In vivo inhibition of JAK-STAT signalling enhances high pathogenicity influenza virus replication in ducks.

While rapid death is the usual outcome of high pathogenicity avian influenza virus (HPAIV) infection in gallinaceous poultry, HPAIV-infected ducks usually present milder clinical signs and shed virus for a prolonged time. The difference in disease severity has been linked to a more rapid type I IFN immune response and reduced proinflammatory cytokine expression in ducks compared to chickens. To investigate the role of the early antiviral innate immune response in controlling viral replication in ducks, we evaluated the effects of ruxolitinib, a Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway inhibitor known to dampen IFN signalling in mammals. We first optimized a treatment protocol in 2-week-old ducklings and showed that repeated intracoelomic injections of ruxolitinib significantly decreased IFN-stimulated gene (ISG) mRNA levels in the lung, while oral administration was ineffective. In subsequent infection experiments with an H5N9 HPAIV strain, ruxolitinib treatment led to an earlier peak of viral shedding and increased viral RNA levels in respiratory tissues, which however was not associated with increased expression of clinical signs. Analysis of host immune gene expression in the respiratory tract confirmed a transient suppression of ISG expression coincident with ruxolitinib treatment, followed by a recovery once treatment was ceased. This work provides the first demonstration of the effectiveness of ruxolitinib at inhibiting the antiviral innate immune response in birds, causing increased levels of virus replication when administered to HPAIV-infected ducks.