Oral tolerance: a new approach to therapy?
There’s a hoary “perfect murder” story in which the villain prepares for months by taking tiny, ever increasing amounts of poison. Then he calmly shares poisoned food or drink with his victim- who drops dead leaving him unscathed. The murderer developed tolerance for a toxin that should have killed him.
The human immune system also develops tolerance for the human body it guards. Immune defense is based on the ability of immune-system cells to tell one protein from another. And it’s remarkably subtle. It’s not only able to tell “self” from “not-self’, it can also discriminate between “okay” and “dangerous”.
The digestive system, for example, encounters foreign proteins every day. Some are bacteria that need to be deactivated. But most are food, and most of the time, the body mounts no unwanted attack, even if it has never before experienced fried squid or a bean burrito. Luckily for us, the immune system in the gut has a lifelong ability to learn.
The phenomenon is called oral tolerance. It was first described scientifically in 1911, when a Dr. Wells showed that allergic reactions to hen’s eggs could be reduced by feeding animals small amounts of hen’s egg proteins. Native Americans understood the principle of oral tolerance long before that. They made themselves resistant to poison ivy by eating tiny portions of the toxic leaves in special ceremonies.
MS specialists believe that MS is an “autoimmune” disease. That is, its symptoms are caused by immune system attacks on the body’s own proteins- attacks initiated by lymph cells called T-cells.
In MS, T-cells that should react only to unwanted foreigners begin to respond to proteins that are part of myelin sheathing, the material that encloses individual nerve fibers. The T-cells’ job is to orchestrate attacks on targets they “recognize”- ultimately calling in other cells that disable and devour proteins they have identified as enemies. Many scientists believe myelin basic protein, or MBP, may be one of several of the targets of immune attacks in MS.
What if tolerance to MBP – which is missing for unknown reasons- could be recreated by eating small doses of it? Could the immune system’s ability to learn tolerance to edible foreign proteins be harnessed to teach tolerance to proteins that belong in the body?
“The premise is so simple, it’s beautiful,” said Dr. Caroline Whitacre, a professor of medical microbiology and immunology at Ohio State University, who is in the midst of laboratory research on oral tolerance, supported by funds from the National Multiple Sclerosis Society.
The first took the concept to the laboratory in 1982. She tried inducing tolerance to myelin in a specially bred strain of laboratory rat that is susceptible to experimental allergic encephalomyelitis, or EAE. Animals do not get MS, but EAE is also a selfagainst-self immune disease, in which myelin is destroyed. Because it bears so many similarities, it is widely used as a “model” for MS.
The first results were overwhelming, Whitacre remembers. After a regimen of myelin feeding, the test animals were completely resistant to EAE. But the next ten experiments didn’t work at all- and Whitacre spent the better part of a year trying to figure out what was different about experiment number one.
She came to believe that the answer was embedded in the digestive process. The MBP molecule can survive being boiled, frozen, or chewed by teeth, but digestive enzymes break it into fragments. Her experiments were successful again only after she combined MBP with special buffers to protect it from digestion.
At almost the same time, and without, initially, knowing of Whitacre’s work, another researcher began to study the same question. Dr. Howard Weiner, a professor of neurology at Harvard Medical School, who heads the Multiple Sclerosis Unit at Boston’s Brigham and Women’s Hospital, also fed lab rats MBP and then tried to give them EAE. Although he used no special buffer for the MBP in his research, he too found that MBP-fed animals could resist developing the MS-like disease.
The two research groups have gone on to mirror the human MS situation more closely, by feeding MBP to animals that already have EKE. Both the Weiner and Whiracre groups now have a body of data that show myelin destruction can be slowed, even halted, in animals with a controlled laboratory disease. But their results are sufficiently different for them to think they have uncovered two different mechanisms for suppressing autoimmune reactions. “In fact, I believe there may be still other mechanisms involved,” Weiner said.
“A T-cell can ‘see’ a target protein only if the T-cell possesses a docking site on its surface that fits exactly around a section of the target protein,” Whitacre explained. “Only a few of the hundreds of thousands of T-cells in an EAE rat have a docking site that fits MBR This docking site is called a receptor. But once a T-cell with an MBP receptor contacts MBP-reactions begin. Rapid replication of that particular T-cell is one of the reactions – and soon there are hundreds of MBP-sensitive T-cells in the area.”
Whitacre’s research has led her to think that tolerance to MBP develops through a mechanism called T-cell “anergy” which reverses that process.
It works this way: When MBP is digested it is taken up by special cells that line the digestive tract- and there it is “stamped” okay, in the same manner as are other foreign food proteins. When T-cells with MBP receptors meet this version of their target, they are silenced. Instead of initiating an immune response, they lose their ability to replicate and to produce their powerful signalling chemicals. This is the same state reached by other T-cells that might have prompted allergies to cheese or wheat. Sooner or later, anergic T-cells die. Thus anergy can eventually eliminate all the MBP-sensitive T-cells, while leaving the rest of the immune system healthy and ready to protect the body.
Weiner’s research leads him to the idea that tolerance occurs through “active suppression”. He and his colleagues have been able to transfer myelin tolerance from one animal to another by transplanting lymph nodes or spleen tissues. This seems to show that tolerance is an active process, for transferring paralyzed or dying T-cells wouldn’t affect the active anti-myelin T-cells in the recipient.
“Active suppression” works this way: T-cells that have been sensitized by the MBP stamped “okay” by the digestive system will secrete a chemical called “transforming growth factor beta” or TGFBeta when they see MBP again. In the brain, where MBP is abundant, they pump out quantities of this powerful signal. And there it has a potentially important effect, for TGFF3 can calm down or stop any immune reaction taking place in brain tissues nearby.
Does the mechanism really matter, if tolerizing works?
“If oral tolerance works only through T-cell anergy, we might have to find the right part of myelin to feed each person with MS,” Weiner said. EAE animals are so inbred that genetic differences between them are just about nil, he pointed out. But there is enormous individual variation in peopleand different forms of MS. It’s possible that each person with MS has T-cells that recognize different myelin proteins. If T-cells create tolerance by secreting TGF BEta, then the road to a therapy for people might be smoother, Weiner thinks.
“If oral tolerance works only through active suppression driven by TGFBeta, we might have down-regulation of the immune system following any oral antigen,” Whitacre said. “The answer may lie in involving both mechanisms.”
Whitacre has begun to study oral tolerance in another strain of lab rodent that develops a less predictable “chronic relapsing” form of EAE. Weiner and his colleagues have made the leap to clinical trials.
“Oral tolerance is a system nature put into place,” he said. “We expected it to be safe, and the animal studies confirmed that.” His group has undertaken a small double-blind pilot study in which people with early relapsingremitting MS were given specially prepared myelin proteins by mouth, daily, for one year. Studies that will involve multiple centers and larger numbers of people should begin sometime next year.
Better understanding of the mechanisms and results from detailed clinical trials are both needed to answer the big question: Can oral tolerance help people with MS? The two research teams are in close contact with each other- and are sharing protocols, which are detailed plans of experimental treatments- in their mutual search for the facts.
COPYRIGHT 1992 National Multiple Sclerosis Society
COPYRIGHT 2004 Gale Group