Impedance-based monitoring of titration and neutralization assays with VSV-G and SARS-CoV-2-spike pseudoviruses

Since cell-based virus neutralization assays are still the gold standard to assess a patient's immune protection against a given virus, they are of utmost importance for serodiagnosis, convalescent plasma therapy, and vaccine development. Monitoring the emergence and characteristics of neutralizing antibodies in an outbreak situation, confirming neutralizing antibodies as correlates of protection from infection and testing vaccine-induced potency of neutralizing antibody responses, quests for automated, fast, and parallel neutralization assays. We developed an impedance-based sensor platform (electric cell-substrate impedance sensing, ECIS) providing time-resolved monitoring of the host cell response to viral pseudotypes. For validation, the impedance assay was compared with state-of-the-art quantification of virus-induced reporter protein expression as an independent indicator of virus infection and neutralization. Vesicular stomatitis virus (VSV) derived pseudoviruses encoding the green fluorescent protein (GFP) as reporter and the autologous G protein (VSV-G) for the initial binding to the host cell membrane were used for monitoring of HEK293T cell infection and neutralization with both, impedance and optical readout. Virus-induced cytopathic effects (CPE) were detectable for low pseudotype concentrations (multiplicity of infection 1) in time-resolved impedance profiles as soon as 5–10 h after infection in a concentration-dependent manner. Neutralization efficacy of α-VSV-G antibodies was determined from impedance time courses and IC₅₀ values compared favorably with fluorescence measurements of virus-borne GFP expression. Sera of convalescent COVID-19 patients were tested successfully for SARS-CoV-2 neutralizing antibodies by incubating VSV, pseudotyped with the SARS-CoV-2 spike protein, with different sera before host cell exposure and impedance recordings. In summary: (i) ECIS monitoring was successfully applied to detect virus-mediated cell infection and neutralization; (ii) Impedance-based monitoring allows reducing the assay time to 5–10 h; and (iii) the platform is easily adapted to other virus-based diseases and scalable to high-throughput.