The zinc-finger transcription factors Gata3 and ThPOK have both been implicated in positive selection of double positive (DP) thymocytes towards the CD4 lineage. As in the absence of Gata3, expression of ThPOK is lacking, Gata3 may directly regulate ThPOK expression. As ThPOK failed to promote CD4+lineage differentiation of Gata3-defcient cells, ThPOK cannot be the only Gata3 target gene essential for the induction of the CD4+lineage program. Therefore, it is conceivable that Gata3 is essential for selected DP T cells to reach the developmental stage at which ThPOK expression is induced. Here, we show that Gata3 overexpression does not affect ThPOK expression levels in DP or CD4+thymocytes, providing evidence that Gata3 does not directly regulate ThPOK. To identify additional target genes that clarify Gata3 function at the DP thymocyte stage, we performed gene expression profiling assays in wild-type mice and transgenice mice with enforced expression of Gata3, in the presence or absence of the MHC class II-restricted DO11.10 TCR. We found that Gata3 expression in DP cells undergoing positive selection was associated with downregulation of the V(D)J-recombination machinery genes Rag1, Rag2 and TdT. Moreover, Gata3 overexpression was associated with downregulation of many signaling molecules and the induction of modulators of TCR signaling, including Ctla-4 and thrombospondin 2. Together with our previous finding that Gata3 reduces expression of CD5, a negative regulator of TCR signaling, and upregulates TCR expression, these findings indicate that Gata3 in DP cells mainly functions to (i) terminate TCRα gene rearrangement, and (ii) regulate TCR signal intensity or duration in cells undergoing positive selection towards the CD4 lineage.

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Molecular Immunology
Erasmus MC: University Medical Center Rotterdam