Electron microscopical investigation of surface alterations on Giardia lamblia trophozoites after exposure to a cytotoxic monoclonal antibody

The present study describes a transmission electron microscopical investigation of trophozoites from Giardia lamblia clone GS/M-83-H7 after exposure to monoclonal antibody (MAb) G10/4. From previous studies it is known that this antibody immunoreacts with the parasite's major surface antigen VSP (variable surface protein) and exhibits a complement-independent cytotoxic effect on trophozoites of clone GS/M-83-H7. Our investigations revealed that cytotoxicity of MAb G10/4 is associated with shedding of VSP-containing membrane vesicles from the parasite surface and a concomitant partial disruption of the cellular membrane. These morphological alterations depend on the cross-linking capacity of the antibody because the immunoreactivity of respective monovalent F(ab)' has no significant influence on the cell-surface structure. These findings indicate that the membrane-disintegrative activity of MAb G10/4 may be responsible for the parasitocidal function of the antibody.