Directional movement of variable surface glycoprotein-antibody complexes in Trypanosoma brucei

Binding of antibody, antibody fragments (Fab and F(ab)2) and biotin molecules to variable surface glycoprotein (VSG) of Trypnnosoma brucei was studied by both light microscopy and fluorescence activated cell sorter (FACS) analysis. Antibodies, antibody fragments and biotin molecules were distributed over the entire parasite surface after incubation at 0°C. Upon warming to 37°C, surface bound Fab and F(ab)₂ fragments showed different rates of clearance from the parasite surface. Clearance, which in both cases followed double exponential decay kinetics, resulted from a directional movement of VSG-bound antibody complexes from both the surface of the flagellum and the cell body towards the cellular site of active endocytosis, the flagellar pocket (FP), even in the absence of antibody-mediated crosslinking of VSG. Immunofluorescence on trypanosomes permeabilized after binding, clearance and internalization, indicated the location of small amounts of antibody intracellularly, between the nucleus and the flagellar pocket. However, if a cocktail of protease inhibitors was added to the medium, larger amounts of internalized antibody could be detected within vacuoles situated between the nucleus and the flagellar pocket. Movement of antibody-VSG complexes was reversibly inhibited at temperatures below 37°C and by increasing the NaCl concentration in the medium to 200 mM.