Researchers’ Lofty Goal: Transplants Minus Drugs
Using the new techniques of biotechnology, immunologists are zeroing in on a number of new approaches for disarming the immune system so that it can no longer mount an attack on a transplanted heart, kidney or other organ.
For the short term, these new approaches will complement anti-rejection drugs such as cyclosporine, both in the initial phases of a transplant operation and when the patient undergoes a severe rejection episode.
But for the longer term, the researchers have an even more ambitious goal: to eliminate drugs entirely. Research being conducted in animals and humans suggests that it may be possible with one short treatment at the time of the transplant to render the body permanently receptive, or tolerant, to the donor organ.
At one stroke, researchers might eliminate major side effects, such as kidney damage and an increased incidence of cancer, that are associated with the use of anti-rejection drugs, while leaving the immune system fully intact to fight off infections and cancer. They also would greatly reduce the cost of transplant surgery, which is usually more than $100,000. In addition, recipients now must take immunosuppressive drugs for the rest of their lives at a cost of $6,000 to $10,000 per year. That cost would be eliminated.
Producing tolerance to transplanted organs “is more than just a pipe dream,” said Dr. Vaughn A. Starnes of the Stanford University Medical Center. “We think it’s possible.”
The techniques being used may have other applications as well. The process of organ rejection is similar to the so-called autoimmune reactions characteristic of arthritis, diabetes and some other diseases, in which the immune system attacks cells of its own body. The same techniques used to prevent organ rejection may work equally well to shut off that attack and prevent the progression of these diseases.
But researchers have to proceed carefully because these immune reactions also are necessary to protect the body from viral and bacterial infections. Physicians must thus shut down organ rejection processes without impairing the body’s immunity to disease.
Overall, researchers are studying three ways of manipulating the immune system. One involves using decoys to intercept naturally occurring molecules that activate immune defenses, thereby preventing the rejection mechanism from beginning. The other two employ different ways of destroying the white blood cells called T-lymphocytes that attack foreign organs, either by selective irradiation or with specialized laboratory-created antibodies called monoclonal antibodies to destroy the offending T-cells.
Preliminary results have been quite impressive. Studies in rodents, dogs and primates show that monoclonal antibodies and irradiation can induce lifelong tolerance to transplanted organs. In South Africa, one woman who received radiation for a kidney transplant has lived for five years without taking anti-rejection drugs, and at least two others have gone without drugs for shorter periods.
Monoclonal antibodies already have been widely used in humans to prevent rejection of organs, and preliminary attempts to induce tolerance with the antibodies are expected to start later this year. Scientists also hope to begin using the decoys in humans within the next year.
Although researchers have not yet traced all the intricate mechanisms by which the immune system is activated, it has become clear that a key component is the hormone interleukin-1, which is produced naturally by white blood cells in response to infections, inflammation or the presence of foreign tissues.
It binds to specific receptors on the surface of other white blood cells and stimulates them to proliferate, to produce other chemicals that also stimulate the immune system and to produce antibodies that attack foreign materials.
Molecular biologist William C. Fanslow and his colleagues at Immunex Corp. in Seattle have developed a way to intercept the interleukin molecules before they can stimulate the immune system. Using genetic engineering techniques, they have isolated and cloned part of the cellular receptor for interleukin-1.
When Fanslow injected a small amount of the receptor into rodents and then attempted to transplant foreign tissue into the animals’ ears, the receptor nearly doubled the length of time the transplants survived--increasing it from 10 days to an average of 17.
The experiment was designed to test the “worst case” transplant scenario. “This is a very, very severe model of graft rejection, like purposefully trying to use a donor that is as different from the recipient as possible,” said Steven Gillis, director of research and development at Immunex. “It’s a model in which a number of drugs, like cyclosporine, won’t work at all.”
Immunex plans to begin clinical trials of the receptor in humans in 1991. But first, Gillis said, the company wants to improve the manufacturing process so it can produce it more efficiently and cheaply. According to Food and Drug Administration rules, any testing done before that process is finalized would have to be repeated before approval could be granted.
The other approaches involve an attack on T-lymphocytes, especially the killer T-lymphocytes that actually attack the foreign tissue and the helper T-lymphocytes that orchestrate the attack. (A third type of white cell, called suppressor T-lymphocytes, suppresses the attack.)
This technique has a twofold benefit. Of primary importance, destruction of the T-cells immediately halts the attack on the transplanted organ, and can end a rejection episode.
But researchers have observed a curious phenomenon. As the immune system produces new T-lymphocytes to replace those that have been destroyed, the new helper and killer T-lyphocytes seem to consider the transplanted organ as part of the body and do not attack it. Once this tolerance develops, it seems to last indefinitely.
One of the new techniques being used to destroy the T-cells actually is a variant of one of the oldest procedures used to suppress the immune system. In the 1940s, researchers at Harvard Medical School used whole body irradiation with X-rays to suppress the immune system for kidney transplants. The transplanted organs survived, but the recipients didn’t; their reduced immunity allowed them to be overwhelmed by infections.
This problem can be overcome, however, by a more selective irradiation of just parts of the immune system, a process called total lymphoid irradiation or TLI. In this process, only the lymph glands, where immature T-lymphocytes are processed to become one of the three functional types, are irradiated. The bone marrow and other crucial parts of the body are protected.
The treatment rids the body of all T-lymphocytes, according to immunologist Samuel Strober of the Stanford University Medical Center. The suppressor T-cells reappear in about a month, he said, while the helper and killer T-cells reappear over a period of six to 12 months.
Experiments in many laboratories have shown that the technique will induce tolerance to transplanted organs in rats, mice, dogs and baboons. Research at the University of Johannesberg in South Africa has shown, for example, that “TLI given to baboons will produce tolerance in the majority of the animals,” Strober said.
Strober estimated that about 200 people around the world have received transplants with TLI, but virtually all are taking small doses of anti-rejection drugs as a precautionary measure.
Strober is monitoring 25 patients who received TLI for kidney transplants performed by surgeon Barry Levin of the Pacific Medical Center in San Francisco five to seven years ago. Most, he said, are doing very well on low doses of a single anti-rejection drug, the steroid prednisolone, which is most commonly given only in conjunction with cyclosporine.
Two of the patients developed problems with the steroid, and Strober decided to withdraw drugs completely. One patient’s kidney was not rejected, but he lost it after 10 months because of a surgical complication. He got a second transplant and then began conventional therapy. The second patient, a 60-year-old woman, has not taken anti-rejection drugs for 18 months and has not had any problems.
Strober also is monitoring a third patient who received her kidney in South Africa seven years ago and has been off drugs for five without any problems.
Strober and Levin are planning another series of transplants after they refine the procedure somewhat. In particular, they want to perform the radiation treatment after the transplant rather than before, so they will have greater flexibility in choosing patients.
“The work I am doing has not revolutionized the field, primarily because I have only three cases to report,” Strober said. “I don’t know whether it will usher in a new era, but it is my guess that this is the beginning of a lot more . . . ways to perform transplants without follow-up drugs.”
By far the greatest amount of work is devoted to the use of monoclonal antibodies to prevent rejection. In fact, one such product, called OKT-3, already is being marketed. OKT-3 kills all T-lymphocytes in the body and is widely used both as an adjunct to anti-rejection drugs at the time of a transplant and to overcome rejection episodes.
It has two primary drawbacks, however. One is that it kills all T-cells, even those involved in defending against viruses and bacteria, thereby increasing the patient’s risk of infection. The second problem, which affects all monoclonal antibodies now being studied, is that they are made in mouse cells. They can thus be used in a patient only once because the body creates antibodies that destroys them.
Two newer antibodies, called OKT-4 and OKT-4a, are very similar to OKT-3, but attack a smaller subset of T-lymphocytes that are directly involved in organ rejection. These are “very effective at immunosuppression in animals,” according to Dr. Benjamin Cosimi, a surgeon at Massachusetts General Hospital in Boston.
Cosimi will report this month in the journal Surgery that single doses of OKT-4a in 30 monkeys who received kidney transplants prolonged survival from about 10 days with no treatment to as long as 10 weeks. “To get that kind of survival from just one injection is quite dramatic,” he said. He and others are applying to the Food and Drug Administration to begin trials in humans later this year.
He already has begun clinical trials with another monoclonal antibody developed by immunologist Robert Rothlein of Boehringer Ingelheim Pharmaceuticals Inc., of Ridgefield, Conn. This antibody is directed against a molecule called intercellular adhesion molecule-1, also known as ICAM-1.
ICAM-1 is a receptor molecule that is displayed on the surface of certain tissues--such as those of donor organs--during an inflammation reaction. This molecule must be present for killer T-lymphocytes to attack the tissue. When ICAM-1 is enveloped and obscured by the monoclonal antibody (called anti-ICAM-1), the T-cells can no longer attack it.
In experiments on 16 monkeys, Rothlein and Cosimi showed that a single injection of anti-ICAM-1 could triple the survival time of transplanted kidneys. In separate studies, they showed that it could also reverse a rejection episode triggered when the amount of cyclosporine given to the animals was reduced below minimal levels.
Cosimi has just started trials in human kidney recipients. “We only have patients who have gone as far as 10 weeks, but so far it looks encouraging,” Cosimi said.
The antibody is particularly useful for kidney transplants because cyclosporine can impair functioning of kidneys, especially those removed from cadavers--the most common source of donor kidneys. Anti-ICAM-1 can be used at the time of the transplant to allow the kidney to begin functioning normally, Rothlein said, and the patient then switched over to cyclosporine.
Researchers are studying “hundreds and hundreds” of other monoclonal antibodies, Cosimi said. “You can make them very simply. The question is which ones to study in humans,” he said. That problem is complicated, he added, because most must be studied in primates, which makes preliminary studies expensive. (Because each antibody is highly specific, one designed to attack mouse lymphocytes will not attack primate cells, and vice versa.)
One of the most promising of the new antibodies has been developed by immunologist C. Garrison Fathman of the Stanford University Medical Center. It is directed against a molecule, called CD-4, that is present on the surface of helper T-lymphocytes. The antibody is similar to OKT-4, he said, but more effective in ridding the body of all helper T-lymphocytes.
The anti-CD-4 antibody has produced very dramatic results in mice and primates. When the antibody is administered to diabetic mice before a transplant of insulin-secreting pancreas cells, for example, they become permanently tolerant to the donor cells and need no further treatment.
Similar studies in rats, mice and monkeys show that tolerance is also developed to transplanted kidneys. And because only the helper T-lymphocytes are affected, immunity to infections returns quickly, in about 30 days, Fathman said.
Fathman hopes to begin using the anti-CD-4 antibody in human kidney transplant patients later this year. He and Stanford oncologist Ron Levy are testing it for safety in patients with mycosis fungoides, a lymphoma (cancer) of T-lymphocytes.
“When they have failed conventional therapy, we hope that monoclonal antibody therapy might be useful against this nasty and aggressive (cancer),” he said.
They have found that the antibodies are safe, Fathman said, but they have not yet shown they are effective.
