Influenza Virus: A Single Noninfectious Interferon Induction-Suppressing Particle Blocks Expression of Interferon-Inducing Particles

Influenza virus subpopulations: exchange of lethal H5N1 virus NS for H1N1 virus NS triggers de novo generation of defective-interfering particles and enhances interferon-inducing particle efficiency

Reassortment of influenza A viruses is known to affect viability, replication efficiency, antigenicity, host range, and virulence, and can generate pandemic strains. In this study, we demonstrated that the specific exchange of the NS gene segment from highly pathogenic A/HK/156/97 (H5N1) [E92 or E92D NS1] virus for the cognate NS gene segment of A/PR/834(H1N1) [D92 NS1] virus did not cause a significant change in the sizes of infectious particle subpopulations. However, it resulted in 2 new phenotypic changes: (1) de novo generation of large subpopulations of defective-interfering particles (DIPs); and (2) enhancement of interferon (IFN)-inducing particle efficiency leading to an order of magnitude or higher quantum (peak) yield of IFN in both avian and mammalian cells. These changes were attributed to loss of function of the H5N1-NS gene products. Most notably, the NS exchange obliterated the usual IFN-induction-suppressing capacity associated with expression of full-size NS1 proteins, and hence functionally mimicked deletions in the NS1 gene. The loss of NS1-mediated suppression of IFN induction, de novo generation of DIPs, and the concomitant enhancement of IFN-inducing particle efficiency suggest that in an attenuated background, the H5N1-NS could be used to formulate a self-adjuvanting live attenuated influenza vaccine similar to viruses with deletions in the NS1 gene.

Influenza virus interferon-inducing particle efficiency is reversed in avian and mammalian cells, and enhanced in cells co-infected with defective-interfering particles

Naturally selected variants of influenza virus encoding truncated NS1 proteins were tested in chickens as candidate live-attenuated influenza vaccines. Their effectiveness correlated with the amount of interferon (IFN) induced in chicken cells. Effective variants induced large amounts of IFN and contained subpopulations with high ratios of defective-interfering particles:IFN-inducing particles (DIP:IFP). Ineffective variants induced less IFN and contained lower ratios of DIP:IFP. Unexpectedly, there was a reversal of phenotypes in mammalian cells. Variants that induced low amounts of IFN and had low DIP:IFP ratios in chicken cells were excellent IFN inducers with high DIP:IFP ratios in mammalian cells, and vice versa. The high DIP:IFP ratios and computer-simulated dynamics of infection suggested that DIP, as an individual particle, did not function as an IFP. The higher efficiency of IFPs in the presence of DIPs was attributed to reduced amounts of newly synthesized viral polymerase known to result from out-competition by defective-interfering RNAs, and the subsequent failure of that polymerase to turn-off cellular mRNA transcription-including IFN-mRNA.