Comparison of cysteine peptidase activities inTrichobilharzia regentiandSchistosoma mansonicercariae

Differential proteomic analysis of laser-microdissected penetration glands of avian schistosome cercariae with a focus on proteins involved in host invasion

Schistosome invasive stages, cercariae, leave intermediate snail hosts, penetrate the skin of definitive hosts, and transform to schistosomula which migrate to the final location. During invasion, cercariae employ histolytic and other bioactive products of specialized holocrine secretory cells - postacetabular (PA) and circumacetabular (CA) penetration glands. Although several studies attempted to characterize protein composition of the in vitro-induced gland secretions in Schistosoma mansoni and Schistosoma japonicum, the results were somewhat inconsistent and dependent on the method of sample collection and processing. Products of both gland types mixed during their secretion did not allow localization of identified proteins to a particular gland. Here we compared proteomes of separately isolated cercarial gland cells of the avian schistosome Trichobilharzia szidati, employing laser-assisted microdissection and shotgun LC-MS/MS, thus obtaining the largest dataset so far of the representation and localization of cercarial penetration gland proteins. We optimized the methods of sample processing with cercarial bodies (heads) first. Alizarin-pre-stained, chemically non-fixed samples provided optimal results of MS analyses, and enabled us to distinguish PA and CA glands for microdissection. Using 7.5 x 10⁶ μm³ sample volume per gland replicate, we identified 3347 peptides assigned to 792 proteins, from which 461 occurred in at least two of three replicates in either gland type (PA = 455, 40 exclusive; CA = 421, six exclusive; 60 proteins differed significantly in their abundance between the glands). Peptidases of five catalytic types accounted for ca. 8% and 6% of reliably identified proteins in PA and CA glands, respectively. Invadolysin, nardilysin, cathepsins B2 and L3, and elastase 2b orthologs were the major gland endopeptidases. Two cystatins and a serpin were highly abundant peptidase inhibitors in the glands. While PA glands generally had rich enzymatic equipment, CA glands were conspicuously abundant in venom allergen-like proteins.

Secreted cathepsin L-like peptidases are involved in the degradation of trapped antibodies on the surface of Echinostoma caproni

Antibody trapping is a recently described strategy for immune evasion observed in the intestinal trematode Echinostoma caproni, which may aid to avoiding the host humoral response, thus facilitating parasite survival in the presence of high levels of local-specific antibodies. Parasite-derived peptidases carry out the degradation of trapped antibodies, being essential for this mechanism. Herein, we show that cathepsin-like cysteine endopeptidases are active in the excretory/secretory products (ESPs) of E. caproni and play an important role in the context of antibody trapping. Cysteine endopeptidase activity was detected in the ESPs of E. caproni adults. The affinity probe DCG-04 distinguished a cysteine peptidase band in ESPs, which was specifically recognized by an anti-cathepsin L heterologous antibody. The same antibody localized this protein in the gut and syncytial tegument of adult worms. Studies with cultured parasites showed that in vivo-bound antibodies are removed from the parasite surface in the absence of peptidase inhibitors, while addition of cathepsin L inhibitor prevented their degradation. These results indicate that cathepsin L-like peptidases are involved in the degradation of surface-trapped antibodies and suggest that cysteine peptidases are not only crucial for tissue-invading trematodes, but they can be equally relevant at the parasite-host interface in gut-dwelling flukes.

Serine proteases in schistosomes and other trematodes

Trematodes, also known as flukes, are phylogenetically ancient parasitic organisms. Due to their importance as human and veterinary parasites, their proteins have been investigated extensively as drug and vaccine targets. Among those, proteases, as crucial enzymes for parasite survival, are considered candidate molecules for anti-parasitic interventions. Surprisingly however, trematode serine proteases, in comparison with other groups of proteases, are largely neglected. Genes encoding serine proteases have been identified in trematode genomes in significant abundance, but the biological roles and biochemical functions of these proteases are poorly understood. However, increasing volumes of genomic and proteomic studies, and accumulated experimental evidence, indicate that this class of proteases plays a substantial role in host-parasite interactions and parasite survival. Here, we discuss in detail serine proteases at genomic and protein levels, and their known or hypothetical functions.

Papain-Based Vaccination Modulates Schistosoma mansoni Infection-Induced Cytokine Signals

We have previously shown that immunization of outbred rodents with cysteine peptidases-based vaccine elicited type 2-biased immune responses associated with consistent and reproducible protection against challenge Schistosoma mansoni. We herein start to elucidate the molecular basis of C57BL/6 mouse resistance to S. mansoni following treatment with the cysteine peptidase, papain. We evaluated the early cytokine signals delivered by epidermal, dermal, and draining lymph node cells of naïve, and S. mansoni -infected mice treated 1 day earlier with 0 or 50 μg papain, or immunized twice with papain only (10 μg/mouse), papain-free recombinant S. mansoni glyceraldehyde 3-phosphate dehydrogenase and 2-Cys peroxiredoxin peptide (10 and 15 μg/mouse, respectively = antigen Mix), or papain-adjuvanted antigen Mix. Schistosoma mansoni infection induced epidermal and lymph node cells to release type 1, type 2 and type 17 cytokines, known to counteract each other. Expectedly, humoral immune responses were negligible until patency. Papain pretreatment or papain-based vaccination diminished or shut off S. mansoni infection early induction of type 1, type 17 and type 2 cytokines except for thymic stromal lymphopoietin and programmed the immune system towards a polarized type 2 immune milieu, associated with highly significant (P < 0.005 - <0.0001) resistance to S. mansoni infection.

Comparative Transcriptomic Exploration Reveals Unique Molecular Adaptations of Neuropathogenic Trichobilharzia to Invade and Parasitize Its Avian Definitive Host

To date, most molecular investigations of schistosomatids have focused principally on blood flukes (schistosomes) of humans. Despite the clinical importance of cercarial dermatitis in humans caused by Trichobilharzia regenti and the serious neuropathologic disease that this parasite causes in its permissive avian hosts and accidental mammalian hosts, almost nothing is known about the molecular aspects of how this fluke invades its hosts, migrates in host tissues and how it interacts with its hosts’ immune system. Here, we explored selected aspects using a transcriptomic-bioinformatic approach. To do this, we sequenced, assembled and annotated the transcriptome representing two consecutive life stages (cercariae and schistosomula) of T. regenti involved in the first phases of infection of the avian host. We identified key biological and metabolic pathways specific to each of these two developmental stages and also undertook comparative analyses using data available for taxonomically related blood flukes of the genus Schistosoma. Detailed comparative analyses revealed the unique involvement of carbohydrate metabolism, translation and amino acid metabolism, and calcium in T. regenti cercariae during their invasion and in growth and development, as well as the roles of cell adhesion molecules, microaerobic metabolism (citrate cycle and oxidative phosphorylation), peptidases (cathepsins) and other histolytic and lysozomal proteins in schistosomula during their particular migration in neural tissues of the avian host. In conclusion, the present transcriptomic exploration provides new and significant insights into the molecular biology of T. regenti, which should underpin future genomic and proteomic investigations of T. regenti and, importantly, provides a useful starting point for a range of comparative studies of schistosomatids and other trematodes.

Despite the clinical importance of Trichobilharzia regenti in bird hosts and as a cause of cercarial dermatitis in humans, almost nothing is known about the molecular aspects of this fluke and its interactions with its hosts. Here, we sequenced, assembled and annotated the transcriptome representing two life stages (cercariae and schistosomula) of T. regenti involved in the first phases of infection of the bird host. We identified key biological and metabolic pathways specific to each of these two developmental stages and also undertook comparative analyses using data available for related flukes. Detailed analyses showed the unique involvement of carbohydrate metabolism, translation and amino acid metabolism, and calcium in T. regenti cercariae during invasion and in growth and development, as well as cell adhesion molecules, microaerobic metabolism (citrate cycle and oxidative phosphorylation), peptidases (cathepsins) and other histolytic and lysozomal proteins in schistosomula during migration in neural tissues. These molecular insights into T. regenti biology should support future genomic and proteomic investigations of T. regenti, and comparative studies of flatworms.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Induction of protective immune responses against schistosomiasis using functionally active cysteine peptidases

Each year schistosomiasis afflicts up to 600 million people in 74 tropical and sub-tropical countries, predominantly in the developing world. Yet we depend on a single drug, praziquantel, for its treatment and control. There is no vaccine available but one is urgently needed especially since praziquantel-resistant parasites are likely to emerge at some time in the future. The disease is caused by several worm species of the genus Schistosoma. These express several classes of papain-like cysteine peptidases, cathepsins B and L, in various tissues but particularly in their gastrodermis where they employ them as digestive enzymes. We have shown that sub-cutaneous injection of recombinant and functionally active Schistosoma mansoni cathepsin B1 (SmCB1), or a cathepsin L from a related parasite Fasciola hepatica (FhCL1), elicits highly significant protection (up to 73%) against an experimental challenge worm infection in murine models of schistosomiasis. The immune modulating properties of this subcutaneous injection can boost protection levels (up to 83%) when combined with other S. mansoni vaccine candidates, glyceraldehyde 3-phosphate dehydrogenase (SG3PDH) and peroxiredoxin (PRX-MAP). Here, we discuss these data in the context of the parasite's biology and development, and provide putative mechanism by which the native-like cysteine peptidase induce protective immune responses.

Cercarial dermatitis, a neglected allergic disease

Cercarial dermatitis (swimmer's itch) is a common non-communicable water-borne disease. It is caused by penetration of the skin by larvae (cercariae) of schistosomatid flukes and develops as a maculopapular skin eruption after repeated contacts with the parasites. The number of outbreaks of the disease is increasing, and cercarial dermatitis can therefore be considered as an emerging problem. Swimmer's itch is mostly associated with larvae of the bird schistosomes of Trichobilharzia spp. Recent results have shown that mammalian infections (including man) manifest themselves as an allergic reaction which is able to trap and eliminate parasites in the skin. Studies on mammals experimentally infected by bird schistosome cercariae revealed, however, that during primary infection, parasites are able to escape from the skin to the lungs or central nervous system. This review covers basic information on detection of the infectious agents in the field and the clinical course of the disease, including other pathologies which may develop after infection by cercariae, and diagnosis of the disease.

β-Lapachone: a naphthoquinone with promising antischistosomal properties in mice

The activity of β-lapachone (3,4-dihydro-2,2-dimethyl-2H-naphthol[1,2-b]pyran-5,6-dione, β-lap) against different stages of Schistosoma mansoni was investigated in mice. Mice infected with 50 cercariae (BH strain) were intraperitoneally treated at a dose of 50 mg/kg for 5 consecutive days, starting on the 1st, 14th, 28th and 45th days after infection, to evaluate the effect of β-lap on skin schistosomula, lung schistosomula, young worms (before oviposition) and adult worms (after oviposition), respectively. All animals were euthanized 60 days after infection. β-Lap significantly reduced (p<0.001) the number of worms in 29.78%, 37.2%, 24.2% and 40.22% when administered during the phases of skin schistosomula, lung schistosomula, young worms and adult worms, respectively. Significant reduction was also achieved in terms of female burden. In all groups, there was significant reduction in the number of eggs and granulomas in the hepatic tissue. When the intervention was performed during the phase of adult worms, β-lap reduced the size of hepatic granulomas and changed the oogram pattern, lowering the percentage of immature eggs and increasing the percentage of mature and dead eggs. Our data indicate that β-lap has moderate antischistosomal properties. Its molecule may also be used as a prototype for synthesis of new naphthoquinone derivatives with potential schistosomicidal properties. Further studies with different formulations containing β-lap are needed to clearly establish the best dose and route of administration and its mechanism of action against schistosomes.

Cysteine protease is a major component in the excretory/secretory products of Euclinostomum heterostomum (Digenea: Clinostomidae)

Cysteine proteases of parasite organisms play numerous indispensable roles in tissue penetration, feeding, immunoevasion, virulence, egg hatching and metacercarial excystment. They are critical key enzymes in the biology of parasites and have been exploited as serodiagnostic markers, therapeutic and vaccine targets. In the present study, the cysteine proteases in the in vitro released excretory/secretory (E/S) products of the digenetic trematode parasite, Euclinostomum heterostomum have been analysed. The encysted progenetic metacercariae of E. heterostomum collected from the infected liver and kidney of Channa punctatus were excysted in vitro and incubated in phosphate buffer at 37 ± 1 °C, and the E/S products released were analysed. The spectrophotometric analysis of the proteases revealed active hydrolysis of chromogenic substrate, azocoll, in a time-, temperature- and pH-dependent manner. Optimum activity was observed at pH 7.0 at 37 ± 1 °C, and with 1 mM each of various protease inhibitors (Mini Protease Inhibitor Cocktail, ethylene diaminetetraacetic acid, phenyl methyl sulphonyl fluoride, iodoacetamide and 1,10-phenanthroline) used, significant inhibition was observed by iodoacetamide and 85% of inhibition at a concentration of 2 mM, suggesting that cysteine protease is a major component in the E/S of this parasite. Four discrete protease bands of Mr 36, 39, 43 and 47 kDa were identified by gelatin-substrate zymography. Maximum gelatinolytic activity was observed at pH 7.0, and among various inhibitors used, almost complete disappearance of protease bands was observed by 2 mM iodoacetamide. The proteolytic cleavage of bovine serum albumin, bovine haemoglobin and human haemoglobin in vitro were also studied.

Characterization and differential expression of cathepsin L3 alleles from Fasciola hepatica

Fasciola hepatica infections cause significant global problems in veterinary and human medicine, including causing huge losses in cattle and sheep production. F. hepatica host infection is a multistage process and flukes express papain-like cysteine proteases, termed cathepsins, which play pivotal roles in virulence through host entry, tissue migration and immune evasion. Expression of these proteases is developmentally regulated. Recent studies indicate that excystment of infective larvae is dependent on cysteine proteases and together FhCL3 and FhCB account for over 80% of total protease activity detectable in newly excysted juvenile (NEJ) fluke. This paper focuses on members of the cathepsin L gene family, specifically those belonging to the CL3 clade. The cDNA of two novel cathepsin L3 proteases--FhCL3-1 and FhCL3-2 were cloned. The mRNA transcript expression levels for these enzymes were significantly different at various time points in life development stages obtained in vitro, from dormant metacercariae to NEJ 24h after excystment. Maximum expression levels were observed in NEJ immediately after excystment. In all stages examined by Real Time PCR, FhCL3-2 was expressed at a higher level compared to FhCL3-1 which was expressed only at very low levels. Western blot and immunohistochemical analysis also indicated higher expression of the FhCL3-2 allele and its secretory nature. The ability of antibody responses from rats and sheep challenged with F. hepatica to recognize recombinant FhCL3-1 and FhCL3-2 was shown to differ. Differences were also confirmed through the use of anti-rFhCL3-1 and anti-rFhCL3-2 sera in Western blot analysis of juvenile excretory/secretory (ES) material separated by 2D electrophoresis. These results indicate analysis of relative expression of parasite virulence factors from different populations is required, as this will likely impact the effectiveness of vaccines based on these antigens.

Proteolytic activity in the adult and larval stages of the human roundworm parasite Angiostrongylus costaricensis

Angiostrongylus costaricensis is a nematode that causes abdominal angiostrongyliasis, a widespread human parasitism in Latin America. This study aimed to characterize the protease profiles of different developmental stages of this helminth. First-stage larvae (L1) were obtained from the faeces of infected Sigmodon hispidus rodents and third-stage larvae (L3) were collected from mollusks Biomphalaria glabrata previously infected with L1. Adult worms were recovered from rodent mesenteric arteries. Protein extraction was performed after repeated freeze-thaw cycles followed by maceration of the nematodes in 40 mM Tris base. Proteolysis of gelatin was observed by zymography and found only in the larval stages. In L3, the gelatinolytic activity was effectively inhibited by orthophenanthroline, indicating the involvement of metalloproteases. The mechanistic class of the gelatinases from L1 could not be precisely determined using traditional class-specific inhibitors. Adult worm extracts were able to hydrolyze haemoglobin in solution, although no activity was observed by zymography. This haemoglobinolytic activity was ascribed to aspartic proteases following its effective inhibition by pepstatin, which also inhibited the haemoglobinolytic activity of L1 and L3 extracts. The characterization of protease expression throughout the A. costaricensis life cycle may reveal key factors influencing the process of parasitic infection and thus foster our understanding of the disease pathogenesis.

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.

Secretory glands in cercaria of the neuropathogenic schistosome Trichobilharzia regenti - ultrastructural characterization, 3-D modelling, volume and pH estimations

BACKGROUND

Cercariae of schistosomes employ bioactive molecules for penetration into their hosts. These are released from specialized unicellular glands upon stimuli from host skin. The glands were previously well-described in the human pathogen Schistosoma mansoni. As bird schistosomes can also penetrate human skin and cause cercarial dermatitis, our aim was to characterize the architecture and ultrastructure of glands in the neurotropic bird schistosome Trichobilharzia regenti and compare it with S. mansoni. In the context of different histolytic enzymes used by these two species, we focused also on the estimations of gland volumes and pH in T. regenti.

RESULTS

The architecture and 3-D models of two types of acetabular penetration glands, their ducts and of the head gland are shown here. We characterized secretory vesicles in all three gland types by means of TEM and confirmed accuracy of the models obtained by confocal microscopy. The results of two independent approaches showed that the glands occupy ca. one third of cercarial body volume (postacetabular glands ca. 15%, circumacetabular 12% and head gland 6%). The inner environment within the two types of acetabular glands differed significantly as evidenced by dissimilar ability to bind fluorescent markers and by pH value which was higher in circumacetabular (7.44) than in postacetabular (7.08) glands.

CONCLUSIONS

As far as we know, this is the first presentation of a 3-D model of cercarial glands and the first exact estimation of the volumes of the three gland types in schistosomes. Our comparisons between T. regenti and S. mansoni implied that the architecture and ultrastructure of the glands is most likely conserved within the family. Only minor variations were found between the two species. It seems that the differences in molecular composition have no effect on general appearance of the secretory cells in TEM. Fluorescent markers employed in this study, distinguishing between secretory vesicles and gland types, can be useful in further studies of mechanisms used by cercariae for host invasion. Results of the first attempts to estimate pH within schistosome glands may help further understanding of regulation of enzymatic activities present within the glands.

Cathepsins B1 and B2 of Trichobilharzia SPP., bird schistosomes causing cercarial dermatitis

Trichobilharzia regenti and T. szidati are schistosomes that infect birds. although T. regenti/T. szidati can only complete their life cycle in specific bird hosts (waterfowl), their larvae-cercariae are able to penetrate, transform and then migrate as schistosomula in nonspecific hosts (e.g., mouse, man). Peptidases are among the key molecules produced by these schistosomes that enable parasite invasion and survival within the host and include cysteine peptidases such as cathepsins B1 and B2. These enzymes are indispensable bio-catalysts in a number of basal biological processes and host-parasite interactions, e.g., tissue invasion/migration, nutrition and immune evasion. Similar biochemical and functional characteristics were observed for cathepsins B1 and B2 in bird schistosomes (T. regenti, T. szidati) and also for their homologs in human schistosomes (Schistosoma mansoni, S. japonicum). Therefore, data obtained in the research of bird schistosomes can also be exploited for the control of human schistosomes such as the search for targets of novel chemotherapeutic drugs and vaccines.

The phylogeny, structure and function of trematode cysteine proteases, with particular emphasis on the Fasciola hepatica cathepsin L family

Helminth parasites (nematodes, flatworms and cestodes) infect over 1 billion of the world's population causing high morbidity and mortality. The large tissue-dwelling worms express papain-like cysteine peptidases, termed cathepsins that play important roles in virulence including host entry, tissue migration and the suppression of host immune responses. Much of our knowledge of helminth cathepsins comes from studies using flatworms or trematode (fluke) parasites. The developmentally-regulated expression of these proteases correlates with the passage of parasites through host tissues and their encounters with different host macromolecules. Recent phylogenetic, biochemical and structural studies indicate that trematode cathepsins exhibit overlapping but distinct substrate specificities due to divergence within the protease active site. Here we provide an overview of the evolution, biochemistry and structure of these important enzymes and highlight how recent advances in proteomics and gene silencing techniques are allowing researchers to probe their biological functions. We focus mainly on members of the cathepsin L gene family of the animal and human pathogen, Fasciola hepatica, because of our deep understanding of their function, biochemistry and structure.

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.

An integrated transcriptomics and proteomics analysis of the secretome of the helminth pathogen Fasciola hepatica: proteins associated with invasion and infection of the mammalian host

To infect their mammalian hosts, Fasciola hepatica larvae must penetrate and traverse the intestinal wall of the duodenum, move through the peritoneum, and penetrate the liver. After migrating through and feeding on the liver, causing extensive tissue damage, the parasites move to their final niche in the bile ducts where they mature and produce eggs. Here we integrated a transcriptomics and proteomics approach to profile Fasciola secretory proteins that are involved in host-pathogen interactions and to correlate changes in their expression with the migration of the parasite. Prediction of F. hepatica secretory proteins from 14,031 expressed sequence tags (ESTs) available from the Wellcome Trust Sanger Centre using the semiautomated EST2Secretome pipeline showed that the major components of adult parasite secretions are proteolytic enzymes including cathepsin L, cathepsin B, and asparaginyl endopeptidase cysteine proteases as well as novel trypsin-like serine proteases and carboxypeptidases. Proteomics analysis of proteins secreted by infective larvae, immature flukes, and adult F. hepatica showed that these proteases are developmentally regulated and correlate with the passage of the parasite through host tissues and its encounters with different host macromolecules. Proteases such as FhCL3 and cathepsin B have specific functions in larvae activation and intestinal wall penetration, whereas FhCL1, FhCL2, and FhCL5 are required for liver penetration and tissue and blood feeding. Besides proteases, the parasites secrete an array of antioxidants that are also highly regulated according to their migration through host tissues. However, whereas the proteases of F. hepatica are secreted into the parasite gut via a classical endoplasmic reticulum/Golgi pathway, we speculate that the antioxidants, which all lack a signal sequence, are released via a non-classical trans-tegumental pathway.

Antibody responses induced by Trichobilharzia regenti antigens in murine and human hosts exhibiting cercarial dermatitis

Cercariae of bird schistosomes (genus Trichobilharzia) are able to penetrate the skin of mammals (noncompatible hosts), including humans, and cause a Th2-associated inflammatory cutaneous reaction termed cercarial dermatitis. The present study measured the antibody reactivity and antigen specificity of sera obtained after experimental infection of mice and natural infection of humans. Sera from mice re-infected with T. regenti showed a bias towards the development of antigen-specific IgM and IgG1 antibodies and elevated levels of total serum IgE, indicative of a Th2 polarized immune response. We also demonstrate that cercariae are a source of antigens triggering IL-4 release from basophils collected from healthy human volunteers. Analysis of sera from patients with a history of cercarial dermatitis revealed elevated levels of cercarial-specific IgG, particularly for samples collected from adults (> 14 years old) compared with children (8-14 years old), although elevated levels of antigen-specific IgE were not detected. In terms of antigen recognition, IgG and IgE antibodies in the sera of both mice and humans preferentially bound an antigen of 34 kDa. The 34 kDa molecule was present in both homogenate of cercariae, as well as cercarial excretory/secretory products, and we speculate it may represent a major immunogen initiating the Th2-immune response associated with cercarial dermatitis.

The functional expression and characterisation of a cysteine peptidase from the invasive stage of the neuropathogenic schistosome Trichobilharzia regenti

A transcriptional product of a gene encoding cathepsin B-like peptidase in the bird schistosome Trichobilharzia regenti was identified and cloned. The enzyme was named TrCB2 due to its 77% sequence similarity to cathepsin B2 from the important human parasite Schistosoma mansoni. The zymogen was expressed in the methylotropic yeast Pichia pastoris; procathepsin B2 underwent self-processing in yeast media. The peptidolytic activity of the recombinant enzyme was characterised using synthetic fluorogenic peptide substrates at optimal pH 6.0. Functional studies using different specific inhibitors proved the typical cathepsin B-like nature of the enzyme. The S(2) subsite specificity profile of recombinant TrCB2 was obtained. Using monospecific antibodies against the recombinant enzyme, the presence of cathepsin B2 was confirmed in extracts from cercariae (infective stage) and schistosomula (early post-cercarial stage) of T. regenti on Western blots. Also, cross-reactivity was observed between T. regenti and S. mansoni cathepsins B2 in extracts of cercariae, schistosomula or adults. In T. regenti, the antisera localised the enzyme to post-acetabular penetration glands of cercariae implying an important role in the penetration of host skin. The ability of recombinant TrCB2 to degrade skin, serum and nervous tissue proteins was evident. Elastinolytic activity suggests that the enzyme might functionally substitute the histolytic role of the serine class elastase known from S. mansoni and Schistosoma haematobium but not found in Schistosoma japonicum or in bird schistosomes.

A distinctive repertoire of cathepsins is expressed by juvenile invasive Fasciola hepatica

Secreted cysteine proteases are relevant actors in parasite biology, taking part in critical host colonization roles such as traversing tissue barriers, immune evasion and nutrient digestion. In the trematode Fasciola hepatica, the initial step to successful infection of the mammalian host is the excystment of metacercariae and the invasion through the intestinal wall by the newly excysted juveniles (NEJ). While the cathepsin L-like cysteine proteinases secreted by the adult fluke have been extensively characterized, the cataloguing and description of the cathepsins B and L reported in the invasive stages is only sketchy. To identify the cathepsins expressed during excystment and early invasion we constructed cDNA libraries encoding NEJ cathepsins B and L. We found two cathepsin L-like cysteine proteinases (CL3, CL4) and three cathepsins B (CB1, CB2, CB3) which are predominantly expressed in NEJ. Phylogenetic analysis showed that NEJ-expressed cathepsins L constitute a well-defined clade separate from the adult enzymes. Excystment induction resulted in a significant increment in activity towards cathepsin-specific fluorogenic substrates in metacercariae homogenates, consistent with the detection of precursor and mature forms of cathepsins B and L before and after induction. In NEJ culture supernatants, protein and relative activity profiles show subtle changes during the first 48 h, with prevalence of cathepsin L-like activity, although cathepsins CB3 and CL3 were detected by mass spectrometry. Noticeably, the hydrolysis of a substrate with proline in the P2 position was predominant, a property only shared with adult CL2 and vertebrate cathepsin K among the C1A subfamily of cysteine proteases. Collectively these mRNA, protein and enzymatic data demonstrate the existence of a NEJ-specific repertoire of cathepsins expressed early in invasion, distinct to those used by other trematodes, potentially relevant for specific vaccine and chemotherapy design. The diversity of proteases employed by trematodes in the invasion process is discussed.