Severe acute respiratory syndrome (SARS) is a newly emerging infectious disease caused by a novel coronavirus, SARS-coronavirus (SARS-CoV). The SARS-CoV spike (S) protein is composed of two subunits; the S1 subunit contains a receptor-binding domain that engages with the host cell receptor angiotensin-converting enzyme 2 and the S2 subunit mediates fusion between the viral and host cell membranes. The S protein plays key parts in the induction of neutralizing-antibody and T-cell responses, as well as protective immunity, during infection with SARS-CoV. In this Review, we highlight recent advances in the development of vaccines and therapeutics based on the S protein. SARS-CoV is an enveloped, single and positive-stranded RNA virus2. Its genome RNA encodes a non-structural replicase polyprotein and structural proteins, including spike (S), envelope (E), membrane (M) and nucleocapsid (N) proteins3-5. SARS-CoV, a zoonotic virus, resides in hosts that form its natural reservoir, such as bats, but can also infect intermediate hosts, such as small animals (for example, palm civets), before being transmitted to humans6-8. SARS-CoV can infect and replicate in several cell types in the human body and causes serious pathological changes Zoonotic virusA virus that normally exists in vertebrate animals, but can also be transmitted to humans and can cause disease in both animals and humans. Box 1 Pathology of SARS and the life cycle of SARS-CoV infectionSevere acute respiratory syndrome-coronavirus (SARS-CoV) spreads primarily through droplets (respiratory secretions) and close person-to-person contact. After the virus enters into the body, it binds to primary target cells that express abundant virus receptor, the angiotensin-converting enzyme 2 (ACE2), including pneumocytes and enterocytes in the respiratory system. The virus enters and replicates in these cells. The matured virions are then released to infect new target cells121 (FIG. 1). SARS-CoV can also infect mucosal cells of intestines, tubular epithelial cells of kidneys, epithelial cells of renal tubules, cerebral neurons and immune cells122,123. Infectious viral particles in patients with SARS can be excreted through respiratory secretions, stool, urine and sweat. SARS-CoV infection damages lung tissues owing to elevated levels of production and activation of proinflammatory chemokines and cytokines124, resulting in atypical pneumonia with rapid respiratory deterioration and failure.Neutralizing antibodies and/or T-cell immune responses can be raised directly against several SARS-CoV proteins21-23, but mainly target the S protein20,24-26, suggesting that S protein- Structure of the SARS-CoV S proteinThe spikes of SARS-CoV are composed of trimers of S protein, which belongs to a group of class I viral fusion glycoproteins that also includes HIV glyco-protein 160 (Env), influenza haemagglutinin (HA), paramyxovirus F and Ebola virus glycoprotein28. The SARS-CoV S protein encodes a surface glycoprotein precursor that is predicted to be 1,255 amino acids in l...

The outbreak of Coronavirus Disease 2019 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 crossreact 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 ongoing COVID-19 pandemic has prioritized the development of small animal models for SARS-CoV-2. Herein, we adapted a clinical isolate of SARS-CoV-2 by serial passaging in the respiratory tract of aged BALB/c mice. The resulting mouse-adapted strain at passage 6 (termed MASCp6) showed increased infectivity in mouse lung, and led to interstitial pneumonia and inflammatory responses in both young and aged mice following intranasal inoculation. Deep sequencing revealed a panel of adaptive mutations potentially associated with the increased virulence. In particular, the N501Y mutation is located at the receptor binding domain (RBD) of the spike protein. The protective efficacy of a recombinant RBD vaccine candidate was validated using this model. Thus, this mouse-adapted strain and associated challenge model should be of value in evaluating vaccines and antivirals against SARS-CoV-2.

Currently, COVID-19 caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been widely spread around the world; nevertheless, so far there exist no specific antiviral drugs for treatment of the disease, which poses great challenge to control and contain the virus. Here, we reported a research finding that SARS-CoV-2 invaded host cells via a novel route of CD147-spike protein (SP). SP bound to CD147, a receptor on the host cells, thereby mediating the viral invasion.Our further research confirmed this finding. First, in vitro antiviral tests indicated Meplazumab, an anti-CD147 humanized antibody, significantly inhibited the viruses from invading host cells, with an EC 50 of 24.86 μg/mL and IC 50 of 15.16 μg/mL. Second, we validated the interaction between CD147 and SP, with an affinity constant of 1.85×10 -7 M. Co-Immunoprecipitation and ELISA also confirmed the binding of the two proteins. Finally, the localization of CD147 and SP was observed in SARS-CoV-2 infected Vero E6 cells by immuno-electron microscope. Therefore, the discovery of the new route CD147-SP for SARS-CoV-2 invading host cells provides a critical target for development of specific antiviral drugs.

Antibody-dependent enhancement (ADE) of viral entry has been a major concern for epidemiology, vaccine development, and antibody-based drug therapy. However, the molecular mechanism behind ADE is still elusive. Coronavirus spike protein mediates viral entry into cells by first binding to a receptor on the host cell surface and then fusing viral and host membranes. In this study, we investigated how a neutralizing monoclonal antibody (MAb), which targets the receptorbinding domain (RBD) of Middle East respiratory syndrome (MERS) coronavirus spike, mediates viral entry using pseudovirus entry and biochemical assays. Our results showed that MAb binds to the virus surface spike, allowing it to undergo conformational changes and become prone to proteolytic activation. Meanwhile, MAb binds to cell surface IgG Fc receptor, guiding viral entry through canonical viral-receptordependent pathways. Our data suggest that the antibody/Fc-receptor complex functionally mimics viral receptor in mediating viral entry. Moreover, we characterized MAb dosages in viral-receptor-dependent, Fc-receptor-dependent, and both-receptorsdependent viral entry pathways, delineating guidelines on MAb usages in treating viral infections. Our study reveals a novel molecular mechanism for antibody-enhanced viral entry and can guide future vaccination and antiviral strategies. IMPORTANCE 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.KEYWORDS antibody-dependent enhancement of viral entry, MERS coronavirus, SARS coronavirus, spike protein, neutralizing antibody, viral receptor, IgG Fc receptor, antibody-dependent enhancement of viral entry A ntibody-dependent enhancement (ADE) occurs when antibodies facilitate viral entry into host cells and enhance viral infection in these cells (1, 2). ADE has been observed for a variety of viruses, most notably flaviviruses (e.g., dengue virus) (3-6). It has been shown that when patients are infected by one serotype of dengue virus (i.e., Downloaded fromprimary infection), they produce neutralizing antibodies targeting the same serotype of the virus. However, if they are later infected by another serotype of dengue virus (i.e., secondary infection), the preexisting antibodies cannot fully neutralize the virus. Instead, the antibodies first bind to the virus and then bind to the IgG Fc rec...

The spike (S) protein of severe acute respiratory syndrome (SARS) coronavirus (CoV), a type I transmembrane envelope glycoprotein, consists of S1 and S2 domains responsible for virus binding and fusion, respectively. The S1 contains a receptor-binding domain (RBD) that can specifically bind to angiotensin-converting enzyme 2 (ACE2), the receptor on target cells. Here we show that a recombinant fusion protein (designated RBD-Fc) containing 193-amino acid RBD (residues 318-510) and a human IgG1 Fc fragment can induce highly potent antibody responses in the immunized rabbits. The antibodies recognized RBD on S1 domain and completely inhibited SARS-CoV infection at a serum dilution of 1:10,240. Rabbit antisera effectively blocked binding of S1, which contains RBD, to ACE2. This suggests that RBD can induce highly potent neutralizing antibody responses and has potential to be developed as an effective and safe subunit vaccine for prevention of SARS.

Human intestinal tract as an alternative route to acquire MERS-CoV infection.

The spike (S) protein of severe acute respiratory syndrome associated coronavirus (SARS-CoV) is a major antigenic determinant capable of inducing protective immunity. Recently, a small fragment on the SARS-CoV S protein (residues 318–510) was characterized as a minimal receptor-binding domain (RBD), which mediates virus binding to angiotensin-converting enzyme 2, the functional receptor on susceptible cells. In this study, we demonstrated that a fusion protein containing RBD linked to human IgG1 Fc fragment (designated RBD-Fc) induced high titer of RBD-specific Abs in the immunized mice. The mouse antisera effectively neutralized infection by both SARS-CoV and SARS pseudovirus with mean 50% neutralization titers of 1/15,360 and 1/24,737, respectively. The neutralization determinants on the RBD of S protein were characterized by a panel of 27 mAbs isolated from the immunized mice. Six groups of conformation-dependent epitopes, designated as Conf I–VI, and two adjacent linear epitopes were identified by ELISA and binding competition assays. The Conf IV and Conf V mAbs significantly blocked RBD-Fc binding to angiotensin-converting enzyme 2, suggesting that their epitopes overlap with the receptor-binding sites in the S protein. Most of the mAbs (23 of 25) that recognized the conformational epitopes possessed potent neutralizing activities against SARS pseudovirus with 50% neutralizing dose ranging from 0.005 to 6.569 μg/ml. Therefore, the RBD of SARS S protein contains multiple conformational epitopes capable of inducing potent neutralizing Ab responses, and is an important target site for developing vaccines and immunotherapeutics.