key: cord-0042999-w1ep0wq8 authors: Ku, Zhiqiang; Ye, Xiaohua; Toa Salazar, Georgina; Zhang, Ningyan; An, Zhiqiang title: Antibody therapies for the treatment of COVID-19 date: 2020-04-30 journal: Antib Ther DOI: 10.1093/abt/tbaa007 sha: 0d3b945d04dc343fec0b31ceda717f31155a8795 doc_id: 42999 cord_uid: w1ep0wq8 An outbreak of COVID-19, the disease caused by infection of the coronavirus SARS-CoV-2, that began in December 2019 in Wuhan, China has caused more than 2,990,559 confirmed human infections and 207,446 deaths as of April 27, 2020 (Coronavirus COVID-19 Global Cases by the Center for Systems Science and Engineering (CSSE) at Johns Hopkins University). Scientists are working quickly on multiple aspects of the pandemic. Genetic analyses are conducted to reveal the source and evolution of SARS-CoV-2, providing knowledge that can be used to contain it and to avoid future outbreaks. Epidemiological studies which incorporates lessons learned from outbreaks of previous related viral diseases can guide development of public health measures effective to contain the current and future outbreaks. Basic virology studies reveal viral structure and function. Pathology studies inform development of strategies to interfere with infection. COVID-19 prevention and treatment strategies are being developed in preclinical and clinical studies. Antibody-based therapy is one viable treatment option. Here, we discuss some of the most active areas of developing strategies to treat COVID-19, focusing on approaches to generate neutralizing antibodies against SARS-CoV-2 for prophylactic and therapeutic treatment of COVID-19. SIGNIFICANCE: Development of SARS-CoV-2 neutralizing antibodies with the desired efficacy and safety profile is a critical part of the toolbox of therapies for the treatment of COVID-19. We discuss in this review the current state of discovery and development of such antibodies. and nucleocapsid (N) proteins 4 . The spike protein comprises an N-terminal S1 subunit responsible for receptor binding and a C-terminal S2 subunit responsible for membrane fusion ( Figure 1C ) 5 . The S1 subunit is further divided into the N-terminal domain (NTD), the receptor-binding domain (RBD), subdomain 1 (SD1) and subdomain 2 (SD2). The S2 subunit is further divided into the fusion peptide (FP), the heptad repeat 1 (HR1) and heptad repeat 2 (HR2) ( Figure 1C ) 6 . Like SARS-CoV, SARS-CoV-2 enters cells through binding of the host cellular receptor angiotensin-converting enzyme 2 (ACE2) via its spike protein 2 . In contrast, MERS-CoV enters cells through binding of the host receptor dipeptidyl peptidase 4 (DPP4) via its spike protein 7 . These host receptors are indispensable for virus infection 2, 4 . Receptor binding triggers a conformational change of the spike protein to an activated state 8 . The activated spike is cleaved by a protease (TMPRSS2 for SARS-CoV and SARS-CoV-2) at the S1/S2 site, releasing the S1 subunit and exposing the FP on the S2 subunit 9 . The FP inserts into target cell membrane, HR1 and HR2 refold to form a postfusion conformation that drives viral membrane fusion with target cells ( Figure 1D ) 8, 10 . The cryo-EM structure of the SARS-CoV-2 spike in the prefusion conformation was recently published 11 . The interaction between SARS-CoV-2 spike RBD and the full-length human ACE2 has also been revealed by cryo-EM or crystallization 6, [12] [13] [14] . Similar to the spike protein of SARS-CoV, the SARS-CoV-2 spike protein also possesses extensive glycosylation, which may be important for virus binding to cells and facilitate immune evasion through epitope masking 15, 16 . The non-structural proteins encoded by coronaviruses are essential for virus replication inside cells 1 Figure 1D ). However, the coronavirus protein nsp14 has a unique exoribonuclease (ExoN) function, which provides the proofreading capability. This mechanism may affect the susceptibility of coronaviruses to nucleoside analogues 19 . Approved viral protease inhibitors, such as the HIV aspartic protease inhibitors lopinavir and ritonavir, have shown efficacy in a clinical trial 20 . It is hypothesized that lopinavir and ritonavir exhibit their efficacy by inhibiting the 3-chymotrypsin-like protease 20 ( Figure 1D ). In addition to repurposing existing antiviral drugs, drugs used to modulate the host The spike protein of SARS-CoV-2 plays an essential role in virus entry into host cells and is a primary target of neutralizing antibodies 5, 9 (Figures 1C,D) . Due to the functionality and high immunogenicity of the coronavirus S1 subunit, most neutralizing antibodies characterized for coronaviruses to date target S1, in particular the S1-RBD [44] [45] [46] . Two MERS-CoV neutralizing mAbs, G2 and 7D10, target the S1-NTD region and function by blocking spike protein interaction with the host receptor DPP4 47, 48 . Compared to the S1 subunit, the coronavirus S2 subunit is more conserved and bears epitopes that could potentially be targeted by broadly neutralizing antibodies 10, 49 . Generation of antibodies with broad neutralizing activity against different coronaviruses, or at least SARS-related coronaviruses, would be of great value for confronting future waves of coronavirus-related disease. However, broadly neutralizing antibodies against different human coronaviruses are very rare, probably related to the sequence variance of spike protein and cryptic nature of the highly conserved epitopes 28, 50 . The S2 conformation is highly dynamic during membrane fusion, presenting a major challenge in preparing antigens for discovery of antibodies against this protein 10 . Antigen stabilizing strategies used in discovery of antibodies against HIV and RSV proteins may be explored in the design of stable coronavirus S2 proteins 51-53 . To target the whole spike protein or domain proteins, several well-established methods have been used for isolating SARS-CoV-2 monoclonal antibodies (Table 1) . Xiong et al. A structural study indicates that CR3022 recognizes a highly conserved cryptic epitope on RBD that is distinct from the receptor-binding site 50 . Antibodies that bind to the spike protein with high affinity can be first tested for their ability to block spike interaction with ACE2 or directly tested in cell based viral neutralization assays in vitro. Two in vitro assay systems are commonly used to evaluate the neutralization activity of antibodies against coronavirus 56 Monkey and mouse models have been reported for testing SARS-CoV-2-neutralizing antibodies [58] [59] [60] . In the monkey study, researchers found that rhesus macaques infected with SARS-CoV-2 through the intratracheal route had mild illness, and their lungs showed signs of pneumonia similar to those in humans with COVID-19 58 . After 3 or 6 days of infection, the virus could be isolated from bronchus, lung tissues, and oropharyngeal swabs. However, no monkey developed severe symptoms during the study 58 . In a study using a mouse model, transgenic mice with human ACE2 expression, but not wild-type mice, were found to be susceptible to SARS-CoV-2 infection 59 . Mice inoculated with SARS-CoV-2 through the intranasal route had a 5~10% loss of body weight and histopathology in the lung, such as interstitial pneumonia, and infiltration of immune cells. No mice died during the study 59 . To establish a model that can mimic more severe human infections, different animal models and other experimental factors must be considered and tested. Antibody-dependent enhancement (ADE) is the phenomenon of non-neutralizing or subneutralizing antibodies facilitating virus infection, leading to more severe disease. The most widely known example is dengue ADE, which has various mechanisms 61, 62 . In one mechanism, certain immune cells, which don't express the receptor for virus entry but should take the risk of ADE into consideration. The unprecedented impact of the current COVID-19 pandemic on the human race calls for unprecedented response from all aspects of society to combat the disease. To address the immediate need for therapies, repurposing existing drugs and strategies is a logical first step. This includes testing antivirals, drugs used to modulate the host immune system, and CP transfusion as therapies for COVID-19. 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