Advancing Methodologies to Determine Protective Humoral Responses Generated by Recombinant Protein Subunit Filovirus Vaccines

Abstract

Zaire ebolavirus (EBOV) and other related viruses, Sudan ebolavirus (SUDV) and Marburg marburgvirus (MARV) are endemic in have caused sporadic and deadly outbreaks in Central and West Africa with case fatality rates of up to 90%. The largest Ebola Virus Disease (EVD) outbreak on record caused over 28,000 cases. There is a clear need for comprehensive disease intervention to prevent future outbreaks. Licensed vaccines, Ervebo® and Zabdeno + Mvabea, utilize virally vectored platforms and are contraindicated in pregnant and nursing women. To address the needs of these and of immunocompromised individuals, our lab has developed protein subunit vaccines that have shown protection against all three filoviruses in non-human primate (NHP) models. There are no known correlates of protection for any filovirus-induced disease and therefore no defined goals for what a protective immune response of a successful vaccine should be comprised of. The goal of this project is to study protective antibody responses in murine and NHP models in order to understand what aspects of immunity are important for protection in both the general population as well as during pregnancy and nursing to potentially induce maternal immunization. We developed methods for analyzing the breadth of antibody responses to our vaccine by utilizing a Peptide-Array based ImmunoSignature (IS) approach, as well as conducting a more thorough characterization of vaccine induced antibody function through avidity and surrogate neutralization assays. The IS in conjunction with avidity assays will elucidate both the antibody specificity and the strength of antibody binding that comprises a protected humoral response. Immunotherapies have provided insight to the importance of antibody binding strength, however there are conflicting findings in regard to the role of antibody avidity as a correlate of protection, our studies contribute to the overall ongoing research into EBOV vaccine correlates of protection. These experiments utilized pre challenge NHP sera, which is key for developing tools to determine vaccine efficacy in humans where challenge is not a possibility. Here we demonstrate that an analysis at the repertoire level as well as at the paratope-epitope binding level are capable of differentiating survivors from non survivors. While the avidity assays did not correlate with survival day they informed our understanding of eliciting antigen-specific maturation with three doses of vaccine as well as with increasing valency of vaccine formulations. To assess maternal immunization, we utilized a mouse model to analyze antibody transfer from immunized mothers to nurslings. The maternal immunization studies demonstrated that vaccine induced maternal antibodies are not only transferred in high concentration to suckling pups but are functional in virus neutralization. These experiments provide specific knowledge on vaccine induced immunity to EBOV. We demonstrate that vaccine failure can be detected in a genetically diverse animal model, that antibody avidity is not a correlate of protection but may provide clues to defining a vaccine correlate of protection, and that vaccine specific IgG can be passed mother to child and function to neutralize virus in pups. To our knowledge this is the first instance in defining vaccine failure and partial protection using pre challenge serum samples as well as the first instance of mother to child transfer of neutralizing EBOV vaccine antibody in milk.