Lanying Du

The outbreak of Coronavirus Disease 2019 (COVID-19) has posed a serious threat to global public health, calling for the development of safe and effective prophylactics and therapeutics against infection of its causative agent, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), also known as 2019 novel coronavirus (2019-nCoV). The CoV spike (S) protein plays the most important roles in viral attachment, fusion and entry, and serves as a target for development of antibodies, entry inhibitors and vaccines. Here, we identified the receptor-binding domain (RBD) in SARS-CoV-2 S protein and found that the RBD protein bound strongly to human and bat angiotensin-converting enzyme 2 (ACE2) receptors. SARS-CoV-2 RBD exhibited significantly higher binding affinity to ACE2 receptor than SARS-CoV RBD and could block the binding and, hence, attachment of SARS-CoV-2 RBD and SARS-CoV RBD to ACE2-expressing cells, thus inhibiting their infection to host cells. SARS-CoV RBD-specific antibodies could cross-react with SARS-CoV-2 RBD protein, and SARS-CoV RBD-induced antisera could cross-neutralize SARS-CoV-2, suggesting the potential to develop SARS-CoV RBD-based vaccines for prevention of SARS-CoV-2 and SARS-CoV infection.

The recent outbreak of coronavirus disease (COVID-19) caused by SARS-CoV-2 infection in Wuhan, China has posed a serious threat to global public health. To develop specific anti-coronavirus therapeutics and prophylactics, the molecular mechanism that underlies viral infection must first be defined. Therefore, we herein established a SARS-CoV-2 spike (S) protein-mediated cell-cell fusion assay and found that SARS-CoV-2 showed a superior plasma membrane fusion capacity compared to that of SARS-CoV. We solved the X-ray crystal structure of six-helical bundle (6-HB) core of the HR1 and HR2 domains in the SARS-CoV-2 S protein S2 subunit, revealing that several mutated amino acid residues in the HR1 domain may be associated with enhanced interactions with the HR2 domain. We previously developed a pan-coronavirus fusion inhibitor, EK1, which targeted the HR1 domain and could inhibit infection by divergent human coronaviruses tested, including SARS-CoV and MERS-CoV. Here we generated a series of lipopeptides derived from EK1 and found that EK1C4 was the most potent fusion inhibitor against SARS-CoV-2 S protein-mediated membrane fusion and pseudovirus infection with IC50s of 1.3 and 15.8 nM, about 241- and 149-fold more potent than the original EK1 peptide, respectively. EK1C4 was also highly effective against membrane fusion and infection of other human coronavirus pseudoviruses tested, including SARS-CoV and MERS-CoV, as well as SARSr-CoVs, and potently inhibited the replication of 5 live human coronaviruses examined, including SARS-CoV-2. Intranasal application of EK1C4 before or after challenge with HCoV-OC43 protected mice from infection, suggesting that EK1C4 could be used for prevention and treatment of infection by the currently circulating SARS-CoV-2 and other emerging SARSr-CoVs.

Coronavirus (CoV) disease 2019 (COVID-19) caused by severe acute respiratory syndrome (SARS)-CoV-2 (also known as 2019-nCoV) is threatening global public health, social stability, and economic development. To meet this challenge, this article discusses advances in the research and development of neutralizing antibodies (nAbs) for the prevention and treatment of infection by SARS-CoV-2 and other human CoVs. Coronavirus (CoV) disease 2019 (COVID-19) caused by severe acute respiratory syndrome (SARS)-CoV-2 (also known as 2019-nCoV) is threatening global public health, social stability, and economic development. To meet this challenge, this article discusses advances in the research and development of neutralizing antibodies (nAbs) for the prevention and treatment of infection by SARS-CoV-2 and other human CoVs. Three emerging, highly pathogenic human CoVs are SARS-CoV, Middle East respiratory syndrome (MERS)-CoV, and COVID-19 virus, which was previously named 2019-nCoV by the World Health Organization (WHO), and is also known as hCoV-19 or SARS-CoV-2 [1.Coronaviridae Study Group of the International Committee on Taxonomy of Viruses The species Severe acute respiratory syndrome-related coronavirus: classifying 2019-nCoV and naming it SARS-CoV-2.Nat. Microbiol. 2020; (Published online March 2, 2020. https://doi.org/10.1038/s41564-020-0695-z)Crossref Scopus (5066) Google Scholar]. Atypical pneumonia (SARS) was first reported from Guangdong Province, China in late 2002. SARS caused a global pandemic in 2003 with approximately 10% (774/8098) case fatality rate (CFR) [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar]. SARS-CoV has not circulated in humans since 2004. MERS-CoV was first reported from Saudi Arabia in 2012 and has continued to infect humans with limited human-to-human transmission, leading to a CFR of approximately 34.4% (858/2494) in 27 countries, according to the most recent WHO reporti. Both SARS-CoV and MERS-CoV are zoonotic viruses. They use bats as their natural reservoirs and transmit from bats to intermediate hosts (e.g., palm civets for SARS-CoV, dromedary camels for MERS-CoV), leading to infection in humans [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar,3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]. Different from SARS-CoV and MERS-CoV, SARS-CoV-2 was first reported in Wuhan, China in December 2019 and is characterized by its rapid spread and virulent human-to-human transmission [4.Zhou P. et al.A pneumonia outbreak associated with a new coronavirus of probable bat origin.Nature. 2020; 579: 270-273Crossref PubMed Scopus (14613) Google Scholar], resulting in 125 048 confirmed cases including 4613 deaths (CFR 3.7%), particularly in Wuhan, China and in at least 117 other countries, territories, or areas as of March 12, 2020. With no vaccines or treatments on the horizon, researchers are exploring various medical interventions, including nAbs, to control the continuous spread of SARS-CoV-2 and the global COVID-19 pandemic [5.Jiang S. et al.An emerging coronavirus causing pneumonia outbreak in Wuhan, China: calling for developing therapeutic and prophylactic strategies.Emerg. Microbes Infect. 2020; 9: 275-277Crossref PubMed Scopus (237) Google Scholar]. SARS-CoV-2 is also a zoonotic virus with bats as its natural reservoir [4.Zhou P. et al.A pneumonia outbreak associated with a new coronavirus of probable bat origin.Nature. 2020; 579: 270-273Crossref PubMed Scopus (14613) Google Scholar], but its intermediate hosts have not been identified. SARS-CoV-2 infection mainly results in pneumonia and upper/lower respiratory tract infection. Fever and cough are two major clinical symptoms, but others include shortness of breath, muscle pain (myalgias)/fatigue, confusion, headache, sore throat, and even acute respiratory distress syndrome, leading to respiratory or multiorgan failure [6.Huang C. et al.Clinical features of patients infected with 2019 novel coronavirus in Wuhan, China.Lancet. 2020; 395: 497-506Abstract Full Text Full Text PDF PubMed Scopus (33292) Google Scholar]. For elderly people with underlying comorbidities such as diabetes, hypertension, or cardiovascular disease, SARS-CoV-2 infection may result in severe and fatal respiratory diseases. So far, its effects on children have been generally mild. The virus can be transmitted through respiratory droplets or close contact with infected surfaces or objects and is detectable in multiple samples, including saliva, stool, and blood [7.Young B.E. et al.Epidemiologic features and clinical course of patients infected with SARS-CoV-2 in Singapore.JAMA. 2020; (Published online March 3, 2020. https://doi.org/10.1001/jama.2020.3204)Crossref PubMed Scopus (1471) Google Scholar]. To develop vaccines and therapeutics, we must understand the behavior of key proteins in SARS-CoV-2. Similar to SARS-CoV and MERS-CoV, SARS-CoV-2 is an enveloped, single-stranded, and positive (+)-sense RNA virus, belonging to the beta-CoV genera in the family Coronaviridae [4.Zhou P. et al.A pneumonia outbreak associated with a new coronavirus of probable bat origin.Nature. 2020; 579: 270-273Crossref PubMed Scopus (14613) Google Scholar]. The genome of this and other emerging pathogenic human CoVs encodes four major structural proteins [spike (S), envelope (E), membrane (M), and nucleocapsid (N)], approximately 16 nonstructural proteins (nsp1–16), and five to eight accessory proteins. Among them, the S protein plays an essential role in viral attachment, fusion, entry, and transmission. It comprises an N-terminal S1 subunit responsible for virus–receptor binding and a C-terminal S2 subunit responsible for virus–cell membrane fusion [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar,3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]. S1 is further divided into an N-terminal domain (NTD) and a receptor-binding domain (RBD). SARS-CoV-2 and SARS-CoV bind angiotensin-converting enzyme 2 (ACE2) while MERS-CoV binds dipeptidyl peptidase 4 (DPP4), as receptors on the host cell expressing ACE2 (e.g., pneumocytes, enterocytes) or DPP4 (e.g., liver or lung cells including Huh-7, MRC-5, and Calu-3) [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar,3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar,8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. Phylogenetically, SARS-CoV-2 is closely related to SARS-CoV, sharing approximately 79.6% genomic sequence identity [4.Zhou P. et al.A pneumonia outbreak associated with a new coronavirus of probable bat origin.Nature. 2020; 579: 270-273Crossref PubMed Scopus (14613) Google Scholar]. During infection, CoV first binds the host cell through interaction between its S1-RBD and the cell membrane receptor, triggering conformational changes in the S2 subunit that result in virus fusion and entry into the target cell (see human CoV life cycle in Figure 1A ) [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar,3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]. Virus nAbs induced by vaccines or infected virus play crucial roles in controlling viral infection. Currently developed SARS-CoV- and MERS-CoV-specific nAbs include monoclonal antibodies (mAbs), their functional antigen-binding fragment (Fab), the single-chain variable region fragment (scFv), or single-domain antibodies [nanobodies (Nbs)] [8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. They target S1-RBD, S1-NTD, or the S2 region, blocking the binding of RBDs to their respective receptors and interfering with S2-mediated membrane fusion or entry into the host cell, thus inhibiting viral infections [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar,5.Jiang S. et al.An emerging coronavirus causing pneumonia outbreak in Wuhan, China: calling for developing therapeutic and prophylactic strategies.Emerg. Microbes Infect. 2020; 9: 275-277Crossref PubMed Scopus (237) Google Scholar]. The putative targets and mechanisms of these SARS-CoV and MERS-CoV nAbs are shown in Figure 1B. Representative SARS-CoV and MERS-CoV RBD-specific nAbs are summarized in Table 1. No SARS-CoV-2-specific nAbs have been reported, but we herein introduce SARS-CoV- and MERS-CoV-specific nAbs in the context of their potential cross-neutralizing activity against SARS-CoV-2 infection.Table 1Representative SARS-CoV RBD- and MERS-CoV RBD-Targeting nAbsaAbbreviations: Ab, antibody; Ad5/hDPP4-transduced mice, adenovirus serotype 5-hDPP4-transduced mice; hDPP4-Tg mice, human DPP4-transgenic mice; NA, not applicable; rGD03 or rSZ16, recombinant SARS-CoVs bearing the S protein of GD03 or SZ16; S, spike.Ab nameSourceNeutralizing activityNeutralizing mechanismProtective efficacyRefsbNote: Due to space limitations, some review articles, rather than original research papers reporting the antibodies, are cited.S230.15m396mAbsHumanNeutralize human (strains GD03, Urbani, Tor2) and palm civet (strains SZ3, SZ16) SARS-CoV infectionRecognize epitopes (residues 408, 442, 443, 460, 475) on SARS-CoV S1 protein, interfering with RBD–ACE2 receptor interactionProtect mice against challenge of SARS-CoV (strains Urbani, rGD03, or rSZ16)[9.Zhu Z. et al.Potent cross-reactive neutralization of SARS coronavirus isolates by human monoclonal antibodies.Proc. Natl. Acad. Sci. U. S. A. 2007; 104: 12123-12128Crossref PubMed Scopus (245) Google Scholar]S109.8S227.14S230.15mAbsHumanNeutralize human (Urbani, GZ02, CUHK-W1), palm civet (HC/SZ/61/03), and raccoon dog (A031G) SARS-CoV infectious clones containing S variantsInhibit the binding of SARS-CoV RBD–ACE2 receptorProtect mice against challenge of SARS-CoV infectious clones (Urbani, GZ02, HC/SZ/61/03) or mouse-adapted strain (MA15)[10.Rockx B. et al.Structural basis for potent cross-neutralizing human monoclonal antibody protection against lethal human and zoonotic severe acute respiratory syndrome coronavirus challenge.J. Virol. 2008; 82: 3220-3235Crossref PubMed Scopus (109) Google Scholar]80RscFv, mAbHumanNeutralize live SARS-CoV (strain Urbani) infectionRecognize epitopes on SARS-CoV S1 (residues 261–672), blocking RBD–ACE2 binding and inhibiting syncytium formationNA[11.Sui J. et al.Potent neutralization of severe acute respiratory syndrome (SARS) coronavirus by a human mAb to S1 protein that blocks receptor association.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 2536-2541Crossref PubMed Scopus (505) Google Scholar]CR3022CR3014scFv, mAbHumanNeutralize live SARS-CoV (strain HKU-39849) infection; CR3022 could neutralize CR3014 escape variantsRecognize epitopes on SARS-CoV RBD (residues 318–510); CR3022 binds SARS-CoV-2 RBD with high affinityCR3014 protects ferrets against SARS-CoV (strain HKU-39849) infection[13.Tian X. et al.Potent binding of 2019 novel coronavirus spike protein by a SARS coronavirus-specific human monoclonal antibody.Emerg. Microbes Infect. 2020; 9: 382-385Crossref PubMed Scopus (940) Google Scholar]33G435B530F9mAbsMouseNeutralize human (strains GD03, Tor2) and palm civet (SZ3) pseudotyped SARS-CoV infectionRecognize epitopes on SARS-CoV RBD, blocking RBD–ACE2 receptor bindingNA[12.He Y. et al.Cross-neutralization of human and palm civet severe acute respiratory syndrome coronaviruses by antibodies targeting the receptor-binding domain of spike protein.J. Immunol. 2006; 176: 6085-6092Crossref PubMed Scopus (96) Google Scholar]MERS-27m336MERS-GD27MCA1mAbs, FabsHumanNeutralize divergent strains of pseudotyped and live (strain EMC2012) MERS-CoV infectionRecognize a number of key epitopes on MERS-CoV RBD protein, blocking RBD–DPP4 receptor bindingProphylactically and therapeutically prevent and treat MERS-CoV (strain EMC2012) challenge in hDPP4-Tg mice, rabbits, or common marmosets[3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar,8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]4C2 hhMS-1mAbsHumanizedNeutralize divergent strains of pseudotyped and live (strain EMC2012) MERS-CoV infectionRecognize epitopes (residues 510, 511, 553) on MERS-CoV RBD protein, blocking RBD–DPP4 receptor bindingPrevent MERS-CoV (strain EMC2012) challenge in Ad5/hDPP4-transduced or hDPP4-Tg mice[3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]Mersmab14C2D12mAbsMouseNeutralize pseudotyped and live (strain EMC2012) MERS-CoV infectionRecognize a number of key epitopes on MERS-CoV RBD protein, blocking RBD–DPP4 receptor bindingNA[3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]HCAb-83NbDromedary camelNeutralizes live MERS-CoV (strain EMC2012) infectionRecognizes epitope (residue 539) on MERS-CoV RBD proteinProphylactically prevents MERS-CoV (strain EMC2012) challenge in hDPP4-Tg mice[8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]NbMS10-FcNbLlamaNeutralizes multiple strains of pseudotyped and live (strain EMC2012) MERS-CoV infectionRecognizes epitope (residue 539) on MERS-CoV RBD proteinProphylactically and therapeutically prevents and treats MERS-CoV (strain EMC2012) challenge in hDPP4-Tg mice[8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]a Abbreviations: Ab, antibody; Ad5/hDPP4-transduced mice, adenovirus serotype 5-hDPP4-transduced mice; hDPP4-Tg mice, human DPP4-transgenic mice; NA, not applicable; rGD03 or rSZ16, recombinant SARS-CoVs bearing the S protein of GD03 or SZ16; S, spike.b Note: Due to space limitations, some review articles, rather than original research papers reporting the antibodies, are cited. Open table in a new tab All currently developed anti-SARS-CoV nAbs target the viral S protein. Most target the RBD, while a few target regions in the S2 subunit or the S1/S2 proteolytic cleavage site. For example, the human neutralizing mAbs S230.15 and m396 were isolated from SARS-CoV-infected individuals. They neutralize human and palm civet SARS-CoV infection by interacting with the RBD, thus blocking binding between the viral RBD and the cellular ACE2 receptor [9.Zhu Z. et al.Potent cross-reactive neutralization of SARS coronavirus isolates by human monoclonal antibodies.Proc. Natl. Acad. Sci. U. S. A. 2007; 104: 12123-12128Crossref PubMed Scopus (245) Google Scholar]. Other human mAbs, such as S109.8 and S227.14, have cross-neutralizing activity against multiple human, palm civet, and raccoon dog SARS-CoV infectious clones, protecting mice against four different homologous and heterologous SARS-CoV strains [10.Rockx B. et al.Structural basis for potent cross-neutralizing human monoclonal antibody protection against lethal human and zoonotic severe acute respiratory syndrome coronavirus challenge.J. Virol. 2008; 82: 3220-3235Crossref PubMed Scopus (109) Google Scholar]. Human nAb 80R (scFv or mAb) neutralizes SARS-CoV infection by blocking the RBD–ACE2 interaction, although its protective efficacy has not yet been reported [11.Sui J. et al.Potent neutralization of severe acute respiratory syndrome (SARS) coronavirus by a human mAb to S1 protein that blocks receptor association.Proc. Natl. Acad. Sci. U. S. A. 2004; 101: 2536-2541Crossref PubMed Scopus (505) Google Scholar]. A variety of SARS-CoV RBD-specific mouse neutralizing mAbs are sufficiently potent to block RBD–ACE2 binding, thus neutralizing viral infection in ACE2-transfected HEK293T cells [12.He Y. et al.Cross-neutralization of human and palm civet severe acute respiratory syndrome coronaviruses by antibodies targeting the receptor-binding domain of spike protein.J. Immunol. 2006; 176: 6085-6092Crossref PubMed Scopus (96) Google Scholar]. Despite their strong neutralizing activity and/or protection in cells or animal models, none of these SARS-CoV nAbs has ever been evaluated in clinical studies. Thus, to determine potential cross-neutralizing activity against SARS-CoV-2 infection, such studies should be vigorously undertaken. A number of MERS-CoV-specific nAbs have been reported, most of which target the RBD in the S protein [3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar,8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. A few recognize epitopes on the S1-NTD and regions of the S2 subunit [3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar]. Among these nAbs, human mAbs or Fabs (MERS-27, m336, MERS-GD27, or MCA1 isolated from humans), humanized mAbs (hMS-1, 4C2 h), mouse mAbs (Mersmab1, 4C2, or D12 isolated from mice), and Nbs (HCAb-83 or NbMS10-Fc isolated from dromedary camels or llamas) recognize epitopes on the RBD and have been demonstrated to neutralize pseudotyped and/or live MERS-CoVs [3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar,8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. Several human/humanized mAbs and Nbs can protect mice, rabbits, or common marmosets from MERS-CoV infection [3.Du L. et al.MERS-CoV spike protein: a key target for antivirals.Expert Opin. Ther. Targets. 2017; 21: 131-143Crossref PubMed Scopus (218) Google Scholar,8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. So far, only one MERS-CoV nAb isolated from transchromosomic cattle has been evaluated in Phase I trials (SAB-301)ii [8.Zhou Y. et al.Advances in MERS-CoV vaccines and therapeutics based on the receptor-binding domain.Viruses. 2019; 11E60Crossref PubMed Scopus (86) Google Scholar]. No other nAbs have gone to clinical trials, again suggesting the urgency of developing nAbs with potential cross-neutralizing activity against SARS-CoV-2 infection. Currently, polyclonal antibodies from recovered SARS-CoV-2-infected patients have been used to treat SARS-CoV-2 infection, but no SARS-CoV-2-specific neutralizing mAbs have been reported. Researchers are working hard to develop such mAbs and/or their functional fragments as putative prophylactic or therapeutic agents to prevent or treat COVID-19. Once such antibodies are produced, the next steps will involve in vitro testing for neutralizing and/or cross-neutralizing activity, in vivo evaluation in available COVID-19 animal models for protective efficacy, preclinical studies, and clinical trials testing the safety and efficacy before they are approved for clinical application. Therefore, it may take one to several years for such SARS-CoV-2 neutralizing mAbs or their fragments to be ready for human use. However, since SARS-CoV-2 is closely related to SARS-CoV and since their S proteins have high sequence identity [4.Zhou P. et al.A pneumonia outbreak associated with a new coronavirus of probable bat origin.Nature. 2020; 579: 270-273Crossref PubMed Scopus (14613) Google Scholar], researchers have attempted to discover SARS-CoV nAbs with potential cross-reactivity and/or cross-neutralizing activity against SARS-CoV-2 infection. Notably, a SARS-CoV RBD-specific human neutralizing mAb, CR3022, could bind SARS-CoV-2 RBD with high affinity and recognize an epitope on the RBD that does not overlap with the ACE2-binding site [13.Tian X. et al.Potent binding of 2019 novel coronavirus spike protein by a SARS coronavirus-specific human monoclonal antibody.Emerg. Microbes Infect. 2020; 9: 382-385Crossref PubMed Scopus (940) Google Scholar]. In addition, sera from convalescent SARS patients or from animals specific for SARS-CoV S1 may cross-neutralize SARS-CoV-2 infection by reducing S protein-mediated SARS-CoV-2 entry [14.Hoffmann M. et al.SARS-CoV-2 cell entry depends on ACE2 and TMPRSS2 and is blocked by a clinically proven protease inhibitor.Cell. 2020; (Published online March 4, 2020. https://doi.org/10.1016/j.cell.2020.02.052)Abstract Full Text Full Text PDF PubMed Scopus (13672) Google Scholar]. Moreover, SARS-CoV RBD-specific polyclonal antibodies have cross-reacted with the SARS-CoV-2 RBD protein and cross-neutralized SARS-CoV-2 infection in HEK293T cells stably expressing the human ACE2 receptor, opening avenues for the potential development of SARS-CoV RBD-based vaccines that might eventually prevent SARS-CoV-2 and SARS-CoV infection [15.Tai W. et al.Characterization of the receptor-binding domain (RBD) of 2019 novel coronavirus: implication for development of RBD protein as a viral attachment inhibitor and vaccine.Cell. Mol. Immunol. 2020; (Published online March 19, 2020. https://doi.org/10.1038/s41423-020-0400-4)Crossref Scopus (1176) Google Scholar]. It is also possible that SARS-CoV RBD-targeting nAbs might be applied for prophylaxis and treatment of SARS-CoV-2 infection in the current absence of SARS-CoV-2-specific vaccines and antibodies. However, robust testing lies ahead. SARS-CoV-2 continues to infect people globally with the concomitant urgency to develop effective nAbs as prophylactic and therapeutic agents to prevent and treat its infection and control its spread. Studies from SARS-CoV and MERS-CoV have demonstrated that many fragments (S1-NTD, RBD, S2) in S proteins can be used as targets to develop nAbs. Still, RBD-specific antibodies have greater potency to neutralize infection with divergent virus strains, suggesting that the RBD of SARS-CoV-2 can also serve as an important target for the development of potent and specific nAbs. Cocktails comprising antibodies specific for RBD and other regions in the S protein may further improve the breadth and potency of nAbs against SARS-CoV-2 and its escape-mutant strains. Human sera from convalescent patients have been used to treat COVID-19, but lessons learned from SARS show that some non-nAbs targeting the non-RBD regions in the S protein may an on viral and disease, as as other [2.Du L. et al.The spike protein of SARS-CoV – a target for vaccine and therapeutic development.Nat. Rev. Microbiol. 2009; 7: 226-236Crossref PubMed Scopus (1241) Google Scholar]. a positive some anti-SARS-CoV nAbs have shown cross-reactivity or cross-neutralizing activity against SARS-CoV-2 infection in Thus, research on SARS-CoV- and MERS-CoV-specific nAbs should important for the rapid and development of SARS-CoV-2-specific nAbs. was by of Health and against SARS-CoV-2 and Other Human in et in Figure 1A of the original online the an in the of S, and proteins in the and the The have Figure 1A in the this does not the of the the to for this and it may have caused PDF

Antibody-dependent enhancement (ADE) of viral entry has been observed for many viruses. It was shown that antibodies target one serotype of viruses but only subneutralize another, leading to ADE of the latter viruses. Here we identify a novel mechanism for ADE: a neutralizing antibody binds to the surface spike protein of coronaviruses like a viral receptor, triggers a conformational change of the spike, and mediates viral entry into IgG Fc receptor-expressing cells through canonical viral-receptor-dependent pathways. We further evaluated how antibody dosages impacted viral entry into cells expressing viral receptor, Fc receptor, or both receptors. This study reveals complex roles of antibodies in viral entry and can guide future vaccine design and antibody-based drug therapy.