CD34⁺ cells home, proliferate, and participate in capillary formation, and in combination with CD34_- cells enhance tube formation in a 3-dimensional matrix

Objective - Emerging evidence suggests that human blood contains bone marrow (BM)-derived endothelial progenitor cells that contribute to postnatal neovascularization. Clinical trials demonstrated that administration of BM-cells can enhance neovascularization. Most studies, however, used crude cell populations. Identifying the role of different cell populations is important for developing improved cellular therapies. Methods and Results - Effects of the hematopoietic stem cell-containing CD34⁺ cell population on migration, proliferation, differentiation, stimulation of, and participation in capillary-like tubule formation were assessed in an in vitro 3-dimensional matrix model using human microvascular endothelial cells. During movement over the endothelial monolayer, CD34⁺ cells remained stuck at sites of capillary tube formation and time- and dose-dependently formed cell clusters. Immunohistochemistry confirmed homing and proliferation of CD34⁺ cells in and around capillary sprouts. CD34⁺ cells were transduced with the LNGFR marker gene to allow tracing. LNGFR gene-transduced CD34 ⁺ cells integrated in the tubular structures and stained positive for CD31 and UEA-1. CD34⁺ cells alone stimulated neovascularization by 17%. Coculture with CD34^- cells led to 68% enhancement of neovascularization, whereas CD34^- cells displayed a variable response by themselves. Cell-cell contact between CD34⁺ and CD34^- cells facilitated endothelial differentiation of CD34⁺ cells. Conclusions - Our data suggest that administration of CD34⁺-enriched cell populations may significantly improve neovascularization and point at an important supportive role for (endogenous or exogenous) CD34^- cells.