INTRACELLULAR MAGNETIC LABELING OF LYMPHOCYTES FOR IN-VIVO TRAFFICKING STUDIES

Lymphocyte adhesion and trafficking is difficult to observe in vivo ov er time. We used magnetic resonance imaging (MRI) to identify magnetic ally labeled lymphocytes in phantom experiments and in tissue. A metho d of lymphocyte labeling was developed that is based on fluid-phase en docytosis of nanometer-sized biocompatible superparamagnetic particles . The maximum cell uptake in culture was 0.11 ng Fe/cell corresponding to 5 x 10(6) particles/lymphocyte. Cells stably retained the label an d were fully viable for at least 3 days. Labeled lymphocytes showed ad hesion to human endothelial cells similar to unlabeled cells, indicati ng no effect of labeling on cell surface expression of adhesion protei ns. No particle-mediated cytotoxicity could be observed. The detection threshold of MRI for detecting labeled lymphocytes in the current stu dy was 2.5 x 10(6) cells/30 mu L sampling volume. Following intravenou s injection of labeled lymphocytes into rats, cells accumulated in spl een, lymph nodes and liver with a similar bio-distribution as unlabele d cells. Lymphocyte accumulation in the spleen resulted in MRI signal intensity changes readily detectable by MRI. These findings suggest th at intracellular lymphocyte labeling with superparamagnetic particles is feasible, does not alter the viability or tissue distribution of la beled cells and allows the detection of labeled lymphocytes by MRI.