Enforced STAT3 and PU.1 Expression Activates Type I Interferon-Producing Cell and Dendritic Cell Differentiation Programs in Megakaryocyte/Erythrocyte Progenitors.

A standing question in early hematopoiesis is whether cytokine signaling is sufficient to induce cell fate decisions. Previously, we have reported that enforced expression of human Flt3 in Flt3 negative megakaryocyte/erythrocyte progenitors (MEPs) rescues their interferon producing cell (IPC) and dendritic cell (DC) developmental potential: Human Flt3-signaling in MEPs leads to up-regulation of IPC, DC, and myelomonocytic development affiliated genes such as, STAT3, PU.1, and G-CSFR/M-CSFR/GM-CSFR, and activates differentiation to these lineages. To test whether single Flt3 downstream activated genes would be sufficient to cause the same effects, we transduced murine STAT3 and PU.1 cDNA into MEPs using retrovirus expression vectors, respectively. Both STAT3- and PU.1-transduced MEPs were capable to differentiate into IPCs and DCs in SCF and TPO supplemented media, and, with even higher efficiency, in SCF, TPO, and Flt3L supplemented cultures. While human Flt3-transduced MEPs maintained megakaryocyte-erythrocyte developmental potential and gained IPC, DC, and myelomonocytic cell potential, STAT3- and PU.1-transduced MEPs lost both megakaryocyte and erythrocyte developmental options, indicating that strong signaling of these transcription factors fully converted MEPs to the IPC, DC, and myelomonocytic lineages. Consistently, GATA-1 expression was down-regulated, and C/EBPα mRNA was up-regulated in STAT3- and PU.1-transduced MEPs. Interestingly, STAT3- and PU.1 over-expression in MEPs led to up-regulation of Flt3 mRNA levels, suggesting a self-sustaining effect of Flt3 signaling-cascade induced Flt3 expression. Thus, enforced expression of STAT3 and PU.1 in MEPs reprogrammed them to differentiate into IPCs, DCs and myelomonocytic cell lineages and inhibit Meg/E-lineage potential. Based on these data, we propose a model where Flt3 positive progenitor cells that locate in Flt3L rich environments will be directed to develop into IPCs and DCs. This process might be enhanced by a self-sustaining mechanism where Flt3 downstream transcription factors as STAT3 and PU.1 in turn maintain Flt3 expression.