Mice Immunized with the Vaccine Candidate HexaPro Spike Produce Neutralizing Antibodies against SARS-CoV-2

Abstract

Updated and revised versions of COVID-19 vaccines are vital due to genetic variations of the SARS-CoV-2 spike antigen. Furthermore, vaccines that are safe, cost-effective, and logistic-friendly are critically needed for global equity, especially for middle- to low-income countries. Recombinant protein-based subunit vaccines against SARS-CoV-2 have been reported using the receptor-binding domain (RBD) and the prefusion spike trimers (S-2P). Recently, a new version of prefusion spike trimers, named HexaPro, has been shown to possess two RBD in the “up” conformation, due to its physical property, as opposed to just one exposed RBD found in S-2P. Importantly, this HexaPro spike antigen is more stable than S-2P, raising its feasibility for global logistics and supply chain. Here, we report that the spike protein HexaPro offers a promising candidate for the SARS-CoV-2 vaccine. Mice immunized by the recombinant HexaPro adjuvanted with aluminum hydroxide using a prime-boost regimen produced high-titer neutralizing antibodies for up to 56 days after initial immunization against live SARS-CoV-2 infection. Also, the level of neutralization activity is comparable to that of convalescence sera. Our results indicate that the HexaPro subunit vaccine confers neutralization activity in sera collected from mice receiving the prime-boost regimen.

SARS-CoV-2 Vaccines Based on the Spike Glycoprotein and Implications of New Viral Variants

“…HexaPro is a stabilized S containing four proline substitutions (F817P, A892P, A899P, A942P) as well as the two proline substitutions in S-2P (K986P/V987P), also incorporates mutations in the furin cleavage site (682-GSAS-685) and a C-terminal trimerization motif ( 84 ). HexaPro protein S turns out to be an ideal candidate for the development of new vaccines, due to its high stability, excellent expression and improved solubility, as well as preserved antigenicity ( 84 , 85 ). Other research groups have proposed the rational design of vaccines using stabilized spike proteins with improved immunogenicity and antigenicity ( 86 ).…”

Prime-pull immunization of mice with a BcfA-adjuvanted vaccine elicits mucosal immunity and prevents SARS CoV-2 infection and pathology

“…Generation of S-specific antibodies by vaccination is correlated with protection against SARS-CoV-2 (DiPiazza et al, 2021; Seephetdee et al, 2021). We determined whether the prime-pull immunization regimen generated both mucosal and systemic antibodies.…”

“…However, it is unclear whether these vaccines elicit tissue-resident memory T cell responses (T RM ) that are critical for preventing transmission and providing sustained protection against disease. Another important consideration in vaccine design is the possibility of vaccine associated enhanced respiratory disease (VAERD), which is the development of a more severe form of disease manifested after vaccination against the causative agent (DiPiazza et al, 2021; Halstead and Katzelnick, 2020; Liang et al, 2020; Seephetdee et al, 2021; Sekine et al, 2020) VAERD, which correlates with Th2 immune responses, is elicited by whole cell inactivated vaccines or Th1/Th2 skewing adjuvants such as alum. VAERD is reported as a side effect for inactivated vaccines against multiple pathogens, including respiratory syncytial virus (Graham et al, 1993; Kim et al, 1969), measles (Fulginiti et al, 1967; Nader et al, 1968), and SARS-CoV and MERS-CoV which are closely related to SARS-CoV-2 (Agrawal et al, 2016; Czub et al, 2005; See et al, 2006; Tseng et al, 2012).…”

A circular mRNA vaccine prototype producing VFLIP-X spike confers a broad neutralization of SARS-CoV-2 variants by mouse sera

“…The microneutralization assay was performed as described previously with some modifications ( Seephetdee et al, 2021 ). Briefly, serially diluted heat-inactivated sera (starting with a dilution of 1:20) were pre-incubated with equal volumes of 100 TCID 50 of SARS-CoV-2 for 1 h at 37 °C.…”