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.