Studies on the mechanisms of immunity in human schistosomiasis

Cross-disciplinary approaches for measuring parasitic helminth viability and phenotype

Parasitic worms (helminths) within the Phyla Nematoda and Platyhelminthes are responsible for some of the most debilitating and chronic infectious diseases of human and animal populations across the globe. As no subunit vaccine for any parasitic helminth is close to being developed, the frontline strategy for intervention is administration of therapeutic, anthelmintic drugs. Worryingly, and unsurprising due to co-evolutionary mechanisms, many of these worms are developing resistance to the limited compound classes currently being used. This unfortunate reality has led to a renaissance in next generation anthelmintic discovery within both academic and industrial sectors. However, a major bottleneck in this process is the lack of quantitative methods for screening large numbers of small molecules for their effects on the whole organism. Development of methodologies that can objectively and rapidly distinguish helminth viability or phenotype would be an invaluable tool in the anthelmintic discovery pipeline. Towards this end, we describe how several basic techniques currently used to assess single cell eukaryote viability have been successfully applied to parasitic helminths. We additionally demonstrate how some of these methodologies have been adopted for high-throughput use and further modified for assessing worm phenotype. Continued development in this area is aimed at increasing the rate by which novel anthelmintics are identified and subsequently translated into everyday, practical applications.

Schistosoma mansoni host-exposed surface antigens characterized by sera and recombinant antibodies from schistosomiasis-resistant rats

Antibodies from Schistosoma mansoni-infected rats, unlike mice, show a higher titer for schistosome apical tegumental antigens compared with non-apical membrane antigens. These antibodies bind to the surface of living lung-stage worms and to formaldehyde-fixed adult worms. We produced a single-chain antibody Fv domain (scFv) phage library displaying the antibody repertoire of rats highly immune to schistosome infection and we selected for scFvs that recognize the host-exposed surface of worms. Five unique rat scFvs (Teg1, Teg4, Teg5, Teg20 and Teg37) were obtained which recognize schistosome surface epitopes. Each of the scFvs recognizes the surface of living schistosomula and lung-stage schistosomules and/or the surface of formaldehyde-fixed adult worms. None of these scFvs reproducibly stained living adult worms. This suggests that a change occurs during the transition from lung schistosomules to 4-week adults such that at least some surface antigens, although remaining on the surface in living adult worms, can no longer be immunologically stained. Teg1 and Teg4 scFvs both recognize specific bands on Western blots. No bands were observed for the other three scFvs, suggesting that these scFvs may recognize non-protein or conformationally-dependent epitopes. Teg1 was unambiguously identified as recognizing the S. mansoni tetraspanin antigen, SmTSP-2, within the large extracellular domain. Teg4 recognizes a 35kDa band tentatively identified as Sm29 by proteomic analysis. These scFvs can now be used to characterize schistosome epitopes at the host-parasite interface, to target worms in vivo, and to study the mechanisms by which these worms naturally evade immune damage to the tegument within permissive hosts.

Oxidative response of phagocytes to parasite invasion

Phagocytes destroy intracellular pathogens and extracellular targets in part by the production of toxic oxygen metabolites--namely, superoxide, hydrogen peroxide, hydroxyl radicals and possibly singlet molecular oxygen. The toxicity of hydrogen peroxide is increased greatly by peroxidase and a halide. A peroxidase that can be used for this purpose is present in neutrophils and monocytes (myeloperoxidase), but is lost when the monocyte matures into a macrophage; a different peroxidase is present in eosinophils. The latter enzyme, because of its strong positive charge, binds to the surface of parasites; any phagocyte in the region, when appropriately stimulated, may provide the hydrogen peroxide required for completion of the peroxidase system. Further, peroxidase-coated organisms are more readily killed when ingested by macrophages than are uncoated organisms. Oxygen-dependent toxicity requires the production of toxic oxygen products by phagocytes in amounts sufficient to overcome the protective capacity of endogenous scavengers in the parasite. The latter include catalase and glutathione peroxidase, which degrade hydrogen peroxide, and superoxide dismutase which dissipates superoxide. The host defence against parasites appears to depend in part on this balance between toxic oxygen metabolites and scavengers.

No overt cellular inflammation around intravascular schistosomes in vivo

Schistosomes are intravascular platyhelminth parasites that are exposed in the blood stream to host immunological effectors. Antibody-dependent cell-mediated cytotoxicity (ADCC) can act in vitro to kill the parasites, and this has been proposed as one important mechanism of antiworm immunity in vivo. In this study, we examined sections of adult Schistosoma mansoni in situ, within the vasculature of strains of mice that exhibit low (Balb/c) or high (CBA) pathology, and in the vasculature of infected chimpanzees, for evidence of cellular inflammation around the worms. In both mouse strains, we observe robust cellular inflammation around the parasite eggs in the intestines and liver tissue. However, we detect no overt cellular inflammation around the mature parasites in vivo. Likewise in the vasculature of infected chimpanzees, no immune cell accumulations are detected around adult schistosomes in situ. These data suggest that the parasites can promote a polar immune response that targets eggs (and assists the eggs to exit the host and continue the life cycle) but that does not effectively target the source of those eggs, namely, the adult worms.

Complement and parasitic trematodes

The complement (C) system acts as a barrier to protect our bodies against invading pathogens. It may react to cytophilic antibodies or directly to foreign molecules presented by the intruder. As well as their cytotoxic activity, C components can attract and attach leucocytes to the surface of the foreign body, and activate them to kill it. Zvi Fishelson describes various strategies used by a parasitic trematode to escape immune damage in the face of potent immune surveillance by C and other effector mechanisms.

Cloning of a cDNA encoding SjIrV1, a Schistosoma japonicum calcium-binding protein similar to calnexin, and expression of the recombinant protein in Escherichia coli

Membrane-associated proteins were isolated from adult Philippine strain Schistosoma japonicum by partitioning into the detergent phase of Triton X-114. A rabbit polyclonal antiserum raised against these proteins was used to screen an S. japonicum expression cDNA library. Positive clones were identified which encoded the species orthologue of SmIrV1, a Schistosoma mansoni protein which was initially identified by screening with sera from mice protectively vaccinated with irradiated cercariae [Hawn et al., J. Biol. Chem. 268 (1993) 7692-7698]. The S. japonicum molecule, which we term SjIrV1, is 83% identical to SmIrV1 at the predicted amino acid level and is a member of the calreticulin family of non-EF-hand, calcium-binding proteins. The Chinese strain S. japonicum orthologue of SjIrV1 was obtained by screening with the radiolabelled insert of the Philippine strain clone. Northern blot analysis revealed a single message of around 2.4 kb and gave no indication of alternative splicing. Southern blot analysis gave a simple pattern, indicating a single-copy gene, and showed a single restriction fragment length polymorphism between the genomes of Chinese and Philippine strains of S. japonicum. Recombinant, full-length SjIrV1 was expressed with a hexahistidine tag in Escherichia coli and the recombinant protein isolated by nickel-chelate chromatography. Recombinant SjIrV1 was shown to exhibit calcium-dependent, differential electrophoretic migration and to bind ruthenium red in the absence but not in the presence of calcium ions. The presence of conserved Ca(2+)-binding motifs predicted from the primary sequence, together with the Ca(2+)-dependent electrophoretic mobility of recombinant SjIrV1, confirmed that SjIrV1 was a functional calcium-binding protein.

Schistosoma mansoni: host cell adhesion to the different stages of the parasite, in vivo

The peritoneal cavity of laboratory mice was used to study the phenomenon of host cell adhesion to different evolutive stages of the Schistosoma mansoni (cercaria, adult worm, developing and mature eggs, miracidium, young and mature daughter sporocysts). Material recovered from the peritoneal cavity 30 and 180 min after the inoculation of each evolutive form was examined with the help of a stereomicroscope. The free swimming larvae (cercaria and miracidium), and the evolutive forms producing such larvae (mature egg and mature daughter sporocyst) elicited the host cell adhesion phenomenon. In all forms but cercariae the adherent cells remained as so till 180 minutes after inoculation.

Antibody isotype responses to the Schistosoma mansoni schistosomulum in the CBA/N mouse induced by different stages of the parasite life cycle

During infection with Schistosoma mansoni the extent and nature of immune reactions against schistosomula may be influenced by responses to cross-reactive antigens in eggs or adult worms, and there is now extensive evidence for cross-reactivity between the different stages of the parasite life cycle. In this study IgM and IgG subclass antibodies produced in (CBA/N x Balb/c) F1 male and female mice were measured over a period of time following exposure to a chronic infection, to unisexual male cercariae or to irradiated larvae. Antibody levels were also measured following immunization with antigen preparations derived from adult worms, schistosomula or eggs. (CBA/N x Balb/c) F1 male mice exhibit an X-linked immune deficiency which results in an inability to respond to T-independent (TI) type 2 polysaccharides. Isotype levels were measured by ELISA to detergent-soluble schistosomulum antigen. Results showed that antigens on the different stages of the parasite life cycle have a qualitative influence on the antibody response to the larval surface, and that T-independent type 2 polysaccharides, particularly abundant in egg, exhibit antigen-directed isotype restriction in the form of IgM and IgG3 antibodies.

Complement regulation on the surface of cultured schistosomula and adult worms of Schistosoma mansoni

Cercaria and freshly prepared schistosomula of Schistosoma mansoni are highly sensitive to complement. However, early in their maturation, the schistosomula become resistant to complement killing. This conversion is preceded by a rapid and massive release of several acetabular proteases and of the glycocalyx coat. Thus, shedding of the glycocalyx which is a major immunogen and a strong activator of the alternative pathway of complement permits the parasite to escape immune damage. Mechanically transformed schistosomula, which were cultured in a defined synthetic medium and developed complement resistance, could be converted by proteolysis to complement sensitivity. Trypsin and pronase markedly increased the susceptibility of cultured schistosomula to complement. The trypsin-induced complement sensitivity persisted for at least 19 h without recovery of resistance. Similar treatment with trypsin produced complete killing of adult worms by complement in absence of antibodies. Efficient killing was obtained with normal human serum (NHS), with normal guinea pig serum (GpS), and with C4-depleted HS and C4-deficient GpS indicating that the killing was mediated by the cytolytic alternative pathway of complement. Larger quantities of C3b with intact alpha' chain could be demonstrated on trypsin-treated than on non-treated schistosomula. Antibodies which were raised in rabbits by immunization with the trypsin-released material bound to cultured (non-treated) schistosomula and to adult worms, and induced their killing in GpS and C4-deficient GpS. These results suggest that following release of the glycocalyx, the transforming schistosomula of S. mansoni spontaneously express a complement regulatory protein(s). A similar regulator is postulated to be present on the surface of adult worms. Such regulatory molecules may serve as good targets for immunotherapy, since antibodies directed to them will inhibit their regulatory activity and thus potentiate in vivo the lytic action of complement.

Parasitic protozoa and helminths: biological and immunological challenges

Parasitic protozoans and helminths pose considerable medical as well as scientific challenges. Investigations of the complex and very different life cycles of these organisms, their adaptation to the obligate parasitic mode of life, and their ability to face the hostile host environment have resulted in many exciting discoveries. Invasion of host erythrocytes by plasmodial sporozoites and intact skin by schistosomal cercariae are outlined as examples of the elaborate mechanisms of parasitism. Isolation and characterization of single protective antigens or subunit vaccines from these two organisms are examined as models for vaccine development. Finally, developments in exploring gene regulation in protozoans and free and parasitic nematodes are briefly outlined.

Induction of resistance to Schistosoma mansoni infection in mice by purified parasite paramyosin

Freeze-thaw (FT)-disrupted schistosomula or their membrane extract induced significant resistance in mice to Schistosoma mansoni infection (34 and 25%, respectively) without the use of adjuvant. Antigens identified in schistosome extracts by sera from immunized animals were then evaluated for protective potential. Immunization with schistosomal antigens of 97 and 68-70 kD resulted in significant protection that was equivalent to that obtained by FT schistosomula. Since the 97-kD antigen was suggested to be parasite paramyosin, we used a biochemical technique to purify this muscle protein. Purified schistosome paramyosin ran as a single band on 10% SDS-PAGE and was recognized both by sera from mice immunized with FT schistosomula and a polyclonal antiserum raised against the 97-kD parasite protein. Preincubation of schistosome paramyosin with sera from mice immunized with FT schistosomula resulted in the removal of reactivity with the 97-kD protein in crude worm extracts. Paramyosin was identified by Western blotting to be in the tegument of schistosomula. The purified schistosome paramyosin resulted in significant protection in three separate experiments (24, 46, and 53%) without the use of adjuvant. Addition of BCG to paramyosin resulted in enhanced protection.

Echinostoma caproni in mice: shedding of antigens from the surface of an intestinal trematode

The surface antigens, which induce a serum antibody response during infection of mice with the intestinal trematode Echinostoma caproni, were examined. It was demonstrated that antigens are shed from the surface of juvenile and 4-week old adult E. caproni during in vitro culture. SDS-PAGE and Western blot analysis of in vitro shed and detergent solubilized surface antigens indicated that the four major antigens released from the surface of adult parasites had molecular masses of approximately 26,000, 66,000, 75,000 and 88,000. A modified ELISA technique showed the in vitro turn-over rate of the surface antigens to be very high, with a half-life of 8-15 min in both juvenile and adult E. caproni trematodes. Transmission electron microscopy of the surface of adult parasites revealed a highly active secreting tegument which was densely packed with membrane-bound vesicles, reflecting the high rate of shedding of the surface antigens. An attempt to immunize mice with detergent solubilized adult surface antigens failed to induce resistance to infection with metacercariae of E. caproni.

Longitudinal studies on human schistosomiasis

A major difficulty in understanding the epidemiology of human schistosomiasis has been to distinguish between acquired immunity and reduced exposure as possible reasons for an observed decline, in older individuals, of levels of superinfection or of reinfection after chemotherapy. A series of studies of Schistosoma mansoni infections in Kenya has been undertaken to approach this problem, by investigation of intensities of reinfection after treatment of individuals whose levels of contact with contaminated water is subsequently observed. Intensities of reinfection are highest among younger children, thereafter declining sharply. This decline can be attributed only in part to age-related changes in the duration and nature of exposure; there is also evidence for the development of an acquired resistance to reinfection that is dependent both on age and on previous experience of infection, and that may be immunologically mediated. Evidence has been obtained that the slow development of this acquired immunity with age may be associated with the early development and subsequent slow decline of inappropriate immune responses that 'block' the effect of potentially protective responses. Implications of these findings for immunological intervention through vaccination are discussed.

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.

Surface proteins and antigens of adult Schistosoma mansoni tegumental membranes detached onto poly-lysine coated beads

Poly-lysine coated beads attached readily onto Schistosoma mansoni. On detachment, the beads removed membranes from the surface of the tegument. Analysis of the proteins of the detached membranes showed that three major proteins of 94, 73 and 62 kDa were present in contrast to a more complex range of proteins present in the phosphate-buffered saline released membranes. The membranes attached to beads were radio-iodinated and the antigens examined in immunoprecipitates by sodium dodecyl sulphate-polyacrylamide gel electrophoresis using various antisera. In addition to the well-established 32 and 20 kDa antigens of the tegument, other major antigens of 200, 25 and 11-12 kDa were iodinated in the membranes attached to the beads. The results suggest that the major antigens studied in the tegument may not correspond to the major proteins identified. The present approach shows promise for deducing the topography of the surface antigens and proteins of schistosomes.

Antibodies of the IgE and IgG isotype, serum IgE and circulating immune complexes in schistosomiasis intercalatum

Individuals in an advanced phase of infection by Schistosoma intercalatum in whom viable ova are still present in the rectal mucosa but not excreted in detectable quantity, can be distinguished clinically from a younger group of individuals excreting S. intercalatum ova in their faeces. This clear-cut distinction is underlined by findings in parameters of humoral response. The ratio of anticercarial to anti-adult worm antibodies was higher in the group excreting eggs with probably more recent infection. Levels of total serum immunoglobulin were higher in this group, with the exception of serum IgA, which was lower as long as eggs were being excreted. In a later stage of the disease, relatively more IgG seems to be bound in circulating immune complexes. It is postulated that these stage specific patterns of humoral immune response represent the evolution of the host-parasite relationship during the infection.

A histological study of skin reactions of baboons to Schistosoma mansoni schistosomula

A histological study was undertaken of the cellular reaction in the skin of baboons challenged with cercariae of Schistosoma mansoni. Control animals with no previous schistosome infection were compared with baboons that had infections of varying duration. In animals infected for 10 weeks no striking inflammatory reaction was seen, the histological picture being very similar to that of the control animals. After 8 months of infection the hosts developed a strong cellular response to the invading schistosomula. The predominant cells in these reactions were eosinophil leucocytes which sometimes closely adhered to degenerating schistosomula. However, some unaffected schistosomula were found close to these intense cellular reactions, suggesting that schistosomula possess varying ability to resist the host's immunological attack.

Immunity in human schistosomiasis mansoni. Regulation of protective immune mechanisms by IgM blocking antibodies

After the demonstration of blocking antibodies during rat experimental schistosomiasis, the existence of such factors was investigated in human schistosomiasis. The depletion, in sera from S. mansoni-infected patients, of a given isotype (IgM) either by protein A-Sepharose (PAS) absorption or by fast protein liquid chromatography (FPLC) induced a significant increase in IgG-mediated killing of S. mansoni schistosomula by human eosinophils. Inhibition experiments showed that IgM-enriched fractions (PAS effluents) were able to inhibit eosinophil-dependent cytotoxicity mediated by IgG fractions (total sera or PAS eluates). Both IgG and IgM antibodies from infected human sera immunoprecipitated antigens of 30,000-40,000 Mr in the labeled detergent extracts of schistosomulum surface. The specificity of IgG and IgM for the 38,000 Mr antigen was suggested by competition experiments using two radiolabeled mAbs (IPLSm1, IPLSm3) directed against this antigen. Moreover, crossinhibition between IgG and IgM antibodies for the Mr 38,000 antigen could be directly demonstrated. The in vivo relevance of such IgM blocking antibodies in the context of human immunity to schistosomiasis was evaluated in two groups of children classified as resistant or susceptible to posttreatment reinfection. IgM antibodies specifically directed against the 38,000 Mr antigen were measured by a capture assay. The mean levels of IgM antibodies were significantly higher in the susceptible than in the resistant group both before and after treatment. These results are consistent with the idea that immunity to schistosomiasis could be attributable not only to the existence of antibodies with defined effector function, but also to the absence of blocking antibodies. The description of the existence in human schistosomiasis of antibody isotypes blocking the effector response against defined surface targets might lead to a new understanding of the mechanisms regulating immunity to reinfection against schistosomes and possibly other parasites.

In vivo activation of macrophages by T cell-derived lymphokines: killing of tumor cells and schistosomula of Schistosoma mansoni

We investigated the role of T cell-derived lymphokines for macrophage activation in vivo. We show for the first time that macrophages from casein-pretreated mice can be primed in vivo by intraperitoneal injection of immune interferon (IFN-gamma) and can be triggered by lipopolysaccharide (LPS) in vitro to kill schistosomula of S. mansoni. Similar results were obtained for the activation of tumoricidal macrophages. Injection of casein-pretreated mice with concanavalin A (Con A)-induced supernatant of a long-term T cell clone containing IFN-gamma and macrophage cytotoxicity inducing factor 2 (MCIF2), however, induced macrophage activation in vivo without further addition of LPS in vitro. These experiments show that macrophages can be activated by lymphokines in vivo. In addition, the data suggest that a combination of IFN-gamma with MCIF2 might be more effective than IFN-gamma alone. These data may be relevant for the strategy of treating cancer and infectious diseases with lymphokines.

Toxocara canis: interaction of human blood eosinophils with the infective larvae

This study was carried out to investigate the nature of the immunological responses which took place in a child who had recently recovered from toxocariasis. She had developed a marked eosinophilia and had high titers of toxocara antibodies. Experiments were performed to examine whether Toxocara canis infective larvae could be killed in the presence of her serum and human eosinophils. Eosinophils with human complement, or this patient's serum, adhered to the surface of the larvae within 10 min. By 40 min, using both light and electron microscopy, it was shown that the cells had flattened against the cuticle and degranulated. However, by 3 hr, eosinophils had begun to detach, and the larvae remained alive for at least 1 week afterward. Further addition of serum or of eosinophils, which were shown to be able to immobilize T. spiralis infective larvae, failed to kill the T. canis larvae. It was concluded that, in this patient, the development of an inflammatory response to a T. canis infection was not associated with the appearance of antibodies capable of inducing eosinophil dependent toxicity to the larvae in vitro. Eosinophil dependent killing mechanisms may be less important than other components of the immune response, in immunity to this parasite in humans.

Killing of schistosomula of Schistosoma mansoni by macrophages: induction by T-cell clone-derived lymphokines and interferon-gamma

The induction of schistosomulicidal activity of peritoneal macrophages by concanavalin A-stimulated supernatants from long-term T-cell clones and by interferon-gamma (IFN-gamma) was investigated in detail. Optimal conditions of in vitro macrophage activation by T-cell clone supernatants were established. Macrophages from 13-week S. mansoni-infected mice responded to lymphokine activation as well as resident macrophages from uninfected mice. IFN-gamma was shown to play an essential role in induction of schistosomulicidal macrophage activity: recombinant IFN-gamma at high concentration could induce schistosomula killing, and an anti-IFN-gamma antiserum inhibited the induction of schistosomulicidal activity by T-cell clone supernatants. Our data also indicate that macrophage activation could be obtained by IFN-gamma in synergy with other lymphokines in the supernatant of long-term T-cell clones. Macrophages from mice injected with T-cell clone supernatants were primed in vivo and triggered to kill schistosomula in vitro in the presence of lipopolysaccharide (LPS). The data demonstrate that lymphokines produced by T-cell clones and, in particular, IFN-gamma can participate in the activation of schistosomulicidal macrophages.

Ion fluxes changes during early stages of Schistosoma mansoni. Evaluation of complement effect

The relationship between the average membrane potential (delta psi av) and sensitivity to complement action of the Schistosoma mansoni parasite was explored. The average membrane potential was estimated by measuring the uptake of [3H]tetraphenyl phosphonium ([3H]Ph4P+). The parasites take up Ph4P+ indicating the existence of a negative internal plasma potential which is in part dependent on the transmembrane K+ gradient, maintained by an active Na+/K+-ATPase. Values for Ph4P+ uptake could be corrected for mitochondrial accumulation by employing the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP), which collapses the mitochondrial potential. The plasma membrane potential derived by this technique was in the range of -60 mV. Transformation of this parasite, from its early cercaria stage to the adult worm, was associated with changes in the average membrane potential. The apparent hyperpolarization, which accompanies transformation, may be related to changes in ionic permeability and morphology which occur concomitantly. Complement acting through both the classical and alternative pathways was found to affect the potential of the parasite in its early development stages. The correlation between effects on delta psi av and sensitivity to complement action, indicates that the complement-induced changes in delta psi av are indeed tightly associated with its mode of action. Treatment of the parasite with complement resulted in net hyperpolarization of the membrane indicating that hyperpolarization rather than depolarization of the membrane is linked to the primary non-lethal action of complement.

Schistosoma mansoni: killing of transformed schistosomula by the alternative pathway of human complement

The interaction of mechanically transformed schistosomula of Schistosoma mansoni with the alternative pathway of human complement was studied in vitro. To detect early changes in transformation, the schistosomula were prepared at a low temperature and used immediately. As shown previously, freshly transformed schistosomula were highly susceptible to killing by normal human serum and by C4-depleted normal human serum. This serum activity was concentration dependent and was markedly reduced on a twofold serum dilution. Upon incubation at 37 C in defined synthetic medium, schistosomula rapidly became refractory to killing by the alternative pathway of complement. After 1 hr of incubation at 37 C, the percentage of schistosomula which were resistant to killing increased from 16 to 85. This conversion was accompanied by a fivefold decrease in deposition of C3b on schistosomula which had been exposed to 37 C for 1 hr and then further incubated with C4-depleted normal human serum. The following events occurred concomitantly during incubation of freshly transformed schistosomula at 37 C with a half-life of 30-60 min: (1) Decrease in activation and consumption of the alternative pathway of complement by schistosomula; (2) appearance of a strong complement consuming activity in the supernatant of incubating schistosomula; and (3) shedding of protein- and carbohydrate-containing substances from the surface of schistosomula into the supernatant. Isolated external membranes of freshly transformed schistosomula consumed the alternative pathway of complement to a greater extent than membranes of schistosomula preincubated in medium at 37 C. The results demonstrate that transformed schistosomula acquire resistance to complement killing via the alternative pathway by shedding complement-activating substances.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of schistosome genes

As part of an integrated programme investigating human schistosomiasis, work which involves epidemiological surveys and detailed immunological studies as well as biochemical investigations, we have, over the last three years, been cloning schistosome genes in a variety of plasmid and lambda vector systems. In this lecture we present a review of some selected aspects of work primarily aimed at production of experimental vaccines against the disease but which, on a broader front, is also concerned with developmental regulation of gene expression around the parasite's life-cycle. Specifically, we are interested in cloning three groups of genes. First, those encoding surface antigens; second, those associated with sexual maturity and egg production; and third, antigens which may provide a basis for a specific immunodiagnostic test.

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

Monoclonal antibody M.1 was generated from mice immunized with membrane enriched extracts of mechanically transformed schistosomula. M.1 bound to the surface membranes of cercariae and young (0-24 h post-transformation) schistosomula but did not bind to older schistosomula or cultured worms. M.1 immunoprecipitated an antigen of approximate molecular weight 27-28 kDa from schistosomula. Experiments using metabolic labeling showed that the antigen was actively synthesized by developing schistosomula. Further M.1 immunoprecipitated a similar 27-28 kDa antigen from membrane-enriched extracts of miracidia, lung and adult worms as well as from schistosomula.

Population dynamics of Schistosoma mansoni in mice repeatedly exposed to infection

Studies of host resistance to parasite infection are usually based on experimental designs involving a primary infection and subsequent challenge exposure, resistance being recorded as the percentage reduction in parasite establishment in challenged hosts when compared with that in uninfected animals. Few studies have focused on the dynamic nature of helminth establishment and mortality (and their presumed dependency on the rate of current exposure and past experiences of infection) in hosts repeatedly exposed to low levels of infection. Here, we report the results of population studies on the dynamics of resistance to Schistosoma mansoni infection (a helminth parasite) in mice repeatedly exposed to cercarial invasion. Parasite burdens created by different levels and durations of exposure to infection reflect a dynamic interplay between rates of helminth establishment and mortality. Depending on the intensity of exposure, changes in worm load with duration of host infection vary from monotonic growth to a stable average parasite burden to convex curves in which the average load attains a maximum value before decaying in old animals. These trends are similar to observed patterns of S. mansoni infection in human communities.

Attempts to manipulate specific responses to induce resistance to Schistosoma mansoni in Kenyan baboons (Papio anubis)

Attempts were made to manipulate specific responses of baboons to protect them from infection with Schistosoma mansoni. In Experiment 1, eosinophilia was induced in naive baboons with Trichinella spiralis larvae given intravenously before intraperitoneal injection of globulin fractions from S. mansoni-infected baboon sera and subsequent percutaneous exposure to S. mansoni cercariae. In Experiment 2, baboons with 8- or 32-week-old primary S. mansoni infections received T. spiralis i.v. before an S. mansoni challenge. In experiments 3 to 5 respectively, naive baboons received intramuscularly before challenge: formalin-fixed S. mansoni schistosomula, with Bordetella pertussis as an adjuvant; a preparation of S. mansoni adult worm teguments; and a preparation of IgE-immune complexes obtained from S. mansoni-infected rat sera, with Freunds Complete Adjuvant. Minor, but statistically insignificant, protection was obtained in Experiments 2 (32-week infections) and 3, but was far less than that given by intact, irradiated living vaccines. There are signs on the horizon of non-living vaccines protecting rodents against S. mansoni infection and it would be prudent, as with drugs, to test these in primates before proceeding to man. The results of our experiments, though essentially negative, should help the design of any future vaccine trials in primates.

Factors affecting the levels of antibody- and complement-dependent eosinophil-mediated damage to schistosomula of Schistosoma mansoni in vitro

In experiments designed to test why high levels of antibody-dependent, eosinophil-mediated killing of schistosomula are routinely observed in this laboratory, several factors that may contribute to variations in eosinophil activity were examined. The most important factors were: (1) the source of eosinophils, with marked variation being demonstrated not only, as previously shown, between individuals, but also between different cell preparations from a single individual; (2) the serum used as a source of anti-schistosomulum antibodies and (3) the age of the schistosomula at the time of assay. In contrast, addition of fresh normal serum as a source of complement had a relatively slight effect when the killing assay was carried out in round bottomed tubes. A more marked enhancement was observed in flat bottomed microtitre plates, and it is suggested that this enhancement may be attributable to the release of chemotactic complement components. No difference was observed between a laboratory maintained and a recently derived isolate of Schistosoma mansoni, either in initial susceptibility or in loss of susceptibility after 3.5 h of culture. In contrast to the marked effects of eosinophils under most conditions tested, there was no evidence for extensive neutrophil-mediated damage under the same conditions.

Vaccination against Schistosoma mansoni with purified surface antigens

Two surface antigens were isolated from young or adult schistosomes by affinity chromatography with monoclonal antibodies. Vaccination with an antigen having a molecular weight of 155,000 gave partial protection against challenge in some batches of mice and in a group of cynomolgus monkeys. Vaccination with an antigen having a molecular weight of 53,000 gave similar levels of protection in mice. The results demonstrate that protection can be obtained with single antigens, but the precise requirements for reproducible vaccination are as yet unknown.

Immunity after treatment of human schistosomiasis mansoni. II. Identification of resistant individuals, and analysis of their immune responses

Intensities of re-infection were monitored at three-monthly intervals after treatment of Schistosoma mansoni infections in a group of 119 Kenyan schoolchildren, whose levels of water contact were also observed. 22 children showed high reinfection intensities (greater than 100 eggs per gram of faeces) by 12 months after treatment, and were considered to be susceptible. Out of 70 children who showed low reinfection intensities during the same period (less than 30 eggs per gram), 35 showed high levels both of total water contact and of contact with sites containing infected snails. In these children, the relative lack of reinfection could not be attributed to a lack of exposure, and they were classified as resistant to reinfection. Comparison of the two groups, resistant and susceptible, revealed no difference in pretreatment intensities of infection. However, there was a marked difference in age, the mean age of the resistant group being two years greater than that of the susceptible group, within a restricted starting age range. These findings indicated that resistance was an acquired and age-dependent phenomenon, not obviously related to previous egg-induced pathology. Studies of immune responses revealed no clearcut correlate of resistance, but there were interesting differences between the two groups. Whereas anti-egg antigen responses declined after treatment to a greater extent in the resistant than in the susceptible group, antibodies mediating eosinophil-dependent killing of schistosomula rose markedly in both groups, strongly suggesting that the resistant children were being exposed to cercariae. Anti-adult worm antibodies rose sharply in both groups immediately after treatment, and thereafter declined to pretreatment levels. Although some individual children showed high levels of IgE anti-schistosomulum antibodies, there were no significant differences between the two groups. Since all children showed detectable levels of antibodies mediating eosinophil-dependent killing of schistosomula, the possibility was considered that such antibodies might be a necessary, but not a limiting, factor in immunity. Instead, the functional state of the effector cells mediating antibody-dependent killing might be limiting. Eosinophil levels, measured as an indirect estimate of eosinophil functional activity, did not differ between the two groups. There were, however, marked differences between different individuals in their capacity to produce eosinophil-stimulating monocyte mediators, and although this cannot yet be related to resistance, this aspect is worth further study.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo protection against S. mansoni infection by monoclonal antibodies

A monoclonal antibody of the IgG1 class (27.21), that was previously shown to be cytotoxic in vitro to the schistosomula, conferred partial protection in vivo against challenge infection with S. mansoni. Monoclonal antibodies of the IgE class (54.10) were not effective in these experiments, but were capable of causing specific degranulation of basophilic cells upon contact with the young larva (3-6 h). The relevance of the IgE antibodies to protection observed in immunized mice is discussed.

Cell-mediated damage to helminths

Although it is difficult to draw any sweeping conclusions that would be applicable to all helminth infections, the main features that are emphasized in this review may be summarized briefly. Pathogenic helminths, although extremely diverse in structure and behaviour, have one common feature, namely that they present to the host's defenses large, non-phagocytosable surfaces. Because of this, they are susceptible to a range of effector mechanisms differing either quantitatively or qualitatively from those that are active against other parasites or against normal or abnormal host cells. As an extreme example, the various types of cytotoxic lymphocyte, with one interesting exception, are inactive against helminths. Instead, helminth infections are characterized by high IgE responses and increased numbers of circulating eosinophils. Such eosinophils are activated, and show a marked capacity to kill a variety of target helminths in vitro. Further activation may occur in response to mast cell mediators released as a result of IgE-dependent degranulation; and IgE, as well as IgG and complement, can mediate eosinophil attachment and killing. It may therefore be suggested that the eosinophil/IgE/mast cell axis represents a powerful host defense against helminth infections. IgE can also mediate macrophage-dependent killing of several helminths, a process which involves a functional change in the macrophage, resembling activation. Although eosinophil-mediated and IgE-dependent macrophage-mediated effects are particularly potent, other effector cells are not excluded: in certain circumstances, neutrophils and conventionally activated macrophages may be equally or more effective. Neutrophils appear to act solely by oxidative killing mechanisms, whereas degranulation and the release of toxic granule contents is equally or more important in eosinophil-mediated damage. Different stages of different helminths vary in their degree of susceptibility to different mechanisms. Eosinophils appear to be somewhat less active than neutrophils against ensheathed nematodes, whereas trematodes and exsheathed nematodes are highly susceptible to eosinophil attack. In many experimental helminth infections, studies in vivo suggest a role for antibody-dependent cell-mediated immune effector mechanisms. The identity of the effector cell is difficult to establish because of a lack of techniques for specific manipulation of individual cell types, but histological studies frequently point to a strong eosinophil or macrophage involvement. The development and analysis of in vitro assays allows the study of immune effector mechanisms in man.(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and antigen analysis of surface tegument membranes from schistosomula of Schistosoma mansoni

The outer tegument membranes of Schistosoma mansoni schistosomula have been removed by mechanical disruption in a hypotonic salt solution and partially purified by differential and sucrose density gradient centrifugation. Fractionation was monitored by measuring increase in specific activity of bound [125I]wheat-germ agglutinin (WGA), alkaline phosphatase and calcium-stimulated ATPase. Two-dimensional IEF/SDS polyacrylamide gel electrophoresis was used to analyse the peptide composition of the isolated membranes and to compare and contrast with lactoperoxidase/glucose oxidase surface labelled peptides. At least 35 surface-labelled peptides were resolved on the two-dimensional maps: all were also present in the membrane material recovered from the sucrose gradient. Western blot analysis demonstrated a marked heterogeneity in the antibody response of infected human patients to individual membrane antigens. The antigenic profile of membranes isolated from cercariae, 18 and 96 h schistosomula were compared using Western blots: some minor differences were observed between the three preparations.

Isolation and partial characterization of the tegumental outer membrane of schistosomula of Schistosoma mansoni

Separation of the external membranes from freshly converted mechanical schistosomula of Schistosoma mansoni was achieved by osmotic shock under hypertonic conditions, followed by mechanical shearing and ultracentrifugation. Prior to treatment, the schistosomula were surface labeled by introduction of N-DNP-epsilon-aminocaproylphosphatidylethanolamine molecules into their lipid bilayer followed by anti-DNP antibodies and stained with either 125I-protein-A or ferritin labeled secondary anti-DNP antibodies. This label provided a membrane marker by which the purity of the preparation could be assessed at each stage. Fluorescence staining with FITC-conjugated secondary antibodies prior to treatment revealed that the homogeneously stained membrane of the intact schistosomula became swollen and ruptured after the osmotic shock. The isolated membrane pellet was intensely fluorescent. Electron microscopical examination revealed mostly vesicles, some of them with organized multilayer assembly. The vesicles were ferritin labeled, indicating that they originated from the outer surface membrane of the schistosomula. A 100 fold enrichment in the alkaline phosphatase activity and about 300 fold enrichment in acetylcholinesterase activity in the membrane preparations, as compared to the intact schistosomula, was found. The isolated tegument was analyzed by SDS-polyacrylamide gel electrophoresis. The pattern obtained showed three major bands, of molecular weights 69 000, 45 000 and 12 000 alongside with a large number of minor bands. Immunoprecipitation of the isolated 125I-labeled membrane antigens with antisera from chronically infected mice revealed these three major bands together with three other bands of molecular weight 38 000, 23 000 and 16 000.

Monoclonal antibodies against microorganisms

The recent spread of hybridoma technology among laboratories has promoted the development of monoclonal antibodies against a wide variety of infectious disease agents. While monoclonal antibodies theoretically represent an excellent (perhaps superior) alternative to conventional antisera as diagnostic, therapeutic or laboratory reagents, traditional antisera may be preferable to monoclonal antibody in some circumstances because of the fixed affinity and specificity as well as the limited functional capacities of some antibodies. The acceptance of monoclonal antibodies by the clinical microbiologist and physician must await proof of their reliability, safety and efficacy.

Schistosoma mansoni: escape from complement-mediated parasiticidal mechanisms following percutaneous primary infection

Schistosomula were recovered from the skin of mice following primary infections. On the surface of such "infecting schistosomula", mouse C3 could not be detected by immunofluorescence. Subsequent incubation in vitro with fresh mouse serum led to the effective deposition of mouse C3 on schistosomula only when they were recovered within a few hours but not after one or two days following infection. In vitro deposited murine C3c was lost from i.v. injected schistosomula in the mouse circulation within one day as was human C3c. Infecting schistosomula exhibited a close to complete resistance to the lytic in vitro activity of human complement. This resistance was complete in older parasites. It existed in spite of the presence of parasite-bound human C9, which was detectable on all developmental stages of schistosomes following incubation in fresh, but not inactivated human serum. Lung schistosomula, 3-week and 6-week-old schistosomes were resistant to cellular cytotoxicity upon incubation with fresh human serum and rat peritoneal exudate cells although cell adherence mediated by human C3b was demonstrated with lung worms. The data suggest that schistosomula may evade in vivo the lytic activity of complement and also complement-mediated cellular cytotoxicity. Depending on the species of serum, this can be demonstrated in vitro by lack of opsonization or by resistance to lytic and cellular attack mechanisms.

Genetic engineering and a schistosome vaccine

Early attempts to develop a vaccine against schistosomiasis have been largely empirical, relying on use of live, attenuated larvae or homogenates and extracts of the various life-cycle stages of the parasite. Although some success has been reported using attenuated larvae, by and large the early attempts at vaccination have proved unsuccessful. Recently, we have begun to apply recombinant DNA technology to the problem of schistosome vaccine development. A majority of the evidence pertaining to immunity against schistosome infection suggests that the schistosomula is the primary target of the immune attack. Therefore, it follows that antigens found on the surface of the young larvae would be prime candidates for inclusion in an experimental vaccine. This lecture summarizes work aimed at the identification of schistosomula surface antigens and molecular cloning of their corresponding genes. cDNA clones have been prepared from both egg and adult mRNA. Relevant clones have been identified by "message" selection. This approach relies on identification of relevant antigen precursors among in vitro translation products. At least 5 schistosomula surface-antigen precursors have been recognised among in vitro translation products directed by adult mRNA. Some of these peptides are also encoded by mRNA isolated from eggs. One egg-specific peptide has been cloned and there is evidence that this is immunodiagnostic for Schistoma mansoni infections. A long-term aim is hoped that such molecules would contribute to a schistosome vaccine as well as providing immunodiagnostic reagents.

Monoclonal antibodies distinguish between storage and secreted forms of eosinophil cationic protein

The toxic effects of eosinophils on parasites and cells are due largely to the secretion of various granule proteins, following stimulation. In order to study this secretory process (degranulation) further, we have raised mouse monoclonal antibodies against both human eosinophil granule extracts and secretion products. From immunocytochemical studies it appears that one antibody, EG1 , recognized both the storage and secreted forms of eosinophilcationic protein (ECP), whereas antibody EG2 only bound to ECP during secretion (and extraction). This antibody also bound to eosinophil protein-X (EP-X). As both antibodies stained eosinophils in formalin-fixed tissues, they were used to demonstrate sites of eosinophil activation and secretion in chronic urticaria. The capacity of monoclonal antibodies to detect differences between storage and secreted forms of proteins is an important property of these reagents with many potential applications in cell biology.