July 6, 2020
Antibodies Reject Viral Entry by Exploiting the SARS-CoV-2 Spike Protein
One of the key strengths of SARS-CoV-2 is its spike (S) protein, which enables the virus to bind tightly to its cellular receptor, ACE2. As scientists gain a better understanding of the spike’s characteristics, however, the spike may soon become the virus’ weakness. This week, we discuss new research on antibodies that are leading the attack on the viral spike protein.
Anti-RBD Antibodies Block Spike Binding and Fusion
The spike protein of SARS-CoV-2 has two functional components, S1 and S2. A region on S1 that is extremely valuable to the virus is the receptor-binding domain (RBD), since it is responsible for engaging the spike with ACE2. This initial interaction is what allows the virus to attach to the cell and eventually gain entry. Because RBD binding is the first step in viral infection, it is thought that immunotherapies aimed at neutralizing the virus should target the RBD.
Scientists are now trying to define the exact sections of the RBD that are required for ACE2 binding. Antibodies that target these specific regions within the RBD would likely be the most effective at blocking the spike-ACE2 interaction. To find the precise points of contact, researchers at the Fred Hutchinson Cancer Research Center systematically mutated single amino acids within the RBD sequence and then tested the ability of each mutant to bind to ACE2 . They identified the mutations that had a detrimental impact on binding, which allowed them to map the RBD sections indispensable for ACE2 attachment. The researchers suggest that these particular sections within the RBD are prime targets for immunotherapies, since they are absolutely required for cellular binding.
Other research suggests that preventing RBD binding to ACE2 isn’t the only way to block infection. Anti-RBD antibodies may also block the second step of viral entry: membrane fusion. A study in Cell Host and Microbe reports that a previously identified antibody against the original SARS-CoV RBD cross-reacts with the SARS-CoV-2 RBD, and can block infection of SARS-CoV-2 in cell culture . The mechanism by which this antibody neutralizes the virus, however, is not through physical obstruction of ACE2 binding. Rather, this antibody triggers conformational changes of the spike. Using cryogenic electron microscopy, the scientists show that binding with this particular antibody alters the structure of the spike protein to a fusion-incompetent shape. This prevents the virus from fusing its membrane with the cellular membrane, and stops the virus from entering the cell even if it docks onto ACE2.
A Multipronged Approach to Neutralization
Even as scientists begin to identify which anti-RBD antibody has the most potential as a drug candidate, it is clear that a single anti-RBD antibody may not be enough to stop SARS-CoV-2. The virus can undergo natural mutations, which form new viral variants or strains that escape recognition by individual antibodies. This raises the need for a multipronged approach to neutralizing the virus.
A recent study published in Science found that growth of SARS-CoV-2 in cell culture under the selective pressure of single anti-RBD antibodies led to the rapid emergence of escape mutants . These mutants were protected from neutralization because their RBD sequences had changed such that they were no longer recognizable by the individual antibodies. However, when the virus was grown in the presence of a cocktail of antibodies targeting unique sites on the RBD, escape mutants were not generated. The researchers conclude that attacking multiple areas of the RBD simultaneously with different antibodies can prevent viral escape. This antibody cocktail is currently being tested in clinical trials.
Inducing Natural Antibody Responses Against the Spike
Development of antibody therapies targeting the spike, though important, is not the ideal end goal. To fully stop the spread of SARS-CoV-2, a vaccine is needed. New research suggests that our immune system, if stimulated correctly, is capable of generating anti-RBD antibodies that neutralize the virus. This bodes well for vaccine candidates using the spike protein to elicit immune responses.
Anti-RBD antibodies blocking spike binding to ACE2. Image source: Suthar et al. Cell Reports Medicine 2020
An analysis of 44 hospitalized COVID-19 patients at Emory University found that anti-RBD antibodies were detectable in the blood of >80% of patients studied . Lack of anti-RBD antibodies in several individuals may have been due to the early timepoints at which their blood was examined – when antibody responses may not yet have peaked. When the researchers tested antibody-containing plasma from these patients in virus-neutralization assays, they found that neutralization ability correlated with anti-RBD antibody concentrations. Their findings indicate that anti-RBD antibodies may be a strong predictor of protective immunity.
Ultimately, vaccines that equip our immune system with the right antibodies will offer the most protection. More knowledge on the spike protein and its points of vulnerability will help scientists achieve this. Research on the spike will also prepare us for a constantly adapting coronavirus family. As new coronaviral spikes evolve, so should our weapons that target them.
BioLegend now offers recombinant SARS-CoV-2 spike proteins and anti-spike antibodies to aid in vaccine, antibody, and viral research. Find them here.
- Starr T et al. Deep mutational scanning of SARS-CoV-2 receptor binding domain reveals constraints on folding and ACE2 binding. bioRxiv (2020). DOI: 10.1101/2020.06.17.157982
- Huo J et al. Neutralization of SARS-CoV-2 by Destruction of the Prefusion Spike. Cell Host and Microbe (2020). DOI: 10.1016/j.chom.2020.06.010
- Baum A et al. Antibody cocktail to SARS-CoV-2 spike protein prevents rapid mutational escape seen with individual antibodies. Science (2020). DOI: 10.1126/science.abd0831
- Suthar M et al. Rapid Generation of Neutralizing Antibody Responses in COVID-19 Patients. Cell Reports Medicine (2020). DOI: 10.1016/j.xcrm.2020.100040