Adaptive Amino Acid Substitutions Enhance the Virulence of a Novel Swine-Origin Canine H1N1 Influenza Virus in Mice

Abstract Swine-origin H1N1 influenza virus caused an outbreak in human in 2009 (H1N1/pdm09). We isolated several novel reassortant influenza A viruses bearing the H1N1/pdm09 segments (PB2, PB1, PA and NP) from dogs, which showed high similarity with H1N1 swine influenza virus in the previous study. To understand the molecular basis of swine-origin H1N1 reassortant canine influenza virus adaptation to new host species, the mouse-adapted variants of H1N1 canine virus were obtained by serial lung-to-lung passages in BALB/c mice. After thirteen passages, the virus significantly enhanced replication in MDCK cells and became lethal to mice. Six adaptive mutations mapped to two in the ribonucleoprotein (RNP) complex (PB2-E578D, PA-T97I), two in hemagglutinin (HA-N198D and A227E), two in the non-structural protein 1 and nuclear export protein (NS1-A53D and NEP-R42K), indicating that they may contribute to the increased virulence and replication. The results provided valuable insights into how the swine-origin influenza virus adapt to a new host that deserves more attentions.