Pre-exposure of cattle to drug-abbreviated Fasciola hepatica infections: the effect upon subsequent challenge infection and the early immune response

Omics tools enabling vaccine discovery against fasciolosis

In the past decade significant advances in our understanding of liver fluke biology have been made through in-depth interrogation and analysis of evolving Fasciola hepatica and Fasciola gigantica omics datasets. This information is crucial for developing novel control strategies, particularly vaccines necessitated by the global spread of anthelmintic resistance. Distilling them down to a manageable number of testable vaccines requires combined rational, empirical, and collaborative approaches. Despite a lack of clear outstanding vaccine candidate(s), we must continue to identify salient parasite-host interacting molecules, likely in the secretory products, tegument, or extracellular vesicles, and perform robust trials especially in livestock, using present and emerging vaccinology technologies to discover that elusive liver fluke vaccine. Omics tools are bringing this prospect ever closer.

Current Status for Controlling the Overlooked Caprine Fasciolosis

Simple Summary

Retaining sustainable agricultural systems is essential to feed the expanding population. Helminth parasite infections impact livestock production values and yields, including infections in goats, which are often overlooked. There is a requirement to collate caprine-specific knowledge surrounding fasciolosis, caused by Fasciola hepatica and Fasciola gigantica (liver flukes). Current liver fluke control methods include drug application and pasture management. This review aims to outline goat-specific fasciolosis vaccine development and the potential for alternative control methods.

Abstract

The disease fasciolosis is caused by the liver flukes Fasciola hepatica and F. gigantica, which infect a wide range of mammals and production livestock, including goats. These flatworm parasites are globally distributed and predicted to cost the livestock industry a now conservative USD 3 billion per year in treatment and lowered on-farm productivity. Infection poses a risk to animal welfare and results in lowered fertility rates and reduced production yields of meat, milk and wool. This zoonotic disease is estimated to infect over 600 million animals and up to 2.4 million humans. Current and future control is threatened with the global emergence of flukes resistant to anthelmintics. Drug resistance calls for immediate on-farm parasite management to ensure treatments are effective and re-infection rates are kept low, while a sustainable long-term control method, such as a vaccine, is being developed. Despite the recent expansion of the goat industry, particularly in developing countries, there are limited studies on goat-focused vaccine control studies and the effectiveness of drug treatments. There is a requirement to collate caprine-specific fasciolosis knowledge. This review will present the current status of liver fluke caprine infections and potential control methods for application in goat farming.

The influence of liver fluke infection on production in sheep and cattle: a meta-analysis

Liver flukes (Fasciola spp.) are important parasites of ruminant livestock worldwide, causing profound damage to animal health and productivity. Many reviews have discussed the results of decades of research on the impact of fluke on livestock traits such as weight gain and milk production, but there have been no known attempts to collate previous research in a quantitative manner or to determine the factors that vary between studies that find substantial effects of fluke and others concluding that effects of fluke are negligible. Here, we use meta-analysis to provide quantitative "global" estimates of the impact of liver fluke on animal performance, and to identify elements of study design ("moderators") that influence variation between studies in their outcome. A literature search provided 233 comparisons of performance in fluke-infected and uninfected animals. We standardised these data as log response ratios and calculated effect size variances to weight studies by the accuracy of their estimates. We performed multi-level meta-analysis to estimate effects of fluke infection on daily weight gain, live weight, carcass weight, total weight gain and milk production. There were statistically-supported negative effects of fluke infection on daily weight gain, live weight and carcass weight (9%, 6% and 0.6% reductions, respectively), but not on total weight gain or milk production. A mixed-effects meta-analysis revealed that studies of younger animals found more severe effects of fluke on weight gain and that effects on live weight increased with time since infection. Limitations to the data that could be analysed, including a lack of statistical reporting in older papers and variation in the outcome variables measured, may have explained the relatively limited influence of modifiers that we detected. Our results provide, to our knowledge, the first quantitative estimate of the impact of liver fluke on performance across studies and highlight some elements of study design that can influence conclusions.

Fasciolosis in India: An overview

Fasciolosis in ruminants is a relentless constraint in the livestock industry across the world. Immuno-prophylactic vaccines against fasciolosis may not come up in near future, rendering the control of this scourge with chemotherapy and snail population control. With the alarming threats of anti-fasciolid drug resistance reported from certain parts of the world; the control of fasciolosis should be directed towards the development of rapid and reliable diagnostic tools to execute the specific and discrete treatment. Understanding the epidemiology of Fasciola, its genomics and proteomics, host-parasite interplay, and advances in drug design research is vital for improving animal health that would ultimately succour to meet the ever-increasing demand for food. Due to possible differences in immune response depending on the species of the host and parasite, immuno-prophylactic studies in India should aim at achieving protective efficacy in buffalo against F. gigantica as workers from other countries concentrate primarily on vaccination of cattle and sheep against F. hepatica. This manuscript focused on the research that has been carried out in India for understanding the epidemiology, genetic diversity, immuno-diagnosis, and possible control measure in terms of immuno-prophylaxis and drug designing against tropical fasciolosis caused by Fasciola gigantica.

Differences in the antibody response to adult Fasciola hepatica excretory/secretory products in experimentally and naturally infected cattle and sheep

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Antibody response is different in animals experimentally and naturally infected with F. hepatica.

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Experimentally infected animals specifically recognised cathepsin proteins.

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Naturally infected animals showed poor recognition of a recombinant cathepsin L1.

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Antibody response of naturally infected animals is against multiple antigens.

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Diagnostic tests based on a single antigen may not be suitable for use in field.

Fasciola hepatica (the liver fluke) is a common, global parasite of livestock. It can be highly pathogenic and has health and welfare implications for infected individuals. Typically, in ruminants, infections are sub-clinical, but if undiagnosed, they can lead to significant production losses. Accurate diagnosis is crucial to identify infection. Antibody detection ELISAs are commonly used to diagnose infection due to their high sensitivity and specificity and are typically based on native fluke excretory/secretory (ES) products or cathepsin L1 (CL1), the immunodominant antigen within ES products. These tests have been developed based on the antibody response of experimentally infected animals; however, this response has not been well characterised in naturally infected animals. We compared the antibody recognition of a recombinant CL1 (rCL1) antigen and native adult fluke ES products. Whilst samples from experimentally infected animals showed strong recognition of rCL1, serum antibodies from naturally infected animals did not. These results were confirmed by peptide array. Immunoblotting sera against ES products showed that experimentally infected animals had a strong, specific response to CL1/CL2 proteins whilst antibodies from naturally infected animals recognised multiple proteins and had a variable response to CL1/CL2. Mass spectrometry of proteins separated by 2D SDS PAGE, identified several antigens recognised by serum antibodies from a naturally infected cow, including cathepsins L1, L2 and L5, glutathione S-transferase and a dihydrolipoyl dehydrogenase. Overall, these results show that the antibody response in naturally infected animals to adult fluke ES products is qualitatively different to experimentally infected animals. This suggests that a diagnostic test based on CL1 alone may not be appropriate for diagnosis of natural F. hepatica infections in sheep and cattle.

Vaccination against Fasciola hepatica using cathepsin L3 and B3 proteases delivered alone or in combination

No licensed vaccine is currently available for prevention of Fasciola hepatica infections. However, considering the alarming increase in drug resistance, there is an urgent need for a safe and fully effective vaccine against fasciolosis. Here, we tested if cathepsins L (FhCL3-1, FhCL3-2) and B (FhCB3) secreted by juvenile liver flukes are viable vaccine targets when delivered alone or in combination in a rat model. Since control over the early immune response is crucial for parasite's establishment in its host, it was hypothesised that targeting fluke juvenile stages may prove beneficial. Moreover, it was assumed that selected antigens will act in a cumulative manner to interfere with liver fluke migration and thereby will reduce F. hepatica infection. Recombinant FhCL3-1 and FhCL3-2 delivered alone reduced liver fluke burdens by 47 % and 63 %, respectively. A trivalent vaccine containing rFhCL3-1/CL3-2/CB3 did not increase the protective vaccine efficacy compared to the rFhCL3-2 vaccinated group (53 %), although, reductions in liver fluke wet weight (statistically significant) and liver damage score were most pronounced. Further, the highest IgG1 and IgG2a levels were seen in rFhCL3-2 vaccinated rats, the group for which the highest reduction in worm burden was demonstrated. Moreover, IgG1 and IgG2a levels in vaccinated rats were significantly elevated compared to those reported for control groups up to 4 week post-infection. While the mechanism of protection remains unknown, it appears that it depends on vaccine-induced antibodies directed against cathepsins. The obtained results imply that F. hepatica juvenile-specific cathepsins are promising vaccine candidates that induce responses that successfully target early migratory liver fluke stages. Now, the challenge is to evaluate these juvenile-specific cathepsins for use in livestock.

Fasciola hepatica vaccine: we may not be there yet but we're on the right road

Major advances have been made in identifying potential vaccine molecules for the control of fasciolosis in livestock but we have yet to reach the level of efficacy required for commercialisation. The pathogenesis of fasciolosis is associated with liver damage that is inflicted by migrating and feeding immature flukes as well as host inflammatory immune responses to parasite-secreted molecules and tissue damage alarm signals. Immune suppression/modulation by the parasites prevents the development of protective immune responses as evidenced by the lack of immunity observed in naturally and experimentally infected animals. In our opinion, future efforts need to focus on understanding how parasites invade and penetrate the tissues of their hosts and how they potentiate and control the ensuing immune responses, particularly in the first days of infection. Emerging 'omics' data employed in an unbiased approach are helping us understand liver fluke biology and, in parallel with new immunological data, to identify molecules that are essential to parasite development and accessible to vaccine-induced immune responses.

Liver fluke vaccines in ruminants: strategies, progress and future opportunities

The development of a vaccine for Fasciola spp. in livestock is a challenge and would be advanced by harnessing our knowledge of acquired immune mechanisms expressed by resistant livestock against fluke infection. Antibody-dependent cell-mediated cytotoxicity directed to the surface tegument of juvenile/immature flukes is a host immune effector mechanism, suggesting that antigens on the surface of young flukes may represent prime candidates for a fluke vaccine. A Type 1 immune response shortly after fluke infection is associated with resistance to infection in resistant sheep, indicating that vaccine formulations should attempt to induce Type 1 responses to enhance vaccine efficacy. In cattle or sheep, an optimal fluke vaccine would need to reduce mean fluke burdens in a herd below the threshold of 30-54 flukes to ensure sustainable production benefits. Fluke infection intensity data suggest that vaccine efficacy of approximately 80% is required to reduce fluke burdens below this threshold in most countries. With the increased global prevalence of triclabendazole-resistant Fasciolahepatica, it may be commercially feasible in the short term to introduce a fluke vaccine of reasonable efficacy that will provide economic benefits for producers in regions where chemical control of new drug-resistant fluke infections is not viable. Commercial partnerships will be needed to fast-track new candidate vaccines using acceptable adjuvants in relevant production animals, obviating the need to evaluate vaccine antigens in rodent models.

Immunomodulatory molecules of Fasciola hepatica: candidates for both vaccine and immunotherapeutic development

The liver fluke, Fasciola hepatica, causes fascioliasis in domestic animals (sheep, cattle), a global disease that is also an important infection of humans. As soon as the parasite invades the gut wall its interaction with various host immune cells (e.g. dendritic cells, macrophages and mast cells) is complex. The parasite secretes a myriad of molecules that direct the immune response towards a favourable non-protective Th2-mediate/regulatory environment. These immunomodulatory molecules, such as cathepsin L peptidase (FhCL1), are under development as the first generation of fluke vaccines. However, this peptidase and other molecules, such as peroxiredoxin (FhPrx) and helminth defence molecule (FhHDM-1), exhibit various immunomodulatory properties that could be harnessed to help treat immune-related conditions in humans and animals.

Characterization and expression of cathepsin B2 in Fasciola gigantica

Fasciola gigantica cathepsin B belongs to a family of cysteine proteases which is involved in invasion of host tissues. In this study, the recombinant cathepsin B2 (rFgCatB2), synthesized in Pichia pastoris, showed enzymatic activity on a fluorometric substrate Z-Phe-Arg-AMC and gelatin. Furthermore, this recombinant enzyme could degrade IgG and type I collagen. Mouse antiserum against rFgCatB2 reacted with the native FgCatB2 in whole body (WB) extracts of metacercariae (MET), newly excysted juveniles (NEJ) and 2week-old juveniles, but not in 3, 4 week-old juveniles and adult flukes. Immunolocalization showed the presence of cathepsin B2 only in the caecal epithelium of MET, NEJ and 2 week-old juveniles. Co-localization of FgCatB2 and a prominent antigen of NEJ, FgCatB3, revealed that these proteins were expressed at the same regions in the caecal epithelium. Anti-rFgCatB2 showed no cross reaction with the other parasites' antigens by Western blotting. These findings suggest that CatB2 is expressed only in early stages of the parasite and may be involved in digestion of host connective tissues and evasion of the host immune system during their penetration and migration. Thus, CatB2 could be considered as an immunodiagnostic and vaccine candidate for fasciolosis.

Protective role of purified cysteine proteinases against Fasciola gigantica infection in experimental animals

Fascioliasis is one of the public health problems in the world. Cysteine proteinases (CP) released by Fasciola gigantica play a key role in parasite feeding, migration through host tissues, and in immune evasion. There has been some evidence from several parasite systems that proteinases might have potential as protective antigens against parasitic infections. Cysteine proteinases were purified and tested in vaccine trials of sheep infected with the liver fluke. Multiple doses (2 mg of CP in Freund's adjuvant followed by 3 booster doses 1 mg each at 4 week intervals) were injected intramuscularly into sheep 1 week prior to infect orally with 300 F. gigantica metacercariae. All the sheep were humanely slaughtered 12 weeks after the first immunization. Changes in the worm burden, ova count, and humoral and cellular responses were evaluated. Significant reduction was observed in the worm burden (56.9%), bile egg count (70.7%), and fecel egg count (75.2%). Immunization with CP was also found to be associated with increases of total IgG, IgG₁, and IgG₂ (P<0.05). Data showed that the serum cytokine levels of pro-inflammatory cytokines, IL-12, IFN-γ, and TNF-α, revealed significant decreases (P<0.05). However, the anti-inflammatory cytokine levels, IL-10, TGF-β, and IL-6, showed significant increases (P<0.05). In conclusion, it has been found that CP released by F. gigantica are highly important candidates for a vaccine antigen because of their role in the fluke biology and host-parasite relationships.

Exploring the Fasciola hepatica tegument proteome

The surface tegument of the liver fluke Fasciola hepatica is a syncytial cytoplasmic layer bounded externally by a plasma membrane and covered by a glycocalyx, which constitutes the interface between the parasite and its ruminant host. The tegument's interaction with the immune system during the fluke's protracted migration from the gut lumen through the peritoneal cavity and liver parenchyma to the lumen of the bile duct, plays a key role in the fluke's establishment or elimination. However, little is known about proteins of the tegument surface or its secretions. We applied techniques developed for the blood fluke, Schistosoma mansoni, to enrich a tegument surface membrane preparation and analyse its composition by tandem mass spectrometry using new transcript databases for F. hepatica. We increased the membrane and secretory pathway components of the final preparation to ∼30%, whilst eliminating contaminating proteases. We identified a series of proteins or transcripts shared with the schistosome tegument including annexins, a tetraspanin, carbonic anhydrase and an orthologue of a host protein (CD59) that inhibits complement fixation. Unique to F. hepatica, we also found proteins with lectin, cubulin and von Willebrand factor domains plus 10 proteins with leader sequences or transmembrane helices. Many of these surface proteins are potential vaccine candidates. We were hampered in collecting tegument secretions by the propensity of liver flukes, unlike blood flukes, to vomit their gut contents. We analysed both the 'vomitus' and a second supernatant released from haematin-depleted flukes. We identified many proteases, some novel, as well as a second protein with a von Willebrand factor domain. This study demonstrates that components of the tegumental surface of F. hepatica can be defined using proteomic approaches, but also indicates the need to prevent vomiting if tegument secretions are to be characterised.

Proteolytic activity inFasciola hepaticais reduced by the administration of cathepsin L mimotopes

The objective of this study was to assess the proteolytic activity ofFasciola hepaticacathepsins in liver sections from mice vaccinated with phage clones of cathepsin L mimotopes, using the filmin situzymography technique. Female BALB/c mice were immunized three times with 2.5 × 1011phage particles without adjuvant. Animals vaccinated with phage clones produced high titres of anti-mimotope antibodies and a significant reduction in fluke burden was observed following challenge with metacercariae ofF. hepatica. The proteolytic activity in hepatic tissue was reduced after the immunization with phage clones.

Kinetics of anti-Fasciola IgG antibodies in serum and milk from dairy cows during lactation, and in serum from calves after feeding colostrum from infected dams

A study was carried out to compare anti-Fasciola hepatica IgG levels in blood serum and mammary secretions during the entire lactation period in dairy cows experimentally infected with different numbers of F. hepatica metacercariae. The kinetics of specific antibodies passively transferred to the offspring was also studied. The MM3-SERO ELISA, a specific and sensitive method of detecting antibodies against F. hepatica, was used to detect antibodies in milk and serum samples. The progress of infection was monitored by use of the MM3-COPRO ELISA, an immunoassay for detecting Fasciola antigens in faecal samples. The optical density of serum and milk from uninfected control cows remained low throughout the study. In the infected animals, a similar pattern of anti-F. hepatica IgG kinetics was observed in serum and milk throughout the entire observation period. This IgG response was characterized by the early appearance of high levels of specific antibodies in serum (detectable 1-4 weeks pi) and in milk (detectable at the beginning of lactation) and remained invariably high throughout the entire lactation period in cows infected with low-to-moderate infective doses (>or=50 metacercariae). However, in animals administered very low infective doses (<or=25 metacercariae) the levels of specific antibodies were variable, and would be difficult to interpret if only sporadic analyses were carried out. The MM3-SERO ELISA proved to be highly sensitive for use with milk samples, as it enabled detection of antibodies in cows infected with very low infective doses (<or=25 metacercariae) and therefore harbouring very few flukes (probably<10). Moreover, specific antibodies were able to be detected at any stage of lactation, in milk from cows infected with >or=50 metacercariae, even when samples were diluted al least 1:8. In newborn calves fed colostrum from infected dams, the levels of anti-Fasciola antibodies increased rapidly, then decreased sharply and were no longer detected in calves >12 weeks old. The results indicate that the detection of antibodies in milk samples may provide useful information about the status of F. hepatica infection in dairy herds if repeated analyses are carried out. Analysis of bulk samples may also be an inexpensive way of identifying herds infected with F. hepatica, provided highly sensitive tests are used.

Identification, expression and immunolocalization of cathepsin B3, a stage-specific antigen expressed by juvenile Fasciola gigantica

To identify antigens that could potentially be developed as vaccines against Fasciola gigantica, somatic antigens were analyzed by immunoprecipitation using pooled sera from rats infected with F. gigantica metacercariae. A prominent antigen of the newly excysted juveniles (NEJ), cathepsin B3 protease (FgCatB3), was identified by N-terminal sequencing and PCR screening of a cDNA library. Recombinant FgCatB3 (rFgCatB3) was expressed in Pichia pastoris, and shown to catalyse the digestion of gelatin, the fluorometric substrate Z-Phe-Arg-AMC and native fibronectin, suggesting that this enzyme may be involved in digesting host connective tissues during the fluke's penetration and migration in the host. Rabbit polyclonal sera against rFgCatB3 was produced and used to determine the distribution of the native cathepsin B3 protease in various developmental stages of F. gigantica. By Western blotting, cathepsin B3 was detected in the whole body (WB) extract of metacercariae and NEJ but not in 4-week-old juveniles or adult parasites which confirmed the stage-specific characteristics of cathepsin B3. Immunolocalization of cathepsin B3 protease in each parasite stage showed that high levels of FgCatB3 were present in the caecal epithelium of the metacercariae and NEJ. The differential distribution of FgCatB3 in the different life cycle stages suggests that this protease is functionally important for the juvenile stage of F. gigantica.

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.

Vaccines against the zoonotic trematodesSchistosoma japonicum,Fasciola hepaticaandFasciola gigantica

Schistosoma japonicum,Fasciola hepaticaandF. giganticaare digenetic trematodes and, therefore, possess similar life cycles. While schistosomiasis japonica has for a long time been recognised as a major disease of both humans and animals, infection with fasciolids has only been considered of relevance to animals. However, a number of recent reports indicate that fasciolosis is becoming a serious public health problem, especially in South America, Egypt and Iran (sporadic cases are also on the increase throughout Europe). Vaccines targeted at animals could play an important role in controlling these three diseases in animals and, by blocking transmission of infection, have a concurrent beneficial effect on disease in humans. Approaches towards identifying and producing vaccines against these parasites are similar and are discussed in this reveiw.

Identification of immunodominant excretory–secretory cysteine proteases of adultParagonimus westermani by proteome analysis

Paragonimus westermani causes inflammatory lung disease in humans. The parasite excretes a host of biologically active molecules, which are thought to be involved in pathophysiological and immunological events during infection. Analyses of the 2-DE protein profiles of the excretory-secretory products (ESP) of adult P. westermani revealed approximately 147 protein spots, at least 15 of which were identified as cysteine proteases (CPs), at pHs between 4.5 and 8.5, and molecular weights (MWs) between 27 and 35 kDa. An additional three CPs (designated as PwCP-3, -8 and -11) were newly recognized by TOF/TOF MS. Their molecular biological information, which shared a high level sequence homology, was elucidated. The majority of the CPs reacted strongly with sera from paragonimiasis patients. When we observed the chronological changes in the antibody responses of the respective CPs against canine sera collected serially at 1, 3, 5, 7, 11 and 14 wk after experimental infection, these molecules exhibited a multiplicity of distinct immune recognition patterns. Our results clearly showed that P. westermani adult ESP were principally composed of excretory-secretory CPs, and that these CPs may exert effects not only on host tissue degradation and nutrient uptake, but also on the immune-regulating cells via synergistic and independent interactions.

Immunology of the host–parasite relationship in fasciolosis (Fasciola hepaticaandFasciola gigantica)

The protective resolution of liver fluke (Fasciola hepatica and Fasciola gigantica) infection is a dynamic interplay between the host's effector responses and the parasite's defence and immunomodulatory systems. The evidence suggests that the juvenile or immature parasite is the target of protective host immune responses but the effector mechanisms employed vary between hosts. Moreover, F. hepatica and F. gigantica differ in their susceptibility to these killing mechanisms. In the rat, in vitro killing of juvenile F. hepatica involves an antibody-dependent cell cytotoxicity mediated by nitric oxide produced by activated monocytes and (or) macrophages. However, monocytes and (or) macrophages from Indonesian sheep do not produce nitric oxide yet can effectively kill juvenile F. gigantica in vitro and in vivo by a mechanism that is ineffective against F. hepatica. These data show that disease progression or resolution in fasciolosis is determined both by biochemical differences between Fasciola species and by host-dependent factors. Understanding the genetic basis for these differences is a key question for the future. Fasciola hepatica and F. gigantica actively modulate the host immune response, downregulating type 1 responses during infection. It is important to determine whether such modulation of the immune response by Fasciola spp. directly leads to enhanced parasite survival in the various hosts.

Fasciola hepatica cathepsin L-like proteases: biology, function, and potential in the development of first generation liver fluke vaccines

Fasciola hepatica secretes cathepsin L proteases that facilitate the penetration of the parasite through the tissues of its host, and also participate in functions such as feeding and immune evasion. The major proteases, cathepsin L1 (FheCL1) and cathepsin L2 (FheCL2) are members of a lineage that gave rise to the human cathepsin Ls, Ks and Ss, but while they exhibit similarities in their substrate specificities to these enzymes they differ in having a wider pH range for activity and an enhanced stability at neutral pH. There are presently 13 Fasciola cathepsin L cDNAs deposited in the public databases representing a gene family of at least seven distinct members, although the temporal and spatial expression of each of these members in the developmental stage of F. hepatica remains unclear. Immunolocalisation and in situ hybridisation studies, using antibody and DNA probes, respectively, show that the vast majority of cathepsin L gene expression is carried out in the epithelial cells lining the parasite gut. Within these cells the enzyme is packaged into secretory vesicles that release their contents into the gut lumen for the purpose of degrading ingested host tissue and blood. Liver flukes also express a novel multi-domain cystatin that may be involved in the regulation of cathepsin L activity. Vaccine trials in both sheep and cattle with purified native FheCL1 and FheCL2 have shown that these enzymes can induce protection, ranging from 33 to 79%, to experimental challenge with metacercariae of F. hepatica, and very potent anti-embryonation/hatch rate effects that would block parasite transmission. In this article we review the vaccine trials carried out over the past 8 years, the role of antibody and T cell responses in mediating protection and discuss the prospects of the cathepsin Ls in the development of first generation recombinant liver fluke vaccines.

The immune response of regional lymph nodes during the early stages of Fasciola hepatica infection in cattle

In this study we examined regional immune responses to Fasciola hepatica infection in the natural ruminant host. Naïve cattle and those pre-exposed to a drug-abbreviated infection were subsequently challenged and lymph nodes extracted at slaughter. In vitro proliferation and cytokine production by mononuclear cells isolated from hepatic and mesenteric lymph nodes were measured after culture with whole fluke antigen (WFA). Hepatic lymph node cells had a significantly greater response to parasite antigen than mesenteric lymph node cells (P < 0.02), although there was no difference in the magnitude of the proliferative response between naïve and pre-exposed challenged cattle. Mononuclear cells from hepatic lymph nodes produced interferon gamma, interleukin 2 and interleukin 4 after culture with parasite antigen, indicative of a mixed, T helper type 0, response. Comparison of the hepatic node response to a variety of F. hepatica antigens showed that proliferation was lower after culture with cathepsin-L, than with a high molecular weight fraction, WFA or excretory-secretory antigen. Cell culture supernatant fluid from unstimulated hepatic lymph node cells showed an IgG1 response to antigens of 48, 52-70, 82, 96 and 120-190 kDa on Western blot in pre-exposed, but not naïve, challenged animals.