Effectiveness of SARS-CoV-2 Antibodies in Tackling Novel Virus Mutants

Comments

Emma Easthope is the founder and director of Cambridge Technical Content Ltd, based in the U.K. Since graduating with a bachelor’s degree in biology from the University of Kent at Canterbury in 2000, she has gained extensive experience developing and running immunoassays within companies including Millennium Pharmaceuticals, AstraZeneca and Cellzome. She now produces a wide range of scientific content, including regular features for Biocompare.

April 26, 2021

In late December 2019, a small cluster of pneumonia cases with unknown etiology was reported in Wuhan City, Hubei Province, China. Just days later, these were attributed to a novel coronavirus—SARS-CoV-2, the causative agent of COVID-19—which has since claimed almost 3 million lives globally. Although recent advances in recombinant genetic technology have enabled vaccines based on SARS-CoV-2 mRNA to be developed in record times, the virus is constantly mutating, giving rise to new lineages able to evade the immune response triggered by these vaccines. This highlights the need for additional therapeutics to tackle the ongoing pandemic. In this article, we note several SARS-CoV-2 lineages that are currently causing concern and comment on the use of neutralizing antibodies as therapeutic agents.

SARS-CoV-2 lineages of concern

At present, SARS-CoV-2 lineages of particular interest to researchers include B.1.1.7, B.1.351, and P.1, often referred to as the UK, South African, and Brazilian variants, respectively. Each of these is characterized by a specific set of mutations, with all three lineages containing mutations within the angiotensin-converting enzyme 2 (ACE2) receptor-binding domain (RBD) of the viral spike protein. These variants have sparked concern for several reasons; the B.1.1.7 variant has been reported to be more infectious than ancestral lineages, including those containing the spike protein D614G mutation (the first notable change compared to the canonical SARS-CoV-2 genome), while B.1.351 has demonstrated resistance to neutralization by convalescent plasma and neutralizing antibodies, thus jeopardizing the effectiveness of some vaccines. The P.1 lineage has been found in documented cases of SARS-CoV-2 re-infection and, like B.1.351, is associated with viral escape from neutralizing antibodies.

Role of B and T cell immunity

Various approaches have been taken to tackle the SARS-CoV-2 pandemic. As well as developing vaccines based on viral mRNA, researchers have focused on generating and characterizing neutralizing antibodies. The REGN-COV2 antibody cocktail, established by Regeneron, combines two monoclonal antibodies (casirivimab and imdevimab) that bind the viral spike protein to prevent SARS-CoV-2 from entering host cells. However, because some novel SARS-CoV-2 lineages—including B.1.351 and P.1—are able to avoid certain neutralizing antibodies targeting the RBD, other efforts have concentrated on understanding the importance of the T cell response. A recent study has shown that most SARS-CoV-2 T cell epitopes are unaffected by the mutations found in B.1.1.7, B.1.351, and P.1, and it is now known that milder cases of acute COVID-19 are associated with coordinated antibody, CD4+, and CD8+ T cell responses, while severe cases are correlated with a lack of coordination.

Significance and structure of the spike protein

The structure of the SARS-CoV-2 spike protein was first determined in March 2020. At that time, the protein was also found to bind host cells at least 10 times more tightly than the spike protein of an existing coronavirus, SARS–CoV. The homotrimeric SARS-CoV-2 spike protein has a molecular weight of approximately 3 x 142kDa, with each monomer comprising two sub-units, S1 and S2. While S1, including the RBD, functions to recognize and bind the host cell receptor ACE2, S2 mediates viral cell membrane fusion to enable host cell entry; once inside the cell, the viral RNA is released and viral replication can begin. Because the spike protein is essential to the viral life cycle, it offers significant potential as a target for therapeutic intervention. Several monoclonal antibodies—including REGN-COV2, and CR3022 that was isolated from a convalescent SARS-CoV patient—have shown promising results in neutralizing SARS-CoV-2.

Image. 3D structure of the SARS-CoV-2 spike protein with the RBD of one monomer in the “up” (yellow) and two in the “down” position based on PDB 6VSB (PMID 32075877).

Epitopes of neutralizing antibodies

Neutralizing antibodies confer protection by preventing SARS-CoV-2 from binding and entering host cells. Many neutralizing antibodies isolated from human COVID-19 patients have been found to target a region of the S1 sub-unit involved in binding the ACE2 RBD of the spike protein. Some of these antibodies bind the RBD only when it is an ‘up’ conformation, others only when it is ‘down’. Additionally, neutralizing antibodies have been discovered that can bind either conformation. By assigning neutralizing antibodies into distinct classes based on SARS-CoV-2 epitope recognition, it may be possible to suggest effective antibody combinations for therapeutic use and better understand the immune response to the virus.

Neutralization assays

Laboratory-based neutralization assays have seen widespread use by researchers investigating SARS-CoV-2. These include assays using the live virus, or a pseudotype, to infect mammalian cells—that must be performed under strict biosafety level 3 (BSL-3) conditions—and a broad range of ‘surrogate’ ELISAs. The clone CR3022 is frequently employed as a reference in these studies, where it provides an essential benchmark against which potential therapeutic candidates can be assessed.

antibodies-online offers a diverse portfolio of reagents for studying SARS-CoV-2, including the CR3022 clone. These are complemented by a SARS-CoV-2 Mutations Tracker for sourcing the very latest mutation-specific antibodies, proteins, and assays. To learn more visit antibodies-online.com