NEGATIVE SELECTION BY ENDOGENOUS ANTIGEN AND SUPERANTIGEN OCCURS AT MULTIPLE THYMIC SITES

The site of negative selection in the thymus has been inferred from a range of different experiments, Analysis of thymic deletion of V(beta) 5(+), V(beta)11(+) or V(beta)17a(+) cells in H-2E transgenic mice led to the theory that negative selection occurs predominantly in the medu lla (specifically, through presentation by medullary dendritic cells). Other experiments investigated whether transgenic TCR are deleted at the double-positive (DP) or single-positive stage following encounter with peptide ligand: by flow cytometric analysis deletion is generally found to occur at the Up thymocyte stage and as these cells are found predominantly in the cortex, it has been inferred that this is the ke y site of negative selection. The visualization of apoptotic thymocyte s in situ has recently been reported for specific examples of negative selection. Using a panel of TCR transgenic lines in which negative se lection occurs at different stages of thymocyte development, we have u sed TUNEL staining to analyse the anatomical sites of thymocyte apopto sis, For the first time we have been able to compare directly the site s of deletion induced by the endogenous cognate peptides or by endogen ous superantigen, We show that generalization from the medullary delet ion of V(beta)5(+), V(beta)11(+) or V(beta)17a(+) cells by the endogen ous superantigens Mtv 8 and 9 and from limited examples of cortical de letion by exogenous peptide administered to TCR transgenic mice is ove r-simplified. Apoptotic thymocytes in mice lacking Mtv superantigens a re indeed localized in the cortex. However, when deletion is induced b y cognate self peptide, apoptosis can occur in the cortex, the medulla or at the junction between the two.