The Complement System: A Prey of Trypanosoma cruzi

Abstract

Trypanosoma cruzi is a protozoan parasite known to cause Chagas disease (CD), a neglected sickness that affects around 6-8 million people worldwide. Originally, CD was mainly found in Latin America but more recently, it has been spread to countries in North America, Asia, and Europe due the international migration from endemic areas. Thus, at present CD represents an important concern of global public health. Most of individuals that are infected by T. cruzi may remain in asymptomatic form all lifelong, but up to 40% of them will develop cardiomyopathy, digestive mega syndromes, or both. The interaction between the T. cruzi infective forms and host-related immune factors represents a key point for a better understanding of the physiopathology of CD. In this context, the complement, as one of the first line of host defense against infection was shown to play an important role in recognizing T. cruzi metacyclic trypomastigotes and in controlling parasite invasion. The complement consists of at least 35 or more plasma proteins and cell surface receptors/regulators, which can be activated by three pathways: classical (CP), lectin (LP), and alternative (AP). The CP and LP are mainly initiated by immune complexes or pathogen-associated molecular patterns (PAMPs), respectively, whereas AP is spontaneously activated by hydrolysis of C3. Once activated, several relevant complement functions are generated which include opsonization and phagocytosis of particles or microorganisms and cell lysis. An important step during T. cruzi infection is when intracellular trypomastigotes are release to bloodstream where they may be target by complement. Nevertheless, the parasite uses a sequence of events in order to escape from complement-mediated lysis. In fact, several T. cruzi molecules are known to interfere in the initiation of all three pathways and in the assembly of C3 convertase, a key step in the activation of complement. Moreover, T. cruzi promotes secretion of plasma membrane-derived vesicles from host cells, which prevent the activity of C3 convertase C4b2a and thereby may hinder complement. In this review, we aim to present an overview on the strategies used by T. cruzi in order to circumvent the activation of complement and, consequently, its biological effects.

Keywords: Trypanosoma cruzi; complement regulatory proteins; complement system; evasion mechanism; innate immunity.

FIGURE 1

Life cycle of Trypanosoma cruzi. Transmission is initiated by insect vectors that defecate after a blood meal and release metacyclic trypomastigotes near the bite wound. This infective stage is characterized by the invasion of host cells by trypomastigotes forming the parasitophorous vacuole, from which they subsequently escape, differentiate into amastigotes, and replicate in the cytosol. The amastigotes then divide, differentiate into trypomastigotes, and upon rupture of the cell spread the infection to tissues. Trypomastigotes reach the bloodstream, where they are eventually taken up by the insect vector or infect new cells. In the triatomine bugs, trypomastigotes differentiate into spheromastigotes becoming initially short epimastigotes (mid-log). After migration to the bug’s hindgut, elongate epimastigotes (late-log) attach to the waxy gut cuticle and give rise to infectious metacyclic trypomastigotes, completing the parasite life cycle.

FIGURE 2

Natural course of human T. cruzi infection. T. cruzi transmission can occur by (1) vectorial (2) blood transfusion or organ transplantation, (3) oral, or (4) congenital routes. The incubation period lasts for 5–10 days after human contamination with T. cruzi from triatomines, which is followed by the acute phase of Chagas disease that lasts 4–8 weeks. This phase is characterized by circulating trypomastigotes, which can be visualized in the blood and IgM anti-T. cruzi antibodies can be detected after 10 days of infection. Most patients have non-specific symptoms, such as fever and anorexia, or are asymptomatic, and may develop inflammation and swelling at the site of inoculation in the skin or conjunctiva, characterizing chagoma and Romaña’s signal, respectively. The chronic phase begins once parasitemia falls below detectable levels by microscopy, usually 4 to 8 weeks after the onset of infection, and therefore diagnosis is based on the detection of anti-T. cruzi IgG antibodies or molecular tests. In this phase, most infected people enter a prolonged asymptomatic form known as the indeterminate form and will never develop Chagas-related symptoms. However, after 10–30 years around 30–40% of chronically infected people will present some clinical manifestations including cardiac, digestive, or cardiodigestive complaints.

FIGURE 3

Activation of complement system by Trypanosoma cruzi epimastigote forms. MBL and ficolins recognize and bind to glycoproteins and carbohydrates present on the T. cruzi epimastigote surface initiating the LP. This interaction leads to conformational changes in MBL and ficolins and activation of MASP-1 followed by MASP-2 which cleavages C4 into C4a and C4b and C2 into C2a and C2b, generating C3 convertase (C4b2a). In presence of specific anti-T. cruzi antibodies, C1q molecules recognize and bind to IgM and IgG on the parasite’s surface initiating the CP. Subsequently, C1q suffers conformational changes and activates C1r, which cleaves and activates C1s that subsequently cleaves C4 and C2 generating C3 convertase (C4b2a), as in the LP convertase. When C3b binds to C4b2a the complex forms C4b2aC3b, with C5 convertase activity. The AP is initiated by the generation of C3(H2O) after spontaneous hydrolysis of C3, or as a consequence of loop amplification triggered during LP and CP activation,. Since FB binds to C3(H2O), FB can be cleaved by FD in Ba and Bb. The Bb fragment remains bound to C3(H2O) forming the first C3 convertase of AP (C3(H2O)Bb), which cleaves C3 into C3a and C3b. C3b also has a binding site for FB, allowing cleavage by FD, resulting in the second C3 convertase of AP, C3bBb. Additional C3b molecules are able to bind to the C3bBb complex to form C3bBb3bn, which exerts C5 convertase activity. Both C5 convertases, LP/CL and AP, cleave the C5 component into C5a and C5b. Newly formed C5b reacts with C6 to form the stable C5b6 complex that recruits C7 resulting in a hydrophobic complex that targets the membrane (mC5b-7). Membrane insertion is initiated upon binding of C8 (C5b-8) and 12–18 copies of C9 polymerize to form the membrane attack complex (MAC) that induces lysis of target membranes. In addition, as a product of all pathways being activated, the small fragments C4a, C3a, and C5a are formed, which are important anaphylatoxins, attracting and activating inflammatory cells to the activation site, such as neutrophils, monocytes, macrophages, and dendritic cells.

FIGURE 4

Complement evasion by strategies of T. cruzi trypomastigotes forms. TcCRT blocks CP and LP binding to C1q, MBL and Ficolin-2; T-DAF and Gp160/TcCRP block AP and CP C3 convertase assembly; TcCRIT blocks CP and LP binding to C2; gp58/68 blocks AP C3 convertase binding to factor B; and MV inhibit CP and LP C3 convertase assembly. AP, Alternative pathway; LP, Lectin pathway; and CP, Classical pathway.