THE INTEGRAL PART OF TREATMENTS FOR MS
Monoclonal antibody therapy has become an integral aspect of the armamentarium of treatments for MS. It is helpful to consider how we got to this point with these tools and how these antibodies work in MS in particular.
The 100th monoclonal antibody (MAB) for treatment has been recently been approved by the FDA; so that it is fitting to look back on how we got to where we are over the 35yr since the first MAB was approved by the FDA in 1986.
Until 1975 polyclonal antibodies (a mixture of different antibodies) were the only tool available to use experimentally to react with a specific immune target on or in a cell (an antigen). This was a blunt edged sword because there could be unwanted cross-reactions with other targets which muddied the waters. At that time Cesar Milstein and George Kohler working in the UK developed a technique (the hybridoma technique) which produced monoclonal antibodies. Here they were able to immunize mice and then isolate individual B cells, the antibody producing cells, from the spleen. To prevent these cells from dying they fused them with a tumor cell line, which in fact immortalized the antibody-producing B cells. By testing each cell line produced from single hybrid B cells they could specifically identify the antibodies reacting with the target of interest. They coupled the antibodies with a fluorochrome (a chemical which fluoresces in a defined light wave length) so they could identify the cell bearing the antigen of interest by microscopy or by using a flow cytometer (this technique is still widely used today, including in our laboratory). The unconjugated antibody could be used for diverse purposes such as blocking an immune reaction or destroying a cell to which the antibody was found. This technique revolutionized Immunology and in recognition of this phenomenal technical advance as well as its potential as a clinical therapeutic tool Milstein and Kohler were awarded the Nobel prize in 1984.
Later the antibodies needed to be modified because mouse antibodies injected into humans produced severe reactions. It is helpful to understand the structure of an antibody molecule and its modifications to recognize how they can be used therapeutically.
The antibody molecule is made up of two pairs of protein molecules which results in a Y-like structure. The larger of the proteins (heavy chain) is coupled to a smaller protein (light chain). This is then coupled to an identical partner to produce the Y shape of the whole molecule. Most of the molecule is made up of a standard sequence of protein building blocks (amino acids) found in all antibodies of a particular type. The outer part of the heavy and light chains, however, is comprised of different amino acids in each different antibody (the hypervariable region). This alteration from a “normative” or standard amino acid sequence is the result of gene mixing in individual B cells. This process occurs to tailor make an antibody to best fit the antigen to which it binds.
The basic structure can also be modified to make a monoclonal antibody for therapeutic use.
The starting point is the antibody molecule from the mouse (murine). The outer ends of the heavy and light chains can be substituted with the same part of the heavy and light chains from a human antibody. This reduces the chance of an immune reaction when used therapeutically. One can then get more focused and just substitute the hypervariable regions from a human antibody. And finally, a human cell making a defined antibody can be used instead of the original mouse cell. This markedly reduces the adverse reactions to the therapeutic antibody. The kind of antibody is denoted by different endings of the name used (see above).
All sorts of variations on this theme produce an array of different antibody molecules. For example, a drug or toxin can be connected to the antibody which will then deliver the drug or toxin to the defined target. Two halves from different antibodies can be combined so for example, two different cells can be brought together. And finally modified portions of the antibody molecule can be used.
Fig. 2 | Antibody formats. Antibody formats include canonical (part a), antibody–drug conjugates (part b), bispecifics (part c) and fragments (part d). Fragments include antigen-binding fragments (Fabs), single-chain variable region (scFv) constructs and domain antibodies. Radiolabelled antibodies and antibody–immunotoxins are not shown. These formats can be further subcategorized, and antibodies can span classifications. There are at least 30 different bispecific formats, for example, some of which include fragments. Modified from Nature Reviews Drug Discovery.
The figure below demonstrates the exponential growth of monoclonal therapies which have been FDA approved. In addition, there are literally hundreds of monoclonal antibodies at different stages of development.
Only 8 of the 100 monoclonal antibodies are approved for neurological disease though this number is likely to increase.
In the past, most of the monoclonal antibodies used to treat MS were repurposed from their original intended use. Since we do not know the driver mechanisms in the immune system in MS the selection of monoclonal antibodies for use in MS is a hit or miss affair. This can lead to unintended consequences in some cases.
Alemtuzumab was originally used in oncology. It is directed against a ubiquitous molecule (labelled CD52) on the surface of immune cells. The effect of using this monoclonal antibody is to break up cells which are bound by the antibody. This depletes the immune system fully. When the immune system re-constitutes there is a diminution of immune cells which react with myelin, delaying MS inflammation. Ultimately re-treatment is needed in most to keep the state of depletion. After treatment, B cells reconstitute faster than T cells, and this can lead to antibody-mediated autoimmune diseases apart from MS.
Natalizumab is an example of a monoclonal antibody which works in a model for MS, Experimental Allergic Encephalomyelitis (EAE). This can be induced in a variety of animals. The monoclonal antibody binds alpha-4 integrin, a molecule on the surface of activated T cells. The effectiveness in EAE predicted correctly, that it would work in MS by blocking immune cells from sticking to the inner surface of blood vessels; the initial stage for immune cells to enter the nervous system. This prevents their transit into the brain, interrupting the cycle of inflammation in MS. Unfortunately, the use of Natalizumab is complicated by the occurrence of a JC viral infection (Progressive Multifocal Leukoencephalopathy).
MS has been considered an autoimmune disease caused by disordered T cells. It was surprising to find that the elimination of B cells (targeting CD20 a molecule on the surface of B cells) had therapeutic benefit for MS. However, elimination of B cells leads to their re-constitution; a process which induces the development of both regulatory T and B cells; which is probably the mechanism by which the immune system is controlled in MS. The original molecule in this class is Rituximab, a chimeric molecule. It was initially used to treat lymphoma and leukemia. It lacked efficacy in the treatment of Lupus and had an increase of infections in Rheumatoid Arthritis. It was then tested in MS. A humanized monoclonal anti-CD20 antibody was subsequently used (Ocrelizumab) with significant benefit in reducing relapses, MRI change and disease progression. A fully humanized antibody has now been developed (Ofatumumab) with similar success in MS. One downside of these antibodies is that killing immune cells in the circulation stimulates cytokine release (these are protein inflammatory molecules) which causes infusion reactions.
Unfortunately, a number of monoclonal antibodies have been unsuccessful in development as MS therapies. In part this is due to the fact that their mechanism of action is not relevant to, or harmful to immune processes in MS.
Daclizumab is a humanized monoclonal antibody developed for cancer therapy. It binds to one part of the Interleukin 2 receptor (Interleukin 2 is a growth stimulating cytokine) which is expressed in activated T cells. The bound cells are then removed. The mechanism by which this antibody acts in MS is not clear. One possibility is that it stimulates development of NK (natural killer) cells which have regulatory effects. The monoclonal antibody reduced relapses and had MRI benefit but a severe autoimmune encephalitis developed in some patients and it was withdrawn from the market.
Thus, monoclonal antibodies have come to play a very important role in medicine in general, and in MS therapy in particular. Knowledge of these antibodies and their actions should help making more informed treatment decisions.