Influenza virus A/California/04/2009 (pdmH1N1) were propagated in embryonated chicken eggs and purified by ultracentrifugation with minor modification of our previous work [4, 24] . Influenza virus A/Quail/Hong Kong/G1/97 (H9N2/G1) was grown in Madin-Darby canine kidney (MDCK) cells with modified Eagle's medium (Invitrogen) containing 2 lg/mL N-p-Tosyl-L-phenylalanine chloromethyl ketone (TPCK)treated trypsin (Sigma-Aldrich). Virus stocks were purified by adsorption to and elution from turkey red blood cells and stored at 280°C until use as previously described [25] . The determination of virus titer was performed by titrating virus in MDCK cells, with daily observation of cytopathic effect and confirmation by hemagglutination assay. The tissue culture infective dose affecting 50% of the cultures (TCID 50 ) was calculated by the Reed-Muench formula. Ultraviolet (UV)-irradiated virus was prepared by irradiation with energy of 0.2 J in a UV crosslinker as described previously [26] .