Novel Antibody Tests Reveal Complexity of Immune Response to COVID-19

Researchers have developed two assays that reveal the ability of antibodies to target SARS-CoV-2, particularly the spike protein essential for viral entry into cells, may help determine disease susceptibility and progression in patients.

The interaction of SARS-CoV-2, the novel coronavirus causing the global COVID-19 pandemic, and the human immune system is the focus of intensive research at The Jackson Laboratory (Bar Harbor, ME USA), leading to the development of two assays. The first one is a highly sensitive and specific antibody test to determine the magnitude of total and different types of antibodies against the virus surface (Spike) and nuclear (nucleocapsid) proteins, while the second determines how well anti-SARS-CoV-2 antibodies are able to neutralize binding with ACE-2, the human receptor for the virus. In the latter assay, the team used a non-infectious pseudo-virus with the SARS-CoV-2 spike protein on the external membrane, meaning the assay does not require a BSL-3-level biosecurity facility yet detects the presence of neutralizing antibodies with extremely high sensitivity and specificity.

In their study, the researchers have shown that the assay is able to detect neutralizing antibody in patient plasma even at high dilutions, up to a 100 thousand-fold. Working with samples from 115 subjects with confirmed SARS-Cov-2 infection as well as healthy controls (56 subjects from year-old frozen samples to ensure non-infection), the team was able to determine accurate antibody levels in every patient. The assay is also sufficiently sensitive to distinguish background levels of antibody in control samples which may have been from anti-SARS-CoV-2 antibodies in patient plasma. In addition, the antibody assay provides the ability to identify antibody isotopes and assess the ratio of IgA, IgG and IgM present in each sample.

In the analysis of the patient antibody levels and neutralization, the team had several key findings. First, samples from patients with severe disease - in the ICU or deceased - had almost 100-fold higher neutralizing antibody levels than those with cases mild enough not to require hospitalization. Why, then, did they get so sick? Could extremely high levels of antibody even be harmful? And how would that influence the use of convalescent plasma as a therapy in severely ill patients who already have high antibody levels? Second, most convalescent plasma samples obtained to treat severe patients had much lower antibody levels, suggesting plasma therapy of hospitalized patients would not benefit them. Indeed, recent findings, including those using synthetic antibodies, strongly support this result. Third, there was big difference - almost a thousand-fold - in the level of neutralizing capacity of antibodies among different subjects.

The results suggest that some individuals with low neutralizing antibodies may be protected for a shorter period of time than others, an important finding given the recent emergence of antibody-evading mutant viruses in South Africa and Brazil. Interestingly, some of patient plasma was also able to effectively neutralize the original SARS virus, and there was no particular correlation with the levels of neutralization with SARS-Cov-2. This could be due to the fact that these two viruses have similar Spike surface protein structures and both use ACE2 as a receptor to enter into cells. It remains to be determined whether antibodies that can block both viruses can be more effective in neutralizing SARS-Cov-2.

The researchers will continue to use these assays to follow some subjects six months to a year post-infection, in both adults and children as well as post-vaccination. These studies will be critical for understanding the precise relationship between antibody levels and protection from reinfection, as well as protection acquired through vaccination. They also suggest the use of therapies that target excessive antibodies generated during the severe disease-causing immune pathology.

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The Jackson Laboratory