Recently, two new influenza A-like viruses have been discovered in bats, HL17NL10 and HL18NL11. The hemagglutinin-like (HL) and neuraminidase like (NL) proteins of these viruses lack hemagglutination and neuraminidase activities despite their sequence and structural homologies with the HA and NA proteins of conventional influenza A virus. We now have investigated whether the NS1 proteins of HL17NL10 and HL18NL11 viruses can functionally substitute the NS1 protein of a conventional influenza A virus. For this purpose we generated recombinant influenza A/PR8/34 (PR8) H1N1 viruses containing the NS1 protein of PR8 WT, HL17NL10 and HL18NL11 viruses. These viruses (r/NS1PR8, r/NS1HL17, r/NS1HL18) were tested for replication in bat and non-bat mammalian cells and in mice. Our results demonstrate that r/NS1HL17 and r/NS1HL18 viruses are attenuated in vitro and in vivo However, bat NS1 recombinant viruses showed similar phenotypes as r/NS1PR8 virus in STAT1-/- human A549 cells and mice, unable to respond to IFN. Interestingly, multiple mouse passages of r/NS1HL17 and r/NS1HL18 viruses resulted in selection of mutant viruses containing single amino acid mutations in the viral PB2 protein. In contrast to the parental viruses, the selected PB2 mutants restored virulence and IFN antagonism. Our results indicate that the NS1 protein of bat influenza A-like viruses is less efficient than its conventional influenza A virus NS1 counterpart in antagonizing the IFN response, and that this deficiency can be overcome by the influenza virus PB2 protein.ImportanceSignificant gaps are still uncovered in our understanding of the basic features of the recently discovered bat influenza A-like viruses, HL17NL10 and HL18NL11. These unique viruses display both similarities and differences in basic biology compared to conventional influenza A viruses. In here, we show that recombinant influenza A viruses containing the NS1 protein from HL17NL10 and HL18NL11 are attenuated. This attenuation was mediated by their inability to antagonize the type I IFN response. However, this deficiency could be compensated by single amino acid replacements in the PB2 gene. Our results unravel a functional divergence between the NS1 proteins of bat influenza A-like and conventional influenza A viruses, and demonstrate an interplay between the viral PB2 and the NS1 protein to antagonize IFN.