A study of proteases throughout the life cycle of Trichinella spiralis

Oral Vaccination of Mice With Trichinella spiralis Putative Serine Protease and Murine Interleukin-4 DNA Delivered by Invasive Lactiplantibacillus plantarum Elicits Protective Immunity

Trichinellosis is a serious zoonotic parasitic disease caused by Trichinella spiralis (T. spiralis) that causes considerable economic losses for the global pig breeding and food industries. As such, there is an urgent need for a vaccine that can prevent T. spiralis infection. Previous studies have reported that recombinant invasive Lactococcus lactis (LL) expressing Staphylococcus aureus fibronectin binding protein A (LL-FnBPA+) can transfer DNA vaccines directly to dendritic cells (DCs) across an epithelial cell monolayer, leading to significantly higher amounts of heterologous protein expression compared to non-invasive Lactococcus lactis. In this study, the invasive bacterium Lactiplantibacillus plantarum (L. plantarum) expressing FnBPA was used as a carrier to deliver a novel oral DNA vaccine consisting of T. spiralis adult putative serine protease (Ts-ADpsp) and murine interleukin (IL)-4 DNA to mouse intestinal epithelial cells. Experimental mice were orally immunized 3 times at 10-day intervals. At 10 days after the last vaccination, mice were challenged with 350 T. spiralis infective larvae by oral inoculation. Immunization with invasive L. plantarum harboring pValac-Ts-ADpsp/pSIP409-FnBPA induced the production of anti-Ts-ADpsp-specific IgG of serum, type 1 and 2 helper T cell cytokines of mesenteric lymph node (MLN) and spleen, secreted (s) IgA of intestinal lavage, and decreased T. spiralis burden and intestinal damage compared to immunization with non-invasive L. plantarum expressing Ts-ADpsp (pValac-Ts-ADpsp/pSIP409). Thus, invasive L. plantarum expressing FnBPA and IL-4 stimulates both mucosal and cellular immune response to protect against T. spiralis infection, highlighting its therapeutic potential as an effective DNA vaccine for trichinellosis.

Proteomic analysis of hydrolytic proteases in excretory/secretory proteins from Trichinella spiralis intestinal infective larvae using zymography combined with shotgun LC-MS/MS approach

The critical step of Trichinella spiralis infection is that the muscle larvae (ML) are activated to intestinal infective larvae (IIL) which invade the intestinal columnar epithelium to further develop. The IIL excretory/secretory (ES) proteins play an important role in host-parasite interaction. Proteolytic enzymes are able to mediate the tissue invasion, thereby increasing the susceptibility of parasites to their hosts. The aim of the current study was to screen and identify the natural active proteases in T. spiralis IIL ES proteins using Western blot and gel zymography combined with liquid chromatography tandem mass spectrometry (LC-MS/MS). The T. spiralis ML and IIL ES proteins were collected from the in vitro cultures and their enzymatic acitvities were examined by gelatin zymography and azocasein degradation. The protease activities were partially inhibited by PMSF, E-64 and EDTA. Three protein bands (45, 118 and 165 kDa) of T. spiralis IIL ES proteins were identified by shotgun LC-MS/MS because they have hydrolytic activity to gelatin compared to the ML ES proteins. Total of 30 T. spiralis proteins were identified and they are mainly serine proteinases (19), but also metalloproteinases (7) and cysteine proteinases (3). The qPCR results indicated that transcription levels of four T. spiralis protease genes (two serine proteases, a cathepsin B-like cysteine proteinase and a zinc metalloproteinase) at IIL stage were obviously higher than at the ML stage. These proteolytic enzymes are directly exposed to the host intestinal milieu and they may mediate the worm invasion of enteral epithelium and escaping from the host's immune responses. The results provide the new insights into understanding of the interaction of T. spiralis with host and the invasion mechanism.

Effect of recombinant serine protease from newborn larval stage of Trichinella spiralis on 2,4,6-trinitrobenzene sulfonic acid-induced experimental colitis in mice

Inflammatory bowel disease (IBD) is a complex immune-mediated disease of gastrointestinal tract that is mainly driven by Th1/Th17 immune response. "Helminth therapy" has emerged, and helminth-derived immunoregulatory molecules are being used as safe and new therapeutic antigens for IBD. Recombinant serine protease (SP) from newborn Trichinella spiralis (T. spiralis) larvae (NBL) was expressed and purified. BALB/c mice were immunized with NBL-SP at 100 µg three times at an interval of 5 days. Experimental colitis was induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS) administration. The disease activity index (DAI) and macroscopic and microscopic scores of the colon were assessed to identify the effect of NBL-SP on experimental colitis. Cytokine production in the serum was analysed by meso scale discovery (MSD). Cytokine production in the colon was detected by ELISA. CD4⁺T cell differentiation was measured by flow cytometry. NBL-SP alleviated TNBS-induced colitis in mice. The DAI, macroscopic and microscopic scores and colon length all showed a positive intervention effect of NBL-SP on experimental colitis. NBL-SP can weaken the increase in IFN-γ, TNF-α and IL-17 production as well as CD4⁺ IFN-γ⁺T cell and CD4⁺IL-17⁺T cell populations induced by colitis. Furthermore, the levels of Th2-related cytokines (IL-4, IL-5) and regulatory cytokines (IL-10, TGF-β) were elevated meanwhile the ratio of regulatory T cells (Tregs) and CD4⁺ IL-4 ⁺ T cells were increased by NBL-SP. NBL-SP of T. spiralis had a potential protective effect against IBD. NBL-SP skewed the Th1 and Th17-mediated response towards the Th2 and Treg response.

Extracellular Vesicles Derived From Trichinella spiralis Muscle Larvae Ameliorate TNBS-Induced Colitis in Mice

Helminths are masters at modulating the host immune response through a wide variety of versatile mechanisms. These complex strategies facilitate parasite survival in the host and can also be exploited to prevent chronic immune disorders by minimizing excessive inflammation. Extracellular vesicles (EVs) are small membrane-bound structures secreted by helminths which mediate immune evasion during parasite infection. The goal of this study was to investigate the immunoregulatory properties of Trichinella spiralis EVs (Ts-EVs) in a murine model of colitis. We found that Ts-EVs significantly ameliorated 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in mice. Ts-EVs alleviated intestinal epithelium barrier damage, markedly reduced pro-inflammatory cytokine secretion and neutrophil infiltration, and upregulated immunoregulatory cytokine expression in colon tissue. Ts-EVs also modulated the adaptive immune response by influencing T-cell composition. The numbers of Th1 and Th17 cells in MLNs, as well as the expression levels of Th1/Th17-associated cytokines and transcription factors in colon were reduced. In contrast, Th2 and Treg cells were increased after Ts-EVs treatment. Furthermore, sequencing of EV-derived microRNAs (miRNAs) indicated that an array of miRNAs was involved in the regulation of the host immune response, including inflammation. These findings expand our knowledge of host-parasite interactions, and may help design novel and effective strategies to prevent parasite infections or to treat inflammatory diseases like IBD. Further studies are needed to identify the specific cargo molecules carried by Ts-EVs and to clarify their roles during T. spiralis infection.

Immune responses in mice vaccinated with a DNA vaccine expressing serine protease-like protein from the new-born larval stage of Trichinella spiralis

Trichinella spiralis is a parasitic helminth that can infect almost all mammals, including humans. Trichinella spiralis infection elicits a typical type 2 immune responses, while suppresses type 1 immune responses, which is in favour of their parasitism. DNA vaccines have been shown to be capable of eliciting balanced CD4⁺ and CD8⁺ T cell responses as well as humoral immune responses in small-animal models, which will be advantage to induce protective immune response against helminth infection. In this study, serine protease (Ts-NBLsp) was encoded by a cDNA fragment of new-born T. spiralis larvae, and was inserted after CMV promoter to construct a DNA vaccine [pcDNA3·1(+)-Ts-NBLsp]. Ts-NBLsp expression was demonstrated by immunofluorescence. Sera samples were obtained from vaccinated mice, and they showed strong anti-Ts-NBLsp-specific IgG response. Mice immunized with the pcDNA3·1(+)-Ts-NBLsp DNA vaccine showed a 77·93% reduction in muscle larvae (ML) following challenge with T. spiralis ML. Our results demonstrate that the vaccination with pcDNA3·1(+)-Ts-NBLsp plasmid promoted the balance of type 1 and 2 immune responses and produced a significant protection against T. spiralis infection in mice.

Serine proteases of parasitic helminths

Serine proteases form one of the most important families of enzymes and perform significant functions in a broad range of biological processes, such as intra- and extracellular protein metabolism, digestion, blood coagulation, regulation of development, and fertilization. A number of serine proteases have been identified in parasitic helminths that have putative roles in parasite development and nutrition, host tissues and cell invasion, anticoagulation, and immune evasion. In this review, we described the serine proteases that have been identified in parasitic helminths, including nematodes (Trichinella spiralis, T. pseudospiralis, Trichuris muris, Anisakis simplex, Ascaris suum, Onchocerca volvulus, O. lienalis, Brugia malayi, Ancylostoma caninum, and Steinernema carpocapsae), cestodes (Spirometra mansoni, Echinococcus granulosus, and Schistocephalus solidus), and trematodes (Fasciola hepatica, F. gigantica, and Schistosoma mansoni). Moreover, the possible biological functions of these serine proteases in the endogenous biological phenomena of these parasites and in the host-parasite interaction were also discussed.

Trichinella spiralis newborn larvae: characterization of a stage specific serine proteinase expression, NBL1, using monoclonal antibodies

Trichinella spiralis is an intracellular parasitic nematode of mammalian skeletal muscle, causing a serious zoonotic disease in humans and showing a high economic impact mainly in pig breeding. Serine proteinases of T. spiralis play important roles in the host-parasite interactions mediating host invasion. In this study, we have focused on newborn larvae (NBL-1), the first identified serine proteinase from the NBL stage of T. spiralis. Five monoclonal antibodies (mAbs) directed against the C-terminal part of NBL1, were produced. These mAbs were IgG1κ isotype and specifically recognized as a common motif of 10 amino acids (PSSGSRPTYP). Selected mAbs were further characterized using antigens from various developmental stages of T. spiralis. Western blot revealed that selected mAbs reacted with the native NBL1 at Mr 50 kDa in the adult and NBL mixed antigens and NBL stage alone. Indirect immunofluorescence analysis revealed that selected mAbs intensely stained only the embryos within the gravid females and the NBL. Thus, the produced mAbs are useful tools for the characterization of NBL1 as a major antigen of Trichinella involved in the invasion of the host but also for the development of new serological tests with an early detection of T. spiralis infection.

Screening of early antigen genes of adult-stage Trichinella spiralis using pig serum from different stages of early infection

The goal of this work was to identify novel, early antigens present in Trichinella spiralis. To this end, a cDNA library generated from 3-day old adult worms (Ad3) was immunologically screened using serum from a pig infected with 20,000 muscle larvae. The serum was obtained from multiple, time course bleeds coinciding with early worm development. Seventeen positive clones were isolated using serum obtained at 20 days post infection (dpi). All clones corresponded to one gene that exhibited high sequence identity with the T. spiralis ATP-dependent RNA helicase DDX19B which is involved in parasite growth and development. In addition, nine additional positive clones representing 5 unique genes were identified when the library was screened with 30 dpi serum; four of these five genes displayed high similarity with members of a putative T. spiralis serine protease family known to be involved in host invasion and host-parasite interactions. The remaining gene aligned with the T. spiralis hypothetical ORF 11.30. The identification of these antigens provides potential candidates for the early diagnosis of trichinellosis and for the development of a vaccine against this parasite.

Identification of Trichinella spiralis early antigens at the pre-adult and adult stages

Three expression cDNA libraries from Trichinella spiralis worms 14 h, 20 h and 48 h post-infection (p.i.) were screened with serum from pigs experimentally infected with 20,000 T. spiralis muscle larvae. Twenty-nine positive clones were isolated from the 14 h p.i. cDNA library, corresponding to 8 different genes. A putative excretory-secretory protein similar to that of T. pseudospiralis was identified. Three clones corresponded to a T. spiralis serine proteinase inhibitor known to be involved in diverse functions such as blood coagulation and modulation of inflammation. Screening of the 20 h p.i. cDNA library selected 167 positive clones representing 12 different sequences. The clone with the highest redundancy encoded a small polypeptide having no sequence identity with any known proteins from Trichinella or other organisms. Fourteen clones displayed sequence identity with the heat shock protein (HSP) 70. HSPs are produced as an adaptive response of the parasite to the hostile environment encountered in the host intestine but their mechanism of action is not yet well defined. From the 48 h p.i. T. spiralis cDNA library, 91 positive clones were identified representing 7 distinct sequences. Most of the positive clones showed high similarity with a member of a putative T. spiralis serine protease family. This result is consistent with a possible major role for serine proteases during invasive stages of Trichinella infection and host-parasite interactions.

Quantitative risk assessment for the annual risk of exposure to Trichinella spiralis in imported chilled pork meat from New Zealand to Singapore

AIMS

To determine the annual likelihood of exposure to an infectious dose of Trichinella spiralis from consuming imported pork meat from New Zealand to Singapore.

METHODS

Input values specific for chilled pork meat imported into Singapore from New Zealand were used in a quantitative risk-assessment model. The model, designed to allow any combination of importing and exporting countries, was divided into two components, viz the release assessment, and the exposure assessment that assessed the annual risk of exposure to the consumer (ARC). The former estimated the likelihood that a contaminated fresh meat product from New Zealand would arrive at Singapore's border, and took into consideration the prevalence of disease on different types of farms. The latter determined the likelihood over a year that a person in Singapore would consume one or more servings of imported fresh meat from New Zealand that contained a burden of greater than or equal to one larva(e) of T. spiralis per gram after preparation for consumption.

RESULTS

The ARC for offal was 2.41 x 10(-7), which was below the pre-selected safety threshold of 1.00 x 10(-6). The ARC for lean meat was 2.39 x 10(-5), which was above the acceptable safety threshold.

CONCLUSIONS

The study demonstrated that continued routine testing at slaughter is unnecessary for pig offal produced commercially, and provided a model with which to further assess management of the risk of exposure to T. spiralis in lean meat.

CLINICAL RELEVANCE

The potential of Trichinella species to cause disease in humans is a public health concern, and has created adverse effects on the international trade of fresh lean meat without regard to the surveillance measures employed by particular pork-producing countries.

Lagochilascaris minor third-stage larvae secrete metalloproteases with specificity for fibrinogen and native collagen

Dissemination of parasitic infections depends on migration through tissues and evasion from both hemostatic processes and immune responses from hosts. Metalloproteases play major roles in these mechanisms of pathogen-host interactions and, thus, are considered drug targets. In this study, we characterized metalloprotease activities in excretory/secretory (ES) products from third stage larvae (L3) of the ascarid Lagochilascaris minor, the causative agent of lagochilascariosis, which demonstrates an impressive migrating capacity across host tissues, including bone. Gel enzymography showed that ES products of L3 display two major gelatinolytic activities. Optimal proteolytic activity was found to occur at neutral/alkaline pH and was associated with two L. minor-secreted metalloproteases of 59 (SM59(Lm)) and 114kDa (SM114(Lm)). We next showed that ES products of L3 were able to hydrolyze fibrinogen and collagen I at neutral pH, but not BSA, in an extensive manner. Furthermore, we demonstrated that ES products of L3 mediate hydrolysis of the triple helical structure of collagen I fibers in mouse mesentery. These results suggest that ES proteases of L3 might facilitate both L. minor migration through host tissues by hydrolyzing collagens of the extracellular matrix and evasion from host hemostatic mechanisms by degrading fibrinogen.

Cloning and analysis of a cDNA encoding a putative serine protease comprising two trypsin-like domains of Trichinella spiralis

The cDNA encoding a putative serine protease, TsSerP, was cloned by degenerative polymerase chain reaction and screening of the cDNA library from Trichinella spiralis adult-newborn larvae stage. Sequence analysis revealed the presence of two trypsin-like serine protease domains flanking a hydrophilic domain, with the catalytic triad residue histidine in the alpha domain substituted by an arginine residue. Southern blots indicated that this was a single copy gene in the parasite genome. Northern blots demonstrated a single 2.3-kb transcript during the muscle larvae and adult stages of T. spiralis. The recombinant protein from the TsSerP beta domain (betaSerP) was produced but not recognised by T. spiralis-infected swine serum. An anti-betaSerP polyclonal serum detected a 69-kDa polypeptide in the soluble antigens of T. spiralis muscle larvae. Immunolocalisation analysis located TsSerP on the inner layer of the cuticle and oesophagus of the parasite, suggesting a potential role in its moulting and/or digestive functions.