Schistosoma japonicum migration through mouse skin compared histologically and immunologically with S. mansoni

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.

Enzyme activity of Schistosoma japonicum cercarial elastase SjCE-2b ascertained by in vitro refolded recombinant protein

Cercarial elastase (CE) secreted from cercariae is evinced to play a pivotal role in initial skin penetration of mammalian host. SjCE-2b, a Schistosoma japonicum CE orthologous to SmCE-2b in S. mansoni, was previously found present in cercarial stage to aid skin invasion, but its enzyme activity has not been validated due to the insolubility and altered conformation when expressed recombinantly in bacteria as inclusion bodies. We report here for the first time a bioactive and soluble recombinant SjCE-2b recovered successfully from inclusion bodies by refolding approaches, enabling our biochemical and immunological investigation of this enzyme. Using a "two-step-denaturing and refolding" method, we recovered an 83% yield with 90% purity of refolded protein. Proteolytic activity of rSjCE-2b was demonstrated and characterized by enzymatic assay, showing a K_m of 0.116 mM and a specific activity of 1900 nmol p-nitroaniline/min/mg protein. A significant immunoprotective response was evidenced in mice immunized with refolded rSjCE-2b. The result of immunoprotection test is at apparent variance with previously reported findings using S. mansoni CE preparation, which was poorly immunogenic in immunized animals. This work extends the knowledge of schistosome cercarial protease, and presents a bioactive form of S. japonicum recombinant CE with high yield and good quality. This will allow further biochemical and biological investigations to explore schistosome CE activity and better understand the molecular mechanisms associated with cercarial skin invasion of the mammalian host.

Local Antiglycan Antibody Responses to Skin Stage and Migratory Schistosomula of Schistosoma japonicum

Schistosomiasis is a tropical disease affecting over 230 million people worldwide. Although effective drug treatment is available, reinfections are common, and development of immunity is slow. Most antibodies raised during schistosome infection are directed against glycans, some of which are thought to be protective. Developing schistosomula are considered most vulnerable to immune attack, and better understanding of local antibody responses raised against glycans expressed by this life stage might reveal possible glycan vaccine candidates for future vaccine research. We used antibody-secreting cell (ASC) probes to characterize local antiglycan antibody responses against migrating Schistosoma japonicum schistosomula in different tissues of rats. Analysis by shotgun Schistosoma glycan microarray resulted in the identification of antiglycan antibody response patterns that reflected the migratory pathway of schistosomula. Antibodies raised by skin lymph node (LN) ASC probes mainly targeted N-glycans with terminal mannose residues, Galβ1-4GlcNAc (LacNAc) and Galβ1-4(Fucα1-3)GlcNAc (LeX). Also, responses to antigenic and schistosome-specific glycosphingolipid (GSL) glycans containing highly fucosylated GalNAcβ1-4(GlcNAcβ1)n stretches that are believed to be present at the parasite's surface constitutively upon transformation were found. Antibody targets recognized by lung LN ASC probes were mainly N-glycans presenting GalNAcβ1-4GlcNAc (LDN) and GlcNAc motifs. Surprisingly, antibodies against highly antigenic multifucosylated motifs of GSL glycans were not observed in lung LN ASC probes, indicating that these antigens are not expressed in lung stage schistosomula or are not appropriately exposed to induce immune responses locally. The local antiglycan responses observed in this study highlight the stage- and tissue-specific expression of antigenic parasite glycans and provide insights into glycan targets possibly involved in resistance to S. japonicum infection.

The developing schistosome worms elicit distinct immune responses in different tissue regions

Schistosome parasites follow a complex migration path through various tissues, changing their antigenic profile as they develop. A thorough understanding of the antibody response in each tissue region could help unravel the complex immunology of these developing parasites and aid vaccine design. Here we used a novel strategy for analysing the local antibody responses induced by Schistosoma japonicum infection at each site of infection. Cells from rat lymph nodes draining the sites of larval migration (the skin and lungs), the liver-lymph nodes where adults reside and the spleens were cultured to allow the in vivo-induced antibody-secreting cells to release antibody into the media. The amount and isotype of antibodies secreted in the supernatants differed significantly in the different lymph nodes and spleen, corresponding with the migration path of the schistosome worms. In addition, there were significant differences in binding specificity, as determined by surface labelling, western blots and by screening a glycan array. Through capturing the local antibody response, this study has revealed dramatic differences in the quality and specificity of the immune response at different tissue sites, and highlighted the existence of stage-specific protein and carbohydrate antigens. This will provide a valuable tool for the isolation of novel vaccine targets against the larval stages of schistosomes.

Pathogenicity of Trichobilharzia spp. for Vertebrates

Bird schistosomes, besides being responsible for bird schistosomiasis, are known as causative agents of cercarial dermatitis. Cercarial dermatitis develops after repeated contact with cercariae, mainly of the genus Trichobilharzia, and was described as a type I, immediate hypersensitivity response, followed by a late phase reaction. The immune response is Th2 polarized. Primary infection leads to an inflammatory reaction that is insufficient to eliminate the schistosomes and schistosomula may continue its migration through the body of avian as well as mammalian hosts. However, reinfections of experimental mice revealed an immune reaction leading to destruction of the majority of schistosomula in the skin. Infection with the nasal schistosome Trichobilharzia regenti probably represents a higher health risk than infections with visceral schistosomes. After the skin penetration by the cercariae, parasites migrate via the peripheral nerves, spinal cord to the brain, and terminate their life cycle in the nasal mucosa of waterfowl where they lay eggs. T. regenti can also get over skin barrier and migrate to CNS of experimental mice. During heavy infections, neuroinfections of both birds and mammals lead to the development of a cellular immune response and axonal damage in the vicinity of the schistosomulum. Such infections are manifest by neuromotor disorders.

Wnt4, the first member of the Wnt family identified in Schistosoma japonicum, regulates worm development by the canonical pathway

The Wnt signaling pathway is an evolutionarily conserved signal transduction pathway used extensively during animal development. We aim, by increasing our understanding of the Wnt signaling pathway, to find a key gene or protein present in schistosomes that can be developed into vaccine candidate or drug target. We therefore isolated the Wnt4 gene from Schistosoma japonicum. Wnt4 encodes a putative protein of 558 amino acids which contains the conserved functional domain of the Wnt gene family. We suppressed the expression of Wnt4 mRNA in 10-day schistosomulae by RNA interference. Quantitative PCR analysis showed that Wnt4 displayed a 73% reduction in the transcript level. And GSK-3beta and beta-catenin, which are involved in Wnt canonical pathway, showed a 45% and 39% reduction in mRNA levels, respectively. PLC, CaMKII, DVL, and JNK, which are involved in Wnt non-canonical pathway, showed no reduction. These results suggest that the Wnt4 signal protein in S. japonicum regulates downstream genes by a canonical pathway. Wnt4 is the first member of the Wnt family to be identified in S. japonicum. An increased understanding of the Wnt signal transduction pathway will allow us to elucidate further the molecular mechanism of development in schistosomes.

Fluorescent imaging of antigen released by a skin-invading helminth reveals differential uptake and activation profiles by antigen presenting cells

Infection of the mammalian host by the parasitic helminth Schistosoma mansoni is accompanied by the release of excretory/secretory molecules (ES) from cercariae which aid penetration of the skin. These ES molecules are potent stimulants of innate immune cells leading to activation of acquired immunity. At present however, it is not known which cells take up parasite antigen, nor its intracellular fate. Here, we develop a technique to label live infectious cercariae which permits the imaging of released antigens into macrophages (MΦ) and dendritic cells (DCs) both in vitro and in vivo. The amine reactive tracer CFDA-SE was used to efficiently label the acetabular gland contents of cercariae which are released upon skin penetration. These ES products, termed ‘0-3hRP’, were phagocytosed by MHC-II⁺ cells in a Ca⁺ and actin-dependent manner. Imaging of a labelled cercaria as it penetrates the host skin over 2 hours reveals the progressive release of ES material. Recovery of cells from the skin shows that CFDA-SE labelled ES was initially (3 hrs) taken up by Gr1⁺MHC-II⁻ neutrophils, followed (24 hrs) by skin-derived F4/80⁺MHC-II^lo MΦ and CD11c⁺ MHC-II^hi DC. Subsequently (48 hrs), MΦ and DC positive for CFDA-SE were detected in the skin-draining lymph nodes reflecting the time taken for antigen-laden cells to reach sites of immune priming. Comparison of in vitro-derived MΦ and DC revealed that MΦ were slower to process 0-3hRP, released higher quantities of IL-10, and expressed a greater quantity of arginase-1 transcript. Combined, our observations on differential uptake of cercarial ES by MΦ and DC suggest the development of a dynamic but ultimately balanced response that can be potentially pushed towards immune priming (via DC) or immune regulation (via MΦ).

Schistosomiasis is caused by the parasitic worm Schistosoma with over 200 million people infected across 76 countries. The parasitic larvae (called cercariae) infect mammalian hosts via the skin, but the exact mechanisms by which dermal cells interact with molecules released by invading larvae are unclear. A better understanding of the infection process and stimulation of the early immune response would thus enable a targeted approach towards the development of drugs and vaccines. Here, we have used the fluorescent tracer CFDA-SE to label infectious cercariae and, together with confocal microscopy, have for the first time tracked in real time the parasite infecting via the epidermis and depositing excretory/secretory material in its wake. Phagocytic macrophages and dendritic cells in the skin internalised excretory/secretory molecules released by the larvae, and both cell types were subsequently located in the draining lymph nodes where priming of the acquired immune response occurs. In vitro studies determined that macrophages were slower to process released parasite material than dendritic cells; they also secreted lower levels of pro-inflammatory cytokines but greater quantities of regulatory IL-10. The relative abundance of macrophages versus dendritic cells in the skin infection site and their differential rates of antigen processing may be crucial in determining the success of adaptive immune priming in response to infection.

Penetration of cercariae into the living human skin: Schistosoma mansoni vs. Trichobilharzia szidati

We studied the skin invasion of Schistosoma mansoni cercariae by placing gamma-irradiated and nonirradiated cercariae onto the living human skin and timing the behavior of 53 individuals. The skin invasion of S. mansoni was less efficient compared to the bird schistosome Trichobilharzia szidati. S. mansoni cercariae crept longer on the skin after attachment until they started penetration movements (median of 43 s [range of 15 s-6.58 min]; T. szidati, median of 8 s [range of 0-80 s]). Subsequent to this longer exploratory phase, 74% penetrated into wrinkles (T. szidati 84%), 22% into the smooth skin surface (T. szidati 0%), and 4% into hair follicles (T. szidati 16%). The S. mansoni cercariae needed, on average, 6.58 min (range of 1.57-13.13 min) for full entry, while T. szidati needed 4.0 min (range of 1.38-13.34 min); the fastest S. mansoni cercaria entered the skin within 94 s, while T. szidati entered within 83 s. Sixty percent of the S. mansoni cercariae had the tails still attached when the bodies disappeared in the skin whereas all T. szidati cercariae shed their tails within 0-105 s after the onset of penetration movements. The faster invasion of T. szidati may result from the more sophisticated host-finding mechanisms of this species. Regarding S. mansoni, cercarial dermatitis, as immediate skin response, developed after a sensitization period of 19 days.

Schistosoma japonicum and S. mansoni cercariae: different effects of protein in medium, of mechanical stress, and of an intact complement system on in vitro transformation to schistosomula

The cercariae of Schistosoma japonicum were subjected in vitro to treatments known for Schistosoma mansoni to generate schistosomula-like organisms. As a technical prerequisite to pipette or to otherwise handle the sticky cercariae of S. japonicum, the addition of protein to water or medium was found to abolish the stickiness of cercariae of this species. Shearing forces exerted in vitro by syringe (22 G) passage are known since long to fully transform S. mansoni cercariae, but this treatment was found to be much less efficient with S. japonicum. Thus, even with very narrow needles (27 G), complete transformation of cercariae was not obtained with S. japonicum. Complement, provided by fresh human serum, is also well known to induce rapid transformation of S. mansoni cercariae with subsequent killing of the schistosomula. This treatment of S. japonicum cercariae induced degeneration of the tails and strongly promoted the transformation to schistosomula-like organisms, but at a much slower pace. These effects were absent from sera either heat-inactivated or depleted of factor B or of complement component C8, but were restored after adding the purified respective complement components. The schistosomula-like organisms of S. japonicum were not susceptible to lysis after 1 day of in vitro culture in the presence of 50% fresh human serum, although both cercariae and schistosomula of S. mansoni were killed under these conditions. In conclusion, the dynamics of in vitro transformation of S. japonicum cercariae differ significantly from those of S. mansoni, and complement has a major transformation-promoting activity.

Modulation of the host's immune response by schistosome larvae

Schistosomes appear to have evolved several strategies to down-regulate the host's immune response in order to promote their own survival. For the host, down-regulation is also beneficial as it can limit the extent of pathology. It is widely accepted that schistosomes modulate the immune response during the chronic phase of infection after egg deposition has started. However, there is increasing evidence that modulation of the immune response can occur much earlier at the time infective cercariae penetrate the host skin. In this review, we explore the various lines of evidence that excretory/secretory (ES) molecules from cercariae down-regulate the host's immune response. We highlight the immunological factors that are produced and may be involved in regulating the immune system (e.g. IL-10, and eicosanoids), as well as speculating on possible mechanisms of immune modulation (e.g. mast-cell activation, T-cell apoptosis, and/or the skewed activation of antigen-presenting cells [APCs]). Finally, we draw attention to several molecules of schistosome origin that have the potential to stimulate the regulatory response (e.g. glycans) and link these to potential host receptors (e.g. TLRs and C-type lectins).