Downregulated Expression of Ly-6-ThB on Developing T Cells Marks CD4(+)CD8(+) Subset Undergoing Selection in the Thymus

Interaction of TCRs on CD4(+)CD8(+) immature T cell with MHC-peptide complexes on stromal cells is required for positive and negative selection in the thymus. Identification and characterization of a subpopulation of CD4(+)CD8(+) thymocytes undergoing selection in the thymus will aid in understanding the mechanisms underlying lineage commitment and thymic selection. Herein, we describe the expression of Ly-6 ThB on developing thymocytes. The majority of CD4(+)CD8(+) thymocytes express Ly-6 ThB at high levels. Its expression is downregulated in a subset of CD4(+)CD8(+) thymocytes as well as in mature CD4(+)CD8(-) and CD4(-)CD8(+) T cells. More importantly, interaction of TCR/coreceptor with the self-MHC-peptide contributes to the downregulation of ThB expression on developing thymocytes. These findings indicate that downregulation of ThB on CD4(+)CD8(+) thymocytes identifies a unique subset (CD4+CD8+ThB(neg–low)) of thymocytes that has received the initial signals for thymic selection but have not yet downregulated the CD4 and CD8 cell surface expression. In addition, these results also indicate that a high frequency (Ÿ20–40%) of CD4(+)CD8(+) immature thymocytes receive these initial signals during thymic selection.