Llama Antibodies Show Promise as Probable Coronavirus Treatment
Right from the very beginning of the coronavirus outbreak, scientists have amassed vital intelligence on the SARS-CoV-2 strain. With that information, many countries around the world have taken extensive measures to counter the COVID-19 pandemic. Currently, more than 90 vaccine candidates and numerous repurposed drugs are in development or different phases of testing.
In that vein, researchers from the University of Texas at Austin, the National Institute of Health, and Ghent University in Belgium have together come up with a potential riposte to the coronavirus in the form of antibodies derived from Llama blood. The bispecific antibody, VHH-72, is a fusion of two smaller antibodies that can potently neutralize the SARS-CoV-2. Their study published in the journal Cell on May 5 finds that these antibodies block the receptor-binding domain (RBD) of spike proteins and trap them in a conformation not suitable for host cell interaction.
“This is one of the first antibodies known to neutralize SARS-CoV-2,” said Jason McLellan, associate professor of molecular biosciences at UT Austin and co-senior author, referring to the virus that causes COVID-19. Since the antibody is small in size, it can be nebulized and used in an inhaler. “That makes them potentially really interesting as a drug for a respiratory pathogen because you’re delivering it right to the site of infection,” said Daniel Wrapp, a graduate student in McLellan’s lab and co-first author of the paper.
The team intends to develop an antibody-based treatment and is now planning to conduct preclinical studies in hamsters or non-human primates before moving to clinical trials in humans. If it works, this therapy could immediately benefit elderly and healthcare workers who are at a higher risk of contracting the infection.
The Llama Antibodies – The Origin
Llamas, unlike other mammals, are unique for one reason- their tiny antibodies. Back in 1989, a student working on a serodiagnostic test for trypanosome infection in camels reported the presence of antibodies that were smaller in size than the usual antibodies and constituted 75% of all serum IgGs in camel. Upon detailed characterization, these antibodies were found devoid of light chains. These heavy chain-only antibodies soon gained commercial traction for their high thermostability, excellent solubility, small size, low production cost, and a high penetration rate in the tissue.
The bispecific antibody produced by the collaborative effort of researchers from UT Austin and Ghent University is a product of years of research on coronaviruses led by the team from Belgium. In 2016, while working on coronaviruses, SARS, and MERS, the team had injected a 9-month-old Llama, Winter, with spike proteins of the virus in an attempt to generate neutralizing heavy chain only antibodies against the protein. What they got in return was a nanobody, VHH-72, that could neutralize SARS-CoV-1 efficiently.
Daniel Wrapp commented, “That was exciting to me because I’d been working on this for years. But there wasn’t a big need for a coronavirus treatment then. This was just basic research. Now, this can potentially have some translational implications, too.”
Killing Two Birds with One Stone
Since COVID-19 turned into a pandemic, the researchers from Belgium revived the old nanobody to check if it can be used against the novel coronavirus. They discovered that VHH-72 did bind to SARS-CoV-2’s spike protein but failed to attach readily due to the rapid rate of dissociation.
This limitation sparked the development of two bispecific antibodies-one where the original nanobody was fused head to tail with its clone and another where nanobody was humanized by fusion with Fc domain of human IgG. Upon examination, these duplex antibodies could offset the high rate of detachment and showed strong binding to the spike protein of both SARS-CoV-1 and SARS-CoV-2. Moreover, these antibodies could neutralize the SARS-CoV-2 pseudovirus with high potency.
Mechanism of SARS-CoV-2 Neutralization
Once the group obtained the neutralizing antibody, they sought to determine the exact molecular points of virus interaction. Using X-ray crystallography, the researchers visualized clear interaction between CDR-2 and three domains of the antibody and the receptor-binding domain of the virus.
However, the most interesting feature was the antibody’s ability to disrupt an important aspect of viral infection. It has been observed previously that dynamic conformational rearrangements of the receptor-binding domain alternately hide and present binding sites to host molecules. The VHH-72 antibody upsets these dynamic rearrangements by forcing the receptor to remain in an unstable conformation leading to disruption of viral-host interaction.
While conformational trapping is one of the mechanisms proposed, the authors also believe that the antibody might work by blocking the interaction between the receptor-binding domain and host cell receptors.
This work was supported by the National Institute of Allergy and Infectious Diseases (NIAID), VIB, The research foundation-Flanders (Belgium), Flanders Innovation, and Entrepreneurship (Belgium), and the Federal Ministry of Education and Research (Germany).
Editor: Rajaneesh K. Gopinath, Ph.D.