Mechanisms by which Factor H protects Trypanosoma cruzi from the alternative pathway of complement.

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Abstract Chagas disease is a neglected tropical disease caused by the protozoan Trypanosoma cruzi (T. cruzi). Complement (C) is a critical arm of the immune system. Various pathogens, including T. cruzi, hijack host regulators such as Factor H (FH) to inactivate C. FH regulates the alternative pathway (AP) of C in blood and on cell surfaces. FH contains 20 short consensus repeat (SCR) domains and binds to C activation products (i.e., C3b/C3d) or polyanionic host cell markers (e.g., sialic acid, glycosaminoglycans) on cell surfaces, and uses its 4 N-terminal SCRs to inactivate the AP. T. cruzi potentially binds FH by transferring host sialic acids onto its surface using its trans-sialidase enzyme. This study asked whether FH binds directly to T. cruzi in the absence of C3b and defines the FH domains involved. Only C-resistant infective trypomastigotes, but not C-sensitive noninfective epimastigotes, bound FH directly in a dose-dependent manner. Although SCR 19–20 was hypothesized as important for this interaction given it is the only known sialic acid-binding region, domain mapping indicated SCR 5–8 competitively inhibited FH binding by >35% with lower contributions of other regions. FH related protein-5 (FHR-5), whose SCRs bear sequence homology to all known polyanion-binding regions (6–7, 10–14, 19–20), fully competitively inhibited FH binding to trypomastigotes. In addition, competitive survival experiments with serum and FHR-5 led to >80% killing when the AP or all C pathways were active. This is the first study to show consequences of completely inhibiting FH protection on T. cruzi and to suggest multiple FH domains are involved in the interaction. These data may be harnessed for developing therapeutics and vaccine candidates against T. cruzi. This work was supported by The University of Toledo Biomedical Research Innovation Program (VF), and The American Association of Immunologists Careers in Immunology Fellowship Program (VF and SSM).