"Then why aren't you looking over there?" asks the exasperated cop. The drunk looks up and replies, "Because the light is better over here."
We know what we do about AIDS from two basic kinds of experience. Soon after the syndrome was first reported, clinical observation quickly revealed that people with the disease tended to waste away or were stricken with any of a number of unusual infections. Most eventually died. At the same time research scientists started looking for abnormalities in blood and tissue samples from AIDS patients, searching for a cause. But the kinds of tests available at that time were limited and the conclusions researchers could draw from their laboratory experiments were general and few. They were simply looking where they could shine a light. After twenty years, new assays and experiments, producing high quality streams of data, continue to illuminate the underlying causes of this disease. Nevertheless, the workings of immunity are so complex and so difficult to observe in living persons that darkness still shrouds many crucial aspects of what goes wrong when HIV interacts with the human immune system.
Since the first recognized AIDS cases involved very sick people, early clinical reports detailed mostly non-specific markers of infection and impaired immunity such as skin antigen recall tests, erythrocyte sedimentation rates, or increased white blood cell counts. A few curious researchers working at the frontiers of intercellular signaling reported elevated levels of interferon and tumor necrosis factor in people with AIDS. Others, specializing in virology and antigen recognition, reported increased levels of cytomegalovirus and herpes virus particles in their patients' blood. These observations were made using the best tools available at the time and the data obtained trickled into the sketchy literature that was slowly accumulating about the mysterious syndrome.
When first described in 1981, it was recognized that the opportunistic diseases attacking a growing number of young, homosexual men, were also those that afflicted people with genetic or chemotherapy-induced deficits in a wing of the immune system called cellular immunity. This branch of immunity depends on T-cells -- white blood cells that earned their name because they were observed to develop in the thymus -- and is responsible for eliminating diseased cells that have been invaded by outside pathogens.
In the seventies, someone noticed that T-cells could be identified and separated from other white blood cells by mixing them with red blood cells (erythrocytes) from sheep. The sheep erythrocytes would then cluster around the T-cells and form rosettes (E-rosettes) that could be viewed and counted under a microscope. An assumption eventually emerged that the sheep cells attached themselves to certain cell-surface proteins that were characteristic of a subtype of T-cells called the T-helper cell. The various receptors and coreceptors we now know as CD4, CD8, etc., were then simply thought of as cell surface antigens -- as late as the mid eighties some papers still referred to the newly identified CD4 receptor as the E-rosette receptor. But even this rudimentary technique revealed that, in AIDS, T-helper cells were disappearing.
The need for a quicker and more reliable measurement of disease progression was urgent. Fatefully, AIDS started to appear just as a new wave of technology for marking and automatically counting immune cells was becoming more available. CD4 cell staining with fluorescent antibodies and flow cytometry machines that could rapidly and accurately count the tagged cells soon led to the establishment of the CD4 cell count as a standard marker for the severity of HIV disease. The CD4 count eventually proved to be useful for predicting disease progression, warning of risk for opportunistic infections (OI), serving as a diagnostic milestone for the onset of AIDS, and, after an initial debate, as a surrogate for demonstrating that an experimental antiretroviral therapy was likely to have clinical benefit. Today, most guidelines for making decisions about starting and stopping treatment and initiating OI prophylaxis continue to rely heavily on CD4 counts.
However, absolute CD4 counts (along with the ratio of the number of CD4 cells to CD8 cells, another important immune system metric, still preferred by some clinicians) are at best somewhat crude measurements of the effects of HIV on an individual's immune system. Within the broad category of CD4 cells, there are many subtypes, each with particular characteristics and roles to play in the orchestra of immunity. And while simple CD4 counts are invaluable for making treatment decisions, they cannot describe which subsets of cells are affected by HIV infection or how well they are functioning. Despite the ability of HAART to raise CD4 counts for many people, concerns about drug toxicities, resistance and the difficulties of sustaining adherence to HAART have brought new urgency to improving the ways we have of measuring and understanding the complexities of the immune system and for finding new ways of treating it.
For example, observations about the density of E-rosettes fueled some of the first speculation about the possibility of immune-based treatments for AIDS. A 1982 paper reporting on in vitro findings announced, "Drugs which are able to modulate T-cell functions, such as thymosin, transfer factor, isoprinosine É, also increase the percentage of active T-rosettes." (Wybran J) These agents and other purported immune modulators were experimentally used and periodically tested throughout the rest of the decade, although they failed to show any consistent clinical value. Interest in IL-2, another immune regulatory messenger recognized during the first years of AIDS, has never gone out of favor and large trials continue to this day. Nevertheless, some have argued that the early, vigorous, focus on antiviral therapy for HIV may have slowed work on immune-based studies.
In the early eighties the basic narrative of immunity went something like this: During a cellular immune response, T-helper cells sound the alarm for antigen-specific immune cells to start multiplying. When the attack has gone far enough, other T-cells, called T-suppressor cells, sound the retreat and start eliminating the now superfluous antigen-specific attackers. Although some early researchers hypothesized that HIV-mediated autoimmune processes were responsible for the resulting immune dysregulation, by mid-decade, conventional wisdom held that AIDS was directly caused by the virus, which, in the words of one famous researcher, "kills T-cells like a Mack truck."
In recent years, we've learned that most depleted T-cells are never actually infected with HIV, and a more sophisticated view of pathogenesis is emerging. There is growing understanding that immunity is a system of give and take, which, when regulated, works very well. We know that over five to ten years or more, T-cell counts decline at a slow but steady pace. Yet the picture of what happens to the turnover of the body's immune cell inventory is still developing. Although it's obvious that something is seriously wrong with the system of immune regulation in AIDS, surprisingly little is known about what actually causes T-cell stores to dwindle. One theory says that T-cells are destroyed at a consistently high rate but are also replaced at a similar rate, thus maintaining a rough balance. Over time, the theory goes, the body's capacity to replace T-cells starts to wear down, leading to their eventual depletion. But which part of the T-cell replacement mechanism is wearing out and what is responsible for wiping out the T-cells that disappear? New research tools are slowly getting at these questions.
In this issue, Daniel Raymond catalogs some of the latest assays that immune system scientists are using to pick apart the mysteries of T-cell depletion. He also describes what their experiments are telling us about how HIV might be doing its damage. Although these studies are still in fairly early stages, it's possible to imagine that, one day, a new approach to treating HIV could be developed, perhaps using a drug that interrupts a crucial, very specific step in HIV-mediated immune dysregulation.
Yet despite accelerating progress, new assays are not born and accepted overnight. It may take years to validate initial observations in competing labs. Then standards must be established before test results can be used diagnostically or correlated with other assays. So, although new assays may be casting new light onto the workings of T-cell creation, destruction and replenishment, many of the explanations constructed from this emerging evidence remain tentative and sometimes contradictory. In 2002 we continue to look where the light is best and make up stories for what we find that fit with what we already know. Hopefully this dawning generation of new immune assays will help us locate the sorely needed key to immune control of HIV.
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