Mutations in the P1A epitope abolished or drastically reduced T cell recognition of the antigen. (a) The AA sequences of the peptides used in the study, with the altered AA shown in bold. (b) Proliferation of transgenic T cells in response to either wild-type or mutant P1A peptide. (c) Cytolysis of P388D1 target cells pulsed with given concentrations of wild-type, mutant, or unrelated control H-2L^d–binding peptides. E/T = 10. (d) P1A mutants lack antagonistic activity for the P1CTL. Activated P1A-specific T cells were used as effectors. The target cells were preincubated with a mixture of wild-type P1A (10 ng/ml) and the indicated concentrations of mutant or wild-type P1A or control peptides. E/T = 10. Data shown were from a 6-hour ⁵¹Cr-release assay and are representative of three to five independent experiments. Ctrl, control.