Receptor‐mediated endocytosis of iron‐oxide particles provides efficient labeling of dendritic cells for in vivo MR imaging

Dendritic cells (DCs) function as antigen presenting cells in vivo and play a fundamental role in numerous diseases. New methods are described for high‐efficiency intracellular labeling of DCs with superparamagnetic iron‐oxide (SPIO) utilizing a receptor‐mediated endocytosis (RME) mechanism. Bone marrow‐derived DCs or a fetal skin‐derived DC line were incubated with SPIO conjugated to anti‐CD11c monoclonal antibody (mAb) under conditions favoring RME. These cells exhibited approximately a 50‐fold increase in uptake relative to DCs incubated with SPIO without the mAb. Flow cytometry studies assaying cell surface markers showed a down‐modulation of CD11c, but no other changes in phenotype. Immunological function of the DCs was unmodified by the labeling, as determined by cytokine secretion assays. The RME mechanism was confirmed using electron microscopy, endocytosis inhibition assays, and incubation experiments with SPIO conjugated to mAbs against accessory molecules that are not expressed on DCs. Labeled DCs were injected into murine quadriceps and monitored in vivo for several days using MR microimaging at 11.7 T. DCs were observed to remain within the muscle for >24 hr. The use of RME is an efficient way to label immune cells for in vivo MRI and can be applied to a wide variety of cell types. Magn Reson Med 49:1006–1013, 2003. © 2003 Wiley‐Liss, Inc.