Janaki, Scott, Ryan, Rebecca
10/14/97
What are monoclonal antibodies?
In 1975 Georges Kohler and Cesar Milstein developed a technique for fusing antibody-producing B cells from a mouse to immortalized cancer cells. These hybridoma clones are capable of producing multiple copies of a single specific antibody. They are referred to as monoclonal antibodies, which distinguishes them from polyclonal antibodies which are produced in a normal immune response.
A major drawback of using monoclonal antibodies in the treatment of human disease was that since the monoclonal antibodies were derived from mice, the human bodys immune system inactivated any therapeutic effect. Also, more than one treatment with any specific monoclonal antibody was impossible once the bodys immune system was stimulated against the mouse monoclonal antibodies.1
How are monoclonal antibodies made?
The difficulty in obtaining antigen-primed human B cells has led researchers to discover new ways of producing monoclonal antibodies. Techniques have recently been developed using genetic engineering to overcome these problems. This technology involves development of a chimeric monoclonal antibody that has mostly human components. As a result, the human immune system has a more tolerable response and the overall therapeutic effect is more beneficial.2
Transgenic mice have been studied over the past few years for use in monoclonal antibody production. This year a group of researchers, headed by Aya Jakobovirs, have created a new transgenic mouse that proves most useful in development of more appropriate monoclonal antibodies for use in humans. The mouse is named Xeromouse II, and contains the largest transfer of the human immunoglobin genes. Using yeast artificial chromosomes (YAC), researchers inserted most of the heavy and kappa light chains, the V, D, and J regions, and pairs of the constant domain containing the IgM, IgD, and IgG regions into the mice stem cells in the embryo. These mice produce totally human antibodies which are created through natural gene rearrangement and somatic mutations. The mice can then be introduced to human proteins such as cytokines and autoimmune diseases which will be recognized as foreign and produce antibodies that can be used therapeutically.3,4
Another recently developed method of producing useful monoclonal antibodies had been through the use of phage libraries. The production of monoclonal antibodies using bacteria has been possible due to three basic discoveries. First, it has been found that E.coli cells are able to express the heavy and light domains of antibodies. This expression of the heavy and light chains can be done using Fab fragments or single chain Fv fragments. Next, it is known that a diverse amount of Fab or Fv fragments can be obtained from human plasma cells and using polymerase chain reaction. The third discovery is that viruses that infect E.coli cells expressing the Fas or Fv fragments, in turn express the fragments on their cells surface. This is an extremely fast method to display monoclonal antibodies. This as a result gives the phage (or virus) the appearance of mimicking a B lymphocyte.5
Selection of the virus expressing the desired monoclonal antibody can be easily done using immobilized antigen and washing away non-binding virus. The bound phages can then be recovered and used to infect more E.coli cells to produce more of the desired phage expressing the desired monoclonal antibody. With this technique, monoclonal antibodies that were present in extremely minute amounts can be isolated and amplified to a huge extent, creating a huge variety of monoclonal antibodies with potential therapeutic or diagnostic uses.6
What type of monoclonal antibody treatments are useful in combatting human
disease?
Monoclonal antibodies are now in development or clinical testing for diagnosis and treatment of human disorders and diseases such as cancer and infectious diseases, rheumatoid arthritis, asthma, psoriasis, lupus, inflammatory bowel diseases, and multiple sclerosis as well as regulation of immune responses. Current research involves a theory for treating malignancies, in which a monoclonal antibody is produced to identify the cancer-specific antigens and then bind to antigens on the patients cancer cells. Theoretical results would be elimination of the cancer cells.
Two recent accomplishments have surfaced in the past year that demonstrate the effectiveness of monoclonal antibodies in fighting cancer. The first shows how monoclonal antibodies are effective in activating lymphocytes, which induces regression of human tumors. The study was done on mice, and the monoclonal antibody BAT was selected for its ability to induce cytotoxicity in human and mouse lymphocytes. By administering BAT, various murine natural killer-sensitive and -resistant tumors went into regression and the mice survived longer than those that went untreated. The study also showed that if T lymphocytes and natural killer cell levels were deficient in the mouse, that ability of BAT to combat the tumor cells was decreased. The study concluded by saying, Engraftment of human lymphocytes into SCID (severely combined immunodeficient) mice bearing human melanoma xenografts rendered them responsive to the antitumor effect of BAT. The efficacy of Bat in the regression of human tumors by activation of human lymphocytes indicates its potential clinical use.7
The second study, also done on mice, considers the effects of homodimerization of tumor reactive monoclonal antibodies in inducing growth arrests or apoptosis of tumor cells. Monoclonal antibodies that were once thought of as incapable of inducing appropriate cytotoxic effects on tumor cells were made useful by conversion into homodimers. This was done by introducing a thioether bond between two selected IgG molecules. Studies were done to compare the effects of monomers on tumor cells to the effects of homodimers on tumor cells. The evidence showed that the survival rate of mice injected with homodimers and a chemotherapeutic drug (doxorubicin) completely prevented tumor cells in the organs where the tumor cells grew. Monomer monoclonal antibodies, on the other hand showed no tumor regression effects.8
Another recent advancement involving therapeutic applications of monoclonal antibodies to cancer is the development of the monoclonal antibody IDEC-C2B8 (Rituximab). In late July, 1997, this monoclonal antibody was unanimously recommended for approval by a US Food and Drug Administration advisory committee for treatment of patients with low-grade follicular B cell non-Hodgkin lymphoma, (NHL) whose tumors had not responded to other therapies. Treatment with Rituximab works by depleting B cells for up to 9-12 months.9 This monoclonal antibody is made using recent genetic engineering technology which produces chimeric monoclonal antibody that has mostly human components. Treatment with this monoclonal antibody, according to Grillo-Lopez of IDEC pharmaceuticals, represents a new approach to cancer treatment since it allows the immune response to focus on destruction of the cancer cells specifically. The treatment course with Rituximab is relatively short (only 21 days compared with 6 months of chemotherapy), and there are fewer side effects than other current cancer treatments as ...it has almost no significant toxicity.10 Results of clinical trials indicate that half of the responding patients remained in remission a year after therapy.11
Rituximab is the first monoclonal antibody for cancer treatment that may soon become available worldwide. Further research and clinical trials are currently underway in designing and modifying monoclonal antibodies suitable for diagnostic and therapeutic use in humans.