Binding of the specific ligand to Fc receptors on Trypanosoma cruzi increases the infective capacity of the parasite.

The infective capacity of Trypanosoma cruzi was significantly increased after treatment with monoclonal IgG1 antibodies, whether or not specific for the parasite; minimal or no change in infectivity was seen after treatment with IgG2a, IgG2b or IgG3 monoclonal antibodies. The stimulatory effect was evidenced by elevated numbers of trypanosomes invading mammalian host cells in vitro compared to parasites treated with medium alone. Greater infectivity was also induced by pure human Fc, suggesting a role for Fc receptors on the organism. This inference received support in the fact that protein A inhibited the stimulatory effect of Fc. In addition, Fc-treated parasites incubated with fluorescein-labelled F(ab')2 from goat anti-human IgG exhibited fluorescence detectable by both ultraviolet microscopy and flow cytometry. 125I-Fc binding to T. cruzi was found to be saturable at 0 degrees and was inhibited by cold Fc but not by bovine serum albumin (BSA) or orosomucoid. Interestingly, 125I-Fc binding was greater at 37 degrees and it was not saturable with the concentrations that did saturate at 0 degrees. Possibly, Fc might up-regulate expression of its own receptor and greater endocytosis could take place at 37 degrees. Significant increases in infectivity were detectable after a 40 min pretreatment with Fc--hinting that Fc could trigger a chain of biochemical events underlying the phenomenon--and were reversible, becoming undetectable 2 hr after Fc removal. The average number of Fc receptors per parasite, determined at 0 degrees (at which binding saturation was possible), was estimated as 5 x 10(5), the dissociation constant was of the order of 10(-6)-10(7)M. The present results define an important biological role for an Fc-binding T. cruzi surface component and expose the capacity of this organism to exploit even elements of the immune system in its quest to attain intracellular localization, required for multiplication.