Involvement of MKK6 in TCR  intCD69lo: a target population for apoptotic cell death in thymocytes

By analyzing real-time caspase activity and DNA fragmentation in live thymocytes, we found that apoptosis occurs predominantly in a TCRalphabeta(int/hi)CD69(lo) population. The number of caspase-active cells and DNA-fragmented cells in MKK6-deficient mice, which were originally generated in our laboratory by gene targeting, was decreased in the TCRalphabeta(int)CD69(lo) population but not in the TCRalphabeta(hi)CD69(lo) population. The percentage of caspase-active cells in the H-Y-specific TCRint population was more clearly decreased in male MKK6-deficient H-Y TCR-transgenic mice. Furthermore, the absolute number of TCR(hi)CD4(lo)CD8(lo) cells, which are developmentally next to TCR(int)CD4(hi)CD8(hi) cells, was increased in MKK6-deficient H-Y TCR-transgenic mice. Deletion of TCRalphabeta(int)CD4(hi)CD8(hi) cells by injecting antigenic lymphocytic chorio-meningitis virus (LCMV) peptide into LCMV-specific TCR-transgenic mice was incomplete in MKK6-deficient mice. Cellular death of TCRalphabeta(int) fetal thymocytes induced by adding an antigenic peptide into an in vitro fetal thymic organ culture system was also diminished in MKK6-deficient TCR-transgenic thymi. These results indicate that MKK6 plays a role in the developing thymocytes, especially in the population of TCRalphabeta(int)CD69(lo) cells, which possibly undergo negative selection.