DIFFERENT FATES OF PHAGOCYTOSED PARTICLES AFTER DELIVERY INTO MACROPHAGE LYSOSOMES

Phagocytosis in macrophages is often studied using inert polymer micro spheres. An implicit assumption in these studies is that such particle s contain little or no specific information in their structure that af fects their intracellular fate. We tested that assumption by examining macrophage phagosomes containing different kinds of particles and fou nd that although all particles progressed directly to lysosomes, their subsequent fates varied. Within 15 min of phagocytosis, >90% of phago somes containing opsonized sheep erythrocytes, poly-e-caprolactone mic rospheres, polystyrene microspheres (PS), or polyethylene glycol-conju gated PS merged with the lysosomal compartment. After that point, howe ver, the characteristics of phagolysosomes changed in several ways tha t indicated differing degrees of continued interaction with the lysoso mal compartment. Sheep erythrocyte phagolysosomes merged together and degraded their contents quickly, poly-e-caprolactone phagolysosomes sh owed intermediate levels of interaction, and PS phagolysosomes became isolated within the cytoplasm. PS were relatively inaccessible to an e ndocytic tracer, Texas red dextran, added after phagocytosis. Moreover , immunofluorescent staining for the lysosomal protease cathepsin L de creased in PS phagolysosomes to 23% by 4 h after phagocytosis, indicat ing degradation of the enzyme without replacement. Finally, PS surface labeled with fluorescein-labeled albumin showed a markedly reduced ra te of protein degradation in phagolysosomes, when compared to rates me asured for proteins in or on other particles. Thus, particle chemistry affected both the degree of postlysosomal interactions with other org anelles and, consequently, the intracellular half-life of particle-ass ociated proteins. Such properties may affect the ability of particles to deliver macromolecules into the major histocompatibility complex cl ass I and II antigen presentation pathways.