Circulating CD14+ monocytes are known to be precursors of phagocytes, such as macrophages and dendritic cells. We have recently identified a novel CD14+CD45+CD34+type I collagen+ cell fraction derived from human circulating CD14+ monocytes, monocyte-derived mesenchymal progenitor (MOMP), which contains progenitors capable of differentiating into a variety of mesenchymal cells, including bone, cartilage, fat and skeletal muscle (

J Leukoc Biol 2003;74:833
). Here, we investigated a differentiation potential of human MOMPs along endothelial, cardiomyocytic, and neuronal lineages. MOMPs treated with angiogenic factors for 7 days underwent a change in their morphology from spindle-shaped to caudated. Transmission electron microscopic analysis revealed that these cells displayed rod-shaped microtubulated structures corresponding to Weibel-Palade bodies. Almost every cell expressed CD31, VE-vadherin, VEGFR2, Tie-2, von Willeband factor (vWF), eNOS and CD146, but CD14/CD45 expression was markedly down-regulated. Functional characteristics, including vWF release upon histamine stimulation, acetylated LDL uptake, and up-regulated expression of VEGFR1 in response to hypoxia, were indistinguishable between MOMP-derived endothelial-like cells and human umbilical vein endothelial cells. We further performed xenogenic transplantation studies using a SCID mouse model, in which syngeneic colon carcinoma cells were injected subcutaneously with or without human MOMPs. Tumors generated from carcinoma cells alone showed central necrosis and less blood vessel formation, but co-transplantation with MOMPs resulted in promotion of blood vessel formation and no areas of necrosis. Immunohistochemical analysis using human specific antibodies to CD31 and vWF demonstrated that >50% of blood vessels incorporated MOMP-derived endothelial cells. To investigate whether MOMPs were able to differentiate along cardiomyocytic and neuronal lineages, pre-labeled human MOMPs were co-cultivated with primary cultures of rat cardiomyocytes or neurons. Shortly after co-cultivation with rat cardiomyocytes, the majority of MOMPs expressed cardiomyocyte-specific transcription factors, Nkx2.5, GATA-4, eHAND and MEF2, together with CD14/CD45. Subsequently, a subpopulation of MOMPs expressed troponin I and atrial natriuretic peptide and lost CD14/CD45 expression. Spontaneously beating cells formed gap junctions with adjacent rat cardiomyocytes and exhibited electrophysiological properties of ventricular myocytes. MOMPs co-cultured with rat neurons for 3 days expressed neuron-specific transcription factors, Ngn-2, NeuroD, Mash1 and nestin. At day 7, these cells expressed neuron-specific markers, NeuN and Hu. At day 18, a subpopulation of the cells exhibited a neuron-like morphology, including characteristic axons and a refractile round cell body, and expressed MAP2 and β3-tubulin. Co-cultivation of MOMPs with rat cells induced to express GFP by adenoviral gene transfer resulted in appearance of human cardiomyoocytes and neurons without GFP staining, suggesting that our observations are not solely explained by cell fusion. In summary, human MOMPs are capable of differentiating along endothelial and cardiomyocytic lineages as well as a neuronal lineage of an ectoderm-origin. Circulating CD14+ monocytes can be an abundant and easily accessible source for autologous cell transplantation for tissue regeneration.

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