Identification of tumor rejection antigen peptide recognized by cytotoxic T lymphocytes on murine leukemia RL♂1 and the effect of vaccination

BALB/c radiation-induced leukemia RL♂1 is an immunogenic tumor. We identified the antigen peptides pRL1a and pRL1b which are recognized by RL♂1-specific cytotoxic T lymphocyte (CTL) by acid extraction, HPLC purification, and direct sequencing. pRL1 peptides were shown to be derived from an untranslated region of the normally expressed akt gene. The murine leukemia virus (MuLV) long terminal repeat (LTR) sequence is inserted directly into the exon of c-akt in RL♂1 leukemia and transcription starts from the cap site of the LTR. Translation appeared to start from the ATG codon created in the six nucleotides of unknown origin which had been inserted into the LTR/akt junction. We demonstrated that in vivo and in vitro sensitization with pRL1a multiple antigen peptide (MAP), but not pRL1a peptide itself, generated pRL1a-specific CTL in spleen cells from BALB/c mice. Furthermore, in vivo sensitization of BALB/c mice with pRL1a MAP, but not pRL1a peptide itself, showed a marked inhibitory effect on RL♂1 tumor growth. Depletion of macrophages by treatment with carrageenan on in vivo sensitization with pRL1a MAP abrogated in vitro CTL generation. Selective depletion of CD4⁺ T cells from mice by treatment with anti-CD4 monoclonal antibody (mAb) on in vivo sensitization also abrogated in vitro CTL generation.