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In Vivo Mononuclear Cell Tracking Using Superparamagnetic Particles of Iron Oxide

Feasibility and Safety in Humans
Originally publishedhttps://doi.org/10.1161/CIRCIMAGING.112.972596Circulation: Cardiovascular Imaging. 2012;5:509–517

Abstract

Background—

Cell therapy is an emerging and exciting novel treatment option for cardiovascular disease that relies on the delivery of functional cells to their target site. Monitoring and tracking cells to ensure tissue delivery and engraftment is a critical step in establishing clinical and therapeutic efficacy. The study aims were (1) to develop a Good Manufacturing Practice–compliant method of labeling competent peripheral blood mononuclear cells with superparamagnetic particles of iron oxide (SPIO), and (2) to evaluate its potential for magnetic resonance cell tracking in humans.

Methods and Results—

Peripheral blood mononuclear cells 1–5×109 were labeled with SPIO. SPIO-labeled cells had similar in vitro viability, migratory capacity, and pattern of cytokine release to unlabeled cells. After intramuscular administration, up to 108 SPIO-labeled cells were readily identifiable in vivo for at least 7 days using magnetic resonance imaging scanning. Using a phased-dosing study, we demonstrated that systemic delivery of up to 109 SPIO-labeled cells in humans is safe, and cells accumulating in the reticuloendothelial system were detectable on clinical magnetic resonance imaging. In a healthy volunteer model, a focus of cutaneous inflammation was induced in the thigh by intradermal injection of tuberculin. Intravenously delivered SPIO-labeled cells tracked to the inflamed skin and were detectable on magnetic resonance imaging. Prussian blue staining of skin biopsies confirmed iron-laden cells in the inflamed skin.

Conclusions—

Human peripheral blood mononuclear cells can be labeled with SPIO without affecting their viability or function. SPIO labeling for magnetic resonance cell tracking is a safe and feasible technique that has major potential for a range of cardiovascular applications including monitoring of cell therapies and tracking of inflammatory cells.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov; Unique identifier: NCT00972946, NCT01169935.

Introduction

Stem cell and other cell-based therapies have emerged as potential, novel treatment options for a wide range of diseases including acute myocardial infarction and severe heart failure.1–5 Ensuring the delivery of a sufficient number of cells to the target site is critical to the development and assessment of these therapies. Several methods have been proposed for tracking cells in vivo, but their translation into the clinical setting has been hampered for many reasons including limitations of the imaging modality in humans and the failure of reagents to comply with Good Manufacturing Practice (GMP) standards: a prerequisite for clinical use.

Clinical Perspective on p 517

Radio-imaging techniques, such as scintigraphy and positron emission tomography, can be sensitive but offer poor spatial resolution and expose patients to ionizing radiation. In addition, although some radiotracers have a long half-life (eg, 89-Zircuronium 78.4 h, 124-Iodine 100.3 h) those that are most widely available have a short half-life (eg, 18-Fluorine), limiting the duration of follow-up. By virtue of its high spatial resolution, excellent soft tissue contrast, and avoidance of ionizing radiation, magnetic resonance imaging (MRI) is ideally suited to human cell-tracking studies. MRI contrast agents consisting of dextran-coated superparamagnetic particles of iron oxide (SPIO) are available for use in humans and have been used for clinical liver imaging. SPIO possess intense superparamagnetism and, by inducing local magnetic field inhomogeneities, cause rapid dephasing of spinning protons. The T2* relaxation time is shortened, causing a profound reduction in signal intensity on T2- and particularly T2*-weighted (T2*W) imaging. Compared with paramagnetic agents such as gadolinium, SPIO are detectable at much lower concentrations and are readily taken up by resident macrophages in the reticuloendothelial system. SPIO are not toxic to cells and are biodegradable in vivo.6–11 It is important to note, however, that the safety of intravenous administration of SPIO-labeled cells in humans has yet to be demonstrated, and would be critical for the development and Food and Drug Administration approval of novel clinical cell-tracking agents.

An effective method has been described for incorporating SPIO into phagocytic cells using protamine sulphate as a polycationic transfection agent to enhance its uptake through electrostatic interactions.12 We have modified and translated this method into a GMP-compliant protocol and successfully (1) labeled human peripheral blood mononuclear cells (PBMC), (2) confirmed normal cell viability and function, and (3) delivered and tracked SPIO-labeled cells in vivo in humans.

Methods

Ethical and Regulatory Considerations

Clinical studies (NCT00972946; NCT01169935) were approved by the local research ethics committee and conducted in accordance with the Declaration of Helsinki with the written informed consent of all participants.

Cell Labeling With SPIO

Cell Collection

Blood was collected by venesection into citrate or by leukapheresis (COBE Spectra; Gambro BCT, Lakewood, CO) to yield 1–5×108 or 1–5×109 PBMC, respectively.

Edinburgh Protocol for Labeling Mononuclear Cells With SPIO

A GMP-compliant protocol for labeling PBMC with SPIO was optimized (online-only Data Supplement Methods and online-only Data Supplement Figure I). PBMC were isolated by density-gradient separation and suspended in 0.9% saline (4×106 cells/mL). Endorem (100 µg/mL; Guerbet, Villepinte, France) and protamine sulphate (4 µg/mL; Prosulf) were added to the cell suspension, which was incubated at room temperature for 2 hours with continuous agitation.

For clinical studies, cell preparation and labeling was undertaken in a GMP facility. At the end of the incubation period, labeled cells were supplied in a low-adherence bag (Cryocyte; Baxter, Deerfield, IL) for reinfusion.

Evaluation of the Effects of SPIO Labeling on Cell Viability and Function

Isolation of Monocytes From PBMC

Monocytes were isolated from PBMC suspensions using indirect immunomagnetic selection (Monocyte Isolation Kit II; Miltenyi Biotec, Auburn, CA).

Cellular Iron Uptake

Cellular SPIO uptake was confirmed by Prussian blue staining of methanol-fixed cytospin preparations. Cellular iron content was determined using the Ferrozine assay (Sigma-Aldrich, Gillingham, Dorset, United Kingdom).

Cell Viability

Monocytes were allowed to adhere for 1 hour in 24-well tissue culture plates (106 cells per well; 37°C; 5% CO2) after which the supernatant was harvested and exchanged to prevent continued uptake of SPIO and to avoid interference of excess SPIO with assays.

SPIO-labeled and unlabeled cells were harvested at 1, 24, and 72 hours, exposed to fluorescein isothiocyanate-annexin V and propidium iodide, and analyzed by flow cytometry. The percentages of cells positive for annexin V binding, propidium iodide staining, or both were combined to give the percentage cytotoxicity. Viability was also assessed at 24, 48, and 72 hours using a colorimetric lactate dehy­drogenase assay (Roche Diagnostics Ltd, West Sussex, United Kingdom).

Cytokine Release

Cells were plated out as described in the preceding section. The concentration of the proinflammatory cytokines released from cells 24, 48, and 72 hours postlabeling was measured using a cytometric bead array kit (Becton Dickinson, Franklin Lakes, NJ).

Migratory Capacity

In vitro migratory capacity was assessed using a modified Boyden chamber. Labeled and unlabeled purified monocytes (2.5×105 cells per well) were allowed to migrate for 2 hours through a microporous (5 µmol/L) membrane (Corning Costar, Corning, NY) toward monocyte chemoattractant protein-1 (R&D Systems, Abingdon, United Kingdom). Transmigration was quantified by counting the cells adherent to the undersurface of the membrane.

Clinical Studies

Participants underwent formal clinical assessment and were excluded if they had renal dysfunction (estimated glomerular filtration rate <25 mL/min), hepatic dysfunction (Childs-Pugh score B or C), positive virology screen, pregnancy, breastfeeding, blood dyscrasia, anemia (hemoglobin concentration <12 g/dL), iron storage disorder, active malignancy, chronic inflammatory condition, risk factors for protamine allergy, intercurrent illness, or contraindication to MRI scanning.

Intramuscular Administration

Six healthy volunteers received three 2-mL intramuscular thigh injections of unlabeled autologous PBMC (107), SPIO-labeled autologous PBMC (107), and SPIO alone. Participants underwent MRI scanning in a 1.5 T scanner immediately and at 7 days.

Phased-Dose Intravenous Administration of Labeled Cells

To establish the safety of intravenous administration of SPIO-labeled cells, we undertook a phased-dosing study in which 2 volunteers each received 6 increasing doses of SPIO-labeled autologous PBMC (104–109 cells) over 4 study visits. Cell infusions were performed in a clinical research facility with noninvasive physiological monitoring (pulse oximetry, noninvasive blood pressure, and temperature) and clinical observation during cell reinfusion and the subsequent observation period. Cells were administered through a blood giving set with a 200-µmol/L macroaggregate filter. Each cell infusion lasted 30 to 60 minutes (depending on volume) and was followed by a 60-minute observation period.

Blood was sampled at each visit before reinfusion and 1 hour postinfusion of labeled cells. Blood was drawn for full blood count, routine biochemistry, and coagulation parameters including prothrombin time, activated partial thromboplastin time, fibrinogen, and D-dimer.

Intravenous Administration of Labeled Cells

Twelve healthy volunteers were recruited and received a low dose of SPIO-labeled autologous PBMC obtained from a 250-mL venesection (≈1–5×108 cells; n=6) or a high dose of cells obtained from a standard leukapheresis collection (≈1–5×109 cells; n=6). Intravenous cell infusions were performed, and blood samples were obtained as described earlier in the article. These participants underwent MRI scanning in a 3T scanner before and 2 hours, 24 hours, 48 hours, and 7 days after administration of cells.

Mantoux Test

A local cutaneous inflammatory response was induced in the thigh of healthy volunteers using the Mantoux test: intradermal injection of 2 units of tuberculin purified protein derivative. In previously exposed individuals, tuberculin purified protein derivative stimulates a type IV delayed, cell-mediated hypersensitivity reaction involving an influx of neutrophils initially (≈6 hours), and subsequently of monocytes (12–48 hours) and lymphocytes (12–96 hours) into the dermal skin layer.13,14 Twenty-four hours after the Mantoux test, participants underwent a standard leukapheresis collection and later the same day received an infusion of ≈109 SPIO-labeled autologous PBMC. T2-weighted and T2*W imaging was performed before and 2, 24, and 48 hours after the administration of cells. A skin biopsy was obtained from the site of the Mantoux test 96 hours after administration of cells (120 hours after the Mantoux test), fixed in 4% paraformaldehyde and stained with Prussian blue.

Magnetic Resonance Imaging

Volunteers receiving cells intramuscularly underwent MRI scanning of the thigh in a 1.5 T Phillips scanner using a T2-weighted sequence with a long echo time (TE; 137 ms).

Volunteers undergoing intravenous infusions were scanned using a 3T Magnetom Verio scanner (Siemens, Erlangen, Germany). Routine localizer sequences were applied after which T2-weighted (TE 87 ms; repetition time 1800 ms; matrix 256×192; field of view 380×285 mm) and T2*W (multiecho TE, 4.1–22.1 ms; repetition time, 100 ms; flip angle, 15°; matrix, 282×512; field of view, 282×410 mm; slice width, 5 mm) breath-hold sequences were used to acquire axial images of the upper abdomen.

T1-weighted (3-dimensional volumetric interpolated breath-hold examination; TE/repetition time, 2.0/5.5 ms; matrix, 416×416; field of view, 210×210 mm; slice width, 5 mm) and T2*W (2-dimensional multislice gradient echo; TE, 9.8, 12.3, 14.8 ms; repetition time, 120.0 ms; matrix size, 312×384; field of view, 203×250 mm; slice width, 5 mm) axial images of the thigh were acquired after Mantoux testing.

Image Analysis

After local administration of SPIO or cells, T2-weighted axial images of the thigh were inspected for evidence of a change in signal intensity.

After systemic administration of SPIO-labeled cells, transaxial abdominal images were analyzed on the basis of R2*. T2* is the decay constant for the decay of signal intensity with increasing TE, and its inverse R2* (T2*=1/R2*) has been used to evaluate tissue iron concentration.15

From the multiecho T2*W data, a 6-echo R2* map was generated. Regions of interest were selected on the initial echo (4.1 ms) taking care to avoid major blood vessels and motion artifact. Five regions of consistent size (0.5 cm2) were selected in the spleen, liver, skeletal muscle (control), and background noise, and copied to the R2* map. Relationship to distinctive anatomical features was used to ensure that regions chosen were consistent across the 5 study visits.

For cell tracking to the site of the Mantoux test, higher spatial resolution was required to allow visualization of the skin and subcutaneous tissue. The individual echoes were acquired separately and had to be registered together for R2* map generation using in-house software. Regions of interest were selected in the inflamed skin, normal unaffected skin (control), skeletal muscle (control), liver, and spleen.

Statistical Analysis

In vitro data were analyzed by Mann-Whitney U test. Blood results and physiological monitoring data were compared by Wilcoxon signed rank test (SPSS Inc, Chicago, IL). R2* MRI data were analyzed by the Friedman test with Dunn correction. Exact testing was used throughout, and 2-sided P<0.05 was regarded as statistically significant.

Results

In Vitro Evaluation of the Effect of SPIO Labeling on Cell Viability and Function

It is important that the behavior of SPIO-labeled mononuclear cells remains unchanged in terms of cell viability and function so that the results of in vivo cell-tracking studies accurately reflect the biological processes being observed. See online-only Data Supplement Methods section for more detail.

Confirmation of SPIO Uptake

Prussian blue staining of labeled cells demonstrated intracellular accumulation of iron nanoparticles (Figure 1). The entire PBMC population, including both lymphocytes and monocytes, was exposed to SPIO but SPIO uptake was predominantly confined to phagocytic monocytes (online-only Data Supplement Figure II). The median (interquartile range) iron content of labeled monocytes was 17 (14–22) pg per cell compared with 1.3 (0.7–1.4) pg per cell for unlabeled monocytes (P=0.03).

Figure 1.

Figure 1. Cellular uptake of superparamagnetic particles of iron oxide (SPIO). Cytospin preparation of peripheral blood mononuclear cells (PBMC) labeled with SPIO and stained with Prussian blue and Nuclear Fast red. Iron nanoparticles are seen within the cytoplasm.

Assessment of Cytotoxicity

SPIO labeling had no effect on cellular release of inflammatory cytokines (Figure 2). Viability of labeled and unlabeled cells was equivalent up to 72 hours postlabeling (Figure 3A and 3B).

Figure 2.

Figure 2. Effect of superparamagnetic particles of iron oxide (SPIO) labeling on cytokine release. SPIO labeling did not affect the cytokine (interleukin-1 [IL-1], IL-6, IL-10, tumor necrosis factor [TNF], IL-12p70) release profile up to 72 hours postlabeling (P>0.05 for all cytokines, at all time points; n=4).

Figure 3.

Figure 3. Effect of superparamagnetic particles of iron oxide (SPIO) labeling on cell viability and chemotaxis. Labeling mononuclear cells with SPIO had no effect on cell viability measured either by flow cytometric analysis of annexin V binding in combination with propidium iodide (PI) staining (A) or a colorimetric assay for lactate dehydrogenase release (B). There was no difference in viability between unlabeled and labeled cells at any time point investigated (P>0.05 for both methods at all time points; n=3). In an in vitro migration assay, cells were allowed to migrate through a 5-µmol/L microporous membrane toward 0 or 50 ng/mL of the chemokine human recombinant monocyte chemoattractant protein-1 (MCP-1). C, Quantification of chemotaxis demonstrated that SPIO labeling did not affect migratory capacity compared with unlabeled cells (P>0.05 for both 0 and 50 ng/mL MCP-1; n=6). Migrated cells adhered to the undersurface of the membrane and were stained with DiffQuik physiological stain (staining nuclei purple) and counterstained with Prussian blue causing SPIO particles carried in the cytoplasm to appear blue. Comparison was made of cells labeled according to the original protocol, which clumped and were unable to migrate (D and E), with cells labeled according to the Edinburgh protocol (F and G) that remained in single cell suspension and migrated in equivalent numbers to unlabeled cells (H and I).

Effect of SPIO Labeling on Chemotaxis

Cell-tracking studies are critically dependent on the ability of labeled cells to migrate to the target site. Cells labeled according to the optimized Edinburgh protocol migrated in equivalent numbers to unlabeled cells (154 [150–184] SPIO-labeled cells versus 112 [105–188] unlabeled cells at 50-ng/mL monocyte chemoattractant protein-1; median [interquartile range]; P=0.29). Prussian blue staining confirmed the presence of SPIO in the cytoplasm of migrating labeled cells. (Figure 3C3I)

Clinical Studies

Intramuscular Administration

Injections were performed successfully in all participants without complication. A signal deficit was observed at the site of administration of SPIO alone and SPIO-labeled cells (Figure 4). Although an edematous change was noted at the site of injection of unlabeled cells, no dropout of signal was seen. These findings were consistent for all 6 participants and persisted for at least 7 days.

Figure 4.

Figure 4. Magnetic resonance imaging scanning after intramuscular administration of cells. T2-weighted imaging of the thigh after intramuscular administration of labeled cells. A signal deficit is seen in relation to injection of superparamagnetic particles of iron oxide (SPIO) alone (A) and SPIO-labeled cells (B) but not unlabeled cells (C). These findings were consistent for all 6 participants.

Safety of Intravenous Administration of SPIO-Labeled Cells

In the phased-dosing study, all cell infusions in both participants were well tolerated with no early or late clinical side effects. A further 12 volunteers then received a low dose (2.03±0.18×108 cells; n=6) or a high dose of SPIO-labeled autologous PBMC obtained from a standard leukapheresis collection (2.23±0.51×109 cells; n=6).

Cell infusions had no effect on heart rate, blood pressure, or temperature (Table 1), and there were no clinically significant microembolic events after cell administration. Peripheral perfusion, monitored continuously by digital pulse oximetry during and for an hour after cell infusion, was unaffected. Demonstrating further that labeled cells did not have a procoagulant effect in vivo, infusion of SPIO-labeled cells did not affect any coagulation variable, and there were no clinically significant effects on routine clinical hematological and biochemical variables (Table 2).

Table 1. Noninvasive Physiological Monitoring Parameters (Median and IQR) Before and at the End of Infusion of 1–5×109 SPIO-Labeled PBMC (n=12)

Preinfusion Median (IQR)Postinfusion Median (IQR)P
Systolic blood pressure, mm Hg126 (120–133)121 (114–130)0.13
Diastolic blood pressure, mm Hg74 (68–85)74 (67–79)0.69
Heart rate, beats per minute67 (58–78)68 (58–76)0.20
Temperature, °C36.9 (36.4–37.0)36.9 (36.7–37.2)0.42
Oxygen saturations on air, %99 (98–100)99 (98–100)1.00

IQR indicates interquartile range; PBMC, peripheral blood mononuclear cells; and SPIO, superparamagnetic particles of iron oxide.

Table 2. Hematological, Biochemical, and Coagulation Variables Before and After Infusion of 1–5×109 SPIO-Labeled PBMC (n=12)

Normal RangePreinfusion Median (IQR)Postinfusion Median (IQR)P
Hemoglobin, g/L115–165133 (120–148)130 (120–140)0.06
White blood count,×109/L4.0–11.05.9 (5.2–7.6)7.4 (5.4–9.7)0.28
Platelets,×109/L150–350259 (195–274)235 (194–280)0.33
Prothrombin time, s10.5–13.511 (11–12)11 (11–12)0.18
Activated partial thromboplastin time, s26–3629 (29–32)30 (27–32)0.67
Fibrinogen, g/L1.5–42.9 (2.3–3.4)2.7 (2.3–3.2)0.10
D-Dimer, ng/mL0–23067 (52–200)
Urea, mmol/L2.5–6.64.4 (3.6–5.2)4.4 (4.0–5.1)0.68
Creatinine, μmol/L60–12064 (58–78)67 (52–78)0.67
Sodium, mmol/L135–145140 (139–141)140 (138–141)0.40
Potassium, mmol/L3.6–5.04.1 (3.8–4.4)4.0 (3.9–4.2)0.26
TCO2, mmol/L20–3027 (25–28)27 (25–30)0.16
Bilirubin, μmol/L3–1611 (9–15)10 (8–15)0.31
Alanine amino transferase, U/L10–5520 (16–22)19 (14–30)0.57
Alkaline phosphatase, U/L40–12572 (53–82)68 (48–89)0.67

PBMC indicates peripheral blood mononuclear cells; SPIO, superparamagnetic particles of iron oxide; IQR indicates interquartile range; and TCO2, total CO2.

MRI Scanning

After administration of labeled cells, the signal intensity was reduced in the liver and spleen on T2- (Figure 5) and T2*W imaging.

Figure 5.

Figure 5. Magnetic resonance imaging of the upper abdomen after systemic administration of cells. T2-weighted transaxial slice through the upper abdomen before (A) and after (B) intravenous administration of superparamagnetic particles of iron oxide (SPIO)-labeled cells. Signal intensity is reduced in the liver (L) and spleen (S) after administration of cells. Change in R2* from baseline 2 hours, 24 hours, 48 hours, and 7 days after intravenous administration of a low dose (C; 1–5×108 peripheral blood mononuclear cells [PBMC]; n=6) or a high dose (D; 1–5×109 PBMC; n=6) of SPIO-labeled cells. An increase in R2* is seen in the spleen (P<0.001) and liver (P<0.001) but not skeletal muscle (control; P>0.05).

The use of 6 TEs for R2* value calculation was validated by plotting signal intensity against TE. Even at longer TE, the signal was clearly distinct from the background noise level, and exponential decay of signal intensity with time was confirmed.

There was a marked increase in R2* in the liver and spleen (P<0.001) after administration of cells (Figure 5) reflecting dose-dependent accumulation of SPIO-labeled cells at these sites. There was no change in R2* in skeletal muscle (negative control).

Cell Tracking to an Inflammatory Focus

At 24 and 48 hours after administration of SPIO-labeled cells, an increase in R2* was observed in inflamed skin (P<0.001) but not adjacent normal skin or skeletal muscle (P>0.05; Figure 6). Prussian blue staining of skin biopsies obtained from the site of the Mantoux test 96 hours after administration of cells, revealed occasional iron-laden cells in the inflamed skin consistent with the diminishing monocyte/macrophage cell numbers by this stage in a type IV hypersensitivity response. In controls (negative Mantoux response), there was no change in R2* in the skin at the site of the Mantoux test, in adjacent normal skin, or in skeletal muscle (P>0.05; Figure 7).

Figure 6.

Figure 6. Cell tracking to a region of cutaneous inflammation. A, T1-weighted axial image of the thigh 24 hours after the Mantoux test and (B) a magnified area of normal unaffected skin/subcutaneous tissue. A thick layer of aqueous jelly was applied to the skin to minimize artifact occurring at the interface between the outer layer of skin and the surrounding air. R2* maps before (C) and 2 hours (D), 24 hours (E), and 48 hours (F) after infusion of superparamagnetic particles of iron oxide (SPIO)–labeled cells show no change in R2* in normal skin/subcutaneous tissue on the medial aspect of the thigh after cell administration. The skin and subcutaneous tissue thickens in the region of the Mantoux test on the anterior thigh (G). Corresponding R2* maps of this inflamed skin (H) show a marked increase in R2* 2 hours (I), 24 hours (J), and 48 hours (K) after administration of SPIO-labeled cells. Consistent with accumulation of SPIO-labeled cells, a significant increase in R2* is observed 24 and 48 hours after cell administration in inflamed skin in the region of the Mantoux test (L; P<0.001) as well as in the liver and the spleen (P<0.001), but not in unaffected normal skin or skeletal muscle (control; P>0.05). No change in R2* was seen if labeled cells were administered in the absence of a positive Mantoux response (M). The strongly positive cutaneous response to the Mantoux test is clearly visible in the anterior thigh (N). SPIO-loaded cells are seen on Prussian blue staining of a skin biopsy from this area 96 hours after cell infusion (O and P).

Figure 7.

Figure 7. Negative Mantoux response (control). R2* maps before (A) and 2 hours (B), 24 hours (C), and 48 hours (D) after infusion of superparamagnetic particles of iron oxide (SPIO)–labeled cells show no change in R2* in skin and subcutaneous tissue at the site of the Mantoux test after cell administration, although there is some artifact at the skin interface in all 4 images. There is no evidence of an inflammatory response in the skin at the site of the Mantoux test (E). F, R2* increased significantly in the liver and spleen (P<0.001) but not in the skin at the site of the Mantoux test or skeletal muscle (P>0.05) after administration of SPIO-labeled cells.

Discussion

In the present study, we have demonstrated, for the first time, that human mononuclear cells can be labeled with SPIO under GMP-compliant conditions without affecting cell viability or migratory capacity. Furthermore, labeled cells can be visualized in vivo at clinically relevant field strengths after local or systemic administration, and it is important to note that SPIO-labeled cells can be tracked to a site of focal inflammation in humans. This technique therefore has the potential to track the distribution, time course, and fate of cells administered systemically as part of cell therapy: a critical component of the development of cellular therapeutic strategies, and ultimately of monitoring the success of treatment.

It is essential that techniques for labeling cells for cell-tracking studies do not impair cell function. We were able to demonstrate in vitro that cells labeled according to the Edinburgh protocol were viable and exhibit normal in vitro migration toward the chemokine monocyte chemoattractant protein-1.

There are several mechanisms through which SPIO might logically be expected to affect cell viability and function. However, it would appear that monocytes and macrophages, as professional scavengers, have an excellent capacity to deal with ingested iron and resist potentially toxic effects.16 We have demonstrated in vitro that viability, cytokine production, and migratory capacity are unaffected by SPIO labeling. This is consistent with the majority of studies that have shown preservation of both viability and function after SPIO labeling of a variety of cell types including mesenchymal, hematopoietic, neural, and adipose-derived stem cells,6–11 with only a few reporting adverse effects and 1 showing increased proliferation of SPIO-labeled mesenchymal stem cells.17–19 It is worth recognizing the diversity in cell-labeling protocols used across these studies that may explain such findings.

It is important to note that we have shown for the first time that intravenous administration of SPIO-labeled cells in humans is safe. Although several SPIO agents have been approved for intravenous administration, the Food and Drug Administration has not approved the intravenous administration of cells labeled with SPIO ex vivo. These safety data are therefore critical for the further development and subsequent Food and Drug Administration approval of novel SPIO-based cell-tracking agents, facilitating translation into the clinical setting. Although ferumoxide (Endorem, Feridex) is no longer clinically available, several alternative agents are currently being developed for clinical use. We have here shown the feasibility of SPIO cell tracking and established the potential use of SPIO as reporters for novel cell-tracking agents.20

Several recent publications have reported tracking of labeled cells to a site of inflammation after both local and systemic administration in the preclinical setting.21–26 Migration of cells to a target site has also been demonstrated in humans after local administration. de Vries et al27 labeled human dendritic cells with ultrasmall SPIO (20 nm diameter), injected them directly into peripheral lymph nodes in patients with melanoma, and demonstrated their migration to regional lymph nodes on MRI scanning and histology of surgical resection specimens. SPIO-labeled pancreatic islet cells have been detected in the liver after portal venous administration, and SPIO-labeled mesenchymal stem cells have been tracked to the site of injury after intracerebral injection in a patient with a traumatic brain injury.28,29 However, this is the first report of successful tracking of SPIO-labeled cells to a target site after systemic intravenous administration. This finding substantially broadens the potential applications for SPIO-labeled cell tracking.

We have shown that at clinically relevant field strengths, it is readily possible to image ≈108 mononuclear cells (containing ≈1–5×107 monocytes) after local or intravenous administration, and others have imaged as few as 1.5×105 cells in a target site after local delivery of 7.5×106 cells.27 If there was any concern regarding the effect of SPIO labeling on the likely success of cell-based treatments, it would therefore be possible to label a small but representative proportion of the total number of cells given, allowing visualization of the fate and distribution of cells, but leaving the majority of cells for engraftment or other desired function. For nontherapeutic applications such as inflammatory cell tracking, a much higher dose of labeled cells could be administered to increase the sensitivity for detecting labeled cells in the target site. We have demonstrated that 1–5×109 SPIO-labeled autologous PBMC can be given safely: 100-fold greater than that needed for detection on MRI scanning.

An alternative control for the Mantoux component of the present study would have been to administer saline or unlabeled cells to participants with a positive Mantoux response, and this could be undertaken as part of a future study. It is acknowledged that although MRI can detect accumulation of SPIO-labeled cells at a target site, further quantification of cell number is currently challenging because clustering of iron nanoparticles magnifies their magnetic effects, confounding attempts at quantification. However, novel positive imaging sequences may address this issue in the future. Further work is required to establish the duration of specificity of SPIO accumulation in tissues to indicate the continued presence of delivered cells at the target site. It would also be important to define the time course of SPIO clearance from tissues if serial administration of cells was considered.

In conclusion, this is the first report of successful magnetic resonance cell tracking in humans after systemic administration of cells. Using a GMP-compliant protocol, human mononuclear cells can be labeled with SPIO on a clinical scale without affecting their viability or migratory capacity, and intravenous administration of SPIO-labeled cells in humans is safe. Cells can be imaged at clinical MRI field strengths after local or systemic administration. This technology for in vivo cell tracking has many potential cardiovascular applications that include monitoring of cell-based therapies for cardiovascular disease and dynamic leukocyte trafficking.

Acknowledgments

We thank the staff of the Wellcome Trust Clinical Research Facility, the Scottish National Blood Transfusion Service, the Cell Separator Unit, and the Clinical Research Imaging Centre for their contribution to the study, along with the Orthotics Department, the nursing staff of the Tuberculosis Clinic and nurses of the Royal Infirmary of Edinburgh.

Footnotes

*Drs Richards and Shaw contributed equally to this work.

The online-only Data Supplement is available with this article at http://circimaging.ahajournals.org/lookup/suppl/doi:10.1161/CIRCIMAGING.112.972596/-/DC1.

Correspondence to Jennifer M.J. Richards, MD, Centre of Cardiovascular Science, The University of Edinburgh, The Chancellor’s Building, Little France Crescent, Edinburgh EH16 5SA, United Kingdom. E-mail

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Clinical Perspective

In the coming years, regenerative medicine and cell-based therapies may become important options to limit and even regress a wide range of diseases. In clinical trials, however, it is challenging to evaluate the efficacy of cell-based treatments because there is no currently available method of determining the proportion of administered cells reaching and remaining in the target therapeutic site. As a result, there is considerable interest in the development of strategies that would enable noninvasive clinical cell tracking to determine the distribution and time course of infused cells. Magnetic resonance imaging is ideal for this purpose because it combines high-spatial resolution with excellent soft tissue contrast while avoiding ionizing radiation. We have labeled autologous peripheral blood mononuclear cells with a magnetic resonance imaging contrast agent containing superparamagnetic particles of iron oxide (SPIO). We have shown the safety and feasibility of SPIO cell labeling and tracking in humans. Furthermore, this is the first study to demonstrate that SPIO-labeled cells can be tracked to a target site of inflammation following intravenous administration. SPIO cell tracking could therefore be a useful tool for the assessment and further development of cell-based treatment options. In addition, SPIO cell tracking could be applied to dynamic leukocyte trafficking to investigate the contribution of inflammatory processes to disease pathology and to evaluate the effects of novel pharmacological interventions.