Effects of Notch signaling on the lineage commitment of human peripheral blood monocyte trilineage progenitor under inflammatory conditions

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disease, causing significant morbidity and disability. Inflammation-induced activation of myeloid cells is involved in disease pathogenesis and contributes to joint destruction. Due to the significant plasticity of myeloid lineage, peripheral blood (PBL) monocytes have the potential to differentiate into a variety of mature cells, including macrophages, osteoclasts, and dendritic cells (DCs), depending on the environmental cues and activated signaling pathways. Therefore, we aimed to determine how the Notch pathway affects differentiation of human PBL monocyte progenitor under inflammatory conditions. We first determined the frequency of monocyte subsets and identified a common trilineage monocyte progenitor (TMP), expressing the phenotype CD45⁺CD15^-CD3^-CD19^-CD56^-CD11b⁺CD14⁺, in the PBL of healthy controls and RA patients. To assess the effect of Notch-pathway activation on TMP differentiation, we then coated culture plates with the immobilized Notch ligands Jagged 1 (JAG1) and Delta-like ligand 1 (DLL1). Macrophages, osteoclasts, and DCs were differentiated from TMPs of control subjects by the appropriate cytokines (M-CSF, M-CSF/RANKL, or GM-CSF/IL4, respectively), whereas the addition of bacterial lipopolysaccharides mimicked an inflammatory environment. We observed that the TMP population is expanded in RA PBL and expresses Notch receptors, implicating its susceptibility to Notch regulation. Our results further suggest that in the context of inflammation, Notch signaling, especially via DLL1, polarizes TMP differentiation in favor of the pro-inflammatory and antigen-presenting capacity of DCs and macrophages, while suppressing phagocytosis and matrix degradation by macrophages and osteoclasts. Specifically, these Notch effects are seen as higher IL1B expression and enhanced T lymphocyte stimulation by DCs, higher HLA-DR expression and suppressed phagocytosis by macrophages, as well as lower CTSK expression and suppressed TRAP activity by osteoclasts. In conclusion, we demonstrated that the Notch-axis effectively regulates the commitment of common TMP into myeloid cell subtypes. Therefore, modulation of Notch signaling may be an important complementary approach to treating RA pathogenesis.