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The Classical Activation Pathway of the Human Complement System Is Specifically Inhibited by Calreticulin from Trypanosoma cruzi¹

Viviana Ferreira^*, Carolina Valck^*, Gittith Sánchez, Alexandre Gingras, Sotiria Tzima, María Carmen Molina^*, Robert Sim, Wilhelm Schwaeble and Arturo Ferreira^2,*

^* Immunology and Molecular Biology Disciplinary Programs, Institute of Biomedical Sciences, Faculty of Medicine, University of Chile, Santiago, Chile; Department of Microbiology and Immunology, University of Leicester, Leicester, United Kingdom; and Medical Research Council Immunochemistry Unit, Department of Biochemistry, University of Oxford, Oxford, United Kingdom

The high resistance of Trypanosoma cruzi trypomastigotes, the causal agent of Chagas’ disease, to complement involves several parasite strategies. In these in vitro studies, we show that T. cruzi calreticulin (TcCRT) and two subfragments thereof (TcCRT S and TcCRT R domains) bind specifically to recognition subcomponents of the classical and lectin activation pathways (i.e., to collagenous tails of C1q and to mannan-binding lectin) of the human complement system. As a consequence of this binding, specific functional inhibition of the classical pathway and impaired mannan-binding lectin to mannose were observed. By flow cytometry, TcCRT was detected on the surface of viable trypomastigotes and, by confocal microscopy, colocalization of human C1q with surface TcCRT of infective trypomastigotes was visualized. Taken together, these findings imply that TcCRT may be a critical factor contributing to the ability of trypomastigotes to interfere at the earliest stages of complement activation.

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