SARS-CoV-2 S1 is superior to the RBD as a COVID-19 subunit vaccine antigen

Abstract

Since its emergence in December 2019, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has developed into a global pandemic within a matter of months. While subunit vaccines are one of the prominent options for combating coronavirus disease 2019 (COVID-19), the immunogenicity of spike protein-based antigens remains unknown. When immunized in mice, the S1 domain induced much higher IgG and IgA antibody levels than the receptor-binding domain (RBD) and more efficiently neutralized SARS-CoV-2 when adjuvanted with alum. It is inferred that a large proportion of these neutralization epitopes are located in the S1 domain but outside the RBD and that some of these are spatial epitopes. This finding indicates that expression systems with posttranslational modification abilities are important to maintain the natural configurations of recombinant spike protein antigens and are critical for effective COVID-19 vaccines. Further, adjuvants prone to a Th1 response should be considered for S1-based subunit COVID-19 vaccines to reduce the potential risk of antibody-dependent enhancement of infection.

Keywords: COVID-19; S1; SARS-CoV-2 subunit vaccine; antibody-dependent enhancement; receptor-binding domain; spike protein.

Figure 1

Production and characterization of E. coli‐expressed SARS‐CoV‐2 S1 and RBD proteins. (A) Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS‐PAGE) of norovirus shell domain‐RBD (S‐RBD, lane 1, indicated by arrows pointing to the left) and norovirus shell domain‐S1 (S‐S1, lane 2, indicated by arrows pointing to the right). (B) Western blot characterization of S‐RBD (lane 1, indicated by arrows pointing to the left) and S‐S1 (lane 2, indicated by arrows pointing to the right). (C, D) Transmission electron microscopy of the (C) S‐RBD and (D) S‐S1 fusion proteins. Particles with diameters ∼30 to 60 nm are indicated by arrows. RBD, receptor‐binding domain; SARS‐CoV‐2, severe acute respiratory syndrome coronavirus 2

Figure 2

Immunization schedule and humoral responses of various immunogens in mice. A, Immunization schedule. I.M.: intramuscular. W: week. B, S1‐ and RBD‐specific IgG titers. C, S1‐ and RBD‐specific IgA titers. RBD, receptor‐binding domain

Figure 3

Th1‐Th2 balance analysis. A, S1‐ and RBD‐specific IgG1 titers. B, S1‐ and RBD‐specific IgG2a titers. C, IgG1/IgG2a ratios. RBD, receptor‐binding domain

Figure 4

SARS‐CoV‐2 neutralization titers of serum from mice vaccinated with various immunogens. PBS: serum from mice immunized with PBS. PBS, phosphate buffered saline; SARS‐CoV‐2, severe acute respiratory syndrome coronavirus 2