Identification of a naturally occurring ligand for thymic positive selection

In the thymus, positive and negative selection shape the T cell repertoire. it has previously been shown that positive selection, like negative selection, is the result of the interaction of the TCR with self-peptides bound to MHC. However, lime is known about the number or nature of the self-peptide ligands that mediate positive selection in vivo. We devised a novel assay with enhanced sensitivity for low affinity TCR ligands to identify self-peptides that may be biologically relevant. At least eight K-b-bound serf-peptides were detected by this assay using thymocytes bearing the OT-I TCR (specific for OVAp/K-b). The sequence of one of these peptides was determined using the recently developed technique of membrane preconcentration-capillary electrophoresis-tandem mass spectrometry. This peptide, CP alpha 1, has limited sequence similarity to OVAp, yet was found to induce positive selection of OT-I thymocytes in fetal thymic organ culture.