Summary Zika virus (ZIKV) is a mosquito-transmitted flavivirus that has caused devastating congenital Zika syndromes (CZS), including microcephaly, congenital malformation, and fetal demise in human newborns in the 2015 - 2016 epidemics. In addition, ZIKV infection can cause Guillain-Barré syndrome (GBS) and meningoencephalitis in adults. Currently, ZIKV pathogenesis is incompletely understood, and there is no approved therapeutic or vaccine available. Although the number of human Zika cases has dropped since 2017, ZIKV will likely become endemic, and a vaccine that can prevent CZS and adult Zika diseases remains urgently needed to prepare for future outbreaks. Our research group recently developed a new live-attenuated ZIKV strain (named Z7) by inserting a 50-RNA-nucleotides (nt) hairpin into the 5’ untranslated region (UTR) of a pre-epidemic ZIKV Cambodian strain, FSS13025, which is attenuated in neurovirulence, immune antagonism, and mosquito infectivity compared with the American epidemic isolates. Our preliminary data demonstrate that Z7 replicates efficiently and produces high titers in Vero cells without causing apparent cytopathic effects (CPE) or losing the insert sequence even after ten consecutive passages. Interestingly, we identified a mutation S1417A in NS2B protein in Z7 after the 7th passage, indicating Z7 may have adapted a fitness mutation through the passaging. Importantly, Z7 induces sterilizing immunity that completely prevents viremia after a challenge with a high dose of an American epidemic strain (PRVABC59) in mice. These results suggest that modification of ZIKV 5’UTR is a novel strategy to develop live-attenuated vaccine candidates for ZIKV and potentially for other flaviviruses. In this application, we will first characterize Z7-induced humoral and cell-mediated immunity in a mouse model (Specific Aim 1), and then evaluate the safety features of Z7 in mice (Specific Aim 2). In Specific Aim 3, we will determine the mechanisms of action of Z7 attenuation. We believe that this project is highly significant because it will characterize the immunity and safety of a novel live-attenuated ZIKV vaccine candidate, Z7, which was developed for the first time by introducing a hairpin loop into a viral 5’ UTR. In addition, this R15 project will also provide a valuable and unique training opportunity to both undergraduate and graduate students at the University of Southern Mississippi.

Narrative Zika virus has recently caused devastating congenital Zika syndromes in human newborns. In addition, Zika virus infection can cause Guillain-Barré syndrome (GBS) and meningoencephalitis in adults. This project will characterize the immunity and safety of a novel live-attenuated Zika vaccine candidate, which was created by modifying the virus genome 5’ untranslated region (UTR).