The major 85-kD surface antigen of the mammalian form of Trypanosoma cruzi is encoded by a large heterogeneous family of simultaneously expressed genes

The Elusive Trypanosoma cruzi Disperse Gene Protein Family (DGF-1)

Chagas disease, caused by Trypanosoma cruzi infections, is included in the group of neglected diseases, and efforts to develop new therapeutic or immunoprevention approaches have not been successful. After the publication of the T. cruzi genome, the number of molecular and biochemical studies on this parasite has increased considerably, many of which are focused on families of variant surface proteins, especially trans-sialidases, mucins, and mucin-associated proteins. The disperse gene protein 1 family (DGF-1) is one of the most abundant families in the T. cruzi genome; however, the large gene size, high copy numbers, and low antibody titers detected in infected humans make it an unattractive study target. However, here we argue that given the ubiquitous presence in all T. cruzi species, and physicochemical characteristics, the DGF-1 gene family may play and important role in host-parasite interactions.

Long-Term Immunity to Trypanosoma cruzi in the Absence of Immunodominant trans-Sialidase-Specific CD8+ T Cells

Trypanosoma cruzi infection drives the expansion of remarkably focused CD8(+) T cell responses targeting epitopes encoded by variant trans-sialidase (TS) genes. Infection of C57BL/6 mice with T. cruzi results in up to 40% of all CD8(+) T cells committed to recognition of the dominant TSKB20 and subdominant TSKB18 TS epitopes. However, despite this enormous response, these mice fail to clear T. cruzi infection and subsequently develop chronic disease. One possible reason for the failure to cure T. cruzi infection is that immunodomination by these TS-specific T cells may interfere with alternative CD8(+) T cell responses more capable of complete parasite elimination. To address this possibility, we created transgenic mice that are centrally tolerant to these immunodominant epitopes. Mice expressing TSKB20, TSKB18, or both epitopes controlled T. cruzi infection and developed effector CD8(+) T cells that maintained an activated phenotype. Memory CD8(+) T cells from drug-cured TSKB-transgenic mice rapidly responded to secondary T. cruzi infection. In the absence of the response to TSKB20 and TSKB18, immunodominance did not shift to other known subdominant epitopes despite the capacity of these mice to expand epitope-specific T cells specific for the model antigen ovalbumin expressed by engineered parasites. Thus, CD8(+) T cell responses tightly and robustly focused on a few epitopes within variant TS antigens appear to neither contribute to, nor detract from, the ability to control T. cruzi infection. These data also indicate that the relative position of an epitope within a CD8(+) immunodominance hierarchy does not predict its importance in pathogen control.

Mechanisms of host cell invasion by Trypanosoma cruzi

One of the more accepted concepts in our understanding of the biology of early Trypanosoma cruzi-host cell interactions is that the mammalian-infective trypomastigote forms of the parasite must transit the host cell lysosomal compartment in order to establish a productive intracellular infection. The acidic environment of the lysosome provides the appropriate conditions for parasite-mediated disruption of the parasitophorous vacuole and release of T. cruzi into the host cell cytosol, where replication of intracellular amastigotes occurs. Recent findings indicate a level of redundancy in the lysosome-targeting process where T. cruzi trypomastigotes exploit different cellular pathways to access host cell lysosomes in non-professional phagocytic cells. In addition, the reversible nature of the host cell penetration process was recently demonstrated when conditions for fusion of the nascent parasite vacuole with the host endosomal-lysosomal system were not met. Thus, the concept of parasite retention as a critical component of the T. cruzi invasion process was introduced. Although it is clear that host cell recognition, attachment and signalling are required to initiate invasion, integration of this knowledge with our understanding of the different routes of parasite entry is largely lacking. In this chapter, we focus on current knowledge of the cellular pathways exploited by T. cruzi trypomastigotes to invade non-professional phagocytic cells and to gain access to the host cell lysosome compartment.

cDNA cloning and partial characterization of amastigote specific surface protein from Trypanosoma cruzi

Trypanosoma cruzi amastigote surface proteins are the target of both humoral and cell-mediated immune responses; however, few such molecules have been thoroughly studied. In order to study a T. cruzi amastigote-specific protein (SSP4), we used antibodies against the deglycosylated form of this molecule to clone cDNA. The selected cDNA clone (2070 bp) encodes for a 64 kDa protein product whose sequence analysis revealed no N-glycosylation signal. The DNA sequence showed high homology with a member of a previously reported dispersed repetitive gene family of T. cruzi. Antibodies against the recombinant protein reacted strongly with a 66 kDa protein and weakly with an 84 kDa protein in amastigote extracts. Immunoelectron microscopy studies showed that intracellular amastigotes express the native protein on their surfaces and flagellar pockets. The antibody label was also associated with an amorphous material present in the parasitic cavity and in direct contact with the parasite surface, which suggest that amastigotes are releasing this material. On cell-free amastigotes, the antibody showed strong decoration of the cell surface and labeling of intracellular vesicles. Immunofluorescence analysis showed that the superficial protein is expressed shortly after trypomastigotes begin to transform into amastigotes. Anti-recombinant protein antibodies recognized proteins of 100 kDa and 50-60 kDa in protein extracts of rat heart and skeletal muscle, respectively.

Lateral flow immunoassay for diagnosis of Trypanosoma cruzi infection with high correlation to the radioimmunoprecipitation assay

The incidence of blood donors seropositive for Trypanosoma cruzi in North America has increased with population migration and more rigorous surveillance. The United States, considered nonendemic for T. cruzi, could therefore be at risk to exposure to parasite transmission through blood or organ donations. Current tests show variable reactivity, especially with Central American sera. Here we describe the development of a lateral flow immunoassay for the rapid detection of T. cruzi infection that has a strong correlation to the radioimmunoprecipitation assay (RIPA) "gold standard" in the United States. Such a test could have utility in small blood banks for prescreening donors, as well as in cardiac transplantation evaluation. T. cruzi consensus and/or RIPA-positive sera from Central and South America were evaluated in enzyme immunoassays (EIAs). These included commercial panels from Boston Biomedica, Inc. (BBI) (n = 14), and HemaBio (n = 21). Other sources included RIPA-positive sera from the American Red Cross (ARC) (n = 42), as well as from Chile. Sera were tested with the multiepitope recombinant TcF. All but one of the BBI samples were positive and 7 of 21 HemaBio samples and 6 of 42 ARC samples were low positive or negative. This observation indicated the need for additional antigens. To complement TcF reactivity, we tested the sera with peptides 30, 36, SAPA, and 1.1, 1.2, and 1.3 His fragments of 85-kDa trans-sialidase. We identified a promising combination of the tested antigens and constructed a single recombinant protein, ITC6, that enhanced the relative sensitivity in U.S. blood donor sera compared to that of TcF. The data on its evaluation using RIPA-confirmed positive sera in EIA and lateral flow immunoassay studies are presented, along with an additional recombinant protein, ITC8.2, with two additional sequences for peptide 1 and Kmp-11. The latter, when evaluated in a dipstick assay with consensus positive sera, had a sensitivity of 99.2% and a specificity of 99.1%.

An exploration of the genetic robustness landscape of surface protein families in the human protozoan parasite Trypanosoma cruzi

The ability of genes to be robust to mutations at the codon level has been suggested as a key factor for understanding adaptation features. It has been proposed that genes relevant to host-parasite interactions will tend to exhibit high volatility or "antirobust" patterns, which may be related to the capacity of the parasite to evade the host immune system. We compared two superfamilies of surface proteins, trans-sialidase (TS)-like proteins and putative surface protein dispersed gene family-1 (DGF-1), in the parasite Trypanosoma cruzi in terms of a measure of gene volatility. We proposed alternative codon robustness indicators based on cross entropy and impurity of amino acids encoded by point-mutations, which were compared to a volatility estimator previously published. This allowed us to present a more detailed description of the differences between families. A significant difference was observed in terms of these scores, with the TS-MVar1 and the DGF-1 families showing the highest and lowest gene volatility values respectively. The cross entropy and impurity estimators suggest that the MVar1 levels of volatility are linearly correlated with their capacity to generate diverse sets of amino acids as a consequence of potential mutations. This study indicates the feasibility of applying different measures of genetic robustness to detect variations between potential drug targets at the protein level.

In silico, biologically-inspired modelling of genomic variation generation in surface proteins of Trypanosoma cruzi

BACKGROUND

Protozoan parasites improve the likelihood of invading or adapting to the host through their capacity to present a large repertoire of surface molecules. The understanding of the mechanisms underlying the generation of antigenic diversity is crucial to aid in the development of therapies and the study of evolution. Despite advances driven by molecular biology and genomics, there is a need to gain a deeper understanding of key properties that may facilitate variation generation, models for explaining the role of genomic re-arrangements and the characterisation of surface protein families on the basis of their capacity to generate variation. Computer models may be implemented to explore, visualise and estimate the variation generation capacity of gene families in a dynamic fashion. In this paper we report the dynamic simulation of genomic variation using real T. cruzi coding sequences as inputs to a computational simulation system. The effects of random, multiple-point mutations and gene conversions on genomic variation generation were quantitatively estimated and visualised. Simulations were also implemented to investigate the potential role of pseudogenes as a source of antigenic variation in T. cruzi.

RESULTS

Computational models of variation generation were applied to real coding sequences from surface proteins in T. cruzi: trans-sialidase-like proteins and putative surface protein dispersed gene family-1. In the simulations the sequences self-replicated, mutated and re-arranged during thousands of generations. Simulations were implemented for different mutation rates to estimate the relative robustness of the protein families in the face of DNA multiple-point mutations and sequence re-arrangements. The gene super-families and families showed distinguishing evolutionary responses, which may be used to characterise them on the basis of their capacity to generate variability. The simulations showed that sequences from T. cruzi nuclear genes tend to be relatively more robust against random, multiple-point mutations than those obtained from surface protein genes. Simulations also showed that a gene conversion model may act as an effective variation generation mechanism. Differential variation responses can be used to characterise the sequence groups under study. For example, unlike other families, sequences from the DGF1 family have the capacity to maximise variation at the amino acid level under relatively low mutation rates and through gene conversion. However, in relation to the other protein families, they exhibit more robust behaviour in response to more severe modifications through intra-family genomic sequence exchange. Independent simulations indicate that DGF1 pseudogenes might play a role in the generation of greater genomic variation in the DFG1 gene family through gene conversion under different experimental conditions.

CONCLUSION

Digital, dynamic simulations may be implemented to characterise gene families on the basis of their capacity to generate variation in the face of genomic perturbations. Such simulations may be useful to explore antigenic variation mechanisms and hypotheses about robustness at the genomic level. This investigation illustrated how sequences derived from surface protein genes and computer simulations can be used to investigate variation generation mechanisms. Such in silico experiments of self-replicating sequences undergoing random mutations and genomic re-arrangements can offer insights into the diversity generation potential of the genes under study. Biologically-inspired simulations may support the study of genomic variation mechanisms in pathogens whose genomes have been recently sequenced.

CD8+ T-Cell responses to Trypanosoma cruzi are highly focused on strain-variant trans-sialidase epitopes

CD8⁺ T cells are crucial for control of a number of medically important protozoan parasites, including Trypanosoma cruzi, the agent of human Chagas disease. Yet, in contrast to the wealth of information from viral and bacterial infections, little is known about the antigen specificity or the general development of effector and memory T-cell responses in hosts infected with protozoans. In this study we report on a wide-scale screen for the dominant parasite peptides recognized by CD8⁺ T cells in T. cruzi–infected mice and humans. This analysis demonstrates that in both hosts the CD8⁺ T-cell response is highly focused on epitopes encoded by members of the large trans-sialidase family of genes. Responses to a restricted set of immunodominant peptides were especially pronounced in T. cruzi–infected mice, with more than 30% of the CD8⁺ T-cell response at the peak of infection specific for two major groups of trans-sialidase peptides. Experimental models also demonstrated that the dominance patterns vary depending on the infective strain of T. cruzi, suggesting that immune evasion may be occurring at a population rather than single-parasite level.

The authors of this paper conducted a broad screen to identify the major proteins in Trypanosoma cruzi, the causative agent of Chagas disease, that allow for detection and control of this intracellular pathogen by CD8⁺ T cells. This study is the first to show that a complex pathogen such as T. cruzi elicits a T-cell response focused on a few peptides, despite having a genome of >12,000 genes capable of encoding hundreds of thousands of potential target epitopes. The immunodominant CD8⁺ T-cell targets in both murine and human T. cruzi infection are almost exclusively peptides within multiple trans-sialidase proteins that are encoded by the large and diverse trans-sialidase gene family. trans-sialidase genes show great potential for variation, and the frequency of individual trans-sialidase epitopes appears to vary significantly in different parasite strains, giving rise to distinct patterns of T-cell responses to different T. cruzi isolates. The authors hypothesize that the massive expansion of this gene family under immunological pressure and the resulting variable expression of specific T-cell epitopes provides a mechanism of immune escape for T. cruzi.

Trypanosoma cruzi-infected individuals demonstrate varied antibody responses to a panel of trans-sialidase proteins encoded by SA85-1 genes

Chronic infection with Trypanosoma cruzi causes significant morbidity and mortality. The parasite expresses on its surface and sheds into the extracellular milieu a large superfamily of trans-sialidase proteins. Previous studies have demonstrated that during T. cruzi infection, the trans-sialidase superfamily stimulates an antibody response, but how individuals respond to different proteins of the trans-sialidase superfamily remain poorly defined. In this report, we present an analysis of the antibody response of chronically infected individuals and inbred strains of mice to a panel of 11 different trans-sialidase proteins encoded by surface antigen 85 kD (SA85-1) genes. These data indicate that: (1) 90% of the individuals tested generated antibodies to one or more trans-sialidase proteins; (2) the individuals develop different patterns of antibody responsiveness to the panel of trans-sialidase proteins; (3) three inbred strains of mice develop trans-sialidase antibody responses, but each strain develops a different pattern of antibody response to the panel of trans-sialidase proteins; (4) the differences in the pattern of antibody response by the mouse strains are independent of MHC differences; and (5) trans-sialidase proteins that do not stimulate an antibody response during T. cruzi infection can stimulate a response following immunization. Together these data indicate that during T. cruzi infection individuals develop a diverse trans-sialidase antibody response that appears to be affected by genetic and environmental factors.

Critical proinflammatory and anti-inflammatory functions of different subsets of CD1d-restricted natural killer T cells during Trypanosoma cruzi infection

Trypanosoma cruzi infects 15 to 20 million people in Latin America and causes Chagas disease, a chronic inflammatory disease with fatal cardiac and gastrointestinal sequelae. How the immune response causes Chagas disease is not clear, but during the persistent infection both proinflammatory and anti-inflammatory responses are critical. Natural killer T (NKT) cells have been shown to regulate immune responses during infections and autoimmune diseases. We report here that during acute T. cruzi infection NKT-cell subsets provide distinct functions. CD1d(-/-) mice, which lack both invariant NKT (iNKT) cells and variant NKT (vNKT) cells, develop a mild phenotype displaying an increase in spleen and liver mononuclear cells, anti-T. cruzi antibody response, and muscle inflammation. In contrast, Jalpha18(-/-) mice, which lack iNKT cells but have vNKT cells, develop a robust phenotype involving prominent spleen, liver, and skeletal muscle inflammatory infiltrates comprised of NK, dendritic, B and T cells. The inflammatory cells display activation markers; produce more gamma interferon, tumor necrosis factor alpha, and nitric oxide; and show a diminished antibody response. Strikingly, most Jalpha18(-/-) mice die. Thus, in response to the same infection, vNKT cells appear to augment a robust proinflammatory response, whereas the iNKT cells dampen this response, possibly by regulating vNKT cells.

Paraflagellar rod protein-specific CD8+ cytotoxic T lymphocytes target Trypanosoma cruzi-infected host cells

Our previous studies show that in mice immunized with the paraflagellar rod (PFR) proteins of Trypanosoma cruzi protective immunity against this protozoan parasite requires MHC class I-restricted T cell function. To determine whether PFR-specific CD8+ T cell subsets are generated during T. cruzi infection, potential CTL targets in the PFR proteins were identified by scanning the amino acid sequences of the four PFR proteins for regions of 8-10 amino acids that conform to predicted MHC class I H-2b binding motifs. A subset of the peptide sequences identified were synthesized and tested as target antigen in 51Cr-release assays with effector cells from chronically infected T. cruzi mice. Short-term cytotoxic T lymphocyte (CTL) lines specific for two of the peptides, PFR-1(164-171) and PFR-3(123-130), showed high levels of lytic activity against peptide-pulsed target cells, secreted interferon (IFN)-gamma in response to parasite-infected target cells, and were found to be CD8+, CD4-, CD3+, TCRalphabeta+ cells of the Tc1 subset. Challenge of PFR immunized CD8-/- and perforin-deficient (PKO) mice confirmed that while CD8+ cells are required for survival of T. cruzi challenge infection, perforin activity is not required. Furthermore, while lytic activity of PFR-specific CD8+ T cell lines derived from PKO mice was severely impaired, the IFN-gamma levels secreted by CTLs from PKO mice were equivalent to that of normal mice, suggesting that the critical role played by CD8+ T cells in immunity to the parasite may be secretion of type 1 cytokines rather than lysis of parasite infected host cells.

Characterization of transsulfuration and cysteine biosynthetic pathways in the protozoan hemoflagellate, Trypanosoma cruzi. Isolation and molecular characterization of cystathionine beta-synthase and serine acetyltransferase from Trypanosoma

Sulfur-containing amino acids play an important role in a variety of cellular functions such as protein synthesis, methylation, and polyamine and glutathione synthesis. We cloned and characterized cDNA encoding cystathionine beta-synthase (CBS), which is a key enzyme of transsulfuration pathway, from a hemoflagellate protozoan parasite Trypanosoma cruzi. T. cruzi CBS, unlike mammalian CBS, lacks the regulatory carboxyl terminus, does not contain heme, and is not activated by S-adenosylmethionine. T. cruzi CBS mRNA is expressed as at least six independent isotypes with sequence microheterogeneity from tandemly linked multicopy genes. The enzyme forms a homotetramer and, in addition to CBS activity, the enzyme has serine sulfhydrylase and cysteine synthase (CS) activities in vitro. Expression of the T. cruzi CBS in Saccharomyces cerevisiae and Escherichia coli demonstrates that the CBS and CS activities are functional in vivo. Enzymatic studies on T. cruzi extracts indicate that there is an additional CS enzyme and stage-specific control of CBS and CS expression. We also cloned and characterized cDNA encoding serine acetyltransferase (SAT), a key enzyme in the sulfate assimilatory cysteine biosynthetic pathway. Dissimilar to bacterial and plant SAT, a recombinant T. cruzi SAT showed allosteric inhibition by l-cysteine, l-cystine, and, to a lesser extent, glutathione. Together, these studies demonstrate the T. cruzi is a unique protist in possessing both transsulfuration and sulfur assimilatory pathways.

Genomic clustering of the Trypanosoma cruzi nonlong terminal L1Tc retrotransposon with defined interspersed repeated DNA elements

We have analyzed the genomic distribution and organization of the long interspersed nucleotide element (LINE) L1Tc, a nonlong terminal repeat (LTR) retrotransposon of Trypanosoma cruzi. The results indicate that the L1Tc element is dispersed along the parasite genome and that in some regions it is organized in tandem repeats. The data allowed us to define the existence of short direct-repeated sequences flanking the genomic L1Tc elements. Relevant is the finding that the LINE L1Tc is located in genomic regions rich in short interspersed nucleotide elements (SINE)-like sequences. In particular, the L1Tc element is found associated to E13-related sequences, redefined in this work and renamed RS13Tc, and to a newly described RS1Tc sequence. The RS1Tc sequence is present, per haploid genome, in about 3,200 copies. Northern blot analysis showed that the RS1Tc is being transcribed into RNAs of different sizes. The analysis of the chromosomal distribution of these elements in various strains of T. cruzi suggested that this type of clustering might be a common feature of the genome of these parasites.

The SA85-1.1 protein of the Trypanosoma cruzi trans-sialidase superfamily is a dominant T-cell antigen

Trypanosoma cruzi currently infects 18 million people, and 30% of those infected develop a chronic inflammatory process that causes significant morbidity or mortality. The major histocompatibility complex class II (MHC-II)-restricted T-cell response is critical to the control of the infection and to the ensuing inflammatory pathology. The specific epitopes or major antigens of this response have not been identified. The parasite simultaneously expresses variant members of the trans-sialidase superfamily. To begin to analyze the MHC-II response to these variant proteins, the response to a single surface protein, SA85-1.1, was initiated. These studies have demonstrated that a biased gamma interferon (IFN-gamma) response to the SA85-1.1 protein develops during T. cruzi infection. In addition, adoptive transfer of a CD4 clone that recognizes an SA85-1.1 epitope, named epitope 1, and immunization with a peptide encoding epitope 1 were protective and suggested that epitope 1 may be immunodominant. In this report IFN-gamma intracellular staining demonstrated that splenocytes from acutely and chronically infected mice, incubated with SA85-1.1 protein or peptides that encode epitope 1, result in IFN-gamma synthesis by 4 to 6% of the splenic CD4 cells. These data indicate that during T. cruzi infection epitope 1 is a major epitope and that 4 to 6% of the CD4 cells are stimulated by a single trans-sialidase superfamily epitope and suggest that a combination of trans-sialidase superfamily proteins combines to stimulate a majority of CD4 cells. These data suggest that during T. cruzi infection the CD4 response to the trans-sialidase superfamily is critical to the protective response and to the ensuing chronic inflammatory pathology.

Trypanosoma cruzi: the effect of nitric oxide synthesis inhibition on the CD4 T cell response to the trans-sialidase superfamily

During Trypanosoma cruzi infection the trans-sialidase superfamily stimulates the development of a large population of CD4 T lymphocytes that produces IFNgamma. These CD4 T cells fail to proliferate when stimulated in vitro. Why they fail to proliferate remains unclear. Nitric oxide is a critical component of the host immune response against T. cruzi, and to determine if NO inhibits trans-sialidase superfamily-specific proliferative responses, mice were fed either N(G)-nitro-L-arginine methylester (L-NAME), an inhibitor of inducible nitric oxide synthase (iNOS), or N(G)-nitro-D-arginine methyl ester (D-NAME), an inactive analog of L-NAME. The L-NAME-fed mice had increased parasitemia and mortality compared to the D-NAME-fed mice. Following stimulation with a T. cruzi trans-sialidase superfamily protein, splenocytes from both groups of mice failed to proliferate but continued to make similar amounts of IFNgamma, suggesting that the development of the trans-sialidase superfamily-specific CD4 response was not affected by iNOS inhibition. In addition, IL-2 receptor (IL-2R) expression was increased on T cells isolated from L-NAME-fed mice. These data suggest that during T. cruzi infection NO causes downregulation of IL-2R expression, but does not cause inhibition of trans-sialidase superfamily-specific CD4 T cell proliferation. Rather, the trans-sialidase superfamily proliferation may be inhibited by epitope variation.

Characterization of the cell adhesion site of Trypanosoma cruzi metacyclic stage surface glycoprotein gp82

The surface glycoprotein gp82, expressed in the insect-stage metacyclic trypomastigotes of Trypanosoma cruzi, has been implicated in mammalian cell invasion. Here we have characterized the cell adhesion site of gp82 by using recombinant proteins and synthetic peptides based on gp82. The recombinant protein Del-4/8, lacking 65 amino acids of gp82 central domain (at positions 257 to 321), was virtually devoid of cell-binding activity and lacked the ability to inhibit parasite invasion, in contrast to J18, the construct containing the full-length gp82 sequence (amino acids 1 to 516). Constructs with shorter deletions, i.e., Del-4 (deleted from 257 to 271) and Del-8 (deleted from 293 to 321), bound to target cells to a significantly lesser degree than did J18. The sites deleted in recombinant proteins Del-4 and Del-8 contained acidic amino acids critical for cell adhesion. Thus, the cell-binding capacity of protein Del-E/D, lacking the glutamic acid (259/260) and aspartic acid (303/304) pairs, was negligible, as was its capacity to inhibit parasite internalization. Of a set of synthetic peptides spanning the gp82 central domain, a 22-mer hybrid peptide, p4/8, formed by two noncontiguous sequences (at positions 257 to 273 and 302 to 306) and containing the four acidic residues, competed with the binding of J18 protein to target cells and significantly inhibited ( approximately 60%) the penetration of parasites. This peptide, generated by the juxtaposition of sequences that are separated by a hydrophobic stretch in the linear molecule, appears to be mimicking a conformation-dependent cell-binding site of gp82. Experiments of antibody competition with a set of 20-mer overlapping peptides mapped the epitope for 3F6, a monoclonal antibody directed to gp82 that inhibits parasite invasion, to the sequence represented by peptide p3 (244 to 263), which has a partial overlap with the cell adhesion site.

Physical mapping of a 670-kb region of chromosomes XVI and XVII from the human protozoan parasite Trypanosoma cruzi encompassing the genes for two immunodominant antigens

As part of the Trypanosoma cruzi Genome Initiative, we have mapped a large portion of the chromosomal bands XVI (2.3 Mb) and XVII (2.6 Mb) containing the highly repetitive and immunodominant antigenic gene families h49 and jl8. Restriction mapping of the isolated chromosomal bands and hybridization with chromosome specific gene probes showed that genes h49 and jl8 are located in a pair of size-polymorphic homologous chromosomes. To construct the integrated map of the chromosomes harboring the h49 and jl8 loci, we used YAC, cosmid, and lambda phage overlapping clones, and long range restriction analysis using a variety of probes (i.e., known gene sequences, ESTs, polymorphic repetitive sequences, anonymous sequences, STSs generated from the YAC ends). The total length covered by the YAC contig was approximately 670 kb, and its map agreed and was complementary to the one obtained by long-range restriction fragment analysis. Average genetic marker spacing in a 105 kb region around h49 and jl8 genes was estimated to be 6.2 kb/marker. We have detected some polymorphism in the H49/JL8 antigens-encoding chromosomes, affecting also the coding regions. The physical map of this region, together with the isolation of specific chromosome markers, will contribute in the global effort to sequence the nuclear genome of this parasite.

Expression of Trypanosoma cruzi surface antigen FL-160 is controlled by elements in the 3' untranslated, the 3' intergenic, and the coding regions

The FL-160 surface antigen gene family of T. cruzi consists of hundreds of members of 160 kDa glycoproteins expressed in trypomastigotes, but not in epimastigotes. Steady-state levels of FL-160 mRNA were 80 to 100-fold higher in trypomastigotes than in epimastigotes, yet transcription rates were equivalent between the lifecycle stages. Luciferase reporter constructs demonstrated that the 3' untranslated region (UTR) and intergenic region (IR) following the coding sequence of FL-160 was sufficient to generate 8-fold higher luciferase expression in trypomastigotes compared with epimastigotes. Transfection of 3' UTR/IR deletion constructs revealed cis-acting elements which conferred a trypomastigote-specific expression pattern similar to that of FL-160. Parasites treated with translation and transcription inhibitors, cyclohexamide and Actinomycin D, respectively, displayed a stage-specific pattern of FL-160 mRNA degradation. Epimastigotes, but not trypomastigotes, treated with the inhibitors accumulated a 1.4 Kb FL-160 cleavage product. The cleavage site mapped to a 31 base poly-purine tract in the FL-160 coding region. The first 526 aa of FL-160, containing the 31 base poly-purine tract and several smaller tracts, were fused to green fluorescent protein (GFP) and expressed from the T. cruzi tubulin locus. Stable transformants expressed 4-fold more FL-160:GFP fusion mRNA and 12-fold more fusion protein in the trypomastigote stage than in the epimastigote stage suggesting post-transcriptional and translational control elements. These data reveal at least two distinct control mechanisms for trypomastigote-specific expression of FL-160 surface glycoproteins, one involving the 3' UTR/IR and one involving the coding region of FL-160.

Organization of telomeric and sub-telomeric regions of chromosomes from the protozoan parasite Trypanosoma cruzi

We present here a characterization of the telomeric and subtelomeric regions of Trypanosoma cruzi chromosomes, using three types of recombinants: cosmids from a genomic library, clones obtained by a vector-adaptor protocol, and a recombinant fragment cloned by a Bal31 trimming protocol. The last nine nucleotides of the T. cruzi overhang are 5'-GGGTTAGGG-3', and there are from 9 to 50 copies of the hexameric repeat 5'-TTAGGG-3', followed by a 189-bp junction sequence common to all recombinants. The subtelomeric region is made of sequences associated with the gp85/sialidase gene family, and/or sequences derived from SIRE, a retrotransposon-like sequence, and also the retrotransposon L1Tc. We discuss the possible implications of this genome organization.

The Surface Protein Superfamily of Trypanosoma cruzi Stimulates a Polarized Th1 Response That Becomes Anergic

Trypanosoma cruzi is an obligate intracellular parasite that chronically infects mammals. Extracellular mammalian stage trypomastigotes simultaneously express and release multiple members of the parasite’s surface protein superfamily; these extracellular proteins should stimulate MHC class II-restricted CD4 T cells. The surface protein superfamily, however, encodes variant epitopes that may inhibit this CD4 response. In this report the surface protein-specific CD4 response was investigated. CD4 cells isolated from acutely and chronically infected mice did not proliferate when stimulated with surface proteins. Adoptive transfer of surface protein-specific CD4 clones or immunization with a peptide encoding a surface protein T cell epitope protected mice during T. cruzi infection. These data strongly suggested that surface proteins were expressed and presented to CD4 cells during infection. Limiting dilution analysis identified an expanded population of surface protein-specific CD4 cells during the acute and chronic infection. These surface protein-specific CD4 cells did not produce IL-2 or IL-4, but did produce IFN-γ. Enzyme-linked immunospot analyses confirmed that many of the surface protein-specific CD4 cells produce IFN-γ. Together these results suggest that during T. cruzi infection a potentially protective CD4 response becomes anergic. It is possible that this anergy is induced by variant T cell epitopes encoded by the surface protein superfamily.

Trypanosoma cruzi: monoclonal antibodies to the surface glycoprotein superfamily differentiate subsets of the 85-kDa surface glycoproteins and confirm simultaneous expression of variant 85-kDa surface glycoproteins

Most surface glycoproteins expressed by mammalian-stage forms of Trypanosoma cruzi are homologous to the parasite's trans-sialidase and therefore are members of the parasite's trans-sialidase superfamily. Few members of this superfamily have trans-sialidase activity. The SA85-1 family is a subfamily of the trans-sialidase superfamily whose members lack trans-sialidase activity. The function of these non-trans-sialidase members remains unknown. In this report a series of monoclonal and polyclonal antibodies to the SA85-1 glycoproteins is presented. The mAbs define distinct subgroups of SA85-1 glycoproteins, and these distinct subgroups are simultaneously expressed by individual trypomastigotes, supporting previous studies indicating that multiple SA85-1 glycoproteins and trans-sialidase superfamily glycoproteins are simultaneously expressed by each trypomastigote. In addition, the antibodies define two major subsets of the SA85-1 family (subset 1 and subset 2) based on differences in migration in SDS-PAGE; the subsets do not appear to be created by differences in glycosylation. Subset 1 migrates slower and is spontaneously released or shed preferentially from the parasite surface compared to subset 2. In addition, subset 1 is attached to the trypomastigote surface by a GPI linkage. Since these glycoprotein subsets are differentially expressed, they may have different functions.

Cloning of a surface membrane glycoprotein specific for the infective form of Trypanosoma cruzi having adhesive properties to laminin

Trypomastigotes of Trypanosoma cruzi express a set of surface glycoproteins known, collectively, as Tc-85. A monoclonal antibody to these proteins, named H1A10, inhibits (50-90%) in vitro parasite interiorization into host cells, thus implicating these glycoproteins in the infection process. Two DNA inserts, a genomic DNA fragment and a full-length cDNA encoding the H1A10 epitope, have now been cloned and characterized. Results show that both have high sequence identity with all reported members of the gp85/trans-sialidase gene family, although the H1A10 epitope exists only in the Tc-85 subset of the family. The epitope has been mapped by competition of antibody binding to a Tc-85 recombinant protein with peptides having sequences predicted by the Tc-85 DNA sequence, which contains also putative N-glycosylation sites and COOH-terminal glycosylphosphatidylinositol anchor insertion sites, as expected, since an N-glycan chain and a glycosylphosphatidylinositol anchor have been characterized previously in the Tc-85 subset. The protein encoded by the full-length cDNA insert binds to cells and in vitro to laminin, but not to gelatin or fibronectin, in a saturable manner. For the first time it was possible to assign a defined ligand to a sequenced glycoprotein belonging to the gp85 family. This fact, together with the reported binding of family members to cell surfaces, reinforces the hypothesis that this family encodes glycoproteins with similar sequences but differing enough as to bind to different ligands and thus forming a family of adhesion glycoproteins enabling the parasite to overcome the barriers interposed by cell membranes, extracellular matrices, and basal laminae.

Virulence in Trypanosoma cruzi infection correlates with the expression of a distinct family of sialidase superfamily genes

The overall success of Trypanosoma cruzi depends on its ability to invade the host and establish a long-term infection. Little is known of the genetic factors responsible for observed differences in virulence from strain to strain in T. cruzi. A virulent T. cruzi line was derived from an attenuated parental line by two passages through mice. To identify virulence genes a subtraction library was constructed and screened for cDNA expressed exclusively in the virulent line. One cDNA hybridized to 3.5 and 4.5 Kb RNA present in virulent trypomastigotes but absent in attenuated trypomastigotes. Sequence analysis showed the cDNA to encode an 85 kDa protein with homology to members of the sialidase/trans-sialidase superfamily and has been designated vp85.1. The highest amino acid sequence similarity was to a previously described T. cruzi sialidase-homologue pseudogene [Takle, G.B., O'Conner, J., Young, A.J. and Cross, G.A.M. (1992) Mol. Biochem. Parasitol. 56, 117-128]. The vp85.1 amino acid sequence has higher homology to members of the 160 kDa flagellar-associated antigen family, FL-160, than to other 85 kDa expressed sialidase superfamily members. Southern blot analysis of virulent and attenuated lines demonstrated a complex hybridization pattern consistent with a multiple gene copy family that was identical in both lines. Antibody directed against recombinant vp85.1 peptide recognized proteins between 95 and 115 kDa in total virulent parasite lysates which were absent in attenuated lysates. Peptide N-glycosidase F treatment reduced the high molecular weight bands to 85 kDa, indicating vp85 is an N-linked glycoprotein. Immunofluorescence with anti-vp85.1 demonstrated surface localization of vp85.1 on virulent, but not attenuated, trypomastigotes. We postulate this new subfamily of trans-sialidases may play a role in virulence.

Trypanosoma rangeli sialidase: kinetics of release and antigenic characterization

The epimastigote stage of Trypanosoma rangeli release a sialidase with a high sialic acid hydrolysis capacity. We demonstrate that sialidase secretion is an active process that is reduced at low temperatures and in the presence of sodium azide. The enzyme is continuously released until certain maximally active concentrations are attained in the BHI culture medium when the parasite density reaches 2-3 x 10(6) cells. When introduced into culture medium already containing such enzyme levels, freshly harvested parasites do not secrete additional sialidase. These findings suggest a self-regulating mechanism and a biological role for the secreted T. rangeli sialidase. The secreted enzyme was purified to homogeneity by fractionation with ammonium sulphate and affinity chromatography. Antibodies raised against the purified molecule recognized antigens of similar molecular weights (73 kDa) in western immunoblotting analyses of T. rangeli and T. cruzi whole cell lysates. No antigenic recognition was recorded against T. cruzi active sialidase/trans-sialidase polypeptides or Clostridium perfringens and Vibrio cholerae commercial sialidases. These observations may indicate the expression of different antigenic domains in T. rangeli, T. cruzi and bacterial sialidases.

Amastigote Surface Proteins of Trypanosoma cruzi Are Targets for CD8+ CTL

Amastigotes of Trypanosoma cruzi express surface proteins that, when released into the host cell cytoplasm, are processed and presented on the surface of infected cells in the context of MHC class I molecules to be recognized by CD8+ CTL. To further understand the role of CTL in T. cruzi infection, we used the available MHC class I peptide binding motifs to identify potential CTL target epitopes in two recently described T. cruzi amastigote surface proteins, ASP-1 and ASP-2. The predicted amino acid sequences of ASP-1 and ASP-2 were screened for H-2b allele-specific class I peptide motifs, and four peptides (PA11, PA12, PA13, and PA14) and six peptides (PA5, PA6, PA7, PA8, PA9, and PA10) were synthesized from ASP-1 and ASP-2, respectively. The majority of the peptides bound to some degree to H-2b class I MHC molecules, and six of 10 of the peptides stimulated spleen cells from T. cruzi-infected mice to lyse target cells sensitized with the homologous peptides. Short term T cell lines specific for three of these peptides also lysed T. cruzi-infected target cells. These results demonstrate that ASP-1 and ASP-2 are targets of in vivo generated CTLs and that this CTL response induced by T. cruzi infection is parasite and peptide specific, MHC restricted, and CD8 dependent.

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39: CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.

Molecular cloning of the gene encoding the 83 kDa amastigote surface protein and its identification as a member of the Trypanosoma cruzi sialidase superfamily

Amastigote surface proteins of Trypanosoma cruzi are likely targets of both humoral and cell-mediated immune responses, however, few such molecules have been well studied. In this study, we have used modified RACE (rapid amplification of cDNA ends) and SOE (gene splicing by overlap extension) polymerase chain reaction strategies to clone the gene for the previously described 83 kDa amastigote surface protein of T. cruzi. Of the several clones obtained, only one clone, clone 4, was found to encode the 20 amino acid sequence originally reported by Pan and McMahon-Pratt (J Immunol 1989;143:1001-1008). The identity of the cloned gene with the 83 kDa amastigote surface protein was further confirmed by the reactivity of polyclonal antisera against the purified 83 kDa protein with the gene product expressed in E. coli. Sequence analyses revealed that this amastigote surface protein (ASP-2) has two conserved aspartic acid box motifs and the highly conserved VTVxNVxLYNR motif characteristic of bacterial and viral sialidases and the type III module of fibronectin, respectively. ASP-2 thus joins ASP-1 as a member of the amastigote surface expressed family of sialidase-like molecules having strong homology with family 2 of the sialidase/trans-sialidase gene superfamily of T. cruzi.

The major surface glycoprotein of Trypanosoma cruzi amastigotes are ligands of the human serum mannose-binding protein

Trypanosoma cruzi, an obligate intracellular protozoan parasite, chronically infects mammals and causes Chagas' disease in humans. T. cruzi evasion of the mammalian immune response and establishment of chronic infection are poorly understood. During T. cruzi infection, amastigotes and trypomastigotes disseminate in the mammalian host and invade multiple cell types. Parasite surface carbohydrates and mammalian lectins have been implicated in the invasion of mammalian cells. A recent study has demonstrated that the human mannose-binding protein and the macrophage mannose receptor, two mammalian C-type lectins, bind to T. cruzi (S. J. Kahn, M. Wleklinski, A. Aruffo, A. Farr, D. Coder, and M. Kahn, J. Exp. Med. 182:1243-1258,1995). In this report we identify the major surface glycoproteins, including the SA85-1 glycoproteins, as T. cruzi ligands of the mannose-binding protein. Further characterization of the interaction between the mannose-binding protein and T. cruzi demonstrates that (i) the SA85-1 glycoproteins are expressed by amastigotes and trypomastigotes but only amastigotes express the mannose-binding protein ligand, (ii) treatment of amastigotes with alpha-mannosidase inhibits the binding of mannose-binding protein, and (iii) amastigote binding of mannose-binding protein is stable despite the spontaneous shedding of some glycoproteins from its surface. Together, the data indicate that developmentally regulated glycosylation of surface glycoproteins controls the expression of ligands that affect the interactions between T. cruzi and mannose-binding protein. It has been established that the binding of mannose-binding protein to microorganisms facilitates their uptake into phagocytic cells. Preferential opsonization of amastigotes with mannose-binding proteins may account for their clearance from the circulation and may contribute to the parasite's ability to invade different cell types.

Identification of a domain of Trypanosoma cruzi metacyclic trypomastigote surface molecule gp82 required for attachment and invasion of mammalian cells

Recombinant proteins and synthetic peptides representing various sequences of gp82, a surface glycoprotein of Trypanosoma cruzi metacyclic trypomastigotes implicated in mammalian cell invasion, were used in this study aiming at the identification of the domain(s) of this molecule required for interaction with target cells. Invasion of cultured HeLa cells by metacyclic trypomastigotes was inhibited by about 80% in the presence of native gp82 or the corresponding recombinant construct J18. Inhibition by recombinant proteins J18a and J18b, containing respectively the N-terminal and the C-terminal portions of gp82, was on the order of 30% and 65%. As compared to J18b (amino acids 224-516), the truncated gp82 fragments J18b1 (amino acids 303-516) and J18b2 (amino acids 357-516) displayed lower inhibitory effect (approximately 40% and approximately 15%, respectively). Compatible with these observations, we found that the recombinant protein J18b, but not J18a or J18b2, binds to HeLa cells in a dose-dependent and saturable fashion. Experiments with ten overlapping synthetic peptides, representing the gp82 portion spanning amino acids 224-333, showed that peptides 4 (amino acids 254-273) and 8 (amino acids 294-313) have significant inhibitory activity on HeLa cell invasion by metacyclic forms. All these results indicate that the portion of gp82 required for mammalian cell attachment and invasion is located in the central domain of the molecule.

A recombinant protein based on the Trypanosoma cruzi metacyclic trypomastigote 82-kilodalton antigen that induces and effective immune response to acute infection

To further investigate the immunological properties of the stage-specific 82-kDa glycoprotein (gp82) of Trypanosoma cruzi metacyclic trypomastigotes, previously shown to induce antigen-specific humoral and T-cell responses in mice, we performed a series of experiments with recombinant proteins containing sequences of gp82 fused to glutathione S-transferase. Of five fusion proteins tested, only J18b and J18b1, the carboxyproximal peptides containing amino acids 224 to 516 and 303 to 516, respectively, were recognized by monoclonal antibody 3F6 as well as by various anti-T. cruzi antisera and, when administered to mice, were capable of eliciting antibodies directed to the native gp82. The amino-terminal peptide and other carboxyterminal recombinant proteins lacking the central domain of gp82 (amino acids 224 to 356), which is exposed on the surface of live metacyclic forms, did not display any of these properties. Spleen cells derived from mice immunized with any of the five recombinant proteins proliferated in vitro in the presence of native gp82.J18b was the most stimulatory, whereas J18b3, the peptide containing amino acids 408 to 516, elicited the weakest response. When BALB/c mice immunized with J18b antigen plus A1(OH)3 as adjuvant were challenged 10 5 metacyclic trypomastigotes, 85% of them resisted acute infection, in comparison with control mice that received glutathione S-transferase plus adjuvant. Antibodies induced by J18b protein lacked agglutinating or complement-dependent lytic activity and failed to neutralize parasite infectivity. On the other hand, CD4+T cells from the spleens of J18b-immunized mice displayed an intense proliferative activity upon stimulation with 1.25 microgram of native gp82 per ml, which resulted in increased production of gamma interferon, a cytokine associated with resistance to T. cruzi infection.

Effects of 3' untranslated and intergenic regions on gene expression in Trypanosoma cruzi

The effects of 3' untranslated regions (UTR) and intergenic regions (INT), from various Trypanosoma cruzi stage-specific and constitutive genes, on the expression of the reporter firefly luciferase gene (luc), were studied using stable episomal transformation. The 3' UTR influenced luciferase expression by changing the steady-state level and/or the translation efficiency of luc mRNA. Glycoprotein 72 gene (gp72), glycoprotein 85 gene (gp85) or amastin gene (ama) 3' UTR decreased the luc mRNA level 6- to 14-fold, compared to the glyceraldehyde 1-phosphate dehydrogenase gene (gapdh) 3' UTR, in epimastigotes. Luciferase activity decreased in parallel with the luc mRNA level in transformants utilizing the gp85 or ama 3' UTR, whereas luc mRNA containing the gp72 3' UTR showed approximately 5-fold higher translation efficiency than luc mRNA containing a minimal 3' UTR. In amastigotes, the inhibitory effect of the ama 3' UTR observed in other life cycle stages was abolished and luciferase expression was stimulated 16-fold. The overall stage-specific difference mediated by the ama 3' UTR, between epimastigotes and amastigotes, was approximately 100-fold. INT, which was expected to influence polyadenylation efficiency, of gapdh, gp72, or heat shock protein 60 gene inserted after gapdh 3' UTR increased luc mRNA 2- to 8-fold, whereas gp85 INT slightly decreased luc mRNA. By separating effects attributable to the 3' UTR and INT, this study shows the effects of 3' UTR on RNA levels and translational efficiency in T. cruzi.

Trypanosoma cruzi amastigote adhesion to macrophages is facilitated by the mannose receptor

Trypanosoma cruzi is an obligate intracellular protozoan parasite. The mammalian stage of the parasite life cycle describes amastigotes as an intracellular form that replicates, and trypomastigotes as an extracellular form that disseminates and invades cells. Recent studies, however, have demonstrated that amastigotes circulate in the blood of infected mammals and can invade mammalian cells. In this report, a T. cruzi surface glycoprotein gene, SA85-1.1, was expressed as an immunoglobulin chimera, and this recombinant globulin was used to screen normal mouse tissues for adhesive interactions. This approach identified a subset of macrophages in the skin and peripheral lymph node that bind the T. cruzi surface glycoproteins through the mannose receptor. To further examine the T. cruzi mannose receptor carbohydrate ligands, the interaction between T. cruzi and the mannose-binding protein, a mammalian lectin with similar carbohydrate binding specificities as the mannose receptor, was examined. These studies demonstrated that the mannose-binding protein recognized amastigotes, but not trypomastigotes or epimastigotes, and suggested that amastigotes would also be recognized by the mannose receptor. Therefore, amastigote adhesion to macrophages was investigated, and these experiments demonstrated that the mannose receptor contributes to amastigote adhesion. The data identify the first mammalian lectins that bind to T. cruzi, and are involved in T. cruzi invasion of mammalian cells. The data suggest that amastigotes and trypomastigotes may have developed different mechanisms to adhere to and invade host cells. In addition, it has been established that IFN-gamma-activated macrophages express low levels of the mannose receptor and are trypanocidal; this suggests that the interaction between amastigotes and the mannose receptor enables amastigotes to increase their adherence with a population of macrophages that are nontrypanocidal and permissive for their intracellular replication.

Cloning and characterization of a gene for the stage-specific 82-kDa surface antigen of metacyclic trypomastigotes of Trypanosoma cruzi

We have cloned and sequenced a cDNA clone coding for a metacyclic trypomastigote-specific surface glycoprotein with a molecular mass of 82 kDa (MTS-gp82). By immunoblotting and immunoprecipitation, antibodies against the recombinant protein recognized an 82-kDa protein of metacyclic trypomastigotes, without any detectable reaction towards amastigotes, epimastigotes or tissue culture-derived trypomastigotes. The insert of the MTS-gp82 cDNA clone strongly hybridized with a single 2.2-kb metacyclic trypomastigote mRNA, suggesting that the steady-state levels of mRNAs for MTS-gp82 are developmentally regulated. MTS-gp82 is encoded by a multigene family whose members are distributed in several chromosomes. Sequence analysis revealed 40-56% identity at amino acid level between MTS-gp82 and members of Trypanosoma cruzi gp85/sialidase family (TSA-1, Tt34c1, SA85-1.1). MTS-gp82 showed several amino acid motifs that are characteristic of gp85/sialidase family, such as the Asp box (SxDxGxTW), the subterminal (VTVxNVFLYNR) motif and the putative GPI-anchor sequence. On the basis of its structural features, the MTS-gp82 gene could be included in the T. cruzi gp85/sialidase family, but constituting a distinct group which is preferentially expressed in metacyclic trypomastigotes.

Characterization of multiple unique cDNAs encoding the major surface glycoprotein of rat-derived Pneumocystis carinii

The major surface glycoprotein (MSG) of rat-derived Pneumocystis carinii represents a group of related molecules that are encoded by multiple genes. We isolated seven unique MSG cDNAs from a library prepared from a single infected rat lung. The cDNAs displayed both conserved and variant regions to previously described cDNAs. These clones contained inserts that ranged in size from 0.4 to 1.8 kb and all contained a poly(A) tail. The largest clone, Pc1410, hybridized to all 15 chromosomes resolved by pulsed-field gel electrophoresis (PFGE). Protein produced by in vitro translation from Pc1410 was immunoprecipitated with affinity-purified MSG antibodies. The clones were characterized by DNA sequencing of their 3' and 5' ends. Analysis of the untranslated and coding regions demonstrated that the clones contained unique and conserved regions of sequence, but none of the clones were identical. Isolation of seven additional unique clones picked from a single screening of a cDNA library suggests that numerous MSG transcripts exist within a population of P. carinii.

Expression and evolution of members of the Trypanosoma cruzi trypomastigote surface antigen multigene family

The trypomastigote specific surface antigens of Trypanosoma cruzi are encoded by a supergene family which includes the TSA family. The TSA family is characterized by the presence of a 27-bp tandem repeat array in the coding region. Here, we report the characterization and analysis of the three TSA family members in the Esmeraldo strain of the parasite. In this strain 2 distinct telomeric members are expressed abundantly as 3.7-kb mRNAs, while the remaining member is located at an internal chromosomal site and is expressed at less than 2% of the level seen for the telomeric members. Based on hybridization to DNA separated by PFGE, 3 chromosomes of sizes 1.8 Mb, 0.98 Mb, and 0.90 Mb each contain one of the telomeric members. In addition, the two smaller chromosomes also contain the single internal member. Since both chromosomes contain similar TSA family members, and vary only slightly in size, we suggest that they are homologues. Comparisons of the nucleotide sequences of the different members of the family show that the internal gene differs from the telomeric genes primarily in sequences found 3' of the repeat array. These comparisons also reveal that the three genes are analogous, supporting the hypothesis that short segments between the family members are exchanged by gene conversion events. We propose that similar conversion events between members of different gene families may generate some of the diversity found within the supergene family.

The glycosylphosphatidylinositol anchor of the trypomastigote-specific Tc-85 glycoprotein from Trypanosoma cruzi. Metabolic-labeling and structural studies

The Tc-85 glycoprotein, specific for the infective stage of Trypanosoma cruzi, is anchored via glycosylphosphatidylinositol. The protein was purified from parasites, labeled metabolically with palmitic acid, by immunoprecipitation with the H1A10 monoclonal antibody or by affinity column chromatography on wheat germ agglutinin. Antisera to the soluble form of the variant surface glycoprotein of Trypanosoma brucei brucei cross-reacted with Tc-85 when the immunoprecipitate was analysed by Western blotting. The reaction was intensified upon previous incubation of the glycoprotein with phosphatidylinositol-specific phospholipase C. Such recognition was abolished when the cyclic phosphate was opened by mild acid treatment. The lipid cleaved by phospholipase C digestion, was identified as 1-O-hexadecylglycerol by reverse-phase thin-layer chromatography. The glycan core was deaminated and chemically labeled by reduction with NaB3H4. The labeled glycoprotein was exhaustively treated with pronase and dephosphorylated with 50% HF. Although microheterogeneity of the oligosaccharide moiety was apparent, by thin layer chromatography, a main spot coincident with Man(alpha 1-2) Man(alpha 1-6) Man(alpha 1-4) anhydromannitol was shown, consistent with the conserved core structure of all glycosylphosphatidylinositol anchors analysed to date.

Characterization of a cDNA clone encoding the carboxy-terminal domain of a 90-kilodalton surface antigen of Trypanosoma cruzi metacyclic trypomastigotes

We have cloned and sequenced a cDNA for a metacyclic trypomastigote-specific glycoprotein with a molecular mass of 90 kDa, termed MTS-gp90. By immunoblotting, antibodies to the MTS-gp90 recombinant protein reacted exclusively with a 90-kDa antigen of metacyclic trypomastigotes. The insert of the MTS-gp90 cDNA clone strongly hybridized with a single 3.0-kb mRNA of metacyclic forms, whereas the hybridization signal with epimastigote mRNA was weak and those with RNAs from other developmental stages were negative, indicating that transcription of the MTS-gp90 gene is developmentally regulated. A series of experiments showed that the MTS-gp90 gene is present in multiple copies in the Trypanosoma cruzi genome, arranged in a nontandem manner, and that there are at least 40 copies of the gene per haploid genome. Sequence analysis of recombinant MTS-gp90 revealed 40 to 60% identity at the amino acid level with members of a family of mammalian stage-specific, 85-kDa surface antigens of T. cruzi. However, there are considerable differences in the amino acid compositions outside the homology region.

A partial cDNA clone of trypomastigote decay-accelerating factor (T-DAF), a developmentally regulated complement inhibitor of Trypanosoma cruzi, has genetic and functional similarities to the human complement inhibitor DAF

Resistance to complement-mediated lysis in Trypanosoma cruzi is due to the expression of complement-regulatory factors by the virulent developmental forms of this protozoan parasite. An 87- to 93-kDa molecule, which we have termed T-DAF (trypomastigote decay-accelerating factor), is present on the surface of the parasite and inhibits complement activation in a manner functionally similar to the mammalian complement regulatory component, decay-accelerating factor. In this report, we characterized monospecific polyclonal and monoclonal antibodies which were obtained from mice and rabbits immunized with fast protein liquid chromatography-purified T-DAF. These polyclonal antibodies were shown to inhibit T-DAF activity and were capable of inducing lysis of the parasites. Both the polyclonal and monoclonal antibodies were used to screen a cDNA expression library prepared from T. cruzi trypomastigote mRNA. From this library, we obtained a partial lambda gt11 cDNA clone which showed genetic and functional similarity to the human C3 convertase inhibitor DAF (A. Nicholson-Weller, J. Burge, D. T. Fearon, P. F. Weller, and K. F. Austen, J. Immunol. 129:184-189, 1982).

Further characterization of protective Trypanosoma cruzi-specific CD4+ T-cell clones: T helper type 1-like phenotype and reactivity with shed trypomastigote antigens

We previously reported the isolation from immune mice of a panel of murine clonal T-cell lines which specifically recognize antigens expressed by the trypomastigote stage of the protozoan parasite Trypanosoma cruzi, the causative agent of human Chagas' disease. Our analysis indicated that distinct clones which recognize common as well as strain-specific antigenic determinants were represented. The immunoprotective potential of several of these T-cell clones was demonstrated by adoptive transfer of protection to naive syngeneic recipients. Here we report that these T-cell clones are all of the TH1 phenotype, as determined from their lymphokine secretion patterns. Significant levels of stimulatory activity for each clone were detected in trypomastigote supernatants, and the release of this activity was time and temperature dependent. Seven of 10 T-cell clones tested responded to nitrocellulose-immunoblotted trypomastigote proteins in the range of 90 to 47 kDa; no fewer than six distinct epitopes residing on at least five distinct polypeptide species were recognized by this panel of clones. Two clones (2G8 and 4B10) previously shown to protect in vivo responded to immunoblotted proteins in the range of 65 to 53 and 90 to 80 kD, respectively. Stimulatory activity for the latter clone was shown to be expressed on the surface of trypomastigotes and to bind specifically to wheat germ agglutinin, indicating that its target antigen is an 85-kDa trypomastigote surface glycoprotein.

FL-160 proteins of Trypanosoma cruzi are expressed from a multigene family and contain two distinct epitopes that mimic nervous tissues

The partial sequence of a gene encoding the COOH terminus of a protein of apparent molecular weight of 160 kD associated with the flagellum of trypomastigotes of Trypanosoma cruzi (FL-160 now renamed to FL-160-1) has been previously reported. The COOH terminus of FL-160-1 has an epitope, defined by 12 amino acids, which molecularly miMics a nervous tissue antigen of 48 kD found in myenteric plexus, sciatic nerve, and a subset of cells in the central nervous system. We now report that FL-160 is a family of highly related genes. The sequence has been determined for the entire open reading frame (ORF) of one of the members of the FL-160 gene family (FL-160-2) and three other partial ORFs. Sequence analysis reveals the various members of the FL-160 gene family to be approximately 80% homologous in the predicted amino acid sequence, but all retain the 12-amino acid molecular mimicry epitope on the COOH terminus. Comparison of the sequence of FL-160-2 to other sequences demonstrates amino acid homology to bacterial sialidase (27%), members of the SA85 gene family (25-30%) and the shed acute-phase antigen/neuraminidase/trans-sialidase gene family (25-30%). Quantitative hybridization at high stringency suggests 750 copies of FL-160 are present in the DNA of each parasite. Reverse transcription and sequence analysis demonstrates that at least five of the members of the FL-160 gene family are transcribed. The NH2 terminus of one of the FL-160 gene products was expressed and antibodies prepared. Antibodies directed to either the COOH or the NH2 terminus of FL-160 bind a 160-kD T. cruzi protein. Both antibodies bind the surface membrane in the flagellar pocket of the trypomastigote. Antibodies to the NH2 terminus bind epineurium and scattered linear densities in sciatic nerve in a pattern distinct from the pattern with antibodies to the COOH terminus. Thus, there are at least two distinct molecular mimicry epitopes on the FL-160 molecule and both mimic epitopes found in nervous tissues. FL-160 may be involved in the generation of autoimmunity to nervous tissues by molecular mimicry, observed in chronic Chagas' disease.

Trypanosoma cruzi trans-sialidase and cell invasion

Trypanosoma cruzi does not synthesize sialic acid but does contain a trans-sialidase, an enzyme capable of transferring sialic acid between host glycoconjugates and the parasite. Sialic acids are negatively charged carbohydrates attached to the terminal non-reducing end of glycoproteins and glycolipids, and their presence can dramatically influence many cell-surface recognition processes. Since sialic acids have been implicated in several ligand-receptor interactions, including the interaction of pathogenic viruses, bacteria and protozoans with their hosts, the expression of trans-sialidase and the acquisition of sialic acid by T. cruzi may be relevant to the interaction of the parasite with the host, and consequently may influence the pathobiology of Chagas disease. In this review, Sergio Schenkman and Daniel Eichinger discuss recent data about the structure and function of T. cruzi trans-sialidase.

Polyreactive autoantibodies to negatively charged epitopes following Trypanosoma cruzi infection

During the course of many human autoimmune diseases, antibodies which recognize negatively charged epitopes on self antigens are detected. Trypanosoma cruzi, an intracellular protozoan parasite capable of infecting a wide variety of vertebrates, is the cause of Chagas disease in humans. Infection with the parasite frequently results in autoimmune and inflammatory pathology. We report here on an affinity-purified population of antibodies that bind to a broad class of antigens that contain runs of acidic amino acids, including tubulin. Although these antibodies can be isolated from both uninfected and T. cruzi chronically infected C3H/He mice, the antibodies from the normal mice (the natural autoantibodies) bind to tubulin poorly at physiological pH, whereas the antibodies isolated from the infected animals bind well at physiological pH. We propose that similar processes may occur in humans following other infections accounting for the detection of antibodies to negatively charged epitopes in a variety of autoimmune diseases.

Sequence homology and absence of mRNA defines a possible pseudogene member of the Trypanosoma cruzi gp85/sialidase multigene family

A genomic clone, pTt21, containing DNA apparently transcribed specifically in Trypanosoma cruzi trypomastigotes, was obtained by differentially screening a genomic library with trypomastigote and epimastigote cDNA. This 3444-bp clone contained open reading frames at each end, separated by a 1.8-kb non-coding region. The translated polypeptide from the 3' open reading frame (ORF2) of 1037 bp had 25-30% identity with 5 recently published T. cruzi gp85/sialidase sequences, and 20-25% identity with bacterial sialidases. Rabbit antiserum raised against an Escherichia coli fusion protein derived from the 5' open reading frame (ORF1) identified a surface antigen of 160 kDa, specifically expressed in trypomastigotes. A probe containing the first 211 bp from ORF1 was used to obtain a complete copy (c1821) of a gene that was closely related to ORF1, and encoded another member of the gp85/sialidase family. c1821 encodes a protein of 897 amino acids, but assignment of the N-terminus of the polypeptide was not possible. The 5'-most start codon is an unfavourable context to act as a translation initiator, it does not align with the initiator methionines of other gp85/sialidase sequences, nor is it followed by a signal peptide sequence characteristically found in other gp85/sialidase sequences. Although homology with the 5' ends of other gp85/sialidase sequences decays towards the 5' end of c1821, alignment of c1821 with 4 other gp85/sialidases indicated that the coding sequence should extend upstream at least 160 amino acids. In this region of c1821 there are multiple stop codons in each frame. The presence of the stop codons, the alignment data and our inability to amplify reverse transcribed mRNA using four internal primers, suggest that c1821 may not be present as a mature mRNA and is a pseudogene. Comparison of the apparently non-repetitive 3' coding domain of c1821 with the corresponding repetitive domains of two other members of the gp85/sialidase family revealed a high degree of similarity in nucleotide but not in amino acid sequence, and c1821 may thus represent an evolutionary intermediate between sub-families of the gp85/sialidase superfamily.

Neutralization-sensitive merozoite surface antigens of Babesia bovis encoded by members of a polymorphic gene family

Monospecific antibodies against native and recombinant versions of the major merozoite surface antigen (MSA-1) of Babesia bovis neutralize the infectivity of merozoites from Texas and Mexico strains in vitro. Sequence analysis shows that MSA-1 and a related, co-expressed 44 kDa merozoite surface protein (MSA-2) are encoded by members of a multigene family previously designated BabR. BabR genes, originally described in Australia strains of B. bovis, are notable because their marked polymorphism is apparently mediated by chromosomal rearrangements, but protein products of BabR genes have not previously been identified. The 3' terminal 173 nucleotides of the MSA-1 gene, including 60 nucleotides of untranslated sequence, are highly similar to the 3' terminal sequences of BabR 0.8 (84% identity) and MSA-2 (94% identity). Alignment of the predicted protein sequences demonstrates significant overall homology between MSA-1 and MSA-2, and between both proteins and the amino terminal BabR sequence. MSA-1 nucleic acid probes also hybridize weakly to genomic DNA from the Australia 'L' strain, even though this strain does not express merozoite surface epitopes cross-reactive with MSA-1 or MSA-2. Hybridization of these same probes to genomic DNA from the cloned Mexico strain reveals a pattern of bands compatible with two copies each of MSA-1 and MSA-2. Proteins encoded by this B. bovis gene family have been designated variable merozoite surface antigens (VMSA). The extent and mechanism of VMSA polymorphism among strains will be important when evaluating the role these surface proteins have in the host-parasite interaction, including immunity to blood stages.