Respiratory Syncytial Virusand Influenza are examples of viruses currently being developed as IFN-sensitive attenuated Regorafenib vaccine candidates. Deletion of RSV IFN antagonists NS1 and NS2 impairs virus Z-VAD-FMK growth in MRC5 cellsbut Ruxolitinib increased plaque size formation in both viruses to that equivalent of MRC5/PIV5-V cells. Therefore IFN inhibitors could be useful in the industrial production of IFNsensitive attenuated RSV vaccine candidates particularly in light of our previous data demonstrating that higher yields of RSV can be achieved in human-derived PIV5-V expressing cells rather than Vero cells. In addition the ability to grow RSV in a cell-line other than Vero cells could be important for vaccine production because virions produced from Vero cells contain a C-terminally truncated 55KDa G glycoprotein which is responsible for a significant reductionin initial infectivity particularly in primary respiratory epithelial target cells. Therefore the use of IFN inhibitors to facilitate the production of candidate RSV vaccines in a cell-line other than Veros would not only increase virus yield but could also reduce the required vaccine inoculum. Plaque size of wild-typeRSV also increased in the presence of Ruxolitinib. This supports our previous observation that inhibiting the IFN response aids the growth of some intrinsically slow growing virusesand could potentially facilitate more rapid isolation of viruses from clinical viral samples. Wild-type influenzavirus plaque size was not increased by Ruxolitinib, presumably because Influenza virus is a fast growing virus that encodes a powerful IFN antagonist the NS1 protein. However, Ruxolitinib significantly increased the plaque size of a recombinant A/PR/8/34 DNS1 virus that does not encode NS1. We also tested two traditional vaccine strains, measlesEdmonson and the MumpsEnders, which have been generated empirically using nonsystematic attenuation methods. Plaque size of the MeV and MuV vaccine strains were significantly increased in the presence of Ruxolitinib. MeV vaccine strains contain attenuating mutations in the P, V and C proteins that contribute to IFN antagonism. However, MuV 
Enders contains a functional V protein IFN antagonist, providing evidence that IFN inhibitors can boost the yield of viruses with reduced replication rates due to attenuating mutations that do not affect viral IFN antagonists, presumably due to the balance between kinetics of virus replication and induction of the IFN response. This is in agreement with our previous work, which demonstrated that RSV viruses with mutations in G and SH proteins whose functions are not directly relevant to the IFN response grew better in PIV5-V expressing cells. We have demonstrated that several IFN inhibitors increased virus growth in vitro. In the initial plaque formation screen the JAK1/2 inhibitor Ruxolitinib was the most effective and hence was taken forward for further study. Moreover, all the results obtained for Ruxolitinib were essentially mirrored with the IKK2 inhibitor TPCA-1. The plaque assays and growth curves performed required incubation with the inhibitor for multiple days. To ensure our results were not affected by loss of activity of the drug, we used the A549/pr.GFP and A549/ prGFP reporter cell-lines to measure the activity of the drug over time; confirming that the inhibitory effect of both Ruxolitinib and TPCA-1 was stable up to at least 7 days in tissue-culture. These results provide proof of principle that supplementing tissue-culture medium with IFN inhibitors provides a simple, effective and flexible approach to enhance virus growth in cell-lines of choice.