Abstract
A few months ago, the availability of a reliable and cost-effective testing capacity for COVID-19 was a concern for many countries. With the emergence and circulation of new SARS-CoV-2 variants, another layer of challenge can be added for COVID-19 testing at both molecular and serological levels. This is particularly important for the available tests principally designed to target the S gene/protein where multiple mutations have been reported. Herein, the SARS-CoV-2 NP recombinant protein was utilized to develop a simple and reliable COVID-19 NP human IgG ELISA. The optimized protocol was validated against a micro-neutralization (MN) assay, in-house S-based ELISA, and commercial chemiluminescence immunoassay (CLIA). The developed assay provides 100% sensitivity, 98.9% specificity, 98.9% agreement, and high overall accuracy with an area under curve equal to 0.9998 ± 0.0002 with a 95% confidence interval of 0.99 to 1.00. The optical density values of positive samples significantly correlated with their corresponding MN titers. The assay specifically detects IgG antibodies to the SARS-CoV-2 NP protein and does not cross-detect IgG to the viral S protein. Moreover, it does not cross-react with antibodies related to other coronaviruses (e.g., the Middle East respiratory syndrome coronavirus or human coronavirus HKU1). The availability of this reliable COVID-19 NP IgG ELISA protocol is highly valuable for its diagnostic and epidemiological applications.
Highlights
It has been over a year since COVID-19 was declared a pandemic
The S protein is cleaved into S1 and S2 subunits that are responsible for binding to angiotensin-converting enzyme 2 (ACE2) and membrane fusion, respectively [11,12]
Additional samples from individuals who received the S-based COVID-19 vaccination (n = 6) and COVID-19 recovered patients (n = 6) were utilized in order to assess the cross-reactivity with antibodies generated to the SARS-CoV-2 S protein
Summary
SARS-CoV-2, the leading cause of COVID-19, belongs to the β-coronaviruses [1,2]. Comprises the nucleocapsid protein and a positive-sense single-stranded RNA [2,3]. The viral nucleocapsid is encased within phospholipid bilayers that contain the membrane, envelope, hemagglutinin-esterase, and spike (S) proteins [2,3]. The mucosal epithelium of the upper respiratory tract is the primary site of viral replication [1,4,5]. The virus can replicate in several organs including the lower respiratory tract (e.g., lung and bronchus), kidneys, and stomach because of the broad expression of its cellular receptor angiotensin-converting enzyme 2 (ACE2) [6,7,8,9,10]. Upon viral entry into target cells, the viral transcription, translation, and genome replication, virion assembly and maturation take place, leading to the production of infectious progeny virions
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