Early endothelial progenitor cells as a source of myeloid cells to improve the pre-vascularisation of bone constructs.

According to present knowledge, blood derived endothelial progenitor cells (EPC) might act as proangiogenic myeloid cells, which play a fundamental role in the regulation of angiogenesis and blood vessel reorganisation. In this context, we have evaluated the contribution of endogenous myeloid cells in co-cultures of blood derived outgrowth endothelial cells (OEC) and osteogenic cells. In addition, we investigated the role of EPC as a potential source of myeloid cells in the formation of vascular structures in an in vitro model consisting of mesenchymal stem cells (MSC) and OEC. For this purpose, we added EPCs to co-cultures of MSC and OECs. Vascular structures and the co-localisation of myeloid cells were analysed by confocal laser microscopy (CLSM) for endothelial and myeloid markers and quantitative image analysis. The molecular effects of myeloid cells were evaluated by quantitative real time PCR, ELISA and protein arrays from cell culture supernatants and lysates. Endogenous myeloid cells were significantly co-localised with angiogenic structures in co-cultures of OEC and osteogenic cells. The active addition of EPC to co-cultures of OEC and MSC resulted in a statistically approved increase in the formation of prevascular structures at early stages of the co-culture process. In addition, we observed an increase of endothelial markers, indicating beneficial effects of EPC or myeloid cells on endothelial cell growth. Furthermore, real time PCR indicated high expression levels of CD68, CD11b and CD163 in co-cultures of EPC and MSC indicating that EPC act at least partly as macrophage like-cells.