Evaluation of the immunogenicity of an inactivated vaccine prototype against SARS-COV- 2 virus in vitro

ABSTRACT

Background: Belarus started developing a vaccine against SARS-CoV- 2 in 2021. The aim of the first stage of investigation was to evaluate the immunogenicity of the vaccine prototypes (VP) in vitro. Method(s) SARS-CoV- 2 strains (n = 7) were isolated using Vero E6 cells, inactivated with beta-propiolactone and purified. Antigens (Ag) were adsorbed on adjuvants Al(OH)3 (Ag+AH group) or Al3PO4 (Ag+AP group). The single dose of VP (500 ul) was composed of 10 mug of Ag adsorbed on adjuvants (200 mug of Al3+). Blood samples from SARS-CoV- 2 recovered donors (n = 18) and healthy controls having no history of COVID-19 infection (n = 5) were used. Whole blood and Tag-it Violet labeled PBMCs were cultivated with VP, pure Ag, adjuvants (0.25-1 mug of Ag, 20 mug of Al3+ for probe) or pool of peptides, covering sequence of SARS-CoV- 2 N, S, M-proteins (PP), for 6 h and 7 days respectively. INF-gamma production and proliferation of CD3+ T-cells were assayed by FACS. Result(s) Counts of CD3+IFN-gamma+ T cells were 3.14(2.72-5.13)/ 1x105 CD3+ T cells in negative control (NC), and 12.73(10.09-33.95)/ 1x105 CD3+ T cells in specific positive control (PP) (n = 18, p < 0.0001), proving presence of antigen-specific T cells (ASCs) in donor blood. Samples were considered positive for VP and Ag immunogenicity when numbers of CD3+IFN-gamma+ T cells were 1.5 times greater compared with NC. Both VP types (Ag+AP, Ag+AH) and pure SARS-CoV- 2 isolates stimulated the production of INF-gamma by ASCs, responses ranged from 1 to 4 isolates of 7 studied per donor. Immunogenicity of Ag+AP, Ag+AH was confirmed by proliferation assay. Proliferation level was 1.07(0.97-2.38)% in Ag group with no differences from NC (n = 7, p > 0.5). Proliferation was significantly greater in VP groups compared with Ag 2.47(1.65-2.68)% in Ag+AH, 4.03(2.56-4.61)% in Ag+AP (p = 0.009 and 0.002, respectively), stimulation of T cell was stronger by Ag+AP compared with Ag+AH (p = 0.009, M-U test). Pure adjuvants did not induce T cells response. There was no T-cell stimulation by Ag and VP in samples obtained from COVID-19 negative donors. Conclusion(s) The VP against SARS-CoV- 2 infection composed of inactivated virus adsorbed on Al(OH)3 and Al3PO4 adjuvants has immunogenic properties proven in two different immunological assays. VP stimulated activation and proliferation of ASCs in vitro suggesting this VP can be used for further preclinical in vivo evaluation.