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

During 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 the newly recombined TCRβ is assessed at the β-selection checkpoint. Most cells fail this checkpoint and die, but the coordination of the complex events that control fate at the β-selection checkpoint is not yet understood. We shed new light on fate determination during β-selection using a selective inhibitor of histone deacetylase 6, ACY1215. ACY1215 disrupted the β-selection checkpoint. Characterising the basis for this disruption revealed a new, pivotal stage in β-selection, bookended by upregulation of the TCR co-receptors, CD28 and CD2 respectively. Within this ‘DN3b^Pre’stage, CD5 and Lef1 are upregulated to reflect pre-TCR signalling and their expression correlates with proliferation. During this phase, ACY1215-mediated disruption of the organisation of the β-selection immunological synapse is associated with a breakdown in connectivity of expression of pre-TCR, CD5 and Lef1. Subsequent deregulation of pre-TCR-induced proliferation leads to bypass of the β-selection checkpoint and subsequent failure to progress. We propose that the progressive expression of CD28, CD5 and Lef1, then CD2 reports and modulates the pre-TCR signal to orchestrate passage through the β-selection checkpoint. These findings suggest a refined model of β-selection in which a coordinated increase in expression of pre-TCR, CD28, CD5 and Lef1 allows for modulating TCR signalling strength, and culminates in the expression of CD2 to enable exit from the β-selection checkpoint.