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.
Bibliographical noteFunding Information:
We thank I. Williams and L. Ignatowicz for communicating unpublished data and S. Jameson for review of the manuscript. M. Bevan is supported by the Howard Hughes Medical Institute and grants from the National Institute of Allergy and Infectious Diseases.