MHC class I-restricted presentation of Tag epitopes by murine gastric carcinoma cell lines. (A) Epitope-specific CTL generated in C57BL/6 mice by DNA immunization with a Tag expression vector and subsequent expansion by in vitro stimulation with Tag peptide-loaded RBL5 cells were incubated with irradiated 424GC, 424 fibroblasts, RBL5 and Tag T1, T2/3 or T4 peptide-pulsed RBL5 cells for 24 h and their IFNγ secretion into the culture media was determined by ELISA. Secretion of IFNγ by CTL stimulated with 424GC cells indicates that these cells present SV40Tag-specific peptides in an MHCI-restricted manner. (B) Coculture of 424GC and 424 fibroblasts were treated with 107 Tag-specific CTL in a petri dish for 48 h. Thereafter, non-adherent (dead) cells were removed. Left, coculture before CTL treatment; right, coculture after treatment. Arrows indicate the position of the tumor cells before addition of CTL.