IDENTIFICATION OF A NATURALLY-OCCURRING LIGAND FOR THYMIC POSITIVE SELECTION

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