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.