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

Recombinant Enolase of Trypanosoma cruzi as a Novel Vaccine Candidate against Chagas Disease in a Mouse Model of Acute Infection

Trypanosoma cruzi is the protozoan parasite that causes Chagas disease, which is considered by the World Health Organization to be a neglected tropical disease. Two drugs exist for the treatment of Chagas disease, nifurtimox and benznidazole; they are only effective in the acute phase, and a vaccine is currently not available. In this study, we used the recombinant enolase from T. cruzi H8 strain (MHOM/MX/1992/H8 Yucatán) (rTcENO) and its encoding DNA (pBKTcENO) to immunize mice and evaluate their protective effects in an experimental murine model of acute phase infection. Our results showed that mice vaccinated with rTcENO or its encoding DNA were able to generate typical specific antibodies (IgG1, IgG2a, and IgG2b), suggesting that a mixed Th1/Th2 immune response was induced. The parasite burden in the blood was reduced to 69.8% and 71% in mice vaccinated with rTcENO and pBKTcENO, respectively. The group vaccinated with rTcENO achieved 75% survival, in contrast to the group vaccinated with pBKTcENO that showed no survival in comparison to the control groups. Moreover, rTcENO immunization elevated the production of IFN-γ and IL-2 after the parasite challenge, suggesting that the Th1-type immune response was polarized. These results indicated that rTcENO could be used as a vaccine against Chagas disease.

A Novel Vaccine Approach for Chagas Disease Using Rare Adenovirus Serotype 48 Vectors

Due to the increasing amount of people afflicted worldwide with Chagas disease and an increasing prevalence in the United States, there is a greater need to develop a safe and effective vaccine for this neglected disease. Adenovirus serotype 5 (Ad5) is the most common adenovirus vector used for gene therapy and vaccine approaches, but its efficacy is limited by preexisting vector immunity in humans resulting from natural infections. Therefore, we have employed rare serotype adenovirus 48 (Ad48) as an alternative choice for adenovirus/Chagas vaccine therapy. In this study, we modified Ad5 and Ad48 vectors to contain T. cruzi’s amastigote surface protein 2 (ASP-2) in the adenoviral early gene. We also modified Ad5 and Ad48 vectors to utilize the “Antigen Capsid-Incorporation” strategy by adding T. cruzi epitopes to protein IX (pIX). Mice that were immunized with the modified vectors were able to elicit T. cruzi-specific humoral and cellular responses. This study indicates that Ad48-modified vectors function comparable to or even premium to Ad5-modified vectors. This study provides novel data demonstrating that Ad48 can be used as a potential adenovirus vaccine vector 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.

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.

Recombinant yellow fever viruses elicit CD8+ T cell responses and protective immunity against Trypanosoma cruzi

Chagas’ disease is a major public health problem affecting nearly 10 million in Latin America. Despite several experimental vaccines have shown to be immunogenic and protective in mouse models, there is not a current vaccine being licensed for humans or in clinical trial against T. cruzi infection. Towards this goal, we used the backbone of Yellow Fever (YF) 17D virus, one of the most effective and well-established human vaccines, to express an immunogenic fragment derived from T. cruzi Amastigote Surface Protein 2 (ASP-2). The cDNA sequence of an ASP-2 fragment was inserted between E and NS1 genes of YF 17D virus through the construction of a recombinant heterologous cassette. The replication ability and genetic stability of recombinant YF virus (YF17D/ENS1/Tc) was confirmed for at least six passages in Vero cells. Immunogenicity studies showed that YF17D/ENS1/Tc virus elicited neutralizing antibodies and gamma interferon (IFN-γ) producing-cells against the YF virus. Also, it was able to prime a CD8⁺ T cell directed against the transgenic T. cruzi epitope (TEWETGQI) which expanded significantly as measured by T cell-specific production of IFN-γ before and after T. cruzi challenge. However, most important for the purposes of vaccine development was the fact that a more efficient protective response could be seen in mice challenged after vaccination with the YF viral formulation consisting of YF17D/ENS1/Tc and a YF17D recombinant virus expressing the TEWETGQI epitope at the NS2B-3 junction. The superior protective immunity observed might be due to an earlier priming of epitope-specific IFN-γ-producing T CD8⁺ cells induced by vaccination with this viral formulation. Our results suggest that the use of viral formulations consisting of a mixture of recombinant YF 17D viruses may be a promising strategy to elicit protective immune responses against pathogens, in general.

Trypanosoma cruzi SSP4 Amastigote Protein Induces Expression of Immunoregulatory and Immunosuppressive Molecules in Peripheral Blood Mononuclear Cells

The acute phase of Chagas' disease in mice and human is marked by states of immunosuppression, in which Trypanosoma cruzi replicates extensively and releases immunomodulatory molecules that delay parasite-specific responses mediated by effector T cells. This mechanism of evasion allows the parasite to spread in the host. Parasite molecules that regulate the host immune response during Chagas' disease have not been fully identified, particularly proteins of the amastigote stage. In this work, we evaluated the role of the GPI anchored SSP4 protein of T. cruzi as an immunomodulatory molecule in peripheral blood mononuclear cells (PBMCs). rMBP::SSP4 protein was able to stimulate nitric oxide (NO) production. Likewise, rMBP::SSP4 induced the expression of genes and production of molecules involved in the inflammatory process, such as, cytokines, chemokines, and adhesion molecules (CAMs) as determined by RT-PCR and ELISA. These results suggest that the amastigote SSP4 molecule could play a key role in the immunoregulatory and/or immunosuppressive process observed in the acute phase of infection with T. cruzi.

Neurodegeneration and neuroregeneration in Chagas disease

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

Vaccine development against Trypanosoma cruzi and Chagas disease

The pathology of Chagas disease presents a complicated and diverse picture in humans. The major complications and destructive evolutionary outcomes of chronic infection by Trypanosoma cruzi in humans include ventricular fibrillation, thromboembolism and congestive heart failure. Studies in animal models and human patients have revealed the pathogenic mechanisms during disease progression, pathology of disease and features of protective immunity. Accordingly, several antigens, antigen-delivery vehicles and adjuvants have been tested to elicit immune protection to T. cruzi in experimental animals. This review summarizes the research efforts in vaccine development against Chagas disease during the past decade.

Testing the efficacy of a multi-component DNA-prime/DNA-boost vaccine against Trypanosoma cruzi infection in dogs

Background

Trypanosoma cruzi, the etiologic agent of Chagas Disease, is a major vector borne health problem in Latin America and an emerging infectious disease in the United States.

Methods

We tested the efficacy of a multi-component DNA-prime/DNA-boost vaccine (TcVac1) against experimental T. cruzi infection in a canine model. Dogs were immunized with antigen-encoding plasmids and cytokine adjuvants, and two weeks after the last immunization, challenged with T. cruzi trypomastigotes. We measured antibody responses by ELISA and haemagglutination assay, parasitemia and infectivity to triatomines by xenodiagnosis, and performed electrocardiography and histology to assess myocardial damage and tissue pathology.

Results

Vaccination with TcVac1 elicited parasite-and antigen-specific IgM and IgG (IgG2>IgG1) responses. Upon challenge infection, TcVac1-vaccinated dogs, as compared to non-vaccinated controls dogs, responded to T. cruzi with a rapid expansion of antibody response, moderately enhanced CD8⁺ T cell proliferation and IFN-γ production, and suppression of phagocytes’ activity evidenced by decreased myeloperoxidase and nitrite levels. Subsequently, vaccinated dogs controlled the acute parasitemia by day 37 pi (44 dpi in non-vaccinated dogs), and exhibited a moderate decline in infectivity to triatomines. TcVac1-immunized dogs did not control the myocardial parasite burden and electrocardiographic and histopatholgic cardiac alterations that are the hallmarks of acute Chagas disease. During the chronic stage, TcVac1-vaccinated dogs exhibited a moderate decline in cardiac alterations determined by EKG and anatomo-/histo-pathological analysis while chronically-infected/non-vaccinated dogs continued to exhibit severe EKG alterations.

Conclusions

Overall, these results demonstrated that TcVac1 provided a partial resistance to T. cruzi infection and Chagas disease, and provide an impetus to improve the vaccination strategy against Chagas disease.

Immunization of dogs with DNA-prime/DNA-boost vaccine (TcVac1) enhanced the Trypanosoma cruzi-specific type 1 antibody and CD8⁺ T cell responses that resulted in an early control of acute parasitemia and a moderate decline in pathological symptoms during chronic phase. Further improvement of vaccine-induced immunity would be required to achieve clinical and epidemiological benefits and prevent transmission of parasites from vaccinated/infected dogs to triatomines.

Biological and immunological characterization of recombinant Yellow Fever 17D viruses expressing a Trypanosoma cruzi Amastigote Surface Protein-2 CD8+ T cell epitope at two distinct regions of the genome

BACKGROUND

The attenuated Yellow fever (YF) 17D vaccine virus is one of the safest and most effective viral vaccines administered to humans, in which it elicits a polyvalent immune response. Herein, we used the YF 17D backbone to express a Trypanosoma cruzi CD8+ T cell epitope from the Amastigote Surface Protein 2 (ASP-2) to provide further evidence for the potential of this virus to express foreign epitopes. The TEWETGQI CD8+ T cell epitope was cloned and expressed based on two different genomic insertion sites: in the fg loop of the viral Envelope protein and the protease cleavage site between the NS2B and NS3. We investigated whether the site of expression had any influence on immunogenicity of this model epitope.

RESULTS

Recombinant viruses replicated similarly to vaccine virus YF 17D in cell culture and remained genetically stable after several serial passages in Vero cells. Immunogenicity studies revealed that both recombinant viruses elicited neutralizing antibodies to the YF virus as well as generated an antigen-specific gamma interferon mediated T-cell response in immunized mice. The recombinant viruses displayed a more attenuated phenotype than the YF 17DD vaccine counterpart in mice. Vaccination of a mouse lineage highly susceptible to infection by T. cruzi with a homologous prime-boost regimen of recombinant YF viruses elicited TEWETGQI specific CD8+ T cells which might be correlated with a delay in mouse mortality after a challenge with a lethal dose of T. cruzi.

CONCLUSIONS

We conclude that the YF 17D platform is useful to express T. cruzi (Protozoan) antigens at different functional regions of its genome with minimal reduction of vector fitness. In addition, the model T. cruzi epitope expressed at different regions of the YF 17D genome elicited a similar T cell-based immune response, suggesting that both expression sites are useful. However, the epitope as such is not protective and it remains to be seen whether expression of larger domains of ASP-2, which include the TEWETGQI epitope, will elicit better T-CD8+ responses to the latter. It is likely that additional antigens and recombinant virus formulations will be necessary to generate a protective response.

Effect of a combination DNA vaccine for the prevention and therapy of Trypanosoma cruzi infection in mice: role of CD4+ and CD8+ T cells

Chagas disease is a major public health problem, with about 10 million infected people, and DNA vaccines are a promising alternative for the control of Trypanosoma cruzi, the causing agent of the disease. We tested here a new DNA vaccine encoding a combination of two leading parasite antigens, TSA-1 and Tc24, for the prevention and therapy of T. cruzi infection. Immunized Balb/c mice challenged by T. cruzi presented a significantly lower parasitemia and inflammatory cell density in the heart compared to control mice. Similarly, the therapeutic administration of the DNA vaccine was able to significantly reduce the parasitemia and inflammatory reaction in acutely infected Balb/c and C57BL/6 mice, and reduced cardiac tissue inflammation in chronically infected ICR mice. Therapeutic vaccination induced a marked increase in parasite-specific IFNγ producing CD4(+) and CD8(+) T cells in the spleen as well as an increase in CD4(+) and CD8(+) T cells in the infected cardiac tissue. In addition, some effect of the DNA vaccine could still be observed in CD4-knockout C57BL/6 mice, which presented a lower parasitemia and inflammatory cell density, but not in CD8-deficient mice, in which the vaccine had no effect. These results indicate that the activation of CD8(+) T cells plays a major role in the control of the infection by the therapeutic DNA vaccine, and to a somewhat lesser extent CD4(+) T cells. This observation opens interesting perspectives for the potentiation of this DNA vaccine candidate by including additional CD8(+) T cell antigens/epitopes in future vaccine formulations.

Perforin and gamma interferon expression are required for CD4+ and CD8+ T-cell-dependent protective immunity against a human parasite, Trypanosoma cruzi, elicited by heterologous plasmid DNA prime-recombinant adenovirus 5 boost vaccination

A heterologous prime-boost strategy using plasmid DNA, followed by replication-defective recombinant adenovirus 5, is being proposed as a powerful way to elicit CD4(+) and CD8(+) T-cell-mediated protective immunity against intracellular pathogens. We confirmed this concept and furthered existing research by providing evidence that the heterologous prime-boost regimen using the gene encoding amastigote surface protein 2 elicited CD4(+) and CD8(+) T-cell-mediated protective immunity (reduction of acute parasitemia and prolonged survival) against experimental infection with Trypanosoma cruzi. Protective immunity correlated with the presence of in vivo antigen-specific cytotoxic activity prior to challenge. Based on this, our second goal was to determine the outcome of infection after heterologous prime-boost immunization of perforin-deficient mice. These mice were highly susceptible to infection. A detailed analysis of the cell-mediated immune responses in immunized perforin-deficient mice showed an impaired gamma interferon (IFN-gamma) secretion by immune spleen cells upon restimulation in vitro with soluble recombinant antigen. In spite of a normal numeric expansion, specific CD8(+) T cells presented several functional defects detected in vivo (cytotoxicity) and in vitro (simultaneous expression of CD107a/IFN-gamma or IFN-gamma/tumor necrosis factor alpha) paralleled by a decreased expression of CD44 and KLRG-1. Our final goal was to determine the importance of IFN-gamma in the presence of highly cytotoxic T cells. Vaccinated IFN-gamma-deficient mice developed highly cytotoxic cells but failed to develop any protective immunity. Our study thus demonstrated a role for perforin and IFN-gamma in a number of T-cell-mediated effector functions and in the antiparasitic immunity generated by a heterologous plasmid DNA prime-adenovirus boost vaccination strategy.

Identification of novel vaccine candidates for Chagas' disease by immunization with sequential fractions of a trypomastigote cDNA expression library

The protozoan Trypanosoma cruzi is the etiological agent of Chagas' disease, a major chronic infection in Latin America. Currently, there are neither effective drugs nor vaccines for the treatment or prevention of the disease. Several T. cruzi surface antigens are being tested as vaccines but none of them proved to be completely protective, probably because they represent only a limited repertoire of all the possible T. cruzi target molecules. Taking into account that the trypomastigote stage of the parasite must express genes that allow the parasite to disseminate into the tissues and invade cells, we reasoned that genes preferentially expressed in trypomastigotes represent potential targets for immunization. Here we screened an epimastigote-subtracted trypomastigote cDNA expression library by genetic immunization, in order to find new vaccine candidates for Chagas' disease. After two rounds of immunization and challenge with trypomastigotes, this approach led to the identification of a pool of 28 gene fragments that improved in vivo protection. Sequence analysis of these putative candidates revealed that 19 out of 28 (67.85%) of the genes were hypothetical proteins or unannotated T. cruzi open reading frames, which certainly would not have been identified by other methods of vaccine discovery.