The world as we knew it was turned on its head by COVID-19. As more and more lock-downs began in an effort to contain it, we have seen COVID-19, the disease caused by the virus SARS-CoV-2, overwhelm our hospitals and bring many healthcare systems to their knees. As of May 26, 2020, this novel coronavirus has caused nearly 350,000 deaths across the world. Schools and businesses have been shuttered, wearing masks and vigilant sanitization are the new normal, and millions are quarantining to protect themselves and those around them. And despite our knowledge of similar viruses and the availability of antiviral drugs currently on the market, a true therapeutic to help treat or stop the spread of COVID-19 has remained elusive.
That is, until, the llamas showed up.
Recently, a team of scientists from Texas, New Hampshire, Maryland, Germany, and Belgium collaborated to isolate single-domain antibodies from llamas, which bind to specific proteins expressed by SARS-CoV-2. While more work will need to be done to verify their findings, it could represent a step in the right direction towards developing treatments to help fight COVID-19.
For years it has been known that llamas, as well as other members of the camelid family, produce single-domain antibodies (also known as nanobodies) which bind very strongly to their protein targets. But what does all of this mean, exactly? To understand that, we first have to break down exactly what an antibody is. Antibodies are small, Y-shaped proteins composed of a constant region and a variable region. The variable region is located at the “arms” of the Y, and is the part responsible for allowing the antibody to interact with and stick to specific proteins expressed on the cell surfaces. This variable region is made up of two types of protein strings, a heavy chain and a light chain. Meanwhile, the constant region of an antibody is the “tail” of the Y, and consists of a pair of heavy chains. Nanobodies are unique antibodies composed of only a single heavy-chain variable region, and do not have a light chain (Figure 1).
Nanobodies are small in size, incredibly heat-stable, interact very strongly with their target proteins, and will not be attacked by a person’s own immune system. These characteristics have caused nanobodies to be an increasingly important tool for scientists, who have begun to employ them to target drugs to specific cells and diagnose and treat various diseases and cancers. Because they are so stable, nanobodies can also be engineered to be administered to patients in the form of an inhaler or nebulizer treatment–making them a strong therapeutic candidate in the race to find a treatment for COVID-19.
To this end, the team injected a llama with the spike (S) proteins found on the surface of SARS-CoV-1 (the virus responsible for the first SARS epidemic, 2002-2003). When an infection with this coronavirus occurs, the virus’ S proteins bind with a protein receptor known as ACE2, expressed on the surface of cells in a person’s airway. This interaction allows the virus to enter cells in the respiratory system, driving the infection. By targeting nanobodies to the viral S protein, the scientists hoped to prevent these interactions and stop the virus from entering cells in the airway, thereby blocking the initiation of an infection.
The scientists isolated the nanobodies from the S-protein treated llamas and combined them in a dish with harmless viruses engineered to express SARS-CoV-1 S protein. One nanobody in particular, which they called SARS VHH-72, bound strongly to the SARS S protein and “neutralized” it, rendering it ineffective at binding its target receptor. Assessing SARS VHH-72’s structure demonstrated a very strong interaction with one specific region of the SARS S protein, positioning the nanobody to perfectly overlap with a region where ACE2 normally binds. Blocking this region on this S protein is enough to prevent it from binding with ACE2, stopping viral entry into airway cells.
One of the paper’s most critical findings came when the scientists explored whether SARS VHH-72 interacted with the SARS-CoV-2 S protein. Although not identical, the S proteins from SARS-Cov-1 and SARS-CoV-2 are very closely related and share the same airway cell receptor: ACE2. If this nanobody interacted with the SARS-CoV-2 S protein, it could be a potential therapeutic for those infected with COVID-19. While SARS VHH-72 bound to the SARS-CoV-2 S protein, this interaction was weaker and not as long-lasting as the one formed between it and the SARS-CoV-1 S protein. The team also determined that SARS VHH-72 “trapped” the viral S proteins in an unstable state and directly prevented their interaction with host ACE2 receptors. They hypothesized this was the reason why SARS VHH-72 neutralized the S proteins from both SARS-CoV strains.
Subsequently, the team created two different “fusion proteins,” with SARS VHH-72. One consisted of two SARS VHH-72 nanobodies linked together, while the other linked SARS VHH-72 to the “tail” region of another human antibody known as IgG1. These two-part, or bivalent, antibodies bound viruses engineered to express both SARS-CoV-1 and SARS-CoV-2 S proteins. Making these nanobodies bivalent also compensated for the weaker binding interactions the group had previously noted between SARS VHH-72 and the SARS-CoV-2 S protein. Finally, the bivalent nanobody made from SARS VHH-72 and IgG1 neutralized viruses engineered to express the SARS-CoV-2 S protein, demonstrating the potential for these novel nanobodies as stepping stones towards the development of COVID-19 treatments.
While the potential for SARS VHH-72 to bind to and neutralize SARS-CoV-2 still needs to be verified further, nanobodies offer a possible avenue of exploration and could change the fields of public health and medicine as we know them. Findings that attempt to solve as complex a problem as COVID-19 are a beacon of hope during these times of fear and great uncertainty, and serve to remind us that science always strives to find a way.