Recombinant attenuated Toxoplasma gondii expressing the Plasmodium yoelii circumsporozoite protein provides highly effective priming for CD8+ T cell-dependent protective immunity against malaria

Specific Cell Targeting by Toxoplasma gondii Displaying Functional Single-Chain Variable Fragment as a Novel Strategy; A Proof of Principle

Toxoplasma gondii holds significant therapeutic potential; however, its nonspecific invasiveness results in off-target effects. The purpose of this study is to evaluate whether T. gondii specificity can be improved by surface display of scFv directed against dendritic cells' endocytic receptor, DEC205, and immune checkpoint PD-L1. Anti-DEC205 scFv was anchored to the T. gondii surface either directly via glycosylphosphatidylinositol (GPI) or by fusion with the SAG1 protein. Both constructs were successfully expressed, but the binding results suggested that the anti-DEC-SAG1 scFv had more reliable functionality towards recombinant DEC protein and DEC205-expressing MutuDC cells. Two anti-PD-L1 scFv constructs were developed that differed in the localization of the HA tag. Both constructs were adequately expressed, but the localization of the HA tag determined the functionality by binding to PD-L1 protein. Co-incubation of T. gondii displaying anti-PD-L1 scFv with tumor cells expressing/displaying different levels of PD-L1 showed strong binding depending on the level of available biomarker. Neutralization assays confirmed that binding was due to the specific interaction between anti-PD-L1 scFv and its ligand. A mixed-cell assay showed that T. gondii expressing anti-PD-L1 scFv predominately targets the PD-L1-positive cells, with negligible off-target binding. The recombinant RH-PD-L1-C strain showed increased killing ability on PD-L1+ tumor cell lines compared to the parental strain. Moreover, a co-culture assay of target tumor cells and effector CD8+ T cells showed that our model could inhibit PD1/PD-L1 interaction and potentiate T-cell immune response. These findings highlight surface display of antibody fragments as a promising strategy of targeting replicative T. gondii strains while minimizing nonspecific binding.

Novel Murine Pancreatic Tumor Model Demonstrates Immunotherapeutic Control of Tumor Progression by a Toxoplasma gondii Protein

Pancreatic ductal adenocarcinoma is the fourth leading cause of cancer-related death in the United States, with few effective treatments available and only 10% of those diagnosed surviving 5 years. Although immunotherapeutics is a growing field of study in cancer biology, there has been little progress in its use for the treatment of pancreatic cancer. Pancreatic cancer is considered a nonimmunogenic tumor because the tumor microenvironment does not easily allow for the immune system, even when stimulated, to attack the cancer. Infection with the protozoan parasite Toxoplasma gondii has been shown to enhance the immune response to clear cancer tumors. A subset of T. gondii proteins called soluble Toxoplasma antigen (STAg) contains an immunodominant protein called profilin. Both STAg and profilin have been shown to stimulate an immune response that reduces viral, bacterial, and parasitic burdens. Here, we use STAg and profilin to treat pancreatic cancer in a KPC mouse-derived allograft murine model. These mice exhibit pancreatic cancer with both Kras and P53 mutations as subcutaneous tumors. Pancreatic cancer tumors in C57BL/6J mice with a wild-type background showed a significant response to treatment with either profilin or STAg, exhibiting a decrease in tumor volume accompanied by an influx of CD4⁺ and CD8⁺ T cells into the tumors. Both IFN-γ^-/- mice and Batf3^-/- mice, which lack conventional dendritic cells, failed to show significant decreases in tumor volumes when treated. These results indicate that gamma interferon (IFN-γ) and dendritic cells may play critical roles in the immune response necessary to treat pancreatic cancer.

Plasmodium falciparum pre-erythrocytic stage vaccine development

Worldwide strategies between 2010 and 2017 aimed at controlling malarial parasites (mainly Plasmodium falciparum) led to a reduction of just 18% regarding disease incidence rates. Many biologically-derived anti-malarial vaccine candidates have been developed to date; this has involved using many experimental animals, an immense amount of work and the investment of millions of dollars. This review provides an overview of the current state and the main results of clinical trials for sporozoite-targeting vaccines (i.e. the parasite stage infecting the liver) carried out by research groups in areas having variable malaria transmission rates. However, none has led to promising results regarding the effective control of the disease, thereby making it necessary to complement such efforts at finding/introducing new vaccine candidates by adopting a multi-epitope, multi-stage approach, based on minimal subunits of the main sporozoite proteins involved in the invasion of the liver.

Vaccination With Sporozoites: Models and Correlates of Protection

Despite continuous efforts, the century-old goal of eradicating malaria still remains. Multiple control interventions need to be in place simultaneously to achieve this goal. In addition to effective control measures, drug therapies and insecticides, vaccines are critical to reduce mortality and morbidity. Hence, there are numerous studies investigating various malaria vaccine candidates. Most of the malaria vaccine candidates are subunit vaccines. However, they have shown limited efficacy in Phase II and III studies. To date, only whole parasite formulations have been shown to induce sterile immunity in human. In this article, we review and discuss the recent developments in vaccination with sporozoites and the mechanisms of protection involved.

Previous Infection with Plasmodium berghei Confers Resistance to Toxoplasma gondii Infection in Mice

Both Plasmodium spp. and Toxoplasma gondii are important apicomplexan parasites, which infect humans worldwide. Genetic analyses have revealed that 33% of amino acid sequences of inner membrane complex from the malaria parasite Plasmodium berghei is similar to that of Toxoplasma gondii. Inner membrane complex is known to be involved in cell invasion and replication. In this study, we investigated the resistance against T. gondii (ME49) infection induced by previously infected P. berghei (ANKA) in mice. Levels of T. gondii-specific IgG, IgG1, IgG2a, and IgG2b antibody responses, CD4⁺ and CD8⁺ T cell populations were found higher in the mice infected with P. berghei (ANKA) and challenged with T. gondii (ME49) compared to that in control mice infected with T. gondii alone (ME49). P. berghei (ANKA) + T. gondii (ME49) group showed significantly reduced the number and size of T. gondii (ME49) cysts in the brains of mice, resulting in lower body weight loss compared to ME49 control group. These results indicate that previous exposure to P. berghei (ANKA) induce resistance to subsequent T. gondii (ME49) infection.

Long-Term Relationships: the Complicated Interplay between the Host and the Developmental Stages of Toxoplasma gondii during Acute and Chronic Infections

Toxoplasma gondii represents one of the most common parasitic infections in the world. The asexual cycle can occur within any warm-blooded animal, but the sexual cycle is restricted to the feline intestinal epithelium. T. gondii is acquired through consumption of tissue cysts in undercooked meat as well as food and water contaminated with oocysts. Once ingested, it differentiates into a rapidly replicating asexual form and disseminates throughout the body during acute infection. After stimulation of the host immune response, T. gondii differentiates into a slow-growing, asexual cyst form that is the hallmark of chronic infection. One-third of the human population is chronically infected with T. gondii cysts, which can reactivate and are especially dangerous to individuals with reduced immune surveillance. Serious complications can also occur in healthy individuals if infected with certain T. gondii strains or if infection is acquired congenitally. No drugs are available to clear the cyst form during the chronic stages of infection. This therapeutic gap is due in part to an incomplete understanding of both host and pathogen responses during the progression of T. gondii infection. While many individual aspects of T. gondii infection are well understood, viewing the interconnections between host and parasite during acute and chronic infection may lead to better approaches for future treatment. The aim of this review is to provide an overview of what is known and unknown about the complex relationship between the host and parasite during the progression of T. gondii infection, with the ultimate goal of bridging these events.

Prospects for immunotherapy and vaccines against Cryptosporidium

Cryptosporidium spp is a ubiquitous parasite that has long been recognized as a frequent cause of protozoal diarrhea in humans. While infections in immunocompetent hosts are usually self-limiting, immunocompromised individuals can develop severe, chronic, and life-threatening illness. Vaccine development or immunotherapy that prevents disease or reduces the severity of infection is a relevant option since efficacious drug treatments are lacking. In particular, children in developing countries might benefit the most from a vaccine since cryptosporidiosis in early childhood has been reported to be associated with subsequent impairment in growth, physical fitness, and intellectual capacity. In this review, immunotherapies that have been used clinically are described as well as experimental vaccines and their evaluation in vivo.

Toxoplasma gondii upregulates interleukin-12 to prevent Plasmodium berghei-induced experimental cerebral malaria

A chronic infection with the parasite Toxoplasma gondii has previously been shown to protect mice against subsequent viral, bacterial, or protozoal infections. Here we have shown that a chronic T. gondii infection can prevent Plasmodium berghei ANKA-induced experimental cerebral malaria (ECM) in C57BL/6 mice. Treatment with soluble T. gondii antigens (STAg) reduced parasite sequestration and T cell infiltration in the brains of P. berghei-infected mice. Administration of STAg also preserved blood-brain barrier function, reduced ECM symptoms, and significantly decreased mortality. STAg treatment 24 h post-P. berghei infection led to a rapid increase in serum levels of interleukin 12 (IL-12) and gamma interferon (IFN-γ). By 5 days after P. berghei infection, STAg-treated mice had reduced IFN-γ levels compared to those of mock-treated mice, suggesting that reductions in IFN-γ at the time of ECM onset protected against lethality. Using IL-10- and IL-12βR-deficient mice, we found that STAg-induced protection from ECM is IL-10 independent but IL-12 dependent. Treatment of P. berghei-infected mice with recombinant IL-12 significantly decreased parasitemia and mortality. These data suggest that IL-12, either induced by STAg or injected as a recombinant protein, mediates protection from ECM-associated pathology potentially through early induction of IFN-γ and reduction in parasitemia. These results highlight the importance of early IL-12 induction in protection against ECM.

Determination of stage interconversion in vitro and in vivo by construction of transgenic Toxoplasma gondii that stably express stage-specific fluorescent proteins

Detection of Toxoplasma gondii conversion from the tachyzoite stage to the bradyzoite stage in living brain tissue is difficult because the parasites are small and conversion and reactivation of the parasites are transient events. To better understand the mechanisms of T. gondii stage conversion between tachyzoites and bradyzoites, and to recognize stage conversion in an intermediate host, we constructed a transgenic cyst-forming strain (PLK) of T. gondii. The parasites stably expressed enhanced green fluorescence protein (EGFP) in the tachyzoite stage and red fluorescence protein (RFP) in the bradyzoite stage, under the control of the SAG1 and BAG1 promoters, respectively. The resulting transgenic parasite was designated as PLK/Bi. The PLK/Bi zoites expressed only green fluorescence in the tachyzoite stage and only red fluorescence in the bradyzoite stage in vitro and in vivo. Fluorescence analyses showed that recombinant GFP and RFP were located to the intracellular vacuolar spaces. In addition, an analysis of growth and culture conditions of transgenic T. gondii was performed in vitro and the virulence was evaluated in vivo. Our data suggested that the stage-specific fluorescence expression by PLK/Bi may be rationally designed for in vitro and in vivo studies on stage conversion and reactivation of T. gondii.

Evaluation of Toxoplasma gondii as a live vaccine vector in susceptible and resistant hosts

Background

Toxoplasma gondii has been shown to trigger strong cellular immune responses to heterologous antigens expressed by the parasite in the inbred mouse model [1]. We studied the immune response induced by T. gondii as an effective vaccine vector in chickens and rabbits.

Results

T. gondii RH strain was engineered to express the yellow fluorescent protein (YFP) in the cytoplasm. A subcutaneous injection of the transgenic T. gondii YFP in chickens afforded partial protection against the infection of transgenic E. tenella YFP. T. gondii YFP induced low levels of antibodies to YFP in chickens, suggesting that YFP specific cellular immune response was probably responsible for the protective immunity against E. tenella YFP infection. The measurement of T-cell response and IFN-γ production further confirmed that YFP specific Th1 mediated immune response was induced by T. gondii YFP in immunized chickens. The transgenic T. gondii stimulated significantly higher YFP specific IgG titers in rabbits than in chickens, suggesting greater immunogenicity in a T. gondii susceptible species than in a resistant species. Priming with T. gondii YFP and boosting with the recombinant YFP can induce a strong anti-YFP antibody response in both animal species.

Conclusions

Our findings suggest that T. gondii can be used as an effective vaccine vector and future research should focus on exploring avirulent no cyst-forming strains of T. gondii as a live vaccine vector in animals.

Toxoplasma gondii: 25 years and 25 major advances for the field

This article is an attempt to identify the most significant highlights of Toxoplasma research over the last 25 years. It has been a period of enormous progress and the top 25 most significant advances, in the view of this author, are described. These range from the bench to the bedside and represent a tremendous body of work from countless investigators. And, having laid out so much that has been discovered, it is impossible not to also reflect on the challenges that lie ahead. These, too, are briefly discussed. Finally, while every effort has been made to view the field as a whole, the molecular biology background of the author almost certainly will have skewed the relative importance attached to past and future advances. Despite this, it is hoped that the reader will agree with, or at least not disagree too strongly with, most of the choices presented here.

Construction of Neospora caninum stably expressing TgSAG1 and evaluation of its protective effects against Toxoplasma gondii infection in mice

Toxoplasma gondii and Neospora caninum are closely related apicomplexan parasites. The surface antigen 1 of T. gondii (TgSAG1) is a major immunodominant antigen and, therefore, is considered to be a good candidate for the development of an effective recombinant vaccine against toxoplasmosis. In this study, N. caninum stably expressing the TgSAG1 gene (Nc/TgSAG1) was constructed using pyrimethamine-resistant DHFR-TS and GFP genes as double-selection markers. The expression level, molecular weight, and antigenic property of recombinant TgSAG1 expressed by the Nc/TgSAG1 were similar to those of the native TgSAG1. The mice immunized with Nc/TgSAG1 induced TgSAG1-specific Th1-dominant immune responses and protected the mice from a lethal challenge infection with T. gondii. These results indicate that N. caninum may provide a new tool for the production of a live recombinant vector vaccine against toxoplasmosis in animals. To our knowledge, this is the first report to evaluate the usefulness of N. caninum-based live vaccine.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

Stable expression of Cryptosporidium parvum glycoprotein gp40/15 in Toxoplasma gondii

Cryptosporidium is a cause of diarrheal disease worldwide. Parasite glycoproteins involved in invasion of Cryptosporidium into host cells have been investigated as possible targets for effective interventions against this parasite. One of these, Cpgp40/15, is expressed as a precursor protein that is cleaved by a parasite-derived furin-like protease activity into gp15, a glycophosphatidyl inositol anchored surface protein, and gp40, that associates with gp15 and binds to host cells. Investigation of the functions of these glycoproteins requires an expression system that can produce similar glycosylation patterns to the native antigens. Previous work demonstrated that Cpgp40/15 transiently expressed in Toxoplasma gondii was appropriately localized and glycosylated. In this study, T. gondii stable transfectants expressing gp40/15, gp15, gp40 and hemagglutinin (HA) tagged gp40 were generated. T. gondii recombinant gp40HA and gp40/15 (recTggp40HA and recTggp40/15) were isolated from infected cells by HA affinity chromatography and Helix pomatia lectin affinity chromatography, respectively. Mass spectrometry confirmed that recTggp40-HA and native Cpgp40 were similarly glycosylated. Like native Cpgp40/15, recTggp40/15 could be cleaved into the gp40 and gp15 products by human furin or by a furin-like protease activity in T. gondii tachyzoite lysates. However, processing was inefficient in intact tachyzoites. Unlike the N-terminus of native Cpgp40/15, which appears to be processed following signal peptide cleavage, the N-terminus of recTggp40/15 began at the predicted signal sequence cleavage site, 11 amino acids upstream of the N-terminus of native Cpgp40. The ability to express and isolate appropriately glycosylated Cryptosporidium glycoproteins will enable further investigations into host-parasite interactions of this important pathogen.

Protective and pathogenic roles of CD8+ T cells during malaria infection

CD8+ T cells play a key role in protection against pre-erythrocytic stages of malaria infection. Many vaccine strategies are based on the idea of inducing a strong infection-blocking CD8+ T cell response. Here, we summarize what is known about the development, specificity and protective effect of malaria-specific CD8+ T cells and report on recent developments in the field. Although work in mouse models continues to make progress in our understanding of the basic biology of these cells, many questions remain to be answered - particularly on the roles of these cells in human infections. Increasing evidence is also emerging of a harmful role for CD8+ T cells in the pathology of cerebral malaria in rodent systems. Once again, the relevance of these results to human disease is one of the primary questions facing workers in this field.

Expression of P23 of Cryptosporidium parvum in Toxoplasma gondii and Evaluation of its Protective Effects

In this study, P23 of Cryptosporidium parvum sporozoites, an immunodominant surface protein, was stably expressed in Toxoplasma gondii (Tg/P23) and its protective effects were evaluated in a mouse model. The molecular weight and antigenic property of P23 expressed by Tg/P23 were similar to those of the native P23. Mice immunized with lysed Tg/P23 tachyzoites produced specific neutralizing antibodies against C. parvum. These findings indicate that the T. gondii vector may provide a new tool for the production of a recombinant vaccine against cryptosporidiosis in animals.

Effector and memory CD8+ T cells as seen in immunity to malaria

Transgenic (Tg) mice carrying a T-cell receptor (TCR) specific for a CD8(+) T-cell epitope expressed in pre-erythrocytic stages of Plasmodium yoelii has proven to be a valuable tool to advance our understanding of this anti-parasite T-cell response, as it occurs in vivo. The visualization of CD8(+) T cells in vivo and ex vivo greatly facilitated research aimed at characterizing basic features of this T-cell response such as the kinetics of differentiation and proliferation and the in vivo antigen presentation. Importantly, this research unveiled the existence of early self-regulatory mechanisms controlling the magnitude of the CD8(+) T-cell response and also identified CD4(+) T cells as critical elements in the development of memory populations. This review discusses our recent research using Tg mice and highlights our progress in understanding the CD8(+) T-cell-mediated immunity against malaria liver stages.

Development of a system to study CD4+-T-cell responses to transgenic ovalbumin-expressing Toxoplasma gondii during toxoplasmosis

The study of the immune response to Toxoplasma gondii has provided numerous insights into the role of T cells in resistance to intracellular infections. However, the complexity of this eukaryote pathogen has made it difficult to characterize immunodominant epitopes that would allow the identification of T cells with a known specificity for parasite antigens. As a consequence, analysis of T-cell responses to T. gondii has been based on characterization of the percentage of T cells that express an activated phenotype during infection and on the ability of these cells to produce cytokines in response to complex mixtures of parasite antigens. In order to study specific CD4(+) T cells responses to T. gondii, recombinant parasites that express a truncated ovalbumin (OVA) protein, in either a cytosolic or a secreted form, were engineered. In vitro and in vivo studies reveal that transgenic parasites expressing secreted OVA are able to stimulate T-cell receptor-transgenic OVA-specific CD4(+) T cells to proliferate, express an activated phenotype, and produce gamma interferon (IFN-gamma). Furthermore, the adoptive transfer of OVA-specific T cells into IFN-gamma(-/-) mice provided enhanced protection against infection with the OVA-transgenic (but not parental) parasites. Together, these studies establish the utility of this transgenic system to study CD4(+)-T-cell responses during toxoplasmosis.

Toxoplasma gondii: the model apicomplexan

Toxoplasma gondii is an obligate intracellular protozoan parasite which is a significant human and veterinary pathogen. Other members of the phylum Apicomplexa are also important pathogens including Plasmodium species (i.e. malaria), Eimeria species, Neospora, Babesia, Theileria and Cryptosporidium. Unlike most of these organisms, T. gondii is readily amenable to genetic manipulation in the laboratory. Cell biology studies are more readily performed in T. gondii due to the high efficiency of transient and stable transfection, the availability of many cell markers, and the relative ease with which the parasite can be studied using advanced microscopic techniques. Thus, for many experimental questions, T. gondii remains the best model system to study the biology of the Apicomplexa. Our understanding of the mechanisms of drug resistance, the biology of the apicoplast, and the process of host cell invasion has been advanced by studies in T. gondii. Heterologous expression of apicomplexan proteins in T. gondii has frequently facilitated further characterisation of proteins that could not be easily studied. Recent studies of Apicomplexa have been complemented by genome sequencing projects that have facilitated discovery of surprising differences in cell biology and metabolism between Apicomplexa. While results in T. gondii will not always be applicable to other Apicomplexa, T. gondii remains an important model system for understanding the biology of apicomplexan parasites.

Characterisation of Toxoplasma gondii engineered to express mouse interferon-gamma

Recent studies have shown the feasibility of using Toxoplasma gondii as an expression system for heterologous protein. For better understanding of the mechanism of interferon-gamma (IFN-gamma) dependent immunity to T. gondii, the parasites were stably transfected with IFN-gamma gene, under control of the GRA1 promoter. Immunofluorescence analyses showed that recombinant mouse IFN-gamma localised to discrete punctuate structures consistent with dense granules and secreted into the vacuolar space. The production of IFN-gamma was detectable in both extracellular parasites and the parasite-infected cells. Growth of the recombinant parasites was inhibited in the mouse macrophage cell line (J774A.1 cells), but not in monkey kidney adherent fibroblasts (Vero cells), demonstrating the species-specificity of IFN-gamma. Addition of anti-mouse IFN-gamma antibody resulted in growth recovery of the recombinant parasites, suggesting that IFN-gamma, secreted from the parasitised host cells across the parasitophorous vacuole membrane, acted in a paracrine manner. Reverse transcription (RT)-PCR analysis revealed significant expression of inducible nitric oxide synthase mRNA and high levels of nitric oxide production in recombinant parasite-infected J774A.1 cells. A competitive inhibitor of the L-arginine-dependent effector pathway, N(G)-monomethyl-L-arginine, inhibited the reduction of recombinant parasite growth in J774A.1 cells. Taken together, our data suggest that the T. gondii expression system may provide a new tool for cytokine gene expression and that parasites engineered to express a cytokine gene may be rationally designed for use in studies on immune responses to T. gondii.

Expression of Cpgp40/15 in Toxoplasma gondii: a surrogate system for the study of Cryptosporidium glycoprotein antigens

Cryptosporidium parvum is a waterborne enteric coccidian that causes diarrheal disease in a wide range of hosts. Development of successful therapies is hampered by the inability to culture the parasite and the lack of a transfection system for genetic manipulation. The glycoprotein products of the Cpgp40/15 gene, gp40 and gp15, are involved in C. parvum sporozoite attachment to and invasion of host cells and, as such, may be good targets for anticryptosporidial therapies. However, the function of these antigens appears to be dependent on the presence of multiple O-linked alpha-N-acetylgalactosamine (alpha-GalNAc) determinants. A eukaryotic expression system that would produce proteins bearing glycosylation patterns similar to those found on the native C. parvum glycoproteins would greatly facilitate the molecular and functional characterization of these antigens. As a unique approach to this problem, the Cpgp40/15 gene was transiently expressed in Toxoplasma gondii, and the expressed recombinant glycoproteins were characterized. Antisera to gp40 and gp15 reacted with the surface membranes of tachyzoites expressing the Cpgp40/15 construct, and this reactivity colocalized with that of antiserum to the T. gondii surface protein SAG1. Surface membrane localization was dependent on the presence of the glycophosphatidylinositol anchor attachment site present in the gp15 coding sequence. The presence of terminal O-linked alpha-GalNAc determinants on the T. gondii recombinant gp40 was confirmed by reactivity with Helix pomatia lectin and the monoclonal antibody 4E9, which recognizes alpha-GalNAc residues, and digestion with alpha-N-acetylgalactosaminidase. In addition to appropriate localization and glycosylation, T. gondii apparently processes the gp40/15 precursor into the gp40 and gp15 component glycopolypeptides, albeit inefficiently. These results suggest that a surrogate system using T. gondii for the study of Cryptosporidium biology may be useful.

The induction and kinetics of antigen-specific CD8 T cells are defined by the stage specificity and compartmentalization of the antigen in murine toxoplasmosis

Toxoplasma gondii forms different life stages, fast-replicating tachyzoites and slow-growing bradyzoites, in mammalian hosts. CD8 T cells are of crucial importance in toxoplasmosis, but it is unknown which parasite stage is recognized by CD8 T cells. To analyze stage-specific CD8 T cell responses, we generated various recombinant Toxoplasma gondii expressing the heterologous Ag beta-galactosidase (beta-gal) and studied whether 1) secreted or cytoplasmic Ags and 2) tachyzoites or bradyzoites, which persist intracerebrally, induce CD8 T cells. We monitored the frequencies and kinetics of beta-gal-specific CD8 T cells in infected mice by MHC class I tetramer staining. Upon oral infection of B6C (H-2(bxd)) mice, only beta-gal-secreting tachyzoites induced beta-gal-specific CD8 T cells. However, upon secondary infection of mice that had received a primary infection with tachyzoites secreting beta-gal, beta-gal-secreting tachyzoites and bradyzoites transiently increased the frequency of intracerebral beta-gal-specific CD8 T cells. Frequencies of splenic and cerebral beta-gal-specific CD8 T cells peaked at day 23 after infection, thereafter persisting at high levels in the brain but declining in the spleen. Splenic and cerebral beta-gal-specific CD8 T cells produced IFN-gamma and were cytolytic upon specific restimulation. Thus, compartmentalization and stage specificity of an Ag determine the induction of CD8 T cells in toxoplasmosis.

De novo pyrimidine biosynthesis is required for virulence of Toxoplasma gondii

Toxoplasma gondii is a ubiquitous protozoan parasite that is responsible for severe congenital birth defects and fatal toxoplasmic encephalitis in immunocompromized people. Fundamental aspects of obligate intracellular replication and pathogenesis are only now beginning to emerge for protozoan parasites. T. gondii has a fragmented pathway for salvaging pyrimidine nucleobases derived from the parasite or host cell, and this limited pyrimidine salvage capacity is funnelled exclusively through uracil phosphoribosyltransferase. Disrupting the function of this enzyme does not affect the growth of T. gondii tachyzoites, which suggests that the de novo pyrimidine biosynthesis pathway may be necessary for growth. We have examined the virulence of T. gondii mutants that lack carbamoyl phosphate synthetase II (uracil auxotrophs) to determine whether de novo pyrimidine biosynthesis is required in vivo. Here we show that T. gondii uracil auxotrophs are completely avirulent not only in immune-competent BALB/c mice but also in mice that lack interferon-gamma. A single injection of the uracil auxotroph into BALB/c mice induces long-term protective immunity to toxoplasmosis. Our findings indicate the significance of the de novo pyrimidine biosynthesis pathway for the virulence of parasitic protozoa, and suggest routes for developing vaccines and chemotherapy.

Vaccine development against Neospora caninum infection

Neospora caninum is a recognized protozoan parasite of a wide range of mammalian hosts, and was reported for the first time in 1988. The isolation of its oocysts in dog's faeces in 1998 led to its establishment as a parasitic species undergoing typical coccidian life cycle. Infection with N. caninum causes paralysis and death in young livestock and companion animals, and is associated with abortions and stillbirth in cattle, and neurologic disease in calves. Considering the economic and agricultural importance of neosporosis, there is the urgent need to develop biological control measures aimed at preventing its transmission, infection, as well as reducing severity of the disease. In this paper, we have reviewed the progress made to date on the parasite-host immunology and on vaccine development including its prospects, and discussed possible strategies in the formulation of vaccine(s) against neosporosis.

Attenuated Toxoplasma gondii ts-4 mutants engineered to express the Leishmania antigen KMP-11 elicit a specific immune response in BALB/c mice

In order to test recombinant Toxoplasma as adjuvant and live vaccine carrier in the infectious disease model of murine experimental leishmaniasis, we engineered the attenuated, temperature-sensitive Toxoplasma gondii strain ts-4 to express the heterologous Leishmania antigen kinetoplastid membrane protein-11 (KMP-11). Transgenic ts-4 clones were obtained which express KMP-11 as cytoplasmatic protein or target it to the secretory pathway of the tachyzoites. Immunization of BALB/c mice with these stably transformed parasites elicited proliferative responses to both T. gondii antigen and recombinant KMP-11. When challenged with Leishmania major, we observed significant protection in animals that had been vaccinated with the KMP-11-expressing ts-4 mutants. The adjuvant attenuated only the onset of the Leishmania infection, but animals were ultimately not able to control the disease. Thus, our findings demonstrate that recombinant Toxoplasma has the potential to serve as an efficient vaccine carrier for cutaneous leishmaniasis. Furthermore, they establish a protective role for the antigen KMP-11 when given in such a vaccine formulation.

Complete, long-lasting protection against malaria of mice primed and boosted with two distinct viral vectors expressing the same plasmodial antigen

We report that complete protection against malaria and total inhibition of liver stage development and parasitemia was obtained in 100% of BALB/c mice primed with a replication-defective recombinant adenovirus expressing the circumsporozoite (CS) protein of Plasmodium yoelii (AdPyCS), followed by a booster with an attenuated recombinant vaccinia virus, expressing the same malaria antigen, VacPyCS. We found increased levels of activated CS-specific CD8(+) and CD4(+) T cells, higher anti-sporozoite antibody titers, and greater protection in these mice, when the time between priming and boosting with these two viral vectors was extended from 2 to 8 or more weeks. Most importantly, by using this immunization regimen, the protection of the immunized mice was found to be long-lasting, namely complete resistance to infection of all animals 3 1/2 months after priming. These results indicate that immunization with AdPyCS generates highly effective memory T and B cells that can be recalled long after priming by boosting with VacPyCS.

Progress toward a malaria vaccine: efficient induction of protective anti-malaria immunity

Malaria can be a very severe disease, particularly in young children, pregnant women (mostly in primipara), and malaria naïve adults, and currently ranks among the most prevalent infections in tropical and subtropical areas throughout the world. The widespread occurrence and the increased incidence of malaria in many countries, caused by drug-resistant parasites (Plasmodium falciparum and P. vivax) and insecticide-resistant vectors (Anopheles mosquitoes), indicate the need to develop new methods of controlling this disease. Experimental vaccination with irradiated sporozoites can protect animals and humans against the disease, demonstrating the feasibility of developing an effective malaria vaccine. However, developing a universally effective, long lasting vaccine against this parasitic disease has been a difficult task, due to several problems. One difficulty stems from the complexity of the parasite's life cycle. During their life cycle, malaria parasites change their residence within the host, thus avoiding being re-exposed to the same immunological environment. These parasites also possess some distinct antigens, present at different life stages of the parasite, the so-called stage-specific antigens. While some of the stage-specific antigens can induce protective immune responses in the host, these responses are usually genetically restricted, this being another reason for delaying the development of a universally effective vaccine. The stage-specific antigens must be used as immunogens and introduced into the host by using a delivery system that should efficiently induce protective responses against the respective stages. Here we review several research approaches aimed at inducing protective anti-malaria immunity, overcoming the difficulties described above.