RELATIONSHIP BETWEEN PHAGOSOME ACIDIFICATION, PHAGOSOME-LYSOSOME FUSION, AND MECHANISM OF PARTICLE INGESTION

The fate of pathogens ingested by macrophages is dependent on phagosom e acidification and fusion with different intracellular vesicles. Wher eas the mode of particle recognition by the phagocyte seems the main d eterminant of phagosome-lysosome fusion, the influence of membrane reo rganization, fusion events, and cell activation in phagosome acidifica tion is not well known. We looked for a relationship between the natur e of receptors involved in phagocytosis, phagosome acidification, and phagosome-lysosome fusion. Murine macrophage-like P388D1 cells were ma de to ingest sheep erythrocytes coated with immunoglobulin G (EIgG) or IgM and complement (EIgMC) or treated with glutaraldehyde and perioda te (EGP). The following results were obtained: (1) As expected, the ad hesion of the three particle types was differentially inhibited by mon oclonal antibodies specific for Fc gamma RII and CD11b/CD18. (2) The p hagosomes containing all three particle types displayed similar acidif ication kinetics with a pH decrease to 6 within the first 10 min after ingestion. (3) Only phagosomes containing EIgG or EIgMC were fused wi th peroxidase-loaded secondary lysosomes. (4) Coating EGP with IgG onl y partially restored fusion, even when the surface density of IgG was markedly higher than found on EIgG. It is concluded that phagosome aci dification and fusion are regulated by different mechanisms. Also, the lack of fusion observed with EGP is not entirely accounted for by the absence of stimulation of suitable receptors on the phagocyte membran e, because it cannot be restored by providing such a stimulus.