In a new study available on the preprint server bioRxiv*, a multinational research group demonstrated how monoclonal antibodies strongly inhibit severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in vitro at even sub-nanomolar concentrations. This makes them a prime candidate for the treatment of coronavirus disease (COVID-19).
A newly discovered SARS-CoV-2 belongs to the group of betacoronaviruses and is the causative agent of the ongoing COVID-19 pandemic. The development of immunity through natural infection with this virus is a multi-step process that typically takes place over 1-2 weeks, resulting in antibodies in the blood.
Most of the published studies in the medical literature show that individuals who have recovered from COVID-19 have antibodies to the virus; nonetheless, some of them have rather low levels of neutralizing antibodies, and as of May 2020 no study has evaluated whether the presence of SARS-CoV-2 antibodies actually confers immunity to subsequent infections.
Currently, there is no approved vaccine for SARS-CoV-2 or drug against COVID-19, so management strategies mostly aim to delay viral spread and disease impact. Taking into account disease severity, but also the rapid spread of the virus in the population, more effective treatments and vaccines are direly needed to tackle this pandemic.
SARS-CoV-2 antigens and antibodies
SARS-CoV-2 utilizes a glycosylated spike protein (S-protein) that sticks out from the viral surface to bind to angiotensin-converting enzyme 2 (ACE2) during the cell entry process. The receptor-binding domain (RBD) of the S-protein, which recognizes the host cell ACE2 receptor, presents a panoply of neutralizing epitopes for COVID-19.
Many studies have demonstrated that the high antigenic capacity of the S-protein is highly dependent on the presence of manifold epitopes responsible for generating the immune response – both in COVID-19 asymptomatic individuals and in convalescent patients.
A new study by researchers from the University of Toronto (Canada), Washington University (US), University of Nevada (US), Nevada System of Higher Education (US), Tor Vergata University of Rome (Italy), University of Turin (Italy), IRCCS Neuromed (Italy) and Intonation Research Laboratories (India) described a comprehensive panel of synthetic monoclonal antibodies.
A synthetic antibody panel
The aforementioned synthetic antibody panel was built on a human framework known to bind SARS-CoV-2 S-protein, compete for binding with ACE2, and strongly inhibit the infection with this virus. In short, antibodies were isolated by panning phage-displayed antigen-binding fragment libraries against S-protein RBD of the virus in multiple rounds.
Although the enzyme-linked immunosorbent assay (ELISA) was used in a large part of this research, a thorough characterization of antibody binding kinetics was pursued with the use of biolayer interferometry – an optical method for studying macromolecular interactions by analyzing interference patterns of white light that are reflected from the biosensor tip.
Finally, in order to appraise the effects of the IgG antibodies on virus infection, a microneutralization assay measuring the infection of ACE2-expressing Vero E6 cells (isolated from the kidney of an African green monkey) with wildtype SARS-CoV-2 has been used.
Dose-dependent neutralization of SARS-CoV-2
In this study, antigen-binding fragment phage clones that successfully blocked ACE2 were converted to full-length human IgG proteins, with an accompanying framework engineered to harbor high thermostability and low immunogenicity for treatment purposes.
Furthermore, several types of IgG antibodies exhibited sub-nanomolar affinities for the SARS-CoV-2 S-protein and were able to neutralize the wildtype virus in live virus assays efficiently. In a nutshell, these synthetic antibodies blocked host receptor binding.
All four subclasses of IgG antibodies exhibited dose-dependent neutralization of viral infection, corroborating their neutralization propensity and the absence of any adverse cytopathic effects. Moreover, all tested antibodies strongly inhibited the infection of Vero E6 cells with SARS-CoV-2.
"These results confirm that these human antibodies are prime candidates for an anti-viral drug that will block the virus from entering host cells and will thus prevent the virus from replicating and causing disease," study authors clarify their findings.
Potential uses and implications
Even though therapeutic, antibody-mediated safeguarding against SARS-CoV-2 is still not demonstrated in humans, the proof of concept is evident, as this and other studies suggest that convalescent plasma transfusion may indeed offer benefits to certain patients.
"Passive virus neutralization appears to not only reduce inflammation, and lung damage associated with it but also works by reducing viral load and preventing dependence on mechanical ventilation," said study authors. "Therefore, therapies with SARS-CoV-2 antibodies must be explored, and the monoclonal antibodies reported here represent valid candidates", they add.
The main advantages of antibodies are long half-life in the serum and adequate tolerance by the human body, especially if based on human frameworks like described in this seminal paper. Cocktails comprised of antibodies specific for RBD and other S-protein regions may further improve treatment potency.
Along with potential therapeutic utility, specific monoclonal antibodies against SARS-CoV-2 are also needed for the development of novel diagnostic methods and research endeavors. Therefore, further insights into humoral responses to COVID-19 are needed.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.