A mutation in the receptor binding site enhances infectivity of 2009 H1N1 influenza hemagglutinin pseudotypes without changing antigenicity

The 2009 H1N1 pandemic highlights the need to better understand influenza A infectivity and antigenicity. Relative to other recent seasonal H1N1 influenza strains, the 2009 H1N1 virus grew less efficiently in eggs, which hindered efforts to rapidly supply vaccine. Using lentiviral pseudotypes bearing influenza hemagglutinin (HA-pseudotypes) we evaluated a glutamine to arginine mutation at position 223 (Q223R) and glycosylation at residue 276 in HA for their effects on infectivity and neutralization. Q223R emerged during propagation in eggs and lies in the receptor binding site. We found that the Q223R mutation greatly enhanced infectivity of HA-pseudotypes in human cells, which was further augmented by inclusion of the viral neuraminidase (NA) and M2 proteins. Loss of glycosylation at residue 276 did not alter infectivity. None of these modifications affected neutralization. These findings provide information for increasing 2009 H1N1HA-pseudotype titers without altering antigenicity and offer insights into receptor use.