Cellular uptake of magnetic nanoparticles imaged and quantified by magnetic particle imaging

Abstract

Magnetic particle imaging (MPI) is a non-invasive, non-ionizing imaging technique for the visualization and quantification of magnetic nanoparticles (MNPs). The technique is especially suitable for cell imaging as it offers zero background contribution from the surrounding tissue, high sensitivity, and good spatial and temporal resolutions. Previous studies have demonstrated that the dynamic magnetic behaviour of MNPs changes during cellular binding and internalization. In this study, we demonstrate how this information is encoded in the MPI imaging signal. Through MPI imaging we are able to discriminate between free and cell-bound MNPs in reconstructed images. This technique was used to image and quantify the changes that occur in-vitro when free MNPs come into contact with cells and undergo cellular-uptake over time. The quantitative MPI results were verified by colorimetric measurements of the iron content. The results showed a mean relative difference between the MPI results and the reference method of 23.8% for the quantification of cell-bound MNPs. With this technique, the uptake of MNPs in cells can be imaged and quantified directly from the first MNP cell contact, providing information on the dynamics of cellular uptake.

Figure 1

Light microscopy images of THP-1 cells after iron blue staining incubated without (a) and with (b) Synomag at an iron concentration of 0.5 mM for 15 min. Iron was visualized by Prussian Blue staining and nuclei with Nuclear Fast Red staining.

Figure 2

Reconstructed MPI images of the MNP distribution as a function of time of free (a) and cell-bound (b) MNPs in contact with THP-1 cells. Displayed are summed up intensities along the z-axis to visualize the 3D datasets. The same scaling was used for all images. The first column displays the reconstruction before injection of the cells (number of cells displayed on the left), in which only MNPs diluted in PBS were located in the FOV. The following columns show representative time frames after the injection with an increasing intensity of cell-bound MNPs at the same location compared to free MNPs.

Figure 3

Quantified iron quantities of free (a) and cell-bound MNPs during initial MNP-THP-1 cell contact. With an increasing number of injected cells, a stronger increase of cell-bound MNPs and decrease of free MNPs was detected. (c) displays the relative deviation of the total iron amount (free+cell-bound MNPs) from the initially quantified amount of free MNPs before the injection. For better visualization, only a fraction of all available data points was incorporated in the figure.

Figure 4

Quantified iron amounts acquired via the phenanthroline-based iron assay method for varying incubation times. Each data point represents the mean value from three independent measurements with the standard deviation visualized as error bars. Note that the lines connecting the dots do not represent measurement data and are only shown for better visualization. Due to the required washing steps during the phenanthroline-based iron assay method, some MNPs are lost, leading to an underestimation of the total iron mass by up to −22%, especially regarding the free MNPs.