Oral vaccination with Trichinella spiralis DNase II DNA vaccine delivered by attenuated Salmonella induces a protective immunity in BALB/c mice

Application of a recombinant novel trypsin from Trichinella spiralis for serodiagnosis of trichinellosis

BACKGROUND

The excretory/secretory (ES) antigen of Trichinella spiralis muscle larvae (ML) is currently the most widely used diagnostic antigen to detect T. spiralis infection. However, this antigen has certain drawbacks, such as a complicated ES antigen preparation process and lower sensitivity during the early phase of infection. The aim of this study was to investigate the features of a novel T. spiralis trypsin (TsTryp) and evaluate its potential diagnostic value for trichinellosis.

METHODS

The TsTryp gene was cloned and recombinant TsTryp (rTsTryp) expressed. Western blotting and an enzyme-linked immunosorbent assay (ELISA) were performed to confirm the antigenicity of rTsTryp. The expression pattern and distribution signature of TsTryp at various life-cycle stages of T. spiralis were analyzed by quantitative PCR, western blotting and the immunofluorescence test. An ELISA with rTsTryp and ML ES antigens was used to detect immunoglobulins G and M (IgG, IgM) in serum samples of infected mice, swine and humans. The seropositive results were further confirmed by western blot with rTsTryp and ML ES antigens.

RESULTS

TsTryp expression was observed in diverse T. spiralis life-cycle phases, with particularly high expression in the early developmental phase (intestinal infectious larvae and adults), with distribution observed mainly at the nematode outer cuticle and stichosome. rTsTryp was identified by T. spiralis-infected mouse sera and anti-rTsTryp sera. Natural TsTryp protease was detected in somatic soluble and ES antigens of the nematode. In mice infected with 200 T. spiralis ML, serum-specific IgG was first detected by rTsTryp-ELISA at 8 days post-infection (dpi), reaching 100% positivity at 12 dpi, and first detected by ES-ELISA at 10 dpi, reaching 100% positivity at 14 dpi. Specific IgG was detected by rTsTryp 2 days earlier than by ES antigens. When specific IgG was determined in serum samples from trichinellosis patients, the sensitivity of rTsTryp-ELISA and ES antigens-ELISA was 98.1% (51/52 samples) and 94.2% (49/52 samples), respectively (P = 0.308), but the specificity of rTsTryp was significantly higher than that of ES antigens (98.7% vs. 95.4%; P = 0.030). Additionally, rTsTryp conferred a lower cross-reaction, with only three serum samples in total testing positive from 11 clonorchiasis, 20 cysticercosis and 24 echinococcosis patients (1 sample from each patient group).

CONCLUSIONS

TsTryp was shown to be an early and highly expressed antigen at intestinal T. spiralis stages, indicating that rTsTryp represents a valuable diagnostic antigen for the serodiagnosis of early Trichinella infection.

The protective immunity induced by Trichinella spiralis galectin against larval challenge and the potential of galactomannan as a novel adjuvant

Previous studies showed that recombinant Trichinella spiralis galectin (rTsgal) promoted larval invasion of gut epithelial cells, while anti-rTsgal antibodies inhibited the invasion. Galactomannan (GM) is a polysaccharide capable of regulating immune response. The aim of this study was to evaluate protective immunity induced by rTsgal immunization and the potential of GM as a novel adjuvant. The results showed that vaccination of mice with rTsgal+ISA201 and rTsgal+GM elicited a Th1/Th2 immune response. Mice immunized with rTsgal+ISA201 and rTsgal+GM exhibited significantly higher levels of serum anti-rTsgal antibodies, mucosal sIgA and cellular immune responses, but level of specific antibodies and cytokines of rTsgal+GM group was lower than the rTsgal+ISA201 group. Immunization of mice with rTsgal+ISA201 and rTsgal+GM showed a 50.5 and 40.16% reduction of intestinal adults, and 52.04 and 37.53% reduction of muscle larvae after challenge. Moreover, the numbers of goblet cells and expression level of mucin 2, Muc5ac and pro-inflammatory cytokines (TNF-α and IL-1β) in gut tissues of vaccinated mice were obviously decreased, while Th2 inducing cytokine (IL-4) expression was evidently increased. Galactomannan enhanced protective immunity, alleviated intestinal and muscle inflammation of infected mice. The results indicated that rTsgal+ISA201 vaccination induced a more prominent gut local as well as systemic immune response and protection compared to rTsgal+GM vaccination. The results suggested that Tsgal could be considered as a candidate vaccine target against Trichinella infection and galactomannan might be a potential novel candidate adjuvant of anti-Trichinella vaccines.

Molecular characterization of a novel serine proteinase from Trichinella spiralis and its participation in larval invasion of gut epithelium

BACKGROUND

A novel serine proteinase of Trichinells spiralis (TsSPc) has been identified in the excretion/secretion (ES) antigens, but its role in larval invasion is unclear. The aim of this study was to clone and express TsSPc, identify its biological and biochemical characteristics, and investigate its role on larval invasion of gut epithelium during T. spiralis infection.

METHODOLOGY/PRINCIPAL FINDINGS

TsSPc has a functional domain of serine proteinase, and its tertiary structure consists of three amino acid residues (His88, Asp139 and Ser229) forming a pocket like functional domain. Recombinant TsSPc (rTsSPc) was expressed and purified. The rTsSPc has good immunogenicity. On Western blot analysis, rTsSPc was recognized by infection serum and anti-rTsSPc serum, natural TsSPc in crude and ES antigens was identified by anti-rTsSPc serum. The results of qPCR, Western blot and indirect immunofluorescence test (IIFT) showed that TsSPc was expressed at diverse stage worms, and mainly localized at cuticle, stichosome and intrauterine embryos of this nematode. The rTsSPc had enzymatic activity of native serine protease, which hydrolyzed the substrate BAEE, casein and collagen I. After site directed mutation of enzymatic active sites of TsSPc, its antigenicity did not change but the enzyme activity was fully lost. rTsSPc specifically bound to intestinal epithelium cells (IECs) and the binding sites were mainly localized in cell membrane and cytoplasm. rTsSPc accelerated larval invasion of IECs, whereas anti-rTsSPc antibodies and TsSPc-specific dsRNA obviously hindered larval invasion.

CONCLUSIONS

TsSPc was a surface and secretory proteinase of the parasite, participated in larval invasion of gut epithelium, and may be considered as a candidate vaccine target molecule against Trichinella intrusion and infection.

Oral non-viral gene delivery platforms for therapeutic applications

Since gene therapy can regulate gene and protein expression directly, it has a great potential to prevent or treat a variety of genetic or acquired diseases through vaccines such as viral infections, cystic fibrosis, and cancer. Owing to their high efficacy, in vivo gene therapy trials are usually conducted intravenously, which is usually costly and invasive. There are several advantages to oral drug administration over intravenous injections, such as better patient compliance, ease of use, and lower cost. However, gene therapy is successful if the oligonucleotides can cross the cell membrane easily and reach the nucleus after the endosomal escape. In order to accomplish this task and deliver the cargo to the intended location, appropriate delivery systems should be introduced. This review summarizes oral delivery systems developed for effective gene delivery, vaccination, and treatment of various diseases. Studies have also shown that oral delivery approaches are potentially applicable to treat various diseases, especially inflammatory bowel disease, stomach, and colorectal cancers. Also, the current review provides an update overview on the development of non-viral and oral gene delivery techniques for gene therapy and vaccination purposes.

In vitro knockdown of TsDNase II-7 suppresses Trichinella spiralis invasion into the host's intestinal epithelial cells

Trichinella spiralis (T. spiralis) adult-specific deoxyribonuclease II-7 (TsDNase II-7), a member of the DNase II-like nuclease family with no DNase II activity, was identified in the excretory-secretory (ES) products of adult worms (AWs). However, its biological functions are still unclear. Our previous study revealed that TsDNase II-7 is located around the infection site in the intestinal tissue, speculating that it was involved in the T. spiralis invasion of host intestinal epithelial cells (IECs). This study aimed to use RNA interference to verify our speculation that TsDNase II-7 in 3-day old adult T. spiralis (Ad3) plays a role in intestinal invasion. TsDNase II-7-specific small interfering RNAs (siRNAs) were delivered into muscle larvae (MLs) to knockdown TsDNase II-7 expression by electroporation. Twenty-four hours later, the MLs transfected with 2 μM siRNA-841 exhibited decreased in TsDNase II-7 transcription and expression as compared to the control MLs. The knockdown of TsDNase II-7 expression did not affect ML viability, and the low expression of TsDNase II-7 still maintained in Ad3 recovered from TsDNase II-7-RNAi-ML infected mice, resulting in a weakened ability of Ad3 to invade intestinal epithelial cells (IECs). These results indicated that knockdown of TsDNase II-7 gene expression via RNA interference (RNAi) suppressed adult worm invasion and confirmed that TsDNase II-7 plays a crucial role during the intestinal phase of T. spiralis infections, which provided new candidate for vaccine development of T. spiralis.

Mannose facilitates Trichinella spiralis expulsion from the gut and alleviates inflammation of intestines and muscles in mice

Trichinellosis is a major zoonotic parasitosis which is a vital risk to meat food safety. It is requisite to exploit new strategy to interdict food animal Trichinella infection and to obliterate Trichinella from food animals to ensure meat safety. Mannose is an oligosaccharide that specifically binds to the carbohydrate-recognition domain of C-type lectin; it has many physiological functions including reliving inflammation and regulating immune reaction. The purpose of this study was to investigate the suppressive role of mannose on T. spiralis larval invasion and infection, its effect on intestinal and muscle inflammation, and immune responses after challenge. The results showed that compared to the saline-treated infected mice, the mannose-treated infected mice had less intestinal adult and muscle worm burdens, mild inflammation of intestine and muscle of infected mice. The levels of specific anti-Trichinella IgG (IgG1/IgG2a), IgA and sIgA in mannose-treated infected mice were obviously inferior to saline-treated infected mice (P < 0.01). Furthermore, the levels of two cytokines (IFN-γ and IL-4) in mannose-treated infected mice were also significantly lower than the saline-treated infected mice (P < 0.01). The protective effect of the mannose against Trichinella infection might be not related to specific antibody and cellular immune responses. The above results demonstrated that mannose could be considered as a novel adjuvant therapeutic agent for anti-Trichinella drugs to block larval invasion at early stage of Trichinella infection.

Oral immunization of mice with recombinant Lactobacillus plantarum expressing a Trichinella spiralis galectin induces an immune protection against larval challenge

BACKGROUND

Trichinella spiralis is an important foodborne parasite that presents a severe threat to food safety. The development of an anti-Trichinella vaccine is an important step towards controlling Trichinella infection in food animals and thus ensure meat safety. Trichinella spiralis galectin (Tsgal) is a novel protein that has been identified on the surface of this nematode. Recombinant Tsgal (rTsgal) was found to participate in larval invasion of intestinal epithelium cells (IECs), whereas anti-rTsgal antibodies impeded the invasion.

METHODS

The rTsgal/pSIP409- pgsA' plasmid was constructed and transferred into Lactobacillus plantarum strain NC8, following which the in vitro biological properties of rTsgal/NC8 were determined. Five groups of mice were orally immunized three times, with a 2-week interval between immunizations, with recombinant NC8-Tsgal, recombinant NC8-Tsgal + α-lactose, empty NC8, α-lactose only or phosphate-buffered saline (PBS), respectively. The vaccinated mice were infected orally with T. spiralis larvae 2 weeks following the last vaccination. Systemic and intestinal local mucosal immune responses and protection were also assessed, as were pathological changes in murine intestine and skeletal muscle.

RESULTS

rTsgal was expressed on the surface of NC8-Tsgal. Oral immunization of mice with rTsgal vaccine induced specific forms of serum immunoglobulin G (IgG), namely IgG1/IgG2a, as well as IgA and gut mucosal secretion IgA (sIgA). The levels of interferon gamma and interleukin-4 secreted by cells of the spleen, mesenteric lymph nodes, Peyer's patches and intestinal lamina propria were significantly elevated at 2-6 weeks after immunization, and continued to rise following challenge. Immunization of mice with the oral rTsgal vaccine produced a significant immune protection against T. spiralis challenge, as demonstrated by a 57.28% reduction in the intestinal adult worm burden and a 53.30% reduction in muscle larval burden, compared to the PBS control group. Immunization with oral rTsgal vaccine also ameliorated intestinal inflammation, as demonstrated by a distinct reduction in the number of gut epithelial goblet cells and mucin 2 expression level in T. spiralis-infected mice. Oral administration of lactose alone also reduced adult worm and larval burdens and relieved partially inflammation of intestine and muscles.

CONCLUSIONS

Immunization with oral rTsgal vaccine triggered an obvious gut local mucosal sIgA response and specific systemic Th1/Th2 immune response, as well as an evident protective immunity against T. spiralis challenge. Oral rTsgal vaccine provided a prospective approach for control of T. spiralis infection.

Oral vaccination of mice with attenuated Salmonella encoding Trichinella spiralis calreticulin and serine protease 1.1 confers protective immunity in BALB/c mice

BACKGROUND

Trichinella spiralis is a foodborne parasitic nematode which is a serious risk to meat safety. Development of anti-Trichinella vaccine is needed to control Trichinella infection in food animals. In this study, two novel T. spiralis genes (calreticulin and serine protease 1.1) in combination were used to construct oral DNA vaccines, and their induced protective immunity was evaluated in a murine model.

METHODOLOGY/PRINCIPAL FINDINGS

TsCRT+TsSP1.1, TsCRT and TsSP1.1 DNA were transformed into attenuated Salmonella typhimurium ΔcyaSL1344. Oral vaccination of mice with TsCRT+TsSP1.1, TsCRT and TsSP1.1 DNA vaccines elicited a gut local mucosal sIgA response and systemic Th1/Th2 mixed response. Oral vaccination with TsCRT+TsSP1.1 induced obviously higher level of serum specific antibodies, mucosal sIgA and cellular immune response than either of single TsCRT or TsSP1.1 DNA vaccination. Oral vaccination of mice with TsCRT+TsSP1.1 exhibited a 53.4% reduction of enteral adult worms and a 46.05% reduction of muscle larvae, conferred a higher immune protection than either of individual TsCRT (44.28 and 42.46%) or TsSP1.1 DNA vaccine (35.43 and 29.29%) alone. Oral vaccination with TsCRT+TsSP1.1, TsCRT and TsSP1.1 also obviously ameliorated inflammation of intestinal mucosa and skeletal muscles of vaccinated mice after challenge.

CONCLUSIONS

TsCRT and TsSP1.1 might be regarded the novel potential targets for anti-Trichinella vaccines. Attenuated Salmonella-delivered DNA vaccine provided a prospective approach to control T. spiralis infection in food animals.

Characterization of a novel pyruvate kinase from Trichinella spiralis and its participation in sugar metabolism, larval molting and development

Background

Pyruvate kinase widely exists in many parasites and plays an important role in the energy production for the parasites. Pyruvate kinase might be a potential drug target for killing the parasites. The aim of the present study was to evaluate the biological characteristics and roles of T. spiralis pyruvate kinase M (TsPKM) in sugar metabolism, larval molting and development of T. spiralis.

Methodology/Principal findings

TsPKM has two functional domains of pyruvate kinase and the tertiary structure of TsPKM is tetramer which has the enzyme active site constituted by 8 amino-acid residues (Arg71, Asn73, Asp110, Phe241, Lys267, Glu269, Asp293 and Thr325). Recombinant TsPKM (rTsPKM) was expressed and purified. The rTsPKM had good immunogenicity. RT-PCR and Western blot showed that TsPKM was transcribed and expressed at various developmental stages in T. spiralis lifecycle. Immunofluorescence test showed that TsPKM was principally located in the cuticle, muscle, stichosome, intestine and the intrauterine embryos of female adults. rTsPKM catalyzed the reaction of phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP) to produce pyruvic acid and adenosine triphosphate (ATP). TsPKM played an important role in the metabolism and energy production of T. spiralis. After silencing of TsPKM gene by specific dsRNA-TsPKM2, protein expression and enzyme activity of TsPKM decreased by 50.91 and 26.06%, respectively. After treatment with RNAi, natural TsPKM enzyme activity, larval molting, sugar metabolism, growth and development of T. spiralis were significantly reduced.

Conclusions

TsPKM participates in the larval molting, sugar metabolism, growth and development of T. spiralis and it might be a candidate target of therapeutic drug of trichinellosis.

Pyruvate kinases belong to transferases and can transfer the high-energy phosphate bond of phosphoenolpyruvate (PEP) to adenosine diphosphate (ADP) to produce pyruvic acid and adenosine triphosphate (ATP). Pyruvate kinases play a significant biological role in the parasite survival in hosts. Our results revealed that TsPKM was expressed at various T. spiralis developmental stages, and principally located in the cuticle, stichosome, intestine and the intrauterine embryos of female adults. rTsPKM catalyzed the reaction of phosphoenolpyruvate (PEP) and adenosine diphosphate (ADP) to produce pyruvic acid and adenosine triphosphate (ATP). TsPKM played an important role in the metabolism and energy production of T. spiralis. Protein expression and enzyme activity of TsPKM were decreased by 50.91 and 26.06% respectively through silencing of TsPKM gene using specific dsRNA-TsPKM2. After treatment with RNAi and inhibitor tannin, natural TsPKM activity, larval molting, sugar metabolism, growth and development of T. spiralis were obviously inhibited. Our results showed that TsPKM participates in T. spiralis molting, sugar metabolism and development, and it might be a candidate target for anti-Trichinella drugs.

A novel C-type lectin from Trichinella spiralis mediates larval invasion of host intestinal epithelial cells

The aim of this study was to investigate the characteristics of a novel type C lectin from Trichinella spiralis (TsCTL) and its role in larval invasion of intestinal epithelial cells (IECs). TsCTL has a carbohydrate recognition domain (CRD) of C-type lectin. The full-length TsCTL cDNA sequence was cloned and expressed in Escherichia coli BL21. The results of qPCR, Western blotting and immunofluorescence assays (IFAs) showed that TsCTL was a surface and secretory protein that was highly expressed at the T. spiralis intestinal infective larva (IIL) stages and primarily located at the cuticle, stichosome and embryos of the parasite. rTsCTL could specifically bind with IECs, and the binding site was localized in the IEC nucleus and cytoplasm. The IFA results showed that natural TsCTL was secreted and bound to the enteral epithelium at the intestinal stage of T. spiralis infection. The rTsCTL had a haemagglutinating effect on murine erythrocytes, while mannose was able to inhibit the rTsCTL agglutinating effect for mouse erythrocytes. rTsCTL accelerated larval intrusion into the IECs, whereas anti-rTsCTL antibodies and mannose significantly impeded larval intrusion in a dose-dependent manner. The results indicated that TsCTL specifically binds to IECs and promotes larval invasion of intestinal epithelium, and it might be a potential target of vaccines against T. spiralis enteral stages.

Characterization of a novel aminopeptidase P from Trichinella spiralis and its participation in the intrusion of intestinal epithelial cells

Aminopeptidases P are metalloproteases belonging to the M24 peptidase family. It specifically hydrolyzes the N-terminus of polypeptides free of acidic amino acids, and plays an important role in the nutrition, metabolism and growth of parasites. The aim of this study was to characterize a novel Trichinella spiralis aminopeptidase P (TsAPP) and to investigate its functions in the invasion of T. spiralis. TsAPP contained two domains of creatinase (a creatinase N and creatinase N2) and a domain of peptidase M24C and APP. The complete TsAPP sequence was cloned and expressed in Escherichia coli BL21 cells. The recombinantly produced TsAPP was used to raise polyclonal antibodies that were subsequently used to detect the expression of the protein in the different life stages of T. spiralis. TsAPP was expressed in various T. spiralis stages. TsAPP was primarily localized in the cuticle, stichosome and intrauterine embryos of this nematode. rTsAPP has an enzymatic activity of a natural aminopeptidase P to hydrolyze the substrate H-Ala-Pro-OH. rTsAPP promoted the larval intrusion of intestinal epithelium cells (IECs). The results showed that TsAPP is involved in the T. spiralis intrusion of IECs and it might be a potential candidate vaccine target against Trichinella infection.

Oral immunization with attenuated Salmonella encoding an elastase elicits protective immunity against Trichinella spiralis infection

Elastase belongs to the serine protease family. Previous studies showed that Trichinella spiralis elastase (TsE) was highly expressed in intestinal infective larvae (IIL). Recombinant TsE (rTsE) promoted the larval intrusion of enteral epithelium cells (IECs), whereas anti-rTsE antibodies and siRNA impeded larval intrusion. Subcutaneous vaccination of mice with rTsE showed a partial protective immunity, suggesting that TsE might be a promising vaccine target against Trichinella infection. In this study, complete TsE cDNA sequence was cloned into pcDNA3.1, and the rTsE DNA was transformed into attenuated S. typhimurium strain ΔcyaSL1344. Oral vaccination of mice with TsE DNA elicited a systemic Th1/Th2/Treg mixed immune response and gut local mucosal sIgA response. Immunized mice exhibited a significant immune protection against T. spiralis larval challenge, as demonstrated by a 52.48% reduction of enteral adult worms and a 69.43% reduction of muscle larvae. The protection might be related to the TsE-induced production of intestinal mucus, specific anti-TsE sIgA and IgG, and secretion of IFN-γ, IL-2, IL-4 and IL-10, which protected gut mucosa from larval intrusion, suppressed worm development and impeded female reproduction. The results demonstrated that attenuated Salmonella-delivered TsE DNA vaccine provided a prospective strategy for the control of Trichinella infection in food animals.

Vaccination of mice with recombinant novel aminopeptidase P and cathepsin X alone or in combination induces protective immunity against Trichinella spiralis infection

Trichinella spiralis is a major foodborne zoonotic parasitic nematode which has a serious threat to meat food safety. Development of anti-Trichinella vaccine is requisite for control and elimination of Trichinella infection in food animals to ensure meat safety. Aminopeptidase P (TsAPP) and cathepsin X (TsCX) are two novel proteins identified in T. spiralis intestinal infectious L1 larvae (IIL1). The objective of this study was to investigate the protective immunity elicited by immunization with TsAPP and TsCX alone and TsAPP-TsCX in combination in a mouse model. The results demonstrate that subcutaneous vaccination of mice with rTsAPP, rTsCX or rTsAPP + rTsCX elicited a systemic humoral response (high levels of serum IgG, IgG1/IgG2a and IgA) and significant local gut mucosal sIgA responses. The vaccination with rTsAPP, rTsCX or rTsAPP + rTsCX also induced a systemic and local mixed Th1/Th2 response, as demonstrated by clear elevation levels of IFN-γ and IL-4 in vaccinated mice. Vaccination of mice with rTsAPP+rTsCX exhibited a 63.99 % reduction of intestinal adult worms and 68.50% reduction of muscle larva burdens, alleviated inflammation of intestinal mucosal and muscle tissues, and provided a higher immune protection than that of vaccination with rTsAPP or rTsCX alone. The results demonstrated that TsAPP and TsCX might be considered novel candidate target molecules for anti-Trichinella vaccines.

Molecular characterization of a novel cathepsin L from Trichinella spiralis and its participation in invasion, development and reproduction

Cathepsin L is one member of cysteine protease superfamily and widely distributed in parasitic organisms, it plays the important roles in worm invasion, migration, nutrient intake, molting and immune evasion. The objective of this study was to investigate the biological characteristics of a novel cathepsin L from Trichinella spiralis (TsCL) and its role in larval invasion, development and reproduction. TsCL has a functional domain of C1 peptidase, which belongs to cathepsin L family. The complete TsCL sequence was cloned and expressed in Escherichia coli BL21. The rTsCL has good immunogenicity. RT-PCR and Western blotting analysis showed that TsCL was transcribed and expressed at different T. spiralis phases (e.g., muscle larvae, intestinal infectious larvae, adult worms and newborn larvae). Immunofluorescence test revealed that TsCL was principally localized in the cuticle, stichosome, midgut and female intrauterine embryos of the nematode. rTsCL has the capacity to specially bind with intestinal epithelial cells (IECs) and the binding sites was located in the cytoplasm. rTsCL promoted larval penetration into IEC, while anti-rTsCL antibodies inhibited the invasion. The silencing of TsCL gene by specific dsRNA significantly reduced the TsCL expression and enzyme activity, and also reduced larval invasive ability, development and female reproduction. The results showed that TsCL is an obligatory protease in T. spiralis lifecycle. TsCL participates in worm invasion, development and reproduction, and may be regarded as a potential candidate vaccine/drug target against T. spiralis infection.

Oral vaccination with recombinant Lactobacillus plantarum encoding Trichinella spiralis inorganic pyrophosphatase elicited a protective immunity in BALB/c mice

Background

Trichinellosis is a serious zoonotic disease distributed around the world. It is needed to develop a safe, effective and feasible anti-Trichinella vaccine for prevention and control of trichinellosis. The aim of this study was to construct a recombinant Lactobacillus plantarum encoding Trichinella spiralis inorganic pyrophosphatase (TsPPase) and investigate its immune protective effects against T. spiralis infection.

Methodology/Principal findings

The growth of recombinant L. plantarum was not affected by TsPPase/pSIP409-pgsA′ plasmid, and the recombinant plasmid was inherited stably in bacteria. Western blot and immunofluorescence assay (IFA) indicated that the rTsPPase was expressed on the surface of recombinant L. plantarum. Oral vaccination with rTsPPase induced higher levels of specific serum IgG, IgG1, IgG2a and mucosal secretory IgA (sIgA) in BALB/c mice. ELISA analysis revealed that the levels of IFN-γ and IL-4 released from spleen, mesenteric lymph nodes and Peyer’s patches were evidently increased at 2–4 weeks following vaccination, compared to MRS (De Man, Rogosa, Sharpe) medium control group (P < 0.05). Immunization of mice with rTsPPase exhibited a 67.18, 54.78 and 51.91% reduction of intestinal infective larvae, adult worms and muscle larvae at 24 hours post infection (hpi), 6 days post infection (dpi) and 35 dpi, respectively (P < 0.05), and the larval molting and development was significantly inhibited by 45.45% at 24 hpi, compared to the MRS group.

Conclusions

TsPPase plays a crucial role in T. spiralis molting and development, oral vaccination with rTsPPase induced a significant local mucosal sIgA response and systemic Th1/Th2 immune response, and immune protection against T. spiralis infection in BALB/c mice.

In the previous study, a Trichinella spiralis inorganic pyrophosphatase (TsPPase) was expressed and its role in larval molting and development was observed. In this study, a recombinant TsPPase/pSIP409-pgsA′ plasmid was constructed and transferred into Lactobacillus plantarum NC8, the rTsPPase was expressed on the surface of recombinant L. plantarum NC8. Oral immunization of mice with rTsPPase DNA vaccine elicited a high level of specific serum IgG, IgG1, IgG2a and mucosal secretory IgA (sIgA). The levels of IFN-γ and IL-4 released from spleen, mesenteric lymph nodes and Peyer’s patches were evidently increased at 2–4 weeks following vaccination. Immunization of mice with rTsPPase showed a significant reduction of intestinal infective larvae, adult worms and muscle larvae, and intestinal larval molting and development was significantly suppressed. The results indicated that oral vaccination with rTsPPase elicited a significant local mucosal sIgA response and specific systemic Th1/Th2 immune response, and an obvious protective immunity against T. spiralis infection.

Characterization of a Novel Glutamine Synthetase From Trichinella spiralis and Its Participation in Larval Acid Resistance, Molting, and Development

Trichinella spiralis is a major foodborne parasite worldwide. After the encapsulated muscle larvae (ML) in meat are ingested, the ML are liberated in the stomach of the host and activated into intestinal infectious larvae (IIL), which develop into adult worm after molting four times. A novel glutamine synthetase (TsGS) was identified from T. spiralis IIL at 10 h post-infection, but its biological role in T. spiralis life cycle is not clear. The aim of this study was to investigate the biological characteristics of TsGS and its functions in larval acid resistance, molting, and development. TsGS has a glutamine synthetase (GS) catalytic domain. Complete TsGS sequence was cloned and expressed in Escherichia coli BL21. rTsGS has good immunogenicity. qPCR and Western blotting showed that TsGS was highly expressed at IIL stage, and immunofluorescence revealed that TsGS was principally localized at the cuticle and intrauterine embryos of this nematode. rTsGS has enzymatic activity of natural GS to hydrolyze the substrate (Glu, ATP, and NH₄⁺). Silencing of TsGS gene significantly reduced the IIL survival at pH 2.5, decreased the IIL burden, and impeded larval molting and development. The results demonstrated that TsGS participates in T. spiralis larval acid resistance, molting and development, and it might be a candidate vaccine target against Trichinella molting and development.

Oral vaccination with attenuated Salmonella encoding the Trichinella spiralis 43-kDa protein elicits protective immunity in BALB/c mice

A vaccine against Trichinella spiralis infection is urgently needed to interrupt its transmission from domestic animals to humans. However, no vaccine against T. spiralis is currently available. Our previous study demonstrated that the use of the 43-kDa glycoprotein present in excretory-secretory (ES) proteins of muscle larvae (ML) as an intramuscular DNA vaccine led to a 52.1% protection rate against T. spiralis infection. Attenuated Salmonella strains have the advantage of eliciting mucosal immunity, which is important for controlling T. spiralis infections at the intestinal stage and can be provided as vaccines via oral or intranasal routes. Therefore, in this study, complete 43-kDa glycoprotein (Ts43) sequences of T. spiralis were cloned into the vector pYA3681, and the recombinant plasmid pYA3681-Ts43 was transformed into the attenuated Salmonella typhimurium strain χ11802. The results showed that oral vaccination of mice with attenuated Salmonella carrying the recombinant plasmid pYA3681-Ts43 induced an evident elevation of the local intestinal mucosal sIgA and serum IgG antibody responses. The flow cytometry results showed that the percentages of CD4⁺ T cells and secreted IFN-γ, IL-4, and IL-17A in CD4⁺ T cells were significantly increased in the spleen and mesenteric lymph node (MLN) lymphocytes of the vaccinated groups. In addition, increased levels of the IFN-γ, IL-4, and IL-17A cytokines were also observed in the serum of the immunized groups. The above immune response results in the immunized groups demonstrated that protective immunity was elicited in this study. Finally, vaccinated mice demonstrated a significant 45.9% reduction in ML burden after infection with T. spiralis. This study demonstrated that oral vaccination with Ts43 delivered by attenuated Salmonella elicited local and systemic concurrent Th1/Th2/Th17 immune responses and provided partial protection against T. spiralis infection in BALB/c mice. This is a prospective strategy for the prevention and control of trichinellosis.

Oral vaccination with invasive Lactobacillus plantarum delivered nucleic acid vaccine co-expressing SS1 and murine interleukin-4 elicits protective immunity against Trichinella spiralis in BALB/c mice

Trichinellosis is a foodborne zoonosis caused by Trichinella spiralis (T. spiralis) that not only causes considerable economic losses for the global pig breeding and food industries, but also seriously threats the health of human. Therefore, it is very necessary to develop an effective vaccine to prevent trichinellosis. In this study, the invasive Lactobacillus plantarum (L. plantarum) expressing fibronectin-binding protein A (FnBPA) was served as a live bacterial vector to deliver DNA to the host to produce a novel oral DNA vaccine. Co-expressing T. spiralis SS1 and murine interleukin-4 (mIL-4) of DNA vaccine were constructed and subsequently delivered to intestinal epithelial cells via invasive L. plantarum. At 10 days after the third immunization, the experimental mice were challenged with 350 T. spiralis infective larvae. The results found that the mice orally vaccinated with invasive L. plantarum harboring pValac-SS1/pSIP409-FnBPA not only stimulated the production of anti-SS1-specific IgG, Th1/Th2 cell cytokines, and secreted(s) IgA but also decreased worm burden and intestinal damage. However, the mice inoculated with invasive L. plantarum co-expressing SS1 and mIL-4 (pValac-SS1-IL-4/pSIP409-FnBPA) induced the highest protective immune response against T. spiralis infection. The DNA vaccine delivered by invasive L. plantarum provides a novel idea for the prevention of T. spiralis infection.

Functional characterization of a glutathione S-transferase in Trichinella spiralis invasion, development and reproduction

The purpose of this study was to determine the biological function of a Trichinella spiralis glutathione S-transferase (TsGST) in larval invasion and development by RNA interference (RNAi). The TsGST-specific siRNA 366 was transfected into T. spiralis muscle larvae (ML) via electroporation. At 1 day following transfection, the larval TsGST mRNA and protein expressions were reduced by 40.09 and 65.22 % (P < 0.05), respectively. The enzymatic activity of natural TsGST in siRNA-transfected ML was also suppressed by 45% compared with PBS group (P < 0.05). Silencing of the TsGST significantly inhibited the ability of larvae to invade intestinal epithelium cells (IECs) and isolated intestine. After challenge with siRNA-366-treated ML, the infected mice exhibited a 62.82% reduction of intestinal adult worms, and 65.03 % reduction of muscle larvae compared to the PBS group. Besides, the length of adults, newborn larvae and muscle larvae was significantly shorter than that of control siRNA and PBS group; the female fecundity of siRNA 366 group was lower than those of control siRNA and PBS group (P <  0.05). The results revealed that the specific RNAi significantly reduced the expression and enzymatic activity of TsGST, inhibited the larval invasive and developmental capacity, and impaired the female fecundity. The results further confirmed that TsGST plays a crucial role in the T. spiralis life cycle and it might be a potential molecular target for anti-Trichinella vaccines.

Vaccination with a DNase II recombinant protein against Trichinella spiralis infection in pigs

Trichinellosis caused by Trichinella spiralis (T. spiralis) is an important public health problem. DNase II is an acidic endonuclease that catalyzes the degradation of DNA into oligonucleotides. DNase II-7 has been detected at the adult stage of T. spiralis and has been examined in excretory/secretory products. Previous studies have indicated that the DNase II-7 recombinant protein has a high rate of protection against T. spiralis infection in mice. In the present study, the protective effect of DNase II-7 recombinant protein against T. spiralis infection in Large White pigs was further explored. The humoral and cellular immune responses to the DNase II-7 recombinant protein were evaluated, including the dynamic trends of specific IgG, IgG1, IgG2a and IgM antibodies levels, as well as the levels of Th1 (IFN-γ and IL-2) and Th2 (IL-10 and IL-4) cytokines in serum. Our results showed that a Th1 dominated Th1/Th2 mixed immune response was induced by the DNase II-7 recombinant protein for all the time or a short period after vaccination. And the DNase II-7 recombinant protein induced partial protection against T. spiralis infection in pigs, compared to the control group. Our results showed that the DNase II-7 recombinant protein group displayed a 45.7 % reduction in the muscle larvae burden five weeks after being challenged. This study suggested that DNaseII-7 recombinant protein could be used as a potential candidate vaccine against T. spiralis infection in pigs.

Characterization of a novel cysteine protease in Trichinella spiralis and its role in larval intrusion, development and fecundity

The aim of this study was to investigate the biological properties of a novel gut-specific cysteine protease in Trichinella spiralis (TsGSCP) and its role in larval intrusion, development and fecundity. TsGSCP has a functional C1 peptidase domain; C1 peptidase belongs to cathepsin B family. The TsGSCP gene cloned and expressed in Escherichia coli BL21 showed intensive immunogenicity. qPCR and Western blotting revealed that TsGSCP mRNA and protein were expressed at various T. spiralis stages, but their expression levels in intestinal infectious larvae (IIL) were clearly higher than those in muscle larvae (ML), adult worms (AWs) and new-born larvae (NBL). Indirect immunofluorescence (IIF) analysis showed that TsGSCP was primarily located at the outer cuticle and the intrauterine embryos of this parasite. rTsGSCP showed the ability to specifically bind with IECs, and the binding site is within the IEC cytoplasm. rTsGSCP accelerated larval intrusion into host intestinal epithelial cells (IECs), whereas anti-rTsGSCP antibodies suppressed larval intrusion; the acceleration and suppression was induced by rTsGSCP and anti-rTsGSCP antibodies, respectively, in a dose-dependent manner. When ML were transfected with TsGSCP-specific dsRNA, TsGSCP expression and enzymatic activity were reduced by 46.82 and 37.39%, respectively, and the capacity of the larvae to intrude into IECs was also obviously impeded. Intestinal AW burden and adult female length and fecundity were significantly decreased in the group of mice infected with dsRNA-transfected ML compared to the control dsRNA and PBS groups. The results showed that TsGSCP plays a principal role in gut intrusion, worm development and fecundity in the T. spiralis lifecycle and might be a candidate target for vaccine development against Trichinella intrusion and infection.

Immunoprotective effects of invasive Lactobacillus plantarum delivered nucleic acid vaccine coexpressing Trichinella spiralis CPF1 and murine interleukin-4

Trichinellosis is a very important food-borne parasitic disease, that seriously endangers animal husbandry and food safety. Therefore, it is necessary to develop a safe and effective vaccine against Trichinella spiralis infection. In this experiment, invasive Lactobacillus plantarum carrying the FnBPA gene served as a live bacterial vector to deliver nucleic acids to the host to produce a novel oral nucleic acid vaccine. Coexpression of the T. spiralis cathepsin F-like protease 1 gene (TsCPF1) and murine IL-4 (mIL-4) by the nucleic acid vaccine was constructed and subsequently delivered to intestinal epithelial cells via invasive L. plantarum. Thirty-seven days after the first immunization, the experimental mice were challenged with 350 T. spiralis infective larvae by oral gavage. The results showed that mice orally immune-stimulated with invasive L. plantarum pValac-TsCPF1/pSIP409-FnBPA not only produce anti-TsCPF1-specific IgG antibodies, sIgA, Th1/Th2 cytokine distinctly increased but also intestinal damage and worm burden relieved compare to non-invasive TsCPF1 group (pValac-TsCPF1/pSIP409). Most notably, experimental mice immunized with invasive L. plantarum coexpressing TsCPF1 and mIL-4 (pValac-TsCPF1-IL-4/pSIP409-FnBPA) exhibited the highest protection efficiency against T. spiralis infection. The above results reveal that invasive L. plantarum-expressing the FnBPA protein improved mucosal and cellular immunity and enhanced resistance to T. spiralis. The nucleic acid vaccine delivered by invasive L. plantarum described in this study offers a novel idea for the prevention of T. spiralis.

Oral immunization with recombinant Lactobacillus plantarum expressing Nudix hydrolase and 43 kDa proteins confers protection against Trichinella spiralis in BALB/c mice

Trichinellosis is a significant food-borne zoonotic parasitic disease caused by parasite Trichinella. Given the side effects of anti-Trichinella drugs (e.g., Mebendazole) aroused in the course of treatments, an effective vaccine against the parasite is called for. The therapies available to date are in most instances targeting a single stage of Trichinella, resulting in an incomplete protective immunity against the parasite in terms of the complexity of its developmental stages. In this study, a recombinant dual-expression double anchor vector NC8-pLp-TsNd-S-pgsA'-gp43 was constructed carrying two antigen genes from Trichinella spiralis (T. spiralis), encoding the gp43 and T. spiralis Nudix hydrolase (TsNd) proteins which were mainly expressed in muscle larva (ML) and intestinal infective larva stages of the parasite respectively. These two proteins were to be expressed by Lactobacillus plantarum NC8 (L. plantarum NC8) which was designed to express the two anchored peptides, a truncated poly-γ-glutamic acid synthetase A (pgsA') and the surface layer protein of Lactobacillus acidophilus (SlpA), on its surface for attaching expressed foreign proteins. Oral immunization with the above recombinant vaccine induced higher levels of specific serum IgG and mucosal secretory IgA (SIgA) in BALB/c mice. In addition, cytokines, interferon-γ (IFN- γ), interleukin-4 (IL-4) and IL-17 released by lymphocytes, and CD4+ levels displayed on the surfaces of splenic and mesenteric lymph cells were significantly enhanced by the vaccination. Moreover, after larval challenges, a 75.67 % reduction of adult worms (AW) at 7 days post-infection (dpi) and 57.14 % reduction of ML at 42 dpi were observed in mice immunized with the recombinant vaccine. Furthermore, this oral vaccination reduced the counts of encysted larvae presented in tongue and masseter muscles after infected with T. spiralis in mice. The overall results demonstrated that the recombinant vaccine developed in this study could induce specific humoral, mucosal, and cellular immune responses, and provides protections against different stages (adult worms and muscle larva) of T. spiralis infections in BALB/c mice, which could make it a promising oral vaccine candidate against trichinellosis.

Biological properties and roles of a Trichinella spiralis inorganic pyrophosphatase in molting and developmental process of intestinal larval stages

Inorganic pyrophosphatase (PPase) participates in energy cycle and plays a vital role in hydrolysis of inorganic pyrophosphate (PPi) into inorganic phosphate (Pi). The aim of this study was to investigate the biological properties of a Trichinella spiralis PPase (TsPPase) and its role in larval molting and developmental process. The predicted TsPPase consisted of 367 amino acids with a molecular mass of 41.48 kDa and a pI of 5.76. Amino acid sequence alignment and phylogenetic analysis showed that the TsPPase gene encodes a functional family I soluble PPase with the same characteristics as prokaryotic, plant and animal/fungal soluble PPase. The rTsPPase was expressed and purified, it has the activity to catalyze the hydrolysis of PPi to Pi, and the activity was dependent on Mg²⁺, pH and temperature. The enzymatic activity of rTsPPase was significantly inhibited after its metal binding sites mutation. TsPPase was transcribed and expressed in all T. spiralis phases, especially in muscle larvae (ML) and intestinal infective larvae (IIL). Immunofluorescence assay (IFA) revealed that TsPPase was mainly located in cuticle and stichosome. When the ML and IIL were treated with TsPPase-specific siRNA-279, TsPPase expression and enzymatic activity were obviously reduced, the larval molting and development were also impeded. Intestinal IIL as well as AW burden, IIL molting rates from mice infected with siRNA-treated ML were obviously suppressed. The results indicated that rTsPPase possesses the enzymatic activity of native inorganic pyrophosphatase, and TsPPase plays an important role in development and molting process of intestinal T. spiralis larval stages.

Molecular characterization of a novel aspartyl protease-1 from Trichinella spiralis

The aim of this study was to characterize the biological properties of a novel aspartic protease-1 from Trichinella spiralis (TsASP1) and evaluate its potential in inducing immune response. TsASP1 gene was cloned and expressed in Escherichia coli BL21 (DE3). On Western blotting analysis with anti-rTsASP1 serum, native TsASP1 was detected in various T. spiralis phases other than newborn larvae (NBL). qPCR results showed that TsASP1 transcription was the highest in intestinal infective larvae (IIL) and the lowest in the NBL stage. Immunofluorescence test result shows that native TsASP1 was principally localized in stichosome, muscle cells of muscle larvae (ML) and IIL, and surrounded intrauterine embryos in female adult worms (AW). After silencing TsASP1 gene of the ML by siRNA, the worm development was significantly inhibited, showed by shorter AW and more wrinkles and longitudinal crack on epicuticle of AW on scanning electron microscopy; the AW and ML burdens were reduced by 41.82 and 56.36% respectively, compared with the control siRNA or PBS group (P < 0.001). Immunization of mice with rTsASP1 elicited an evident antibody response (serum IgG, IgG1/IgG2a and enteral sIgA), and systemic (spleen) and intestinal local mucosal (mesenteric lymph node) cellular immune response, demonstrated by a prominent elevation of IFN-γ and IL-4. The results suggested TsASP1 participated in T. spiralis development and survival in host, and immunization of mice with rTsASP1 induced systemic/intestinal local mucosal humoral and cellular immune response against Trichinella.

Immunoproteomic analysis of Trichinella spiralis and Trichinella britovi excretory-secretory muscle larvae proteins recognized by sera from humans infected with Trichinella

The present study compares the immunogenic patterns of muscle larvae excretory-secretory proteins (ML E-S) from T. spiralis and T. britovi recognized by Trichinella-infected human sera. Samples were analyzed using two-dimensional electrophoresis (2-DE) coupled with 2D-immunoblot and liquid chromatography-tandem mass spectrometry LC-MS/MS analysis, two ELISA procedures and a confirmatory 1D-immunoblot test. Sera were obtained from nine patients with a history of ingestion of raw or undercooked meat who presented typical clinical manifestations of trichinellosis and from eleven healthy people. Specific anti-Trichinella IgG antibodies were detected in all samples tested with the Home-ELISA kits, but in only four samples for the commercially-available kit. The 1D-immunoblot results indicated that all nine serum samples were positive for T. spiralis ML E-S antigens, expressed as the presence of specific bands. In contrast, eight of the serum samples with T. britovi E-S ML antigens were positive, with one serum sample taken from a patient at 33dpi (days post infection) being negative. To identify immunoreactive proteins that are specifically recognized by host antibodies, both species of ML E-S proteins were subjected to 2D-immunoblotting with human serum taken at 49 dpi. The sera recognized 22 protein spots for T. spiralis and 18 for T. britovi in 2D-immunoblot analysis. Their molecular weights (MW) ranged from 50 to 60 kDa. LC-MS/MS analysis identified both common and specifically-recognized immunoreactive proteins: transmembrane serine protease 9, serine protease, antigen targeted by protective antibodies and Actin-1 partial were shared for both Trichinella species; hypothetical protein T01_7775 and P49 antigen, partial those specific to T. spiralis; deoxyribonuclease-2-alpha and hypothetical protein T03_17187/T12_7360 were specific to T. britovi. Our results demonstrate the value of 2-DE and 2D-immunblot as versatile tools for pinpointing factors contributing to the parasite-host relationship by comparing the secretomes of different Trichinella species.

Vaccination with novel low-molecular weight proteins secreted from Trichinella spiralis inhibits establishment of infection

Trichinella spiralis muscle stage larvae (mL1) produce excretory-secreted products (ESPs), a complex mixture of protein, which are believed to be important for establishing or maintaining an infection niche within skeletal muscle and the intestine. Studies of both whole ESPs and individual cloned proteins have shown that some ESPs are potent immunogens capable of eliciting protective immune responses. Here we describe two novel proteins, Secreted from Muscle stage Larvae SML-4 and SML-5 which are 15 kDa and 12 kDa respectively. The genes encoding these proteins are highly conserved within the Trichinellids, are constituents of mL1 ESP and localized in the parasite stichosome. While SML-5 is only expressed in mL1 and early stages of adult nematode development, SML-4 is a tyvosylated glycoprotein also produced by adult nematodes, indicating it may have a function in the enteral phase of the infection. Vaccination with these proteins resulted in an impaired establishment of adult stages and consequently a reduction in the burden of mL1 in BALB/c mice. This suggests that both proteins may be important for establishment of parasite infection of the intestine and are prophylactic vaccine candidates.

Molecular characterization of a Trichinella spiralis serine proteinase

The aim of this study was to investigate the biological characteristics and functions of a Trichinella spiralis serine proteinase (TsSerp) during larval invasion and development in the host. The full-length TsSerp cDNA sequence was cloned and expressed in Escherichia coli BL21. The results of RT-PCR, IFA and western blotting analyses showed that TsSerp was a secretory protein that was highly expressed at the T. spiralis intestinal infective larva and muscle larva stages and primarily located at the cuticle, stichosome and intrauterine embryos of the parasite. rTsSerp promoted the larval invasion of intestinal epithelial cells (IECs) and the enteric mucosa, whereas an anti-rTsSerp antibody impeded larval invasion; the promotion and obstruction roles were dose-dependently related to rTsSerp and the anti-rTsSerp antibodies, respectively. Vaccination of mice with rTsSerp elicited a remarkable humoral immune response (high levels of serum IgG, IgG1/IgG2a, IgE and IgM), and it also triggered both systemic (spleen) and local intestinal mucosal mesenteric lymph node (MLN) cellular immune responses, as demonstrated by a significant elevation in Th1 cytokines (IFN-γ) and Th2 cytokines (IL-4) after the spleen and MLN cells from vaccinated mice were stimulated with rTsSerp. Anti-TsSerp antibodies participated in the killing and destruction of newborn larvae via ADCC. The mice vaccinated with rTsSerp exhibited a 48.7% reduction in intestinal adult worms and a 52.5% reduction in muscle larvae. These results indicated that TsSerp participates in T. spiralis invasion and development in the host and might be considered a potential candidate target antigen to develop oral polyvalent preventive vaccines against Trichinella infection.

Molecular cloning and characterization of a novel peptidase from Trichinella spiralis and protective immunity elicited by the peptidase in BALB/c mice

In our previous studies, a novel T. spiralis peptidase (TsP) was identified among the excretory/secretory (ES) proteins of T. spiralis intestinal infective larvae (IIL) and T. spiralis at the adult worm (AW) stage using immunoproteomics, but the biological function of TsP in the life cycle of T. spiralis is not clear. The objective of this study was to investigate the biological properties and functions of TsP in larval intrusion and protective immunity induced by immunization with rTsP. The complete TsP cDNA sequence was cloned and expressed. The results of RT-PCR, indirect immunofluorescence assay (IIFA) and western blotting revealed that TsP is a surface and secretory protein expressed in T. spiralis at different stages (muscle larvae, IIL, AWs and newborn larvae) that is principally localized at the epicuticle of the nematode. rTsP facilitated the larval intrusion of intestinal epithelial cells (IECs) and intestinal mucosa, whereas anti-rTsP antibodies suppressed larval intrusion; these facilitative and suppressive roles were dose-dependently related to rTsP or anti-rTsP antibodies. Immunization of mice with rTsP triggered an obvious humoral immune response (high levels of IgG, IgG1/IgG2a, and sIgA) and also elicited systemic (spleen) and intestinal local mucosal (mesenteric lymph node) cellular immune responses, as demonstrated by an evident increase in the cytokines IFN-γ and IL-4. Immunization of mice with rTsP reduced the numbers of intestinal adult worms by 38.6% and muscle larvae by 41.93%. These results demonstrate that TsP plays a vital role in the intrusion, development and survival of T. spiralis in hosts and is a promising candidate target molecule for anti-Trichinella vaccines.

Vaccination of Mice with a Novel Trypsin from Trichinella spiralis Elicits the Immune Protection against Larval Challenge

Trichinella spiralis is a major foodborne parasite and has a serious threat to meat safety. Development of anti-Trichinella vaccines is prospective to eliminate Trichinella infection in food animal. The aim of this study was to assess the biological properties of a novel T. spiralis trypsin (TsT) and its elicited immune protection against larval challenge. The cDNA sequence of TsT gene was cloned and expressed. Western blotting showed rTsT was identified by infection serum and anti-TsT serum. RT-PCR results revealed that TsT gene was transcribed at diverse T. spiralis lifecycle stages. The IIFT results showed that natural TsT was principally expressed at epicuticle of 5-6 day adult worms, indicating that TsT is a worm somatic antigen and adult-stage specific surface antigen. Vaccination of mice with rTsT triggered an evident humoral immune response (high levels of serum IgG, IgG1/IgG2a, and enteral sIgA), and it also induced the systemic and enteral local cellular immune response, demonstrated by an significantly elevation of cytokines IFN-γ and IL-4. The mice vaccinated with rTsT exhibited a 33.17% reduction of enteral adult worms and a 37.80% reduction of muscle larvae after larval challenge. The results showed that TsT might be considered as a candidate target antigen for anti-T. spiralis vaccines.

Characterization of a Trichinella spiralis aminopeptidase and its participation in invasion, development and fecundity

A Trichinella spiralis aminopeptidase (TsAP) has been identified in intestinal infectious larvae (IIL) and adult worms (AW), but its biological function in the T. spiralis life cycle is unknown. The aim of this study was to characterize TsAP and ascertain its functions in the invasion, development and fecundity of T. spiralis. Recombinant TsAP (rTsAP) was expressed and purified. rTsAP has strong immunogenicity. qPCR and western blotting show that TsAP was transcribed and expressed at all T. spiralis lifecycle stages, but the expression level of TsAP mRNA and proteins at IIL and AW stages was obviously higher than those in muscle larvae (ML) and newborn larvae (NBL). The IFT results reveal that TsAP was principally located at the cuticle and the intrauterine embryos of this nematode. rTsAP had the enzymatic activity of natural aminopeptidase to hydrolyze the substrate Leu-pNA with an optimal temperature of 50 °C and optimal pH of 8.0. rTsAP promoted the larval penetration into intestinal epithelial cells, whereas anti-rTsAP antibodies suppressed the larval intrusion; the promotion and suppression was dose-dependently related to rTsAP or anti-rTsAP antibodies. TsAP protein expression level and enzymatic activity were reduced by 50.90 and 49.72% through silencing of the TsAP gene by specific siRNA 842. Intestinal AW and muscle larval burdens, worm length and female reproductive capacity were significantly declined in mice infected with siRNA-transfected ML compared to the control siRNA and PBS group. These results indicate that TsAP participates in the invasion, development and fecundity of T. spiralis and it might be a candidate target for anti-Trichinella vaccines.

Protective immunity in mice vaccinated with a novel elastase-1 significantly decreases Trichinella spiralis fecundity and infection

Trichinella spiralis is an important foodborne parasitic nematode that represents an enormous threat to the food safety of pork meat. The development of a preventive vaccine is valuable for the prevention and control of Trichinella infection in domestic pigs to ensure pork safety. Elastase is a trypsin-like serine protease that hydrolyzes the host's diverse tissue components and participates in parasite penetration, and it might be a novel vaccine target molecule. The aim of this study was to assess the protective immunity produced by vaccination with a novel Trichinella spiralis elastase-1 (TsE) in a mouse model. The results demonstrate that subcutaneous vaccination of mice with rTsE elicited a systemic humoral response (high levels of serum IgG and subclass IgG1/IgG2a and IgA) and significant local enteral mucosal sIgA responses. Anti-rTsE IgG recognized the native TsE at the cuticle, stichosome of intestinal infective larvae and adult worm (AW), and intrauterine embryos of female AW. The rTsE vaccination also produced a systemic and local mixed Th1/Th2 response, as demonstrated by clear elevation levels of Th1 cytokines (IFN-γ, IL-2) and Th2 cytokines (IL-4, IL-10) after spleen, mesenteric lymph node and Peyer's patch cells from immunized mice were stimulated with rTsE. The immunized mice exhibited a 52.19% reduction in enteral AW and a 64.06% reduction in muscle larvae after challenge infection. The immune response triggered by rTsE vaccination protected enteral mucosa from larval intrusion, suppressed larval development and reduced female fecundity. The results indicate that TsE may represent a novel target molecule for anti-T. spiralis vaccines.

Primary characterization of the immune response in pigs infected with Trichinella spiralis

Trichinellosis, which is caused by Trichinella spiralis (T. spiralis), is a serious zoonosis. Pigs play an important role in the transmission of human trichinellosis. Characterizing the immune response to T. spiralis infection is key to elucidating host–parasite interactions. However, most studies on the immune response to T. spiralis infection have employed murine models. In this study, we investigated the immune response to T. spiralis infection in pigs. The results showed that the average numbers of larvae per gram (lpg) for the 100-muscle larvae (ML), 1000-ML, and 10 000-ML groups were 1.502, 35.947, and 398.811, respectively. The percentages of CD3+ T cells, B cells, CD4+ T cells, Treg cells, and Th17 cells were elevated in the infection groups compared to the control animals. In contrast, CD8+ T cell percentages were reduced after infection in the low-dose group. The number of neutrophils was increased at 3–17 days post-infection (dpi). Th1 cytokine IL-2 levels were significantly decreased at 7 dpi, and Th2 cytokine IL-4 levels were significantly elevated at 3 dpi. Treg cytokine IL-10 levels were significantly elevated between 7 dpi and 30 dpi. Th17 cytokine IL-17A levels were significantly increased beginning at 11 dpi. These results confirmed that pigs infected with T. spiralis predominantly induced Th2 and Treg immune responses, which suppress the Th1 immune responses. This study provides novel insights into the immune response of pigs infected with T. spiralis.

Label-free quantitative proteomic analysis of molting-related proteins of Trichinella spiralis intestinal infective larvae

Molting is a key step for body-size expansion and environmental adaptation of parasitic nematodes, and it is extremely important for Trichinella spiralis growth and development, but the molting mechanism is not fully understood. In this work, label-free LC-MS/MS was used to determine the proteome differences between T. spiralis muscle larvae (ML) at the encapsulated stage and intestinal infective larvae (IIL) at the molting stage. The results showed that a total of 2885 T. spiralis proteins were identified, 323 of which were differentially expressed. These proteins were involved in cuticle structural elements, regulation of cuticle synthesis, remodeling and degradation, and hormonal regulation of molting. These differential proteins were also involved in diverse intracellular pathways, such as fatty acid biosynthesis, arachidonic acid metabolism, and mucin type O-glycan biosynthesis. qPCR results showed that five T. spiralis genes (cuticle collagen 14, putative DOMON domain-containing protein, glutamine synthetase, cathepsin F and NADP-dependent isocitrate dehydrogenase) had significantly higher transcriptional levels in 10 h IIL than ML (P < 0.05), which were similar to their protein expression levels, suggesting that they might be T. spiralis molting-related genes. Identification and characterization of T. spiralis molting-related proteins will be helpful for developing vaccines and new drugs against the early enteral stage of T. spiralis.

In vitro silencing of a serine protease inhibitor suppresses Trichinella spiralis invasion, development, and fecundity

In a previous study, immunoproteomics was used to identify a serine protease inhibitor (TsSPI) of T. spiralis excretory/secretory (ES) proteins that exhibited an inhibitory effect on trypsin enzymatic activity, but the precise role of TsSPI on worm infection and development in its host is not well understood. The objective of the present study was to use RNA interference to ascertain the function of TsSPI in larval invasion and growth. TsSPI-specific small interference RNAs (siRNAs) were delivered to muscle larvae (ML) to silence TsSPI expression by electroporation. Four days after electroporation, the ML transfected with 2 μM siRNA-653 exhibited a 75.75% decrease in TsSPI transcription and a 69.23% decrease in TsSPI expression compared with control ML. Although the silencing of TsSPI expression did not decrease worm viability, it significantly suppressed the larval invasion of intestinal epithelium cells (IEC) (P < 0.01), and the suppression was siRNA dose-dependent (r = 0.981). The infection of mice with siRNA-653-treated ML produced a 63.71% reduction of adult worms and a 72.38% reduction of muscle larvae. In addition, the length of the adults, newborn larvae, and ML and the fecundity of female T. spiralis from mice infected with siRNA-treated ML were obviously reduced relative to those in the control siRNA or PBS groups. These results indicated that the silencing of TsSPI by RNAi suppressed larval invasion and development and decreased female fecundity, further confirming that TsSPI plays a crucial role during the T. spiralis lifecycle and is a promising molecular target for anti-Trichinella vaccines.

Molecular Characterization of Fructose-1,6-bisphosphate Aldolase From Trichinella spiralis and Its Potential in Inducing Immune Protection

Trichinella spiralis is a major food-borne parasite worldwide. Trichinellosis caused by T. spiralis is not only a public health problem, but also an economic hazard in food safety. The development of effective vaccines to prevent Trichinella infection in domestic animals and humans is urgently needed for controlling of this zoonosis. Fructose-1, 6-bisphosphate aldolase (FBPA) is involved in energy production in glycolysis and is also associated with many non-glycolysis functions in the parasite, such as adhesion to host cells, plasminogen binding, and invasion. FBPA has been considered as a potential vaccine candidate or as a target for chemotherapeutic treatment. Here, we report for the first time the characterization of FBPA of T. spiralis and an evaluation of its potential as a vaccine candidate antigen against T. spiralis infection in mice. The results of qPCR and western blot analysis showed that the Ts-FBPA gene was expressed at various developmental stages of T. spiralis and was also detected in excretory–secretory products (ES) of T. spiralis muscle larvae (ML). Immunostaining with anti-Ts-FBPA mouse sera indicated that it localized principally to the surface and embryos of this parasitic nematode. Vaccination of mice with recombinant Ts-FBPA (rTs-FBPA) resulted in a Th1/Th2 mixed humoral and cellular immune response with Th2 predominant, as well as remarkably elevated IgE levels. Moreover, mice vaccinated with rTs-FBPA displayed a 48.7% reduction in adult worm burden and 52.5% reduction in muscle larval burden. These studies indicated that Ts-FBPA is a promising target for developing an effective vaccine to prevent and control Trichinella infection.