Progression from the Common Lymphoid Progenitor to B/Myeloid PreproB and ProB Precursors during B Lymphopoiesis Requires C/EBPα.

The C/EBPα transcription factor is required for myelopoiesis, with prior observations suggesting additional contributions to B lymphopoiesis. expression is evident in common lymphoid progenitor (CLP) and preproB cells but is absent in proB and preB cells. We previously observed that marrow lacking the +37 kb enhancer is impaired in producing B cells upon competitive transplantation. Additionally, a enhancer/promoter-hCD4 transgene is expressed in B/myeloid CFU. Extending these findings, pan-hematopoietic murine enhancer deletion using Mx1-Cre leads to expanded CLP, fewer preproB cells, markedly reduced proB and preB cells, and reduced mature B cells, without affecting T cell numbers. In contrast, enhancer deletion at the proB stage using Mb1-Cre does not impair B cell maturation. Further evaluation of CLP reveals that the transgene is expressed almost exclusively in Flt3 multipotent CLP versus B cell-restricted Flt3 CLP. In vitro, hCD4 preproB cells produce both B and myeloid cells, whereas hCD4 preproB cells only produce B cells. Additionally, a subset of hCD4 preproB cells express high levels of RAG1-GFP, as seen also in proB cells. Global gene expression analysis indicates that hCD4 preproB cells express proliferative pathways, whereas B cell development and signal transduction pathways predominate in hCD4 preproB cells. Consistent with these changes, enhancer-deleted preproB cells downmodulate cell cycle pathways while upregulating B cell signaling pathways. Collectively, these findings indicate that C/EBPα is required for Flt3 CLP maturation into preproB cells and then for proliferative B/myeloid preproB cells to progress to B cell-restricted preproB cells and finally to proB cells.