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.