A steric block to SARS-CoV-2: In response to infection by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the immune system makes antibodies, many of which target the spike protein, a key player in host cell entry. Antibodies that potently neutralize the virus hold promise as therapeutics and could inform vaccine design. Lv et al. report a humanized monoclonal antibody that protected against SARS-CoV-2 in a mouse model. The cryo–electron microscopy structure, together with biochemical, cellular, and virological studies, showed that the antibody acts by binding to the receptor-binding domain of the spike and blocking its attachment to the host receptor. Science , this issue p. 1505

The re-emerging mpox (formerly monkeypox) virus (MPXV), a member of Orthopoxvirus genus together with variola virus (VARV) and vaccinia virus (VACV), has led to public health emergency of international concern since July, 2022. Inspired by the unprecedent success of coronavirus disease 2019 (COVID-19) mRNA vaccines, the development of a safe and effective mRNA vaccine against MPXV is of high priority. Based on our established lipid nanoparticle (LNP)-encapsulated mRNA vaccine platform, we rationally constructed and prepared a panel of multicomponent MPXV vaccine candidates encoding different combinations of viral antigens including M1R, E8L, A29L, A35R and B6R. In vitro and in vivo characterization demonstrated that two immunizations of all mRNA vaccine candidates elicit a robust antibody response as well as antigen specific Th1-biased cellular response in mice. Importantly, the penta- and tetra-component vaccine candidates AR-MPXV5 and AR-MPXV4a showed superior capability of inducing neutralizing antibodies as well as of protecting from VACV challenge in mice. Our study provides critical insights to understand the protection mechanism of MPXV infection and direct evidence supporting further clinical development of these multicomponent mRNA vaccine candidates.

Background: Viral self-replicating sub-genomic replicons represent a powerful tool for studying viral genome replication, antiviral screening and chimeric vaccine development. Many kinds of flavivirus replicons have been developed with broad applications. Findings: The replicon system of JEV live vaccine strain SA14-14-2 was successfully developed in this study. Two kinds of replicons that express enhanced green fluorescent protein (EGFP) and Renilla luciferase (R.luc) were constructed under the control of SP6 promoter, respectively. Robust EGFP and R.luc signals could be detected in the replicon-transfected BHK-21 cells. Furthermore, the potential effects of selected amino acids in the C-terminal of envelope protein on replication were characterized using the replicon system. Conclusions: Our results provide a useful platform not only for the study of JEV replication, but also for antiviral screening and chimeric vaccine development.

The ongoing SARS-CoV-2 pandemic has brought an urgent need for animal models to study the pathogenicity of the virus. Herein, we generated and characterized a novel mouse-adapted SARS-CoV-2 strain, named MASCp36, that causes severe acute respiratory symptoms and mortality in standard laboratory mice. Particularly, this model exhibits age and gender related skewed distribution of mortality akin to severe COVID-19, and the 50% lethal dose (LD50) of MASCp36 was 58 PFU in 9-month-old, male BALB/c mice. Deep sequencing identified three amino acid substitutions, N501Y, Q493H, and K417N, subsequently emerged at the receptor binding domain (RBD) of MASCp36, during in vivo passaging. All three mutations in RBD significantly enhanced the binding affinity to its endogenous receptor, mouse ACE2 (mACE2). Cryo-electron microscopy (cryo-EM) analysis of human ACE2 (hACE2) or mACE2 in complex with the RBD of MASCp36 at 3.1 to 3.7 angstrom resolution elucidates molecular basis for the receptor-binding switch driven by specific amino acid substitutions. Interestingly, N501Y and Q493H enhanced the binding affinity to human ACE2 (hACE2); while triple mutations N501Y/Q493H/K417N decreased affinity to hACE2, thus led to the reduced infectivity of MASCp36 to human cells. Our study not only provides a robust platform for studying the pathogenesis of severe COVID-19 and rapid evaluation of coutermeasures against SARS-CoV-2, but also unveils the molecular mechanism for the rapid adaption and evolution of SARS-CoV-2 in human and animals. One sentence summary: A mouse adapted SARS-CoV-2 strain that harbored specific amino acid substitutions in the RBD of S protein showed 100% mortality in aged, male BALB/c mice.

Mutations and transient conformational movements of receptor binding domain (RBD) that make neutralizing epitopes momentarily unavailable, present immune escape routes to SARS-CoV-2. To mitigate viral escape, we developed a cocktail of neutralizing antibodies (NAbs) targeting epitopes located on different domains of spike (S) protein. Screening of a library of monoclonal antibodies generated from peripheral blood mononuclear cells of COVID-19 convalescent patients yielded potent NAbs, targeting N-terminal domain (NTD) and RBD domain of S, effective at nM concentrations. Remarkably, combination of RBD-targeting NAbs and NTD-binding NAb, FC05, dramatically enhanced the neutralization potency in cell-based assays and animal model. Results of competitive SPR assays and cryo-EM structures of Fabs bound to S unveil determinants of immunogenicity. Combinations of immunogens, identified in NTD and RBD of S, when immunized in rabbits elicited potent protective immune responses against SARS-CoV-2. These results provide a proof-of-concept for neutralization-based immunogen design targeting SARS-CoV-2 NTD and RBD. One sentence summary: Immunogens identified in the NTD and RBD of the SARS-CoV-2 spike protein using a cocktail of non-competing NAbs when injected in rabbits elicited a potent protective immune response against SARS-CoV-2.

Usutu virus (USUV) is an emerging arthropod-borne flavivirus that has expanded into multiple European countries during the past several decades. USUV infection in human has been linked to severe neurological complications, and no vaccine is now available against USUV. In this work, we develop a live-attenuated chimeric USUV vaccine (termed ChinUSUV) based on the full-length infectious cDNA clone of the licensed Japanese encephalitis virus (JEV) vaccine strain SA14-14-2. In vitro studies demonstrate that ChinUSUV replicates efficiently and maintains its genetic stability. Remarkably, ChinUSUV exhibits a significant attenuation phenotype in multiple mouse models even compared with the licensed JEV vaccine. A single immunization with ChinUSUV elicits potent IgG and neutralizing antibody responses as well as T cell response. Passive transfer of sera from ChinUSUV-immunized mice confers significant protection against lethal homologous challenge in suckling mice. Taken together, our results suggest that ChinUSUV represents a potential USUV vaccine candidate that merits further development.