Determining the Fate of Bone Marrow Mononuclear Cells in Tissue Engineered Vascular Grafts using MRI

The most widely used method of creating tissue engineered vascular grafts (TEVGs) for in vivo implantation consists of seeding autologous bone marrow cells (BMCs) onto biodegradable scaffolds. In this model of TEVG development it has traditionally been thought that stem cells and endothelial progenitor cells (EPCs) within the seeded bone marrow population gave rise to the cells of the neovessel. Recent work in our lab indicates that the seeded BMCs are not incorporated into the neovessel and are actually rapidly lost from the implanted scaffold. Here we show the feasibility of noninvasively monitoring this process by tracking ultrasmall superparamagnetic iron oxide (USPIO) labeled macrophages with MRI. Murine macrophages were labeled with USPIO through in vitro culture in media containing 2mg/ml of USPIO. The USPIO-labeled macrophages were seeded onto polyglycolic acid (PGA) scaffolds that were surgically implanted as inferior vena cava interposition grafts in SCID/bg mice. Images were then obtained using a 4.7T Bruker horizontal bore scanner with an optimized RARE spin echo sequence and a multislice-multiecho sequence to determine the T2 relaxation time with serial imaging. The T2 signal was found to be significantly lower immediately following implantation of the USPIO labeled scaffolds (T2 = 44+/-6.8 vs. 71+/-10.2), but increased rapidly to a value identical to that of control implants seeded with unlabeled macrophages (T2 = 63+/-12 vs. 63+/-14). This strongly indicates the rapid loss of seeded cells from the scaffolds, a finding verified using Prussian blue staining for iron containing macrophages on histological sections of explanted TEVGs. Our findings provide further support for the paradigm shift away from BMC neovessel incorporation towards the host cell based population of implanted TEVGs. Furthermore, we demonstrate one of the first successful applications of noninvasive MR imaging for serial study of cellular level processes in tissue engineering.