Identification of a CD4 T‐cell epitope in tumor rejection antigen RLakt on BALB/c radiation‐leukemia RL male 1

We have previously shown that the RLakt antigen was predominantly recognized by CD8 cytotoxic T lymphocytes (CTL) in RL male 1‐bearing or ‐rejected syngeneic BALB/c mice. CD8 CTL were directed to the octamer pRL1a peptide IPGLPLSL of which recognition was H‐2Ld‐restricted. In this study, we identified a CD4 T‐cell epitope peptide in the tumor rejection antigen RLakt on BALB/c radiation‐leukemia RL male 1. Analyses of the recognition of a bulk CD4 T‐cell line using several recombinant RLakt proteins suggested the presence of multiple CD4 T‐cell epitopes in the molecule. However, cloning from a bulk CD4 T‐cell line resulted in only two clones from 200 wells seeded at three cells per well, and those two CD4 T‐cell clones recognized the same epitope peptide in RLakt. The epitope peptide was 14‐mer p12‐25, AYREETLSIIPGLP, and its recognition was H‐2IAd‐restricted. This sequence overlapped with the CD8 T‐cell epitope pRL1a in its N‐terminal 5 amino acid residues. The relationship of the epitope to the pRL1a peptide predominantly recognized by CD8 CTL suggests that the 14‐mer epitope is predominantly recognized by CD4 T‐cells. (Cancer Sci 2008; 99: 1441–1447)