Identification by monoclonal antibody of a major (28 kDa) surface membrane antigen of Schistosoma mansoni

Novel immunomic technologies for schistosome vaccine development

Schistosomiasis remains one of the most common human helminthiases, despite the availability of an effective drug against the causative parasites. Drug treatment programmes have several limitations, and it is likely that a vaccine is required for effective control. While decades of vaccine development have seen the discovery and testing of several candidate antigens, none have shown consistent and acceptable high levels of protection. The migrating larval stages are susceptible to immunity, however few larval-specific antigens have been discovered. Therefore, there is a need to identify novel larval-specific antigens, which may prove to be more efficacious than existing targets. Immunomics, a relatively new field developed to cope with the recent large influx of biological information, holds promise for the discovery of vaccine targets, and this review highlights some immunomic approaches to schistosome vaccine development. Firstly, a method to focus on the immune response elicited by the important and vulnerable larval stage is described, which allows a targeted study of the immunome at different tissue sites. Then, two high-throughput arrays are discussed for the identification of protein and carbohydrate antigens. It is anticipated that these approaches will progress vaccine development against the schistosomes, as well as other parasites.

Novel therapeutic and prevention approaches for schistosomiasis: review

Schistosomiasis is a debilitating disease affecting approximately 600 million people in 74 developing countries, with 800 million, mostly children at risk. To circumvent the threat of having praziquantel (PZQ) as the only drug used for treatment, several PZQ derivatives were synthesized, and drugs destined for other parasites were used with success. A plethora of plant-derived oils and extracts were found to effectively kill juvenile and adult schistosomes, yet none was progressed to pre- and clinical studies except an oleo-gum resin extracted from the stem of Commiphora molmol, myrrh, which action was challenged in several trials. We have proposed an essential fatty acid, a component of our diet and cells, the polyunsaturated fatty acid arachidonic acid (ARA) as a remedy for schistosomiasis, due to its ability to activate the parasite tegument-bound neutral sphingomyelinase, with subsequent hydrolysis of the apical lipid bilayer sphingomyelin molecules, allowing access of specific antibody molecules, and eventual worm attrition. This concept was convincingly supported using larval and adult Schistosoma mansoni and Schistosoma haematobium worms in in vitro experiments, and in vivo studies in inbred mice and outbred hamsters. Even if ARA proves to be an entirely effective and safe therapy for schistosomiasis, it will not prevent reinfection, and accordingly, the need for developing an effective vaccine remains an urgent priority. Our studies have supported the status of S. mansoni calpain, glutathione-S-transferase, aldolase, triose phosphate isomerase, glyceraldehyde 3-phosphate dehydrogenase, enolase, and 2-cys peroxiredoxin as vaccine candidates, as they are larval excreted-secreted products and, contrary to the surface membrane molecules, are entirely accessible to the host immune system effector elements. We have proposed that the use of these molecules, in conjunction with Th2 cytokines-inducing adjuvants for recruiting and activating eosinophils and basophils, will likely lead to development and implementation of a sterilizing vaccine in a near future.

Induction of type 2 responses by schistosome worms during prepatent infection

During natural schistosome infection, the induction of T helper type 2 (Th2) responses has been ascribed to parasite eggs, because exposure of the host to this life-cycle stage elicits a polarized Th2 response to egg antigens. In the present study, we show that schistosome worms also elicit systemic, antigen-specific type 2 responses during prepatent infection, before egg deposition begins. CD4(+) T cells producing interleukin (IL)-4 were induced by both male and female worms during single-sex infections, demonstrating that this response is independent of exposure to eggs. The Th2 response was accompanied by production of immunoglobulin E and the sensitization of circulating basophils to produce additional IL-4 in response to schistosome antigens. Together, our data show that schistosome worms establish an immunologic milieu where CD4(+) T cells and basophils are both primed to produce IL-4 before eggs are laid, suggesting that worms play a role in establishment of the Th2 response that is critical for host survival and parasite transmission.

Schistosoma mansoni triose phosphate isomerase peptide MAP4 is able to trigger naïve donor immune response towards a type-1 cytokine profile

We evaluated the ability of naïve monocyte-derived dendritic cells (DC) to sensitize autologous peripheral blood mononuclear cells (PBMC) to the schistosome vaccine candidate MAP4 using a priming in vitro (PIV) assay. MAP4 is a multiple antigen peptide containing B- and T-cell epitopes derived from the glycolytic enzyme triose phosphate isomerase. PBMC primed and restimulated with MAP4 first and secondary recalls (MAP4 PIV cells) were examined for cell phenotype and cytokine production. We found that after the first recall stimulation with MAP4, the major cell population was predominantly CD4(+) T-cell subsets (68.5%), CD8(+high) (16%) and CD19(+) (10%). Additionally, MAP4 PIV cells significantly expressed CD4(+)-HLA-DR(+), -CD54(+), -CD45RO(+) (P < 0.0001) and -CD25(+) (P < 0.0004) together with significant expression of CD80(+) on CD19(+) B cells (P < 0.007). Cytokine production from activated MAP4 PIV cells was predominantly Th1-like, consisting mainly of IFN-gamma. Interestingly, IFN-gamma production was suppressed when Schistosoma mansoni-soluble egg antigen (SEA) was added to a MAP4 PIV cell culture. Furthermore, addition of MAP4 to a SEA PIV cell culture significantly reduced secretion of IL-10. The present findings add to the knowledge gained from studies in the mouse model, and our results show that naïve donor DC, sensitized with MAP4, were able to prime and clonally expand MAP4-specific T cells towards a Th1-type response.

Schistosoma japonicum triose-phosphate isomerase plasmid DNA vaccine protects pigs against challenge infection

The protective efficacy of a Schistosoma japonicum, Chinese strain, triose-phosphate isomerase (TPI) plasmid DNA vaccine was examined in naïve pigs. Pigs were vaccinated with the TPI DNA-plasmid alone, or in conjunction with IL-12 as pcDNA3.1-P35, pcDNA3.1-P40 plasmids via intramuscular injection. Control pigs were immunized with equivalent amounts of pcDNA3.1. Pigs were immunized 3 times at 21-day intervals and challenged 30 days after the final boost. Forty-five days post-challenge, pigs were sacrificed and perfused to compare adult worm burdens, female worm burdens, liver egg burdens and granuloma size. We found that pigs vaccinated with SjCTPI DNA alone had adult worm burdens reduced by 48.3% and that a further decrease in adult worm burdens was not seen in the group vaccinated with SjCTPI DNA in conjunction with IL-12 (46.2% reduction). The SjCTPI DNA vaccines had a more pronounced effect on reducing female worm burdens i.e. 53.6% SjCTPI alone and 59.6% for SjCTPI+IL-12. Vaccination with SjCTPI-DNA reduced liver eggs by 49.4% and this response was significantly enhanced by the addition of IL-12 (65.8% reduction in liver eggs). In addition to the dramatic protective effects seen in vaccinated pigs, we also noted that granuloma size was reduced by 42% in both groups. Thus, vaccination of pigs and other large animals in China with SjCTPI DNA vaccine will likely reduce transmission by reducing adult worm burdens and worm egg output and simultaneously reduce hepatic egg-associated pathology.

Protective immunity and vaccines to schistosomiasis

Human schistosomiasis is a major health problem in endemic areas and it is estimated that world-wide more than 200 million people are infected. Active infection, exposure to attenuated parasites, or appropriate immunization with parasite extracts or purified antigens can lead to partial protection against challenge infections. Studies of the humoral and cellular mechanisms responsible for these forms of resistance, and the definition, purification or production of recombinant moieties responsible for the induction of immunity are currently the focus of considerable research efforts.

Proteomic identification, nucleotide sequence, heterologous expression and immunological reactivity of the triosephosphate isomerase of Paracoccidioides brasiliensis

An antigen of Paracoccidioides brasiliensis (Pb) was gel isolated and characterized. Endoproteinase Lys-C-digested peptides of the purified protein, which presented a molecular mass of 29 kDa and pI of 5.8, were subjected to sequence analysis of their amino acids. Searches at databases comparing the sequence of amino acids from the three peptides of the native protein revealed strong homology to triosephosphate isomerase (TPI: E.C. 5.3.1.1) from several sources. The complete cDNA and gene encoding PbTPI were obtained and both contained an open reading frame predicted to encode a 249-amino acid protein that presented all the peptides characterized in the native PbTPI. The Pbtpi gene contained six exons interrupted by five introns. Analysis performed with the deduced PbTPI suggested its usefulness in providing phylogenetic relatedness, as well as evidencing the correlation between the phylogeny provided by the deduced protein and intron positions in the cognate genes. The immunological reactivity of PbTPI was examined. The complete coding cDNA of PbTPI was overexpressed in an Escherichia coli host to produce high levels of recombinant fusion protein with glutathione S-transferase (GST) that had been purified by affinity chromatography. The purified recombinant TPI was recognized by sera of patients with confirmed paracoccidioidomycosis and not by sera of healthy individuals. Thus, recombinant PbTPI can be a valuable addition to the still small arsenal of P. brasiliensis immunoreactive proteins, which could be tested for incorporation into assays for serodiagnosis of the disease.

Human immune responses to Schistosoma mansoni vaccine candidate antigens

To determine the naturally occurring immunological responses to the Schistosoma mansoni antigens paramyosin, IrV-5, Sm-23 (MAP-3), and triose phosphate isomerase (MAP-4), a total of 119 subjects from an area of endemicity for schistosomiasis, including "resistant" subjects (n = 17) were evaluated. Specific immunoglobulin G1 (IgG1), IgG2, IgG3, IgG4, and IgA levels for each of the antigens and the cytokine profile in culture supernatants from antigen-stimulated peripheral blood mononuclear cells (PBMC) were determined. Although all the subjects had a high degree of contaminated water exposure, their infection levels were variable (0 to 1,128 eggs/g of stool). There were direct correlations between infection levels and levels of SWAP- and paramyosin-specific IgG1 and IgG4 (P < 0.05). However, an inverse correlation between infection levels and specific IgG2 to IrV-5 (P < 0.01) was observed. The evaluation of the cytokine profile (interleukin 5 [IL-5], IL-10, gamma interferon [IFN-gamma], and tumor necrosis factor alpha) in response to these antigens showed inverse correlations between the degree of infection and IFN-gamma levels in PBMC supernatants stimulated with paramyosin (P < 0.05) and IrV-5 (P < 0.01). Additionally, inverse correlations between the degree of infection and IL-5 levels in MAP-3- and MAP-4-stimulated PBMC supernatants (P < 0.01) were found. Logistic regression analysis was performed to adjust the results of cytokine profile by age. IL-5 production in MAP-3-stimulated PBMC supernatants was associated with lower infection levels (odds ratio = 11.2 [95% confidence interval, 2.7 to 45.8]).

Identification of a candidate vaccine peptide on the 37 kDa Schistosoma mansoni GAPDH

A previous study performed in adolescents living in an area endemic for Schistosoma mansoni in Brazil has shown that a 37 kDa schistosome surface antigen is a selective target for antibodies in sera from those who were resistant to reinfection. This antigen was shown by molecular cloning to be the schistosome GAPDH. The aim of the present work was to assess whether peptides corresponding to GAPDH antigenic determinants could be used in a subunit vaccine. Five B cell and two T cell epitopic regions were identified on Sm37-GAPDH. One of the B cell determinants (Sm37-5, aa 268-289) is highly antigenic in human infections and antibody reactivity toward this determinant is associated with resistance to reinfection. Mice and rats immunized with Sm37-5 were partially protected against a challenge infection, indicating that this peptide can induce protective immunity. Analysis of Sm37-5 amino acid sequence indicated that this antigenic determinant is likely conserved among other pathogenic strains of schistosome (S. haematobium, S. intercalatum and S. japonicum), although it shows major amino acid differences with the corresponding human GAPDH sequence. All together these results indicate that Sm37-5 should be considered as a candidate component for an anti-schistosome subunit vaccine.

A vaccine against Asian schistosomiasis: the story unfolds

The development of an effective vaccine against the Asian schistosome is at a critical stage. Despite the fact that progress has been relatively slow, the successful use in animals of attenuated vaccines combined with recent encouraging results using defined native and recombinantly derived Schistosoma japonicum antigens, suggests that development of a safe and effective vaccine is feasible. This review examines current progress aimed at achieving this objective, and a summary is provided of recent results obtained with the most encouraging vaccine antigens. When available for wide-scale use, it is envisaged that the vaccine would be applied in the first instance, at least in China, in the veterinary context (to impact on human transmission) and then, perhaps, if required, clinically (to prevent or reduce disease). The search for the final product is likely to be demanding, and funding issues pertaining to Good Manufacturing Practice-scale-up of the vaccine for the required extensive veterinary coverage, and to support any future human trials, will need to be resolved. As such, we may still have to wait some time before the ultimate vaccine, possibly comprising a cocktail of several molecules, is available. Even then, the vaccine would probably be used optimally as one component of an integrated programme of schistosomiasis control that would include effective and well-tested approaches, such as health education and targeted chemotherapy.

Bacterial expression and characterization of functional recombinant triosephosphate isomerase from Schistosoma japonicum

The dimeric enzyme triosephosphate isomerase (TPI) converts glyceraldehyde-3-phosphate to dehydroxyacetone phosphate, a key reaction in glycolysis. Previous studies of the native enzyme in the human blood-flukes belonging to the genus Schistosoma have indicated that TPI is a promising anti-schistosome vaccine antigen. However, a recombinant form of the enzyme is required as an alternative to the impractical option of using biochemically purified TPI obtained from worm tissue for large-scale vaccine use. We previously cloned and sequenced a full-length cDNA encoding the TPI of the Asian (Chinese strain) schistosome Schistosoma japonicum (SjcTPI). We now report very high level bacterial expression of this cDNA and the subsequent purification of the recombinant protein to >98% homogeneity under nondenaturing conditions. The recombinant SjcTPI (re-SjcTPI) was shown to be enzymatically active with a specific activity of 7687 units/mg protein, an activity higher than that of commercially obtained porcine TPI tested concurrently under the same assay conditions. The K(m) value for the re-SjcTPI using glyceraldehyde-3-phosphate as substrate was 406.7 microM, which is similar to the K(m) values reported for the yeast enzyme and various mammalian TPIs. With the availability of substantial amounts of enzymatically active and readily purified re-SjcTPI made in bacteria we can now test whether the recombinant protein can induce a similar level of protection in vaccination/challenge experiments as the native, biochemically purified enzyme.

Induction of protective immunity against Schistosoma mansoni infection by antigens purified from PIII, a fraction of adult worm, associated to the downregulation of granuloma formation

This study was performed in order to define Schistosoma mansoni antigens able to function as modulator agents in BALB/c mice granulomatous hypersensitivity to parasite egg. The antigens P-24, P-35 and P-97 were purified by affinity chromatography from a fraction of S. mansoni adult worm antigenic preparation, denominated PIII, involved in the inhibition of granulomatous response to eggs. Immunization of mice with these antigens, in the presence of Corynebacterium parvum and Al(OH)3 as adjuvant, induced a significant protection degree against challenge infection, as observed by the decrease on worm burden recovered from portal system. In vitro blastogenesis assays revealed that purified antigens were able to induce significant proliferation of spleen cells from S. mansoni-infected mice. This protection was correlated to significant decrease in granuloma size induced by PIII. From these results, we concluded that PIII preparation contains antigens capable of mediating protective anti-parasite immunity and down-regulating granulomatous hypersensitivity to S. mansoni eggs.

Characterization of fractionated Schistosoma mansoni soluble adult worm antigens that elicit human cell proliferation and granuloma formation in vitro

Soluble adult worm antigens (SWAP) of Schistosoma mansoni were fractionated by fast protein liquid chromatography (FPLC) system, using Q-Sepharose anion-exchange resin, in order to characterize antigenic fractions that may elicit cell responses in human schistosomiasis. SWAP fractions were eluted by 20 nM Tris-HCl solution (pH 9.6) with an increasing gradient of 1 M NaCl. The FPLC system was able to resolve 6 fractions, enumerated I to VI, according to the NaCl gradient. The analysis of each fraction on SDS-PAGE showed that fractions I to IV were constituted by multiple protein bands with M, ranging from 21 to > 200 kDa. Large amounts of nucleic acids were evidenced in fractions V and VI, as revealed by ethidium bromide staining of agarose electrophoresis gels. Using ELISA, it was shown that sera from chronic schistosomiasis patients contained antibodies that recognized antigens in practically all fractions. Studies were designed to investigate the capacity of these fractions to induce cell proliferation and granuloma formation. It was demonstrated that fraction III stimulated significant proliferative response of peripheral blood mononuclear cells (PBMC) from chronic schistosomiasis patients. However, fraction III coupled to polyacrylamide beads induced small granuloma formation in vitro, whereas beads coated with fractions I, II and V were able to induce significant granuloma reactions.

Cloning of the gene for phosphoglycerate kinase from Schistosoma mansoni and characterization of its gene product

As molecules on the surface or associated with the outer covering (tegument) of Schistosoma mansoni are a major focus as potential vaccine candidates, affinity purified antibodies which are specific to the tegumental antigens were used to immunoscreen a lambda gt11 S. mansoni cercarial cDNA library. One of the identified clones was found to encode the glycolytic enzyme phosphoglycerate kinase (PGK, EC 2.7.2.3). The 1.5-kb cDNA clone has a single open reading frame encoding 416 amino acids and exhibits over 60% identity to PGKs from a number of eukaryotic species. Recombinant S. mansoni PGK (SmPGK) was overexpressed in Escherichia coli, purified, and shown to have PGK enzyme activity. Native protein affinity purified from S. mansoni adult worms was shown by microsequencing to have the same amino-acid sequence as deduced from the cDNA sequence, thus confirming the cDNA clone we identified encodes S. mansoni phosphoglycerate kinase. Antibodies localize the native SmPGK to various tissues including the tegument of 3-h schistosomula and 42-day adult worms.

Effect of chemotherapy on immune response to egg antigens of Schistosoma mansoni in chronically infected children from areas of low transmission

In an attempt to identify antigenic molecules from Schistosoma mansoni eggs, a serological study was performed on children of a Venezuelan town (Caraballeda) in which the transmission of schistosomiasis had been interrupted two years prior to sera sampling. Infected children received treatment with Praziquantel and, based on the disappearance of eggs in the stools plus negativization of the circumoval precipitin test (COPT) one year after treatment, they were classified as either responders or non-responders to chemotherapy. Western blots of soluble egg antigen (SEA) with a very sensitive chemiluminescent substrate were performed. Sera from responder children recognized a 25 kDa band of SEA which diminished significantly after treatment. This was less frequent in non-responder children. When the sera of responder and non-responder children were compared before treatment, we found that the recognition of the 40 and 41 kDa proteins could be predictive of response to chemotherapy. All these antigens, used in ELISA-type techniques, might be of importance in the evaluation and follow-up of large scale schistosome control programmes.

Isotype responses to candidate vaccine antigens in protective sera obtained from mice vaccinated with irradiated cercariae of Schistosoma mansoni

In experimental schistosomiasis, sera of mice multiply vaccinated with radiation-attenuated cercariae of Schistosoma mansoni passively transfer resistance against cercarial challenge to naive mice. To further characterize these sera, we tested their protective capacities in two mouse strains (C57BL/6J and CBA/J) and compared the antigen-specific isotype compositions of the different protective sera by means of the enzyme-linked immunosorbent assay. By using an array of purified schistosomal antigens, the patterns of antibody titers and isotypes differed for each experimental group and antigen. In the most-protective C57BL/6J sera, high levels of immunoglobulin G1 (IgG1), IgG2a, and IgG2b bound to heat shock protein 70 and the integral membrane protein Sm23, whereas recognition of these antigens by less-protective CBA/J sera was lower. Glutathione S-transferase (GST) was recognized predominantly by IgM antibodies of all vaccinated groups, and a significant portion of this response was directed against carbohydrate epitopes. Antibodies specific for triosephosphate isomerase, paramyosin, and Sm32 (hemoglobinase) were present in less-protective sera and thus seem less relevant for passive transfer of resistance. The results of this study suggest a contribution of IgG antibodies specific for heat shock protein 70 and Sm23, and possibly a contribution of GST-specific IgM antibodies, to the protective effect of sera from C57BL/6J mice vaccinated with irradiated cercariae.

Fractionation of schistosome antigens by high performance electrophoretic chromatography and their screening for the ability to induce Th1 lymphocyte activity

The technique of high performance electrophoretic chromatography (HPEC) has been used to fractionate soluble antigens from adult Schistosoma mansoni worms on the basis of molecular weight (MW), prior to screening for their ability to stimulate T lymphocyte activity. Approximately 250 micrograms of protein were separated by continuous electrophoresis through an SDS polyacrylamide gel into 30-50 aqueous samples of minimal volume (80 microliters). Each consecutive sample contained a limited number of proteins of progressively greater MW, although the resolution of the fractionation was affected by a number of factors including acrylamide concentration, gel length, gel diameter and electrophoretic current. Following the extraction of SDS using Calbiosorb resin, the aqueous fractions were used directly to stimulate cultures of lymphocytes taken from the lymph nodes of infected or vaccinated mice. The most promising fractions were those containing proteins which induced the release of high levels of interferon-gamma relative to the extent of proliferation. This suggests that these proteins are good inducers of Th1 lymphocyte activity.

cDNA cloning and functional expression of the Schistosoma mansoni protective antigen triose-phosphate isomerase

M.1 monoclonal antibody has previously been shown to passively transfer partial resistance to schistosome infection within mice and to recognize a 28-kDa antigen that has peptide sequence homology with triose-phosphate isomerase (TPI; D-glyceraldehyde-3-phosphate ketol-isomerase, EC 5.3.1.1). We have now isolated the complete coding DNA for Schistosoma mansoni TPI and confirmed that this cDNA encodes the 28-kDa antigen recognized by M.1. The predicted translation product has strong homology with other TPIs, particularly from higher eukaryotes, and the sequence homology is greatest in regions known to form the active site. The complete coding DNA has been expressed within an Escherichia coli host to produce high levels of soluble, recombinant S. mansoni TPI protein. The product is recognized and purified by the M.1 antibody and is a functional TPI with an intrinsic specific activity comparable to that of rabbit and yeast TPI.

Schistosome vaccines

Schistosomiasis control currently relies primarily on chemotherapy which is both expensive and temporary. There is an urgent need for an effective vaccine. Studies in animal models and man have demonstrated the existence of protective immunity. Antibody-dependent cell-mediated cytotoxicity mechanisms involving eosinophils and macrophages have been implemented in destruction of the parasites. Antigens expressed on the surface of the schistosomulum are among the targets of protective immune responses. Vaccines comprising recombinant antigens are now being tested in vivo for their capacity to evoke protective responses. Live oral vaccines based on attenuated Salmonella expressing schistosomular surface antigens are being developed.

The functional and immunological significance of some schistosome surface molecules

The molecules discussed in this review include some of the leading vaccine candidates in schistosomiasis: the glutathione S-transferases, triose-phosphate isomerase, glyceraldehyde-3-phosphate dehydrogenase and the 23 and 25 kDa surface integral membrane proteins. Mark Wright, Kathy Davem and Graham Mitchell highlight the possible biological roles and immunological relevance of these molecules.

Production and characterization of human monoclonal antibodies against Schistosoma mansoni

We have produced a panel of human monoclonal antibodies (MoAb) from patients infected with Schistosoma mansoni in order to analyse more carefully the human immune response to this helminth infection. This study describes the production, characterization and analysis of these MoAbs. Briefly, peripheral blood mononuclear cells from chronically infected patients were (1) isolated and stimulated with parasite antigens in vitro, (2) positively selected for B-cells on anti-Ig columns, and (3) then transformed with Epstein-Barr virus (EBV). Once EBV cell lines were established, they were selected for anti-S. mansoni antibodies using an ELISA, cloned, retested and then fused with the mouse-human heteromyeloma SHM-D33. In this study, we describe five MoAbs which have different antigenic specificities for life-cycle stages based on ELISA to soluble crude antigen preparations, membrane immunofluorescence on whole intact organisms, and immunofluorescent staining of cryostat frozen sections. The importance of these reagents with regard to the human immune response to S. mansoni is currently being evaluated.

Schistosoma mansoni: identification and characterization of schistosomula polypeptides

Schistosomula proteins separated by a two-dimensional (NEPHGE) gel system identify 94 major silver-stained polypeptides. When compared to polypeptides similarly separated from cercariae and adult worms; cercariae share the same polypeptides as schistosomula, adult worms share ca. 60% of the polypeptides. A group of five schistosomula polypeptides 15-31 kDa (apparent pI 8.2-8.9) was not found in adult worm extracts. To identify which polypeptides were immunogens, Western blots of the NEPHGE gels were probed with sera either from humans with chronic schistosomiasis or from mice vaccinated with irradiated cercariae. For characterization studies, polyclonal antibodies were made against the five schistosomula-specific and selected immunogenic polypeptides by immunizing mice with silver-stained spots removed from NEPHGE gels. We show that the polyclonal serum against a polypeptide of 12.5 kDa and an apparent pI of 6.70 mediated complement and eosinophil-dependent killing of schistosomula in an in vitro assay. Epitopes recognized by antibody against the 12.5-kDa polypeptide show a diffuse distribution and are found on flame cells of the excretory system of the schistosomula.

Purification and immunochemical characterization of a 22 kilodalton surface antigen from Schistosoma mansoni

A 22 kDa antigen (Sm22) was purified from schistosomula membrane extracts by immunoaffinity chromatography with monoclonal antibody M.2. Western blotting suggested that the epitope bound by M.2 required a specific conformational folding of the molecule, which was sensitive to reducing agents. Two-dimensional electrophoresis of purified Sm22 demonstrated that the single 22 kDa protein recognized by M.2 on one-dimensional gel analysis was composed of at least two isomorphs. Additional Western blotting showed that Sm2 was one of the major antigens recognized by mouse anti-irradiated cercariae serum, and that this same serum recognized at least one epitope which was not sensitive to reducing agents. The mice vaccinated with irradiated cercariae were shown to be 75% protected from cercarial challenge. Sera from a rabbit immunized with Sm22 contained antibodies which bound to the surface of schistosomula and detected a single protein at 22 kDa by immunoprecipitation or Western blot. The rabbit anti-Sm22 sera also immunoprecipitated a 22 kDa in vitro translation product, indicating that at least one epitope on Sm22 is not dependent on glycosylation.

Immunity to schistosomes: progress toward vaccine

Among the major parasitic infections, schistosomiasis may be the most promising candidate for human vaccination. Information about mechanisms of immunity, gained mainly from experimental models but likely to be relevant to human infection, indicates a dynamic balance between protective and regulatory (blocking) mechanisms. Besides cell-mediated responses leading to macrophage activation, antibody-dependent cell-mediated cytotoxicity systems involving precise antibody isotypes and nonlymphoid cells (mononuclear phagocytes, eosinophils, and platelets) appear to be essential effectors of immune attack. The slow development of immunity in humans seems related to the production of antibodies that cross-react with schistosomulum surface antigen and block the binding of antibodies of the effector isotype. Schistosomes that survive in the bloodstream and produce chronic infections may evade the immune system as a result of intrinsic changes in membrane susceptibility and of transient expression of target antigens; at other stages of the parasite life cycle, cross-reactive molecules may be secreted that play an essential role in the induction of immunity. Several schistosome proteins have been characterized as candidates for vaccination. Among these, an antigen of 28 kilodaltons has been cloned and shown to be immunogenic in humans and protective in mice, rats, and baboons.

Schistosoma spp.: progress toward a defined vaccine

During the last 2 decades, much was learned concerning the nature of acquired immunity in schistosomiasis, under experimental as well as field conditions. The knowledge is being applied now to design of a defined vaccine against this major parasitic disease. Several Schistosoma spp. antigens have been purified and shown to potentially protect in vivo. Determination of a method for presenting these antigens that will induce an optimal combination of cellular and humoral immune responses remains a critical step in vaccine development.

Molecular cloning of a protective antigen of schistosomes

The complementary DNA sequence encoding the Mr 28,000 antigen of Schistosoma mansoni has been isolated and expressed in Escherichia coli. Experimental vaccination of rats, hamsters and monkeys with a recombinant fusion protein induces a strongly cytotoxic antibody response. Immunization of rats and hamsters with this protein leads to significant protection against a natural challenge infection with live cercariae.