Abstract 1383: Iron-labeled Stem Cells Seen by Magnetic Resonance Imaging: Dead or Alive?
Stem cell labeling with iron oxide particles causes labeled cells to appear as dark spots on MR imaging and is commonly used to track stem cell engraftment. However, the validity of this approach for detecting long-term cell survival is not known
Aim. To determine the relationship of MRI signals to cell survival in a rat model 3 weeks after injection of iron-labeled cardiac-derived stem cells (CDCs).
Methods. Rats were imaged at 2 days and 3 weeks after syngeneic or xenogeneic CDC injection. The rationale is that all xenogeneic, but not syngeneic CDCs are destroyed by the immune system. CDCs from humans (xenogeneic) and Wistar Kyoto (WK, syngeneic) rats were expanded in vitro, incubated with Feridex and Poly-L-Lysine for 16hrs (25μg/ml and 0.75μg/ml of medium, respectively) and then injected intramyocardially into immunocompetent WK rats. Sensitivity for detecting labeled cells was assessed by in vitro imaging of different cell concentrations and by in vivo imaging of rat hearts (n=8) injected with varying numbers of CDCs. Gradient echo MR images were acquired on a clinical Philips Achieva 3T scanner and analyzed with Image J software.
Results. Labeling efficiency (by Prussian Blue staining) was >80% and did not affect cell proliferation or induce cell death/apoptosis, as assessed by the WST-8 assay and flow cytometry. A minimum of 2× 105 cells could be detected in vitro and in vivo. At 2 days, xenogenic and syngeneic CDCs were identified as large intramyocardial dark spots (26.4±6.3% and 23.8±9.5% of the apical ventricular volume, respectively). Contrast to noise ratio (CNR) was 9.4±0.5 for the xenogeneic and 13.6±2.8 for the syngeneic group. At 3 weeks, intramyocardial dark spots were large (20±9.3%) and comparable to the ones of 2 day scans in animals injected with syngeneic cells. However, at 3 weeks in the xenogeneic model, where there were no surviving human cells by histology, a dark spot of significant size (14.3±5.5%) was still present. CNR of syngeneic and xenogeneic regions was similar (10.5±0.2 vs 11.6±4.9, respectively).
Conclusion. Persistence of iron-dependent MRI signals (dark spots) after 3 weeks at the injection site despite cell death indicates that iron oxide particles are not reliable reporters of long-term CDC engraftment and survival.