PHAGOSOME MATURATION AND FUSION WITH LYSOSOMES IN RELATION TO SURFACE-PROPERTY AND SIZE OF THE PHAGOCYTIC PARTICLE

Phagosomes with pathogenic mycobacteria, or with hydrophobic polystyre ne beads of 1 mu m in size, do not mature, but remain fusogenic toward s early endosomes and do not fuse with lysosomes (de Chastellier et al , Fur. a, Cell Biol. 68, 167-182 (1995)), Both types of phagocytic par ticles display a close apposition to the phagosome membrane, We have p ostulated that due to the absence of tubule formation of the phagosoma l membrane, efficient recycling of hypothetical fusion-mediating facto rs is impaired thus keeping the phagosome fusogenic to early endosomes and unable to fuse with lysosomes, To test this hypothesis, we now an alyzed phagosome maturation for particles which were expected to displ ay a less dose particle-membrane apposition, in addition to confirming our previous results for non-maturing phagosomes as a direct comparis on, In contrast to hydrophobic latex beads as before, we now used bead s with a more hydrophilic surface, being carboxylated, with and withou t additional coating by protein (horseradish peroxidase, HRP; or bovin e serum albumin, BSA), In addition, we used hydrophobic beads of small er sizes (0.5, 0.3, 0.1 mu m), in order to determine the limiting size at which the particle no Longer determined the size and the fate of t he phagosome, As predicted, all the above particles displayed a less t ight interaction with the phagosome membrane. Tubule formation was obs erved to a similar extent as for early endosomes, Morphological eviden ce showed that phagosomes rapidly lost their ability to fuse with earl y endosomes, after which they could be seen fusing with lysosomes labe led with gold-conjugated BSA, Functional evidence for the formation of phagolysosomes was based on the kinetic observation that subsequently endocytosed contents marker (HRP) was acquired by phagosomes only aft er a lag of about 5 min as is typical for lysosomes, The present obser vations could be explained in terms of a model which suggests that myc obacteria can prevent phagosome maturation and, therefore, fusion with lysosomes, by a tight interaction with constituents of the phagosomal membrane, Furthermore, these results show that it is important to cho ose artificial phagocytic particles according to the appropriate surfa ce properties when using them as a model system to study phagosome pro cessing.