Evasion of alternative complement pathway by Trypanosoma cruzi results from inefficient binding of factor B.

Abstract

During its differentiation in the insect vector to a stage infective for the mammalian host, Trypanosoma cruzi becomes resistant to lysis by the alternative pathway of complement. To elucidate the mechanism of complement evasion, we studied control of complement activation on the surface of the noninfective epimastigote and the infective culture-derived metacyclic trypomastigote stages (CMT) of T. cruzi. It was found that the predominant form of complement component C3 on epimastigotes is C3b, whereas the majority of C3 on CMT is in the form of the hemolytically inactive fragment iC3b, which cannot participate in C5 convertase formation or lead to deposition of the lytic C5b-9 complex. Our results also showed that C3 binds by a covalent ester linkage to surface molecules of different molecular weight in the epimastigote stage and CMT. Binding studies with purified complement components indicated that CMT do not support efficient formation of an alternative pathway C3 convertase. C3b on the parasite surface fails to bind the amplification component, factor B, rather than showing enhanced binding of the control component, factor H. These results identify the biochemical basis for evasion of complement-mediated killing in T. cruzi and reveal a mechanism for developmental regulation of complement activation.Protozoan parasites have developed a variety of mechanisms to evade the immunologic defense mechanisms of the mammalian host. Thus, vertebrate-stage parasites and infective vector-stage parasites typically resist direct serum killing and can also evade ingestion or intracellular killing by the phagocytic cells of the host. In contrast, most noninfective vector-stage parasites are susceptible to direct lysis by serum and to phagocytosis and destruction by monocytes or macrophages. For example, the epimastigote stage (Epi) of Trypanosoma cruzi, which multiplies in the gut ofthe vector, is efficiently lysed in human serum by means of alternative complement pathway (ACP) activation (1). In contrast, the infecting metacyclic trypomastigote stage, the amastigote stage (which multiples intracellularly in the vertebrate host), and the disseminating bloodstream trypomastigote form are not lysed when incubated in human serum (1-3). The membrane changes that control the transformation of these parasites from activators of the ACP to nonactivators are incompletely understood, as are the molecular interactions of the ACP with the parasite surface of various life-cycle stages.Activation of the ACP is currently thought to be mediated and regulated by the following mechanisms: continuous low-grade fluid-phase generation of complement component C3b or water-hydrolyzed C3 occurs normally in serum and results in the random deposition of C3b on all particles (4). The subsequent interaction of this randomly deposited C3b with proteins of the ACP is determined by the nature of the particle surface (5-7). On alternative pathway activators, C3b binds factor B in preference to factor H. C3b that bears B cannot interact with the i...

Discovery and Verification of Osteopontin and Beta-2-microglobulin as Promising Markers for Staging Human African Trypanosomiasis

“…Trypanosomes are able to activate the alternative complement pathway in blood (35), whereas decreased complement activation was reported during the 1980s in infective cultures of T. cruzi, responsible for Chagas' disease (36). Complement factor H is the principal inhibitor of the alternative pathway, and is also involved in the protection of epithelial, endothelial, and some cancer cells against complement action (37).…”

Infectious diseases associated with complement deficiencies

“…The basis for this difference is the ability of trypomastigotes but not of epimastigotes to N-glycosylate an 87-to 93-kDa surface protein which has DAF-like activity (180,182,325). Although C3b is deposited on both forms, the presence of the glycosylated protein reduces the affinity of C3b for factor B and leads to more rapid C3b decay (181).…”

Chagas' disease

“…Trypomastigotes are refractory to complement-mediated lysis, whereas epimastigotes activate the alternate complement pathway. The mechanism of the resistance of trypomastigotes to complement is unclear, and several different mechanisms have been proposed (104,114,115,235). Amastigotes activate complement but are not lysed.…”