Breeding pairs of MBL wild-type (WT) and MBL knockout (KO) mice on C57B6/J were provided by Dr Takahashi [27] . They were maintained under specific pathogen-free conditions in the animal facilities of the Laboratory Animal Unit, The University of Hong Kong. Female mice were used at 6-10 weeks of age. They were anesthetized and inoculated intranasally with 30 lL of 10 3 TCID 50 pdmH1N1 virus, 10 5 TCID 50 H9N2/G1 virus, or PBS at day 0. Virus-infected or mocktreated mice were weighed daily. All animal care and experiments were conducted in accordance with the Committee on the Use of Live Animals in Teaching and Research guidelines of the University of Hong Kong.