Human T-cell clones against Toxoplasma gondii: production of interferon-gamma, interleukin-2, and strain cross-reactivity

Key Limitations and New Insights Into the Toxoplasma gondii Parasite Stage Switching for Future Vaccine Development in Human, Livestock, and Cats

Toxoplasmosis is a parasitic disease affecting human, livestock and cat. Prophylactic strategies would be ideal to prevent infection. In a One Health vaccination approach, the objectives would be the prevention of congenital disease in both women and livestock, prevention/reduction of T. gondii tissue cysts in food-producing animals; and oocyst shedding in cats. Over the last few years, an explosion of strategies for vaccine development, especially due to the development of genetic-engineering technologies has emerged. The field of vaccinology has been exploring safer vaccines by the generation of recombinant immunogenic proteins, naked DNA vaccines, and viral/bacterial recombinants vectors. These strategies based on single- or few antigens, are less efficacious than recombinant live-attenuated, mostly tachyzoite T. gondii vaccine candidates. Reflections on the development of an anti-Toxoplasma vaccine must focus not only on the appropriate route of administration, capable of inducing efficient immune response, but also on the choice of the antigen (s) of interest and the associated delivery systems. To answer these questions, the choice of the animal model is essential. If mice helped in understanding the protection mechanisms, the data obtained cannot be directly transposed to humans, livestock and cats. Moreover, effectiveness vaccines should elicit strong and protective humoral and cellular immune responses at both local and systemic levels against the different stages of the parasite. Finally, challenge protocols should use the oral route, major natural route of infection, either by feeding tissue cysts or oocysts from different T. gondii strains. Effective Toxoplasma vaccines depend on our understanding of the (1) protective host immune response during T. gondii invasion and infection in the different hosts, (2) manipulation and modulation of host immune response to ensure survival of the parasites able to evade and subvert host immunity, (3) molecular mechanisms that define specific stage development. This review presents an overview of the key limitations for the development of an effective vaccine and highlights the contributions made by recent studies on the mechanisms behind stage switching to offer interesting perspectives for vaccine development.

T cell mediated immune responses to Toxoplasma gondii in pregnant women with primary toxoplasmosis

The aim of this study was to investigate T cell immunity to Toxoplasma gondii (Tg) in pregnant women with primary toxoplasmosis. This issue has never been addressed before in humans and available information derives from murine models. Peripheral blood mononuclear cells (PBMC) from pregnant women with primary Tg infection were stimulated with Tg tachyzoites, excretory-secretory antigens (ESA) or recombinant surface antigen-1 (rSAG-1), and tested for proliferation, immunophenotype, cytokine production and antigen specific cytotoxic activity. Pregnant women with primary toxoplasmosis displayed a significant decrease of the CD4/CD8 T cell ratio and a significant increase of circulating T cell receptor (TCR) gammadelta+ cells as compared to their uninfected counterparts. T cells from Tg infected pregnant women proliferated to Tg tachyzoites, ESA or rSAG-1. Most tachyzoite and ESA specific T cell blasts were CD4+, whereas SAG-1 specific blasts were CD4+ and CD8+. ESA and tachyzoite specific T cell blasts displayed a Th1 or Th0 cytokine profile with overexpression of IFN-gamma. This pattern was unchanged upon in vitro exposure of T cells to progesterone, tested at a concentration close to that reached in vivo at the maternal-fetal interface. Finally, tachyzoite or ESA specific T cell blasts lysed, through a granule exocytosis dependent mechanism, autologous lymphoblastoid cell lines presenting Tg antigens. In conclusion, pregnant women with primary toxoplasmosis mounted in vitro Tg-specific Th1/Th0 responses whose impact on neonatal infection warrants further investigation.

Protective effect of a naked DNA vaccine cocktail against lethal toxoplasmosis in mice

BALB/c mice were intramuscularly immunized with low doses (25-50microg) of DNA cocktail containing plasmids encoding the full-length SAG1/P30 and the 196-561 terminal sequence of ROP2 genes. This immunization resulted in a Th1-type response with predominance of IgG2a and a specific T-cell proliferation with high levels of interferon-gamma (IFN-gamma) secretion, whereas no IL-4 was detected. Moreover, DNA cocktail immunization induced a long-lasting protection against a lethal challenge with the highly virulent Toxoplasma gondii RH strain, whereas low doses of single genes were not protective. These results support further investigations to achieve a multigene anti-T. gondii DNA vaccine.

T cell clones raised from chronically infected healthy humans by stimulation with Toxoplasma gondii excretory-secretory antigens cross-react with live tachyzoites: characterization of the fine antigenic specificity of the clones and implications for vaccine development

Excreted-secreted Ags (ESA) of Toxoplasma gondii (Tg) play an important role in the stimulation of the host immune system in both acute and chronic infections. To identify the parasite Ag(s) involved in the maintenance of T cell-mediated long term immunity, 40 ESA-specific T cell clones were derived from three chronically infected healthy subjects. All the clones were CD4+ and recognized both ESA and live tachyzoites in a HLA-DR-restricted manner. Conversely, CD4+ tachyzoite-specific T cell clones from the same subjects proliferated in response to ESA, pointing to shared immunodominant Ags between ESA and Tg tachyzoites. By T cell blot analysis using SDS-PAGE-fractionated parasite extracts, the following patterns of reactivity were detected. Of 25 clones, 6 recognized Tg fractions in the 24- to 28-kDa range and proliferated to purified GRA2, 5 reacted with Tg fractions in the 30- to 33-kDa range; and 4 of them proved to be specific for rSAg1. Although surface Ag (SAg1) is not a member of ESA, small amounts of this protein were present in ESA preparation by Western blot. Of 25 clones, 8 responded to Tg fractions in the 50- to 60-kDa range but not to the 55-kDa recombinant rhoptries-2 parasite Ag, and 6 did not react with any Tg fraction but proliferated in response to either ESA or total parasite extracts. In conclusion, CD4+ T cells specific for either ESA (GRA2) or SAg1 may be involved in the maintenance of long term immunity to Tg in healthy chronically infected individuals.

A switch towards Th2 during serological rebound in children with congenital toxoplasmosis

Serological rebounds occur frequently in patients with congenital toxoplasmosis, but remain poorly understood. A link between Th1 and Th2 cytokines and the pathophysiology of infectious diseases has been reported. Production of interferon-gamma (IFN-gamma) and IL-4 in supernatants of whole blood after in vitro specific Toxoplasma gondii stimulation and serum-specific IgE levels were studied in 31 congenitally infected children. IFN-gamma was produced at higher levels by lymphocytes from children with stable congenital toxoplasmosis (n = 18) than from children showing serological rebound (n = 13) (P < 0.04). Conversely, supernatants from children with serological rebound showed higher levels of IL-4 than those from children with stable congenital toxoplasmosis (P < 0.03). The polarized Th2 response was confirmed by a greater (IL-4:IFN-gamma) x 100 ratio (P < 0.0001) and production of T. gondii-specific IgE in six out of 13 children showing serological rebound. These results suggest a role of Th2 cytokines in destabilization of congenital toxoplasmosis and perhaps in local reactivation of the parasite.

The conformation of purified Toxoplasma gondii SAG1 antigen, secreted from engineered Pichia pastoris, is adequate for serorecognition and cell proliferation

A truncated form of SAG1, the immunodominant surface antigen of Toxoplasma gondii, has been produced in the methylotrophic yeast, Pichia pastoris. By construction, the recombinant protein lacks C-terminal residues 308-336 which, in native SAG1, encompass the glycosylphosphatidylinositol anchorage site. Secretion of anchor-less SAG1 proceeded via the yeast prepro alpha-mating factor signal peptide and yielded two immunoreactive protein species having apparent molecular masses of 31.5 and 34.5 kDa, respectively, and differing only by N-glycosylation of the single Asn-X-Ser site present in the molecule. Purification of the anchor-less SAG1 was achieved by a combination of ion-exchange and size-exclusion chromatographies. N-terminal amino acid sequencing of the products indicated the presence of additional residues glutamic acid--alanine at the N-terminal end of the products. Despite incomplete processing and unnatural glycosylation, anchor-less SAG1 proteins apparently adopted a suitable conformation recognized by monoclonal and human serum-derived antibodies, specific for the native SAG1. In addition, the recombinant anchor-less SAG1 proved competent for inducing proliferation, in vitro, of mononuclear cells from seropositive individuals. Finally, properly adjuvanted anchor-less SAG1 was able to induce protection of mice against a lethal challenge with T. gondii tachyzoites.

αβ T Cell Response to Toxoplasma gondii in Previously Unexposed Individuals

The mechanisms by which T cells from previously unexposed hosts respond in vitro to certain intracellular pathogens remain to be fully understood. We report and characterize the in vitro reactivity to Toxoplasma gondii of human αβ T cells from T. gondii-seronegative individuals. Resting αβ T cells from these individuals proliferated in response to PBMC infected with T. gondii or pulsed with T. gondii lysate Ags. This was accompanied by an increase in the percentage of CD4+ αβ T cells. Purified CD4+ αβ T cells but not CD8+ αβ T cells proliferated in response to these T. gondii preparations. Both CD4+ αβ T cells with naive (CD45RA+) and memory (CD45RO+) phenotypes from adults as well as αβ T cells from T. gondii-seronegative newborns proliferated after incubation with T. gondii. This αβ T cell response to the parasite was inhibited by anti-HLA-DR mAb and to a lesser degree by anti-HLA-DQ mAb. Use of paraformaldehyde-fixed PBMC completely abrogated the proliferation of αβ T cells, indicating the need for processing of T. gondii Ags. Analysis of the TCR Vβ expression did not show evidence for restriction in TCR Vβ usage during T. gondii stimulation of αβ T cells. αβ T cells secreted significant amounts of IFN-γ after incubation with T. gondii-infected monocytes. This rapid and remarkable αβ T cell response may play an important role in the early events of the immune response to T. gondii.

Epitopes recognized by human T lymphocytes in the ROP2 protein antigen of Toxoplasma gondii

The ROP2 protein of Toxoplasma gondii possesses immunological and biological properties which have led to its proposal as a vaccine candidate. To identify epitopes recognized by human T cells in the ROP2 antigen, we submitted the sequence of this protein to three reported T-cell epitope prediction algorithms. Three sequences that were predicted by all three methods were selected (sequences 197 to 216, 393 to 410, and 501 to 524), and the corresponding peptides were synthesized. The peptides were first tested in a proliferation assay with a DPw4-restricted, ROP2-specific human T-cell clone, and the peptide corresponding to residues 197 to 216 was shown to stimulate the T-cell clone. The three peptides were further tested in proliferation assays with peripheral blood mononuclear cells from a panel of T. gondii-seropositive and -seronegative individuals. We found that cells from a high proportion of the seropositive donors (64%) recognized at least one of the three peptides. The most frequently recognized ones were peptides 197 to 216 (45%) and 501 to 524 (36%). None of the seronegative donors responded to any peptide. These results show that the ROP2 antigen of T. gondii contains T-cell epitopes recognized by a high percentage of the immune population and further strengthen its potential as a vaccine candidate.

Human CD4+ and CD8+ T lymphocytes are both cytotoxic to Toxoplasma gondii-infected cells

Studies to determine if Toxoplasma gondii-specific human T cells lyse parasite-infected cells have yielded conflicting results. Furthermore, attempts to obtain human cytotoxic CD8+ T lymphocytes have been difficult because of the lack of a reproducible system for their generation. By using paraformaldehyde-fixed, T. gondii-infected peripheral blood mononuclear cells as antigen-presenting cells, we developed a method whereby T. gondii-specific T-cell lines can be reproducibly generated. Six T. gondii-specific T-cell lines were generated from an individual chronically infected with T. gondii. Cytofluorometric analysis of these lines revealed > 99% CD3+, 85 to 95% CD3+ alpha beta T-cell-receptor-positive (TCR+), 5 to 9% CD3+ gamma delta TCR+, 50 to 70% CD4+, and 20 to 40% CD8+ cells when cells were examined during the first 3 weeks of stimulation and >99% CD3+, >99% CD3+ alpha beta TCR+, < 1% CD3+ gamma delta TCR+, 20 to 40% CD4+, and 60 to 80% CD8+ cells when cells were examined between 5 and 11 weeks. Both CD4+ and CD8+ T cells had remarkable cytotoxic activity against T. gondii-infected target cells (30 to 50% specific Cr release at an effector-to-target ratio of 30:1) but not against uninfected target cells ( < 10% at an effector-to-target ratio of 30:1). Cytotoxic activity by the whole T-cell lines was not T. gondii strain specific. Whole T-cell lines were cytotoxic for target cells infected with the C56 and ME49 strains and the RH strain (which was used to infect peripheral blood mononuclear cells). T. gondii-specific T-cell lines displayed the predominant expression of V beta 7 TCR. The CDR3 regions of the V beta 7 TCRs of these T-cell lines showed a striking degree of sequence identity (oligoclonality). T-cell lines obtained by the method reporter here can be used to characterize functional activity of T-lymphocyte subsets in humans infected with T. gondii.

A new, simple, bioassay for human IFN-gamma

IFN-gamma induces the production of N-formyl-kynurenine from L-tryptophan in various cell types by the induction of the enzyme indoleamine 2,3-dioxygenase (IDO). The IFN-gamma induced IDO activity in the glioblastoma cell line 86HG39 and cells of clone 2D9 derived from this cell line was found to be greater than that in Hela cells and U373MG cells. Consequently 2D9 cells were used in all subsequent experiments. The determination of kynurenine in the supernatant of IFN-gamma activated cells was performed photometrically using a microplate reader. It was found that the amount of kynurenine produced was directly proportional to the amount of IFN-gamma used to activate cells. The detection limit for IFN-gamma of this assay was 20 U/ml. The induction of L-tryptophan degradation was specific for IFN-gamma since neither IFN-alpha, IFN-beta, IL-1, IL-2, IL-6, GM-CSF nor TNF alpha induced the production of detectable amounts of kynurenine by 86HG39 and 2D9 cells. Furthermore, a mab directed against IFN-gamma was able to completely block the IFN-gamma induced IDO activation. This bioassay was used to determine the IFN-gamma content of supernatants harvested from toxoplasma antigen specific human T cell lines and clones. This assay gave reproducible results which correlated well with the IFN-gamma content detected in the same samples using a commercially available ELISA kit. Furthermore in the case of T cell supernatant stimulated 2D9 cells a mab directed against IFN-gamma was able to completely block IDO induction. We conclude that the measurement of kynurenine production induced by IFN-gamma can be used to determinate IFN-gamma content. This is a simple bioassay which can be performed with standard laboratory equipment.