Clues from coronavirus’ past

From the University of Washington and the McGuire lab, Vaccine and Infectious Disease Division

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the novel coronavirus responsible for the current ongoing pandemic, is similar to the 2002 severe acute respiratory syndrome coronavirus (SARS-CoV). Both viruses originated in bats before circulating in an intermediate animal host where they gained the ability to infect humans and cause widespread deadly disease. The 2002 SARS-CoV enters cells through host cell surface receptor angiotensin-converting enzyme 2 (ACE-2), and recent research suggests that SARS-CoV-2 cell entry is mediated through the same pathway.  Lexi Walls and Young-Jun Park from the Veesler lab at the University of Washington, along with colleagues from the McGuire lab in the Vaccine and Infectious Disease Division, recently interrogated the degree of similarities between the two  virus’ ability to enter cells, leveraging what is already known about the earlier SARS-CoV to better inform ongoing vaccine development for SARS-CoV-2, the current pandemic coronavirus.

Coronaviruses are eponymously named for their crown-like surface arrangement of spike glycoproteins (S), which are known to facilitate SARS-CoV cellular entry through binding of host receptor ACE-2. Given its exposed localization in the virus surface and critical role in viral entry, S protein is the primary binding site of neutralizing antibodies and a target for therapeutic and vaccine design. The work by Walls et al., published in Cell, explores the role of the S protein in SARS-CoV-2. The SARS-CoV-2 S glycoprotein shares 80% amino acid sequence identity with SARS-CoV S, leading the authors to hypothesize that SARS-CoV-2 likely also uses ACE2 to enter and establish infection in cells.

The authors used a murine leukemia virus (MLV) pseudotyping system—a technique that exploits a non-replicating virus backbone to express SARS-CoV-2 surface proteins—to test the S-mediated cellular entry of SARS-CoV-2 in the laboratory. In a cell line known to express ACE-2 and support SARS-CoV replication, both the current and former pseudotyped coronavirus strains entered cells with equal efficiency. To confirm that entry was mediated by ACE-2, the authors transiently transfected ACE-2 into a cell line that lacks the receptor, which allowed conditional entry of SARS-CoV-2 S-MLV when ACE-2 was present. These results validated recent findings suggesting that ACE-2 is the functional receptor for SARS-CoV-2 entry into human cells.

The strength of the binding interaction between coronavirus ligand and host cell receptor correlates with virus transmissibility and disease severity. To understand how the S glycoprotein-ACE-2 relationship affects SARS-CoV-2 transmission, the authors tested the binding kinetics of both former and current coronavirus S glycoproteins with ACE-2, finding that SARS-CoV-2 S and observed that the novel virus binds ACE-2 with comparable rate to the former virus, suggesting that like the SARS-CoV, SARS-CoV-2 S glycoprotein is highly adapted to mediate entry through ACE-2 and perpetuate rapid viral transmission in people. 

Graphical abstract of ACE-2-mediated entry of SARS-CoV-2 with and without SARS-CoV S immunization.
Graphical abstract of ACE-2-mediated entry of SARS-CoV-2 with and without SARS-CoV S immunization. Image from publication.

This research also revealed through cryo-electron microscopy (cryoEM) that the structures of SARS-CoV and SARS-CoV-2 S glycoproteins are closely related, prompting the authors to hypothesize that antibodies against the original virus might also neutralize cellular entry of SARS-CoV-2. Indeed, when they immunized mice with SARS-CoV S glycoprotein and added the resulting sera to cell lines transduced with SARS-CoV-2 S-MLV, all mice generated polyclonal neutralizing antibodies and prevented virus entry into cells in vitro.

These findings demonstrate a strategy to identify conserved antibody binding sites on SARS-CoV-2 S glycoprotein, which can be subsequently targeted in vaccine design. Furthermore, the polyclonal antibody response resulting from S glycoprotein immunization could potentially neutralize other viral variants that arise from animal reservoirs. Further studies are needed to confirm that immunization with SARS-CoV S glycoprotein will neutralize replication-competent SARS-CoV-2 virus in vivo.

This work was supported by the National Institute of General Medical Sciences, the National Institute of Allergy and Infectious Diseases, the Pew Biomedical Scholars Award, an Investigators in the Pathogenesis of Infectious Disease Award from the Burroughs Wellcome Fund, the University of Washington Arnold and Mabel Beckman cryoEM center, the Washington Research Foundation, and the Pasteur Institute.

UW/Fred Hutch Cancer Consortium member Andrew McGuire contributed to this work.

Walls AC, Park YJ, Tortorici MA, Wall A, McGuire AT, Veesler D. 2020. Structure, Function, and Antigenicity of the SARS-CoV-2 Spike Glycoprotein. Cell. 2020 Apr 16;181(2):281-292.e6. doi: 10.1016/j.cell.2020.02.058. Epub 2020 Mar 9.