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Antibodies are a principal determinant of immunity for most RNA viruses and have 54 promise to reduce infection or disease during major epidemics. The novel 55 coronavirus SARS-CoV-2 has caused a global pandemic with millions of infections 56 and hundreds of thousands of deaths to date 1,2 . In response, we used a rapid 57 antibody discovery platform to isolate hundreds of human monoclonal antibodies 58 (mAbs) against the SARS-CoV-2 spike (S) protein. We stratify these mAbs into five 59 major classes based on their reactivity to subdomains of S protein as well as their 60 cross-reactivity to SARS-CoV. Many of these mAbs inhibit infection of authentic 61 SARS-CoV-2 virus, with most neutralizing mAbs recognizing the receptor-binding 62 domain (RBD) of S. This work defines sites of vulnerability on SARS-CoV-2 S and 63 demonstrates the speed and robustness of new antibody discovery methodologies. 64 65 Human mAbs to the viral surface spike (S) glycoprotein mediate immunity to other 66 betacoronaviruses including SARS-CoV 3-7 and Middle East respiratory syndrome 67 (MERS) 8-17 . Because of this, we and others have hypothesized that human mAbs may 68 have promise for use in prophylaxis, post-exposure prophylaxis, or treatment of SARS-69 CoV-2 infection 18 . MAbs can neutralize betacoronaviruses by several mechanisms 70 including blocking of attachment of the S protein RBD to a receptor on host cells (which 71 for SARS-CoV and SARS-CoV-2 1 is angiotensin-converting enzyme 2 [ACE2]) 12 . We 72 hypothesized that the SARS-CoV-2 S protein would induce diverse human neutralizing 73 antibodies following natural infection. While antibody discovery usually takes months 74 to years, there is an urgent need to both characterize the human immune response to 75 SARS-CoV-2 infection and to develop potential medical countermeasures. Using Zika 76 virus as a simulated pandemic pathogen and leveraging recent technological advances 77in synthetic genomics and single-cell sequencing, we recently isolated hundreds of 78 was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission.

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GII.4 noroviruses (NoVs) are the primary cause of epidemic viral acute gastroenteritis. One primary obstacle to successful NoV vaccination is the extensive degree of antigenic diversity among strains. The major capsid protein of GII.4 strains is evolving rapidly, resulting in the emergence of new strains with altered blockade epitopes. In addition to characterizing these evolving blockade epitopes, we have identified monoclonal antibodies (MAbs) that recognize a blockade epitope conserved across time-ordered GII.4 strains. Uniquely, the blockade potencies of MAbs that recognize the conserved GII.4 blockade epitope were temperature sensitive, suggesting that particle conformation may regulate functional access to conserved blockade non-surface-exposed epitopes. To map conformation-regulating motifs, we used bioinformatics tools to predict conserved motifs within the protruding domain of the capsid and designed mutant VLPs to test the impacts of substitutions in these motifs on antibody cross-GII.4 blockade. Charge substitutions at residues 310, 316, 484, and 493 impacted the blockade potential of cross-GII.4 blockade MAbs with minimal impact on the blockade of MAbs targeting other, separately evolving blockade epitopes. Specifically, residue 310 modulated antibody blockade temperature sensitivity in the tested strains. These data suggest access to the conserved GII.4 blockade antibody epitope is regulated by particle conformation, temperature, and amino acid residues positioned outside the antibody binding site. The regulating motif is under limited selective pressure by the host immune response and may provide a robust target for broadly reactive NoV therapeutics and protective vaccines. IMPORTANCEIn this study, we explored the factors that govern norovirus (NoV) cross-strain antibody blockade. We found that access to the conserved GII.4 blockade epitope is regulated by temperature and distal residues outside the antibody binding site. These data are most consistent with a model of NoV particle conformation plasticity that regulates antibody binding to a distally conserved blockade epitope. Further, antibody "locking" of the particle into an epitope-accessible conformation prevents ligand binding, providing a potential target for broadly effective drugs. These observations open lines of inquiry into the mechanisms of human NoV entry and uncoating, fundamental biological questions that are currently unanswerable for these noncultivatable pathogens.

The COVID-19 pandemic is a major threat to global health for which there are only 50 limited medical countermeasures, and we lack a thorough understanding of mechanisms of 51 humoral immunity 1,2 . From a panel of monoclonal antibodies (mAbs) targeting the spike 52 (S) glycoprotein isolated from the B cells of infected subjects, we identified several mAbs 53 that exhibited potent neutralizing activity with IC50 values as low as 0.9 or 15 ng/mL in 54 pseudovirus or wild-type (wt) SARS-CoV-2 neutralization tests, respectively. The most 55 potent mAbs fully block the receptor-binding domain of S (SRBD) from interacting with 56 human ACE2. Competition-binding, structural, and functional studies allowed clustering 57 of the mAbs into defined classes recognizing distinct epitopes within major antigenic sites 58 on the SRBD. Electron microscopy studies revealed that these mAbs recognize distinct 59 conformational states of trimeric S protein. Potent neutralizing mAbs recognizing unique 60 sites, COV2-2196 and COV2-2130, bound simultaneously to S and synergistically 61 neutralized authentic SARS-CoV-2 virus. In two murine models of SARS-CoV-2 infection, 62 passive transfer of either COV2-2916 or COV2-2130 alone or a combination of both mAbs 63 protected mice from severe weight loss and reduced viral burden and inflammation in the 64 lung. These results identify protective epitopes on the SRBD and provide a structure-based 65 framework for rational vaccine design and the selection of robust immunotherapeutic 66 cocktails. 67 68 The S protein of SARS-CoV-2 is the molecular determinant of viral attachment, fusion, and 69 entry into host cells 3 . The cryo-EM structure of a prefusion-stabilized trimeric S protein 70 ectodomain (S2Pecto) for SARS-CoV-2 reveals similar features to that of the SARS-CoV S 71 protein 4 . This type I integral membrane protein and class I fusion protein possesses an N-72