Transcription factor C/EBPα is required for the development of Ly6C^hi monocytes but not Ly6C^lo monocytes

Monocytes comprise two major subsets, Ly6C^hi classical monocytes and Ly6C^lo nonclassical monocytes. Notch2 signaling in Ly6C^hi monocytes triggers transition to Ly6C^lo monocytes, which require Nr4a1, Bcl6, Irf2, and Cebpb. By comparison, less is known about transcriptional requirements for Ly6C^hi monocytes. We find transcription factor CCAAT/enhancer-binding protein alpha (C/EBPα) is highly expressed in Ly6C^hi monocytes, but down-regulated in Ly6C^lo monocytes. A few previous studies described the requirement of C/EBPα in the development of neutrophils and eosinophils. However, the 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 found a severe and selective loss of Ly6C^hi monocytes, while preserving Ly6C^lo monocytes. We find that BM progenitors from Cebpa +37^-/- mice rapidly progress through the monocyte progenitor stage to develop directly into Ly6C^lo monocytes even in the absence of Notch2 signaling. These results identify a previously unrecognized role for C/EBPα in maintaining Ly6C^hi monocyte identity.