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The problem of the three players.

The Problem of the Three Players

Most medical scientists today believe that multiple sclerosis is an autoimmune disease, in which the body's immune system treats the myelin of the central nervous system (brain and spinal cord) as foreign. Our immune system is designed by nature to eliminate foreign invaders, such as viruses and bacteria, and so it mounts an attack on myelin. This leads to the inflammatory lesions with destruction of myelin characteristic of MS.

Details of the mechanism leading to myelin breakdown are being intensively pursued by immunologists on several continents. If what they think is happening is true, it may well be possible to block the damaging chain of events in those who have MS. It may even become possible to develop a vaccine to prevent MS in high-risk persons.

The story concerns myelin, immune reactions and genes. How these dovetail makes for an intriguing research drama which could have far-reaching implications. There are three principal players in this drama:

The Three Players

1) The myelin component (antigen) which the immune system recognizes as foreign, possibly MBP. Although the myelin component could be any of several proteins, myelin basic protein (MBP) has received the most attention. It can play the role of antigen in experimental animal models of MS and may well play this role in the human disease.

2 The T-cell receptor (TCR) for MBP is the second player. The T-cell or lymphocyte, is the cell of the immune system which recognizes the MBP antigen and thus starts the attack. It has many thousands of specific receptors for antigen on its surface. Different T-cells have different receptors but, on one cell, these are all of one kind.

(How T-cells in a person with MS first become immunized against MBP is another story. This may happen during adolescence as a result of exposure to viruses whose structure may resemble parts of the MBP Molecule. Suffice it to say that this person from then on has immune T-cells cruising the blood and tissues looking for MBP, the "enemy" they are trained to recognize with their TCR).

3) HLA-D, a cell surface component, is the third player. T-cells can only recognize antigen when it is associated with a cell surface component, which they must see at the same time as the antigen. This "dual recognition," similar to the two-key requirement for opening a safety deposit box, is Nature's way of guaranteeing that the T-cell will only discharge its function in close relation to an infected cell of the body and not at a distance. HLA-D is the cell surface component which plays this role. It is found only on a limited number of cells throughout the body which, as a result, can "present" antigen and HLA-D together to T-cells.

Where the Three Players Meet

In the brain, three quite different cells play the role of "antigen-presenting cell." The endothelial cells that line blood vessels are the first cells the cruising T-cells meet in the brain. For T-cells that actually leave the blood stream and enter the brain tissue, macrophages (the large scavenger cells found throughout the body) can play this role. Finally, the astrocytes (star cells) of the brain itself can, when excited, express HLA-D to present antigen, in this case MBP, to the T-cells. Whether one, two, or all three of these cells is involved, the encounter results in a turn-on of the T-cells and the start of inflammation and the attack on myelin.

MBP, one of the players, is a normal component of myelin. It undergoes constant renewal, like other tissue components, and is taken up by scavenger cells and other cells in the neighborhood. With the use of refined electron microscopic methodology, MBP has been demonstrated in or on all these three types of antigen-presenting cells in the brain.

How the Three Players Interact

How the three players interact is shown in the accompanying drawing. The HLA-D molecule is a protein made up of two different peptides, (peptides are chains of amino acids, the basic building blocks of all proteins) called alpha and beta. The TCR also has alpha and beta chains, plus some additional chains that do not concern us here. A small peptide of the MBP nestles into a groove in the HLA-D molecule on an antigen-presenting cell and the TCR, on an approaching immune T-cell, locks on to both.

Immunologists, who study these things, like to compare the exact fit of the MBP with HLA-D, and of the MBP-HLA-D complex with the TCR, with the perfect fit of keys in the locks for which they are designed. Without this perfect fit, no turn-on can occur. That means there would be no inflammation, no attack on myelin, and no MS lesions. If one could somehow block the three players' interaction, this should effectively block the MS process in a very specific way.

How close, then, are we to this ideal goal? It all depends on the exact identification of the players. In the last three years we have moved rapidly to identify two of them by looking behind the cell surface at the genes.

The Genes

Genes, of course, decide who and what we are. They consist of DNA molecules that contain the genetic code governing all our physical and mental characteristics. Among the hundreds of thousands of genes contained within the 46 chromosomes resident in each cell of our body, for the purposes of this story we are interested in just four. These are the genes that correspond to the alpha and beta chains of HLA-D and the alpha and beta chains of the TCR.

It is well to recall at this point that, in each case the DNA of the gene is an exact blueprint for the molecule we are interested in; the chain of bases in the DNA corresponds point by point to the chain of amino acids in the peptide. The techniques of molecular biology permit us to read the DNA text--that is, its sequence of bases--much faster and much more easily than it was possible previously to analyze the peptides for their sequence of amino acids.

In order to know which genes to sequence one must start with old-fashioned epidemiologic and family studies. Tissue typing techniques, first developed as an adjunct to organ transplantation programs, began to be applied in the early 1970s to comparisons of MS patients and their families with the surrounding normal population. This approach showed an association between MS and an HLA-D marker known as DR2 (and less frequently DR4 and DR6).

Dr. Daniel Cohen and his colleagues, at the Research Unit for Immunogenetics of Human Transplantation in the St. Louis Hospital in Paris, were the first to apply the new molecular biological techniques to the MS problem. Their results, published in 1984, suggested that MS was much more closely associated with another HLA-D marker called DQw1. The most striking validation of this finding has come from a recent study in Norway.

Professor Bodvar Vandvik of the University of Oslo in Norway, who was interviewed recently, observed: "We don't know the genes that determine a person's susceptibility to MS, but we have good evidence that at least part of the susceptibility may be located in the HLA system. We know that other diseases such as juvenile diabetes, rheumatoid arthritis, and ankylosing spondylitis show a strong association with HLA."

In the last few months, Dr. Frode Vartdal, a neurologist and student of Dr. Vandvik, now working in the transportation laboratory of the University of Oslo, decided to look at the relationships between MS and HLA-DQ. He first used standard tissue typing methods to study blood samples from a series of 61 Norwegian MS patients, and found that 59 of them were positive for DR2, 4 or 6. He was able to confirm this with gene hybridization techniques and made the exciting additional observation that all 59 were also positive for DQw1.

Specially, the beta chain of the DQw1 molecule was essentially identical for long stretches in all 59. Yet this same market was also found in 70% of healthy Norwegians. Dr. Vandvik comments, "What this suggests is that the DQw1 particularity could be one prerequisite for MS susceptibility but it almost certainly isn't the only one. There could be other genes that have to function together with this one in order to lay the groundwork. In addition, there are certainly some exogenous factors that trigger MS--an infection or viral invasion or something else."

The next step will be to look for the presence of DQw1 in other ethnic groups to determine whether that high percentage in Norwegian patients was due to the high prevalence of MS in Scandinavia.

"What we have to find out now," Dr. Vandvik says, "is whether DQw1 can be found in other MS populations in countries like Japan, for instance, where MS is rare. If MS patients in Japan or North Africa carry the DQw1 antigen, this would strengthen our case immeasurably."

Of the remaining genes of interest, the alpha and beta chains of the TCR are being pursued with great intensity by Dr. Stephen Hauser at the Massachusetts General Hospital in Boston, Dr. Lawrence Steinman at Stanford University, and cooperating teams of scientists at the National Institutes of Health and the California Institute of Technology. There are unique sequences in each of these chains, as well, associated with MS. As with the HLA-DQw1 beta chain, the findings must be validated in various genetically diverse populations.

What of the future?

Knowing the actual genes of two out of the three players, the alpha and beta chains of the TCR and the beta chain of HLA-DQw1, will permit us in the future to assess the level of MS risk in an individual.

This question is of the greatest importance to members of families in which there already are cases of MS and families in which other autoimmune diseases have occurred. Epidemiology and family studies thus far show that each of the genes we have discussed is associated with an increase in the risk of getting MS by 2 to 12 times, relative to the normal population. (The risk in the overall population of the U.S. and Canada is about 1 in 1000). Taken together, however, these genes may account for a risk of more than 1 in 10. This figure is sufficiently high to become a significant element of genetic counseling for those who are planning families. If additional important genes are found, even higher risk figures may become predictable.

The other question concerns possible treatments: Is it possible to block the chain of events triggered by a meeting of the three players in the brain? There are at least two possible methods. One might introduce a mono-clonal antibody against the HLA molecule or against the TCR, designed to bind to it in such a way as to prevent it from recognizing MBP.

Dr. Lawrence Steinman of Stanford University is working toward clinical trials of such an antibody (anti-DR2) in MS. Other groups have developed antibodies against DQw1 and are embarking on the necessary preliminary work before they can be tested in their patients.

Another approach is to synthesize an artificial peptide designed to fit that groove in the DQw1 or the one in the T-cell receptor (shown in red in the drawing). If a peptide fits either groove perfectly, it will keep anything else from moving in. This is a big project, because thousands of peptides might have to be synthesized to get the one that's just the right shape to bind tightly. Teams at Stanford and at the Imperial Cancer Research Fund in London are working on this problem.

But the best way to sabotage the whole process of T-cell triggering and demyelination would be by using a vaccine to induce a suppressor T-cell response that turns off other T-cells. To do that, however, one needs to know the identity of the triggering antigen. The day we know, we can go to work immediately on that vaccine.

As we know more and more about DQw1 and the T-cell receptor, we can figure out ways to block their action. But if we really knew the antigen, that would be the "jewel in the crown."
COPYRIGHT 1989 National Multiple Sclerosis Society
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Title Annotation:includes related information; myelin, T-cell receptor, HLA-D; mechanisms of myelin destruction
Author:Waksman, Byron
Publication:Inside MS
Date:Jan 1, 1989
Previous Article:Do you see yourself in this picture?
Next Article:Getting a grip on gait.

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