Induction of thymocyte deletion by purified single peptide/major histocompatibility complex ligands

We report a novel in vitro approach that allows study of the consequences of TCR ligation on thymocytes in the absence of thymic stromal cells. Hence, thymocytes were incubated either in the presence of recombinant antigenic peptide/MHC complexes, which represent ligands of physiologic affinities, or with anti-TCR mAb, a ligand of supraphysiologic affinity. Whereas TCR cross-linking with mAb led to thymocyte deletion, incubation with peptide/MHC ligands did not trigger cellular apoptosis. However, the addition of a costimulatory signal (provided by anti-CD28 mAb) allowed the induction of apoptosis following TCR binding to peptide/MHC ligands, and it increased the levels of cell death obtained through mAb-mediated TCR cross-linking. Requirement for accessory signals seen with TCR stimulation by peptide/MHC complexes argues in favor of qualitative differences between TCR engagement by ligands of either physiologic or supraphysiologic affinity.