Migration of the schistosomula of Schistosoma mansoni from skin to lungs

Immunological mechanisms involved in macrophage activation and polarization in schistosomiasis

Human schistosomiasis is caused by helminths of the genus Schistosoma. Macrophages play a crucial role in the immune regulation of this disease. These cells acquire different phenotypes depending on the type of stimulus they receive. M1 macrophages can be ‘classically activated’ and can display a proinflammatory phenotype. M2 or ‘alternatively activated’ macrophages are considered anti-inflammatory cells. Despite the relevance of macrophages in controlling infections, the role of the functional types of these cells in schistosomiasis is unclear. This review highlights different molecules and/or macrophage activation and polarization pathways during Schistosoma mansoni and Schistosoma japonicum infection. This review is based on original and review articles obtained through searches in major databases, including Scopus, Google Scholar, ACS, PubMed, Wiley, Scielo, Web of Science, LILACS and ScienceDirect. Our findings emphasize the importance of S. mansoni and S. japonicum antigens in macrophage polarization, as they exert immunomodulatory effects in different stages of the disease and are therefore important as therapeutic targets for schistosomiasis and in vaccine development. A combination of different antigens can provide greater protection, as it possibly stimulates an adequate immune response for an M1 or M2 profile and leads to host resistance; however, this warrants in vitro and in vivo studies.

Molecular insights into the heat shock proteins of the human parasitic blood fluke Schistosoma mansoni

Background

Heat shock proteins (HSPs) are evolutionarily conserved proteins, produced by cells in response to hostile environmental conditions, that are vital to organism homeostasis. Here, we undertook the first detailed molecular bioinformatic analysis of these important proteins and mapped their tissue expression in the human parasitic blood fluke, Schistosoma mansoni, one of the causative agents of the neglected tropical disease human schistosomiasis.

Methods

Using bioinformatic tools we classified and phylogenetically analysed HSP family members in schistosomes, and performed transcriptomic, phosphoproteomic, and interactomic analysis of the S. mansoni HSPs. In addition, S. mansoni HSP protein expression was mapped in intact parasites using immunofluorescence.

Results

Fifty-five HSPs were identified in S. mansoni across five HSP families; high conservation of HSP sequences were apparent across S. mansoni, Schistosoma haematobium and Schistosoma japonicum, with S. haematobium HSPs showing greater similarity to S. mansoni than those of S. japonicum. For S. mansoni, differential HSP gene expression was evident across the various parasite life stages, supporting varying roles for the HSPs in the different stages, and suggesting that they might confer some degree of protection during life stage transitions. Protein expression patterns of HSPs were visualised in intact S. mansoni cercariae, 3 h and 24 h somules, and adult male and female worms, revealing HSPs in the tegument, cephalic ganglia, tubercles, testes, ovaries as well as other important organs. Analysis of putative HSP protein-protein associations highlighted proteins that are involved in transcription, modification, stability, and ubiquitination; functional enrichment analysis revealed functions for HSP networks in S. mansoni including protein export for HSP 40/70, and FOXO/mTOR signalling for HSP90 networks. Finally, a total of 76 phosphorylation sites were discovered within 17 of the 55 HSPs, with 30 phosphorylation sites being conserved with those of human HSPs, highlighting their likely core functional significance.

Conclusions

This analysis highlights the fascinating biology of S. mansoni HSPs and their likely importance to schistosome function, offering a valuable and novel framework for future physiological investigations into the roles of HSPs in schistosomes, particularly in the context of survival in the host and with the aim of developing novel anti-schistosome therapeutics.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05500-7.

Making sense of sensory behaviors in vector-borne helminths

Migrations performed by helminths are impressive and diverse, and accumulating evidence shows that many are controlled by sophisticated sensory programs. The migrations of vector-borne helminths are particularly complex, requiring precise, stage-specific regulation. We review the contrasting states of knowledge on snail-borne schistosomes and mosquito-borne filarial nematodes. Rich observational data exist for the chemosensory behaviors of schistosomes, while the molecular sensory pathways in nematodes are well described. Recent investigations on the molecular mechanisms of sensation in schistosomes and filarial nematodes have revealed some features conserved within their respective phyla, but adaptations correlated with parasitism are pronounced. Technological developments are likely to extend these advances, and we forecast how these technologies may be applied.

Characterization of MicroRNA Cargo of Extracellular Vesicles Isolated From the Plasma of Schistosoma japonicum-Infected Mice

Schistosoma is a genus of parasitic trematodes that undergoes complex migration in final hosts, finally developing into adult worms, which are responsible for egg production and disease dissemination. Recent studies documented the importance of extracellular vesicles (EVs) in the regulation of host-parasite interactions. Herein, we investigated the microRNA (miRNA) profiles of EVs isolated from host plasma at different stages of Schistosoma japonicum infection (lung stage: 3 days post-infection (dpi), and liver stages: 14 and 21 dpi) to identify miRNA cargo potentially involved in the pathogenesis and immune regulation of schistosomiasis. Characterization of the isolated plasma EVs revealed their diameter to be approximately 100 nm, containing typical EV markers such as Hsp70 and Tsg101. Deep sequencing analysis indicated the presence of 811 known and 15 novel miRNAs with an increasing number of differential miRNAs from the lung stage (27 miRNAs) to the liver stages (58 and 96 miRNAs at 14 and 21 dpi, respectively) in the plasma EVs of infected mice compared to EVs isolated from the uninfected control. In total, 324 plasma EV miRNAs were shown to be co-detected among different stages of infection and the validation of selected miRNAs showed trends of abundance similar to deep sequencing analysis. For example, miR-1a-3p and miR-122-5p showed higher abundance, whereas miR-150-3p and miR-126a showed lower abundance in the plasma EVs of infected mice at 3, 14, and 21 dpi as compared to those of uninfected mice. In addition, bioinformatic analysis combined with PCR validation of the miRNA targets, particularly those associated with the immune system and parasitic infectious disease, indicated a significant increase in the expression of Gbp7and Ccr5 in contrast to the decreased expression of Fermt3, Akt1, and IL-12a. Our results suggested that the abundance of miRNA cargo of the host plasma EVs was related to the stages of Schistosoma japonicum infection. Further studies on the roles of these miRNAs may reveal the regulatory mechanism of the host-parasite interaction. Moreover, the differentially abundant miRNA cargo in host EVs associated with S. japonicum infection may also provide valuable clues for identifying novel biomarkers for schistosomiasis diagnosis.

Schistosome migration in the definitive host

Schistosomes are parasitic blood flukes that infect >200 million people around the world. Free-swimming larval stages penetrate the skin, invade a blood vessel, and migrate through the heart and lungs to the vasculature of the liver, where maturation and mating occurs. From here, the parasite couples migrate to their preferred egg laying sites. Here, we compare and contrast what is known about the migration patterns within the definitive host of the three major species of human schistosome: Schistosoma mansoni, S. japonicum, and S. haematobium. We conclude that intravascular schistosomes are inexorable colonizers whose migration and egg laying strategy is profligate; all three species (and their eggs) can be found throughout the mesenteric venules, the rectal venous plexus, and, to a greater or lesser extent, the urogenital venous plexuses. In addition, it is common for parasite eggs to be deposited in locations that lack easy access to the exterior, further demonstrating the relentless exploratory nature of these intravascular worms.

A novel cell-free method to culture Schistosoma mansoni from cercariae to juvenile worm stages for in vitro drug testing

Background

The arsenal in anthelminthic treatment against schistosomiasis is limited and relies almost exclusively on a single drug, praziquantel (PZQ). Thus, resistance to PZQ could constitute a major threat. Even though PZQ is potent in killing adult worms, its activity against earlier stages is limited. Current in vitro drug screening strategies depend on newly transformed schistosomula (NTS) for initial hit identification, thereby limiting sensitivity to new compounds predominantly active in later developmental stages. Therefore, the aim of this study was to establish a highly standardized, straightforward and reliable culture method to generate and maintain advanced larval stages in vitro. We present here how this method can be a valuable tool to test drug efficacy at each intermediate larval stage, reducing the reliance on animal use (3Rs).

Methodology/Principal findings

Cercariae were mechanically transformed into skin-stage (SkS) schistosomula and successfully cultured for up to four weeks with no loss in viability in a commercially available medium. Under these serum- and cell-free conditions, development halted at the lung-stage (LuS). However, the addition of human serum (HSe) propelled further development into liver stage (LiS) worms within eight weeks. Skin and lung stages, as well as LiS, were submitted to 96-well drug screening assays using known anti-schistosomal compounds such as PZQ, oxamniquine (OXM), mefloquine (MFQ) and artemether (ART). Our findings showed stage-dependent differences in larval susceptibility to these compounds.

Conclusion

With this robust and highly standardized in vitro assay, important developmental stages of S. mansoni up to LiS worms can be generated and maintained over prolonged periods of time. The phenotype of LiS worms, when exposed to reference drugs, was comparable to most previously published works for ex vivo harvested adult worms. Therefore, this in vitro assay can help reduce reliance on animal experiments in search for new anti-schistosomal drugs.

Schistosomiasis remains a major health threat, predominantly in developing countries. Even though there has been some progress in search of new drugs, praziquantel remains the only available drug. Probably the most important advance in the search for new drugs was in vitro transformation of cercariae and their subsequent culture. However, hit identification in compound screenings is exclusively tested in skin stage parasites and is only confirmed for more mature worms in a subsequent step. This is in part due to the lack of an easy culture system for advance-stage parasites. We present here a reliable and highly standardized way to generate LiS worms in vitro in a cell-free culture system. The inclusion of in vitro drug tests on advanced-stage parasites in initial hit identification will help to identify compounds that might otherwise be overlooked. Furthermore, the ability to continuously observe the parasite’s development in vitro will provide an important platform for a better understanding of its maturation in the human host. Taken together, this opens up new avenues to investigate the influence of specific cell types or host proteins on the development of Schistosoma mansoni and provides an additional tool to reduce animal use in future drug discovery efforts (3Rs).

Portal serum constituents possible determinants for anatomical localization ofSchistosoma mansoni during maturation and reproduction

Coupled adult pairs ofSchistosoma mansoni were incubated in medium containing either peripheral or portal serum from rat, rabbit, hamster, or man, and egg production was measured daily. In all cases egg production was significantly increased for pairs in the presence of portal sera compared with that in the presence of peripheral sera. Fractionation of rabbit portal serum according to molecular weight demonstrated that the most active component(s) were in the range of 2000 to 50,000. Similarly, a rat portal serum fraction in the range of 2000 to 30,000 molecular weight was most stimulatory. These fractions were as effective in stimulating oviposition as whole portal serum.

CONCLUSIONS

(1) portal serum factor(s) exist that stimulateS. mansoni oviposition in vitro; (2) they are present in susceptible and nonsusceptible hosts; and (3) the molecular weight range for the active components is larger than would be expected for simple carbohydrates, amino acids, or free fatty acids absorbed from the gastrointestinal tract.

Schistosoma mansoni: schistosomicidal effect of mefloquine and primaquine in vitro

We investigated the effects of the anti-malarials mefloquine and primaquine against the juvenile and adult life stages of Schistosoma mansoniin vitro. Cercariae were incubated with 0.5 μg/ml, 1 μg/ml and 2 μg/ml mefloquine or primaquine and with 1 μg/ml praziquantel for 12h. Schistosomula, pre-adults and adults were incubated with 0.5 μg/ml, 1 μg/ml and 2 μg/ml mefloquine or primaquine and with 1 μg/ml praziquantel for 7 days. The viability status was classified as viable, damaged or dead and was checked every 3h for cercariae and every 12h for schistosomula, pre-adults and adults. Both, mefloquine and primaquine show time and dose-dependent schistosomicidal effects on the four life stages of S. mansoni. The promising in vitro effects on all stages of the blood fluke S. mansoni warrants further evaluation of both anti-malarials and their derivatives for their prophylactic and therapeutic values in early and late schistosomiasis in field trials.

Schistosomula, pre-adults and adults of Schistosoma mansoni ingest fluorescence-labelled albumin in vitro and in vivo: implication for a drug-targeting model

SUMMARY OBJECTIVE

Bilharziosis is one of the most important helminthal infections in humans and is caused by blood flukes of the genus Schistosoma. Three different life stages of the parasite occur within the mammalian host: schistosomula located in the skin, pre-adults located in the lung and adult worms located in the portal venous system. Erythrocytes are a major source of nutrient supply for adults. However, sources of nutrition for the developing stages are still unclear.

METHODS

To investigate whether schistosomula, pre-adults and adults of Schistosoma mansoni ingest human serum albumin (HSA) in vitro, these life stages were incubated with aminofluorescein-labelled human serum albumin (Afl-HSA) for 5 h. To test the uptake of albumin in vivo, the albumin conjugate was given intravenously to S. mansoni infected NMRI mice 24 h before harvesting the 3 life stages.

RESULTS

In comparison to the control group schistosomula, pre-adults, and adults showed an accumulation of Afl-HSA within the oesophagus and intestinal caecum in vitro and in vivo.

CONCLUSION

Our findings suggest that albumin seems to be a major source of energy supply for the early schistosomal life stages and an additive energy support for adult worms. Since albumin has been used successfully as a drug carrier for chemotherapeutic substances against malignant disorders, further studies will focus on albumin as a carrier for anthelminthics in a drug-targeting model.

Imaging schistosomes in vivo

Schistosomes are intravascular, parasitic helminths that cause a chronic, often debilitating disease afflicting over 200 million people in over 70 countries. Here we describe novel imaging methods that, for the first time, permit visualization of live schistosomes within their living hosts. The technology centers on fluorescent agent uptake and activation in the parasite's gut, and subsequent detection and signal quantitation using fluorescence molecular tomography (FMT). There is a strong positive correlation between the signal detected and parasite number. Schistosoma mansoni parasites of both sexes recovered from infected experimental animals exhibit vivid fluorescence throughout their intestines. Likewise, the remaining important human schistosome parasites, S. japonicum and S. hematobium, also exhibit gut fluorescence when recovered from infected animals. Imaging has been used to efficiently document the decline in parasite numbers in infected mice treated with the antischistosome drug praziquantel. This technology will provide a unique opportunity both to help rapidly identify much-needed, novel antischistosome therapies and to gain direct visual insight into the intravascular lives of the major schistosome parasites of humans.

The saga of schistosome migration and attrition

Schistosomes infect the mammalian host by direct penetration of the skin and must then undergo a protracted migration to the site of parasitization, for Schistosoma mansoni the hepatic portal vasculature. This article reviews the work published roughly between 1976 and 1986 that clarified our understanding of the process in the laboratory mouse. A combination of histopathology, larval injection experiments and autoradiographic tracking revealed that migration involved one to several circuits of the pulmonary-systemic vasculature before chance delivery in cardiac output to splanchnic arteries that lead indirectly to the portal tract. The kinetics of migration through different capillary beds was established, with the lungs of naïve mice not the skin proving the greatest obstacle; a proportion of schistosomula entered the alveoli from where they did not recover. The 'immunity' displayed by mice with a chronic infection was shown to be an artefact of a 'leaky' hepatic portal system, generated as a result of egg-induced hepatic pathology. The blockade of pulmonary migration was exacerbated in mice vaccinated with irradiated cercariae by immune-mediated inflammatory foci that developed around lung schistosomula thus decreasing the proportion that matured, but parasite elimination was a prolonged process, not an acute cytolytic 'hit.'

Schistosomiasis prophylaxis in vivo using N,N-diethyl-m-toluamide (DEET)

Topical N,N-Diethyl-m-toluamide (DEET) was studied as a schistosomiasis prophylactic in vivo for the use of individuals with limited exposure. Fifteen subjects, on a 3-week expedition to Lake Malawi in September 2001, applied 50% DEET to their skin after exposure to lake water. No subjects developed evidence of a new infection at 3-month follow-up.

SCHISTOSOMA MANSONI: ANTIGEN-PRESENTING CELLS EMIGRATING FROM SKIN EXPOSED TO ATTENUATED CERCARIAE ACTIVATE LYMPHOID CELLS AND TRANSFER PROTECTION IN C57Bl/6 MICE

C57B1/6 mice develop significant levels of protection to a challenge infection after percutaneous exposure to irradiated Schistosoma mansoni cercariae. Although some circumstantial evidence has suggested that antigen-presenting cells (APCs) within the skin play a role in priming anti-schistosomula effector mechanisms, no direct evidence has been presented. In this study, we describe efforts to directly test whether skin-resident APCs exposed to irradiated cercariae are capable of mediating responses consistent with previously proposed mechanisms associated with delayed-type hypersensitivity reactions. We demonstrate that a population of APCs emigrates from the skin after percutaneous vaccination and that these cells are able to induce proliferation of S. mansoni-specific lymphocytes. We describe our experiments conducted to confirm that proliferation is dependent on major histocompatibility complex (MHC) Class-II interactions and cell-to-cell contact between APCs and lymphocytes. Immunohistological staining of emigrating cells revealed a population of large MHC Class-II+ cells with a morphology characteristic of mature dendritic cells. On recovery and adoptive transfer into naive mice, these cells demonstrated the ability to mediate protection to a challenge infection at levels similar to those in percutaneously vaccinated controls. This confirms that cutaneous APCs can initiate anti-schistosomula effector mechanisms in C57B1/6 mice after percutaneous vaccination.

Platelets as an innate defence mechanism against Schistosoma mansoni infections in mice

The interaction between Schistosoma mansoni and platelets of non-immune mice has been studied in vivo and in vitro. A moderate thrombocytopaenia was observed in mice 2 days after they had been infected percutaneously with 200 cercariae. A rabbit anti-mouse platelet antiserum, 25 microL of which injected subcutaneously induced a nearly 900% reduction in blood platelet count 24 h later, was used to investigate the effects of severe thrombocytopaenia on S. mansoni infections. In replicate experiments worm burdens were significantly increased in mice that were thrombocytopaenic at the time of infection when compared with untreated mice. Induction of thrombocytopaenia on day 4 after infection had no effect on worm count. Platelets isolated from non-immune mice were shown to adhere to the surfaces of and kill mechanically transformed schistosomula in vitro. Platelets may thus be an innate mechanism of defence against schistosome infection, and the thrombocytopaenia that occurs during patent schistosome infections may be a strategy that helps secondarily incoming parasites evade this type of host defensiveness.

Trematode behaviours and the perceptual worlds of parasites

There is a great deal of empirical data and theoretical predictions on the patterns and processes of trematode behaviour, particularly in relation to host-finding activities by the free-living stages and site-finding migrations by the parasitic stages within their hosts. Ecological and evolutionary models of trematode life histories often make explicit assumptions about how these organisms must perceive and respond to signals in their worlds as they move from host to host and as they parasitize each host. Nevertheless, it is unclear how natural selection shapes the parasites' behavioural strategies. In addition, at each stage in their life cycle, trematodes are adorned with elaborate sensory organs and possess sophisticated neuromuscular systems, but it is not clear how they use these complex machinery to perceive their worlds. The purpose of this review is to address this question through insights gathered from a century of research on trematode behaviour. Core theoretical assumptions from modern animal behaviour are used to provide the context for this analysis; a key concept is that all animals have unique perceptual worlds that may be inferred from their behaviours. A critical idea is that all animals possess complex patterns of innate behaviour which can be released by extremely specific signals from the environment. The evidence suggests that trematode parasites live in ecologically predictable aquatic and internal host environments where they perceive only small subsets of the total information available from the environment. A general conclusion is that host finding in miracidia and cercaria, and site-finding by trematodes migrating within their definitive hosts, is accomplished through the release of innate patterns of behaviours which are adaptive within the context of conditions in the worm's environment. Examples from empirical studies are used to support the contention that, despite the apparent complexity of their free-living and parasitic environments, the perceptual worlds of trematodes are impoverished, and complex patterns of behaviour may be released by only a few signals in their environment.

Invasion of skin by schistosome cercariae: some neglected facts

The process of skin invasion by Schistosoma cercariae was reviewed in a recent Trends Research Update, accompanied by a computer animation. Some aspects of that article were misleading and perpetuated misconceptions about parasite migration through the skin that should by now have been dispelled. This article sets out a different interpretation of events, taking account of the extensive data on migration and larval structural changes that have been documented for Schistosoma mansoni over the past 20 years.

Dermal Endothelial Cells and Keratinocytes Produce IL-7 In Vivo After Human Schistosoma mansoni Percutaneous Infection

The parasite Schistosoma mansoni infects its definitive mammalian host through an obligatory cutaneous penetration. In this work, we studied early immune response following migration of larvae through human skin, the first immunocompetent organ encountered by the parasite. For this purpose we used an experimental model of severe combined immunodeficient mice engrafted with human skin and injected with autologous PBL. Six days after percutaneous infection, we observed an infiltration of lymphocytes within the human skin, predominantly composed of CD4+ T cells. Moreover, among the cytokines potentially present in the infected skin, immunohistochemistry analysis revealed an in vivo expression of IL-7 in the epidermal layers and strikingly at the level of vascular endothelium. Using an in vitro coculture system, we showed that the S. mansoni larvae directly trigger IL-7 production by human dermal microvascular endothelial cells but not by keratinocytes. Finally, measurements of IL-7 concentrations in plasma of 187 S. mansoni-infected individuals showed that the youngest, which are also the most infected, displayed the highest IL-7 levels. Together, these findings describe dermal endothelial cells as a novel source of IL-7, a cytokine particularly important in schistosomiasis.

Cloning and characterization of a Schistosoma mansoni cDNA clone with a specific antigenic expression during development in a vertebrate host

Approximately 2.0 x 10 cDNA clones of an Schistosoma mansoni lambda gt11 cDNA library were screened in duplicate with serum from infected mice corresponding to distinct phases of infection. A cDNA clone (7/1) was isolated and recognized only by seven week serum. The clone was subcloned in pGEX-2T and Western-blot studies showed a specific antigenic expression confirming that only serum from the chronic phase is capable of recognizing this antigen. Dot-hybridization with RNA from different developmental phases of the parasite showed that the corresponding 7/1 RNA is expressed in all phases of parasite development in vertebrate hosts.

Involvement of regional lymph nodes after penetration of Schistosoma mansoni cercariae in naive and infected mice

The parotid lymph nodes of naive and previously infected Balb/c mice were studied after, respectively, infection and re-infection with cercariae of Schistosoma mansoni via the ears. Schistosomula were able to pass through the lymph node by following the lymph flow or by penetrating the veins of the medullary cords. The number of nodal mast cells was higher from day 2 to 6 of primary infection; and from day 5 to 11 of re-infection. The amount of degranulating mast cells was significantly higher at day 4 of infection and at day 1 of re-infection. Eosinophils characterized the nodal inflammatory processes observed after day 5 in both primarily-infected and re-infected mice. However, only in the latter the eosinophils were able to adhere to the larval surface. In primarily-infected mice, no intranodal larva presented signs of degeneration. In contrast, in re-infected animals, some degenerating larvae were found inside eosinophilic infiltrates. The eosinophils reached the nodal tissue by migrating through the high endothelial venules and their collecting veins.

Comparative phenotypic analysis of lymph node cells in mice after infection or vaccination with normal or ultraviolet-attenuated cercariae of Schistosoma japonicum or S. mansoni

Mice were infected with 200 untreated or vaccinated with 500 ultraviolet-attenuated cercariae of either Schistosoma japonicum or S. mansoni. For three weeks, cell numbers in axillary and mediastinal lymphnodes were counted and cell populations typed by cytofluorometry. In the axillary lymphnodes, numbers of B-cells and CD3+CD4+ T-cells but not CD3+CD8+ T-cells increased. Following vaccination with either species, parasite migration was apparently delayed in the skin and interrupted at the lungs, the lymphnodes gained weight, and cell numbers of axillary lymph nodes increased more than after infection. In mediastinal lymphnodes, only immunization with S. japonicum but not S. mansoni cercariae led to an increase of CD3+CD4+ T-cells. Following infection, both schistosome species induced higher CD3+CD4+, but not CD3+CD8+ T-cells in mediastinal nodes, and the peak was earlier with S. japonicum (about seven days after infection) than with S. mansoni (about 10 days). In analogy to T-cell observations by others using a gamma-attenuated cercarial vaccine in S. mansoni, the present results suggest that CD3+CD4+ cells also play a role in the ultraviolet-attenuated vaccine against S. japonicum.

Schistosoma japonicum and S. mansoni: comparison of larval migration patterns in mice

Mice were infected percutaneously with cercariae of Schistosoma japonicum or S. mansoni and parasites recovered by tissue-mincing from the skin or lungs or by perfusion of the mesenteric veins. S. japonicum had a narrow peak of recovery (up to 30%) from the lungs 3 days after infection, whereas lung recovery of S. mansoni peaked only on day 6 and levelled off during the following week. Infection with S. japonicum induced lung petechiae, but only after most of the parasites had left the lungs. The axillary lymph nodes draining the infection site increased in weight after infection and this effect was much greater and longer with S. mansoni than with S. japonicum. S. japonicum was perfusable from the mesenteric veins earlier (from day 3 onwards) and in higher number (40-60% from days 6 to 10) than S. mansoni (20% on day 20). The percentage of cercariae developing to adult worms was 57% for S. japonicum and 33% for S. mansoni. The data demonstrate that S. japonicum might escape from local tissue reactions in the skin and lungs and, due to its rapid migration, might induce only poor lymphocyte proliferation. As a possible consequence, S. japonicum may establish more efficiently in mice than S. mansoni.

Phenotypic and functional properties of Th lines and clones recognizing larval antigens of Schistosoma mansoni

Five T cell clones and two lines were derived from the lymph nodes (LN) of C57BL/6 mice immunized with radiation-attenuated lung-stage larvae of Schistosoma mansoni. All seven clones/lines were CD4+, CD8- and expressed high levels of CD44 and CD45RB surface markers. After prolonged maintenance in-vitro, with soluble antigen from 18 h schistosomula (SSP), five retained the ability to proliferate readily and release IFNg in response to concanavalin A (Con-A) and to SSP and/or soluble adult worm antigen (SWAP). These Th clones/lines induced significant footpad DTH reactions when injected with SWAP, but were unable to confer protective immunity after transfer to naïve recipient mice. This result could be explained by the antigen specificity of the clones/lines, since they were not able to release IFNg when cultured in-vitro with living lung-stage larvae. A second possibility is that the high level of CD45RB expression, which is not seen on the surface of pulmonary CD4+ memory/effector cells isolated directly from protectively-vaccinated mice, alters the ability of the clones/lines to release IFNg and to induce a DTH response in the lungs when they encounter antigen released from migrating schistosomula.

[Relationship between the pathogenicity of Schistosoma mansoni in mice and the susceptibility of the vector mollusc. I. Cercariae infectivity and worm burden]

The possible influence of the immune mechanisms of the molluscs on the development of the sporocysts and the resultant behavior of the adult worm in the vertebrate host were studied. Two strains of Schistosoma mansoni (BH and SJ) were used. These were genetically selected for their susceptibility and maintained, respectively, in Biomphalaria glabrata and Biomphalaria tenagophila, the natural invertebrate hosts. Experimental groups of mice infected with cercariae proceeding from the two species of molluscs, belonging to generations P, F1, F2, F3 and F4, were formed. The infection rates of the snails, the number of the penetrant cercariae and the worm burden in mice were evaluated. It was concluded that the higher susceptibility of Biomphalaria tenagophila results in a greater ability on the part of the cercarial to develop into adult worms. The higher susceptibility of B. glabrata results in greater power of penetration into mice tegument on the part of the cercariae.

Use of 75Se tracer and autoradiographic techniques in the study of schistosomiasis

The paper provides an overview of recent studies on the use of 75Se to tag larval schistosomes and to monitor their migration and distribution patterns in naive mice and those previously exposed to cercariae. The principles and techniques of radioassay and autoradiography in studying various aspects of 75Se-labelled larval schistosomes are described. The main shortcoming of radioassay in monitoring location and movement of labelled schistosomula is that some of the label dissociates from the schistosomula and accumulates in host tissues, notably the liver. Dissociated label is indistinguishable from schistosomula-bound label making monitoring of parasite migration extremely difficult. This difficulty is overcome by compressed tissue autoradiography where labelled schistosomula can be seen as reduced silver foci on an autoradiographic film, whereas dissociated label is too diffusely distributed to produce such reduced silver foci. Furthermore, using autoradiography, quantitative information on parasite migration in normal and immunized laboratory animals can be obtained that would be impossible using traditional recovery techniques. In addition to using 75Se tracer in migration studies, the radio-isotope has potential for elucidating various aspects of schistosome transmission ecology and snail population dynamics in natural waters.

Schistosoma mansoni: an ultrastructural examination of pulmonary migration

The migration of schistosomula through the lungs of mice has been investigated at the ultrastructural level. On Days 4 and 7 post-infection schistosomula were located only within the vasculature. Newly arrived Day 4 schistosomula were highly convoluted and completely occluded vessels measuring approximately 15-25 micron in diameter. By Day 7 parasites were located in pulmonary capillaries, often causing considerable distension. The subtegumentary fibrous interstitial layer was absent in elongated lung schistosomula. The minimum diameter of such parasites was 8 micron, and their tegument was usually unridged and flattened against the vascular endothelium, thus resulting in close apposition of endothelial and tegumental membranes. The tight fit of schistosomula in the pulmonary capillaries implies that migration through them is both a strenuous and slow process. The migratory delay in the lungs after arrival from the skin occurs because dimensional changes are necessary for successful capillary migration.

Migration of the schistosomula of Schistosoma mansoni in mice vaccinated with radiation-attenuated cercariae, and normal mice: an attempt to identify the timing and site of parasite death

The migration of the schistosomula of Schistosoma mansoni labelled with [75Se]methionine, has been followed from the skin to the hepatic portal system. Parasites were detected in all mouse tissues by compressed organ autoradiography. Two separate experiments were performed to track parasites in normal mice, and in mice previously vaccinated with irradiated cercariae. In normal mice, the profile of numbers of autoradiographic foci detected in the skin, lungs, systemic and splanchnic organs was described with time post-infection. The distribution of parasites to systemic organs, following exit from the lungs, paralleled the fractional distribution of cardiac output. Accumulation of schistosomula in the hepatic portal system was complete by day 21 post-infection. Only 2-3 passes of parasites around the vascular system would be required to produce the hepatic portal population. No significant decline in total foci was detected in the first 12 days post-infection. The majority of parasite elimination appeared to occur in the lungs as late as day 21, with lesser proportions in the systemic organs and skin infection site. The pattern of migration in vaccinated mice was similar to that in normal animals. One difference observed was the longer duration of stay in the skin; however, the majority of parasites eventually reached the lungs. The systemic phase of migration occurred on a reduced scale, as did accumulation of parasites in the hepatic portal system. The decline in total foci in vaccinated mice commenced approximately 7 days earlier than in normal mice and proceeded to a lower end-point. Again the majority of parasite elimination appeared to occur in the lungs with lesser proportions in the systemic organs and skin infection site. It is suggested that resistance to reinfection in vaccinated mice has two additive components which combine to retard the migration of schistosomula within the vasculature, preventing them from reaching the hepatic protal system.

Schistosoma mansoni: dynamics of migration through the vascular system of the mouse

Autoradiography of compressed mouse tissues has been used to estimate the numbers of 75Se-labelled schistosomula present in different mouse organs. Day 7 parasites extracted from the lungs of donor mice were delivered by injection to the lungs, systemic organs and liver of recipient mice as a discrete pulse. The numbers detected in various locations with time post-injection were then used to analyse the dynamics of intravascular migration. Approximately 98% of cercaria-associated label was lost during the first 14 days of parasite life, two-thirds of this in the first 7 h post-infection. Nevertheless, 99-113% of schistosomula could be detected 30 min post-injection into the locations chosen. The efficiency of the parasite delivery system was 95%. The time required for the number of foci in the lungs to decline to 50%, after injection of parasites via the femoral vein, was 55 h. Adjustment of this data to allow for parasites returning to the lungs after passage round the systemic vasculature gave a value of 30-35 h for the true mean time of lung transit. The distribution of parasites to systemic organs after their exit from the lungs was proportional to the fractional distribution of cardiac output. The probability (P) of a schistosomulum being distributed to splanchnic beds was estimated at 0.32 and its P of being trapped in the hepatic portal distributaries within the liver as 0.72-0.86. On this basis, the entire hepatic portal population of adult schistosomes would be recruited during 2-3 circuits of parasites around the pulmonary--systemic vasculature. The mean transit time of schistosomula through intestinal capillaries was 6.5 h whilst that through other systemic organs combined (muscles, kidneys, brain, etc) was 16 h, considerably more rapid than lung transit. The time taken for schistosomula to pass between organs, in arterial and venous blood, was shown to be less than 30 min in both cases (probably much less).

Schistosoma mansoni: an ultrastructural examination of skin migration in the hamster cheek pouch

The hamster cheek pouch has been used to investigate the skin migration of schistosomula at the ultrastructural level. Parasites have been observed up to 72 h post-infection. Despite rapidly reaching the epidermal basement membrane schistosomula remain in an epidermal location for at least 40 h post-infection, by which time the acetabular glands and their ducts have been lost and the transformation from cercaria to schistosomulum completed. Entry into the dermis and exit from the skin via the blood vessels is therefore by mechanical means or by lytic secretions emanating from a source other than the acetabular glands. The head gland which persists in the dermal schistosomula is a probable source of such secretions. The observations suggest that the apical area and musculature of the head capsule are important in gaining access to the lumen of blood vessels.

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.

Schistosoma mansoni: eicosanoid production by cercariae

Cercariae of Schistosoma mansoni are stimulated to penetrate skin by certain free fatty acids. The cercariae have an active arachidonate cascade, presumably using host skin essential fatty acids as cascade precursors. Exposing cercariae to 3.3 mM linoleate for 1, 10, and 60 min resulted in production of a wide variety of eicosanoids. Using high-performance liquid chromatography, eicosanoids coeluting with prostaglandin E2, D2, and A2, leukotriene B4, and 5-hydroxyeicosatetraenoic acid standards were identified, as well as unidentified peak positions. Radioimmunoassay confirmed the presence of immunoreactive prostaglandin E1, and E2, and 5- and 15-hydroxyeicosatetraenoic acids in cercarial extracts. No eicosanoid production occurred when cercariae were exposed to 3.3 mM oleate and 1 or 330 microM linoleate. Both high-performance liquid chromatography and radioimmunoassay data indicated that cercariae regulate the production of eicosanoids through time. It is postulated that arachidonate metabolism and subsequent eicosanoid production are required for successful cercarial penetration.

Schistosoma mansoni: cellular reactions to challenge infections in the cheek pouch skin of chronically infected Chinese hamsters

The cellular reactions to the migration of challenge schistosomula of Schistosoma mansoni through the cheek pouch skin in Chinese hamsters immunized 20 weeks previously via the abdomen has been investigated at the ultrastructural level. The composition of the leucocytic infiltrate into the dermis and epidermis changed with time after infection. At 24 and 48 h the infiltrate was composed mainly of neutrophils and mononuclear cells. At 72 h fewer neutrophils but more eosinophils were present. The interaction of the leucocytes with the challenge schistosomula is described. Degranulation of eosinophils at the cell-parasite interface has been observed. Despite the fact that approximately 30% of 48 h schistosomula and 50% of 72 h schistosomula examined had adherent leucocytes only a few of the parasites exhibited signs of damage, consisting of tegumental vacuolation and changes in the electron opacity of the tegumental cytoplasm. There was no strong evidence of increased attrition of challenge schistosomula in the cheek pouch skin relative to the primary infection, although some dead challenge parasites were observed. It is concluded that a substantial proportion of the attrition of challenge infections in the Chinese hamster occurs at a post-dermal location.

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

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

Interactions between eosinophils and antibodies: in vivo protective role against rat schistosomiasis

An original protocol of cell transfer from Schistosoma mansoni-infected rats to normal recipient rats is used to investigate the protective role of phagocytic cell populations, described as effector cells in vitro, against a challenge infection with S. mansoni. Nonadherent, eosinophil-enriched and -adherent, macrophage-rich cell preparations, injected via intradermal and subcutaneous routes at the precise site of exposure to cercariae, were able to significantly protect the recipient rats. The time-course study of this protective effect according to the time after infection of donor rats revealed that eosinophils were the major cell population involved in the early phase of infection (4 to 5 weeks), whereas macrophages could also be incriminated thereafter. A rosette assay using anti-immunoglobulin-coated erythrocytes indicated a sequence of the various antibody isotypes under study (IgG1, IgG2a, IgE) on the eosinophil surface, during the course of infection. As previously shown in vitro, cytophilic antibodies seemed to participate in the protective effect of eosinophils, since eosinophil-enriched cells from normal rats, sensitized in vitro with immune complexes present in infected rat serum, could also confer significant protection. These observations establish therefore the relevance between our previous in vitro studies and rat resistance to a challenge infection with S. mansoni, underlining the major role played by the interaction between antibodies and phagocytic cells (eosinophils and macrophages).

The relationship between the age of Schistosoma mansoni cercariae and their ability to penetrate and infect the mammalian host

The ability of the cercariae of Schistosoma mansoni to penetrate the tails of mice was shown to remain constant throughout their lives. However, their capacity to establish themselves and then reach maturity decreased as they aged. The abdominal route of penetration produced consistently higher maturation rates than the tail route. Significantly different maturation rates were obtained by modifying the standard tail infection technique. Evidence is presented that age-related mortality of schistosomula occurs within 24 h of penetration and may be associated with the exhaustion of energy reserves during the penetration of the stratum corneum. The relationship of this age-related mortality to 'mass mortality' is discussed.

Schistosoma mansoni: migration and attrition of irradiated and challenge schistosomula in the mouse

The fate of irradiated, immunizing cercariae and challenge schistosomula was investigated in mice using a quantitative, histological technique which would appear to be more efficient in estimating parasite numbers in skin and lungs than does the tissue mincing and incubation recovery technique used previously by other workers. There was evidence for slight retardation of irradiated (20 krad.) parasite migration in skin, but death of schistosomula in the skin appeared negligible. It was observed that the majority of irradiated parasites remained in the lungs until at least day 21 after infection, and that most schistosomula observed at this time were dead. In mice immunized with irradiated cercariae there was no evidence of attrition of the challenge infection in the skin. The migration profile of challenge parasites in the lungs of immunized animals was not significantly different from that of normal parasites in the lungs of naive animals. Dead challenge parasites were observed in the lungs but their numbers were not considered great enough to account for the final attrition measured by perfusion. It is suggested that the remainder of the attrition of challenge schistosomula may occur in the systemic and/or hepatic portal circulation.

Schistosoma mansoni: complement activation in human and rodent sera by living parasites of various developmental stages

Living Schistosoma mansoni of various developmental stages were studied with respect to their ability to activate the complement system in sera of humans, mice and rats. Immunofluorescence assays demonstrated that binding of human C3 occurred on fresh schistosomula as well as on schistosomula prepared from mouse lymph-nodes or lungs and on adult schistosomes. However, rodent C3 was deposited only on fresh schistosomula. Deposition of human C3 on the worms' surface required activation of the complement system. The alternative pathway was shown to be involved in deposition of human C3 on schistosomes of all ages, whereas activation of the classical pathway was demonstrable only with fresh schistosomula. Immunoelectrophoretic studies demonstrated a dose-dependent cleavage of human C3 and conversion of factor B by living adult schistosomes. The results demonstrate that the ability of living schistosomes to activate complement in vitro is dependent not only on their development stage but also on the species of the serum.

Recovery of parasites at different stages of migration following infection of rats with Schistosoma mansoni

The technique of tail amputation is utilized as a method for interrupting the migration process of Schistosoma mansoni schistosomula from the skin of Fischer rats infected by exposure of the tail to cercariae. The yields of schistosomula recovered from the lungs at different times post-infection are compared, using rats with or without tail amputation. Residence times of schistosomula in skin and lungs, as well as their transit time and efficiency of migration between these sites, are estimated. At least one-third of the infecting cercariae migrate from skin to lung in rats. Amputation of the tail on days 4 or 5 post-infection isolates a definable number of schistosomula in the lung and their migration to the portal circulation can be followed. The kinetics of this migration in rats and mice is compared and a significant difference is revealed.

Schistosoma mansoni: temporal distribution of radioselenium-labelled schistosomula in lungs of mice during the first two weeks of infection

The number of schistosomula in lungs was determined by compressed organ autoradiography at intervals up to 14 days after exposure of mice to 75Se-labelled cercariae by tail immersion. Probit analysis of compressed lung autoradiogram focus counts, expressed as percentages of initial infection level, yielded estimates of the average time of arrival, peak accumulation in the lungs and average time of departure of schistosomula: 4.5 +/- 0.87, 6.3 +/- 0.45 and 11 +/- 0.58 days, respectively. At peak accumulation 92 +/- 3.5% of the initial number of schistosomula were found in the lungs. It thus appears that little or no significant attrition of schistosomula occurred in the skin and, instead, that most of the 50-70% of penetrant cercariae that fail to reach adulthood are lost somewhere between the pulmonary and hepatic phases of development. Loss of 75Se label from schistosomula during the first 14 days was exponential, with an average half-life of 4.5 +/- 0.81 days. However, the high sensitivity of autoradiography tended to compensate for this rather rapid rate of label loss. It was pointed out that autoradiographic detection of schistosomula as discrete loci of radioactivity can also be expected to overcome the problem posed by the accumulation in such tissues as liver and kidney of 75Se label that has become separated from larvae.

Schistosoma mansoni: quantification of skin penetration and early migration by differential external radioassay and autoradiography

Eleven gamma-emitting radionuclides (49Sc, 54Mn, 59Fe (+2 and +3), 60Co, 65Zn, 75Se (as selenomethionine, selenocystine, selenite and selenate), 109Cd, 125Sb, 133Ba, 137Cs and 203Hg) were screened as labelling agents for Schistosoma mansoni cercariae by incubation of infected Biomphalaria glabrata snails in radioactive solution according to the technique of Christensen (1977). Only [75Se]methionine yielded satisfactorily labelled cercariae. Multiple regression analysis of volume, number of cercariae and radioactivity from a series of 10 aliquots of unwashed cercarial suspensions yielded estimates of unbound and cercarial bound radioactivity that were equivalent or superior to estimates based on assay of washed cercariae and eliminated loss of cercariae. Washing of cercarial suspensions over 8 micrometers pore diameter Millipore filters was found to result in entanglement of 60-90% of the cercariae on the filter disc. Differential external radioassay, a new technique employing partial body shielding within a total body counter, permitted separate estimation of tail and body radioactivity of conscious mice previously exposed by tail immersion to 75Se-labelled cercariae, with measurements repeated as often as desired. Approximately 39% of the 75Se present in emergent cercariae was retained by schistosomula transformed in vitro but this was subject to considerable variation, especially in schistosomula transformed in vivo. Secreted or catabolized label from penetrant cercariae and schistosomula was rapidly removed from the skin by the bloodstream. Numbers of schistosomula in tail skin were directly proportional to the number of reduced silver foci counted on tail autoradiograms; only a very small fraction of tail radioactivity represented unbound ('spurious') label. Migration of schistosomula away from skin was 50% complete at 3.8-4.3 days, as determined by probit analysis of autoradiographic data.

Schistosoma mansoni: a scanning electron microscope study of the developing schistosomulum

The surface morphology of schistosomula extracted from the skin, lungs and hepatic portal system (hps) of mice was investigated from Days 0 to 18 post-infection. Skin schistosomula and newly arrived schistosomula from the hps were of similar dimensions but were morphologically distinct. Lung schistosomula were considerably elongated with an estimated 53% increase in surface area compared to skin schistosomula. The pitted tegument of lung schistosomula was formed into ridges and troughs. These were compressed together in contracted individuals recovered from the hps on Day 10. The annular ridges were cross-linked by longitudinal septae which possibly prevent further elongation of the body. A regression of the spines between the mouth and the ventral sucker was observed in Day 2 skin schistosomula. In lung schistosomula only the spines at the anterior and posterior of the body remained. New spines were formed after the schistosomula reached the hps. It is suggested that the spines in the mid-region of the body are selectively disassembled and that their loss facilities migration along the lumina of capillaries with the residual spines acting as anterior and posterior anchors. The mouth opening was enlarged in schistosomula from the hps recovered from Day 10 onwards. Skin schistosomula lost the cercarial apical tegumentary ridges between 24 and 48 h after penetration but a spineless protrusible area remained. After arrival in the hps this area became integrated into the anterior surface as the oral sucker developed around the sub-terminal mouth. The cercarial ciliated papillae were lost on penetration. The migrating schistosomulum had few visible sensory papillae but following arrival in the hps new papillae were observed.