Zika virus (ZIKV) has emerged as a cause of congenital brain anomalies and a range of placenta-related abnormalities, highlighting the need to unveil the modes of maternal-fetal transmission. The most likely route of vertical ZIKV transmission is via the placenta. The earliest events of ZIKV transmission in the maternal decidua, representing the maternal uterine aspect of the chimeric placenta, have remained unexplored. Here, we show that ZIKV replicates in first-trimester human maternal-decidual tissues grown ex vivo as three-dimensional (3D) organ cultures. An efficient viral spread in the decidual tissues was demonstrated by the rapid upsurge and continued increase of tissue-associated ZIKV load and titers of infectious cellfree virus progeny, released from the infected tissues. Notably, maternal decidual tissues obtained at midgestation remained similarly susceptible to ZIKV, whereas fetus-derived chorionic villi demonstrated reduced ZIKV replication with increasing gestational age. A genome-wide transcriptome analysis revealed that ZIKV substantially upregulated the decidual tissue innate immune responses. Further comparison of the innate tissue response patterns following parallel infections with ZIKV and human cytomegalovirus (HCMV) revealed that unlike HCMV, ZIKV did not induce immune cell activation or trafficking responses in the maternal-fetal interface but rather upregulated placental apoptosis and cell death molecular functions. The data identify the maternal uterine aspect of the human placenta as a likely site of ZIKV transmission to the fetus and further reveal distinct patterns of innate tissue responses to ZIKV. Our unique experimental model and findings could further serve to study the initial stages of congenital ZIKV transmission and pathogenesis and evaluate the effect of new therapeutic interventions.IMPORTANCE In view of the rapid spread of the current ZIKV epidemic and the severe manifestations of congenital ZIKV infection, it is crucial to learn the fundamental mechanisms of viral transmission from the mother to the fetus. Our studies of ZIKV infection in the authentic tissues of the human maternal-fetal interface unveil a route of transmission whereby virus originating from the mother could reach the fe-

The initial events of viral infection at the primary mucosal entry site following horizontal person-to-person transmission have remained ill defined. Our limited understanding is further underscored by the absence of animal models in the case of human-restricted viruses, such as human cytomegalovirus (HCMV) – a leading cause of congenital infection and a major pathogen in immunocompromised individuals. Here we established a novel ex vivo model of HCMV infection in native human nasal turbinate tissues. Nasal-turbinate tissue viability and physiological functionality were preserved for at least seven days in culture. We found that nasal mucosal tissues were susceptible to HCMV infection, with predominant infection of ciliated respiratory epithelial cells. A limited viral spread was demonstrated, involving mainly stromal- and vascular endothelial cells within the tissue. Importantly, functional antiviral and pro-leukocyte-chemotactic signaling pathways were significantly upregulated in the nasal mucosa in response to infection. Conversely, HCMV downregulated the expression of nasal epithelial-cell related genes. We further revealed tissue-specific innate immune response patterns to HCMV, comparing infected human nasal-mucosal and placental tissues, representing the viral entry- and the maternal-to-fetal transmission site, respectively. Taken together, our studies provide insights into the earliest stages of HCMV infection. Studies in this model could help evaluate new interventions against the horizontal transmission of HCMV. IMPORTANCE HCMV is a ubiquitous human pathogen, causing neurodevelopmental disabilities in congenitally infected children and severe disease in immunocompromised patients. The earliest stages of HCMV infection in the human host have remained elusive in the absence of a model for the viral entry site. Here we describe the establishment and use of a novel nasal turbinate organ culture, to study the initial steps of viral infection and the consequent innate immune responses, within the natural complexity and the full cellular repertoire of human nasal mucosal tissues. This model can be applied to examine new antiviral interventions against the horizontal transmission of HCMV and potentially other viruses.

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The data sets related to this article have been deposited in the GEO database under accession number GSE81637.

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