The ongoing coronavirus disease 2019 (COVID-19) pandemic, which has been caused by the highly contagious severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has claimed more than 4.5 million lives worldwide.
Scientists and healthcare policymakers have implemented many non-pharmaceutical interventions, such as the use of masks and frequent hand washing, as well as several public health measures like city lockdowns, travel restrictions, and social distancing, to contain the COVID-19 pandemic.
Study: Nanobody-Functionalized Cellulose for Capturing and Containing SARS-CoV-2. Image Credit: NECHAPHAT / Shutterstock.com
The threat of VoCs
The development of vaccines and the commencement of vaccination programs have significantly reduced the mortality rate of COVID-19 worldwide. Scientists hoped that rapid vaccination of the majority of the population of the world would promote herd immunity. However, the emergence of SARS-CoV-2 variants has threatened the efficacy of the available vaccines.
Some of the SARS-CoV-2 variants have been classified as variants of concern (VoC), as they are more virulent than the original SARS-CoV-2 strain. Scientists have reported that VoCs can escape immune responses elicited by either vaccines or natural infection.
In the given scenario, where the efficacy of vaccines has been challenged as a result of SARS-CoV-2 variants, there is an urgent need for effective, low-cost, and off-the-shelf agents that can assist in the rapid diagnosis of COVID-19. Further, there is also a need for an efficient SARS-CoV-2 decontaminating agent that can eliminate the virus from frequently contacted environmental surfaces.
Characteristic features of SARS-CoV-2
SARS-CoV-2 belongs to the family Coronaviridae of the genus β-coronavirus and shares the same subfamily (Orthocoronaviridae) with SARS-CoV. Both these coronaviruses cause severe respiratory tract illness in humans.
SARS-CoV-2 is a single-stranded ribonucleic acid (RNA) virus, which encodes four major structural proteins including the spike (S), membrane (M), envelope (E), and nucleocapsid (N) proteins. The main function of the S protein is to initiate the virus-host interaction and, subsequently, allow the virus to gain entry into the host cell.
The S protein, which contains the receptor-binding domain (RBD), binds to the angiotensin-converting enzyme 2 (ACE2) receptor of the host via the S1 domain of the S protein. The S2 domain of the S protein promotes fusion of the membranes. Owing to this, the S protein has been largely explored as a potential agent for the development of antiviral antibodies.
What are nanobodies?
Scientists have developed nanobodies (Nbs), which are regarded as a unique class of antibodies. Nbs are single-domain nanosized heavy chain-only antibodies (hcAbs) that are developed from variable fragments of Camelidae. Members of Camelidae include camels and llamas.
Some of the advantages of Nbs as compared to other antibodies, with regard to their diagnostic potential, include their small nanometer size, deep penetration capacity in the tissues, high affinity and specificity, ease of mass production, and low immunogenicity. Scientists have stated that as compared to conventional human immunoglobin (IgG) based antibodies, Nbs are easily produced using Escherichia coli fermentation.
Researchers have successfully identified many high-affinity neutralizing Nbs against the S protein of SARS-CoV-2. Among these, Ty1 has been reported to possess a nanomolar binding affinity with effective neutralization.
These nanobodies can specifically target the RBD of SARS-CoV-2 with high affinity and directly block ACE2 engagement. Hence, Ty1 could potentially be used for diagnostics and the treatment of COVID-19 disease.
Repurposing Nb Ty1 to detect and neutralize SARS-CoV-2
Scientists have repurposed the Nb Ty1 for the detection and neutralization of SARS-CoV-2, particularly when it is present on the surface and in biologically relevant fluids. The development of this economical approach, which is based on cellulose materials, is discussed in a recent study published on the preprint server bioRxiv*.
In the current study, researchers developed a generic strategy to contain SARS-CoV-2 through the use of cellulose materials. Herein, the authors designed a bifunctional fusion protein that contains cellulose-binding domain (CBD) and Ty1 for cellulose immobilization and SARS-CoV-2 capturing, respectively.
The CBD can escalate the absorption of CBD-containing fusion proteins to cellulose in molar quantities. The immobilization of the fusion proteins on cellulose substrates improves the capture efficiency of Nbs against SARS-CoV-2 pseudoviruses of the wildtype and D614G variant.
As a proof-of-concept, the researchers conducted an immunoassay on cellulose-based filter paper for the detection of the SARS-CoV-2 RBD using bifunctional proteins. Further, they were able to integrate the fusion protein into a customizable chromatography containing highly porous cellulose for neutralizing viruses from contaminated fluids in a continuous and low-cost approach.
Conclusion and future research
The authors of this study developed cost-effective point-of-care (POC) diagnostic and preventative measures, as well as therapeutic strategies against COVID-19 disease. The newly developed bifunctional fusion protein technology can capture SARS-CoV-2 using cellulose substrates.
These cellulose-based POC diagnostics and functionalized facemasks can effectively reduce airborne virus transmission. The technology can also decontaminate fluids containing SARS-CoV-2. In the future, researchers could experiment using advanced fermentation technologies to improve the production of fusion protein.
Although this study holds many promises, it is limited due to the fact that only used pseudovirus-containing culture media were used to characterize the fusion proteins for proof-of-concept. Thus, it is imperative to investigate this technology using real specimens, such as blood from COVID-19 patients, for future study.
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.