Immune responses in sheep after immunization with Toxoplasma gondii antigens incorporated into iscoms

Advances in vaccine development and the immune response against toxoplasmosis in sheep and goats

Toxoplasmosis is a zoonotic, parasitic infection caused by the intracellular, apicomplexan parasite Toxoplasma gondii, which infects all homeothermic animals including humans. The parasite has a major economic impact on the livestock industry. This is especially true for small ruminants (sheep, goats) as it is one of the most likely reasons for reproductive disorders in these animals. Primary infection in sheep and goats can result in a fetus that is mummified or macerated, fetal embryonic death, abortion, stillbirth, or the postnatal death of neonates, all of which threaten sheep and goat rearing globally. Humans can also become infected by ingesting bradyzoite-containing chevon or mutton, or the contaminated milk of sheep or goats, highlighting the zoonotic significance of this parasite. This article reviews the advances in vaccine development over recent decades and our current understanding of the immune response to toxoplasmosis in small ruminants (sheep, and goats).

Identification of Oocyst-Driven Toxoplasma gondii Infections in Humans and Animals through Stage-Specific Serology-Current Status and Future Perspectives

The apicomplexan zoonotic parasite Toxoplasma gondii has three infective stages: sporozoites in sporulated oocysts, which are shed in unsporulated form into the environment by infected felids; tissue cysts containing bradyzoites, and fast replicating tachyzoites that are responsible for acute toxoplasmosis. The contribution of oocysts to infections in both humans and animals is understudied despite being highly relevant. Only a few diagnostic antigens have been described to be capable of discriminating which parasite stage has caused an infection. Here we provide an extensive overview of the antigens and serological assays used to detect oocyst-driven infections in humans and animals according to the literature. In addition, we critically discuss the possibility to exploit the increasing knowledge of the T. gondii genome and the various 'omics datasets available, by applying predictive algorithms, for the identification of new oocyst-specific proteins for diagnostic purposes. Finally, we propose a workflow for how such antigens and assays based on them should be evaluated to ensure reproducible and robust results.

Promising Plant-Derived Adjuvants in the Development of Coccidial Vaccines

Coccidial parasites cause medical and veterinary diseases worldwide, frequently leading to severe illness and important economic losses. At present, drugs, chemotherapeutics and prophylactic vaccines are still missing for most of the coccidial infections. Moreover, the development and administration of drugs and chemotherapeutics against these diseases would not be adequate in livestock, since they may generate unacceptable residues in milk and meat that would avoid their commercialization. In this scenario, prophylactic vaccines emerge as the most suitable approach. Subunit vaccines have proven to be biologically safe and economically viable, allowing researchers to choose among the best antigens against each pathogen. However, they are generally poorly immunogenic and require the addition of adjuvant compounds to the vaccine formulation. During the last decades, research involving plant immunomodulatory compounds has become an important field of study based on their potential pharmaceutical applications. Some plant molecules such as saponins, polysaccharides, lectins and heat shock proteins are being explored as candidates for adjuvant/carriers formulations. Moreover, plant-derived immune stimulatory compounds open the possibility to attain the main goal in adjuvant research: a safe and non-toxic adjuvant capable of strongly boosting and directing immune responses that could be incorporated into different vaccine formulations, including mucosal vaccines. Here, we review the immunomodulatory properties of several plant molecules and discuss their application and future perspective as adjuvants in the development of vaccines against coccidial infections.

Evaluation of the Western blotting method for the diagnosis of congenital toxoplasmosis

OBJECTIVE

To evaluate the Western blotting method for the detection of IgG anti-Toxoplasma gondii (T. gondii) (IgG-WB) in the serum of children with suspected congenital toxoplasmosis.

METHODS

We accompanied 47 mothers with acquired toxoplasmosis in pregnancy and their children, between June of 2011 and June of 2014. The IgG-WB was done in house and the test was considered positive if the child had antibodies that recognized at least one band on IgG blots different from the mother's or with greater intensity than the corresponding maternal band, during the first three months of life.

RESULTS

15 children (15.1%) met the criteria for congenital toxoplasmosis and 32 (32.3%) had the diagnosis excluded. The symptoms were observed in 12 (80.0%) children and the most frequent were cerebral calcification in 9 (60.0%), chorioretinitis in 8 (53.3%), and hydrocephalus in 4 (26.6%). IgM antibodies anti-T. gondii detected by chemiluminescence (CL) were found in 6 (40.0%) children and the polymerase chain reaction (PCR) for detection of T. gondii DNA was positive in 5 of 7 performed (71.4%). The sensitivity of IgG-WB was of 60.0% [95% confidence interval (CI) 32.3-83.7%] and specificity 43.7% (95% CI 26.7-62.3%). The sensitivity of IgG-WB increased to 76.0 and 89.1% when associated to the research of IgM anti-T. gondii or PCR, respectively.

CONCLUSIONS

The IgG-WB showed greater sensitivity than the detection of IgM anti-T. gondii; therefore, it can be used for the diagnosis of congenital toxoplasmosis in association with other congenital infection markers.

Veterinary vaccines against toxoplasmosis

Toxoplasma gondii is a cosmopolitan protozoan parasite that infects a wide range of mammal and bird species. Common infection leads to high economic (e.g., abortions in sheep) and human (e.g., congenital toxoplasmosis or neurotoxoplasmosis in humans) losses. With one exception (Toxovax for sheep), there are no vaccines to prevent human or animal toxoplasmosis. The paper presents the current state and challenges in the development of a vaccine against toxoplasmosis, designed for farm animals either bred for consumption or commonly kept on farms and involved in parasite transmission. So far, the trials have mostly revolved around conventional vaccines and, compared with the research using laboratory animals (mainly mice), they have not been very numerous. However, the results obtained are promising and could be a good starting point for developing an effective vaccine to prevent toxoplasmosis.

Molecular cloning, sequencing, and biological characterization of GRA4 gene of Toxoplasma gondii

In the present study, GRA4 (dense granule antigen) gene of Toxoplasma gondii was cloned, sequenced, and biologically characterized. The nucleotide sequence data obtained were analyzed and submitted in GenBank database (accession no. EU660037). Analysis of nucleotide sequence of GRA4 gene revealed 99.2 % homology with the published sequence (accession no. M76432). The gene segment (open reading frame) of 1,054 bp was further amplified and re-cloned in expression vector pET-32a. The recombinant protein obtained following the expression in prokaryotic system had a molecular mass of approx. 50 kDa and showed good immunoreactivity with T. gondii sera collected from infected goats. The immunization study of the recombinant protein performed in laboratory mice and live challenge with T. gondii revealed a high level of IgG response against the tachyzoite lysate antigen (TLA) by an indirect ELISA. Protection against T. gondii challenge infection was not evident in immunized mice except for the prolongation of survival period by 2 days. Humoral immune response profile revealed initially a high level of IgG antibody, but at 1 week post-challenge, a sudden drop in the level of the antibody was appreciable. Cytokine profiling by enzyme-linked immunosorbent spot method revealed relatively high level of IFN-γ production by the rodent spleen cells followed by IL-10 and IL-4. Increase in IFN-γ production by spleen cells of immunized mice following TLA stimulation suggested direct correlation to the up-regulated Th1 cells. However, the present immunization trial failed to show any positive relationship with the protection of mice following T. gondii challenge infection.

IGM and IGG response to 29-35-kDa Toxoplasma gondii protein fractions in experimentally infected goats

The evolution of the humoral responses of IgG and IgM against 29-35-kDa Toxoplasma gondii fractions from experimentally infected goats were studied and compared by ELISA with the use of whole T. gondii soluble extracts and 29-35-kDa electroeluted proteins as antigens. The IgM response to electroeluted proteins was detected from wk 1 to wk 3 postinfection, showing a similar evolution to that observed when T. gondii crude extracts were used as antigens. These results suggest that this group of proteins could be used for a more specific detection of anti-T. gondii IgM. In the same way, the IgG response was equivalent in both cases, although when 29-35-kDa T. gondii fractions were used as antigens, the level of specific IgGs reached a peak 2 wk before than when T. gondii crude extract was used. The detection by ELISA of anti-T. gondii IgM in goats does not seem to be affected by the presence of specific IgG in serum samples when 29-35-kDa protein fractions were used as antigens.

A new perspective on and re-assessment of SAG2 locus as the tool for genetic analysis of Toxoplasma gondii isolates

SAG2 locus, the coding gene of the P22 protein, has been widely used for the molecular epidemiology of Toxoplasma gondii and characterization of the parasite isolates with two separate polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) processes. To re-assess the resolution power and suitability of this genetic marker for molecular characterization of the parasite isolates, a number of 27 Toxoplasma strains from different zymodeme patterns were used in the present study. Both codon and non-codon regions of the SAG2 locus of all 27 strains were amplified and subjected to sequencing and nucleotide alignment. Nucleotide variations clustered the three major genotypes (I, II and III). Some minor genotypes, unidentifiable by SAG2-RFLP, could be identified by sequence comparison. However, there were other genotypes that could not be differentiated from the major types due to having identical sequences. This suggests that a remarkable number of field isolates representing several minor types will be miss-clustered with the major types by using the traditional SAG2-PCR-RFLP method. It was concluded that this technique seems not to be suitable for Toxoplasma population study. Thus, the utilization of more variable markers and other discriminatory methods are also recommended.

Evaluation of the immune response induced by multiantigenic DNA vaccine encoding SAG1 and ROP2 of Toxoplasma gondii and the adjuvant properties of murine interleukin-12 plasmid in BALB/c mice

The heavy incidence and severe or lethal damages of toxoplasmosis clearly indicate the need for the development of a more effective vaccine. In the present study, we constructed a multiantigenic DNA vaccine, eukaryotic plasmid pcDNA3.1-SAG1-ROP2, expressing surface protein SAG1 and rhoptry protein ROP2 of Toxoplasma gondii, and examined the expression ability of the DNA vaccine in HeLa cells by Western blot. Afterwards, we investigated the efficacy of pcDNA3.1-SAG1-ROP2 with or without co-administration of a plasmid encoding murine interleukin-12 (pIL-12) as a genetic adjuvant to protect Bagg albino/c mice against toxoplasmosis. After T. gondii RH strain challenge, mice immunized with pcDNA3.1-SAG1-ROP2 displayed significant high survival rates. Moreover, the protection was markedly enhanced by pIL-12 co-administration. The results of lymphocyte proliferation assay, cytokine, and antibody determinations show that mice immunized with pcDNA3.1-SAG1-ROP2 elicited stronger humoral and Th1-type cellular immune responses than those immunized with single-gene plasmids, empty plasmid, or phosphate-buffered saline. Furthermore, co-immunization with IL-12 genes resulted in a dramatic enhancement of these responses. Our study indicates that the introduction of multiantigenic DNA vaccine is more powerful and efficient than single-gene vaccine, and the co-delivery of pIL-12 further enhanced the potency of multiantigenic DNA vaccine.

Particulate delivery systems for animal vaccines

The requirements for veterinary vaccines are different to those of human vaccines. Indeed, while more side effects can be tolerated in animals than in humans; there are stricter requirements in terms of cost, ease of delivery (including to wildlife), and a need to develop vaccines in species for which relatively little is known in terms of molecular immunology. By their nature particulate vaccine delivery systems are well suited to address these challenges. Here, we review particulate vaccine delivery systems, ranging from cm-sized long-distance ballistic devices to nano-bead technology for veterinary species and wildlife.

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.

Oral immunization with a live recombinant attenuated Salmonella typhimurium protects mice against Toxoplasma gondii

The natural site of infection for T. gondii is the mucosal surface of the intestine, so the protective immunity obtained after natural infection with T. gondii points to the importance of developing a vaccine that stimulates mucosal defences. In this study, an aroA- and aroD- attenuated strain of Salmonella typhimurium (BRD509) has been used to deliver the recombinant eukaryotic plasmid pSAG(1-2)/CTA2/B expressing a multi-antigenic gene encoding SAG1 and SAG2 of T. gondii linked to A2/B subunits of cholera toxin as a candidate oral T. gondii vaccine. Immunoblot analysis showed compound gene expression in HeLa cells in vitro and intragastric immunization of mice with the recombinant salmonella resulted in the induction of humoral and Th1 type cellular immune responses and afforded protection against RH strain T. gondii challenge. Anti-T. gondii IgG values increased markedly in the BRD509/pSAG(1-2)-CTA2/B immunized group; these values were significantly higher than those in the negative controls (P = 0.008). With CTA2/B genetic adjuvant, the T. gondii-specific response was predominantly Th1, indicating that the CTA(2)/B genetic adjuvant was able to overcome the strong Th2-bias of the antigen (IgG2a >> IgG1). Antigen-specific T cell proliferative responses and CTL activity were significantly enhanced when cholera toxin CTA2/B genetic adjuvant was used (P = 0.009; P = 0.006). Culture supernatants from antigen-stimulated splenocytes from mice in these groups were also examined by ELISA for Th1- and Th2-type cytokines; mean IFN-gamma levels produced after oral immunization with BRD509/pSAG(1-2)-CTA2/B were about nine-fold higher than after immunization with BRD509/pSAG(1-2) (P = 0.007). On the other hand, the levels of IL-4 were low for all groups and no increase was seen in the presence of CTA2/B genetic adjuvant. When the immunized mice were intraperitoneally challenged with 10(3) tachyzoites of the highly virulent RH strain, the survival time of the mice immunized with BRD509/pSAG(1-2)-CTA2/B was markedly longer than other groups (P = 0.003) and a 40% survival rate was achieved. This is the first report that demonstrates that an oral attenuated salmonella DNA vaccine can induce protective immunity against the acute phase of T. gondii infection.

Toxoplasma gondii: isolation of tachyzoites rhoptries and incorporation into Iscom

Rhoptries have been isolated from Toxoplasma gondii tachyzoites by subcellular fractionation in isopynic density sucrose gradient. Five bands were observed, and transmission electron microscopy of these indicated that rhoptries were in band 3. This band had a density of 1.17 g/cm(3). Fraction 1 had membrane structures of the parasite. Fraction 2 contained membranes and mitochondria. Fraction 4 had mostly conoid structure and fraction 5 showed ghosts. The electrophoretic and Western blotting analysis of the fractions indicated the presence of a number of proteins. Iscoms were constructed from band 3, which contained the rhoptry structures. Iscom showed a only protein incorporated of 55 kDa. Isolation of the parasite organelles has got in this work is necessary to identification, characterization, and function elucidation of the organelle proteins.

Vaccination of pigs with Toxoplasma gondii antigens incorporated in immunostimulating complexes (iscoms)

Immunity to Toxoplasma gondii was studied in pigs, after vaccination with T. gondii antigens incorporated into immunostimulating complexes. Nine pigs (group 1 - G1) were inoculated subcutaneously with T. gondii iscoms (LIV-5 sample) and three doses were given at 21 and 13 day-intervals. The results were compared in other three groups of nine pigs each: animals in group 2 (G2) were immunized with the LIV-5 antigens without Quil A, animals in group 3 (G3) were inoculated with tachyzoites of RH T. gondii isolate, and animals in group 4 (G4) received no vaccination. Four animals were neither vaccinated nor challenged with T. gondii (group 5 - G5). Thirty days after vaccination, pigs were challenged orally with 5´10(4) oocysts at AS-28 T. gondii isolate. Euthanasia was carried out 47 days after challenge and specimens of the heart, muscle, brain, liver, tongue and retina were inoculated into mice. Three out of nine pigs from G2 and one out of nine pigs from G4 showed hypertermia after the challenge. Antibody response was analysed by indirect fluorescent antibody test. The first iscom immunization (G1) induced low antibody levels, the second and third produced high antibody levels, similarly to the RH isolate infection (G3). Western blotting analysis indicated that the antibody response in animals in G1, after challenge, was more intense than in animals in the non-vaccinated group. T. gondii was not isolated by bioassays from tissues of iscom vaccinated pigs, while recovery was obtained from four animals in G4, one in G2 and one in G3.

Risk factors for mortality associated with respiratory disease among Menz and Horro sheep in Ethiopia

A retrospective case-control study was conducted on 6718 sheep of two breeds (2772 Horro and 3946 Menz) on risk factors for mortality associated with respiratory disease (MARD) in Ethiopia, based on data collected between October 1993 and December 1997. Potential risk factors examined were breed, gender, age, month, and air temperatures.Fifty-four per cent of total deaths in the flock could be attributed to respiratory disease and the annual MARD rate ranged between 6.3 and 19.0%. There was significant breed (P<0.0001) and gender (P<0.0001) difference in MARD. The Horro breed had a higher (P<0.0001) annual MARD than the Menz breed (16.5+/-0.18 vs. 12.4+/-0.15%). A higher (P<0.0001) proportion of males suffered than females (15.1+/-0.23% vs. 13.8+/-0.13%). Age was also an important risk factor for MARD: there was a strong polynomial relationship (R(2)=0.91, P<0.0001) between MARD and age; the risk of being young if a sheep was a MARD case was high. MARD was high between October and March but relatively low between the months of April and September. There was a significant (P<0.05) negative cubic relationship (R(2)=0.49) between monthly MARD and monthly average minimum air temperatures. There was also a significant (P<0.01) positive exponential relationship (R(2)=0.61) between monthly MARD and average monthly daily deviation between maximum and minimum air temperatures. Timely health and management interventions focusing on these factors are necessary to alleviate losses from MARD. Understanding variations in MARD risk within a population can enhance early response to potential outbreaks, reducing losses.

Complete protection against lethal Toxoplasma gondii infection in mice immunized with a plasmid encoding the SAG1 gene

Infection with the protozoan parasite Toxoplasma gondii is transmitted to humans from infected animals by tissue cysts and oocysts excreted by cats. Immunization with inactivated parasites or recombinant proteins has at best shown partial protection. We constructed a plasmid expressing the SAG1 surface antigen of T. gondii, p1tPASAG1, and showed that animals immunized with the plasmid produce anti-SAG1 antibodies which recognize the native SAG1. Mice immunized with p1tPASAG1 showed 80 to 100% protection against challenge with the non-cyst-producing, virulent RH isolate, compared to an 80% mortality in mice immunized with empty plasmid, which is the greatest efficacy of any vaccine against T. gondii produced so far. The SAG1 molecule was analyzed for potential cytotoxic T-lymphocyte (CTL) epitopes, and four peptides with the best fit were synthesized. The ability of the peptides to stimulate gamma interferon production by CD8(+) T cells from p1tPASAG1-immunized mice was tested in an ELISPOT assay, and one new CTL epitope was identified. Adoptive transfer of CD8(+) T cells from p1tPASAG1-immunized to naïve mice showed partial protection. In conclusion, DNA vaccination with p1tPASAG1 gave effective protection in mice against T. gondii infection and the protection could be adoptively transferred by purified CD8(+) T cells.

Vaccine delivery to animals

For many years vaccination of animals has been practiced to prevent infectious diseases using inactivated organisms or modified live organisms. The live vaccines were effective but lacked safety. The vaccines made with inactivated organisms required an adjuvant to induce an immune response that was not as effective as either the clinical disease or live vaccines. An 'ideal' vaccine would induce effective immunity specific for the type of infection, have long duration, require minimal or no boosters, have impeccable safety, would not induce adverse reactions, and be easy to administer. The desire to meet these criteria, and especially safety, has resulted in the development of vaccines that do not depend on the use of the viable disease agent. The emphasis on subunit or inactivated vaccines that meet the desired criteria of a perfect vaccine has resulted in a critical need for better adjuvants and delivery systems. This has resulted in a technological innovation revolution with development of a wide array of different technologies to generate effective vaccines. This review will describe the historical relevance of adjuvants used for parenterally administered inactivated/subunit vaccines as well as describe some of the exciting technological advances including adjuvants (ISCOMS), delivery systems (recombinant vectors, microparticles), and novel approaches (transgenic plants, naked DNA) that are currently being, or will be used in the future, in the search for better, more effective vaccines that meet the current and future needs of veterinary medicine.

Application of iscom antigen preparations in ELISAs for diagnosis of Neospora and Toxoplasma infections

Immunostimulating complexes (iscoms) are cage-like structures of about 40 nm composed of Quil A, cholesterol, phospholipids and antigen. Their main area of use has been as adjuvants and carriers of immunogens in vaccines. Iscoms can also be used for selection of surface membrane proteins of micro-organisms for use in immunoassays, thus decreasing the number of internal proteins that might cause problems with non-specific binding and cross-reactivity. Enzyme-linked immunoassays (ELISAs) utilising parasite antigens incorporated into iscoms have been developed for demonstration of antibodies directed to the intracellular coccidian parasites Toxoplasma gondii and Neospora caninum. These iscom ELISAs have proved very reliable, with high sensitivity and specificity. The preparation of T. gondii and N. caninum iscoms is described, and ELISAs based on iscom antigen preparations that have so far been used for diagnosis of protozoal infections are reviewed.

Iscoms in parasitological research

During the history of vaccine development, a number of adjuvants and adjuvant formulations have been tested and evaluated for their ability to increase the immunogenicity of different antigens. In this review, Anna Lundén, Karin Lövgren Bengtsson, Anders Sjölander and Arvid Uggla focus on iscoms (immune stimulating complexes), their characteristics and applications to different types of parasitic antigens.