The Autoimmune Disease Process
Multiple sclerosis is referred to as an autoimmune disease. The general theory for the development of MS is that a genetically damaged immune system is unable to distinguish between virus proteins and the bodys own myelin and so produces antibodies that attack. In other words, the body becomes allergic to itself, a condition known as autoimmunity.
Theoretically, this condition develops when the body's immune system is damaged by genetic or environmental factors or both, causing it to attack its own tissues. In the case of MS, the immune system attacks the tissues that make up myelin:
- Myelin is made from layers of cell membranes that are produced in the brain and spinal cord by specialized cells called oligodendrocytes. The destruction of this myelin sheath during the disease process is the hallmark for multiple sclerosis.
- The myelin coat is distributed in segments along the axons, the long filaments that carry electric impulses away from a nerve cell.
- The segments are separated from each other by tiny clusters called nodes of Ranvier, which house channels for sodium ions. These sodium ions are important for boosting the electrical charge required to pass signals from one nerve to another.
- As the myelin insulation is destroyed, signals transmitted from nerve cell to nerve cell throughout the central nervous system are disrupted.
- Experts once believed that axons themselves were spared during the disease process. Research, however, has shown that many are severed in MS and, in fact, axon destruction appears to start at an early stage in the disease and may be a major cause of its irreversibility.
The body often makes corrective actions to offset the effects of the nerve cell destruction:
- For example, researchers have observed an increase in the density of the sodium channels, which carry electric charges. By increasing their numbers, the nerve cells can continue to communicate, in spite of the loss of myelin.
- The nerves also retain some capacity to remyelinate (to restore the insulating myelin).
Such processes are probably responsible for the remissions that most MS patients experience. Unfortunately, the disease process nearly always eventually outpaces these corrective actions.
Onset of Multiple Sclerosis: The Autoimmune Process and the Inflammatory Response
The Normal Immune Response.
- The most important critical immune factors in the disease process are white blood cells called lymphocytes, which consist of T-cells and B-cells. These cells are the warriors in the immune defense system.
- Receptors on T-cells acquire the ability to recognize specific molecules called antigens. Antigens are typically proteins from infecting organisms, such as bacteria or viruses, and perceived as a threat to the body.
- Once the antigen is identified, specific T-cells, called helper T-cells, trigger the B-cells to release antibodies. These molecules are designed to attach to and destroy the targeted antigen.
Autoimmunity.
- Multiple sclerosis, and probably all autoimmune diseases, involves an error in the education of T-cells, which makes them unable to distinguish self from non-self.
- In multiple sclerosis, the miseducated T-cells mistake molecules in the body's own myelin as a foreign antigen. (Such targets, then, are referred to as self-antigens.)
- In response to detection of these self-antigens, the T-cells set off the usual cascading immune events, including the release of B-lymphocytes, to rid the body of the perceived threat.
- The B-lymphocytes fire off antibodies as usual, but in this case they are referred to as autoantibodies, because they are attacking antigens that belong to the body's own self.
- In MS, the immune system is tricked into targeting self-antigens that are myelin proteins, the fatty insulation covering the nerve fibers. Another autoantibody target may be the oligodendrocytes themselves--the specialized cells that make up myelin.
- To make matters worse, the process perpetuates through a cascading series of events in which the B-cells and T-cells continue to interact, creating numerous different self-antigens. The attacks continue and, in the process, the original self-antigen is unrecognizable.
Cytokines and the Inflammatory Response. The inflammatory response is the product of an overactive immune system and is a major destructive force in an autoimmune disease.
- Once the lymphocytes have launched a response to an antigen, they also release masses of other white blood cells to gather at the injured or infected site.
- The major players in this response are white blood cells called leukocytes. Researchers are particularly interested in leukocytes called cytokines. These are small powerful proteins that, in tiny amounts, are indispensable for healing. When they are overproduced, however, which occurs in MS, they play a major role in the destructive process.
- Their intensive convergence on the affected area causes it to be become inflamed and injurious to the very cells they are designed to protect. Under normal conditions, this inflammatory process is controlled and self-limiting, but in people with autoimmune diseases such as multiple sclerosis, the process persists and damage occurs in the surrounding tissues.
Important cytokines in MS appear to be tumor necrosis factors, interleukin-12, and interferon-gamma. (Other cytokines, including interleukin-10 and transforming growth factor beta, may play a protective role and help block inflammatory activity.)
Axon Destruction and Progression of MS
The inflammatory response may trigger the disease, but afterward a progressive course takes over that does not appear to be related to inflammation. Experts have now found that destruction of axons, the long filaments that carry electric impulses away from a nerve cell, is a major feature of multiple sclerosis. In fact, it may be the major cause of permanent disability that occurs with this disease. Microscopic studies reveal that axons are injured early on as "bystanders" while myelin is being peeled off. As the disease progresses, these weakened and exposed axons degenerate further. Most of the damage occurs early in the disease process and decreases over time, although some destruction can still be observed years and decades afterward. Such evidence is having significant effect on approaches to treatment and research.
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