What is SARS-CoV-2 IgG Neutralizing Antibodies ( Spike Protein)?

SARS-CoV-2 causing the worldwide pandemic has changed people’s life in multiple aspects dramatically since it’s first identified in Wuhan, China at the end of 2019. While the numbers of infected patients and death toll keep vigorous increasing, curbing the progression of the pandemic is an urgent goal. Efforts have been made to search for prophylactic and therapeutic approaches including neutralizing antibodies development. By reviewing dozens of studies on anti-spike antibodies identification, we concluded that promising therapeutic antibodies are being fished out by various approaches, such as screening of single B cells of convalescent patients, recombinant antibody library and B cells of immunized animals; the epitopes are mainly RBD, but also some non-RBD domains, without the requisite of overlapping with ACE2 binding sites; Neutralizing antibodies are convergent to a few germline genes, including IGHV3-30, IGHV3-53, IGHV3-66, with varying levels of somatic mutations. This review summarizes the progress in neutralizing antibodies development and the germline enrichment of effective antibodies, which will shed light on COVID-19 treatment and vaccine design.

A better understanding of the anti-SARS-CoV-2 immune response is necessary to finely evaluate commercial serological assays but also to predict protection against reinfection and to help the development of vaccines. For this reason, we monitored the anti-SARS-CoV-2 antibody response in infected patients. In order to assess the time of seroconversion, we used 151 samples from 30 COVID-19 inpatients and monitored the detection kinetics of anti-S1, anti-S2, anti-RBD and anti-N antibodies with in-house ELISAs. We observed that specific antibodies were detectable in all inpatients 2 weeks post-symptom onset and that the detection of the SARS-CoV-2 Nucleocapsid and RBD was more sensitive than the detection of the S1 or S2 subunits. Using retroviral particles pseudotyped with the spike of the SARS-CoV-2, we also monitored the presence of neutralizing antibodies in these samples as well as 25 samples from asymptomatic individuals that were shown SARS-CoV-2 seropositive using commercial serological tests. Neutralizing antibodies reached a plateau 2 weeks post-symptom onset and then declined in the majority of inpatients but they were undetectable in 56% of asymptomatic patients. Our results indicate that the SARS-CoV-2 does not induce a prolonged neutralizing antibody response. They also suggest that induction of neutralizing antibodies is not the only strategy to adopt for the development of a vaccine. Finally, they imply that anti-SARS-CoV-2 neutralizing antibodies should be titrated to optimize convalescent plasma therapy.

Introduction

The Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) has recently emerged and caused a human pandemic of coronavirus disease 2019 (COVID-19) (Wu et al., 2020b; Zhou et al., 2020; Zhu et al., 2020). Most infected patients showed mild symptoms, but around 10% had severe symptoms, such as dyspnea, high respiratory rate, and low blood oxygen saturation which can lead to death due to respiratory or multiple organ failure. There is currently no specific treatment and vaccine and thus patients are treated with supportive care.

Among the coronaviruses structural proteins, the Spike (S) and the Nucleocapsid (N) proteins are the main immunogens (Meyer et al., 2014). The S protein consists of two subunits, S1 which contains the Receptor Binding Domain (RBD) and S2. Commercial SARS-CoV-2 serological assays that detect antibodies specific to these viral proteins/domains have become available but they need to be finely evaluated. Some manufacturers have decided to target the S1 and/or S2 subunits whereas others chose the RBD or the N protein. Furthermore, neutralizing antibodies (NAbs) are considered key to recovery and protection against viral disease but the SARS-CoV-2 NAb response remains poorly documented and it is still unknown how long cured patients will be protected against new infection (Kirkcaldy et al., 2020; Ota, 2020).

In this study, we aimed at monitoring the anti-SARS-CoV-2 antibody response in infected patients. Our results will help to better understand the SARS-CoV-2 humoral immune response and will be useful to evaluate commercial serological assays.

Materials and Methods

Study Population and Specimen
Thirty patients diagnosed SARS-CoV-2 positive by RT-PCR on a nasopharyngeal swab sample, between 25 February and 23 March 2020 at the Amiens University Medical Center, were enrolled in the study. The general information was extracted from electronic medical records and the clinical characteristics of the 30 inpatients are described in Supplementary Table 1. Inpatients were considered as having mild disease when needing non-intensive care or severe disease when needing intensive care. Samples from patients diagnosed positive for other human coronaviruses [OC43 (n = 5), 229E (n = 4), NL63 (n = 2) or HKU1 (n = 1)] were also tested as negative controls (Supplementary Table 2). Finally, we also used samples from 25 asymptomatic individuals (Table 1) that were shown SARS-CoV-2 seropositive using commercial serological tests (LIAISON® SARS-CoV-2 IgG from DiaSorin and/or ELISA SARS-CoV-2 (IgG) from EUROIMMUN). All plasmas were decomplemented at 56°C for 30 min. The study was approved by the institutional review board of the Amiens University Medical Center (number PI2020_843_0046, 21 April 2020).