Influenza virus B/Osaka/32/2009 was kindly provided by Osaka Prefectural Institute of Public Health. Madin–Darby canine kidney (MDCK) cells were obtained from the American Type Culture Collection (Manassas, VA) and were grown in minimum essential medium (MEM; Invitrogen, Carlsbad, CA) supplemented with 10% fetal bovine serum (Invitrogen) and 100 μg/ml kanamycin sulfate (Invitrogen) in a humidified atmosphere of 5% CO2 at 37 °C. Approximately 7- to 8-month-old female ferrets were purchased from Marshall Bioresources Japan Inc. (Ibaragi, Japan) and Japan SLC Inc. (Shizuoka, Japan). The experiments were performed under applicable laws and guidelines and after approval

from the Shionogi Animal Care and Use Committee. Under anesthesia, at least 1 week before virus inoculation, a data logger (DS1921H-F5;

Maxim Integrated Products, Inc., KRX-0401 molecular weight Sunnyvale, CA) was subcutaneously implanted into each learn more ferret to monitor body temperature as previously reported [14]. The absence of influenza A/California/7/2009 (H1N1), A/Victoria/210/2009 (H3N2), and B/Brisbane/60/2008 virus-specific antibody in serum from each ferret was confirmed by hemagglutination inhibition (HI) test before the first immunization. HI assay was performed according to the protocol previously reported [14]. Serum was treated with receptor-destroying enzyme (RDEII; Denka Seiken, Tokyo, Japan). Serially diluted sera were mixed with 4 HA units of virus antigen for 1 h at room temperature. The mixture was then incubated with 0.5% chicken red blood cells for 30 min at room temperature. The HI titers were expressed as reciprocals of the highest dilution of serum samples that completely inhibited hemagglutination. Ferrets were subcutaneously see more immunized with 22.5 μg of SV, 22.5 μg of SV adjuvanted with 50–800 μg of sHZ (SV/sHZ (50–800 μg)) or premix solution Fluad, which

is composed of 22.5 μg of SV and MF59. Second immunizations were conducted 28 days after the first immunization. Serum was collected by vena cava puncture on the day of the first immunization and 7, 14, 21, 28, and 35 days after the first immunization, and HI titers against three HA antigens, A/California/7/2009 (H1N1), A/Victoria/210/2009 (H3N2), and B/Brisbane/60/2008, were determined. Ferrets were subcutaneously immunized with saline or 22.5 μg of SV adjuvanted with 800 μg of sHZ. Body temperatures were monitored every 15 min with the data logger implanted in the ferrets. Under anesthesia, ferrets were inoculated intranasally with B/Osaka/32/2009 (1.0 × 104 TCID50) in 400 μl of phosphate-buffered saline (PBS). To monitor virus replication in nasal cavities, nasal washes were collected from infected ferrets on days 1 to 6 after infection. The collected samples were stored at below −80 °C until use. For virus titration, serial dilutions of nasal washes were inoculated onto confluent MDCK cells in 96-well plates. After 1 h incubation, the suspension was removed, and the cells were cultured in MEM including 0.