Immune function is helped by two kinds of white blood cells. The “B cells” (so-called because they develop in bone marrow) produce antibodies. The “T cells” (so-called because they develop in a small organ called the thymus gland) are responsible for a variety of other immune responses. These responses include: 1) direct attacks on foreign substances such as bacteria, viruses, or foreign tissues; 2) augmenting the B-cell response; and 3) producing substances called cytokines that direct responses and activities in other immune cells.
Three Broad Categories of T cells
There are three broad categories of T cells.
- Helper T cells augment the immune response by recognizing the presence of a foreign antigen and then stimulating antibody production and producing cytokines that “turn on” or activate other T cells.
- Regulatory T cells function in an opposite manner. They dampen or turn off the immune response.
- Cytotoxic or “killer” T cells directly attack and destroy cells bearing antigenic material.
Individual T cells are able to recognize only certain antigens. Their ability to discriminate between antigens is conferred by protein molecules on the cell surface called receptors. The receptor and the antigen fit together like a lock and a key only when their shapes match perfectly. The number and specificity of T-cell receptors appears to be determined by the cell’s genes.
It is now generally accepted that the demyelination seen in MS is caused by an abnormal autoimmune process—that is, by activation of T cells (and perhaps B cells) against some component of central nervous system myelin (the fatty sheath that surrounds and insulates nerve fibers). Demyelination—the destruction of myelin—causes nerve impulses to be slowed or halted and produces the symptoms of MS.
Much More Now Known about Roles of T Cells
Over the last 15 years, much knowledge has been gained about the specific roles of T cells. Among the activities that have been observed are:
- Decreased regulatory T-cell function in the peripheral blood of MS patients during an acute exacerbation (also known as an attack, relapse, or flare);
- Increased numbers of helper T cells in the spinal fluid;
- Increased numbers of activated T cells passing into the brain from peripheral blood (which then attract other immune cells into the brain);
- Presence of T cells in MS plaques; and
- Increased frequency of activated T cells against the myelin seen in MS patients compared to healthy controls.
Therapies Directed Against Specific T cells or T-cell Receptors
The abnormal autoimmune process that is likely to be responsible for the central nervous system demyelination in MS appears to involve selective activation of helper T cells and killer T cells, with a corresponding decrease in regulatory T-cells. These findings suggest the rationale for therapies that target only specific T cells or T-cell receptors that are sensitized to myelin. Some of these approaches, such as the use of monoclonal antibodies directed against certain T cells, have proven successful in treating animals with experimental allergic encephalomyelitis (EAE), an animal model of MS. Pilot clinical trials are in progress. Future experimental therapies may involve antibodies directed against the cytokines that turn on the T cells or therapy designed to desensitize or inhibit T-cell activity.
While much more information is needed before the exact nature of the autoimmune response in MS is explained, it appears that T cells and their cytokines are the keys to this process. Ongoing research in these areas may provide new, specific immunotherapies that will stop the progression of MS, without harming any immune cells that are not involved in the process of myelin destruction.