Recognition of Giardia lamblia cyst-specific antigens by monoclonal antibodies.

Immunization of BALB/c mice with a sonicated extract of in vitro-generated Giardia lamblia cysts produced six cyst-specific monoclonal antibodies (MoAbs). Two MoAbs (8C5.C11 and 5A4.G6), which recognize proteinaceous cyst antigens, were selected for further study. In indirect immunofluorescence (IFA), MoAb 8C5.C11 reacted with encystation-specific vesicles in trophozoites beginning 3 h after the induction of encystation in vitro. This MoAb also recognized cysts which began to appear at 12 h. In contrast, MoAb 5A4.G6 stained only cyst walls. In Western blots, both MoAbs also reacted with cyst antigens, but not trophozoite antigens. MoAb 8C5.C11 first recognized cyst antigen from 3 h encysting cultures, reacting with 26, 28, 42 and 46 kD bands. MoAb 5A4.G6 reacted with a 38 kD band, beginning with 12 h encysting cultures. When added to G. lamblia encysting cultures before the appearance of cysts (0 to 9 h) and in the presence of a source of complement, MoAb 8C5.C11 caused a significant reduction in the numbers of water-resistant cysts produced in vitro compared to the control. MoAb 5A4.G6 did not affect in vitro encystation. These findings confirm the heterogeneity of cyst antigens, and also indicate that the process of encystation in vitro can be interrupted by antibodies and complement.