Immunisation of ferrets and mice with recombinant SARS-CoV-2 spike protein formulated with Advax-SM adjuvant protects against COVID-19 infection

Abstract

The development of a safe and effective vaccine is a key requirement to overcoming the COVID-19 pandemic. Recombinant proteins represent the most reliable and safe vaccine approach but generally require a suitable adjuvant for robust and durable immunity. We used the SARS-CoV-2 genomic sequence and in silico structural modelling to design a recombinant spike protein vaccine (Covax-19). A synthetic gene encoding the spike extracellular domain (ECD) was inserted into a baculovirus backbone to express the protein in insect cell cultures. The spike ECD was formulated with Advax-SM adjuvant and first tested for immunogenicity in C57BL/6 and BALB/c mice. The Advax-SM adjuvanted vaccine induced high titers of binding antibody against spike protein that were able to neutralise the original wildtype virus on which the vaccine was based as well as the variant B.1.1.7 lineage virus. The Covax-19 vaccine also induced potent spike-specific CD4+ and CD8+ memory T-cells with a dominant Th1 phenotype, and this was shown to be associated with cytotoxic T lymphocyte killing of spike labelled target cells in vivo. Ferrets immunised with Covax-19 vaccine intramuscularly twice 2 weeks apart made spike receptor binding domain (RBD) IgG and were protected against an intranasal challenge with SARS-CoV-2 virus 2 weeks after the second immunisation. Notably, ferrets that received two 25 or 50ug doses of Covax-19 vaccine had no detectable virus in their lungs or in nasal washes at day 3 post-challenge, suggesting the possibility that Covax-19 vaccine may in addition to protection against lung infection also have the potential to block virus transmission. This data supports advancement of Covax-19 vaccine into human clinical trials.

An adjuvanted subunit SARS-CoV-2 spike protein vaccine provides protection against Covid-19 infection and transmission

“…It is interesting that in our study the 2.5 µg dose induced higher levels of antibodies than the 5 µg dose. Indeed, similar observations of inverse spike protein dose effects on antibody production was seen in a previous ferret study using the same spike protein as used here 11 and was also reported in the Novavax Phase I study where the lower 5 μg dose in the study was associated with higher antibody levels than the much higher 25 μg dose 18 . This suggests that excess doses of spike protein may in some way modulate and suppress the humoral immune response, thereby explaining why lower doses could result in high antibody responses.…”

“…The spike ECD has an amino acid sequence corresponding to wild-type SARS-CoV-2 (Wuhan strain) together with some stabilizing mutations. This spike ECD protein has been previously characterized in mice and ferrets as part of a vaccine called COVAX-19/SpikoGen 11 that has progressed through phase III clinical trials and recently received emergency use authorization in Iran 5 . Adjuvants are generally needed to increase the effectiveness of subunit vaccines 12 .…”

Evaluation of a Novel Adjuvanted Vaccine for Ultrashort Regimen Therapy of Artemisia Pollen-Induced Allergic Bronchial Asthma in a Mouse Model

“…Advax can be used in combination with a CpG oligonucleotide (Advax-CpG) that stimulates TLR-9, which results in even stronger activation of humoral and cellular immunity (41). Advax-CpG adjuvant efficacy has been demonstrated to be effective in vaccines against tuberculosis (41,42), Alzheimer's disease (43), and coronavirus disease 2019 (COVID)-19 (44), among others. This study also tested the SWE squalene emulsion adjuvant that mimics the wellknown MF59 adjuvant (Sequirus, USA) included in commercial influenza vaccines (45,46).…”

A Spike Protein-Based Subunit SARS-CoV-2 Vaccine for Pets: Safety, Immunogenicity, and Protective Efficacy in Juvenile Cats

“…The vaccines are based on wild-type SARS-CoV-2 (Wuhan strain) spike protein ECD with a histidine tag to assist with purification and stabilization that is expressed in insect cells using the baculovirus transfection system. This spike protein ECD antigen when combined with Vaxine's proprietary Advax-CpG55.2 has previously been shown to be immunogenic, protective, and safe in mice and ferrets in an Australian-developed vaccine called COVAX-19 or SpikoGen ( 28 ) that has already successfully passed phase III clinical trials and in October 2021 received emergency use authorization for human adult use from the Iranian FDA ( 23 ). This same spike protein ECD when formulated with a squalene O/W (SWE) adjuvant (NARUVAX-C19 vaccine) provided complete protection of Syrian hamsters against SARS-CoV-2 infection and prevented transmission to naïve animals placed in the same cage as the challenged animals ( 21 ).…”

“…The detailed methodology of obtaining recombinant spike protein ECD has been described previously ( 23 ). Briefly, the spike protein was identified from the SARS-CoV-2 genomic sequence in NCBI (access number: NC 045512) ( 24 ).…”