PROJECT SUMMARY/ABSTRACT Zika virus (ZIKV) is a mosquito-transmitted flavivirus that has been implicated in devastating birth defects such as microcephaly, ventriculomegaly, cerebellar hypoplasia, and fetal akinesia deformation sequence (arthrorgryposis). An ongoing Zika epidemic in Central and South America reached the continental U.S. when autochthonous cases were first documented in Florida in June 2016. To mitigate morbidity following Zika infection during pregnancy, a clearer understanding of how maternal-fetal transmission occurs is needed. We hypothesize that ZIKV infects placental cells with an impaired innate immune response, while cells that have active innate immune defenses are resistant to ZIKV. Once ZIKV is able to propagate through the placenta, transplacental passage of the virus to the fetus may occur. Here we propose to define the placental cell types vulnerable to ZIKV infection utilizing a human placental explant model. We will characterize innate immune responses to ZIKV infection by determining infection rates under conditions of innate immune signaling blockade, assess for changes in toll-like receptor (TLR) and retinoic acid inducible-gene-I-like receptor (RLR) expression in placental cells exposed to ZIKV, and evaluate TLR- and RLR agonists for ability to suppress ZIKV infection. Our recent studies demonstrate robust expression of TLRs in first trimester villous cytotrophoblast that breaks down with advancing gestation. We will leverage our placental expertise in collaboration with virus experts to determine the consequences of ZIKV infection of placenta from early gestation, the time of highest risk for ZIKV-associated microcephaly. The proposed studies will improve our understanding of the conditions under which the placenta is able to resist ZIKV infection and transfer to the fetus.