The endocytic pathway for H-ferritin established in live MOLT-4 cells by laser scanning confocal microscopy.

We have established the intracellular destination of the putative immunoregulatory protein, human recombinant H (heavy)-ferritin, in the transformed T-cell line MOLT-4, by laser scanning confocal microscopy of live cells. A series of confocal images was collected over a 60 min time course using indirect immunofluorescence of H-ferritin and transferrin, their respective monoclonal antibodies, and fluorescein isothiocyanate (FITC)-labelled IgG. A marked drop in FITC fluorescence after 40 min of H-ferritin internalization, indicative of an acidic environment, and co-localization with tetramethylrhodamine isothiocyanate-labelled-dextran strongly suggests that H-ferritin is transferred to the lysosome. In contrast, transferrin was observed to return to the cell surface. Electron microscopy confirmed that H-ferritin was transferred to the lysosome. The receptor-mediated endocytosis and lysosomal delivery of H-ferritin may thus potentiate its putative immunoregulatory activity.