Myasthenia gravis therapies: an interview with Professor Daniel Drachman
Please could you give a brief introduction to myasthenia gravis?
Myasthenia gravis (MG) is an autoimmune disease that produces weakness and fatiguability of muscles. It affects between 1 and 7 people per 10,000, according to the best statistics.
Typically, it may begin with double vision and droopy eyelids. Weakness then often progresses to involve the face, the arms and legs, and the muscles concerned with breathing and swallowing. The name “myasthenia gravis” means serious (grave) muscle weakness.
Before the development of current treatments of the immune system, about two thirds of MG patients did very poorly or died. With present treatment, nearly all MG patients can be brought back to a nearly normal or completely normal life, although there may be problems with the side effects of the treatments.
What is known about the causes of myasthenia gravis?
MG is due to a mistaken response of the immune system that attacks the acetylcholine receptors of muscles. Every muscle movement requires the release of the transmitter acetylcholine (ACh) from motor nerves, which then crosses a very narrow gap to reach the ACh receptors (AChRs), and triggers contraction of muscles. Failure of transmission causes weakness or paralysis.
In MG, there is an attack by the immune system’s antibodies on the AChRs, which are decreased in number. This produces weakness, fatigue, and may cause paralysis and death.
The actual cause of the mistake by the immune system is not known, but there is evidence that a combination of a genetic susceptibility plus some environmental trigger leads to the autoimmune attack. The thymus gland is probably involved in the origin of MG.
What therapies are currently available for myasthenia gravis?
There are now a number of treatments for MG. The most immediate treatment is a drug (pyridostigmine, Mestinon) that blocks the enzyme acetylcholinesterase that normally hydrolyzes ACh after the event of neurotransmission. This drug allows ACh to persist longer, and to interact repeatedly with the reduced number of AChRs. It has a temporary beneficial effect, and is considered a sort of “band-aid” for MG.
Agents that suppress the immune system as a whole are used to treat the underlying autoimmune problem in MG. These include adrenal corticosteroids (Prednisone), and other general immunosuppressive agents, such as azathioprine, tacrolimus and cyclosporine. These agents suppress the whole immune system; properly used they are quite effective. However, they may have adverse side effects, including increased susceptibility to infection, and rarely malignancies, as well as other side effects.
How did your research into developing a gene-based therapy to stop the rodent equivalent of the myasthenia gravis originate? What did your research involve?
The goal of my research is to develop a treatment that will eliminate the specific abnormality that is responsible for the autoimmune problem without otherwise affecting the immune system.
To accomplish this, it is necessary to target those cells that are specifically involved in the autoimmune response. I set out to target the T lymphocytes which provide help for production of antibodies to AChR. So the first aspect of our research involved genetically engineering antigen presenting cells (the most efficient of which are Dendritic cells) so that they would target the T cells that specifically interact with AChRs.
A special issue involved in this is that each individual (or for that matter each mouse) with MG has its own unique set of T cells specific for different aspects of the antigen AChR (different epitopes). To deal with that complication, we would use dendritic cells from the same individual, which of course are capable of targeting the entire repertoire of that individual’s AChR-specific T cells. The targeting gene complex consists of a gene for the most important part of the AChR linked to a complex that directs it to the endosomal processing compartment, so that it is presented on the surface of the dendritic cell.
Once we had perfected the targeting method, we chose a “warhead” to destroy the T cells, and for this purpose we supplied our dendritic cells with the gene for Fas Ligand (FasL). FasL interacts with “Fas”, a protein on the surface of activated T cells, causing death by apoptosis.
The next step was to test the engineered dendritic cells in vitro. We found that they did indeed target and kill AChR-specific T cells, but not T cells that were specific for another unrelated antigen. Having been able to demonstrate the specificity of these “guided missile” dendritic cells, we then tested their ability to kill AChR-specific T cells in living mice.
We immunized the mice with two antigens – AChR and an unrelated antigen (KLH). We injected the mice with the “guided missile” engineered dendritic cells, and evaluated the T cells from the mice, and the production of antibodies in the mice. AChR-specific T cells from the treated mice were markedly reduced, whereas T cells specific for KLH were not reduced at all. Similarly, the antibodies against AChR were highly significantly lower in the Guided Missile treated mice than in those not treated with Guided Missile dendritic cells, whereas the antibody response to KLH was not affected by the AChR-specific Guided Missiles.
There are several special features to these results:
It is important to be able to use the individual’s own dendritic cells, so as to target the entire repertoire of that individual’s autoreactive T cells.
The targeting complex and the “warhead” must both be expressed by all of the dendritic cells. If only the targeting mechanism were present, the dendritic cells would stimulate – rather than eliminating – the T cells. On the other hand, if only the “warhead” were present without the targeting complex, the dendritic cells could damage other T cells, and damage other tissues, such as liver or lungs.
What impact do you think your research will have on Myasthenia gravis therapies and do you think your research will have any impact on therapies for other autoimmune diseases?
Our experiments have demonstrated a “proof of principle” i.e. they showed that the “Guided Missile” strategy of specific immunotherapy can be carried out in vivo to treat the autoimmune response in a model of MG.
Actually, a similar approach could be used for any autoimmune disease for which the antigen is known. I am hopeful that this, or a similar strategy will be translated for treatment of human MG and other autoimmune diseases. Of course, each part of the process must be developed according to “Best Clinical Practices” standards, which will require considerable development.
Do you have any plans for further research into this area?
We are currently studying the genetics of MG by carrying out a Genome Wide Association Study (GWAS). We have collected DNA from 1100 MG patients and some 3,000 non-affected control individuals of similar ethnic background.
In collaboration with Dr. Bryan Traynor at the NIH, we are comparing single nucleotide polymorphisms (SNPs) from MG patients with those from controls. So far we have found 3 SNPs that are highly significantly different in our MG population, and many more that are promising. These are being sequenced, and appear to involve strongly relevant genes.
Where can readers find more information?
The original artricle was published in J. Neuroimmunology 2012 251:25 – 32.
Further information on MG can be found in: