The nature of the peptide/MHC ligand involved in positive selection

Positive and negative selection in the thymus occur when the TCR on an immature thymocyte engages self peptide/MHC complexes present on epithelial cells. The signaling mechanism which dictates the selection outcome is currently a matter of intense investigation. Here we review experiments that defined the peptide ligands for positive selection. A comparison of these to the peptide ligands that induce negative selection (antigenic peptides) sheds some insight into how a thymocyte interprets peptide/MHC interactions leading to life versus death of the cell.