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 tripotent differentiation of human PBL monocyte progenitor under inflammatory conditions. After obtaining ethical approval and informed consent, we first determined the frequency of common trilineage monocyte progenitors (TMPs), expressing the phenotype CD45+CD15−CD3−CD19−CD56−CD11b+CD16−CD14+CCR2+, in the PBL of healthy controls and RA patients. To assess the effect of Notch-pathway activation on TMPs differentiation potential, we then coated culture plates with the immobilized Notch-ligands Jagged-1 (JAG1) and Delta-like ligand-1 (DLL1). Macrophages, osteoclasts and DCs were induced by the appropriate cytokines (M-CSF, M-CSF and RANKL or GM-CSF and IL-4, respectively), whereas the addition of bacterial lipopolysaccharides mimicked an inflammatory environment. We observed that 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 TMPs toward the pro-inflammatory and antigen-presenting capacity of DCs and macrophages, in contrast to suppressing phagocytosis and matrix degradation by macrophages and osteoclasts. 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 behavior of mature myeloid lineages derived from common TMP. Therefore, modulation of Notch-signaling may be an important complementary approach to treating RA pathogenesis.