THE REPERTOIRE OF T-CELLS SHAPED BY A SINGLE MHC PEPTIDE LIGAND/

Although the thymus produces many immature thymocytes, few of these ce lls mature. Positive selection has been thought to limit thymocyte dev elopment. In thymuses expressing a single MHC/peptide combination, how ever, surprisingly large numbers of thymocytes are selected to mature. Many of these react with the selecting MHC, bound to other self-pepti des. Therefore, the number of thymocytes that mature is limited by the fact that positively selected cells die because they react too well w ith MHC bound to self-peptides that are not identical to those involve d in positive selection. T cells that mature in thymuses expressing a single MHC/peptide ligand react frequently with foreign MHC, suggestin g that the repertoire of alpha beta receptors may be more biased towar d reaction with MHC than was previously thought.