In vitro imaging of human monocytic cellular activity using superparamagnetic iron oxide

The aim is to evaluate if a difference in activity of monocytic cells can be described on superparamagnetic iron oxide (SPIO)-magnetic resonance imaging (MRI) in vitro when a low concentration of SPIO is administered. Human monocytic cells were stimulated with phorbol 12-myristate 13-acetate (PMA) or left non-stimulated for 24 h and then treated with SPIO with a final concentration of 7.2 μg Fe/ml for 48 h. They were collected, and an MRI phantom was prepared by suspending them in 4% gelatin and scanned using gradient-echo T2*-weighted and spin-echo T2-weighted imaging with two different echo times. The signal intensity and T2*/T2 relaxation times of each layer were also assessed. The number and degree of intracellular filling of cells positively stained with Berlin blue were evaluated microscopically. In addition, the heme oxygenase-1 expression in non-stimulated monocytic cells or monocytic cells stimulated with PMA was evaluated using semiquantitative reverse transcription and polymerase chain reaction. The signal reduction of cells stimulated with PMA was 3.9- to 5.0-fold and 2.7- to 3.1-fold greater than that of non-stimulated cells on the T2*- and T2-weighted images, respectively. These signal reductions were also visually confirmed. Estimated T2*/T2 relaxation times of cells stimulated with PMA and non-stimulated cells were 31.3/137.8 and 122.8/182.6 ms, respectively. Cells positively stained with Berlin blue were observed in 4.5% and 25.3% of non-stimulated cells and those stimulated with PMA, respectively (p < 0.05). The intracellular filling in cells stimulated with PMA was significantly larger than that in non-stimulated cells (p < 0.05). The expression of the heme oxygenase-1 gene in monocytic cells stimulated with PMA was 2.9-fold greater than that in non-stimulated monocytic cells. SPIO-MRI can visually describe a difference in activity of monocytic cells in vitro when a low concentration of SPIO is administered.