Cytotoxic T cell recognition of a human melanoma derived peptide with a carboxyl-terminal alanine-proline sequence.

Recently, we defined the antigenic epitope recognized by the human monoclonal antibody L94 to be a protein with a C-terminal sequence of alanine-proline (AP). An antigenic peptide no. 707 (RVAALARDAP), which was identified by the use of cDNA libraries of an antigen positive melanoma cell line M14, was evaluated for cellular immune responses in melanoma patients. PBMC from 16 of 19 melanoma patients were shown to lyse autologous B lymphoblastoid cell lines (BCL) pulsed with synthetic peptide no. 707 (hereafter no. 707). This specific cytotoxicity to the peptide significantly increased in 84% of melanoma patients after in vivo immunization with a melanoma cell vaccine (MCV). In contrast, peptide specific cytotoxicity was observed in only one of 19 normal volunteer donors. In vitro restimulation of MCV treated patients' PBMC with no. 707 augmented cytotoxicity against autologous no. 707-pulsed BCL. This cytotoxicity was specific to the C-terminal sequence AP, since the removal of C-terminal AP completely abolished the specific lysis. no. 707 restimulation of PBMC enhanced cytotoxicity against autologous melanomas. Autologous melanoma and peptide-pulsed BCL targets were lysed by CD8+CTL in a HLA class I-restricted manner. The strong cytotoxicity was obtained from patients of HLA A24. CTL lysis of autologous no. 707-pulsed BCL was partially blocked by unlabeled autologous melanomas in a cold target inhibition test. This suggested that the epitope identical or cross-reactive to no. 707 may be presented on the melanoma cell surface by HLA class I antigens. Our findings suggest that peptide no. 707 presented on human melanoma cells is recognized by CTL and that C-terminal AP plays a critical role in both antibody and T cell recognition.