A team of Swiss-German researchers has developed a method of testing for neutralizing antibodies against SARS-CoV-2 that produces a result in half the time of conventional tests. A traditional neutralization test to determine immunity to SARS-CoV-2 usually takes two to three days and must be carried out with infectious coronaviruses in a laboratory complying to biosafety level 3. Now, a team from the University of Bern, in cooperation with colleagues from Ruhr-Universität Bochum (RUB), have developed a test that takes only 18 hours and doesn’t have high biosafety requirements. Disguising a harmless virus as SARS-CoV-2 In order to detect antibodies against SARS-CoV-2 in serum samples from covid-19 patients, the researchers used another virus that doesn’t propagate. They exchanged the envelope protein of this virus for the spike protein of the novel coronavirus, which mediates virus entry and infection. “As a result, the viruses can be identified by antibodies against Sars-Cov-2,” explains lead author Toni-Luise Meister from Ruhr-Universität Bochum. “The antibodies bind to the viruses that have been altered in this way and neutralize them so that no longer can penetrate the host cells.” Luminescence helps determine immunity Since the virus pseudotyped in this way can’t propagate in host cells, no elaborate biosafety precautions are necessary for the test. In order to determine the amount of antibodies, the researchers genetically modified the virus so that green fluorescent protein and a luciferase, an enzyme from fireflies, will be produced by infected cells. “After a single round of infection, we can then determine how many cells show green fluorescence,” says lead author Ferdinand Zettl from the Institute of Virology and Immunology in Bern. The green fluorescence is an indicator of infection with the pseudotyped virus. The fewer green cells the researchers are finding, the more neutralizing antibodies are present which blocked the virus. In addition, a luminometer may be used to read the luminescence signal produced by the luciferase enzyme—another way of evaluating the test. Quick and reliable In order to check the reliability and comparability with the conventional neutralization test, the researchers applied it to blood samples from covid-19 patients. “The direct comparison showed a good correlation between the two test systems,” explains corresponding author Professor Stephanie Pfänder, PhD, from the Department of Molecular and Medical Virology at RUB. Compared to 56 hours for the conventional test, the new test is much faster, with only 18 hours to the test result. “Another great advantage is that it can be carried out in almost all medical labs, because no sophisticated safety precautions are necessary,” points out Gert Zimmer, PhD, from the Institute of Virology and Immunology in Bern, corresponding author of the study. Reference 1 Zettl F, Meister TL, Vollmer T, et al. Rapid quantification of SARS-CoV-2-neutralizing antibodies using propagation-defective vesicular stomatitis virus pseudotypes. Vaccines, 2020;8(3):386. doi: 3390/vaccines8030386. Featured image: Stephanie Pfänder, PhD, of Ruhr-Universität Bochum. © RUB, Marquard.