How Camels Revolutionized the Antibody Engineering Industry
Our bodies are fighting a constant arms race. As pathogens infiltrate our first line of defense, our immune system responds by sending out weapons to destroy infectious invaders: hundreds of thousands of potential infectious pathogens are destroyed by your immune system every day, without you noticing a thing.
One such potent weapon is the antibody, also known as immunoglobulins (IgGs) Antibodies are small Y-shaped proteins made in a family of white blood cells—the effector B cells—and they are notoriously confusing. This blog will clear the fog, as well as show how an antibody’s structure can be re-invented by synthetic biologists.
These proteins have modular structures, as well as three regions where the amino acid sequence is not static, but highly variable. The effector B cells engineer these variable regions to bind to any specific target (an antigen), like puzzle pieces fitting together. Once an antibody has been engineered to a specific antigen, it can be retained in another family of white blood cell as the ‘affinity matured memory B cells’. When the same antigen is encountered again, the body already has the tools to fight infection, so a pathogen is destroyed before it can even begin to cause damage.
This is how immunization works. Your body is presented with just the antigens that belong to the pathogen (not the pathogen itself). The next time that pathogen infiltrates the blood stream it is instantly recognized and quickly destroyed.
Antibodies have several methods of protection. If the antigen is a toxin that is secreted by the pathogen, antibody binding can lead to a masking of that toxic molecule, and the toxin-bound antibody is often passed in urine. If the antigen binds to a pathogen’s surface receptor protein, that cell cannot function correctly and will naturally die. Alternatively, the antibody can bind one portion of the pathogen, and hand it over to immune cells that then take on the job of destroying the cell.
But pathogens can exploit this system too—that’s why the flu virus is so dangerous, especially to the young and elderly. The flu virus has no ‘error correction’ when it multiplies, so the DNA sequences that encode the virus’ surface proteins mutate often. When your body encounters the flu virus a second time its antigens are entirely different, and the previously memorized antibodies are useless. This is why physicians recommend immunizations every few years for people with compromised immune systems.
Antibodies consist of four main pieces: two long sections (the heavy chains), and two short sections (the light chains). Two heavy chains come together to form the Y shape of the antibody, and two light chains then bind to the outside of each arm of the Y.
The four main units that make up an antibody, and the variable regions at the end which bind antigens (Source).
Each heavy chain contains four specific domains, and each light chain contains two. The two domains at the base of the heavy chain facilitate their binding to one another, creating the antibody’s characteristic Y shape. The first domain in each arm of the Y facilitates binding to the light chain’s lower domain. Both the light chain and the heavy chain have three highly variable regions in their uppermost domains that make up the tops of the arms. It is these three highly variable regions that allow for specific antigen binding.
The individual defined regions which make up an antibodies heavy and light chains. For an image please click here.
This structure can be broken up even further to hinge regions and chain isoforms, but those structures are way beyond the scope of this blog. If this is still confusing, take a read of this review and this book chapter for more information.
Twist Bioscience recently attended the Antibody Engineering & Therapeutics conference in San Diego. Over 100 speakers showcased how such modular properties allow researchers to engineer antibodies without effector B cells in the lab, with the aim of training our immune systems to treat diseases that our bodies can’t usually handle—cancer for example. See Twist Bioscience’s recent post, about how our technology is accelerating antibody engineering, for a primer on this field.
It was clear during the conference that researchers have moved far beyond conventional antibodies, making them more compact and more complex in the hopes of improving upon current treatments.
The discovery that smaller antibodies were even possible came completely by chance. In the late 1980s, a lab in Brussels was performing antibody extractions from human blood samples, and happened to find some old camel blood at the back of a freezer (I am sure this is not the strangest thing lurking in the depths of lab freezers).
When antibody extraction was performed on the camel blood, researchers found that alongside normal antibodies, a small antibody could also be extracted that contained only heavy chain fragments. These heavy chains were also cut short, containing only three domains, and completely lacking the domain that binds the heavy chains to the light chains.
This obscure finding opened up new doors for antibody engineering. By creating a smaller camelid antibody—now named a heavy chain antibody or nanobody—a number of new properties could be accessed. The small size of a nanobody allows it to rapidly penetrate the targeted tissue. The hope is that a drug could be attached to the antibody, and problematic tissues could be directly targeted. This would significantly reduce the toxic fallout from chemotherapy for diseases like cancer, for example.
A camelid nanobody (left) compared to a conventional antibody (right)
Smaller antibodies can also be removed from the blood stream much faster than larger types. Thus, if the nanobody is engineered to bind a toxin, that toxin can be rapidly removed from the blood stream, significantly reducing treatment side-effects.
Small antibodies also offer the potential to target brain tumors. Currently, 98% of small molecule drugs cannot pass the blood-brain barrier—the wall that prevents potentially harmful substances circulating in the blood from entering the brain1. Regular antibodies are also too large to pass the barrier, but nanobodies have the potential to pass, and possibly deliver drugs to target tumors.
Biopharmaceutical company Ablynx is currently leading the development of nanobodies. Their FDA approved nanobody Ozoralizmab is in its phase-II clinical trial. It works by neutralizing the inflammatory response by masking a protein called TNFα in joints associated with rheumatoid arthritis. This debilitating disorder affects 1% of the world’s population, causing chronic and progressive joint inflammation. Patients in the trial saw around a 70% reduction in joint swelling after a 12-week treatment course.
Today, the full potential of smaller antibodies has been realized. Not only are the naturally occurring heavy-chain antibodies being utilized, but individual, or pairs of variable regions of the full size antibodies or the camelid antibodies are being bound together with synthetic linkers.
The main advantage of having an antibody-based targeting molecule containing just variable regions is even higher specificity. Antibodies don’t just bind to molecules or proteins with their variable regions. The heavy chain stem of the Y shape can also bind to a group of proteins in the body called Fc receptors. This mis-targeting could be disastrous if the job of an antibody is to deliver a drug. Since they lack the region to which the Fc receptors bind, antibody-based fragments can be delivered with high fidelity.
Different flavors of antibody based targeting molecules that have been developed
A swath of novel shapes and designs have been engineered based upon antibody parts. One such molecule is the captivatingly named AFM13, produced by Affimed Theraputics, which is in phase-II of clinical trials. AFM13 treats Hodgkin’s lymphoma, a cancer that destroys the immune system.
Antibody therapy is at fever pitch, and research in this field looks to only get more complex, creative and promising in the next few years. It is already one of the most hopeful curative agents for a wide number of cancers, as well as other untreatable or dangerous autoimmune diseases, or pathogens. It is amazing to think such diseases could be cured in the near future, thanks to some frozen camel blood, an accidental discovery and one of the greatest worldwide engineering efforts to date.
(1) Partridge W. M. The Blood-Brain Barrier: Bottleneck in Brain Drug Development. 2005.