Lytic rabbit IgG for tissue culture trypomastigotes of Trypanosoma cruzi alters the extent and form of complement deposition

Infective and vertebrate stages of Trypanosoma cruzi are resistant to lysis by the alternative pathway of complement. To further elucidate the mechanism of complement evasion and to study how some immune sera render the infective stage sensitive to lysis, we compared the interaction of complement components C3 and C9 with the surface of complement susceptible, vector stage epimastigotes and vertebrate stage trypomastigotes of T. cruzi. Our studies showed that, upon incubation in human serum, complement resistant tissue culture trypomastigotes (TCT) bound five- to eightfold less C3 or C9 than complement sensitive epimastigotes (Epi). C3 bound to Epi is mainly in the hemolytically active C3b form, while TCT bear predominantly the hemolytically inactive iC3b fragment, which cannot participate in C5 convertase formation or lead to deposition of the lytic C5b-9 complex. Three- to sixfold more C3 and two- to threefold more C9 were deposited on TCT when lytic rabbit immune IgG with broad specificity was used to sensitize the parasites, and nearly one-half of bound C3 was present as C3b. In contrast, a comparison of three different sources of IgG from immune human serum showed a less clear correlation between the titer or specificity of anti-T. cruzi antibody, enhancement of C3 or C9 deposition, change in the form of bound C3, or killing. These results show that lytic rabbit IgG for T. cruzi changes the form and amount of bound complement components in anticipated fashion, but that human immune IgG does not give predictable changes in the extent or form of C3 or C9 deposition.