Influenza A virus strains that circulate in humans differ in the ability of their NS1 proteins to block the activation of IRF3 and interferon-β transcription.

We demonstrate that influenza A virus strains that circulate in humans differ markedly in the ability of their NS1 proteins to block the activation of IRF3 and interferon-β transcription. Strong activation occurs in cells infected with viruses expressing NS1 proteins of seasonal H3N2 and H2N2 viruses, whereas activation is blocked in cells infected with viruses expressing NS1 proteins of some, but not all seasonal H1N1 viruses. The NS1 proteins of the 2009 H1N1 and H5N1 viruses also block these activations. The difference in this NS1 function is mediated largely by the C-terminal region of the effector domain, which contains the only amino acid (K or E at position 196) that covaries with the functional difference. Further, we show that TRIM25 binds the NS1 protein whether or not IRF3 activation is blocked, demonstrating that binding of TRIM25 by the NS1 protein does not necessarily lead to the blocking of IRF3 activation.