PHAGOCYTIC PROCESSING OF THE MACROPHAGE ENDOPARASITE, MYCOBACTERIUM-AVIUM, IN COMPARISON TO PHAGOSOMES WHICH CONTAIN BACILLUS-SUBTILIS OR LATEX BEADS

The intraphagosomal survival strategy of pathogenic mycobacteria was s tudied in bone marrow-derived mouse macrophages. These bacteria surviv e inside phagosomes by interfering in an unknown manner with phagosome processing which normally would lead to digestion of the phagocytic p article in phagolysosomes. Here, phagosome processing was compared for different phagocytic particles: live Mycobacterium avium, degradable Bacillus subtilis, or indigestible latex beads. We show detailed elect ron microscopic morphological observations which characterize various phases of interaction between endocytic organelles and phagosomes. We measured fusion of phagosomes with early endosomes or with lysosomes b y using newly internalized endocytic contents (horseradish peroxidase, HRP) and membrane marker (plasma membrane glycoconjugates labeled wit h [H-3]galactose via exoglycosylation). Morphometric analysis of these observations showed that the nature of the phagocytic particle affect s phagosome processing: As long as particles remain undigested, matura tion of phagosomes is prevented and they remain fusogenic towards earl y endosomes; concurrent to particle digestion, phagosome processing pr oceeds towards transfer of phagocytic contents to phagolysosomes which display kinetic and compositional characteristics of lysosomes. As an intact phagocytic particle, M. avium remains in non-matured phagosome s which fuse with early endosomes, but not with lysosomes. Fusion with early endosomes is reduced, thereby indicating the stage where this e ndoparasite exerts its effect.