Stepwise progression of β-selection during T cell development as revealed by histone deacetylation inhibition

Abstract

AbstractDuring T cell development, the first step in creating a unique T Cell Receptor (TCR) is the genetic recombination of the TCRβ chain. The quality of this newly recombined gene is assessed at the β-selection checkpoint, and most cells fail this checkpoint and are removed. The coordination of the complex events that combine to control fate at the β-selection checkpoint is not yet understood. We assessed the impact on T cell development of a selective inhibitor to histone deacetylase 6, ACY1215, currently in clinical use. ACY1215 led to bypass of the β-selection checkpoint such that cells in the DN4 stage often lacked expression of TCRβ, and failed to progress to the DP stage. Characterisation of the molecular basis for this bypass revealed a new, pivotal stage in β-selection, the beginning and end of which were defined by the upregulation of the TCR co-receptors, CD28 and CD2 respectively. Within this stage, termed DN3bPre, CD5 and Lef1 are upregulated to reflect pre-TCR signalling. We propose that the progressive expression of CD28, CD5 then CD2 reports and modulates the pre-TCR signal to orchestrate passage through the β-selection checkpoint. By disrupting the functional connection between CD5 and pre-TCR, ACY1215 allows cells to inappropriately bypass the β-selection checkpoint. These findings implicate a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD5 and Lef1 provides for an escalating test of TCR signalling strength, and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.