The CD8+T cells response is sufficient for protection with a CCHFV M-segment based DNA vaccine and GP38 enhances vaccine immunogenicity

Introduction: Crimean-Congo hemorrhagic fever virus (CCHFV) is the most widely distributed tick-borne virus of human medical importance, with outbreaks occurring in the Middle East, Asia, Africa, and parts of Western Europe. Our team developed and tested a DNA vaccine expressing the full-length codon-optimized M-segment, which encodes the structural and non-structural viral glycoproteins. Methods: Several mouse models and non-human primates (NHPs) were vaccinated with our DNA vaccine, CCHFV-MAfg09, and challenged with CCHFV. The humoral and cellular immune responses were examined following vaccination and challenge. Results: The CCHFV-MAfg09 vaccine was designed against the clinically relevant CCHFV-Afg09-2990 strain of CCHFV. In both species, the CCHFV-M-Afg09 vaccine is highly immunogenic, eliciting both antigen-specific humoral and cellular immunity when delivered by intramuscular (IM) electroporation (EP). CCHFV-M-Afg09 is completely protective against CCHFV-Afg09-2990 challenge in mice, and 83% protective against measurable viremia in NHPs. We found that CCHFV-M-Afg09 vaccinated mice lacking B-cells (µMT knockout mice) were still protected against lethal challenge with CCHFV, whereas mice lacking T-cells (TCRα/β knockout mice) and CD8+ T-cell deficient mice (CD8-/-) succumbed to disease. Additionally, the team examined several M-segment deletion mutants to determine the components of the vaccine that influence protection. The vaccine encoding the non-structural protein, GP38, provided significant protection alone. In contrast, the vaccine encoding the two structural glycoproteins, GN and GC, did not provide protection. Discussion: The CCHFV-M-Afg09 vaccine is highly immunogenic and protective in mice and NHPs. The data suggests that the CD8+ T-cells play a significant role in the protective response provided by the vaccine. Finally, the data shows that the non-structural M-segment protein, GP38, influences CCHF vaccine immunogenicity and provides significant protection from homologous CCHFV challenge. In contrast, the immune responses to vaccination with the structural glycoproteins alone were not sufficient for protection. Our results demonstrate that M-segment DNA vaccines elicit protection primarily through the CD8+ T cell response and illustrates the immunorelevance of GP38. Conclusion: The significant efficacy of our M-segment based DNA vaccine makes it an ideal candidate to be combined with an efficacious nucleocapsid-based vaccine to produce a potentially broadly cross-protective vaccine effective against the multiple clades of CCHFV. Alternative delivery modalities of the vaccine are being explored.