Background-From October 2013 to April 2014, French Polynesia experienced the largest Zika virus (ZIKV) outbreak ever described at that time. During the same period, an increase in GuillainBarré syndrome (GBS) was reported, suggesting a possible association between ZIKV and GBS.

Zika virus (ZIKV) is an emerging arbovirus of the Flaviviridae family, which includes dengue, West Nile, yellow fever, and Japanese encephalitis viruses, that causes a mosquito-borne disease transmitted by the Aedes genus, with recent outbreaks in the South Pacific. Here we examine the importance of human skin in the entry of ZIKV and its contribution to the induction of antiviral immune responses. We show that human dermal fibroblasts, epidermal keratinocytes, and immature dendritic cells are permissive to the most recent ZIKV isolate, responsible for the epidemic in French Polynesia. Several entry and/or adhesion factors, including DC-SIGN, AXL, Tyro3, and, to a lesser extent, TIM-1, permitted ZIKV entry, with a major role for the TAM receptor AXL. The ZIKV permissiveness of human skin fibroblasts was confirmed by the use of a neutralizing antibody and specific RNA silencing. ZIKV induced the transcription of Toll-like receptor 3 (TLR3), RIG-I, and MDA5, as well as several interferonstimulated genes, including OAS2, ISG15, and MX1, characterized by strongly enhanced beta interferon gene expression. ZIKV was found to be sensitive to the antiviral effects of both type I and type II interferons. Finally, infection of skin fibroblasts resulted in the formation of autophagosomes, whose presence was associated with enhanced viral replication, as shown by the use of Torin 1, a chemical inducer of autophagy, and the specific autophagy inhibitor 3-methyladenine. The results presented herein permit us to gain further insight into the biology of ZIKV and to devise strategies aiming to interfere with the pathology caused by this emerging flavivirus. IMPORTANCEZika virus (ZIKV) is an arbovirus belonging to the Flaviviridae family. Vector-mediated transmission of ZIKV is initiated when a blood-feeding female Aedes mosquito injects the virus into the skin of its mammalian host, followed by infection of permissive cells via specific receptors. Indeed, skin immune cells, including dermal fibroblasts, epidermal keratinocytes, and immature dendritic cells, were all found to be permissive to ZIKV infection. The results also show a major role for the phosphatidylserine receptor AXL as a ZIKV entry receptor and for cellular autophagy in enhancing ZIKV replication in permissive cells. ZIKV replication leads to activation of an antiviral innate immune response and the production of type I interferons in infected cells. Taken together, these results provide the first general insights into the interaction between ZIKV and its mammalian host.

In December 2013, during a Zika virus (ZIKV) outbreak in French Polynesia, a patient in Tahiti sought treatment for hematospermia, and ZIKV was isolated from his semen. ZIKV transmission by sexual intercourse has been previously suspected. This observation supports the possibility that ZIKV could be transmitted sexually.

Zika virus (ZIKV) was discovered in 1947 and was thought to lead to relatively mild disease. The recent explosive outbreak of ZIKV in South America has led to widespread concern with reports of neurological sequelae ranging from Guillain Barré syndrome to microcephaly. ZIKV infection has occurred in areas previously exposed to dengue, a flavivirus closely related to ZIKV. Here we investigate the serological crossreaction between the two viruses. Dengue immune plasma substantially crossreacted with ZIKV and could drive antibody-dependent enhancement of ZIKV infection. Using a panel of human anti-dengue monoclonal antibodies we showed that most antibodies reacting to dengue envelope protein also reacted to ZIKV. Antibodies to linear epitopes including the immunodominant fusion loop epitope while able to bind ZIKV could not neutralize the virus but instead promoted ADE. These data indicate that dengue immunity may drive higher ZIKV replication and have clear implications for disease pathogenesis and future ZIKV and dengue vaccine programs.

Zika virus (ZIKV) is an arthropod-borne enveloped virus belonging to the Flavivirus genus in the family Flaviviridae, which also includes the human pathogenic yellow fever, dengue, West Nile and tick-borne encephalitis viruses 1 . Flaviviruses have two structural glycoproteins, prM and E (for precursor membrane and envelope proteins, respectively), which form a heterodimer in the endoplasmic reticulum (ER) of the infected cell and drive the budding of spiky immature virions into the ER lumen. These particles transit through the cellular secretory pathway, during which the trans-Golgi-resident protease furin cleaves prM. This processing is required for infectivity, and results in the loss of a large fragment of prM and reorganization of E on the virion surface. The mature particles have a smooth aspect, with 90 E dimers organized with icosahedral symmetry in a 'herringbone' pattern 2,3 .Three-dimensional cryo-electron microscopy (cryo-EM) structures of the mature ZIKV particles have recently been reported to near atomic resolution (3.8 Å) 4,5 , showing that the virus has essentially the same organization as the other flaviviruses of known structure, such as dengue virus (DENV) 3 and West Nile virus 6,7 . The E protein is about 500 amino acids long, with the 400 N-terminal residues forming the ectodomain essentially folded as β-sheets with three domains, named I, II and III, aligned in a row with domain I at the centre. The conserved fusion loop is at the distal end of the rod in domain II, buried at the E dimer interface. At the C terminus, the E ectodomain is followed by the 'stem' , featuring two α-helices lying flat on the viral membrane (the stem helices), which link to two C-terminal transmembrane α-helices. The main distinguishing feature of the ZIKV virion is an insertion within a glycosylated loop of E (the '150' loop), which protrudes from the mature virion surface 4,5 .Flaviviruses have been grouped into serocomplexes based on cross-neutralization studies with polyclonal immune sera 8 . The E protein is the main target of neutralizing antibodies, and is also the viral fusogen; cleavage of prM allows E to respond to the endosomal pH by undergoing a large-scale conformational change that catalyses membrane fusion and releases the viral genome into the cyotosol. Loss of the precursor fragment of prM lets the E protein fluctuate from its tight packing at the surface of the virion, transiently exposing otherwise buried surfaces. One surface exposed by this 'breathing' is the fusionloop epitope (FLE), which is a dominant cross-reactive antigenic site 9 . Although antibodies to this site can protect by complement-mediated mechanisms, as shown in a mouse model for West Nile virus 10 , they are poorly neutralizing and lead to antibody-dependent enhancement 11-15 , thereby aggravating Flavivirus pathogenesis and complicating the development of safe and effective vaccines.We recently reported the functional and structural characterization of a panel of antibodies isolated from patients with dengue disease 13,16 . ...

Since October 2013, French Polynesia has experienced the largest documented outbreak of Zika virus (ZIKAV) infection. To prevent transmission of ZIKAV by blood transfusion, specific nucleic acid testing of blood donors was implemented. From November 2013 to February 2014: 42 (3%) of 1,505 blood donors, although asymptomatic at the time of blood donation, were found positive for ZIKAV by PCR. Our results serve to alert blood safety authorities about the risk of post-transfusion Zika fever.

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