Even more efficient nanobodies

Although vaccines against SARS-CoV-2 do exist, they cannot fully protect immunocompromised patients for example. Other prophylaxes or therapeutics need to be developed for high-risk patients. Therapeutic neutralizing antibodies may provide an alternative. However, they are complicated and costly to produce. In addition, these “conventional” antibodies may, in certain cases, facilitate infection as previously described for SARS-CoV-2. However, their substitution by nanobodies could provide an additional solution.

Nano-antibodies or nanobodies are small antibodies derived from the antibodies of camelids (notably lamas) possessing a simplified architecture (see the  28 December-3 January 2021 letter at News-COVID-19.info). They are monomeric (composed of a single molecule) and can bind to an antigen. Their size means they can reach targets inaccessible to classic therapeutic antibodies. They also have the advantage of being simpler to produce (in bacteria), more stable, and therefore less costly.

How do these nanobodies act? A large number of antibodies bind with the RBD domain of the spike (S) surface viral protein, preventing attachment of the S protein to the ACE2 cellular receptor and so blocking the entry of the viral particle into the cell. The RBD (Receptor Binding Domain) domain is the region of bonding to the ACE2 receptor. The S protein is homotrimeric (an assemblage of 3 S proteins) and possesses a certain flexibility enabling it to exist in 2 conformations: a “closed” conformation that makes the cell receptor link site inaccessible, and an “open” conformation, that can trigger the viral and cellular membrane fusion mechanism for entry of the virus into the cell (see the 18-24 january 2021 letter at news-COVID-19.info).

A study carried out by German and American researchers has enabled the identification of 4 nanobodies capable of fixing themselves to this domain, using alpaca immunization. These nanobodies target 2 epitopes, distinct from RBD, determined using imaging techniques such as X-ray crystallography or cryo-electron microscopy. The scientists were curious as to whether these 4 nanobodies could have a synergic effect. They therefore studied a combination of 2 nanobodies and observed that if this combination contained 2 nanobodies targeting the same epitope, there was no synergic effect. However, if the 2 nanobodies targeted 2 different epitopes, the neutralization of the virus was twice as effective.

The researchers then experimented with different constructions of multivalent nanobodies (assembly of several nanobodies). They developed 2 types and observed that they were a hundred times more efficient as compared to single nanobodies. Certain multivalent nanobodies, fusing two identical nanobodies, targeted the RBD domain and prevented its binding with the ACE2 cellular receptor and thus the entry of the virus into the cell. Other biparatopical nanobodies (resulting from the fusion of 2 nanobodies each targeting a single epitope of the same antigen) recognized a post-fusion conformation of the S protein, without triggering the membrane-fusion mechanism. In addition, use of these biparatopical nanobodies delays the selection of viral variants resistant to this treatment.

For all of these reasons, these nanobodies may constitute an effective alternative treatment against SARS-CoV-2.