Combined implantation of CD34+ and CD14+ cells increases neovascularization through amplified paracrine signalling

Cell therapy strategies that use adult peripheral blood-derived CD34⁺ progenitor cells are hampered by low cell numbers and the infrequent cellular incorporation into the neovasculature. Hence, the use of CD34⁺ cells to treat ischaemic diseases is under debate. Interaction between CD34⁺ cells and CD14⁺ cells results in superior endothelial differentiation of CD14⁺ cells in vitro, indicating that cell therapy approaches utilizing both CD34⁺ and CD14⁺ cells may be advantageous in therapeutic neovascularization. Here, human CD34⁺ and CD14⁺ cells were isolated from adult peripheral blood and implanted subcutaneously into nude mice, using matrigel as the carrier. Combined implantation of human CD34⁺ and CD14⁺ cells resulted in superior neovascularization, compared to either cell type alone, albeit incorporation of human cells into the murine vasculature was not observed. Human CD34⁺ and CD14⁺ cells produced and secreted a pentad of pro-angiogenic mediators, such as HGF, MCP-1 and IL-8, bFGF and VEGFa in monoculture. The production and secretion of pro-angiogenic mediators by CD14⁺ cells was highly amplified upon incubation with conditioned medium from CD34⁺ cells. In vivo, neovascularization of matrigel implants did not rely on the endothelial differentiation and incorporation of CD34⁺ or CD14⁺ cells, but depended on the paracrine effects of IL-8, MCP-1, HGF, bFGF and VEGFa secreted by implanted cells. Administration of this growth factor/cytokine pentad using matrigel as a carrier results in cell recruitment and microvessel formation equal to progenitor cell-induced neovascularization. These data provide new insights on neovascularization by cell therapy and may contribute to new strategies for the treatment of ischaemic diseases.