Delta-Vac’ subunit vaccine induces strong humoral immune response against SARS-CoV-2

The COVID-19 pandemic has focused the worldwide attention on the fight against this infection. Development of preventive vaccines based on different platforms, including DNA and RNA vaccines, genetic vectors and subunit vaccines A has been the key approach to combating COVID-19. The subunit vaccines were among these platforms, primarily, due to their unique safety profile. However, the safety of these vaccines is often associated with low efficiency. Hence, their application often requires usage of immune adjuvants, along with more complex immunization regimens. Meanwhile, an important advantage of subunit vaccines is their scalability and relative ease of production, since there is no need to work with live virus or viral vectors during the production process. The purpose of our work was to develop a candidate vaccine based on the recombinant receptor-binding domain (RBD) of the SARS-CoV-2 spike S-protein from the Delta variant (B.1.617.2). The study used immunological techniques employing methods of genetic engineering and biotechnology. In the course of this work, we have developed a producer of recombinant RBD based on the CHO-K1 mammalian cells. To obtain a protein meeting the requirements of injectable drugs, a chromatographic purification scheme was developed, including affinity and ion exchange chromatography. We have proposed a variant of the subunit vaccine “Delta-Vac” developed on this recombinant protein. Immunogenicity of “Delta-Vac” vaccine was tested in the BALB/c mice. The animals were immunized twice at a dose of 50 μg of RBD combined with Al(OH)3, at a two-week interval. The ability of the candidate vaccine was assessed by induced production of specific IgG and neutralizing antibodies in BALB/c mice. Specific antibody titers of immunized animals ranged from 1/10 5 to 1/10 6 . Moreover, the blood sera showed neutralizing activity against SARS-CoV-2 (variant Delta, B.1.617.2) at a titer of up to 1/2000. The developed vaccine “Delta-Vac” is highly immunogenic and induces production of neutralizing antibodies against homologous Delta variant as well as heterologous Wuhan and Omicron SARS-CoV-2 variants. Hence, “Delta-Vac” may act as a vaccine candidate and serve as a prototype for the development of subunit vaccines against COVID-19.