Development of a candidate vaccine against severe fever with thrombocytopenia syndrome virus using Gn/Gc glycoprotein via multiple expression vectors delivered by attenuated Salmonella confers effective protection in hDC-SIGN transduced mice

In this study, we developed two plasmid constructs, pJHL270 and pJHL305, for the dual expression of Severe Fever with Thrombocytopenia Syndrome Virus (SFTSV) Gn and Gc glycoproteins in both prokaryotic and eukaryotic systems. The constructs feature a prokaryotic expression controlled by the Ptrc promoter and a eukaryotic expression driven by the cytomegalovirus promoter and Semliki Forest Virus RNA-dependent RNA polymerase. The Gn/Gc antigenic epitope was derived from consensus sequences of 12 SFTSV M segments collected in South Korea and designed for optimal antigen expression. The full antigen was expressed eukaryotically for post-translational modifications, while the epitope construct was expressed prokaryotically. These constructs were electroporated into an attenuated Salmonella Typhimurium strain (JOL2500) for plasmid delivery, resulting in JOL3042 and JOL3045. Successful expression was confirmed via qRT-PCR and western blot analysis. Mice immunized with JOL3042 showed antibody responses as early as two weeks, while JOL3045 elicited responses at six weeks, skewed toward a Th1 response initially, later balancing with Th2. Flow cytometry revealed significant CD3⁺CD4⁺ and CD3⁺CD8⁺ T-cell responses. Both constructs generated neutralizing antibodies, and a challenge study indicated significant reductions in viral loads in the serum, liver, and spleen of vaccinated mice, demonstrating the effectiveness of the Salmonella-mediated delivery system against SFTSV infection. The outcome of the current study may pave the way to develop a safer and more effective Salmonella-mediated vaccine against lethal SFTSV infection in vulnerable populations.