Host and parasite apoptosis following Trypanosoma cruzi infection in in vitro and in vivo models

Immunopathogenesis in Trypanosoma cruzi infection: a role for suppressed macrophages and apoptotic cells

During Trypanosoma cruzi infection, macrophages phagocytose parasites and remove apoptotic cells through efferocytosis. While macrophage 1 (M1) produces proinflammatory cytokines and NO and fights infection, M2 macrophages are permissive host cells that express arginase 1 and play a role in tissue repair. The regulation of M1 and M2 phenotypes might either induce or impair macrophage-mediated immunity towards parasite control or persistence in chronic Chagas disease. Here, we highlight a key role of macrophage activation in early immune responses to T. cruzi that prevent escalating parasitemia, heart parasitism, and mortality during acute infection. We will discuss the mechanisms of macrophage activation and deactivation, such as T cell cytokines and efferocytosis, and how to improve macrophage-mediated immunity to prevent parasite persistence, inflammation, and the development of chagasic cardiomyopathy. Potential vaccines or therapy must enhance early T cell-macrophage crosstalk and parasite control to restrain the pathogenic outcomes of parasite-induced inflammation in the heart.

Extracellular vesicles of trypomastigotes of Trypanosoma cruzi induce changes in ubiquitin-related processes, cell-signaling pathways and apoptosis

Chagas disease is caused by the protozoan parasite Trypanosoma cruzi. The disease has an acute and a chronic phase in which approximately 30% of the chronic patients suffer from heart disease and/or gastrointestinal symptoms. The pathogenesis of the disease is multifactorial and involves the virulence of the strains, immunological factors and extracellular vesicles (EV) shed by the parasite which participate in cell-cell communication and evasion of the immune response. In this work, we present a transcriptomic analysis of cells stimulated with EV of the trypomastigote stage of T. cruzi. Results after EV-cell incubation revealed 322 differentially expressed genes (168 were upregulated and 154 were downregulated). In this regard, the overexpression of genes related to ubiquitin-related processes (Ube2C, SUMO1 and SUMO2) is highlighted. Moreover, the expression of Rho-GTPases (RhoA, Rac1 and Cdc42) after the interaction was analyzed, revealing a downregulation of the analyzed genes after 4 h of interaction. Finally, a protective role of EV over apoptosis is suggested, as relative values of cells in early and late apoptosis were significantly lower in EV-treated cells, which also showed increased CSNK1G1 expression. These results contribute to a better understanding of the EV-cell interaction and support the role of EV as virulence factors.

Modulation of STAT-1, STAT-3, and STAT-6 activities in THP-1 derived macrophages infected with two Trypanosoma cruzi strains

Trypanosoma cruzi is the causative protozoan of Chagas’ Disease, a neglected tropical disease that affects 6−7 million people worldwide. Interaction of the parasite with the host immune system is a key factor in disease progression and chronic symptoms. Although the human immune system is capable of controlling the disease, the parasite has numerous evasion mechanisms that aim to maintain intracellular persistence and survival. Due to the pronounced genetic variability of T. cruzi, co-infections or mixed infections with more than one parasite strain have been reported in the literature. The intermodulation in such cases is unclear. This study aimed to evaluate the co-infection of T. cruzi strains G and CL compared to their individual infections in human macrophages derived from THP-1 cells activated by classical or alternative pathways. Flow cytometry analysis demonstrated that trypomastigotes were more infective than extracellular amastigotes (EAs) and that strain G could infect more macrophages than strain CL. Classically activated macrophages showed lower number of infected cells and IL-4-stimulated cells displayed increased CL-infected macrophages. However, co-infection was a rare event. CL EAs decreased the production of reactive oxygen species (ROS), whereas G trypomastigotes displayed increased ROS detection in classically activated cells. Co-infection did not affect ROS production. Monoinfection by strain G or CL mainly induced an anti-inflammatory cytokine profile by decreasing inflammatory cytokines (IFN-γ, TNF-α, IL-1β) and/or increasing IL-4, IL-10, and TGF-β. Co-infection led to a predominant inflammatory milieu, with reduced IL-10 and TGF-β, and/or promotion of IFN-γ and IL-1β release. Infection by strain G reduced activation of intracellular signal transducer and activator of transcription (STAT) factors. In EAs, monoinfections impaired STAT-1 activity and promoted phosphorylation of STAT-3, both changes may prolong cell survival. Coinfected macrophages displayed pronounced activation of all STATs examined. These activations likely promoted parasite persistence and survival of infected cells. The collective results demonstrate that although macrophages respond to both strains, T. cruzi can modulate the intracellular environment, inducing different responses depending on the strain, parasite infective form, and co-infection or monoinfection. The modulation influences parasite persistence and survival of infected cells.

Administration of artinm lectin reduces the severity of the acute phase infection with Trypanosoma cruzi

The acute phase of experimental Trypanosoma cruzi infection is associated with a strong inflammatory reaction, physiological changes, amastigote nests in tissues, and hematological alterations. ArtinM, a lectin extracted from Artocarpus heterophyllus seeds, is a homotetramer exhibiting immunomodulatory properties that promotes Th1 immune responses against intracellular pathogens, including the induction of neutrophil migration and increase in IL‐12 production. This study aimed to evaluate the effects of ArtinM on experimental Chagas disease in mice. We evaluated mouse survival curves, parasitemia, hematological parameters including quantification of inflammatory infiltrates, and amastigote nests in cardiac tissue during infection. The results showed a reduced number of parasites in the blood, an increase in animal survival, improvements in hematological parameters, and decrease in inflammatory infiltrates and amastigote nests in the group treated with ArtinM. Collectively, these data suggest that the administration of ArtinM can lower the number of parasites in peak parasitemia caused by the Colombian strain of T. cruzi and can increase survival of infected mice. The observed reduction in cardiac tissue injury may be due to fewer T. cruzi amastigote nests and lower levels of inflammatory infiltrates. This study highlights the need for further investigation into the use of ArtinM as a potential alternative therapeutic for treating Chagas disease.

Trypanosoma cruzi Promotes Transcriptomic Remodeling of the JAK/STAT Signaling and Cell Cycle Pathways in Myoblasts

Chagas disease is responsible for more than 10,000 deaths per year and about 6 to 7 million infected people worldwide. In its chronic stage, patients can develop mega-colon, mega-esophagus, and cardiomyopathy. Differences in clinical outcomes may be determined, in part, by the genetic background of the parasite that causes Chagas disease. Trypanosoma cruzi has a high genetic diversity, and each group of strains may elicit specific pathological responses in the host. Conflicting results have been reported in studies using various combinations of mammalian host-T. cruzi strains. We previously profiled the transcriptomic signatures resulting from infection of L6E9 rat myoblasts with four reference strains of T. cruzi (Brazil, CL, Y, and Tulahuen). The four strains induced similar overall gene expression alterations in the myoblasts, although only 21 genes were equally affected by all strains. Cardiotrophin-like cytokine factor 1 (Clcf1) was one of the genes found to be consistently upregulated by the infection with all four strains of T. cruzi. This cytokine is a member of the interleukin-6 family that binds to glycoprotein 130 receptor and activates the JAK/STAT signaling pathway, which may lead to muscle cell hypertrophy. Another commonly upregulated gene was tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein theta (Ywhaq, 14-3-3 protein Θ), present in the Cell Cycle Pathway. In the present work, we reanalyzed our previous microarray dataset, aiming at understanding in more details the transcriptomic impact that each strain has on JAK/STAT signaling and Cell Cycle pathways. Using Pearson correlation analysis between the expression levels of gene pairs in biological replicas from each pathway, we determined the coordination between such pairs in each experimental condition and the predicted protein interactions between the significantly altered genes by each strain. We found that although these highlighted genes were similarly affected by all four strains, the downstream genes or their interaction partners were not necessarily equally affected, thus reinforcing the idea of the role of parasite background on host cell transcriptome. These new analyses provide further evidence to the mechanistic understanding of how distinct T. cruzi strains lead to diverse remodeling of host cell transcriptome.

Cardiomyocyte diffusible redox mediators control Trypanosoma cruzi infection: role of parasite mitochondrial iron superoxide dismutase

Chagas disease (CD), caused by the protozoa Trypanosoma cruzi, is a chronic illness in which parasites persist in the host-infected tissues for years. T. cruzi invasion in cardiomyocytes elicits the production of pro-inflammatory mediators [TNF-α, IL-1β, IFN-γ; nitric oxide (^·NO)], leading to mitochondrial dysfunction with increased superoxide radical (O₂^·-), hydrogen peroxide (H₂O₂) and peroxynitrite generation. We hypothesize that these redox mediators may control parasite proliferation through the induction of intracellular amastigote programmed cell death (PCD). In this work, we show that T. cruzi (CL-Brener strain) infection in primary cardiomyocytes produced an early (24 h post infection) mitochondrial dysfunction with H₂O₂ generation and the establishment of an oxidative stress evidenced by FoxO3 activation and target host mitochondrial protein expression (MnSOD and peroxiredoxin 3). TNF-α/IL-1β-stimulated cardiomyocytes were able to control intracellular amastigote proliferation compared with unstimulated cardiomyocytes. In this condition leading to oxidant formation, an enhanced number of intracellular apoptotic amastigotes were detected. The ability of H₂O₂ to induce T. cruzi PCD was further confirmed in the epimastigote stage of the parasite. H₂O₂ treatment induced parasite mitochondrial dysfunction together with intra-mitochondrial O₂^·- generation. Importantly, parasites genetically engineered to overexpress mitochondrial Fe-superoxide dismutase (Fe-SODA) were more infective to TNF-α/IL-1β-stimulated cardiomyocytes with less apoptotic amastigotes; this result underscores the role of this enzyme in parasite survival. Our results indicate that cardiomyocyte-derived diffusible mediators are able to control intracellular amastigote proliferation by triggering T. cruzi PCD and that parasite Fe-SODA tilts the process toward survival as part of an antioxidant-based immune evasion mechanism.

Identification of Anti-Trypanosoma cruzi Lead Compounds with Putative Immunomodulatory Activity

In seeking substitutions for the current Chagas disease treatment, which has several relevant side effects, new therapeutic candidates have been extensively investigated. In this context, a balanced interaction between mediators of the host immune response seems to be a key element for therapeutic success, as a proinflammatory microenvironment modulated by interleukin-10 (IL-10) is shown to be relevant to potentiate anti-Trypanosoma cruzi drug activity. This study aimed to identify the potential immunomodulatory activities of the anti-T. cruzi K777, pyronaridine (PYR), and furazolidone (FUR) compounds in peripheral blood mononuclear cells (PBMC) from noninfected (NI) subjects and chronic Chagas disease (CD) patients. Our results showed low cytotoxicity to PBMC populations, with 50% cytotoxic concentrations (CC50) of 71.0 μM (K777), 9.0 μM (PYR), and greater than 20 μM (FUR). In addition, K777 showed no impact on the exposure index (EI) of phytohemagglutinin-stimulated leukocytes (PHA), while PYR and FUR treatments induced increased EI of monocytes and T lymphocytes at late stages of apoptosis in NI subjects. Moreover, K777 induced a more prominent proinflammatory response (tumor necrosis factor alpha-positive [TNF-α+] CD8+/CD4+, gamma interferon-positive [IFN-γ+] CD4+/CD8+ modulated by interleukin-10-positive [IL-10+] CD4+ T/CD8+ T) than did PYR (TNF-α+ CD8+, IL-10+ CD8+) and FUR (TNF-α+ CD8+, IL-10+ CD8+). Signature analysis of intracytoplasmic cytokines corroborated the proinflammatory/modulated (K777) and proinflammatory (PYR and FUR) profiles previously found. In conclusion, the lead compound K777 may induce beneficial changes in the immunological profile of patients presenting the chronic phase of Chagas disease and may contribute to a more effective therapy against the disease.

Trypanosoma cruzi activates mouse cardiac fibroblasts in vitro leading to fibroblast-myofibroblast transition and increase in expression of extracellular matrix proteins

Background

Cardiac fibrosis is a consequence of chronic chagasic cardiomyopathy (CCC). In other cardiovascular diseases, the protagonist role of fibroblasts in cardiac fibrosis is well established. However, the role of cardiac fibroblasts (CFs) in fibrosis during the CCC is not clear. Here, our aim was to investigate the effect of Trypanosoma cruzi, the etiological agent of Chagas disease on CFs activation.

Methods

Cardiac fibroblasts were purified from primary cultures of mouse embryo cardiac cells. After two passages, cells were infected with T. cruzi (Y strain) and analyzed at different times for determination of infectivity, activation and production of extracellular matrix components (fibronectin, laminin and collagen IV) by immunofluorescence and western blot.

Results

At second passage, cultures were enriched in CFs (95% of fibroblasts and 5% of cardiomyocytes), as revealed by presence of alpha-smooth muscle actin (α-SMA) and discoidin domain receptor 2 (DDR2) and absence of sarcomeric tropomyosin (ST) protein expression. Trypanosoma cruzi infection induced fibroblast-myofibroblast transition, with increased expression of α-SMA after 6 and 24 h post-infection (hpi). Fibronectin was increased at 6, 24 and 48 hpi, laminin was increased at 6 and 24 hpi and collagen IV was increased at 6 hpi.

Conclusions

Our results showed that T. cruzi activates CFs, inducing activation and exacerbates ECM production. Furthermore, our data raise the possibility of the involvement of CFs in heart fibrosis during Chagas disease.

New Insights into the Immunobiology of Mononuclear Phagocytic Cells and Their Relevance to the Pathogenesis of Cardiovascular Diseases

Macrophages are the primary immune cells that reside within the myocardium, suggesting that these mononuclear phagocytes are essential in the orchestration of cardiac immunity and homeostasis. Independent of the nature of the injury, the heart triggers leukocyte activation and recruitment. However, inflammation is harmful to this vital terminally differentiated organ with extremely poor regenerative capacity. As such, cardiac tissue has evolved particular strategies to increase the stress tolerance and minimize the impact of inflammation. In this sense, growing evidences show that mononuclear phagocytic cells are particularly dynamic during cardiac inflammation or infection and would actively participate in tissue repair and functional recovery. They respond to soluble mediators such as metabolites or cytokines, which play central roles in the timing of the intrinsic cardiac stress response. During myocardial infarction two distinct phases of monocyte influx have been identified. Upon infarction, the heart modulates its chemokine expression profile that sequentially and actively recruits inflammatory monocytes, first, and healing monocytes, later. In the same way, a sudden switch from inflammatory macrophages (with microbicidal effectors) toward anti-inflammatory macrophages occurs within the myocardium very shortly after infection with Trypanosoma cruzi, the causal agent of Chagas cardiomyopathy. While in sterile injury, healing response is necessary to stop tissue damage; during an intracellular infection, the anti-inflammatory milieu in infected hearts would promote microbial persistence. The balance of mononuclear phagocytic cells seems to be also dynamic in atherosclerosis influencing plaque initiation and fate. This review summarizes the participation of mononuclear phagocyte system in cardiovascular diseases, keeping in mind that the immune system evolved to promote the reestablishment of tissue homeostasis following infection/injury, and that the effects of different mediators could modulate the magnitude and quality of the immune response. The knowledge of the effects triggered by diverse mediators would serve to identify new therapeutic targets in different cardiovascular pathologies.

Death of adrenocortical cells during murine acute T. cruzi infection is not associated with TNF-R1 signaling but mostly with the type II pathway of Fas-mediated apoptosis

Earlier studies from our laboratory demonstrated that acute experimental Trypanosoma cruzi infection promotes an intense inflammation along with a sepsis-like dysregulated adrenal response characterized by normal levels of ACTH with raised glucocorticoid secretion. Inflammation was also known to result in adrenal cell apoptosis, which in turn may influence HPA axis uncoupling. To explore factors and pathways which may be involved in the apoptosis of adrenal cells, together with its impact on the functionality of the gland, we carried out a series of studies in mice lacking death receptors, such as TNF-R1 (C57BL/6-^{Tnfrsf1a tm1Imx} or TNF-R1^-/-) or Fas ligand (C57BL/6 Fas-deficient lpr mice), undergoing acute T. cruzi infection. Here we demonstrate that the late hypercorticosterolism seen in C57BL/6 mice during acute T. cruzi infection coexists with and hyperplasia and hypertrophy of zona fasciculata, paralleled by increased number of apoptotic cells. Apoptosis seems to be mediated mainly by the type II pathway of Fas-mediated apoptosis, which engages the mitochondrial pathway of apoptosis triggering the cytochrome c release to increase caspase-3 activation. Fas-induced apoptosis of adrenocortical cells is also related with an exacerbated production of intra-adrenal cytokines that probably maintain the late supply of adrenal hormones during host response. Present results shed light on the molecular mechanisms dealing with these phenomena which are crucial not only for the development of interventions attempting to avoid adrenal dysfunction, but also for its wide occurrence in other infectious-based critical illnesses.

A local innate immune response against Trypanosoma cruzi in the human placenta: The epithelial turnover of the trophoblast

Congenital Chagas disease, caused by Trypanosoma cruzi, is partially responsible for the progressive globalization of Chagas disease despite of its low transmission rate. The probability of congenital transmission depends on complex interactions between the parasite, the maternal and fetus/newborn immune responses and placental factors, being the latter the least studied one. During transplacental transmission, the parasite must cross the placental barrier where the trophoblast, a continuous renewing epithelium, is the first tissue to have contact with the parasite. Importantly, the epithelial turnover is considered part of the innate immune system since pathogens, prior to cell invasion, must attach to the surface of cells. The trophoblast turnover involves cellular processes such as proliferation, differentiation and apoptotic cell death, all of them are induced by the parasite. In the present review, we analyze the current evidence about the trophoblast epithelial turnover as a local placental innate immune response.

Immunoregulatory mechanisms in Chagas disease: modulation of apoptosis in T-cell mediated immune responses

Background

Chronic Chagas disease presents different clinical manifestations ranging from asymptomatic (namely indeterminate) to severe cardiac and/or digestive. Previous results have shown that the immune response plays an important role, although no all mechanisms are understood. Immunoregulatory mechanisms such as apoptosis are important for the control of Chagas disease, possibly affecting the morbidity in chronic clinical forms. Apoptosis has been suggested to be an important mechanism of cellular response during T. cruzi infection. We aimed to further understand the putative role of apoptosis in Chagas disease and its relation to the clinical forms of the disease.

Methods

Apoptosis of lymphocytes, under antigenic stimuli (soluble T. cruzi antigens – TcAg) where compared to that of non-stimulated cells. Apoptosis was evaluated using the expression of annexin and caspase 3⁺ by T cells and the percentage of cells positive evaluated by flow cytometry. In addition activation and T cell markers were used for the identification of TCD4⁺ and TCD8⁺ subpopulations. The presence of intracellular and plasma cytokines were also evaluated. Analysis of the activation status of the peripheral blood cells showed that patients with Chagas disease presented higher levels of activation determined by the expression of activation markers, after TcAg stimulation. PCR array were used to evaluate the contribution of this mechanism in specific cell populations from patients with different clinical forms of human Chagas disease.

Results

Our results showed a reduced proliferative response associated a high expression of T CD4⁺CD62L⁻ cells in CARD patients when compared with IND group and NI individuals. We also observed that both groups of patients presented a significant increase of CD4⁺ and CD8⁺ T cell subsets in undergoing apoptosis after in vitro stimulation with T. cruzi antigens. In CARD patients, both CD4⁺ and CD8⁺ T cells expressing TNF-α were highly susceptible to undergo apoptosis after in vitro stimulation. Interestingly, the in vitro TcAg stimulation increased considerably the expression of cell death TNF/TNFR superfamily and Caspase family receptors genes in CARD patients.

Conclusions

Taken together, our results suggest that apoptosis may be an important mechanism for the control of morbidity in T. cruzi infection by modulating the expression of apoptosis genes, the cytokine environment and/or killing of effector cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12879-016-1523-1) contains supplementary material, which is available to authorized users.

Trypanosoma cruzi parasites fight for control of the JAK-STAT pathway by disarming their host

The zoonotic Chagas' disease is caused by infections with the hemoflagellate Trypanosoma cruzi (T. cruzi) which is endemic in Latin America. Despite recent advances in our understanding of the pathogenesis of the disease, the underlying molecular processes involved in host-parasite interactions are only poorly understood. In particular, the mechanisms for parasite persistence in host cells remain largely unknown. Cytokine-driven transcription factors from the family of STAT (signal transducer and activator of transcription) proteins appear to play a central role in the fight against T. cruzi infection. However, amastigotes proliferating in the cytoplasm of infected host cells develop effective strategies to circumvent the attack executed by STAT proteins. This review highlights the interactions between T. cruzi parasites and human host cells in terms of cytokine signaling and, in particular, discusses the impact of STATs on the balance between parasite invasion and clearance.

Immunomodulation by Trypanosoma cruzi: toward understanding the association of dendritic cells with infecting TcI and TcII populations

Dendritic cells (DCs) are major immune components, and depending on how these cells are modulated, the protective host immune response changes drastically. Trypanosoma cruzi is a parasite with high genetic variability and modulates DCs by interfering with their capacity for antigen recognition, migration, and maturation. Despite recent efforts, the association between DCs and T. cruzi I (TcI) and TcII populations is unknown. Herein, it was demonstrated that AQ1.7 and MUTUM TcI strains present low rates of invasion of bone marrow-derived DCs, whereas the 1849 and 2369 TcII strains present higher rates. Whereas the four strains similarly induced the expression of PD-L1, the production and expression of IL-10 and TLR-2, respectively, in DCs were differentially increased. The production of TNF-α, IL-12, IL-6, and CCL2 and the expression of CD40, CD80, MHC-II, CCR5, and CCR7 changed depending on the strain. The 2369 strain yielded the most remarkable results because greater invasion correlated with an increase in the levels of anti-inflammatory molecules IL-10 and PD-L1 but not with a change in the levels of TNF-α, MHC-II, or CD40 molecules. These results suggest that T. cruzi strains belonging to different populations have evolved specific evasion strategies that subvert DCs and consequently the host response.

Natural sesquiterpene lactones induce programmed cell death in Trypanosoma cruzi: a new therapeutic target?

BACKGROUND

Chagas disease or American Trypanosomiasis is caused by the flagellated protozoan parasite Trypanosoma cruzi (T. cruzi) and is recognized by the WHO as one of the world's 17 neglected tropical diseases. Only two drugs (Benznidazol, Bz and Nifurtimox, Nx) are currently accepted for treatment, however they cause severe adverse effects and their efficacy is still controversial. It is then important to explore for new drugs.

PURPOSE

Programmed cell death (PCD) in parasites offers interesting new therapeutic targets. The aim of this work was to evaluate the induction of PCD in T. cruzi by two natural sesquiterpene lactones (STLs), dehydroleucodine (DhL) and helenalin (Hln) as compared with the two conventional drugs, Bz and Nx.

MATERIAL AND METHODS

Hln and DhL were isolated from aerial parts of Gaillardia megapotamica and Artemisia douglassiana Besser, respectively. Purity of compounds (greater than 95%) was confirmed by (13)C-nuclear magnetic resonance, melting point analysis, and optical rotation. Induction of PCD in T. cruzi epimastigotes and trypomastigotes by DhL, Hln, Bz and Nx was assayed by phosphatidylserine exposure at the parasite surface and by detection of DNA fragmentation using the TUNEL assay. Trypanocidal activity of natural and synthetic compounds was assayed by measuring parasite viability using the MTT method.

RESULTS

The two natural STLs, DhL and Hln, induce programmed cell death in both, the replicative epimastigote form and the infective trypomastigote form of T. cruzi. Interestingly, the two conventional antichagasic drugs (Bz and Nx) do not induce programmed cell death. A combination of DhL and either Bz or Nx showed an increased effect of natural compounds and synthetic drugs on the decrease of parasite viability.

CONCLUSION

DhL and Hln induce programmed cell death in T. cruzi replicative epimastigote and infective trypomastigote forms, which is a different mechanism of action than the conventional drugs to kill the parasite. Therefore DhL and Hln may offer an interesting option for the treatment of Chagas disease, alone or in combination with conventional drugs.

Trypanosoma cruzi antioxidant enzymes as virulence factors in Chagas disease

SIGNIFICANCE

Chagas disease (CD) affects several million people in Latin America and is spreading beyond its classical boundaries due to the migration of infected host and insect vectors, HIV co-infection, and blood transfusion. The current therapy is not adequate for treatment of the chronic phase of CD, and new drugs are warranted.

RECENT ADVANCES

Trypanosoma cruzi is equipped with a specialized and complex network of antioxidant enzymes that are located at different subcellular compartments which defend the parasite against host oxidative assaults. Recently, strong evidence has emerged which indicates that enzyme components of the T. cruzi antioxidant network (cytosolic and mitochondrial peroxiredoxins and trypanothione synthetase) in naturally occurring strains act as a virulence factor for CD. This precept is recapitulated with the observed increased resistance of T. cruzi peroxirredoxins overexpressers to in vivo or in vitro nitroxidative stress conditions. In addition, the modulation of mitochondrial superoxide radical levels by iron superoxide dismutase (FeSODA) influences parasite programmed cell death, underscoring the role of this enzyme in parasite survival.

CRITICAL ISSUES

The unraveling of the biological significance of FeSODs in T. cruzi programmed cell death in the context of chronic infection in CD is still under examination.

FUTURE DIRECTIONS

The role of the antioxidant enzymes in the pathogenesis of CD, including parasite virulence and persistence, and their feasibility as pharmacological targets justifies further investigation.

Combined approaches for drug design points the way to novel proline racemase inhibitor candidates to fight Chagas' disease

Chagas' disease is caused by Trypanosoma cruzi, a protozoan transmitted to humans by blood-feeding insects, blood transfusion or congenitally. Previous research led us to discover a parasite proline racemase (TcPRAC) and to establish its validity as a target for the design of new chemotherapies against the disease, including its chronic form. A known inhibitor of proline racemases, 2-pyrrolecarboxylic acid (PYC), is water-insoluble. We synthesized soluble pyrazole derivatives, but they proved weak or inactive TcPRAC inhibitors. TcPRAC catalytic site is too small and constrained when bound to PYC to allow efficient search for new inhibitors by virtual screening. Forty-nine intermediate conformations between the opened enzyme structure and the closed liganded one were built by calculating a transition path with a method we developed. A wider range of chemical compounds could dock in the partially opened intermediate active site models in silico. Four models were selected for known substrates and weak inhibitors could dock in them and were used to screen chemical libraries. Two identified soluble compounds, (E)-4-oxopent-2-enoic acid (OxoPA) and its derivative (E)-5-bromo-4-oxopent-2-enoic acid (Br-OxoPA), are irreversible competitive inhibitors that presented stronger activity than PYC on TcPRAC. We show here that increasing doses of OxoPA and Br-OxoPA hamper T. cruzi intracellular differentiation and fate in mammalian host cells. Our data confirm that through to their binding mode, these molecules are interesting and promising as lead compounds for the development of chemotherapies against diseases where active proline racemases play essential roles.

Cell death and serum markers of collagen metabolism during cardiac remodeling in Cavia porcellus experimentally infected with Trypanosoma cruzi

We studied cell death by apoptosis and necrosis in cardiac remodeling produced by Trypanosoma cruzi infection. In addition, we evaluated collagen I, III, IV (CI, CIII and CIV) deposition in cardiac tissue, and their relationship with serum levels of procollagen type I carboxy-terminal propeptide (PICP) and procollagen type III amino-terminal propeptide (PIIINP). Eight infected and two uninfected guinea pigs were necropsied at seven time points up to one year post-infection. Cell death by necrosis and apoptosis was determined by histopathological observation and terminal deoxynucleotidyl transferase dUTP nick end labeling, respectively. Deposition of cardiac collagen types was determined by immunohistochemistry and serum levels of PICP, PIIINP, and anti-T. cruzi IgG1 and IgG2 by ELISA. IgG2 (Th1 response) predominated throughout the course of infection; IgG1 (Th2 response) was detected during the chronic phase. Cardiac cell death by necrosis predominated over apoptosis during the acute phase; during the chronic phase, both apoptosis and necrosis were observed in cardiac cells. Apoptosis was also observed in lymphocytes, endothelial cells and epicardial adipose tissue, especially in the chronic phase. Cardiac levels of CI, CIII, CIV increased progressively, but the highest levels were seen in the chronic phase and were primarily due to increase in CIII and CIV. High serum levels of PICP and PIIINP were observed throughout the infection, and increased levels of both biomarkers were associated with cardiac fibrosis (p = 0.002 and p = 0.038, respectively). These results confirm the role of apoptosis in cell loss mainly during the chronic phase and the utility of PICP and PIIINP as biomarkers of fibrosis in cardiac remodeling during T. cruzi infection.

Chronic Chagas heart disease (CHHD) caused by the infection with the parasite Trypanosoma cruzi is the most important infectious heart disease in the world. The typical manifestations are dilated cardiomyopathy and congestive heart failure; they result from death of cardiomyocytes and their replacement by collagen. Knowing the mechanisms of cardiomyocyte death is important for the development of therapies that prevent them. The contribution of apoptosis in cardiomyocyte death was evaluated in the guinea pig model of T. cruzi infection, and the detection of serum levels of collagen precursors were evaluated as biomarkers of cardiac fibrosis. We observed apoptosis of lymphocytes, cardiomyocytes, endothelial cells and epicardial adipose tissue in cardiac tissue of infected guinea pigs. The increase of serum levels of collagen precursors PICP and PIIINP were associated with cardiac fibrosis. Areas of inflammation and apoptosis of epicardial adipose tissue were associated with cardiac pathology, which suggests the importance of epicardial adipose tissue in CCHD. These results show that apoptosis is an important characteristic of cardiac cell death during CCHD and serum levels of PICP and PIIINP could be used as biomarkers of cardiac fibrosis.

Challenge of chronically infected mice with homologous trypanosoma cruzi parasites enhances the immune response but does not modify cardiopathy: implications for the design of a therapeutic vaccine

Chagas disease is a Trypanosoma cruzi-induced zoonosis that has no natural cure. Local damage induced by the parasite and the immune response causes chronic heart and digestive lesions. Efforts to develop a therapeutic vaccine that boosts the immune response to completely clear the parasite are needed because there is no effective treatment for chronically infected patients. In an attempt to modify the host-parasite equilibrium to increase parasite destruction, we analyzed cardiopathy and the immune response in chronically infected mice that were challenged with live homologous parasites. Challenge with a single dose of parasite increased CD4(+) and CD8(+) T cell populations, gamma interferon (IFN-γ) production, and serum-specific IgG levels. However, subpatent parasitemias and cardiac tissue were not affected. Because of the short duration of the immune boost after a single challenge, we next evaluated the impact of four parasite doses, administered 3 weeks apart. At 1 to 2 months after the last dose, the numbers of CD4(+) T cells and IFN-γ-producing CD4(+) memory cells and the CD4(+) T cell proliferative response to T. cruzi antigen were increased in the spleen. The frequency of IFN-γ-producing CD8(+) memory cells in the blood was also increased. However, the sustained challenge did not favor TH1 development; rather, it induced an increase in serum-specific IgG1 levels and mixed TH1/TH2 cytokine production. Moreover, there were no significant changes in cardiac lesions and subpatent parasitemias. In conclusion, we believe that this study may help in elucidating the necessary elements for a successful therapeutic vaccine which may reduce cardiomyopathy in chronically infected human patients.

Current understanding of the Trypanosoma cruzi-cardiomyocyte interaction

Trypanosoma cruzi, the etiological agent of Chagas disease, exhibits multiple strategies to ensure its establishment and persistence in the host. Although this parasite has the ability to infect different organs, heart impairment is the most frequent clinical manifestation of the disease. Advances in knowledge of T. cruzi-cardiomyocyte interactions have contributed to a better understanding of the biological events involved in the pathogenesis of Chagas disease. This brief review focuses on the current understanding of molecules involved in T. cruzi-cardiomyocyte recognition, the mechanism of invasion, and on the effect of intracellular development of T. cruzi on the structural organization and molecular response of the target cell.

Oxidative stress fuels Trypanosoma cruzi infection in mice

Oxidative damage contributes to microbe elimination during macrophage respiratory burst. Nuclear factor, erythroid-derived 2, like 2 (NRF2) orchestrates antioxidant defenses, including the expression of heme-oxygenase-1 (HO-1). Unexpectedly, the activation of NRF2 and HO-1 reduces infection by a number of pathogens, although the mechanism responsible for this effect is largely unknown. We studied Trypanosoma cruzi infection in mice in which NRF2/HO-1 was induced with cobalt protoporphyrin (CoPP). CoPP reduced parasitemia and tissue parasitism, while an inhibitor of HO-1 activity increased T. cruzi parasitemia in blood. CoPP-induced effects did not depend on the adaptive immunity, nor were parasites directly targeted. We also found that CoPP reduced macrophage parasitism, which depended on NRF2 expression but not on classical mechanisms such as apoptosis of infected cells, induction of type I IFN, or NO. We found that exogenous expression of NRF2 or HO-1 also reduced macrophage parasitism. Several antioxidants, including NRF2 activators, reduced macrophage parasite burden, while pro-oxidants promoted it. Reducing the intracellular labile iron pool decreased parasitism, and antioxidants increased the expression of ferritin and ferroportin in infected macrophages. Ferrous sulfate reversed the CoPP-induced decrease in macrophage parasite burden and, given in vivo, reversed their protective effects. Our results indicate that oxidative stress contributes to parasite persistence in host tissues and open a new avenue for the development of anti-T. cruzi drugs.

The Trypanosoma cruzi genome; conserved core genes and extremely variable surface molecule families

The protozoan parasite Trypanosoma cruzi is an important but neglected pathogen that causes chagas disease, which affects millions of people, mainly in latin America. The population structure and epidemiology of the parasite are complex, with much variability among strains. The genome sequence of a reference strain, CL Brener, was published in 2005, and the availability of this sequence has both revealed the complexity of the parasite genome and greatly facilitated research into parasite biology and pathogenesis, by making the sequences of more than 8000 core genes available. The T. cruzi genome is highly repetitive, which has resulted in inaccuracies in the genome sequence, and attempts have been made to provide a deeper analysis of repeated genes as a complement to the genome sequence. The genome was found to be organized in stable core regions containing housekeeping and other genes, surrounded by highly repetitive, often sub-telomeric highly variable regions containing multiple members of large families of surface molecule genes. Comparative sequencing of T. cruzi strains has been initiated and the results show that the core gene content of the parasite is highly conserved, but that much sequence variability, 3-4% difference at the DNA level on average between strains in coding regions, is present. The additional genomes will improve the understanding of parasite biology and epidemiology.

Shotgun sequencing analysis of Trypanosoma cruzi I Sylvio X10/1 and comparison with T. cruzi VI CL Brener

Trypanosoma cruzi is the causative agent of Chagas disease, which affects more than 9 million people in Latin America. We have generated a draft genome sequence of the TcI strain Sylvio X10/1 and compared it to the TcVI reference strain CL Brener to identify lineage-specific features. We found virtually no differences in the core gene content of CL Brener and Sylvio X10/1 by presence/absence analysis, but 6 open reading frames from CL Brener were missing in Sylvio X10/1. Several multicopy gene families, including DGF, mucin, MASP and GP63 were found to contain substantially fewer genes in Sylvio X10/1, based on sequence read estimations. 1,861 small insertion-deletion events and 77,349 nucleotide differences, 23% of which were non-synonymous and associated with radical amino acid changes, further distinguish these two genomes. There were 336 genes indicated as under positive selection, 145 unique to T. cruzi in comparison to T. brucei and Leishmania. This study provides a framework for further comparative analyses of two major T. cruzi lineages and also highlights the need for sequencing more strains to understand fully the genomic composition of this parasite.

Chagas disease is a major health problem in Latin America and it is caused by the protozoan parasite Trypanosoma cruzi. The genome sequence of the T. cruzi strain CL Brener (TcVI) has revealed a genome with large repertoires of genes for surface antigens, among other features. In the present study, we sequenced the genome of a representative member of TcI, the predominant agent of Chagas disease North of the Amazon and performed comparative analyses with CL Brener. Genetic variation between strains can potentially explain differences in disease pathogenesis, host preferences and aid the identification of drug targets. Our analysis showed that the two genomes have very similar sets of genes, but contain large differences in the relative size of several important gene families. Moreover, an abundance of allelic sequence variation was found in a large fraction of genes, and an evolutionary analysis indicated that many genes have evolved at different rates.

Trypanosoma cruzi infection induces a global host cell response in cardiomyocytes

Chagas' disease, caused by the hemoflagellate protozoan Trypanosoma cruzi, affects millions of people in South and Central America. Chronic chagasic cardiomyopathy, the most devastating manifestation of this disease, occurs in approximately one-third of infected individuals. Events associated with the parasite's tropism for and invasion of cardiomyocytes have been the focus of intense investigation in recent years. In the present study, we use murine microarrays to investigate the cellular response caused by invasion of primary murine cardiomyocytes by T. cruzi trypomastigotes. These studies identified 353 murine genes that were differentially expressed during the early stages of invasion and infection of these cells. Genes associated with the immune response, inflammation, cytoskeleton organization, cell-cell and cell-matrix interactions, apoptosis, cell cycle, and oxidative stress are among those affected during the infection. Our data indicate that T. cruzi induces broad modulations of the host cell machinery in ways that provide insight into how the parasite survives, replicates, and persists in the infected host and ultimately defines the clinical outcome of the infection.

Evasion of immune responses by Trypanosoma cruzi, the etiological agent of Chagas disease

Infection with the protozoan parasite Trypanosoma cruzi leads to Chagas disease, which affects millions of people in Latin America. Infection with T. cruzi cannot be eliminated by the immune system. A better understanding of immune evasion mechanisms is required in order to develop more effective vaccines. During the acute phase, parasites replicate extensively and release immunomodulatory molecules that delay parasite-specific responses mediated by T cells. This immune evasion allows the parasite to spread in the host. In the chronic phase, parasite evasion relies on its replication strategy of hijacking the TGF-β signaling pathway involved in inflammation and tissue regeneration. In this article, the mechanisms of immune evasion described for T. cruzi are reviewed.

Impact of protozoan cell death on parasite-host interactions and pathogenesis

PCD in protozoan parasites has emerged as a fascinating field of parasite biology. This not only relates to the underlying mechanisms and their evolutionary implications but also to the impact on the parasite-host interactions within mammalian hosts and arthropod vectors. During recent years, common functions of apoptosis and autophagy in protozoa and during parasitic infections have emerged. Here, we review how distinct cell death pathways in Trypanosoma, Leishmania, Plasmodium or Toxoplasma may contribute to regulation of parasite cell densities in vectors and mammalian hosts, to differentiation of parasites, to stress responses, and to modulation of the host immunity. The examples provided indicate crucial roles of PCD in parasite biology. The existence of PCD pathways in these organisms and the identification as being critical for parasite biology and parasite-host interactions could serve as a basis for developing new anti-parasitic drugs that take advantage of these pathways.

Apoptosis induced by parasitic diseases

Fatalities caused by parasitic infections often occur as a result of tissue injury that results from a form of host-cell death known as apoptosis. However, instead of being pathogenic, parasite-induced apoptosis may facilitate host survival. Consequently, it is of utmost importance to decipher and understand the process and the role of apoptosis induced or controlled by parasites in humans. Despite this, few studies provide definitive knowledge of parasite-induced host-cell apoptosis. Here, the focus is on a consideration of host-cell apoptosis as either a pathogenic feature or as a factor enabling parasite survival and development.

Cell death by apoptotic-like mechanisms could be described as a ride to death with a return ticket, as initiation of the pathway may be reversed, with the potential that it could be manipulated for therapeutic purposes. The management of host-cell apoptosis could thus be an adjunctive factor for parasitic disease treatment. Evidence that the apoptotic process could be reversed by anti-apoptotic drugs has recently been obtained, leading to the possibility of host-cell rescue after injury. An important issue will be to predict the beneficial or deleterious effects of controlling human cell death by apoptotic-like mechanisms during parasitic diseases.

Differential apoptosis-like cell death in amastigote and trypomastigote forms from Trypanosoma cruzi-infected heart cells in vitro

Apoptosis, type-I of programmed cell death (PCD-I), is not restricted to multicellular organisms since many apoptotic features have been described in different trypanosomatids, including Trypanosoma cruzi. Our present aim was to monitor, by different morphological markers, the occurrence of apoptosis-like death in amastigotes and trypomastigotes of T.cruzi (Y strain) during the infection of heart culture cells. We documented the differential occurrence of PCD-I in amastigotes and trypomastigotes, with distinct death rates noticed between these two parasite-distinct forms. Fluorescence microscopy and flow cytometry analysis using different hall markers of apoptosis (phosphatidylserine exposure, collapse of mitochondrial membrane potential and DNA fragmentation) showed that amastigotes present higher levels of apoptosis-like cell death as compared to trypomastigotes. It is possible that the higher levels of PCD-I in these highly multiplicative forms may contribute to the control of the parasite burden within the host cells. On the other hand, the apoptosis-like occurrence in the infective but non-proliferative stage of the parasite (trypomastigotes) may play a role in parasite evasion mechanisms as suggested for other parasites.

Infection by the Sylvio X10/4 clone of Trypanosoma cruzi: relevance of a low-virulence model of Chagas' disease

The physiopathology of Chagas' disease has been largely defined in murine infections with virulent strains which partially represent parasite diversity. This report reviews our studies with Sylvio X10/4 parasites, a Trypanosoma cruzi clone that induces no acute phase but in C3H/He mice leads to chronic myocarditis resembling the human disease.

Pivotal role for TGF-beta in infectious heart disease: The case of Trypanosoma cruzi infection and consequent Chagasic myocardiopathy

This paper summarizes recent data from the literature suggesting that transforming growth factor-beta (TGF-beta) participates at least in four different processes influencing development of myocardiopathy in Chagas disease, a major parasitic illness caused by Trypanosoma cruzi infection: (a) invasion of cardiac fibroblasts and myocytes; (b) intracellular parasite cycle; (c) regulation of inflammation and immune response; (d) fibrosis and heart remodeling during acute and chronic disease. All these effects point to an important role of TGF-beta in Chagas disease myocardiopathy and suggest that monitoring the circulating levels of this cytokine could be of help in clinical prognosis and management of patients. Moreover, TGF-beta-interfering therapies appear as interesting adjuvant interventions during acute and chronic phases of T. cruzi infection.

Agglutination of Trypanosoma cruzi in infected cells treated with serum from chronically infected mice

The protozoan parasite Trypanosoma cruzi is the causative agent of Chagas disease. The chronic stage of infection is characterized by a production of neutralizing antibodies in the vertebrate host. A polyclonal antibody, anti-egressin, has been found to inhibit egress of parasites from the host cell late in the intracellular cycle, after the parasites have transformed from the replicative amastigote into the trypomastigote. It has also been found that BALB/c mouse fibroblasts in the late stages of parasite infection become permeable to molecules as large as antibodies, leading to the possibility that anti-egressin affects the intracellular parasites. This project addresses the fate of the intracellular trypomastigotes that have been inhibited from egressing the host cell. Extended cultures of infected fibroblasts treated with chronic mouse serum reduced parasite egress at all time points measured. Parasites released from infected fibroblasts treated with chronic serum had a reduced ability to infect fibroblasts in culture, yet did not lose infectivity entirely. Absorption of chronic serum with living trypomastigotes removed the anti-egressin effect. The possibility that the target of anti-egressin is a parasite surface component is further indicated by the agglutination of extracellular trypomastigotes by chronic serum. The possibility that cross-linking by antibody occurs intracellularly, thus inhibiting egress, was reinforced by cleaving purified IgG into Fab fragments, which did not inhibit egress when added to infected cultures. From this work, it is proposed that the current, best explanation of the mechanism of egress inhibition by anti-egressin is intracellular agglutination, preventing normal parasite-driven egress.

Fas ligand-dependent inflammatory regulation in acute myocarditis induced by Trypanosoma cruzi infection

Fas/Fas ligand (Fas-L) engagement, a potent inducer of apoptosis, is also important for cellular activation, regulation of effector and chemotactic activity, and secretion of chemokines and cytokines. We evaluated the relevance of Fas/Fas-L in the regulation of myocarditis induced by Trypanosoma cruzi infection and observed that in Fas-L(-/-) mice (gld/gld), cardiac infiltration was significantly reduced, accordingly showing less cardiomyocyte destruction. Fluorescence-activated cell sorting analysis of cardiac inflammatory cells showed higher numbers of CD8(+) T cells in BALB/c compared with gld/gld mice but similar levels of lymphocyte function-associated antigen-1, intercellular adhesion molecule, CD2, and CD69 expression; MAC-1(+) myeloid cells and mast cells were increased in BALB/c mice, whereas gld/gld mice exhibited an enrichment of CD4(+/low) T cells. Intracellular labeling of cytokines revealed no clear cardiac skewing of Th1 or Th2 responses, but we found a higher number of interleukin-10(+) cells in gld/gld mice and a deficient expression of vascular cell adhesion molecule-1 on cardiac endothelial cells in gld/gld mice. Finally, we found a population of CD3(+) but CD4/CD8 double negative cardiac T cells in both groups of infected mice, but down-regulation of some adhesion molecules and surface receptors was only observed in gld/gld mice, indicating a targeted T-cell population mostly affected by the lack of Fas-L engagement. These results point to a role for myocarditis regulation by Fas/Fas-L beyond its possible direct relevance in cellular death.

SB-431542, a transforming growth factor beta inhibitor, impairs Trypanosoma cruzi infection in cardiomyocytes and parasite cycle completion

The antiinflammatory cytokine transforming growth factor beta (TGF-beta) plays an important role in Chagas disease, a parasitic infection caused by the protozoan Trypanosoma cruzi. In the present study, we show that SB-431542, an inhibitor of the TGF-beta type I receptor (ALK5), inhibits T. cruzi-induced activation of the TGF-beta pathway in epithelial cells and in cardiomyocytes. Further, we demonstrate that addition of SB-431542 greatly reduces cardiomyocyte invasion by T. cruzi. Finally, SB-431542 treatment significantly reduces the number of parasites per infected cell and trypomastigote differentiation and release. Taken together, these data further confirm the major role of the TGF-beta signaling pathway in both T. cruzi infection and T. cruzi cell cycle completion. Our present data demonstrate that small inhibitors of the TGF-beta signaling pathway might be potential pharmacological tools for the treatment of Chagas disease.

Mitochondrial superoxide radicals mediate programmed cell death in Trypanosoma cruzi: cytoprotective action of mitochondrial iron superoxide dismutase overexpression

Trypanosoma cruzi undergo PCD (programmed cell death) under appropriate stimuli, the mechanisms of which remain to be established. In the present study, we show that stimulation of PCD in T. cruzi epimastigotes by FHS (fresh human serum) results in rapid (<1 h) externalization of phosphatidylserine and depletion of the low molecular mass thiols dihydrotrypanothione and glutathione. Concomitantly, enhanced generation of oxidants was established by EPR and immuno-spin trapping of radicals using DMPO (5,5-dimethylpyrroline-N-oxide) and augmentation of the glucose flux through the pentose phosphate pathway. In the early period (<20 min), changes in mitochondrial membrane potential and inhibition of respiration, probably due to the impairment of ADP/ATP exchange with the cytosol, were observed, conditions that favour the generation of O2*-. Accelerated rates of mitochondrial O2*- production were detected by the inactivation of the redox-sensitive mitochondrial aconitase and by oxidation of a mitochondrial-targeted probe (MitoSOX). Importantly, parasites overexpressing mitochondrial FeSOD (iron superoxide dismutase) were more resistant to the PCD stimulus, unambiguously indicating the participation of mitochondrial O2*- in the signalling process. In summary, FHS-induced PCD in T. cruzi involves mitochondrial dysfunction that causes enhanced O(2)(*-) formation, which leads to cellular oxidative stress conditions that trigger the initiation of PCD cascades; moreover, overexpression of mitochondrial FeSOD, which is also observed during metacyclogenesis, resulted in cytoprotective effects.

Mode of action of natural and synthetic drugs against Trypanosoma cruzi and their interaction with the mammalian host

Current knowledge of the biochemistry of Trypanosoma cruzi has led to the development of new drugs and the understanding of their mode of action. Some trypanocidal drugs such as nifurtimox and benznidazole act through free radical generation during their metabolism. T. cruzi is very susceptible to the cell damage induced by these metabolites because enzymes scavenging free radicals are absent or have very low activities in the parasite. Another potential target is the biosynthetic pathway of glutathione and trypanothione, the low molecular weight thiol found exclusively in trypanosomatids. These thiols scavenge free radicals and participate in the conjugation and detoxication of numerous drugs. Inhibition of this key pathway could render the parasite much more susceptible to the toxic action of drugs such as nifurtimox and benznidazole without affecting the host significantly. Other drugs such as allopurinol and purine analogs inhibit purine transport in T. cruzi, which cannot synthesize purines de novo. Nitroimidazole derivatives such as itraconazole inhibit sterol metabolism. The parasite's respiratory chain is another potential therapeutic target because of its many differences with the host enzyme complexes. The pharmacological modulation of the host's immune response against T. cruzi infection as a possible chemotherapeutic target is discussed. A large set of chemicals of plant origin and a few animal metabolites active against T. cruzi are enumerated and their likely modes of action are briefly discussed.

Cross-talk between apoptosis and cytokines in the regulation of parasitic infection

Parasitic diseases have worldwide medical and economical impact. Host T lymphocytes and the cytokines they produce determine the outcome of parasitic infections. Programmed cell death by apoptosis is induced in the course of parasitic infections, and affects cytokine production by removing activated effector T and B cells. In addition, engulfment of apoptotic cells promotes the secretion of cytokines that regulate intracellular replication of protozoan parasites. In this review, we discuss how the cross-talk between apoptosis and cytokines regulates parasitic infection.

Mechanisms of pathogenesis in Chagas disease

Chagas disease, caused by the obligate unicellular parasite Trypanosoma cruzi, presents itself in a diverse collection of clinical manifestations, ranging from severe, fatal heart and digestive tract pathologies to unapparent or minor alterations that do not compromise survival. Over the years, a number of mechanisms have been proposed to explain the pathogenesis of chagasic tissue lesions, all of which have faced some criticism or been received with skepticism. This article excludes the autoimmunity hypothesis for Chagas disease because it has been extensively reviewed elsewhere, and summarizes the various alternative hypotheses that have been advanced over the years. For each of these hypotheses, an outline of its main tenets and key findings that support them is presented. This is followed by the results and comments that have challenged them and the caveats that stand on their way to wider acceptance. It is hoped that this writing will draw attention to our shortcomings in understanding the pathogenesis of Chagas disease, which, unfortunately, continues to figure among the most serious health problems of the American continent.

Programmed cell death in African trypanosomes

Until recently it had generally been assumed that apoptosis and other forms of programmed cell death evolved during evolution of the metazoans to regulate growth and development in these multicellular organisms. However, recent research is adding strength to the original phenotypic observations described almost a decade ago which indicated that some parasitic protozoa may have evolved a cell death pathway analogous to the process described as apoptosis in metazoa. Here we explore the implications of a programmed cell death pathway in the African tsetse-transmitted trypanosomes.

Interaction of natural killer cells with Trypanosoma cruzi-infected fibroblasts

The protozoan parasite Trypanosoma cruzi circulates in the blood as trypomastigotes and invades a variety of cells to multiply intracellularly as amastigotes. The acute phase triggers an immune response that restricts the proliferation of the parasite. However, parasites are able to persist in different tissues causing the pathology of Chagas' disease. Natural killer (NK) cells play an important role in innate resistance to a variety of pathogens. In the present study we demonstrate that NK cells trigger trypanocidal mechanisms in infected L929 cells that are critically dependent on inducible nitric oxide (NO) synthase (iNOS) induction which is, to a major degree, triggered by interferon (IFN)-gamma provided by NK cells. This work provides a more detailed analysis of how NK cells as a part of the innate immune system participate in the control of parasites that reside intracellularly in fibroblast-like L929 cells.