When adult schistosome worm pairs are transferred from experimental hosts to in vitro culture they cease producing viable eggs within a few days. Female worms in unisexual infections fail to mature, and when mature adult females are separated from male partners they regress sexually. Worms cultured from the larval stage are also permanently reproductively defective. The cytokine transforming growth factor beta derived from the mammalian host is considered important in stimulating schistosome female worm maturation and maintenance of fecundity. The means by which schistosomes acquire TGF-β have not been elucidated, but direct uptake in vivo seems unlikely as the concentration of free, biologically active cytokine in host blood is very low. Here we review the complexities of schistosome development and male-female interactions, and we speculate about two possibilities on how worms obtain the TGF-β they are assumed to need: (i) worms may have mechanisms to free active cytokine from the latency-inducing complex of proteins in which it is associated, and/or (ii) they may obtain the cytokine from alpha 2-macroglobulin, a blood-borne protease inhibitor to which TGF-β can bind. These ideas are experimentally testable.

The syncytial cytoplasmic layer, termed the tegument, which covers the entire surface of adult schistosomes, is a major interface between the parasite and its host. Since schistosomes can survive for decades within the host bloodstream, they are clearly able to evade host immune responses, and their ability is dependent on the properties of the tegument surface. We review here the molecular organization and biochemical functions of the tegument, combining the extensive literature over the last three decades with recent proteomic studies. We have interpreted the organization of the tegument surface as bounded by a conventional plasma membrane overlain by a membrane-like secretion, the membranocalyx, with which host molecules can associate. The range of parasite proteins, glycans and lipids found in the surface complex is evaluated, together with the host molecules detected. We consider the way in which the tegument surface is formed after cercarial penetration into the skin, and changes that occur as parasites develop to maturity. Lastly, we review the evidence on surface dynamics and turnover.

Intravascular Schistosoma mansoni worms seem to take up immunoglobulins from blood by surface Fc-receptors, but the process whereby bound immunoglobulins are processed by the parasite is poorly understood. We here present morphological data suggesting that two distinct main processes are involved: Host immunoglobulins were seen at two distinct locations in the parasite: in the frontal part of the enteric tube, the oesophagus, and as a fine granular staining at the surface and in the subtegumental region. The latter staining pattern corresponds to host immunoglobulin localization in discrete organelle-like aggregates tentatively identified as 'discoid or elongate bodies' at the ultrastructural level using immunogold staining. Immunoglobulin uptake by intravascular worms was also demonstrated in vivo after passive administration of 125I-labelled rabbit and mouse immunoglobulins. Radiolabelled immunoglobulins were taken up by the worms and shown to localize as fine strands running perpendicular to the parasite surface. Our results suggest that intravascular schistosomes take up host immunoglobulins both as part of their enteric digestion and by a surface Fc-receptor-mediated mechanism, involving transport and processing within organelles, 'elongate bodies'. Immunoglobulins taken up by intravascular schistosomes form a distinct organelle-like granules, which seem to be processed within the excretory system of the parasite.

Although controversial, schistosomes are believed to cloak themselves in antibody through non-specific interactions with the immunoglobulin (Ig) molecule. The acquisition of host Ig by the schistosome may mask its foreign status and/or interfere with Fc-dependent functions. We report experiments aimed at characterizing the interaction between Ig-Fc and paramyosin, a schistosome Fc-receptor previously reported to bind human IgG. We show that certain Ig classes, in particular murine IgG2b and IgG3, are not only able to bind recombinant paramyosin, but also associate with other parasite proteins. The Fc region of IgG contains four hydrophobic patches, two of which are known to interact with distinct molecules: one in the Cgamma2-Cgamma3 interdomain region bound by protein G, mannose binding lectin (MBL), and the neonatal Fc-receptor (FcRn), and one at the top of the Cgamma2 domain bound by phagocytic FcgammaRs and C1q. We provisionally discounted the involvement of these regions, since IgG binding by paramyosin did not inhibit FcgammaR-mediated NADPH respiratory bursts, and protein G was unable to block IgG binding to paramyosin. Given their apparent low affinity, we postulate hydrogen bonding between reactive residues in a hydrophobic patch at the bottom of the Cgamma3 domain and negatively charged Glu or Asp amino acids in paramyosin.

Activated leukocytes participate in immunity to infection by the parasitic blood fluke Schistosoma mansoni. They attach to the surface of schistosomes and secrete schistosomicidal substances. Cationic proteins, hydrolytic enzymes, and oxidants, produced by the leukocytes, have been implicated in the damage to the schistosomes. To examine the possible involvement of elastase in the killing of schistosomes by leukocytes, young and adult stages of S. mansoni were treated in vitro with pancreatic elastase (PE) and neutrophil elastase (NE). Schistosomula, lung-stage schistosomula (LSS), and adult worms (AW) have been found to be sensitive to both PE and NE. Male AW were more sensitive to PE than female AW. The enzymatic activity of elastase is essential for its toxic effect because heat-inactivation and specific elastase inhibitors prevented elastase-mediated schistosome killing. Thus, alpha1-antitrypsin and the chloromethyl ketone (CMK)-derived tetrapeptides Ala-Ala-Pro-Val-CMK and Ala-Ala-Pro-Ala-CMK but not Ala-Ala-Pro-Phe-CMK and Ala-Ala-Pro-Leu-CMK blocked PE caseinolytic and schistosomulicidal activities. As shown previously, schistosomes are also efficiently killed by hydrogen peroxide. LSS appear to be more resistant than AW and early-stage schistosomula to the lytic effects of hydrogen peroxide. Cotreatment experiments with both elastase and hydrogen peroxide indicated that they exert an additive toxic effect and that hydrogen peroxide sensitizes schistosomula to the toxic effect of elastase but not vice versa. These results demonstrate, for the first time, that elastases may be toxic molecules used by neutrophils, eosinophils, and macrophages to kill various developmental stages of S. mansoni.

The localization of repetitive DNA sequences in the mouse genome such as mouse type 2 Alu sequence (B2) and mouse retrovirus-related sequences was shown in the body of adult Schistosoma japonicum and Schistosoma mansoni by applying an in situ PCR and hybridization technique. Using the same method, mouse major histocompatibility complex (MHC) class I sequence was also found in schistosomes. Furthermore, mouse MHC class I sequence and type A retroviral sequence were detected in S. japonicum and S. mansoni cercarial DNA by blot hybridization. These findings indicated that horizontal and vertical transmission of host DNA sequences occurred in schistosomes. The incorporation and propagation of host sequences in schistosomes and the roles played by such host sequences form the focus of this brief review.

Schistosoma mansoni masks its surface with adsorbed host proteins including erythrocyte antigens, immunoglobulins, major histocompatibility complex class I, and beta(2)-microglobulin (beta(2)m), presumably as a means of avoiding host immune responses. How this is accomplished has not been explained. To identify surface receptors for host proteins, we biotinylated the tegument of live S. mansoni adults and mechanically transformed schistosomula and then removed the parasite surface with detergent. Incubation of biotinylated schistosome surface extracts with human immunoglobulin G (IgG) Fc-Sepharose resulted in purification of a 97-kDa protein that was subsequently identified as paramyosin (Pmy), using antiserum specific for recombinant Pmy. Fc also bound recombinant S. mansoni Pmy and native S. japonicum Pmy. Antiserum to Pmy decreased the binding of Pmy to Fc-Sepharose, and no proteins bound after removal of Pmy from extracts. Fluoresceinated human Fc bound to the surface, vestigial penetration glands, and nascent oral cavity of mechanically transformed schistosomula, and rabbit anti-Pmy Fab fragments ablated the binding of Fc to the schistosome surface. Pmy coprecipitated with host IgG from parasite surface extracts, indicating that complexes formed on the parasite surface as well as in vitro. Binding of Pmy to Fc was not inhibited by soluble protein A, suggesting that Pmy does not bind to the region between the CH2 and CH3 domains used by many other Fc-binding proteins. beta(2)m did not bind to the schistosome Fc receptor (Pmy), a finding that contradicts reports from earlier workers but did bind to a heteromultimer of labeled schistosomula surface proteins. This is the first report of the molecular identity of a schistosome Fc receptor; moreover it demonstrates an additional aspect of the unusual and multifunctional properties of Pmy from schistosomes and other parasitic flatworms.

Contrapsin, a serine protease inhibitor (serpin) present in mouse serum, was compared with that found in adult Schistosoma mansoni worm homogenates, which although immunologically identical to contrapsin in mouse serum, had a higher molecular weight in Western blotting. Immunolocalization studies demonstrated parasite-associated contrapsin on the surface and interstitial cells of adult male worms. After extraction of these parasites with Triton X-114, contrapsin was found in the aqueous phase of the detergent, suggesting it is unlikely to be an integral membrane protein. Treatment of adult worms with deoxycholate resulted in a change in the electrophoretic behaviour of worm-derived contrapsin. Parallel studies with trypsin suggested this was due to interaction of the serpin with a protease. Using porcine pancreatic trypsin as a model for a putative schistosome protease reacting with contrapsin, we have shown that trypsin, following complex formation with contrapsin, loses immunogenicity. Thus, when contrapsin-trypsin complexes were used as immunogen, the resulting antisera contained antibodies to contrapsin and contrapsin-trypsin complexes only, and none to native trypsin. Thus, epitopes characterizing native trypsin were presumably either masked following complex formation with contrapsin, or their processing and presentation to antigen presenting cells was suppressed, so that an antibody response was not mounted against them. These observations encourage speculation that S. mansoni may be elaborating an immune evasion strategy whereby immunologically sensitive proteases are first complexed with host serpins, which would render them immunogenically inert, and then cleared from the circulation by the host's reticulo-endothelial system. In this way the immune system would be unable to 'see' sensitive parasite proteases sufficiently to mount a response against the parasite.

Human alpha-1-antitrypsin (alpha 1-AT) was incubated with an extract of Schistosoma mansoni cercariae or porcine pancreatic elastase and analysed by immunoelectrophoresis and Western blotting. The inhibitor was shown to form complexes with components in S. mansoni cercariae in the same way as elastase. The role of alpha 1-AT in S. mansoni infection is discussed.

The localization of host (mouse)-related DNA sequences in the adults and cercariae of Schistosoma japonicum and S. mansoni was examined by in situ hybridization using 32P-labelled probes. The hybridization signals to the sequences of the mouse type C and type A retroviruses were clearly observed in the subtegumental layer and inner tissues of S. japonicum adults. In contrast, it was hard to find any signals to these sequences in S. mansoni adults. Distinct signals to the env-specific region of the mouse ecotropic type C retrovirus were observed in the subtegumental layer and inner tissues of S. mansoni adults. No signal to the sequence of the mouse type B retrovirus was found on the sections of either schistosome adult. The signals to the sequence of the genomic clone SmM51 derived from male S. mansoni were found in the tissues of both the schistosome adults. The signals to the sequences of the mouse type A and env-specific region of ecotropic type C retroviruses were not seen in the cercariae of each species, whereas the signals to the sequence of SmM51 were detected in S. mansoni cercariae. These observations suggest that host-related DNA sequences might be incorporated unequally into the schistosome genome during development in the final host.

Cancers and parasites have a number of properties in common, particularly those that relate to their respective capacities to evade host defence mechanisms. This review highlights the similarities between metastatic tumours and schistosomes in particular, and describes the role that proteases may have in the migration, growth, survival and transmission of the different stages of the schistosome life-cycle in the vertebrate host. An elastase-like serine protease of schistosome larvae has been particularly well characterized, and its substrate profile and other properties are indicative of a role in facilitating migration of the parasite through skin tissue early after infection. The primary structures of a cathepsin B-like enzyme, and a putative 'haemoglobinase' found in adult worms have also recently been derived, these enzymes being responsible for degradation of haemoglobin in erythrocytes upon which the adults feed. Adult schistosome worms reside and produce eggs intravascularly, and the processes that mediate the extravasation and subsequent migration of the egg through host tissue are dependent on both blood platelets and the immune response. Fibrino(geno)lytic enzymatic activity that is present in the egg could modulate the thrombogenic potential that eggs might have as a result of their capacity to cause platelet aggregation in vitro and in vivo. The roles of other proteases and peptidases that have been found in schistosome larvae, worms and eggs are less clear. Some of these enzymes may modulate immunological and haemostatic defence mechanisms and thus prolong survival of the parasite, and the consequences of the interactions between schistosomes and host protease inhibitors could also be immune modulatory.

Immune reactions to cysticercosis have been extensively studied in mice. The lack of significant lymphocyte infiltration into the livers of infected mice and the obvious role of antibodies in rejection has led to the general conclusion that cellular reactions do not play a role in protection against this disease. In contrast, the present study examining the immune response to cestode infections in a large animal model (sheep) revealed the presence of a massive and highly organized cellular infiltration in the livers after a secondary Taenia hydatigena infection. The majority of the infiltrating lymphocytes were of the CD4+ phenotype with much fewer CD8+ cells present. While most gamma delta-TCR+ cells in peripheral blood are SBU-T19+, the majority of gamma delta-TCR+ lymphocytes in the liver lesions are SBU-T19- suggesting selective migration of these cells into the lesions. In contrast to the diffuse distribution of T cells in the lesions, B cells were present as distinct aggregates. In primary T. hydatigena infections, host class I and class II MHC antigens were shown, for the first time in cestode infections, to be absorbed onto the surface of the metacestode bladderwall indicating their possible involvement in parasite survival. No immune reactions were observed close to the parasite although lymphocytes and eosinophils were infiltrating the adjacent portal tract areas. Most lymphocytes in both primary and secondary infections were positive for MHC class II antigens suggesting selective recruitment of activated cells to the site of infection. Significant changes in relative and absolute numbers of lymphocyte subpopulations were also observed in the draining hepatic lymph nodes dominated by a massive increase of B cells. In contrast, at the peak of local cellular infiltration, no changes in lymphocyte subpopulations were observed in peripheral blood showing the limited usefulness of this compartment in studying cellular changes in localized infections. The vigorous cellular response observed in the livers of sheep contrasts sharply with the lack of lymphocyte infiltration reported in mice indicating that small animal models may not be appropriate to study cellular responses to cysticercosis in large animals and man.

In spite of some remarkable progress in our understanding of the immune response to parasites and in the molecular cloning of dozens of genes encoding for potentially protective proteins, no definitive step has yet been made towards operational vaccines against major human parasitic diseases. The reasons for our present failures are no longer attributable to the lack of appropriate tools but rather to our rather primitive knowledge of the basic mechanisms governing host-parasite relationship. Mainly on the basics of our personal observations, we have attempted to review and discuss some of the prominent factors in host-parasite interactions, such as molecular mimicry, phyletic convergence, molecular mechanisms of infectivity and lures of cell communication. In many respects, the development of efficient vaccines applicable to humans appears closely dependent on a better understanding of the basic biological mechanisms underlying the natural history of parasitic diseases. In this context, the development of new concepts regarding the definition of potentially protective antigens based on the study of non-surface molecules, cross-reactive stage antigens and antibody isotype selection might represent promising alternatives.

An antigen in homogenates of adult Schistosoma mansoni worms grown in mice, and immunoprecipitated by polyspecific rabbit antiserum, has been identified as contrapsin, a mouse serine protease inhibitor (serpin). In the serum of some mice infected with S. mansoni contrapsin was found with altered immunoelectrophoretic characteristics when compared with contrapsin from uninfected mice.

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.

This review covers some of the basic mechanisms whereby parasites evade host responses. These mechanisms include; antigenic variation, repeated antigenic determinants, induction of suppressor cells, acquisition of host proteins or molecular mimicry, proteinase destruction of host effector molecules, proteinase inhibitor-mediated inhibition of humoral and cellular immune effector arms and immunosuppressive products of parasite arachidonic acid metabolism. Vet. Parasitol.

The immunofluorescent antibody test and immunocytochemical method were employed to study the surface antigens of Angiostrongylus cantonensis obtained from infected rats, mice and guinea pigs. Positive results with intense fluorescence and brownish peroxidase staining were observed on the cuticular surface of A. cantonensis recovered from rats 22 days (late cerebral phase) and 34 days (lung phase) post-infection when tested with antisera against host (normal rat serum) antigens as well as crude extracts of A. cantonensis. However, host antigens were not observed on the surface of the nematode recovered from the brain of mice and guinea pigs 15 days post-infection.

The outer and inner bilayers of the apical membrane complex of Schistosoma mansoni were sequentially stripped from adult worms by two incubations in 0.1% digitonin solutions. Membrane removal was evaluated by electron microscopy of worms and bilayer material, using Con A-ferritin as a marker for the outer bilayer. Amounts of Con A removed by the digests were measured with a tritiated Con A marker. To measure the purity of the fractions membrane markers were characterised and quantitated for both bilayers. In the absence of the usual enzymatic markers for plasma membrane diazotised [125I]-iodosulfanilic acid was used as a marker for the outer bilayer. Alkaline phosphatase and a Na+, Mg2+-ATPase were localised to the inner bilayer. From these results we can deduce that the inner bilayer is analogous to the typical, apical plasma membrane of other animal epithelia. The outer bilayer does not share these enzymatic similarities. The integrity of the syncytium after removal of the outer bilayer and the increased levels of lactate dehydrogenase in the supernatant after removal of the inner bilayer suggests that the outer bilayer is secondary in maintaining the permeability barrier of the apical membrane complex, with respect to soluble proteins. The possible significance of these results in terms of the destructive action of complement on the parasite are discussed.

Incorporation of labelled amino acid into tegumental proteins and acquisition of protection by schistosomula against antibody-mediated killing in vitro were simultaneously stimulated by serum factors and inhibited by puromycin. Comparison of polyacrylamide gel electrophoresis patterns with fluorographic autoradiography indicates that the majority of proteins in the parasite tegument were labelled with the isotope after incubation for 3 h. No new, clearly defined band was observed in the autoradiography pattern. During this period a decreasing susceptibility of the schistosomula to antibody plus complement was observed. Quantitative fluorescence assay shows that schistosomula insensitive to antibody plus complement were still able to bind the same amount of antibody as the unprotected parasites. Pre-culture of schistosomula in the presence of inactivated normal rabbit serum also decreased the susceptibility of the parasites to the in vivo killing mechanism.

Intraperitoneal injection of cercariae into pristane (2, 6, 10, 14 tetramethyl pentadecane)-primed Balb/c mice led to greatly diminished numbers of portal and peritoneal worms compared with untreated mice. Schistosomula taken from the peritoneal cavity of pristane-primed mice carried globules of pristane on their surfaces, were contracted and were permeable to Trypan blue. Pristane globules bound also to adult worms in vitro and in vivo causing rapid damage to the surface membrane. Hydrophobic compounds other than hydrocarbons either bound without causing gross damage, or did not bind to the adult worms. 51Cr release studies showed that pristane had no effect on the permeability of human erythrocytes, while causing significant release from both schistosomula and adult worms. The binding of hydrocarbon globules to a variety of other parasites did not occur. The binding of n-[1-14C]hexadecane to adult Schistosoma mansoni was significantly decreased by extraction of the parasite with organic solvents or treatment with staphylococcal delta toxin, which interacts with phospholipids in the membrane. Possible mechanisms of damage of the parasite by the hydrocarbons are discussed.

The multilaminate vesicles present in the tegument cytoplasm appear to fuse with side channels projecting out into the cytoplasm from the base of the surface pits. Their lamellate contents then unroll and spread out to form a trilaminate membranocalyx lining the pits and covering the tegument surface. The plant lectin concanavalin A appears to stabilize the process of vesicle fusion leading to an aggregation of multilaminate vesicles trapped in the lumen of the surface pits. The membranocalyx can be labelled with cationized ferritin. Chase incubations in normal medium indicate that by 4 h most of the label and the membranocalyx to which it is bound have been lost to the medium. This suggests that under the conditions of these experiments the membranocalyx has a half-life of 2-3 h.