Imaging of nanoparticle-labeled stem cells using magnetomotive optical coherence tomography, laser speckle reflectometry, and light microscopy

Abstract

Cell transplantation and stem cell therapy are promising approaches for regenerative medicine and are of interest to researchers and clinicians worldwide. However, currently, no imaging technique that allows three-dimensional in vivo inspection of therapeutically administered cells in host tissues is available. Therefore, we investigate magnetomotive optical coherence tomography (MM-OCT) of cells labeled with magnetic particles as a potential noninvasive cell tracking method. We develop magnetomotive imaging of mesenchymal stem cells for future cell therapy monitoring. Cells were labeled with fluorescent iron oxide nanoparticles, embedded in tissue-mimicking agar scaffolds, and imaged using a microscope setup with an integrated MM-OCT probe. Magnetic particle-induced motion in response to a pulsed magnetic field of 0.2 T was successfully detected by OCT speckle variance analysis, and cross-sectional and volumetric OCT scans with highlighted labeled cells were obtained. In parallel, fluorescence microscopy and laser speckle reflectometry were applied as two-dimensional reference modalities to image particle distribution and magnetically induced motion inside the sample, respectively. All three optical imaging modalities were in good agreement with each other. Thus, magnetomotive imaging using iron oxide nanoparticles as cellular contrast agents is a potential technique for enhanced visualization of selected cells in OCT.

Improved Imaging of Magnetically Labeled Cells Using Rotational Magnetomotive Optical Coherence Tomography

“…All experiments were carried out with a multimodal imaging system for MM-OCT, laser speckle reflectometry and light microscopy, which was presented recently by our group [6]. In brief, the setup shown in Figure 1 consists of a conventional epi-fluorescence microscope (Leica DMRB, Leica Microsystems GmbH, Wetzlar, Germany) for bright-field transmission and multi-color fluorescence microscopy, an integrated MM-OCT probe including an electromagnet for magnetic field generation, and a dark-field laser illumination for laser speckle imaging.…”

“…We dispersed magnetic particles in glycerin (0.5 mg/mL) and investigated them using the bright-field microscopy channel of our multimodal imaging setup in combination with a high-magnification objective (Leica C Plan L 40×/0.50). The particles were identical to the ones used for cell experiments in our previous study [6]; they are sub-micron particles with a nominal size of 200 nm (nano-screen MAG/R-DXS, Chemicell GmbH, Berlin, Germany) consisting of a magnetite core, a fluorescent shell emitting in the red visible spectral range, and a coating made of dextran-sulfate for biomolecule coupling.…”

“…For this, we magnetically labeled mesenchymal stem cells (MSCs) with the fluorescent magnetic particles, as mentioned in Section 3.1, and embedded them in agar scaffolds resembling the elastic properties of soft tissue, as described in [6]. Concerning cellular particle load and its effect on cell function, recent literature indicates that MSCs incubated with 200-nm magnetic particles coated with dextran-sulfate take up ~10 pg of iron oxide per cell, and show a cell viability of ~70% after 24 h [12].…”

“…Hypothetically, the pulse length of 80 ms is not long enough to allow the sub-micron particles to fully respond to the magnetic field. However, we choose this pulse duration to make the results comparable to previous work [6].…”

“…For our experiments, we used a multimodal imaging system presented recently by our group [6]. This system allows MM-OCT imaging in combination with light microscopy and laser speckle reflectometry for structural and motion analysis of single cells.…”