left the thymus. The test looks for TREC (T-cell Receptor Excision Circles), which are small pieces of DNA present almost exclusively in newly-made naïve T-cells.
In a search for factors that influence naïve T-cell increases, immunologist Mike Lederman from Case Western University in Cleveland, Ohio discovered that the pace of naïve T-cell recovery correlated with age, with younger people gaining naïve cells fastest. In children, the effect is most dramatic -- the rate of naïve T-cell recovery is 10-40 times faster than that of adults. This finding is consistent with Douek's recent TREC research showing that the thymus is most active in childhood, but then decreases production of naïve T-cells to a steady (but gradually slowing) daily output that continues into old age.
Brigitte Autran also looked for evidence of improved memory CD4 cell function in people taking HAART. Memory responses to CMV and tuberculosis (TB) antigens were measured using lymphoproliferation tests (see "Measuring Memory T-Cell Responses"). Before therapy, when the average CD4 cell count was 176, study participants showed no response to either antigen. Within three months of starting HAART, significant responses to both TB and CMV became detectable. Since this initial study in 1997, many other reports have confirmed improvements in antigen-specific T-cell responses, some using newer testing technologies. Additional antigens that have been studied include influenza, candida (the fungus that causes thrush) and tetanus. The one exception to this rule appears to be HIV. Responses to HIV antigens can sometimes be detected before starting HAART, but do not improve after beginning therapy. In fact, the reduction in HIV levels caused by HAART seems to cause a decline in HIV-specific T-cells.
Improvements in immune function after HAART can also be assessed more indirectly. Large, widely-publicized cohort studies involving thousands of people have demonstrated that HAART dramatically reduces the occurrence of OIs. Evidence that is even more persuasive comes from studies in which preventive treatments (prophylaxis) for OIs were successfully stopped if HAART boosted CD4 cell counts above certain thresholds. Even treatments for active OIs can sometimes be discontinued without recurrence of disease, a situation unimaginable just a few years ago.*
Although HIV does not appear to directly impair the function of naïve CD4 cells, the decline in overall numbers that occurs during infection is associated with a poor response to vaccinations. This led Mike Lederman's research team to investigate whether HAART-related recovery of naïve CD4 cells improved people's ability to respond to new immunizations. Lederman found that three-quarters of HAART-treated participants developed antibodies to the hepatitis A vaccine, and that the magnitude of these responses was indeed linked to increases in the number of naïve CD4 cells. Similarly, Dutch researchers have shown that HAART is associated with a greatly improved response to influenza vaccines in adults. Extending these findings to children, a recent study from the University of Washington in Seattle found that HIV-infected children on HAART experienced significantly better responses to the MMR (measles-mumps-rubella) vaccine than children who were untreated.
The improvements in immune function associated with HAART appear to occur in parallel with a decline in abnormal immune activation. Brigitte Autran's study measured levels of the T-cell activation markers HLA-DR and CD38 and found they fell steadily, reaching normal or near-normal levels after a year of treatment. The researchers also discovered a strong correlation between these markers and HIV viral load levels. Subsequent research has confirmed these findings, with Israeli scientist Zvi Bentwich going so far as to suggest that changes in markers of immune activation are better at predicting immune restoration than viral load.
Both the effects of HIV infection and responses to HAART treatment vary from individual to individual. The overall trends seen in studies of immune restoration are not experienced by everyone, and cases of increased CD4 cell counts but persistent defects in responses to certain antigens have been reported. For example, Krishna Komanduri from the Gladstone Institute in San Francisco used the intracellular cytokine staining test to look for memory CD4 cell responses to CMV and found they were generally restored after HAART. But recent work by Komanduri has also shown that, despite CD4 cell count increases to over 400, a very few individuals may show persistent defects in CMV-specific responses and develop CMV disease as a result. Fred Valentine from New York University has reported a similar case of persistent defects in PCP-specific CD4 cell responses despite a high CD4 count. This was also associated with the development of disease. Although rare, such cases suggest that individualized monitoring of antigen-specific responses might eventually help provide better guidance for OI prophylaxis than CD4 cell counts alone. So far, most T-cell function tests are only available in research settings.
Studies have also documented so-called "paradoxical" effects of HAART. Around 10% of people taking HAART experience ongoing CD4 cell count increases despite only temporary reductions in viral load. A number of theories have been put forward to explain this phenomenon, such as drug-resistant HIV potentially being less harmful to T-cells and/or the thymus. Another possibility is that a brief reduction in viral load can be sufficient to cause a major redistribution of T-cells from the lymphoid organs. Some researchers have found that this type of paradoxical response is associated with a decrease in immune activation markers that persists after viral load has risen. At the other extreme, perhaps 5% of people on HAART experience sustained reductions in viral load but only minor increases in CD4 cells. Recent studies utilizing the TREC test have found that this problem may relate to a lack of production of naïve T-cells by the thymus.
It's now clear that suppression of HIV replication can allow a phoenix-like reconstitution of the immune system in many people. Researchers are not resting on their laurels, however, but working on new strategies that may help correct remaining defects and aid people who do not experience an ideal response to HAART. Individualized monitoring of antigen-specific memory CD4 cell responses may already be on the horizon. Speeding up the process of naïve T-cell recovery is another highly desirable goal, although it's currently unclear if the activity of the thymus can be increased by any therapy. Interleukin-2 is being studied for its potential to increase overall CD4 cell numbers by stimulating existing cells to copy themselves. The ultimate aim of many immunologists is to find ways of creating vigorous, functional HIV-specific memory CD4 cell responses, since HAART alone cannot accomplish this task.
* The new United States Public Health Service (USPHS) guidelines on preventing OIs, updated in November, include specific criteria for stopping prophylaxis for PCP, MAC and toxoplasmosis. The USPHS guidelines also describe circumstances under which treatment for active MAC, toxoplasmosis, cryptococcal meningitis and CMV might safely be stopped. The guidelines are available on the Internet at http://www.thebody.com/cdc/pdfs/oi_1101.pdf (PDF).
Immune Reconstitution and Immunologic Responses to Antiretroviral Therapies: Implications for Therapeutic Strategies (Plenary Session 3)
This article was provided by AIDS Community Research Initiative of America. It is a part of the publication ACRIA Update. Visit ACRIA's website to find out more about their activities, publications and services.