Universal Vaccines for Tick-Borne Encephalitis Viruses

Background: Tick-borne encephalitis (TBE) viruses are significant, high-consequence pathogens that are present in many localities where Warfighters may be deployed and where others may visit. TBEs of note include classical tick-borne encephalitis virus (TBEV), Omsk hemorrhagic fever virus (OMSK), Kyasanur Forest disease virus (KFDV) and Powassan virus (POWV). No therapeutics exist for these agents, and vaccines tend to be agent-specific and poorly cross-reactive. Hypothesis/Objective: We therefore seek to pursue a novel method for designing vaccine antigens that are broadly cross-reactive and may provide protection from as-yet-undescribed members of the TBEV group. Our overarching hypothesis is that computationally predicted ancestral antigens will provide effective antibody and cell-mediated immune responses to an array of TBEVs. Specific Aims Aim 1. Design ancestral TBEV envelope (E) and nonstructural protein 1 (NS1) using maximum likelihood modeling, engineer them into three distinct vaccine platforms and evaluate their in vitro viability. Aim 2. Assess safety and immunogenicity of downselected putative vaccine constructs in mice. Antibody binding, neutralization, and T-cell assays will be used to measure immunogenicity. Aim 3. Determine the protective efficacy of vaccines against TBEV, POWV, OMSK, and KFDV. Vaccinated animals will be challenged at BSL4, as appropriate, at the National Institute of Allergy and Infectious Diseases Rocky Mountain Laboratories. Study Design: The overall design of this study is fairly simple. Common ancestors of existing TBEVs will be inferred using maximum likelihood modelling. Predicted ancestral E and NS1 coding sequences will then be engineered into three potential vaccine constructs. These include (a) the YF17D vaccine strain (B) an attenuated version of POWV that we will develop during the course of this project, and (c) an alphavirus replicon system that expresses exogenous antigens under the control of the alphavirus subgenomic promoter. These constructs will be assessed for stability and ease of production, and only stable candidates will progress into immunogenicity/safety trials in mice. To assess immunogenicity, mice will be vaccinated with diminishing doses of the produced constructs, boosted as appropriate, and antibody binding and neutralization will be assessed using ELISA and PRNT testing, respectively. Cell-mediated immunity will be measured using CTL and tetramer assays. Safety will be assessed by observing mice daily and noting weight gain during the vaccination process. Finally, vaccinated animals will be challenged with infectious virus and morbidity and mortality monitored. Follow-up studies may include experiments to assess viral load and tissue distribution, and vaccination impact of virus evolution within vaccinated animals. The most important controls for this project include mock vaccinated, and mock inoculated animals in our challenge studies. Other controls (i.e., for molecular biology, etc.) are standard. This study is highly innovative because we are exploring a fairly unique concept in vaccine design: That ancestral viral antigens may be effective at protecting against many or all of their descendants. Impact: Our impact, if successful, will be twofold. First, we will have generated candidate vaccines against problematic emerging viruses. Second, we will have explored a new concept in vaccine design (this impact will occur even if we ultimately determine that ancestral antigens perform poorly as vaccines). Less