Abstract
A recurring obstacle for cell-base strategies in treating ischemic diseases continues to be the significant cell loss during the process of transplantation. Additionally, the recipient microenvironment in the ischemic tissues may confer an elevated state of oxidative stress to the administered cells, thus hindering their adhesion and retention to the therapeutic target, ultimately limiting the scope of therapeutic benefit. In our previous studies, a thermo-responsive methylcellulose (MC) hydrogel system was employed to grow three-dimensional cell aggregates for the treatment of ischemic diseases. By using human umbilical vein endothelial cells (HUVECs) and cord-blood mesenchymal stem cells (cbMSCs), the fabricated cell aggregates have great potential in inducing therapeutic angiogenesis. Using a rat model of myocardial infarction (MI), the cell aggregates that were transplanted intramuscularly via local injection were demonstrated to be entrapped effectively in the interstices of muscular tissues. The engrafted cells subsequently promoted considerable angiogenesis, improving the post-infarcted heart function. Although the therapeutic efficacy of HUVEC/cbMSC aggregates appears to be favorable, the mechanism of their angiogenesis in repairing ischemic tissues remains elusive. In Study I, the process of cell-mediated angiogenesis and its therapeutic effects that are induced by exogenously engrafted HUVEC/cbMSC aggregates in rats with MI are investigated. By maximizing cell‒cell and cell‒ECM communications and establishing a hypoxic microenvironment in their inner cores, these cell aggregates are capable of forming widespread tubular networks together with the angiogenic marker αvβ3 integrin 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 global/regional 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. In Study II, we hypothesize that by concurrent delivery of an antioxidant N-acetylcysteine (NAC), the cell retention following transplantation of HUVEC/cbMSC aggregates in a mouse model with hindlimb ischemia may be significantly augmented. Our in vitro results demonstrate that the antioxidant NAC can successfully restore the reactive oxygen species (ROS)-impaired cell adhesion and recover the reduced angiogenic potential of HUVEC/cbMSC aggregates. In the animal study, we found that by scavenging the ROS generated in ischemic tissues, NAC has great potential to establish a receptive cell environment at the early stage of cell transplantation, thereby promoting the cell adhesion, retention, and survival of engrafted cell aggregates, which subsequently enhances therapeutic angiogenesis and ultimately results in blood flow recovery and limb salvage. The combinatory strategy using an antioxidant and cell aggregates may offer a new opportunity to boost the therapeutic efficacy for the treatment of ischemic diseases.