ROLE OF SIALIC-ACID IN THE RESISTANCE OF TRYPANOSOMA-CRUZI TRYPOMASTIGOTES TO COMPLEMENT

Trypomastigotes of Trypanosoma cruzi, mammalian infective forms of the parasite, express an unusual cell surface trans-sialidase. This enzym e enables the parasite to rapidly sialylate its surface when supplied with alpha(2,3)-linked sialic acid from glycoconjugates in serum or on cell surfaces. Here we used a novel fluorescence-based, trypomastigot e lysis assay to evaluate the role of sialic acid on the parasite's pl asma membrane in providing protection against the complement cascade. Trypomastigotes were desialylated, and sialic acid removal was confirm ed by a chemical assay and also by flow cytometry with the use of a mA b that recognizes a T. cruzi-sialylated epitope. Compared with sialyla ted trypomastigotes, which were completely refractory to lysis by huma n serum, only about 5% of the desialylated trypomastigotes were lysed by complement. However, further analysis revealed that the desialylate d parasites had been resialylated during exposure to serum complement. Next we incubated desialylated trypomastigotes with samples of desial ylated human serum. Although the sialidase-treated serum retained its full hemolytic activity, lysis of trypomastigotes increased only from 5 to 24%. This increase correlated with an enhanced deposition of comp lement protein C3 on the parasite surface. The ratio of C3b to lytical ly inactive iC3b was increased for desialylated, compared with sialyla ted, parasites. We conclude that although parasite sialic acid promote s C3b cleavage into iC3b, this mechanism alone does not account for th e robust resistance of these parasites to complement lysis.