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Circulating blood–derived vasculogenic cells improve neovascularization of ischemic tissue by a broad repertoire of potential therapeutic actions. Whereas initial studies documented that the cells incorporate and differentiate to cardiovascular cells, other studies suggested that short-time paracrine mechanisms mediate the beneficial effects. The question remains to what extent a physical incorporation is contributing to the beneficial effects of cell therapy. By using the inducible suicide gene thymidine kinase to deplete transplanted cells, we determined the contribution of physical incorporation in 3 animal models. After acute myocardial infarction, depletion of cells 14 days after infusion resulted in a reduction of capillary density and a substantial deterioration of heart function. Likewise, neovascularization of Matrigel plugs and ischemic limbs was significantly suppressed when infused cells were depleted 7 days after infusion. Induction of cell death in the previously transplanted cells reduced perfusion and led to vascular leakage as evidenced by Evans blue extravasation. These results indicate that physical incorporation and persistence of cells contribute to cell-mediated improvement of neovascularization and cardiac function. Long-term paracrine activities and/or cell intrinsic mechanisms may have contributed to the maintenance of functional improvement.

Endothelial progenitor cells (EPCs) are recruited to ischemic regions and improve neovascularization. Integrins contribute to EPC homing. High-mobility group box 1 (HMGB1) is a nuclear protein that is released extracellularly on cell necrosis and tissue damage, eliciting a proinflammatory response and stimulating tissue repair. In the present study, we investigated the effects of HMGB1 on EPC homing. EPCs express the HMGB1 receptors RAGE (receptor for advanced glycation end products) and TLR2 (Toll-like receptor 2). EPC migration was stimulated by HMGB1 in a RAGE-dependent manner. In addition, the HMGB1-induced migration of EPCs on fibronectin and fibrinogen was significantly inhibited by antibodies against β 1 and β 2 integrins, respectively. Short-term prestimulation of EPCs with HMGB1 also increased EPC adhesion to endothelial cell monolayers, and this effect was blocked by antibodies to β 2 integrins or RAGE. HMGB1 increased EPC adhesion to the immobilized integrin ligands intercellular adhesion molecule-1 and fibronectin in a RAGE-dependent manner. Strikingly, HMGB1 rapidly increased integrin affinity and induced integrin polarization. Using intravital microscopy in a tumor model of neovascularization, prestimulation of EPCs with HMGB1 enhanced the initial in vivo adhesion of EPCs to microvessels and the recruitment of EPCs in the tumor tissue. In addition, prestimulation of EPCs with HMGB1 increased the homing of EPCs to ischemic muscles. In conclusion, these data represent a link between HMGB1 and integrin functions of EPCs and demonstrate that HMGB1 stimulates EPC homing to ischemic tissues. These results may provide a platform for the development of novel therapeutic approaches to improve EPC homing.

Endothelial progenitor cells (EPCs) and hematopoietic progenitor cells are recruited to ischemic regions, improving neovascularization. β1 and β2 integrins play a crucial role for progenitor cell homing to ischemic tissues. Integrin activity is regulated by chemokines and their respective G protein–coupled receptors. The phosphatidylinositol-3-kinase catalytic subunit γ (PI3Kγ) is the PI3K isoform that selectively transduces signals from G protein–coupled receptors. Here, we investigated the role of PI3Kγ as a signaling intermediate in the chemokine-induced integrin-dependent homing functions of progenitor cells. A pharmacological PI3Kγ inhibitor significantly reduced chemokine-induced chemotaxis and stromal cell–derived factor (SDF)1α-induced transmigration of human EPCs. Moreover, the PI3Kγ inhibitor significantly reduced SDF1α-induced adhesion of EPCs to intercellular adhesion molecule-1 and human umbilical vein endothelial cell monolayers. These findings were corroborated with Lin−bone marrow–derived progenitor cells from PI3Kγ-deficient mice that displayed reduced SDF1α-induced migration and intercellular adhesion molecule-1 adhesion as compared with wild-type cells. Pharmacological inhibition or genetic ablation of PI3Kγ reduced SDF1α-induced integrin activation in human EPCs and in murine Lin−BM-derived progenitor cells, respectively. In vivo, the homing of PI3Kγ-deficient Lin−progenitor cells to ischemic muscles after intravenous infusion in the model of hindlimb ischemia and their neovascularization-promoting capacity was reduced as compared with wild-type cells. In conclusion, PI3Kγ is integral to the integrin-dependent homing of progenitor cells.