Designing and developing a sensitive and specific SARS-CoV-2 RBD IgG detection kit for identifying positive human samples

Abstract

More than 3 y into the coronavirus 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) continues to undergo mutations. In this context, the Receptor Binding Domain (RBD) is the most antigenic region among the SARS-CoV-2 Spike protein and has emerged as a promising candidate for immunological development. We designed an IgG-based indirect enzyme-linked immunoassay (ELISA) kit based on recombinant RBD, which was produced from the laboratory to 10L industry scales in Pichia pastoris. A recombinant-RBD comprising 283 residues (31kDa) was constructed after epitope analyses. The target gene was initially cloned into an Escherichia coli TOP10 genotype and transformed into Pichia pastoris CBS7435 muts for protein production. Production was scaled up in a 10L fermenter after a 1L shake-flask cultivation. The product was ultrafiltered and purified using ion-exchange chromatography. IgG-positive human sera for SARS-CoV-2 were employed by an ELISA test to evaluate the antigenicity and specific binding of the produced protein. Bioreactor cultivation yielded 4g/l of the target protein after 160h of fermentation, and ion-exchange chromatography indicated a purity>95%. A human serum ELISA test was performed in 4 parts, and the ROC area under the curve (AUC) was>0.96 for each part. The mean specificity and sensitivity of each part was 100% and 91.5%, respectively. A highly specific and sensitive IgG-based serologic kit was developed for improved diagnostic purposes in patients with COVID-19 after generating an RBD antigen in Pichia pastoris at laboratory and 10L fermentation scales.