Magnetic resonance hypointensive signal primarily originates from extracellular iron particles in the long-term tracking of mesenchymal stem cells transplanted in the infarcted myocardium

Abstract

Zheyong Huang,1,* Chenguang Li,1,* Shan Yang,2 Jianfeng Xu,1 Yunli Shen,3 Xinxing Xie,4 Yuxiang Dai,1 Hao Lu,1 Hui Gong,5 Aijun Sun,1 Juying Qian,1 Junbo Ge1 1Shanghai Institute of Cardiovascular Diseases, Zhongshan Hospital, Fudan University, Shanghai, People’s Republic of China; 2Department of Radiology, Zhongshan Hospital, Fudan University, Shanghai, People’s Republic of China; 3Department of Cardiology, Shanghai East Hospital, Tongji University, Shanghai, People’s Republic of China; 4Department of Cardiology, Qianfoshan Hospital, Shandong University, Jinan, Shandong Province, People’s Republic of China; 5Institute of Biomedical Science, Fudan University, Shanghai, People’s Republic of China *These authors contributed equally to this work Purpose: The long-lasting hypointensities in cardiac magnetic resonance (CMR) were believed to originate from superparamagnetic iron oxide (SPIO)-engulfed macrophages during long-term stem cell tracking. However, the iron clearance capacity of the ischemic heart was limited. Therefore, we speculated that the extracellular SPIO particles may also be involved in the generation of false-positive signals.Methods and results: Male swine mesenchymal stem cells (MSCs) were incubated with SPIO for 24 hours, and SPIO labeling had no significant effects on either cell viability or differentiation. In vitro studies showed that magnetic resonance failed to distinguish SPIO from living SPIO-MSCs or dead SPIO-MSCs. Two hours after the establishment of the female swine acute myocardial infarction model, 2×107 male SPIO-labeled MSCs (n=5) or unlabeled MSCs (n=5) were transextracardially injected into the infarcted myocardium at ten distinct sites. In vivo CMR with T2 star weighted imaging-flash-2D sequence revealed a signal void corresponding to the initial SPIO-MSC injection sites. At 6 months after transplantation, CMR identified 32 (64%) of the 50 injection sites, where massive Prussian blue-positive iron deposits were detected by pathological examination. However, iron particles were predominantly distributed in the extracellular space, and a minority was distributed within CD68-positive macrophages and other CD68-negative cells. No sex-determining region Y DNA of donor MSCs was detected.Conclusion: CMR hypointensive signal is primarily caused by extracellular iron particles in the long-term tracking of transplanted MSCs after myocardial infarction. Consideration should be given to both the false-positive signal and the potential cardiac toxicity of long-standing iron deposits in the heart. Keywords: superparamagnetic iron oxide nanoparticles, SPIO, cardiac magnetic resonance (CMR), stem cells tracking, extracellular iron particles, myocardial infarction (MI)