Subdominant/Cryptic CD8 T Cell Epitopes Contribute to Resistance against Experimental Infection with a Human Protozoan Parasite

Abstract

During adaptive immune response, pathogen-specific CD8+ T cells recognize preferentially a small number of epitopes, a phenomenon known as immunodominance. Its biological implications during natural or vaccine-induced immune responses are still unclear. Earlier, we have shown that during experimental infection, the human intracellular pathogen Trypanosoma cruzi restricts the repertoire of CD8+ T cells generating strong immunodominance. We hypothesized that this phenomenon could be a mechanism used by the parasite to reduce the breath and magnitude of the immune response, favoring parasitism, and thus that artificially broadening the T cell repertoire could favor the host. Here, we confirmed our previous observation by showing that CD8+ T cells of H-2a infected mice recognized a single epitope of an immunodominant antigen of the trans-sialidase super-family. In sharp contrast, CD8+ T cells from mice immunized with recombinant genetic vaccines (plasmid DNA and adenovirus) expressing this same T. cruzi antigen recognized, in addition to the immunodominant epitope, two other subdominant epitopes. This unexpected observation allowed us to test the protective role of the immune response to subdominant epitopes. This was accomplished by genetic vaccination of mice with mutated genes that did not express a functional immunodominant epitope. We found that these mice developed immune responses directed solely to the subdominant/cryptic CD8 T cell epitopes and a significant degree of protective immunity against infection mediated by CD8+ T cells. We concluded that artificially broadening the T cell repertoire contributes to host resistance against infection, a finding that has implications for the host-parasite relationship and vaccine development.

About the Authors:

Mariana R. Dominguez

Affiliations Centro de Terapia Celular e Molecular (CTCMol), Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil, Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil

Eduardo L. V. Silveira

Current address: Yerkes National Primate Research Center/Emory University, Atlanta, Georgia, United States of America

Affiliations Centro de Terapia Celular e Molecular (CTCMol), Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil, Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil

José Ronnie C. de Vasconcelos

Affiliations Centro de Terapia Celular e Molecular (CTCMol), Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil, Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil

Bruna C. G. de Alencar

Current address: Institut Curie U932, Paris, France

Affiliations Centro de Terapia Celular e Molecular (CTCMol), Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil, Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil

Alexandre V. Machado

Affiliation: Centro de Pesquisas René Rachou, FIOCRUZ, Belo Horizonte, Minas Gerais, Brazil

Oscar Bruna-Romero

Affiliation: Departamento de Microbiologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Ricardo T. Gazzinelli

Affiliations Departamento de Microbiologia, Instituto de Ciências Biológicas, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil, Departamento de Bioquímica e Imunologia, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil, Division of Infectious Disease and Immunology, Department of Medicine, University of Massachusetts Medical School, Worcester, Massachusetts, United States of America

Mauricio M. Rodrigues

* E-mail: [email protected]

Affiliations Centro de Terapia Celular e Molecular (CTCMol), Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil, Departamento de Microbiologia, Imunologia e Parasitologia, Universidade Federal de São Paulo-Escola Paulista de Medicina, São Paulo, Brazil

Introduction

MHC class Ia-restricted CD8+ T cells are important mediators of the adaptive immune response against infections caused by intracellular microorganisms, including the digenetic intracellular protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease (American trypanosomiasis). During experimental infection, this T cell subpopulation has been shown to be critical for host survival even when small doses of parasites are used in challenges [1]–[5]. In spite of the CD8+ T-cell mediated immune response, the parasite survives within the host and establishes a life-long chronic infection. Parasite persistence is considered one of the critical factors in the development of the complex immunopathology caused by T. cruzi that may occur years after the initial infection in ∼30% of infected individuals [6]–[11]. Thus, understanding how the parasites escape the immune response and persist for such long periods may help us to find new means for interventions against Chagaś disease that would improve quality of life for millions of infected individuals in Latin America

Recent studies on the CD8+ T-cell immune responses that occur during experimental T. cruzi infection in inbred mouse strains described a surprising immunodominance of certain epitopes expressed by members of a large family of T. cruzi surface antigens named trans-sialidases (TS) [1], [5], [12]–[20]. How and why this strong pattern of immunodominance is established is still a matter of debate. In general terms, immunodominance can emerge as a result of different mechanisms that regulate the formation of the complex of MHC-I-peptide on the surface of antigen presenting cells (APC) such as antigen concentration, stability or epitope availability after processing and translocation to the endoplasmic reticulum, where the MHC-I-peptide complex is assembled to be transported to the APC surface [21]–[24].

After a stable MHC-I-peptide complex is formed on the surface of the APC, factors related to CD8+ T cells, such as the frequency of precursors, their TCR affinities, their capacity to proliferate in response to antigen and, thus, be incorporated into the pool of responder cells, are factors that shape immunodominance hierarchies. These factors transcend the MHC restriction element and may create T-cell competition for APCs and other resources, enabling certain CD8+ T cells to dominate and suppress others [23]–[29].

By comparing the specificity of CD8+ T cells of homozygous and heterozygous mouse strains, we observed that the immunodominance that occurs during experimental T. cruzi infection could be exerted not only on epitopes restricted by the same MHC molecules but also, unexpectedly, on the immune response to epitopes restricted by different MHC-I molecules. This phenomenon, termed cross-competition, represents a potent means by which T cells with a certain specificity may become immunodominant [30]–[34]. This strong and unusual phenomenon has been shown to be due to T. cruzi infection because following immunization with recombinant adenovirus expressing the same parasite antigens, this pattern of immunodominance was not observed [15].

Based on these observations, we hypothesized that this competition/immunodomination between T cells of different specificities could be a sophisticated strategy that T. cruzi developed to reduce the breath and magnitude of CD8+ T-cell responses, suppressing the immune responses of these T cells with other specificities in order to escape complete elimination by host effector cells. Thus, we expected that artificially broadening the immune response to include T cells specific for subdominant or cryptic epitopes could favor the host, counteracting the restriction imposed by the infection. Here, we tested this hypothesis by using mice genetically immunized with a mutated form of the amastigote surface protein (asp) - 2 gene in which the immunodominant CD8 T cell epitope is no longer functional. The CD8 T cell-mediated immune response of these mice was directed only to the newly described subdominant/cryptic CD8 T cell epitopes of ASP-2. Even in the absence of an immune response directed to the immunodominant epitope, these mice displayed a significant degree of protective immunity, albeit not as strong as the immune response elicited by the original gene expressing both the immunodominant and the subdominants epitopes. These results are compatible with our hypothesis that artificially broadening the immune response favors the host. Indirectly, we suggest that immunodominance may in fact be a mechanism to establish a chronic infection.

Materials and Methods

Ethics Statement

All experimental procedures were approved by the Ethics Committee for Animal Care of the Federal University of São Paulo (Id # CEP 0426/09).

Mice and parasites

Female 8-week-old H-2a mice (B10.A and A/Sn) were purchased from CEDEME (Federal University of São Paulo). Bloodstream trypomastigotes of the Y strain of T. cruzi were obtained from A/Sn mice infected 7–8 days earlier [12]. Each B10.A or A/Sn mouse was challenged i.p. with a final dose containing 104 or 150 parasites, respectively, in a final volume of 0.2 mL. Parasite development was monitored by counting the number of bloodstream trypomastigotes in 5 µL of fresh blood collected from the tail vein [12].

Peptides

Peptides were purchased from Genscript (Piscataway, NJ). Purity was as follows: TEWETGQI (95%); PETLGHEI (97.4%); YEIVAGYI (99.40%); TPTAGLVGF (98.6%); GSRNGNDRL (97.1%); ESKSGDAPL (96.1%); HEHNLFGI (98.7%); ESSTPTAGL (99.1%); ESEPKRPNM (98.7%); VSWGEPKSL (99.2%); YSDGALHLL (97.3%); AESWPSIV (96.5%); and RPNMSRHLF (99.4%).

Recombinant plasmids and adenoviruses

Plasmid pIgSPCl.9 and the human replication-defective adenovirus type 5 containing the asp-2 gene were obtained as described previously [35], [36]. Mutated asp-2 was generated by a series of PCR reactions using DNA encoding the asp-2 clone 9 gene as a template (Genbank Accession Number: AY186572). In the first reaction, the forward and reverse oligonucleotides were as follows:

1. 5′-GGGGGTACCATGCTCTCACGTGTTGCT-3′;

2. 5′-GAACGATCATGAGTGCTTGGCCCGTCTCCCATGCGGTGATGCGGGGATC-3′

In the second reaction, they were as follows:

1. 5′-GATCCCCGCATCACCGCATGGGAGACGGGACAAGCACTCATGATCGTTC-3′

2. 5′-GGGTCTAGATCAGACCATTTTTAGTTCACC-3′.

PCR products were purified, mixed and subjected to a third PCR reaction. Forward and reverse oligonucleotides were respectively

1. 5′-GGGGGTACCATGCTCTCACGTGTTGCT-3′;

2. 5′-GGGTCTAGATCAGACCATTTTTAGTTCACC-3′.

The final PCR product was completely sequenced. The only modifications found were in the nucleotide sequences encoding the immunodominant epitope TEWETGQI. The new sequence encoded the amino acids (AA) TAWETGQA. This plasmid is referred to as pIgSpTAWETGQA. The new gene was also subcloned into the pAdCMV shuttle vector, and the recombinant replication-defective adenovirus human adenovirus 5 was produced by Vectors BioLabs, Philadelphia, USA. This new recombinant adenovirus is referred to as AdTAWETGQA. Viruses and plasmids were purified as described previously [35]–[37]. Mice were inoculated intra-muscularly (i.m.) in each tibialis anterioris muscle with 50 µg of plasmid DNA 3 times every 3 weeks.

Heterologous prime-boost immunization consisted of priming i.m. with a total of 100 µg of plasmid DNA followed by a dose of viral suspension containing 2×108 plaque forming units (pfu) of adenovirus twenty-one days later in the same locations. Immunological assays or challenges were performed 14 days after viral inoculation.

In vivo depletion of CD8+ T cells were performed by treating vaccinated A/Sn mice with 53.6.7 MAb. At days 2 and 3 before challenge with trypomastigotes, mice were injected i.p. with a dose of 1 mg of anti-CD8 or control Rat IgG. Seven days after challenge, each mouse received one more dose of 1 mg of anti-CD8 or Rat IgG. The efficacy of depletion of CD8+ spleen cells before challenge was more than 95% in anti-CD8 treated mice compared to Rat IgG treated ones.

Immunological T cell assays

Ex vivo ELISPOT (IFN-γ) or in vivo cytotoxic assays were performed exactly as described previously [12], [15]. The surface mobilization of CD107a and the intracellular expression of cytokines (IFN-γ, TNF-α, IL-2 and IL-10) was evaluated after in vitro culture of splenocytes in the presence or absence of antigenic stimulus. Cells were washed three times in plain RPMI and re-suspended in cell culture medium consisting of RPMI 1640 medium, pH 7.4, supplemented with 10 mM Hepes, 0.2% sodium bicarbonate, 59 mg/l of penicillin, 133 mg/l of streptomycin, and 10% Hyclone fetal bovine sera (Hyclone, Logan, Utah). The viability of the cells was evaluated using 0.2% Trypan Blue exclusion dye to discriminate between live and dead cells. Cell concentration was adjusted to 5×106 cells/mL in cell culture medium containing anti-CD28 (2 µg/mL), Brefeldin A (10 µg/mL), Monensin (5 µg/mL) and FITC-labeled anti-CD107a (Clone 1D4B, 2 µg/mL, BD Pharmingen). In half of the cultures, a final concentration of 10 µM of the VNHRFTLV peptide was added. The cells were cultivated in flat-bottom 96-well plates (Corning) in a final volume of 200 µl in duplicate, at 37°C in a humid environment. After a 20-h incubation, cells were stained for surface markers with Per-CP or PE-labeled anti-CD8 on ice for 20 min. To detect IFN-γ, TNF-α? IL-2 and IL-10 by intra-cellular staining (ICS), cells were then washed twice in buffer containing PBS, 0.5% BSA and 2 mM EDTA, fixed in 4% PBS-paraformaldehyde solution for 10 minutes and permeabilized for 15 minutes in a PBS, 0.1% BSA, 0.1% saponin solution. After being washed twice, cells were stained for intracellular markers using APC or PE-labeled anti-IFN-γ (Clone XMG1.2) and? PE- labeled anti-TNF-α (clone MP6-XT22), APC-labeled anti-IL-2 (clone JES6-5H4) or APC-labeled anti-IL-10 (JES5-16E3) for 20 minutes on ice. Finally, cells were washed twice and fixed in 1% PBS-paraformaldehyde. At least 300,000 cells were acquired on a BD FacsCanto flow cytometer and then analyzed with FlowJo.

Statistical analysis

Values were expressed as means ± SD. These values were compared using one-way ANOVA followed by Tukey's HSD tests (http://faculty.vassar.edu/lowry/VassarStats.html). The LogRank test was used to compare mouse survival rates after challenge with T. cruzi (http://bioinf.wehi.edu.au/software/russell/logrank/). The differences were considered significant when the P value was <0.05.

Results

During experimental infection of H-2b or H-2a inbred mouse strains with parasites of the Y strain of T. cruzi, two epitopes were identified within the ASP-2 antigen represented by the VNHRFTLV or TEWETGQI peptides. They were recognized by H-2Kb- or H-2Kk-restricted CD8+ cytotoxic T cells, respectively [12], [14], [15], [37], [38]. ASP-2 is a member of the large family of TS surface antigens and is abundantly expressed only in the intracellular forms (amastigotes) of T. cruzi [39]. Fig. 1A and Fig. 1B show, respectively, some of the structural features of ASP-2 antigen and its expression by amastigotes.

[Figure omitted. See PDF.]

Figure 1. Structure and localization of Amastigote Surface Protein-2 (ASP-2).

A- Schematic view of the primary structure of T. cruzi ASP-2. B- HeLa cells were infected for 48 h with trypomastigotes of the Y strain. After fixation, indirect immunofluorescence or DAPI staining were performed as described using MAb K22 and imaged using fluorescence microscopy [39]. Bar, 14 µM.

https://doi.org/10.1371/journal.pone.0022011.g001

To test whether the H-2Kk-restricted epitope TEWETGQI could be an immunodominant epitope, a series of synthetic peptides containing the predicted AA anchor motif for binding to the H-2Kk or H-2Ld alleles of mouse MHC haplotype H-2a was employed. No epitope was predicted to bind to H-2Dd (Table 1). After infection of H-2a mice, antigen-specific IFN-γ producing cells could only be detected in the presence of the peptide TEWETGQI (Fig. 2A). We concluded that this epitope was the immunodominant epitope of ASP-2 during infection of H-2a mice with the Y strain of T. cruzi. H-2a mice used in this experiment were B10.A because they are resistant to infection with T. cruzi.

[Figure omitted. See PDF.]

Figure 2. CD8 T-cell epitope identification during immune responses of H-2a mice infected with T. cruzi or genetically vaccinated with asp-2 gene.

B10.A mice were infected with 104 T. cruzi blood parasites. A/S mice were immunized with pIgSPCl.9 followed by AdASP-2 and injected i.m. at 0 and 3 weeks, respectively. Control mice were naïve or injected with pcDNA3 followed by pcDNA3/Adβ-gal. A- Two weeks after infection or the final immunizing dose, splenic cells were re-stimulated in vitro in the presence of medium or the indicated peptides at a final concentration of 10 µM. The number of splenic IFN-γ spot-forming cells (SFC) was estimated by ex vivo ELISPOT assay. B- In vivo cytotoxic activity was estimated by injecting each mouse with syngeneic CFSE-labeled splenic cells coated with or without 2 µM of the indicated peptide. Results are expressed as means ± SD of 4 mice per group and are representative of experiments performed at least twice with similar results. Asterisks denote that the number of SFC or in vivo cytotoxicity were significantly higher when compared to naïve or pcDNA3/Adβ-gal injected mice (P<0.01, one-way ANOVA).

https://doi.org/10.1371/journal.pone.0022011.g002

[Figure omitted. See PDF.]

Table 1. Peptides used in the study.

https://doi.org/10.1371/journal.pone.0022011.t001

We showed previously that the strong pattern of immunodominance observed following infection with T. cruzi was not duplicated in mice genetically immunized with a recombinant adenovirus expressing ASP-2 (AdASP-2, ref. 15). Then, we determined whether genetically immunized H-2a mice could present a different pattern of immunodominance. We used a genetic immunization approach which consisted of a heterologous prime-boost regimen using plasmid DNA followed by a recombinant adenovirus both containing the same asp-2 gene. This protocol provided strong and long lasting protective immunity against experimental infection mediated by CD8+ T cells [37], [38]. In these experiments, we used H-2a mice of the A/Sn strain. These mice are highly susceptible to infection with T. cruzi, allowing us to perform protective immunity studies [37], [38]. Nevertheless, it is important to mention that the results were similar when we used B10.A mice.

After ex vivo stimulation with our synthetic peptides, as expected, IFN-γ producing cells were detected following stimulation with the TEWETGQI peptide. In addition to this epitope, two other epitopes (PETLGHEI and YEIVAGYI) induced IFN-γ production by immune cells (Fig. 2A). These peptide-specific IFN-γ producing cells were CD8+ T cells as determined by simultaneous staining of intra-cellular IFN-γ and the surface marker CD8 (see below).

These peptides were recognized by cytotoxic cells in H-2a mice as determined by in vivo cytotoxicity assays using target cells coated with each of these peptides. B10.A mice infected with T. cruzi developed strong in vivo cytotoxicity against target cells coated with the peptide TEWETGQI. In contrast, very limited (if any) in vivo cytotoxicity was observed against target cells coated with peptides PETLGHEI and YEIVAGYI. These results were not due to different kinetics of the immune response because we observed the same results 14 or 28 days after an infectious challenge (Fig. 2B).

However, A/Sn (H-2a) mice genetically vaccinated with a heterologous prime-boost vaccination regimen displayed easily detectable in vivo cytotoxic activity against target cells coated with any of these three peptides. The elimination of target cells coated with peptide TEWETGQI was always stronger than the two others, suggesting a pattern of immunodominance.

To determine whether other cytokines and/or effector molecules could be secreted by peptide-specific T cells, we performed staining to detect surface mobilization of CD107a (a marker for exocytosis) or intra-cellular accumulation of IFN-γ, TNF-α, IL-2 or IL-10. When we used splenic cells from B10.A mice infected with T. cruzi, we observed that upon stimulation with TEWETGQI, a large fraction of CD8+ cells mobilize CD107a to the surface (data not shown) and accumulate intra-cellular IFN-γ and TNF-α (Fig. 3B). These CD8+ cells were therefore multifunctional CD107a+IFN-γ+TNF-α+ (∼35%) or IFN-γ+TNF-α+ (∼40%). We were unable to detect the presence of significant numbers of IL-2 or IL-10 expressing cells in these same samples (Fig. 3C and 3D). The expression of these cytokines was dependent on the infection because they were not detected in cells from naive mice (Fig. 3A).

[Figure omitted. See PDF.]

Figure 3. Surface mobilization of CD107a and expression of IFN-γ, TNF-α, IL-2 or IL-10 by specific CD8+ T cells from B10.A mice infected with T. cruzi or A/Sn mice immunized with pIgSCl.9/AdASP-2 vaccine.

B10.A or A/Sn mice were infected or immunized as described in the legend of Fig. 2. Control mice were either naive mice or mice immunized with pCDNA3/Adβ-gal. Twenty one or fourteen days after infection or immunization, respectively, these mice had their splenic cells cultured in the presence of anti-CD107a and anti-CD28, with or without the peptides TEWETGQI, PETLGHEI or YEIVAGYI. After 12 h, cells were stained for CD8, IFN-γ, TNF-α, IL-2 and IL-10. Representative analyses (medians) are shown from four mice performed per experiment. A) Example of splenic CD8+ cells from B10.A naive mice cultivated in vitro in the presence of medium alone (Medium) or with the indicated peptides and stained for expression of IFN-γ and TNF-α. B, C and D) Examples of splenic CD8+ cells from B10.A infected mice cultivated in vitro in the presence of medium alone (Medium) or with the indicated peptides and stained for expression of: B) IFN-γ and TNF-α; C) IFN-γ and IL-2; D) IFN-γ and IL-10. E and F) Examples of splenic CD8+ cells from mice immunized with pcDNA3/Adβ-gal cultivated in vitro in the presence of medium alone (Medium) or with the indicated peptides and stained for surface mobilization of CD107a and expression IFN-γ (panel E) or expression of IFN-γ and TNF-α (panel F). G and H) Examples of splenic CD8+ cells from mice immunized with pIgSPCl.9/AdASP-2 cultivated in vitro in the presence of medium alone (Medium) or with the indicated peptides and stained for surface mobilization of CD107a and expression IFN-γ (panel G) and expression IFN-γ and TNF-α (panel H). I and J) Determination of multifunctional CD8+ cells from mice immunized with pIgSPCl.9/AdASP-2 cultivated in vitro in the presence of the indicated peptides and stained for surface mobilization of CD107a and expression IFN-γ and TNF-α.

https://doi.org/10.1371/journal.pone.0022011.g003

In contrast, a relatively low frequency of CD8+ splenic cells stimulated with peptides PETLGHEI or YEIVAGYI mobilized CD107a to the surface (data not shown) or accumulated intra-cellular IFN-γ or TNF-α (Fig. 3B). We were also unable to detect the presence of significant numbers of IL-2 or IL-10 expressing cells in these same cells (Fig. 3C and 3D).

Splenic cells from A/Sn mice genetically vaccinated with heterologous prime-boost regimen were stimulated with peptides TEWETGQI, PETLGHEI or YEIVAGYI. The results shows that a large fraction of the CD8+ cells of pIgSPCl.9/AdASP-2 immunized mice at the same time mobilize CD107a to the surface and expressed intra-cellular IFN-γ and TNF-α (Fig. 3G to 3J). These cells were therefore multifunctional CD8+ T cells as we have previously described [37]. We were unable to detect the presence of intra-cellular IL-2 or IL-10 in these same cells (data not shown). These results confirmed and extended the ones described in Fig. 2).

The description of these two new epitopes allowed us to test whether immunity to the subdominant/cryptic epitope could participate during protective immunity against T. cruzi infection, a phenomenon that has not previously been tested experimentally. For this purpose, we generated a plasmid DNA and a recombinant adenovirus containing a mutated form of the asp-2 gene in which we modified the nucleotides encoding the anchor residues required for the immunodominant TEWETGQI epitope to bind to the H-2Kk molecule. The mutated gene expressed the AA sequence TAWETGQA, where the alanines (A) replaced glutamic acid (E) or isoleucine (I) of the original epitope. In preliminary experiments, we observed that the synthetic peptide TAWETGQA was not recognized by immune cells from genetically vaccinated H-2a mice (data not shown). Details of the plasmid and recombinant adenovirus containing the mutated form of the asp-2 gene are shown in Table 2.

[Figure omitted. See PDF.]

Table 2. Genetic vectors used in the study.

https://doi.org/10.1371/journal.pone.0022011.t002

Initially, we genetically immunized mice with plasmids containing the original gene (pIgSPCl.9), the mutated gene (pIgSPTAWETGQA) or both plasmids simultaneously. Immune responses were estimated by ELISPOT after ex vivo stimulation with the immunodominant epitope TEWETGQI or with the subdominant/cryptic epitopes PETLGHEI or YEIVAGYI two weeks after challenge with T. cruzi. We chose this protocol because, as the immune response following plasmid DNA vaccination is usually low, it is easier to detect the anamnestic immune responses after challenge [12]. We observed that all mice immunized with asp-2 genes (mutated or not) presented specific IFN-γ producing cells when stimulated with subdominant/cryptic epitopes (PETLGHEI or YEIVAGYI, Fig. 4A). The immune response was specific because mice immunized with control plasmid pcDNA3 failed to recognize these peptides. However, we detected IFN-γ producing cells specific to TEWETGQI only in mice immunized with plasmid pIgSPCl.9. It is noteworthy that the number of cells detected in mice immunized with pIgSPTAWETGQA was similar to the number of cells in pcDNA3 injected mice (Fig. 4A). These numbers reflect cells primed during infection. Analysis of in vivo cytotoxic activity also demonstrated that in mice immunized with the plasmid pIgSPTAWETGQA, the response to the immunodominant epitope TEWETGQI was not different from control mice immunized with pcDNA3 (Fig. 4B). These immunological analyses demonstrated that mice immunized with pIgSPTAWETGQA indeed lost the functional immunodominant TEWETGQI response but had an unaltered ability to elicit immune responses to the subdominant epitopes PETLGHEI and YEIVAGYI.

[Figure omitted. See PDF.]

Figure 4. CD8 immune responses and trypomastigote-induced parasitemia and mortality in A/Sn mice genetically immunized with different plasmid DNA containing the asp-2 gene.

A/Sn mice were immunized with pcDNA3, pIgSPCl.9, pIgSPTAWETGQA or simultaneously with the last two (pIgSPCl.9 and pIgSPTAWETGQA). Immunization consisted of 3 doses of 100 µg of DNA each given by the i.m. route in the tibialis anterioris three weeks apart. A- Two weeks after the final immunizing dose, mice were challenged i.p. with 150 bloodstream trypomastigotes. Two weeks after infection, splenic cells were re-stimulated in vitro in the presence of medium only or the indicated peptides at a final concentration of 10 µM. The number of splenic IFN-γ spot-forming cells (SFC) was estimated by ex vivo ELISPOT assay. B- In vivo cytotoxic activity was estimated by injecting each mouse with syngeneic CFSE-labeled splenic cells coated with or without 2 µM of the indicated peptide. Results are expressed as means ± SD of 4 mice per group and are representative of experiments performed at least twice with similar results. Asterisks denote that the number of SFC, or the in vivo cytotoxicity, were significantly higher when compared to SFC found in naïve or pcDNA3/Adβ-gal-injected mice (P<0.01). C- Parasitemia for each mouse group is represented as mean ± SD (n = 5–7). Asterisks denote that mice from groups immunized with pIgSPCl.9 or pIgSPTAWETGQA or both had significantly lower parasitemia (P<0.01) than animals injected with pcDNA3. The curves of parasitemia of animals immunized with pIgSPCl.9 (squares) or pIgSPTAWETGQA (triangles) are superimposed. D- Kaplan-Meier curves for the survival of mouse groups immunized and challenged as described above (n = 5–7). Mice from groups immunized with pIgSPCl.9 or pIgSPCl.9/pIgSPTAWETGQA survived significantly longer than animals injected with pcDNA3 or pIgSPTAWETGQA (P<0.05, in all cases, LogRank test). Mice immunized with pIgSPTAWETGQA also survived longer than animals injected with pcDNA3 (P<0.05). No animals died after the 30th day until they were euthanized. Results are representative of two independent experiments.

https://doi.org/10.1371/journal.pone.0022011.g004

After challenge, mice immunized with plasmids containing the asp-2 gene (mutated or not) presented significantly lower parasitemia than control mice immunized with pcDNA3 (Fig. 4C). Although the levels of parasitemia were not statistically different when compared to mice vaccinated with the plasmid containing the mutated asp-2 gene, the mortality of these animals was significantly faster (Fig. 4D). We therefore concluded that broadening the CD8+ T cell immune response by vaccination with a plasmid containing epitopes that elicit immune responses to subdominant/cryptic epitopes of ASP-2 could provide some degree of protective immunity. Nevertheless, because protective immunity elicited by vaccination with pIgSPTAWETGQA was not as efficient, the presence of a functional immunodominant epitope was clearly important for effective protective immunity.

We then sought to test the same hypotheses described above using a distinct approach. For that purpose, H-2a mice were primed with plasmid pIgSPCl.9 followed by a booster immunization with AdASP-2 (heterologous prime- boost regimen). Alternatively, mice were primed with plasmid pIgTAWETGQA followed by a booster immunization with AdTAWETGQA. We consider this approach complementary to the one described above because plasmid or adenovirus may used distinct routes for stimulating CD8+ T cells.

Immune responses were estimated 14 days after the booster immunization by ELISPOT following ex vivo stimulation with synthetic peptides encoding the immunodominant epitope TEWETGQI, the subdominant/cryptic epitopes PETLGHEI or YEIVAGYI or the mutated epitope TAWETGQA. We chose this protocol because the immune responses following heterologous prime boost immunization generates strong immune responses that can be easily detected after boosting [37]. We observed that mice immunized with asp-2 genes (mutated or not) had specific IFN-γ producing cells when stimulated with peptides PELTHGEI or YEIVAGYI. The fact that these responses were of similar magnitude strongly argued that the expression/immunogenicity of both genes/antigens were very similar. We could detect IFN-γ producing cells specific for the TEWETGQI epitope only in mice immunized with pIgSPCl.9/AdASP-2. In contrast, none of the immunized mice presented IFN-γ producing cells when stimulated with the TAWETGQA peptide (Fig. 5A). The analysis of in vivo cytotoxic activity showed a similar picture (Fig. 5B). We also performed intra-cellular cytokine staining analysis for IFN-γ and TNF-α after in vitro peptide stimulation. As depicted in Fig. 5C, we detected CD8+ cells expressing IFN-γ and/or TNF-α specific for the TEWETGQI epitope only in mice immunized with pIgSPCl.9/AdASP-2 (Fig. 5C). Similar analyses performed 14 days after an infectious challenge with T. cruzi exhibited the same pattern of response (data not shown). Together, these immunological analyses demonstrated that heterologous prime-boost immunization with pIgSPTAWETGQA/AdTAEWTGQA failed to induce an immune response to the immunodominant epitope TEWETGQI but induced almost unaltered immune responses to the subdominant epitopes PETLGHEI or YEIVAGYI.

[Figure omitted. See PDF.]

Figure 5. CD8 immune responses in A/Sn mice immunized with asp-2 using the heterologous DNA prime-adenovirus boost vaccination regimen.

A/Sn mice were primed i.m. with 100 µg of plasmids pcDNA3, pIgSPCl.9 or pIgSPTAWETGQA. Three weeks later, these mice were boosted i.m. with 2×108 pfu Adβ-gal, AdASP-2 or AdTAWETGQA. A- Two weeks after the last dose, splenic cells were re-stimulated in vitro in the presence of medium only or the indicated peptides at a final concentration of 10 µM. The number of splenic IFN-γ spot forming cells (SFC) was estimated by ex vivo ELISPOT assay. B- In vivo cytotoxic activity was estimated by injecting each mouse with syngeneic CFSE-labeled splenic cells coated with or without 2 µM of the indicated peptide. Results are expressed as mean ± SD of 4 mice per group and are representative of experiments performed at least twice with similar results. Asterisks denote that the number of SFC or in vivo cytotoxicity were significantly higher when compared to SFC found in naïve or pcDNA3/Adβ-gal injected mice (P<0.01). C- Fourteen days after the last dose, these mice had their splenic cells cultured in the presence of anti-CD28 and Medium or the indicated peptides. After 12 h, cells were stained for CD8, IFN-γ and TNF-α. Examples of splenic CD8+ cells from immunized mice. Representative analyses (medians) are shown from four mice performed per experiment.

https://doi.org/10.1371/journal.pone.0022011.g005

After challenge, parasitemia in mice immunized with the asp-2 genes (mutated or not) was significantly lower than in control mice injected with pcDNA3/Adβ-gal (P<0.01, Fig. 6A). Although most of the mice immunized with pIgSPTAWETGQA/AdTAEWTGQA died after challenge, they survived longer than control mice injected with pcDNA3/Adβ-gal (Fig. 6B, P<0.01, LogRank test). In parallel, vaccinated and control mice were challenged by the s.c. route. We observed that the majority of the mice immunized with pIgSPTAWETGQA/AdTAEWTGQA survived the infectious challenge (Fig. 6D). We therefore concluded that broadening the T cell immune response using a genetic vaccination that elicits immune responses to subdominant/cryptic epitopes of ASP-2 provides protective immunity against infection. Nevertheless, likewise in the case of the plasmid DNA, after an i.p. challenge, in terms of survival, immunization with only genes expressing subdominant epitopes was not as effective as immunization with genes expressing both the dominant and the subdominant epitopes.

[Figure omitted. See PDF.]

Figure 6. Trypomastigote-induced parasitemia and mortality in A/Sn mice immunized with asp-2 using the heterologous DNA prime-adenovirus boost vaccination regimen.

A/Sn mice were immunized as depicted in the legend of Fig. 5. Two weeks after the final immunizing dose, mice were challenged i.p. (Panels A and B) or s.c. (Panels C and D) with 150 bloodstream trypomastigotes. Parasitemia for each mouse group is represented as mean ± SD (n = 10 or 11). Asterisks denote that mice from groups immunized with pIgSPCl.9/AdASP-2 or pIgSPTAWETGQA/AdTAWTEGQA had significantly lower parasitemia (P<0.01) than pcDNA3/Adβ-gal-injected animals. Panels B and D represent Kaplan-Meier curves for survival of the mouse groups immunized and challenged as described above (n = 10 or 11). Mice immunized with pIgSPCl.9/AdASP-2 survived significantly longer than animals immunized with pIgSPTAWETGQA/AdTAWETGQA or pcDNA3/Adβ-gal (P = 0.01 or P<0.01, respectively). Mice immunized with pIgSPTAWETGQA/AdTAWETGQA survived significantly longer than pcDNA3/Adβ-gal-injected animals (P<0.01). Results are representative of two pooled experiments. No animals died after the 40th day.

https://doi.org/10.1371/journal.pone.0022011.g006

Finally, to firmly establish that protective immunity was mediated by CD8+ T cells, we performed in vivo depletion experiments in mice vaccinated with the pIgSPCl.9/AdASP-2 or pIgSPTAWETGQA/AdTAEWTGQA. Treatment with anti-CD8 MAb renders these mice more susceptible to infection. CD8 depleted mice presented higher parasitemia (Fig. 7A and B) and shorter survival times (Fig. 7C and D) when compared to A/Sn mice vaccinated with heterologous prime-boost regimen (pIgSPCl.9/AdASP-2 or pIgSPTAWETGQA/AdTAEWTGQA) and treated with rat IgG. CD8 depleted mice had their survival time reduced to the same time as control mice which were injected with pcDNA3/Adβ-gal.

[Figure omitted. See PDF.]

Figure 7. CD8 T cell dependence of protective immunity of A/Sn mice immunized with asp-2 using the heterologous DNA prime-adenovirus boost vaccination regimen.

A/Sn mice were immunized as described in the legend of Fig. 5. pIgSPTAWETGQA/AdTAWTEGQA had significantly lower parasitemia (P<0.01) than pcDNA3/Adβ-gal-injected animals. Before and after challenge, mice were treated as described in Methods section with rat IgG (control) or anti-CD8 MAb. The parasitemia for each mouse group is represented as mean ± SD (n = 6). Asterisks denote that mice from groups immunized with pIgSPCl.9/AdASP-2 or pIgSPTAWETGQA/AdTAWTEGQA and treated with Rat IgG had significantly lower parasitemia (P<0.01) than vaccinated mice treated with anti-CD8 (Panels A and B). Panels C and D represent Kaplan-Meier curves for survival of the mouse groups immunized and challenged as described above (n = 6). Mice immunized with pIgSPCl.9/AdASP-2 or pIgSPTAWETGQA/AdTAWETGQA and treated with Rat IgG survived significantly longer than vaccinated animals treated with anti-CD8 (P<0.01 in both cases).

https://doi.org/10.1371/journal.pone.0022011.g007

Discussion

Here, we initially confirmed and extended our previous observation that experimental infection with the human intracellular pathogen T. cruzi restricted the repertoire of CD8+ T cells. While immune cells of infected H-2a mice recognized a single immunodominant epitope of ASP-2, cells from mice immunized with recombinant genetic vaccines expressing this same T. cruzi antigen recognized, in addition to the immunodominant epitope, two other subdominant/cryptic epitopes. The sub-dominant epitopes not only failed to elicit IFN-γ and in vivo cytotoxicity during infection (Fig. 2), but they also did not stimulate TNF-α, IL-2 or IL-10 secretion by CD8+ T cells (Fig. 3). The precise reason for this strong immunodominant pattern during T. cruzi infection is unknown at present. One possible explanation for this biased immune response could be to provide an advantage to the parasite by avoiding an even higher and broader immune response. This hypothesis is in agreement with earlier studies showing that immunity to subdominant epitopes can provide an important contribution to protective immunity against viral infection [34], [40]–[43]. Our results also corroborated this hypothesis. Immunity to epitopes that are not commonly recognized during infection (cryptic) provided a significant degree of protective immunity. Nonetheless, immune responses against these cryptic epitopes did not substitute completely for CD8+ T cells specific for the immunodominant TEWETGQI epitope. We observed that immunization with plasmids alone or in combination with a recombinant adenovirus that did not express the immunodominant epitope elicited immune responses to the subdominant/cryptic epitopes but failed to provide optimal protective immunity when compared to a plasmid that expresses both the immunodominant and subdominants epitopes (Fig. 4D, 6B and 7D). These results demonstrate that the response to the immunodominant epitope contributes to the immunity elicited by genetic vaccination and is might be required for highly efficient resistance.

In previous studies, we showed that immunization with short proteins in the presence of the TLR9 agonist CpG elicited CD8+ T-cell mediated immunity against T. cruzi infection in A/Sn mice [44]. These short proteins contained only the AA 261 to 500 or 261 to 380. In both cases, they did not express the subdominant/cryptic epitopes. Based on that, we concluded that immunization with the immundominant CD8 epitope alone could provide a high degree of protective immunity even in the absence of the subdominant epitopes [44]. The fact that the immune response directed solely to the immunodominant epitope can provide significant degree of protective immunity against protozoan parasites has been established a long time ago by the use of adoptively transferred T cell clones or heterelogous-prime boost vaccination regimen [45]–[49].

In earlier studies in which we depleted CD8+ T cells from genetically vaccinated mice, we observed that these mice were unable to control parasitemia and died at the same time as control unvaccinated animals [37]. Therefore, although genetic immunization elicits effector CD4+ T cells, these cells do not account for the protection we observed. This concept was further corroborated with experiments of CD8 T cell-depletion performed here (Fig. 7A to D).

Using a different approach, a similar conclusion was also reached by Rosemberg et al., 2010 [17]. In their study, they induced simultaneous tolerance to two immunodominant T. cruzi epitopes in a resistant mouse strain. Following infection, an increased susceptibility to infection was observed. Nevertheless, they were still able to control and survive the experimental infection. This protective immunity was possibly mediated by CD8+ T cells specific to subdominant epitopes that substituted for the immunodominant ones. The AA sequences of these subdominant/cryptic epitopes have yet to be identified. Together with our study, they strongly support the notion that the immune responses to both dominant and subdominant/cryptic epitopes can be important for controlling experimental T. cruzi infection in inbred mouse strains. These results are in agreement with the observation with other parasites such as Plasmodium. Tolerance to or removal of the the immunodominant CD8 epitope of P. yoelii led to the development of immunity to CD8 subdominant epitopes as well [50], [51].

The mechanism operating during T. cruzi infection to restrict the immune response leading to immunodominance has yet to be characterized. We provide initial evidence that it could be explained by T cell competition for APCs by showing that in mice infected simultaneously with two different parasite strains containing different immunodominant epitopes, we could generate maximal responses to both epitopes without immunodominance or competition [15]. Our interpretation was that if the epitopes are presented by different APCs, then the immunodominant pattern is disrupted. However, a more formal demonstration using bone marrow chimeric mice studies is lacking. At the molecular level, this strong immunodominance can be explained by the type of antigen presentation that predominates during T. cruzi infection. In recent studies, important evidence has been provided that subdominant epitopes can only be directly presented by the expressing cells, which might occur in the case of the recombinant adenovirus. However, during indirect priming (cross-priming), these epitopes would be at a disadvantage [52]. T. cruzi may use cross-priming as the dominant route, drastically reducing the priming of subdominant epitopes.

In addition to shedding some light on the host-parasite relationship, our results may have important implications for the development of T cell vaccines against parasitic diseases. In our earlier studies, we observed that genetic vaccination with a heterologous prime-boost regimen employing plasmid DNA and recombinant adenovirus elicited strong, long lasting CD8+ T cell-mediated protective immunity against experimental infection in a mouse strain highly susceptible to T. cruzi infection [37], [38]. Here, we demonstrated that the CD8 T cell-mediated protective immunity observed was directed to three distinct epitopes, two of which are cryptic. The strategy of redirecting immunity to epitopes that are not usually targets of the naturally acquired immune response has been proposed as a possible means to improve immunity against viral infection [40]–[43]. Recently, this strategy was also proven useful to improve vaccination against T. cruzi infection [53].

Finally, our observations may have important implications regarding the basis for the strong immunodominance pattern observed after experimental infection with other intracellular parasites such as Plasmodium, Toxoplasma gondii and Theileria parva [50], [51], [54]-[57].

Acknowledgments

The authors are in debt with Dr. C. Claser for providing the immunofluorescence picture of Fig. 1. We are also in debt with Drs. Laurent Renia (Singapore Immunology Network), Dr. Chris Ibegbu (Emory Vaccine Center), Dr. Fanny Tzelepis (University of Ottawa) and Dr. Silvia B. Boscardin, for careful reviewing the manuscript.

Author Contributions

Conceived and designed the experiments: MRD ELVS JRCdV BCGdA MMR. Performed the experiments: MRD ELVS JRCdV BCGdA. Analyzed the data: MRD ELVS AVM OB-R RTG MMR. Contributed reagents/materials/analysis tools: AVM OB-R RTG. Wrote the paper: MMR.

Citation: Dominguez MR, Silveira ELV, de Vasconcelos JRC, de Alencar BCG, Machado AV, Bruna-Romero O, et al. (2011) Subdominant/Cryptic CD8 T Cell Epitopes Contribute to Resistance against Experimental Infection with a Human Protozoan Parasite. PLoS ONE6(7): e22011. https://doi.org/10.1371/journal.pone.0022011

References

1. Tarleton RL (2007) Immune system recognition of Trypanosoma cruzi. Curr Opin Immunol 19: 430–4.RL Tarleton2007Immune system recognition of Trypanosoma cruzi.Curr Opin Immunol194304

2. Miyahira Y (2008) Trypanosoma cruzi infection from the view of CD8+ T cell immunity–an infection model for developing T cell vaccine. Parasitol Int 57: 38–48.Y. Miyahira2008Trypanosoma cruzi infection from the view of CD8+ T cell immunity–an infection model for developing T cell vaccine.Parasitol Int573848

3. Padilla AM, Bustamante JM, Tarleton RL (2009) CD8+ T cells in Trypanosoma cruzi infection. Curr Opin Immunol 21: 385–90.AM PadillaJM BustamanteRL Tarleton2009CD8+ T cells in Trypanosoma cruzi infection.Curr Opin Immunol2138590

4. Boscardin SB, Torrecilhas AC, Manarin R, Revelli S, Rey EG, et al. (2010) Chagas' disease: an update on immune mechanisms and therapeutic strategies. J Cell Mol Med 14: 1373–84.SB BoscardinAC TorrecilhasR. ManarinS. RevelliEG Rey2010Chagas' disease: an update on immune mechanisms and therapeutic strategies.J Cell Mol Med14137384

5. Junqueira C, Caetano B, Bartholomeu DC, Melo MB, Ropert C, et al. (2010) The endless race between Trypanosoma cruzi and host immunity: lessons for and beyond Chagas disease. Expert Rev Mol Med 12: e29.C. JunqueiraB. CaetanoDC BartholomeuMB MeloC. Ropert2010The endless race between Trypanosoma cruzi and host immunity: lessons for and beyond Chagas disease.Expert Rev Mol Med12e29

6. Kierszenbaum F (2005) Where do we stand on the autoimmunity hypothesis of Chagas disease? Trends Parasitol 21: 513–6.F. Kierszenbaum2005Where do we stand on the autoimmunity hypothesis of Chagas disease?Trends Parasitol215136

7. Marin-Neto JA, Cunha-Neto E, Maciel BC, Simões MV (2008) Pathogenesis of chronic Chagas heart disease. Circulation 115: 1109–23.JA Marin-NetoE. Cunha-NetoBC MacielMV Simões2008Pathogenesis of chronic Chagas heart disease.Circulation115110923

8. Bonney KM, Engman DM (2008) Chagas heart disease pathogenesis: one mechanism or many? Curr Mol Med 8: 510–8.KM BonneyDM Engman2008Chagas heart disease pathogenesis: one mechanism or many?Curr Mol Med85108

9. Gutierrez FR, Guedes PM, Gazzinelli RT, Silva JS (2009) The role of parasite persistence in pathogenesis of Chagas heart disease. Parasite Immunol 31: 673–85.FR GutierrezPM GuedesRT GazzinelliJS Silva2009The role of parasite persistence in pathogenesis of Chagas heart disease.Parasite Immunol3167385

10. Scharfstein J, Gomes Jde A, Correa-Oliveira R (2009) Mem Inst Oswaldo Cruz. Back to the future in Chagas disease: from animal models to patient cohort studies, progress in immunopathogenesis research 104: Suppl 1187–98.J. ScharfsteinA. Gomes JdeR. Correa-Oliveira2009Mem Inst Oswaldo Cruz.Back to the future in Chagas disease: from animal models to patient cohort studies, progress in immunopathogenesis research104Suppl 118798

11. Lannes-Vieira J, Silverio JC, Pereira IR, Vinagre NF, Carvalho CM, et al. (2009) Chronic Trypanosoma cruzi-elicited cardiomyopathy: from the discovery to the proposal of rational therapeutic interventions targeting cell adhesion molecules and chemokine receptors–how to make a dream come true. Mem Inst Oswaldo Cruz 104: Suppl 1226–35.J. Lannes-VieiraJC SilverioIR PereiraNF VinagreCM Carvalho2009Chronic Trypanosoma cruzi-elicited cardiomyopathy: from the discovery to the proposal of rational therapeutic interventions targeting cell adhesion molecules and chemokine receptors–how to make a dream come true.Mem Inst Oswaldo Cruz104Suppl 122635

12. Tzelepis F, de Alencar BC, Penido ML, Gazzinelli RT, Persechini PM, et al. (2006) Distinct kinetics of effector CD8+ cytotoxic T cells after infection with Trypanosoma cruzi in naive or vaccinated mice. Infect Immun 74: 2477–81.F. TzelepisBC de AlencarML PenidoRT GazzinelliPM Persechini2006Distinct kinetics of effector CD8+ cytotoxic T cells after infection with Trypanosoma cruzi in naive or vaccinated mice.Infect Immun74247781

13. Martin DL, Weatherly DB, Laucella SA, Cabinian MA, Crim MT, et al. (2006) CD8+ T-Cell responses to Trypanosoma cruzi are highly focused on strain-variant trans-sialidase epitopes. PLoS Pathog 2(8): e77.DL MartinDB WeatherlySA LaucellaMA CabinianMT Crim2006CD8+ T-Cell responses to Trypanosoma cruzi are highly focused on strain-variant trans-sialidase epitopes.PLoS Pathog28e77

14. Tzelepis F, Persechini PM, Rodrigues MM (2007) Modulation of CD4+ T cell-dependent specific cytotoxic CD8+ T cells differentiation and proliferation by the timing of increase in the pathogen load. PLoS One 2(4): e393.F. TzelepisPM PersechiniMM Rodrigues2007Modulation of CD4+ T cell-dependent specific cytotoxic CD8+ T cells differentiation and proliferation by the timing of increase in the pathogen load.PLoS One24e393

15. Tzelepis F, de Alencar BC, Penido ML, Claser C, Machado AV, et al. (2008) Infection with Trypanosoma cruzi restricts the repertoire of parasite-specific CD8+ T cells leading to immunodominance. J Immunol 180: 1737–48.F. TzelepisBC de AlencarML PenidoC. ClaserAV Machado2008Infection with Trypanosoma cruzi restricts the repertoire of parasite-specific CD8+ T cells leading to immunodominance.J Immunol180173748

16. Bixby LM, Tarleton RL (2008) Stable CD8+ T cell memory during persistent Trypanosoma cruzi infection. J Immunol 181: 2644–50.LM BixbyRL Tarleton2008Stable CD8+ T cell memory during persistent Trypanosoma cruzi infection.J Immunol181264450

17. Rosenberg CS, Martin DL, Tarleton RL (2010) CD8+ T cells specific for immunodominant trans-sialidase epitopes contribute to control of Trypanosoma cruzi infection but are not required for resistance. J Immunol 185: 560–8.CS RosenbergDL MartinRL Tarleton2010CD8+ T cells specific for immunodominant trans-sialidase epitopes contribute to control of Trypanosoma cruzi infection but are not required for resistance.J Immunol1855608

18. Freire-de-Lima L, Alisson-Silva F, Carvalho ST, Takiya CM, Rodrigues MM, et al. (2010) Trypanosoma cruzi subverts host cell sialylation and may compromise antigen-specific CD8+ T cell responses. J Biol Chem 285: 13388–96.L. Freire-de-LimaF. Alisson-SilvaST CarvalhoCM TakiyaMM Rodrigues2010Trypanosoma cruzi subverts host cell sialylation and may compromise antigen-specific CD8+ T cell responses.J Biol Chem2851338896

19. Oliveira AC, de Alencar BC, Tzelepis F, Klezewsky W, da Silva RN, et al. (2010) Impaired innate immunity in Tlr4(-/-) mice but preserved CD8+ T cell responses against Trypanosoma cruzi in Tlr4-, Tlr2-, Tlr9- or Myd88-deficient mice 29; 6(4): e1000870.AC OliveiraBC de AlencarF. TzelepisW. KlezewskyRN da Silva2010Impaired innate immunity in Tlr4(-/-) mice but preserved CD8+ T cell responses against Trypanosoma cruzi in Tlr4-, Tlr2-, Tlr9- or Myd88-deficient mice 29;64e1000870

20. Rodrigues MM, de Alencar BC, Claser C, Tzelepis F (2009) Immunodominance: a new hypothesis to explain parasite escape and host/parasite equilibrium leading to the chronic phase of Chagas' disease? Braz J Med Biol Res 42: 220–3.MM RodriguesBC de AlencarC. ClaserF. Tzelepis2009Immunodominance: a new hypothesis to explain parasite escape and host/parasite equilibrium leading to the chronic phase of Chagas' disease?Braz J Med Biol Res422203

21. Chen W, Antón LC, Bennink JR, Yewdell JW (2000) Dissecting the multifactorial causes of immunodominance in class I-restricted T cell responses to viruses. Immunity 12: 83–93.W. ChenLC AntónJR BenninkJW Yewdell2000Dissecting the multifactorial causes of immunodominance in class I-restricted T cell responses to viruses.Immunity128393

22. Tenzer S, Wee E, Burgevin A, Stewart-Jones G, Friis L, et al. (2009) Antigen processing influences HIV-specific cytotoxic T lymphocyte immunodominance. Nat Immunol 10: 636–46.S. TenzerE. WeeA. BurgevinG. Stewart-JonesL. Friis2009Antigen processing influences HIV-specific cytotoxic T lymphocyte immunodominance.Nat Immunol1063646

23. Yewdell JW (2006) Confronting complexity: real-world immunodominance in antiviral CD8+ T cell responses. Immunity 25: 533–43.JW Yewdell2006Confronting complexity: real-world immunodominance in antiviral CD8+ T cell responses.Immunity2553343

24. Irvine K, Bennink J (2006) Factors influencing immunodominance hierarchies in TCD8+ -mediated antiviral responses. Expert Rev Clin Immunol 2: 135–47.K. IrvineJ. Bennink2006Factors influencing immunodominance hierarchies in TCD8+ -mediated antiviral responses.Expert Rev Clin Immunol213547

25. La Gruta NL, Kedzierska K, Pang K, Webby R, Davenport M, et al. (2006) A virus-specific CD8+ T cell immunodominance hierarchy determined by antigen dose and precursor frequencies. Proc Natl Acad Sci USA 103: 994–9.NL La GrutaK. KedzierskaK. PangR. WebbyM. Davenport2006A virus-specific CD8+ T cell immunodominance hierarchy determined by antigen dose and precursor frequencies.Proc Natl Acad Sci USA1039949

26. Kotturi MF, Scott I, Wolfe T, Peters B, Sidney J, et al. (2008) Naive precursor frequencies and MHC binding rather than the degree of epitope diversity shape CD8+ T cell immunodominance. J Immunol 181: 2124–33.MF KotturiI. ScottT. WolfeB. PetersJ. Sidney2008Naive precursor frequencies and MHC binding rather than the degree of epitope diversity shape CD8+ T cell immunodominance.J Immunol181212433

27. Sacha JB, Reynolds MR, Buechler MB, Chung C, Jonas AK, et al. (2008) Differential antigen presentation kinetics of CD8+ T-cell epitopes derived from the same viral protein. J Virol 82: 9293–8.JB SachaMR ReynoldsMB BuechlerC. ChungAK Jonas2008Differential antigen presentation kinetics of CD8+ T-cell epitopes derived from the same viral protein.J Virol8292938

28. La Gruta NL, Rothwell WT, Cukalac T, Swan NG, Valkenburg SA, et al. (2010) Primary CTL response magnitude in mice is determined by the extent of naive T cell recruitment and subsequent clonal expansion. J Clin Invest 120: 1885–94.NL La GrutaWT RothwellT. CukalacNG SwanSA Valkenburg2010Primary CTL response magnitude in mice is determined by the extent of naive T cell recruitment and subsequent clonal expansion.J Clin Invest120188594

29. Chen W, McCluskey J (2009) Immunodominance and immunodomination: critical factors in developing effective CD8+ T-cell-based cancer vaccines. Adv Cancer Res 95: 203–47.W. ChenJ. McCluskey2009Immunodominance and immunodomination: critical factors in developing effective CD8+ T-cell-based cancer vaccines.Adv Cancer Res9520347

30. Jenkins MR, Webby R, Doherty PC, Turner SJ (2009) Addition of a prominent epitope affects influenza A virus-specific CD8+ T cell immunodominance hierarchies when antigen is limiting. J Immunol 177: 2917–25.MR JenkinsR. WebbyPC DohertySJ Turner2009Addition of a prominent epitope affects influenza A virus-specific CD8+ T cell immunodominance hierarchies when antigen is limiting.J Immunol177291725

31. Willis RA, Kappler JW, Marrack PC (2006) CD8 T cell competition for dendritic cells in vivo is an early event in activation. Proc Natl Acad Sci USA 103: 12063–8.RA WillisJW KapplerPC Marrack2006CD8 T cell competition for dendritic cells in vivo is an early event in activation.Proc Natl Acad Sci USA103120638

32. Newberg MH, McEvers KJ, Gorgone DA, Lifton MA, Baumeister SH, et al. (2006) Immunodomination in the evolution of dominant epitope-specific CD8+ T lymphocyte responses in simian immunodeficiency virus-infected rhesus monkeys. J. Immunol 176: 319–28.MH NewbergKJ McEversDA GorgoneMA LiftonSH Baumeister2006Immunodomination in the evolution of dominant epitope-specific CD8+ T lymphocyte responses in simian immunodeficiency virus-infected rhesus monkeys.J. Immunol17631928

33. Kastenmuller W, Gasteiger G, Gronau JH, Baier R, Ljapoci R, et al. (2007) Cross-competition of CD8+ T cells shapes the immunodominance hierarchy during boost vaccination. J Exp Med 204: 2187–98.W. KastenmullerG. GasteigerJH GronauR. BaierR. Ljapoci2007Cross-competition of CD8+ T cells shapes the immunodominance hierarchy during boost vaccination.J Exp Med204218798

34. Riedl P, Wieland A, Lamberth K, Buus S, Lemonnier F, et al. (2009) Elimination of immunodominant epitopes from multispecific DNA-based vaccines allows induction of CD8 T cells that have a striking antiviral potential. J Immunol 183: 370–80.P. RiedlA. WielandK. LamberthS. BuusF. Lemonnier2009Elimination of immunodominant epitopes from multispecific DNA-based vaccines allows induction of CD8 T cells that have a striking antiviral potential.J Immunol18337080

35. Vasconcelos JR, Hiyane MI, Marinho CR, Claser C, Machado AM, et al. (2004) 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. Hum Gene Ther 15: 878–86.JR VasconcelosMI HiyaneCR MarinhoC. ClaserAM Machado2004Protective immunity against trypanosoma cruzi infection in a highly susceptible mouse strain after vaccination with genes encoding the amastigote surface protein-2 and trans-sialidase.Hum Gene Ther1587886

36. Machado AV, Cardoso JE, Claser C, Rodrigues MM, Gazzinelli RT, et al. (2006) Long-term protective immunity induced against Trypanosoma cruzi infection after vaccination with recombinant adenoviruses encoding amastigote surface protein-2 and trans-sialidase. Hum Gene Ther 17: 898–908.AV MachadoJE CardosoC. ClaserMM RodriguesRT Gazzinelli2006Long-term protective immunity induced against Trypanosoma cruzi infection after vaccination with recombinant adenoviruses encoding amastigote surface protein-2 and trans-sialidase.Hum Gene Ther17898908

37. de Alencar BC, Persechini PM, Haolla FA, de Oliveira G, Silverio JC, et al. (2009) 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. Infect Immun 77: 4383–95.BC de AlencarPM PersechiniFA HaollaG. de OliveiraJC Silverio2009Perforin 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.Infect Immun77438395

38. Haolla FA, Claser C, de Alencar BC, Tzelepis F, de Vasconcelos JR, et al. (2009) Strain-specific protective immunity following vaccination against experimental Trypanosoma cruzi infection. Strain-specific protective immunity following vaccination against experimental Trypanosoma cruzi infection. Vaccine 27: 5644–53.FA HaollaC. ClaserBC de AlencarF. TzelepisJR de Vasconcelos2009Strain-specific protective immunity following vaccination against experimental Trypanosoma cruzi infection. Strain-specific protective immunity following vaccination against experimental Trypanosoma cruzi infection.Vaccine27564453

39. Claser C, Espíndola NM, Sasso G, Vaz AJ, Boscardin SB, et al. (2007) Immunologically relevant strain polymorphism in the Amastigote Surface Protein 2 of Trypanosoma cruzi. Microbes Infect 9: 1011–9.C. ClaserNM EspíndolaG. SassoAJ VazSB Boscardin2007Immunologically relevant strain polymorphism in the Amastigote Surface Protein 2 of Trypanosoma cruzi.Microbes Infect910119

40. Oukka M, Manuguerra JC, Livaditis N, Tourdot S, Riche N, et al. (1996) Protection against lethal viral infection by vaccination with nonimmunodominant peptides. J Immunol 157: 3039–45.M. OukkaJC ManuguerraN. LivaditisS. TourdotN. Riche1996Protection against lethal viral infection by vaccination with nonimmunodominant peptides.J Immunol157303945

41. Friedrich TC, Valentine LE, Yant LJ, Rakasz EG, Piaskowski SM, et al. (2007) Subdominant CD8+ T-cell responses are involved in durable control of AIDS virus replication. J Virol 81: 3465–76.TC FriedrichLE ValentineLJ YantEG RakaszSM Piaskowski2007Subdominant CD8+ T-cell responses are involved in durable control of AIDS virus replication.J Virol81346576

42. Holtappels R, Simon CO, Munks MW, Thomas D, Deegen P, et al. (2008) Subdominant CD8 T-cell epitopes account for protection against cytomegalovirus independent of immunodomination. J Virol 82: 5781–96.R. HoltappelsCO SimonMW MunksD. ThomasP. Deegen2008Subdominant CD8 T-cell epitopes account for protection against cytomegalovirus independent of immunodomination.J Virol82578196

43. Ruckwardt TJ, Luongo C, Malloy AM, Liu J, Chen M, et al. (2010) Responses against a subdominant CD8+ T cell epitope protect against immunopathology caused by a dominant epitope. J Immunol 185: 4673–80.TJ RuckwardtC. LuongoAM MalloyJ. LiuM. Chen2010Responses against a subdominant CD8+ T cell epitope protect against immunopathology caused by a dominant epitope.J Immunol185467380

44. Araújo AF, de Alencar BC, Vasconcelos JR, Hiyane MI, Marinho CR, et al. (2005) 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. Infect Immun 73: 6017–25.AF AraújoBC de AlencarJR VasconcelosMI HiyaneCR Marinho2005CD8+-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.Infect Immun73601725

45. Rodrigues M, Nussenzweig RS, Romero P, Zavala F (1992) The in vivo cytotoxic activity of CD8+ T cell clones correlates with their levels of expression of adhesion molecules. J Exp Med 175: 895–905.M. RodriguesRS NussenzweigP. RomeroF. Zavala1992The in vivo cytotoxic activity of CD8+ T cell clones correlates with their levels of expression of adhesion molecules.J Exp Med175895905

46. Li S, Rodrigues M, Rodriguez D, Rodriguez JR, Esteban M, et al. (1993) Priming with recombinant influenza virus followed by administration of recombinant vaccinia virus induces CD8+ T-cell-mediated protective immunity against malaria. Proc Natl Acad Sci USA 90: 5214–18.S. LiM. RodriguesD. RodriguezJR RodriguezM. Esteban1993Priming with recombinant influenza virus followed by administration of recombinant vaccinia virus induces CD8+ T-cell-mediated protective immunity against malaria.Proc Natl Acad Sci USA90521418

47. Rodrigues M, Li S, Murata K, Rodriguez D, Rodriguez JR, et al. (1994) Influenza and vaccinia viruses expressing malaria CD8+ T and B cell epitopes. Comparison of their immunogenicity and capacity to induce protective immunity. J Immunol 153: 4636–48.M. RodriguesS. LiK. MurataD. RodriguezJR Rodriguez1994Influenza and vaccinia viruses expressing malaria CD8+ T and B cell epitopes. Comparison of their immunogenicity and capacity to induce protective immunity.J Immunol153463648

48. Sedegah M, Jones TR, Kaur M, Hedstrom R, Hobart P, et al. (1998) Boosting with recombinant vaccinia increases immunogenicity and protective efficacy of malaria DNA vaccine. Proc Natl Acad Sci USA 95: 7648–53.M. SedegahTR JonesM. KaurR. HedstromP. Hobart1998Boosting with recombinant vaccinia increases immunogenicity and protective efficacy of malaria DNA vaccine.Proc Natl Acad Sci USA95764853

49. Schneider J, Gilbert SC, Blanchard TJ, Hanke T, Robson KJ, et al. (1998) Enhanced immunogenicity for CD8+ T cell induction and complete protective efficacy of malaria DNA vaccination by boosting with modified vaccinia virus Ankara. Nat Med 4: 397–402.J. SchneiderSC GilbertTJ BlanchardT. HankeKJ Robson1998Enhanced immunogenicity for CD8+ T cell induction and complete protective efficacy of malaria DNA vaccination by boosting with modified vaccinia virus Ankara.Nat Med4397402

50. Kumar KA, Sano G, Boscardin S, Nussenzweig RS, Nussenzweig MC, et al. (2006) The circumsporozoite protein is an immunodominant protective antigen in irradiated sporozoites. Nature 444: 937–40.KA KumarG. SanoS. BoscardinRS NussenzweigMC Nussenzweig2006The circumsporozoite protein is an immunodominant protective antigen in irradiated sporozoites.Nature44493740

51. Mauduit M, Grüner AC, Tewari R, Depinay N, Kayibanda M, et al. (2009) A role for immune responses against non-CS components in the cross-species protection induced by immunization with irradiated malaria sporozoites. PLoS One 4(11): e7717.M. MauduitAC GrünerR. TewariN. DepinayM. Kayibanda2009A role for immune responses against non-CS components in the cross-species protection induced by immunization with irradiated malaria sporozoites.PLoS One411e7717

52. Pavelic V, Matter MS, Mumprecht S, Breyer I, Ochsenbein AF (2009) CTL induction by cross-priming is restricted to immunodominant epitopes. Eur J Immunol 39: 704–16.V. PavelicMS MatterS. MumprechtI. BreyerAF Ochsenbein2009CTL induction by cross-priming is restricted to immunodominant epitopes.Eur J Immunol3970416

53. Cazorla SI, Frank FM, Becker PD, Arnaiz M, Mirkin GA, et al. (2010) Redirection of the immune response to the functional catalytic domain of the cystein proteinase cruzipain improves protective immunity against Trypanosoma cruzi infection. J Infect Dis 202: 136–44.SI CazorlaFM FrankPD BeckerM. ArnaizGA Mirkin2010Redirection of the immune response to the functional catalytic domain of the cystein proteinase cruzipain improves protective immunity against Trypanosoma cruzi infection.J Infect Dis20213644

54. Blanchard N, Gonzalez F, Schaeffer M, Joncker NT, Cheng T, et al. (2008) Immunodominant, protective response to the parasite Toxoplasma gondii requires antigen processing in the endoplasmic reticulum. Nat Immunol 9: 937–44.N. BlanchardF. GonzalezM. SchaefferNT JonckerT. Cheng2008Immunodominant, protective response to the parasite Toxoplasma gondii requires antigen processing in the endoplasmic reticulum.Nat Immunol993744

55. Frickel EM, Sahoo N, Hopp J, Gubbels MJ, Craver MP, et al. (2008) Parasite stage-specific recognition of endogenous Toxoplasma gondii-derived CD8+ T cell epitopes. J Infect Dis 198: 1625–33.EM FrickelN. SahooJ. HoppMJ GubbelsMP Craver2008Parasite stage-specific recognition of endogenous Toxoplasma gondii-derived CD8+ T cell epitopes.J Infect Dis198162533

56. Wilson DC, Grotenbreg GM, Liu K, Zhao Y, Frickel EM, et al. (2010) Differential regulation of effector- and central-memory responses to Toxoplasma gondii Infection by IL-12 revealed by tracking of Tgd057-specific CD8+ T cells. PLoS Pathog 6(3): e1000815.DC WilsonGM GrotenbregK. LiuY. ZhaoEM Frickel2010Differential regulation of effector- and central-memory responses to Toxoplasma gondii Infection by IL-12 revealed by tracking of Tgd057-specific CD8+ T cells.PLoS Pathog63e1000815

57. MacHugh ND, Connelley T, Graham SP, Pelle R, Formisano P, et al. (2009) CD8+ T-cell responses to Theileria parva are preferentially directed to a single dominant antigen: Implications for parasite strain-specific immunity. Eur J Immunol 39: 2459–69.ND MacHughT. ConnelleySP GrahamR. PelleP. Formisano2009CD8+ T-cell responses to Theileria parva are preferentially directed to a single dominant antigen: Implications for parasite strain-specific immunity.Eur J Immunol39245969