Stimulation of thymocytes before and after positive selection results in the induction of different NF-κB/Rel protein complexes

Positive selection triggers the differentiation of immature CD4⁺8⁺TCR(low) thymocytes into TCR(high) single-positive CD4⁺ or CD8⁺ cells and is associated with major changes in gene expression. However, little is known about the DNA binding factors controlling these fundamental changes. Here we have examined NF-κB/Rel subunit expression and DNA-binding activity in developing thymocytes before and after the induction of positive selection. We show that positive selection is accompanied by the strong up- regulation of c-rel mRNA expression and the constitutive activation of p50/p65 and p50/c-Rel NF-κB/Rel complexes, confirming the activation-like status of cells undergoing positive selection. Moreover, CD69⁺ cells that have initiated positive selection (but not their preselection CD4⁺B⁺TCR^- precursors) respond to stimulation by the preferential activation of c-Rel- containing DNA-binding complexes. Because the different NF-κB/Rel dimers have distinct transcriptional activities and binding site preferences, this preferential activation of c-Rel-containing DNA-binding complexes may well have implications for the changes in gene expression and functional response associated with positive selection.