TCR-mediated ThPOK induction promotes development of mature (CD24-) γ δ thymocytes

T lymphocytes develop into two major lineages characterized by expression of the αβ and γ δ T cell receptor (TCR) heterodimers. Within each major lineage, further specialization occurs, resulting in distinct subsets that differ in TCR specificity, phenotype and functional attributes. Thus, in the murine thymus, two distinct subsets of mature (CD24-) γ δ cells have been identified, that is NK1.1+ cells, which are enriched for VÎ 31.1 usage and selectively produce IFNÎ 3 on stimulation, and CCR6+ cells, which are enriched for VÎ 32 usage produce IL17. The upstream signals and transcriptional pathways that promote development of these distinct γ δ subsets remain relatively poorly understood. Here, we show that the Zn-finger transcription factor ThPOK has a critical function in the development of γ δ thymocytes. Thus, lack of functional ThPOK causes a marked reduction in the percentage and absolute number of mature γ δ thymocytes, and a particularly severe reduction of NK1.1+ cells. Conversely, constitutive ThPOK expression leads to a striking increase in mature NK1.1+ γ δ thymocytes. Further, we show that ThPOK induction in γ δ thymocytes is induced by strong TCR signals mediated by engagement with antibody or high-affinity endogenous ligands, and that an important ThPOK cis-acting element, the distal regulatory element (DRE), is sufficient for this TCR-dependent induction. These results show that ThPOK expression in γ δ thymocytes is regulated in part by the strength of TCR signalling, identify ThPOK as an important mediator of γ δ T cell development/maturation, and lend strong support to the view that development of a significant fraction of γ δ T cells depends on TCR engagement/signalling.