Differential Expression of PU.1 and Key T Lineage Transcription Factors Distinguishes Fetal and Adult T Cell Development.

The PU.1 transcription factor plays a critical role in the regulation of T cell development, so a report that it is dispensable for fetal thymopoiesis is puzzling. To understand this paradox, we examined the requirement for PU.1, encoded by Spi1, during fetal, neonatal, and adult thymopoiesis in a PU.1 hypomorphic mouse generated by deletion of the Spi1 14-kb upstream regulatory element and by analysis of patterns of gene expression in fetal and adult T cell progenitors. Our data demonstrate that the initiation of thymopoiesis during early gestation is less dependent on PU.1 compared with T cell differentiation in adults and that fetal T cell progenitors express lower levels of Spi1 compared with their adult counterparts. We also show that expression of the core network of T lineage transcription factors regulated by PU.1 differs in fetal and adult T cell progenitors. In particular, PU.1-regulated genes that promote T cell differentiation are differentially expressed in fetal versus adult early T lineage progenitors. These results indicate that the transcriptional differences between the fetal and adult T cell developmental programs are driven in part by differential levels of PU.1 expression and that this likely underlies the differences in the properties of fetal and adult T cell progenitors.