Transport of phagosomal components to an endosomal compartment

The participation of phagosomes in interorganellar protein and membrane exchange is important to the maturation of phagosomes into phagolysosomes. To investigate this process, we have developed an assay to measure protein transport from phagosomes to other vesicle populations. J774-E clone macrophages phagocytosed 125I-anti-dinitrophenol IgG-coated Staphylococcus aureus for 3 min followed by chase for intervals of 0-30 min. Following cell fractionation, the intracellular distribution of radioiodinated protein was assayed. We observed a time-dependent increase radioiodinated protein in a non-phagosome vesicle fraction which displayed endosome characteristics. Concomitantly, radioiodinated protein within phagosomes decreased over the chase period. As assessed via Percoll density gradient fractionation, the phagocytosed radioiodinated protein migrated to both heavy (lysosome density) and light (endosome density) vesicle populations. Characterization of the fusogenic properties of the transport vesicles demonstrated that they are capable of in vitro fusion with early endosomes. Furthermore, this fusion event shares many of the biochemical requirements identified for phagosome-endosome and endosome-endosome fusion. Morphological analysis of phagosome maturation provides additional evidence for phagosome to endosome transport. These results suggest phagocytosed material is transferred from phagosomes to endosomes and then recycled out of the cell.