Production of murine leukemia RL♂1 rejection antigen peptide pRL1a by proteolysis of natural precursor pRL1b

In this study, we demonstrated that NH₂-terminal Ser and Ile residues of pRL1b (SI-pRL1a) (SIIPGLPLSL) are not involved in the recognition by RL♂l-specific cytotoxic T lymphocyte. The sensitization activity observed with pRL1b (SI-pRL1a) was not greater than that of peptides substituted with irrelevant amino acids at these positions. In serum-free medium, pRL1a retained sensitization activity, but pRL1b (SI-pRL1a) did not. Furthermore, addition of bestatin to serum-containing medium blocked sensitization by pRL1b (SI-pRL1a). On the other hand, the addition of captopril enhanced it, probably by inhibiting the degradation of pRL1a by ACE, pRL1a-D peptide with D-Ile in place of the L-Ile residue of pRL1a (IPGLPLSL) showed sensitization, but SI-pRL1a2,3D peptide, which has D-Iles in place of the L-Ile residues of pRL1b (SI-pRL1a), and which was not cleaved between the two D-Iles, did not. The findings suggest that pRL1a is the antigenic peptide bound to L(d) molecules and pRL1b (SI-pRL1a) peptide is its natural precursor, which generates pRL1a via proteolysis.