Cell tracking using iron oxide fails to distinguish dead from living transplanted cells in the infarcted heart

Abstract

Recently, debate has arisen about the usefulness of cell tracking using iron oxide-labeled cells. Two important issues in determining the usefulness of cell tracking with MRI are generally overlooked; first, the effect of graft rejection in immunocompetent models, and second, the necessity for careful histological confirmation of the fate of the labeled cells in the presence of iron oxide. Therefore, both iron oxide-labeled living as well as dead epicardium-derived cells (EPDCs) were investigated in ischemic myocardium of immunodeficient non-obese diabetic (NOD)/acid: non-obese diabetic severe combined immunodeficient (NOD/scid) mice with 9.4T MRI until 6 weeks after surgery, at which time immunohistochemical analysis was performed. In both groups, voids on MRI scans were observed that did not change in number, size, or localization over time. Several studies demonstrated that iron-labeled cellular transplants were visible with MRI and could be followed over time (3,4,6,7), even when a magnet of clinical field strength was used (4,8), although the detection limit was low and only high numbers of iron-loaded cells were visible (9). It was only assumed that these hypointense spots on the MRI images actually represented the transplanted living iron-loaded cells. It was not verified whether the transplanted cells were indeed present in histological sections, and whether MRI signal was really generated by the original transplant and not by, e.g., macrophages that had phagocytosed the iron-containing cells. The first study (5) that addressed this issue reported that the MRI signal originally generated by iron in transplanted MSCs appeared to be present regardless of the existence of these cells, which was later confirmed by Terrovitis et al. (10), who demonstrated, like Amsalem et al. (5) had done before, that iron-loaded macrophages created the signal rather than the iron-loaded living transplanted cells.As suggested by Sadek and Garry (11) in a comment on the study of Amsalem et al. (5), the results should be extended by studies in immunocompromised animals. In the previous studies, cardiac-derived stem cells (CDCs) or MSCs, which are not immunoprivileged, were harvested from "syngeneic" rats and transplanted into other inbred immunocompetent rats (5,10). However, rats cannot, unlike mouse donors from same inbred strain, be considered really syngeneic but rather allogeneic (5), implying that the cells could simply be rejected by the host, and thus that the concern regarding the utility of SPIOs in cardiac cell therapy applies only to comparable studies with nonautologous cells and immunocompetent animals. Sadek and Garry (11), therefore, pleaded for similar experiments with either autologous cell transplantation in normal animals, or with allogeneic stem cell transplantation into immunodeficient animals (11). It could very well be that iron-loaded transplanted cells are not rejected in those settings or that the clearance pattern is altered in such a way that the hypointense signals from iron-oxide largely correspon...

The long-term fate of mesenchymal stem cells labeled with magnetic resonance imaging-visible polymersomes in cerebral ischemia

“…Hypointensities detected on MRI refer only to iron oxide particles themselves, but cannot give any information on the type or viability of labeled cells. 15 Besides iron particles retained in labeled cells, the persistence of hypointensities may be affected by the imbalance between the massive iron oxide particles released from the dead labeled stem cells and the relatively restricted iron oxide clearance capacity of the brain. 17 It is well known that the iron oxide clearance was mainly through endogenous macrophages/ microglia engulfment, which are then taken away by cerebral blood flow.…”

“…13,14 The ability of SPION-based MRI to reliably evaluate long-term cell engraftment and survival remains controversial. Further studies have shown that long-lasting hypo-signal intensity on MRI mainly originated from macrophages that engulfed SPION, 15,16 or from extracellular iron particles that persisted in the interstitial space after the grafted labeled cells died. 17 However, almost all of these investigations mainly focused on cell fate in myocardial infarction.…”

Nanoparticles and clinically applicable cell tracking

“…The released nanoparticles can reside for a long time or be taken up by other cells and thus create a contrast agentrelated signal that is not related to the presence of the transplanted cell itself. 51,71,[110][111][112][113] In the study by Guenoun et al, 71 it was shown that prolonged retention of released cell label is dependent on nanoparticle characteristics. In this study, a comparison was made between iron oxide nanoparticles and Gdliposomes (liposomes containing gadopentetate dimeglumine in the water phase) as cell labelling agents for monitoring the in vivo fate of transplanted cells.…”

“…20 In this study, tumour antigen-containing DCs were labelled with iron oxide nanoparticles or 111 In-oxine and coinjected into lymph nodes of 10 patients with melanoma under ultrasound guidance. The patients subsequently underwent MRI and scintigraphy, by which the labelled DC vaccine could be visualized.…”

In vivo MRI tracking of iron oxide nanoparticle-labeled human mesenchymal stem cells in limb ischemia

“…Subsequently, macrophages may phagocytose dead cells and iron particles, which may become secondarily iron labeled. 33 Second, cell viability was different after transplantation, which was due to the different injection sites. 34 Finally, although we used PBS to remove loosely attached or extracellular particles, it is possible that a small amount of iron particles was attached to the surface of the labeled cells.…”