With the COVID-19 pandemic raging into its second year, it has become increasingly clear that completely eradicating SARS-CoV-2 will not be possible. Instead, containing the virus and mitigating its impact will be key to resuming ‘normal’ life, making the routine detection of SARS-CoV-2 background levels important.
Serology-based tests, which detect antibodies against the virus rather than the virus itself, will play a crucial role in identifying asymptomatic patients and those who have recovered. However, existing serology tests typically use only a fragment of the virus’ most exposed protein, the spike protein, detecting antibodies specific to each fragment rather than the full range of antibodies produced against the virus.
Now, a team of researchers led by Laurent Renia, Executive Director of A*STAR’s Infectious Diseases Labs (ID Labs), has developed a serology test that not only captures the full repertoire of antibodies against the spike protein that covers the surface of SARS-CoV-2, but also distinguish between the antibody subclasses detected.
“Being able to distinguish between IgG and IgM subclasses, for example, might inform us about which stage of infection the patient is at. This could be particularly important for asymptomatic patients,” said Renia. “Similarly, the assay could be used to detect functional IgG1 and IgG2, which are important in virus neutralization.”
To develop their assay, the researchers first introduced the full-length spike protein gene into cells, causing them to stably express the protein on their surface. They then incubated the cells with patient plasma, allowing any antibodies present to bind to the spike protein. Finally, fluorescent-labeled secondary antibodies that can distinguish between the antibody subclasses were added and detected using a technique called flow cytometry.
When tested on samples that were confirmed to be positive using the gold-standard polymerase chain reaction test, the assay was able to detect 97 percent of the infections in pre-symptomatic and asymptomatic patients, despite the lower antibody levels in these patients. In contrast, commercially available serology tests only indicated results in 32–35 percent of pre- or asymptomatic patients.
The team is currently using the assay to support Singapore’s COVID-19 surveillance efforts and investigate the immune response to SARS-CoV-2 over time, with studies planned to follow patients for up to 18 months after the onset of illness. “As the assay is based on full-length spike protein, it will likely still be able to detect antibodies against the spike protein even in cases where there are point mutations,” said Renia, adding that the group is currently developing a parallel assay to detect both UK and South African variants.