Vaccination of mice with the protective F3G3 antigen of Toxoplasma gondii activates CD4+ but not CD8+ T cells and induces Toxoplasma specific IgG antibody

Partial protective effect of intranasal immunization with recombinant Toxoplasma gondii rhoptry protein 17 against toxoplasmosis in mice

Toxoplasma gondii (T. gondii) is an obligate intracellular protozoan parasite that infects a variety of mammals, including humans. An effective vaccine for this parasite is therefore needed. In this study, RH strain T. gondii rhoptry protein 17 was expressed in bacteria as a fusion with glutathione S-transferase (GST) and the recombinant proteins (rTgROP17) were purified via GST-affinity chromatography. BALB/c mice were nasally immunised with rTgROP17, and induction of immune responses and protection against chronic and lethal T. gondii infections were investigated. The results revealed that mice immunised with rTgROP17 produced high levels of specific anti-rTgROP17 IgGs and a mixed IgG1/IgG2a response of IgG2a predominance. The systemic immune response was associated with increased production of Th1 (IFN-γand IL-2) and Th2 (IL-4) cytokines, and enhanced lymphoproliferation (stimulation index, SI) in the mice immunised with rTgROP17. Strong mucosal immune responses with increased secretion of TgROP17-specific secretory IgA (SIgA) in nasal, vaginal and intestinal washes were also observed in these mice. The vaccinated mice displayed apparent protection against chronic RH strain infection as evidenced by their lower liver and brain parasite burdens (59.17% and 49.08%, respectively) than those of the controls. The vaccinated mice also exhibited significant protection against lethal infection of the virulent RH strain (survival increased by 50%) compared to the controls. Our data demonstrate that rTgROP17 can trigger strong systemic and mucosal immune responses against T. gondii and that ROP17 is a promising candidate vaccine for toxoplasmosis.

Intranasal immunisation with recombinant Toxoplasma gondii actin partly protects mice against toxoplasmosis

Toxoplasma gondii is a ubiquitous protozoan intracellular parasite, the causative agent of toxoplasmosis, and a worldwide zoonosis for which an effective vaccine is needed. Actin is a highly conserved microfilament protein that plays an important role in the invasion of host cells by T. gondii. This study investigated the immune responses elicited by BALB/c mice after nasal immunisation with a recombinant T. gondii actin (rTgACT) and the subsequent protection against chronic and lethal T. gondii infections. We evaluated the systemic response by proliferation, cytokine and antibody measurements, and we assessed the mucosal response by examining the levels of TgACT-specific secretory IgA (SIgA) in nasal, vaginal and intestinal washes. Parasite load was assessed in the liver and brain, and the survival of mice challenged with a virulent strain was determined. The results showed that the mice immunised with rTgACT developed high levels of specific anti-rTgACT IgG titres and a mixed IgG1/IgG2a response with a predominance of IgG2a. The systemic immune response was associated with increased production of Th1 (IFN-γ and IL-2), Th2 (IL-4) and Treg (IL-10) cytokines, indicating that not only Th1-type response was induced, but also Th2- and Treg-types responses were induced, and the splenocyte stimulation index (SI) was increased in the mice immunised with rTgACT. Nasal immunisation with rTgACT led to strong mucosal immune responses, as seen by the increased secretion of SIgA in nasal, vaginal and intestinal washes. The vaccinated mice displayed significant protection against lethal infection with the virulent RH strain (survival increased by 50%), while the mice chronically infected with RH exhibited lower liver and brain parasite loads (60.05% and 49.75%, respectively) than the controls. Our data demonstrate, for the first time, that actin triggers a strong systemic and mucosal response against T. gondii. Therefore, actin may be a promising vaccine candidate against toxoplasmosis.

Comparison of protective immune responses to apicomplexan parasites

Members of the phylum Apicomplexa, which includes the species Plasmodium, Eimeria, Toxoplasma, and Babesia amongst others, are the most successful intracellular pathogens known to humankind. The widespread acquisition of antimicrobial resistance to most drugs used to date has sparked a great deal of research and commercial interest in the development of vaccines as alternative control strategies. A few antigens from the asexual and sexual stages of apicomplexan development have been identified and their genes characterised; however, the fine cellular and molecular details of the effector mechanisms crucial for parasite inhibition and stimulation of protective immunity are still not entirely understood. This paper provides an overview of what is currently known about the protective immune response against the various types of apicomplexan parasites and focuses mainly on the similarities of these pathogens and their host interaction. Finally, the evolutionary relationships of these parasites and their hosts, as well as the modulation of immune functions that are critical in determining the outcome of the infection by these pathogenic organisms, are discussed.

Vaccines against Toxoplasma gondii: challenges and opportunities

Development of vaccines against Toxoplasma gondii infection in humans is of high priority, given the high burden of disease in some areas of the world like South America, and the lack of effective drugs with few adverse effects. Rodent models have been used in research on vaccines against T. gondii over the past decades. However, regardless of the vaccine construct, the vaccines have not been able to induce protective immunity when the organism is challenged with T. gondii, either directly or via a vector. Only a few live, attenuated T. gondii strains used for immunization have been able to confer protective immunity, which is measured by a lack of tissue cysts after challenge. Furthermore, challenge with low virulence strains, especially strains with genotype II, will probably be insufficient to provide protection against the more virulent T. gondii strains, such as those with genotypes I or II, or those genotypes from South America not belonging to genotype I, II or III. Future studies should use animal models besides rodents, and challenges should be performed with at least one genotype II T. gondii and one of the more virulent genotypes. Endpoints like maternal-foetal transmission and prevention of eye disease are important in addition to the traditional endpoint of survival or reduction in numbers of brain cysts after challenge.

The dense granule protein GRA2, a new marker for the serodiagnosis of acute Toxoplasma infection: comparison of sera collected in both France and Iran from pregnant women

GRA2 is a highly immunogenic protein secreted from the dense granules of Toxoplasma gondii. Recent success in purifying full-length, soluble GRA2 from bacteria as a thioredoxin (TRX)-(Hisx6) fusion protein led to investigate the antigenicity of the recombinant protein against human sera. On immunoblots, TRX-(Hisx6)-GRA2 was recognized by sera collected in Iran from T. gondii-infected pregnant women. An IgG enzyme-linked immunosorbent assay was developed to evaluate the reactivity of sera, collected from pregnant women both in France and Iran, to the TRX-(Hisx6)-GRA2 fusion protein. Specificity of the test was 96.4%. Sensitivity of the GRA2 enzyme-linked immunosorbent assay ranged from 95.8% (sera collected in France) to 100% (sera collected in Iran) for sera of acute infection and from 65.7% (sera collected in France) to 71.4% (sera collected in Iran) for sera of chronic infection. The recombinant GRA2 could thus advantageously complement previously described T. gondii antigens for the serodiagnosis of acute Toxoplasma infection.

Toxoplasma gondii: expression and characterization of a recombinant protein containing SAG1 and GRA2 in Pichia pastoris

Toxoplasma gondii is an obligate intracellular protozoan which infects most species of warm-blooded animals and causes toxoplasmosis. Previous immunological and immunization studies have demonstrated the potential role of T. gondii antigens SAG1 and GRA2 as a vaccine candidate. In the present study, we have cloned, expressed, and purified a recombinant protein SAG1-GRA2 in Pichia pastoris. Results showed that P. pastoris was a robust system producing a large amount of highly purified and biological activity protein. BALB/c mice immunized with SAG1-GRA2 elicited stronger humoral and cellular responses in comparison to control groups. This immunization resulted in an enhanced Th1 immune response as measured by IgG2a antibody production and increased splenocyte IFN-gamma production, whereas no IL-4 was detected. After a lethal challenge with the highly virulent T. gondii RH strain, a prolonged survival time in SAG1-GRA2-immunized mice was observed in comparison to control groups. Our data demonstrate that SAG1-GRA2 triggered a protective response against toxoplasmosis. Therefore, SAG1-GRA2 protein might be a good candidate for the further development of a multiantigenic vaccine.

Evidence for a compartmentalized B cell response as characterized by IgG epitope specificity in human ocular toxoplasmosis

Infectious agents in the eye induce both a local and a systemic humoral immune response. Previously, differences in Ag recognition were observed between systemic and ocular derived IgG of patients with ocular toxoplasmosis. This finding implied a nonrandom distribution of IgG-producing B cells in the inflamed eye. In the present study, we compared the intraocular and systemic B cell responses of patients with ocular toxoplasmosis to a single Toxoplasma gondii Ag. Two series of C-terminally deleted recombinant T. gondii GRA-2 proteins were constructed to delineate IgG B cell epitopes of paired ocular and serum samples. Differences in epitope region recognition between the ocular and systemic compartment were detected in 9 of 13 patients. The difference in distribution of GRA-2 epitopes between paired samples is indicative of a local GRA-2 specific B cell population functionally different from the systemic GRA-2-specific B cell population. Our results suggest a selective activation of a subset of B cells locally in nonlymphoid tissue.

Advances and prospects for subunit vaccines against protozoa of veterinary importance

Protozoa are responsible for considerable morbidity and mortality in domestic and companion animals. Preventing infection may involve deliberate exposure to virulent or attenuated parasites so that immunity to natural infection is established early in life. This is the basis for vaccines against theilerosis and avian coccidiosis. Vaccination may not be effective or practical with diseases, such as cryptosporidiosis, that primarily afflict the immune-compromised or individuals with an incompletely developed immune system. Strategies for combating these diseases often rely on passive immunotherapy using serum or colostrums containing antibodies to parasite surface proteins. Subunit vaccines offer an attractive alternative to virulent or attenuated parasites for several reasons. These include the use of bacteria or lower eukaryotes to produce recombinant proteins in batch culture, the relative stability of recombinant proteins compared to live parasites, and the flexibility to incorporate only those antigens that elicit "protective" immune responses. Although subunit vaccines offer many theoretical advantages, our lack of understanding of immune mechanisms to primary and secondary infection and the capacity of many protozoa to evade host immunity remain obstacles to developing effective vaccines. This review examines the progress made on developing recombinant proteins of Eimeria, Giardia, Cryptosporidium, Toxoplasma, Neospora, Trypanosoma, Babesia, and Theileria and attempts to use these antigens for vaccinating animals against the associated diseases.

Targeted disruption of the GRA2 locus in Toxoplasma gondii decreases acute virulence in mice

Following invasion into the host cell, the protozoan Toxoplasma gondii secretes a variety of proteins that modify the parasitophorous vacuole. Within the vacuole, the 28-kDa dense granule protein known as GRA2 is specifically targeted to the tubulovesicular network which forms connections with the vacuolar membrane. To investigate the importance of GRA2, we derived from strain RH a mutant T. gondii line in which GRA2 was disrupted by replacement with the marker Ble (selecting for phleomycin resistance). The Deltagra2 mutant invaded and grew normally in both fibroblasts and macrophages in vitro; however, it was less virulent during acute infection in mice. The survival rate of mice inoculated with Deltagra2 was significantly higher; some infected mice survived the acute infection, whereas all mice infected with the wild-type strain RH succumbed to early death. Chronic infection by Deltagra2 was detected by positive serology, immunohistochemical detection of parasites and cysts in the brain, and reisolation of parasites by bioassay at 6 weeks postinfection. Thus, absence of GRA2 partially attenuates the virulence of T. gondii during the acute phase of infection and allows for establishment of chronic infection by the otherwise highly virulent RH strain. These results establish that GRA2 plays an important role during in vivo infection and provide a potential model for examining acute pathogenesis by T. gondii.

Detection of a novel 40,000 MW excretory Toxoplasma gondii antigen by murine Th1 clone which induces toxoplasmacidal activity when exposed to infected macrophages

To analyse target molecules of the CD4+ T-cell response to toxoplasma infection, a panel of Toxoplasma gondii-specific murine CD4+ T-cell clones has been established. Clone 3Tx15, belonging to the T helper 1 (Th1) subtype, abolished intracellular parasite growth when co-cultured with macrophages and live toxoplasma at a ratio of 2:2:1. This effect results from macrophage toxoplasmicidal activity induced upon parasite-dependent cellular interaction, an irrelevant Th1 clone failed in this three-party system. Clone 3Tx15 detects its corresponding antigen in the supernatant of infected cells and also reacts with a host cell-free preparation of T. gondii-excreted/secreted antigens. T-cell blot analysis of two-dimensionally separated toxoplasma lysate revealed a molecular weight of about 40,000 for the fractions stimulating clone 3Tx15. As checked in parallel enzyme-linked immunosorbent assay, the 40,000 MW T-cell antigen co-migrates with the excretory protein GRA4, the sole 40,000 MW T. gondii antigen hitherto known to be recognized by T lymphocytes. Nevertheless, neither recombinant GRA4 nor immunoaffinity-purified natural GRA4 was stimulatory for clone 3Tx15. Our findings thus demonstrate that Th1 clone 3Tx15 which induces toxoplasmicidal activity during antigenic interaction with infected macrophages defines a new 40,000 MW excretory T. gondii antigen.

Intranasal immunization with SAG1 protein of Toxoplasma gondii in association with cholera toxin dramatically reduces development of cerebral cysts after oral infection

SAG1 protein of Toxoplasma gondii was evaluated as a protective antigen in mucosal immunization with cholera toxin as an adjuvant. CBA/J mice intranasally immunized with a combination of SAG1 and cholera toxin exhibited significantly fewer cysts in the brain after oral infection with the 76K strain of T. gondii than control mice. This acquired protection lasted at least 5 months. Protected mice developed high levels of serum anti-SAG1 immunoglobulin G antibodies as well as an enhanced systemic cellular response, as assessed by the proliferation of splenocytes in response to SAG1 restimulation in vitro. This cellular proliferation was associated with an increase of interleukin-2 and interleukin-5 synthesis and with barely detectable gamma interferon production. Splenic immune T cells were shown to convey modest protection to recipients against development of brain cysts following oral infection with T. gondii. Significant production of anti-SAG1 immunoglobulin A was induced in intestinal secretions of protected mice. These results indicate that intranasal immunization with SAG1 and cholera toxin can induce mucosal and systemic immune responses and affords partial and long-lasting resistance against the establishment of chronic toxoplasmosis.

Antigen processing: the gateway to the immune response

The T-lymphocyte response to an antigen is governed by the source of that antigen and the way in which it is processed. Before recognition by T lymphocytes, proteins must be degraded to peptides by antigen-presenting cells. The peptides are then presented on major histocompatibility complex (MHC) molecules for recognition by the T cells. Antigens arising outside the cell (e.g., bacteria) are phagocytosed and processed by the exogenous pathway for presentation on MHC class II molecules (e.g., DR) to CD4+ cells. Antigens derived from the cytoplasm (e.g., viral proteins) are processed by the endogenous pathway for presentation by MHC class I molecules (e.g., HLA-A, -B, -C) to CD8+ cells. The response to a hapten or drug is a function of the antigen processing pathway and is determined by its chemical properties. Antigen processing also governs the T-cell response to pathogens, vaccines, and autoimmune conditions.