Hybrid Proteins with Short Conformational Epitopes of the Receptor-Binding Domain of SARS-CoV-2 Spike Protein Promote Production of Virus-Neutralizing Antibodies When Used for Immunization.

Based on the previously developed approach, hybrid recombinant proteins containing short conformational epitopes (a.a. 144-153, 337-346, 414-425, 496-507) of the receptor-binding domain (RBD) of SARS-CoV-2 Spike protein (S protein) were synthesized in Escherichia coli cells as potential components of epitope vaccines. Selected epitopes are involved in protein-protein interactions in the S protein complexes with neutralizing antibodies and ACE2 (angiotensin-converting enzyme 2). The recombinant proteins were used for immunization of mice (three doses with 2-week intervals), and the immunogenicity of protein antigens and ability of the resulting sera to interact with inactivated SARS-CoV-2 and RBD produced in eukaryotic cells were examined. All recombinant proteins showed high immunogenicity; the highest titer in the RBD binding assay was demonstrated by the serum obtained after immunization with the protein containing epitope 414-425. At the same time, the titers of sera obtained against other proteins in the RBD and inactivated virus binding assays were significantly lower than the titers of sera obtained with the previously produced four proteins containing the loop-like epitopes 452-494 and 470-491, the conformation of which was fixed with a disulfide bond. We also studied activation of cell-mediated immunity by the recombinant proteins that was monitored as changes in the levels of cytokines in the splenocytes of immunized mice. The most pronounced increase in the cytokine synthesis was observed in response to the proteins containing epitopes with disulfide bonds (452-494, 470-491), as well as epitopes 414-425 and 496-507. For some recombinant proteins with short conformational epitopes, adjuvant optimization allowed to obtained mouse sera displaying virus-neutralizing activity in the microneutralization assay with live SARS-CoV-2 (hCoV-19/Russia/StPetersburg-3524/2020 EPI_ISL_415710 GISAID). The results obtained can be used to develop epitope vaccines for prevention of COVID-19 and other viral infections.

Development of a Platform for Producing Recombinant Protein Components of Epitope Vaccines for the Prevention of COVID-19.

A new platform for creating anti-coronavirus epitope vaccines has been developed. Two loop-like epitopes with lengths of 22 and 42 amino acid residues were selected from the receptor-binding motif of the Spike protein from the SARS-CoV-2 virus that participate in a large number of protein-protein interactions in the complexes with ACE2 and neutralizing antibodies. Two types of hybrid proteins, including one of the two selected epitopes, were constructed. To fix conformation of the selected epitopes, an approach using protein scaffolds was used. The homologue of Rop protein from the Escherichia coli ColE1 plasmid containing helix-turn-helix motif was used as an epitope scaffold for the convergence of C- and N-termini of the loop-like epitopes. Loop epitopes were inserted into the turn region. The conformation was additionally fixed by a disulfide bond formed between the cysteine residues present within the epitopes. For the purpose of multimerization, either aldolase from Thermotoga maritima, which forms a trimer in solution, or alpha-helical trimerizer of the Spike protein from SARS-CoV-2, was attached to the epitopes incorporated into the Rop-like protein. To enable purification on the heparin-containing sorbents, a short fragment from the heparin-binding hemagglutinin of Mycobacterium tuberculosis was inserted at the C-terminus of the hybrid proteins. All the obtained proteins demonstrated high level of immunogenicity after triplicate parenteral administration to mice. Sera from the mice immunized with both aldolase-based hybrid proteins and the Spike protein SARS-CoV-2 trimerizer-based protein with a longer epitope interacted with both the inactivated SARS-CoV-2 virus and the Spike protein receptor-binding domain at high titers.

Safety and Immunogenicity of Betuvax-CoV-2, an RBD-Fc-Based SARS-CoV-2 Recombinant Vaccine: Preliminary Results of the First-in-Human, Randomized, Double-Blind, Placebo-Controlled Phase I/II Clinical Trial.

COVID-19, being a life-threatening infection that evolves rapidly, remains a major public health concern calling for the development of vaccines with broad protection against different pathogenic strains and high immunogenicity. Aside from this, other concerns in mass immunization settings are also the scalability of production and relative affordability of the technology. In that regard, adjuvanted protein vaccines with particles mimicking the virus stand out among known vaccine technologies. The "Betuvax-CoV-2" vaccine, developed on the basis of a recombinant protein and an adjuvant, has already been tested in preclinical studies and has advanced to clinical evaluation. Open, double-blinded, placebo-controlled, randomized phase I/II clinical trial of the "Betuvax-CoV-2," recombinant protein subunit vaccine based on the S-protein RBD fused with the Fc-fragment of IgG, was conducted to evaluate safety and immunogenicity in response to the vaccination. METHODS: In the phase I/II clinical trial, 116 healthy adult men and women, ages 18-58, were enrolled: 20 in Stage I, and 96 in Stage II. In Stage I, 20 µg of the vaccine was administered intramuscularly on day 2, and either 5 µg (group 1) or 20 µg (group 2) on day 30. In Stage II, 20 µg of the vaccine was administered intramuscularly on day 2, and either 5 µg (group 3) or 20 µg (group 4) on day 30. In group 5, both injections were replaced with placebo. The primary outcome measures were safety (number of participants with adverse events throughout the study) and antigen-specific humoral immunity (SARS-CoV-2-specific antibodies measured by ELISA and CMIA). Antigen-specific cell-mediated immunity and changes in neutralizing antibodies (detected with a SARS-CoV-2 neutralization assay) were measured as a secondary outcome. The trial is registered with ClinicalTrials.gov (Study Identifier: NCT05270954). FINDINGS: Both vaccine formulations (20 µg + 5 µg and 20 µg + 20 µg) were safe and well tolerated. Most adverse events were mild, and no serious adverse events were detected. On day 51,anti-SARS-CoV-2 total and IgG antibody titers and anti-SARS-CoV-2 neutralizing antibodies were significantly higher in the vaccine groups (both formulations) than in the placebo. A more pronounced CD4+-mediated immune response was observed in the group of volunteers administered with the 20 + 20 µg vaccine formulation. INTERPRETATIONS: RBD-Fc-based COVID-19 "Betuvax-CoV-2" vaccine in doses (20 + 5 µg and 20 + 20 µg) demonstrated an excellent safety profile and induced a strong humoral response. Further research on the protective effectiveness of the "Betuvax-CoV-2" vaccine for the prevention of COVID-19 is on its way.

Design and Immunological Properties of the Novel Subunit Virus-like Vaccine against SARS-CoV-2.

The COVID-19 pandemic is ongoing, and the need for safe and effective vaccines to prevent infection and to control spread of the virus remains urgent. Here, we report the development of a SARS-CoV-2 subunit vaccine candidate (Betuvax-CoV-2) based on RBD and SD1 domains of the spike (S) protein fused to a human IgG1 Fc fragment. The antigen is adsorbed on betulin adjuvant, forming spherical particles with a size of 100-180 nm, mimicking the size of viral particles. Here we confirm the potent immunostimulatory activity of betulin adjuvant, and demonstrate that two immunizations of mice with Betuvax-CoV-2 elicited high titers of RBD-specific antibodies. The candidate vaccine was also effective in stimulating a neutralizing antibody response and T cell immunity. The results indicate that Betuvax-CoV-2 has good potential for further development as an effective vaccine against SARS-CoV-2.

SARS-CoV-2 Subunit Virus-like Vaccine Demonstrates High Safety Profile and Protective Efficacy: Preclinical Study.

Public health threat coming from a rapidly developing COVID-19 pandemic calls for developing safe and effective vaccines with innovative designs. This paper presents preclinical trial results of "Betuvax-CoV-2", a vaccine developed as a subunit vaccine containing a recombinant RBD-Fc fusion protein and betulin-based spherical virus-like nanoparticles as an adjuvant ("Betuspheres"). The study aimed to demonstrate vaccine safety in mice, rats, and Chinchilla rabbits through acute, subchronic, and reproductive toxicity studies. Along with safety, the vaccine demonstrated protective efficacy through SARS-CoV-2-neutralizing antibody production in mice, rats, hamsters, rabbits, and primates (rhesus macaque), and lung damage and infection protection in hamsters and rhesus macaque model. Eventually, "Betuvax-CoV-2" was proved to confer superior efficacy and protection against the SARS-CoV-2 in preclinical studies. Based on the above results, the vaccine was enabled to enter clinical trials that are currently underway.

Comparative safety, immunogenicity, and efficacy of several anti-H5N1 influenza experimental vaccines in a mouse and chicken models (Testing of killed and live H5 vaccine).

OBJECTIVE: Parallel testing of inactivated (split and whole virion) and live vaccine was conducted to compare the immunogenicity and protective efficacy against homologous and heterosubtypic challenge by H5N1 highly pathogenic avian influenza virus. METHOD: Four experimental live vaccines based on two H5N1 influenza virus strains were tested; two of them had hemagglutinin (HA) of A/Vietnam/1203/04 strain lacking the polybasic HA cleavage site, and two others had hemagglutinins from attenuated H5N1 virus A/Chicken/Kurgan/3/05, with amino acid substitutions of Asp54/Asn and Lys222/Thr in HA1 and Val48/Ile and Lys131/Thr in HA2 while maintaining the polybasic HA cleavage site. The neuraminidase and non-glycoprotein genes of the experimental live vaccines were from H2N2 cold-adapted master strain A/Leningrad/134/17/57 (VN-Len and Ku-Len) or from the apathogenic H6N2 virus A/Gull/Moscow/3100/2006 (VN-Gull and Ku-Gull). Inactivated H5N1 and H1N1 and live H1N1 vaccine were used for comparison. All vaccines were applied in a single dose. Safety, immunogenicity, and protectivity against the challenge with HPAI H5N1 virus A/Chicken/Kurgan/3/05 were estimated. RESULTS: All experimental live H5 vaccines tested were apathogenic as determined by weight loss and conferred more than 90% protection against lethal challenge with A/Chicken/Kurgan/3/05 infection. Inactivated H1N1 vaccine in mice offered no protection against challenge with H5N1 virus, while live cold-adapted H1N1 vaccine reduced the mortality near to zero level. CONCLUSIONS: The high yield, safety, and protectivity of VN-Len and Ku-Len made them promising strains for the production of inactivated and live vaccines against H5N1 viruses.