THE AGENT OF HUMAN GRANULOCYTIC EHRLICHIOSIS RESIDES IN AN ENDOSOMAL COMPARTMENT

The composition of cytoplasmic vacuoles containing the agent of Human Granulocytic Ehrlichiosis (HGE) was studied to investigate how this pa thogen exists within infected host cells. Electron microscopy demonstr ated that the HGE organism resides in a membrane-bound compartment wit hin HL-60 cells: early forms of the HGE agent have a round reticular a ppearance while later structures are small and dense. Vacuoles contain ing HGE bacteria incorporated endocytosed colloidal gold particles, su ggesting that they are part of the endocytic pathway. Antibodies direc ted to the mannose-6-phosphate receptor labeled vacuole membranes. Ant ibodies to the transferrin receptor and to the lysosomal membrane glyc oprotein LAMP 1 did not. Moreover, ,4-dinitroanilino)-3'-amino-N-methy ldipropylamine, which normally accumulates in compartments with low pH , was not present inside these vacuoles. These results suggest that va cuoles containing the agent of HGE fail to mature into phagolysosomes. We conclude that the agent of HGE appears to enter and modify part of the endocytic pathway.