The pandemic (H1N1) 2009 influenza virus is resistant to mannose-binding lectin

BACKGROUND: Mannose-binding lectin (MBL) is an important component of innate immunity because it promotes bacterial clearance and neutralization of human influenza A viruses. Since a majority of humans have no neutralizing antibody against the pandemic (H1N1) 2009 influenza (pandemic 2009) virus, innate immunity may be crucial and MBL susceptibility may therefore influence viral pathogenesis. RESULTS: We examined MBL susceptibility of influenza A viruses and observed that the pandemic 2009 virus was resistant to MBL, whereas all seasonal influenza A viruses tested were susceptible. The mortality of mice infected with a seasonal H1N1 influenza virus was evidently enhanced on transient blockage of MBL activity by simultaneous inoculation of mannan, whereas mannan inoculation had no effect on mice infected with a pandemic 2009 virus. This indicates that MBL protects mice against infection with the seasonal virus but not against that with the pandemic 2009 virus. CONCLUSIONS: These results indicate that the pandemic 2009 virus is not susceptible to MBL, an important component of innate immunity.