GP100 PMEL-17 IS A MURINE TUMOR REJECTION ANTIGEN - INDUCTION OF SELF-REACTIVE, TUMORICIDAL T-CELLS USING HIGH-AFFINITY, ALTERED PEPTIDE LIGAND/

Many tumor-associated antigens are nonmutated, poorly immunogenic tiss ue differentiation antigens. Their weak immunogenicity may be due to ' 'self'-tolerance. To induce autoreactive T cells, we studied immune re sponses to gp100/pmel 17, an antigen naturally expressed by both norma l melanocytes and melanoma cells. Although a recombinant vaccinia viru s (rVV) encoding the mouse homologue of gp100 was nonimmunogenic, immu nization of normal C57BL/6 mice with the rVV encoding the human gp100 elicited a specific CD8+ T cell response. These lymphocytes were cross -reactive with mgp 100 in vitro and treated established Bib melanoma u pon adoptive transfer. To understand the mechanism of the greater immu nogenicity of the human version of gp100, we characterized a 9-amino a cid (AA) epitope, restricted by H-2D(b), that was recognized by the T cells. The ability to induce specific T cells with human but not mouse gp100 resulted from differences within the major histocompatibility c omplex (MHC) class I-restricted epitope and not from differences elsew here in the molecule, as was evidenced by experiments in which mice we re immunized with rVV containing minigenes encoding these epitopes. Al though the human (hgp100(25-33)) and mouse (mgp 100(25-33)) epitopes w ere homologous, differences in the three NH2-terminal AAs resulted in a 2-log increase in the ability of the human peptide to stabilize ''em pty'' Db On RMA-S cells and a 3-log increase in its ability to trigger interferon gamma release by T cells. Thus, the fortuitous existence o f a peptide homologue with significantly greater avidity for MHC class I resulted in the generation of self-reactive T cells. High-affinity, altered peptide ligands might be useful in the rational design of rec ombinant and synthetic vaccines that target tissue differentiation ant igens expressed by tumors.