Toxoplasma gondii : Antibody-independent Binding of Human Complement Subcomponent C1q to the Parasite

Abstract

The binding capacity of C1q onto Toxoplasma gondii (T. gondii) parasites at each infectious stage was examined by using the immunofluorescence assay (IFA) with purified human C1q, rabbit anti-human C1q antibodies and fluorescein -isothiocynate conjugated goat anti-rabbit IgG antibodies in parallel with the IgM-binding to the parasite.
Any possibly contaminated natural antibodies to the parasite were completely removed from the anti-human C1q-preparations by repeated absorption with the parasite in advance. Parasites incubated with C1q before adding anti-C1q antibodies have shown the specific fluorescence on their outer surface membrane, whereas those without any addition of C1q have no specific fluorescence. The intensity of fluorescence differed in stages, and cystozoites and trophozoites seemed to have significantly higher C1q-binding activities than sporozoites. Furthermore, C1q-binding sites have been differently located from IgM-binding sites, or anterior poles, on the parasite.
These observations indicate that T. gondii parasite has the antibody- independent binding capacity to human C1q on its outer surface membrane.