Cutting edge: Positive selection induced by a self-peptide with TCR antagonist activity

Antagonist-like engagement of the TCR has been proposed to induce T cell se lection in the thymus. However, no natural TCR ligand with TCR antagonist a ctivity is presently known. Using a combination of bioinformatics and funct ional testing we identified the first self-peptide that can both deliver an tagonist-like signals and promote T cell selection in the thymus. The pepti de is presented by appropriate MHC class I molecules in vivo. Thus, endogen ous antagonist peptides exist and may be involved in TCR repertoire selecti on.