CD3-dependent regulation of early TCRβ gene expression in mainstream αβ and NKαβ T cell development

Abstract

During development in the thymus, mainstream αβ lineage thymocytes repeatedly pass through checkpoints at which they are selected for molecular properties that make them useful for further maturation. Selection of a useful repertoire of specific αβ T cell receptors (TCR) takes place at the CD4⁺8⁺double positive (DP) stage, i.e. relatively late in thymic development (reviewed in^1,2). Earlier checkpoints which select cells for generic functions occur in the CD4^-8^-double negative (DN) stage of thymocyte maturation (reviewed in^3,4). This stage is divided sequentially into the subsets DN1 (CD44⁺25^-), DN2 (CD44⁺25⁺), DN3 (CD44^-25⁺), and DN4 (CD44^-25^-)⁵. Most of TCRβ locus VDJ rearrangement takes place in DN3⁶. TCRβ polypeptide chains are then synthesized and assemble with pre-Tα 7,8 and the clonotype independent CD3 complexes (CIC) 9 to form the pre-TCR which signals TCRβ⁺cells to continue maturation to the DP stage, a process referred to as “β-selection” (reviewed in^4,10-12). A number of mouse strains genetically manipulated to have severe defects in the molecular composition or signaling function of the pre-TCR/CD3 complex have drastically reduced numbers of cells in DN4, suggesting a block at the DN3 stage. Moreover, we have previously shown that expression of TCRβ polypeptide chains is impaired in DN3 cells of mice deficient in CD3 and/or p56^1ck(Lck)^13,14While TCRβlocus rearrangement appeared to proceed independently of CD3 signaling, TCRβ mRNA and protein levels were 5-10 fold reduced in DN cells of CD3 signaling-deficient mice^13-15