The Trypanosoma cruzi trans-sialidase, through its COOH-terminal tandem repeat, upregulates interleukin 6 secretion in normal human intestinal microvascular endothelial cells and peripheral blood mononuclear cells

Parasite-host glycan interactions during Trypanosoma cruzi infection: trans-Sialidase rides the show

Many important pathogen-host interactions rely on highly specific carbohydrate binding events. In the case of the protozoan Trypanosoma cruzi, the causative agent of Chagas disease, glycointeractions involving sialic acid (SA) residues are pivotal for parasite infectivity, escape from immune surveillance and pathogenesis. Though unable to synthesize SA de novo, T. cruzi displays a unique trans-Sialidase (TS) enzyme, which is able to cleave terminal SA residues from host donor glycoconjugates and transfer them onto parasite surface mucins, thus generating protective/adhesive structures. In addition, this parasite sheds TS into the bloodstream, as a way of modifying the surface SA signature, and thereby the signaling/functional properties of mammalian host target cells on its own advantage. Here, we discuss the pathogenic aspects of T. cruzi TS: its molecular adaptations, the multiplicity of interactions in which it is involved during infections, and the array of novel and appealing targets for intervention in Chagas disease provided by TS-remodeled sialoglycophenotypes.

Proteomic analysis reveals different composition of extracellular vesicles released by two Trypanosoma cruzi strains associated with their distinct interaction with host cells

Trypanosoma cruzi, the aetiologic agent of Chagas disease, releases vesicles containing a wide range of surface molecules known to affect the host immunological responses and the cellular infectivity. Here, we compared the secretome of two distinct strains (Y and YuYu) of T. cruzi, which were previously shown to differentially modulate host innate and acquired immune responses. Tissue culture-derived trypomastigotes of both strains secreted extracellular vesicles (EVs), as demonstrated by electron scanning microscopy. EVs were purified by exclusion chromatography or ultracentrifugation and quantitated using nanoparticle tracking analysis. Trypomastigotes from YuYu strain released higher number of EVs than those from Y strain, enriched with virulence factors trans-sialidase (TS) and cruzipain. Proteomic analysis confirmed the increased abundance of proteins coded by the TS gene family, mucin-like glycoproteins, and some typical exosomal proteins in the YuYu strain, which also showed considerable differences between purified EVs and vesicle-free fraction as compared to the Y strain. To evaluate whether such differences were related to parasite infectivity, J774 macrophages and LLC-MK2 kidney cells were preincubated with purified EVs from both strains and then infected with Y strain trypomastigotes. EVs released by YuYu strain caused a lower infection but higher intracellular proliferation in J774 macrophages than EVs from Y strain. In contrast, YuYu strain-derived EVs caused higher infection of LLC-MK2 cells than Y strain-derived EVs. In conclusion, quantitative and qualitative differences in EVs and secreted proteins from different T. cruzi strains may correlate with infectivity/virulence during the host-parasite interaction.

Pathogenesis of Chronic Chagas Disease: Macrophages, Mitochondria, and Oxidative Stress

PURPOSE OF REVIEW

Trypanosoma cruzi is the causative agent of Chagas disease. Decades after initial infection, ~30% of individuals can develop chronic chagasic cardiomyopathy. There are several proposed mechanisms for pathogenesis of Chagas disease, including parasite persistence, immune responses against parasite or self that continue in the heart, vascular compromise, and involvement of autonomous and central nervous system. Herein, we will focus on the significance of macrophages, mitochondrial dysfunction, and oxidative stress in progression of chagasic cardiomyopathy.

RECENT FINDINGS

The current literature suggests that T. cruzi prevents cytotoxic activities of the innate immune cells and persists in the host, contributing to mitochondrial oxidative stress. We discuss how the neoantigens generated due to cellular oxidative damage contribute to chronic inflammatory stress in chagasic disease.

SUMMARY

We propose that metabolic regulators, PARP-1/SIRT1, determine the disease outcome by modulating the mitochondrial and macrophage stress and antioxidant/oxidant imbalance, and offer a potential new therapy against chronic Chagas disease.

Th-17 cytokines are associated with severity of Trypanosoma cruzi chronic infection in pediatric patients from endemic areas of Mexico

In Chagas disease the clinical, acute and chronic manifestations are the result of the interaction between the parasite and the host factors. The balance between inflammatory and anti-inflammatory immune responses is essential for the increase or resolution of the manifestations in individuals infected with T. cruzi. To identify if children with chronic Chagas disease and heart injury is related with non-regulated Th1, Th2 and Th17 responses. We included 31 children with T. cruzi confirmed chronic infection from endemic areas of Mexico. Subsequently, they were separated according to their ECHO and ECG results into three groups according to the severity of cardiac involvement. Circulating Th1, Th2 and Th17 cytokine profiles were performed by Luminex assays and the results were analyzed by bivariate and multivariable analysis. Patients were classified in asymptomatic chronic (group 1, N=12); individuals with IRBBB in ECG and incipient lesions in ECHO (Group 2, N=8) and Patients with severe chronic symptomatic disease (Group 3, N=11). The analysis of immune mediators revealed that patients with severe cardiac manifestations had significant higher levels (p <0.05) of Th17 related cytokines including IL-17 and IL-6 as well as IFN-γ and IL-2. Also patients with severe cardiomyopathy exhibit increased levels of IL-13 (p <0.05) after multivariate analysis. High levels of Th17 related cytokines including IL-17, IFN-γ, IL-6 and IL-2 and pro-fibrotic factors such as IL-13 could be associated to the severity of cardiac involvement in children with chronic T. cruzi infection. These cytokines could be useful as indicators for the early identification of cardiac damage associated to the T. cruzi infection.

Regulatory role of Toll-like receptor 2 during infection with Trypanosoma cruzi

Previous studies have emphasized the role of Toll-like receptors (TLRs) and myeloid differentiation factor 88 (MyD88) during infection with protozoan parasites. TLR2 was shown to be important for induction of cytokine synthesis by macrophages exposed to the purified glycosylphosphatidylinositol (GPI)-anchored mucin-like glycoproteins of Trypanosoma cruzi trypomastigotes (tGPIm). On the other hand, MyD88 —/— mice, but not TLR2 —/— mice, showed impaired cytokine production and resistance to infection with T. cruzi parasites. Here we evaluate the importance of MyD88 and TLR2 in MAPK activation and cytokine synthesis by macrophages exposed to live T. cruzi parasites and compared to tGPIm. The absence of MAPK phosphorylation in TLR2- and MyD88-deficient macrophages exposed to tGPIm correlated with the incapacity to induce cytokine release in these cells. In contrast, activation of MAPK and synthesis of pro-inflammatory cytokines were not abrogated in TLR2-deficient macrophages exposed to live T. cruzi parasites. We also showed that pretreatment with tGPIm significantly reduces cytokine release by macrophages in response to T. cruzi in a TLR2-dependent manner. Consistently, TLR2-/- mice were shown to produce enhanced levels of cytokines upon in vivo challenge with T. cruzi parasites. Together, these results suggest the involvement of additional TLR(s) in the pro-inflammatory response of macrophages to whole parasites, and that, in vivo, TLR2 may have a predominant immunoregulatory role during acute infection with T. cruzi parasites .

Current understanding of immunity to Trypanosoma cruzi infection and pathogenesis of Chagas disease

Chagas disease caused by Trypanosoma cruzi remains an important neglected tropical disease and a cause of significant morbidity and mortality. No longer confined to endemic areas of Latin America, it is now found in non-endemic areas due to immigration. The parasite may persist in any tissue, but in recent years, there has been increased recognition of adipose tissue both as an early target of infection and a reservoir of chronic infection. The major complications of this disease are cardiomyopathy and megasyndromes involving the gastrointestinal tract. The pathogenesis of Chagas disease is complex and multifactorial involving many interactive pathways. The significance of innate immunity, including the contributions of cytokines, chemokines, reactive oxygen species, and oxidative stress, has been emphasized. The role of the components of the eicosanoid pathway such as thromboxane A(2) and the lipoxins has been demonstrated to have profound effects as both pro- and anti-inflammatory factors. Additionally, we discuss the vasoconstrictive actions of thromboxane A(2) and endothelin-1 in Chagas disease. Human immunity to T. cruzi infection and its role in pathogen control and disease progression have not been fully investigated. However, recently, it was demonstrated that a reduction in the anti-inflammatory cytokine IL-10 was associated with clinically significant chronic chagasic cardiomyopathy.

Bioactive lipids in Trypanosoma cruzi infection

Chagas disease is caused by Trypanosoma cruzi, a protozoan parasite. Chagas disease remains a serious health problem in large parts of Mexico and Central and South America, where it is a major cause of morbidity and mortality. This disease is being increasingly recognized in non-endemic regions due to immigration. Heart disease develops in 10-30% of infected individuals. It is increasingly clear that parasite- and host-derived bioactive lipids potently modulate disease progression. Many of the changes that occur during acute and chronic Chagas disease can be accounted for by the effects of arachidonic acid (AA)-derived lipids such as leukotrienes, lipoxins, H(P)ETEs, prostaglandins (PGs) and thromboxane. During the course of infection with T. cruzi, changes in circulating levels of AA metabolites are observed. Antagonism of PG synthesis with cyclooxygenase (COX) inhibitors has both beneficial and adverse effects. Treatment with COX inhibitors during acute infection may result in increased parasite load and mortality. However, treatment instituted during chronic infection may be beneficial with no increase in mortality and substantial improvement with cardiac function. Recently, T. cruzi infection of mice deficient in AA biosynthetic enzymes for various pathways has yielded more insightful data than pharmacological inhibition and has highlighted the potential deleterious effects of inhibitors due to "off-target" actions. Using COX-1 null mice, it was observed that parasite biosynthesis is dependent upon host metabolism, that the majority of TXA(2) liberated during T. cruzi infection is derived from the parasite and that this molecule may act as a quorum sensor to control parasite growth/differentiation. Thus, eicosanoids present during acute infection may act as immunomodulators aiding the transition to, and maintenance of, the chronic stage of the disease. It is also likely that the same mediators that initially function to ensure host survival may later contribute to cardiovascular damage. Collectively, the eicosanoids represent a new series of targets for therapy in Chagas disease with defined potential therapeutic windows in which to apply these agents for greatest effect. A deeper understanding of the mechanism of action of non-steroidal anti-inflammatory drugs may provide clues to the differences between host responses in acute and chronic T. cruzi infection.

Neurodegeneration and neuroregeneration in Chagas disease

Autonomic dysfunction plays a significant role in the development of chronic Chagas disease (CD). Destruction of cardiac parasympathetic ganglia can underlie arrhythmia and heart failure, while lesions of enteric neurons in the intestinal plexuses are a direct cause of aperistalsis and megasyndromes. Neuropathology is generated by acute infection when the parasite, though not directly damaging to neuronal cells, elicits immune reactions that can become cytotoxic, inducing oxidative stress and neurodegeneration. Anti-neuronal autoimmunity may further contribute to neuropathology. Much less clear is the mechanism of subsequent neuronal regeneration in patients that survive acute infection. Morphological and functional recovery of the peripheral neurons in these patients correlates with the absence of CD clinical symptoms, while persistent neuronal deficiency is observed for the symptomatic group. The discovery that Trypanosoma cruzi trans-sialidase can moonlight as a parasite-derived neurotrophic factor (PDNF) suggests that the parasite might influence the balance between neuronal degeneration and regeneration. PDNF functionally mimics mammalian neurotrophic factors in that it binds and activates neurotrophin Trk tyrosine kinase receptors, a mechanism which prevents neurodegeneration. PDNF binding to Trk receptors triggers PI3K/Akt/GSK-3β and MAPK/Erk/CREB signalling cascades which in neurons translates into resistance to oxidative and nutritional stress, and inhibition of apoptosis, whereas in the cytoplasm of infected cells, PDNF represents a substrate-activator of the host Akt kinase, enhancing host-cell survival until completion of the intracellular cycle of the parasite. Such dual activity of PDNF provides sustained activation of survival mechanisms which, while prolonging parasite persistence in host tissues, can underlie distinct outcomes of CD.

ROS signalling of inflammatory cytokines during Trypanosoma cruzi infection

Inflammation is a host defence activated by exogenous (e.g. pathogen-derived, pollutants) or endogenous (e.g. reactive oxygen species-ROS) danger signals. Mostly, endogenous molecules (or their derivatives) have well-defined intracellular function but become danger signal when released or exposed following stress or injury. In this review, we discuss the potential role of ROS in chronic evolution of inflammatory cardiovascular diseases, using our experiences working on chagasic cardiomyopathy as a focus-point.

Inflammatory and Prothrombotic Activation With Conserved Endothelial Function in Patients With Chronic, Asymptomatic Chagas Disease

Previously, our group showed a prothrombotic state in asymptomatic patients with chronic Chagas disease. The current paper studies the inflammatory status and endothelial function in these patients. Methods: In 40 patients and 40 healthy volunteers, we evaluated prothrombotic state, blood parasitemia (molecular biology: polymerized chain reaction [PCR]-amplification), tissue factor pathway inhibitor antibodies (aTFPI), interleukin 6 (IL-6), and vascular cell adhesion molecule-1 (VCAM-1). Endothelial function was determined by reactive hyperemia (pulse plethysmography). Results: In patients, prothrombin fragment 1 + 2, d-dimer, PAI-1, and fibrinogen were higher. Amplification of 121/122 primers ( Trypanosoma cruzi) was positive in 45% of the patients. Patients presented higher values of aTFPI- immunoglobulin G (IgG; P < .05), aTFPI-IgM ( P < .001), IL-6 ( P = .004), and VCAM-1 ( P = .00001). In both groups, endothelial function was preserved. Conclusions: We found that asymptomatic patients with chronic Chagas disease presented a prothrombotic/inflammatory status. The fact that endothelial function is still preserved suggests that prothrombosis and inflammation are primarily implicated in the beginning of cardiovascular damage.

Lack of association between IL-6-174G/C gene polymorphism and Chagas disease

The aim of this study was to investigate the role of the IL-6-174G/C gene polymorphism in susceptibility/resistance to Trypanosoma cruzi infection in two independent cohorts from Colombia and Peru. We determined the IL-6-174G/C genotypes in a sample of 399 seronegative individuals and 317 serologically positive patients from Colombia and Peru. All individuals are from regions where T. cruzi infection is endemic. No statistically significant differences in the frequency of IL-6-174G/C gene polymorphism between chagasic patients and controls or between asymptomatic and individuals with cardiomyopathy were observed. Our results do not support an evidence for a major role contribution of this IL-6 gene polymorphism in the susceptibility to or clinical manifestations of Chagas disease in these studied cohorts.

Trypanosoma cruzi-Derived Neurotrophic Factor: Role in Neural Repair and Neuroprotection

Some patients infected with the parasite Try-panosoma cruzi develop chronic Chagas' disease, while others remain asymptomatic for life. Although pathological mechanisms that govern disease progression remain unclear, the balance between degeneration and regeneration in the peripheral nervous system seems to contribute to the different clinical outcomes. This review focuses on certain new aspects of host-parasite interactions related to regeneration in the host nervous system induced by the trans-sialidase of T. cruzi, also known as a parasite-derived neurotrophic factor (PDNF). PDNF plays multiple roles in T. cruzi infection, ranging from immunosuppression to functional mimicry of mammalian neurotrophic factors and inhibition of apoptosis. PDNF affinity to neurotrophin Trk receptors provide sustained activation of cellular survival mechanisms resulting in neuroprotection and neuronal repair, resistance to cytotoxic insults and enhancement of neuritogenesis. Such unique PDNF-elicited regenerative responses likely prolong parasite persistence in infected tissues while reducing neuropathology in Chagas' disease.

The surface-anchored NanA protein promotes pneumococcal brain endothelial cell invasion

In humans, Streptococcus pneumoniae (SPN) is the leading cause of bacterial meningitis, a disease with high attributable mortality and frequent permanent neurological sequelae. The molecular mechanisms underlying the central nervous system tropism of SPN are incompletely understood, but include a primary interaction of the pathogen with the blood–brain barrier (BBB) endothelium. All SPN strains possess a gene encoding the surface-anchored sialidase (neuraminidase) NanA, which cleaves sialic acid on host cells and proteins. Here, we use an isogenic SPN NanA-deficient mutant and heterologous expression of the protein to show that NanA is both necessary and sufficient to promote SPN adherence to and invasion of human brain microvascular endothelial cells (hBMECs). NanA-mediated hBMEC invasion depends only partially on sialidase activity, whereas the N-terminal lectinlike domain of the protein plays a critical role. NanA promotes SPN–BBB interaction in a murine infection model, identifying the protein as proximal mediator of CNS entry by the pathogen.

Innate and Acquired Immunity in the Pathogenesis of Chagas Disease

The apparent discrepancy between the intensity of inflammatory reaction and scarce number of parasites in chronic chagasic myocarditis prompt several investigators to hypothesize that an autoimmune process was involved in the pathogenesis of Chagas disease. Here, we recapitulate diverse molecular and cellular mechanisms of innate and acquired immunity involved in the control of parasite replication and in the build up of myocarditis observed during infection with Trypanosoma cruzi. In addition, we review the immunoregulatory mechanisms responsible for preventing excessive immune response elicited by this protozoan parasite. Ongoing studies in this research area may provide novel therapeutic strategies that could enhance the immunoprotective response while preventing the deleterious parasite-elicited responses observed during Chagas disease.

Auto-antibodies to receptor tyrosine kinases TrkA, TrkB and TrkC in patients with chronic Chagas' disease

The Chagas' disease parasite Trypanosoma cruzi promotes survival and differentiation of neurones by binding and activating nerve growth factor (NGF) receptor TrkA. The functional mimic of NGF in T. cruzi is a surface-bound and shed immunogenic protein [neurotrophic factor/trans-sialidase (TS)], which raised the possibility that immune response to T. cruzi in general and to neurotrophic factor/TS in particular leads to loss of immunological tolerance to host NGF and/or the NGF-binding partner TrkA. In testing this hypothesis, we found that sera of individuals with chronic Chagas' disease bear unique IgG2 autoantibodies that bind TrkA and TrkA family members TrkB and TrkC (ATA). Binding of ATA to Trk receptors is specific because the autoantibodies did not cross-react with five other growth factor receptors, NGF and other neurotrophins, and T. cruzi. Thus, individuals with chronic Chagas' disease produce unique antibodies that react with pan-Trk receptors, one of which (TrkA) T. cruzi exploits to inhibit host cell apoptosis and to promote cellular invasion.

Transcriptome profile of Trypanosoma cruzi-infected cells: simultaneous up- and down-regulation of proliferation inhibitors and promoters

As Trypanosoma cruzi, the etiological agent of Chagas disease, multiplies in the cytoplasm of nucleated host cells, infection with this parasite is highly likely to affect host cells. We performed an exhaustive transcriptome analysis of T. cruzi-infected HeLa cells using an oligonucleotide microarray containing probes for greater than 47,000 human gene transcripts. In comparison with uninfected cells, those infected with T. cruzi showed greater than threefold up-regulation of 41 genes and greater than threefold down-regulation of 23 genes. Real-time reverse transcriptase-polymerase chain reaction (RT-PCR) of selected, differentially expressed genes confirmed the microarray data. Many of these up- and down-regulated genes were related to cellular proliferation, including seven up-regulated genes encoding proliferation inhibitors and three down-regulated genes encoding proliferation promoters, strongly suggesting that T. cruzi infection inhibits host cell proliferation, which may allow more time for T. cruzi to replicate and produce its intracellular nests. These findings provide new insight into the molecular mechanisms by which intracellular T. cruzi infection influences the host cell, leading to pathogenicity.

Endothelial cell signalling induced by trans-sialidase from Trypanosoma cruzi

The protozoan responsible for Chagas' disease, Trypanosoma cruzi, expresses on its surface an unusual trans-sialidase enzyme thought to play an important role in host-parasite interactions. Trans-sialidase is the product of a multigene family encoding both active and inactive proteins. We have demonstrated that despite lacking enzymatic activity due to a single mutation, Tyr342-His, inactive trans-sialidase displays sialic acid binding activity, with identical specificity to that of its active analogue. In this work we demonstrate that binding of a recombinant inactive trans-sialidase to molecules containing alpha2,3-linked sialic acid on endothelial cell surface triggers NF-kappaB activation, expression of adhesion molecules and upregulation of parasite entry into host cells. Furthermore, inactive recombinant trans-sialidase blocks endothelial cell apoptosis induced by growth factor deprivation. These results suggest that inactive members of the trans-sialidase family play a role in endothelial cell responses to T. cruzi infection.

Expression of trypomastigote trans-sialidase in metacyclic forms of Trypanosoma cruzi increases parasite escape from its parasitophorous vacuole

Trypanosoma cruzi actively invades mammalian cells by forming parasitophorous vacuoles (PVs). After entry, the parasite has to escape from these vacuoles in order to replicate inside the host cell cytosol. Trans-sialidase (TS), a parasite enzyme that is used to obtain sialic acid from host glycoconjugates, has been implicated in cell invasion and PV exit, but how the enzyme acts in these processes is still unknown. Here we show that trypomastigotes derived from infected mammalian cells express and release 20 times more TS activity than axenic metacyclic trypomastigotes, which correspond to the infective forms derived from the insect vector. Both forms have the same capacity to invade mammalian cells, but cell derived trypomastigotes exit earlier from the vacuole. To test whether high TS expression is responsible for this increased exit from the PV, trypomastigote TS was expressed on the surface of metacyclic forms. Transfected and non-transfected metacyclics attached to and invaded HeLa or CHO cells equally. In contrast, metacyclics expressing TS on the surface escaped earlier from the vacuole than non-transfected metacyclics, or metacyclics expressing TS in their cytoplasm. Sialic acid may act as a barrier, which is removed by surface and/or secreted TS, because all types of parasites escaped earlier from the vacuoles of sialic acid-deficient Lec 2 cells than wild-type CHO cells. In addition, trypomastigotes and metacyclic forms expressing TS differentiated earlier into amastigotes. These results indicate that the increased expression of TS in cell-derived trypomastigotes is responsible for the earlier exit from the PV to the cytoplasm and their subsequent differentiation into amastigotes.

Downregulation of Selected Cytokines in Amebiasis

A 220-kDa E. histolytica lectin is capable of downregulating some inflammatory cytokines (IL-5, IL-6, INF-gamma and TNF-alpha) and thus of inducing an overall anti-inflammatory Th-phenotype in leucocytes of selected, perhaps constitutionally predisposed, individuals irrespective of their HLA-DR3 profile (i.e., in this study patients long recovered from amebic abscess of the liver). This probably inhibited cytokine response pattern could increase the risk for developing amebic abscess of the liver in the course of invasive intestinal amebiasis.

A synthetic peptide modeled on PDNF, Chagas' disease parasite neurotrophic factor, promotes survival and differentiation of neuronal cells through TrkA receptor

The human parasite Trypanosoma cruzi, the agent of Chagas' disease, expresses a membrane-bound neuraminidase/trans-sialidase, also known as parasite-derived neurotrophic factor, PDNF, because it binds and activates nerve growth factor (NGF) receptor TrkA in neuronal cells. Here, we identify a 21 amino acid region (425GNASQNVWEDAYRCVNASATAN445) of PDNF that reproduces its neurotrophic activities. Synthetic peptide Y21, modeled on this sequence, induces survival and neurite outgrowth in primary dorsal root ganglion neurons. Y21 but not other PDNF-based peptides promotes survival and neurite extension in TrkA-expressing but not in TrkA-deficient PC12 cells. Y21 also enhances phosphorylation of TrkA in PC12 cells and activation of Erk1/2 and Akt kinases with kinetics distinct from that of PDNF. In addition, Y21 stimulates phosphorylation of cAMP response element-binding protein, CREB. Peptide Y21, therefore, reproduces several TrkA-dependent activities of PDNF and NGF. However, Y21 inhibits the binding of PDNF but not NGF to TrkA. Similarly, Y21 blocks PDNF- but not NGF-dependent phosphorylation of Erk1/2. These findings raise the possibility that Y21 reacts with a TrkA site required for the binding of PDNF but not NGF. The functioning of Y21 as TrkA agonist reproducing TrkA-dependent biological activities of PDNF should help elucidate the mechanism of PDNF activation of TrkA-expressing cells and the design of small drugs for the treatment of Chagas' and other neurodegenerative diseases.

Parasite-derived trans-sialidase binds to heart tissue in Trypanosoma cruzi-infected animals

Trypanosoma cruzi is an obligate intracellular protozoan parasite that actively penetrates into non-phagocytic mammalian cells. To accomplish this, the parasite relies on the binding of cell surface ligands. It is reported herein that the T. cruzi trans-sialidase (TS), which is exposed on the parasite surface, binds to mouse heart cells, and should therefore be further studied as a possible cell penetration-related ligand. In addition, as has been proposed elsewhere, the binding of T. cruzi to tissues may turn them into targets for parasite-specific immune reactions. Washed heart sections from T. cruzi-infected mice were subjected to immunoenzymatic staining with antisera against whole T. cruzi and with polyclonal or monoclonal antibodies against TS. The anti-TS antibodies stained both parasites and uninfected heart cells in the vicinity of T. cruzi nest remains/trypomastigotes. On the other hand, an anti-T. cruzi serum, which did not recognize TS, only stained the parasites. In addition, normal heart sections from uninfected nude mice were shown to react with both enzymatically active and inactive recombinant TS molecules, probably through their amino-terminal region, since a recombinant TS lacking this region failed to bind.

Role of the Toll/interleukin-1 receptor signaling pathway in host resistance and pathogenesis during infection with protozoan parasites

Different studies have illustrated the activation of the innate immune system during infection with protozoan parasites. Experiments performed in vivo also support the notion that innate immunity has a crucial role in resistance as well as pathogenesis observed during protozoan infections such as malaria, leishmaniasis, toxoplasmosis, and trypanosomiasis. While major advances have been made in the assignment of bacterial molecules as Toll-like receptors (TLRs) agonists as well as defining the role of the Toll/interleukin-1 receptor (TIR) signaling pathway in host resistance to bacterial infection, this research area is now emerging in the field of protozoan parasites. In this review, we discuss the recent studies describing parasite molecules as TLR agonists and those studies indicating the essential role of the TIR-domain bearing molecule named myeloid differentiation factor 88 in host resistance to infection with protozoan parasites. Together, these studies support the hypothesis that the TIR signaling pathway is involved in the initial recognition of protozoan parasites by the immune system of the vertebrate host, early resistance to infection, development of acquired immunity, as well as pathology observed during acute infection with this class of pathogens.

Diversity of microbial sialic acid metabolism

Sialic acids are structurally unique nine-carbon keto sugars occupying the interface between the host and commensal or pathogenic microorganisms. An important function of host sialic acid is to regulate innate immunity, and microbes have evolved various strategies for subverting this process by decorating their surfaces with sialylated oligosaccharides that mimic those of the host. These subversive strategies include a de novo synthetic pathway and at least two truncated pathways that depend on scavenging host-derived intermediates. A fourth strategy involves modification of sialidases so that instead of transferring sialic acid to water (hydrolysis), a second active site is created for binding alternative acceptors. Sialic acids also are excellent sources of carbon, nitrogen, energy, and precursors of cell wall biosynthesis. The catabolic strategies for exploiting host sialic acids as nutritional sources are as diverse as the biosynthetic mechanisms, including examples of horizontal gene transfer and multiple transport systems. Finally, as compounds coating the surfaces of virtually every vertebrate cell, sialic acids provide information about the host environment that, at least in Escherichia coli, is interpreted by the global regulator encoded by nanR. In addition to regulating the catabolism of sialic acids through the nan operon, NanR controls at least two other operons of unknown function and appears to participate in the regulation of type 1 fimbrial phase variation. Sialic acid is, therefore, a host molecule to be copied (molecular mimicry), eaten (nutrition), and interpreted (cell signaling) by diverse metabolic machinery in all major groups of mammalian pathogens and commensals.

PDNF, a human parasite-derived mimic of neurotrophic factors, prevents caspase activation, free radical formation, and death of dopaminergic cells exposed to the Parkinsonism-inducing neurotoxin MPP+

The neuraminidase/trans-sialidase of Trypanosoma cruzi, the agent of Chagas' disease, promotes differentiation and survival of growth factor-deprived neuronal and glial cells. To gain further insights into the possible neuroprotection of this parasite-derived counterpart of neurotrophic factors (PDNF), we sought to determine whether it mimics growth factors in a cellular model of neurodegenerative diseases. Ascertaining cell viability by morphology, vital dye exclusion, mitochondrial reducing function, and absence of DNA fragmentation, we show here that PDNF rescues from death two dopaminergic neuronal cell lines and one differentiated immortalized mesencephalic neurons exposed to the neurotoxin 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) and its toxic metabolite, 1-methyl-4-phenylpyridinium (MPP+), both widely used in models of Parkinson's disease. We further show that PDNF promoted survival at concentrations comparable to bona fide growth factors in a MAPK/Erk activation-dependent manner. PDNF also strongly suppresses the overproduction of MPTP-induced reactive oxygen species (ROS), and the activation of both initiator caspase-9 and effector caspase-3. This down-regulation of ROS and caspases explains, at least in part, the PDNF-induced salvaging of the dopaminergic cells from the Parkinsonism-promoting toxin, confirming the novel and striking functional mimicry by the trypanosome neuraminidase of host growth factors in a cellular model of neurodegeneration.

Impaired Production of Proinflammatory Cytokines and Host Resistance to Acute Infection withTrypanosoma cruziin Mice Lacking Functional Myeloid Differentiation Factor 88

Studies performed in vitro suggest that activation of Toll-like receptors (TLRs) by parasite-derived molecules may initiate inflammatory responses and host innate defense mechanisms against Trypanosoma cruzi. Here, we evaluated the impact of TLR2 and myeloid differentiation factor 88 (MyD88) deficiencies in host resistance to infection with T. cruzi. Our results show that macrophages derived from TLR2 (-/-) and MyD88(-/-) mice are less responsive to GPI-mucin derived from T. cruzi trypomastigotes and parasites. In contrast, the same cells from TLR2(-/-) still produce TNF-alpha, IL-12, and reactive nitrogen intermediates (RNI) upon exposure to live T. cruzi trypomastigotes. Consistently, we show that TLR2(-/-) mice mount a robust proinflammatory cytokine response as well as RNI production during the acute phase of infection with T. cruzi parasites. Further, deletion of the functional TLR2 gene had no major impact on parasitemia nor on mortality. In contrast, the MyD88(-/-) mice had a diminished cytokine response and RNI production upon acute infection with T. cruzi. More importantly, we show that MyD88(-/-) mice are more susceptible to infection with T. cruzi as indicated by the higher parasitemia and accelerated mortality, as compared with the wild-type mice. Together, our results indicate that T. cruzi parasites elicit an alternative inflammatory pathway independent of TLR2. This pathway is partially dependent on MyD88 and necessary for mounting optimal inflammatory and RNI responses that control T. cruzi replication during the early stages of infection.

Trans-sialidase-like sequences from Trypanosoma congolense conserve most of the critical active site residues found in other trans-sialidases

Trypanosoma congolense is the agent of Nagana, the trypanosomiasis in African ruminants. Trypanosomes express an enzyme called trans-sialidase, which is believed to play an important role in maintaining pathogenicity of the parasites. Thus far, only two complete trans-sialidase sequences have been characterised, one from the American trypanosome T. cruzi and one from the African trypanosome T. brucei brucei. Although the crystal structure of T. cruzi trans-sialidase has recently been published [Buschiazzo et al., Mol. Cell 10 (2002), pp. 757-768], a number of questions concerning the exact transfer mechanism remain unanswered. The availability of further trans-sialidase sequences will ensure a better understanding of how transfer activity can be achieved and will provide the opportunity to develop highly specific, structure-based trans-sialidase inhibitors. Utilising a PCR-based approach two different trans-sialidase gene copies from T. congolense were identified, which share only 50% identity with each other, but show significant similarity with known viral, bacterial and trypanosomal sialidases and trans-sialidases. In both partial sequences most of the critical active site residues common to other trypanosomal sialidases and trans-sialidases are conserved. This is further illustrated by modelling the active site of the longer of the two partial gene sequences.

Two trans-sialidase forms with different sialic acid transfer and sialidase activities from Trypanosoma congolense

Trypanosomes express an enzyme called trans-sialidase (TS), which enables the parasites to transfer sialic acids from the environment onto trypanosomal surface molecules. Here we describe the purification and characterization of two TS forms from the African trypanosome Trypanosoma congolense. The purification of the two TS forms using a combination of anion exchange chromatography, isoelectric focusing, gel filtration, and subsequently, antibody affinity chromatography resulted, in both cases, in the isolation of a 90-kDa monomer on SDS-PAGE, which was identified as trans-sialidase using micro-sequencing. Monoclonal antibody 7/23, which bound and partially inhibited TS activity, was found in both cases to bind to a 90-kDa protein. Both TS forms possessed sialidase and transfer activity, but markedly differed in their activity ratios. The TS form with a high transfer-to-sialidase activity ratio, referred to as TS-form 1, possessed a pI of pH 4-5 and a molecular mass of 350-600 kDa. In contrast, the form with a low transfer-to-sialidase activity ratio, referred to as TS-form 2, exhibited a pI of pH 5-6.5 and a molecular mass of 130-180 kDa. Both TS forms were not significantly inhibited by known sialidase inhibitors and revealed no significant differences in donor and acceptor substrate specificities; however, TS-form 1 utilized various acceptor substrates with a higher catalytic efficiency. Interestingly, glutamic acid-alanine-rich protein, the surface glycoprotein, was co-purified with TS-form 1 suggesting an association between both proteins.

Mimicry of tandem repeat peptides against cell surface carbohydrates

Our approach to multivalent peptide construction relies on tentacle peptides, also known as a multiple antigenic peptides, which contain two and four repeats of a selected peptide. In this communication, we report the results of preliminary studies aimed at (1) the selection of short peptides against the carbohydrate, sLeX, (2) the synthesis of tentacle dimers and tetramers of the selected peptides, and (3) the determination of affinities and specificities of the peptides to several related carbohydrates by using the surface plasmon resonance (SPR) and the equilibrium dialysis techniques. Binding affinity studies, as well as assays of in vitro binding of the peptides to a sLeX-specific cell line, have shown that the tetrameric peptides bind to the cell surface sugars.

The Trypanosoma cruzi trans-sialidase is a T cell-independent B cell mitogen and an inducer of non-specific Ig secretion

Polyclonal lymphocyte activation and hypergammaglobulinemia characterize the acute phase of many parasitic diseases, including Chagas' disease, a debilitating condition caused by Trypanosoma cruzi. Polyclonal lymphocyte activation correlates with disease susceptibility inT. cruzi infection. Thus, identifying factors that drive such reactivities should provide insight into mechanisms of parasite evasion from host immunity and of disease pathogenesis. Sensitization of mice with small doses of T. cruzi trans-sialidase (TS) turns the mice into highly susceptible hosts to T. cruzi. In addition, TS heterologously expressed in Leishmania major greatly enhances virulence of the parasite to mice. In attempt to study the mechanism of TS-induced virulence, we found that TS and its C-terminal long tandem repeat (LTR) are T-independent polyclonal activators for mouse B cells. While B cells deficient/defective in L-6, CD40 or Toll-like receptor-4 are similarly activated by TS as compared to wild-type cells, B cells from Bruton's tyrosine kinase-defective X-linked immunodeficient mice are remarkably insensitive to TS activation. TS-induced B cell activation in vitro is accompanied by Ig secretion independent of T cells. Furthermore, administration of TS into normal mice leads to non-specific Ig secretion that peaks 4-6 days after injection. Thus TS, through its LTR, induces abnormal polyclonal B cell activation and Ig secretion, which could explain in part its virulence-enhancing activity.

Interleukin-6 is required for parasite specific response and host resistance to Trypanosoma cruzi

Infection with Trypanosoma cruzi, the agent of Chagas' disease, results in elevated levels of interleukin-6 (IL-6) in serum and infected tissues. However, it remains unknown whether IL-6 plays a role in host defence against T. cruzi. To determine whether IL-6 underlies disease progression, we followed the time course of T. cruzi-infected mice bearing IL-6 +/+ and minus sign/minus sign genotypes, respectively. We found that IL-6 minus sign/minus sign mice were more susceptible to T. cruzi infection as they exhibited about 3-fold higher parasitaemia and died earlier than wild-type animals. Unlike what might be expected, T. cruzi-infected IL-6 minus sign/minus sign mice did not show at peak infection a decrease in the secretion of IFN-gamma, a Th1 cytokine crucial for controlling the parasite. Instead, they exhibited a much reduced splenocyte recall response to T. cruzi antigens. Our results suggest that IL-6 mediates anti-parasite protective responses against T. cruzi.

Immediate/early response to Trypanosoma cruzi infection involves minimal modulation of host cell transcription

Host cell infection by the intracellular pathogen, Trypanosoma cruzi, involves activation of signaling pathways, cytoskeletal reorganization, and targeted recruitment of host cell lysosomes. To determine the consequences of T. cruzi invasion on host cell gene expression, high density microarrays consisting of approximately 27,000 human cDNAs were hybridized with fluorescent probes generated from T. cruzi-infected human fibroblasts (HFF) at early time points following infection (2-24 h). Surprisingly, no genes were induced > or =2-fold in HFF between 2 and 6 h post-infection (hpi) in repeated experiments while immediate repression of six host cell transcripts was observed. A significant increase in transcript abundance for 106 host cell genes was observed at 24 hpi. Among the most highly induced is a set of interferon-stimulated genes, indicative of a type I interferon (IFN) response to T. cruzi. In support of this, T. cruzi-infected fibroblasts begin to secrete IFNbeta at 18 hpi following the induction of IFNbeta transcripts. As compared with global transcriptional responses evoked by other intracellular pathogens, T. cruzi is a stealth parasite that elicits few changes in host cell transcription during the initiation of infection.

Trypanosoma cruzi trans-sialidase: a potent and specific survival factor for human Schwann cells by means of phosphatidylinositol 3-kinase/Akt signaling

Patients infected with Trypanosoma cruzi may remain asymptomatic for decades and show signs of neuroregeneration in the peripheral nervous system (PNS). In the absence of such neuroregeneration, patients may die in part by extensive neuronal destruction in the gastrointestinal tract. Thus, T. cruzi may, despite their invasion of the PNS, directly prevent cell death to keep nerve destruction in check. Indeed, T. cruzi invasion of Schwann cells, their prime target in PNS, suppressed host-cell apoptosis caused by growth-factor deprivation. The trans-sialidase (TS) of T. cruzi and the Cys-rich domain of TS reproduced the antiapoptotic activity of the parasites at doses (> or =3.0 nM) comparable or lower than those of bona fide mammalian growth factors. This effect was blocked by LY294002, an inhibitor of phosphatidylinositol 3-kinase (PI3K). TS also activated Akt, a downstream effector of PI3K. Ectopic expression of TS in an unrelated parasite, Leishmania major, turned those parasites into activators of Akt in Schwann cells. In contrast, the Cys-rich domain of TS did not block apoptosis in Schwann cells overexpressing dominant-negative Akt or constitutively active PTEN, a negative regulator of PI3K/Akt signaling. The results demonstrate that T. cruzi, through its TS, triggers the survival of host Schwann cells via the PI3K/Akt pathway, suggesting a role for PI3K/Akt in the pathogenesis of Chagas' disease.

Activation of Toll-like receptor-2 by glycosylphosphatidylinositol anchors from a protozoan parasite

Glycosylphosphatidylinositol (GPI) anchors and glycoinositolphospholipids (GIPLs) from parasitic protozoa have been shown to exert a wide variety of effects on cells of the host innate immune system. However, the receptor(s) that are triggered by these protozoan glycolipids has not been identified. Here we present evidence that Trypanosoma cruzi-derived GPI anchors and GIPLs trigger CD25 expression on Chinese hamster ovary-K1 cells transfected with CD14 and Toll-like receptor-2 (TLR-2), but not wild-type (TLR-2-deficient) Chinese hamster ovary cells. The protozoan-derived GPI anchors and GIPLs containing alkylacylglycerol and saturated fatty acid chains or ceramide were found to be active in a concentration range of 100 nM to 1 microM. More importantly, the GPI anchors purified from T. cruzi trypomastigotes, which contain a longer glycan core and unsaturated fatty acids in the sn-2 position of the alkylacylglycerolipid component, triggered TLR-2 at subnanomolar concentrations. We performed experiments with macrophages from TLR-2 knockout and TLR-4 knockout mice, and found that TLR-2 expression appears to be essential for induction of IL-12, TNF-alpha, and NO by GPI anchors derived from T. cruzi trypomastigotes. Thus, highly purified GPI anchors from T. cruzi parasites are potent activators of TLR-2 from both mouse and human origin. The activation of TLR-2 may initiate host innate defense mechanisms and inflammatory response during protozoan infection, and may provide new strategies for immune intervention during protozoan infections.

Maternal Trypanosoma cruzi infection upregulates capacity of uninfected neonate cells To produce pro- and anti-inflammatory cytokines

The possibility of maternal in utero modulation of the innate and/or adaptive immune responses of uninfected newborns from Trypanosoma cruzi-infected mothers was investigated by studying the capacity of their whole blood cells to produce cytokines in response to T. cruzi lysate or lipopolysaccharide-plus-phytohemagglutinin (LPS-PHA) stimulation. Cells of such newborns occasionally released gamma interferon (IFN-gamma) and no interleukin-2 (IL-2) and IL-4 upon specific stimulation, while their mothers responded by the production of IFN-gamma, IL-2, and IL-4. Infection in mothers was also associated with a hyperactivation of maternal cells and also, strikingly, of cells of their uninfected neonates, since their release of proinflammatory (IL-1beta, IL-6, and tumor necrosis factor alpha [TNF-alpha]) as well as of anti-inflammatory (IL-10 and soluble TNF receptor) cytokines or factors was upregulated in the presence of LPS-PHA and/or parasite lysate. These results show that T. cruzi infection in mothers induces profound perturbations in the cytokine response of their uninfected neonates. Such maternal influence on neonatal innate immunity might contribute to limit the occurrence and severity of congenital infection.

Heterologous expression of Trypanosoma cruzi trans-sialidase in Leishmania major enhances virulence

Earlier studies showed that mice primed for a few hours with the trans-sialidase (TS) of Trypanosoma cruzi, the agent of Chagas' disease, become highly susceptible to trypanosomal infection. These studies suggest that TS affects parasite virulence independent of antigenic stimulation. Potentially, TS could enhance or reduce the virulence of heterologous microbes depending on the mechanism of TS action and on the type of immune response elicited by the particular parasite. We tested this hypothesis by expressing heterologous TS in Leishmania major, a protozoan parasite that causes cutaneous leishmaniasis and lacks TS and the TS product alpha2-3-linked sialic acid. Leishmania cells transfected with a T. cruzi TS expression construct made high levels of active enzyme, which was present in the promastigotes and shed into the extracellular milieu. TS expression did not affect L. major binding to and entry into cultured macrophages or its tropism for macrophage infection in vivo. However, TS-expressing L. major exhibited elevated virulence in BALB/c mice, as determined by lesion progression, parasite numbers, and macro- and microscopic examination of cutaneous lesions. Several genetic tests proved that the enhanced virulence was directly attributable to TS expression. The results are consistent with TS functioning to sabotage the mouse immune system to confer a growth advantage on T. cruzi and transgenic L. major. These data suggest that heterologous expression of T. cruzi virulence factors in Leishmania may provide a new approach for dissecting their function in vivo.

A trypanosomal protein synergizes with the cytokines ciliary neurotrophic factor and leukemia inhibitory factor to prevent apoptosis of neuronal cells

Despite the neuronal degeneration in the chronic stage of Chagas' disease, neuron counts actually increase in the preceding, asymptomatic stage, in contrast to the age-related decrease in neuron counts in age-matched normal individuals. Relevant to this observation, we found that the trans-sialidase (TS) of Trypanosoma cruzi, the etiologic agent of Chagas' disease, induces neurite outgrowth and rescues PC12 cells from apoptotic death caused by growth factor deprivation. These properties, novel for a parasite protein, were independent of catalytic activity and were mapped to the C terminus of the catalytic domain of TS. TS activated protein kinase Akt in a phosphoinositide-3 kinase-inhibitable manner, suggesting a molecular mechanism for the TS-induced neuroprotection. TS also triggered bcl-2 gene expression in growth factor-deprived cells, an effect consistent with TS protecting against apoptosis. Ciliary neurotrophic factor and leukemia inhibitory factor, two cytokines critical to the repair of injured motor neurons, specifically potentiated the TS action. The results suggest that TS acts in synergy with host ciliary neurotrophic factor or leukemia inhibitory factor to promote neuronal survival in T. cruzi-infected individuals.

Cell-specific activation of nuclear factor-kappaB by the parasite Trypanosoma cruzi promotes resistance to intracellular infection

The transcription factor nuclear factor-kappaB (NF-kappaB) is central to the innate and acquired immune response to microbial pathogens, coordinating cellular responses to the presence of infection. Here we demonstrate a direct role for NF-kappaB activation in controlling intracellular infection in nonimmune cells. Trypanosoma cruzi is an intracellular parasite of mammalian cells with a marked preference for infection of myocytes. The molecular basis for this tissue tropism is unknown. Trypomastigotes, the infectious stage of T. cruzi, activate nuclear translocation and DNA binding of NF-kappaB p65 subunit and NF-kappaB-dependent gene expression in epithelial cells, endothelial cells, and fibroblasts. Inactivation of epithelial cell NF-kappaB signaling by inducible expression of the inhibitory mutant IkappaBaM significantly enhances parasite invasion. T. cruzi do not activate NF-kappaB in cells derived from skeletal, smooth, or cardiac muscle, despite the ability of these cells to respond to tumor necrosis factor-alpha with NF-kappaB activation. The in vitro infection level in these muscle-derived cells is more than double that seen in the other cell types tested. Therefore, the ability of T. cruzi to activate NF-kappaB correlates inversely with susceptibility to infection, suggesting that NF-kappaB activation is a determinant of the intracellular survival and tissue tropism of T. cruzi.