Rabies virus entry into endosomes in IMR-32 human neuroblastoma cells

Early events in rabies virus entry into cultured IMR-32 human neuroblastoma cells were investigated. After adsorption of rabies virus to the cell surface in the cold and warming to 37 degrees C in the presence of tracers for early endosomes, rabies virus and tracers were localized by immunofluorescence microscopy. After 5 min, rabies virus colocalized with Lucifer Yellow, Texas Red-dextran, rhodamine-wheat germ agglutinin, and transferrin receptor in puncta in the cell body, neurites, and nerve terminals. Rabies virus did not colocalize with lysosomal glycoprotein. An acidotropic probe revealed that some of the virus-containing puncta were acidified. Rabies virus also colocalized with synapsin I, a synaptic vesicle marker, in swellings along processes, indicating some virus enters nerve terminals. Electron microscopy revealed the presence of rabies virus within irregular membrane compartments located near the cell surface in the cell body and neurites. The membrane of the virus particle was often continuous with that of the vacuole. It is concluded that rabies virus enters IMR-32 neuroblastoma cells by adsorptive endocytosis and that, shortly after entry, rabies virus is located within and fuses with acidic endosomes.