GREGG GONSALVES: Now that we've achieved some degree of control of viral replication with HAART (highly active antiretroviral therapy), a lot of people are thinking about how to harness the immune system to fight HIV or how to repair some of the damage done to it by the virus. I wanted to know what each of you thought about the prospects of accomplishing this. Are you more or less optimistic about this sort of idea than you were a year ago? Who'd like to go first?

BRUCE WALKER: I am clearly more optimistic than I was a year ago. There are very compelling data now to indicate that the immune system plays a major role in determining what level of virus there is in the bloodstream in any given individual. Those data come in part from some very important studies that were performed in chronically-infected monkeys showing that removal of CD8 T-cells led to increases in viral load, and as the CD8 T-cells came back, viral load declined. Those results and other newer assay techniques done in humans have shown that there's a relationship between the level of virus in the bloodstream and the amount of immune defense against the virus. The stronger the immune response, the lower the viral load. It's clear that the immune system is still doing something in chronically-infected people even though it's not fully effective.

The way the immune system deals with lots of viruses is not by eradicating them but by maintaining constant long-term control. This is a situation that actually happens in HIV infection where we have people who have now been infected for 20 years, have never been treated, and maintain extremely low viral loads. We know that this control is associated with a strong immune response against the virus, suggesting that HIV is a virus like others, for example, infectious mononucleosis, that can be chronically controlled.

GONSALVES: Anybody else?

MIKE McCUNE: Let me add several thoughts based on observations about the natural progression of HIV disease. One is the observation that a weak immune system is associated with rapid disease progression: In kids that are infected in utero, born without a fully developed immune system, disease progression is much more rapid than in kids infected after birth. On the other end of the spectrum are studies showing that in older individuals, who tend to have immune responses that are less robust, disease progression is also rapid. Finally, there are observations that effective immune responses might occur in some individuals that are exposed but not infected and that there are some individuals who are infected but who don't progress after long periods of time. All of these observations put together make me optimistic that the immune system can control HIV disease.

GONSALVES: Melissa, do you have something to add?

MELISSA POPE: One question that still needs to be addressed is how we are going to induce the appropriate CD4 and CD8 T-cell responses needed to fight HIV infection. An additional problem, particularly in the face of a damaged immune system, is maintaining immune activation.

LOUIS PICKER: I was just going to add that recent experiments this year in the SIV (simian immunodeficiency virus) macaque model system have shown both the promise of the immune system and its limitations in controlling virus. These experiments clearly demonstrated that the immune system has the power to suppress the virus because it forced the virus to make very specific changes in its genes that were clearly aimed at escaping the immune response–that's the good news. The bad news is the virus did manage to change sufficiently to outrun the immune system, and therefore continue its ability to cause disease. We still have a lot to learn about what it will take for the immune system to "keep up" with the virus.

GONSALVES: I wanted to keep talking about immune-based therapies for just a few more minutes. Each of you works on different cells or organs of the immune system and I want to know what are the implications for immune-based therapies in terms of the role of dendritic cells, the thymus, CD4 and CD8 T-cells?

POPE: There have been some terrific advances in our understanding of dendritic cells' function and their potential application in immune-based therapies (see related story). Dendritic cells are the most effective antigen-presenting cells and are therefore critical in initiating specific T-cell and B-cell immune responses. However, we need to learn how to properly harness the dendritic cells to enable them to most efficiently activate the immune system. In vivo applications of dendritic cells in cancer therapies have been initiated. These are promising but are in the early stages. There are some major hurdles that still face us, including which antigen should we use, and how do we get the dendritic cells to present this antigen most effectively to activate the immune system? Several studies are being carried out ex vivo where dendritic cells are directly handled, mixed with the antigen of interest, and then re-injected into the individual. This allows many of these basic questions to be addressed.

Considerable progress is also coming from the use of animal models, especially the macaque system. The ultimate issue of whether an immune response can control existing and new infections can be readily tested in animal systems. Dendritic cells represent an amazing opportunity to effectively activate broad and potent immunity in both therapeutic and vaccine programs.

GONSALVES: Louis, you've done a lot of work on T-cell homeostasis, the mechanisms that regulate the production of T-cells of different classes. Does this work have any implications for immune-based therapies?

PICKER: The body has evolved a number of mechanisms to develop and maintain sufficient numbers of immune cells to effectively respond to chronic pathogens. These mechanisms involve a balance between maintaining an appropriately large and diverse cohort of cells able to combat one pathogen, while leaving "room" in the system for similar responses to distinct pathogens. To be truthful, we don't understand these mechanisms very well yet–the forces that determine which T-cells, out of the many possible, are selected to be expanded and maintained in antiviral immune responses. What we know suggests that immune control of HIV will require that a large and diverse population of memory T-cells, both regulatory and effector, be continuously maintained so as to prevent the virus from evolving faster than the immune system can respond. We need T-cells that see lots of different parts of the virus so if the virus changes one part of itself, the virus can still be recognized by T-cells by some of its other parts. Only continued basic work in animal models in the study of infected humans will shed light on these issues and provide clues as to how we may better manipulate the immune system for therapeutic benefit. I am now very optimistic that the immune system can be harnessed to combat HIV–this wasn't the case a few years ago. We know the direction to travel, but we still have a long journey ahead.

GONSALVES: Mike, could you talk about the role of the thymus in boosting production of T-cells and the implications for immune-based therapies?

McCUNE: From extensive studies in rodents, we know that the thymus is an organ that receives progenitor cells from the bone marrow and then generates cells that have a diverse T-cell receptor repertoire. We don't know much about the function of the thymus in humans, but humans infected with HIV actually represent the first case wherein thymic function can be studied in the context of chronic T-cell depletion. We've been surprised to find, contrary to what we would have anticipated from prior studies in humans and in rodents, that the thymus in HIV-infected adults actually appears to be more abundant than in HIV-uninfected, age-matched controls.

This has two implications. One is that there might be some feedback control from the periphery [blood] to the thymus and/or the bone marrow which asks the thymus, in essence, to produce more cells in the setting of T-cell depletion. Further analysis of that possibility might provide us with clues about the physiologic mechanisms which can effect such control. We believe that the feedback control might involve soluble cytokines or other mediators and these, together with HAART, might prove to be of therapeutic benefit in those patients who have HIV disease and T-cell depletion. The other implication is that, for those who do not have abundant thymus, we need to understand where the lesion (defect in the immune response) may lie. Then, we can begin to explore treatment options aimed at correcting those lesions.

GONSALVES: Bruce, you've done a lot of work on HIV specific CD4 cell responses and CTL (cytotoxic T-lymphocyte, or cellular immune) responses in the context of HAART. What are the next steps for your work in this area?

WALKER: CD8 cells are the cells that actually do the work in the trenches in that they provide a defense against infection and kill off virus-infected cells directly. They work effectively by receiving signals from CD4 cells that are specifically trained to respond to and coordinate a defense against HIV. We know that people who have a strong cadre of these HIV specific T-helper cells can mount a more effective immune response and that their CTLs, their CD8 cells, do a better job of killing virus-infected cells and keeping the viral load down. Our hope with immune-based therapy is that by correcting the defect in the CD4 helper cell response we will actually allow the CD8 cells or the killer cells to function more efficiently. We've been looking in the very early stages of infection to try to understand why most people lose their HIV-specific helper cells. We still don't fully understand why.

GONSALVES: Louis, in your Nature Medicine paper, you said you saw HIV-specific immune responses in most patients that only started to wane once there was substantial viral suppression achieved through the use of HAART. This seems a bit different than what Bruce has described. What's going on here?

PICKER: When you take a step back and look at it, I think that we were just better able to see the anti-HIV CD4 T-cell responses. The technique that Bruce used isn't really very good at detecting low responses or distinguishing low from medium responses very well. I think what our data shows, using a more sensitive and quantitative assay, is that most but not all infected subjects do indeed manifest a detectable CD4 T-cell response to HIV and maintain it in chronic disease. It is not correct to say these responses are completely absent in subjects with progressive infection. However, if you compare the level of these responses to the CD4 T-cell responses to other chronic viruses like CMV (cytomegalovirus), they are not impressive. So, I wouldn't argue with the hypothesis that something about the virus negates the full potential of this response, and that if you were to prevent this immunosuppressive effect by controlling the virus very early in infection, you'd probably end up with a more impressive response. What such higher CD4 T-cell responses might mean to ultimate viral control of HAART is, however, not clear.

Our feeling that these cells might be of critical importance really derives from data in select animal models looking at very different viruses. There is no direct evidence in humans or even monkeys that CD4 T-cells have a role in controlling HIV/SIV infection (as opposed to CD8 T-cells where such evidence exists). The associations that we, Dr. Walker, and other groups have made between the frequency of virus specific CD4 T-cells and clinical course (for example, higher responses in nonprogressive infection) are not proof of such a role. An individual might be infected with a less pathogenic virus, and this lack of aggressiveness on the virus' part might result in both a favorable clinical course and sparing of anti-viral CD4 T-cell responses–analogous to the classic conundrum of which came first, the chicken or the egg. However, while we are looking for this evidence, it is very reasonable to assume that these cells are going to be important and to include them in our plans for vaccines and other immune-based therapies–just as long as we don't forget to continue our quest to figure out what exactly they do!

GONSALVES: I want to go back to Melissa for a second, and talk about the transmission of HIV. Some studies have said that the dendritic cells are the first to be infected and other studies have said that T-cells are the first. What is your view?

POPE: Dendritic cells are found throughout the body, particularly at the body surfaces, like the mucosa, that are exposed to virus during sexual or perinatal transmission. Therefore, by virtue of their physical location, dendritic cells are one of the first cells that may be encountered by virus that crosses the mucosa. Dendritic cells at the periphery function to survey the tissue and pick up incoming foreign organisms, to traffic them to the lymphoid tissues and initiate immunity. This provides a perfect way for the virus to be transported around the body. Even if the dendritic cells themselves do not necessarily produce large amounts of virus, they can rapidly facilitate virus growth upon interacting with CD4 T-cells. Recent reports using the macaque model revealed that CD4 T-cells appear to be the main source of virus growth in the first days following mucosal transmission. Interestingly, virus is not detected immediately and may, therefore, be trapped by other cells (notably dendritic cells) before being "presented" and subsequently transmitted to the CD4 T-cells in which the virus most effectively replicates. In addition to a potential low-level infection of dendritic cells, the identification of the virus-binding molecule, DC-SIGN, provides another mechanism through which dendritic cells can very efficiently capture small amounts of virus and amplify infection in the CD4 T-cell pool. (see "Show Me a DC-SIGN")

GONSALVES: Mike, we were just talking about the thymus. There's some controversy as to what's driving the CD4 cell depletion in HIV disease. Can you explain what you've found in regard to the production of T-cells and the thymus?

McCUNE: We've been looking at this from both a quantitative and a qualitative standpoint, both of which are difficult to measure; hence, the controversy. With respect to quantitation first, the question is: how many cells are being made and destroyed per day? Working over the past several years with my colleague Marc Hellerstein at UC San Francisco, we've developed a technique using deuterated glucose which we believe provides for more accurate estimates of T-cell division than other techniques. This technique shows us that, indeed, the fraction of T-cells being destroyed per day in the HIV-infected host is higher than normal; however, we also observe that the absolute production of such cells does not increase to compensate for loss. This suggests that the loss is due to two reasons: accelerated destruction of mature cells and decreased production of new cells to replace the ones that are destroyed.

This brings us to consideration of questions related to qualitative aspects of production. The organs of production are the bone marrow, the thymus, and lymph nodes. We know that each of these organs is affected and can be destroyed by the disease. We believe that production from each of these organs is important in the adult and that the quality of response to HAART will be satisfactory to the extent that intra-thymic maturation can occur. Our studies suggest that such intra-thymic production is possible in some individuals.

GONSALVES: For our last question, I wanted to get your opinions on some of the immune-based therapies or strategies in development. There's IL-2, therapeutic immunization, pulsing dendritic cells with HIV antigens and CD40 ligand (receptor) and strategic treatment interruptions (drug holidays). I'd like a response from each of you.

MCCUNE: We've been interested in understanding whether circulating mediators of thymic activity might be present and have studied this in thymic organ culture and in individuals who have varying degrees of T-cell depletion. Of all the different candidates that might be found, and there are a number of them, IL-7 appears to be the one that is the most evident. IL-7 in thymic organ culture prevents the death of thymic progenitors and also enhances the proliferation of them and their progeny, including T-cells in the periphery. IL-7 is not available for clinical studies at this point and for good reason: It's been associated with a variety of adverse effects. So, although it seems to be an important molecule and might actually be of benefit to HIV-infected patients who are on HAART, it can only be used with great care and probably not in the near future.

GONSALVES: Louis and Bruce, we're down to therapeutic immunization or structured treatment interruptions, both strategies for boosting HIV-specific immune responses.

WALKER: I think therapeutic immunization got a bad name when it was initially introduced a number of years ago because if you activate cells in the presence of virus they can become infected, destroyed or can undergo activation-induced cell death. The availability of drugs now that can control virus means that we can activate the immune system without the newly activated cells being destroyed by the virus itself. Perhaps we can boost the immune system to the point where it can actually gain the upper hand over the virus. There are a number of approaches now in development to augment both CD4 and CD8 T-cell responses and I think it is an area that has the potential for some real benefit. I think that the concept of structured treatment interruption is teaching us a lot about how the immune system deals with the virus. However, there are concerns about taking people on and off therapy because of the risk of the development of resistance during washout periods when they're going to be exposed to sub optimal doses of drug. I think that is of potentially more concern in people that are chronically infected where a greater viral diversity has already developed. I also wonder whether some combination of therapeutic immunization and structured treatment interruption might be able to provide the equivalent of a prime-boost vaccine strategy in people that are already infected. I hope that within the next year or so we'll have some better answers.

GONSALVES: Louis, I'm going to give you the last word.

PICKER: Well, I basically agree with what Bruce has just said. The idea of structured treatment interruptions is to auto-immunize the subject to his or her own virus. HIV viruses are quite variable, and an individual's particular virus may or may not match up well with a generic vaccine virus. Auto-immunization with one's own virus during structured treatment interruptions obviates this mismatch issue. The negative aspect of this, of course, is the potential for the subject's rebounding virus to damage his or her immune system, and the possibility that such treatment withdrawal could lead to drug resistant viruses. Again, we are limited by our lack of specific knowledge, and we must pursue both approaches until we learn the specifics of which approach works best for which patients.

Gregg Gonsalves is Policy Director for the Treatment Action Group (TAG), an AIDS research advocacy organization, and works on policy issues concerning the basic laboratory science of HIV infection, vaccines, and immune-based therapies.