Negative tissue parasitism in mice injected with a noninfective clone of Trypanosoma cruzi

Gene expression network analyses during infection with virulent and avirulent Trypanosoma cruzi strains unveil a role for fibroblasts in neutrophil recruitment and activation

Chagas disease is caused by Trypanosoma cruzi, a protozoan parasite that has a heterogeneous population composed of a pool of strains with distinct characteristics, including variable levels of virulence. In previous work, transcriptome analyses of parasite genes after infection of human foreskin fibroblasts (HFF) with virulent (CL Brener) and non-virulent (CL-14) clones derived from the CL strain, revealed a reduced expression of genes encoding parasite surface proteins in CL-14 compared to CL Brener during the final steps of the intracellular differentiation from amastigotes to trypomastigotes. Here we analyzed changes in the expression of host genes during in vitro infection of HFF cells with the CL Brener and CL-14 strains by analyzing total RNA extracted from cells at 60 and 96 hours post-infection (hpi) with each strain, as well as from uninfected cells. Similar transcriptome profiles were observed at 60 hpi with both strains compared to uninfected samples. However, at 96 hpi, significant differences in the number and expression levels of several genes, particularly those involved with immune response and cytoskeleton organization, were observed. Further analyses confirmed the difference in the chemokine/cytokine signaling involved with the recruitment and activation of immune cells such as neutrophils upon T. cruzi infection. These findings suggest that infection with the virulent CL Brener strain induces a more robust inflammatory response when compared with the non-virulent CL-14 strain. Importantly, the RNA-Seq data also exposed an unexplored role of fibroblasts as sentinel cells that may act by recruiting neutrophils to the initial site of infection. This role for fibroblasts in the regulation of the inflammatory response during infection by T. cruzi was corroborated by measurements of levels of different chemokines/cytokines during in vitro infection and in plasma from Chagas disease patients as well as by neutrophil activation and migration assays.

Trypanosoma cruzi is the causative agent of Chagas disease, a debilitating and often life-threatening illness that affects 6 to 7 million people mainly in Latin America. The parasite, transmitted to humans by an insect vector, needs to invade different cells from the infected person in order to multiply and spread the infection to various organs, including the heart and the gut. In this study, we investigated how the host cell responds to the infection by analyzing changes in the expression of human genes in fibroblasts infected with the CL Brener and CL-14 strains, which are strains that present highly distinct virulent phenotypes during infection in mice. We showed that human fibroblasts build a strong immune response upon infection by T. cruzi and that this response is different depending on the parasite strain: infection with the virulent CL Brener strain induces a more robust inflammatory response compared with the infection with the avirulent CL-14 strain. We also showed that, in response to the infection, fibroblasts produce molecules that can recruit and activate neutrophils, which are important immune cells that controls the infection.

New Vaccine Formulations Containing a Modified Version of the Amastigote 2 Antigen and the Non-Virulent Trypanosoma cruzi CL-14 Strain Are Highly Antigenic and Protective against Leishmania infantum Challenge

Visceral leishmaniasis (VL) is a major public health issue reported as the second illness in mortality among all tropical diseases. Clinical trials have shown that protection against VL is associated with robust T cell responses, especially those producing IFN-γ. The Leishmania amastigote 2 (A2) protein has been repeatedly described as immunogenic and protective against VL in different animal models; it is recognized by human T cells, and it is also commercially available in a vaccine formulation containing saponin against canine VL. Moving toward a more appropriate formulation for human vaccination, here, we tested a new optimized version of the recombinant protein (rA2), designed for Escherichia coli expression, in combination with adjuvants that have been approved for human use. Moreover, aiming at improving the cellular immune response triggered by rA2, we generated a recombinant live vaccine vector using Trypanosoma cruzi CL-14 non-virulent strain, named CL-14 A2. Mice immunized with respective rA2, adsorbed in Alum/CpG B297, a TLR9 agonist recognized by mice and human homologs, or with the recombinant CL-14 A2 parasites through homologous prime-boost protocol, were evaluated for antigen-specific immune responses and protection against Leishmania infantum promastigote challenge. Immunization with the new rA2/Alum/CpG formulations and CL-14 A2 transgenic vectors elicited stronger cellular immune responses than control groups, as shown by increased levels of IFN-γ, conferring protection against L. infantum challenge. Interestingly, the use of the wild-type CL-14 alone was enough to boost immunity and confer protection, confirming the previously reported immunogenic potential of this strain. Together, these results support the success of both the newly designed rA2 antigen and the ability of T. cruzi CL-14 to induce strong T cell-mediated immune responses against VL in animal models when used as a live vaccine vector. In conclusion, the vaccination strategies explored here reveal promising alternatives for the development of new rA2 vaccine formulations to be translated human clinical trials.

Comparative transcriptome profiling of virulent and non-virulent Trypanosoma cruzi underlines the role of surface proteins during infection

Trypanosoma cruzi, the protozoan that causes Chagas disease, has a complex life cycle involving several morphologically and biochemically distinct stages that establish intricate interactions with various insect and mammalian hosts. It has also a heterogeneous population structure comprising strains with distinct properties such as virulence, sensitivity to drugs, antigenic profile and tissue tropism. We present a comparative transcriptome analysis of two cloned T. cruzi strains that display contrasting virulence phenotypes in animal models of infection: CL Brener is a virulent clone and CL-14 is a clone that is neither infective nor pathogenic in in vivo models of infection. Gene expression analysis of trypomastigotes and intracellular amastigotes harvested at 60 and 96 hours post-infection (hpi) of human fibroblasts revealed large differences that reflect the parasite’s adaptation to distinct environments during the infection of mammalian cells, including changes in energy sources, oxidative stress responses, cell cycle control and cell surface components. While extensive transcriptome remodeling was observed when trypomastigotes of both strains were compared to 60 hpi amastigotes, differences in gene expression were much less pronounced when 96 hpi amastigotes and trypomastigotes of CL Brener were compared. In contrast, the differentiation of the avirulent CL-14 from 96 hpi amastigotes to extracellular trypomastigotes was associated with considerable changes in gene expression, particularly in gene families encoding surface proteins such as trans-sialidases, mucins and the mucin associated surface proteins (MASPs). Thus, our comparative transcriptome analysis indicates that the avirulent phenotype of CL-14 may be due, at least in part, to a reduced or delayed expression of genes encoding surface proteins that are associated with the transition of amastigotes to trypomastigotes, an essential step in the establishment of the infection in the mammalian host. Confirming the role of members of the trans-sialidase family of surface proteins for parasite differentiation, transfected CL-14 constitutively expressing a trans-sialidase gene displayed faster kinetics of trypomastigote release in the supernatant of infected cells compared to wild type CL-14.

Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, is an infection that occurs in several Latin American countries, resulting in a mild illness or in severe damage of the heart and intestinal tract. Such a broad spectrum of clinical manifestations observed in Chagas disease patients is likely due to differences in host susceptibility as well as to a large heterogeneity among T. cruzi isolates. The identification of virulence factors that are differentially expressed in the parasite population is a valuable strategy for understanding of the distinct interactions that occur between this pathogen and its host, which may or may not lead to pathogenesis. By comparing the gene expression profiles of two T. cruzi strains that display contrasting virulence phenotypes in animal models of infection, we identified a central role for genes encoding surface proteins that is associated with the differentiation from intracellular replicative amastigotes to infective trypomastigotes. We showed that the expression of these genes occurs differentially within the two strains and this difference may be a factor that impacts parasite survival and dissemination in the mammalian host.

Aspirin treatment exacerbates oral infections by Trypanosoma cruzi

Oral transmission of the protozoan parasite Trypanosoma cruzi, the etiological agent of Chagas disease, has been documented in Latin American countries. The reported cases of infection were due to the ingestion of contaminated fresh fruit, juices, or sugar cane juice. There have been few studies on the physiopathology of the disease in oral transmission cases. Gastritis is a common ailment that can be caused by poor dietary habits, intake of alcohol or other gastric irritants, bacterial infection, or by the widespread use of non-steroidal anti-inflammatory drugs (NSAIDs). This study investigated in a mouse model whether gastric mucosal injury, induced by aspirin, would affect the course of disease in animals infected with T. cruzi by the oral route. The CL14 and G strains of T. cruzi, both of low infectivity, were used. To this end, groups of BALB/c mice were treated during 5 days with aspirin (100 mg kg(-1)) before oral infection with T. cruzi metacyclic forms (4 × 10(5) or 5 × 10(7) parasites/mouse). Histological analysis and determination of nitric oxide and TNF-α were performed in gastric samples obtained 5 days after infection. Parasitemia was monitored from the thirteenth day after infection. The results indicate that aspirin treatment of mice injured their gastric mucosa and facilitated invasion by both CL14 and G strains of T. cruzi. Strain CL14 caused more severe infection compared to the G strain, as larger numbers of amastigote nests were found in the stomach and parasitemia levels were higher. Our study is novel in that it shows that gastric mucosal damage caused by aspirin, a commonly used NSAID, facilitates T. cruzi infection by the oral route.

Vesicles from different Trypanosoma cruzi strains trigger differential innate and chronic immune responses

Trypomastigote forms of Trypanosoma cruzi, the causative agent of Chagas Disease, shed extracellular vesicles (EVs) enriched with glycoproteins of the gp85/trans-sialidase (TS) superfamily and other α-galactosyl (α-Gal)-containing glycoconjugates, such as mucins. Here, purified vesicles from T. cruzi strains (Y, Colombiana, CL-14 and YuYu) were quantified according to size, intensity and concentration. Qualitative analysis revealed differences in their protein and α-galactosyl contents. Later, those polymorphisms were evaluated in the modulation of immune responses (innate and in the chronic phase) in C57BL/6 mice. EVs isolated from YuYu and CL-14 strains induced in macrophages higher levels of proinflammatory cytokines (TNF-α and IL-6) and nitric oxide via TLR2. In general, no differences were observed in MAPKs activation (p38, JNK and ERK 1/2) after EVs stimulation. In splenic cells derived from chronically infected mice, a different modulation pattern was observed, where Colombiana (followed by Y strain) EVs were more proinflammatory. This modulation was independent of the T. cruzi strain used in the mice infection. To test the functional importance of this modulation, the expression of intracellular cytokines after in vitro exposure was evaluated using EVs from YuYu and Colombiana strains. Both EVs induced cytokine production with the appearance of IL-10 in the chronically infected mice. A high frequency of IL-10 in CD4+ and CD8+ T lymphocytes was observed. A mixed profile of cytokine induction was observed in B cells with the production of TNF-α and IL-10. Finally, dendritic cells produced TNF-α after stimulation with EVs. Polymorphisms in the vesicles surface may be determinant in the immunopathologic events not only in the early steps of infection but also in the chronic phase.

Blockade of CTLA-4 promotes the development of effector CD8+ T lymphocytes and the therapeutic effect of vaccination with an attenuated protozoan expressing NY-ESO-1

The development of cancer immunotherapy has long been a challenge. Here, we report that prophylactic vaccination with a highly attenuated Trypanosoma cruzi strain expressing NY-ESO-1 (CL-14-NY-ESO-1) induces both effector memory and effector CD8(+) T lymphocytes that efficiently prevent tumor development. However, the therapeutic effect of such a vaccine is limited. We also demonstrate that blockade of Cytotoxic T Lymphocyte Antigen 4 (CTLA-4) during vaccination enhances the frequency of NY-ESO-1-specific effector CD8(+) T cells producing IFN-γ and promotes lymphocyte migration to the tumor infiltrate. As a result, therapy with CL-14-NY-ESO-1 together with anti-CTLA-4 is highly effective in controlling the development of an established melanoma.

Trypanosomatid comparative genomics: Contributions to the study of parasite biology and different parasitic diseases

In 2005, draft sequences of the genomes of Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, also known as the Tri-Tryp genomes, were published. These protozoan parasites are the causative agents of three distinct insect-borne diseases, namely sleeping sickness, Chagas disease and leishmaniasis, all with a worldwide distribution. Despite the large estimated evolutionary distance among them, a conserved core of ~6,200 trypanosomatid genes was found among the Tri-Tryp genomes. Extensive analysis of these genomic sequences has greatly increased our understanding of the biology of these parasites and their host-parasite interactions. In this article, we review the recent advances in the comparative genomics of these three species. This analysis also includes data on additional sequences derived from other trypanosmatid species, as well as recent data on gene expression and functional genomics. In addition to facilitating the identification of key parasite molecules that may provide a better understanding of these complex diseases, genome studies offer a rich source of new information that can be used to define potential new drug targets and vaccine candidates for controlling these parasitic infections.

The genome and its implications

Trypanosoma cruzi has a heterogeneous population composed of a pool of strains that circulate in the domestic and sylvatic cycles. Genome sequencing of the clone CL Brener revealed a highly repetitive genome of about 110Mb containing an estimated 22,570 genes. Because of its hybrid nature, sequences representing the two haplotypes have been generated. In addition, a repeat content close to 50% made the assembly of the estimated 41 pairs of chromosomes quite challenging. Similar to other trypanosomatids, the organization of T. cruzi chromosomes was found to be very peculiar, with protein-coding genes organized in long polycistronic transcription units encoding 20 or more proteins in one strand separated by strand switch regions. Another remarkable feature of the T. cruzi genome is the massive expansion of surface protein gene families. Because of the high genetic diversity of the T. cruzi population, sequencing of additional strains and comparative genomic and transcriptome analyses are in progress. Five years after its publication, the genome data have proven to be an essential tool for the study of T. cruzi and increasing efforts to translate this knowledge into the development of new modes of intervention to control Chagas disease are underway.

Trypanosoma cruzi as an effective cancer antigen delivery vector

One of the main challenges in cancer research is the development of vaccines that induce effective and long-lived protective immunity against tumors. Significant progress has been made in identifying members of the cancer testis antigen family as potential vaccine candidates. However, an ideal form for antigen delivery that induces robust and sustainable antigen-specific T-cell responses, and in particular of CD8(+) T lymphocytes, remains to be developed. Here we report the use of a recombinant nonpathogenic clone of Trypanosoma cruzi as a vaccine vector to induce vigorous and long-term T cell-mediated immunity. The rationale for using the highly attenuated T. cruzi clone was (i) the ability of the parasite to persist in host tissues and therefore to induce a long-term antigen-specific immune response; (ii) the existence of intrinsic parasite agonists for Toll-like receptors and consequent induction of highly polarized T helper cell type 1 responses; and (iii) the parasite replication in the host cell cytoplasm, leading to direct antigen presentation through the endogenous pathway and consequent induction of antigen-specific CD8(+) T cells. Importantly, we found that parasites expressing a cancer testis antigen (NY-ESO-1) were able to elicit human antigen-specific T-cell responses in vitro and solid protection against melanoma in a mouse model. Furthermore, in a therapeutic protocol, the parasites expressing NY-ESO-1 delayed the rate of tumor development in mice. We conclude that the T. cruzi vector is highly efficient in inducing T cell-mediated immunity and protection against cancer cells. More broadly, this strategy could be used to elicit a long-term T cell-mediated immunity and used for prophylaxis or therapy of chronic infectious diseases.

Requirement of UNC93B1 reveals a critical role for TLR7 in host resistance to primary infection with Trypanosoma cruzi

UNC93B1 associates with TLR3, 7, and 9, mediating their translocation from the endoplasmic reticulum to the endolysosome, thus allowing proper activation by microbial nucleic acids. We found that the triple-deficient 3d mice, which lack functional UNC93B1 as well as functional endosomal TLRs, are highly susceptible to infection with Trypanosoma cruzi. The enhanced parasitemia and mortality in 3d animals were associated with impaired proinflammatory response, including reduced levels of IL-12p40 and IFN-γ. Importantly, the phenotype of 3d mice was intermediary between MyD88(-/-) (highly susceptible) and TLR9(-/-) (moderately susceptible), indicating the involvement of an additional UN93B1-dependent TLR(s) on host resistance to T. cruzi. Hence, our experiments also revealed that TLR7 is a critical innate immune receptor involved in recognition of parasite RNA, induction of IL-12p40 by dendritic cells, and consequent IFN-γ by T lymphocytes. Furthermore, we show that upon T. cruzi infection, triple TLR3/7/9(-/-) mice had similar phenotype than 3d mice. These data imply that the nucleic acid-sensing TLRs are critical determinants of host resistance to primary infection with T. cruzi.

The endless race between Trypanosoma cruzi and host immunity: lessons for and beyond Chagas disease

Infection with the protozoan parasite Trypanosoma cruzi, the agent of Chagas disease, is characterised by a variable clinical course - from symptomless cases to severe chronic disease with cardiac and/or gastrointestinal involvement. The variability in disease outcome has been attributed to host responses as well as parasite heterogeneity. In this article, we review studies indicating the importance of immune responses as key determinants of host resistance to T. cruzi infection and the pathogenesis of Chagas disease. Particular attention is given to recent studies defining the role of cognate innate immune receptors and immunodominant CD8+ T cells that recognise parasite components - both crucial for host-parasite interaction and disease outcome. In light of these studies we speculate about parasite strategies that induce a strong and long-lasting T-cell-mediated immunity but at the same time allow persistence of the parasite in the vertebrate host. We also discuss what we have learned from these studies for increasing our understanding of Chagas pathogenesis and for the design of new strategies to prevent the development of Chagas disease. Finally, we highlight recent studies employing a genetically engineered attenuated T. cruzi strain as a vaccine shuttle that elicits potent T cell responses specific to a tumour antigen and protective immunity against a syngeneic melanoma cell line.

Role of the gp85/trans-sialidase superfamily of glycoproteins in the interaction of Trypanosoma cruzi with host structures

Invasion of mammalian cells by T. cruzi trypomastigotes is a multi-step and complex process involving several adhesion molecules, signaling events and proteolytic activities. From the blood to the cell target in different tissues the parasite has to interact with different cells and the extracellular matrix (ECM). The review focus on the role of the gp85/ trans-sialidase superfamily members in the interaction of the parasite with the host cell, particularly with ECM components, with emphasis on the significant variability among the ligands and receptors involved. Use of the SELEX technique to evolve nuclease-resistant RNA aptamers for receptor identification is briefly discussed.

Enucleated L929 cells support invasion, differentiation, and multiplication of Trypanosoma cruzi parasites

Cell infection with Trypanosoma cruzi, the agent of Chagas' disease, begins with the uptake of infective trypomastigotes within phagosomes and their release into the cytosol, where they transform into replicating amastigotes; the latter, in turn, differentiate into cytolytically released and infective trypomastigotes. We ask here if the T. cruzi infection program can develop in enucleated host cells. Monolayers of L929 cells, enucleated by centrifugation in the presence of cytochalasin B and kept at 34 degrees C to extend the survival of cytoplasts, were infected with parasites of the CL strain. Percent infection, morphology, stage-specific markers, and numbers of parasites per cell were evaluated in nucleated and enucleated cells, both of which were present in the same preparations. Parasite uptake, differentiation and multiplication of amastigotes, development of epimastigote- and trypomastigote-like forms, and initial cytolytic release of parasites were all documented for cytoplasts and nucleated cells. Although the doubling times were similar, parasite loads at 48 and 72 h were significantly lower in the cytoplasts than in nucleated cells. Similar results were obtained with the highly virulent strain Y as well as with strains CL-14 and G, which exhibit low virulence for mice. Cytoplasts could also be infected with the CL strain 24 or 48 h after enucleation. Thus, infection of cells by T. cruzi can take place in enucleated host cells, i.e., in the absence of modulation of chromosomal and nucleolar gene transcription and of RNA modification and processing in the nucleus.

Molecular basis of non-virulence of Trypanosoma cruzi clone CL-14

We investigated the properties of metacyclic trypomastigotes of non-virulent Trypanosoma cruzi clone CL-14, as compared to the parental isolate CL. In contrast to the CL isolate, which produces high parasitemias in mice, metacyclic forms of clone CL-14 failed to produce patent infection. In vitro, the number of clone CL-14 parasites that entered epithelial HeLa cells, after 1 h incubation, was approximately four-fold lower than that of the CL isolate and at 72 h post-infection intracellular replication was not apparent whereas cells infected with the CL isolate contained large number of parasites replicating as amastigotes. CL isolate metacyclic forms were long and slender, with the kinetoplast localised closer to the nucleus than to the posterior end, whereas clone CL-14 parasites were shorter, with the kinetoplast very close to the posterior end. Cysteine proteinase cruzipain and trans-sialidase activities were lower in CL isolate than in clone CL-14. The surface profile was similar, except that the expression of gp82, the stage-specific glycoprotein that promotes CL isolate mucosal infection in vivo and host cell invasion in vitro, was greatly reduced on the surface of clone CL-14 metacyclic forms. Genistein, a specific inhibitor of protein tyrosine kinase, which is activated in CL isolate by binding of gp82 to its host cell receptor, did not affect host cell entry of clone CL-14. In contrast with CL isolate, the infectivity of clone CL-14 was not affected by phospholipase C inhibitor U73122 but was diminished by a combination of ionomycin plus NH(4)Cl, which releases Ca(2+) from acidic vacuoles. Internalisation of clone CL-14, but not of CL isolate, was significantly increased by treating parasites with neuraminidase, which removes sialic acid from the mucin-like surface molecule gp35/50. Taken together, our data suggest an association between the non-virulence of clone CL-14 metacyclic forms and the reduced expression of gp82, which precludes the activation of signal transduction pathways leading to effective host cell invasion.

Balanced cytokine-producing pattern in mice immunized with an avirulent Trypanosoma cruzi

We have previously demonstrated that inoculation of BALB/c mice with trypomastigotes of CL-14, an avirulent Trypanosoma cruzi clone, prevents the development of parasitemia and mortality after challenge with virulent CL strain. In this report, we investigated the cytokine and antibody profiles induced by inoculation with CL-14 clone. Groups of mice were inoculated with trypomastigotes of CL-14 clone and challenged with infective CL strain. Challenged CL-14-inoculated mice had lower levels of IFN-gamma and higher production of IgG1 antibodies as compared to CL strain-infected mice. Previous inoculation with CL-14 clone partially prevented the suppression of IL-2 production caused by CL strain infection. No significant differences were found regarding IL-4 production by splenocytes from CL-14-inoculated or control groups after challenge with CL-strain. Our results show that protection against acute T. cruzi infection induced by CL-14 inoculation correlates with a balanced T1/T2 cytokine production, a profile likely to be beneficial for the host.

Trypanosoma cruzi sensitizes mice to fulminant SEB-induced shock: overrelease of inflammatory cytokines and independence of Chagas' disease or TCR Vbeta-usage

Trypanosoma cruzi-infected mice display increased susceptibility to shock induced by injection of lipopolysaccharide (LPS), anti-CD3, or resulting from interleukin (IL)-10-defective response to the parasite itself, but the basis of such susceptibility remains unknown. Herein, we tested the susceptibility of mice inoculated with virulent and avirulent T. cruzi to staphylococcal enterotoxins (SE), potent inducers of inflammatory cytokine secretion. Mice infected with T. cruzi CL-strain or inoculated with the avirulent clone CL-14, a clone that does not induce disease or polyclonal lymphocyte activation, succumb suddenly to low doses of staphylococcal enterotoxin B (SEB), but not to staphylococcal enterotoxin A (SEA). High plasma levels of TNF, IFN-gamma, and liver transaminases alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were found in these mice, indicating lethal toxic shock. Sensitization to shock required inoculation of live avirulent trypomastigotes and a time interval before challenge with SEB. We found no prior skewing of T cell receptor (TCR) Vbeta-repertoire in CL-14-inoculated mice that could be responsible for sensitization. Splenocytes from CL-14-inoculated mice proliferated more under anti-Vbeta8 than anti-TCRbeta stimulation when compared with normal mice, but were suppressed to SEB stimulation. Both SEB and anti-Vbeta8 antibodies stimulated splenocytes from T. cruzi-inoculated mice to secrete higher levels of inflammatory cytokines than normal controls. Taken together, our results show that T. cruzi inoculation can sensitize mice to lethal SEB-induced shock even in the absence of tissue damage, polyclonal lymphocyte activation, or previously increased levels of inflammatory cytokines, and they suggest that altered reactivity of Vbeta8 lymphocytes may be involved in the phenomenon.

Trypanosoma cruzi: requirements for induction and maintenance of protective immunity conferred by immunization

Immunization with CL-14-trypomastigotes generates efficient humoral and cellular responses against infective challenge. Herein, we investigated the relevance of these mechanisms in vivo. Immunization with live CL-14-trypomastigotes protected only part of beta2m(-/-) mice but efficiently protected perforin-knockout mice. Fixed CL-14-trypomastigotes could successfully immunize BALB/c, though live trypomastigotes lowered the requirements for doses and time intervals. Post-immune depletion of CD4 or CD8 subsets did not affect protection conferred by immunization, but switched the production of anti-Trypanosoma cruzi antibodies to IgG2a. Sublethal irradiation partially broke the resistance of immune mice, leading to development of late parasitemia. Passive serum transfer from immune mice conferred protection to nai;ve mice. Our results indicate that presentation of cytosolic antigens by MHC class I molecules is involved in the generation of immunity and suggest that the humoral response contributes to a great extent to keep CL-14-immunized mice protected against infective challenge.

Chagas disease etiology: autoimmunity or parasite persistence?

The question of the cause and the mechanisms of disease in chronic Trypanosoma cruzi infection continues to attract debate. Chagas disease, characterized by cardiomyopathy and/or megasyndrome involving the esophagus or colon, occurs in approximately 30% of individuals with chronic T. cruzi infections. Although the pathogenesis of Chagas disease is often attributed to autoimmune mechanisms, definitive proof of anti-self responses as the primary cause of disease in T. cruzi-infected hosts is lacking. Rick Tarleton and Lei Zhang here consider an alternative view that the primary cause of chronic Chagas disease is the failure of the host to clear the infection, resulting in infection-induced, immune-mediated tissue damage.

Detection of live Trypanosoma cruzi in tissues of infected mice by using histochemical stain for beta-galactosidase

The pathogenesis of tissue damage in chronic Trypanosoma cruzi infection has been a subject of long-standing debate. Conventional staining methods reveal a paucity of parasites in tissues from chronically infected individuals, which has led to the theory that the pathologic findings may be primarily autoimmune in origin. Immunostaining for T. cruzi antigens or in situ PCR methods show evidence for parasite components in chronic tissues; however, these methods do not address whether the stained material represents parasite debris or live organisms. An improved method for detecting intact T. cruzi in tissues was developed by making a genetically engineered strain that expresses Escherichia coli beta-galactosidase. The expression of this enzyme allows the detection of T. cruzi in tissues by using the histochemical stain 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside (X-Gal). The technique was used to monitor tissue parasitism and its relation to pathologic findings in the mouse model of Chagas' disease. Parasites were easily visible as bright blue structures in skeletal muscle, heart, bladder, peripheral nerve, liver, spleen, adrenal gland, brain, and adipose tissue in acutely infected mice. The number of viable parasites diminished >100-fold when tissues from 3-week-infected mice were compared with those from 10-month-infected mice. However, even at the lower level, parasites were clearly recognizable in sections of skeletal muscle and bladder at the 10-month time point. Inflammation remained robust in skeletal muscle, bladder, and sciatic nerve despite the near disappearance of parasites, suggesting three possibilities: exuberant host reactions to the few remaining parasites, autoimmune inflammation, or reactions to retained parasite antigens in the tissues.

Trypanosoma cruzi: protective response of vaccinated mice is mediated by CD8+ cells, prevents signs of polyclonal T lymphocyte activation, and allows restoration of a resting immune state after challenge

Currently, there is no vaccine available against Chagas' disease. Immune abnormalities induced by T. cruzi pose particular difficulties for vaccine development, since immunological memory must be able to overcome them to prevent spread of infection/sequelae. We have previously demonstrated that experimental vaccination with live CL-14 trypomastigotes does not induce polyclonal lymphocyte activation, immunosuppression, or pathology and efficiently immunizes against virulent T. cruzi. Herein we show that: (1) expansion of CD4+ and CD8+ subsets peaks 2 weeks after infective challenge in both challenged-vaccinated mice and infected controls, but the former exhibit a smaller increase in blastogenesis and in the numbers of activated CD11a(hi)CD4+ and CD11a(hi)CD8+ cells; (2) in long-term-vaccinated mice, expansion of activated subsets (CD62Llo/- and CD11a(hi)) is accelerated among CD8+ PBL 1 week after challenge; (3) challenged-vaccinated mice retract the CD8+-activated subset 5 weeks after challenge, different from infected controls; (4) protection conferred by CL-14 immunization can be adoptively transferred to naïve recipients with lymphocyte suspensions, and prior depletion of CD8+ (but not of CD4+) cells abolishes protective immunity. Our findings indicate that protective immunity generated by CL-14 immunization involves a transient CD8+ recall response and is capable of preventing the signs of polyclonal lymphocyte activation induced by virulent challenge.

"Autoimmune rejection" of neonatal heart transplants in experimental Chagas disease is a parasite-specific response to infected host tissue

Infection with the protozoan parasite Trypanosoma cruzi often results in chronic heart- and gut-associated disease known as Chagas disease. In this study we show that contrary to previous reports, neonatal hearts transplanted into mice chronically infected with T. cruzi do not exhibit signs of autoimmune-type rejection or any significant inflammatory response. In addition to an absence of inflammation, these syngeneic heart transplants survive for more than 1 year and are absolutely free of parasites as determined by in situ PCR analysis. However, if well-established transplanted hearts in chronically infected mice are directly injected with live parasites, a rapid and dramatic inflammatory response ensues that results in cessation of heart function. Likewise, transplanted hearts established in mice prior to systemic infection with T. cruzi or hearts transplanted into mice during the acute stage of T. cruzi infection become parasitized and develop inflammatory foci. In these cases where the transplanted hearts become parasitized, the ensuing inflammatory response is nearly identical to that observed in the native hearts of T. cruzi-infected mice in terms of cell types present and adhesion molecules and cytokines expressed. Importantly, this response is strikingly different from that observed in the allogeneic heart rejection. These results clearly document that parasitization of heart tissue is both necessary and sufficient for the induction of tissue damage in Chagas disease and strongly argue against a principal autoimmune etiology for this disease.