Molecular and immunological characterisation of a polymorphic cytosolic fatty acid binding protein from the human blood fluke of humans, Schistosoma japonicum

Recent Advances and Methodological Considerations on Vaccine Candidates for Human Schistosomiasis

Schistosomiasis remains a neglected tropical disease of major public health concern with high levels of morbidity in various parts of the world. Although considerable efforts in implementing mass drug administration programs utilizing praziquantel have been deployed, schistosomiasis is still not contained. A vaccine may therefore be an essential part of multifaceted prevention control efforts. In the 1990s, a joint United Nations committee promoting parasite vaccines shortlisted promising candidates including for schistosomiasis discussed below. After examining the complexity of immune responses in human hosts infected with schistosomes, we review and discuss the antigen design and preclinical and clinical development of the four leading vaccine candidates: Sm-TSP-2 in Phase 1b/2b, Sm14 in Phase 2a/2b, Sm-p80 in Phase 1 preparation, and Sh28GST in Phase 3. Our assessment of currently leading vaccine candidates revealed some methodological issues that preclude a fair comparison between candidates and the rationale to advance in clinical development. These include (1) variability in animal models - in particular non-human primate studies - and predictive values of each for protection in humans; (2) lack of consensus on the assessment of parasitological and immunological parameters; (3) absence of reliable surrogate markers of protection; (4) lack of well-designed parasitological and immunological natural history studies in the context of mass drug administration with praziquantel. The controlled human infection model - while promising and unique - requires validation against efficacy outcomes in endemic settings. Further research is also needed on the impact of advanced adjuvants targeting specific parts of the innate immune system that may induce potent, protective and durable immune responses with the ultimate goal of achieving meaningful worm reduction.

Recent Developments in Recombinant Proteins for Diagnosis of Human Fascioliasis

Fascioliasis is an important neglected tropical disease that causes severe injury to the bile ducts and liver. Therefore, a rapid and accurate method for detection of Fasciola hepatica infection plays a vital role in early treatment. Currently, the diagnosis of fascioliasis is mainly conducted via serological tests using the excretory/secretory (E/S) products, which might cross-react with antigens from other helminth parasitic diseases. Hence, the development of serodiagnosis test using recombinant antigens may contribute to differentiate fascioliasis from other helminth infections. In the past 20 years, many attempts have been made to exert different F. hepatica recombinant antigens to obtain a well-established standard assay with high accuracy. In this review, we address recent studies that refer to the development of serodiagnosis tests for diagnosis of human fascioliasis based on the candidate recombinant antigens produced by different approaches. Meanwhile, in the present review, some main factors have been highlighted to improve the accuracy of diagnostic tests such as the effect of refolding methods to recover antigens' tertiary structure as well as applying a mixture of recombinant antigens with the highest sensitivity and specificity to improve the accuracy of diagnostic tests.

Evaluation of anti-Cathepsin L1: a more reliable method for serodiagnosis of human fasciolosis

BACKGROUND

Coprological examinations are commonly used for diagnosis of fasciolosis. However, these methods are not useful during the acute phase of the infection and also show poor sensitivity during its chronic phase. In this study we compared the immunoreactivity of the native and recombinant forms of Fasciola hepatica excretory/secretory antigens and determined the most appropriate one for development of F. hepatica-specific immunoassays.

METHODS

The coding sequences of previously-determined immunogenic proteins including cathepsin L1 (CL1), fatty acid binding protein (FABP) and glutathione-S-transferase (GST) were cloned and expressed in E. coli BL-21 cells. Native forms of FABP and GST were also purified. We evaluated the immunoreactivity of the native and recombinant proteins by ELISA using sera from 40 healthy individuals, 15 fasciolosis patients, and 57 patients with other infectious diseases.

RESULTS

All of the studied proteins showed high sensitivity and specificity for F. hepatica serodiagnosis. However, CL1 was more sensitive and specific (100%) than the others for the detection of F. hepatica-specific antibodies. Notably, both FABP and GST showed significant cross-reactivity with hydatidosis patients' sera while CL1 did not.

CONCLUSIONS

Cathepsin L1 has acceptable sensitivity and specificity for serodiagnosis of F. hepatica and its application could be advantageous in immunoassay development.

Structural and binding properties of two paralogous fatty acid binding proteins of Taenia solium metacestode

Background

Fatty acid (FA) binding proteins (FABPs) of helminths are implicated in acquisition and utilization of host-derived hydrophobic substances, as well as in signaling and cellular interactions. We previously demonstrated that secretory hydrophobic ligand binding proteins (HLBPs) of Taenia solium metacestode (TsM), a causative agent of neurocysticercosis (NC), shuttle FAs in the surrounding host tissues and inwardly transport the FAs across the parasite syncytial membrane. However, the protein molecules responsible for the intracellular trafficking and assimilation of FAs have remained elusive.

Methodology/Principal Findings

We isolated two novel TsMFABP genes (TsMFABP1 and TsMFABP2), which encoded 133- and 136-amino acid polypeptides with predicted molecular masses of 14.3 and 14.8 kDa, respectively. They shared 45% sequence identity with each other and 15–95% with other related-members. Homology modeling demonstrated a characteristic β-barrel composed of 10 anti-parallel β-strands and two α-helices. TsMFABP2 harbored two additional loops between β-strands two and three, and β-strands six and seven, respectively. TsMFABP1 was secreted into cyst fluid and surrounding environments, whereas TsMFABP2 was intracellularly confined. Partially purified native proteins migrated to 15 kDa with different isoelectric points of 9.2 (TsMFABP1) and 8.4 (TsMFABP2). Both native and recombinant proteins bound to 11-([5-dimethylaminonaphthalene-1-sulfonyl]amino)undecannoic acid, dansyl-DL-α-amino-caprylic acid, cis-parinaric acid and retinol, which were competitively inhibited by oleic acid. TsMFABP1 exhibited high affinity toward FA analogs. TsMFABPs showed weak binding activity to retinol, but TsMFABP2 showed relatively high affinity. Isolation of two distinct genes from an individual genome strongly suggested their paralogous nature. Abundant expression of TsMFABP1 and TsMFABP2 in the canal region of worm matched well with the histological distributions of lipids and retinol.

Conclusions/Significance

The divergent biochemical properties, physiological roles and cellular distributions of the TsMFABPs might be one of the critical mechanisms compensating for inadequate de novo FA synthesis. These proteins might exert harmonized or independent roles on lipid assimilation and intracellular signaling. The specialized distribution of retinol in the canal region further implies that cells in this region might differentiate into diverse cell types during metamorphosis into an adult worm. Identification of bioactive systems pertinent to parasitic homeostasis may provide a valuable target for function-related drug design.

Neurocysticercosis (NC), an infection of the central nervous system with Taenia solium metacestode (TsM), constitutes a leading cause of adult-onset seizures in endemic areas. Like other helminths, TsM is incapable of synthesizing lipid molecules. It should be equipped with a specialized system for lipid transportation from the host to ensure its long-survival. Such a transport system may be a target for function-associated drug design. We characterized two novel fatty-acid (FA)-binding TsM proteins (TsMFABP1 and TsMFABP2). Native and recombinant proteins bound to several FA analogs and retinol at micromolar and millimolar concentrations. Their binding was specifically inhibited by oleic acid. TsMFABP1exhibited high affinity toward FA analogs, while TsMFABP2 showed preferential affinity to retinol. Both TsMFABPs were predominantly expressed in the canal region of the worm, where lipids and retinol were abundantly distributed. The two paralogous TsMFABPs have undergone (or are still undergoing) structural diversification and following functional divergence to act as FABP or retinol binding protein, similar to the intracellular lipid binding proteins of deuterostomian animals. The canal region specific distribution of lipids, retinol and FABPs further suggested that cells in this area might differentiate into diverse cells to compose huge numbers of the proglottids, thereby playing vital roles in the parasite growth and development.

Comparative analysis of two fatty acid binding proteins fromFasciola gigantica

Fatty acid binding proteins are considered to be promising vaccine candidates against trematodiasis. In order to provide additional information about their function inFasciola giganticawe performed a comparative analysis of FgFABP1 and FgFABP3, two isoforms with quite different isoelectric points of 4·9 and 9·9 and 67% sequence identity. Both are expressed in the juvenile and adult parasite but differ in their tissue-specific distribution. In addition, the sequence of FABP3 is identical inF. hepaticaandF. giganticaindicating the protein's functional importance in this genus. Immune sera produced against soluble recombinant FgFABPs reacted with 14 kDa antigens in crude worm, soluble egg, cirrus sac extracts, and excretion/secretion product. Both FgFABPs were located in the parenchyma of the parasite but in addition, FgFABP1 was abundant in testes and spermatozoa while FgFABP3 was abundant in vitelline cells, eggs, and caecal epithelium. Mass spectrometry identified FgFABP1 and FgFABP3 in the ES product whereas only FgFABP3 was identified in egg extract. Serum samples of an experimentally infected rabbit reacted from week 6 post-infection with FgFABP3 and from week 12 with FgFABP1 while sera of infected sheep were not reactive. The results suggest differences in the biological functions of these 2 isoforms and differences in the host/parasite interaction that should be considered for their potential as vaccines against fascioliasis.

IL-18 enhances protective effect in mice immunized with a Schistosoma japonicum FABP DNA vaccine

Two recombinant plasmids, pVAX/SjFABP and pVAX/mIL-18 containing Schistosoma japonicum 14 kDa fatty acid binding protein (SjFABP) and murine IL-18, were constructed and evaluated for their ability to induce immune responses and to protect against S. japonicum challenge in mice. Mice were intramuscularly immunized twice at three-weekly intervals, and challenged with S. japonicum cercariae at 4 weeks after the last vaccination. All animals vaccinated with pVAX/SjFABP alone or plus pVAX/mIL-18 developed specific anti-SWAP ELISA antibody and T lymphocyte proliferation. Co-injection of pVAX/mIL-18 significantly increased the production of IFN-gamma and IL-2 compared with pVAX/SjFABP alone, indicating that IL-18 enhances the Th1-dominant immune response. The challenge experiment showed that co-injection of plasmid encoding IL-18 significantly enhances protective effect against S. japonicum infection, as demonstrated by worm reduction rates and the hepatic egg reduction rates 45 days post-challenge. These results indicated that IL-18 may become a novel vaccine adjuvant for development of vaccines against schistosomiasis.

Excretory/secretory proteome of the adult developmental stage of human blood fluke, Schistosoma japonicum

Schistosomes are the causative agents of schistosomiasis, one of the most prevalent and serious of the parasitic diseases that currently infects approximately 200 million people worldwide. Schistosome excretory/secretory (ES) proteins have been shown to play important roles in modulating mammalian host immune systems. In our current study, we performed a global proteomics identification of the ES proteins from adult worms of Schistosoma japonicum, one of the three major schistosome species. Our results unambiguously identified 101 proteins, including 53 putatively secreted proteins. By quantitative analysis, we revealed fatty acid-binding protein as a major constituent of the in vitro ES proteome. Strikingly the heat shock proteins HSP70s, HSP90, and HSP97 constituted the largest protein family in the ES proteome, implying a central role for these proteins in immunomodulation in the host-parasite relationship. Other important S. japonicum ES proteins included actins, 14-3-3, aminopeptidase, enolase, and glyceraldehyde-3-phosphate dehydrogenase, some of which have been considered as viable vaccine candidates and therapeutic targets. A comparison with previous studies suggests that 48.5% of S. japonicum ES proteins are common to other parasite ES products, indicating that the molecular mechanisms involved in evading the host immune response may be conserved across different parasites. Interestingly seven host proteins, including antimicrobial protein CAP18, immunoglobulins, and a complement component, were identified among in vitro S. japonicum ES products likely originating from the schistosome tegument or gut, indicating that host innate and acquired immune systems could defend against schistosome invasion. Our present study represents the first attempt at profiling S. japonicum ES proteins, provides an insight into host-parasite interactions, and establishes a resource for the development of diagnostic agents and vaccines for the control of schistosomiasis.

Invertebrate intracellular fatty acid binding proteins

Fatty acid binding proteins are multigenic cytosolic proteins largely distributed along the zoological scale. Their overall identity at primary and tertiary structure is conserved. They are involved in the uptake and transport of hydrophobic ligands to different cellular fates. The precise functions of each FABP type remain imperfectly understood, since sub-specialization of functions is suggested. Evolutionary studies have distinguished major subfamilies that could have been derived from a common ancestor close to vertebrate/invertebrate split. Since the isolation of the first invertebrate FABP from Schistocerca gregaria in 1990, the number of FABPs isolated from invertebrates has been increasing. Differences at the sequence level are appreciable and relationships with vertebrate FABPs are not clear, and lesser among invertebrate proteins, introducing some uncertainty to infer functional relatedness and phylogenetic relationships. The objective of this review is to summarize the information available on invertebrate FABPs to elucidate their mutual relationships, the relationship with their vertebrate counterparts and putative functions. Structure, gene structure, putative functions, expression studies and phylogenetic relationships with vertebrate counterparts are analyzed. Previous suggestions of the ancestral position concerning the heart-type of FABPs are reinforced by evidence from invertebrate models.

Development of a vaccine against Schistosoma japonicum in China: a review

Significant progress has been made over the past 50 years in the control of schistosomiasis japonica in China. However, recent data suggest that the disease is re-emerging. By the end of 2003, Schistosoma japonicum was still endemic in 110 counties in seven provinces in the southern part of China where the long-term reduction of the disease has been replaced by an increase in the number of people infected and areas infested by the intermediate host snail, i.e. Oncomelania hupensis. Explanations are multifactorial, including the construction of the Three Gorges dam, major flooding events, recovery of the Dongting Lake and the possible impact of climate change. An efficacious vaccine against S. japonicum would represent a significant addition to the current arsenal of control tools, particularly in the framework of an integrated control approach. The vaccine could be targeted either towards the prevention of infection or towards the reduction of parasite fecundity. Although progress in this field has been relatively slow, encouraging results have been obtained in recent years using defined native and recombinantly derived S. japonicum antigens. These findings suggest that development of a safe and efficacious vaccine is feasible. This paper reviews the progress in the development of a vaccine against S. japonicum in China, and includes also data from foreign researchers who are engaged in collaborative work with Chinese scientists.

Host humoral immune response to Leishmania lipid-binding protein

SUMMARY We report on the use of Leishmania donovani lipid-binding proteins (LBPs) as antigens capable of being recognized by serum from immunocompetent patients from southern Spain suffering from visceral leishmaniasis and from Peruvian patients with localized cutaneous leishmaniasis caused by Leishmania braziliensis. The absorbance found by immunoenzymatic techniques gave significantly different results for the serum samples from patients with and without leishmaniasis. Specificity by ELISA testing was 93.2% and sensibility 100%. Dot blots from human patient serum samples or naturally infected dogs from Spain gave similarly significant results. All the human serum samples from individuals with visceral leishmaniasis and the Leishmania-positive canine samples recognized two bands, with molecular weights of 8 and 57 kDa. The serum from individuals with cutaneous leishmaniasis caused by L. braziliensis recognized an additional band of 16 kDa. We discuss the role of Leishmania FABP and compare the immunological reactions found with serum samples from other protozoan infections such as toxoplasma and Chagas as well as bacterial infections such as tuberculosis and syphilis.

Gene cloning, expression and vaccine testing of Schistosoma japonicum SjFABP

A 600 bp DNA fragment was amplified by PCR from an adult Schistosoma japonicum cDNA library. Sequence analysis confirmed that this fragment contained an S. japonicum Chinese mainland strain fatty acid binding protein (Sj14FABP) gene. This gene was subsequently expressed in Escherichia coli (E. coli) and in Baculovirus/silkworm systems. The recombinant protein from E. coli was a 41 kDa GST fusion protein (rSj14/GST), which could be purified by glutathione agarose affinity chromatography, with a yield of 25 mg/L E. coli culture. The recombinant protein from the Baculovirus/silkworm system was an 18 kDa fusion protein (rSj14/His), which could be purified by Ni-NTA resin chromatography column with a yield of 3.5 mg per silkworm larva. Both rSj14/GST and rSj14/His could be recognized by S. japonicum-infected mouse sera and anti-rSj14/GST mouse sera in Western blotting. The purified recombinant protein was immunogenic in mice, rats and sheep, and 34.3%, 31.9% and 59.2% worm reductions, respectively, were obtained in vaccinated Kunming mice, Wistar rats and sheep vaccinated with Sj14/GST, compared to non-vaccinated control groups. Worm reductions of 48.8% and 49.0% were recorded in Balb/c mice immunized with Sj14/His, compared to non-vaccinated and BCG-vaccinated groups, respectively. These results indicate that rSj14FABP is a promising candidate vaccine for schistosomiasis japonica, particularly as in the rat and sheep vaccination experiments, no adjuvant was used.

The crystal structure of Echinococcus granulosus fatty-acid-binding protein 1

We describe the 1.6 A crystal structure of the fatty-acid-binding protein EgFABP1 from the parasitic platyhelminth Echinococcus granulosus. E. granulosus causes hydatid disease, which is a major zoonosis. EgFABP1 has been implicated in the acquisition, storage, and transport of lipids, and may be important to the organism since it is incapable of synthesising most of its lipids de novo. Moreover, EgFABP1 is a promising candidate for a vaccine against hydatid disease. The crystal structure reveals that EgFABP1 has the expected 10-stranded beta-barrel fold typical of the family of intracellular lipid-binding proteins, and that it is structurally most similar to P2 myelin protein. We describe the comparison of the crystal structure of EgFABP1 with these proteins and with an older homology model for EgFABP1. The electron density reveals the presence of a bound ligand inside the cavity, which we have interpreted as palmitic acid. The carboxylate group of the fatty acid interacts with the protein's P2 motif, consisting of a conserved triad R em leader R-x-Y. The hydrophobic tail of the ligand assumes a fairly flat, U-shaped conformation and has relatively few interactions with the protein.We discuss some of the structural implications of the crystal structure of EgFABP1 for related platyhelminthic FABPs.

Gene structure and M20T polymorphism of the Schistosoma mansoni Sm14 fatty acid-binding protein. Molecular, functioanl, and immunoprotection analysis

The Schistosoma mansoni Sm14 antigen belongs to the fatty acid-binding protein family and is considered a vaccine candidate against at least two parasite worms, Fasciola hepatica and S. mansoni. Here the genomic sequence and the polymorphism of Sm14 have been characterized for the first time. We found that the conserved methionine at position 20 is polymorphic, being exchangeable with threonine (M20T). To evaluate the function of the amino acid residue at this position, we have also constructed the mutant Sm14-A20 besides the two native isoforms (Sm14-M20 and Sm14-T20). The three purified recombinant His(6)-tagged Sm14 proteins (rSm14-M20, rSm14-T20, and rSm14-A20) present a predominant beta-barrel structure as shown by CD spectroscopy. Thermal and urea unfolding studies evidenced a higher structural stability of rSm14-M20 over the other forms (rSm14-M20>rSm14-T20>rSm14-A20). All of the Sm14 proteins were able to bind 11-(dansylamino)undecanoic acid (DAUDA) without substantial difference in the binding affinity. However, rSm14-M20 exhibited a higher affinity for natural fatty acids than the rSm14-T20 and rSm14-A20 proteins as judged by competitive experiments against DAUDA (rSm14-M20>rSm14-T20>rSm14-A20). The rSm14-M20 or rSm14-T20 isoforms but not the rSm14-A20 mutant was able to induce significant protection against S. mansoni cercariae challenge in immunized mice. The level of protection efficacy correlates with the extent of structure stability of the recombinant Sm14 isoforms and mutant.