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By Jamshed Arslan, Pharm. D., PhD.
The immune system protects living organisms against harmful substances. B cells ward off infections by producing antibodies to invading pathogens. Conventional antibodies are stable only at a narrow range of temperature and pH. They are also too big to penetrate tissues or to bind with many active sites on proteins. Thankfully, sharks and camelids like llamas produce smaller antibodies called heavy chain-only antibodies (HCAbs). HCAbs contain a single variable domain that can itself be used as an antibody, called VHH single domain antibodies. VHHs are free of the aforementioned problems associated with conventional antibodies.
Taking advantage of these properties, a team affiliated with USA , German and Belgian institutes employed VHHs to successfully neutralize notorious coronaviruses, MERS-CoV, SARS-CoV-1 and SARS-CoV-22. Their discovery may be crucial in humanity’s fight against the COVID-19 pandemic.
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The surface of coronaviruses contains spike glycoproteins with two subunits: S1 and S2. The S1 subunit in SARS coronaviruses has a receptor-binding domain (RBD) which binds with host receptor ACE-2 to trigger fusion, while S2 makes the virus-host fusion possible. To isolate coronavirus-neutralizing VHHs, researchers immunized a llama with MERS-CoV and SARS-CoV-1 spike proteins. The genes for small antibodies were obtained from llama blood and expressed in yeast. ELISA and in vitro neutralization assay using pseudotyped viruses confirmed that the binding of purified VHHs to spike’s RBD neutralized MERS-CoV and SARS-CoV-1. Due to the high sequence similarity between the SARS-CoV-1 and SARS-CoV-2 derived spike protein epitopes, the next step was to determine whether selected VHHs showed cross-reactivity against SARS-CoV-2.
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Based on the structural homology of the spike proteins, the team tested binding kinetics of anti-SARS-CoV-1 spike protein VHHs against SARS-CoV-2 spike protein using surface plasma resonance. VHHs were able to trap SARS-CoV-2 spike’s RBD in a conformation that reduced viral activity. However, it was not the monomeric VHHs, but the bivalent VHHs (VHHs fused to Fc domain of human IgG) that efficiently neutralized pseudotyped SARS-CoV-2. In other words, VHHs offer biophysical features and neutralization capacity against notorious respiratory coronaviruses that make them a potential countermeasure for COVID-19.
This research shows that VHHs are not only useful reagents, but also a potential candidate against pathogenic coronaviruses. Turning anti-SARS VHHs into bivalent anti-SARS-CoV-2 VHH-Fc might be the candidate we need in human trials. After establishing safety, we must explore both monomeric and bivalent VHHs as prophylactic and therapeutic treatments during the current and future outbreaks of coronaviruses.
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Jamshed Arslan, Pharm D, PhD
Dr Arslan is an Assistant Professor at Barrett Hodgson University, Pakistan,
where he uses various pedagogical methods to teach Pharm D students.
Research in focus
