Background
Influenza virus infection activates NF-κB and is a general prerequisite for a productive influenza virus infection. On the other hand, non-structural protein 1 (NS1) suppresses this viral activated NF-κB, presumably to prevent expression of NF-κB mediated anti-viral response. NS1 proteins of influenza A viruses are divided into two groups, known as allele A and allele B. The possible functional relevance of this NS1 division to viral pathogenicity is lacking.
Findings
The ability of NS1 protein from two avian influenza subtypes, H6N8 and H4N6, to inhibit NF-κB promoter activation was assessed. Further, efforts were made to characterize the genetic basis of this inhibition. We found that allele A NS1 proteins of H6N8 and H4N6 are significantly better in preventing dsRNA induced NF-κB promoter activation compared to allele B of corresponding subtypes, in a species independent manner. Furthermore, the ability to suppress NF-κB promoter activation was mapped to the effector domain while the RNA binding domain alone was unable to suppress this activation. Chimeric NS1 proteins containing either RNA binding domain of allele A and effector domain of allele B or vice versa, were equally potent in preventing NF-κB promoter activation compared to their wt. NS1 protein of allele A and B from both subtypes expressed efficiently as detected by Western blotting and predominantly localized in the nucleus in both A549 and MiLu cells as shown by in situ PLA.
Conclusions
Here, we present another aspect of NS1 protein in inhibiting dsRNA induced NF-κB activation in an allele dependent manner. This suggests a possible correlation with the virus’s pathogenic potential.
Introduction
Within hours of host-pathogen interaction, the type 1 interferons (IFNs), an essential arm of innate immune response, are induced to initiate a range of antiviral processes. The binding of dsRNA, produced as a viral by-product (or administered externally such as poly I:C) to helicases or toll-like receptors (TLR), initiates a series of events culminating in the activation of two kinase complexes: TANK-binding kinase 1-inhibitor of kappa B-kinase ε (TBK1-IKK-ε) and IKK-α/β/γ. TBK1-IKK-ε phosphorylates interferon regulatory factor 3 and 7 (IRF3 and IRF7) while IKK-α/β/γ phosphorylates and hence activates nuclear factor-κB (NF-κB) transcription factor. Activated NF-κB translocates to the nucleus where it induce the transcription of IFN-α and IFN-β as well as other pro-inflammatory cytokines together with ATF2/c-Jun (AP-1), p300 and CBP. NF-κB consists of a family of transcription factors that play indispensable roles in mediating inflammation, immune responses to pathogen infection, proliferation, apoptosis, and other cellular activities. Because of the essential role of NF-κB in stimulation of IFN-α/β synthesis, many viruses have evolved different strategies to subvert this system. The non-structural protein 1 (NS1) of influenza A viruses is one of best example having ability to prevent NF-κB activation.