HPLC fingerprinting of natural peptide reactivity of the CTL lines. Total MHC class I-bound peptides were isolated from approximately 10⁹ PCI-13 (filled bars) or PCI-1 (open bars) tumor cells and separated individually on RP-HPLC as described in Materials and Methods. T2 cells (expressing only the HLA-A2 class I allele) were pulsed with peptides from individual HPLC fractions and analyzed for their ability to promote IFN-gamma secretion from CD8+ CTLs (CTL-DC#1 or CTL-DC#2) in 20 h ELISPOT assays. The assays were performed in triplicate and spots were counted using a Zeiss autoimager. The data are reported as the mean ± SD for one representative analysis of 3 independent assays performed.