Bcl6 , Irf2 , and Notch2 promote nonclassical monocyte development.

Ly6C lo monocytes are a myeloid subset that specializes in the surveillance of vascular endothelium. Ly6C lo monocytes have been shown to derive from Ly6C hi monocytes. NOTCH2 signaling has been implicated as a trigger for Ly6C lo monocyte development, but the basis for this effect is unclear. Here, we examined the impact of NOTCH2 signaling of myeloid progenitors on the development of Ly6C lo monocytes in vitro. NOTCH2 signaling induced by delta-like ligand 1 (DLL1) efficiently induced the transition of Ly6C hi TREML4 - monocytes into Ly6C lo TREML4 + monocytes. We further identified two additional transcriptional requirements for development of Ly6C lo monocytes. Deletion of BCL6 from myeloid progenitors abrogated development of Ly6C lo monocytes. IRF2 was also required for Ly6C lo monocyte development in a cell-intrinsic manner. DLL1-induced in vitro transition into Ly6C lo TREML4 + monocytes required IRF2 but unexpectedly could occur in the absence of NUR77 or BCL6. These results imply a transcriptional hierarchy for these factors in controlling Ly6C lo monocyte development.