Humoral Immune Response to SARS-CoV-2

Pauline H. Herroelen, MSc; Geert A. Martens, MD, PhD; Dieter De Smet, MD; Koen Swaerts, MSc; An-Sofie Decavele, MSc

Disclosures

Am J Clin Pathol. 2020;154(5):610-619. 

In This Article

Abstract and Introduction

Abstract

Objectives: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) serology tests are clinically useful to document prior SARS-CoV-2 infections. Data are urgently needed to select assays with optimal sensitivity at acceptable specificity for antibody detection.

Methods: A comparative evaluation was performed of 7 commercial SARS-CoV-2 serology assays on 171 sera from 135 subjects with polymerase chain reaction–confirmed SARS-CoV-2 infection (71 hospitalized patients and 64 paucisymptomatic individuals). Kinetics of IgA/IgM/IgG seroconversion to viral N and S protein epitopes were studied from 0 to 54 days after onset of symptoms. Cross-reactivity was verified on 57 prepandemic samples.

Results: Wantai SARS-COV-2 Ab ELISA and Orient Gene COVID-19 IgG/IgM Rapid Test showed superior overall sensitivity for detection of SARS-CoV-2 antibodies. Elecsys Anti-SARS-CoV-2 assay and EUROIMMUN Anti-SARS-CoV-2 combined IgG/IgA showed acceptable sensitivity (>95%) vs the consensus result of all assays from 10 days post onset of symptoms. Wantai SARS-COV-2 Ab ELISA, Elecsys Anti-SARS-CoV-2 assay, and Innovita 2019-nCoV Ab rapid test showed least cross-reactivity, resulting in an optimal analytical specificity greater than 98%.

Conclusions: Wantai SARS-COV-2 Ab ELISA and Elecsys Anti-SARS-CoV-2 assays are suitable for sensitive and specific detection of SARS-CoV-2 antibodies from 10 days after onset of symptoms.

Introduction

Coronavirus disease 2019 (COVID-19) is an infectious disease caused by a newly discovered coronavirus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Severe cases show excessive activity of proinflammatory immune cells, causing acute respiratory distress syndrome, septic shock, and bleeding and coagulation dysfunction.[1–3] Due to high human-to-human transmissibility, COVID-19 rapidly became a pandemic threat in which clinical laboratory testing from diagnosis and treatment to epidemiologic surveillances are indispensable.

The gold standard for diagnosis of COVID-19 lung disease is nucleic acid amplification testing of SARS-CoV-2 virus-specific sequences coding for spike (S), envelope (E), and nucleocapsid (N) proteins; RNA dependent RNA polymerase (RdRP) gene; and open reading frame 1ab (ORF1ab) region.[4] Diagnostic sensitivity of the most commonly used technique, reverse transcription polymerase chain reaction (PCR), on nasopharyngeal swabs is currently unknown. When compared to chest computed tomography analysis of lesions characteristic for viral pneumonia, estimates vary from lower than 70% to 90%,[5,6] likely depending on COVID-19 disease stage, intensity of viral replication, sampling quality, and analytical properties of the amplification assay. In addition, insufficient PCR capacity during peak infection rate in overwhelmed health care systems left many patients with milder clinically suspected SARS-CoV-2 infections as well as asymptomatic infections untested.

Serology testing for COVID-19, comprising detection of IgM, IgA, or IgG antibodies to SARS-CoV-2–specific epitopes, might represent an interesting tool to document past SARS-CoV-2 infections, both in individual patients with suspected COVID-19 symptoms or late-stage complications who had no (conclusive) PCR test and at a population level to guide infection control policies. In addition, measuring SARS-CoV-2 antibodies might harbor prognostic value and convey information on protective immunity in vaccination trials.

SARS-CoV-2 shares a 80% overall nucleotide homology with SARS-CoV.[1,7,8] In SARS-CoV, the S and N proteins contain the highest density of B-cell epitopes,[9,10] and in silico analysis indicated that dominant B-cell epitopes share 69% to 100% homology to SARS-CoV-2. It was therefore a logical choice of many commercial developers of SARS-CoV-2 serology kits to target S and N proteins. Antibodies against S protein, composed of a S1 subunit with the receptor-binding domain (RBD) and a S2 subunit that mediates membrane fusion for viral entry, appear additionally interesting because of their proposed correlation with neutralizing antibodies and protective immunity to both SARS-CoV[8] and, based on emerging data, also to SARS-CoV-2.[11,12]

Data on kinetics of humoral immune responses to SARS-CoV-2 are rapidly emerging, but relative sensitivity for detection of antibodies of various commercial assays is questioned. In our study, a cross-platform comparison of 7 commercially available SARS-CoV-2 serology assays was conducted. N and S protein epitopes and different combinations of antibody isotypes in PCR-confirmed COVID-19 patients, with critical and mild disease course at various time points, were targeted. Acceptable performance was defined as minimal sensitivity of 95% for detection of SARS-CoV-2 antibodies vs a consensus estimate and minimal cross-reactivity, defined as analytical specificity of 98%.

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