The effect of C3 depletion on the clearance of Trypanosoma cruzi induced by IgG antibodies.

Passive transfer of homologous immune serum or IgG anti-Trypanosoma cruzi antibodies to normal mice containing circulating T. cruzi bloodstream trypomastigotes (Btrys) induces a very fast clearance of the parasites. Previous work from this laboratory has shown that F(ab')2 fragments obtained from IgG anti-T. cruzi antibodies retain the ability to induce lysis of the Btrys but are unable to induce clearance of the parasites. This suggests that the clearance is dependent on the Fc region. The removal of the Btrys may then be effected by attachment of the antibodies to the parasites and removal of the opsonized parasites by the mononuclear phagocytic system. Attachment of the opsonized parasites to macrophages may be effected either through the Fc receptor that binds specifically to the heavy chain of IgG or through C3b fragments after cleavage of C3 by C3-convertases. In order to find out the possible role of C3b in this phenomenon the clearance of Btrys was determined in mice depleted of C3 by previous treatment with cobra venom factor. The results of these experiments showed that depletion of C3 completely abolished the immune clearance induced by anti-T. cruzi antibodies. It is suggested that C3 is required for the clearance of Btrys from circulation.