In vivo comparison of cat alveolar and pulmonary intravascular macrophages: phagocytosis, particle clearance, and cytoplasmic motility.

The phagocytic and particle clearance function as well as the intracellular motility of cat pulmonary intravascular macrophages (PIMs) and pulmonary alveolar macrophages (PAMs) in situ were compared using ferrimagnetic gammaFe2O3 tracer particles and magnetometry. Submicrometric particles were injected intravenously (phagocytized by PIMs) or inhaled as respirable aerosols (phagocytized by PAMs). At various times post administration, the particles were magnetized with an external magnet. Then, the rate of decay of the remanent magnetic field (relaxation) reflecting the motion of particle-containing phagosomes was measured. There was a gradual increase in relaxation in PAMs, but a decrease in PIMs over the initial 7 hours, suggesting differences in rates of phagocytosis. From 7 to 42 days, relaxation parameters were constant in both cell types although faster in PAMs than in PIMs. Disappearance of retained particles in PIMs depended on the administered dose. Particles in PAMs cleared faster than particles in PIMs. Particle clearance from PIMs, but not from PAMs, was accompanied by translocation of particles to the liver and elsewhere. Our data show that PAMs have a slower rate of in vivo phagocytosis, higher cell motility, and faster particle clearance than PIMs.