Identification of a tumor-specific shared antigen derived from an eph receptor and presented to CD4 T cells on HLA class II molecules

We obtained a lytic CD4 T-cell clone that recognized an antigen presented b y HLA-DRB1*1101 on the tumor cells of a melanoma patient who enjoyed an unu sually favorable clinical evolution. The antigen appeared to be shared betw een several melanoma cell lines. To identify the encoding gene, we used a n ew method, based on the cotransfection into human embryonal kidney cell lin e 293 of a cDNA library from the tumor together with a cDNA clone encoding the class II transactivator, which induces the expression of HLA class LI m olecules. The product of the gene coding for the antigenic peptide is EphA3 , a member of the Eph family of tyrosine kinase receptors, which mediate th e repulsion of neural cells by cells carrying the ligand Ephrins on their s urface. EphA3 is expressed at a high level in the retina and fetal brain, a t a lower level in several normal tissues, and not at all in hematopoietic cells, the only cells that constitutively express HLA class II molecules. I t is overexpressed in several types of tumors, including melanoma, lung car cinoma, and sarcoma, On the basis of this pattern of expression, EphA3 may be a source of tumor-specific antigens recognized on tumor cells that expre ss HLA class II molecules. Anti-EphA3 T cells may have participated in a tu mor rejection response in the patient, because the cells of metastases coll ected several years later than the metastasis used to characterize the anti gen had lost expression of HLA-DR or EphA3, therefore escaping recognition by these lymphocytes.