Project Summary The long-term goal of this project is to develop a vaccine that confers robust and durable protection against transplacental transmission of Zika virus (ZIKV). Accomplishing this goal may be challenging in that the vaccine may need to induce both antibody and T cell responses to confer highly effective protection against ZIKV at the maternal-fetal interface (MFI). Studies with pregnant women in Brazil have suggested that antibody responses may contribute to pathogenesis of congenital Zika syndrome (CZS) and neutralizing antibody responses may not correlate with protection against CZS. Recent human studies also suggest that the anti-ZIKV antibody response may be less durable than T cell response. However, ongoing ZIKV vaccine development efforts are focused on eliciting mainly antibody responses. Based on our published data demonstrating a critical role for CD8 T cells in protecting against ZIKV infection in multiple mouse models, we will test our central hypothesis that a robust ZIKV vaccine-induced CD8 T cell response in mothers is required to provide strong and durable protection against transplacental transmission of ZIKV. Our replicon RNA vaccine expressing ZIKV premembrane (prM) and envelope (E) or nonstructural protein 3 (NS3) induces robust protection against ZIKV infection in pregnant mice but only partial protection in their fetuses. Therefore, we will use these replicon RNA vaccines and mouse models to achieve the following Specific Aims: 1) Improve vaccine-induced protection against ZIKV infection during pregnancy, and identify the maternal immune responses associated with the most protective and durable vaccines. 2) Test the role of CD8 T cells in vaccine-induced protection, and determine precise features of MFI CD8 T cells elicited by the most protective and durable vaccine. We have expertise in examining flaviviral pathogenesis and immunity using mouse models. We also have a longstanding collaboration with colleagues at our institute and UC San Diego to investigate virus-host interactions using genomics and histopathology-informed approaches.