| T lymphocytes develop in the thymus through multiple stages (DN, DP and SP) during which their survival and lineage are determined. During thymocyte maturation, T-cell Receptor (TCR) signaling induces important downstream mediators of differentiation. ThPOK (T-helper inducing POZ-Kruppel factor) has been identified as a master regulator of lineage commitment at the CD4/CD8 branch point, and is selectively induced in class II-restricted thymocytes, suggesting that it is upregulated by strong TCR signals. In order to identify upstream signaling pathways responsible for ThPOK induction, we mapped key transcriptional control elements at the ThPOK locus. These studies show that a dual silencer/enhancer element is primarily responsible for selective induction of ThPOK. We are currently attempting to identify key factors that bind to and regulate activity of this element during thymocyte development. Given that gammadelta lineage commitment, like CD4 commitment, may be decided by relative TCR signal strength, it is possible that the same downstream effectors may be involved in both processes. However, the potential role of ThPOK in gammadelta commitment has not yet been explored. By carrying out in vitro stimulation of gammadelta thymocytes with anti-TCR antibody and varying ligand affinity for the transgenic KN6 gammadeltaTCR in vivo, we have now established that ThPOK induction in gammadeltaTCR+ thymocytes is controlled by TCR signaling and specifically requires a strong/ high affinity TCR ligand. Strikingly, ThPOK deficient mice show a severe reduction in mature gammadelta thymocytes as well as altered V region usage in gammadelta thymocytes, showing that ThPOK is important for maturation/selection of gammadelta cells. ThPOK is particularly important for development of a subset of gammadelta thymocytes that express Vgamma1.1 and the NK1.1 surface marker, which has been previously referred to as NKTgammadelta cells and appears to be enriched for self-reactive specificities. Conversely, in mice that express ThPOK constitutively, the proportion of mature gammadelta thymocytes is greatly increased, as is the proportion of Vgamma1.1+ thymocytes. These results demonstrate a new role for ThPOK in development of gammadelta thymocytes. Future work will distinguish whether ThPOK is important for differentiation, proliferation and our lineage commitment of the gammadelta subset. |
