Microparticulate uptake mechanisms of in-vitro cell culture models of the respiratory epithelium

The objective of this study was to examine the uptake mechanisms of fluorescent polystyrene microspheres of various diameters and surface chemistry by two human cell lines derived from the respiratory epithelium, A549 and Calu-3. Briefly, A549 and Calu-3 cells were grown to confluence in 12-well cluster plates and the uptake of fluorescent microspheres by the cells was determined at various time points. The amount of microspheres internalized by the cells was determined by correcting for non-specific binding to the cell surface. The data showed that A549 cells appeared to have more phagocytic activity than Calu-3 cells. Albumin-coated microspheres as large as 3 mum diameter can be internalized by A549 cells. The amount of internalization by A549 cells observed for 0.5-mum diameter albumin-coated microspheres was approximately 10-times greater than that observed for 1-mum diameter spheres and approximately 100-times greater than values observed for 2- and 3-mum diameter beads. Transmission electron micrographs confirmed that the microspheres were internalized by the cells. Uptake experiments conducted with Calu-3 cells indicated that albumin-coated microspheres were neither bound nor internalized by the cells. The effect of microsphere surface chemistry on the uptake mechanism indicated that amidine microspheres were internalized more rapidly and to a greater extent by both A549 and Calu-3 cells than carboxylate microspheres and non-coated microspheres. This phenomenon is thought to be attributed to masking of the negative polystyrene core by the positive amidine functional group; this effect was less marked for the carboxylate microspheres. These results suggest that A549 and Calu-3 cells can internalize microspheres and that size and effective charge played an important role in the uptake process.