Therefore, XG005 induces a rearrangement of RBDs to the up state, which is essential for receptor attachment and further conformational changes required for membrane fusion and viral entry. The online version consists of supplementary material available at 10.1007/s13238-021-00871-6. Keywords:SARS-CoV-2, neutralizing antibody, receptor-binding website, XG014, antibody-dependent cell-cell fusion == Intro == The pandemic of coronavirus disease of 2019 (COVID-19) is definitely caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) with more than 190 million confirmed infections and a death toll exceeding 4 million (https://www.who.int/). Since the outbreak of COVID-19, great attempts have been made globally to develop effective countermeasures against SARS-CoV-2 illness, such as vaccines and restorative antibodies (Krammer,2020; Lurie et al.,2020; Baden et al.,2021; Weinreich et al.,2021; Chen et al.,2021a). To day, several vaccines and monoclonal antibodies have been tested in medical trials or have been granted authorization for emergency use in many countries. Several types of vaccines, including mRNA-, adenoviral vector-based and inactivated computer virus vaccines, induce strong immune reactions (Corbett et al.,2020; Gao et al.,2020; Krammer,2020; Mercado et al.,2020; Poland et al.,2020), while several human being neutralizing monoclonal antibodies isolated from convalescent individuals, Ingenol Mebutate (PEP005) such as REGN10987, REGN10933, LY-CoV555, C144 and C135, efficiently neutralize SARS-CoV-2 for prophylaxis and treatment (Alsoussi et al.,2020; Baum et al.,2020; Robbiani et al.,2020; Rogers et al.,2020; Shi et al.,2020; Zost et al.,2020; Wang et al.,2020a; Andreano et al.,2021; Group et al.,2021; Weinreich et al.,2021; Chen et al.,2021a). Over the past few months, the emergence and blood circulation of several SARS-CoV-2 lineage B variants, such as B.1.1.7 (Alpha, WHO label), B.1.351 (Beta, WHO label), P.1 (Gamma, WHO label) and B.1.617.2 (Delta, Who also label) identified in the United Kingdom, South Africa, Brazil and India, respectively, have posed a new challenge (Grubaugh et al.,2021; Harvey et al.,2021; Lopez Bernal et al.,2021; Planas et al.,2021; Supasa et al.,2021; Xie et al.,2021; Wang et al.,2021b). Current studies have shown the mutations in these SARS-CoV-2 variants could EFNA3 increase the affinity of the receptor-binding website (RBD) to its cellular receptor angiotensin-converting enzyme-2 (ACE2) and confer resistance to vaccine sera and many monoclonal antibodies (Hou et al.,2020; Korber et al.,2020; Starr et al.,2020; Yurkovetskiy et al.,2020; Greaney et al.,2021; Gupta,2021; Liu et al.,2021; Plante et al.,2021; Wang et al.,2021c,d). Convalescent plasma or sera from individuals vaccinated by mRNA vaccines or inactivated-virus vaccines display a significant reduction in neutralizing activity against these growing circulating variants (Weisblum et al.,2020; Wibmer et al.,2021; Chen et al.,2021b; Wang et al.,2021a,2021d). Such immune escapes have also been reported for a number of monoclonal antibodies that are in the medical center such as LY-CoV555, CB6 and REGN10933 (Wang et al.,2021b,d). These variants indicate continuous antigenic drift of SARS-CoV-2 and spotlight its adaptability to the human being sponsor (Wang et al.,2021b). Consequently, the recognition of highly conserved neutralizing epitopes could help generate more broadly protecting vaccines and restorative drugs, but little is known about the identity and biology of such cross-neutralizing epitopes. Here, we evaluated the neutralizing activity of four monoclonal antibodies focusing on four non-overlapping RBD epitopes that we isolated from a convalescent individual previously infected with SARS-CoV-2 (Zhou et al.,2021b). We found that one of these antibodies, XG014, is definitely ultrapotent and broadly neutralizing, without enhancing S protein-mediated membrane fusion. Cryo-electron microscopy (Cryo-EM) showed that XG014 recognizes a conserved epitope outside of the receptor-binding Ingenol Mebutate (PEP005) motif (RBM) in RBD, and that it locks all three RBDs of the S trimer in the down or closed conformation, sterically hindering receptor binding. Epitope assessment with another antibody, XG005, which belongs Ingenol Mebutate (PEP005) to the same epitope group with XG014 but enhances S protein-mediated membrane fusion, provides a structural explanation for the potent and broad cross-neutralizing activity of XG014. Finally, XG014 was effective for SARS-CoV-2 prophylaxis and treatment in human being ACE2-transgenic (hACE2-Tg) mice, suggesting the potential for XG014 to be developed like a broadly effective restorative agent. == Results == == Four groups of neutralizing epitopes on SARS-CoV-2 RBD == We have previously recognized monoclonal antibodies realizing four groups of non-overlapping RBD epitopes (Zhou et al.,2021b). Four representative neutralizing antibodies (XG011, XG014, XG017, XG025) from each epitope group (Fig.1A) were chosen.
