1% Tween 20 (v/v) (PBST) and 3% (w/v) non-fat dry milk powder. After three washes with PBST, the blots were incubated for 3 h with convalescent serum obtained from mice sublethally infected with SH1 at a dilution of 1:1000. Membranes were washed three times with PBST and incubated for

1 h at room temperature with a horseradish-peroxidase-conjugated goat anti-mouse IgG (H + L) secondary antibody (Santa Cruz Biotechnology, Inc., Dallas, TX) at a dilution of 1:2000. Then, membranes were rinsed again and protein bands were visualised using the two-component Western Lightning® Plus-ECL GW3965 research buy enhanced chemiluminescence substrate kit (PerkinElmer, Inc., Waltham, MA) and Ultra Cruz™ Autoradiography Blue Films (Santa Cruz Biotechnology, Inc., Dallas, TX). Radiographs were RO4929097 concentration developed on a SRX-101A processor (Konica Minolta, Osaka, Japan). HA content of the VLP samples was determined densitometrically against known concentrations of the SH1-HA protein using ImageJ (National Institutes of Health). Two-fold serial dilutions of PR8:AH1, PR8:SH1, PR8:malNL00, PR8:malAlb01 and PR8:chickJal12 recombinant reassortant virus strains in PBS (50 μL) were prepared in Nunc® 96-well polystyrene

V-bottom microwell plates (Thermo Fisher Scientific, Waltham, MA), followed by the addition of 50 μL 0.5% (v/v) chicken or turkey red blood cells (RBCs) (Lampire Biological Laboratory, Pipersville, PA) in PBS into each well. RBCs were allowed to settle for 45–60 min at 4 °C and the HA titre was determined by visual inspection. Hemagglutination units (HAU) are read as the reciprocal of the last dilution, giving rise to hemagglutination of red blood cells. Baculovirus titres in the VLP vaccine doses were determined by plaque assay on Sf9 cells with minor modifications as described in [24]. Briefly, the assay was carried out in 6-well plates in duplicates. After seeding 1 × 106 cells per well, the cells were allowed to attach to the

surface, Thiamine-diphosphate kinase medium was removed and 200 μL of the diluted VLP vaccine formulations (10-fold dilutions in TNM-FH unsupplemented) were added and incubated for 1 h at 27 °C with periodic shaking. After infection, the samples were removed and cells were overlaid with 2 mL of a solution containing 1% agarose in TNM-FH, 10% (v/v) foetal bovine serum, Penicillin–Streptomycin antibiotic mixture pre-warmed to 37 °C. The plates were incubated at 27 °C for 6 days and plaques were counted after live-cell staining with 200 μL of 5 mg/mL Thiazolyl blue formazan MTT (Sigma, St. Louis, MO) for 3–4 h. SH1-VLPs were prepared in three different concentrations in PBS as per HA content (3 μg, 0.3 μg and 0.03 μg SH1-HA per 50 μL vaccine dose). The AH1-VLP vaccine was prepared at a single concentration (0.3 μg AH1-HA per 50 μL). M1-VLPs served as a negative control and were adjusted to a total protein concentration equal to that of SH1-VLP (0.