IL-2 Low Dose and Treatment Interruption: Interview with Kendall A. Smith, M.D.

Stopping antiretroviral treatment, even after long periods of suppression of HIV to below detectable levels, usually results in high levels of virus returning rapidly--although there have been a few exceptions, like the famous "Berlin patient," who stopped antiretrovirals and had very little HIV return, apparently because the immune system had become able to keep the virus under control.

At the recent large medical conference in San Francisco (ICAAC, the 39th Interscience Conference on Antimicrobial Agents and Chemotherapy), there was great interest in a small study which reported partial success in all nine volunteers who stopped antiretrovirals under certain conditions(1). HIV did return and reach high levels, but then was partly controlled by the immune system, without the patient restarting antiretrovirals. And unlike the Berlin patient, these volunteers were not recently infected.

This result surprised everyone, including the researchers who did the study. Although it is widely agreed that this protocol is not ready for general use, the research is an important step toward treatments which could enable the body to control the virus, reducing or possibly eliminating dependence on antiretroviral drugs for some patients, or improving the results of antiretroviral therapy.

This study was presented by Kendall A. Smith, M.D., whose laboratory first identified the IL-2 molecule and the IL-2 receptor in the late 1970s and early 1980s. We asked Dr. Smith to explain the new study to our readers.

Dr. Smith: Our dose was equivalent to about 2,000,000 units per day of the Chiron IL-2 preparation, for an averaged-sized adult, given by a single subcutaneous injection every day. At the NIH (the U.S. National Institutes of Health), the dose is 15,000,000 units per day--about seven and a half times larger. [Note: Dr. Smith's abstract published at ICAAC lists a dose of 250,000 units of Amgen's IL-2, which is not on the market in the U.S. The Amgen preparation is more potent, so more Chiron IL-2 units are necessary for an equivalent effect.]

ATN: To what extent do people have IL-2 side effects at the low dose?

Dr. Smith: We adjust the dose so that there are no systemic side effects, no fevers or flu-like symptoms; if people do get these effects, we reduce the dose. Then the only side effect is local inflammation at the site of injection-- which is bothersome, but has not been limiting; people have not discontinued their treatment because of the local inflammation.

ATN: What is the theory behind the low dose, and why it can be effective without the side effects?

Dr. Smith: IL-2 is a natural hormone produced by the body; it is one of the most critical hormones for the immune system. Studies in mice have found that if they lack IL-2, they have a severely compromised immune system.

The theory behind the low dose is based upon the IL-2 receptor. After we discovered the IL-2 receptor in 1981, we spent more than 10 years trying to understand the relationship between IL-2 binding to its receptor and IL-2 promotion of T cell growth. Then, within the past 5 years, in human volunteers we have studied carefully the IL-2 doses required to achieve the IL-2 concentrations sufficient to fully saturate the IL-2 receptors and to promote a maximal response from the T cells.

IL-2 interacts with lymphocytes which express IL-2 receptors on their cell surface. T cells express IL-2 receptors that have a very high affinity for IL-2, which means that you don't need much IL-2 to bind to them, because they hold onto it very well. Therefore low doses will produce low concentrations which are still enough to bind to the IL-2 receptors on activated T cells.

Another cell involved is the natural killer (NK) cell. About 90% of the natural killer cells lack one of the components of the IL-2 receptor, which lowers their affinity about a hundred fold. Therefore they are only affected if you use higher doses; then you can bind to the IL-2 receptor on all these NK cells. That causes toxicity, because there are about a billion circulating NK cells; if IL-2 binds to the receptors on all these cells, they become activated and make their own cytokines.

And the cytokines they make are the so-called pro- inflammatory cytokines; the prototypic one, and the worst source of side effects, is tumor necrosis factor. This excessive TNF causes the constitutional symptoms--fever, fatigue, and the so-called capillary leak syndrome--that are associated with high-dose IL-2. These are due to secondary cytokines released by NK cells, and TNF is the major one.

So the thing to do is to keep your doses low enough so that you bind to the high-affinity receptors on T cells, but avoid activating most of the natural killer cells. Also, our data indicate that constant low doses of IL-2 are better than intermittent high doses of IL-2, in that the low, continuous IL-2 concentrations prevent loss of cells due to apoptosis (programmed cell death).

ATN: In your study, when did patients give themselves the daily injection? And does the medication need to be refrigerated?

Dr. Smith: You can take the IL-2 whenever you want to; most of our patients have been taking it in the evening, before going to bed. It should be refrigerated, so they need access to a refrigerator.

ATN: What patients qualified for your study?

Dr. Smith: They had to have a CD4 count between 200 and 500, and be on successful antiretroviral treatment, with a viral load remaining less than 400. Then in our study, they continued their HAART treatment, and also received low-dose IL-2, for periods ranging from 3 months to one year. Then they could choose to enter the treatment-interruption part of the study, where they would stop HAART but continue the IL-2.

The purpose of the first part [the treatment with IL-2 plus HAART] was to see if we could use low-dose IL-2 to accelerate the recovery of CD4 T cells and other lymphocytes. We found that we could do that. During our studies, a group of patients essentially came back to normal in certain immunological parameters; and they had had undetectable virus for some time, because of the effectiveness of the HAART.

The second question was, is it possible to discontinue the HAART, and continue the IL-2, to augment and help the immune system, so that the immune system can help control any residual HIV in the body?

Also, there are other virus infections that, once you become infected, you never rid the body of the very last virus; instead, the remaining virus goes into a dormant or latent state. This is common in viral infections of the herpes family; examples are chicken pox, herpes simplex, and CMV. When you become infected, a normal immune system can suppress the replicating virus; you don't need any drugs. However, the body does not eradicate the very last virus; rather, it keeps the residual virus dormant. If a patient is later immunocompromised, either through drugs, or AIDS, or something else that might happen to them, then those viruses that had been kept dormant are no longer controlled, and they come back again.

The immune system is quite effective in keeping some viruses dormant. Therefore it might be possible to help it do the same thing with HIV as it already does with CMV.

ATN: When you gave IL-2 plus HAART for a year, did the CD4 count return to normal in all the patients, or only some?

Dr. Smith: It was a matter of time; some people took longer than others. But in a year or two of continuous HAART and IL- 2, you can get almost all of these individuals [starting with CD4 counts of at least 200, in this trial] back into the normal range of circulating CD4 cells.

Then the key question is whether they have specific immunity to HIV itself after their CD4 counts have returned to normal. The most important test to demonstrate specific immunity is to discontinue the antiretroviral drugs, and see if the immune system can either keep the virus dormant, or control its resurgence if it comes back.

So far we have only 9 individuals who have tried discontinuing the HAART, but continuing the IL-2. We found that in all these individuals the virus came back, in an average time of about two and a half to three weeks, and then it increased rapidly for two weeks, to a peak level. But during the next two weeks, the viral load fell precipitously, and arrived at a level approximately 10% of the peak. Our mean peak viral load was about 350,000 copies, which decreased to a mean of about 26,000--more than an order of magnitude (10-fold) decrease, which happened without any antiretrovirals. Apparently the body responded to the virus and brought the levels down.

CD8 Cells and IL-2

Dr. Smith: How does the body do this? When we studied the immune system, we found that at the same time the virus is coming back and increasing, there was an increase in the level of the killer T cells, the CD8-positive T cells [which are different from the natural killer (NK) cells mentioned earlier]. It is well known in immunology that these CD8 killer cells are the ones that fight off viral infections. And over the last several years, animal studies have shown very well that when you inject a virus, these CD8 T cells multiply very rapidly.

Two things make the cells multiply. One is the presence of the virus itself; the T cells recognize the virus. And when they recognize it, they express IL-2 receptors, and the CD4 T cells make IL-2, and then IL-2 is the agent that causes the CD8 T cells to multiply.

So the rapid expansion of CD8 T cells in our patients was mediated, I think, by the same kind of two-fold mechanism-- the cells recognizing the virus, and then responding to the IL-2 we were administering.

ATN: Is IL-2 known to be lower than normal in people with AIDS?

Dr. Smith: We don't yet have all the data on that. But there have been reports that the answer is yes, that there is a relative, but not absolute, deficiency of IL-2 producing capacity in HIV infection. We also have our own data, where we studied 50 HIV-positive individuals and compared them to uninfected, and we found evidence for a selective deficiency of IL-2 production.

I think that even healthy people, if confronted with a systemic infection, have difficulty making enough IL-2 fast enough to be able to handle that infection rapidly. So the therapeutic administration makes up for any deficiency of IL- 2 in persons with HIV, but also goes beyond that and corrects any deficiency that might be present normally.

ATN: And the CD8 levels stayed up?

Dr. Smith: That's a second part of it. In experimental animals, when the virus comes down and you no longer have it stimulating the T cells, then the CD8-positive T-cell count also comes down. The striking thing about our data is that the CD8 count did not come down, it stayed up. From animal tests, we now know that it comes down because there is not enough IL-2 to sustain the population; these CD8 cells then die due to apoptosis [programmed cell death]. The therapeutic administration of IL-2 seems to prevent that.

So IL-2 first expands the number of CD8 cells, but then it also keeps them alive and functioning, so they can more effectively react against any residual virus-infected cells.

ATN: Did these 9 patients end up with a stable viral load lower than they started with, before any treatment?

Dr. Smith: We have data on their viral loads before they started on HAART [from their medical records, since these patients started antiretroviral treatment before the IL-2 study began], and the level was lower after the treatment than before they started. The mean viral load before HAART was about 70,000 copies; afterwards it is about 25,000. This change was statistically significant. However, I think that we now need to do a prospective trial to answer the question as to whether our approach will lead to a lower viral set- point. That was not really the aim of this initial study, but our data provide hope that additional stimulation of the immune system may ultimately control viral replication altogether, just as it does in the herpes virus family.

ATN: How long have these 9 patients been off antiretrovirals?

Dr. Smith: Each patient consulted with his or her physician to decide whether to restart HAART treatment. So far 5 have done so. The other 4 are still off HAART, but receiving IL-2. So far the maximum time off HAART is 9 months.

ATN: What was the biggest surprise of this study?

Dr. Smith: We expected the HIV viral load to rise when antiretrovirals were discontinued, but we did not expect to then see it fall more than 10 fold without any antiretroviral therapy. Everyone though the viral load would go up and then reach a plateau [a steady level]. But from the literature on animal studies, we should not have been surprised, we should have anticipated this fall.

We expected the rise and plateau based on anecdotal reports in the past few years, of people stopping their HAART and the virus coming back again. But those patients were not followed as frequently and as carefully as our volunteers. People may have missed the viral load peak altogether. We did viral loads twice a week, and CD4 and CD8 counts once a week. If we had not done those twice weekly viral loads, we would have missed the peaks in some of the individuals--it could go up and down so fast that we could have missed it if we had only measured viral loads weekly. So it could look like patients plateaued, just because the researchers missed the peak.

ATN: Did the virus return more slowly with your volunteers than reported by others?

Dr. Smith: A report from Barcelona earlier this summer on ten patients treated for two years with HAART, without IL-2, noted that all ten had viral load return within two or three days after antiretrovirals were discontinued. And Tony Fauci and Rick Davey from his lab reported on 18 patients at the recent ICAAC meeting that had received HAART and had extremely low levels of virus, but in 17 of the 18 the viral load came back in about 10 days when treatment was discontinued, vs. our average time of 19 days.

Seventeen of these 18 went up to a high level and then plateaued. They did have one patient that looked like all 9 of ours--where the viral load went up and then came back down again.

ATN: I remember the joking reference at ICAAC to "the Bethesda patient" [from the well-known "Berlin patient" who discontinued antiretroviral therapy and was able keep the virus under control].

Are the big IL-2 trials coming up--one funded by NIAID and the other by Chiron Corporation--set up to address the information you have learned?

Dr. Smith: They are not. They have been working with a different hypothesis than we have, with a very different dosing regimen; it's like trying to compare apples and oranges.

ATN: My understanding is that the big NIAID trial is to see if those increases in T cells provide clinical benefit [in patients starting with a CD4 count of at least 300]--and the Chiron trial will study somewhat more advanced patients [CD4 T-cell counts between 50 and 300, but viral load under 10,000 copies]?

Dr. Smith: Yes, and the Chiron study is using a somewhat lower dose, about 10 million units a day instead of 15 million. All these people will get toxicity--which is the reason you can only give the treatment for five days, particularly with individuals who are trying to work and carry on a normal life.

The purpose of their experiments is to try to accelerate the repair of the immune system with IL-2. I believe IL-2 will do that, but that you don't have to subject people to the toxicity. You can lower the dose almost 10 fold and give it every day, instead of giving higher doses every two months.

Each of those studies could include a low-dose IL-2 arm, if they want to. Then there would be a comparison of the immune recovery, but without the toxicity.

ATN: Are there data on whether or not any CD8 count increase on the intermittent schedule will be lost in the seven weeks off IL-2?

Dr. Smith: The high dose, intermittent regimen causes first a decrease in all circulating lymphocyte subsets, including CD4 and CD8 T cells, NK cells, B cells, and monocytes, during the 5 day treatment interval. Later, there is a rebound when the IL-2 therapy is stopped, and all of the cells increase to about 10-fold the pre-therapy levels. Subsequently, over the next 7 weeks, there is a gradual decline of all of these cells to pre-therapy levels. So it is not clear whether there is simply a redistribution of cells from the circulation to the tissues during the treatment interval, then back again into the circulation after the treatment is stopped.

ATN: What was the role of your laboratory in the discovery of IL-2?

Dr. Smith: My laboratory discovered the IL-2 molecule, in the late 1970s--this was one of three phases of the discovery of IL-2.

First, the activity of IL-2 as a T cell growth promoter was discovered in 1976 by Doris Morgan working with Frank Ruscetti in Robert Gallo's lab at the U.S. National Cancer Institute. They found that something released from growing lymphocytes would make other lymphocytes grow--but they did not know what was responsible for the activity. This group did not follow up on their findings because they were interested in leukemia and retroviruses.

Later, we picked up this work and purified and characterized the active molecule, IL-2, in a series of experiments in the late '70s and early '80s.

Still later, Tadatsugu Taniguchi and others in Japan cloned the gene for IL-2, making it possible to use genetic engineering to manufacture it in large enough quantities that it could be used in treating patients.

ATN: And your new research isn't the end of the story, but a step along the road?

Dr. Smith: Exactly, it is just a beginning. However, I think that we now have seen a light to illuminate our way.

For More Information

A literature review of IL-2 therapy for restoration of immunity, by Dr. Smith and others, is scheduled for publication in THE AIDS READER, November 1999(2). A detailed technical background paper on IL-2 will soon be available online(3); also see clinical papers on the use of the low dose in therapy(4,5).

References

1. Jacobson EL, Emert R, Giordano M, Kovacs E, Mumneh N, Sohn T, Warren D, and Smith KA. Restoration of immunity after HIV infection. 39th Annual Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, September 1999 [abstract #1828].

2. Smith KA, Jacobson EL, Emert R and others. Restoration of Immunity with Interleukin-2 Therapy. The AIDS Reader (in press).

3. Smith, Kendall A. Interleukin 2 (IL-2). Cytokine Database (in press).

4. Jacobson, EL, Pilaro, F, & Smith, KA, Rational interleukin therapy for HIV positive individuals: Daily low doses enhance immune function without toxicity. Proceedings of the National Academy of Sciences (USA). 1996; volume 93, pages 10405- 10410.

5. Smith, KA, Rational interleukin 2 therapy.Cancer Journal. 1997; volume 3, pages S137-S140.