Rapid antibody test to gauge immune response to SARS-CoV-2 variants

COVID-19 infections are once again on the rise as our immune systems struggle to combat new variants. 

That’s according to a University of Toronto study that found the antibodies generated by people who were vaccinated and/or recovered from COVID-19 prior to 2022 failed to neutralize the variants circulating today.

Furthermore, the researchers expect that the antibody test they developed to measure immunity in the study’s participants will become a valuable tool for deciding who needs a booster and when, helping to save lives and avoid future lockdowns.

"The truth is we don’t yet know how frequent our shots should be to prevent infection," said  Igor Stagljar , a professor of biochemistry and molecular genetics at the Donnelly Centre for Cellular and Biomolecular Research and at the Temerty Faculty of Medicine. "To answer these questions, we need rapid, inexpensive and quantitative tests that specifically measure Sars-CoV-2 neutralizing antibodies, which are the ones that prevent infection."

The study was led by Stagljar and Shawn Owen , an associate professor of pharmaceutics and pharmaceutical chemistry, at the University of Utah.

The journal Nature Communications recently published their findings.

Many antibody tests have been developed over the past two years. But only a few of the authorized ones are designed to monitor neutralizing antibodies, which coat the viral spike protein so that it can no longer bind its receptor and enter cells.

It’s an important distinction, as only a fraction of all Sars-CoV-2 antibodies generated during infection are neutralizing. And while most vaccines were specifically designed to produce neutralizing antibodies, it’s not clear how much protection they give against variants.

"Our method, which we named Neu-SATiN, is as accurate as - but faster and cheaper than - the gold standard, and it can be quickly adapted for new variants as they emerge," Stagljar said.

Neu-SATiN stands for Neutralization Serological Assay based on split Tri-part Nanoluciferase, and it is a newer version of SATiN, which monitors the complete IgG pool they developed last year.

The development of Neu-SATiN was spearheaded by  Zhong Yao , a senior research associate in Stagljar’s lab, and Sun Jin Kim , a post-doctoral researcher in Owen’s lab, who are the co-first authors on the paper.

The pinprick test is powered by the fluorescent luciferase protein from a deepwater shrimp. It measures the binding between the viral spike protein and its human ACE2 receptor, each of which is attached to a luciferase fragment. The engagement of the spike protein with ACE2 pulls the fragments close, catalyzing reconstitution of the full length luciferase with a concomitant glow of light captured by the luminometer instrument. When a patient’s blood sample is added into the mixture, the neutralizing antibodies bind to - and mop up - all spike protein, while ACE2 remains in unengaged state. Consequentially, the luciferase remains in pieces and the light signal drops. The researchers say the plug-and-play design of the test means it can be adapted to emerging variants by engineering mutations in the spike protein.

The researchers applied Neu-SATiN to blood samples collected from 63 patients with different histories of COVID-19 and vaccination up to November 2021. Patient neutralizing capacity was assessed against the original Wuhan strain and the following variants: Alpha, Beta, Gamma, Delta and Omicron.

"We thought it would be important to monitor people that have been vaccinated to see if they still have protection and how long it lasts," said Owen, who did his post-doctoral training in the Donnelly Centre with distinguished bioengineer and University Professor Molly Shoichet of the Faculty of Applied Science & Engineering. "But we also wanted to see if you were vaccinated against one variant, does it protect you against another variant?"

The neutralizing antibodies were found to last about three to four months before their levels would drop by about 70 per cent irrespective of infection or vaccination status. Hybrid immunity, acquired through both infection and vaccination, produced higher antibody levels at first, but these too dropped significantly four months later.

Most worryingly, infection and/or vaccination provided good protection against the previous variants, but not Omicron or its sub-variants BA.4 and BA.5.

The data match those from a recent U.K. study that showed that both neutralizing antibodies and cellular immunity - a type of immunity provided by memory T cells - from either infection, vaccination, or both, offered no protection from catching Omicron. In a surprising twist, the U.K. group also found that infections with Omicron boosted immunity against earlier strains, but not against Omicron itself for reasons that remain unclear.

"It’s important to stress that vaccines still confer significant protection from severe disease and death," said Stagljar. Still, he added that the findings from his team and others call for vigilance in the coming period given that the more transmissible BA.4 and BA.5 sub-variants can escape immunity acquired from earlier infections with Omicron, as attested by rising reinfections.

"There will be new variants in the near future for sure," Stagljar said. "Monitoring and boosting immunity with respect to circulating variants will become increasingly important and our method could play a key role in this since it is fast, accurate, quantitative and cheap."

He is already collaborating with the Canadian vaccine maker Medicago to help determine the efficacy of their candidates against Omicron and its sub-variants. Meanwhile, University of Toronto is negotiating to license Neu-SATiN to a company which will scale it up for real world uses such as population immunosurveillance and vaccine development.