Development of immuno-PCR for sensitive quantification of SARS-CoV-2 nucleocapsid protein

Abstract

Accurately diagnosing people with suspected SARS-CoV-2 infection is essential to help manage COVID-19. Currently available SARS-CoV-2 diagnostics detect either RNA of the virus by RT-PCR or the presence of viral antigens in biological fluids by ELISA or similar techniques. Low sensitivity of antigen tests could lead to the risk of false negative results. Therefore, this study aimed to develop a highly sensitive immuno-PCR method for quantifying SARS-CoV-2 nucleocapsid (N) protein that combines the specificity of sandwich ELISA with the sensitivity of PCR. Recombinant N protein fragment was produced in E. coli as an expression system and purified using immobilized metal ion affinity chromatography. The antibodies against the N protein were raised in rabbits and mice. High-affinity polyclonal mice and rabbit N protein-specific antisera were purified using ammonium sulfate precipitation and used to develop sandwich ELISA for the quantification of N protein. Mice polyclonal serum was used as a capture for N protein. N protein bound to mice antibodies was detected with rabbit polyclonal sera. A double-stranded amino-DNA molecule of 77 base pairs was PCR-synthesized, covalently conjugated to a secondary goat anti-rabbit antibody and subsequently amplified and quantified by real-time PCR. The results were compared to analogous sandwich ELISA consisting of alkaline phosphatase-labeled goat anti-rabbit antibody. The sensitivity of immuno-PCR for quantification of N protein was increased by up to 7-fold compared to analogous ELISA, having a limit of detection of 92 pg/mL and a limit of quantification of 840 pg/mL. The developed immuno-PCR method thus has the potential to be used as a new antigen test for COVID-19 and beyond.