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.

In face of the everlasting battle toward COVID-19 and the rapid evolution of SARS-CoV-2, no specific and effective drugs for treating this disease have been reported until today. Angiotensin-converting enzyme 2 (ACE2), a receptor of SARS-CoV-2, mediates the virus infection by binding to spike protein. Although ACE2 is expressed in the lung, kidney, and intestine, its expressing levels are rather low, especially in the lung. Considering the great infectivity of COVID-19, we speculate that SARS-CoV-2 may depend on other routes to facilitate its infection. Here, we first discover an interaction between host cell receptor CD147 and SARS-CoV-2 spike protein. The loss of CD147 or blocking CD147 in Vero E6 and BEAS-2B cell lines by anti-CD147 antibody, Meplazumab, inhibits SARS-CoV-2 amplification. Expression of human CD147 allows virus entry into non-susceptible BHK-21 cells, which can be neutralized by CD147 extracellular fragment. Viral loads are detectable in the lungs of human CD147 (hCD147) mice infected with SARS-CoV-2, but not in those of virus-infected wild type mice. Interestingly, virions are observed in lymphocytes of lung tissue from a COVID-19 patient. Human T cells with a property of ACE2 natural deficiency can be infected with SARS-CoV-2 pseudovirus in a dose-dependent manner, which is specifically inhibited by Meplazumab. Furthermore, CD147 mediates virus entering host cells by endocytosis. Together, our study reveals a novel virus entry route, CD147-spike protein, which provides an important target for developing specific and effective drug against COVID-19.

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 COVID-19 pandemic caused by the SARS-CoV-2 virus has resulted in an unprecedented public health crisis. There are no approved vaccines or therapeutics for treating COVID-19. Here we reported a humanized monoclonal antibody, H014, efficiently neutralizes SARS-CoV-2 and SARS-CoV pseudoviruses as well as authentic SARS-CoV-2 at nM level by engaging the S receptor binding domain (RBD). Importantly, H014 administration reduced SARS-CoV-2 titers in the infected lungs and prevented pulmonary pathology in hACE2 mouse model. Cryo-EM characterization of the SARS-CoV-2 S trimer in complex with the H014 Fab fragment unveiled a novel conformational epitope, which is only accessible when the RBD is in open conformation. Biochemical, cellular, virological and structural studies demonstrated that H014 prevents attachment of SARS-CoV-2 to its host cell receptors. Epitope analysis of available neutralizing antibodies against SARS-CoV and SARS-CoV-2 uncover broad cross-protective epitopes. Our results highlight a key role for antibody-based therapeutic interventions in the treatment of COVID-19.

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

The pathogenesis of highly pathogenic Middle East respiratory syndrome coronavirus (MERS-CoV) remains poorly understood. In a previous study, we established an hDPP4-transgenic (hDPP4-Tg) mouse model in which MERS-CoV infection causes severe acute respiratory failure and high mortality accompanied by an elevated secretion of cytokines and chemokines. Since excessive complement activation is an important factor that contributes to acute lung injury after viral infection, in this study, we investigated the role of complement in MERS-CoV-induced lung damage. Our study showed that complement was excessively activated in MERS-CoV-infected hDPP4-Tg mice through observations of increased concentrations of the C5a and C5b-9 complement activation products in sera and lung tissues, respectively. Interestingly, blocking C5a production by targeting its receptor, C5aR, alleviated lung and spleen tissue damage and reduced inflammatory responses. More importantly, anti-C5aR antibody treatment led to decreased viral replication in lung tissues. Furthermore, compared with the sham treatment control, apoptosis of splenic cells was less pronounced in the splenic white pulp of treated mice, and greater number of proliferating splenic cells, particularly in the red pulp, was observed. These data indicate that (1) dysregulated host immune responses contribute to the severe outcome of MERS; (2) excessive complement activation, triggered by MERS-CoV infection, promote such dysregulation; and (3) blockade of the C5a–C5aR axis lead to the decreased tissue damage induced by MERS-CoV infection, as manifested by reduced apoptosis and T cell regeneration in the spleen. Therefore, the results of this study suggest a new strategy for clinical intervention and adjunctive treatment in MERS-CoV cases.

A novel human Middle East respiratory syndrome coronavirus (MERS-CoV) caused outbreaks of severe acute respiratory syndrome (SARS)-like illness with a high mortality rate, raising concerns of its pandemic potential. Dipeptidyl peptidase-4 (DPP4) was recently identified as its receptor. Here we showed that residues 377 to 662 in the S protein of MERS-CoV specifically bound to DPP4-expressing cells and soluble DPP4 protein and induced significant neutralizing antibody responses, suggesting that this region contains the receptor-binding domain (RBD), which has a potential to be developed as a MERS-CoV vaccine. In 2003, Farzan and colleagues successfully identified the receptor of SARS-CoV, angiotensin-converting enzyme 2 (ACE2) (7), and a 193-amino-acid fragment in the spike (S) protein (residues 318 to 510) as the receptor-binding domain (RBD) (8). We found that SARS-CoV S-RBD contains a critical neutralizing site (9) which induces potent neutralizing antibodies and protection against SARS-CoV infection in an animal model (10).Since MERS-CoV is genetically related to SARS-CoV (1), we compared their S protein sequences and predicted that the RBD of MERS-CoV might be located in the region spanning residues 377 to 662 of the S1 subunit (Fig. 1). Using the Swiss-Model Workplace homology modeling server (11) and basing our work on the X-ray crystal structure of the SARS-CoV S-RBD (Protein Data Bank [PDB] identification no. 2DD8) (12), we predicted the conformational structure of the region consisting of residues 377 to 662 in the S1 subunit of the MERS-CoV S protein (13). We noticed that the SARS-CoV S-RBD and the predicted MERS-CoV S-RBD possessed similar core structures but had an extended secondary structure consisting predominantly of the receptor-binding motifs (RBM) (12,14). The extended region in MERS-CoV S-RBD is much longer than that in SARS-CoV S-RBD, suggesting that MERS-CoV and SARS-CoV use different receptors. Indeed, it has been proven that dipeptidyl peptidase-4 (DPP4; also known as CD26) is the functional receptor of MERS-CoV (15).We then constructed MERS-CoV S-RBD based on the synthesized codon-optimized MERS-CoV S sequences (GenBank accession no. AFS88936.1) and fused it to Fc of human IgG using pFUSE-hIgG1-Fc2 expression vector (here named Fc) (InvivoGen, San Diego, CA). The SARS-CoV S-RBD-Fc was constructed by fusing RBD of codon-optimized SARS-CoV S sequence into the Fc vector referred to above as a control (Fig. 1) (16). The S-RBD-Fc proteins were expressed in 293T cell culture supernatant and purified by protein A affinity chromatography (GE Healthcare, Piscataway, NJ) (17). We found that both MERS-CoV and SARS-CoV S-RBD-Fc proteins were highly purified from transfected culture supernatants ( Fig. 2A, panel a). MERS-CoV S-RBD-Fc could be recognized by an MERS-CoV S-specific polyclonal antibody (1:1,000), while SARS-CoV S-RBD-Fc could not react with this antibody, as detected by Western blotting (Fig. 2A, panel b).Using analysis performed by Western blotting, we found that DPP4 was highly express...

Prophylactic and therapeutic strategies are urgently needed to combat infections caused by the newly emerged Middle East respiratory syndrome coronavirus (MERS-CoV