## 1 Introduction
Whether YFV vaccination occurs in childhood, in adulthood, or during pregnancy, cross-reactive antibodies that complex with other flaviviruses could be a source for the enhancement of infection. Furthermore, non-neutralizing antibodies have been shown to contribute significantly to antibody-dependent enhancement [22; 23; 24]. Since ZIKV infections can cause severe congenital syndromes, it is crucial to understand any potential enhancement or protection offered by cross-reactive antibodies [8]. Studying the vertical transmission of ZIKV has posed some challenges to researchers. Results produced in mouse models are difficult to translate directly to a human or non-human primate model since mice placentas are structurally different [25; 26; 27]. Ovine and non-human primate models have proved to be promising, but these too have their limitations, such as increased costs, small sample sizes, and labor intensiveness [28; 29]. To address these roadblocks, recent advances in biotechnology have generated co-culture models that use primary human cell lines and stem cells to replicate cellular interfaces. Co-culture models have been used to simulate the blood-brain barrier, the pulmonary barrier, and the maternal-fetal axis in nanoparticle translocation studies [30; 31; 32; 33; 34]. The transwell co-culture model was utilized in this study to determine if the cross-reactivity of YFV antibodies could impact ZIKV pathogenesis in utero during early pregnancy. This in vitro model offers multiple benefits, such as reproducibility and standardization, and excels in simulating the physiological boundary of the maternal-fetal axis [35; 36]. Syncytiotrophoblasts and umbilical vein cells (BeWo and HUVEC) were used in our transwell co-culture, following established placental models [34; 35; 37]. The BeWo cell line was derived from a human placenta and best simulates the structure and function of the syncytiotrophoblasts layer of the placenta that forms the continuous outer layer of the placenta [38; 39]. These cells directly contact maternal blood and regulate the exchange of nutrients and particles to a developing fetus [25; 40; 41]. Any virus or antibodies moving across the placental barrier would first have to cross the syncytiotrophoblast layer to reach a fetus, and previous studies have determined the translocation rates across a BeWo layer to replicate the rates found in ex vivo placental perfusions [27; 42]. Furthermore, we employed a ZIKV isolate derived from human placenta that we felt would be more relevant than utilizing a lab-adapted strain with unknown tissue tropism in humans. The paper by [PERSON] et al. (2018) was followed by including a differentiating embryoid body (EB) in the basolateral chamber of the transwell co-culture, which mimics an early-stage developing embryo [37]. An EB is generated by inducing stem cells to differentiate and self-organize in the three germ layers: the endoderm, mesoderm, and ectoderm [43; 44; 45]. By including an EB in the co-culture model, the goal was to determine if there were differences in the translocation of the virus and/or virus-antibody complexes that cross the placental barrier and infect an EB. Here, a co-culture model (Figure 1) is described, which can be utilized to study the enhancement or neutralization of a virus by maternal antibodies at the maternal-fetal axis. This study showed that YFV post-vaccination antibodies can enhance the ZIKV infection of an EB, which could impact the development of congenital syndromes.