Limited Immune Recovery After Treatment with Antiretrovirals, IL-2; Interview with Clifford Lane, M.D.

A person's "CD4 count" (the number of CD4+ T-cells per cubicmillimeter of blood) is used as a rough indicator of immune system health. But the CD4 cells which are counted are not all the same. In a healthy individual, there are a billion ormore different kinds of CD4+ T-lymphocytes -- each programmed to recognize only one specific antigen (foreign substance, such as part of a protein produced by a bacterium or virus).

In HIV infection, when the CD4 count becomes low (especiallyunder 200), some of this "repertoire" of cells is lost; it is probably not immediately restored when the CD4 count rises inresponse to antiretroviral treatment (or to treatment with antiretrovirals plus IL-2 (Proleukin(R)), which causes growth of the CD4 cells which are still there). Eventually there may be some natural recovery of the repertoire -- or researchers might learn how to restore the repertoire by cell transplantation. Meanwhile, it may be possible for the immune system to function adequately even though some of itsdiversity is lost.

On May 31 AIDS TREATMENT NEWS interviewed Clifford Lane,M.D., Clinical Director of the U.S. National Institute of Allergy and Infectious Disease. Dr. Lane is senior investigator of a research team which published a recent paper on immune recovery after treatment of HIV with antiretroviral therapy, or with antiretrovirals plus IL-2. (1)To introduce the interview, we wrote the following background section to explain some of the immunology involved.

Background: Different Kinds of CD4+ T-Cells

AIDS TREATMENT NEWS often uses the term "CD4 cells" as a less complicated way of saying "CD4+ T-lymphocytes" -- which is technically more accurate, since there are CD4 cells which are not T-cells at all. This article is only concerned with T-cells, however.

T-lymphocytes are often named according to the molecules on their surface. CD4+ T-lymphocytes are those immune-system T-cells which have the CD4 molecule on their surface; CD8+ T-lymphocytes have the CD8 molecule. Both cells also have antigen-specific receptors on their surface. Each cell has approximately 10,000 or more copies of its antigen-specific receptor molecules on the cell surface.

The billion or more different kinds of these cells are due to a billion or more variations in the T-cell receptor. These variations occur when the cell is formed (usually near the beginning of life, in the womb or in early childhood). T-cells begin as undifferentiated stem cells -- special cells which are able to mature into any kind of blood cell. Certain stem cells are programmed by the thymus gland to become T-cells; when this happens, certain genes of the cell combine randomly, forming the immense number of different possible combinations. Many of the resulting receptors would cause the cells to attack the body itself, leading to autoimmune disease; these unwanted cells are destroyed in the thymus shortly after they are formed. A billion or more combinations remain -- probably enough to recognize some parts of all of the viruses, bacteria, or other disease-causing organisms the person will ever encounter.

Once a T-cell has been programmed, it can only recognize one antigen; its receptor cannot change further. It begins as a"naive" T-cell (one which has never encountered its antigen). If it does encounter its antigen (many cells never do), it changes into what is known as a memory cell. In a healthy adult, about half of the CD4+ T-cells are naive and half are memory cells; in people with HIV disease, the naive cells are more rapidly depleted, for reasons which are not entirely known.

Both naive and memory cells can divide and reproduce in the blood. This increases the number of cells, which can improve the immune response. But this kind of cell reproduction cannot expand the repertoire, because the new cells, whether naive or memory, are programmed the same as their parent (to recognize the same antigen). Only new CD4+ T-cells formed in a thymic environment can add to the repertoire -- and in adults, the thymus is largely inactive, so these new kinds of cells are usually formed very slowly.

Any group of cells -- usually just a few cells -- which have the identical receptor are referred to as a clonal population, since they probably all arose from a single cell(or clone). Since there can be a billion or more different clones in a healthy adult, it is not possible to count and study them all individually and know which ones are effective against which diseases. To make it possible to study this system, the clones have been divided into various families of related cells; one common system is to divide the many clones into "V-beta" families (the name is from a region of thegenes which provide a major part of the variability). The recently published work of Dr. Lane and others(1) studied 22 of these families (which probably account for at least 95% ofthe total diversity in this cell population).

In the following interview, we occasionally added explanatory comments; these are shown [in brackets].


Interview with Clifford Lane, M.D.

ATN: What has been learned about the loss of diversity of CD4+ T-cells in advanced HIV disease? How much of the immunity comes back when the total CD4 count increases after treatment with antiretrovirals, or with IL-2 in addition to antiretrovirals?

Dr. Lane: The ability of the immune system to come back, following antiretroviral therapy, will depend on how much has been destroyed. The reason for this limited recovery is that the immune system is initially generated by stem cell differentiation through a thymic environment -- and this particular pathway [involving the thymus] appears to play avery small role in the mature adult human.

This has been confusing to immunologists because much of the work on this aspect of immunology has been done with mice and rats -- in which the lifespan of the animal and the life span of the thymus are not that different. But in humans, thymic function drops off greatly in childhood and then drops again in puberty, and the adult has very little remaining. There is some left; how much varies between people.

To understand this, it helps to look first at what happens to the adult human immune system following a major insult when HIV is not there -- for example, when the system is decreased through radiation or chemotherapy. The immune system can come back, but generally not all the way back to where it was.

With HIV, it is now clear that as immune health declines, the diversity in the repertoire of cells declines. You can block the virus and allow the total count to come up -- but what comes up, at least immediately, is mainly an expansion of the cells that were already there. The number of cells can increase to a new plateau, which may be higher or lower depending on the patient, and depending on the level of ongoing viral replication.

Even though the count does not go all the way back to normal,and some of the original diversity of cells is missing, you might still have an adequately functioning immune system. This is because the immune system has great redundancy in it. If the virus can be stopped, then with repeated antigenic challenge, what had been in effect a second-line set of clones can expand and become a first-line defense. The T-cell immune system's ability to adapt to the environment is phenomenal; this adaptability is its key characteristic.

Animal studies suggest that the number of different T-cell receptors you need [to maintain good health] is much smaller than the number you can make. You do not necessarily need agreat number to be protected. In studies in rats, in which cells were transferred from a healthy animal to an ablated animal [one with its immune system destroyed], it was found that the best protection was from the memory cells compared to the naive cells -- and that it only takes a transfer of about a thousand clones to provide a good defense [in the animal; no similar experiment could ethically be done with humans]. The immune system can adapt, and low-affinity clones [those which are not especially good in recognizing a disease-causing organism] can expand to meet the needs of the environment.

ATN: And in HIV disease, the loss of diversity begins even at CD4 counts above 200, but becomes worse below 200?

Dr. Lane: It appears to be more pronounced as the count drops below 200.

ATN: Is there a similar repertoire with CD8 cells as with CD4 cells?

Dr. Lane: The CD4 repertoire is quite similar over time for a person, almost like a fingerprint; we can look at a diagram of the repertoire even after a year or more, and often tell whose blood it is. The patterns do change if a person has HIV infection, but the changes tend to be gradual over time. CD8 T-cells, on the other hand, are always coming and going in the blood, and they have a very chaotic repertoire. This difference may reflect the role of the CD4 cells as the control center of the immune system, vs. some of the more effector limbs of the system [such as the CD8 cells], that need to change more rapidly to deal with whatever the emergency is. The CD4 cells are more for surveillance, keeping an eye on things.

ATN: I heard that in cancer, after chemotherapy there could be some slow immune recovery over years.

Dr. Lane: That is true, but usually the system does not get all the way back to normal. Part of this slow recovery overtime may be a trickling of cells through a thymic environment, in patients who have some remaining degree of thymic function.

ATN: What does this mean for prophylaxis of opportunistic infections -- pneumocystis, for example?

Dr. Lane: To me the first message is that if you are following a patient with a CD4 count of 50 who is on Bactrim,and start combination antiretroviral treatment and a month later the count is over 200, I would not stop prophylaxis at that point, as [immune recovery] takes time. Six months out, it would be a good question for a clinical trial. We simply do not know the answer today. Each physician needs to use his or her own judgment about the risks and benefits of prophylaxis [when it might or might not still be necessary, after partial immune recovery due to antiretroviral treatment]. The risk of an opportunistic infection is probably somewhat less [after the T-cell count rise] -- but how much less is unclear. It's not black and white; the immune system is not static but is actively changing, to try to do its best in host defense. Once the negative influence of HIV is taken away, what is left of the immune system can probably do a pretty good job.

IL-2 and CD4 Cell Increase

[Note: Dr. Lane suggested that AIDS TREATMENT NEWS disclose that, as part of his government duties, he received a patent for his work with IL-2.]

ATN: In the recent paper(1), your team studied not only antiretroviral therapies to increase T-cell counts, but also IL-2 [which stimulates the growth of T-cells]. Is it the same picture with IL-2, of increasing only the cells that are already there?

Dr. Lane: Yes, with IL-2, the cell increases you get are an expansion of the cells already present. But there is one slight difference. The paper (1) looks at the increase in two different ways. Besides the V-beta families, our study also measured naive vs. memory cells, and found that in the setting of antiretroviral therapy alone they increased in parallel. (Naive cells are depleted most rapidly in HIV, and some patients with CD4 counts under 50 may have almost no naive cells.)

With IL-2 there is some preferential increase in naive cells. But though this makes the ratio of naive to memory cells more normal, it is probably not a benefit -- it may make the increase with IL-2 a little less desirable than the increase with antiretrovirals. Because with antiretrovirals, the cells that increase will tend to be in proportion to what antigens, what pathogens, are there. So, with antivirals alone, you may get a cell expansion which is directed more appropriately to each individual patient -- while with IL-2 you may get more of an expansion of whatever clones are there.

I do believe that the new cells [produced by IL-2 treatment] are totally functional CD4 T-cells.

ATN: But the only way to prove that is a large clinical trial-- since we do not have an accepted marker, like viral load, for immune treatments?

Dr. Lane: I feel very strongly that a phase III trial is needed. Before using IL-2 on a large scale for HIV treatment, we need to be sure that its benefits outweigh its disadvantages. There is often a burst of virus when IL-2 stimulates T-cell growth. And there are side effects of this drug. The T-cell increases are substantial and impressive. But unless they provide clinical benefit, they would not be very meaningful.

The problem is that to deliver IL-2 easily, you want to work with an earlier patient population -- because then you can use lower doses and give the drug less often. But to obtain clinical endpoints in an early cohort requires many patients to be followed for a long time.

This is a central challenge now in HIV medicine -- how do we determine the best way to manage patients? While short studies looking at changes in viral load will be helpful in determining the most potent antiviral drugs, I am not sure they will give us a clear picture on how best to manage patients over years.

ATN: What patient population should be studied with IL-2?

Dr. Lane: We are talking about those with a CD4 count of 350 and above -- and letting the antiretroviral strategy be at the discretion of the physician-investigator.

ATN: In that case, if the antiretrovirals work well enough, the trial will not get clinical end points, and it will not prove anything. But in case the antivirals do not work well enough over the long period, then you will see if there is any additional benefit from the IL-2. Is that the rationale of this trial?

Dr. Lane: Exactly. You are trying to increase your margin, increase the safety net. If drug resistance eventually develops, the patient will probably be better off with a CD4 count of perhaps 1,000 than with a CD4 count of 200. But if we can totally block the virus and maintain a healthy immune system with antiretrovirals alone, there will be no role for IL-2.

We now know how to use IL-2 with less toxicity than before and how to generate similar results with self-administered subcutaneous dosing (2), especially in early patients (in advanced patients you need higher doses). In early patients,my guess is that after three 5-day cycles of IL-2 over a six month period of time, we will be able to almost double the CD4 count. Then we can spread out the cycles of IL-2 -- maybe once a year, or once every two years.

ATN: Measure the CD4 count and give IL-2 only when they need it?

Dr. Lane: Yes. This is like the induction vs. Maintenance used with many different kinds of therapies.

ATN: But with advanced patients you would have to treat more often?

Dr. Lane: Probably. It would become a more difficult clinical trial to carry out. And the answer for one group of patients should apply to the other as well -- since the key question is whether increasing the CD4 count as a result of IL-2 therapy is of clinical benefit. I think that the strategy of running a trial with more patients for a longer period of time, but with a regimen which is easier to administer, is probably more effective [than a smaller trial which uses a higher dose of IL-2 with more side effects].

ATN: The ethics and acceptability of this trial look quite good. It would not interfere with peoples' antiretroviral treatment at all; they would use whatever they want, and the IL-2 treatment would not be onerous.

Dr. Lane: This trial would find out if the benefit of T-cell expansion with IL-2 is enough to warrant the cost and trouble, and the side effects.

ATN: Is such a trial likely?

Dr. Lane: I believe we are going to do it -- but am not yet entirely sure of all the researchers and organizations who will be there when we start. A group of European and U.S.investigators who have expressed strong interest in such a trial will be at a meeting in mid June to make some final decisions on a draft protocol for review by the entire group. Hopefully at a meeting of the CPCRA (NIAID's Community Program for Clinical Research on AIDS) in July, we will come to final decisions on the protocol. In addition to the CPCRAand intramural NIAID, a consortium of European and Australian investigators, and the Canadian trials network, are currently part of this effort. My hope is that we will build the support close to a critical mass, and then get the additional help that we need so that this trial can happen.

The data are quite solid. The clinical data we have fit with the immunologic data. The viral levels certainly have not been going up in the randomized study. Our knowledge of many of the immunological and virological subtleties is in place as well.

[Note: An earlier clinical trial in 60 patients (3) found that when IL-2 was added to an antiretroviral regimen for 12months, the average CD4 count increased from 428 to 916. In the control group with the same treatment except for the IL-2, the average CD4 count fell from 406 to 349. But this trial was too small to show whether or not increasing the CD4 count in this way was beneficial to patients. In the long-term followup of this cohort there have been seven patients with AIDS-defining events among the patients originally randomized to the control arm, compared to two AIDS-defining events among the patients originally randomized to IL-2.]

References

1. Connors M, Kovacs JA, Krevat S, Gea-Banacloche JC, Sneller MC, Flanigan M, Metcalf JA, Walker RE, Falloon J, Baseler M, Stevens R, Feuerstein I, Masur H, and Lane HC. HIV Infection induces changes in CD4+ T-cell phenotype and depletions within the CD4+ T-cell repertoire that are not immediately restored by antiviral or immune-based therapies. NATURE MEDICINE May 1997; volume 3, number 5, pages 533-540.

2. Davey, Jr., RT, Chaitt DG, Piscitelli SC and others. Subcutaneous administration of interleukin-2 in HIV-1 infected individuals. JOURNAL OF INFECTIOUS DISEASES 1997; volume 175, number 4, pages 781-789.

3. Kovacs JA, Vogel S, Albert JM, and others. Controlled trial of interleukin-2 infusions in patients infected with the human immunodeficiency virus. NEW ENGLAND JOURNAL OF MEDICINE October 31 1996; volume 335, number 18, pages 1350-1356.