During an analysis of T-cell responses against human renal cell carcinoma (RCC), we identified a CD4(+) T-cell line that showed TCR-mediated recognition and lysis of nearly all RCC lines regardless of MHC type. We have now elucidated the nature of the ligand for this α/β TCR, and it contains no MHC-related moiety and does not involve classic peptide processing. First, matrix metalloproteinase 14 (MMP14) expressed on RCC cells releases membrane-bound TRAIL expressed by the T cell; then, soluble TRAIL binds to its receptor DR4 (TRAIL-R1), which is expressed on tumor cells, and this TRAIL-DR4 complex is recognized by the TCR through a complementarity-determining region 3α (CDR3α)-mediated interaction. Direct and specific antigen-TCR interaction was demonstrated when the immobilized recombinant TRAIL/DR4 complex stimulated the TCR. In addition, amino acid substitutions in the CDR3α of the TCR either obliterated or enhanced target-specific recognition. This description of the molecular nature of a non-MHC target structure recognized by a naturally occurring α/β TCR not only broadens our concept of what the TCR can recognize, but also raises the question of whether such a T cell could be of clinical utility against RCC.