DNA sequences encoding CD4+ and CD8+ T-cell epitopes are important for efficient protective immunity induced by DNA vaccination with a Trypanosoma cruzi gene

The high identity of the Trypanosoma cruzi Group-I of trans-sialidases points them as promising vaccine immunogens

Trans-sialidase (TS) superfamily of proteins comprises eight subgroups, being the proteins of Group-I (TS-GI) promising immunogens in vaccine approaches against Trypanosoma cruzi. Strikingly, TS-GI antigenic variability among parasite lineages and their influence on vaccine development has not been previously analyzed. Here, a search in GenBank detects 49 TS-GI indexed sequences, whereas the main infecting human different parasite discrete typing units (DTU) are represented. In silico comparison among these sequences indicate that they share an identity above 92%. Moreover, the antigenic regions (T-cell and B-cell epitopes) are conserved in most sequences or present amino acid substitutions that scarcely may alter the antigenicity. Additionally, since the generic term TS is usually used to refer to different immunogens of this broad family, a further in silico analysis of the TS-GI-derived fragments tested in preclinical vaccines was done to determine the coverage and identity among them, showing that overall amino acid identity of vaccine immunogens is high, but the segment coverage varies widely. Accordingly, strong H-2K, H-2I, and B-cell epitopes are dissimilarly represented among vaccine TS-derived fragments depending on the extension of the TG-GI sequence used. Moreover, bioinformatic analysis detected a set of 150 T-cell strong epitopes among the DTU-indexed sequences that strongly bind human HLA-I supertypes. In all currently reported experimental vaccines based on TS-GI fragments, mapping these 150 epitopes showed that they are moderately represented. However, despite vaccine epitopes do not present all the substitutions observed in the DTUs, these regions of the proteins are equally recognized by the same HLAs.  Interestingly, the predictions regarding global and South American population coverage estimated in these 150 epitopes are similar to the estimations in experimental vaccines when the complete sequence of TS-GI is used as an antigen. In silico prediction also shows that a number of these MHC-I restricted T-cell strong epitopes could be also cross-recognized by HLA-I supertypes and H-2Kb or H-2Kd backgrounds, indicating that these mice may be used to improve and facilitate the development of new TS-based vaccines and suggesting an immunogenic and protective potential in humans. Further molecular docking analyses were performed to strengthen these results. Taken together, different strategies that would cover more or eventually fully of these T-cell and also B-cell epitopes to reach a high level of coverage are considered.

Epitopes in the Glycosylphosphatidylinositol Attachment Signal Peptide of Trypanosoma cruzi Mucin Proteins Generate Robust but Delayed and Nonprotective CD8+ T Cell Responses

Infection with the protozoan parasite Trypanosoma cruzi elicits substantial CD8+ T cell responses that disproportionately target epitopes encoded in the large trans-sialidase (TS) gene family. Within the C57BL/6 infection model, a significant proportion (30-40%) of the T. cruzi-specific CD8+ T cell response targets two immunodominant TS epitopes, TSKb18 and TSKb20. However, both TS-specific CD8+ T cell responses are dispensable for immune control, and TS-based vaccines have no demonstrable impact on parasite persistence, a determinant of disease. Besides TS, the specificity and protective capacity of CD8+ T cells that mediate immune control of T. cruzi infection are unknown. With the goal of identifying alternative CD8+ T cell targets, we designed and screened a representative set of genome-wide, in silico-predicted epitopes. Our screen identified a previously uncharacterized, to our knowledge, T cell epitope MUCKb25, found within mucin family proteins, the third most expanded large gene family in T. cruzi. The MUCKb25-specific response was characterized by delayed kinetics, relative to TS-specific responses, and extensive cross-reactivity with a large number of endogenous epitope variants. Similar to TS-specific responses, the MUCKb25 response was dispensable for control of the infection, and vaccination to generate MUCK-specific CD8+ T cells failed to confer protection. The lack of protection by MUCK vaccination was partly attributed to the fact that MUCKb25-specific T cells exhibit limited recognition of T. cruzi-infected host cells. Overall, these results indicate that the CD8+ T cell compartment in many T. cruzi-infected mice is occupied by cells with minimal apparent effector potential.

Use of Leishmania major parasites expressing a recombinant Trypanosoma cruzi antigen as live vaccines against Chagas disease

Introduction

Trypanosoma cruzi is the protozoan parasite causing Chagas disease, a Neglected Tropical Disease that affects 8 million people and causes 12,000 deaths per year, primarily because of cardiac pathology. Effective vaccination for T. cruzi remains an elusive goal. The use of a live vaccine vector, especially one that mimics the pathogen target, may be superior to the use of recombinant protein or DNA vaccine formulations.

Methods

We generated recombinant Leishmania major, a related trypanosomatid parasite, as a vaccine vehicle to express the immunogenic T. cruzi trans-sialidase (TS) antigen. The induction of T cell and antibody responses, as well as T. cruzi protective immunity generated by these vaccines were assessed in vivo.

Results

We demonstrate that mice inoculated with these recombinant TS-expressing L. major parasites mount T cell and antibody responses directed against TS and are protected against future T. cruzi infection. We also show that the partially attenuated dhfr-ts- CC1 L. major strain, previously found to induce protective immunity to virulent L. major infection without causing pathology, can also be engineered to express the TS antigen. This latter recombinant may represent a safe and effective option to explore for ultimate use in humans.

Discussion

Altogether, these data indicate that L. major can stably express a T. cruzi antigen and induce T. cruzi-specific protective immunity, warranting further investigation of attenuated Leishmania parasites as vaccine.

Recombinant Mycobacterium bovis BCG is a promising platform to develop vaccines against Trypansoma cruzi infection

Chagas disease, caused by the hemoflagelate parasite Trypanosoma cruzi, is one of the most prevalent endemic parasitoses, affecting 7-8 million people. Due to the complexity of the infection, no vaccines are available at present. The extraordinary adjuvant capacity of bacille Calmette-Guérin (BCG) was explored in this work to develop a vaccine candidate to protect against T. cruzi infection using the recombinant BCG (rBCG) vaccine platform. Three antigens of the parasite corresponding to the N and C terminal fragments of the enzyme trans-sialidase (NT-TS and CT-TS, respectively) and a fragment of the cruzipain enzyme (CZf) were cloned into the vectors pUS997 and pUS2000 and transformed into the BCG Pasteur strain. In vaccinated mice, rBCG expressing NT-TS in pUS2000 plasmid provided the highest protection and the lowest parasitemia after challenging BALB/c mice with a 50% lethal dose of parasites. When mice vaccinated with pUS2000-NT-TS were challenged with a 100% lethal dose of parasite, high levels of protection were also obtained, together with a low degree of cardiac lesions 120 days after infection. In immunized mice with pUS2000-NT-TS/rBCG clone, the proliferation of CD4⁺ cells from splenocytes stimulated with the TS antigen was significant; this stimulation increased interferon (IFN)-γ and interleukin (IL)-17 within CD4⁺ T lymphocytes (LTCD4⁺ ) cells and IFN-γ and CD107 expression within LTCD8⁺ cells. Therefore, pUS2000-NT-TS/rBCG conferred high levels of protection, which correlated with an immune response orientated towards a T helper type 1 (Th1)/Th17 profile, together with an LTC-specific response, indicating that rBCG is a promising platform to develop vaccines against T. cruzi.

Production of recombinant TSA-1 and evaluation of its potential for the immuno-therapeutic control of Trypanosoma cruzi infection in mice

A therapeutic vaccine for human Chagas disease (American trypanosomiasis caused by Trypanosoma cruzi) is under development based on the success of vaccinating mice with DNA constructs expressing the antigens Tc24 and Tc-TSA-1. However, because DNA and nucleic acid vaccines produce less than optimal responses in humans, our strategy relies on administering a recombinant protein-based vaccine, together with adjuvants that promote Th1-type immunity. Here we describe a process for the purification and refolding of recombinant TSA-1 expressed in Escherichia coli. The overall yield (20–25%) and endotoxin level of the purified recombinant TSA-1 (rTSA-1) is suitable for pilot scale production of the antigen for use in phase 1 clinical trials. Mice infected with T. cruzi were treated with rTSA-1, either alone or with Toll-like receptor 4 (TLR-4) agonist adjuvants including monophosphoryl lipid A (MPLA), glucopyranosyl lipid A (GLA, IDRI), and E6020 (EISEI, Inc). TSA-1 with the TLR-4 agonists was effective at reducing parasitemia relative to rTSA-1 alone, although it was difficult to discern a therapeutic effect compared to treatment with TLR-4 agonists alone. However, rTSA-1 with a 10 ug dose of MPLA optimized reductions in cardiac tissue inflammation, which were significantly reduced compared to MPLA alone. It also elicited the lowest parasite burden and the highest levels of TSA-1-specific IFN-gamma levels and IFN-gamma/IL-4 ratios. These results warrant the further evaluation of rTSA-1 in combination with rTc24 in order to maximize the therapeutic effect of vaccine-linked chemotherapy in both mice and non-human primates before advancing to clinical development.

Th17 Cells Are More Protective Than Th1 Cells Against the Intracellular Parasite Trypanosoma cruzi

Th17 cells are a subset of CD4⁺ T cells known to play a central role in the pathogenesis of many autoimmune diseases, as well as in the defense against some extracellular bacteria and fungi. However, Th17 cells are not believed to have a significant function against intracellular infections. In contrast to this paradigm, we have discovered that Th17 cells provide robust protection against Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas disease. Th17 cells confer significantly stronger protection against T. cruzi-related mortality than even Th1 cells, traditionally thought to be the CD4⁺ T cell subset most important for immunity to T. cruzi and other intracellular microorganisms. Mechanistically, Th17 cells can directly protect infected cells through the IL-17A-dependent induction of NADPH oxidase, involved in the phagocyte respiratory burst response, and provide indirect help through IL-21-dependent activation of CD8⁺ T cells. The discovery of these novel Th17 cell-mediated direct protective and indirect helper effects important for intracellular immunity highlights the diversity of Th17 cell roles, and increases understanding of protective T. cruzi immunity, aiding the development of therapeutics and vaccines for Chagas disease.

Chronic infection with the intracellular parasite Trypanosoma cruzi results in Chagas disease, an illness endemic in more than 20 countries that leads to life-threatening cardiac and gastrointestinal dysfunction. Although CD4⁺ Th1 cells are known to be protective against T. cruzi infection, less is known about the role of other CD4⁺ T cell subsets. We demonstrate that CD4⁺ Th17 cells are also highly protective against T. cruzi infection, even outperforming Th1 cells in protection from T. cruzi-related death, despite the standard conception that Th17 cells are only important in the defense against extracellular pathogens. The novel discovery that Th17 cells are significantly more protective than Th1 cells against T. cruzi infection has increased our understanding of the advantageous immune responses for this major human pathogen, and may guide future efforts toward vaccine development.

Costimulatory Effects of an Immunodominant Parasite Antigen Paradoxically Prevent Induction of Optimal CD8 T Cell Protective Immunity

Trypanosoma cruzi infection is controlled but not eliminated by host immunity. The T. cruzi trans-sialidase (TS) gene superfamily encodes immunodominant protective antigens, but expression of altered peptide ligands by different TS genes has been hypothesized to promote immunoevasion. We molecularly defined TS epitopes to determine their importance for protection versus parasite persistence. Peptide-pulsed dendritic cell vaccination experiments demonstrated that one pair of immunodominant CD4⁺ and CD8⁺ TS peptides alone can induce protective immunity (100% survival post-lethal parasite challenge). TS DNA vaccines have been shown by us (and others) to protect BALB/c mice against T. cruzi challenge. We generated a new TS vaccine in which the immunodominant TS CD8⁺ epitope MHC anchoring positions were mutated, rendering the mutant TS vaccine incapable of inducing immunity to the immunodominant CD8 epitope. Immunization of mice with wild type (WT) and mutant TS vaccines demonstrated that vaccines encoding enzymatically active protein and the immunodominant CD8⁺ T cell epitope enhance subdominant pathogen-specific CD8⁺ T cell responses. More specifically, CD8⁺ T cells from WT TS DNA vaccinated mice were responsive to 14 predicted CD8⁺ TS epitopes, while T cells from mutant TS DNA vaccinated mice were responsive to just one of these 14 predicted TS epitopes. Molecular and structural biology studies revealed that this novel costimulatory mechanism involves CD45 signaling triggered by enzymatically active TS. This enhancing effect on subdominant T cells negatively regulates protective immunity. Using peptide-pulsed DC vaccination experiments, we have shown that vaccines inducing both immunodominant and subdominant epitope responses were significantly less protective than vaccines inducing only immunodominant-specific responses. These results have important implications for T. cruzi vaccine development. Of broader significance, we demonstrate that increasing breadth of T cell epitope responses induced by vaccination is not always advantageous for host immunity.

Chagas disease, driven by persistent infection with the intracellular protozoan parasite Trypanosoma cruzi, continues to be a major cause of morbidity and mortality in the Americas. Infections with complex pathogens induce T cell responses to a variety of proteins and epitopes in a specific and sometimes predictable pattern of immune dominance. Extensive research has been conducted with the goal of broadening vaccine-induced T cell responses, for example to better protect against pathogen immune escape due to epitope mutation. We show that T. cruzi trans-sialidase (TS) induces complex immunodominant and subdominant T cell responses after both T. cruzi infection and TS vaccination. In order to study the importance of immunodominant and subdominant TS epitopes, we generated TS vaccines designed to abolish T cell responses to the immunodominant TS CD8⁺ T cell epitope. These vaccines indeed abolished CD8⁺ T cell responses to the immunodominant epitope, but interestingly also eliminated T cell responses to many subdominant epitopes. Furthermore, vaccines inducing both immunodominant and subdominant epitope responses were significantly less protective than vaccines inducing only immunodominant-specific responses. Thus, increasing the breadth of T cell epitope recognition may not necessarily enhance protective immunity, and in fact, may be detrimental to the desired goals.

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

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

An immunoinformatic approach for identification of Trypanosoma cruzi HLA-A2-restricted CD8(+) T cell epitopes

Chagas disease is a major neglected tropical disease caused by persistent chronic infection with the protozoan parasite Trypanosoma cruzi. An estimated 8 million people are infected with T. cruzi, however only 2 drugs are approved for treatment and no vaccines are available. Thus there is an urgent need to develop vaccines and new drugs to prevent and treat Chagas disease. In this work, we identify T cell targets relevant for human infection with T. cruzi. The trans-sialidase (TS) gene family is a large family of homologous genes within the T. cruzi genome encoding over 1,400 members. There are 12 highly conserved TS gene family members which encode enzymatically active TS (functional TS; F-TS), while the remaining TS family genes are less conserved, enzymatically inactive and have been hypothesized to be involved in immune evasion (non-functional TS; NF-TS). We utilized immunoinformatic tools to identify HLA-A2-restricted CD8(+) T cell epitopes conserved within F-TS family members and NF-TS gene family members. We also utilized a whole-genome approach to identify T cell epitopes present within genes which have previously been shown to be expressed in life stages relevant for human infection (Non-TS genes). Thirty immunogenic HLA-A2-restricted CD8(+) T cell epitopes were identified using IFN-γ ELISPOT assays after vaccination of humanized HLA-A2 transgenic mice with mature dendritic cells pulsed with F-TS, NF-TS, and Non-TS peptide pools. The immunogenic HLA-A2-restricted T cell epitopes identified in this work may serve as potential components of an epitope-based T cell targeted vaccine for Chagas disease.

Antigen targeting to dendritic cells allows the identification of a CD4 T-cell epitope within an immunodominant Trypanosoma cruzi antigen

Targeting antigens to dendritic cells (DCs) by using hybrid monoclonal antibodies (mAbs) directed against DC receptors is known to improve activation and support long-lasting T cell responses. In the present work, we used the mAb αDEC205 fused to the Trypanosoma cruzi amastigote surface protein 2 (ASP-2) to identify a region of this protein recognized by specific T cells. The hybrid αDEC-ASP2 mAb was successfully generated and preserved its ability to bind the DEC205 receptor. Immunization of BALB/c mice with the recombinant mAb in the presence of polyriboinosinic: polyribocytidylic acid (poly (I:C)) specifically enhanced the number of IFN-γ producing cells and CD4+ T cell proliferation when compared to mice immunized with a mAb without receptor affinity or with the non-targeted ASP-2 protein. The strong immune response induced in mice immunized with the hybrid αDEC-ASP2 mAb allowed us to identify an ASP-2-specific CD4+ T cell epitope recognized by the BALB/c MHCII haplotype. We conclude that targeting parasite antigens to DCs is a useful strategy to enhance T cell mediated immune responses facilitating the identification of new T-cell epitopes.

Immunization with Hexon modified adenoviral vectors integrated with gp83 epitope provides protection against Trypanosoma cruzi infection

BACKGROUND

Trypanosoma cruzi is the causative agent of Chagas disease. Chagas disease is an endemic infection that affects over 8 million people throughout Latin America and now has become a global challenge. The current pharmacological treatment of patients is unsuccessful in most cases, highly toxic, and no vaccines are available. The results of inadequate treatment could lead to heart failure resulting in death. Therefore, a vaccine that elicits neutralizing antibodies mediated by cell-mediated immune responses and protection against Chagas disease is necessary.

METHODOLOGY/PRINCIPAL FINDINGS

The "antigen capsid-incorporation" strategy is based upon the display of the T. cruzi epitope as an integral component of the adenovirus' capsid rather than an encoded transgene. This strategy is predicted to induce a robust humoral immune response to the presented antigen, similar to the response provoked by native Ad capsid proteins. The antigen chosen was T. cruzi gp83, a ligand that is used by T. cruzi to attach to host cells to initiate infection. The gp83 epitope, recognized by the neutralizing MAb 4A4, along with His6 were incorporated into the Ad serotype 5 (Ad5) vector to generate the vector Ad5-HVR1-gp83-18 (Ad5-gp83). This vector was evaluated by molecular and immunological analyses. Vectors were injected to elicit immune responses against gp83 in mouse models. Our findings indicate that mice immunized with the vector Ad5-gp83 and challenged with a lethal dose of T. cruzi trypomastigotes confer strong immunoprotection with significant reduction in parasitemia levels, increased survival rate and induction of neutralizing antibodies.

CONCLUSIONS/SIGNIFICANCE

This data demonstrates that immunization with adenovirus containing capsid-incorporated T. cruzi antigen elicits a significant anti-gp83-specific response in two different mouse models, and protection against T. cruzi infection by eliciting neutralizing antibodies mediated by cell-mediated immune responses, as evidenced by the production of several Ig isotypes. Taken together, these novel results show that the recombinant Ad5 presenting T. cruzi gp83 antigen is a useful candidate for the development of a vaccine against Chagas disease.

CD8(+) T cell-mediated immunity during Trypanosoma cruzi infection: a path for vaccine development?

MHC-restricted CD8⁺ T cells are important during infection with the intracellular protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease. Experimental studies performed in the past 25 years have elucidated a number of features related to the immune response mediated by these T cells, which are important for establishing the parasite/host equilibrium leading to chronic infection. CD8⁺ T cells are specific for highly immunodominant antigens expressed by members of the trans-sialidase family. After infection, their activation is delayed, and the cells display a high proliferative activity associated with high apoptotic rates. Although they participate in parasite control and elimination, they are unable to clear the infection due to their low fitness, allowing the parasite to establish the chronic phase when these cells then play an active role in the induction of heart immunopathology. Vaccination with a number of subunit recombinant vaccines aimed at eliciting specific CD8⁺ T cells can reverse this path, thereby generating a productive immune response that will lead to the control of infection, reduction of symptoms, and reduction of disease transmission. Due to these attributes, activation of CD8⁺ T lymphocytes may constitute a path for the development of a veterinarian or human vaccine.

Genetic vaccination against experimental infection with myotropic parasite strains of Trypanosoma cruzi

In earlier studies, we reported that a heterologous prime-boost regimen using recombinant plasmid DNA followed by replication-defective adenovirus vector, both containing Trypanosoma cruzi genes encoding trans-sialidase (TS) and amastigote surface protein (ASP) 2, provided protective immunity against experimental infection with a reticulotropic strain of this human protozoan parasite. Herein, we tested the outcome of genetic vaccination of F1 (CB10XBALB/c) mice challenged with myotropic parasite strains (Brazil and Colombian). Initially, we determined that the coadministration during priming of a DNA plasmid containing the murine IL-12 gene improved the immune response and was essential for protective immunity elicited by the heterologous prime-boost regimen in susceptible male mice against acute lethal infections with these parasites. The prophylactic or therapeutic vaccination of resistant female mice led to a drastic reduction in the number of inflammatory infiltrates in cardiac and skeletal muscles during the chronic phase of infection with either strain. Analysis of the electrocardiographic parameters showed that prophylactic vaccination reduced the frequencies of sinus arrhythmia and atrioventricular block. Our results confirmed that prophylactic vaccination using the TS and ASP-2 genes benefits the host against acute and chronic pathologies caused by T. cruzi and should be further evaluated for the development of a veterinary or human vaccine against Chagas disease.

Differential immune response in mice immunized with the A, R or C domain from TcSP protein of Trypanosoma cruzi or with the coding DNAs

In a murine model of experimental Trypanosoma cruzi (H8 strain) infection, we investigated the induction of protective immunity against the domains [amino (A), repeats (R) and carboxyl (C)] of the surface protein (SP), a member of the trans-sialidase (TS) superfamily. Recombinant proteins and plasmid DNA coding for the respective proteins were used to immunize BALB/c mice, and the humoral response and cytokine levels were analysed. Immunization with the recombinant proteins induced higher levels of anti-TcSP antibodies than immunization with the corresponding DNAs, and analysis of serum cytokines showed that immunization with both recombinant proteins and naked DNA resulted in a Th1-Th2 mixed T-cell response. Mice immunized with either recombinant proteins or plasmid DNA were infected with blood trypomastigotes. The recombinant protein-immunized mice showed a variable reduction in peak parasitemia, and most died by day 60. Only the pBKTcSPR-immunized mice exhibited a significant reduction in peak parasitemia and survived the lethal challenge. DNA-based immunization with DNA coding for the repeats domain of TcSP is a good candidate for the development of a vaccine against experimental T. cruzi infection.

DNA Vaccines against Protozoan Parasites: Advances and Challenges

Over the past 15 years, DNA vaccines have gone from a scientific curiosity to one of the most dynamic research field and may offer new alternatives for the control of parasitic diseases such as leishmaniasis and Chagas disease. We review here some of the advances and challenges for the development of DNA vaccines against these diseases. Many studies have validated the concept of using DNA vaccines for both protection and therapy against these protozoan parasites in a variety of mouse models. The challenge now is to translate what has been achieved in these models into veterinary or human vaccines of comparable efficacy. Also, genome-mining and new antigen discovery strategies may provide new tools for a more rational search of novel vaccine candidates.

The role of Toll-like receptors and adaptive immunity in the development of protective or pathological immune response triggered by the Trypanosoma cruzi protozoan

Trypanosoma cruzi, the causal agent of Chagas disease, is an intracellular protozoan parasite that predominantly invades macrophages and cardiomyocytes, leading to persistent infection. Several members of the Toll-like receptor family are crucial for innate immunity to infection and are involved in maintaining tissue homeostasis. This review focuses on recent experimental findings of the innate and adaptive immune response in controlling the parasite and/or in generating heart and liver tissue injury. We also describe the importance of the host’s genetic background in the outcome of the disease and emphasize the importance of studying the response to specific parasite antigens. Understanding the dual participation of the immune response may contribute to the design of new therapies for Chagas disease.

A DNA vaccine encoding for TcSSP4 induces protection against acute and chronic infection in experimental Chagas disease

Immunization of mice with plasmids containing genes of Trypanosoma cruzi induces protective immunity in the murine model of Chagas disease. A cDNA clone that codes for an amastigote-specific surface protein (TcSSP4) was used as a candidate to develop a DNA vaccine. Mice were immunized with the recombinant protein rTcSSP4 and with cDNA for TcSSP4, and challenged with bloodstream trypomastigotes. Immunization with rTcSSP4 protein makes mice more susceptible to trypomastigote infection, with high mortality rates, whereas mice immunized with a eukaryotic expression plasmid containing the TcSSP4 cDNA were able to control the acute phase of infection. Heart tissue of gene-vaccinated animals did not show myocarditis and tissue damage at 365 days following infection, as compared with control animals. INF-γ was detected in sera of DNA vaccinated mice shortly after immunization, suggesting the development of a Th1 response. The TcSSP4 gene is a promising candidate for the development of an anti-T. cruzi DNA vaccine.

Nonimmune Cells Contribute to Crosstalk between Immune Cells and Inflammatory Mediators in the Innate Response to Trypanosoma cruzi Infection

Chagas myocarditis, which is caused by infection with the intracellular parasite Trypanosoma cruzi, remains the major infectious heart disease worldwide. Innate recognition through toll-like receptors (TLRs) on immune cells has not only been revealed to be critical for defense against T. cruzi but has also been involved in triggering the pathology. Subsequent studies revealed that this parasite activates nucleotide-binding oligomerization domain- (NOD-)like receptors and several particular transcription factors in TLR-independent manner. In addition to professional immune cells, T. cruzi infects and resides in different parenchyma cells. The innate receptors in nonimmune target tissues could also have an impact on host response. Thus, the outcome of the myocarditis or the inflamed liver relies on an intricate network of inflammatory mediators and signals given by immune and nonimmune cells. In this paper, we discuss the evidence of innate immunity to the parasite developed by the host, with emphasis on the crosstalk between immune and nonimmune cell responses.

Immune responses to gp82 provide protection against mucosal Trypanosoma cruzi infection

The potential use of the Trypanosoma cruzi metacyclic trypomastigote (MT) stage-specific molecule glycoprotein-82 (gp82) as a vaccine target has not been fully explored. We show that the opsonization of T. cruzi MT with gp82-specific antibody prior to mucosal challenge significantly reduces parasite infectivity. In addition, we investigated the immune responses as well as the systemic and mucosal protective immunity induced by intranasal CpG-adjuvanted gp82 vaccination. Spleen cells from mice immunized with CpG-gp82 proliferated and secreted IFN-γ in a dose-dependent manner in response to in vitro stimulation with gp82 and parasite lysate. More importantly, these CpG-gp82-immunized mice were significantly protected from a biologically relevant oral parasite challenge.

Cross-presentation of peptides from intracellular pathogens by MHC class I molecules

Many prokaryotic and eukaryotic parasites multiply in specialized subcellular niches in the host cell. The invading microbes hijack key cellular functions to establish the intracellular niches but, unlike viruses, do not need the protein synthesis machinery of host cells to replicate. Circulating CD8+ T cells provide protective immunity by recognizing pathogen-derived peptide major histocompatibility complex class I molecules (pMHC I) expressed by infected cells. Here, we review studies on the complex and varied pathways that produce the appropriate pMHC I as ligands for the CD8+ T cells. We also discuss possible explanations for the curious observations that CD8+ T cells are specific for fewer pMHC I ligands in parasite infections compared to the diversity of pMHC I ligands in viral infections.

Intranasal vaccinations with the trans-sialidase antigen plus CpG Adjuvant induce mucosal immunity protective against conjunctival Trypanosoma cruzi challenges

Trypanosoma cruzi is an intracellular protozoan parasite capable of infecting through mucosal surfaces. Our laboratory has previously elucidated the anatomical routes of infection after both conjunctival and gastric challenge in mice. We have shown that chronically infected mice develop strong immune responses capable of protecting against subsequent rechallenge with virulent parasites through gastric, conjunctival, and systemic routes of infection. We have also shown that intranasal immunizations with the unique T. cruzi trans-sialidase (TS) antigen protect against gastric and systemic T. cruzi challenge. In the current work we have investigated the ability of purified TS adjuvanted with CpG-containing oligonucleotides to induce immunity against conjunctival T. cruzi challenge. We confirm that intranasal vaccinations with TS plus CpG induce TS-specific T-cell and secretory IgA responses. TS-specific secretory IgA was detectable in the tears of vaccinated mice, the initial body fluid that contacts the parasite during infectious conjunctival exposures. We further show that intranasal vaccinations with TS plus CpG protect against conjunctival T. cruzi challenge, limiting local parasite replication at the site of mucosal invasion and systemic parasite dissemination. We also provide the first direct evidence that mucosal antibodies induced by intranasal TS vaccination can inhibit parasite invasion.

Genetic immunization converts the trypanosoma cruzi B-Cell mitogen proline racemase to an effective immunogen

Trypanosoma cruzi is the etiologic agent of Chagas' disease. Acute T. cruzi infection results in polyclonal B-cell activation and delayed specific humoral immunity. T. cruzi proline racemase (TcPRAC), a T. cruzi B-cell mitogen, may contribute to this dysfunctional humoral response. Stimulation of murine splenocytes with recombinant protein (rTcPRAC) induced B-cell proliferation, antibody secretion, interleukin-10 (IL-10) production, and upregulation of CD69 and CD86 on B cells. Marginal zone (MZ) B cells are more responsive to T-cell-independent (TI) rTcPRAC stimulation than are follicular mature (FM) B cells in terms of proliferation, antibody secretion, and IL-10 production. During experimental T. cruzi infection, TcPRAC-specific IgG remained undetectable when responses to other T. cruzi antigens developed. Conversely, intradermal genetic immunization via gene gun (GG) delivered TcPRAC as an immunogen, generating high-titer TcPRAC-specific IgG without B-cell dysfunction. TcPRAC GG immunization led to antigen-specific splenic memory B-cell and bone marrow plasma cell formation. TcPRAC-specific IgG bound mitogenic rTcPRAC, decreasing subsequent B-cell activation. GG immunization with rTcPRAC DNA was nonmitogenic and did not affect the generation of specific IgG to another T. cruzi antigen, complement regulatory protein (CRP). These data demonstrate the utility of genetic immunization for the conversion of a protein mitogen to an effective antigen. Furthermore, coimmunization of TcPRAC with another T. cruzi antigen indicates the usefulness of this approach for multivalent vaccine development.

HLA Class I-T cell epitopes from trans-sialidase proteins reveal functionally distinct subsets of CD8+ T cells in chronic Chagas disease

Background

Previously, we identified a set of HLA-A020.1-restricted trans-sialidase peptides as targets of CD8⁺ T cell responses in HLA-A0201⁺ individuals chronically infected by T. cruzi.

Methods and Findings

Herein, we report the identification of peptides encoded by the same trans-sialidase gene family that bind alleles representative of the 6 most common class I HLA-supertypes. Based on a combination of bioinformatic predictions and HLA-supertype considerations, a total of 1001 epitopes predicted to bind to HLA A01, A02, A03, A24, B7 and B44 supertypes was selected. Ninety-six supertype-binder epitopes encoded by multiple trans-sialidase genes were tested for the ability to stimulate a recall CD8⁺ T cell response in the peripheral blood from subjects with chronic T. cruzi infection regardless the HLA haplotype. An overall hierarchy of antigenicity was apparent, with the A02 supertype peptides being the most frequently recognized in the Chagas disease population followed by the A03 and the A24 supertype epitopes. CD8⁺ T cell responses to promiscuous epitopes revealed that the CD8⁺ T cell compartment specific for T. cruzi displays a functional profile with T cells secreting interferon-γ alone as the predominant pattern and very low prevalence of single IL-2-secreting or dual IFN-γ/IL-2 secreting T cells denoting a lack of polyfunctional cytokine responses in chronic T. cruzi infection.

Conclusions

This study identifies a set of T. cruzi peptides that should prove useful for monitoring immune competence and changes in infection and disease status in individuals with chronic Chagas disease.

At present, 16–20 million people in Central and South America are infected with Trypanosoma cruzi, the causative agent of Chagas disease in humans. The primary clinical consequence of the infection is a cardiomyopathy, which manifests in approximately 30% of infected individuals, many years after the initial infection. Our work in Chagas disease patients began as an effort to assess the range and specificity of antigens that were recognized by T cells, in particular CD8⁺ T cells, in individuals with long-term infections with Trypanosoma cruzi. Trans-sialidase proteins from T. cruzi are major surface and released proteins that are targets of humoral and cellular immune responses. We previously, identified a set of trans-sialidase peptides that were recognized by a very low frequency of chronically T. cruzi-infected subjects. Based on bioinformatic predictions, herein we report the identification of new trans-sialidase epitopes that are recognized by a higher proportion of T. cruzi-infected people. The functional profile of T cells specific for these peptides is characteristic of an infection with long term stimulation of the immune system, with high levels of IFN-γ-secreting T cells and low levels of IL-2 production. This set of T. cruzi peptides should prove useful for monitoring immune competence and changes in infection and disease status in individuals with chronic Chagas disease.

Novel protective antigens expressed by Trypanosoma cruzi amastigotes provide immunity to mice highly susceptible to Chagas' disease

Earlier studies have demonstrated in A/Sn mice highly susceptible to Chagas' disease protective immunity against lethal Trypanosoma cruzi infection elicited by vaccination with an open reading frame (ORF) expressed by amastigotes. In our experiments, we used this mouse model to search for other amastigote-expressed ORFs with a similar property. Fourteen ORFs previously determined to be expressed in this developmental stage were individually inserted into a eukaryotic expression vector containing a nucleotide sequence that encoded a mammalian secretory signal peptide. Immunization with 13 of the 14 ORFs induced specific antibodies which recognized the amastigotes. Three of those immune sera also reacted with trypomastigotes and epimastigotes. After a lethal challenge with Y strain trypomastigotes, the vast majority of plasmid-injected mice succumbed to infection. In some cases, a significant delay in mortality was observed. Only two of these ORFs provided protective immunity against the otherwise lethal infection caused by trypomastigotes of the Y or Colombia strain. These ORFs encode members of the trans-sialidase family of surface antigens related to the previously described protective antigen amastigote surface protein 2 (ASP-2). Nevertheless, at the level of antibody recognition, no cross-reactivity was observed between the ORFs and the previously described ASP-2 from the Y strain. In immunofluorescence analyses, we observed the presence of epitopes related to both proteins expressed by amastigotes of seven different strains. In conclusion, our approach allowed us to successfully identify two novel protective ORFs which we consider interesting for future studies on the immune response to Chagas' disease.

Immunization with an engineered mutant trans-sialidase highly protects mice from experimental Trypanosoma cruzi infection: a vaccine candidate

Chagas' disease is a major tropical disease for which a cure for chronic phase does not exist yet. Trypanosoma cruzi trans-sialidase (TS) seems to be involved in relevant processes such as infectivity, host survival and, very importantly, disease pathogenesis. In this study, we show that mice vaccinated with an engineered enzymatically deficient mutant TS containing the catalytic domain without the immunodominant SAPA (Shed Acute Phase Antigen) repeats, were highly protected against T. cruzi infection. Adult male BALB/c mice were immunized with mutant protein, purified from Pichia pastoris yeast, using three inoculations in Freund's adjuvant. All immunized mice were protected against challenge with a lethal dose of T. cruzi trypomastigotes. The protected immunized mice developed no clinical or tissue evidence of infection throughout the study. In contrast, 60-90% mortality and 100% occurrence of myocardial lesions were observed in the non-immunized counterparts. Titers of circulating antibody against TS did not correlate with protection, while anti-SAPA antibodies were coincident with disease severity. Further studies indicated that a single inoculation of mutant recombinant protein in Freund's complete adjuvant was not associated with blood or organic alterations, per se. Mutant TS vaccination seems to be a promising tool for immune intervention strategies in Chagas' disease, aimed at preventing T. cruzi-related heart tissue damage.

Trans-sialidase recombinant protein mixed with CpG motif-containing oligodeoxynucleotide induces protective mucosal and systemic trypanosoma cruzi immunity involving CD8+ CTL and B cell-mediated cross-priming

The Trypanosoma cruzi trans-sialidase (TS) is a unique enzyme with neuraminidase and sialic acid transfer activities important for parasite infectivity. The T. cruzi genome contains a large family of TS homologous genes, and it has been suggested that TS homologues provide a mechanism of immune escape important for chronic infection. We have investigated whether the consensus TS enzymatic domain could induce immunity protective against acute and chronic, as well as mucosal and systemic, T. cruzi infection. We have shown that: 1) TS-specific immunity can protect against acute T. cruzi infection; 2) effective TS-specific immunity is maintained during chronic T. cruzi infection despite the expression of numerous related TS superfamily genes encoding altered peptide ligands that in theory could promote immune tolerization; and 3) the practical intranasal delivery of recombinant TS protein combined with a ssDNA oligodeoxynucleotide (ODN) adjuvant containing unmethylated CpG motifs can induce both mucosal and systemic protective immunity. We have further demonstrated that the intranasal delivery of soluble TS recombinant Ag combined with CpG ODN induces both TS-specific CD4(+) and CD8(+) T cells associated with vaccine-induced protective immunity. In addition, optimal protection induced by intranasal TS Ag combined with CpG ODN requires B cells, which, after treatment with CpG ODN, have the ability to induce TS-specific CD8(+) T cell cross-priming. Our results support the development of TS vaccines for human use, suggest surrogate markers for use in future human vaccine trials, and mechanistically identify B cells as important APC targets for vaccines designed to induce CD8(+) CTL responses.

Evaluation of immune responses raised againstTc13 antigens ofTrypanosoma cruziin the outcome of murine experimental infection

We have previously reported that genetic immunization withTc13Tul antigen ofTrypanosoma cruzi, the aetiological agent of Chagas' disease, triggers harmful effects and non-protective immune responses. In order to confirm the role ofTc13 antigens duringT. cruziinfection, herein we studied the humoral and cellular immune responses to theTc13Tul molecule and its EPKSA C-terminal portion in BALB/cT. cruzi-infected mice or mice immunized with recombinantTc13Tul. Analysis of the antibody response showed that B-cell epitopes that stimulate a sustained IgM production along the infection and high levels of IgG in the acute phase are mainly located at theTc13 N- and C-terminal domains, respectively. DTH assays showed that T-cell epitopes are mainly at theTc13 N-terminal segment and that they do not elicit an efficient memory response. RecombinantTc13Tul did not induce IFN-γ secretion in either infected or immunized mice. However, a putative CD8+Tc13Tul-derived peptide was found to elicit IFN-γ production in chronically infected animals. Immunization with recombinantTc13Tul did not induce pathology in tissues and neither did it protect against the infection. Our results show that in the outcome ofT. cruziinfection theTc13 family protein mainly triggers non-protective immune responses.

Role of Trypanosoma cruzi autoreactive T cells in the generation of cardiac pathology

Chagas disease, caused by Trypanosoma cruzi, affects several million people in Central and South America. About 30% of chronic patients develop cardiomyopathy probably caused by parasite persistence and/or autoimmunity. While several cross-reactive antibodies generated during mammal T. cruzi infection have been described, very few cross-reactive T cells have been identified. We performed adoptive transfer experiments of T cells isolated from chronically infected mice. The results showed the generation of cardiac pathology in the absence of parasites. We also transferred cross-reactive SAPA-specific T cells and observed unspecific alterations in heart repolarization, cardiac inflammatory infiltration, and tissue damage.

Trypanosoma cruzi infection from the view of CD8+ T cell immunity--an infection model for developing T cell vaccine

Chagas' disease is caused by Trypanosoma cruzi (T. cruzi) which was once prevalent in Central and South America. Although the recent success in Triatoma vector control has made the disease being possibly "extinct" in the near future, the development of effective preventive and therapeutic vaccines is still necessary to prevent the resurgence of the neglected infection. In addition to the importance for containing the disease, T. cruzi infection presents unique features for elucidating hosts' immune responses against intracellular infectious agents. Due to its biological capacity for invading into principally any types of cells and for causing systemic infection which damages particularly muscle and neural cells, T cell immunity is critical for resolving its infection. Although T cell-mediated immune responses have been, so far, extensively investigated in viral and bacterial infections, parasitic infection such as malaria has presented epoch-making discovery in T cell immunity. Recent advances in the analyses of T cell-mediated immune responses against T. cruzi infection now make this infectious disease potentially more suitable for detecting subtle immunological changes in hosts' immune defense upon modifying immune system. The current review focuses on the usefulness of T. cruzi infection as a model for developing effective CD8(+) T cell-mediated vaccine against intracellular infectious agents.

Parasite-induced chronic inflammation is not exacerbated by immunotherapy before or during Trypanosoma cruzi Infection

Trypanosoma cruzi infection causes Chagas' disease, a chronic inflammatory disease. The specific inflammatory responses that cause Chagas' disease remain unclear, but data argue that parasites that persist in the host stimulate chronic self-damaging immune responses. Because T. cruzi appears to stimulate self-damaging responses, the enthusiasm to develop vaccines that boost antiparasite responses that might increase self-damaging responses has been limited. We previously demonstrated that immunization with a T. cruzi trans-sialidase protein or adoptive transfer of trans-sialidase-specific T-cell clones decreased parasitemia, morbidity, and mortality. Here we report that immunization or adoptive transfer with the protein or clones, before or during T. cruzi infection, boosts the anti-T. cruzi immune response without exacerbating acute or chronic tissue inflammation. These results argue that prophylactic and therapeutic immunotherapy for Chagas' disease can be developed safely.

Comparative evaluation of therapeutic DNA vaccines against Trypanosoma cruzi in mice

Chagas disease, caused by the protozoan parasite Trypanosoma cruzi, is a major public health problem in most of Latin America. A key priority is the development of new treatments, due to the poor efficacy of current ones. We report here the comparative evaluation of therapeutic DNA vaccines encoding various T. cruzi antigens. ICR mice infected with 500 parasites intraperitoneally were treated at 5 and 12 days postinfection with 20 microg of plasmid DNA encoding T. cruzi antigens TSA-1, TS, ASP-2-like, Tc52 or Tc24. Treatment with plasmid encoding TS and/or ASP-2-like antigens had no significant effect on parasitemia or survival. Treatment with Tc52 DNA significantly reduced parasitemia, as well as cardiac parasite burden, and improved survival, although myocarditis was not significantly affected. Finally, treatment with plasmids encoding Tc24 and TSA-1 induced the most complete control of disease as evidenced by significant reductions in parasitemia, mortality, myocarditis and heart parasite burden. These data demonstrate that therapeutic vaccine efficacy is dependent on the antigen and suggest that DNA vaccines encoding Tc24, TSA-1, and Tc52 represent the best candidates for further studies of a therapeutic vaccine against Chagas disease.

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

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

Anatomical route of invasion and protective mucosal immunity in Trypanosoma cruzi conjunctival infection

Trypanosoma cruzi is a protozoan parasite that can initiate mucosal infection after conjunctival exposure. The anatomical route of T. cruzi invasion and spread after conjunctival parasite contamination remains poorly characterized. In the present work we have identified the sites of initial invasion and replication after contaminative conjunctival challenges with T. cruzi metacyclic trypomastigotes using a combination of immunohistochemical and real-time PCR confirmatory techniques in 56 mice between 3 and 14 days after challenge. Our results demonstrate that the predominant route of infection involves drainage of parasites through the nasolacrimal duct into the nasal cavity. Initial parasite invasion occurs within the ductal and respiratory epithelia. After successive waves of intracellular replication and cell-to-cell spread, parasites drain via local lymphatic channels to lymph nodes and then disseminate through the blood to distant tissues. This model of conjunctival challenge was used to identify immune responses associated with protection against mucosal infection. Preceding mucosal infection induces mucosal immunity, resulting in at least 50-fold reductions in recoverable tissue parasite DNA in immune mice compared to controls 10 days after conjunctival challenge (P < 0.05). Antigen-specific gamma interferon production by T cells was increased at least 100-fold in cells harvested from immune mice (P < 0.05). Mucosal secretions containing T. cruzi-specific secretory immunoglobulin A harvested from immune mice were shown to protect against mucosal parasite infection (P < 0.05), demonstrating that mucosal antibodies can play a role in T. cruzi immunity. This model provides an important tool for detailed studies of mucosal immunity necessary for the development of mucosal vaccines.

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

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

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

Long-Term Protective Immunity Induced AgainstTrypanosoma cruziInfection After Vaccination with Recombinant Adenoviruses Encoding Amastigote Surface Protein-2 andTrans-Sialidase

Protection against protozoan parasite Trypanosoma cruzi has been shown to be dependent on the induction of type 1 immune responses. Replication-deficient human type 5 recombinant adenoviruses have an unsurpassed ability to induce type 1 immune responses. Thus, we constructed two type 5 recombinant adenoviruses encoding parasite antigens trans-sialidase (rAdTS) and amastigote surface protein-2 (rAdASP2). Both antigens were genetically engineered to secrete recombinant products in order to induce both optimal antibody and T cell responses. Immunizations of mice with rAdASP2 and rAdTS induced high levels of serum antibodies specific for their recombinant products. In addition, both recombinant viruses were able to elicit a biased helper T cell type 1 (Th1) cellular immune response and a substantial CD8+ T cell-mediated immune response. Moreover, individual immunization with rAdASP2 or rAdTS induced high levels of protection against a challenge with live parasites. CD8+ T cells mediated, at least in part, such protection. Furthermore, when combined in the same inoculum, rAdTS plus rAdASP2 induced complete protection in all animals tested, even when challenges were performed 14 weeks after the last immunization. Taking together, these results show that recombinant adenoviruses expressing TS and ASP-2 antigens of T. cruzi are interesting candidates for the development of a vaccine against Chagas' disease.

Adjuvant effect of the Propionibacterium acnes and its purified soluble polysaccharide on the immunization with plasmidial DNA containing a Trypanosoma cruzi gene

In the present work we investigated the role of killed Propionibacterium acnes or a soluble polysaccharide extracted from bacterium cell wall in modulated experimental immunization with plasmidial DNA. We used a plasmid, p154/13, containing a gene-encoding catalytic domain of Trypanosoma cruzi (T. cruzi) trans-sialidase. As previously described, immunization of BALB/c mice with p154/13 elicited humoral, cell-mediated and protective immune responses against T. cruzi infection. In this study we describe that both P. acnes and its soluble polysaccharide fraction have the ability to modulate the immune response elicited by p154/13. Treatment with these adjuvants enhanced specific trans-sialidase Th1 immune response, as revealed by a lower IgG1/IgG2a ratio and stronger in vitro IFN-gamma synthesis by CD4+ T cells. The most important fact was that treatment with P. acnes or its soluble polysaccharide fraction in the presence of p154/13 significantly reduced the peak of parasitemia observed 7 to 8 days after T. cruzi challenge. These data suggest that P. acnes or its soluble polysaccharide fraction may improve the protective potential of a DNA vaccine against experimental T. cruzi infection.

The non-palindromic adaptor-PCR method for the identification of the T-cell receptor genes of an interferon-gamma-secreting T-cell hybridomaspecific for trans-sialidase, an immunodominant Trypanosoma cruzi antigen

Cloning of the T-cell receptor genes is a critical step when generating T-cell receptor transgenic mice. Because T-cell receptor molecules are clonotypical, isolation of their genes requires reverse transcriptase-assisted PCR using primers specific for each different Valpha or Vbeta genes or by the screening of cDNA libraries generated from RNA obtained from each individual T-cell clone. Although feasible, these approaches are laborious and costly. The aim of the present study was to test the application of the non-palindromic adaptor-PCR method as an alternative to isolate the genes encoding the T-cell receptor of an antigen-specific T-cell hybridoma. For this purpose, we established hybridomas specific for trans-sialidase, an immunodominant Trypanosoma cruzi antigen. These T-cell hybridomas were characterized with regard to their ability to secrete interferon-gamma, IL-4, and IL-10 after stimulation with the antigen. A CD3+, CD4+, CD8- interferon-gamma-producing hybridoma was selected for the identification of the variable regions of the T-cell receptor by the non-palindromic adaptor-PCR method. Using this methodology, we were able to rapidly and efficiently determine the variable regions of both T-cell receptor chains. The results obtained by the non-palindromic adaptor-PCR method were confirmed by the isolation and sequencing of the complete cDNA genes and by the recognition with a specific antibody against the T-cell receptor variable beta chain. We conclude that the non-palindromic adaptor-PCR method can be a valuable tool for the identification of the T-cell receptor transcripts of T-cell hybridomas and may facilitate the generation of T-cell receptor transgenic mice.

CD8+-T-cell-dependent control of Trypanosoma cruzi infection in a highly susceptible mouse strain after immunization with recombinant proteins based on amastigote surface protein 2

We previously described that DNA vaccination with the gene encoding amastigote surface protein 2 (ASP-2) protects approximately 65% of highly susceptible A/Sn mice against the lethal Trypanosoma cruzi infection. Here, we explored the possibility that bacterial recombinant proteins of ASP-2 could be used to improve the efficacy of vaccinations. Initially, we compared the protective efficacy of vaccination regimens using either a plasmid DNA, a recombinant protein, or both sequentially (DNA priming and protein boosting). Survival after the challenge was not statistically different among the three mouse groups and ranged from 53.5 to 75%. The fact that immunization with a recombinant protein alone induced protective immunity revealed the possibility that this strategy could be pursued for vaccination. We investigated this possibility by using six different recombinant proteins representing distinct portions of ASP-2. The vaccination of mice with glutathione S-transferase fusion proteins representing amino acids 261 to 500 or 261 to 380 of ASP-2 in the presence of the adjuvants alum and CpG oligodeoxynucleotide 1826 provided remarkable immunity, consistently protecting 100% of the A/Sn mice. Immunity was completely reversed by the in vivo depletion of CD8(+) T cells, but not CD4(+) T cells, and was associated with the presence of CD8(+) T cells specific for an epitope located between amino acids 320 and 327 of ASP-2. We concluded that a relatively simple formulation consisting of a recombinant protein with a selected portion of ASP-2, alum, and CpG oligodeoxynucleotide 1826 might be used to cross-prime strong CD8(+)-T-cell-dependent protective immunity against T. cruzi infection.

Type 1 immunity provides both optimal mucosal and systemic protection against a mucosally invasive, intracellular pathogen

It has been hypothesized that optimal vaccine immunity against mucosally invasive, intracellular pathogens may require the induction of different types of immune responses in mucosal and systemic lymphoid tissues. Mucosal type 2/3 responses (producing interleukin-4 [IL-4], IL-6 and/or transforming growth factor beta) could be necessary for optimal induction of protective secretory immunoglobulin A responses. On the other hand, systemic type 1 responses (including gamma interferon [IFN-gamma], tumor necrosis factor alpha, and optimal cytotoxic T-cell responses) are likely to be critical for protection against the disseminated intracellular replication that occurs after mucosal invasion. Despite these predictions, we recently found that vaccines inducing highly polarized type 1 immunity in both mucosal and systemic tissues provided optimal mucosal and systemic protection against the protozoan pathogen Trypanosoma cruzi. To further address this important question in a second model system, we now have studied the capacity of knockout mice to develop protective immune memory. T. cruzi infection followed by nifurtimox treatment rescue was used to immunize CD4, CD8, beta2-microglobulin, inducible nitric oxide synthase (iNOS), IL-12, IFN-gamma, and IL-4 knockout mice. Despite the previously demonstrated importance of CD4(+) T cells, CD8(+) T cells, and nitric oxide for T. cruzi immunity, CD4, CD8, and iNOS knockout mice developed mucosal and systemic protective immunity. However, IL-12, IFN-gamma, and beta2-microglobulin-deficient mice failed to develop mucosal or systemic protection. In contrast, IL-4 knockout mice developed maximal levels of both mucosal and systemic immune protection. These results strongly confirm our earlier conclusion from studies with polarizing vaccination protocols that type 1 immunity provides optimal mucosal and systemic protection against a mucosally invasive, intracellular pathogen.

The genome sequence of Trypanosoma cruzi, etiologic agent of Chagas disease

Whole-genome sequencing of the protozoan pathogen Trypanosoma cruzi revealed that the diploid genome contains a predicted 22,570 proteins encoded by genes, of which 12,570 represent allelic pairs. Over 50% of the genome consists of repeated sequences, such as retrotransposons and genes for large families of surface molecules, which include trans-sialidases, mucins, gp63s, and a large novel family (>1300 copies) of mucin-associated surface protein (MASP) genes. Analyses of the T. cruzi, T. brucei, and Leishmania major (Tritryp) genomes imply differences from other eukaryotes in DNA repair and initiation of replication and reflect their unusual mitochondrial DNA. Although the Tritryp lack several classes of signaling molecules, their kinomes contain a large and diverse set of protein kinases and phosphatases; their size and diversity imply previously unknown interactions and regulatory processes, which may be targets for intervention.

Antigen-specific T cells maintain an effector memory phenotype during persistent Trypanosoma cruzi infection

Infection with the protozoan parasite Trypanosoma cruzi is a major cause of morbidity and mortality in Central and South America. Control of acute experimental infection with T. cruzi is dependent on a robust T cell and type 1 cytokine response. However, little evidence exists demonstrating the development and persistence of a potent antiparasite T cell memory response, and there has been much speculation that the majority of the immune response to T. cruzi infection is not directed against the parasite. In this study, we used an experimental mouse model of T. cruzi infection to test both the Ag specificity and the functional and phenotypic characteristics of the responding T cell population. We observed a vigorous antiparasite response from both CD4(+) and CD8(+) T cells that was maintained in the face of persistent infection. T cells from infected mice also proliferated in response to re-exposure to Ag, and CD8(+) T cells underwent spontaneous proliferation when transferred to naive congenic mice, both characteristic of central memory T cells. Interestingly, T cells from infected mice showed significant down-regulation of CD62L, a characteristic associated with an effector memory phenotype. These results suggest that T cells maintained in mice with chronic T. cruzi infection are fully functional memory cells that cannot be easily categorized in the current central/effector memory paradigm.

Protective immunity against trypanosoma cruzi infection in a highly susceptible mouse strain after vaccination with genes encoding the amastigote surface protein-2 and trans-sialidase

Protective immunity against lethal infection is developed when BALB/c or C57BL/6 mice are immunized with plasmids containing genes from the protozoan parasite Trypanosoma cruzi. However, genetic vaccination of the highly susceptible mouse strain A/Sn promoted limited survival after challenge. This observation questioned whether this type of vaccination would be appropriate for highly susceptible individuals. Here, we compared the protective efficacy and the immune response after individual or combined genetic vaccination of A/Sn mice with genes encoding trans-sialidase (TS) or the amastigote surface protein-2 (ASP-2). After challenge, a significant proportion of A/Sn mice immunized with either the asp-2 gene or simultaneously with asp-2 and ts genes, survived infection. In contrast, the vast majority of mice immunized with the ts gene or the vector alone died. Parasitological and histological studies performed in the surviving mice revealed that these mice harbored parasites; however, minimal inflammatory responses were seen in heart and striated muscle. We used this model to search for an in vitro correlation for protection. We found that protective immunity correlated with a higher secretion of interferon- by spleen cells on in vitro restimulation with ASP-2 and the presence of ASP-2-specific CD8 cells. Depletion of either CD4 or CD8 or both T-cell subpopulations prior to the challenge rendered the mice susceptible to infection demonstrating the critical contribution of both cell types in protective immunity. Our results reinforce the prophylactic potential of genetic vaccination with asp-2 and ts genes by describing protective immunity against lethal T. cruzi infection and chronic tissue pathology in a highly susceptible mouse strain.

Generation, specificity, and function of CD8+ T cells in Trypanosoma cruzi infection

CD8(+) T cells are crucial to the control of Trypanosoma cruzi infection and probably act via multiple mechanisms, the most important being the production of interferon-gamma (IFN-gamma). In the absence of CD8(+) T cells, mice quickly succumb to the infection or develop a more severe chronic disease. Reduced production of IFN-gamma by CD8(+) T cells is also associated with increased severity of chagasic disease in humans. CD8(+) T cells in chronic T. cruzi infection are maintained as effector memory cells, undergo rapid expansion, and demonstrate effector functions following re-exposure to antigen. However, the initial generation of T. cruzi-specific CD8(+) T-cell responses appears to be relatively slow to develop. In addition, the expression of the effector function of the CD8(+) T cells is compromised in some tissues, particularly in muscle. The targets of effective CD8(+) T-cell responses in T. cruzi infection are multiple and varied, and they represent some of the best vaccine candidates described to date. Further analysis of CD8(+) T cells will provide insight into the disease process in T. cruzi infection and should identify methods to assess and enhance immunity to T. cruzi infection and protection from the symptoms of Chagas' disease.

Use of a purified Trypanosoma cruzi antigen and CpG oligodeoxynucleotides for immunoprotection against a lethal challenge with trypomastigotes

The crucial role played by Ag163B6/cruzipain, the major cystein proteinase of Trypanosoma cruzi, in the process of parasite internalization into mammalian cells and IgG hydrolysis, signals this antigen as a potential target for raising a protective immune response against Chagas' disease. On the other hand, synthetic oligodeoxynucleotides containing CpG-motifs (CpG-ODN) are capable of driving immunity toward a Th1 bias. Considering the importance of Th1 mechanisms in resistance against this intracellular parasite, we analyzed the ability of Ag163B6/cruzipain plus CpG-ODN to induce immunoprotection against a lethal challenge with trypomastigotes. Mice were immunized with Ag163B6+CpG-ODN showing high specific antibody titers, mostly IgG2a. Spleen cells from these mice strongly proliferated and presented significant increase of IL-2 and IFN-gamma concentrations in their supernatant upon antigen stimulation. Trypomastigote challenge rendered elevated parasitemia and mortality in all control groups, meanwhile Ag163B6+CpG-ODN mice displayed the lowest level of blood parasites and 100% survival to acute infection. Besides, we demonstrated that other parasite antigens introduced into mice when challenged, and consequently never seen before by the immune system, also elicited a Th1 immune response. Taken together, these results plus others provide the basis for the design of a multicomponent anti-T. cruzi vaccine which may ultimately be used not only to protect humans at risk of infection, but also may alleviate or prevent the pathogenic responses characteristic of chronic Chagas' disease by reducing or perhaps eliminating tissue parasites from infected patients.