Humoral immune response to the Trypanosoma cruzi complement regulatory protein as an indicator of parasitologic clearance in human Chagas' disease

Antigenic diversity of MASP gene family of Trypanosoma cruzi

Trypanosoma cruzi, the etiological agent of Chagas disease (CD), is a heterogeneous species with high genetic and phenotypic diversity. MASP is the second largest multigene family of T. cruzi. The high degree of polymorphism of the family associated with its location at the surface of infective forms of T. cruzi suggests that MASP participates in mechanisms of host-parasite interaction. In this work, MASP members were divided into 7 subgroups based on protein sequence similarity, and one representative member from each subgroup was chosen to be expressed recombinantly. Immunogenicity of recombinant MASP proteins (rMASP) was investigated using different sera panels from T. cruzi infected mice. To mimic a natural condition in which different MASP members are expressed at the same time in the parasite population, a multiplex bead-based flow cytometry assay was also standardized. Results showed that rMASPs are poorly recognized by sera from mice infected with Colombiana strain, whereas sera from mice infected with CL Brener and Y display high reactivity against the majority of rMASPs tested. Flow cytometry showed that MASP recognition profile changes 10 days after infection. Also, multiplex assay suggests that MASP M1 and M2 are more immunogenic than the other MASP members evaluated that may play an immunodominant role during infection.

Epidemiology and pathogenesis of maternal-fetal transmission of Trypanosoma cruzi and a case for vaccine development against congenital Chagas disease

Trypanos o ma cruzi (T. cruzi or Tc) is the causative agent of Chagas disease (CD). It is common for patients to suffer from non-specific symptoms or be clinically asymptomatic with acute and chronic conditions acquired through various routes of transmission. The expecting women and their fetuses are vulnerable to congenital transmission of Tc. Pregnant women face formidable health challenges because the frontline antiparasitic drugs, benznidazole and nifurtimox, are contraindicated during pregnancy. However, it is worthwhile to highlight that newborns can be cured if they are diagnosed and given treatment in a timely manner. In this review, we discuss the pathogenesis of maternal-fetal transmission of Tc and provide a justification for the investment in the development of vaccines against congenital CD.

Immunity and vaccine development efforts against Trypanosoma cruzi

Trypanosoma cruzi (T. cruzi) is the causative agent for Chagas disease (CD). There is a critical lack of methods for prevention of infection or treatment of acute infection and chronic disease. Studies in experimental models have suggested that the protective immunity against T. cruzi infection requires the elicitation of Th1 cytokines, lytic antibodies and the concerted activities of macrophages, T helper cells, and cytotoxic T lymphocytes (CTLs). In this review, we summarize the research efforts in vaccine development to date and the challenges faced in achieving an efficient prophylactic or therapeutic vaccine against human CD.

The Complement System: A Prey of Trypanosoma cruzi

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.

Roles of Trypanosoma cruzi calreticulin in parasite-host interactions and in tumor growth

In Latin America, there are about 10-12 million people infected with Trypanosoma cruzi, the agent of Chagas' disease, one of the most important neglected tropical parasitism. Identification of molecular targets, specific for the aggressor or host cells or both, may be useful in the development of pharmacological and/or immunological therapeutic tools. Classic efforts in Chagas' disease explore those strategies. Although the immune system frequently controls parasite aggressions, sterile immunity is seldom achieved and chronic interactions are thus established. However, laboratory-modified immunologic probes aimed at selected parasite targets, may be more effective than their unmodified counterparts. Calreticulin (CRT) from vertebrates is a calcium binding protein, present mainly in the endoplasmic reticulum (ER), where it directs the conformation of proteins and controls calcium levels. We have isolated, gene-cloned, expressed and characterized T. cruzi calreticulin (TcCRT). Upon infection, the parasite can translocate this molecule from the ER to the surface, where it inhibits both the classical and lectin complement pathways. Moreover, by virtue of its capacity to bind and inactivate first complement component C1, it promotes parasite infectivity. These two related properties reside in the central domain of this molecule. A different domain, amino terminal, binds to endothelial cells, thus inhibiting their angiogenic capacity. Since tumor growth depends, to a large extent on angiogenesis, their growth is also inhibited.

Sequence diversity of the Trypanosoma cruzi complement regulatory protein family

As a central component of innate immunity, complement activation is a critical mechanism of containment and clearance of microbial pathogens in advance of the development of acquired immunity. Several pathogens restrict complement activation through the acquisition of host proteins that regulate complement activation or through the production of their own complement regulatory molecules (M. K. Liszewski, M. K. Leung, R. Hauhart, R. M. Buller, P. Bertram, X. Wang, A. M. Rosengard, G. J. Kotwal, and J. P. Atkinson, J. Immunol. 176:3725-3734, 2006; J. Lubinski, L. Wang, D. Mastellos, A. Sahu, J. D. Lambris, and H. M. Friedman, J. Exp. Med. 190:1637-1646, 1999). The infectious stage of the protozoan parasite Trypanosoma cruzi produces a surface-anchored complement regulatory protein (CRP) that functions to inhibit alternative and classical pathway complement activation (K. A. Norris, B. Bradt, N. R. Cooper, and M. So, J. Immunol. 147:2240-2247, 1991). This study addresses the genomic complexity of the T. cruzi CRP and its relationship to the T. cruzi supergene family comprising active trans-sialidase (TS) and TS-like proteins. The TS superfamily consists of several functionally distinct subfamilies that share a characteristic sialidase domain at their amino termini. These TS families include active TS, adhesions, CRPs, and proteins of unknown functions (G. A. Cross and G. B. Takle, Annu. Rev. Microbiol. 47:385-411, 1993). A sequence comparison search of GenBank using BLASTP revealed several full-length paralogs of CRP. These proteins share significant homology at their amino termini and a strong spatial conservation of cysteine residues. Alternative pathway complement regulation was confirmed for CRP paralogs with 58% (low) and 83% (high) identity to AAB49414. CRPs are functionally similar to the microbial and mammalian proteins that regulate complement activation. Sequence alignment of mammalian complement control proteins to CRP showed that these sequences are distinct, supporting a convergent evolutionary pathway. Finally, we show that a clonal line of T. cruzi expresses multiple unique copies of CRP that are differentially recognized by patient sera.

The classical activation pathway of the human complement system is specifically inhibited by calreticulin from Trypanosoma cruzi

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.

Role of calreticulin from parasites in its interaction with vertebrate hosts

Although parasites range from protozoan to complex, evolutionary advanced arthropods, in general, a hallmark of parasite life cycles is their ability to adapt to changes in temperature, pH and host defense strategies. Calreticulin, a calcium-binding protein, highly conserved and multifunctional, is present in every cell of higher organisms, except erythrocytes. The surprising array of calreticulin-associated functions include lectin-like chaperoning, calcium storage and signaling, modulation of gene expression, cell adhesion, enhancement of phagocytosis of C1q or collectin opsonized apoptotic cells, inhibition of angiogenesis and tumoral growth, inhibition of perforin pore formation in T and NK cells, and inhibition of C1q-dependent complement activation. Likewise, calreticulin is present in a wide spectrum of sub cellular compartments. Parasite calreticulin shows a surprisingly high degree of conservation within the framework of its functional domains. Its role within the parasite/host relationship needs to be assessed further, in particular with regard to its impact on parasite infectivity, by helping to evade from its hosts' immune response. With special emphasis on calreticulin from Trypanosoma cruzi, the intracellular protozoan agent of American trypanosomiasis (Chagas' disease), we wish to exemplify and highlight the various implications of parasite calreticulin, within the pathophysiology of parasite-mediated human and animal disease.

Use of the Trypanosoma cruzi recombinant complement regulatory protein to evaluate therapeutic efficacy following treatment of chronic chagasic patients

One of the greatest concerns in Chagas' disease is the absence of reliable methods for the evaluation of chemotherapy efficacy in treated patients. The tests available to evaluate cure after the specific treatment are the complement-mediated lysis (CoML) and flow cytometry tests, but they are not feasible for routine clinical use. In this study, we evaluated an enzyme-linked immunosorbent assay (ELISA) based on the recombinant Trypanosoma cruzi complement regulatory protein (rCRP) as a method to determine parasite clearance in comparison to the CoML and other methods such as conventional serology, hemoculture, and PCR in serum samples of 31 patients collected before and after the treatment, monitored for an average of 27.7 months after chemotherapy. The results showed that the percentage of patient samples that were positive by rCRP ELISA was reduced from 100 to 70.3, 62.5, 71.4, and 33.4% in the first, second, third, and fourth years after treatment, respectively, while the samples positive by CoML were reduced to 85.2, 81.2, 71.4, and 33.4% during the same period, demonstrating the same significant tendency in the reduction of positive samples. On the other hand, the conventional serology (CS) tests did not present this reduction. The percentage of samples positive by PCR was initially 77.4% and decreased to 55.5, 68.7, 47.7, and 50.0% at the fourth year after treatment, confirming the drastic clearance of circulating parasites after treatment. Our results strongly suggest that the rCRP ELISA was capable of detecting the early therapeutic efficacy in treated patients and confirmed its superiority over the CS tests and parasitologic methods.

Expression and purification of functional, recombinant Trypanosoma cruzi complement regulatory protein

The complement regulatory protein (CRP) of Trypanosoma cruzi is a developmentally regulated glycosylphosphatidylinositol (GPI)-anchored membrane protein that protects the parasite from complement-mediated killing, and is an important virulence determinant of the microorganism. CRP binds human complement components C3b and C4b to restrict activation of the complement cascade. Here, we report production of functional, recombinant T. cruzi CRP in mammalian cells and a one-step purification of the recombinant protein. Exchange of the crp DNA sequence encoding the carboxy-terminal GPI signal sequence with the corresponding sequence of decay accelerating factor (DAF) was necessary for recognition, cleavage, and addition of GPI in mammalian cells. CRP production was assessed in two mammalian cell lines with crp-daf gene expression driven by three different transcription control regions: Rous sarcoma virus long terminal repeat, cytomegalovirus (CMV) immediate early gene, and chicken beta-actin promoter/CMV enhancer. We present evidence that CRP produced in transfected Chinese hamster Ovary (CHO) cells was functional and protected the cells from complement-mediated lysis. To facilitate purification of the recombinant protein, a hexahistidyl tag was incorporated at 3(') end of the cDNA upstream of the GPI anchor addition sequence. An additional histidine fusion construct was made that allowed for secretion and recovery of recombinant protein from culture supernatant fluid. Both membrane and secreted forms of the protein were purified in one step by nickel nitrilotriacetic acid. The production and purification of functionally active CRP in a non-infectious expression system will allow for structure and function studies aimed at identifying the active site(s) of this protein.

Trypanosoma cruzi recombinant complement regulatory protein: a novel antigen for use in an enzyme-linked immunosorbent assay for diagnosis of Chagas' disease

Currently, diagnosis of Chagas' disease is based on serological methods, but due to the high occurrence of inconclusive results, more reliable methods are needed. The use of recombinant antigens for serodiagnosis of Chagas' disease is recommended in order to increase the sensitivity and specificity of the serological tests. The Trypanosoma cruzi complement regulatory protein (CRP) is a surface glycoprotein present on the trypomastigote forms of the parasite, and the recombinant CRP (rCRP) was cloned in a mammalian expression system and purified by affinity chromatography. The purified recombinant protein was used as an antigen in an enzyme-linked immunosorbent assay (rCRP ELISA) in order to verify its sensitivity and specificity compared with other established methods. In this evaluation, a panel of 184 serum samples distributed among chronic chagasic patients (n = 65), blood bank donors (n = 100), and patients infected with Leishmania spp. (n = 19) was used. The sensitivity and specificity of the rCRP ELISA were 100% when compared to conventional serology and complement-mediated lysis tests from these groups. When hemoculture and PCR tests were evaluated for diagnosis of chronic chagasic patients, using the rCRP ELISA as a reference test, the positivities were found to be 64.62 and 81.54%, respectively, showing a higher degree of sensitivity of the test. The data demonstrate that rCRP ELISA was able to discriminate between chronic chagasic patients and nonchagasic individuals, such as blood donors and patients with leishmaniasis. Thus, the rCRP is an excellent antigen for use in Chagas' disease diagnosis, due to the absence of false-negative or false-positive results.

Trypanosoma cruzi: isolation of an immunodominant peptide of TESA (Trypomastigote Excreted-Secreted Antigens) by gene cloning

Trypomastigote forms of Trypanosoma cruzi excrete-secrete several molecules, which are immunodominant during the human infection. This complex antigenic mixture termed TESA (Trypomastigote Excreted-Secreted Antigens) presents a 150-160 kDa band that shows excellent specificity and sensitivity in Chagas' disease diagnosis by immunoblotting. Here we describe the isolation and the antigenic characterization of a recombinant peptide (TESA-1) containing a 10 kDa T. cruzi peptide that belongs to the 150-160 kDa TESA fraction. The clone was isolated by screening a T. cruzi genomic expression library with chagasic antibodies reactive to the 150-160 kDa band of TESA immunoblots. After expression, the recombinant peptide TESA-1 was purified and used to immunize rabbits. Anti-TESA-1 immunesera specifically recognized the 150-160 kDa fraction of TESA-blots from eight different T. cruzi strains. The TESA-1 peptide reacted with 82.2% of chagasic patient sera by immunoblotting, showing that it harbors most of the antigenic epitopes that account for the high reactivity of the 150-160 kDa band of TESA.

DNA-Based immunization with Trypanosoma cruzi complement regulatory protein elicits complement lytic antibodies and confers protection against Trypanosoma cruzi infection

A complement regulatory protein (CRP) of Trypanosoma cruzi was evaluated as a vaccine candidate in a murine model of experimental T. cruzi infection. Recombinant CRP derived from an Escherichia coli expression system and a plasmid encoding the full-length crp structural gene under the control of a eukaryotic promoter were used to immunize BALB/c mice. Immunization with both protein and DNA vaccines resulted in a Th1-type T-cell response, comparable antibody titers, and similar immunoglobulin G isotype profiles. Only mice immunized with the crp DNA plasmid produced antibodies capable of lysing the parasites in the presence of complement and were protected against a lethal challenge with T. cruzi trypomastigotes. These results demonstrate the superiority of DNA immunization over protein immunization with the recombinant CRP. The work also supports the further investigation of CRP as a component of a multigene, anti-T. cruzi DNA vaccine.

The targets of the lytic antibody response against Trypanosoma cruzi

Trypanosoma cruzi trypomastigotes, but not epimastigotes, are normally resistant to the lytic effects of complement from vertebrate hosts susceptible to infection. This resistance facilitates parasite survival and infectivity. During the course of chronic infections, however, the vertebrate hosts produce antibodies that render the trypomastigotes sensitive to lysis, primarily via the alternative complement cascade and amplified by the classical pathway. Here, Greice Krautz, Jessica Kissinger and Antoniana Krettli summarize research on lytic antibodies, and on their respective target(s) on the T. cruzi surface. These targets are useful in tests aimed at the diagnosis of chronic Chagas disease for control of cure after specific treatment and for vaccine development.

Stable transfection of Trypanosoma cruzi epimastigotes with the trypomastigote-specific complement regulatory protein cDNA confers complement resistance

Trypanosoma cruzi blood stage trypomastigotes are highly resistant to complement-mediated killing in normal serum. A previously described trypomastigote surface glycoprotein was shown to have binding affinity for human complement components C3b and C4b and restrict activation of the complement cascade, thus preventing lysis of the parasites. Insect stage epimastigotes do not produce detectable levels of this 160-kDa complement regulatory protein (CRP) and are highly sensitive to the lytic effects of complement. Epimastigotes were stably transfected with a T. cruzi expression vector carrying the trypomastigote CRP cDNA and produced fully functional recombinant CRP. The recombinant CRP had binding affinity for C3b, and the transfected epimastigotes were protected from complement-mediated lysis. These results demonstrate for the first time that a developmentally regulated gene of T. cruzi trypomastigotes can be expressed in noninfectious epimastigotes and that production of CRP by epimastigotes is sufficient to confer a virulence-associated trait. Furthermore, these studies demonstrate the critical role that trypomastigote CRP plays in the protection of parasites from the deleterious effects of complement, thus establishing the protein as a virulence factor of T. cruzi.

Identification of the gene family encoding the 160-kilodalton Trypanosoma cruzi complement regulatory protein

Trypanosoma cruzi trypomastigotes are exquisitely resistant to the lytic effects of vertebrate complement, and this characteristic contributes to the survival of the parasites in the host bloodstream. Trypomastigotes avoid complement-mediated lysis by the production of a surface glycoprotein that inhibits the formation of the alternative and classical C3 convertase, thus preventing activation and amplification of the complement cascade at the parasite surface. We have developed a monoclonal antibody to the 160-kDa T. cruzi complement regulatory protein (CRP) and describe a one-step immunoaffinity purification procedure. The CRP was purified to homogeneity and subjected to amino-terminal peptide sequence analysis. Based on the protein sequence obtained, the CRP was identified as a member of a large family of trypomastigote-specific genes, and a complete cDNA was isolated and sequenced. The complete coding sequence was cloned in Escherichia coli, and antibodies raised against the full-length recombinant protein reacted specifically with a 160-kDa protein in trypomastigote membrane protein preparations as well as with native, purified CRP. Indirect immunofluorescence revealed that the protein is uniformly expressed at the cell surfaces of trypomastigotes.

Total and specific anti-Trypanosoma cruzi immunoglobulin E in pericardial fluid samples from patients with chronic Chagas disease

Levels of total and specific anti-Trypanosoma cruzi immunoglobulin E (IgE) were determined by immunoenzymatic assay among 101 samples of pericardial fluid from patients who had died in one trypanosomiasis endemic area in central Brazil. These samples were divided into 6 groups. Group I, 17 samples from patients with the cardiac form of Chagas disease; group II, 11 samples from patients with the digestive form of Chagas disease, presenting megaoesophagus and/or megacolon; group III, 41 samples from patients with the indeterminate form of Chagas disease; group IV, 4 samples from patients with both cardiac and digestive forms of Chagas disease; group V, 5 samples from patients who suddenly died and were seropositive for T. cruzi antibodies; group VI, 23 samples, used as a control group, which came from patients seronegative for T. cruzi antibodies. Significantly high levels of total IgE were observed in groups I, II, III, IV and V when compared with group VI (mean concentrations 708-1157 iu/mL compared with 394 iu/mL). In groups I-V, 32 samples (41%) had specific anti-T. cruzi IgE antibodies. The individual percentage positivity rates in these groups were 64.7% (group I), 45.4% (group II), 34.1% (group III), nil (group IV), and 40.0% (group V). A significant correlation between total IgE and specific anti-T. cruzi IgE was observed only in the samples from patients with the cardiac form of Chagas disease (group I).

Use of a 24-kilodalton Trypanosoma cruzi recombinant protein to monitor cure of human Chagas' disease

A 24-kDa recombinant protein from Trypanosoma cruzi (rTc24) was evaluated by enzyme-linked immunosorbent assay (ELISA) and Western blot (immunoblot) tests to identify treated chagasic patients considered parasitologically cured on the basis of persistently negative tests of hemocultures and lytic antibodies. Some of these patients were termed dissociated because their sera, although negative by the complement-mediated lysis test, were positive by conventional serology. The negative lysis test indicates the absence of active infection after specific treatment, but this assay requires live and infectious parasites and cannot be used easily in a laboratory routine. Here we tested rTc24 by ELISA and Western blotting as an alternative for the complement-mediated lysis test. For the group of patients with active infection despite the treatment (uncured patients), all the sera tested recognized rTc24 in both tests. For the dissociated patients, approximately 80% of the sera did not react with rTc24 in the ELISA or in Western blots, in agreement with the negative complement-mediated lysis tests. Thus, the 24-kDa T. cruzi recombinant antigen, when used for initial trials to evaluate cure of chagasic patients submitted to specific treatment, will allow the identification of most, but not all, cases.