Knockdown of Pu.1 by Small Interfering RNA in CD34+ Embryoid Body Cells Derived from Mouse ES Cells Turns Cell Fate Determination to Pro-B Cells

The factors that regulate murine ES cell-derived hematopoietic progenitor cell (HPC) commitment to the B lymphocyte lineage remain unclear. Pu.1 plays an essential role in the development of all lymphoid lineages; however, it also regulates commitment to other blood cell lineages. In this study, we found evidence for early B cell lineage commitment as determined by coexpression of CD19 and CD45R (B220) when Pu.1 expression was knocked down in HPC by specific small interfering RNA (siRNA); moreover, the expression of early B cell factor (Ebf) and paired box protein 5 (Pax-5) transcription factors was induced when cells were treated by Pu.1 siRNA, but not by control siRNA. We also found that siRNA-mediated knockdown of Pu.1 expression was more efficient in generating progenitor B cells (pro-B cells) compared with the more common in vitro method of B lymphoid development by means of coculture of CD34+ embryoid body (EB) cells with OP9 stromal cells. To investigate whether this phenomenon also exists in HPC from other sources, we then knocked down Pu.1 gene expression in CD34+ murine bone marrow cells and found a similar effect of increased production of CD19+CD43+CD45R+ progenitor B cells upon the siRNA-mediated decrease in Pu.1 expression. We conclude that, in early B cell development from ES cell-derived HPC, constitutive Pu.1 expression inhibits the earliest B cell development through repressing early B cell factor and paired box protein 5 expression, although lower levels of Pu.1 expression in HPC play a key role in promoting B cell fate determination.