Lung Particle Retention and Lung Macrophage Function Evaluated Using Magnetic Aerosols

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Abstract
Iron oxide can be used as a non-toxic test aerosol for studies of particle deposition and clearance in lungs. These particles can be seen in the light microscope either unstained (due to their intrinsic color) or stained (by the Prussian blue reaction) and in the electron microscope (due to electron absorption). Magnetic forms of iron oxide can be detected in the body by the remanent magnetism measured at the body surface of animals or humans. The remanent magnetic field (RMF) outside the chest, immediately following magnetization, is related to lung content of magnetic material; measurements repeated over weeks or months, describe lung clearance curves. Particle motions cause the RMF to decay after magnetization, a phenomenon called "relaxation"; the initial magnitude of the RMF can be restored by remagnetization, and relaxation begins again. Lung macrophages are one major cause of relaxation. These cells reside on lung surfaces and ingest the micrometer-sized magnetic particles into intracellular vesicles; cytoplasmic motions move the phagocytized particles, causing decay of the RMF. Magnetic particles can also be twisted by an applied external field, and the response can be observed magnetometrically; this allows measurement of mucus, surfactant, or cytosol viscoelasticity. Both for isolated cells and for cells in situ, one can examine cytoplasmic motion and viscosity by magnetometry and thus evaluate the motile status of a population of cells without optical observation.