Fasciola hepatica cathepsin L proteinase cleaves fibrinogen and produces a novel type of fibrin clot

Pathogenicity and virulence of the liver flukes Fasciola hepatica and Fasciola Gigantica that cause the zoonosis Fasciolosis

Fasciolosis caused by the liver flukes Fasciola hepatica and Fasciola gigantica is one of the most important neglected parasitic diseases of humans and animals. The ability of the parasites to infect and multiply in their intermediate snail hosts, and their adaptation to a wide variety of mammalian definitive hosts contribute to their high transmissibility and distribution. Within the mammalian host, the trauma caused by the immature flukes burrowing through the liver parenchyma is associated with most of the pathogenesis. Similarly, the feeding activity and the physical presence of large flukes in the bile ducts can lead to anemia, inflammation, obstruction and cholangitis. The high frequency of non-synonymous polymorphisms found in Fasciola spp. genes allows for adaptation and invasion of a broad range of hosts. This is also facilitated by parasite’s excretory-secretory (ES) molecules that mediate physiological changes that allows their establishment within the host. ES contains cathepsin peptidases that aid parasite invasion by degrading collagen and fibronectin. In the bile ducts, cathepsin-L is critical to hemoglobin digestion during feeding activities. Other molecules (peroxiredoxin, cathepsin-L and Kunitz-type inhibitor) stimulate a strong immune response polarized toward a Treg/Th2 phenotype that favors fluke’s survival. Helminth defense molecule, fatty acid binding proteins, Fasciola-specific glycans and miRNAs modulate host pro-inflammatory responses, while antioxidant scavenger enzymes work in an orchestrated way to deter host oxidant-mediated damage. Combining these strategies Fasciola spp. survive for decades within their mammalian host, where they reproduce and spread to become one of the most widespread zoonotic worm parasites in the world.

A unique group of scabies mite pseudoproteases promotes cutaneous blood coagulation and delays plasmin-induced fibrinolysis

BACKGROUND

Scabies, a highly contagious skin disease affecting more than 200 million people worldwide at any time, is caused by the parasitic mite Sarcoptes scabiei. In the absence of molecular markers, diagnosis requires experience making surveillance and control challenging. Superficial microthrombi in the absence of vasculitis in scabies-affected skin are a recognised, yet unexplained histopathological differential of scabies infection. This study demonstrates that a family of Scabies Mite Inactivated Cysteine Protease Paralogues (SMIPP-Cs) excreted by the mites plays a role in formation of scabies-induced superficial microthrombi.

METHODOLOGY/PRINCIPAL FINDINGS

A series of in vitro and ex vivo experiments involving two representative recombinant SMIPP-Cs was carried out. In the presence of SMIPP-Cs, the thrombin clotting time (TCT), fibrin formation and plasmin induced fibrinolysis were monitored in vitro. The ultrastructure of the SMIPP-C-modulated fibrin was analysed by Scanning Electron Microscopy (SEM). Immuno-histological analyses were performed ex vivo, to localise the SMIPP-C proteins within scabies infected skin biopsies. SMIPP-Cs displayed pro-coagulant properties. They bound calcium ions, reduced the thrombin clotting time, enhanced the fibrin formation rate and delayed plasmin-induced fibrinolysis. The SMIPP-Cs associated with fibrin clots during fibrinogen polymerisation and did not bind to preformed fibrin. Scanning electron microscopy revealed that the fibrin clots formed in the presence of SMIPP-Cs were aberrant and denser than normal fibrin clots. SMIPP-Cs were detected in microthrombi which are commonly seen in scabietic skin.

CONCLUSIONS/SIGNIFICANCE

The SMIPP-Cs are the first scabies mite proteins found in sub-epidermal skin layers and their pro-coagulant properties promote superficial microthrombi formation in scabetic skin. Further research is needed to evaluate their potential as diagnostic or therapeutic target.

Blood anticlotting activity of a Rhipicephalus microplus cathepsin L-like enzyme

In parasites, cathepsins are implicated in mechanisms related to organism surveillance and host evasion. Some parasite cathepsins have fibrinogenolytic and fibrinolytic activity, suggesting that they may contribute to maintain blood meal fluidity for extended feeding periods. Here, it is shown that BmGTI (Rhipicephalus [Boophilus] microplus Gut Thrombin Inhibitor), a protein previously described as an inhibitor of fibrinogen hydrolysis and platelet aggregation by thrombin, and BmCL1 (Rhipicephalus [Boophilus] microplus Cathepsin-L like 1) are the same protein, hereinafter referred to using the earliest name (BmCL1). To further characterize BmCL1, Rhipicephalus microplus native and recombinant (rBmCL1) proteins were obtained. Native BmCL1 was isolated using thrombin-affinity chromatography, and it displays thrombin inhibition activity. We subsequently investigated rBmCL1 interaction with thrombin. We show that rBmCL1 and thrombin have a dissociation constant (Κ_D) of 130.2 ± 11.2 nM, and this interaction likely occurs due to a more electronegative surface of BmCL1 at pH 7.5 than at pH 5.0, which may favor an electrostatic binding to positively charged thrombin exosites. During BmCL1-thrombin interaction, thrombin is not degraded or inhibited. rBmCL1 impairs thrombin-induced fibrinogen clotting via a fibrinogenolytic activity. Fibrinogen degradation by BmCL1 occurs by the hydrolysis of Aα- and Bβ-chains, generating products similar to those produced by fibrinogenolytic cathepsins from other organisms. In conclusion, BmCL1 likely has an additional role in R. microplus blood digestion, besides its role in hemoglobin degradation at acid pH. BmCL1 fibrinogenolytic activity indicates a proteolytic activity in the neutral lumen of tick midgut, contributing to maintain the fluidity of the ingested blood, which remains to be confirmed in vivo.

Human cathepsins K, L, and S: Related proteases, but unique fibrinolytic activity

BACKGROUND

Fibrin formation and dissolution are attributed to cascades of protease activation concluding with thrombin activation, and plasmin proteolysis for fibrin breakdown. Cysteine cathepsins are powerful proteases secreted by endothelial cells and others during cardiovascular disease and diabetes. Their fibrinolytic activity and putative role in hemostasis has not been well described.

METHODS

Fibrin gels were polymerized and incubated with recombinant human cathepsins (cat) K, L, or S, or plasmin, for dose-dependent and time-dependent studies. Dissolution of fibrin gels was imaged. SDS-PAGE was used to resolve cleaved fragments released from fibrin gels and remnant insoluble fibrin gel that was solubilized prior to electrophoresis to assess fibrin α, β, and γ polypeptide hydrolysis by cathepsins. Multiplex cathepsin zymography determined active amounts of cathepsins remaining.

RESULTS

There was significant loss of α and β fibrin polypeptides after incubation with cathepsins, with catS completely dissolving fibrin gel by 24 h. Binding to fibrin stabilized catL active time; it associated with cleaved fibrin fragments of multiple sizes. This was not observed for catK or S. CatS also remained active for longer times during fibrin incubation, but its association/binding did not withstand SDS-PAGE preparation.

CONCLUSIONS

Human cathepsins K, L, and S are fibrinolytic, and specifically can degrade the α and β fibrin polypeptide chains, generating fragments unique from plasmin.

GENERAL SIGNIFICANCE

Demonstration of cathepsins K, L, and S fibrinolytic activity leads to further investigation of contributory roles in disrupting vascular hemostasis, or breakdown of fibrin-based engineered vascular constructs where non-plasmin mediated fibrinolysis must be considered.

True Lies: Using Proteomics to Assess the Accuracy of Transcriptome-Based Venomics in Centipedes Uncovers False Positives and Reveals Startling Intraspecific Variation in Scolopendra Subspinipes

Centipede venoms have emerged as a rich source of novel bioactive compounds. However, most centipede species are commonly considered too small for venom extraction and transcriptomics is likely to be an attractive way of probing the molecular diversity of these venoms. Examining the venom composition of Scolopendra subspinipes, we test the accuracy of this approach. We compared the proteomically determined venom profile with four common toxin transcriptomic toxin annotation approaches: BLAST search against toxins in UniProt, lineage-specific toxins, or species-specific toxins and comparative expression analyses of venom and non-venom producing tissues. This demonstrated that even toxin annotation based on lineage-specific homology searches is prone to substantial errors compared to a proteomic approach. However, combined comparative transcriptomics and phylogenetic analysis of putative toxin families substantially improves annotation accuracy. Furthermore, comparison of the venom composition of S. subspinipes with the closely related S. subspinipes mutilans revealed a surprising lack of overlap. This first insight into the intraspecific venom variability of centipedes contrasts the sequence conservation expected from previous findings that centipede toxins evolve under strong negative selection. Our results highlight the importance of proteomic data in studies of even comparably well-characterized venoms and warrants caution when sourcing venom from centipedes of unknown origin.

Across intra-mammalian stages of the liver f luke Fasciola hepatica: a proteomic study

Fasciola hepatica is the agent of fasciolosis, a foodborne zoonosis that affects livestock production and human health. Although flukicidal drugs are available, re-infection and expanding resistance to triclabendazole demand new control strategies. Understanding the molecular mechanisms underlying the complex interaction with the mammalian host could provide relevant clues, aiding the search for novel targets in diagnosis and control of fasciolosis. Parasite survival in the mammalian host is mediated by parasite compounds released during infection, known as excretory/secretory (E/S) products. E/S products are thought to protect parasites from host responses, allowing them to survive for a long period in the vertebrate host. This work provides in-depth proteomic analysis of F. hepatica intra-mammalian stages, and represents the largest number of proteins identified to date for this species. Functional classification revealed the presence of proteins involved in different biological processes, many of which represent original findings for this organism and are important for parasite survival within the host. These results could lead to a better comprehension of host-parasite relationships, and contribute to the development of drugs or vaccines against this parasite.

Clotting of mammalian fibrinogens by papain: a re-examination

Papain has long been known to cause the gelation of mammalian fibrinogens. It has also been reported that papain-fibrin is insoluble in dispersing solvents like strong urea or sodium bromide solutions, similar to what is observed with thrombin-generated clots in the presence of factor XIIIa and calcium. In those old studies, both the gelation and subsequent clot stabilization were attributed to papain, although the possibility that the second step might be due to contaminating factor XIII in fibrinogen preparations was considered. I have revisited this problem in light of knowledge acquired over the past half-century about thiol proteases like papain, which mostly cleave peptide bonds, and transglutaminases like factor XIIIa that catalyze the formation of ε-lysyl-γ-glutamyl cross-links. Recombinant fibrinogen, inherently free of factor XIII and other plasma proteins, formed a stable gel when treated with papain alone. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that the intermolecular cross-linking in papain-fibrin leads to γ-chain dimers, trimers, and tetramers, just as is the case with thrombin-factor XIIIa-stabilized fibrin. Mass spectrometry of bands excised from gels showed that the cross-linked material is quite different from what occurs with factor XIIIa, however. With papain, the cross-linking occurs between γ chains in neighboring protofibrils becoming covalently linked in a "head-to-tail" fashion by a transpeptidation reaction involving the α-amino group of γ-Tyr1 and a papain cleavage site at γ-Gly403 near the carboxy terminus, rather than by the (reciprocal) "tail-to-tail" manner that occurs with factor XIIIa and that depends on cross-links between γ-Lys406 and γ-Gln398.

Production and characterization of a monoclonal antibody against recombinant cathepsin L1 of Fasciola gigantica

Monoclonal antibodies (MoAbs) against a recombinant cathepsin L1 of Fasciola gigantica (rFgCatL1) were produced in vitro by fusion of BALB/c mice spleen cells immunized with rFgCatL1 and mouse myeloma cells. Reactivity and specificity of these MoAbs were evaluated by indirect ELISA and immunoblotting techniques. Seven MoAb clones were selected from the stable hybridoma clones, namely 1E10, 1F5, 3D11, 4B10, 4D3, 4E3 and 5E7. Clones 1E10, 1F5 and 3D11 were IgM, whereas clones 4B10, 4D3, 4E3 and 5E7 were IgG1. All MoAbs had kappa light chain isotypes. All MoAbs reacted with rCatL1 at molecular weight (MW) 30kDa and with the native CatL1 at MW 27kDa in whole body (WB) extracts of metacercariae (Met), newly excysted juveniles (NEJ), 1, 3, 5-week-old juveniles (Ju), adult WB and adult excretory-secretory (ES) fractions, but not with adult tegumental antigens (TA). All of these MoAbs showed no cross-reactions with antigens of other parasites commonly found in ruminants and human, including Paramphistomum cervi, Eurytrema pancreaticum, Gigantocotyle explanatum, Schistosoma spindale, Schistosoma mansoni, Moniezia benedeni, Avitellina centripunctata, Trichuris sp., Haemonchus placei and Setaria labiato-papillosa. Localization of CatL1 in each developmental stages of F. gigantica by immunoperoxidase technique, using these MoAbs as probes, indicated that CatL1 was present at high concentration in the caecal epithelium and caecal lumen of metacercariae, NEJ, 1, 3, 5-week-old juveniles and adult fluke. This finding indicated that CatL1 is a copiously expressed parasite protein that is released into the ES, thus CatL1 and its MoAb could be a good candidate for immunodiagnosis of fasciolosis in ruminant and human.

41.5-kDa Cathepsin L protease from Clonorchis sinensis: expression, characterization, and serological reactivity of one excretory-secretory antigen

Cysteine proteases (CPs) were associated with the pathogenicity and excystment of Clonorchis sinensis. Most of them were potential antigens for the immunodiagnosis of clonorchiasis. More researches on CPs will let us know more about their functions, and further employ them for the development of more efficient diagnostic reagent and prevention strategies. In the current study, a full-length sequence encoding cathepsin L from C. sinensis (CsCL41.5) was identified from our adult cDNA library. Bioinformatic analysis showed that CsCL41.5 included typical motifs of cathepsin L (ERFNIN and GNFD motifs) and conserved amino acid positions which constituted the active center of the enzyme. The identity of its amino acid sequence with the cathepsin L of Schistosoma japonicum was 49.6 %. Recombinant CsCL41.5 (rCsCL41.5) was highly expressed in the form of inclusion body in Escherichia coli, and soluble rCsCL41.5 was obtained after purification and renaturation. Western blotting analysis indicated that CsCL41.5 is an excretory-secretory antigen of C. sinensis adult. Immunolocalization demonstrated that CsCL41.5 is distributed in the intestine and eggs in the uterus of adult worm, tegument of metacercaria, oral suck, and tail of cercaria. ELISA assays showed that IgG4 was the predominant IgG isotype responding to rCsCL41.5 in sera from clonorchiasis patients. The sensitivity and specificity of specific IgG4 detection with rCsCL41.5 was 62.5 % (15/24) and 81.7 % (49/60), respectively. It was concluded that there were differences in biological function, efficiency of serodiagnosis, and characterization of immune reactivity between CsCL41.5 and other CPs of C. sinensis, combining with previous studies.

Fasciola gigantica: histology of the digestive tract and the expression of cathepsin L

The digestive tract of Fasciola gigantica is composed of the oral sucker, buccal tube, pharynx, esophagus, and caecum. The tegumental-type epithelium lines the first four parts of the digestive tract while the caecal-type epithelium lines the remaining parts from the caecal bifurcation. The caecal-epithelial cells are classified into 3 types according to their staining properties and ultrastructural characteristics, as related to the amount of food contents in the caecal lumen. All caecal-type epithelial cells synthesize and secrete cathepsin L, a major group of enzymes in the digestive tract, as detected by in situ hybridization and immunolocalization. Moreover, the secreted cathepsin L is also adsorbed on the outer surface of the tegument and the glycocalyx coating of the surface of the tegument, whereas the tegumental cells and tegumental syncytium covering the parasite's body and lining the proximal part of the digestive tract exhibit no in situ hybridization signal and immunostaining for cathepsin L.

Eosinophilia and thrombosis in parasitic diseases: an overview

It is known that peripheral blood eosinophilia (PBE) is a normal hematopoietic response to several parasitic diseases, but it is less known that PBE promotes a hypercoagulable state that may favor thrombosis. Scope of this article is to explore which parasitic infestations are most likely to be complicated by thrombosis and to highlight the pathogenetic contribution of PBE to vascular occlusions in this setting. A review of the world literature revealed 18 cases in which PBE was associated with vascular occlusion though no specific surveys were dedicated to this topic. The eosinophil exerts its thrombogenic potential by inhibition of the natural anticoagulant pathways and release of tissue factor with enhanced coagulation activation leading to vascular occlusion. It is hoped that this review contributes to the awareness of the link between PBE and thrombosis in parasitic disorders to foster research in this area.

A major cathepsin B protease from the liver fluke Fasciola hepatica has atypical active site features and a potential role in the digestive tract of newly excysted juvenile parasites

The newly excysted juvenile (NEJ) stage of the Fasciola hepatica lifecycle occurs just prior to invasion into the wall of the gut of the host, rendering it an important target for drug development. The cathepsin B enzymes from NEJ flukes have recently been demonstrated to be crucial to invasion and migration by the parasite. Here we characterize one of the cathepsin B enzymes (recombinant FhcatB1) from NEJ flukes. FhcatB1 has biochemical properties distinct from mammalian cathepsin B enzymes, with an atypical preference for Ile over Leu or Arg residues at the P(2) substrate position and an inability to act as an exopeptidase. FhcatB1 was active across a broad pH range (optimal activity at pH 5.5-7.0) and resistant to inhibition by cystatin family inhibitors from sheep and humans, suggesting that this enzyme would be able to function in extracellular environments in its mammalian hosts. It appears, however, that the FhcatB1 protease functions largely as a digestive enzyme in the gut of the parasite, due to the localization of a specific, fluorescently labeled inhibitor with an Ile at the P(2) position. Molecular modelling and dynamics were used to predict the basis for the unusual substrate specificity: a P(2) Ile residue positions the substrate optimally for interaction with catalytic residues of the enzyme, and the enzyme lacks an occluding loop His residue crucial for exopeptidase activity. The unique features of the enzyme, particularly with regard to its specificity and likely importance to a vital stage of the parasite's life cycle, make it an excellent target for therapeutic inhibitors or vaccination.

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.

Structural and functional relationships in the virulence-associated cathepsin L proteases of the parasitic liver fluke, Fasciola hepatica

The helminth parasite Fasciola hepatica secretes cysteine proteases to facilitate tissue invasion, migration, and development within the mammalian host. The major proteases cathepsin L1 (FheCL1) and cathepsin L2 (FheCL2) were recombinantly produced and biochemically characterized. By using site-directed mutagenesis, we show that residues at position 67 and 205, which lie within the S2 pocket of the active site, are critical in determining the substrate and inhibitor specificity. FheCL1 exhibits a broader specificity and a higher substrate turnover rate compared with FheCL2. However, FheCL2 can efficiently cleave substrates with a Pro in the P2 position and degrade collagen within the triple helices at physiological pH, an activity that among cysteine proteases has only been reported for human cathepsin K. The 1.4-A three-dimensional structure of the FheCL1 was determined by x-ray crystallography, and the three-dimensional structure of FheCL2 was constructed via homology-based modeling. Analysis and comparison of these structures and our biochemical data with those of human cathepsins L and K provided an interpretation of the substrate-recognition mechanisms of these major parasite proteases. Furthermore, our studies suggest that a configuration involving residue 67 and the "gatekeeper" residues 157 and 158 situated at the entrance of the active site pocket create a topology that endows FheCL2 with its unusual collagenolytic activity. The emergence of a specialized collagenolytic function in Fasciola likely contributes to the success of this tissue-invasive parasite.

Vaccine potential of inclusion bodies containing cysteine proteinase of Fasciola hepatica in calves and lambs experimentally challenged with metacercariae of the fluke

Despite intensive research efforts, progress in the development of effective anti-Fasciola hepatica vaccine has not been satisfactory. However, it has been found that cysteine proteinases of F. hepatica are very important candidates for a vaccine antigen because of their role in fluke biology and in the host-parasite relationship. In our previous experiments we found that recombinant cysteine proteinase which we have cloned from adult F. hepatica (CPFhW) can protect rats against the liver fluke infection when administered intramuscularly or when given intranasally in the form of cDNA. In the present experiments we aimed to evaluate the protectivity of the mucosal vaccination in calves and lambs with inclusion bodies containing recombinant CPFhW using different vaccination doses and various sites of antigen delivery. Female calves vaccinated intranasally with two doses of 300 microg of the recombinant CPFhW showed 54.2% protection against the subsequent challenge of 400 metacercariae (mc). Flukes which developed in vaccinated calves showed a reduction of reproductive potential. Male Corriedale lambs vaccinated at the age of 4 months demanded three doses of the antigen to gain 56.5% of protection to a challenge with 250 mc of F. hepatica. Vaccinated animals showed significantly lower blood eosinophil counts. No correlation was found between serum and mucosal IgG or IgA reacting with F. hepatica ES antigens and the protection level.

Vaccination with DNA encoding cysteine proteinase confers protective immune response to rats infected with Clonorchis sinensis

Cysteine proteinases of C. sinensis are important virulence factors that induce pathological changes associated with larval migration and localized biliary epithelial destruction. This study investigated the immunogenicity and protective efficacy of a DNA vaccine encoding Clonorchis sinensis cysteine proteinase (CsCP). The CsCP cDNA sequence displays significant homology to the mammalian or trematode cathepsin L. Plasmid DNA carrying the CsCP gene (pcDNA3.1-CsCP) was injected into Sprague-Dawley (SD) rats intradermally. Animals injected with pcDNA3.1-CsCP developed CsCP-specific antibodies, which exhibited an IgG2a dominance in sera. In addition, the DNA vaccine elicited the production of IFN-gamma, but not IL-4 in splenocytes, suggesting the induction of a typical Th-1 dominated immune response in rats. The pcDNA3.1-CsCP induced a significant level of protection (31.5%, p<0.05) in SD rats challenged with C. sinensis metacercariae. These results indicate that pcDNA3.1-CsCP induces both humoral and cellular immune responses. The CsCP gene may be a good candidate for use in future studies of vaccination against clonorchiasis.

Proteases in egg, miracidium and adult of Fasciola gigantica. Characterization of serine and cysteine proteases from adult

Proteolytic activity of 0-12 day old eggs, miracidium and adult worm of Fasciola gigantica was assessed and proteases were partially purified by DEAE-Sepharose and CM-cellulose columns. Four forms of protease were separated, PIa, PIb, PIc and PII. Purifications were completed for PIc and PII using Sephacryl S-200 chromatography. A number of natural and synthetic proteins were tested as substrates for F. gigantica PIc and PII. The two proteases had moderate activity levels toward azoalbumin and casein compared to azocasein, while gelatin, hemoglobin, albumin and fibrin had very low affinity toward the two enzymes. Amidolytic substrates are more specific to protease activity. PIc had higher affinity toward BAPNA-HCl (N-benzoyl-arginine-p-nitroanilide-HCl) and BTPNA-HCl (N-benzoyl-tyrosine-p-nitroanilide-HCl) at pH 8.0 indicating that the enzyme was a serine protease. However, PII had higher affinity toward BAPNA at pH 6.5 in the presence of sulfhydryl groups (beta-mercaptoethanol) indicating that the enzyme was a cysteine protease. The effect of specific protease inhibitors on these enzymes was studied. The results confirmed that proteases PIc and PII could be serine and cysteine proteases, respectively. The molecular weights of F. gigantica PIc and PII were 60,000 and 25,000, respectively. F. gigantica PIc and PII had pH optima at 7.5 and 5.5 and K(M) of 2 and 5 mg azocasein/mL, respectively. For amidolytic substrates, PIc had K(M) of 0.3 mM BAPNA/mL and 0.5 mM BTPNA/mL at pH 8.0 and PII had K(M) of 0.6 mM BAPNA/mL at pH 6.5 with reducing agent. F. gigantica PIc and PII had the same optimum temperature at 50 degrees C and were stable up to 40 degrees C. All examined metal cations tested had inhibitory effects toward the two enzymes. From substrate specificity and protease inhibitor studies, PIc and PII could be designated as serine PIc and cysteine PII, respectively.

Identification of a novel Fasciola hepatica cathepsin L protease containing protective epitopes within the propeptide

Cathepsin L (CL)-like proteases are important candidate vaccine antigens for protection against helminth infections. We previously identified an immunogenic 32 kDa protein specifically present in newly excysted juveniles (NEJs) of Fasciola hepatica. Here we show by N-terminal protein sequencing that this protein represents a CL-like protease still containing the propeptide. Two cDNAs encoding this CL were subsequently isolated from NEJs by RT-PCR. The predicted amino acid sequences of these cDNAs showed approximately 70% sequence homology to both CL1 and CL2 sequences isolated from adult stage F. hepatica and are, therefore, referred to as CL3. The CL3 clones encoded asparagine at position P1 of the propeptide cleavage site, suggesting a dependence on asparaginyl endopeptidases for maturation. Recombinant expression of a CL3 cDNA in Saccharomyces cerevisiae resulted in secretion of the proenzyme form. The propeptide of CL-like proteins was predicted to contain important B-cell epitopes. To determine the contribution of the propeptide to protective immunity, rats were vaccinated with Keyhole Limpet Haemocyanin-conjugated synthetic peptides encoding these putative B-cell epitopes derived from the CL1 or CL3 sequence. A subsequent challenge infection resulted in a significant (P < 0.05) reduction of fluke load compared to adjuvant controls. We conclude that the propeptide of CL3 plays an important role in inducing immunity against F. hepatica infection.

The evolution of enzyme specificity in Fasciola spp

Fasciola spp., commonly known as liver fluke, are significant trematode parasites of livestock and humans. They secrete several cathepsin L-like cysteine proteases, some of which differ in enzymatic properties and timing of expression in the parasite's life cycle. A detailed sequence and evolutionary analysis is presented, based on 18 cathepsin L-like enzymes isolated from Fasciola spp. (including a novel clone identified in this study). The enzymes form a monophyletic group which has experienced several gene duplication events over the last approximately 135 million years, giving rise to the present-day enzymatic repertoire of the parasite. This timing of these duplications appears to correlate with important points in the evolution of the mammalian hosts. Furthermore, the dates suggest that Fasciola hepatica and Fasciola gigantica diverged around 19 million years ago. A novel analysis, based on the pattern of amino acid diversity, was used to identify sites in the enzyme that are predicted to be subject to positive adaptive evolution. Many of these sites occur within the active site cleft of the enzymes, and hence would be expected to lead to differences in substrate specificity. Using homology modeling, with reference to previously obtained biochemical data, we are able to predict S2 subsite specificity for these enzymes: specifically those that can accommodate bulky hydrophobic residues in the P2 position and those that cannot. A number of other positions subject to evolutionary pressure and potentially significant for enzyme function are also identified, including sites anticipated to diminish cystatin binding affinity.

Immunization with cathepsin L proteinases CL1 and CL2 secreted by Fasciola hepatica elicit a preferential type 1 response based on IgG2a antibodies in rats

Cathepsin L proteinases (CL1 and CL2), the major components of Fasciola hepatica excretion/secretion products (E/S) are considered potential antigens of a vaccine against fascioliasis. The humoral response elicited by CL1 and CL2 in rats either immunized with the enzymes or infected with F. hepatica has been analysed, examining specific IgE and IgG subclass dynamics. The experiment was continued for 10 weeks and peripheral blood eosinophilia was also determined. Infected rats presented peaks of eosinophilia at weeks 3 and 7 post-infection, while those immunized with CL1 and CL2 were no different from controls. Total IgE in infected rats increased up to week 5, reaching 30 microg(-1) in some cases, then decreased slowly and rising again towards the end of the experiment. Determination of specific IgE, carried out in sera previously absorbed with Protein G-Sepharose, reached a peak in infected rats between weeks 2 and 5, depending on the individual. In immunized rats both total and specific IgE levels remained around the pre-immunization values. With regard to the IgG subclass responses to E/S products, in infected rats IgG1 predominated over IgG2a, and the reverse was true in rats immunized with CL1 and CL2 and tested against the respective antigens. In all cases an increase in IgG1 and IgG2a antibody titres was seen, with maximum levels being reached later (weeks 6-7) in infected rats than in immunized ones (weeks 4-5). No IgG2b or IgG2c responses were detected in any of the groups studied.

Fasciola hepatica cathepsin L suppresses sheep lymphocyte proliferation in vitro and modulates surface CD4 expression on human and ovine T cells

Fasciola hepatica infection has been shown to suppress sheep lymphocyte proliferation in vitro and this is at least partially attributable to excretory/secretory products (ES) released by F. hepatica parasites. We identified a suppressive component in ES by analysing the effect of ES fractions, separated by gel filtration, on the proliferation of sheep T cells in vitro. A major proportion of the suppressive activity in ES was shown to coelute with the cathepsin L proteases: E64, a cysteine protease inhibitor, blocked the suppressive activity of cathepsin L. In order to identify possible mechanisms by which cathepsin L could suppress T cell proliferation, the effect of ES and F. hepatica recombinant cathepsin L (rFhCatL) on the expression of 22 different sheep T cell surface markers was analysed by flow cytometry. Incubation of sheep T cells with ES or two rFhCatL significantly reduced surface CD4 expression and this effect was prevented in the presence of E64. In similar experiments with human T lymphocytes, ES and rFhCatL were shown to down regulate surface CD4 expression. These results show that F. hepatica cathepsin L both suppresses sheep T cell proliferation and reduces surface CD4 expression on both human and ovine T cells.

Characterization and large-scale expression of the recombinant cysteine proteinase from adult Clonorchis sinensis

Cysteine proteinases play important roles in the pathogenesis of several parasitic infections and have been proposed as targets for the structure-based approach of drug design. As the first step toward applying this strategy to design inhibitors as antiparasitic agents for Clonorchis sinensis, we overexpressed and characterized the 24-kDa cysteine proteinase from adult worms. First, the partial cysteine proteinase gene from C. sinensis was cloned by performing reverse transcription polymerase chain reaction (RT-PCR) with degenerate oligonucleotide primers derived from conserved cysteine proteinase sequences. The 5' and the 3' regions of the cysteine proteinase gene were amplified using the PCR protocol for the rapid amplification of cDNA ends-polymerase chain reaction (RACE-PCR). The cDNA has an open reading frame of 981 bp, and the deduced amino acid sequence shares similarity with the cathepsin L-like cysteine proteinases from Schistosoma mansoni, Paragonimus westermani metacercaria, Fasciola hepatica, and human cathepsin L by 52%, 47%, 34%, and 29%, respectively. The cysteine proteinase was then overexpressed in the yeast Pichia pastoris as an active enzyme on a large-scale basis (19.7 mg/L). The active recombinant enzyme was purified from culture media using a Ni2+-NTA-agarose affinity column and gel filtration chromatography. This 24-kDa recombinant protein exhibited a substrate preference for Z-Phe-Arg-AMC (benzyloxycarbonyl-L-phenylalanyl-L-arginine-7-amino-4-methyl-coumarin) compared with Z-Arg-Arg-AMC, and the activity was inhibited by E-64 (L-trans-epoxysuccinylleucylamido(4-quanidino)butane).

The contribution of molecular biology to the development of vaccines against nematode and trematode parasites of domestic ruminants

Rapid developments in molecular biology have had an enormous impact on the prospects for the development of vaccines to control the major nematode and trematode infestations of livestock. Vaccine candidates are purified using conventional protein chemistry techniques but the limitations imposed by the scarcity of parasite material provide an insurmountable barrier for commercial vaccine production by this means. The ability to purify mRNA from different parasite life-cycle stages and to prepare cDNA expression libraries from it has proven central to the identification of immunogenic parasite proteins. Potentially, protective parasite antigens can now be produced in recombinant form in a variety of vectors and this represents a key breakthrough on the road to commercial vaccine production. The contribution of molecular biology to this process is discussed using several examples, particularly in vaccine development against the pathogenic abomasal nematode of sheep and goats, Haemonchus contortus, and the liver fluke of sheep and cattle, Fasciola hepatica. The difficulties of producing recombinant proteins in the correct form, with appropriate post-translational modification and conformation, are discussed as well as emerging means of antigen delivery including DNA vaccination. The opportunities offered by genome and expressed sequence tag analyses programmes for antigen targeting are discussed in association with developing microarray and proteomics technologies which offer the prospect of large scale, rapid antigen screening and identification.

Molecular cloning and characterization of cathepsin L encoding genes from Fasciola gigantica

In this study cDNAs encoding cathepsin L-like proteins of Fasciola gigantica were cloned by the reverse transcription polymerase chain reaction method (RT-PCR) from total RNA of adult specimens. DNA sequence analyses revealed that six different cathepsin L cDNA fragments were isolated, which have DNA sequence identities of 87-99% towards the homologous genes from F. hepatica. Gene expression was studied at the RNA level by Northern and RNA in situ hybridizations. Northern analysis showed the cathepsin L genes to be strongly expressed in adult parasites as a group of 1050 nt sized RNAs. RNA in situ hybridization localized cathepsin L RNA to the cecal epithelial cells. Southern hybridization was used to determine the number of cathepsin L genes and indicated the presence of a family of closely related cathepsin L genes in the genome of F. gigantica.

Fasciola: kinetics and quality of humoral responses to fatty acid binding protein and cathepsin l following delivery as DNA vaccines in mice

The humoral response to DNA vaccination of mice with two important Fasciola antigens has been investigated. Both F. gigantica fatty acid binding protein (FABP) and F. hepatica cathepsin L5 (FhCatL5) were shown to be expressed in COS 7 cells and induced a humoral response when delivered as secretory constructs in mice. FABP induced an IgGl dominant response, with significant IgE, IgG2a, and IgG2b responses also present, indicating a mixed Th1/Th2 response. The total Ig response peaked at 1:24,500 and antibody titers were sustained for at least 32 weeks. In contrast, the delivery of FABP as a nonsecreted construct did not result in the induction of a measurable humoral response. FhCatL5 was delivered as a secretory construct, with secretion mediated by the native F. hepatica signal sequence, which was shown to operate in COS 7 cells. The humoral response peaked at 1:2000 at week 8 and was sustained for at least 20 weeks. Antibody isotype analysis demonstrated a Th2-like response, which was qualitatively different from that obtained for FABP with an IgE dominant response, and lower titers to IgG1 and IgG3. The results demonstrate that Fasciola antigens can be delivered as DNA vaccines, but that the quality of the response varies between antigens and is influenced by the method of vaccine delivery.

A single amino acid substitution affects substrate specificity in cysteine proteinases from Fasciola hepatica

The trematode Fasciola hepatica secretes a number of cathepsin L-like proteases that are proposed to be involved in feeding, migration, and immune evasion by the parasite. To date, six full cDNA sequences encoding cathepsin L preproproteins have been identified. Previous studies have demonstrated that one of these cathepsins (L2) is unusual in that it is able to cleave substrates with a proline in the P2 position, translating into an unusual ability (for a cysteine proteinase) to clot fibrinogen. In this study, we report the sequence of a novel cathepsin (L5) and compare the substrate specificity of a recombinant enzyme with that of recombinant cathepsin L2. Despite sharing 80% sequence identity with cathepsin L2, cathepsin L5 does not exhibit substantial catalytic activity against substrates containing proline in the P2 position. Molecular modeling studies suggested that a single amino acid change (L69Y) in the mature proteinases may account for the difference in specificity at the S2 subsite. Recombinant cathepsin L5/L69Y was expressed in yeast and a substantial increase in the ability of this variant to accommodate substrates with a proline residue in the P2 position was observed. Thus, we have identified a single amino acid substitution that can substantially influence the architecture of the S2 subsite of F. hepatica cathepsin L proteases.

Fasciola hepatica: parasite-secreted proteinases degrade all human IgG subclasses: determination of the specific cleavage sites and identification of the immunoglobulin fragments produced

The study was focused on the relationship of Fasciola hepatica-secreted proteinases and human IgG subclasses. Each IgG was incubated at different pH values and lengths of time with either the adult parasite excretion-secretion products or the purified cysteinyl proteinases cathepsin L1 and cathepsin L2. The Ig fragments produced were isolated and characterized by Western blot analysis, and the specific cleavage sites were determined by amino acid sequence analysis. Parasite excretion-secretion products and both cathepsins L produced similar degradation patterns and cleaved all human IgG subclasses at the hinge region, yielding at pH 7.3 and 37 degrees C Fab and Fc fragments in the case of IgG1 and IgG3 or Fab(2) and Fc in IgG2 and IgG4. While IgG1 and IgG3 were readily degraded by E/S products either in the presence or in the absence of reducing agents, IgG2 and IgG4 were resistant to proteolysis and were only digested in the presence of 0.1 M dithiothreitol. The cathepsins L needed the presence of dithiothreitol to digest IgG1, IgG2, and IgG4 whereas IgG3 was identically cleaved under both reducing and nonreducing conditions. The main cleavage sites produced by E/S products, CL1, or CL2 were located at the positions peptide bonds: His237-Thr238, Glu237-Cys239, Gly233-Asp234, and Ser241-Cys242 for gamma1, gamma2, gamma3, or gamma4, respectively. The enzymes gave additional splitting sites on the middle hinge of IgG3 to produce shorter Fc fragments and also produce Fd degradation of the IgG4. No cleavage specificity differences were found between CL1 and CL2, but they differed in the kinetics of IgG3 degradation. By lowering the pH, only the E/S products produced concomitant destruction of the Fc while preserving the Fab portion. Under all the conditions assayed the enzymes produced an Fc'-like fragment of 14-15 kDa corresponding to the whole CH3 domain of the immunoglobulin. Contrary to the extensive degradation produced by cathepsins on digested proteins, its actions on IgG subclasses were specific and restricted; thus, all the fragments produced could be potentially involved in the mechanisms used by the parasite to evade the host immune response.

Proteinases and associated genes of parasitic helminths

Many parasites have deployed proteinases to accomplish some of the tasks imposed by a parasitic life style, including tissue penetration, digestion of host tissue for nutrition and evasion of host immune responses. Information on proteinases from trematodes, cestodes and nematode parasites is reviewed, concentrating on those worms of major medical and economical importance. Their biochemical characterization is discussed, along with their putative biological roles and, where available, their associated genes. For example, proteinases expressed by the various stages of the schistosome life-cycle, in particular the well-characterized cercarial elastase which is involved in the penetration of the host skin and the variety of proteinases, such as cathepsin B (Sm31), cathepsin L1, cathepsin L2, cathepsin D, cathepsin C and legumain (Sm32), which are believed to be involved in the catabolism of host haemoglobin. The various endo- and exoproteinases of Fasciola hepatica, the causative agent of liver fluke disease, are reviewed, and recent reports of how these enzymes have been successfully employed in cocktail vaccines are discussed. The various proteinases of cestodes and of the diverse superfamilies of parasitic nematodes are detailed, with special attention being given to those parasites for which most is known, including species of Taenia, Echinococcus, Spirometra, Necator, Acylostoma and Haemonchus. By far the largest number of papers in the literature and entries to the sequence data bases dealing with proteinases of parasitic helminths report on enzymes belonging to the papain superfamily of cysteine proteinases. Accordingly, the final section of the review is devoted to a phylogenetic analysis of this superfamily using over 150 published sequences. This analysis shows that the papain superfamily can be divided into two major branches. Branch A contains the cathepin Bs, the cathepsin Cs and a novel family termed cathepsin Xs, while Branch B contains the cruzipains, cathepsin Ls, papain-like and aleurain/cathepsin H-like proteinases. The relationships of the helminth proteinases, and similar proteinases from protozoan parasites and other organisms, within these groups are discussed.

Vaccination with cathepsin L proteinases and with leucine aminopeptidase induces high levels of protection against fascioliasis in sheep

The potential of different parasite proteinases for use as vaccine candidates against fascioliasis in sheep was studied by vaccinating animals with the cathepsin L proteinases CL1 and CL2 and with leucine aminopeptidase (LAP) purified from adult flukes. In the first trial, sheep were immunized with CL1 or CL2 and the mean protection levels obtained were 33 and 34%, respectively. Furthermore, a significant reduction in egg output was observed in sheep vaccinated either with CL1 (71%) or with CL2 (81%). The second trial was performed to determine the protective potential of the two cathepsin L proteinases assayed together, as well as in combination with LAP, and of LAP alone. The combination of CL1 and CL2 induced higher levels of protection (60%) than those produced when these enzymes were administered separately. Those sheep that received the cocktail vaccine including CL1, CL2, and LAP were significantly protected (78%) against metacercarial challenge, but vaccination with LAP alone elicited the highest level of protection (89%). All vaccine preparations induced high immunoglobulin G titers which were boosted after the challenge infection, but no correlations between antibody titers and worm burdens were found. However, the sera of those animals vaccinated with LAP contained LAP-neutralizing antibodies. Reduced liver damage, as assessed by the level of the liver enzyme gamma-glutamyl transferase, was observed in the groups vaccinated with CL1, CL2, and LAP or with LAP alone.

Progress in Development of Liver Fluke Vaccines

Infection of ruminants by Fasciola spp continues to cause large economic losses worldwide. Recent results from several laboratories have demonstrated that animals can be significantly protected against infection by vaccination with defined Fasciola antigens. Apart from reducing fluke burdens, some vaccines can elicit a concurrent reduction in parasite egg production. The expectation of a commercially feasible vaccine that might also reduce parasite transmission in the field is now realistic, although major hurdles still exist. Here, Terry Spithill and John Dalton review the results of several recent vaccine trials and discuss the future prospects for vaccine development.

Isolation of a cDNA encoding Fasciola hepatica cathepsin L2 and functional expression in Saccharomyces cerevisiae

Cathepsin L2 is a major cysteine proteinase secreted by adult Fasciola hepatica. The enzyme differs from other reported cathepsin Ls in that it can cleave peptide substrates that contain proline in the P2 position. A cDNA was isolated from an expression library by immunoscreening with antiserum prepared against purified native cathepsin L2. This cDNA was sequenced and shown to encode a complete preprocathepsin L proteinase. Functionally active recombinant cathepsin L proteinase was expressed and secreted by Saccharomyces cerevisiae transformed with the cDNA. The recombinant enzyme was purified from large-scale fermentation broths using ultrafiltration and gel filtration chromatography on Sephacryl S200 HR columns. NH2-terminal amino acid sequencing showed that the cleavage point for activation of the recombinant pro-enzyme is identical to that of the F. hepatica-produced cathepsin L2. The mature active recombinant proteinase behaved similarly to the native enzyme when analysed by SDS-PAGE, immunoblotting and zymography and also cleaved peptides containing proline in the P2 position. Finally, the recombinant cathepsin L2 cleaved fibrinogen to form a fibrin clot, a property we described for F. hepatica cathepsin L2.