A state-of-the-art engineering approach toward improving the neutralizing activity of a cross-reactive nanobody, previously generated against the original SARS virus, showed it can be an attractive and viable option against the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The study is published in the journal Advanced Therapeutics.
The coronavirus disease 2019 (COVID-19) pandemic, caused by the SARS-CoV-2, has substantially interfered with our way of living. Despite the availability of vaccines, the virus (and especially its emerging variants) continue to pose a severe threat to global health.
Hence, effective neutralizing therapeutic agents are urgently needed. One promising strategy is the development of broadly neutralizing antibodies with the potential to engage evolutionarily conserved antigens (epitopes) on SARS-CoV-2 spike glycoproteins.
And indeed, antibodies elicited following the exposure to other coronavirus antigens (such as SARS-CoV) are actually cross-reactive against SARS-CoV-2. Nonetheless, they have an impaired affinity and neutralizing activity against SARS-CoV-2, which means they necessitate further optimization before their introduction to our treatment armamentarium.
A quest towards multivalent nanobodies
A potential solution is multivalency, a simultaneous interaction of many ligands with multiple receptors that has already been used to ameliorate potency and binding affinity of molecular therapeutics against SARS-CoV-2, but also other viral agents.
Nanobodies, which are single-domain fragments of heavy-chain antibodies from camels, have been developed to target a wide array of viruses and have also been reformatted into multivalent constructs to confer a plethora of advantages; however, their usage against SARS-CoV-2 has not been studied.
In this new paper, researchers from the University of Michigan in Ann Arbor (USA) employed a facile multivalent engineering approach to reach sizeable and synergistic improvements in the neutralizing activity of a specific nanobody against SARS-that was initially generated against the original SARS-CoV.
Facile multivalent engineering approach
The methodological approach included both the engineering and detailed characterization of multivalent nanobodies that exhibit potent and broadly neutralizing activity for wild-type SARS-CoV-2 and the alpha (B.1.1.7) and beta (B.1.351) variants of concern.
A specific SARS-CoV-2 cross-reactive nanobody of interest was VHH-72, which was further evaluated using a competition analysis. This was used to determine whether an antibody's epitope overlaps with the angiotensin-converting enzyme 2 (ACE2) receptor and the epitopes of antibodies that compete with ACE2.
In addition, the researchers used specific measurements to detect antibodies that compete with VHH-72 for binding to the receptor-binding domain (RBD) of SARS-CoV-2 spike glycoprotein – either due to binding to an overlapping epitope or via steric hindrance (which influences active sites and reaction rate in chemical reactions).
Using multivalent reformatting to improve neutralization
The researchers have demonstrated that the neutralization activity of a moderate affinity can be increased substantially when a cross-reactive nanobody (VHH-72) is formatted as a tetravalent and hexavalent construct rather than a bivalent construct. In other words, multivalent reformatting increases VHH-72 neutralization activity.
Moreover, the study has shown how this performance is superior in comparison to multivalent nanobodies targeted against non-conserved epitopes in the RBD of spike glycoprotein. VHH-72 actually recognizes RBD rather distinct in comparison to other common SARS-CoV and SARS-CoV-2 antibodies.
"Importantly, a hexavalent VHH-72 nanobody retains binding to spike proteins from multiple highly transmissible SARS-CoV-2 variants (B.1.1.7 and B.1.351) and potently neutralizes them", say study authors. "Multivalent VHH-72 nanobodies also display drug-like biophysical properties - including high stability, high solubility, and low levels of non-specific binding", they add.
Appealing anti-COVID candidates
In a nutshell, a perfect storm of broad and potent neutralization activity, advantageous biophysical properties, as well as anticipated long half-lives of multivalent VHH-72 fusion proteins makes them very appealing candidates in our fight against COVID-19
"We have demonstrated that engineering multivalent nanobody constructs has the potential to rapidly improve the properties of promising nanobodies to prepare them for use in therapeutic applications", say study authors in this paper published in the journal Advanced Therapeutics.
There are potential barriers to the use of nanobodies as antivirals – such as smaller size in comparison to monoclonal antibodies and non-human origin. Another possible limitation is their propensity to elicit anti-drug immune responses.
However, it is known that nanobodies have been successfully humanized and administered as therapeutics, which is why our cautious optimism to use them not only for combating SARS-CoV-2 but also a plethora of viral variants that may emerge during the current and any future pandemic.