TRANSFER OF PHAGOCYTOSED PARTICLES TO THE PARASITOPHOROUS VACUOLE OF LEISHMANIA-MEXICANA IS A TRANSIENT PHENOMENON PRECEDING THE ACQUISITION OF ANNEXIN-I BY THE PHAGOSOME

The eukaryotic intracellular pathogen Leishmania mexicana resides insi de macrophages contained within a membrane bound parasitophorous vacuo le which, as it matures, acquires the characteristics of a late endoso mal compartment. This study reports the selectivity of fusion of this compartment with other particle containing vacuoles. Phagosomes contai ning zymosan or live Listeria monocytogenes rapidly fused with L. mexi cana parasitophorous vacuoles, while those containing latex beads or h eat killed L. monocytogenes failed to do so. Fusigenicity of phagosome s was not primarily dependent on the receptor utilized for ingestion, as opsonization with defined ligands could not overcome the exclusion of either latex beads or heat killed organisms. However modulation of intracellular pH by pharmacological agents such as chloroquine and amm onium chloride increased delivery of live Listeria and also induced tr ansfer of previously excluded particles. The absence of fusion correla ted with the acquisition of annexin I, a putative lysosomal targeting molecule, on the phagosome membrane, We propose that the acquisition o f cellular membrane constituents such as annexin I during phagosome ma turation can ultimately direct the fusion pathway of the vesicles form ed and have described a model system to further document changes in ve sicle fusigenicity within cells.