Suppression of Schistosoma bovis egg production in cattle by vaccination with either glutathione S-transferase or keyhole limpet haemocyanin

A comprehensive and critical overview of schistosomiasis vaccine candidates

A digenetic platyhelminth Schistosoma is the causative agent of schistosomiasis, one of the neglected tropical diseases that affect humans and animals in numerous countries in the Middle East, sub-Saharan Africa, South America and China. Several control methods were used for prevention of infection or treatment of acute and chronic disease. Mass drug administration led to reduction in heavy-intensity infections and morbidity, but failed to decrease schistosomiasis prevalence and eliminate transmission, indicating the need to develop anti-schistosome vaccine to prevent infection and parasite transmission. This review summarizes the efficacy and protective capacity of available schistosomiasis vaccine candidates with some insights and future prospects.

Schistosoma haematobium Extracellular Vesicle Proteins Confer Protection in a Heterologous Model of Schistosomiasis

Helminth parasites release extracellular vesicles which interact with the surrounding host tissues, mediating host-parasite communication and other fundamental processes of parasitism. As such, vesicle proteins present attractive targets for the development of novel intervention strategies to control these parasites and the diseases they cause. Herein, we describe the first proteomic analysis by LC-MS/MS of two types of extracellular vesicles (exosome-like, 120 k pellet vesicles and microvesicle-like, 15 k pellet vesicles) from adult Schistosoma haematobium worms. A total of 57 and 330 proteins were identified in the 120 k pellet vesicles and larger 15 k pellet vesicles, respectively, and some of the most abundant molecules included homologues of known helminth vaccine and diagnostic candidates such as Sm-TSP2, Sm23, glutathione S-transferase, saponins and aminopeptidases. Tetraspanins were highly represented in the analysis and found in both vesicle types. Vaccination of mice with recombinant versions of three of these tetraspanins induced protection in a heterologous challenge (S. mansoni) model of infection, resulting in significant reductions (averaged across two independent trials) in liver (47%, 38% and 41%) and intestinal (47%, 45% and 41%) egg burdens. These findings offer insight into the mechanisms by which anti-tetraspanin antibodies confer protection and highlight the potential that extracellular vesicle surface proteins offer as anti-helminth vaccines.

Schistosome Vaccines for Domestic Animals

Schistosomiasis is recognized as a tropical disease of considerable public health importance, but domestic livestock infections due to Schistosoma japonicum, S. bovis, S. mattheei and S. curassoni are often overlooked causes of significant animal morbidity and mortality in Asia and Africa. In addition, whereas schistosomiasis japonica is recognized as an important zoonosis in China and the Philippines, reports of viable schistosome hybrids between animal livestock species and S. haematobium point to an underappreciated zoonotic component of transmission in Africa as well. Anti-schistosome vaccines for animal use have long been advocated as part of the solution to schistosomiasis control, benefitting humans and animals and improving the local economy, features aligning with the One Health concept synergizing human and animal health. We review the history of animal vaccines for schistosomiasis from the early days of irradiated larvae and then consider the recombinant DNA technology revolution and its impact in developing schistosome vaccines that followed. We evaluate the major candidates tested in livestock, including the glutathione S-transferases, paramyosin and triose-phosphate isomerase, and summarize some of the future challenges that need to be overcome to design and deliver effective anti-schistosome vaccines that will complement current control options to achieve and sustain future elimination goals.

Schistosomiasis is more prevalent than previously thought: what does it mean for public health goals, policies, strategies, guidelines and intervention programs?

Mapping and diagnosis of infections by the three major schistosome species (Schistosoma haematobium, S. mansoni and S. japonicum) has been done with assays that are known to be specific but increasingly insensitive as prevalence declines or in areas with already low prevalence of infection. This becomes a true challenge to achieving the goal of elimination of schistosomiasis because the multiplicative portion of the life-cycle of schistosomes, in the snail vector, favors continued transmission as long as even a few people maintain low numbers of worms that pass eggs in their excreta. New mapping tools based on detection of worm antigens (circulating cathodic antigen – CCA; circulating anodic antigen – CAA) in urine of those infected are highly sensitive and the CAA assay is reported to be highly specific. Using these tools in areas of low prevalence of all three of these species of schistosomes has demonstrated that more people harbor adult worms than are regularly excreting eggs at a level detectable by the usual stool assay (Kato-Katz) or by urine filtration. In very low prevalence areas this is sometimes 6- to10-fold more. Faced with what appears to be a sizable population of “egg-negative/worm-positive schistosomiasis” especially in areas of very low prevalence, national NTD programs are confounded about what guidelines and strategies they should enact if they are to proceed toward a goal of elimination. There is a critical need for continued evaluation of the assays involved and to understand the contribution of this “egg-negative/worm-positive schistosomiasis” condition to both individual morbidity and community transmission. There is also a critical need for new guidelines based on the use of these more sensitive assays for those national NTD programs that wish to move forward to strategies designed for elimination.

Immunization with Wuchereria bancrofti Glutathione-S-transferase Elicits a Mixed Th1/Th2 Type of Protective Immune Response Against Filarial Infection in Mastomys

Lymphatic filariasis is a mosquito borne parasitic infection and can severely affect the normal working ability of an individual. Currently there is no vaccine available to prevent this infection and the development of a potential vaccine could effectively support the on-going mass drug administration program by World Health Organization (WHO). Filarial parasites have complex mechanisms to modulate the host immune responses against them. The glutathione-S-transferases (GST) are the important enzymes effectively involved to counteract the oxidative free radicals produced by the host. In the present study, we have shown that the mastomys which are fully permissible rodents for Brugia malayi when immunized with Wuchereria bancrofti recombinant GST (rWbGST) could induce 65.5 % in situ cytotoxicity against B. malayi infective (L3) larvae. There was a balanced Th1/Th2 immune response in the vaccinated animals, characterized by higher levels of WbGST-specific IgG1 and IgG2a antibodies and pronounced IFN-γ, IL-10 and IL-4 cytokines production by the spleen cells.

The combination of the aliphatic diamine AA0029 in ADAD vaccination system with a recombinant fatty acid binding protein could be a good alternative for the animal schistosomiasis control

Fatty acid binding proteins (FABP) from Fasciola hepatica have demonstrated immune cross-protection against schistosomes. The present study was conducted to develop a new formulation of the recombinant FABP rFh15 with the synthetic immunomodulator AA0029 in the adjuvant adaptation (ADAD) vaccination system and to evaluate its ability to induce immune response and protection against the challenge with Schistosoma bovis cercariae. Immunization of BALB/c mice showed high levels of TNFα, IFNγ, interleukin (IL)-2, IL-6, and IL-4 in splenocyte supernatant culture and also high levels of serum specific anti-rFh15 IgG, IgG1, IgG2a IgE and IgM antibodies suggesting a mixed Th1/Th2 immune response. Using this approach, high levels of protection against experimental challenge with S. bovis cercariae were observed in the mouse and hamster models. A marked reduction up to 64% in worm burden, as well as in the number of eggs retained in liver (66%) and intestine (77%) and hepatic lesions (42%), was achieved in vaccinated BALB/c mice. Golden hamsters vaccinated and challenged in similar conditions had reductions in recovered worms (83%), liver eggs (90%), intestine eggs (96%), liver lesions (56%) and worm fecundity (48-80%). These data suggest that formulation of rFh15 in the ADAD vaccination system using the AA0029 immunomodulator could be a good option to drive an effective immunological response against schistosomiasis.

A Fasciola hepatica-derived fatty acid binding protein induces protection against schistosomiasis caused by Schistosoma bovis using the adjuvant adaptation (ADAD) vaccination system

Several efforts have been made to identify anti-schistosomiasis vaccine candidates and new vaccination systems. The fatty acid binding protein (FAPB) has been shown to induce a high level of protection in trematode infection. The adjuvant adaptation (ADAD) vaccination system was used in this study, including recombinant FABP, a natural immunomodulator and saponins. Mice immunised with the ADAD system were able to up-regulate proinflammatory cytokines (IL-1 and IL-6) and induce high IgG2a levels. Moreover, there was a significant reduction in worm burden, egg liver and hepatic lesion in vaccinated mice in two independent experiments involving Schistosoma bovis infected mice. The foregoing data shows that ADAD system using FABP provide a good alternative for triggering an effective immune response against animal schistosomiasis.

Human Schistosoma haematobium antifecundity immunity is dependent on transmission intensity and associated with immunoglobulin G1 to worm-derived antigens

Background

Immunity that reduces worm fecundity and, in turn, reduces morbidity is proposed for Schistosoma haematobium, a parasite of major public health importance. Mathematical models of epidemiological trends suggest that antifecundity immunity is dependent on antibody responses to adult-worm-derived antigen.

Methods

For a Malian cohort (age, 5–29 years) residing in high-transmission fishing villages or a moderate-transmission village, worm fecundity was assessed using the ratio of urinary egg excretion to levels of circulating anodic antigen, a Schistosoma-specific antigen that is steadily secreted by adult worms. Fecundity was modeled against host age, infection transmission intensity, and antibody responses specific to soluble worm antigen (SWA), tegument allergen-like 1, and 28-kDa glutathione-S-transferase.

Results

Worm fecundity declined steadily until a host age of 11 years. Among children, host age and transmission were negatively associated with worm fecundity. A significant interaction term between host age and transmission indicates that antifecundity immunity develops earlier in high-transmission areas. SWA immunoglobulin G1 (IgG1) levels explained the effect of transmission on antifecundity immunity.

Conclusion

Antifecundity immunity, which is likely to be protective against severe morbidity, develops rapidly during childhood. Antifecundity immunity is associated with SWA-IgG1, with higher infection transmission increasing this response at an earlier age, leading to earlier development of antifecundity immunity.

Cytokine responses to the anti-schistosome vaccine candidate antigen glutathione-S-transferase vary with host age and are boosted by praziquantel treatment

Background

Improved helminth control is required to alleviate the global burden of schistosomiasis and schistosome-associated pathologies. Current control efforts rely on the anti-helminthic drug praziquantel (PZQ), which enhances immune responses to crude schistosome antigens but does not prevent re-infection. An anti-schistosome vaccine based on Schistosoma haematobium glutathione-S-transferase (GST) is currently in Phase III clinical trials, but little is known about the immune responses directed against this antigen in humans naturally exposed to schistosomes or how these responses change following PZQ treatment.

Methodology

Blood samples from inhabitants of a Schistosoma haematobium-endemic area were incubated for 48 hours with or without GST before (n = 195) and six weeks after PZQ treatment (n = 107). Concentrations of cytokines associated with innate inflammatory (TNFα, IL-6, IL-8), type 1 (Th1; IFNγ, IL-2, IL-12p70), type 2 (IL-4, IL-5, IL-13), type 17 (IL-17A, IL-21, IL-23p19) and regulatory (IL-10) responses were quantified in culture supernatants via enzyme-linked immunosorbent assay (ELISA). Factor analysis and multidimensional scaling were used to analyse multiple cytokines simultaneously.

Principal Findings

A combination of GST-specific type 2 (IL-5 and IL-13) and regulatory (IL-10) cytokines was significantly lower in 10–12 year olds, the age group at which S. haematobium infection intensity and prevalence peak, than in 4–9 or 13+ year olds. Following PZQ treatment there was an increase in the number of participants producing detectable levels of GST-specific cytokines (TNFα, IL-6, IL-8, IFNγ, IL-12p70, IL-13 and IL-23p19) and also a shift in the GST-specific cytokine response towards a more pro-inflammatory phenotype than that observed before treatment. Participant age and pre-treatment infection status significantly influenced post-treatment cytokine profiles.

Conclusions/Significance

In areas where schistosomiasis is endemic host age, schistosome infection status and PZQ treatment affect the cellular cytokine response to GST. Thus the efficacy of a GST-based vaccine may also be shaped by the demographic and epidemiological characteristics of targeted populations.

Schistosomiasis is caused by infection with Schistosoma spp. parasites, for which the main treatment is the drug praziquantel (PZQ). Since PZQ does not prevent reinfection, an anti-schistosome vaccine based on the Schistosoma haematobium enzyme glutathione-S-transferase (GST) is being developed. In this study we investigated the GST-specific immune responses of people naturally exposed to schistosomes and the affect that PZQ has on these responses. We cultured blood samples from a schistosome-exposed community with GST before and six weeks after PZQ treatment and measured a range of soluble proteins (cytokines) in culture supernatants as indicators of blood cell activation and phenotype. Before treatment, GST-specific cytokine responses varied with host age, particularly in children with high intensity schistosome infections. Following treatment, GST activated blood samples from more individuals to produce a broader range of cytokines and the combination of GST-specific cytokine responses reflected a more pro-inflammatory immune phenotype than that observed pre-treatment. Post-treatment responses varied according to host age and pre-treatment infection status. Taken together, our study suggests that current and future GST-based vaccine trials should take host age, schistosome infection status and PZQ treatment history into account since these factors influence GST-specific immune activation.

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.

Persistent immune responses in late infection and after treatment in experimental Schistosoma bovis infections in goats

This study explored host immune responses and their possible relationship to the anti-fecundity phenomenon in Schistosoma bovis-infected goats. The design comprised a primary infection with or without treatment at week (wk) 13, and with or without challenge at wk 36. Necropsy was performed at 36 or 52wk. Serum levels of anti-egg IgG, and anti-worm IgG and IgM, were measured by ELISA. In chronic infection, anti-worm antibodies stayed high, reflecting persisting worm burdens, whereas anti-egg IgG remained high despite minimized egg excretion. After treatment, anti-worm IgM and anti-egg IgG were minimized, but anti-worm IgG remained above the values of the uninfected controls. Histopathology showed lowered numbers of perioval granulomas in chronic infection and resolution of liver fibrosis with time, but intestinal lymphoplasmacytic perivasculitis and hepatic eosinophilic infiltrates were maintained at wk 52. Significant splenic plasmacytosis persisted after treatment. The results indicated that persistent immune responses, in chronically infected and in treated goats, may explain sustained worm fecundity depression at challenge infection.

Treatment efficacy and regulatory host responses in chronic experimental Schistosoma bovis infections in goats

The aim of this study was to elucidate the regulatory responses and the long-term effect of praziquantel treatment in chronically Schistosoma bovis-infected West African Dwarf goats. Forty-two goats were used and the design comprised a primary infection followed by treatment at week 13, challenge infection at week 36 and termination at week 52. Dependent variables included clinico-pathological data, worm numbers, faecal and tissue egg counts, and gross pathology of the liver. The results showed that primary infections remained suppressed for up to 52 weeks and, although challenge infections imposed on 36-week-old primary infections established fully, the impairment of their egg production capacity provided protection against clinico-pathological consequences measured by body weight and haemoglobin levels. The study also confirmed a high efficacy (97.7%) of praziquantel for treatment of S. bovis infection in goats and showed that anthelminthic removal of primary infections does not interfere with the ability of the goat to elicit a marked resistance to a subsequent challenge infection. Although treated goats had more fibrous scarring of livers than untreated goats, no negative effects of liver lesions were reflected in weight gains of treated goats. This study provides strong evidence for the beneficial effects of anthelminthic treatment of young domestic stock as an element of treatment and preventive programmes.

Immunity to schistosomiasis in animals: an update

The present paper reviews the available literature on the development of immunity to animal Schistosoma infections. The majority of the studies on animal schistosomiasis were performed in cattle and pigs and only Schistosoma mattheei, S. bovis and S. japonicum received particular attention, mainly because of their recognized veterinary significance or zoonotic aspect. Although it is an accepted fact that acquired resistance to Schistosoma is of major importance in the regulation of infection intensity in the field, almost nothing is yet known of either the nature of the antigens or of the immune mechanisms involved. The recent studies on immunity development focus in particular on the occurrence of maternal to foetal transfer of immunological substances related to animal Schistosoma infections and possible effects of these transfers on the immunity development of the foetus/newborn. Since congenital infections for Schistosoma species other than S. japonicum are extremely rare, the most plausible route for foetal contact is the transplacental or postnatal transfer of immunological substances. Prenatal transfers of specific antibodies and antigens via placental lesions and postnatal transfers via the colostrum were observed in cattle and pigs, and subsequent modifications of the immune response of the newborn were observed. Placental lesions induced by Schistosoma eggs could allow other pathogens to cross the placenta.

The epidemiology and control of cattle schistosomiasis

Schistosomiasis remains a major health problem in much of the developing world. Despite decades of research, many fundamental questions on the dynamics of infection and immunity development remain unanswered. Schistosomiasis is also a common parasitic infection in cattle, and studies on livestock exposed to their own species of schistosome may help in understanding some aspects of the host-parasite relationship. Here, Jan De Bont and Jozef Vercruysse review the current knowledge on the epidemiology and control of cattle schistosomiasis.

Immunoepidemiology of human schistosomes: taking the theory into the field

Much is known about human immune responses to schistosome infection, but it has proved difficult to determine the impact of these responses on schistosome epidemiology in the field. In this paper, Mark Woolhouse compares epidemiological patterns from field data with the predictions o f simple mathematical models of different immunological processes. The comparison gives some indications as to which types of immune response may be important, and of their strength and duration. The results are consistent with a significant impact of a process similar to 'concomitant' immunity, with a possible role for anti-fecundity effects.

Specific antibody responses to three schistosome-related carbohydrate structures in recently exposed immigrants and established residents in an area of Schistosoma mansoni endemicity

By the use of surface plasmon resonance spectroscopy, immunoglobulin G (IgG) subclass and IgM antibodies against three schistosome-derived carbohydrate structures, FLDN (Fucalpha1-3GalNAcbeta1-4GlcNAcbeta1-3Galalpha1), LDN-DF [GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAcbeta1], and LDNF [GalNAcbeta1-4(Fucalpha1-3)GlcNAcbeta1-3Galalpha1], were measured in 184 previously unexposed Kenyan immigrants who moved into the Masongaleni area, where Schistosoma mansoni is endemic. They were sampled within their first year of exposure and again 2 years later. A cohort selected out of the original residents of the area, who had been exposed for many years, served as controls. Associations with responses to S. mansoni worm, egg (SEA), and cercarial (CERC) antigens were examined. In addition, we measured responses to keyhole limpet hemocyanin, a glycoprotein which carries glycan epitopes that are also expressed by schistosomes. Specific IgG1 responses were most pronounced against FLDN and LDN-DF and strongly associated with those previously measured to SEA and CERC. Similarly to previously published age profiles of IgG1 and IgG2 responses to SEA, levels of IgG1 against LDN-DF decreased with age. In contrast, specific IgM responses against the three schistosome-derived carbohydrate structures were most marked against LDNF. Our results indicate that, of the three glycan structures tested, the acute response against schistosome glycoconjugate antigens in young children is mainly directed against the LDN-DF epitope. The response to LDN-DF in older individuals and the responses to the two other epitopes were similar in the two cohorts, suggesting that these antigens are recognized in the early stages of infection and that the immune response persists. The biological significance of these observations needs further elucidation.

The Trypanosoma cruzi Tc52-released protein induces human dendritic cell maturation, signals via Toll-like receptor 2, and confers protection against lethal infection

The intracellular protozoan parasite Trypanosoma cruzi is the etiological agent of Chagas disease. We have recently identified a T. cruzi-released protein related to thiol-disulfide oxidoreductase family, called Tc52, which is crucial for parasite survival and virulence. In vitro, Tc52 in combination with IFN-gamma activates human macrophages. In vivo, active immunization with Tc52 relieves the immunosuppression associated to acute infection and elicits a specific immune response. As dendritic cells (DC) have a central role in the initiation of immune responses, we investigated whether Tc52 may modulate DC activity. We show that Tc52 induces human DC maturation. Tc52-treated immature DC acquire CD83 and CD86 expression, produce inflammatory chemokines (IL-8, monocyte chemoattractant protein-1, and macrophage-inflammatory protein-1 alpha), and present potent costimulatory properties. Tc52 binds to DC by a mechanism with the characteristics of a saturable receptor system and signals via Toll-like receptor 2. While Tc52-mediated signaling involves its reduced glutathione-binding site, another portion of the molecule is involved in Tc52 binding to DC. Finally, we report that immunization with Tc52 protects mice in vivo against lethal infection with T. cruzi. Together these data evidence complex molecular interactions between the T. cruzi-derived molecule, Tc52, and DC, and suggest that Tc52 and related class of proteins might represent a new type of pathogen-associated molecular patterns. Moreover, the immune protection data suggest that Tc52 is among candidate molecules that may be used to design an optimal multicomponent vaccine to control T. cruzi infection.

Glutathione S-transferases and related proteins from pathogenic human parasites behave as immunomodulatory factors

There is a rapidly expanding interest into the glutathione S-transferases (GSTs) and the structurally related molecules. Many of the latter have been identified as members of conserved protein families sharing structural and some times functional properties being particularly involved in heat-shock response, drug resistance and carcinogenesis. Also, evidence is emerging that members of the GST super family from some pathogens could exert immunomodulatory functions toward the cell of the immune system, involving separate profiles of cytokine gene transcription and different patterns of cell growth, illustrating therefore the 'one gene-dual function' phenomenon. The implication of these biological properties for pathogenesis is discussed.

Immunization of sheep against Fasciola gigantica with glutathione S-transferase

Glutathione S-transferase in Fasciola gigantica (FgGST) was isolated by affinity chromatography, by which highly purified enzyme was obtained. FgGST on the SDS-PAGE showed three protein bands ranging 24.5-26.5kDa. Glutathione S-transferase (GST) activity was determined by HABIG method. FgGST was evaluated as vaccine alone or in combination with either aluminum hydroxide or saponin in sheep against F. gigantica infection. ELISA was used for detection of anti-FgGST IgG. After vaccination, all sheep were challenged with 120 metacercaria of F. gigantica. The results indicated that anti-GST IgG was not elevated after challenge. All sheep were slaughtered 24-26 weeks after challenge. The results indicated that, although after second vaccination, antibody titers rose markedly in GST-Al(OH)(3) and GST-saponin groups, but declined 4 weeks after challenge. No correlation between anti-GST IgG titers and protection was observed. The highest fluke burden reduction was observed in the group vaccinated with GST-saponin (32%), but this reduction was not statistically significant in comparison with the control group.

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

Schistosoma mansoni gene GP22 encodes the tegumental antigen sm25: (1) antibodies to a predicted B-cell epitope of Sm25 cross-react with other candidate vaccine worm antigens; (2) characterization of a recombinant product containing tandem-repeats of this peptide as a vaccine

Monospecific antibodies against two putative epitopes of schistosome protein encoded by gene GP22 (182 codons, no introns) were used to probe worm extracts fractionated by lentil-lectin affinity chromatography or by electrophoresis. Anti-peptide-alpha (codons 70-84) exclusively identifies the N-glycanase-sensitive, 25 kDa tegumental glycoprotein Sm25 in the lectin-bound fraction of detergent-solubilized adult worm extract S3. In contrast, antipeptide-delta (codons 151-162) does not react with Sm25 but cross-reacts with other schistosome proteins, including candidate vaccine antigens paramyosin (Sm97) and glutathione-S-transferases (Sm26, Sm28, Sj26). Recombinant protein r4 x 47, constructed to express multiple copies of codon sequence 117-163 (containing delta), reacts with anti-delta and is uniquely recognized by protective Fischer twice-infected (F-2x) rat antiserum. Immunization with r4 x 47 induces antibodies with cross reactivities similar to anti-delta, but which also recognize Sm25. Despite these cross-reactivities with protective antigens, rodents vaccinated with r4 x 47 were not protected against cercarial infection. On the basis of these data, two hypotheses are proposed: (1) antigenic epitopes other than delta are present within the r4 x 47 sequence which induce antibodies reactive with Sm25 and/or (2) peptide-delta assumes alternative antigenic conformations, dependent upon the context of neighbouring sequences, some of which mimic epitopes of proteins encoded by other schistosome genes. These mimotopes are not targets of protective antibodies.

Keyhole limpet hemocyanin (KLH): a biomedical review

In this review we present a broad survey of fundamental scientific and medically applied studies on keyhole limpet hemocyanin (KLH). Commencing with the biochemistry of KLH, information on the biosynthesis and biological role of this copper-containing respiratory protein in the marine gastropod Megathura crenulata is provided. The established methods for the purification of the two isoforms of KLH (KLH1 and KLH2) are then covered, followed by detailed accounts of the molecular mass determination, functional unit (FU) structure, carbohydrate content, immunological analysis and recent aspects of the molecular genetics of KLH. The transmission electron microscope (TEM) has contributed significantly to the understanding of KLH structure, primarily from negatively stained images. We give a brief account of TEM studies on the native KLH oligomers, the experimental manipulation of the oligomeric states, together with immunolabelling data and studies on subunit reassociation. The field of cellular immunology has provided much relevant biomedical information on KLH and has led to the expansion of use of KLH in experimental immunology and clinically as an immunotherapeutic agent; this area is presented in some detail. The major clinical use of KLH is specifically for the treatment of bladder carcinoma, with efficacy probably due to a cross-reacting carbohydrate epitope. KLH also has considerable possibilities for the treatment of other carcinomas, in particular the epithelially derived adenocarciomas, when used as a carrier for carcinoma ganglioside and mucin-like epitopes. The widespread use of KLH as a hapten carrier and generalised vaccine component represent other major on-going aspects of KLH research, together with its use for the diagnosis of Schistosomiasis, drug assay and the treatment of drug addiction. Immune competence testing, assessment of stress and the understanding of inflammatory conditions are other areas where KLH is also making a useful contribution to medical research.

Egg-hatching inhibition in mice immunized with recombinant Schistosoma bovis 28 kDa glutathione S-transferase

The capacity of a recombinant glutathione S-transferase from Schistosoma bovis (rSb 28GST) to protect BALB/c mice against homologous and heterologous infections with, respectively, S. bovis or Schistosoma mansoni has been studied. Two injections of the rSb 28GST and an intravenous boost resulted in a marked specific IgG response on the day of experimental challenge with S. bovis or S. mansoni cercariae. Immunization of BALB/c mice led to a reduction in egg maturation and egg viability after infection with S. bovis or S. mansoni. Adult worm recoveries after an S. bovis challenge infection and tissue egg densities (intestine and liver) in S. mansoni challenge infection were also reduced in the immunized groups, but these differences were not statistically significant. No association between in vitro inhibition of GST enzymatic activity induced by immunized mouse sera and worm burden reduction was recorded. The analysis of the immune response, on the day of perfusion, showed the production of immunoglobulin (Ig)G1, IgG2a and IgG2b specific antibodies and the production of interleukin (IL)-4 and IL-5 by spleen cells after rSb 28GST stimulation. These data suggest that rSb 28GST immunization induces a moderate effect upon egg maturation and egg hatching, suggesting the involvement of similar mechanisms of action and common, but not exclusive, targets during S. bovis and S. mansoni infections. As a consequence, immunization with rSb 28GST may prove useful in affecting the pathology and transmission of African schistosomes.

A fatty acid binding protein from Fasciola hepatica induced protection in C57/BL mice from challenge infection with Schistosoma bovis

Three strains of mice (NMRI, C57/BL, BALB/c) were each immunized with a 12 kDa purified, native Fasciola hepatica fatty acid binding protein (Fh12) and challenged percutaneously with Schistosoma bovis cercariae. C57/BL mice immunized with Fh12 had significant reductions in S. bovis worm burden recoveries (96 and 87% reductions over controls in two separate experiments). When using NMRI or BALB/c mice, Fh12 alone or in Freund's adjuvant failed to induce significant protection against S. bovis. In C57/BL mice vaccinated against Fh 12, antibodies to the IgG2a isotype, but not to the IgG1 isotype, increased by 2 weeks after the second immunization and remained high through 8 weeks of S. bovis infection. Antibodies to S. bovis increased after 4 weeks of infection. Regarding cytokine production by spleen mononuclear cells, C57/BL mice vaccinated with Fh12 in adjuvant, and having the highest protective response against challenge infection with S. bovis, had an increase of IFN-gamma production with Concanavalin A but no increase of IL-4 in similarly stimulated cells. These results suggest that the protection obtained in this group of mice is mediated by a Th1 immune response.

Schistosoma bovis: vaccine effects of a recombinant homologous glutathione S-transferase in sheep

The economic importance of the trematode Schistosoma bovis in African livestock has justified the development of a specific vaccine. Administered preventively to sheep, rSb28GST--the only molecule cloned from S. bovis which has demonstrated vaccine potentialities in goats and cattle--reduced the mean worm burden in vaccinated animals and improved their health status compared with that of non-vaccinated controls. As in goats, but not in bovines, the fecundity of the settled worm pairs was not modified. Therefore, rSb28GST can be proposed as a universal tool for the prevention of clinical disorders engendered by the main schistosome species affecting domestic ruminants in the African continent.

Vaccine potential of a recombinant glutathione S-transferase cloned from Schistosoma haematobium in primates experimentally infected with an homologous challenge

Patas monkeys were twice immunized with a Schistosoma haematobium-derived recombinant glutathione S-transferase (Sh28GST) then challenged with an homologous calibrated challenge. BCG and Freund's Complete Adjuvant (FCA) were used as adjuvants in two distinct protocols. Specific IgG and IgA antibody responses were intense and homogeneous in the animals receiving Sh28GST in the presence of FCA, whereas BCG could only induce moderate and heterogeneous antibody titres. No significant effect on worm burdens was evidenced 36 weeks post-infection in either group of Sh28GST-immunized animals compared to their matched controls receiving an irrelevant protein. Although not significant, 50% reductions in the numbers of eggs located in all tissues (FCA group) and in the urogenital system (BCG group) were noted. Moreover, the total number of excreted eggs was dramatically diminished by 60% and 77% in the BCG and FCA groups, respectively. These reductions reached 75% and 80% in the urines of vaccinated monkeys. Bladder pathology was also reduced in the animals displaying the lowest urinary egg excretions. There was no clear positive or negative correlate between antibody responses and individual levels of protection. Taken as a whole, our results show that Sh28GST was capable of significantly reducing S. haematobium worm fecundity in experimentally infected primates. Although FCA induced higher levels of protection, the efficacy of BCG as an adjuvant appeared sufficient to justify consideration of the future application of this new formulation as a vaccine against human urogenital schistosomosis.

Glutathione S-transferases of 28kDa as major vaccine candidates against schistosomiasis

For the development of vaccine strategies to generate efficient protection against chronic infections such as parasitic diseases, and more precisely schistosomiasis, controlling pathology could be more relevant than controlling the infection itself. Such strategies, motivated by the need for a cost-effective complement to existing control measures, should focus on parasite molecules involved in fecundity, because in metazoan parasite infections pathology is usually linked to the output of viable eggs. In numerous animal models, vaccination with glutathione S-transferases of 28kDa has been shown to generate an immune response strongly limiting the worm fecundity, in addition to the reduction of the parasite burden. Recent data on acquired immunity directed to 28GST in infected human populations, and new development to draw adapted vaccine formulations, are presented.

Definition and mapping of epitopes recognized by specific monoclonal antibodies to Schistosoma bovis 28 kDa glutathione S-transferase: relation with anti-egg viability immunity

Monoclonal antibodies to the 28kDa glutathione S-transferase of Schistosoma bovis have been constructed in mice and used to characterize the epitope(s) potentially implied in the induction of anti-fecundity and anti-egg viability immune responses. Among the MoAbs produced three were particularly studied: Sb4-50 (IgG2a) and Sb4-56 (IgG1) which inhibited Sb28GST activity and Sb4-10 (IgG1) which did not. The use of overlapping peptides covering the entire amino acid sequence of Sb28GST, allowed us to define the linear epitopes recognized by these anti-Sb28GST MoAbs. Amino acid residues 202-211 were recognized by both MoAbs Sb4-50 and Sb4-56 and MoAb Sb4-10 recognized amino acid residues 58-67. Their capacity to inhibit GST activity suggested binding to the active site or to neighbouring regions, which include the C-terminal domain (a.a. 190-211) of the protein. When passively transferred into BALB/c mice MoAbs induced a significant reduction in egg hatching and an increase in immature eggs. Effects on worm burdens were, however, variable and no clear-cut association between the inhibition of enzyme activity and anti-fecundity or anti-viability activities was recorded. Our data indicate that beside the anti-fecundity and anti-viability immunity related to the impairment of GST activity, immune response to epitopes located in other regions of the molecule also contribute to the reduction of egg viability.

Relationship of impairment of schistosome 28-kilodalton glutathione S-transferase (GST) activity to expression of immunity to Schistosoma mattheei in calves vaccinated with recombinant Schistosoma bovis 28-kilodalton GST

Sera from calves vaccinated with the recombinant Schistosoma bovis-derived 28-kDa glutathione S-transferase (28GST) and subsequently naturally or experimentally exposed to Schistosoma mattheei were studied for their content of specific immunoglobulin G (IgG) and IgA antibodies to recombinant S. bovis 28GST as well as for their capacity to inhibit the enzymatic activity of the antigen. The results were analyzed in regard to the presence (natural infection) or absence (experimental infection) of a protective effect(s) (reductions in worm burden, egg load, fecal egg counts, and excretion of viable eggs) toward S. mattheei challenge. Under such conditions, no differences in the IgG- and IgA-specific antibodies to recombinant S. bovis 28GST or in the ability to block the catalytic function of the antigen between the two groups were recorded. Nevertheless, correlation analysis between the specific antibody responses to recombinant S. bovis 28GST and the inhibition of GST activity suggested an association with IgG in experimentally infected vaccinated animals, while in naturally infected vaccinated calves, the inhibitory activity appeared to be linked to a greater degree with IgA. These results suggest that in contrast to schistosomiasis in humans, IgG antibodies in calves with schistosomiasis may exhibit inhibitory functions toward GST enzymatic activity or have a modulatory effect on IgA antibody properties. Furthermore, sera from animals immunized with recombinant S. bovis 28GST recognized the native S. mattheei 28GST and achieved comparable levels of inhibition of activity of recombinant S. bovis 28GST and S. matthei 28GST, indicating the presence of cross-reactive epitopes on these two molecules.

Murine Schistosoma bovis infection: analysis of parasitic and immune parameters

Humoral and cellular responses to Schistosoma bovis antigens have been evaluated over a period of 11 weeks in mice exposed to S. bovis cercariae and data analysed in the context of the parasitic parameters (worm and egg loads) recorded at days 30, 60 and 80 of the ongoing infection. Results revealed a decrease of worm burden, particularly marked for female worms, between day 60 and day 80 of infection suggesting a higher susceptibility of female schistosomes to attrition mechanisms. The B-cell response, studied by measuring the production of different isotypes, was directed against different stage specific antigens, with a predominance of IgG1 antibodies associated with a significant increase of IgA and IgE antibodies after egg deposition. The T-cell response, assessed after in vitro stimulation of splenocytes, showed a predominant production of Th-2 cytokines (IL-4, IL-5 and IL-10) occurring after egg laying. Interestingly in contrast to S. mansoni infection the Th-2 polarization did not seem to be exclusively triggered by egg-associated antigens since significant amounts of IL-10 were produced after stimulation with adult worm antigen preparation (SWAP) before the beginning of egg deposition.

Evaluation of antigens of Fasciola gigantica as vaccines against tropical fasciolosis in cattle

Vaccine trials were conducted in Brahman cross cattle evaluating the efficacy of 4 native antigens purified from adult Fasciola gigantica flukes, and 1 recombinant F. gigantica antigen, as vaccines against tropical fasciolosis. The antigens tested were native glutathione S-transferase, cathepsin L, paramyosin, fatty acid binding protein (FABP), and a recombinant FABP expressed in E. coli, and were formulated in 1 or more of several adjuvants (Quil A, Squalene Montanide 80, MF59-100, Auspharm, NAGO, polylactoglycolide microspheres, Algammulin, DEAE, Freund's). Vaccination induced low, moderate or high antibody titres to the various antigens which were dependent on the adjuvant. Low but significant reductions in fluke burdens (31%, P < 0.026) and fluke wet weight (36%, P < 0.041) were only observed in cattle vaccinated with the native FABP in Freund's adjuvant. There was no correlation between total antibody titres to FABP and protection. The protection observed in cattle vaccinated with native FABP of F. gigantica supports the notion that this class of proteins is a useful target for protection of animals against Fasciola and extends the efficacy of FABPs to the tropical liver fluke. This is the first report of vaccination of cattle against F. gigantica with a purified protein.

Anti-fecundity immunity to Schistosoma japonicum induced in Chinese water buffaloes (Bos buffelus) after vaccination with recombinant 26 kDa glutathione-S-transferase (reSjc26GST)

We have shown previously that immunisation of mice and pigs with recombinant 26 kDa GST (reSjc26GST) induces a pronounced anti-fecundity effect after experimental infection with Chinese Schistosoma japonicum. We report here that anti-fecundity immunity can also be induced against reSjc26GST in Chinese water buffaloes (Bos buffelus), important reservoir hosts for S. japonicum in China. Anti-Sjc26GST antibodies were produced in immunised buffaloes and, following challenge with S. japonicum cercariae, a 22.3% reduction in worm numbers was evident in vaccinated when compared with control animals. The anti-fecundity effect was characterised by a significant decrease in faecal egg output and eggs deposited in host tissues with those in the liver and intestine being reduced by about 50%. In addition to the anti-fecundity effect, reSjc26GST reduced by nearly 40% the egg-hatching capacity of S. japonicum eggs into viable miracidia. In terms of vaccination strategy, these effects would combine to diminish pathology in animals immunised with reSjc26GST and reduce transmission of schistosomiasis japonica.

Human immunodeficiency virus infection of xenografted SCID-beige mice

The application of C.B-17 SCID-Beige mice as an experimental animal system for the acceptance of human leukocyte xenografts, the establishment of functional human immune responses and infection with HIV has been assessed. Reconstitution efficiencies approaching 100% could be obtained by using 2 x 10(7) human peripheral blood lymphocytes (PBLs). Typical levels of human immunoglobulin in mouse blood reached 120 micrograms/ml within 2 weeks of reconstitution rising to a maximum in excess of 3 mg/ml by 5 weeks. Immunohistological examination of lung, spleen, lymph node and thymus tissue, derived from reconstituted mice, with human leukocyte specific monoclonal antibodies revealed the presence of human macrophages (CD68+), T cells (CD43+) and B cells (CD20+). The establishment of a functional immune system was demonstrated by the ability of reconstituted mice to respond to immunisation with KLH. Finally, reconstituted Hu-PBL-SCID-Beige mice were susceptible to infection with HIV-1 by intraperitoneal injection. These results indicate that SCID-Beige mice are a valuable tool for the generation of human/mouse chimeras and for the establishment of an in vivo HIV infection model. The results are compared with other similar model systems and are discussed in the context of animal models of HIV vaccine studies.

Vaccination of patas monkeys experimentally infected with Schistosoma haematobium using a recombinant glutathione S-transferase cloned from S. mansoni

The capacity of a recombinant glutathione S-transferase from Schistosoma mansoni (rSm28GST) to vaccinate primates (Erythrocebus patas) against a heterologous infection with Schistosoma haematobium has been tested. Two injections of the purified molecule with Muramyl-Di-Peptide (MDP) as adjuvant resulted in a high level antibody response in the five immunized animals and in a significant reduction in worm fecundity compared to the controls which received adjuvant alone. Mean levels of daily egg excretion in urine an faeces were reduced by respectively 55% and 74% although perfusion revealed that worm burdens were similar in both groups. The protective effect was long lasting since it was maintained up to the end of the experiment, 42 weeks after infection. Hatching rates and the numbers of intra-uterine eggs were also significantly affected by the vaccination. Tissue eggs were also drastically diminished in the urogenital system (-80%) but the reduction was not statistically significant. One animal was not protected by the immunization. There was a good correlation between parasitological data and the intensity of bladder lesions assessed by microscopic examination. Polypoid formations together with an intense exudation of the lamina propria were frequently seen in the controls but rarely in the vaccinated group where formation of scar tissue was predominant. These results underline the vaccine potential of the recombinant Sm28GST as a possible valuable prophylactic tool for the control of egg-induced pathology and transmission of African schistosomes.

Vaccination of sheep against Schistosoma japonicum with either glutathione S-transferase, keyhole limpet haemocyanin or the freeze/thaw schistosomula/BCG vaccine

The protective potential of glutathione S-transferase (GST), keyhole limpet haemocyanin (KLH) and the freeze/thaw (F/T) schistosomula/BCG vaccine was evaluated against Schistosoma japonicum in the natural sheep host. Groups of ten sheep each were vaccinated as follows: Group I: 2 x F/T 30,000 schistosomula+BCG 3 x 10(8) organisms, with a 2 week interval between vaccinations (F/T 'Low'). Group II: 3 x F/T 20,000 schistosomula+BCG 3 x 10(8), with 4 week interval (F/T 'High'). Group III: 2 x GST 0.24 mg+FCA (Freund's complete adjuvant) with 2 week interval (GST 'Low'). Group IV: 3 x GST 0.24 mg+FCA, with 4 week interval (GST 'High'). Group V: 2 x KLH 1.0 mg in phosphate-buffered saline (PBS), with 2 week interval (KLH 'Low'). Group VI: 3 x KLH 1.0 mg in PBS, with 4 week interval (KLH 'High'). Group VII: control (not vaccinated). Specific antibody, detected by GST-enzyme-linked immunosorbent assay (ELISA) and KLH-ELISA on the day after the last vaccination and 1, 2 and 3 weeks post-challenge, was found in all GST- or KLH-vaccinated groups. The same was found in F/T schistosomula-vaccinated groups against crude adult worm antigen (AWA). In Western blotting all GST-vaccinated sera recognized 26 kDa and 28 kDa bands on the challenge day and at 3 and 11 weeks post-challenge. Mean faecal egg counts between Weeks 6 and 10 post-challenge were reduced in a statistically significant way at five time points in the four groups, i.e. 83.38% (P < 0.005) in Group II, 49.29% (P < 0.025) in Group III, 47.9% (P < 0.05) and 71.15% (P < 0.01) in Group IV, 52.0% (P < 0.025) and 66.38% (P < 0.025) in Group VI. On autopsy and perfusion 1 week after the last faecal count, adult worm reductions were obtained of 40.36% (P < 0.05) in Group I, 37.26% (P < 0.025) in Group II, 24.73% (not significant) in Group III, 35.93% (P < 0.025) in Group IV, 27.46% (P < 0.05) in Group V and 33.81% (P < 0.01) in Group VI. Mean tissue egg densities were also reduced significantly in Groups III, IV and VI, especially in Group IV vaccinated animals. Mean liver egg granuloma diameters of the vaccinated groups were found to be less than those of the controls but there was no statistical significance.(ABSTRACT TRUNCATED AT 400 WORDS)

Anti-fecundity immunity induced in pigs vaccinated with recombinant Schistosoma japonicum 26kDa glutathione-S-transferase

We have recently reported (Liu et al. 1995) that immunization of mice with recombinant 26kDa GST (reSjc26GST) induces a pronounced anti-fecundity effect after experimental infection with Chinese Schistosoma japonicum. A similar vaccination trial was thus carried out on pigs, important reservoirs for schistosomiasis japonica, using purified, reSjc26GST and reSjp26GST from Schistosoma japonicum with alum as adjuvant; in general, similar results were obtained with the two sources of recombinant 26kDa GST. Some protection in terms of worm reduction, significant with males, against challenge infection was observed in vaccinated pigs. Moreover, prior to challenge, levels of specific anti-re26GST antibodies in the vaccinated pigs were significantly higher than in non-vaccinated pigs as determined by GST-ELISA. The most striking feature of the vaccine trial was the significant reduction in the number of eggs, especially mature eggs, in the livers of vaccinated animals. The results indicate that immunization with recombinant Sj26GST can provide some reduction in worm burden following exposure of pigs to reinfection with S. japonicum. In addition, reSj26GST can induce an anti-fecundity effect, thereby reducing pathology, coupled with a delay or interruption of the development of immature to mature eggs in the liver. As a consequence, vaccination with Sj26GST would also prove useful in affecting the transmission of schistosomiasis japonica.

Immunogenicity of genetically engineered glutathione S-transferase fusion proteins containing a T-cell epitope from diphtheria toxin

Glutathione S-transferase (GST) has been shown to induce a marginal antibody response in experimental animals as well as partial protection against a number of parasitic worms, including Schistosoma and Fasciola species. The objective of our study was to increase the immunogenicity of GST by adding heterologous T-cell epitopes at the carboxy terminus of the protein. We generated recombinant GST proteins by attaching one or three tandem repeats of a T-cell epitope of CRM197, a nontoxic variant of diphtheria toxin. This T-cell epitope encoding the region of amino acids 366 to 383 of CRM197, when contained in a GST fusion protein and/or after purification as a recombinant peptide, retained the ability to induce a CRM197-specific T-cell response. The fusion protein containing a single T-cell epitope induced a strong T-cell proliferative response to GST and also enhanced anti-GST antibody production in mice. The addition of three repeats of the epitope did not augment the responses when compared with the responses of GST itself. The results suggest that the addition of a single T-cell epitope to a larger protein like GST increases the immunogenicity of the protein.

The development of oral vaccines against parasitic diseases utilizing live attenuated Salmonella

Genetically defined, live attenuated Salmonella vaccines are proving useful both as oral vaccines against salmonellosis and for the development of multivalent vaccines based on the expression of heterologous antigens in such strains. Several candidate attenuated S. typhi strains are at present being evaluated as new single dose oral typhoid vaccines in human volunteers. The emergence of such a vaccine will facilitate the development of multivalent vaccines for humans. Many antigens from different infectious organisms have been expressed in attenuated Salmonella. A focus of this work has been on developing vaccines against parasitic diseases. This review will summarize the efforts that have been made in this area.

Construction, expression, and immunogenicity of the Schistosoma mansoni P28 glutathione S-transferase as a genetic fusion to tetanus toxin fragment C in a live Aro attenuated vaccine strain of Salmonella

A vector has been constructed to allow genetic fusions of guest antigens via a hinge domain to the C terminus of the highly immunogenic C fragment of tetanus toxin. A fusion has been constructed with the gene encoding the protective 28-kDa glutathione S-transferase (EC 2.5.1.18) from Schistosoma mansoni. The recombinant vector has been electroporated into the nonvirulent Salmonella typhimurium aroA live vaccine strain SL3261. The corresponding chimeric protein is stably expressed in a soluble form in Salmonella as evaluated by Western blotting with fragment C and glutathione S-transferase antisera. Mice immunized intravenously with a single dose of the live recombinant bacteria elicit antibodies to both fragment C and glutathione S-transferase as detected by enzyme-linked immunosorbent assays. Furthermore, all of the mice were solidly protected when challenged with lethal doses of either tetanus toxin or the virulent Salmonella typhimurium strain C5. Mice have also elicited antibodies to fragment C and glutathione S-transferase after oral immunization. It may be that a live trivalent vaccine against typhoid, tetanus, and schistosomiasis is feasible.

Vaccination of goats against the trematode Schistosoma bovis with a recombinant homologous schistosome-derived glutathione S-transferase

We assayed the vaccine potentialities of a recombinant S. bovis-derived glutathione S-transferase (rSb28GST), member of a molecular family already shown to have protective capacities in the S. mansoni and S. japonicum models. Injection of the rSb28GST in Freund's Complete Adjuvant resulted in good specific IgG responses allowing all the animals to display high antibody titres on the day of experimental challenge with S. bovis cercariae. No statistically significant differences were observed in the faecal egg output. Although tissue egg counts in vaccinated animals were lower than in controls, the difference was not statistically significant, apart from the number of eggs trapped in the liver (P < 0.05). Likewise, PCV values remained parallel between the two groups. However, immunized goats gained 1.4 kg of body weight throughout the experiment whereas controls lost 1.2 kg (P < 0.05). In addition, the mean worm burden, assessed by perfusion 20 weeks after infection, was significantly reduced by 48% in the vaccinated group, the sex ratio being unaffected. It appears that a recombinant homologous protein can affect, in a natural host, the course of an experimental infection with a local strain of S. bovis, by affecting worm viability but not fecundity. These results also point to the striking differences in the effect of vaccination according to animal species. Because it has the capacity to prevent growth impairment due to schistosome pathogenicity, the molecule can be proposed as a valuable tool in the development of vaccine-based control programs in endemic areas.

The effect of multiple transfers of immune serum on maturing Schistosoma bovis infections in calves

To investigate the role of humoral factors in immunity, serum from cattle with naturally acquired immunity to Schistosoma bovis was injected intraperitoneally into calves that had been infected 4 weeks earlier with 10,000 S. bovis cercariae. Serum was injected weekly until 12 weeks post-infection to a total of 4,500 ml per calf and controls received normal serum or saline. No significant difference in worm or in faecal or tissue egg counts were seen in the three groups of recipients in spite of the observation that the serum donors had proved highly resistant to experimental challenge. In a second experiment, pre-infection or 4-, 8- or 12-week post-infection serum from donors given a single experimental infection with 10,000 S. bovis cercariae was injected intraperitoneally into groups of calves that had been infected 4 weeks earlier with 20,000 S. bovis cercariae. Injections were given weekly up to week 10 post-infection to a total of 2000-3500 ml serum per calf. In calves injected with immune serum there was a reduction in faecal and tissue egg counts and in the numbers of worms recovered as compared with the controls. In recipients of 8- and 12-week serum the reductions in faecal and tissue egg counts were higher than those in worm recovery, suggesting that 8- and 12-week post-infection sera contained factors capable of causing, in addition to worm death, suppression of worm fecundity. This provides further evidence of the importance of fecundity suppression in immunity to schistosomiasis.