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

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.

Characterization and cloning of T24, a Taenia solium antigen diagnostic for cysticercosis

The third and final diagnostic antigen of the lentil lectin purified glycoproteins (LLGP) extracted from the larval stage of Taenia solium has been characterized, cloned, and expressed. T24 is an integral membrane protein that belongs to the tetraspanin superfamily. It migrates at a position corresponding to 24-kDa and as a homodimer at 42-kDa. Antibodies from cysticercosis patients recognize secondary structure epitopes that are dependent upon correctly formed disulfide bonds. A portion of T24, the large, extracellular loop domain, was expressed in an immunologically reactive form in insect cells. When tested in a Western blot assay with a large battery of serum samples, this protein, T24H, has a sensitivity of 94% (101/107), for detecting cases of cysticercosis with two or more viable cysts, and a specificity of 98% (284/290). The identification and expression of T24H sets the stage for the development of an ELISA suitable for testing single samples and for large-scale serosurveys that is not dependent upon the isolation and purification of antigens from parasite materials.

Immunization with plasmid DNA encoding the integral membrane protein, Sm23, elicits a protective immune response against schistosome infection in mice

Schistosomes are helminth parasites infecting at least 200 million people worldwide. In this study, we evaluated the feasibility of using a nucleic acid vaccine to induce protective immune responses to the Schistosoma mansoni integral membrane protein Sm23. C57BL/6 mice were immunized by intramuscular injection in three separate vaccination trials. ELISA and Western Blot analyses indicated that mice immunized with a DNA plasmid construct encoding Sm23 (Sm23-pcDNA) generated specific IgG for Sm23, while sera from mice immunized with the control pcDNA plasmid did not. The vaccine elicited IgG(2a), and IgG(1) antibody isotypes. We also tested the adjuvant activity of IL-12 and IL-4 on humoral responses to Sm23. Co-immunization with plasmid encoding IL-12 did not affect the level of anti-Sm23 IgG(2a), but did reduce the IgG(1) level. In contrast, co-injection with a plasmid encoding IL-4 significantly reduced the level of anti-Sm23 IgG(2a), while the level of IgG(1) was largely unchanged. Importantly, the Sm23-pcDNA vaccine provided statistically significant levels of protection against challenge infection (21-44%, P<0.001-0.02). Co-administration of plasmids encoding either IL-12 or IL-4 did not significantly enhance this protective effect.

Molecular characterisation of mouse and human TSSC6: evidence that TSSC6 is a genuine member of the tetraspanin superfamily and is expressed specifically in haematopoietic organs

Previous analyses of the murine and human TSSC6 (also known as Phemx) proteins were not carried out using the full length sequence. Using 5'-RACE and cDNA library screening, we identified an additional 5' sequence for both the murine Tssc6 cDNA and its human homologue TSSC6. This novel sequence encodes a 5' exon encoding an in frame, upstream start codon, an N-terminal cytoplasmic domain and a transmembrane domain. The deduced, and now full length, murine and human TSSC6 proteins contained four hydrophobic regions together with other features characteristic of the tetraspanin superfamily. Computational analyses of the full length sequences show that TSSC6 is a genuine, albeit relatively divergent member of this superfamily. Using RNA from a number of mouse tissues, we identified seven splice variants of Tssc6. Splice variants of the human gene were also detected. Tssc6 expression was detected early in embryogenesis in primitive blood cells and was confined to haematopoietic organs in the adult mouse. Tssc6 expression was detected in many haematopoietic cell lines and was highest in cell lines of the erythroid lineage.

The L6 membrane proteins--a new four-transmembrane superfamily

L6, IL-TMP, and TM4SF5 are cell surface proteins predicted to have four transmembrane domains. Previous sequence analysis led to their assignment as members of the tetraspanin superfamily. In this paper, we identify a new sequence (L6D) that is strikingly similar to L6, IL-TMP, and TM4SF5. Analyses of these four sequences indicate that they are not significantly related to genuine tetraspanins, but instead constitute their own L6 superfamily.

Schistosomiasis

Schistosomiasis is a man-made disease related to water contact in the agricultural fields and affecting millions of people in developing countries in the tropical and subtropical parts of Africa, Asia and South America. It is a bisexual trematode living in the portal blood and perivesical venous plexus. Its life cycle necessitates the presence of an intermediate host - a fresh water mollusc - that differs according to place. The pathogenetic stage is the ova that initiate an immunologically delayed hypersensitivity cell-mediated reaction in the organs where they are deposited. The liver, colon, urinary bladder and ureter are the main organs affected; however, any organ can be affected, even the skin and the brain. The review discusses the pathogenesis, pathology, diagnosis: parasitological and immunological, the clinical picture, treatment and control. The present status of the research work on vaccination is also presented.

Human immunoglobulin E responses to a recombinant 22.6-kilodalton antigen from Schistosoma mansoni adult worms are associated with low intensities of reinfection after treatment

Schistosoma mansoni-infected individuals who have low intensities of reinfection following treatment produce immunoglobulin E (IgE) antibodies against a range of S. mansoni adult-worm antigens. One of the targets of the IgE response is an adult-worm sodium dodecyl sulfate-polyacrylamide gel electrophoresis band of 22 kDa (Sm22), which contains an antigen(s) located within the tegument and gut lining of adult worms and relatively late schistosomula life cycle stages only. A significant negative correlation between the level of anti-Sm22 IgE and the intensity of reinfection following treatment suggests that IgE responses against this antigen(s) are characteristic of individuals who are resistant to reinfection. To identify the antigen(s) in the Sm22 band that are associated with these IgE responses, we have cloned and characterized a recombinant 22-kDa protein (rSm22) that cross-reacts immunologically with Sm22. There was a high correlation between native and recombinant Sm22 isotype responses, indicating that the correct antigen had been cloned and that responses against rSm22 made up the majority of the responses against Sm22. By analyzing human isotype responses to rSm22 with human sera from a longitudinal treatment and reinfection study and correlating the anti-rSm22 isotype responses, retrospectively, with the intensity of reinfection following treatment for each individual, we observed a negative correlation between the IgE response to rSm22 and the intensity of reinfection. This relationship remained significant after allowing for age and other isotype responses to rSm22, in particular IgG4.

The ins and outs of the transmembrane 4 superfamily

The recently discovered transmembrane 4 superfamily comprises a group of cell-surface proteins that are characterized by the presence of four hydrophobic domains, which are presumed to be membrane spanning. At least seven of these molecules are expressed on leukocytes, and it seems likely that they mediate signal transduction events that play a role in the regulation of cell development, activation, growth and motility.

Cloning and characterization of the SmIMP25 integral membrane protein of the parasitic helminth Schistosoma mansoni

The cDNA and genomic clones encoding a 25 kDa integral membrane protein, termed SmIMP25, were isolated from Schistosoma mansoni. The 2.2 kb SmIMP25 mRNA was found in all developmental stages of the parasite tested: miracidium, sporocyst, cercaria and adult worm. The SmIMP25 gene is at least 16 kb long and it is split by four introns ranging in size from 36 bp to > or = 9 kb. Excluding the introns, the gene and the cDNA show 100% sequence identity. The cDNA has an open reading frame encoding a protein 223 amino acids long. The predicted sequence reveals a distinct hydrophobic domain of 20 amino acids located 12 residues from the carboxyl-terminal end. The properties of this domain (marked hydrophobicity, size, flanking by charged residues and C-terminal location) are typical of the transmembrane segments of integral membrane proteins. The presence of three potential N-glycosylation sites is also consistent with membrane proteins that are often glycosylated at the extracellular domain. Accordingly we propose that SmIMP25 is an integral membrane protein in which residues 1-191 are extracellular, residues 192-211 comprise the hydrophobic domain that spans the membrane, and residues 212-223 are intracellular. The SmIMP25 was synthesized as a fusion protein in bacteria and antibodies were elicited in rabbits. Antibodies against SmIMP25 specifically precipitated a 25 kDa protein from cell-free products programmed by schistosome mRNA, in agreement with the size of the protein predicted from the cDNA sequence. Immunofluorescence studies showed SmIMP25 on the surface of the parasite. Surface molecules expressed at the host-parasite interface are likely to provide information on host parasite relationship and may serve as targets for protective immunity.

Uroplakins Ia and Ib, two major differentiation products of bladder epithelium, belong to a family of four transmembrane domain (4TM) proteins

The mammalian bladder epithelium elaborates, as a terminal differentiation product, a specialized plasma membrane called asymmetric unit membrane (AUM) which is believed to play a role in strengthening and stabilizing the urothelial apical surface through its interactions with an underlying cytoskeleton. Previous studies indicate that the outer leaflet of AUM is composed of crystalline patches of 12-nm protein particles, and that bovine AUMs contain three major proteins: the 27- to 28-kD uroplakin I, the 15-kD uroplakin II and the 47-kD uroplakin III. As a step towards elucidating the AUM structure and function, we have cloned the cDNAs of bovine uroplakin I (UPI). Our results established the existence of two isoforms of bovine uroplakin I: a 27-kD uroplakin Ia and a 28-kD uroplakin Ib. These two glycoproteins are closely related with 39% identity in their amino acid sequences. Hydropathy plot revealed that both have four potential transmembrane domains (TMDs) with connecting loops of similar length. Proteolytic digestion of UPIa inserted in vitro into microsomal vesicles suggested that its two main hydrophilic loops are exposed to the luminal space, possibly involved in interacting with the luminal domains of other uroplakins to form the 12-nm protein particles. The larger loop connecting TMD3 and TMD4 of both UPIa and UPIb contains six highly conserved cysteine residues; at least one centrally located cysteine doublet in UPIa is involved in forming intramolecular disulfide bridges. The sequences of UPIa and UPIb (the latter is almost identical to a hypothetical, TGF beta-inducible, TI-1 protein of mink lung epithelial cells) are homologous to members of a recently described family all possessing four transmembrane domains (the "4TM family"); members of this family include many important leukocyte differentiation markers such as CD9, CD37, CD53, and CD63. The tissue-specific and differentiation-dependent expression as well as the naturally occurring crystalline state of uroplakin I molecules make them uniquely suitable, as prototype members of the 4TM family, for studying the structure and function of these integral membrane proteins.

Epitope mapping of anti-rat CD53 monoclonal antibodies. Implications for the membrane orientation of the Transmembrane 4 Superfamily

CD53 is a pan-leukocyte glycoprotein which is a member of the recently described Transmembrane 4 Superfamily (TM4SF) of membrane proteins that are predicted to span the lipid bilayer four times. The major hydrophilic region of murine CD53 was expressed as a glutathione-S-transferase fusion protein, and the epitopes of four mouse anti-rat CD53 monoclonal antibodies (mAb) (OX-44, 2D1, 6E2 and 7D2) were mapped to this region using mouse/rat chimeric fusion proteins. The epitopes of OX-44, 6E2 and 7D2 are restored by the substitution of a single isoleucine residue for threonine at position 154 in the mouse protein. The 2D1 epitope is non-linear and appears to require the juxtaposition of isoleucine at position 154 with one or more of the amino acids arginine (132), methionine (133) and serine (140). All of these epitopes are shown to be sensitive to reduction, thus indicating the importance of disulfide bonding in the correct folding of the CD53 hydrophilic domain. Moreover, as these four mAb recognize CD53 at the cell surface, the data provide direct molecular evidence for the proposed membrane orientation of the TM4SF.

Candidate vaccine antigens identified by antibodies from mice vaccinated with 15- or 50-kilorad-irradiated cercariae of Schistosoma mansoni

In murine schistosomiasis, the highest levels of resistance to cercarial challenge are obtained by vaccination with radiation-attenuated cercariae. To identify candidate vaccine antigens relevant to the vaccine model, we examined parasite antigens recognized by antibodies from mice vaccinated with irradiated cercariae of Schistosoma mansoni. To optimize recognition of a wide spectrum of antigens, several factors that influence the level of protection in this model were varied; specifically, we examined the effect of (i) single versus multiple vaccinations with irradiated cercariae, (ii) the dose of irradiation (15 or 50 kilorads) administered to the cercariae, and (iii) the genetic background of mouse strains, high-responder (C57BL/6J) versus moderate-responder (CBA/J) mice. We found that the number of vaccinations did not alter antibody specificity but modified the relative antibody titers against particular antigens. The dose of irradiation used to attenuate the immunizing cercariae had a similar effect on antibody titers but in addition influenced antibody specificity. Only mice that had been vaccinated with moderately irradiated cercariae recognized cathepsin B (Sm31) and Sm32. Interestingly, when vaccinated mice of the two strains, C57BL/6J and CBA/J, were compared, differences in antibody responses to particular antigens were observed. Both strains recognized the integral membrane protein Sm23, glutathione S-transferase, and cathepsin B, whereas Sm32 and paramyosin were recognized only by CBA/J mice, and heat shock protein 70 was recognized exclusively by C57BL/6J mice. In this study, we conclusively identified six distinct antigens that are specifically recognized by the humoral immune response of vaccinated mice.

Experimental models of immunization against schistosomes: lessons for vaccine development

Schistosomiasis is a debilitating, and sometimes deadly, parasitic infection that afflicts hundreds of millions of people living in developing countries. One of the best hopes for control of this disease is vaccine development. Studies on experimental models of attenuated vaccines have proven that high levels of protective immunity can be achieved. In these systems, resistance has been shown to be directed against the migrating larval stages of the parasite and to have both cellular and humoral components. Several candidate vaccine immunogens have been identified on the basis of antibody reactivity. However, the level of protection induced by immunization with nonliving vaccines has at yet not approached the level observed with attenuated infection. Current challenges to vaccine development include identification of protective T cell immunogens, determination of ways to strengthen the immunogenicity of isolated parasite antigens, and development of methodologies to selectively stimulate protective, as opposed to ineffective or even detrimental, immune responses.

Alternative patterns of MHC-restricted antibody responsiveness following intraperitoneal immunization of inbred mice with different preparations of an 86 kilodalton antigen of Schistosoma mansoni

The investigation of MHC restricted antibody responses to an 86 kDa antigen (p86) during chronic Schistosoma mansoni infection has been extended to immunization with this antigen. In the absence of adjuvant, a similar pattern of responsiveness by mice expressing H-2k and H-2d but not H-2b was observed following immunization with unpurified adult worm homogenate. Adjuvant selectively abrogated the capacity of H-2d mice to respond and this was also the case when purified p86 with adjuvant was injected. Immunization with purified subfragments of p86 again demonstrated MHC restriction in the capacity to immunoprecipitate p86 in vitro translation product, the pattern varying according to the fragment used. Western blot analysis showed that in some, but not all instances of apparent 'non-responsiveness' characterized by immunoprecipitation, antibody specificities capable of recognizing p86 epitopes on the nitrocellulose bound p86 were, indeed, present. Thus the fine specificity as well as the absolute capacity to respond is influenced by both the MHC haplotype of the host and the nature of the immunizing antigen.

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.