Automated imaging and other developments in whole-organism anthelmintic screening

Helminth infections still represent a huge public health problem throughout the developing world and in the absence of vaccines control is based on periodic mass drug administration. Poor efficacy of some anthelmintics and concerns about emergence of drug resistance has highlighted the need for new drug discovery. Most current anthelmintics were discovered through in vivo screening of selected compounds in animal models but recent approaches have shifted towards screening for activity against adult or larval stages in vitro. Larvae are normally available in greater numbers than adults, can often be produced in vitro and are small enough for microplate assays. However, the manual visualization of drug effects in vitro is subjective, laborious and slow. This can be overcome by application of automated readouts including high-content imaging. Incorporated into robotically controlled HTS platforms such methods allow the very large compound collections being made available by the pharmaceutical industry or academic organizations to be screened against helminths for the first time, invigorating the drug discovery pipeline. Here, we review the status of whole-organism screens based on in vitro activity against living worms and highlight the recent progress towards automated image-based readouts.

IFN-gamma is associated with risk of Schistosoma japonicum infection in China

Before the start of the schistosomiasis transmission season, 129 villagers resident on a Schistosoma japonicum-endemic island in Poyang Lake, Jiangxi Province, 64 of whom were stool-positive for S. japonicum eggs by the Kato method and 65 negative, were treated with praziquantel. Forty-five days later the 93 subjects who presented for follow-up were all stool-negative. Blood samples were collected from all 93 individuals. S. japonicum soluble worm antigen (SWAP) and soluble egg antigen (SEA) stimulated IL-4, IL-5 and IFN-gamma production in whole-blood cultures were measured by ELISA. All the subjects were interviewed nine times during the subsequent transmission season to estimate the intensity of their contact with potentially infective snail habitats, and the subjects were all re-screened for S. japonicum by the Kato method at the end of the transmission season. Fourteen subjects were found to be infected at that time. There was some indication that the risk of infection might be associated with gender (with females being at higher risk) and with the intensity of water contact, and there was evidence that levels of SEA-induced IFN-gamma production were associated with reduced risk of infection.

Down-regulation of specific antigen-driven cytokine production in a population with endemic Schistosoma japonicum infection

Schistosome antigen-driven cytokine responses and antischistosome antibody levels of residents of a Schistosoma japonicum endemic island in Poyang Lake, Jiangxi Province were studied before and 45 days after treatment with praziquantel. IL-4, IL-5, IL-10 and INF-gamma were all detected in the supernatants of whole-blood cultures after stimulation with schistosome soluble egg antigen (SEA) and soluble worm antigen preparation (SWAP). The percentages of subjects producing detectable amounts of each cytokine assayed were higher in the group who were negative by stool examination at the start of the study than in those who were initially stool positive. After praziquantel treatment the percentages of subjects producing both type I and type II cytokines increased. This suggests that the production of both types of cytokine was down-regulated in the presence of live, egg-laying S. japonicum adult worms but that this was reversible by treatment. In contrast, the antibody studies showed higher levels of SWAP and SEA-specific antibodies (IgE, total IgG, IgG4, IgM) in subjects who were originally stool-positive than in those who were stool-negative. After treatment specific IgE responses were elevated, but total IgG and IgG4 anti-SEA and IgM anti-SWAP antibody levels all fell significantly.

Vaccination of mice with a cocktail DNA vaccine induces a Th1-type immune response and partial protection against Schistosoma japonicum infection

Several defined vaccine candidate antigens of Schistosoma japonicum have shown promise in large animal vaccination experiments. However, vaccination of mice in the laboratory with either single recombinant antigens or DNA encoding forms of the individual antigens has so far failed to induce significant protection against S. japonicum cercarial challenge infection as judged by worm reduction, although specific antibodies were generated. This is in contrast to the results achieved using radiation-attenuated vaccines which are highly protective. Even in large animal vaccination experiments, the protection levels obtained with single defined antigens were far below those achieved using the attenuated vaccines. One possible interpretation is that the immune responses induced by single antigen vaccination may not be strong enough to combat the challenging infection. We, therefore, carried out mouse vaccination experiments using a cocktail DNA vaccine comprising four DNA plasmids encoding four different S. japonicum antigens, Sj62, Sj28, Sj23 and Sj14-3-3, respectively. We, also investigated whether co-injection of the mouse IL-12 encoding plasmid with the cocktail DNA vaccine was able to enhance the Th1 responses and hence the protective immunity. Three intramuscular injections of the cocktail DNA vaccine induced a significant Th1-type cellular response with high level of IFN-gamma production by splenocytes upon in vitro stimulation with recombinant antigens. Importantly, significant IgG antibody responses were also induced against crude worm antigens. In two out of three experiments, significant resistance (34-37 and 44-45%, respectively) was demonstrated while another experiment did not show any protection against S. japonicum cercarial challenge infection. Co-injection of the IL-12 encoding DNA did not further enhance these responses, nor the level of resistance, compared with the cocktail DNA alone.

Laboratory and field evaluation of Schistosoma japonicum DNA vaccines in sheep and water buffalo in China

Vaccines are needed to control zoonotic Schistosoma japonicum infection and several vaccine candidates have now been identified. Two of these (Sj28GST and Sj23) have shown particular promise in sheep when injected with Freund's adjuvants. The objective of the present work was to find a vaccine formulation which may have potential for widespread use in the field. DNA vaccine formulations of these antigens were produced and tested first in sheep under laboratory conditions and then in both the laboratory and the field in water buffalo. In both host species partial protection as evidenced by a reduction in parasite counts in vaccinated compared with control animals was induced by both vaccines, and in water buffalo the vaccines were shown to be partially protective in the field as well as in the laboratory. These results suggest that the two DNA vaccines tested here may have potential for large-scale field use.

Comparison of the vaccine efficacy of gamma-irradiated Schistosoma japonicum cercariae with the defined antigen Sj62(IrV-5) in pigs

Development of a vaccine against Schistosoma japonicum which can protect both man and the domestic animal zoonotic reservoirs of infection would be an invaluable tool in attempts to control this infection in those areas in which conventional control methods have failed to break transmission. The pig is a natural host of S. japonicum and because of its anatomical and immunological similarities to humans, it is a potentially valuable host for studies on S. japonicum in particular and schistosomes in general. Radiation-attenuated cercariae are highly effective in inducing immunity in experimental schistosomosis and there are promising reports of partial protection against schistosomes with recombinant-derived individual antigens. In the present study we have set out to establish a protocol for inducing protection with gamma-irradiated cercariae in pigs and to assess the protective capacity of recombinant and naked DNA formulations of Sj62, a 62kDa region of S. japonicum myosin. The corresponding S. mansoni version or Sj62, recombinant IrV-5, has previously been implicated in irradiated vaccine immunity in S. mansoni infections and has been shown to induce high levels of immunity in a variety of hosts. Groups of pigs were immunised three times at 2-week intervals with 2000 cercariae irradiated at 20krad, with Sj62 as a recombinant (rSj62) incorporated in Freund's adjuvant, a micellar preparation, or as a naked DNA construct. Vaccination with irradiated cercariae did not induce significant anti-Sj62 antibody but following intramuscular challenge with 2000 cercariae, the vaccinated pigs showed >95% resistance as assessed by reduced faecal egg output, worm tissue egg burdens and also reduced septal fibrosis. Immunisation with each of the Sj62 formulations induced significant anti-Sj62 antibody responses, the highest titre (>12,800) being with the Freund's preparation but none of the Sj62-immunised groups showed significant resistance to challenge. The data suggest that Sj62 shows little promise as a vaccine candidate for schistosomosis.

Analysis of human antibodies to erythrocyte binding antigen 175 of Plasmodium falciparum

Invasion of human erythrocytes by Plasmodium falciparum merozoites is a multistep process. For many strains of the parasite, part of this process requires that the erythrocyte binding antigen 175 (EBA-175) of the merozoite binds to sialic acid residues of glycophorin A on the erythrocyte surface, a receptor-ligand interaction which represents a potential target for inhibition by antibodies. This study characterizes the reactivity of naturally acquired human antibodies with four recombinant proteins representing parts of EBA-175 (region II, regions III to V, and the dimorphic C and F segment region) in populations in which the organism is endemic. Serum immunoglobulin G (IgG) recognizing the recombinant proteins is predominantly of the IgG1 and IgG3 subclasses, and its prevalence increases with age. In a large population study in The Gambia, serum positivity for IgG or IgG1 and IgG3 subclass antibodies to each of the EBA-175 recombinant antigens was not significantly associated with subsequent protection from clinical malaria. However, there was a trend indicating that individuals with high levels of IgG to region II may have some protection.

Cloning of Schistosoma japonicum 14-3-3 epsilon (Sj14-3-3 epsilon), a new member of the 14-3-3 family of proteins from schistosomes

A new member of the 14-3-3 protein family from Schistosoma japonicum has been identified. Phylogenetic analysis showed that this member belongs to the epsilon subfamily of the 14-3-3 proteins, and it is therefore named Sj14-3-3 epsilon. Consistent with the findings for the previously reported S. japonicum 14-3-3 protein (Sj14-3-3), Southern analysis suggested the presence of more than one gene, and/or introns or allelic polymorphism in this epsilon isoform. By RT-PCR, Sj14-3-3 epsilon was shown to be stage-specifically transcribed, being abundant in adults, present in sporocysts but absent in cercariae. Furthermore, mRNA of the epsilon isoform seemed to be much less abundant in the sporocyst stage, compared with Sj14-3-3. This suggests varying requirements of the different 14-3-3 isoforms at different stages of the life cycle.

Schistosoma mansoni phosphofructokinase: immunolocalization in the tegument and immunogenicity

Schistosoma mansoni depends for its survival on glycolysis. Two glycolytic enzymes, glyceraldehyde-3P-dehydrogenase and triose-phosphate dehydrogenase, found in both the adult and schistosomular tegument, have been reported to confer partial protection against cercarial infection. This paper describes the immunogenic properties of phosphofructokinase (PFK), a rate-limiting enzyme of glycolysis, and its localization in the tegument and adjacent tissues. Recombinant schistosome PFK was used as antigen. A polyclonal antibody against purified PFK from Fasciola hepatica was affinity purified using recombinant PFK and used in combination with immunogold labelling to identify PFK by transmission electron microscopy in cryosections. In both adult worms and in schistosomula most immunogold label localized in the cytoplasmic syncytial region with less being found in the tegument. There was no significant PFK localization within or external to the outer membrane. Sera from mice immunized with recombinant S. mansoni PFK with Freund's adjuvant or alum plus rIL-12 demonstrated high titres of anti-PFK IgG, but no protection against cercarial infection. Sera from mice that were acutely or chronically infected or multiply exposed to irradiated cercariae did not recognize recombinant schistosome PFK in either Western blotting or ELISA. Similarly, sera from humans infected with S. mansoni did not recognize PFK. We conclude that in spite of the high immunogenicity of rPFK in mice, it is not a significant immunogen during the course of infection and does not confer protection from schistosomiasis. One main difference between PFK and the other 2 glycolytic enzymes seems to be the inaccessibility of PFK to the outside surface of the tegument.

Immunogenicity of plasmid DNA encoding the 62 kDa fragment of Schistosoma japonicum myosin

The recombinant Schistosoma mansoni 62 kDa myosin fragment, rIrV-5, is highly protective in experimental animals, however, vaccination of mice and rats with the recombinant Schistosoma japonicum homologue, rSj62, did not induce significant resistance against S. japonicum infection. To explore alternative ways of presenting this antigen, we further constructed a plasmid (VRSj62) which encodes Sj62 using the VR1020 vector and tested it in vaccination experiments. Four immunisations with 10 microg VRSj62 DNA alone were sufficient to induce high and progressively increasing levels of IgG antibodies against rSj62 with increasing numbers of injections in CBA/Ca mice (IgG titre > or =1:25000), and three injections with 50 microg VRSj62 DNA alone induced significant IgG responses in C57Bl/6 mice (IgG titre, 1:1600). However, vaccination with plasmid DNA entrapped in cationic liposomes or together with pUC19 DNA as a source of CpG motifs, both of which have been reported to enhance immune responses, did not enhance specific antibody production. In spite of the stimulation of specific antibodies against rSj62 with the naked DNA construct no resistance to challenge was demonstrated.

Molecular cloning and characterization of a novel Schistosoma japonicum "irradiated vaccine-specific" antigen, Sj14-3-3

A Schistosoma japonicum cDNA coding for a full length S. japonicum 14-3-3 protein was obtained by antibody screening of an adult worm cDNA library using sera taken from mice vaccinated with UV-attenuated cercariae, which are capable of transferring high levels of passive immunity to this parasite. The deduced amino acid sequence consists of 254 amino acids and is highly homologous with 14-3-3 family of proteins from a variety of species (55-69% identity). The recombinant S. japonicun 14-3-3 protein (rSj14-3-3) was expressed and purified in pGEX/E. coli, and in Western blotting was strongly recognised by sera from mice, rats and bovines vaccinated with irradiated S. japonicum cercariae. Analysis of mRNA showed that Sj14-3-3 is expressed in sporocysts and adult worms, but not in cercariae, however mouse antisera against rSj14-3-3 recognised a 29 kDa native antigen in antigen preparations made from eggs, cercariae, schistosomula and adult worms of S. japonicum indicating that this antigen is present in all life-cycle stages. The presence of the native antigen in detergent extracts of intact schistosomula suggests that it is also present in the schistosomular tegument which is the most vulnerable target for immune attack. However, antisera against rSj14-3-3 did not recognise a similar band in S. mansoni or S. haematobium antigens, indicating that, like the UV-attenuated vaccines, this protein induced species-specific immune responses. Southern blot analysis suggested that there may exist more than one gene copy and/or polymorphism for Sj14-3-3. Immunoelectron microscopy confirmed that the native antigen is present throughout the body of adult worms including the tegument, but is less abundant in the muscles. The potential of rSj14-3-3 as a vaccine is now under further investigation.

Characterization of a stage-specific Mr16000 schistosomular surface glycoprotein antigen of Schistosoma mansoni

A 16 kDa Schistosoma mansoni schistosomular surface antigen (Sm16) was originally described as the target of a passively protective mAb (B3A). It appeared on the schistosomular surface after transformation of cercariae and was uniquely recognised by sera from animals exposed to attenuated cercariae. In this work sequential extractions of schistosomula with Triton X-114 and sodium dodecyl sulphate showed Sm16 to be an integral membrane structure which did not appear to be glycosylphosphatidylinositol-anchored as judged by experiments using phosphatidyl inositol-specific phospholipase C. The antigen was strongly reactive in Western blotting with rabbit irradiated vaccine sera. Sm16 was demonstrated in the hepatopancreas of S. mansoni-infected snails and was equally abundant in cercariae and mechanically- transformed schistosomula but was undetected in liver stage worms or eggs. Immunoelectron microscopy showed Sm16 to be localised, in cercariae, to what are believed to be subtegumental cell bodies packed with membraneous vesicles. Treatment with proteases and with sodium metaperiodate showed Sm16 to be a glycoprotein of which the epitope recognised by B3A was periodate sensitive. Two-dimensional electrophoresis gave a PI of 6. Neither the size or the recognition by B3A was affected by treatment with N-glycosidase F, endoglycosidase F or endo-alpha-N-acetylgalactosaminidase. Western blotting using a wide range of biotinylated lectins showed recognition only by peanut agglutinin and Ricinus communis agglutinin II (ricin). It is concluded that Sm16 has antigenic surface-exposed O-linked complex oligosaccharides which lack mannose/glucose, GlcNAc, L-fucose and sialic acid but contain terminal Gal beta (1-3) GalNAc and/or galactose.

Vaccination of mice with gamma-irradiated Schistosoma japonicum cercariae

gamma-irradiated cercarial vaccines induce high levels of protection in mice against Schistosoma mansoni infection, however, the same has not been well established for S. japonicum. Here we describe vaccination studies in mice with gamma-irradiated S. japonicum cercariae testing the effectiveness of different irradiation doses, number of vaccinations, and mouse strains. In CBA/Ca mice, a single percutaneous exposure to 500 S. japonicum cercariae previously attenuated by 10, 20, 30, 40 or 50 krad gamma-irradiation induced significant, but comparable levels of protection (34-46%) against challenge infection. In a repeat experiment in C57Bl/6 mice, only groups vaccinated with 10 or 20 krad gamma-irradiated cercariae showed statistically significant, but lower levels of resistance (20-24%). Multiple vaccination of CBA/Ca mice with 500 20 krad gamma-irradiated cercariae did not improve the resistance level (40%). Analysis of IgG responses showed no clear correlation between antibody levels and levels of protection. Western blot analysis suggested that recognition of a 200-kDa antigen might be correlated with protection, that antigens of 42 and 50 kDa may be involved in the protection induced by single vaccination, but that different antigens might be protective in single vs multiple vaccinations. Sera from mice vaccinated with gamma-irradiated cercariae recognized many fewer antigens than more protective sera from mice vaccinated with UV-attenuated cercariae. These results suggest that the mouse may not be a suitable host for studies involving gamma-irradiated S. japonicum vaccines.

Schistosoma japonicum myosin: cloning, expression and vaccination studies with the homologue of the S. mansoni myosin fragment IrV-5

The Schistosoma japonicum homologue of the 62 kDa fragment of S. mansoni myosin (SmIrV-5), which has proved highly protective against S. mansoni infection in mice and rats, has been cloned and expressed as the full length 62 kDa equivalent, Sj62, and a truncated 44 kDa version, Sj44. DNA sequencing showed the Sj62 sequence to be 88.4% identical at the nucleic acid level and 96% identical in deduced amino acid sequence to that of SmIrV-5. The recombinant proteins (rSj44 and rSj62) were strongly recognized in Western blotting by sera from mice multiply vaccinated with UV-irradiated S. japonicum cercariae and weakly recognized by S. japonicum chronic infection mouse sera. Unlike SmIrV-5, mouse antisera against the recombinant S. japonicum proteins did not give positive recognition in immunofluorescence assay with the surface of newly transformed schistosomula of the homologous species, S. japonicum, nor did they react with S. mansoni schistosomula. However, the anti-rSj62 sera clearly localized the native antigen to the subtegumental muscle layers in male adult worm sections by immunoelectron microscopy. Vaccination of several groups of mice and/or rats with rSj44 and rSj62 incorporated into different adjuvants induced high titres of specific IgG but in only one experimental group was there a significant reduction in worm burden (27%, P < 0.05). The possible reasons for the disparity between the vaccination results presented here and those demonstrated in experiments using rSm62 (IrV-5) are discussed.

Rapid purification and characterization of L-dopachrome-methyl ester tautomerase (macrophage-migration-inhibitory factor) from Trichinella spiralis, Trichuris muris and Brugia pahangi

Macrophage-migration-inhibition factor (MIF) is an essential stimulator of mammalian T-lymphocyte-dependent adaptive immunity, hence MIF orthologues might be expressed by infectious organisms as an immunosubversive stratagem. Since MIF actively catalyses the tautomerization of the methyl ester of l-dopachrome (using dopachrome tautomerase), the occurrence of MIF orthologues in several parasitic helminths was investigated by assaying and characterizing such activity. Evidence of MIF orthologues (dopachrome tautomerase) was found in the soluble fraction of the nematodes Trichinella spiralis (stage 4 larvae) and Trichuris muris (adults), and the filarial nematode Brugia pahangi (adults). The MIF orthologues of Tr. muris (TmMIF) and B. pahangi (BpMIF) were purified to homogeneity using phenyl-agarose chromatography, that of T. spiralis (TsMIF) required a further step: cation-exchange FPLC. Retention time on reverse-phase HPLC and Mr on SDS/PAGE of the nematode MIFs were similar to those of human MIF. N-terminal sequences (19 residues) of TsMIF and TmMIF showed 47 and 36% identity, respectively, with human MIF. The N-terminal sequence of BpMIF (14 residues) was identical to that of an MIF orthologue in the genome of B. malayi (Swiss-Prot, P91850) and showed 43% identity to either human or TsMIF. TsMIF had 10-fold higher dopachrome tautomerase activity than MIF from the other sources. The enzyme activities of TsMIF, BpMIF and TmMIF were less sensitive to inhibition by haematin (I50: >15 microM, >15 microM and 2.6 microM, respectively) than that of human MIF (I50 0.2 microM). Significant dopachrome tautomerase or phenyl-agarose-purifiable MIF-like protein was not detected in the soluble fraction of the nematodes Heligmosomoides polygyrus and Nippostrongylus brasiliensis, the cestode Hymenolepis diminuta, or the trematodes Schistosoma mansoni, S. japonicum and S. haematobium, or the free-living nematode, Caenorhabditis elegans, which does contain an MIF-related gene.

Production and testing of Schistosoma japonicum candidate vaccine antigens in the natural ovine host

The objectives of this work were to clone and express Chinese strain Schistosoma japonicum antigens and evaluate their immunogenicity and protective efficacy in the natural ovine host in China. Recombinant antigens selected for testing were: isoforms of glutathione S-transferase Sj28GST and Sj26GST; the large hydrophilic domain of Sj23, the homologue of the protective S. mansoni membrane antigen Sm23; and a 3' fragment of S. japonicum paramyosin. In addition, Chinese strain S. japonicum native paramyosin and GST were purified and used for vaccination. Antigens were co-administered with Freund's adjuvants or BCG. We also examined the effects of co-administration of native unfractionated GSTs with keyhole limpet haemocyanin (KLH), which shares a cross-reactive protective epitope with schistosomes. These are the first side-by-side comparisons of candidate defined-antigen schistosomiasis vaccines in a natural host. Significant partial protection was obtained with each of the antigens tested. Less protection was obtained with a recombinant fragment of S. japonicum paramyosin compared with native paramyosin. Co-administration of native GST and KLH was no more effective than vaccination with either antigen alone. Although encouraging levels of protection against S. japonicum were demonstrated using each of these antigens, further work is needed to optimise vaccine delivery and vaccination schedules.

Higher levels of passive compared with active immunity in rats immunized with larval antigens of Schistosoma mansoni

We have previously shown that it is the lung stage schistosomulum which is both the inducer and the target of immunity in rats vaccinated with irradiated cercariae but that rats can also be partially protected by immunization with detergent extracts of mechanically-transformed schistosomula. In the present study we therefore compared the immunogenicity of these and intermediate schistosomular stages. In the first experiment low levels of protection (13-24%) were induced by extracts of mechanically-transformed schistosomula cultured for 3 hours or 2 days but not by extracts of lung schistosomula. In a repeat experiment insignificant levels of protection were induced by extracts of 3 h schistosomula and lung schistosomula were again non-protective. Nevertheless, high levels of anti-schistosomula antibodies were demonstrated and sera from actively immunized rats conferred significant passive protection in five out of six trials. The levels of protection conferred by passive immunization (26-51%) were in each case higher than the levels of protection demonstrated in the respective serum donor groups, showing that some form of immunological blockade suppression is operating to prevent expression of protective immunity in the actively immunized rats.

Resistance induced by drug abbreviated Schistosoma mansoni infections: treatment with the drug Ro11-3128 leads to enhanced antigen presentation

Treatment of mice with the benzodiazepine derivative Ro11-3128 1-2 days post-infection with Schistosoma mansoni leads to arrest of virtually all schistosomula at the skin stage, and results in the development of protective immunity to challenge infection. A characteristic feature of Ro11-3128 treatment in vitro is the formation of exudates and membranous blebs at the schistosomular surface; other drugs tested, such as Ro15-5458 and oxamniquine which are also effective against the skin stages but relatively ineffective in inducing protection, do not induce this reaction. Here, we have examined whether such in vitro treatment causes enhanced presentation of schistosomular antigens by host antigen-presenting cells (APC) using an in vitro assay with activated peritoneal adherent cells as APC and T cells from S. mansoni antigen-sensitized mice. We have shown that viable mechanically transformed schistosomula (MS) can be processed and presented with similar kinetics to soluble antigen. However, in vitro drug treatment leads to enhanced presentation of MS. Experiments in which membranous blebs and antigen released by Ro11-3128-treated parasites during in vitro culture were separated from the remaining intact schistosomula, demonstrated significant stimulatory activity in the soluble and particulate-released antigen fractions. Filtration, antigen transfer experiments and SDS-PAGE analysis of the released material further suggested that most of the activity resided in the particulate fraction. Thus, quantitative and qualitative changes to antigen presentation by Ro11-3128 treatment early after infection may underlie the immunoprotective efficacy of Ro11-3128-abbreviated infections.

Diagnosis of Schistosoma haematobium infection: evaluation of ELISA using keyhole limpet haemocyanin or soluble egg antigen in comparison with detection of eggs or haematuria

Keyhole limpet haemocyanin (KLH) was compared with Schistosoma haematobium soluble egg antigen (SEA) in the enzyme-linked immunosorbent assay (ELISA) for detection of S. haematobium infection, using 187 human sera collected from the S. haematobium endemic area of Pemba Island, Tanzania, and 30 normal sera from blood donors in Europe. There was a clear separation in terms of immunoglobulin (Ig) G and IgM titres between parasitologically positive patients and the blood donors, but titres of many parasitologically negative individuals in the endemic area were significantly high in comparison with the normal controls. Using as cut-off point the mean optical density +2 SD of sera from the blood donors to define ELISA positivity, and comparing the results with urine egg counts, the sensitivity of IgG-ELISA using KLH or SEA was high (91.11% and 95.56%, respectively) but the specificity was poor (43.30% and 31.90%, respectively. Similar results were obtained with IgM. When the 'gold standard' of haematuria and/or egg positivity as indicative of infection was used, the sensitivity of the ELISAs was similar but the specificity was increased to 59.25% and 44.44%, respectively. These results suggest that the patients with haematuria were probably infected with S. haematobium, which further supported the diagnostic value of haematuria detection for S. haematobium infection in endemic areas, and KLH was found to have a potential use in immunodiagnosis of S. haematobium infection in endemic areas. With both KLH and SEA antigens, the trend of reactivity in ELISA provided a correlate of the egg output (parasite burden) of infected patients.

Use of human infection and vaccine-protected baboon sera for the characterisation of cloned Schistosoma haematobium antigen genes

A Schistosoma haematobium adult worm cDNA library was screened with sera from demonstrably immune baboons (mean worm reduction 94%) vaccinated with highly-irradiated cercariae (VBabS) and also with sera from adult S. haematobium-infected humans (HIS Sh). As species-specificity is a characteristic feature of the immunity induced by irradiated cercariae, S. haematobium species-specific clones were identified by rescreening positives with sera from S. mansoni-vaccinated animals or infected humans. Six species-specific clones, 3 initially detected with vaccinated baboon sera (D11, D14 and D26) and 3 with human infection sera (E1, E2 and E16) were further characterized by Western blotting. The three HIS-selected clones did not react significantly with VBabS or acutely-infected baboon sera (ABabS) while D11, D14 and D26 showed increasing reactivity with successive vaccinations. In addition D11 and D14 but not D26 responded to ABabS. When tested against HIS Sh from 24 patients of varying ages, D11 and D26 reacted most frequently with sera from individuals in the older age groups (> 17 yrs). A species cross reactive clone, D12, which was used as a positive control throughout, was found to react with all schistosome specific sera tested.

Mechanisms involved in the loss of antibody-mediated adherence of macrophages to lung-stage schistosomula of Schistosoma mansoni in vitro

Sera from rabbits vaccinated with irradiated cercariae mediated cell (P388D1 or mouse peritoneal macrophage) adherence to lung-stage schistosomula (LS) but such antibody-mediated cell adherence was short-lived in contrast to cell adherence to mechanically transformed schistosomula (MS). Thus LS lost 50% of their adherent cells within 3-6 h in culture and up to 90% by 24 h, whereas adherence to MS was undiminished during this time. Rapid loss of adherent cells was unique to schistosomula that had developed to the lung stage because schistosomula recovered from the skin up to 3 days post-infection did not exhibit the rapid cell loss shown by 3-day LS. To determine whether cell loss was caused by loss of surface antigenicity during culture LS were cultured on their own for up to 24 h and at various intervals samples of schistosomula were tested for antigenicity by addition of immune serum and cells. Levels of adherence to both MS and LS were maintained throughout the incubation period. When antibody-opsonized schistosomula were washed and indicator cells added at progressive intervals, persistence of adherence was again demonstrated, showing that antibody binding to LS had not promoted surface antigen loss or degradation of bound antibody. It was then shown, by adding fresh macrophages to cultures up to 24 h old that LS which had lost their adherent cells nevertheless retained bound antibody, and comparison of adherence of 'used' and 'fresh' cells to MS and LS showed that the cytoadherence properties of macrophages were not significantly reduced during their culture with LS from which cells had been lost.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of immunity against Schistosoma mansoni by drug (Ro11-3128)-terminated infections: analysis of surface antigen recognition

As with 20 krad-irradiated infections in mice, the present study shows that the immunity induced by Ro11-3128 termination of unattenuated infections at the skin stage is species specific, not operating against S. japonicum. Treatment with the drug Ro15-5458, also effective at the skin stage, however, resulted in significantly lower levels of resistance than Ro11-3128. Sera from mice immunized by infection plus Ro11-3128 treatment on days 1 or 2 (Ro11S) coprecipitated essentially the same pattern of 125I-labelled surface antigens as the 20 krad vaccine serum (VMS), viz. Mr 38,000, 32,000, 23,000 and 15,000. However, recognition by Ro11S was markedly stronger. Sera from the infected and Ro15-5458-treated mice (Ro15S) failed to recognize the Mr 23,000 antigen and produced a weaker response than Ro11S or VMS against the Mr 38,000 or 32,000 antigens but a comparable response to VMS against the Mr 15,000 antigen. Ro11S and VMS also recognized the Mr 16,000 surface antigen seen by Western blotting but its recognition by Ro15S was weaker. Compared with sera from animals treated at the skin stage, sera from animals treated at the lung stage (day + 6) showed weaker recognition of the Mr 32,000 and 15,000 antigens and no recognition of the Mr 23,000 antigen. In contrast, sera from mice treated at 15 days recognized both the Mr 32,000 and 23,000 antigens but not the Mr 15,000 antigen. Mice treated at these times show progressively less immunity than at the skin stage. Infected but untreated animals only showed significant recognition of the Mr 32,000 antigen. Thus compared with infections treated with Ro11-3128 on days 1 or 2, treatment at later times or with the drug Ro15-5458 resulted in selective and differential absence or diminution of response against either the Mr 38,000, 32,000, 23,000, 16,000 or 15,000 antigens. In vitro, Ro11-3128, in contrast to Ro15-5458, caused multiple vesicle formation at the surface of skin stage schistosomula but this was progressively less pronounced with lung and liver stage worms. The vesicles were shown to express surface membrane antigens but were apparently not derived from the existing outer leaflet of the surface membrane. It is suggested that this altered antigen expression might explain the optimum immunity induced.

Antibody-dependent killing of Schistosoma mansoni schistosomula in vitro by starch-elicited murine macrophages. Critical role of the cell surface integrin Mac-1 in killing mediated by the anti-Mr 16,000 mAb B3A

Starch-elicited murine peritoneal macrophages were able to kill schistosomula in vitro in the presence of a variety of immune sera. Dose response experiments revealed the superior "quality" of serum from mice vaccinated four times with highly irradiated cercariae (4xVMS) in mediating killing at titers comparable to the other sera tested. B3A, a partially protective mAb (IgG3) that recognizes a Mr 16,000 schistosomular surface Ag, mediated higher levels of killing than any of the sera at comparable titers. In contrast, H12, a partially protective mAb (IgG2a; anti-Mr 32,000), and C1C9, a nonprotective McAb (IgG3; anti-Mr 38,000) failed to mediate killing. Two anti-Mac-1 alpha-chain mAb (5C6 and M1/70) mediated substantial dose-dependent blocking of 4xVMS and B3A-mediated macrophage killing. In contrast, a mAb to the Mac-1-associated beta-chain was less effective, whereas the mAb F4/80 did not significantly block killing despite being present on this macrophage population. Although whole 5C6 Ig was the most efficient at inhibiting B3A-mediated killing, 5C6 Fab fragments were still effective at concentrations as low as 0.5 microgram/ml (10 nM). On a molar basis 5C6 appeared to be more effective at blocking 4xVMS-mediated killing than M1/70, while only M1/70 was capable of inhibiting macrophage adherence to schistosomula. These findings, together with the observation that anti-alpha chain mAb were far more effective at blocking killing than the anti-beta-chain mAb, rules out the possibility that 5C6 is nonspecifically inhibiting B3A-FcR interaction. The data also imply a functional relationship between Mac-1 and FcRIII, the receptor for B3A, in macrophage killing.

Antibody-dependent killing of Schistosoma mansoni schistosomula in vitro by starch-elicited murine macrophages. Critical role of the cell surface integrin Mac-1 in killing mediated by the anti-Mr 16,000 mAb B3A

Starch-elicited murine peritoneal macrophages were able to kill schistosomula in vitro in the presence of a variety of immune sera. Dose response experiments revealed the superior "quality" of serum from mice vaccinated four times with highly irradiated cercariae (4xVMS) in mediating killing at titers comparable to the other sera tested. B3A, a partially protective mAb (IgG3) that recognizes a Mr 16,000 schistosomular surface Ag, mediated higher levels of killing than any of the sera at comparable titers. In contrast, H12, a partially protective mAb (IgG2a; anti-Mr 32,000), and C1C9, a nonprotective McAb (IgG3; anti-Mr 38,000) failed to mediate killing. Two anti-Mac-1 alpha-chain mAb (5C6 and M1/70) mediated substantial dose-dependent blocking of 4xVMS and B3A-mediated macrophage killing. In contrast, a mAb to the Mac-1-associated beta-chain was less effective, whereas the mAb F4/80 did not significantly block killing despite being present on this macrophage population. Although whole 5C6 Ig was the most efficient at inhibiting B3A-mediated killing, 5C6 Fab fragments were still effective at concentrations as low as 0.5 microgram/ml (10 nM). On a molar basis 5C6 appeared to be more effective at blocking 4xVMS-mediated killing than M1/70, while only M1/70 was capable of inhibiting macrophage adherence to schistosomula. These findings, together with the observation that anti-alpha chain mAb were far more effective at blocking killing than the anti-beta-chain mAb, rules out the possibility that 5C6 is nonspecifically inhibiting B3A-FcR interaction. The data also imply a functional relationship between Mac-1 and FcRIII, the receptor for B3A, in macrophage killing.

The role of antibody affinity and titre in immunity to Schistosoma mansoni following vaccination with highly irradiated cercariae

Sera from rabbits and rats vaccinated with highly irradiated cercariae of Schistosoma mansoni (VRabS, VRatS) were found to be of substantially higher affinity than sera from CBA mice vaccinated four times (4 X CVMS), single sex sera (SSS) or chronic infection sera (CIS). In contrast, VRabS and SSS appeared to possess the highest titres of antibody, followed by CIS and VRatS, with 4 X CVMS displaying the lowest titre. Two mouse strains selectively bred for high-affinity (HA) or low-affinity (LA) antibody following vaccination were tested for their ability to resist a challenge infection. LA mice, which produce high titres of low-affinity antibody, manifested significantly more resistance than HA mice, which produce low titres of high-affinity antibody. Immunoprecipitation studies demonstrated that sera from vaccinated LA mice (LVMS) recognized 125I-labelled schistosomular surface antigens more intensely than sera from vaccinated HA mice (HVMS). However, peritoneal macrophages from HA and LA mice in the presence of HVMS, LVMS or 4 X CVMS, and naive macrophages activated in vitro with interferon-gamma (IFN-gamma)/lipopolysaccharide (LPS) mediated comparable levels of schistosomula killing in vitro. The experiments described here provide evidence that the titre of antibody rather than its affinity may be a more critical factor in the development of optimal immunity to S. mansoni.

Immunization of baboons with attenuated schistosomula of Schistosoma haematobium: levels of protection induced by immunization with larvae irradiated with 20 and 60 krad

We have shown previously that baboons (Papio anubis) develop high levels (greater than 80%) of protection against challenge infection following immunization with Schistosoma haematobium cercariae irradiated with 20 krad. In the present study baboons were immunized with schistosomula irradiated with either 20 krad or 60 krad, with variations in the timing and number of larvae comprising each vaccination. Baboons immunized 2 or 3 times with schistosomula irradiated with 20 krad were significantly more protected (85-90%) against challenge infection than baboons similarly immunized with larvae receiving 60 krad (56-50% protection). Baboons immunized with schistosomula irradiated with 20 krad were better protected against challenge infection at 8 weeks after immunization than at 28 weeks after immunization. Protection was manifest by a reduction in worm numbers, tissue and excreta egg counts, gross pathology and, to a lesser extent, by stability of body weight and haematological indices following challenge. Enzyme-linked immunosorbent assay (ELISA) results of selected baboon sera showed few differences related to irradiation dose alone, but titres were higher in baboons receiving booster immunizations, and there was a significant correlation between titres immediately preceding challenge and the degree of resistance. Examination of responses to individual schistosomular surface antigens by immunoprecipitation and sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed no correlation between the pattern of antigens recognized and resistance status. As with the ELISA assay, an anamnestic response was detected after vaccination, while the amount of antibody present declined markedly with increasing time after individual immunizations.

Immunity to Schistosoma mansoni in vivo: contradiction or clarification?

In recent years controversy and contradiction have hindered the elucidation of the immune effector mechanisms that are most effective against Schistosoma mansoni - an essential goal for the development of an effective vaccine. However, recent in-vivo studies have clarified the relative contributions of such mechanisms to protection. Here, Dario Vignali and colleagues summarize current evidence that suggests that both antibody and CD4+ T cells, in cooperation with macrophages, are crucial for the development of an effective response. In addition, a model is presented that may account for some of the discrepancies observed and which could be used as a basis for future research.

A role for CD4+ but not CD8+ T cells in immunity to Schistosoma mansoni induced by 20 krad-irradiated and Ro 11-3128-terminated infections

The role of CD4+ (L3/T4+) and CD8+ (Lyt-2+) T cells in immunity to Schistosoma mansoni induced by 20 krad-irradiated and Ro 11-terminated infections in mice was investigated directly by in vivo depletion of these subsets with cytotoxic rat monoclonal antibodies (mAb). Effective physical depletion was demonstrated by flow cytometric analysis and immunohistochemical staining. Functional depletion of helper activity following anti-CD4 treatment was indicated by an abrogation of concanavalin A(Con A)-induced colony-stimulating factor (CSF) release, while anti-CD8 treatment had no effect in these assays. Pre-existing S. mansoni-specific antibody levels were unaffected by anti-CD4 and anti-CD8 treatment. In vivo depletion of CD4+ T cells resulted in a dramatic reduction in immunity induced by one (up to 100%) and two (up to 70%) vaccinations with 20 krad-irradiated cercariae and also of resistance induced by Ro 11-attenuated infections (up to 100%). Depletion of CD8+ T cells had no effect on resistance induced by any of the vaccination protocols investigated. A correlation was observed between resistance and T cell-induced, macrophage-mediated killing of schistosomula in vitro, both of which were abrogated following anti-CD4 treatment but were unaffected by CD8+ T-cell depletion. The possible role of CD4+ T cells in vivo and the implications for vaccine development are discussed.

Histological examination of the cellular reactions around schistosomula of Schistosoma mansoni in the lungs of sublethally irradiated and unirradiated, immune and control rats

Histopathological data on the cellular reactions (foci) around Schistosoma mansoni schistosomula in the lungs of both irradiated (750 rad) and unirradiated, passively immunized and normal rats were consistent with the idea that a significant proportion of immune-mediated attrition in passively immunized rats occurs in the lungs. In unirradiated rats, immune serum elicited an enhanced (i.e. larger) and accelerated (i.e. more rapidly developing) inflammatory cellular infiltration around lung-stage parasites when administered 5 days post-infection, when the parasites were already in the lungs. This demonstrated the antigenicity of lung-stage schistosomula and their potential as targets for immune attack. In irradiated rats, innate immunity was decreased as judged by an increase in the number of worms recovered by portal perfusion, and was accompanied by an overall decreased percentage of trapped parasites compared with unirradiated controls, suggesting that trapping in the lungs is involved in innate, as well as acquired immunity. In contrast to the results in unirradiated rats, passive transfer of immune serum into irradiated recipients did not result in larger lung foci than in the NRS-recipients. However, there was evidence of an accelerated response resulting in an essentially similar ratio of trapped parasites (VRS- compared with NRS-recipients) in irradiated rats, as compared with unirradiated rats, reflecting the similar levels of resistance manifested in both groups of rats. This also lent credence to the notion that it was the speed of immune recognition of the migrating schistosomula and the establishment of trapping foci that were of greater importance rather than the size of the enveloping granulomata.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Schistosoma mansoni monoclonal antibodies which block in-vitro killing: failure to demonstrate blockage of immunity in vivo

We have characterized various anti-schistosomular surface MoAbs previously shown to partially block in-vitro killing of schistosomula by human sera and eosinophils (Dunne et al. 1987). Immunodiffusion analysis showed that four IgM and one IgG3 MoAbs recognized periodate sensitive epitopes on the same molecular species present in schistosomular antigen but their patterns of reactivity with soluble egg antigen demonstrated that at least three distinct epitopes were involved. SDS-PAGE analysis showed the IgMs to react with 125I-labelled surface antigens of Mr 35,000-38,000 and Mr 20,000, and the IgG3 to react with an Mr 38,000 antigen. In spite of their effect in vitro, transfer of the IgM MoAbs at the time of challenge of mice vaccinated with irradiated cercariae or of mice injected with an unrelated (anti-Mr 16,000) protective MoAb failed to produce in-vivo blocking. Similarly, injection of Schistosoma mansoni eggs, prior infection with worms of one sex, or passive transfer of serum from single-sex infected mice failed to influence the resistance conferred by vaccination with irradiated cercariae.

Studies on immunity to Schistosoma mansoni in vivo: whole-body irradiation has no effect on vaccine-induced resistance in mice

Actively immunized mice, whole-body irradiated with 650 or 525 rad., manifested comparable levels of resistance to Schistosoma mansoni compared with unirradiated, immunized mice in spite of a marked reduction in circulating leucocytes (greater than 90%) and platelets (greater than 85%), and despite an abrogation of delayed-type hypersensitivity (DTH) (Type IV) response to schistosomular antigens (as determined by footpad swelling, 24 h after injection of antigen). However, limited histopathological comparison of lung sections from irradiated and unirradiated mice 7 days post-challenge showed that cellular reactions ('foci') around parasites were essentially similar in size and cellular composition except that in irradiated mice, eosinophils were poorly represented both in the foci and in lung tissue in general. Neither presumed immune complex-mediated (Type III, Arthus reaction) hypersensitivity (as determined by footpad swelling, 5 h after injection of antigen) nor serum anti-schistosomulum extract antibody levels (as determined by ELISA) were affected. In addition, the pattern of 125I-labelled schistosomular surface antigens immunoprecipitated with serum from irradiated and unirradiated mice was essentially similar. These results are consistent with antibody playing an important role in vaccine-induced immunity in mice but suggest that radiosensitive T cell function and radiosensitive cells, such as platelets and polymorphonuclear cells, including eosinophils, may not be essential.

Comparison of the role of complement in immunity to Schistosoma mansoni in rats and mice

In vivo depletion of C3 with cobra venom factor (CoF) was used to demonstrate the participation of complement in the innate immunity to S. mansoni and in the acquired immunity of both actively and passively immunized rats. Complement was shown to play an important role in innate immunity, being more involved later in larval migration (Days 8-13 post-infection) than at earlier times (Days--1-3 and Days 3-8 post-infection). Furthermore, the specific component of immunity conferred by immune serum transferred at the lung-migration stage also required complement for optimal expression. This supports the notion that both innate and acquired immunity act not against the much studied early post-penetration stages, but primarily against the lung stages. Although decomplementation at earlier stages of parasite migration (up to 3 days post-infection) did cause some reduction of innate immunity, there was no evidence of any effect on the levels of resistance actively induced by exposure to irradiated cercariae. This suggests that, while complement may play a role in innate immunity during the skin-migration phase, specific complement-mediated attrition does not play a crucial role at this time. The situation was very different in the mouse model, since no involvement of complement in either innate or irradiated vaccine-induced immunity could be demonstrated within the first 15 days of infection. Thus, there appear to be phases in the parasite migration in rats, but not in mice, during which complement becomes a critical factor in both innate and acquired immunity to S. mansoni.

Comparison of the live vaccine potential of different geographic isolates of Schistosoma mansoni

We have investigated the capacity of irradiated larvae of each of six different isolates of Schistosoma mansoni to protect mice against challenge infections from the homologous or a different isolate. The isolates differed in respect of their geographic origins, and in the lengths of time they had been maintained in the laboratory. All isolates induced significant protection against unattenuated challenge infections, and neither the degree of geographic disimilarity between vaccinating and challenge parasites, nor the numbers of laboratory cycles, appeared to influence the degree of protection induced by irradiation-attenuated larvae.

The isotypes of antibody responsible for the 'late' passive transfer of immunity in rats vaccinated with highly irradiated cercariae

The role of different antibody isotypes in the transfer of protection with serum from rats vaccinated with irradiated cercariae were investigated by immunoadsorption of IgG, IgG2a and IgG2c, and by heating at 56 degrees C to remove IgE. Only depletion of IgG2a reduced the levels of vaccine-induced immunity transferred, whereas immunity transferred by infection serum was reduced by both IgG2a and IgE depletion. It was also shown that exposure to irradiated parasites did not lead to either specific or non-specific induction of an IgE response. The successful passive-transfer experiments using serum from non-resistant 25 week-infected rats into recipient animals demonstrated that the waning of resistance observed in 25 week-infected rats was not due to a failure of the humoral response to protective antigens.

Identification and characterization of a polysaccharide-containing antigen from Schistosoma mansoni eggs which cross-reacts with the surface of schistosomula

Antisera were produced by immunizing rabbits with either a trichloroacetic acid-soluble fraction, or a high molecular weight (Mr) fraction of Schistosoma mansoni SEA (a saline-soluble fraction of homogenized egg). Both of these sera reacted monospecifically in immunoelectrophoresis against unfractionated SEA, recognizing a cathodally migrating antigen. This antigen had been identified previously as being responded to by S. mansoni-infected mouse sera, and has been designated K3 (Kappa 3). The rabbit antisera were used to partially characterize antigen K3 as having Mr in the range greater than 750-70 K, and being resistant to boiling, resistant to the action of proteases, but sensitive to periodate. It partially binds to Concanavalin A. In addition to SEA, the antigen was present in homogenized cercariae and schistosomula, but not adult worms, and it was also present in detergent extracts of intact cercariae and schistosomula. Using an antibody-dependent cell adherence assay, anti-K3 serum was found to react with the surface of live cercariae and with the surface of schistosomula recovered from the skin of mice infected up to 48 h previously. Anti-K3 serum also reacted with the surface of S. bovis, S. haematobium and to a lesser extent S. japonicum schistosomula.

The blocking of human antibody-dependent, eosinophil-mediated killing of Schistosoma mansoni schistosomula by monoclonal antibodies which cross-react with a polysaccharide-containing egg antigen

Three IgM monoclonal antibodies, M22G11P, M7B7 and M22P3G, which reacted with the surface of Schistosoma mansoni schistosomula in an indirect fluorescent antibody assay, were found to recognize a polysaccharide-containing egg antigen previously designated K3. The monoclonal antibodies and a monospecific rabbit anti-K3 serum also recognized a cross-reacting antigen in a crude cercarial antigen preparation. In an eosinophil-mediated schistosomulum killing assay, all three monoclonal antibodies significantly inhibited the level of killing produced by human infection serum. An IgG3 monoclonal antibody, M22C1C, which also recognized the egg antigen K3, did not inhibit eosinophil-mediated killing. However, when lower concentrations of human serum were used in the assay, this monoclonal antibody significantly enhanced the level of killing, despite having no capacity to induce eosinophil-mediated damage in the absence of human infection serum. On the basis of these and other results we suggest the possibility that antibodies to S. mansoni egg antigens which cross-react with the surface of the early post-penetration schistosomulum may influence the effective expression of antibody-dependent, eosinophil-mediated effector mechanisms in human infections.

Immunity to Schistosoma mansoni in congenitally athymic, irradiated and mast cell-depleted rats

Immunity to Schistosoma mansoni was investigated in congenitally athymic (Nu/Nu) rats, irradiated rats and in mast cell-depleted rats. Nu/Nu rats failed to develop significant resistance following vaccination with irradiated cercariae, although Nu/Nu recipients of serum from vaccinated Fischer rats (VRS) manifested resistance comparable to heterozygous controls, suggesting that T-cells were required in the induction of resistance but were not involved in the efferent arm of antibody-dependent elimination. Radiosensitive cells (including eosinophils, basophils, neutrophils, lymphocytes and mast cells) were apparently not essential for the antibody-dependent elimination of lung or post-lung stages since irradiated (700-750 rad.) recipients of VRS manifested comparable degrees of resistance to unirradiated controls in spite of a greater than 85% reduction in total blood leucocyte counts after irradiation. Depletion of 99% of tissue mast cells by treatment of rats with Compound 48/80 had no significant effect on the attrition of a challenge infection in rats rendered immune by vaccination with irradiated cercariae or by transfer of VRS. However, there was a significant increase in worm recovery in unimmunized and mast cell-depleted or irradiated rats, indicating that mast cells and perhaps other radio-isotope sensitive cells may be involved in innate resistance.

Reevaluation of the potential of Fasciola hepatica antigens for immunization against Schistosoma mansoni infection

Previous workers have reported that significant resistance to Schistosoma mansoni cercarial challenge can be induced in mice by immunization with various Fasciola hepatica antigen preparations. We have attempted to reproduce these observations and have also carried out similar experiments in rats. In our hands, however, CBA mice did not develop either antibodies against the schistosomular surface, or significant resistance to challenge after immunization with heterologous antigens. Similar results were obtained in Fischer rats, but rats did develop significant resistance after immunization with homologous (schistosomular) antigens, even without adjuvant, and the homologous 'protective' antigens could be separated by antibody-affinity chromatography using serum from rats vaccinated with irradiated cercariae. Thus, we found no evidence for the existence of shared 'protective' antigens between S. mansoni and F. hepatica, and suggest that this is further evidence that where heterologous resistance can be demonstrated between these two parasites, it is probably due to immunologically non-specific mechanisms.

Schistosoma mansoni: characterization of two protective monoclonal antibodies

Two monoclonal antibodies against the surface of S. mansoni schistosomula were found to confer significant passive protection to mice (M7B3A, range 28-70%; M22H12C, range 14-58%). No additive effect was observed when both were transferred together. Neither McAb bound to the cercarial surface but both bound to the surface of in vitro derived schistosomula and schistosomula recovered from mouse skin up to 3 days after infection. The McAbs were species specific, but not S. mansoni strain specific. M22H12C immunoprecipitated an 125I-labelled surface antigen of relative molecular weight (mol. wt) 32 000. In Western blotting of an NP40 schistosomular extract, M7B3A recognized an antigen smear of 13 000-18 000 with a dominant band at 16 000. This 16 000 antigen was recognized by serum from demonstrably immune mice and rats vaccinated with highly irradiated carcariae but not by sera from mice with chronic single sex or bisexual infections.

Effect of Plasmodium berghei infection on benzoic acid metabolism in mice

The metabolism of benzoic acid was studied in Plasmodium berghei infected mice both in vitro and in vivo. Results of in vitro studies showed a considerable decrease in the ability of the infected liver to detoxify benzoic acid by hippuric acid formation. The in vivo study showed that hippuric acid formation decreases with increasing parasitemia and the emergence of benzoyl-glucuronide. This new pathway stops operating with further increase in parasitemia.

An ultrastructural examination of irradiated, immunizing schistosomula of Schistosoma mansoni during their extended stay in the lungs

An ultrastructural study has been performed on radiation-attenuated parasites fixed in situ during their extended stay in the lungs. At day 7 post-infection all parasites were located within blood vessels. In contrast to normally migrating parasites, at days 13 and 21 post-infection the majority of radiation-attenuated parasites were lying free within alveoli. As far as could be seen, radiation-attenuated parasites had undergone the normal developmental changes associated with the lung stage of migration. These included elongation of the body and loss of mid-body spines and the fibrous layer beneath the tegument. No ultrastructural evidence was found to suggest why schistosomula failed to migrate further and instead burst into alveoli. No parasites observed on day 7 post-infection were associated with a host inflammatory reaction. At day 13, inflammation was noticeable but could have been a response as much to pulmonary tissue damage as to the presence of parasites. At day 21 post-infection some parasites were internally disrupted but were not associated with host inflammatory reactions and had intact tegumental surfaces - all facts consistent with death by autophagy. One parasite was observed at day 21 with an adherent neutrophil infiltrate and extensive tegumental damage. The relevance of persisting lung-stage parasites to the induction of resistance to challenge is discussed.

Induction of resistance to Schistosoma mansoni in mice by chemotherapy: migration of schistosomula in primary and challenge infections

The fate of 75Se-labelled schistosomula in mice treated at 24 h post-infection with either Ro 11-3128 or oxamniquine, compared to untreated controls, was followed by compressed organ autoradiography. No difference in the total percentage of schistosomula detected as foci was found between the three groups at each sampling time. However, the distribution of schistosomula was altered. In oxamniquine-treated mice there was a delayed migration from the skin relative to controls and fewer parasites in total appeared to reach the lungs. In Ro 11-3128 treated mice very few parasites left the skin. Ro 11-3128 treatment induces resistance to challenge whereas oxamniquine does not. The fate of challenge schistosomula in previously infected Ro 11-3128 treated mice was also compared to that in control mice. There was evidence for delayed or reduced migration from the skin of Ro 11-3128 treated mice. Significant death of challenge parasites occurred in the lungs, or post-lung sites in both control and treated mice. It was not possible to conclude that any of the late attrition was immune-dependent in the treated mice and the results appeared to indicate that a significant fraction of parasite death occurred at the skin stage.

Resistance against Schistosoma mansoni induced by highly irradiated infections: studies on species specificity of immunization and attempts to transfer resistance

Significant levels of resistance against Schistosoma mansoni challenge were developed by mice exposed to highly irradiated (20 krad.) cercariae of the homologous species (53-67%), whereas vaccination with S. bovis, S. haematobium or S. japonicum failed to confer significant levels of resistance (-5-12%), thus confirming the specificity of the immunizing procedure. Attempts to transfer resistance to naive recipients by injection of serum and of spleen or lymph node cells from donor mice vaccinated with highly irradiated cercariae were largely unsuccessful. However, significant levels of resistance could be transferred to mice by injection of serum from rabbits exposed to irradiated cercariae. Comparable levels of resistance were conferred by injection of serum at the time of challenge (34-69%) or 5-6 days later (31-56%). In contrast, sera from rabbits injected with soluble egg antigen or homogenized cercariae failed to confer protection upon recipient mice. Sera from vaccinated mice, vaccinated rabbits and antigen-injected rabbits all caused cell adherence to skin-transformed schistosomula but neither the level of adherence nor the serum titre correlated with the ability to confer protection to mice.

The induction of specific immunity against Schistosoma japonicum by exposure of mice to ultraviolet attenuated cercariae

Mice can be partially protected against Schistosoma japonicum by prior exposure to ultraviolet (UV)-attenuated infections which fail to survive to the adult stage and produce no overt pathology in the host. Optimum resistance was induced by parasites exposed to 40 seconds of UV, significantly lower levels of resistance being stimulated by both shorter and longer exposures. No consistent relationship between the degree of resistance induced and the number of irradiated cercariae given could be demonstrated and equivocal results were obtained when comparing the efficacy of single and multiple vaccinations. Vaccinations with UV-attenuated cercariae given intraperitoneally (i.p.) were as efficacious as those given percutaneously but mice were as or more resistant to challenges given by the i.p. route, the possible reasons are discussed. There was no observed delay in the migration of the challenge, vaccinated mice being as resistant when perfused 6 or 3.5 weeks after challenge. Vaccination was species specific since mice exposed to either UV-attenuated S. japonicum cercariae or gamma-attenuated S. mansoni cercariae were resistant to homologous but not heterologous challenge.

Resistance following drug attenuation (Ro 11-3128 or oxamniquine) of early Schistosoma mansoni infections in mice

A single dose of Ro 11-3128 was found to be 98-100% effective against Schistosoma mansoni infections at intervals of 3 h to 15 days following infection, and apparently killed the schistosomula stages soon after administration, thus allowing an assessment of the immunizing potential of progressive larval stages. Following infection with 500 unirradiated cercariae, optimum resistance was manifest by groups drug-treated at 48-96 h (60-75%). Significantly lower levels of resistance occurred with early (3 h) or later (6-15 day) treatments. Superimposition of an infection treated at 15 days on a prior infection which had been treated at 2 days did not reduce the level of resistance caused by the latter, indicating that the infection plus delayed treatment had not induced suppression. Thus the peak resistance manifest during the 48-96 h period may reflect optimum expression of protective antigens. Comparison of irradiated (20 krad.) with unirradiated infections showed that, when drug-terminated 24, 48 or 96 h post-infection, irradiated cercariae induced significantly less resistance than unirradiated cercariae, perhaps indicating a delayed appearance of protective antigens following radiation treatment.

Protection of sheep against Schistosoma bovis using cryopreserved radiation-attenuated schistosomula

Three sheep were vaccinated with two doses of 3 krad-irradiated cryopreserved Schistosoma bovis schistosomula containing 20,000 and 17,000 organisms respectively, injected intramuscularly 23 days apart after storage in liquid nitrogen for between 9 and 46 days. A challenge of 5360 S. bovis cercariae was administered percutaneously approximately four weeks after the last vaccine dose to these animals and to three controls. Post-challenge the vaccinated animals gained significantly more weight (27% v. 9%), produced fewer eggs in their faeces, showed a smaller reduction in PCV values (-18% v. -27%) and were over-all in better condition than control animals. At perfusion 49.1% fewer adult worms were found in the vaccinated sheep than in controls. The tissue egg burdens were similar in both groups. Histopathologically both groups were similar except that fewer and smaller egg lesions were observed in the livers of vaccinated animals.

Passive transfer of resistance and the site of immune-dependent elimination of the challenge infection in rats vaccinated with highly irradiated cercariae of Schistosoma mansoni

The immune-dependent elimination of a challenge infection in rats vaccinated with highly-irradiated cercariae of Schistosoma mansoni was analysed by passive transfer of serum, recovery of the challenge from the lungs and liver and by transferring lung-stage schistosomula. Recipients of serum from rats immunized with either unirradiated, 20 or 40 krad.-irradiated cercariae, most of which die in the liver, lungs and skin, respectively, were equally resistant if the serum was injected on the day of infection or 5-7 days after infection. In addition, vaccinated rat serum transferred to mice and vaccinated rabbit serum transferred to rats conferred comparable protection when injected on day 0 or 5 days after infection of the recipients. This apparent susceptibility of the lung schistosomula to immune attack was confirmed by challenging 20 or 40 krad.-irradiated cercariae vaccinated rats with lung-stage schistosomula derived either from mice or rats. All the detectable attrition of a cercarial challenge in vaccinated rats occurred between 7 and 10 days post-challenge, before the parasites reached the liver. Since there was no evidence of damage or attrition in the skin or lungs before day 7 it was concluded that immune-dependent elimination occurred rapidly following a 'window of sensitivity' coinciding with the migration of the parasites from the lungs to the liver.

Immunization of rats against Schistosoma mansoni using irradiated cercariae, lung schistosomula and liver-stage worms

In PVG rats a single immunizing infection with Schistosoma mansoni cercariae exposed to 0, 5, 10 or 20 krad. gamma radiation failed to induce more than minimal resistance (17-29%) to challenge 4 weeks later, whereas 4 immunizations with 20 krad.-irradiated cercariae, over several months, induced substantial resistance (75%). In contrast, significant protection was induced in Fischer rats by a single immunization with unirradiated cercariae or cercariae irradiated with up to 80 krad. Comparable resistance was induced by unirradiated and by 2, 5 and 20 krad.-irradiated cercariae (67-74%) and lower levels by 10, 40 and 80 krad.-irradiated infections (57, 48 and 33%, respectively). Although the resistance induced by a single dose of 1000 20 krad.-irradiated cercariae could be boosted by a second (88%), further immunizations failed to enhance this resistance. Also, increasing the number of immunizing cercariae in single or multiple vaccinations from 1000 to 3000 failed to increase the resistance. While the resistance induced by 20 krad.-irradiated cercariae persisted undiminished for at least 25 weeks (67%), the resistance induced by normal cercariae declined to insignificant levels by 25 weeks (21%). Comparison of the migration and survival of unirradiated and of 20 and 40 krad.-irradiated cercariae revealed dramatic differences in their fate: parasites exposed to 40 krad. remained in the skin, while the majority of 20 krad.-irradiated parasites died in the lungs after a sojourn of at least 14 days. Thus, although skin schistosomula alone could induce significant protection, optimal resistance was induced only by parasites which migrated to the lungs or beyond. The immunizing potential of these older parasites was investigated by exposing rats to lung- and liver-stage larvae injected into the tail and mesenteric veins, respectively. Irradiated 4-day lung schistosomula induced comparable resistance (79%) to that induced by a complete unattenuated cercarial infection (78%), whereas less resistance was induced by irradiated and unirradiated 11-day-old liver worms (30 and 27%) and by 25-day-old pre-adult worms (48%).