USE OF COMBINATORIAL PEPTIDE LIBRARIES TO CONSTRUCT FUNCTIONAL MIMICSOF TUMOR EPITOPES RECOGNIZED BY MHC CLASS I-RESTRICTED CYTOLYTIC T-LYMPHOCYTES

Identification of cytolytic T lymphocyte (CTL) epitopes presented by m ajor histocompatibility complex (MHC) class I molecules on tumor cells is critical for the design of active immunotherapy. We describe the u se of combinatorial peptide libraries with defined amino acids in two MHC anchor positions to search for epitopes that are recognized by H-2 D(b)- and K-b-restricted CTL specific for the mouse lymphoma EL4. AII iterative strategy was used for screening libraries in which 16 amino acids were divided into 3 groups and 3 subgroups: alpha(AL, VT, FY); b eta(GS, P, DE); gamma(KR, H, NQ). The proportions of each group and su bgroup at individual peptide positions were changed in the library syn thesis, and the effect of these changes on CTL activity was measured i n a sensitive RMA-S cell assay. A single H-2D(b) epitope mimic was ded uced from the original library that contained >2 x 10(8) potential pep tides and was at least 9 logs more potent than the original library. I mmunization of syngeneic mice with this peptide elicited CTL that lyse d EL4 cells as well as RMA-S cells pulsed with pi-prides isolated from Db molecules of EL4 cells, indicating functional similarity between t he mimicking peptide and the naturally processed CTL epitope. Furtherm ore, adoptive transfer of such a CTL line had a therapeutic effect in mice with EL4 established as an ascites tumor. Two H-2K(b)-restricted epitope mimics of the same tumor were also identified. Our method repr esents a novel approach for the construction of MHC class I-restricted targets that can serve as immunogens for active immunotherapy of canc er.