mAbs were then diluted 2-fold over 8 points in 96 well round bottom plates, and an equal volume containing 5×105 293E cells expressing SARS-CoV-2 spike proteins was added to each well. antibodies that were generated in this patient during the first weeks of COVID-19 infection were non-neutralizing and target epitopes outside the RBD. Antibodies that disrupt the SARS-CoV-2 S-ACE2 interaction can potently neutralize the virus without undergoing extensive maturation. Such antibodies have potential preventive and/or therapeutic potential and can serve as templates for vaccine design. Keywords: COVID-19, SARS, SARS-CoV-2, ACE2, antibodies, B cells, spike protein, receptor-binding domain, neutralization, MERS Graphical Abstract Open in a separate window Highlights ? Early B cell responses to SARS-CoV-2 spike protein are analyzed from a COVID-19 patient ? Most antibodies target non-neutralizing epitopes outside the RBD ? A potent neutralizing mAb blocks the interaction of the S protein with ACE2 ? Neutralizing antibodies are minimally mutated Seydoux et?al. analyze B cell responses in a COVID-19 patient and find that SARS-CoV-2 infection expands diverse B cell clones against the viral spike glycoprotein (S). Two neutralizing antibodies were identified that bind S with high affinity despite being minimally mutated. Thus, vaccine-induced neutralizing antibody responses may require activation of specific naive B cells without requiring extensive somatic mutation. Introduction The World Health Organization (WHO) declared the 2020 COVID-19 to be a global pandemic on March 11, 2020 (World Health Organization, 2020). According to data compiled from multiple local and government sources compiled by a team at Johns Hopkins University, MK-0557 as of June 12, 2020, there are MK-0557 currently 7.5 million documented cases of COVID-19 and over 420,000 deaths (Dong et?al., 2020). The infection is caused by SARS-CoV-2, a beta coronavirus, closely related to SARS-CoV (Wan et?al., 2020). Presently, the immune response to COVID-19 is not well understood and preventative measures, such as Rabbit polyclonal to FABP3 vaccines, are not available. It is also unclear which immune responses are required to prevent or control SARS-CoV-2 infection. High-resolution structures of the SARS-CoV-2 prefusion-stabilized spike (S) ectodomain revealed that it adopts multiple conformations with either one receptor-binding domain (RBD) in the up or open conformation or all RBDs in the down or closed conformation, similar to previous reports on both SARS-CoV S and MERS-CoV S (Gui et?al., 2017, Kirchdoerfer et?al., 2018, Pallesen et?al., 2017, Song et?al., 2018, Walls et?al., 2019, Walls et?al., 2020, Wrapp et?al., 2020, Yuan et?al., 2017). Like?SARS-CoV, SARS-CoV-2 utilizes angiotensin-converting enzyme 2 (ACE2) as an entry receptor binding with nM affinity (Li et?al., 2003, Walls et?al., 2020, Wrapp et?al., 2020; Hoffmann et?al., 2020, Letko et?al., 2020, Ou et?al., 2020). Indeed, the S proteins of the two viruses share a high degree of amino acid sequence homology, 76% overall and 74% in RBD (Wan et?al., 2020). Although binding and neutralizing antibody responses are known to develop following MK-0557 SARS-CoV-2 infection (Ni et?al., 2020, Okba et?al., 2020), no information is currently available on the epitope specificities, clonality, binding affinities, and neutralizing potentials of the antibody response. Monoclonal antibodies (mAbs) isolated from SARS-CoV-infected subjects can recognize the SARS-CoV-2?S protein (Yuan et?al., 2020), and immunization with SARS S protein can elicit anti-SARS-CoV-2 neutralizing antibodies in wild-type and humanized mice, as well as llamas (Walls et?al., 2020, Wang et?al., 2020, Wrapp et?al., 2020). However, SARS-CoV-2 infection appears to not elicit strong anti-SARS-CoV neutralizing antibody responses and vice versa (Ou et?al., 2020). Here, we employed diverse but complementary approaches to investigate the serum binding and neutralizing antibody responses to a stabilized ectodomain variant of the SARS-CoV-2 S-protein (S2P) as well as the frequency and clonality of S2P-specific B cells in a SARS-CoV-2-infected individual 21?days following the onset of clinical disease. We isolated anti-SARS-CoV-2?S mAbs and characterized their binding properties and determined their neutralizing potencies. Among all B cells analyzed, no particular variable heavy (VH) or variable light (VL) gene family was expanded, and the isolated antibodies were minimally mutated. Our analysis reveals that only a small fraction of S2P-specific B cells recognized the RBD. Of the forty-five mAbs analyzed, only three displayed neutralizing activity. The most potent mAb, CV30, bound the RBD in a manner that disrupted the S-ACE2 interaction. The other two mAbs, CV1 and CV35, were clonal variants that bound to an epitope distinct from the RBD and were much less potent. Results A SARS-CoV-2 Infected Donor Displays Potent Neutralizing Activity within 3 Weeks of Clinical Disease Onset Serum and peripheral blood.