Latency is a hallmark of herpesviruses, allowing them to persist into their host without virions production. Acute exposure to hypoxia (below 3% O 2 ) was identified as a trigger of latent-to-lytic switch (reactivation) for human oncogenic gamma-herpesviruses (KSHV and EBV). Therefore, we hypothesized that hypoxia could also induce reactivation of Marek’s disease virus (MDV), sharing biological properties with EBV and KSHV (notably oncogenic properties), into lymphocytes. Acute exposure to hypoxia (1% O 2 ) of two MDV-latently infected cell lines derived from MD tumors (3867K and MSB-1) induced MDV reactivation. A bioinformatic analysis of the RB-1B MDV genome revealed 214 putative hypoxia-response element consensus sequences on 119 open reading frames. RT-qPCR analysis showed five MDV genes strongly upregulated early after hypoxia. In 3867K cells under normoxia, pharmacological agents mimicking hypoxia (MLN4924 and CoCl 2 ) increased MDV reactivation, but to a lower level than real hypoxia. Overexpression of wild-type or stabilized human hypoxia inducible factor-1α (HIF-1α) in MSB-1 cells in normoxia also promoted MDV reactivation. In such conditions, lytic cycle was detected in cells with a sustainable HIF-1α expression, but also in HIF-1α negative cells, indicating that MDV reactivation is mediated by HIF-1, in a direct and/or indirect manner. Lastly, we demonstrated by a reporter assay that HIF-1α overexpression induced the transactivation of two viral promoters, shown upregulated in hypoxia. These results suggest that hypoxia may play a crucial role in the late lytic replication phase observed in vivo in MDV-infected chickens exhibiting tumors, since a hypoxic microenvironment is a hallmark of most solid tumors. IMPORTANCE Latent-to-lytic switch of herpesviruses (aka reactivation) is responsible for pathology recurrences and/or viral shedding. Studying physiological triggers of reactivation is therefore important for health to limit lesions and viral transmission. Marek's disease virus (MDV) is a potent oncogenic alpha-herpesvirus establishing latency in T-lymphocytes and causing lethal T-lymphomas in chickens. In vivo , a second lytic phase is observed during tumoral stage. Hypoxia being a hallmark of tumors, we wondered whether hypoxia induces MDV reactivation in latently-infected T-lymphocytes, like previously shown for EBV and KSHV in B-lymphocytes. In this study, we demonstrated that acute hypoxia (1% O2) triggers MDV reactivation in two MDV transformed T-cell lines. We provide some molecular basis of this reactivation by showing that hypoxia inducible factor (HIF-1) overexpression induces MDV reactivation to a similar extend than hypoxia after 24 hours. Hypoxia is therefore a reactivation stimulus shared by mammalian and avian oncogenic herpesviruses of different genus.

Protocols allowing the in vitro culture of human hair follicles in a serum free-medium up to 9 days were developed 30 years ago. By using similar protocols, we achieved the prolonged maintenance in vitro of juvenile feather follicles (FF) microdissected from young chickens. Histology showed a preservation of the FF up to 7 days as well as feather morphology compatible with growth and/or differentiation. The integrity of the FF wall epithelium was confirmed by transmission electron microscopy at Day 5 and 7 of culture. A slight elongation of the feathers was detected up to 5 days for 75% of the examined feathers. By immunochemistry, we demonstrated the maintenance of expression and localization of two structural proteins: scaffoldin and fibronectin. Gene expression (assessed by qRT-PCR) of NCAM, LCAM, Wnt6, Notch1, and BMP4 was not altered. In contrast, Shh and HBS1 expression collapsed, DKK3 increased, and KRT14 transiently increased upon cultivation. This indicates that cultivation modifies the mRNA expression of a few genes, possibly due to reduced growth or cell differentiation in the feather, notably in the barb ridges. In conclusion, we have developed the first method that allows the culture and maintenance of chicken FF in vitro that preserves the structure and biology of the FF close to its in vivo state, despite transcriptional modifications of a few genes involved in feather development. This new culture model may serve to study feather interactions with pathogens or toxics and constitutes a way to reduce animal experimentation.

In vivo bioluminescence imaging facilitates the non-invasive visualization of biological processes in living animals. This system has been used to track virus infections mostly in mice and ferrets; however, until now this approach has not been applied to pathogens in avian species. To visualize the infection of an important avian pathogen, we generated Marek’s disease virus (MDV) recombinants expressing firefly luciferase during lytic replication. Upon characterization of the recombinant viruses in vitro, chickens were infected and the infection visualized in live animals over the course of 14 days. The luminescence signal was consistent with the known spatiotemporal kinetics of infection and the life cycle of MDV, and correlated well with the viral load measured by qPCR. Intriguingly, this in vivo bioimaging approach revealed two novel sites of MDV replication, the beak and the skin of the feet covered in scales. Feet skin infection was confirmed using a complementary fluorescence bioimaging approach with MDV recombinants expressing mRFP or GFP. Infection was detected in the intermediate epidermal layers of the feet skin that was also shown to produce infectious virus, regardless of the animals’ age at and the route of infection. Taken together, this study highlights the value of in vivo whole body bioimaging in avian species by identifying previously overlooked sites of replication and shedding of MDV in the chicken host.