Human Vascular Progenitor Cells Can Guide Mesodermal Lineage Choice of Mesenchymal Stem and Progenitor Cells After Co-Transplantation In Vivo.

Multilineage differentiation potential of mesenchymal stem and progenitor cells (MSPCs) make them attractive candidates for tissue regeneration purposes. Guiding the differentiation of MSPCs towards single lineages would facilitate their application for targeted therapies in vivo. We have previously shown that MSPCs are essential for endothelial colony-forming progenitor cell (ECFC)-derived patent vessel formation in vivo^*[Blood 2009; 113 (26):6716-25]. Preliminary data indicate that the ratio of co-applied cells can change mesenchymal lineage differentiation from vascular support towards either osteogenesis with subsequent bone marrow (BM) ingrowth or chondrogenesis. We hypothesized that environmental conditioning by ECFCs plays an instructive role during the developmental fate decision of MSPCs in vivo.

MSPCs as well as ECFCs were isolated from adult BM, white adipose tissue (WAT), umbilical cord blood (UCB) and perivascular cord tissue^**[J Vis Exp. 2009;(32) pii: 1525]. Proliferation potential and clonogenicity were monitored. Phenotype was analyzed by flow cytometry and immune cytochemistry. Cell function was studied in differentiation assays and during vascular network assembly in vitro. Models for in vivo human vessel as compared to bone, BM or cartilage formation were established in immune-deficient NSG mice (NOD.Cg-Prkdc^scid Il2rg^tm1Wjl/SzJ). Non-invasive imaging was performed using computed tomography (CT), magnetic resonance (MRI) and near-infrared fluorescence imaging to elucidate the time course of heterotopic tissue development. Immune histochemistry was applied for morphologic studies of organogenesis.

Baseline analysis confirmed MSPC and ECFC purity, immune phenotype and sustained proliferation potential. We could show that human BM-derived MSPCs are capable of forming bone in vivo. Osteogenic differentiation and heterotopic ossicle formation was followed by attraction of mouse hematopoiesis and the establishment of entire murine BM including red and white blood cells and megakaryocytes within a human endosteal niche. Co-transplanted human ECFCs could instruct the MSPCs to differentiate also into pericytes or chondrocytes in vivo, depending on the applied MSPC/ECFC cell ratio. Non-invasive imaging and histological staining revealed that ectopic organogenesis had already started after 2–4 weeks and was stable during the observation period of 20 weeks. Non-BM-derived populations, although phenotypically identical, invariably lacked the capacity to build bone and marrow environment in this model in vivo.

These data indicate that human ECFCs can instruct MSPCs and induce developmental fate decisions early in the time course of organ regeneration after transplantation. We suppose that effective regenerative stem cell therapy in vivo requires more than the injection of one single cell population. For vascular repair as compared to bone and marrow environment reconstitution our model is a promising tool to study the therapeutic applicability and risk profile of such ECFC/MSPC-based transplantation strategies.

No relevant conflicts of interest to declare.