Our bodies are in constant battle with the outside world, protecting ourselves from foreign invaders, including parasites, bacteria, viruses, and fungi. When these parasites or germs, or the noxious substances they produce, manage to get past our skin, the acid in our stomach, or other physical and chemical barriers that protect our body, our immune system kicks in. Our immune system has several lines of defense, including cells, molecules, and chemicals, that are produced to neutralize the action of the invaders, or simply destroy them. An important component of the system is the antibodies, which can specifically recognize and bind tightly to foreign substances, targeting them for elimination.
It takes our body several days to produce an effective immune response to harmful germs and other invaders and, if we survive those few days, we will then have antibodies to protect us against those invaders in the future. This is because our immune system has memory. Immune memory results from an actual infection, or vaccination. In some instances, immune memory may last a lifetime.
But if our immune system is weak or dysfunctional, our body is weakened by disease, we are taking a medication that changes our immune system, or if we do not have the time needed to produce an effective immune response (for example, if we are bitten by a snake!), we could still be helped by antibodies from external sources. Depending on the clinical picture, an individual may need antibodies of one type (monoclonal antibodies) to help treat a specific disease or infection, e.g., non-Hodgkins lymphoma, or they may need many different antibodies (polyclonal antibodies) to protect them from different infections.
Most individuals have been exposed to numerous germs during their lifetime, and they have successfully made antibodies against these foreign invaders. Taking blood samples from many individuals and isolating the antibodies from the samples could provide a source of immediate protection against many diseases for another individual. This is the idea behind the use of gamma globulins — the “gamma” fraction obtained through electrophoresis from sera collected from hundreds of donors. That is the fraction that contains the antibodies. The administration of gamma globulins or immune globulins, whether given as an intramuscular shot, intravenous or subcutaneous infusion, has been used as treatment against many diseases, including immunodeficiency disorders, hepatitis A, tetanus, and rabies, as well as various autoimmune disorders, chronic lymphocytic anemia, pediatric HIV infection, and bone marrow and kidney transplants.
Antibodies used for human therapy need not come from human sources. Antibodies used for treatment against snake bites and dog bites are usually horse-derived and those used to prevent rejection of kidney transplants are often rabbit-derived. Unfortunately, these “foreign” antibodies become targets of our immune system and are rendered ineffective by our immune system.
Recent advances in biotechnology provide the means to produce antibodies that are more specific for the disease at hand. Thus, there are now in the market pure antibodies to treat certain forms of breast, colorectal, and other cancers; rheumatoid arthritis, ulcerative colitis, and other autoimmune disorders, organ-transplant rejection, asthma, and various other diseases. Those antibodies are “recombinant” molecules, i.e., they were produced in the lab using the techniques of molecular biology. Most of those monoclonal antibodies are from nonhuman sources (usually mice) and they have to be modified (“engineered”) for use in humans. A non-human antibody has to be first “humanized.”
There are various ways of “humanizing” a non-human antibody, i.e., making it look like a human antibody and thereby less likely to be eliminated by a patient’s immune system.
Antibodies are composed of two distinct regions, a variable region, and a constant region. The variable region differs from antibody to antibody and is the part that is responsible for binding to the foreign substances. The constant region is the same in all antibodies of the same type or subtype and is the part used for recruiting cells or other molecules of the immune system that the system uses to eliminate the foreign objects.
For instance, one technique to make a nonhuman antibody less foreign is simply to preserve the binding properties of the variable regions and to replace the constant region with the human counterpart. The resulting molecule is called a “chimeric” antibody. These monoclonal antibodies are given the suffix –ximab, such as rituximab.
(Monoclonal antibodies are given the generic ending -mab. An unmodified mouse antibody would have the suffix -momab, e.g., tositumomab.)
But not every section of the variable region is involved in the binding — only the parts called complementarity-determining regions, or CDRs. Therefore, one could eliminate most of the nonhuman antibody and retain only the CDRs, replacing everything else with human parts. The resulting molecule is called a “CDR-grafted” antibody. The resulting molecules are called “humanized” antibodies and have the suffix –zumab, such as omalizumab.
Now, not every part of some of the CDRs is involved in the binding. By comparing the structure of antibody molecules, one could see which parts of the CDRs are actually involved and then keep only those during “humanization.” This procedure is referred to as “grafting of abbreviated CDRs.”
Further, only certain amino-acid residues are involved in the binding — the so-called specificity-determining residues, or SDRs. Again, one could just keep those. This procedure is referred to as “SDR-transfer.”
Even further, since antibodies “see” only the outside of a substance, one could replace the surface of the nonhuman antibody with amino acids from a human antibody (sort of like a “cloaking device”). This procedure is called “veneering,” or “resurfacing.”
Now, other cells are involved in mounting an immune response. One of them is the so-called “helper T cell.” Unless helper T cells are activated, a strong antibody response is not generated. Helper T cells recognize segments of the foreign substance and what these cells recognize is known for the most part. Another way to prevent the patient’s immune system from rejecting a foreign antibody is to engineer the antibody so that it no longer has segments that the helper T cells might recognize.
But the best procedure to obtain antibodies that are totally acceptable to the patient’s immune system is to find a human antibody that has the desired binding properties. There are a few “fully human” monoclonal antibodies currently on the market, and they are named with a suffix of –mumab, e.g., adalimumab.
The described procedures of “humanizing” antibodies are used to reduce, or eliminate the possibility of elimination of the therapeutic antibodies by the patient’s immune system. There are other factors that might need to be considered in making an ideal therapeutic antibody. One is size.
A smaller size would allow deeper penetration into tissues. In this regard, antibody molecules from members of the camel family and from sharks are very attractive. Camels and sharks have a type of antibody whose binding part is just half the size of the variable region of typical antibodies. Of course, they still have to be engineered for use in human patients. The features that make them useful despite their small size are being explored to make more typical antibodies smaller and still useful for therapy.
Other engineering strategies are being employed to make better and more powerful therapeutic antibodies. For example, antibodies are being modified to make them more stable, to increase their half-life, to have lesser side effects, to make them bind more strongly, or to have novel functions.
As we advance our knowledge of biotechnology, the parasites and germs get smarter too, evading our immune system and causing disease. We have developed many monoclonal antibodies for clinical practice, but we haven’t even begun to tackle many of the infections and diseases that afflict mankind. Designer monoclonal antibodies for specific individuals are on the horizon. Are we ready for the challenge, or will the germs and parasites beat us to the punch — again?
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LTC Cecilia Padlan Mikita is an allergist/immunologist at the Walter Reed Army Medical Center, Washington, DC. Eduardo A. Padlan is an adjunct professor at the Marine Science Institute, UP Diliman, and is a corresponding member of the NAST. They can be reached at cecilia.mikita@us.army.mil and epadlan@upmsi.ph, respectively.
The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the Department of the Army or the Department of Defense. Neither author has any direct personal or commercial interest in any of the products mentioned in this article. E. A. Padlan is responsible for the “grafting of abbreviated CDRs,” “SDR-transfer,” and “veneering” procedures.