Targeting the SARS-CoV-2 spike: The surface of severe acute respiratory syndrome–coronavirus 2 (SARS-CoV-2) is decorated with trimeric spikes that bind to host cell receptors. These spikes also elicit an antibody response, so understanding antibody recognition may aid in vaccine design. Yuan et al. determined the structure of CR3022, a neutralizing antibody obtained from a convalescent SARS-CoV–infected patient, in complex with the receptor-binding domain of the SARS-CoV-2 spike. The antibody binds to an epitope conserved between SARS-CoV-2 and SARS-CoV that is distinct from the receptor-binding site. CR3022 likely binds more tightly to SARS-CoV because its epitope contains a glycan not present in SARS-CoV-2. Structural modeling showed that the epitope is only revealed when at least two of the three spike proteins are in a conformation competent to bind the receptor. Science , this issue p. 630

Immune profiling of COVID-19 patients: Coronavirus disease 2019 (COVID-19) has affected millions of people globally, yet how the human immune system responds to and influences COVID-19 severity remains unclear. Mathewet al.present a comprehensive atlas of immune modulation associated with COVID-19. They performed high-dimensional flow cytometry of hospitalized COVID-19 patients and found three prominent and distinct immunotypes that are related to disease severity and clinical parameters. Arunachalamet al.report a systems biology approach to assess the immune system of COVID-19 patients with mild-to-severe disease. These studies provide a compendium of immune cell information and roadmaps for potential therapeutic interventions.Science, this issue p.eabc8511, p.1210

The World Health Organization has declared the ongoing outbreak of COVID-19, which is caused by a novel coronavirus SARS-CoV-2, a pandemic. There is currently a lack of knowledge about the antibody response elicited from SARS-CoV-2 infection. One major immunological question concerns antigenic differences between SARS-CoV-2 and SARS-CoV. We address this question by analyzing plasma from patients infected by SARS-CoV-2 or SARS-CoV and from infected or immunized mice. Our results show that, although cross-reactivity in antibody binding to the spike protein is common, cross-neutralization of the live viruses may be rare, indicating the presence of a non-neutralizing antibody response to conserved epitopes in the spike. Whether such low or non-neutralizing antibody response leads to antibody-dependent disease enhancement needs to be addressed in the future. Overall, this study not only addresses a fundamental question regarding antigenicity differences between SARS-CoV-2 and SARS-CoV but also has implications for immunogen design and vaccine development.

Multiple reassortant strains of novel, highly pathogenic avian influenza A(H5) viruses in clade 2.3.4.4 have recently emerged and spread in Asia, Europe, and North America, giving rise to multiple subtypes (H5N2, H5N5, H5N6, and H5N8).^1-3 We report on a 59-year-old man in Guangzhou, China, with influenza A(H5N6) infection and associated illness. He had a 30-year history of smoking and had stopped smoking in January 2014. He had undergone surgery and chemotherapy for colon cancer, the last chemotherapy dose having been administered in April 2014. During the weeks before illness onset, he regularly purchased and handled live poultry in live poultry markets.

A novel avian-origin influenza A/H7N9 virus emerged in 2013 to cause more than 130 cases of zoonotic human disease, with an overall case fatality rate of around 30% in cases detected. It has been shown that an E-to-K amino acid change at residue 627 of polymerase basic protein 2 (PB2) occurred frequently in the H7N9 isolates obtained from humans but not in viruses isolated from poultry. Although this mutation has been reported to confer increased mammalian pathogenicity in other avian influenza subtypes, it has not been experimentally investigated in the H7N9 virus. In this study, we determined the contribution of PB2-E627K in H7N9 virus to its pathogenicity in mammalian hosts. In addition, the compensatory role of the PB2 mutations T271A, Q591K, and D701N in H7N9 virus was investigated. We characterized the activity of polymerase complexes with these PB2 mutations and found that they enhance the polymerase activity in human 293T cells. The rescued mutants enhanced growth in mammalian cells in vitro . Mice infected with the H7N9 mutant containing the avian signature protein PB2-627E showed a marked decrease in disease severity (weight loss) and pathology compared to mice infected with the wild-type strain (PB2-627K) or other PB2 mutants. Also, mutants with PB2-627E showed lower virus replication and proinflammatory cytokine responses in the lungs of the virus-infected mice, which may contribute to pathogenicity. Our results suggest that these amino acid substitutions contribute to mouse pathogenicity and mammalian adaptation. IMPORTANCE A novel avian H7N9 influenza A virus emerged in east China in 2013 to cause zoonotic human disease associated with significant mortality. It is important to understand the viral genetic markers of mammalian adaptation and disease severity in this H7N9 virus. Since many human (but not avian) H7N9 virus isolates have an amino acid substitution at position E627K in the polymerase basic protein 2 (PB2) gene, we investigated the role of this and other functionally related mutations for polymerase activity in vitro , virus replication competence, and pathogenicity in the mouse model. We found that E627K and functionally related mutations are associated with increased polymerase activity, increased viral replication competence, and increased disease severity in mice.