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Infect Immun. May 2003; 71(5): 2744–2757.
PMCID: PMC153249

Immunization with cDNA Expressed by Amastigotes of Trypanosoma cruzi Elicits Protective Immune Response against Experimental Infection

Abstract

Immunization of mice with plasmids containing Trypanosoma cruzi genes induced specific antibodies, CD4+ Th1 and CD8+ Tc1 cells, and protective immunity against infection. In most cases, plasmids used for DNA vaccination contained genes encoding antigens expressed by trypomastigotes, the nonreplicative forms of the parasite. In this study, we explored the possibility of using genes expressed by amastigotes, the form of the parasite which replicates inside host cells, for experimental DNA vaccination. For that purpose, we selected a gene related to the amastigote surface protein 2 (ASP-2), an antigen recognized by antibodies and T cells from infected mice and humans, for our study. Using primers specific for the asp-2 gene, four distinct groups of genes were amplified from cDNA from amastigotes of the Y strain of T. cruzi. At the nucleotide level, they shared 82.3 to 89.9% identity with the previously described asp-2 gene. A gene named clone 9 presented the highest degree of identity with the asp-2 gene and was selected for immunological studies. Polyclonal antisera raised against the C terminus of the recombinant protein expressed by the clone 9 gene reacted with an antigen of approximately 83 kDa expressed in amastigotes of T. cruzi. Immunization of BALB/c mice with eukaryotic expression plasmids containing the clone 9 gene elicited specific antibodies and CD4+ T-cell-dependent gamma interferon secretion. Upon challenge with trypomastigotes, mice immunized with plasmids harboring the clone 9 gene displayed reduced parasitemia and survived lethal infection. We concluded that amastigote cDNA is an interesting source of antigens that can be used for immunological studies, as well as for vaccine development.

Trypanosoma cruzi is an obligate intracellular protozoan parasite and the etiologic agent of Chagas' disease. Despite the significant reduction in transmission observed in several countries in the last 20 years, Chagas' disease is still a major health problem for many Latin American countries, afflicting more than 15 million individuals and causing thousands of deaths every year. The poor prospect of treatment raises the possibility that immune interventions, such as immunization, could be used as an additional weapon to improve disease prevention and treatment efficacy.

Recently, independent groups established that immunization with plasmids containing T. cruzi genes generated immune responses mediated by antibodies, CD4+ and CD8+ T cells. Mice vaccinated with DNA displayed remarkable protective immunity, surviving otherwise lethal infection with T. cruzi (5, 9, 10, 28, 38, 45). Genes used for DNA vaccination against experimental Chagas' disease were mainly expressed by trypomastigotes of T. cruzi. Genes expressed by amastigotes, the form of the parasite that multiplies inside host cells, have only very recently been subject to similar studies (10). Based on the limited information on that subject, the present study was designed to explore the possibility of using a gene expressed by amastigotes for DNA vaccination against experimental Chagas' disease.

Toward that goal, to initiate our studies, we selected the gene encoding amastigote surface protein 2 (ASP-2), a surface antigen expressed by amastigotes of T. cruzi, that has been described previously (19, 20, 26, 47). This protein was initially described by Pan and McMahon-Pratt in 1989 (26) with the aid of a stage- and species-specific monoclonal antibody (MAb) raised against purified membrane components of T. cruzi amastigotes. An 83-kDa antigen was purified with the aid of the MAb, and its N-terminal amino acid sequence was determined. The observation that the purified antigen was specifically recognized by antibodies from individuals with Chagas' disease residing in geographically distant regions of South America was very important (26).

Based on the known N-terminal amino acid sequence, a cDNA corresponding to an 83-kDa surface protein was cloned by reverse transcription-PCR (RT-PCR) from RNA from amastigotes. The protein encoded by this cDNA was named ASP-2 (20). The predicted amino acid sequence encoded by the asp-2 gene contains one aspartic acid box motif (ASP box; SXDXGXTW) present in bacterial sialidases and the highly conserved motif VTVXNVXLYNR characteristic of type three module of fibronectin. On the basis of its predicted primary structure, the asp-2 gene was assigned to subfamily II of the sialidase/trans-sialidase gene superfamily of T. cruzi (6).

Using synthetic peptides based on the predicted amino acid sequence encoded by the asp-2 gene, it was possible to demonstrate that ASP-2 contains multiple major histocompatibility complex (MHC) class I-restricted epitopes recognized by specific CD8+ cytotoxic T lymphocytes (CTL) from mice and humans infected with T. cruzi (19, 47). These CTL specific for ASP-2 lysed target cells infected with T. cruzi, demonstrating that infected host cells process and present ASP-2 to CTL (19). Together with the work by Pan and McMahon-Pratt (26), these studies provided a strong immunological rationale for the selection of ASP-2 to initiate our studies on DNA immunization with genes expressed by amastigotes of T. cruzi.

In the present study, we characterized cDNA genes containing nucleotide sequences with high identity to the asp-2 gene expressed by amastigotes of the Y strain of T. cruzi. A gene named clone 9 shared the highest degree of identity with the previously described asp-2 gene sequence and was selected for immunological studies of experimental genetic vaccination. BALB/c mice were immunized with eukaryotic expression plasmids harboring the clone 9 gene. In these mice, we studied the presence of immune responses by looking at the presence of specific antibodies and gamma interferon (IFN-γ)-secreting spleen cells, as well as protective immunity against infection with bloodstream trypomastigotes.

MATERIALS AND METHODS

Mice and parasites.

Female 5- to 8-week-old BALB/c and C57BL/6 mice used in this study were purchased from the University of São Paulo.

Parasites of the Y strain of T. cruzi were used in this study (41). Epimastigotes were maintained in liver infusion tryptose medium containing 10% (vol/vol) fetal bovine serum (FBS) (Life Technologies) at 28°C. Intracellular amastigotes were obtained from infected L6E6 cells (American Type Culture Collection, Rockville, Md.) grown in RPMI medium (Life Technologies) containing 10% FBS at 37°C in an atmosphere containing 5% CO2. The intracellular forms were obtained from infected cells by scraping the monolayer with the aid of a cell scraper (Costar) in a buffer containing 27 mM K2HPO4, 8 mM Na2HPO4, and 26 mM KH2PO4, pH 7.2. The cells were lysed with the aid of a Potter-Elvehjem homogenizer. The solution containing free parasites and cell debris was placed on top of a Ficoll-Paque gradient (Pharmacia Biotech). After centrifugation at 800 × g for 20 min, the purified parasites were collected from the bottom of the tube. Amastigotes were subsequently washed twice in plain RPMI medium.

Trypomastigotes were collected from the extracellular medium of infected LLC-MK2 cells (kindly provided by S. Schenkman, Universidade Federal de São Paulo-Escola Paulista de Medicina [UNIFESP], São Paulo, Brazil) 5 to 7 days after infection, centrifuged at 2,000 × g for 10 min, and recovered from the supernatant 3 h later.

Bloodstream trypomastigotes were obtained from mice infected 7 days earlier. Blood samples were collected from the axillary vein and transferred to a tube containing heparin. After centrifugation, the parasites were collected with plasma, centrifuged, and washed twice in phosphate-buffered saline (PBS). The number of parasites was estimated and adjusted to a concentration of 75,000 per ml. Each mouse was inoculated intraperitoneally with 0.2 ml (15,000 trypomastigotes). Parasite development was monitored by counting the number of bloodstream trypomastigotes in 5 μl of fresh blood collected from the tail vein (17). When there were more than 105 parasites per ml, the blood was diluted 10 times in a solution containing 0.15 M NH4Cl, 1 mM KHCO3, and 0.1 mM EDTA, pH 7.0, to lyse red blood cells. The concentration of parasites was estimated with the aid of a hematocytometer.

RNA purification and RT-PCR.

Parasite RNA was purified from the different developmental stages using TRIzol (Life Technologies) according to the manufacturer's instructions. Subsequently, the RNA was treated with RNase-free DNase I (Life Technologies) according to the protocol supplied by the manufacturer. The RNA was precipitated with ethanol, washed, and resuspended in nuclease-free water. The cDNA reaction was performed with 2.5 μg of total RNA using the ThermoScript RT-PCR system (Life Technologies). The oligo(dT) primer supplied in the kit was used during the first strand reaction, and the annealing temperature chosen was 60°C for 60 min. Further steps were followed according to the manufacturer's instructions. Control RT reactions were performed in the absence of the enzyme.

Specific oligonucleotides were designed on the basis of the previously published nucleotide sequence of the asp-2 gene (20). All oligonucleotides used in this study were purchased from Life Technologies. The oligonucleotides and their sequences were as follows: ASP-2F forward, 5′-GGGGGTACCATGCTCTCACGTGTTGCT-3′; and ASP-GR reverse, 5′-GGGTCTAGATCAGACCATTTTTAGTTCACC-3′ (underlined nucleotides represent KpnI and XbaI sites, respectively). For a control, we used a pair of primers specific for the sequence of the T. cruzi α-tubulin gene: TUBF forward (5′-ACCGATGTTGCGGCGATGCTTGAC-3′) and TUBR reverse (5′-CCGCGCGCTGCACCTTGGCAA-3′).

PCR was performed using the enzyme PLATINUM Pfx DNA polymerase (Life Technologies) according to the protocol supplied by the manufacturer. The annealing temperature used was 48°C, and the number of cycles was 35. PCR was performed in a Perkin-Elmer 2400 GeneAmp PCR system. Amplification products were electrophoresed in 1% (wt/vol) agarose gels and visualized by ethidium bromide staining and UV illumination. For DNA size markers, we used λ HindIII fragments or 100-bp DNA ladder (both purchased from Life Technologies).

Cloning and sequencing.

A fragment of approximately 2.1 kb was excised from agarose gels, and the DNA was purified with Geneclean II kit (BIO 101). The purified product was cloned into the pMOSBlue vector (Amersham-Pharmacia), and after restriction analysis using restriction enzymes EcoRI and XbaI (Life Technologies), the clones containing inserts with the expected size were selected. Twenty clones were then chosen and submitted to restriction analysis using enzymes EcoRI, HindIII, and BamHI (Life Technologies).

Automatic sequencing of double-stranded DNA was performed using the BigDye Terminator Cycle Sequencing kit (Perkin-Elmer) on an ABI PRISM 377 sequencer (Perkin-Elmer). Initially, U19 and T7 oligonucleotides (35) present in the flanking regions of the pMOSBlue vector were used. Subsequently, to complete the sequencing of each clone, new oligonucleotides were designed to cover the entire sequence.

DNA and predicted amino acid sequences were analyzed using the DNASTAR package version 5.00 (DNASTAR, Inc.). Alignments were produced by using the Clustal W program. Analysis for potential secretory signal peptide (SP) was performed at the SignalP website (http//www.cbs.dtu.dk/services).

Pulsed-field gel electrophoresis, Northern blot analysis, and hybridization conditions.

DNA was prepared from T. cruzi epimastigotes as described previously (23). Agarose blocks of epimastigotes of T. cruzi were prepared, and pulsed-field gel electrophoresis was performed at 18°C in 0.5× Tris-borate-EDTA (TBE) (2), using contour-clamped homogeneous electric field apparatus (Pharmacia Biotech). Chromosomes were fractionated on 1.2% (wt/vol) agarose gels (chromosome-grade agarose; Bio-Rad). Electrophoresis was performed as described previously (2). Gels were stained with ethidium bromide, photographed, and blotted under alkaline conditions to Hybond-N membranes (Amersham). Hansenula wingei DNA fragments (Bio-Rad) (that had been prepared using contour-clamped homogeneous electric field apparatus) ranging in size from 1.05 to 3.13 Mb were used as molecular size markers.

Total RNA was resuspended in RNase-free water, equilibrated in a buffer containing formamide, formaldehyde, and morpholinepropanesulfonic acid (MOPS), heated for 30 min at 70°C, and fractionated in 1% agarose formaldehyde gels (35). RNA ladder ranging from 0.24 to 9.5 kb was used as size marker (Life Technologies). Gels were photographed and transferred to membranes, and the RNA was fixed and cross-linked to Hybond-N membranes by UV irradiation.

Filters were prehybridized for 2 h at 42°C in a hybridization solution containing 50% (vol/vol) formamide (Sigma), 5× SSC (1× SSC is 0.15 M NaCl plus 0.015 M sodium citrate), 5× Denhardt's solution, 10 μg of tRNA (Sigma) per ml, and 100 μg of salmon testes DNA (Sigma) per ml. Filters were hybridized overnight at the same temperature with the indicated probes labeled with [α-32P]dCTP (Amersham) by using the Oligolabeling kit (Pharmacia Biotech) according to the manufacturer's instructions. The filters were washed in SSC solutions (2 to 0.1×) containing 0.1% (wt/vol) sodium dodecyl sulfate (SDS) from 42 to 50°C and exposed to Hyperfilm (Amersham-Pharmacia).

Recombinant proteins based on the sequence of the clone 9 gene: plasmid generation, Escherichia coli expression, and purification and production of specific antisera.

For a bacterial expression vector, we used the pHIS plasmid kindly provided by Peter Sheffield (University of Virginia) (39). Two plasmids were generated based on the sequence of the clone 9 gene: pHIS-65kDa and pHIS-25kDa. We selected these two plasmids for expression because attempts to generate a recombinant protein with the entire clone 9 gene were not successful.

For plasmid pHIS-65kDa, the forward oligonucleotide (5′-GGG AAT TCA TGC CTC AAT GGG TC-3′) was designed to anneal with nucleotides 232 to 246 of the clone 9 gene. For plasmid pHIS-25kDa, the forward oligonucleotide (5′-GGG GAT CCT GGT CTG GTT GGA TTC-3′) was designed to anneal with nucleotides 1474 to 1489 of the clone 9 gene (details in Table Table1)1) (underlined nucleotides represent EcoRI and BamHI sites, respectively). The reverse oligonucleotide used in the PCR was ASP-GR described above. PCR was performed using the enzyme PLATINUM Pfx DNA polymerase, and the annealing temperature used was 48°C. The amplified PCR products were cloned into the pMOSBlue vector, removed by treatment with restriction enzymes EcoRI and SalI, and ligated into the pHIS vector treated with the same enzymes.

TABLE 1.
Characteristics of the plasmids used in this study

Plasmids pHIS-65kDa and pHIS-25kDa were transformed into E. coli BL-21 expression host cells (Novagen, Madison, Wis.). Protein expression was obtained by inoculation of 1 ml of a culture grown overnight in 100 ml of Luria broth containing 100 μg of ampicillin (Sigma) per ml. The culture was grown with continuous shaking at 37°C to an optical density at 600 nm (OD600) of 0.6 and then induced overnight at room temperature in the presence of 0.01 mM isopropyl-l-thio-β-d-galactoside (Life Technologies). Cells were harvested, centrifuged, and resuspended in 5 ml of a solution containing 1% (vol/vol) Triton X-100 (Sigma), 4 mg of lysozyme (Sigma) per ml, and 1 mM phenylmethylsulfonyl fluoride (Life Technologies) dissolved in PBS. The suspension was then transferred to cryogenic tubes and freeze-thawed three times in liquid nitrogen. After centrifugation at 12,500 × g for 20 min, the pellet was resuspended in 500 μl of a denaturing solution containing 8 M urea, 100 mM Tris-HCl (pH 9.0), 20 mM dithiothreitol (Life Technologies), and 3 mM EDTA (Sigma). After centrifugation at 12,500 × g for 10 min, SDS-polyacrylamide gel electrophoresis (SDS-PAGE) sample buffer (35) containing 5% (vol/vol) 2-mercaptoethanol was added to 200 μl of the supernatant, and the sample was boiled for 3 min. These samples were subjected to electrophoresis on gels consisting of 10 or 12.5% (wt/vol) polyacrylamide and 0.1% (wt/vol) SDS. Gels were stained with a solution of 250 mM KCl, and the band corresponding to the recombinant protein was excised using a razor blade. The gel bands were cut in small pieces and eluted overnight in distilled water at room temperature under constant agitation. The volume was reduced with the aid of a Hetovac (Heto, Scandinavia). The purity of recombinant protein was determined by SDS-PAGE. For molecular size standards, SDS-molecular weight marker kit (Sigma) or Benchmarker Prestained Protein Ladder (Life Technologies) was used.

Before recombinant proteins were used in cell cultures, SDS was removed by dialysis against distilled water overnight at 4°C. Water was changed four times. Protein concentration was determined by gel electrophoresis using known concentrations of bovine serum albumin (BSA) (Sigma) as the standard.

C57BL/6 mice were immunized with 100 μg of the purified recombinant protein (His-25kDa) in complete Freund's adjuvant (CFA) in the hind foot. Two weeks later, mice were bled, and their sera were used for immunological assays.

Eukaryotic expression vectors: plasmid generation, purification, and immunization.

Commercially available plasmid pcDNA3 (Invitrogen, San Diego, Calif.) was used as the mammalian expression vector. Plasmid pclone9 was generated by inserting the clone 9 gene into the EcoRI and XbaI sites of plasmid pcDNA3 (details in Table Table1).1). Plasmid pSPclone9 was generated by inserting the nucleotide sequence encoding the signal peptide of hemagglutinin protein from the influenza virus in frame with the 5′ nucleotide sequence of the clone 9 gene into the pclone9 plasmid. The nucleotides encoding the hemagglutinin protein SP (15) were obtained from a plasmid kindly provided by Sang Won Han (UNIFESP, São Paulo, Brazil) and inserted using the HindIII and BamHI sites of the pcDNA3 vector containing the clone 9 gene. Plasmid pIgSPclone9 was obtained by inserting the nucleotide sequence encoding the mouse immunoglobulin κ chain SP (IgSP) in frame with the 5′ nucleotide sequence of the clone 9 gene. The nucleotide sequence encoding IgSP was generated from annealing six different oligonucleotides synthesized on the basis of the sequence described in reference 14. This product was subcloned into the KpnI site from pcDNA3 plasmid. The clone 9 gene was subsequently cloned into KpnI and XbaI sites of pcDNA3. The presence of the nucleotides encoding the hemagglutinin protein SP or IgSP was confirmed by direct sequencing of plasmids pSPclone9 and pIgSPclone9 using the T7 oligonucleotide present in the 5′-flanking region of the pcDNA3 vector. Plasmid p154/13 containing the catalytic domain of T. cruzi trans-sialidase used as a positive control in a few experiments was generated as described previously (5).

Plasmids were produced in E. coli DH5α and purified on cesium chloride density gradients according to standard protocols (35). DNA concentration was estimated to be 260 nM and confirmed by agarose gels stained with ethidium bromide. Each plasmid DNA was diluted in sterile PBS to a concentration of 1 mg/ml.

BALB/c mice were immunized according to a protocol described earlier (5). Both tibialis anterioris muscles were injected with 3.5 μg of cardiotoxin (Sigma). Five days later, 50 μg of plasmid DNA was injected intramuscularly (i.m.) at the same sites as for cardiotoxin injection (a total of 100 μg of plasmid DNA per mouse). The subsequent doses consisted of the same amount of plasmid DNA injected 3, 5, and 7 weeks after the first dose.

Ten days to five weeks after the last dose, mice were used for immunological studies or challenged with bloodstream trypomastigotes.

Immunoblotting.

Parasites of the different stages of T. cruzi were obtained as described above. After the samples were washed in PBS twice, the parasite concentration was estimated by counting them with a hematocytometer and their concentration was adjusted to 109 parasites per ml. Parasite extracts were obtained by treating the parasites with a solution of PBS containing a final concentration of 1% (vol/vol) NP-40 and proteolytic enzyme inhibitors (1 mM phenylmethylsulfonyl fluoride and 1 mM iodoacetamide [Sigma]). After centrifugation for 5 min at 12,500 × g, the supernatants were collected, mixed with SDS-PAGE sample buffer containing 2-mercaptoethanol and boiled for 3 min. The amount of liquid corresponding to 107 parasites was loaded onto SDS-polyacrylamide gels. After electrophoresis, the samples were transferred to nitrocellulose membranes (Millipore) for 1 h at 100 V and 4°C. The membranes were treated with PBS containing 5% (wt/vol) dry nonfat milk and 2% (wt/vol) BSA for at least 2 h and incubated with a pool of sera from mice immunized with the recombinant His-25kDa protein (diluted 1:750). After extensive washes with PBS containing 0.05% (vol/vol) Tween 20 (PBS-Tween), bound antibodies were detected by incubating the immunoblot membranes with peroxidase-conjugated goat anti-mouse immunoglobulin G (IgG) (Life Technologies) (diluted 1:2,000) and visualizing the chemoluminescence reaction with enhanced chemiluminescent reagent (Amersham-Pharmacia) and exposure to Hyperfilm.

ELISA and indirect immunofluorescence assay (IIA).

Antibodies were detected by enzyme-linked immunosorbent assay (ELISA) using microtiter polystyrene plates (high binding; Costar) coated with the recombinant protein His-65kDa. Each well was incubated overnight at room temperature with 0.125 μg of the recombinant protein dissolved in 0.05 M NaHCO3/Na2CO3 buffer, pH 9.6, in a final volume of 50 μl. Unbound antigen was removed by washing the plates three times with PBS-Tween. Wells were blocked with PBS containing 5% (wt/vol) dry nonfat milk and 2% (wt/vol) BSA. After 2 h, 50-μl serum samples (diluted 800 times) from immunized and control mice were incubated for 60 min at room temperature. After five washes with PBS-Tween, wells were incubated for 60 min at room temperature with anti-mouse IgG conjugated to peroxidase (KPL) diluted 1:4,000. Bound immunocomplexes were detected with o-phenylenediamine (Sigma). Plates were read at OD492 on an ELISA reader (Labsystems Multiskan MS).

For IIA, parasites or infected cells were fixed for 30 min in PBS containing the nonpermeabilizing reagent paraformaldehyde (Merck) (3.5% [vol/vol]). After fixation, parasites were washed twice with PBS, and the solution was adjusted to a concentration of 107 parasites per ml. Ten-microliter portions of the parasite suspension were layered onto round, coated coverslips and allowed to stand overnight at room temperature. Coverslips were stored at −20°C until use. Infected cells were permeabilized by treatment with PBS containing 0.2% (vol/vol) Triton X-100 for 15 min at room temperature. Blocking was performed for 30 min at room temperature in PBS containing 3% (wt/vol) BSA. Sera from mice immunized with plasmid pclone9 were diluted to 1:25 in PBS-BSA and allowed to react for 30 min at room temperature. After three washes in PBS-Tween, fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse IgG (Sigma) diluted 1:25 was added for an additional 60 min. Coverslips were covered with 50% (vol/vol) glycerol carbonate-0.5 M bicarbonate buffer and observed using an Optishot-2 fluorescence microscope (Nikon). IIA images were acquired with a Zeiss Axiovert microscope attached to a Bio-Rad 1024UV confocal microscope using a 100× 1.4 (aperture) differential interference contrast PlanApo objective.

Cell cultures and detection of IFN-γ and NO.

Cells were cultured in RPMI medium (Life Technologies) supplemented with 10 mM HEPES, 2 mM l-glutamine, 5 × 10−5 M 2-mercaptoethanol, 1 mM sodium pyruvate, 1% (vol/vol) nonessential amino acid solution, 1% (vol/vol) MEM vitamin solution (all purchased from Life Technologies), 100 U of penicillin and streptomycin (Sigma) per ml, and 2% (vol/vol) normal human serum. The cultures were maintained at 37°C in an atmosphere containing 5% CO2.

Cells were obtained from the spleens of three BALB/c mice immunized with plasmid pclone9, pSPclone9, pIgSPclone9, or pcDNA3. Red blood cells were lysed in a solution of 0.15 M NH4Cl, 1 mM KHCO3, and 0.1 mM EDTA, pH 7.0. These cells were washed three times in plain RPMI medium and resuspended in complete medium to a concentration of 20 × 106 viable cells per ml. The assay was performed in 96-well flat-bottom plates (Costar); 0.2 ml of cell suspension was pipetted into each well, and the antigen was added at the desired concentration. Recombinant His-65kDa protein was added at final concentrations ranging from 10 to 0.1 μg/ml. Recombinant trans-sialidase (TS) containing the catalytic domain of T. cruzi TS used as a positive control in a few experiments was generated as described previously (5, 9, 30). Rat IgG and purified anti-CD4 (GK1.5) and anti-CD8 MAbs (2.43) were added to a final concentration of 10 μg/ml. We collected supernatants for cytokine determination after 5 days of culture. Each determination was done in triplicate; results are reported as means ± standard deviations (SDs).

The concentration of IFN-γ or interleukin-4 (IL-4) was estimated by capture ELISA using commercially available antibodies and recombinant cytokine (Pharmingen, San Diego, Calif.) exactly as described earlier (30). The concentration of cytokine in each sample was determined from standard curves executed in parallel with known concentrations of recombinant IFN-γ or IL-4. The detection limit of the assays was 0.2 ng/ml.

Nitrite production by cultured cells was assessed by incubation of 50 μl of each supernatant with 50 μl of Griess solution (1% [wt/vol] sulfanilamide, 0.1% [wt/vol] naphthylene diamine dihydrochloride, 2% [vol/vol] H3PO4; all purchased from Sigma). The absorbance was measured at OD540 on an ELISA reader. Known concentrations of sodium nitrite diluted in culture medium were used as standards.

ELISPOT assay for the detection of peptide-specific IFN-γ-producing cells.

The enzyme-linked immunospot (ELISPOT) assay was performed essentially as described earlier (25, 29). Briefly, the plates were prepared by coating the wells of 96-well Multiscreen HA nitrocellulose plates (Millipore) with a solution of sterile PBS containing 10 μg of the anti-mouse IFN-γ MAb R4-6A2 (Pharmingen) per ml (60 μl of the PBS-MAb solution was used to coat each well). After overnight incubation at room temperature, the MAb solution was removed by sterile aspiration and the plates were washed three times with plain RPMI medium under sterile conditions. Plates were blocked by incubating wells with 100 μl of RPMI medium containing 10% (vol/vol) FBS for at least 2 h at 37°C.

Responder cells were obtained from the spleens of three BALB/c mice immunized four times with plasmid pclone9, pIgSPclone9, or pcDNA3. For a positive control, we used spleen cells from three BALB/c mice immunized with plasmid p154/13. Responder cells were washed three times in plain RPMI medium and resuspended in complete RPMI medium to a concentration of 5 × 106 viable cells per ml. Complete RPMI medium was produced as described above except that it contained 10% (vol/vol) FBS (HyClone, Logan, Utah), and we added recombinant human IL-2 (kindly provided by Hoffman-LaRoche) (30 U/ml) to the medium.

Antigen-presenting cells were prepared by irradiating A20J cells (kindly provided by Moriya Tsuji, New York University School of Medicine) for 45 min. These cells express MHC class I and II molecules that improve peptide presentation to specific T cells. Cells were then washed three times in plain RPMI medium. Their viability was determined by trypan blue dye exclusion, and their concentration was adjusted to 106 cells/ml in complete RPMI medium. Some cells were incubated with the indicated synthetic peptide at a final concentration of 4 μM for 30 min at 37°C.

One hundred microliters of a suspension containing responders or antigen-presenting cells was pipetted into each well. The plates were incubated (without shaking) for 24 h at 37°C in an atmosphere containing 5% CO2.

After incubation, the majority of the cultured cells were flicked off the plates. To remove residual cells, plates were washed three times with PBS and three times with PBS-Tween. Each well received 75 μl of biotinylated anti-mouse IFN-γ MAb XMG1.2 (Pharmingen) diluted in PBS-Tween to a final concentration of 2 μg/ml. Plates were incubated overnight at 4°C. Unbound antibodies were removed by washing the plates at least six times with PBS-Tween. Peroxidase-labeled streptavidin (KPL) was added at a 1:800 dilution in PBS-Tween in a final volume of 100 μl/well. Plates were incubated for 1 to 2 h at room temperature and then washed three to five times with PBS-Tween and three times with PBS.

The reactions on the plates were developed by adding peroxidase substrate (50 mM Tris-HCl [pH 7.5] containing 3,3′-diaminobenzidine tetrahydrochloride [1 mg/ml] and 30% hydrogen peroxide solution [1 μl/ml] [both from Sigma]) (100 μl/well). After incubation at room temperature for 15 min, the reaction was stopped by discarding the substrate solution and rinsing the plates under running tap water. Plates were dried at room temperature, and spots were counted with the aid of a stereomicroscope (Nikon).

Synthetic peptides.

Synthetic peptides based on the predicted amino acid sequence of the clone 9 gene were purchased from New England Peptides (Fitchburg, Mass.). As estimated by high-pressure liquid chromatography analysis, the purity of the synthetic peptides varied from 78 to 99%. Synthetic peptide TS359-367 (IYNVGQVSI) was purchased from Neosystem (Strasbourg, France). As estimated by high-pressure liquid chromatography analysis, TS359-367 was more than 90% pure.

Statistical analysis.

The Student t test was used to compare the possible differences in the mean values of IFN-γ and nitrite concentrations. The peak parasite loads observed in the different mouse groups were compared by one-way analysis of variance (ANOVA) and Tukey's honestly significant difference (HSD) tests. The Mann-Whitney test was used to compare the OD492s obtained by ELISA using sera from mice immunized with the different plasmids. Log-rank test was used to compare mouse survival times after challenge with T. cruzi. The differences were considered significant when the P value was <0.05.

RESULTS

Identification and characterization of genes related to asp-2 present in the cDNA of amastigotes of the Y strain of T. cruzi.

In an attempt to characterize genes expressed by the Y strain of T. cruzi related to the previously described asp-2 gene, RT-PCR was performed using an oligo(dT) primer, a pair of primers based on the published sequence of asp-2, and RNA extracted from the different developmental stages of the parasite. A single band of ~2.1 kb was obtained by RT-PCR using RNA from amastigotes or epimastigotes (Fig. (Fig.1A).1A). PCR products were not observed in the absence of reverse transcriptase.

FIG. 1.
RT-PCR using RNA purified from different developmental stages of the Y strain of T. cruzi and oligonucleotide primers based on the previously described asp-2 gene sequence. RT-PCR was performed using oligo(dT) primer and total RNA purified from trypomastigotes ...

In contrast, in several independent experiments, specific PCR products were not detected when RT-PCR was performed using trypomastigote RNA. This absence cannot be attributed to the trypomastigote RNA itself, because specific PCR products could be readily obtained in the presence of primers specific for the α-tubulin gene of T. cruzi (Fig. (Fig.1B1B).

DNA products obtained from PCR using cDNA from amastigotes were cloned into the pMOSBlue vector. Twenty clones were subjected to restriction analysis, and four different restriction patterns (named 1 through 4) were observed. Clones belonging to restriction pattern 1 or 2 were more common in the cDNA representing 9 or 5 of the 20 clones studied, respectively. Restriction patterns 3 and 4 were less represented in the amastigote cDNA (each corresponding to 3 of the 20 clones). Similar proportions were observed when we studied clones isolated from different RT-PCR mixtures. The observation made when we performed PCR using an oligonucleotide representing the splice leader sequence of T. cruzi as the forward primer was also important. Using this oligonucleotide as a primer, restriction pattern 1 or 2 represented 12 or 6 of 18 clones studied, respectively. Clones representing restriction pattern 3 or 4 were not observed in this case (data not shown). We consider this result important, because the splice leader sequence is present only in mature mRNA of T. cruzi.

DNA sequences were obtained from two clones representing each restriction pattern. Comparison of the DNA sequences of the two clones representing each pattern showed minor but significant differences, indicating that they were distinct genes (data not shown). We selected clones 9, 13, 117, and 105 to represent restriction patterns 1, 2, 3, and 4, respectively. The clone 9 gene shared the highest degree of identity with the asp-2 gene (89.9 or 82.7% identity at the nucleotide or predicted amino acid sequence levels, respectively). The other genes revealed decreasing degrees of identity with the asp-2 gene sequence.

At the nucleotide level, clone 117 (pattern 3) and clone 105 (pattern 4) presented a high degree of identity when compared to each other (98.4%). The clone 117 and clone 105 genes had stop codons at nucleotides 309 and 285, respectively. Despite the identity with asp-2, all genes isolated from amastigote cDNA with restriction pattern 3 or 4 had stop codons in their 5′ region. These genes most likely represent pseudogenes which are transcribed but not translated in T. cruzi (1). Figure Figure22 shows a Clustal W multiple-sequence alignment of the predicted amino acid sequences of clones 9, 13, 117, 105, and the previously described asp-2 gene. The predicted amino acid sequences of clone 9 or clone 13 contain a sequence encoding a highly probable SP sequence at the N terminus that contains a strongly hydrophobic, noncharged core represented by amino acids (aa) 39 to 66. The predicted cleavage site of SP is between aa 66 to 67 (Fig. (Fig.22).

FIG. 2.
Clustal W multiple-sequence alignment of the predicted amino acid sequences of clone 9, 13, 117, and 105 genes isolated from amastigote cDNA of the Y strain of T. cruzi and the previously described asp-2 gene. The predicted amino acid sequences of clone ...

The predicted amino acid sequence of the clone 13 or clone 9 gene contains, respectively, one or two ASP box motifs (SXDXGXTW) present in bacterial sialidases. Both genes contain the sequence encoding the highly conserved motif VTVXNVXLYNR characteristic of type three module of fibronectin. These sequences are common to members of the sialidases/trans-sialidase gene superfamily of T. cruzi (6).

Because the clone 9 gene shared the highest identity with the asp-2 gene sequence, it was selected for subsequent studies. The genomic distribution of the clone 9 gene was determined by chromosome mapping, and this gene hybridized with many chromosomal bands of the Y strain genome (Fig. (Fig.3A,3A, lane 2). Similar results were observed when we used the clone 13 gene (data not shown). In contrast, the parasite α-tubulin gene used as a control hybridized only with two bands of the Y strain genome (Fig. (Fig.3A,3A, lane 3). This result indicates that the clone 9 and clone 13 genes belong to a relatively large family of genes distributed in different chromosomes of the parasite.

FIG. 3.
Chromosome location and Northern blot analysis using the clone 9 gene as a probe. (A) Chromosome mapping of the clone 9 gene was performed by pulsed-field gel electrophoresis using DNA from epimastigotes. Chromosomes were stained with ethidium bromide ...

By Northern blotting, the clone 9 gene hybridized with two RNA populations of different sizes present in the different developmental stages of the parasite. The shorter RNA population was detected in RNA isolated from amastigotes or epimastigotes. The longer RNA population was detected in trypomastigotes or epimastigotes (Fig. (Fig.3B).3B). Identical results were obtained when the clone 13 gene was hybridized with RNA purified from the different developmental stages of the parasite (data not shown).

Antisera raised against the C terminus of the recombinant protein encoded by the clone 9 gene recognizes an ~83-kDa protein in amastigotes of T. cruzi.

To verify whether the clone 9 gene did in fact encode a protein with characteristics similar to those of ASP-2, a pHIS protein expression construct was designed on the basis of the clone 9 sequence for the production of a fusion protein, development of specific polyclonal antisera, and immunoblot analysis. The pHIS construct His-25kDa (Table (Table1)1) represented the C terminus of the recombinant protein encoded by the clone 9 gene. We selected this region of the protein, because its predicted amino acid sequence shared a high degree of identity (180 of 203 aa) with the same region of ASP-2 (Fig. (Fig.22).

Sera from mice immunized with the fusion protein were employed to develop immunoblots containing extracts of trypomastigotes, amastigotes, and epimastigotes, as well as the recombinant protein. As shown in Fig. Fig.4,4, the recombinant protein His-25kDa (lane 5) and a single band of ~83 kDa present in extracts of amastigotes of T. cruzi (lane 7) were recognized. Two smaller proteins of ~67 and ~47 kDa were recognized in extracts of trypomastigotes and epimastigotes, respectively (lanes 6 and 8). Antibody recognition was specific, because sera from mice immunized with CFA alone failed to react with any protein (lanes 1 to 4).

FIG. 4.
Immunoblot using sera collected from mice immunized with the purified recombinant protein His-25kDa. Purified recombinant protein His-25kDa (0.3 μg) or parasite extracts (equivalent to 107 parasites) were added to each lane of a SDS-polyacrylamide ...

Presence of specific antibodies in the sera of mice immunized with eukaryotic expression vectors containing the clone 9 gene.

In order to study the immune response induced by DNA immunization with different eukaryotic expression plasmids, we used three different plasmids containing the clone 9 gene. These three plasmids differ in the presence in the 5′ region of the clone 9 gene of a DNA sequence encoding a secretory SP. While pclone9 contained only the clone 9 gene, plasmid pSPclone9 or pIgSPclone9 also contained DNA sequences encoding the secretory SP of the influenza virus hemagglutinin or mouse Ig κ light chain, respectively (Table (Table11).

BALB/c mice were immunized i.m. with plasmid pclone9, pSPclone9, or pIgSPclone9. Control mice were immunized with pcDNA3 or pIgSP. The presence of specific antibodies was determined by ELISA using the recombinant protein His-65kDa as an antigen (Table (Table1).1). Specific antibodies were detected in most of the animals immunized with each of the three plasmids containing the clone 9 gene (Fig. (Fig.55).

FIG. 5.
Antibody immune response of mice immunized with eukaryotic expression plasmids containing the clone 9 gene. BALB/c mice were immunized with plasmids containing the clone 9 gene (pclone9, pSPclone9, and pIgSPclone9) or control (empty) plasmids (pcDNA3 ...

By IIA, it was possible to determine that intra- and extracellular amastigotes, but not extracellular trypomastigotes, of T. cruzi were specifically recognized by antibodies from mice immunized with plasmid pclone9 (Fig. 6B and D). IIA results were negative when we used sera from mice immunized with control plasmid pcDNA3 (data not shown).

FIG. 6.
IIA recognition of amastigotes, but not of trypomastigotes, of T. cruzi by immune sera from mice immunized with a plasmid containing the clone 9 gene. Intracellular amastigotes (A and B) or mixtures of extracellular amastigotes and trypomastigotes from ...

Cellular immune responses of mice immunized with eukaryotic expression plasmids containing the clone 9 gene.

Next, we assessed whether immunization with eukaryotic expression plasmids containing the clone 9 gene elicited a cell-mediated immune response, as estimated in vitro by antigen-specific IFN-γ or IL-4 secretion. Upon in vitro stimulation with the recombinant protein His-65kDa but not TS, spleen cells from mice immunized with plasmid pclone9, pSPclone9, or pIgSPclone9 secreted IFN-γ. This response was dependent on in vivo priming, because spleen cells from mice injected with control plasmid pcDNA3 failed to secrete IFN-γ upon stimulation with the recombinant protein (Table (Table2).2). These supernatants were also tested for the presence of the type 2 cytokine IL-4. On repeated occasions, we were unable to detect it (data not shown).

TABLE 2.
IFN-γ secretion by spleen cells from BALB/c mice immunized with different plasmids upon in vitro restimulation with the recombinant protein His-65kDa

Secretion of IFN-γ was dependent on the activation of CD4+ T cells. When compared to culture containing rat IgG, the presence of an anti-CD4 MAb significantly inhibited the antigen-specific IFN-γ production by spleen cells of mice immunized with plasmid pclone9, pSPclone9, or pIgSPclone9 (P < 0.01 in all cases) (Table (Table2).2). In contrast, the addition of an anti-CD8 MAb to the cultures did not cause a significant reduction in the IFN-γ secretion by these spleen cells (P > 0.05 in all cases) (Table (Table2).2). The addition of both MAbs did not further increase the inhibition observed in the presence of an anti-CD4 MAb (Table (Table22).

In addition to IFN-γ production, we measured the presence of NO in the supernatants of spleen cell cultures. Upon in vitro stimulation with the recombinant protein His-65kDa, a significant increase in NO concentration was observed in the supernatants of spleen cells from mice immunized with plasmids containing the clone 9 gene (P < 0.05) (Table (Table3).3). In contrast, NO concentrations in the supernatants of spleen cells from mice immunized with the control plasmid pcDNA3 cultured in the presence or absence of the recombinant protein His-65kDa were not statistically different (P > 0.05) (Table (Table33).

TABLE 3.
NO secretion by spleen cells of DNA-immunized mice upon in vitro restimulation with the recombinant protein His-65kDa

The increase in the NO concentration in the supernatant of spleen cells stimulated by the recombinant protein His-65kDa was dependent on the activation of CD4+ T cells. As found for IFN-γ secretion, the addition of an anti-CD4 MAb significantly reduced the NO accumulation in the supernatant of cultures containing spleen cells from mice immunized with plasmid pclone9 (Table (Table44).

TABLE 4.
Presence of anti-CD4 MAb inhibits NO secretion by spleen cells of pclone9-immunized mice upon in vitro restimulation with the recombinant protein His-65kDa

In order to determine whether we could detect specific CD8+ T cells in the spleens of mice immunized with plasmid pclone9 or pIgSPclone9, we synthesized 14 peptides based on the predicted amino acid sequence of the clone 9 gene. Peptides were selected by the presence of putative sites for binding to the MHC class I H-2Kd or H-2Ld gene product of BALB/c mice (4, 7). Peptides with putative sites for binding to H-2Dd were not present in the predicted amino acid sequence of the clone 9 gene (3).

Spleen cells from mice immunized with DNA were tested by the ELISPOT assay for their ability to secrete IFN-γ upon in vitro stimulation with target A20J cells coated with each of these peptides. This assay was selected because it is very sensitive and specific (25, 29). In repeated experiments, we could not detect the presence of peptide-specific IFN-γ-secreting cells in the spleens of mice immunized with plasmid pclone9 or pIgSPclone9 (Table (Table5).5). This failure could not be attributed to the ELISPOT assay itself or to the responder cells, because IFN-γ secretion could be detected when we used the recombinant protein His-65kDa as the antigen (Table (Table5).5). Also, we detected the presence of IFN-γ-secreting cells in the spleens of mice immunized with plasmid p154/13 which contains the gene encoding the catalytic domain of T. cruzi TS. These cells were specific for the TS359-367 peptide (IYNVGQVSI) described as a CD8+ T-cell epitope in our earlier study (30).

TABLE 5.
ELISPOT assay for detection of peptide-specific IFN-γ-secreting cells

Protective immunity in mice immunized with plasmids containing the clone 9 gene.

In order to determine whether DNA immunization could induce protective immunity against T. cruzi infection, BALB/c mice immunized with plasmid pclone9, pSPclone9, or pcDNA3 were challenged with bloodstream trypomastigotes. In mice immunized with plasmid pclone9 or pSPclone9, we observed a significant reduction in the peak parasite level compared to those in animals immunized with pcDNA3 plasmid (P < 0.0001 by one-way ANOVA) (Fig. (Fig.7A).7A). Most importantly, none of the mice immunized with plasmid pclone9 or pSPclone9 died (Fig. (Fig.7B).7B). While 100% of the animals immunized with plasmid pclone9 or pSPclone9 survived the infection, 90% of mice injected with plasmid pcDNA3 died. This experiment was reproduced with identical results. In both experiments, a total of 36 BALB/c mice were immunized with plasmid pclone9 or pSPclone9. All animals survived the infection. In contrast, 16 of the 17 mice immunized with pcDNA3 died (P < 0.0001 by log-rank test).

FIG. 7.
Trypomastigote-induced parasitemia and mortality in BALB/c mice immunized with plasmids pclone9 and pSPclone9. BALB/c mice were immunized with pclone9, pSPclone9, or pcDNA3. Ten days after the last immunization, mice were challenged intraperitoneally ...

DISCUSSION

The aim of our study was to explore the possibility of using genes expressed by amastigotes, the intracellular replicating forms of T. cruzi, for experimental DNA vaccination. For that purpose, we selected for our study the gene encoding the ASP-2 protein, an antigen recognized by antibodies, as well as potentially protective immune T cells from mice and humans infected with T. cruzi (19, 26, 47).

In a detailed characterization of the asp-2 cDNA expressed by amastigotes of T. cruzi (Y strain), we isolated cDNA genes displaying four distinct restriction patterns. Each restriction pattern may include two or more distinct genes. Sequence analysis of these genes revealed 82.3 to 89.9% identity at the nucleotide level with the previously described asp-2 gene isolated from the cDNA of the Brazil strain of T. cruzi. On the basis of their high degree of identity with the asp-2 gene, they can be assigned as new members of subfamily II of the sialidase/trans-sialidase gene superfamily of T. cruzi. Southern blot hybridization using clone 9 and clone 13 genes confirmed that they belong to a family of genes distributed in multiple chromosomes of the Y strain genome. The isolation of these cDNA clones defined a new group of genes similar to asp-2 expressed in the replicating forms of T. cruzi.

Members of subfamily II have been shown to be expressed in both trypomastigotes and amastigotes of T. cruzi (6, 8, 16, 20, 27, 33, 34, 36). RT-PCR analyses using specific primers showed that the presence of genes related to asp-2 in the Y strain of T. cruzi is restricted to the cDNA of amastigotes and epimastigotes (Fig. (Fig.1A).1A). In contrast, Northern blot analysis reproducibly revealed the presence of mRNA that hybridizes with clone 9 and clone 13 genes in the different developmental stages of the parasite, including trypomastigotes. Unlike amastigotes, trypomastigotes had a longer form of RNA not seen in the intracellular stages (Fig. (Fig.3B).3B). These contrasting results can be explained by the fact that PCR was performed with cDNA generated with an oligo(dT) primer. It is possible that Northern blot analysis detected trypomastigote mRNA which does not contain a poly(A) tail. In agreement with this possibility is the fact that specific amplification products were obtained when PCR was performed with trypomastigote cDNA generated by RT in the presence of the reverse 3′ oligonucleotide ASP-GR (data not shown).

Alternatively, it is possible that clone 9 and clone 13 genes hybridized with mRNA from other members of subfamily II expressed in trypomastigotes of T. cruzi. Indeed, comparison of clone 9 nucleotide sequence with other genes available in GenBank revealed 62 to 63.6% identities with several genes present in trypomastigote cDNA (GenBank accession numbers L31947/Z32744, M58466, U02613, U02615, and L13844 [8, 33, 34]).

The fact that we found four distinct groups of genes in PCR products of cDNA from amastigotes raised the question of which one actually encodes the Y strain ASP-2. Clones representing restriction pattern 1, for example, the clone 9 gene, represented the majority of clones obtained by RT-PCR using RNA from amastigotes. Also, at the level of predicted amino acid sequences, the clone 9 gene exhibited the highest identity with the asp-2 gene (82.7%). The second group of genes most commonly isolated from amastigote cDNA was represented by the clone 13 gene. At the level of predicted amino acid sequence, this gene displayed lower degrees of identity with the asp-2 and clone 9 genes (72 and 71.1%, respectively).

The above observations led us to select the clone 9 gene as the most likely candidate for the gene encoding ASP-2 of T. cruzi (Y strain). In support of this hypothesis, we found that antibodies prepared against a recombinant protein based on the C terminus of the protein encoded by the clone 9 gene recognized a band of ~83 kDa in amastigote extracts on an immunoblot. This result lends further support that the clone 9 gene does indeed encode at least part of the ASP-2 antigen expressed by amastigotes of the Y strain of T. cruzi.

Although it is clear that an 83-kDa protein related to the clone 9 gene is expressed in amastigotes of T. cruzi, it is still not clear whether epimastigotes and trypomastigotes express this antigen. The lack of recognition by specific antibodies of an 83-kDa molecule can be explained by posttranslational modifications. A ~47-kDa band was recognized in epimastigotes by antibodies specific for the His-25kDa protein. This protein had been recognized previously using polyclonal antibodies specific for a recombinant protein encoded by the asp-2 gene of the Brazil strain of T. cruzi (20). We also detected a cross-reaction with a ~67-kDa polypeptide present in trypomastigotes. The nature of the identity of these polypeptides with ASP-2 antigen is unknown and remains to be investigated.

The fact that at the level of the predicted amino acid sequences, the clone 9 gene shared 82.7% identity with the asp-2 gene isolated from the T. cruzi Brazil strain is noteworthy. Sequence conservation was observed in several of the previously described epitopes recognized by mouse and human CD8+ T cells primed during infection. For example, the sequences encoding two epitopes recognized by H-2Kb-restricted CD8+ T cells (DNRQYSFV and VNHRFTLV) were entirely conserved (19). Conservation was also observed for epitope FVNHRFTLV recognized by HLA-A2-restricted human CD8+ T cells. A second HLA-A2-restricted human CD8+ T-cell epitope (WVFPESISPV) had a single amino acid substitution (which is underlined in the sequence) in the clone 9 gene product (WVFPEGISPV). Whether this single amino acid substitution interferes with HLA-A2-restricted CTL recognition will be important to be determined. Also relevant is the fact that the sequence encoding the recently described peptide representing a parasite ligand for host cells (VTVTNVFLYNRPL) is entirely conserved in the clone 9 and asp-2 genes (21).

Using eukaryotic expression vectors harboring the clone 9 gene, we reported that specific immune responses to T. cruzi could be generated after i.m. inoculation of plasmid. Three different plasmids containing the clone 9 gene were used for immunization. The difference between the three plasmids was the presence of sequences encoding a secretory SP in plasmid pSPclone9 or pIgSPclone9. We found that the vast majority of the immunized BALB/c mice developed specific serum antibodies as detected by ELISA.

Cell-mediated immune response was estimated in vitro by the antigen-specific IFN-γ or IL-4 secretion. Upon in vitro stimulation with the recombinant protein His-65kDa, spleen cells from mice immunized with plasmid pclone9, pSPclone9, or pIgSPclone9 secreted comparable levels of IFN-γ. We also failed to observe significant differences among spleen cells of mice injected with the different plasmids in experiments using different concentrations of antigen in culture (0.1 to 10 μg/ml [data not shown]). We concluded that under the conditions tested, the presence of different SPs does not interfere with the priming or activation of IFN-γ-producing cells in vivo.

An important characteristic of DNA immunization is the ability to prime in vivo for specific CD8+ T cells. Indeed, earlier studies have shown that immunization with plasmids containing T. cruzi genes primed CD8+ T cells that were capable of lysing infected cells (19, 46) or inhibiting intracellular parasite development in vitro (30). We attempted to detect the presence of specific CD8+ T cells using 14 different synthetic peptides selected for the presence of putative sites for binding to H-2Kd or H-2Ld. Using a very sensitive assay, the ELISPOT assay, we could not detect the presence of peptide-specific IFN-γ-secreting cells in the spleens of mice immunized with plasmid pclone9 or pIgSPclone9. This negative result could not be attributed to the ELISPOT assay, because we could easily and reproducibly detect the presence of IFN-γ-secreting cells specific for the CD8 T-cell epitope TS359-367 (IYNVGQVSI) in the spleens of mice immunized with plasmid p154/13 (30). Nevertheless, we cannot completely exclude the possibility that the putative epitope(s) has sites for binding to H-2Kd, H-2Dd, or H-2Ld distinct from the sites currently known for other antigens.

The fact that immunization of BALB/c mice with eukaryotic expression plasmids containing the clone 9 gene elicited specific immune responses allowed us to determine whether DNA immunization with plasmid pclone9 or pSPclone9 could provide protective immunity against experimental infection. We found that immunizing the mice with these plasmids significantly reduced the peak parasite levels and that none of these mice died. In the experiments performed, 100% of the mice immunized with plasmid pclone9 or pSPclone9 survived a lethal challenge with bloodstream trypomastigotes. In contrast, 94.4% of the mice immunized with the control pcDNA3 plasmid died after challenge.

Our data on the protective immune response elicited by DNA immunization with the clone 9 gene confirms and extends the results of the recently published study by Garg and Tarleton (10). Their study reported that immunization of C57BL/6 mice with a plasmid harboring the asp-2 gene elicited specific antibodies and CD8 T cells (10). Immunization with the asp-2 gene provided protective immunity against a lethal challenge with bloodstream forms of T. cruzi (Brazil strain), as estimated by the reduction of acute parasitemia and mortality. Also important was the observation that protective immunity induced by genetic vaccination with the asp-2 gene reduced the skeletal muscle inflammation observed in the chronic phase of infection (10). Together, both studies emphasize the importance of the immune response to ASP-2 for the development of protective immunity to T. cruzi infection.

Although the protective mechanisms mediated in vivo by immunization with plasmids containing the clone 9 gene were not addressed in the present study, it is plausible that IFN-γ-secreting T cells have a role in protection. Solid previous work established that T cells and IFN-γ participate in the control of parasitemia and mortality during the natural course of T. cruzi infection in mice (13, 18, 32, 40, 42; reviewed in reference 43). In addition to secreting IFN-γ, spleen cells from mice immunized with plasmids containing the clone 9 gene also produced NO, a potent inhibitor of T. cruzi development in vivo (22, 24, 31, 44). Our results are compatible with those of recent studies showing that a strong Th1, but not Th2, immune response can provide a high degree of systemic immunity against experimental T. cruzi infection in BALB/c mice (11, 12, 37). However, the precise role of these IFN-γ-secreting CD4+ T cells in protective immunity remains to be determined. Until now, there has been no clear evidence that specific CD4+ Th1 cells alone can provide protective immunity against T. cruzi infection (11, 12, 37). These cells most likely have to interact with other T- and B-cell subsets in order to provide optimal protection against systemic infection. Future studies will be required to elucidate the effector immune mechanisms involved during the protective immune response induced by genetic vaccination with the clone 9 gene.

It seems less likely that antibodies to the ASP-2 antigen participate in protective immunity. Amastigotes are predominantly intracellular and have never been described as targets of protective antibodies. On the other hand, extracellular trypomastigotes, which are classically described as targets for protective antibodies, were not recognized by antibodies from mice immunized with plasmids containing the clone 9 gene (Fig. 6C and D).

In summary, our results confirm and extend the results of earlier studies obtained by different research groups showing that DNA immunization can be another way to induce protective immune responses against experimental T. cruzi infection. The fact that DNA immunization with cDNA from amastigotes is a feasible strategy to induce immunity against T. cruzi may greatly facilitate future studies on immunity to these forms of the parasite using individual genes or cDNA libraries. Studies on the immune response to amastigote antigens can provide relevant basic knowledge on the complex immunopathology of Chagas' disease and may be of importance for vaccine development.

Acknowledgments

This work was supported in part by grants from the Fundação de Amparo à Pesquisa do Estado de São Paulo, CNPq, PRONEX, and FINEP (Brazil). M.M.R and S.B.B received fellowships from CNPq and FAPESP, respectively.

We thank Carla Claser, Beatriz Schnabel, Miriam do Carmo, Marcia R. M. Santos, Renato A. Mortara, and J. Franco Da Silveira for help with different aspects of this study.

Notes

Editor: J. M. Mansfield

REFERENCES

1. Allen, C. L., and J. M. Kelly. 2001. Trypanosoma cruzi: mucin pseudogenes organized in a tandem array. Exp. Parasitol. 97:173-177. [PubMed]
2. Cano, M. I., A. Gruber, M. Vazquez, A. Cortes, M. J. Levin, A. Gonzalez, W. Degrave, E. Rondinelli, B. Zingales, J. L. Ramirez, C. Alonso, J. M. Requena, and J. Franco da Silveira. 1995. Molecular karyotype of clone CL Brener chosen for the Trypanosoma cruzi genome project. Mol. Biochem. Parasitol. 71:273-278. [PubMed]
3. Corr, M., L. F. Boyd, E. A. Padlan, and D. H. Margulies. 1993. H-2Dd exploits a four residue peptide binding motif. J. Exp. Med. 178:1877-1892. [PMC free article] [PubMed]
4. Corr, M., L. F. Boyd, S. R. Frankel, S. Kozlowski, E. A. Padlan, and D. H. Margulies. 1992. Endogenous peptides of a soluble major histocompatibility complex class I molecule, H-2Lds: sequence motif, quantitative binding, and molecular modeling of the complex. J. Exp. Med. 176:1681-1692. [PMC free article] [PubMed]
5. Costa, F., G. Franchin, V. L. Pereira-Chioccola, M. Ribeirão, S. Schenkman, and M. M. Rodrigues. 1998. Immunization with a plasmid DNA containing the gene of trans-sialidase reduces Trypanosoma cruzi infection in mice. Vaccine 16:768-774. [PubMed]
6. Cross, G. A., and G. B. Takle. 1993. The surface trans-sialidase family of Trypanosoma cruzi. Annu. Rev. Microbiol. 47:385-411. [PubMed]
7. Falk, K., O. Rotzschke, K. Dres, J. Metzger, G. Jung, and H.-G. Rammensee. 1991. Identification of naturally processed viral nonapeptide allows their quantification in infected cells and suggests an allele-specific T cell epitope forecast. J. Exp. Med. 174:425-434. [PMC free article] [PubMed]
8. Fouts, D. L., B. J. Ruef, P. T. Ridley, R. A. Wrightsman, D. S. Peterson, and J. E. Manning. 1991. Nucleotide sequence and transcription of a trypomastigote surface antigen gene of Trypanosoma cruzi. Mol. Biochem. Parasitol. 46:189-200. [PubMed]
9. Fujimura, A. E., S. S. Kinoshita, V. L. Pereira-Chioccola, and M. M. Rodrigues. 2001. DNA sequences encoding CD4+ and CD8+ T cell epitopes are important for efficient protective immunity induced by DNA vaccination with a Trypanosoma cruzi gene. Infect. Immun. 69:5477-5486. [PMC free article] [PubMed]
10. Garg, N., and R. Tarleton. 2002. Genetic immunization elicits antigen-specific protective immune responses and decreases disease severity in Trypanosoma cruzi infection. Infect. Immun. 70:5547-5555. [PMC free article] [PubMed]
11. Hoft, D. F., and C. S. Eickhoff. 2002. Type 1 immunity provides optimal protection against both mucosal and systemic Trypanosoma cruzi challenges. Infect. Immun. 70:6715-6725. [PMC free article] [PubMed]
12. Hoft, D. F., A. R. Schnapp, C. S. Eickhoff, and S. T. Roodman. 2000. Involvement of CD4 Th1 cells in systemic immunity protective against primary and secondary challenges with Trypanosoma cruzi. Infect. Immun. 68:197-204. [PMC free article] [PubMed]
13. Hölscher, C., G. Kölher, U. Müller, H. Mossman, G. A. Schaub, and F. Brombacher. 1998. Defective nitric oxide effector function lead to extreme susceptibility of Trypanosoma cruzi-infected mice deficient in gamma interferon receptor or inducible nitric oxide synthase. Infect. Immun. 66:1208-1215. [PMC free article] [PubMed]
14. Ishioka, G. Y., J. Fikes, G. Hermanson, B. Livingston, C. Crimi, M. Qin, M. F. del Guercio, C. Oseroff, C. Dahlberg, J. Alexander, R. W. Chesnut, and A. Sette. 1999. Utilization of MHC class I transgenic mice for development of minigene DNA vaccines encoding multiple HLA-restricted CTL epitopes. J. Immunol. 162:3915-3925. [PubMed]
15. Jou, W. M., M. Verhoeyen, R. Devos, E. Saman, R. Fang, D. Huylebroeck, W. Fiers, G. Threlfall, C. Barber, N. Carey, and S. Emtage. 1980. Complete structure of the hemagglutinin gene from the human influenza A/Victoria/3/75 (H3N2) strain as determined from cloned DNA. Cell 19:683-696. [PubMed]
16. Kahn, S., M. Wleklinski, A. Aruffo, A. Farr, D. Coder, and M. Kahn. 1995. Trypanosoma cruzi amastigote adhesion to macrophages is facilitated by the mannose receptor. J. Exp. Med. 182:1243-1258. [PMC free article] [PubMed]
17. Krettli, A. U., and Z. Brener. 1976. Protective effects of specific antibodies in Trypanosoma cruzi infections. J. Immunol. 116:755-760. [PubMed]
18. Kumar, S., and R. L. Tarleton. 2001. Antigen-specific Th1 but not Th2 cells provide protection from lethal Trypanosoma cruzi infection in mice. J. Immunol. 166:4596-4603. [PubMed]
19. Low, H. P., M. A. Santos, B. Wizel, and R. L. Tarleton. 1998. Amastigote surface proteins of Trypanosoma cruzi are targets for CD8+ CTL. J. Immunol. 160:1817-1823. [PubMed]
20. Low, H. P., and R. L. Tarleton. 1997. Molecular cloning of the gene encoding the 83 kDa amastigote surface protein and its identification as a member of the Trypanosoma cruzi sialidase super-family. Mol. Biochem. Parasitol. 88:137-149. [PubMed]
21. Magdesian, M. H., R. Giordano, H. Ulrichi, M. A. Juliano, L. Juliano, R. I. Schumacher, W. Colli, and M. J. M. Alves. 2001. Infection by Trypanosoma cruzi: identification of a parasite ligand and its host cell receptor. J. Biol. Chem. 276:19382-19389. [PubMed]
22. Martins, G. A., S. B. Petkova, F. S. Machado, R. N. Kitsis, L. M. Weiss, M. Wittner, H. B. Tanowitz, and J. S. Silva. 2001. Fas-FasL interaction modulates nitric oxide production in Trypanosoma cruzi-infected mice. Immunology 103:122-129. [PMC free article] [PubMed]
23. Medina-Acosta, E., and G. A. M. Cross. 1993. Rapid isolation of DNA from trypanosomatid protozoa using a simple “mini-prep” procedure. Mol. Biochem. Parasitol. 59:327-330. [PubMed]
24. Michailowsky, V., N. M. Silva, C. D. Rocha, L. Q. Vieira, J. Lannes-Vieira, and R. T. Gazzinelli. 2001. Pivotal role of interleukin-12 and interferon-gamma axis in controlling tissue parasitism and inflammation in the heart and central nervous system during Trypanosoma cruzi infection. Am. J. Pathol. 159:1723-1733. [PMC free article] [PubMed]
25. Miyahira, Y., K. Murata, D. Rodriguez, J. R. Rodriguez, M. Esteban, M. M. Rodrigues, and F. Zavala. 1995. Quantification of antigen specific CD8+ T cells using ELISPOT assay. J. Immunol. Methods 181:145-154. [PubMed]
26. Pan, A. A., and D. McMahon-Pratt. 1989. Amastigote and epimastigote stage-specific components of Trypanosoma cruzi characterized by using monoclonal antibodies. Purification and molecular characterization of an 83-kilodalton amastigote protein. J. Immunol. 143:1001-1008. [PubMed]
27. Peterson, D. S., R. A. Wrightsman, and J. E. Manning. 1986. Cloning of a major surface-antigen gene of Trypanosoma cruzi and identification of a nonapeptide repeat. Nature 322:566-568. [PubMed]
28. Planelles, L., M. C. Thomas, C. Alonso, and M. C. Lopez. 2001. DNA immunization with Trypanosoma cruzi HSP70 fused to the KMP11 protein elicits a cytotoxic and humoral immune response against the antigen and leads to protection. Infect. Immun. 69:6558-6563. [PMC free article] [PubMed]
29. Rodrigues, M., S. Li, K. Murata, D. Rodriguez, J. R. Rodriguez, I. Bacik, J. R. Bennink, J. W. Yewdell, A. Garcia-Sastre, R. S. Nussenzweig, M. Esteban, P. Palese, and F. Zavala. 1994. Influenza and vaccinia virus expressing malaria CD8 T and B cell epitopes: comparison of their immunogenicity and capacity to induce protective immunity against malaria. J. Immunol. 153:4636-4649. [PubMed]
30. Rodrigues, M. M., M. Ribeirão, V. Pereira-Chioccola, L. Renia, and F. Costa. 1999. Predominance of CD4 Th1 and CD8 Tc1 cells revealed by characterization of the cellular immune response generated by immunization with a DNA vaccine containing a Trypanosoma cruzi gene. Infect. Immun. 67:3855-3863. [PMC free article] [PubMed]
31. Rodrigues, M. M., M. Ribeirão, and S. B. Boscardin. 2000. CD4 Th1 but not Th2 clones efficiently activate macrophages to eliminate Trypanosoma cruzi through a nitric oxide dependent mechanism. Immunol. Lett. 73:43-50. [PubMed]
32. Rottenberg, M. E., M. Bakhiet, T. Olsson, K. Kristensson, T. Mak, H. Wigzell, and A. Orn. 1993. Differential susceptibilities of mice genomically deleted of CD4 and CD8 to infections with Trypanosoma cruzi or Trypanosoma brucei. Infect. Immun. 61:5129-5133. [PMC free article] [PubMed]
33. Ruef, B. J., B. D. Dawson, D. Tewari, D. L. Fouts, and J. E. Manning. 1994. Expression and evolution of members of the Trypanosoma cruzi trypomastigote surface antigen multigene family. Mol. Biochem. Parasitol. 63:109-120. [PubMed]
34. Ruef, B. J., and J. E. Manning. 1993. Isolation and characterization of the TSA/trans-sialidase family gene from the Silvio strain of Trypanosoma cruzi. Mol. Biochem. Parasitol. 62:117-120. [PubMed]
35. Sambrook, J., E. F. Fritsch, and T. Maniatis. 1989. Molecular cloning: a laboratory manual, 2nd ed. Cold Spring Harbor Laboratory Press, New York, N.Y.
36. Santos, A. M., N. Garg, and R. L. Tarleton. 1997. The identification and molecular characterization of Trypanosoma cruzi surface protein-1, a member of the trans-sialidase gene super-family. Mol. Biochem. Parasitol. 86:1-11. [PubMed]
37. Schnapp, A. R., C. S. Eickhoff, D. Sizemore, R. Curtiss III, and D. F. Hoft. 2002. Cruzipain induces both mucosal and systemic protection against Trypanosoma cruzi in mice. Infect. Immun. 70:5065-5074. [PMC free article] [PubMed]
38. Sepulveda, P., M. Hontebeyrie, P. Liegeard, A. Mascilli, and K. A. Norris. 2000. DNA-based immunization with Trypanosoma cruzi complement regulatory protein elicits complement lytic antibodies and confers protection against Trypanosoma cruzi infection. Infect. Immun. 68:4986-4991. [PMC free article] [PubMed]
39. Sheffield, P., S. Garrard, and Z. Derewenda. 1999. Overcoming expression and purification problems of RhoGDI using a family of “parallel” expression vectors. Protein Expr. Purif. 15:34-39. [PubMed]
40. Silva, J. S., P. J. Morrisey, K. H. Grabstein, K. M. Moller, D. Anderson, and S. G. Reed. 1992. Interleukin 10 and interferon gamma regulation of experimental Trypanosoma cruzi infection. J. Exp. Med. 175:169-174. [PMC free article] [PubMed]
41. Silva, L. H. P., and V. Nussenzweig. 1953. Sobre uma cepa de Trypanosoma cruzi altamente virulenta para o camundongo branco. Folia Clin. Biol. 20:191-203.
42. Tarleton, R. L., M. J. Grusby, and L. Zhang. 2000. Increased susceptibility of Stat4-deficient and enhanced resistance in Stat6-deficient mice to infection with Trypanosoma cruzi. J. Immunol. 165:1520-1525. [PubMed]
43. Tarleton, R. L. 1995. The role of T cells in Trypanosoma cruzi infection. Parasitol. Today 11:7-12. [PubMed]
44. Vespa, G. N., F. Q. Cunha, and J. S. Silva. 1994. Nitric oxide is involved in control of Trypanosoma cruzi-induced parasitemia and directly kills the parasite in vitro. Infect. Immun. 62:5177-5182. [PMC free article] [PubMed]
45. Wizel, B., N. Garg, and R. Tarleton. 1998. Vaccination with trypomastigote surface antigen 1-encoding plasmid DNA confers protection against lethal Trypanosoma cruzi infection. Infect. Immun. 66:5073-5081. [PMC free article] [PubMed]
46. Wizel, B., M. Nunes, and R. L. Tarleton. 1997. Identification of Trypanosoma cruzi trans-sialidase family members as targets of protective CD8+ TC1 responses. J. Immunol. 159:6120-6130. [PubMed]
47. Wizel, B., M. Palmieri, C. Mendoza, B. Arana, J. Sidney, A. Sette, and R. L. Tarleton. 1998. Human infection with Trypanosoma cruzi induces parasite antigen-specific cytotoxic T lymphocyte responses. J. Clin. Investig. 102:1062-1071. [PMC free article] [PubMed]

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