We describe here the ultrastructural localization of Giardia
cyst antigens in the filaments associated with the outer portion of intact
cysts and on developing
cyst wall filaments in encysting trophozoites. Post-embedding immunogold labeling of thin sections of intact Giardia
cysts with polyclonal and
monoclonal antibodies specific for
cyst wall
antigens (major
protein bands of approximately 29, 75, 88, and 102 KD on Western blots) showed strong labeling of the filamentous
cyst wall, whereas no labeling was seen on the membranous portion. High-resolution field emission scanning electron microscopy (FESEM) of Giardia
cysts revealed that the
cyst wall-specific polyclonal rabbit
antisera and monoclonal mouse antibody produced
gold labeling of 20-nm filaments in the
cyst wall as detected with secondary electron imaging (SEI) and backscatter electron imaging (BEI)
at 10 kV, despite coating of the cells with
platinum by ion sputtering. FESEM studies of encysting Giardia trophozoites demonstrated that immunostaining with
antibodies to
cyst wall
antigens produced
colloidal gold labeling of developing
cyst wall filaments on the cell surface; however, the intervening membrane domains were unlabeled. Substitution of normal serum for
cyst wall-specific
antibodies, or preabsorption of specific
antibodies with Giardia
cysts, eliminated immunolabeling of the filaments.