The Ordered Expression of Transcription Factors Directs Hierarchical Lineage Specification of Eosinophils, Basophils and Mast Cells.

Here we show that eosinophil progenitors (EoPs) and basophil/mast cell progenitors (BMCPs) are prospectively isolatable in normal hematopoiesis, and that their lineage decisions are regulated principally by GATA-2 and C/EBPα. These progenitors were isolated downstream of granulocyte/monocyte progenitors (GMPs), and BMCPs further generated monopotent basophil progenitors (BaPs) and mast cell progenitors (MCPs). Gene expression analysis showed that neither GATA-1 nor GATA-2 was expressed in GMPs, whereas both of them were upregulated in EoPs, BMCPs, BaPs and MCPs. Importantly, C/EBPα was expressed in EoPs and BaPs as well as GMPs, but was downregulated in BMCPs and MCPs. We have reported that GATA-1 is critical primarily for megakaryocyte/erythrocyte commitment or conversion of stem and progenitor cells. We therefore focused on GATA-2 and C/EBPα functions in this study. Since both EoPs and BaPs co-expressed GATA-2 and C/EBPα while GMPs expressed only C/EBPα, we first transduced GATA-2 into GMPs via a GFP-tagged retrovirus. Strikingly, all GATA-2⁺ GMPs gave rise to pure eosinophil colonies but not basophil colonies, indicating that enforced GATA-2 can instruct GMPs to become EoPs. Next, since BMCPs only expressed GATA-2 but not C/EBPα, we maintained the expression of C/EBPα in GMPs by retroviral transduction. Interestingly, the sustained expression of C/EBPα blocked basophil/mast cell differentiation from GMPs, indicating that C/EBPα downregulation is required for GMPs to choose the basophil/mast cell fate. As a reciprocal experiment, we conditionally disrupted C/EBPα gene at the level of GMPs by retrovirally transducing Cre gene into GMPs purified from mice in which C/EBPα gene is flanked by loxP sequences (floxed: F). The frequency of mast cell read-out from C/EBPα-disrupted GMPs was 5-fold higher than that from C/EBPα ^F/F (Cre⁻) GMPs. C/EBPα-disrupted GMPs, however, did not give rise to BaPs. Furthermore, MCPs transduced with C/EBPα were converted into BaPs. Thus, C/EBPα is required to be reactivated during transition from BMCPs to BaPs. We further tested their interplay in specification of these lineages by using common lymphoid progenitors (CLPs), which do not express GATA-2 or C/EBPα. We enforced the expression of each transcription factor in CLPs in different orders by using the two-step retroviral transduction system. Interestingly, C/EBPα transduction reprogrammed CLPs into GM lineages, and subsequently-transduced GATA-2 instructed C/EBPα ⁺ CLPs to select the eosinophil fate. Next, we switched the order of transduction. Strikingly, GATA-2 transduction converted CLPs into BMCPs, and subsequently-transduced C/EBPα specified GATA-2⁺ CLPs to become BaPs. Thus, at the branchpoint for EoPs and BMCPs, GATA-2 upregulation instructed EoP development if C/EBPα was present, whereas it instructed BMCP development if C/EBPα was absent. After the BMCP stage, C/EBPα had to remain suppressed for MCP development, whereas BaPs developed by C/EBPα reactivation. These data collectively suggest that the order of expression of GATA-2 and C/EBPα is critical for their interplay to selectively activate developmental programs for the eosinophil, the basophil and the mast cell lineages.