Strong innate immune response and cell death in chicken splenocytes infected with genotype VIId Newcastle disease virus

Background: Genotype VIId Newcastle disease virus (NDV) isolates induce more severe damage to lymphoid tissues, especially to the spleen, when compared to virulent viruses of other genotypes. However, the biological basis of the unusual pathological changes remains largely unknown. Methods: Virus replication, cytokine gene expression profile and cell death response in chicken splenocytes infected with two genotype VIId NDV strains (JS5/05 and JS3/05), genotype IX NDV strain F48E8 and genotype IV NDV strain Herts/33 were evaluated. Statistical significance of differences between experimental groups was determined using the Independent-Samples T test. Results: JS5/05 and JS3/05 caused hyperinduction of type I interferons (IFNs) (IFN-alpha and -beta) during detection period compared to F48E8 and Herts/33. JS5/05 increased expression level of IFN-gamma gene at 6 h post-inoculation (pi) and JS3/05 initiated sustained activation of IFN-gamma within 24 h pi, whereas transcriptional levels of IFN-gamma remained unchanged at any of the time points during infection of F48E8 and Herts/33. In addition, compared to F48E8 and Herts/33, JS3/05 and JS5/05 significantly increased the amount of free nucleosomal DNA in splenocytes at 6 and 24 h pi respectively. Annexin-V and Proidium iodid (PI) double staining of infected cells showed that cell death induced by JS3/05 and JS5/05 was characterized by marked necrosis compared to F48E8 and Herts/33 at 24 h pi. These results indicate that genotype VIId NDV strains JS3/05 and JS5/05 elicited stronger innate immune and cell death responses in chicken splenocytes than F48E8 and Herts/33. JS5/05 replicated at a significantly higher efficiency in splenocytes than F48E8 and Herts/33. Early excessive cell death induced by JS3/05 infection partially impaired virus replication. Conclusions: Viral dysregulaiton of host response may be relevant to the severe pathological manifestation in the spleen following genotype VIId NDV infection.