Expression of recombinant SARS-CoV-2 nucleocapsid protein in mammalian cells

The SARS-CoV-2 nucleocapsid (N) protein plays a significant role in the coronavirus life cycle and participates in a variety of critical events following viral invasion1. In infected patients, high titers of immunoglobulin G (IgG) targeting N protein were detected and correlated with the clinical course of the disease2. Therefore, N protein and anti-N protein IgGs were recognized as important diagnostic indicators of COVID-19 infection in serological and quick antigen tests3. In this study, we optimized the expression of the recombinant form of SARS-CoV-2 N protein in a mammalian cell line HEK293T by comparing the transfection efficiency between Polyethylenimine (PEI) and Calcium Phosphate (CaP) DNA-complexing agents. Transfection potency was tested at different cell confluency and passage number, in several cell culture media, pre-transfection and post-transfection media change and in conditions of reduced serum. Chloroquine and glycerol treatments were included to enhance transfection efficiency as they might inhibit DNA degradation in lysosomes or increase membrane permeability. Protein expression was monitored in cell supernatants up to 7 days post-transfection in dot-bot and Western blot using anti-N protein antibodies. Both transfection methods have shown moderate to relatively high transfection efficiency dependent on the applied conditions, making them affordable and easy to use techniques for recombinant N protein production on a small-scale in adherent mammalian systems. PEI acts as a good delivery system regardless of the presence of the fetal bovine serum (FBS), while CaP transfection is more dependent on the presence of FBS which in turn favors N protein degradation. However, we have optimized both methods to achieve optimal expression of unfragmented N-protein in serum-free conditions. Apart from setting up a cost-effective platform for expression of N protein in mammalian cells, we plan on investigating the mechanisms behind the PEI and CaP non-viral gene delivery systems as there are still some uncertainties in the scientific community

Sandwich ELISA for the Quantification of Nucleocapsid Protein of SARS-CoV-2 Based on Polyclonal Antibodies from Two Different Species

In this study, a cost-effective sandwich ELISA test, based on polyclonal antibodies, for routine quantification SARS-CoV-2 nucleocapsid (N) protein was developed. The recombinant N protein was produced and used for the production of mice and rabbit antisera. Polyclonal N protein-specific antibodies served as capture and detection antibodies. The prototype ELISA has LOD 0.93 ng/mL and LOQ 5.3 ng/mL, with a linear range of 1.52–48.83 ng/mL. N protein heat pretreatment (56 °C, 1 h) decreased, while pretreatment with 1% Triton X-100 increased analytical ELISA sensitivity. The diagnostic specificity of ELISA was 100% (95% CI, 91.19–100.00%) and sensitivity was 52.94% (95% CI, 35.13–70.22%) compared to rtRT-PCR (Ct < 40). Profoundly higher sensitivity was obtained using patient samples mostly containing Wuhan-similar variants (Wuhan, alpha, and delta), 62.50% (95% CI, 40.59 to 81.20%), in comparison to samples mostly containing Wuhan-distant variants (Omicron) 30.00% (6.67–65.25%). The developed product has relatively high diagnostic sensitivity in relation to its analytical sensitivity due to the usage of polyclonal antibodies from two species, providing a wide repertoire of antibodies against multiple N protein epitopes. Moreover, the fast, simple, and inexpensive production of polyclonal antibodies, as the most expensive assay components, would result in affordable antigen tests