M-CSF Drives Human Cord Blood Monocyte Differentiation along an Alternative Pathway into Suppressive CXCR4^highIL-10^highIL-12^absentTNF-a^defective Dendritic Cells.

M-CSF Drives Human Cord Blood Monocyte Differentiation along an Alternative Pathway into Suppressive CXCR4^highIL-10^highIL-12^absentTNF-α^defective Dendritic cells

Geling Li and Hal E. Broxmeyer. Microbiology and Immunology, Indiana University School of Medicine, Indianapolis, IN, USA; Walther Oncology Center, Indiana University School of Medicine, Indianapolis, IN, USA; Walther Cancer Institute, Indianapolis, IN, USA

Macrophage colony-stimulating factor (M-CSF) in cooperation with TGF-β1 induces CD34⁺ hematopoietic progenitor cells to differentiate into Langerhans cells, a subset of immature dendritic cells (DCs) specifically localized in the epidermis. M-CSF levels are dramatically elevated in immunosuppressive conditions and M-CSF deficient mice are osteopetrotic. Therefore, we hypothesized that M-CSF might be involved in the generation of a novel subtype of suppressive DCs which may be located primary in bone marrow where large amounts of SDF-1 are constitutively produced. We also speculated that M-CSF-induced DCs might express the only known ligand of SDF-1, CXCR4 and migrate towards SDF-1. Highly purified umbilical cord blood monocytes (> 98% purity) cultured in the presence of M-CSF, IL-4 and TGF-β1 differentiated into a highly homologous population of cells with typical dendrites and acquired a langerhan cell (LC) phenotype by up-regulated expression of CD1a and E-cadherin. We termed these cells M-LC to distinguish them from regular LCs cultured in the presence of GM-CSF, IL-4 and TGF-β1. In comparison with LC, M-LCs produced a much higher level of IL-10, no IL-12 (detection limit, 4 pg/ml) and a minimal level of TNF-α (21.6 ± 37.5 pg/ml) upon LPS stimulation. LPS-activated M-LCs induced lower allogeneic mixed lymphocyte reaction (MLR) than LPS-activated LCs. Furthermore, exposure to LPS-activated M-LCs rendered a state of low-responsiveness in cord blood naïve CD4⁺ T cells to further re-stimulation with alloantigens in secondary MLR. M-LCs expressed much higher surface levels of CXCR4 than LCs regardless of their maturation status. M-LCs started to migrate towards SDF-1 at a lower concentration (≥10 fold lower than that required for LCs) and displayed enhanced responsiveness to SDF-1-induced chemotaxis. Our results demonstrate that M-CSF drives monocyte differentiation along an alternative pathway into suppressive CXCR4^highIL-10^highIL-12^absentTNF-α^defective DCs. These novel DCs may be of use to suppress unwanted immune responses in vivo.