Glutathione-binding proteins of Setaria digitata: antibody responses in human infected with Wuchereria bancrofti

Setaria digitata in advancing our knowledge of human lymphatic filariasis

Setaria digitata is a filarial parasite that causes fatal cerebrospinal nematodiasis in goats, sheep and horses, resulting in substantial economic losses in animal husbandry in the tropics. Due to its close resemblance to Wuchereria bancrofti, this nematode is also frequently used as a model organism to study human lymphatic filariasis. This review highlights numerous insights into the morphological, histological, biochemical, immunological and genetic aspects of S. digitata that have broadened our understanding towards the control and eradication of filarial diseases.

Immunoprophylactic potential of filarial glutathione-s-transferase in lymphatic filariaisis

OBJECTIVE

To elucidates the immunoprophylactic potential of glutathion-s-transferase (GST) from cattle filarial parasite Setaria digitata (S. digitata) against lymphatic filariasis.

METHODS

GST was purified through affinity chromatography (SdGST) and chacterized by SDS-PAGE and Nano-LC MS/MS analysis. Antibody isotypes to SdGST were measured by ELISA. Antibody dependant cellular cytotoxicity (ADCC) was performed in vitro using sera from immunized animals and immune individuals. T-cell proliferation and cytokine response to SdGST in different groups of filariasis were measured. Immunoprophylactic potential of SdGST was evaluate in animal model.

RESULTS

SdGST exhibited 30-fold enhancement of enzyme activity over crude parasitic extract. It was found to be 26 kDa by SDS-PAGE. Nano LC-MS/MS analysis followed by blast search showed 100% homology with Dirofilaria immitis (D. immitis) and only 43% with Homo sapiens (H. sapiens). Immunoblotting analysis showed putatively immune individuals carry significant level of antibodies to SdGST as compared with microfilaraemics. Immunized sera and sera endemic normal could neutralize the enzymatic activity of SdGST and inducing in vitro cytotoxicity of microfilariae. Peripheral blood mononuclear cells (PBMC) from endemic normals upon stimulation with SdGST showed a mixed type of Th1/Th2 response. SdGST immunization clear microfilariae from circulation in S. digitata implanted mastomys.

CONCLUSIONS

The heterologous GST could be potentially developed as a vaccine candidate against lymphatic filarial parasite.

Evaluation of Wuchereria bancrofti GST as a vaccine candidate for lymphatic filariasis

BACKGROUND

Lymphatic filarial parasites survive within the lymphatic vessels for years despite the complex immune environment surrounding them. Parasites possibly accomplish this by adopting various immunomodulatory strategies, which include release of glutathione-S-transferases (GSTs) that counteract the oxidative free radicals produced by the host. Since GSTs produced by parasites appear to be critical for the survival of parasites in the host, several studies evaluated the potential of parasite GSTs as vaccine candidates especially against schistosomiasis, fascioliasis and Seteria cervi. However, vaccine potential of GSTs of lymphatic filarial parasites has not been evaluated before.

METHODS/PRINCIPAL FINDINGS

In the present study, the GST gene was cloned from the third stage larval (L3) cDNA libraries of Wuchereria bancrofti, and recombinant GST (WbGST) was expressed and purified. Serum samples from individuals living in an endemic area were analyzed for their reactivity with rWbGST. These findings showed that sera from endemic normal individuals (EN) carry significant levels of anti-WbGST IgG antibodies compared to subjects who are microfilaraemic (Mf) or show symptoms of clinical pathology (CP). Isotype analysis of the anti-WbGST IgG antibodies showed a predominance of IgG1 and IgG3 antibodies in EN individuals. Subsequent functional analysis of the rWbGST showed that the rWbGST protein retained the enzymatic activity of GST and the antibodies in EN sera could inhibit this enzymatic activity. Similar results were obtained when anti-rWbGST antibodies raised in mice were used in the neutralization assay. Brugia malayi GST and WbGST show significant sequence similarity. Therefore, to evaluate the vaccine potential of rWbGST, we used B. malayi L3 as challenge parasites. Vaccine potential of rWbGST was initially evaluated by confirming the role of human and mice WbGST antibodies in an antibody dependent cellular cytotoxicity (ADCC) assay. Subsequent vaccination studies in a jird model showed that approximately 61% protection could be achieved against a B. malayi L3 challenge infection in jirds immunized with rWbGST.

CONCLUSIONS

Results of this study show that rWbGST is a potential vaccine candidate against lymphatic filariasis. Nearly 61% protection can be achieved against a B. malayi challenge infection in a jird model. The study also showed that the WbGST protein retained the enzymatic activity of GST and this enzymatic activity appears to be critical for the survival of the parasite in the host.

Biochemical composition and metabolic pathways of filarial worms Setaria cervi: search for new antifilarial agents

The main problem regarding the chemotherapy of filariasis is that no safe and effective drug is available yet to combat the adult human filarial worms. Setaria cervi, the causal organism of setariasis and lumbar paralysis in cattle, is routinely employed as a model organism for conducting biochemical and enzymatic studies on filarial parasites. In view of the practical difficulties in procuring human strains of Wuchereria bancrofti and Brugia malayi for drug screening, the bovine filarial parasite S. cervi, resembling the human species in having microfilarial periodicity and chemotherapeutic response to known antifilarial agents, is widely used as a model in such studies. For a rational approach to antifilarial chemotherapy, knowledge of the biochemical composition and metabolic pathways of this helminth parasite may be of paramount importance, so that more potent antifilarial agents based on specific drug targets can be identified in drug discovery programmes. The present review provides an update on the biochemistry of the important metabolic pathways functioning within this potentially important bovine parasite, that have so far been studied, and on those that need to be investigated further so as to identify novel drug targets that can be exploited for designing new antifilarial drugs.

MALDI mass sequencing and characterization of filarialglutathione-S-transferase

Glutathione-S-transferase has been detected in the somatic extract and excretory-secretory products of different life stages of Setaria cervi, a bovine filarial parasite. The enzyme was subjected to MALDI-TOF followed by mass spectrometry and the nearest match found was Pleuronectes platessa GST. Molecular mass of the purified enzyme was approximately 26 kDa as determined by SDS-PAGE and MALDI-TOF. Setaria cervi GST exhibited high activity towards 1-chloro-2,4-dinitrobenzene and ethacrynic acid. Kinetic analysis with respect to 1-chloro-2,4-dinitrobenzene and glutathione as substrate revealed a K(m) of 2.22 mM and 0.61 mM, respectively. The activity was inhibited significantly by Cibacron blue and alpha-tocopherol.

Lymphatic filariasis: new insights into an old disease

Lymphatic filariasis has afflicted people in the tropical areas of the world for thousands of years but even up to comparatively recent times it has been poorly understood and its importance under recognised. In the last 2 decades or so there has been a flurry of activity in filariasis research, which has provided new insights into the global problem of filariasis, the pathogenesis of filarial disease, diagnosis and control.

Setaria digitata infections in cattle: parasite load, microfilaraemia status and relationship to immune response

A total of 110 cattle were examined in an area endemic for Bancroftian filariasis for the prevalence of infection of the bovine filarial parasite Setaria digitata. About 12.5% of cattle were found to harbour both adult worms in the peritoneum and microfilariae (mf) in circulation; 70% of the cattle were amicrofilaraemic but with an adult worm infection. A third group of cattle (16.5%) was free of detectable mf and adult worms. The presence of adult worms and/or mf did not influence the antibody levels to any of the four antigen preparations of S. digitata. However, there was a significant inverse relationship between the presence of antibodies to microfilarial sheaths and the absence of circulating mf as shown by the immunoperoxidase assay. Cattle immunoglobulin containing high titres of anti-sheath antibodies cleared circulating microfilariae very effectively in Mastomys coucha thus demonstrating the protective nature of anti-sheath antibodies in eliminating circulating microfilariae in vivo.

Antibody response to a filarial antigen fraction in individuals exposed to Wuchereria bancrofti infection in India

An antigen fraction (DssdI) was isolated from the aqueous-insoluble components of adult Setaria digitata. Rabbit antiserum to DssdI labeled the sheaths of Wuchereria and Setaria microfilariae suggesting DssdI to be surface associated. Antibody responses to DssdI were determined in Wuchereria bancrofti exposed individuals. IgM titre was highest in endemic normals (EN; asymptomatic amicrofilaraemic) followed by chronic filarial patients (CP) and asymptomatic microfilaraemics (AS). IgG titer was high in both chronic filariasis and endemic normals. The microfilaraemic group had the lowest titer. The seropositivity rate in AS was 28% for IgG and 10% for IgM, in CP 100% for IgG and 33% for IgM, and in EN 86% for both antibodies. An age-dependency of IgM but not of IgG antibodies was observed in endemic normals, with IgM prevalence reaching a plateau by 12 years of age. The subclass composition of the antibody response to DssdI appeared to be primarily IgG3 in endemic normals, IgG1 in chronic filariasis and IgG4 in asymptomatic microfilaraemics. Diethylcarbamazine (DEC) treatment led to a significant increase in IgG and IgM levels in microfilaraemic individuals. Of interest, the expression of IgG subclasses altered. There was an increased IgG3 and IgG1 response associated with a reciprocal decrease in IgG4 following DEC therapy.