The Covid-19 pandemic is caused by SARS-COV-2. To date, more than 664 million people have been affected and 6.71 million have died(1–27).
An mRNA vaccine has been developed and has shown remarkable immunopreventive effects.
However, the emergence of SARS-COV-2 variants with increased infectivity, immune evasion, and altered virulence has continued since then in many parts of the world, and pandemics of these variants continue to occur(1–27).
With continued mRNA vaccination, neutralizing antibody titers against the SARS-COV-2 variant could be maintained, but with the emergence of new variants appearing one after another, it becomes difficult to maintain neutralizing antibody titers.
The Omicron variant of the highly infectious SARS-COV-2 mutant, which emerged at the
end of 2021, began as BA.1 and has mutated to BA.2 and BA.5. In January 2023, a BQ.1.1 strain derived from BA.5 and an XBB strain derived from BA.2 have emerged, although BA.5 is still the mainstream strain(1–27), (28–31).
Furthermore, with the elimination of the zero-corona policy in China beginning in December 2022, it is estimated that there is an explosion of more than 900 million more infections at this point, and it is feared that new SARS-COV-2 variants, especially new Omicron lineages, may emerge in the future.
Currently, the majority of cases are of the omicron type, which mainly infects the nasopharynx, but it is feared that the emergence of a new type, such as the delta type, with added lung invasiveness, would be a worldwide catastrophe.
Currently, the Omicron strain-compatible bivalent vaccine is used to address prevention of infection and severe disease. And in addition to the mRNA vaccine, the oral drugs Molnupiravir (Lagevrio, Merck) and nirmatrelvir + ritonavir (Paxlovid, Pfizer) are used for treatment(32–40).
As for antibody drugs, some of them have high therapeutic efficacy, but with the emergence of each new variant, the neutralizing antibody activity against the spike decreases, resulting in a short efficacy period, and it is necessary to constantly develop antibody drugs tailored to the new variant(4, 41–54).
Antibody drugs are very effective in the treatment of immunocompromised patients with mild disease and risk factors such as severe cardiac disease, chronic respiratory disease, and obesity.
Antibody drugs are also expected to be effective in reducing the incidence of the disease in those who cannot be expected to acquire sufficient immunity through vaccination and in facilities with many elderly people with low basic physical ability.
Antibody drugs, by their nature as macromolecular biopharmaceuticals, take a long time to develop, and have problems such as difficulties in production and purification and their cost.
In addition, the constant use of antibody drugs for treatment is difficult because of the decreasing efficacy of antibody drugs with each new variant.
In recent years, attention has focused on the development of universal antibodies with neutralizing activity against several variants, but the emergence of new variants has weakened the neutralizing activity(55–70).
The omicron variant in particular is very strongly immuno evasive, and it is assumed that the spike mutant has an altered structure that makes it difficult for neutralizing antibodies to bind to it.
Antibody drugs bind easily to the convex surface of a molecule, but lose their binding ability when a conformational change occurs due to amino acid mutation on the surface of the molecule.
Molecular concavities, grooves, and clefts are well conserved and sometimes have enzyme active centers.
However, molecular concavities, grooves, and clefts are places where amino acid mutations are less likely to occur, but also where macromolecular antibodies cannot penetrate and bind.
Therefore, in this study, we attempted in vitro screening of "7-amino acid peptide aptamers" that can bind not only to convex surfaces of molecules but also to concave surfaces, grooves, and clefts.
For screening, the ribosomal display method (71–81), which links genes and protein molecules on a one-to-one basis, was employed, allowing for larger library sizes.
In this ribosome display method (71–81), the stop codon of the gene DNA is omitted and in vitro transcription and translation are performed in a cell-free protein synthesis system to create a protein-mRNA-ribosome complex (PRM complex, ternary complex), followed by affinity selection against the target antigen, and clones that bind to the target antigen are purified.
Affinity selection (panning) (71, 72, 81–84) can be done once a day, and even if panning is repeated six times, rapid screening can be done in about a week's process.
A 7-amino acid random peptide ribosome display library(71–81) was constructed and screened for peptide aptamers that bind to the SARS-CoV-2 wild-type spike RBDs. Among them, we selected peptide aptamers that can bind not only wild-type spike RBDs but also delta variant, omicron variant, BA.1, BA.2, and BA.5 spike RBDs.
The peptide aptamers selected in this screening bind to the common epitope of the spike RBD of SARS-CoV-2 variants, including the omicron variant, and are potential universal binding peptide aptamers for all SARS-CoV-2 variants.
These peptide aptamers are approximately 30 amino acids in length, including a 7 amino acid binding site and a surrounding scaffold sequence. They can be easily obtained by peptide synthesis and purification. In addition, peptide aptamer proteins can be easily synthesized and purified in bacteria such as E. coli.
These peptide aptamers can be diagnostic and therapeutic agents.
Recently, in the very early stages of an emerging infectious disease outbreak with pathogenic potential, the identification of the pathogen by Next-generation sequencing is used to identify the pathogen and its component proteins.
While the development of diagnostics can be done quickly with PCR and other genetic diagnostics, the development of diagnostics and therapeutics for pathogen proteins requires the complicated, expensive, and time-consuming process of antibody production.
In such cases, antibody production becomes even more difficult for pathogens that repeatedly mutate, such as the SARS-CoV-2 variant in this case.
In this study, we screened universal binding peptide aptamers against SARS-CoV-2 variants using a 7-amino acid random peptide ribosome display library screening system, demonstrating the utility of this method for screening peptide aptamers as antibody alternatives .