Transcription factor C/EBP⍺ is required for the development of Ly6Chi monocytes but not Ly6Clo monocytes

Monocytes comprise two major subsets, Ly6Chi classical monocytes and Ly6Clo non-classical monocytes. Notch2 signaling in Ly6Chi monocytes triggers transition to Ly6Clo monocytes, which require Nr4a1, Bcl6, Irf2 and Cebpb. By comparison, less is known about transcriptional requirements for Ly6Chi monocytes. We find transcription factor CCAAT/enhancer-binding protein alpha (C/EBPα) is highly expressed in Ly6Chi monocytes, but down-regulated in Ly6Clo monocytes. A few previous studies described the requirement of C/EBPα in the development of neutrophils and eosinophils. However, role of C/EBPα for in vivo monocyte development has not been understood. We deleted the Cebpa +37 kb enhancer in mice, eliminating hematopoietic expression of C/EBPα, reproducing the expected neutrophil defect. Surprisingly, we also discovered a severe and selective loss of Ly6Chi monocytes, while preserving Ly6Clo monocytes. We find that BM progenitors from Cebpa +37–/– mice rapidly progress through the cMoP stage to develop directly into Ly6Clo monocytes even in the absence of Notch2 signaling. These results identify a previously unrecognized role for C/EBPα in maintaining Ly6Chi monocyte identity.