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.