Factor H binds to Trypanosoma cruzi trypomastigotes via its C-terminus and contributes to parasite resistance against the alternative pathway of complement (MPF2P.809)

Abstract

Trypanosoma cruzi (T. cruzi), the causal agent of devastating Chagas disease, possesses multifactorial resistance to the alternative pathway (AP) of complement. AP regulatory protein factor H (fH) uses its C-terminal 19-20 domains to recognize polyanions (i.e. sialic acid) and C3b on host cells, facilitating decay of C3 convertases and inactivation of C3b via the N-terminus of fH. We have shown that recombinant C-terminal fH domains 19-20 (rH19-20) compete with fH for binding to C3b and host cell polyanions, inhibiting protection from the AP. T. cruzi tripomastigotes resist the AP by capturing sialic acid from host cells. The specific role that fH plays in protecting T. cruzi from the AP and the fH domains involved remain unknown. Here we assessed the susceptibility of T. cruzi (MF, Y and Brenner strains) to AP-mediated killing, and the ability of fH to bind directly to the parasite by FACS. For determining the fH domains needed for the interaction with T. cruzi, trypomastigotes were incubated with normal human sera in the presence or absence of equimolar amounts of overlapping recombinant fH proteins (2 to 3 domains each) spanning domains 7 to 20. The ability of each protein to increase AP-mediated killing of T. cruzi was assessed. Only rH19-20 effectively competed with serum fH, inducing a significant decrease in parasite survival (p<0.0001), indicating that the C-terminus of fH is essential for fH-mediated recognition and protection of T. cruzi trypomastigotes from the AP.