Expression of TCRαβ partly rescues developmental arrest and apoptosis of αβ T cells in Bcl11b-/- mice

Bcl11b(-/-) mice show developmental arrest at the CD44(-)CD25(+) double-negative 3 (DN3) or immature CD8(+)single-positive stage of alpha beta T cell. We have performed detailed analysis of sorted subsets of Bcl11b(-/-) thymocytes, DN3 and CD44(-)CD25(-) double-negative 4 (DN4) cells. Surface expression of TCRP proteins was not detected in DN3 thymocytes and markedly reduced in DN4 thyrnocytes, whereas expression within the cell was detected in both, suggesting some impairment in processing of TCR beta proteins from the cytoplasm to the cell surface. This lack of expression, resulting in the absence of pre-TCR signaling, could be responsible for the arrest, but the transgenic TCR beta or TCR alpha beta expression on the cell surface failed to promote transition from the DN3 to CD4(+)CD8(+) double-positive stage of development. This suggests that the pre-TCR signal cannot compensate the deficiency of Bcl11b for development. Bcl11b(-/-) DN3 thymocytes showed normal DNA rearrangements between D beta and J beta segments but limited DNA rearrangements between V beta and DJ beta without effect of distal or proximal positions. Because this impairment may be due to chromatin accessibility, we have examined histone H3 acetylation in Bcl11b(-/-) DN3 cells using chromatin immunoprecipitation assay. No change was observed in acetylation at the V beta and D beta gene locus. Analysis of Bclllb(-/-) DN4 thyrnocytes showed apoptosis, accompanied with lower expression of anti-apoptotic proteins, Bel-x(L) and Bcl-2, than wild-type DN4 thymocytes. Interestingly, the transgenic TCR alpha beta in those cells reduced apoptosis and raised their protein expression without increased cellularity. These results suggest that Bcl11b deficiency affects many different signaling pathways leading to development arrests.