T2 cells (174 · CEM.T2, ATCC, Manassas, VA, USA) were identified as HLA-A*0201 positive, and they were transporters for the antigen-processingdeficient cells. The cells were maintained in IMDM medium (Invitrogen, Carlsbad, CA, USA) supplemented with 5% FBS (Invitrogen), 100 U/mL penicillin and 100 lg/mL streptomycin (Invitrogen) at 37°C with 5% carbon dioxide. In T2-cell binding assays, the cells (1 · 10 5 ) were pulsed with each of the target NP peptides (10 lg/mL), respectively, in serum-free medium containing the human b 2 -microglobulin (5 lg/mL) for 2 h. The cells were washed with 1· PBS, stained with an FITC-conjugated anti-human HLA-A2 antibody (BD Pharmingen, San Diego, CA, USA), and finally subjected to flow cytometrical analysis (BD FASC Aria II, BD Bioscience, San Jose, CA, USA). The fluorescent index was calculated by the formula as follows: FI = [MFI (T2 + peptide)/MFI (T2 only)] À 1. The flu peptide, GILGFVFTL, was used as a positive control [1] . 