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

Stable expression of mucin glycoproteins GP40 and GP15 of Cryptosporidium parvum in Toxoplasma gondii

BACKGROUND

Cryptosporidium spp. are common protozoa causing diarrhea in humans and animals. There are currently only one FDA-approved drug and no vaccines for cryptosporidiosis, largely due to the limited knowledge of the molecular mechanisms involved in the invasion of the pathogens. Previous studies have shown that GP60, which is cleaved into GP40 and GP15 after expression, is an immunodominant mucin protein involved in the invasion of Cryptosporidium. The protein is highly O-glycosylated, and recombinant proteins expressed in prokaryotic systems are non-functional. Therefore, few studies have investigated the function of GP40 and GP15.

METHODS

To obtain recombinant GP40 with correct post-translational modifications, we used CRISPR/Cas9 technology to insert GP40 and GP15 into the UPRT locus of Toxoplasma gondii, allowing heterologous expression of Cryptosporidium proteins. In addition, the Twin-Strep tag was inserted after GP40 for efficient purification of GP40.

RESULTS

Western blotting and immunofluorescent microscopic analyses both indicated that GP40 and GP15 were stably expressed in T. gondii mutants. GP40 localized not only in the cytoplasm of tachyzoites but also in the parasitophorous vacuoles, while GP15 without the GPI anchor was expressed only in the cytoplasm. In addition, a large amount of recTgGP40 was purified using Strep-TactinXT supported by a visible band of ~ 50 kDa in SDS-PAGE.

CONCLUSIONS

The establishment of a robust and efficient heterologous expression system of GP40 in T. gondii represents a novel approach and concept for investigating Cryptosporidium mucins, overcoming the limitations of previous studies that relied on unstable transient transfection, which involved complex steps and high costs.

Lessons Learned from Protective Immune Responses to Optimize Vaccines against Cryptosporidiosis

In developing countries, cryptosporidiosis causes moderate-to-severe diarrhea and kills thousands of infants and toddlers annually. Drinking and recreational water contaminated with Cryptosporidium spp. oocysts has led to waterborne outbreaks in developed countries. A competent immune system is necessary to clear this parasitic infection. A better understanding of the immune responses required to prevent or limit infection by this protozoan parasite is the cornerstone of development of an effective vaccine. In this light, lessons learned from previously developed vaccines against Cryptosporidium spp. are at the foundation for development of better next-generation vaccines. In this review, we summarize the immune responses elicited by naturally and experimentally-induced Cryptosporidium spp. infection and by several experimental vaccines in various animal models. Our aim is to increase awareness about the immune responses that underlie protection against cryptosporidiosis and to encourage promotion of these immune responses as a key strategy for vaccine development. Innate and mucosal immunity will be addressed as well as adaptive immunity, with an emphasis on the balance between T_H1/T_H2 immune responses. Development of more effective vaccines against cryptosporidiosis is needed to prevent Cryptosporidium spp.-related deaths in infants and toddlers in developing countries.

Evaluation of recombinant P23 protein as a vaccine for passive immunization of newborn calves against Cryptosporidium parvum

Cryptosporidiosis is a zoonotic protozoan disease that affects the gastrointestinal tract of animals and humans. Diarrhoea as the most important indication of the infection leads to high economic losses in livestock industries and is a life threatening infection in immunocompromised individuals. In the absence of the effective drugs, vaccine has an effective role in the prevention of infection. For this purpose we developed a vaccine utilizing recombinant P23 protein and immunized pregnant cows four times from 70 days to parturition every 2 weeks. After parturition, each calf received his dam colostrum and challenged with 1 × 10(7) Cryptosporidium parvum oocysts at 12 h of age. Results showed that in contrast with the control group, the antibody titre in the sera and first milking colostra of the immunized cows significantly increased and calves fed hyperimmune colostrum did not show cryptosporidiosis signs. Moreover, enriched colostrum not only reduced significantly the amount of oocyst excretion but also delayed its onset. Our study showed that recombinant P23 protein could be used for passive immunization of newborn calves against Cryptosporidium parvum.

Systemic and Mucosal Immune Responses to Cryptosporidium-Vaccine Development

Cryptosporidium spp is a major cause of diarrheal disease worldwide, particularly in malnourished children and untreated AIDS patients in developing countries in whom it can cause severe, chronic and debilitating disease. Unfortunately, there is no consistently effective drug for these vulnerable populations and no vaccine, partly due to a limited understanding of both the parasite and the host immune response. In this review, we will discuss our current understanding of the systemic and mucosal immune responses to Cryptosporidium infection, discuss the feasibility of developing a Cryptosporidium vaccine and evaluate recent advances in Cryptosporidium vaccine development strategies.

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.

Optimized expression of dual reporter genes in transient transfection of purified Toxoplasma gondii using different promoters

Fluorescent protein and luciferase genes are valuable reporter genes and have been widely used for noninvasive monitoring of gene expression in living tissues and cells. We tested expression of the dual reporter genes in transient transfection of purified Toxoplasma gondii tachyzoites. Two copies of the enhanced yellow fluorescent protein (EYFP) gene were put under the control of 3 representative T. gondii promoters (GRA1, SAG1, and DHFR). Fluorescence from each EYFP reporter was significantly higher than that from a green fluorescent protein (GFP) reporter. The GRA1-EYFP reporter gave the highest fluorescence. Although both fluorescence and luciferase were expressed in the dual reporter system, the luciferase reporter was more efficient than either the EYFP or GFP reporters, and it required fewer parasites to be successfully used.

Systemic antibody responses to the immunodominant p23 antigen and p23 polymorphisms in children with cryptosporidiosis in Bangladesh

Cryptosporidium is a major cause of diarrhea in children in developing countries. However, there is no vaccine available and little is known about immune responses to protective antigens. We investigated antibody responses to p23, a putative vaccine candidate, in children in Bangladesh with cryptosporidiosis and diarrhea (cases) and uninfected children with diarrhea (controls), and p23 gene polymorphisms in infecting species. Serum IgM, IgG, and IgA responses to p23 were significantly greater in cases than controls after three weeks of follow-up. Cases with acute diarrhea had significantly greater serum IgA and IgM responses than those with persistent diarrhea, which suggested an association with protection from prolonged disease. The p23 sequences were relatively conserved among infecting species and subtype families. Although most children were infected with Cryptosporidium hominis, there was a cross-reactive antibody response to C. parvum antigen. These results support further development of p23 as a vaccine candidate.

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.

Construction and analysis of full-length cDNA library of Cryptosporidium parvum

A full-length cDNA library was constructed from the sporozoite of Cryptosporidium parvum. Normalized clones were subjected to Solexa shotgun sequencing, and then complete sequences for 1066 clones were reconfigured. Detailed analyses of the sequences revealed that 13.5% of the transcripts were spliced; the average and median 5' UTR lengths were 213.5 and 122 nucleotides, respectively. There were 148 inconsistencies out of 562 examined genes between the experimentally described cDNA sequence and the predicted sequence from its genome. In addition, we identified 118 sequences that had little homology against annotated genes of C. parvum as prospective candidates for addable genes. These observations should improve the reliability of C. parvum transcriptome and provide a versatile resource for further studies.

Immunogenicity of orally administrated recombinant Lactobacillus casei Zhang expressing Cryptosporidium parvum surface adhesion protein P23 in mice

Cryptosporidium parvum, an intestinal apicomplexan parasite, is a significant cause of diarrheal diseases in both humans and animals. What is more, there is no promising strategy for controlling cryptosporidiosis. In this study, the P23 immunodominant surface protein of C. parvum sporozoites was stably expressed in the Lactobacillus casei Zhang strain and its immunogenicity was evaluated in a mouse model. The molecular weight (23 kDa) and immunogenicity of p23 gene expressed by L. casei Zhang were similar to that of the native P23 protein. Oral immunization with control L. casei Zhang and recombinant L. casei Zhang-p23 activated the mucosal immune system to elicit serum immunoglobulin G (IgG) and mucosal IgA in mice. Furthermore, the expression of cytokines such as IL-4, IL-6, and IFN-γ in splenocytes of mice was detected by real-time PCR after oral immunization. P23-specific immunocyte activation was also verified. These findings indicate that the live L. casei Zhang vector may be a new tool for the production of mucosal vaccines against cryptosporidiosis 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.

Cryptosporidium: genomic and biochemical features

Recent progress in understanding the unique biochemistry of the two closely related human enteric pathogens Cryptosporidium parvum and Cryptosporidium hominis has been stimulated by the elucidation of the complete genome sequences for both pathogens. Much of the work that has occurred since that time has been focused on understanding the metabolic pathways encoded by the genome in hopes of providing increased understanding of the parasite biology, and in the identification of novel targets for pharmacological interventions. However, despite identifying the genes encoding enzymes that participate in many of the major metabolic pathways, only a hand full of proteins have actually been the subjects of detailed scrutiny. Thus, much of the biochemistry of these parasites remains a true mystery.

Recombinant Cryptosporidium parvum p23 as a target for the detection of Cryptosporidium-specific antibody in calf sera

Cryptosporidium parvum is a widely distributed coccidian parasite and causes enteric disease in humans and animals. In addition to being a cause of life-threatening disease in immunodeficient people, mostly AIDS patients, C. parvum has been reported as a common serious primary cause of outbreaks of diarrhea in newborn calves, especially newborn ruminants (De Graaf et al. in Int J Parasitol 29:1269-1287, 1999). To obtain the recombinant P23 protein, we isolated the mRNA from oocyst of C. parvum and amplified the cDNA of P23 gene by reverse transcriptase PCR. Sequencing of cDNA showed 100% homology to the known P23 sequences. The double strand P23-cDNA was then cloned in pGEX-5X-2 expression vector. Western blot analysis of recombinant P23 showed that it could be recognized by the positive C. parvum serum. Since P23 is an immunodominant surface glycoprotein expressed in the early phase of infection (Jakobi and Petry in Microbes Infect 8:2186-2194, 2006) and the immunogenic epitopes are also found in the residual chain of amino acid sequence of this glycoprotein, the recombinant P23 was used for the screening of 437 serum samples collected from calves (#264) and cattle (#173). The dot blot analysis showed that from 264 calf and 173 cattle sera, 33% and 37% sera were positive, respectively. Due to the simple handling and equipment, dot blot analysis with P23 could be recommended for calves screening against cryptosporidiosis.

Molecular basis of Cryptosporidium-host cell interactions: recent advances and future prospects

Host-parasite interactions mediating attachment of Cryptosporidium spp. to host cells and invasion of the cell membrane are complex processes that involve multiple parasite and host molecules. Knowledge of the molecular basis of these processes is crucial for understanding the pathogenic mechanisms underlying infection and for designing strategies to combat cryptosporidiosis. Recent progress in this field has been greatly facilitated by the completion of the genome sequences of Cryptosporidium parvum and Cryptosporidium hominis and by success in heterologous expression of Cryptosporidium genes in the related apicomplexan Toxoplasma gondii. However, although a number of Cryptosporidium proteins implicated in mediating host-parasite interactions have been identified, progress in establishing their functional role has been hindered by the inability to genetically manipulate the parasite and to continuously propagate it in vitro. This article reviews the recent advances in knowledge regarding the Cryptosporidium proteins mediating attachment to and invasion of host epithelial cells, and outlines prospects for future research in this field.

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.

Molecular characterization of the Cryptosporidium cervine genotype

In this study, the 27 kDa immunodominant antigen (CP23), 70 kDa heat shock protein (HSP70), actin and beta-tubulin genes were amplified and sequenced for the first time from human isolates of Cryptosporidium cervine genotype. New primers were designed from reported sequences of other Cryptosporidium species and genotypes as well as the whole genome sequences of C. parvum and C. hominis, which enabled novel gene sequences and regions extending beyond those deposited in GenBank to be determined. In comparison with other species in the Cryptosporidium genus, multiple sequence alignment and phylogenetic analysis revealed that the Cryptosporidium cervine genotype isolates from humans clustered most closely with Cryptosporidium deer mouse genotype and C. suis (n. sp. formerly pig genotype I). The complete coding sequence of CP23 was determined to reveal low (72.4% and 68.0-69.8% respectively) identity to C. parvum and C. hominis sequences and the presence of a unique multiple proline-alanine-proline-valine (PAPV) repeat region.