Transformation of trypomastigote forms of Trypanosoma cruzi into activators of alternative complement pathway by immune IgG fragments.

In this report we examined the capacity of immune IgG fragments to prepare trypomastigote bloodstream forms (TBF) of Trypanosoma cruzi for lysis. F(ab')2 fragments were capable of presensitizing TBF for complement (C) lysis, thus excluding the participation of Fc domains in the C activation process. An intact hinge region of the IgG molecule was not involved either, since the corresponding Fab' were almost as active as the original molecules in preparing TBF for lysis. Fab also retained such activity even after further reduction and alkylation. These findings indicate that neither the portions of heavy chains that make up the hinge region nor the intrachain disulphide bonds are involved in the process. The IgG fragments promoted lysis through the activation of the alternative C pathway (ACP). These results suggest that the immune IgG transforms TBF into ACP activators by blocking the capacity of some parasite cell surface components that are known inhibitors of C activation.