Magnetic Particle Spectroscopy Reveals Dynamic Changes in the Magnetic Behavior of Very Small Superparamagnetic Iron Oxide Nanoparticles During Cellular Uptake and Enables Determination of Cell-Labeling Efficacy

Abstract

In vivo tracking of nanoparticle-labeled cells by magnetic resonance imaging (MRI) crucially depends on accurate determination of cell-labeling efficacy prior to transplantation. Here, we analyzed the feasibility and accuracy of magnetic particle spectroscopy (MPS) for estimation of cell-labeling efficacy in living THP-1 cells incubated with very small superparamagnetic iron oxide nanoparticles (VSOP). Cell viability and proliferation capacity were not affected by the MPS measurement procedure. In VSOP samples without cell contact, MPS enabled highly accurate quantification. In contrast, MPS constantly overestimated the amount of cell associated and internalized VSOP. Analyses of the MPS spectrum shape expressed as harmonic ratio A₅/A₃ revealed distinct changes in the magnetic behavior of VSOP in response to cellular uptake. These changes were proportional to the deviation between MPS and actual iron amount, therefore allowing for adjusted iron quantification. Transmission electron microscopy provided visual evidence that changes in the magnetic properties correlated with cell surface interaction of VSOP as well as with alterations of particle structure and arrangement during the phagocytic process. Altogether, A₅/A₃-adjusted MPS enables highly accurate, cell-preserving VSOP quantification and furthermore provides information on the magnetic characteristics of internalized VSOP.

Uptake of citrate-coated iron oxide nanoparticles into atherosclerotic lesions in mice occurs via accelerated transcytosis through plaque endothelial cells

“…We hypothesized that the VSOP-GAG interaction occurs in a transchelation reaction in which the citrate coat of the cationic iron oxide core of the VSOPs is replaced by a GAG coat. This hypothesis was supported by the observation that the magnetic properties of VSOPs change immediately after contact with cells [31]. Considering that an electrostatic interaction with cell surface GAGs and subsequent endocytosis of the resulting complexes have been reported for polycationic macromolecules and cationic peptides [32], and for phagocytosis of VSOPs into macrophages [15], we propose a similar mechanism for the uptake of VSOPs into the endothelium.…”

“…MPS measurements of VSOP content in organs and blood were performed using a commercial magnetic particle spectrometer (Bruker) as previously described in detail [31]. VSOP calibration curves had high correlation coefficients (R 2 = 0.99) and the limit of detection with a signal-to-noise ratio of 3 was assessed as 91 ng, reflecting the high sensitivity of this method.…”

Biocompatibility of very small superparamagnetic iron oxide nanoparticles in murine organotypic hippocampal slice cultures and the role of microglia

“…So far, VSOP have been tested in vitro and in vivo by means of immortalized or primary cell cultures, animal models, and human probands. 2,17,28,44,45 In our view, further studies and assessments using OHSC are imperative. On the one hand, cell culture experiments exclude questions regarding VSOP interactions on the three-dimensional multicellular level.…”

“…MPS measurements were performed as described before. 44 Statistical analysis was performed with GraphPad Prism software using one-way ANOVA and Tukey post hoc test.…”

“…MPS was recently published as a new, highly accurate technique for quantifying VSOP. 44 We used MPS and found a significant iron uptake in VSOP-treated specimens ( Figure 5). Even though no difference was detectable in the amount of iron between VSOP-R1 and VSOP-R2-incubated samples, DMDP treatment plus SPIO application led to a significantly decreased iron accumulation in OHSC, proving the involvement of microglia.…”