COVID-19 PANDEMIC AND MONOCLONAL ANTIBODIES
In addition to vaccination of the population, which made it possible to contain the Covid-19 epidemic, passive immunization by infusion of monoclonal antibodies constitutes a very interesting complementary approach, in particular for the people most at risk. Several studies have shown that early administration of monoclonal antibodies targeting the viral protein Spike, and more specifically its receptor-binding domain (RBD), blocks the entry of the virus into human cells and prevents the progression to severe disease.
Since 2020, multiple antibodies directed against the RBD domain of the Spike protein of Sars-CoV-2, the agent responsible for the Covid-19 pandemic, have been designed, but these antibodies have several limitations in use (narrow recognition spectrum, modest neutralization capacity, loss of recognition of emerging virus strains...). Alternative strategies to develop monoclonal antibodies with high neutralizing activity, capable of neutralizing circulating strains of SARS-CoV-2 with broad spectrum, are needed.
CONTRIBUTION OF ANTIBODY ENGINEERING
Among the antibodies neutralizing the SARS-CoV-1 virus, developed following the 2003 epidemic in Asian countries, very few have proven to be effective against the SARS-CoV-2 virus. The VHH72 antibody is a molecule derived from the immunization of a llama with the Spike protein of SARS-CoV-1 which is able to recognize the corresponding antigen of SARS-CoV-2 with however, a moderate binding affinity to the RBD of SARS-CoV-2. This nanobody was an extremely valuable starting point for the generation of new anti-SARS-CoV-2 antibodies due to its cross-reactivity to different viral strains.
In the present study, scientists combined a systematic evaluation of the effect of mutations (Deep Mutational Scanning) on the activity of the VHH72 antibody with a high-throughput Yeast Surface Display. This resulted in novel highly neutralizing antibody-candidate variants with significantly improved affinity for SARS-CoV-2 and broad cross-reactivity for SARS-CoV-1 and SARS-CoV-2. The researchers thus identified all individual substitutions of VHH72 that increase its binding to the SARS-CoV-2 RBD, and then screened targeted combinatorial libraries to isolate modified antibodies with optimized properties. The resulting VHH molecules i) exhibit very high affinities (in the picomolar range) for SARS-CoV-2 antigens from various emerging variants and for SARS-CoV-1, ii) block the interaction between the RBD and its cellular receptor, and iii) neutralize the virus with high efficiency thanks to a common motif of three amino acids that gives them a very high recognition specificity.
CONCLUSION
An immediate prospect of this work could be to adapt VHH72, or one of the many antibodies affected by omicron mutations, to these new antigens and thus generate neutralizing antibodies of interest to combat the current pandemic. The power of the approach used here makes it possible to rapidly design, from existing antibodies, novel neutralizing molecules, active against new emerging strains of pathogens.
Contact : Hervé Nozach, herve.nozach@cea.fr
See also the "Antibody Engineering and Immunogenicity" page of our website
- A single-domain antibody, also known as a nanobody, is an antibody fragment composed of a single monomeric variable domain (camelids make these unique antibodies in the animal kingdom, with a less complex structure than those of humans). Like a whole antibody, it is able to bind selectively to a specific antigen, with a high affinity, often in the nanomolar range. Nanobodies are therefore the smallest antigen binding domains. Their format is flexible and scalable, which is an advantage for their engineering. They can be fused to the constant Fc part of antibodies to give them a bivalent format with a long lifetime in the bloodstream.
- Deep Mutational Scanning consists in generating and screening the set of unique mutations in the sequence of a protein. The data obtained in DMS allow the analysis of protein/protein interfaces and are also used to guide the engineering of optimized molecules, notably for affinity maturation or antibody selectivity engineering.
- Yeast Surface Display is a protein engineering technique that uses the expression of recombinant proteins on the surface of yeast cells to screen at high throughput for improved antibodies with the desired functional properties.