Abstract
Cell transplantation via direct intramyocardial injection is a promising therapy for patients with myocardial infarction (MI). Following intramuscular injection, however, retention of these dissociated cells at the cell graft site remains problematic; this poor retention adversely affects the efficacy of cell-transplantation therapy. In our previous study, a method for constructing spherically symmetric three-dimensional (3D) cell aggregates was developed by using a thermos-responsive methylcellulose (MC) hydrogel system. The grown cell aggregates can be harvested without using proteolytic enzymes; consequently, their inherent extracellular matrices (ECMs) and integrative adhesive agents remain well preserved. With an adequate physical size and adhesive ECMs, the 3D cell aggregates can entrap and retain in the muscular interstices after transplantation. Therefore, the remaining amount of transplanted cells is more than their dissociated counterparts. On the other hand, cell aggregates can develop a hypoxic microenvironment in their inner cores at distances that exceed the diffusion capacity of oxygen. By switching on a series of signal transduction mechanisms, hypoxia-inducible factors (HIFs) can cause the transcriptional activation of several pro-angiogenic genes. As a result, we hypothesize that transplantation of internally-hypoxia cell aggregates can trigger robust angiogenesis by HIF-1α-dependent angiogenic mechanisms prior to cell engraftment, thereby enhancing regional blood perfusion. In this work, 3D aggregates of human umbilical vein endothelial cells (HUVECs) and cord-blood mesenchymal stem cells (cbMSCs) are constructed using the methylcellulose hydrogel system. These cell aggregates are capable of forming widespread tubular networks together with the angiogenic marker αvβ3 integrin; they secret multiple pro-angiogenic, pro-survival, and mobilizing factors when grown on Matrigel. The aggregates of HUVECs/cbMSCs are exogenously engrafted into the peri-infarct zones of rats with MI via direct local injection. Multimodality noninvasive imaging techniques, including positron emission tomography, single photon emission computed tomography, and echocardiography, are employed to monitor serially the beneficial effects of cell therapy on angiogenesis, blood perfusion, and ventricular function, respectively. The myocardial perfusion is correlated with ventricular contractility, demonstrating that the recovery of blood perfusion helps to restore regional cardiac function, leading to the improvement in global ventricular performance. These experimental data reveal the efficacy of the exogenous transplantation of 3D cell aggregates after MI and elucidate the mechanism of cell-mediated therapeutic angiogenesis for cardiac repair.