Immune system pathogenesis is prevented by the neutralization of the systemic trans-sialidase from Trypanosoma cruzi during severe infections

During the acute phase of Trypanosoma cruzi infection, strong haematological and immune system alterations are observed. The parasite expresses trans-sialidase, a virulence factor responsible for the sialylation of its surface glycoconjugates. This enzyme is also shed to the bloodstream where it is associated with immune system alterations triggered during the infection. During experimental and human infections, the host elicits antibodies able to neutralize the enzyme activity that would be responsible for restricting systemic trans-sialidase to the early steps of the infection, when major immune alterations are induced. The actual relevance of these antibodies was tested by passive transference of monoclonal neutralizing antibodies in acute infection models displaying extreme sensitivity to the infection. Mice were inoculated with virulent parasite strains that induce high parasitaemia, early mortality and strong immune tissue abnormalities. The trans-sialidase-neutralizing antibodies were able to preserve B cell areas both in ganglia and spleen as well as the thymus architecture even in these extreme models. Although no differences between control and treated mice regarding animal survival were found, a major role for the humoral response in controlling the damage of the immune system induced by a systemically distributed virulence factor was defined in an infection with a eukaryotic pathogen.

A superfamily of Trypanosoma cruzi surface antigens

Several genes o f Trypanosoma cruzi encode surface antigens that include an amino acid motif that is conserved among bacterial neurominidases. Oscar Campetella, Daniel Sdnchez, Juan Jose Cazzulo and Alberto Carlos Frasch here suggest grouping these gene families in a superfamily.

Multiple overlapping epitopes in the repetitive unit of the shed acute-phase antigen from Trypanosoma cruzi enhance its immunogenic properties

The repetitive shed acute-phase antigen (SAPA) from Trypanosoma cruzi was thoroughly mapped by SPOT peptides and phage display strategies, showing that a single SAPA repeat is composed of multiple overlapping B-cell epitopes. We propose that this intricate antigenic structure constitutes an alternative device to repetitiveness in order to improve its immunogenicity.

Epitope mapping of trans-sialidase from Trypanosoma cruzi reveals the presence of several cross-reactive determinants

Trypanosoma cruzi, the agent of Chagas' disease, expresses trans-sialidase, a unique enzyme activity that enables the parasite to invade host cells by transferring sialyl residues from host glyconjugates to the parasite's surface acceptor molecules. The enzyme is also shed into the surrounding environment, causing apoptosis in cells from the immune system. During infections, an antibody response against the catalytic region of the trans-sialidase that is coincident with the control of the parasitemia and survival of the host is observed. This low-titer humoral response is characterized by its persistence for many years in benznidazole-treated patients. Here we analyzed the antigenic structure of the molecule by phage-displayed peptide combinatorial libraries and SPOT synthesis. Several epitopes were defined and located on the three-dimensional model of the enzyme. Unexpectedly, cross-reaction was found among several epitopes distributed in different locations displaying nonconsensus sequences. This finding was confirmed by the reactivity of three monoclonal antibodies able to recognize non-sequence-related peptides that together constitute the surface surrounding the catalytic site of the enzyme. The presence of cross-reacting epitopes within a single molecule suggests a mechanism developed to avoid a strong humoral response by displaying an undefined target to the immune system.

trans-sialidase inhibition assay, a highly sensitive and specific diagnostic test for Chagas' disease

For the diagnosis of Chagas' disease, the trans-sialidase inhibition assay was able to resolve the results for samples with borderline results, to detect as positive 60% of samples that were negative by conventional serology, and to discriminate idiopathic from chagasic megaviscera or cardiopathy. No cross-reaction with sera from patients with other diseases was observed.

Structural basis of sialyltransferase activity in trypanosomal sialidases

The intracellular parasite Trypanosoma cruzi, the etiological agent of Chagas disease, sheds a developmentally regulated surface trans-sialidase, which is involved in key aspects of parasite-host cell interactions. Although it shares a common active site architecture with bacterial neuraminidases, the T.cruzi enzyme behaves as a highly efficient sialyltransferase. Here we report the crystal structure of the closely related Trypanosoma rangeli sialidase and its complex with inhibitor. The enzyme folds into two distinct domains: a catalytic beta-propeller fold tightly associated with a lectin-like domain. Comparison with the modeled structure of T.cruzi trans-sialidase and mutagenesis experiments allowed the identification of amino acid substitutions within the active site cleft that modulate sialyltransferase activity and suggest the presence of a distinct binding site for the acceptor carbohydrate. The structures of the Trypanosoma enzymes illustrate how a glycosidase scaffold can achieve efficient glycosyltransferase activity and provide a framework for structure-based drug design.

Trans-sialidase from Trypanosoma cruzi induces apoptosis in cells from the immune system in vivo

Trypanosoma cruzi, the etiologic agent of Chagas' disease, expresses trans-sialidase, an enzyme able to direct transfer of sialyl residues among macromolecules. The enzyme is shed and can be detected in blood during the acute phase of the disease. Several alterations of the immune response and apoptosis of cellular components of the immune system are observed early in the infection. The possible involvement of bloodstream trans-sialidase on these events was analyzed here. The enzyme induced apoptosis in cells of the immune system in the spleen, thymus, and peripheral ganglia. Both natural and recombinant trans-sialidases induced apoptosis to a similar extent. No effect was detected when enzymatically inactive recombinant molecules were used. In dose-response assays, apoptosis was observed even when an amount of trans-sialidase was administered that was enzymatically undetectable in blood. These findings strongly suggest a role for sialic acid mobilization in T. cruzi-induced apoptosis of immune system cells.

Enzymically inactive members of the trans-sialidase family from Trypanosoma cruzi display beta-galactose binding activity

trans-sialidase is a unique sialidase in that, instead of hydrolizing sialic acid, it preferentially transfers the monosaccharide to a terminal beta-galactose in glycoproteins and glycolipids. This enzyme, originally identified in Trypanosoma cruzi, belongs to a large family of proteins. Some members of the family lack the enzymatic activity. No function has been yet assigned to them. In this work, the gene copy number and the possible function of inactive members of the trans -sialidase family was studied. It is shown that genes encoding inactive members are not a few, but rather, are present in the same copy number (60-80 per haploid genome) as those encoding active trans -sialidases. Recombinant inactive proteins were purified and assayed for sialic acid and galactose binding activity in agglutination tests. The enzymatically inactive trans -sialidases were found to agglutinate de-sialylated erythrocytes but not untreated red blood cells. Assays made with mouse and rabbit red blood cells suggest that inactive trans -sialidases bind to beta, rather than alpha, terminal galactoses, the same specificity required by active trans -sialidases. A recombinant molecule that was made enzymatically inactive through a mutation in a single amino acid also retained the galactose binding activity. The binding was competed by lactose and was dependent on conservation of the protein native conformation. Therefore, at least some molecules in the trans -sialidase family that have lost their enzymatic function still retain their Gal-binding properties and might have a function as lectins in the parasite-host interaction.

Tandem amino acid repeats from Trypanosoma cruzi shed antigens increase the half-life of proteins in blood

Proteins containing amino acid repeats are widespread among protozoan parasites. It has been suggested that these repetitive structures act as immunomodulators, but other functional aspects may be of primary importance. We have recently suggested that tandem repeats present in Trypanosoma cruzi trans-sialidase stabilize the catalytic activity in blood. Because the parasite releases trans-sialidase, this delayed clearance of the enzyme might have implications in vivo. In the present work, the ability of repetitive units from different T. cruzi molecules in stabilizing trans-sialidase activity in blood was evaluated. It is shown that repeats present on T. cruzi shed proteins (antigens 13 and Shed-Acute-Phase-Antigen [SAPA]) increase trans-sialidase half-life in blood from 7 to almost 35 hours. Conversely, those repeats present in intracellular T. cruzi proteins only increase the enzyme half-life in blood up to 15 hours. Despite these results, comparative analysis of structural and catalytic properties of both groups of chimeric enzymes show no substantial differences. Interestingly, antigens 13 and SAPA also increase the persistence in blood of chimeric glutathione S-transferases, thus suggesting that this effect is inherent to these repeats and independent of the carrier protein. Although the molecular basis of this phenomenon is still uncertain, its biotechnological potential can be envisaged.

Structure of the glycosylphosphatidylinositol-anchor of the trans-sialidase from Trypanosoma cruzi metacyclic trypomastigote forms

Both, culture-derived and metacyclic trypomastigotes of Trypanosoma cruzi shed a glycoprotein, the shed acute phase antigen, that is responsible for the trans-sialidase activity. In the present work the structure of the glycosylphosphatidylinositol membrane anchor of the trans-sialidase isolated from metacyclic forms was determined. Parasites were metabolically labelled with [9, 10(n)3H]-palmitic acid and the glycoprotein was purified by immunoprecipitation with a monoclonal antibody directed against the repetitive aminoacid sequence. Treatment of the glycoprotein with phosphatidylinositol phospholipase C (PI-PLC) from Bacillus thuringiensis rendered a lipid that comigrated in TLC with a standard of ceramide. No alkylglycerol was detected in contrast with the results previously obtained for the trans-sialidase isolated from culture-derived trypomastigotes where both lipids were found. Chemical and chromatographic analysis showed that the lipid moiety is palmitoyldihydrosphingosine with a minor amount of stearoyldihydrosphingosine. The glycan constituent of the glycosylphosphatidylinositol-anchor was analysed by nitrous acid deamination of the aqueous phase of the PI-PLC treatment, followed by reduction with NaBH4 and hydrolysis of the phosphodiester with aqueous hydrofluoric acid. A major oligosaccharide was obtained and enzymatic treatment with exoglycosidases and further chromatography in a high pH anion exchange system showed that the trimannosyl core backbone is substituted by an alpha-galactose. A comparison between the lipid constituent of the glycosylphosphatidylinositol anchor of several proteins and their spontaneous shedding by the action of an endogenous PI-PLC was made.

Use of trans-Sialidase inhibition assay in a population serologically negative for Trypanosoma cruzi but at a high risk of infection

trans-Sialidase inhibition assay (TIA) was employed in a population at high risk of Trypanosoma cruzi infection. From 20 serum samples that were negative by conventional serologic and parasitologic assays, 18 (90%) were reactive in TIA, providing further evidence of the higher sensitivity of TIA and suggesting that the actual prevalence of T. cruzi infection might be underestimated.

The NADP+-linked glutamate dehydrogenase from Trypanosoma cruzi: sequence, genomic organization and expression

NADP-linked glutamate dehydrogenase (NADP+-GluDH, EC 1.4.1.4) has been purified to homogeneity from epimastigotes of Trypanosoma cruzi by an improved procedure, and the amino acid sequences of 11 internal peptides obtained by digestion with trypsin, endopeptidase Lys-C, endopeptidase Arg-C or CNBr have been obtained. Using oligonucleotide primers synthesized according to the amino acid sequence of the N-terminus of the mature enzyme and to the nucleotide sequence of a clone corresponding to the C-terminus, obtained by immunological screening of an expression library, two complete open reading frames (TcGluDH1 and TcGluDH2) were isolated and sequenced. The sequences obtained are most similar to that of the NADP+-GluDH of Escherichia coli (70-72% identity), and less similar (50-56%) to those of lower eukaryotes. Using TcGluDH1 as a probe, evidence for the presence of several genes and developmental regulation of the expression of NADP+-GluDH in different parasite stages was obtained. TcGluDH1 encodes an enzymically active protein, since its expression in E. coli resulted in the production of a GluDH activity with kinetic parameters similar to those of the natural enzyme.

The repetitive domain of Trypanosoma cruzi trans-sialidase enhances the immune response against the catalytic domain

Trypanosoma cruzi trans-sialidase consists of a C-terminal domain composed essentially of immunodominant amino acid repeat units (SAPA-repeats) and an amino region responsible for the enzymatic activity (catalytic domain). To investigate the possible function(s) of SAPA-repeats, recombinant trans-sialidases either containing or lacking the C-terminal region were tested in mice. The presence of SAPA-repeats in the intravenously injected protein has two consequences. First, they enhance the persistence of the trans-sialidase activity in blood. Second, SAPA-repeats promoted the production of antibodies directed to the catalytic domain that inhibit trans-sialidase activity. These results suggest that SAPA-repeats modulate the trans-sialidase activity in blood.

Trypanosoma rangeli sialidase: cloning, expression and similarity to T. cruzi trans-sialidase

Sialidases are hydrolytic enzymes present from virus to higher eukaryotes, catalyzing the removal of sialic acid from glycoconjugates. Some protozoa Trypanosomatidae secrete high levels of sialidase into the medium. We have now purified the secreted sialidase from Trypanosoma rangeli. Its N-terminal sequence reveals 100% identity with the corresponding region of the trans-sialidase from T. cruzi. Trans-sialidase, although homologous to viral and bacterial sialidases, displays a novel sialyltransferase activity and is involved in host cell invasion. Several homologous transsialidase-like genes were cloned from genomic DNA of T. rangeli, and grouped in three subfamilies. Active sialidase-encoding genes were found in one of them. The recombinant sialidase shows similar properties to those of the native enzyme, including undetectable trans-sialidase activity. Nevertheless, it has an overall identity of 68.9% with the catalytic domain of T. cruzi trans-sialidase, increasing to 86.7% admitting conservative substitutions. Only three other eukaryotic sialidases have been previously cloned, none of them showing significant homology to trans-sialidase. The isolation of a highly similar sialidase is relevant to further identify the molecular determinants allowing trans-sialidase activity. As a first approach, chimeric constructs between sialidase and trans-sialidase were generated, one of them rendering a sialidase with three times lower Km than the natural enzyme.

Long-lasting antibodies detected by a trans-sialidase inhibition assay of sera from parasite-free, serologically cured chagasic patients

A test based on the inhibition by antibodies of the trans-sialidase was used to analyze infection by Trypanosoma cruzi, the agent of Chagas' disease. Sera collected during the longitudinal follow-up of benznidazole-treated acutely and congenitally infected patients became negative for T. cruzi as determined by tests presently used to assess cure; however, the sera remained positive for T. cruzi by the trans-sialidase inhibition assay (TIA) up to 14 years after treatment. Therefore, TIA is a highly sensitive marker for previous T. cruzi infection.

Medium scale production and purification to homogeneity of a recombinant trans-sialidase from Trypanosoma cruzi

Trypanosoma cruzi, the agent of Chagas' disease, presents an enzyme that catalyzes the transfer of sialic acid among glycoproteins and glycolipids known as trans-sialidase (TS), displaying some interesting features: 1) It differs from all other eucaryotic sialyltransferases, both kinetically and in substrate specificity and 2) it is involved in the parasite's mechanism of mammalian host cell invasion. We report here the production and purification to homogeneity of an enzymatically active recombinant TS (rTS) lacking the C-terminal amino acid repeats, using iminodiacetic metal affinity chromatography. Initial ratios of non-fusion recombinant versus total protein were very low in several expression systems tested, mainly due to high degradation rate. However, after purifying 1,330 times, we were able to obtain an essentially homogeneous preparation of rTS with a final yield of 29%. After minor changes, a modified protocol for a medium scale production was designed obtaining 0.5 mg of homogeneous rTS per liter of bacterial culture. The purified rTS behaved as a homogeneous protein in silver-stained denaturing gels, isoelectrofocusing and N-terminal sequencing having identical pH and temperature optima as the natural enzyme. Conditions to keep the rTS for long periods without a significant loss of activity were identified.

Effect of primary structure modifications in Trypanosoma cruzi neuraminidase/trans-sialidase activities

Neuraminidases have been implicated in various processes involving the interaction of pathogens and their receptor cells. Trypanosoma cruzi, the agent of Chagas disease, has an unusual neuraminidase, able to transfer bound alpha(2-3)sialic acid to a suitable acceptor rather than to water: the trans-sialidase (TcTS). This enzyme is encoded by a family of several homologous genes, some of them rendering inactive the products. We have shown that enzymatically active proteins have Tyr in position 342, whereas inactive TcTS contain a His342. We have now mutated this Tyr residue to Phe or Thr. Both mutant proteins resulted in enzymatically inactive products. Chimeras expressing parts of Salmonella typhimurium neuraminidase (NANH) and TcTS were constructed. Only the construct containing the complete NANH molecule fused to the last 272 residues of TcTS had neuraminidase activity. However this construct did not present TcTS activity. This finding suggests that other residues present in the homology region are required for TcTS activity.

A single tyrosine differentiates active and inactive Trypanosoma cruzi trans-sialidases

Several genes encode members of the Trypanosoma cruzi (Tc) trans-sialidase (TS) family. These proteins contain an enzymatic domain on the N terminus, the only one required for TS activity, and an antigenic domain (SAPA (shed acute phase antigen) amino acid (aa) repeats) on the C terminus. Only some members of this glycoprotein family are enzymatically active. The complete sequence of two clones encoding the enzymatic domain of active and inactive protein from each of two Tc strains has now been obtained. Comparison of these sequences showed a limited divergence among them: 20 out of the 642 deduced aa in the enzymatic domain were found to differ. From these 20 aa, only one was found to be essential for enzymatic activity. A Tyr342 residue is deduced in both active proteins while a His342 is present in both inactive ones. This naturally occurring Tyr342-->His substitution completely abolished the TS activity. In addition to Tyr342, a second deduced aa, Pro231, was found to be necessary for full enzymatic TS activity; a Pro231-->Ala change rendered the TS protein partially active. Fourteen aa residues, including Tyr342, out of the 16 aa in the active site of a sialidase from Salmonella typhimurium are present at the same or very similar positions in the Tc TS.

Antibodies inhibiting Trypanosoma cruzi trans-sialidase activity in sera from human infections

Trans-sialidase, an enzyme that transfers sialic acid among macromolecules, has been implicated in invasion of host cells by Trypanosoma cruzi, the agent of Chagas' disease. Most antibodies produced in natural and experimental infections are directed to the highly antigenic C-terminal domain (shed acute-phase antigen). These antibodies do not inhibit the trans-sialidase activity, which is present in the N-terminal domain of the molecule. Antibodies able to inhibit trans-sialidase in sera from human infections have been found. TIA (trans-sialidase inhibition assay) was positive in sera from patients with acute and chronic infections. Healthy and congenitally infected infants born to mothers with Chagas' disease were also TIA-positive, but the antibody titers diminished within months after birth or after treatment. Thus, antibodies neutralizing trans-sialidase are detectable in most forms of T. cruzi human infections, and TIA may be useful in the diagnosis of Chagas' disease.

The reactivity of sera from chagasic patients against different fragments of cruzipain, the major cysteine proteinase from Trypanosoma cruzi, suggests the presence of defined antigenic and catalytic domains

Three fragments of cruzipain, expressed separately in bacterial vectors, were used to detect antibodies in sera from patients with chronic Chagas' disease. Most antibodies directed against the enzyme were found to react with the C-terminal extension, thus suggesting the presence of immunodominant B-cell epitopes within this protein domain. Immunoprecipitation with these antibodies did not impair enzyme activity. It is suggested that cruzipain consists of an enzymatic domain and a non-enzymatic, immunodominant domain, which corresponds to the C-terminal extension.

The major cysteine proteinase (cruzipain) from Trypanosoma cruzi is encoded by multiple polymorphic tandemly organized genes located on different chromosomes

We demonstrate that cruzipain, the major cysteine proteinase of Trypanosoma cruzi epimastigotes, is encoded by a large number of tandemly arranged genes. Restriction enzyme analysis of 20 clones containing complete repeat units of the gene, as well as sequencing of 2 of these clones, and comparison with previously published partial sequences, indicated that the sequence is conserved among the repeat units, although polymorphisms clearly exist. The repeat units contain an intergenic region of 528 bp and coding regions for pre- and pro-enzyme, a central domain and a C-terminal extension. The predicted amino acid sequences of these regions indicated a sequence identity of 30, 60, 70 and 36%, respectively, when the T. cruzi sequence was compared with the sequence of a similar cysteine proteinase from Trypanosoma brucei. Studies by pulsed field gel electrophoresis, complemented with restriction analysis, indicated that the clusters are located on 2-4 different chromosomes in several parasite isolates.

The major cysteine proteinase (cruzipain) from Trypanosoma cruzi is antigenic in human infections

Antibodies against the major cysteine proteinase (cruzipain) from Trypanosoma cruzi were detected in human sera obtained from patients with chronic Chagas' disease. Not only the intact 60-kDa enzyme but also its 25-kDa self-proteolysis fragment are antigenic in vivo. Although T. cruzi antigens 13 and 36 also reacted with the apparently monospecific antiproteinase serum, the antigenicity of cruzipain to human patients is genuine, since its reactivity was not modified by the adsorption of human sera with the recombinant proteins 13 and 36.

A major cysteine proteinase is developmentally regulated in Trypanosoma cruzi

Epimastigotes of different stocks of Trypanosoma cruzi contain similar levels of proteinase activity on azocasein; amastigotes and trypomastigotes contain 10-fold lower levels of this proteolytic activity, which seems, therefore, to be developmentally regulated. The proteinase could be detected as a broad band, centered at about 60 kDa, which in some cases resolved into two close bands, in (a) SDS-polyacrylamide gels containing fibrinogen, and (b) Western blots probed with a polyclonal rabbit antiserum prepared against purified cysteine proteinase. No proteinase activity was observed at molecular weights lower than 55 kDa. The results show that the enzyme previously purified is the major cysteine proteinase present in epimastigotes of all stocks of T. cruzi tested.