Imbalance between innate antiviral and pro-inflammatory immune responses may contribute to different outcomes involving low-and highly pathogenic avian influenza H5N3 infections in chickens

In order to gain further insight into the early virus host interactions associated with highly pathogenic avian influenza virus infections in chickens, genome-wide expression profiling of chicken lung and brain was carried out at 24 and 72h post inoculation (h p.i.). For this purpose two recombinant H5N3 viruses were utilized, each possessing a polybasic HA cleavage site but differing in pathogenicity. The original rH5N3 PO virus, which has a low-pathogenic phenotype, was passaged six times through chickens to give rise to the derivative rH5N3 P6 virus, which is highly pathogenic (Diederich S, Berhane Y, Embury-Hyatt C, Hisanaga T, Handel K et al. J Virol 2015;89:10724-10734). The gene-expression profiles in lung were similar for both viruses, although they varied in magnitude. While both viruses produced systemic infections, differences in clinical disease progression and viral tissue loads, particularly in brain, where loads of rH5N3 P6 were three orders of magnitude higher than rH5N3 PO at 72.p.i., were observed. Although genes associated with gene ontology (GO) categories INF alpha and INF beta biosynthesis, regulation of innate immune response, response to exogenous dsRNA, defence response to virus, positive regulation of NF-kappa B import into the nucleus and positive regulation of immune response were up-regulated in rH5N3 PO and rH5N3 P6 brains, fold changes were higher for rH5N3 P6. The additional up-regulation of genes associated with cytokine production, inflammasome and leukocyte activation, and cell cell adhesion detected in rH5N3 P6 versus rH5N3 PO brains, suggested that the balance between antiviral and pro-inflammatory innate immune responses leading to acute CNS inflammation might explain the observed differences in pathogenicity.