Doctor of Philosophy / Department of Diagnostic Medicine/Pathobiology / Manuel Borca / Stephen Higgs / African swine fever (ASF) is a lethal hemorrhagic disease of swine caused by a double-stranded DNA virus. ASFV is endemic in Sardinia and Saharan Africa and has been recently expanded from the Caucasus to Eastern Europe. There is no vaccine to prevent the disease and current control measures are limited to culling and restricted animal movement. Swine infected with attenuated strains are protected against challenge with a homologous virulent virus, but there is limited knowledge of the host immune mechanisms generating that protection. Swine infected with Pret4 virus develop a fatal severe disease, while a derivative strain lacking virulence-associated gene 9GL (Pret4Δ9GL virus) is completely attenuated. Swine infected with Pret4 Δ9GL virus and challenged with the virulent parental virus at 7, 10, 14, 21, and 28 dpi showed a progressive acquisition of protection (from 40% at 7 dpi to 80% at 21 and 28 dpi). This animal model was used to associate the presence of host immune response and protection against the challenge. Anti-ASFV antibodies and cytokines in serum, as well as ASFV-specific IFN-γ production in PBMCs, were assessed in each group. Interestingly, with the exception of ASFV-specific antibodies in the surviving swine challenged at 21 and 28 dpi, no solid association between any of the parameters assessed and the extent of protection could be established. These results were corroborated using a similar model based on the use of a rationally attenuated derivative of the highly virulent strain Georgia 2007. These results, encompassing data from 114 immunized swine, underscore the complexity of the system under study where it is very plausible that protection against disease or infection relies heavily on the concurrence and or interaction of different host immune mechanisms.

Ebola virus (EBOV) is a zoonotic pathogen which causes a fulminant hemorrhagic fever and results in up to 90% fatality. Despite efforts over the past 38 years, a licensed prophylactic or post-exposure option remains unavailable. Several experimental vaccines have already demonstrated protection from lethal EBOV disease in nonhuman primates (NHPs). However, attempts to translate this research from bench to bedside have been hampered, since the immune responses that correlate with protection against EBOV are not well-defined. Without this information, it is not possible to reliably predict the efficacy of vaccines or treatments in humans without lethal challenge. The goal of this thesis is to determine the immune parameters that are predictive of protection against EBOV. Due to the rapid speed of EBOV pathogenesis, it was originally hypothesized that a rapid and robust CD8+ T-cell response must be crucial for survival. Using a previously-characterized adenovirus-vectored Ebola vaccine (Ad5-optGP), transgenic/knockout C57BL/6J mice with ablation for selected immune responses were vaccinated and challenged 28 days later. Surprisingly, while CD8+ T-cell knockout mice survived infection, B-cell knockout mice did not, indicating that the antibody response played a critical role in protection. Humoral and cell-mediated responses were compared between survivor and moribund guinea pigs and NHPs from previous vaccination and post-exposure therapy experiments, either with Ad5-optGP or a vesicular stomatitis virus (VSV)-vectored vaccine (VSVΔG/EBOVGP). Circulating EBOV glycoprotein (GP)-specific IgG antibody levels were the best correlation for protection independent of vaccine platform or timing of exposure (p<0.0001), strongly supporting the role of antibody responses in the control of EBOV infection. To demonstrate that antibodies are also responsible for protection, three previously-characterized monoclonal antibodies recognizing different GP epitopes were combined into a cocktail (ZMAb). Initiation of ZMAb treatment 1 and 2 days post infection (dpi) with EBOV protected 100% and 50% of NHPs, respectively. These results provide a reliable measure for predicting protection from EBOV in three commonly used animal models, and present a strong case for the use of antibodies as an effective post-exposure treatment. This knowledge will ultimately help in the development and validation of a clinical product against EBOV infection.