On the use of micron-sized iron oxide particles (MPIOS) to label resting monocytes in bone marrow.

PURPOSE: The use of MRI to monitor immune cell infiltration into various pathologies is well established. In an effort to boost the magnetic material within immune cells, this work attempted to label resting monocytes within bone marrow, in mice, by intravenous administration of micron-sized iron oxide particles (MPIOs), similar in fashion to the administration of (U)SPIO. PROCEDURES: MPIOs were incubated with various immune cells both in culture, and in whole blood. Flow cytometry and histology were used to analyze magnetic cell labeling. Also, MPIOs were injected intravenously into mice. In vivo, high-resolution 3-D MRI was performed on mouse legs, and signal changes were quantified. Flow cytometry and histology were used to analyze magnetic cell labeling of bone marrow resident cells.
RESULTS: It is demonstrated here that monocytes and neutrophils can indeed endocytose MPIOs both in cell culture and ex vivo in whole blood. However, despite rapid accumulation of MPIOs within the bone marrow following injection, MPIOs did not label monocytes or any other hematopoietic cell type in the marrow. Hypotheses are drawn to explain these results in light of recent usage of MPIOs for immune cell tracking.
CONCLUSIONS: Systemic administration of various MPIO formulations showed that MPIOs arrive in bone marrow rapidly following injection and remain there for at least 7 days. Data also shows slow clearance of some particles from the tissue over this period. While MPIOs can efficiently label monocytes in culture and in whole blood ex vivo, they were not found to label bone marrow resident monocytes.