PU.1 is required for the developmental progression of multipotent progenitors to common lymphoid progenitors.. Mus musculus

The transcription factor PU.1 is a central regulator of hematopoiesis, required for the normal differentiation of the myeloid and lymphoid lineages. Due to its essential role in early development and the importance of PU.1 in regulating several defining markers of lymphoid progenitors, its precise function in early lymphopoiesis has remained unclear. Using conditional mutagenesis and alternative lineage identification strategies, we demonstrate the developmental stage restricted function for PU.1 in early lymphopoiesis. PU.1 was required for efficient generation of “lymphoid primed multipotent progenitors (LMPPs)” from hematopoietic stem cells and was essential for the subsequent formation of “common lymphoid progenitors (CLPs)”. In contrast, further differentiation into the B cell lineage was independent of PU.1. The function of PU.1 was dosage sensitive as loss of one allele decreased all stages of early lymphopoiesis. PU.1 expression mirrored its functional role during lineage differentiation. PU.1 peaked at LMPPs and was maintained in CLPs, before being down regulated in committed pro-B cells. Examination of the transcriptional changes in PU.1 conditionally deficient LSK cells revealed that PU.1 activates lymphoid and dendritic cell associated genes in LMPPs, while repressing genes normally expressed in neutrophils. These data identify PU.1 as a critical regulator of lymphoid priming and the transition between LMPPs and CLPs. Overall design: Total RNA was obtained from wild type LSKs and LSKs with Pu1 conditionally deleted