key: cord-0706178-mmh8o0a7
authors: Yang, Yang; Du, Lanying
title: SARS-CoV-2 spike protein: a key target for eliciting persistent neutralizing antibodies
date: 2021-02-26
journal: Signal Transduct Target Ther
DOI: 10.1038/s41392-021-00523-5
sha: 604e887a5337de94dcb19fb65e280e204c945402
doc_id: 706178
cord_uid: mmh8o0a7

nan

A recent paper published in Science describes the detection of IgG antibody responses in individuals infected by severe acute respiratory syndrome coronavirus (SARS-CoV-2). The authors also examined the duration of antibody production and the correlation between IgG antibody titers and neutralizing antibody titers. 1 This study provides information about the kinetics of antibody production, and the functionality and longevity of these antibodies, in patients with Coronavirus Disease 2019 (COVID-19).

The SARS-CoV-2 genome encodes spike (S), nucleocapsid, membrane, and envelope structural proteins. The S protein plays a key role in viral infection and pathogenesis. 2 It comprises subunits S1 and S2: S1 harbors the N-terminal domain (NTD) and the receptor-binding domain (RBD), whereas S2 harbors heptad repeat 1 (HR1) and HR2 (Fig. 1a) . SARS-CoV-2 infection undergoes a series of processes: the RBD first binds its receptor, angiotensin converting enzyme 2 (ACE2), to form an RBD/ACE2 complex. This triggers conformational changes in the S protein, leading to membrane fusion mediated via HR1 and HR2; this process culminates in viral entry into target cells (Fig. 1b) . Different from other structural proteins, the S protein is a critical target for the induction of antibodies, particularly neutralizing antibodies, specific for SARS-CoV-2. Antibodies targeting various regions of S protein have different mechanisms in inhibiting SARS-CoV-2 infection. For example, NTD-targeting antibodies (monoclonal antibodies (mAbs) or their fragments) bind the NTD to form an NTD/mAb complex, thereby preventing conformational changes in the S protein and blocking membrane fusion and viral entry (Fig. 1b) . By contrast, RBD-targeting antibodies such as mAbs and nanobodies (Nbs) form RBD/mAb or RBD/Nb complexes that inhibit binding of the RBD to ACE2, thereby preventing entry of SARS-CoV-2 into target cells (Fig. 1b) . Thus, understanding the aforementioned mechanism underlying SARS-CoV-2 infection and the mode of action of anti-SARS-CoV-2-S antibodies will help elucidate the kinetics of antibody production in SARS-CoV-2infected individuals, and facilitate the development of effective countermeasures. In general, antibodies targeting the viral RBD are more potent than the antibodies targeting other regions (such as NTD) of S protein, but they might be less broad in inhibiting multiple virus strains.

The Science paper shows that anti-SARS-CoV-2-S antibodies are elicited at detectable titers after infection. 1 The authors detected IgG antibodies in convalescent plasma from patients with mild-tomoderate COVID-19 symptoms. They performed an ELISA by coating plates with a stabilized SARS-CoV-2 S trimer protein. They then selected 120 plasma samples with IgG titers varying from non-detectable to ≥1:2880 and examined neutralizing activity against authentic SARS-CoV-2 infection. Plasma with IgG titers of 1:320, 1:960, or ≥1:2880 corresponded to neutralizing antibody titers of 1:30, 1:75, and 1:550, respectively, and all samples with IgG titers ranging from 1:960 to ≥1:2880 neutralized SARS-CoV-2 infection. Overall, these results indicate that SARS-CoV-2 S-targeting IgG antibodies in SARS-CoV-2-infected patients neutralized authentic SARS-CoV-2 infection, and that the induced IgG antibody titer correlated with the neutralizing antibody titer. The data from this study are consistent with those from previous studies on SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV), other coronaviruses with high fatality rates in humans; the latter studies show that, virus-specific antibodies, particularly those targeting S proteins and/or RBD fragments, correlate positively with neutralizing antibody titers. 2 In addition, the Science paper also examined longevity of IgG antibodies specific for the SARS-CoV-2 S protein in 121 volunteers over a period of around 5 months, separated by three mean time points (30, 82, and 148 days after symptom onset). 1 Notably, specific antibodies were maintained for at least 5 months, showing a modest decrease over time (particularly at initial titers of 1:960 to ≥1:2880). Moreover, neutralizing antibody titers still correlated with neutralizing antibody titers at Day 148. As pointed out by the authors, detection and measurement of antibodies generated by SARS-CoV-2 infection should be extended over a longer period of time. This may give indications as to whether people infected with SARS-CoV-2 can be reinfected and, if so, at which antibody level. In the case of SARS-CoV and MERS-CoV, high-titer S/RBD-specific IgG and neutralizing antibodies can be maintained for up to 12 months post-vaccination, providing complete protection against viral infection. 2 It is therefore expected that the higher the titer of specific IgG antibodies with neutralizing activity against SARS-CoV-2 infection, and the longer they persist, the more effective they will be in preventing reinfection.

IgG antibodies induced by SARS-CoV-2 in patients with moderate-to-severe symptoms mainly target the S protein; 1 however, it is unclear which regions (RBD, NTD, or other fragments) of the S protein are targeted, or whether the induced antibodies show cross-reactivity and/or cross-neutralizing activity against other coronaviruses. Some studies indicate that SARS-CoV-2 RBD-specific IgG antibodies are present in convalescent plasma from COVID-19 patients, and that IgG antibodies specific for RBD and S (ectodomain) correlate positively with anti-SARS-CoV-2 neutralizing antibody titers. 3 Other studies demonstrate the generation of persistent, but slightly reduced, IgG antibodies in patients with COVID-19; these antibodies also target SARS-CoV-2 RBD, and the titers correlate with S-specific neutralizing antibody titers. 4 Moreover, these antibodies may cross-react with the S/RBD of SARS-CoV, thereby cross-neutralizing SARS-CoV infection; however, they show low to no cross-reactivity with lowpathogenic human coronaviruses such as HKU1, 229E, OC43, or NL63. 4, 5 Considering the high sequence similarity between SARS-CoV-2, SARS-CoV, and other SARS-like coronaviruses, and their use of the same ACE2 receptor, further studies are needed to understand whether antibodies generated by SARS-CoV-2infected patients cross-react with S proteins or their fragments (S1-RBD/S2), and/or cross-neutralize infections by SARS-like coronaviruses with pandemic potential. Monoclonal antibodies (mAbs) targeting S protein NTD prevent conformational changes of the S protein that are required for S2-mediated membrane fusion, and hence inhibit viral entry into host cells. RBD-targeting neutralizing mAbs or nanobodies (Nbs), on the other hand, bind directly to SARS-CoV-2 S protein RBD and compete with the cellular receptor, angiotensin converting enzyme 2 (ACE2), resulting in neutralization of viral infection and clearance of the virus. The following PDB entries are used for structural illustrations: 7C2L (structure of SARS-CoV-2 S in complex with NTD-targeting mAb 4A8), 7K4N (structure of SARS-CoV-2 S in complex with RBD-targeting mAb S2E12), 7KKK (structure of SARS-CoV-2 S in complex with RBD-targeting nanobody Nb6), and 6VW1 (structure of SARS-CoV-2 RBD in complex with human ACE2). SARS-CoV-2 S NTD is colored in orange, RBD in green, and the rest part of S protein in light blue. ACE2 is colored in purple. This figure was prepared using BioRender (https://biorender.com/) and UCSF ChimeraX (https://www.cgl.ucsf. edu/chimerax/) Studies of highly pathogenic human coronaviruses (SARS-CoV-2, SARS-CoV, and MERS-CoV) indicate that, although full-length and fragments (NTD and RBD) of S proteins induce specific antibodies with neutralizing activity, the RBD is a major target for eliciting highly potent neutralizing antibodies with protective efficacy. 2 The majority of the currently developed anti-SARS-CoV-2 therapeutic antibodies target the S protein, particularly the RBD. Therefore, the S/RBD-specific antibodies have great potential for development as effective therapeutics that prevent COVID-19 spread.

Robust neutralizing antibodies to SARS-CoV-2 infection persist for months

Subunit vaccines against emerging pathogenic human coronaviruses

Convalescent plasma anti-SARS-CoV-2 spike protein ectodomain and receptor-binding domain IgG correlate with virus neutralization

Persistence and decay of human antibody responses to the receptor binding domain of SARS-CoV-2 spike protein in COVID-19 patients

Effect of low-pathogenic human coronavirus-specific antibodies on SARS-CoV-2

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This study was supported by the National Institutes of Health (NIH) grants (R01AI157975, R01AI137472, and R01AI139092).

Competing interests: The authors declare no competing interests.