Rat IgE directed against schistosomula-released products is cytotoxic for Schistosoma mansoni schistosomula in vitro

The immunogenicity of molecules shed by schistosomula into culture medium (antigens present in schistosomula-released products, SRP-A) has been studied. The results obtained show that SRP-A preferentially induce an IgE response when injected into rats, without the need for adjuvants. Moreover, anti-SRP-A IgE is cytotoxic in vitro for the larvae in the presence of macrophages, eosinophils or platelets, which have previously been demonstrated as being the three efficient killer cells for schistosomula in the presence of specific IgE. Immunofluorescence analysis locates the target antigens at the schistosomulum surface. Among the antigens recognized by anti-SRP-A IgE, two molecules of 26 and 22 kDa have been identified by sodium dodecyl sulfate polyacrylamide gel electrophoresis followed by western blotting.     

Cross-linking of immune complexes by human mononuclear phagocytes

Incubation of immune complexes (IC) bound to plastic surfaces with human blood monocytes for 48 hours resulted in the cross-linking of a proportion of antibody molecules. This process was largely inhibited by the addition of sodium azide to the cultures. Cross-linking was defined as the inability of strong chaotropic solutions (3 M MgCl₂ or 5 M guanidine) or acid pH (0.1 N HCl) to solubilize¹²⁵J-labeled rabbit anti-human serum albumin attached to plastic-bound antigen. Addition to the cultures of a suitable hydrogen donor such as catechol (0.5 mM) resulted in a large increase in cross-linking of IC. This process was shown to depend on the presence of viable phagocytic cells because incubation with dead monocytes or with viable T lymphocytes failed to induce cross-linking. Quantitation of rabbit immunoglobulin remaining in the wells by enzyme-linked immunoassy techniques excluded the possibility that the increase in¹²⁵I bound was merely due to a transiodination reaction. Experiments using various oxygen metabolite inhibitors and scavengers indicated that catechol-dependent protein cross-linking depended on the action of hydrogen peroxide and enzyme systems inhibitable by sodium azide, probably monocyte-peroxidase. Superoxide dismutase,¹O₂, and OH · radical scavengers failed to inhibit cross-linking, whereas addition of catalase resulted in almost complete abolition of the process. These observations suggest that catechol-dependent cross-linking of IC may be due to oxidation of catechol to orthoquinone and that this strong oxidant is responsible for nonenzymic chemical action on proteins leading to intermolecular covalent bond formation. Cell-mediated protein cross-Sinking by oxidative mechanisms may be a prominent feature of drug-related reactions and of acute and chronic inflammatory processes in general. The possible mechanisms involved in catechol-dependent and -independent cross-linking of IC by human mononuclear phagocytes are discussed.     

Protection against experimental Schistosoma mansoni schistosomiasis achieved by immunization with schistosomula released products antigens (SRP-A): role of IgE antibodies.

Schistosomula-released products (SRP-A) have been shown to induce preferentially a significant IgE response against Schistosoma mansoni schistosomula when injected into rats, in the absence of adjuvant. The present work provides additional evidence of the in vivo relevance of the anti-SRP-A target antigens. Two strains of rat (Brown Norway and Fischer) were immunized with SRP-A and infected percutaneously. A significant level of protection (up to 83% reduction in worm burden) was observed. Passive transfer experiments carried out with anti-SRP-A or IgE-depleted anti-SRP-A sera suggested the preponderant role of antibodies and particularly of IgE in the protective immunity developed by Fischer rats. Platelets and macrophages recovered from such immunized rats had surface IgE as demonstrated by immunofluorescence analysis with FITC anti-IgE, and have been shown to be directly cytotoxic for schistosomula. The chemiluminescence observed when the macrophages were incubated with anti-IgE suggested the presence of IgE on the surface of these cells.     

Cleavage of human IgE mediated by Schistosoma mansoni

BACKGROUND: IgE-mediated mechanisms are important in protection against helminth parasites. However, schistosomes are long-lived in mammalian hosts, presumably as a result of immune evasion strategies. We sought evidence for one such strategy, namely specific cleavage of host IgE. METHODS: Human IgE, IgA and IgG were incubated with extracts from cercarial and schistosomular stages of Schistosoma mansoni or with schistosomular culture supernatants. The resulting products were analysed by Western blotting with Ig-specific antibodies. Numerous protease inhibitors were assessed for ability to inhibit the observed cleavage of IgE by the extracts. Partial purification of the IgE-proteolytic activity from cercarial extract was achieved by gel filtration. To test IgE function, we compared the abilities of untreated and schistosomular-treated IgE to mediate rosette formation through interaction with Fcepsilon receptors. RESULTS: Cercarial and schistosomular extracts were found to cleave human, mouse and rat IgE but not human IgA1, IgA2 or IgG1. Schistosomular culture supernatants displayed similar proteolytic activity towards IgE. Immunoblotting suggested that cleavage occurred close to the Cepsilon2/Cepsilon3 domain interface of the IgE heavy chain. PMSF and elastatinal inhibited cleavage, suggesting that the protease involved is an elastase-like serine protease, particularly since porcine pancreatic elastase also cleaved IgE to give similar-sized products. Further, the chloromethyl ketone derivatized peptide MeO-Suc-Ala-Ala-Pro-Leu- CMK, known to specifically inhibit the schistosome elastase, prevented IgE cleavage. Cleavage of human IgE rendered the antibody molecule unable to interact with U937 cells expressing FcepsilonRII. CONCLUSIONS: An elastase-like protease in S. mansoni is able to render IgE non-functional.     

Surface antigens on Schistosoma mansoni. II. Adsorption of a Forssman-like host antigen by schistosomula

A study was made of the nature of mouse (host) antigens adsorbed by schistosomula of Schistosoma mansoni. Using the mixed antiglobulin test, extracts of a number of individual mouse tissues were tested for their ability to coat schistosomula. All were effective to some extent, with the greatest activity being found in extracts of the lung and spleen. Antibodies against the schistosomulum-coating antigen as well as surface host antigens of adult Schistosoma mansoni were removed by absorbing with erythrocytes from a number of Forssman-positive but not Forssman-negative animal species. These antibodies were also absorbed by Forssman-positive guineapig kidney extract and methanol soluble (Forssman-positive) but not insoluble fractions of sheep erythrocyte stromata and mouse lungs. Schistosomula could be coated in vitro with methanol soluble fractions of mouse lung and erythrocytes and sheep erythrocytes. Though both mouse and sheep coating antigens reacted with anti-mouse and anti-sheep antibodies, reactions were stronger with the homologous antiserum. It was concluded that schistosomula of Schistosoma mansoni adsorb from mice an antigen similar but not identical to the Forssman antigen of sheep erythrocytes, and that this antigen is also found on the surface of adult worms.     

Signal transduction events in human peripheral blood mononuclear cells stimulated by Schistosoma mansoni antigens

Activation of protein tyrosine kinases (PTKs) is a common step of T cell stimulation. However, the relationship between PTKs and activation of peripheral blood mononuclear cells (PBMC) from intestinal chronic schistosomiasis patients has not been explored yet. In this study, we investigated the participation of Lck and ZAP-70 protein tyrosine kinases (PTKs), as well as PLC-gamma1 and Shc proteins in PBMC activation by Schistosoma mansoni antigens. PBMC were stimulated with SEA (soluble egg antigen) or SWAP (soluble worm preparation), lysed, precipitated with specific antibodies and the level of tyrosine phosphorylation evaluated. Our results show that Lck and Shc were phosphorylated upon stimulation of the cells with SWAP, as well as with SEA. However, the phosphorylation level was more pronounced in SWAP than in SEA-stimulated cells. Phosphorylation of ZAP-70 was observed only in SWAP stimulated cells. Additionally, PLC-gamma1 phosphorylation was not observed in PBMC stimulated with SEA. Together, these results indicate that SEA and SWAP induce PBMC proliferation through distinct intracellular signaling pathways. Moreover, the weaker response of PBMC to SEA compared to SWAP stimulation suggests down-regulation of cells from intestinal chronic schistosomiasis patients to SEA, which may occur during immunomodulation to S. mansoni response.     

Schistosoma mansoni anticomplementary antigens (SACA): Detection in schistosomula and adult worms

In previous studies we have shown that schistosomula of Schistosoma mansoni are able to activate complement (C), in the absence of antibodies, by both the classical (CP) and the alternative C pathway (AP). In the present work we have demonstrated that certain factors present in the antigen extract of schistosomula and adult worms also showed an anticomplementary activity. These schistosome anticomplementary antigens (SACA) were found in the low molecular weight fraction (< 35,000 daltons) of the whole extract of adult schistosomes and were able to deplete C through both the CP and the AP.     

The acquisition of antigens in the intercellular substance of mouse skin by schistosomula of Schistosoma mansoni.

Serum from patients suffering from the autoimmune skin disease pemphigus vulgaris has been used to demonstrate the presence of intercellular substance (ICS) on the surface of these chistosomula of Schistosoma mansoni which has penetrated mouse skin in vitro or during a percutaneous infection. ICS was absent from mechanically transformed schistosomula or those formed in the peritoneal cavity of mice. Schistosomula which penetrated mouse skin rapidly in vitro acquired very little of the ICS. It was found during a percutaneous infection that schistosomula recovered from the skin after 10 min had no detectable ICS, while those recovered after 2 hr and 24 hr gained increasing quantities of the material. It is concluded that schistosomula which are delayed in their exit from the skin acquire more ICS. However, this material must be shed during subsequent migration since schistosomula from lung and liver, and 7-week-old worms do not posses it. The implications of the findings are discussed.     

Circulating antigens, immune complexes and C3d levels in human schistosomiasis: relationship with Schistosoma mansoni egg output.

Circulating schistosome antigens (CSA), circulating immune complexes (CIC) and C3 breakdown product - C3d - were investigated in human schistosomiasis in comparison to the S. mansoni egg count. A close relationship was observed between the mean number of eggs/g of stool and the detection of CSA (evaluated by the radioimmunoprecipitation-PEG assay - Ripega), CIC (Clq-binding test) and C3d levels (quantitated by radial immunodiffusion). All the patients with more than 500 S. mansoni eggs/g of stool also presented antigen '4', specific of the genus Schistosoma, in the serum. A significant correlation was noticed between levels of CSA and CIC. This suggests the involvement of several schistosome antigens in the detected CIC. No relationship was noted between CIC and C3d levels. In contrast, there was a highly significant correlation between levels of CSA and C3d. The interaction between certain schistosome antigens and the complement system is discussed.     

Acquisition of human blood group antigens by Schistosoma mansoni.

Juvenile forms of Schistosoma mansoni (schistosomula) have been cultured in human blood of various specificities and tested for the presence of blood group substances on their surfaces. The tests employed were survival following transfer into rhesus monkeys immunized against human blood substances, mixed agglutination reactions, and immunofluorescence. A, B, H AND Lewisb+ antigens were expressed at the surface when the parasites were cultured in blood of appropriate specificities. Rhesus, M N S, AND Duffy antigens could not be detected on the parasite surface following culture. The evidence suggests that the expressed blood group antigens are of host origin and are acquired by the parasite during culture, probably in the form of glycolipids or megaloglycolipids. It is likely that these substances are also acquired by parasites in the bloodstream of man. They may serve to mask surface parasite antigens, and so enable schistosomes to evade parasite-specific humoral or cellular immune responses.     

Freshly isolated and cultured human monocytes obtained by plasmapheresis kill schistosomula of Schistosoma mansoni.

The efficacy of human peripheral blood monocytes (PBM) in killing of schistosomula is controversial. The purpose of this study was to determine the schistosomulacidal activity of human monocytes isolated by two different techniques. Peripheral blood monocytes were obtained either by venipuncture (PBMv) or plasmapheresis (PBMp), purified on Ficoll-Paque, and cultured briefly. The cells then were incubated with schistosomula (cell parasite ratio of 10(4):1) for 16 to 18 hours with or without interferon-gamma IFN-gamma (600 U/ml) or sera from patients with schistosomiasis as a source of antischistosomal antibodies (HASA). Freshly isolated PBMv treated with IFN-gamma or HASA did not kill schistosomula. Freshly isolated PBMp alone killed 22 +/- 13% (mean +/- standard deviation [SD]; n = 9) of worms over background and after incubation with IFN-gamma and HASA, 30 +/- 17%. PBMp cultured in vitro for 7 days killed 50 +/- 15% (mean +/- SD; n = 12) of the schistosomula. Pretreatment of the cells with IFN-gamma and incubation with HASA did not significantly enhance the parasite killing beyond this level. Electron microscopy showed that freshly isolated PBMp attached to the worms and fused occasionally with the outer tegumental membrane. Granules constituted 1.4% of the cytoplasmic volume. Degranulation onto the parasite surface was not observed. Peripheral blood monocytes obtained by plasmapheresis accumulated glycogen during in vitro culture with the parasite and released threefold more H2O2 than PBMv after exposure to phorbol myristate acetate. Thus plasmapheresis increases the schistosomulacidal activity of PBM, enhances the generation of H2O2 and promotes the accumulation of glycogen.     

IgE-dependent killing of Schistosoma mansoni schistosomula by human platelets: modulation by T cell products

The in vitro stimulation of T lymphocytes is known to induce the release of factors that possess distinct biological activities. In the present report, we describe the presence, in supernatants of Schistosoma mansoni antigen stimulated T cells from S. mansoni infected patients, of a factor able to inhibit the IgE-dependent platelet cytotoxicity of the same individuals toward the young larvae of S. mansoni.     

Eosinophil-enriched inflammatory response to schistosomula in the skin of mice immune to Schistosoma mansoni.

Exposure of the mouse skin to Schistosoma mansoni cercariae gives rise to acute, exudative inflammation in both normal and immune mice, but the immune response is anamnestically accelerated and is oesinophil-enriched, thereby enhancing opportunities for tegumental contact of schistosomula with host leukocytes, particularly with eosinophils. Many of the inflammatory changes occurring within the first 48 hours after exposure are due to cercarial products, e.g., "penetration tracts," but some remain demonstrable when schistosomula metamorphosed in vitro are injected intradermally and are therefore directed against the schistosomula themselves, such as the leukocyte "streaming patterns" seen in their pathways. In contrast to earlier observations in primates, cellular responses to schistosomula in the mouse lung 4 days after penetration are minimal in either normal or immune mice. Thus, immune cellular responses to schistosomula in mice are limited to an early time period after cercarial penetration and are morphologically suggestive of an antibody-mediated response rather than of delayed hypersensitivity. Our observations complement earlier evidence suggesting that antibody-mediated host leukocyte contact with schistosomula initiates the killing of challenge parasites in immune mice, with the eosinophil probably playing a crucial role.     

Ultrastructure of the attack of eosinophils stimulated by blood mononuclear cell products on schistosomula of Schistosoma mansoni.

Purified human eosinophils were treated with peripheral blood mononuclear cell supernatants containing eosinophil cytotoxic enhancing activity (ECEA). Schistosomula of Schistosoma mansoni which had been coated either with antibody (Ab) from the sera of infected patients or with the lectin concanavalin A (Con A) were incubated with ECEA-treated and untreated cells for 2 minutes to 12 hours and examined ultrastructurally. Killing was assayed at 18 hours. ECEA caused an increase in the killing of Ab-coated worms, but Con-A-coated worms were not killed by either ECEA-treated or untreated cells. Eosinophils began to degranulate on Ab-coated worms within 2 minutes and continued to degranulate, so that by 12 hours about half of the parasites had greater than 50% of their surface covered by discharge material. The ECEA-treated cells degranulated more than the untreated cells. There was much less discharge material on Con-A-coated worms than on Ab-coated worms. Eosinophils adhered to discharge material on the surface of both Ab- and Con-A-coated parasites. At 3 and 12 hours, lysed cells and cell fragments were also seen adhering to discharge material. In the absence of discharge material the cells adhered to residual glycocalyx or to the tegumental outer membrane. These studies suggest that eosinophils kill schistosomula by progressively degranulating onto their surface over many hours and that the increased toxicity caused by ECEA is due to an increase in discharge.     

Schistosoma mansoni: phospholipid methylation and the escape of schistosomula from in vitro cytotoxic reaction

The ability of Schistosoma mansoni schistosomula to evade in vitro cytotoxic activity of antibodies plus complement is shown to be increased by incubation with Concanavalin A (Con A) or with non-immune inactivated human serum. This effect was not observed if S-adenosyl-homocysteine (SAH) a methyltransferase inhibitor was added to the incubation medium. Methyl group incorporation occurs in schistosomulum phospholipids if parasites are incubated in Earle's balanced salt solution. This incorporation is increased by Con A addition and this increase is inhibited by SAH. Supernatants of schistosomula incubated in culture media containing Con A were able to promote phospholipid methylation, showing that methyltransferases were liberated into the culture media. The possible roles played by these phenomena in host-parasite interactions are discussed.     

Protection by phospholipids of Schistosoma mansoni schistosomula against the action of cytotoxic antibodies and complement

Schistosoma mansoni schistosomula cultured in the presence of phospholipids showed a decreased sensitivity to the lethal complement-mediated action of anti-schistosome antibodies. Phosphatidyl choline, sphingomyelin and phosphatidyl ethanolamine had a protective action on the schistosomula transformed in vitro by passage through the skin or by a mechanical procedure. Phosphatidyl choline acted regardless of its fatty acid composition. Phosphatidyl serine and phosphatidic acid did not protect. Thus, it appears that phospholipids can play a role in parasite resistance to immune attack by cytotoxic antibodies and complement, and that this role is specific to certain phospholipid types.     

Transgene expression in Schistosoma mansoni: introduction of RNA into schistosomula by electroporation

Despite their significance in human and veterinary medicine, and the ability to maintain the parasites in the mouse, relatively little functional detail is available regarding the biology of schistosomes. This deficit is due largely to the lack of well-developed molecular tools for manipulating gene expression in these parasites. Here, we describe an electroporation protocol that provides a routine approach for efficiently introducing nucleic acids into schistosomes. Using luciferase-encoding RNA for electroporation, and luciferase activity as a read-out, we established 400 microg/ml of RNA, and a 20 ms pulse at 125 V using a square wave electroporation generator to be optimal for electroporating schistosomes. Under these conditions schistosomula from 1 hr to 18 hr old could be successfully electroporated, the majority of parasites within a population expressed the introduced RNA, and acute mortality was negligible. Electroporation, as described here, makes possible experimental studies using transiently expressed constitutively active and/or dominant negative mutant proteins, etc. In addition, the finding that electroporation can be used to introduce RNA into schistosomula raises the possibility of using this approach to introduce either DNA constructs or dsRNA sequences, both of which might be expected to have longer-term, ideally inheritable, effects.     

Surface antigens on Schistosoma mansoni. I. Demonstration of host antigens on schistosomula and adult worms using the mixed antiglobulin test

Using the mixed antiglobulin test it was possible to demonstrate mouse-like antigens on the surface of schistosomula and adult Schistosoma mansoni, but not cercariae. The results indicated that schistosomula incubated with mouse tissue in vitro (newborn mouse extract) or in vivo (peritoneal cavity) adsorb mouse antigens onto their surfaces. Mouse antigens were also demonstrated on the surfaces of adult worms. In contrast, no mouse antigens could be demonstrated on cercariae or cercarial tails which had been incubated with mouse antigens, or on schistosomula or cercariae which had not been exposed to mouse antigens. Adsorption of mouse antigens by formalin-fixed, and therefore, non-viable schistosomula suggested that host surface antigens are passively adsorbed by schistosomula and are not actively produced by the parasite.     

Detection of antibodies against ovum antigens in Schistosoma mansoni bilharziasis

Fifty-five sera from bilharziasis patients from West Indies and thirty-five control sera (from normal subjects and patients with hydatidosis and fascioliasis) were examined by ELISA for antibodies reacting with S. mansoni soluble egg antigen MSA1 . MSA1 antigen was prepared according to Pelley and coupled to isothiocyanate substituted plastic discs. Sensitivity and specificity of reaction were good. Moreover, in experimental infection in mice, sera of mice cured with praziquantel became negative. The sera were also examined by a whole egg antigen in counter immunoelectrophoresis. The results of the two different techniques correspond well although ELISA using purified antigen prove to be more specific and sensitive.     

Purification and characterization of proteases secreted by transforming schistosomula of Schistosoma mansoni

Schistosomula of Schistosoma mansoni which are mechanically transformed at 4 degrees C and are then incubated at 37 degrees C in defined medium spontaneously secrete two proteases, a major one of 28 kDa and a minor one of 60 kDa. These were purified by ion exchange chromatography on DEAE-cellulose and gel filtration on Ultrogel AcA 54 with yields of 33% and 29%, respectively. Both appeared as single bands by silver staining following sodium dodecyl sulphate-polyacrylamide gel electrophoresis analysis. The 28 kDa protease is a glycoprotein that has a pI of 11 or higher and an optimal activity around pH 9.0. It cleaves casein, gelatin and human C3 and C3b. It is metal-ion independent and is inhibited by diisopropyl fluorophosphate, phenylmethanesulfonyl fluoride, soy-bean trypsin inhibitor, alpha 1 antitrypsin, Zn2+ ions, sodium dodecyl sulphate and normal human serum. The 60 kDa protease is a glycoprotein with a pI of 9.2. It can also cleave casein and gelatin and its activity is inhibited by phenylmethanesulfonyl fluoride but not by diisopropyl fluorophosphate or sodium dodecyl sulphate. We suggest that these proteases may play a role during cercarial penetration of the skin and in shedding of the cercarial glycocalyx.