A member of the TGF-beta receptor gene family in the parasitic nematode Brugia pahangi

Cytokines from parasites: manipulating host responses by molecular mimicry

Helminth parasites have evolved sophisticated methods for manipulating the host immune response to ensure long-term survival in their chosen niche, for example, by secreting products that interfere with the host cytokine network. Studies on the secretions of Heligmosomoides polygyrus have identified a family of transforming growth factor-β (TGF-β) mimics (TGMs), which bear no primary amino acid sequence similarity to mammalian TGF-β, but functionally replicate or antagonise TGF-β effects in restricted cell types. The prototypic member, TGM1, induces in vitro differentiation of Foxp3+ T regulatory cells and attenuates airway allergic and intestinal inflammation in animal models. TGM1 is one of a family of ten TGM proteins expressed by H. polygyrus. It is a five-domain modular protein in which domains 1-2 bind TGFBR1, and domain 3 binds TGFBR2; domains 4-5 increase its potency by binding a co-receptor, CD44, highly expressed on immune cells. Domains 4-5 are more diverse in other TGMs, which bind co-receptors on cells such as fibroblasts. One variant, TGM6, lacks domains 1-2 and hence cannot transduce a signal but binds TGFBR2 through domain 3 and a co-receptor expressed on fibroblasts through domains 4-5 and blocks TGF-β signalling in fibroblasts and epithelial cells; T cells do not express the co-receptor and are not inhibited by TGM6. Hence, different family members have evolved to act as agonists or antagonists on various cell types. TGMs, which function by molecularly mimicking binding of the host cytokine to the host TGF-β receptors, are examples of highly evolved immunomodulators from parasites, including those that block interleukin (IL)-13 and IL-33 signalling, modulate macrophage and dendritic cell responses and modify host cell metabolism. The emerging panoply and potency of helminth evasion molecules illustrates the range of strategies in play to maintain long-term infections in the mammalian host.

Importance of TGFβ in Cancer and Nematode Infection and Their Interaction-Opinion

Historically, there has been little interaction between parasitologists and oncologists, although some helminth infections predispose to the development of tumours. In addition, both parasites and tumours need to survive immune attack. Recent research suggests that both tumours and parasites suppress the immune response to increase their chances of survival. They both co-opt the transforming growth factor beta (TGFβ) signalling pathway to modulate the immune response to their benefit. In particular, there is concern that suppression of the immune response by nematodes and their products could enhance susceptibility to tumours in both natural and artificial infections.

The 'nuclear option' revisited: Confirmation of Ss-daf-12 function and therapeutic potential in Strongyloides stercoralis and other parasitic nematode infections

Mechanisms governing morphogenesis and development of infectious third-stage larvae (L3i) of parasitic nematodes have been likened to those regulating dauer development in Caenorhabditis elegans. Dauer regulatory signal transduction comprises initial G protein-coupled receptor (GPCR) signaling in chemosensory neurons of the amphidial complex that regulates parallel insulin- and TGFβ-like signaling in the tissues. Insulin- and TGFβ-like signals converge to co-regulate steroid signaling through the nuclear receptor (NR) DAF-12. Discovery of the steroid ligands of DAF-12 opened a new avenue of small molecule physiology in C. elegans. These signaling pathways are conserved in parasitic nematodes and an increasing body of evidence supports their function in formation and developmental regulation of L3i during the infectious process in soil transmitted species. This review presents these lines of evidence for G protein-coupled receptor (GPCR), insulin- and TGFβ-like signaling in brief and focuses primarily on signaling through parasite orthologs of DAF-12. We discuss in some depth the deployment of sensitive analytical techniques to identify Δ7-dafachronic acid as the natural ligand of DAF-12 homologs in Strongyloides stercoralis and Haemonchus contortus and of targeted mutagenesis by CRISPR/Cas9 to assign dauer-like regulatory function to the NR Ss-DAF-12, its coactivator Ss-DIP-1 and the key ligand biosynthetic enzyme Ss-CYP-22a9. Finally, we present published evidence of the potential of Ss-DAF-12 signaling as a chemotherapeutic target in human strongyloidiasis.

Elucidating the molecular and developmental biology of parasitic nematodes: Moving to a multiomics paradigm

In the past two decades, significant progress has been made in the sequencing, assembly, annotation and analyses of genomes and transcriptomes of parasitic worms of socioeconomic importance. This progress has somewhat improved our knowledge and understanding of these pathogens at the molecular level. However, compared with the free-living nematode Caenorhabditis elegans, the areas of functional genomics, transcriptomics, proteomics and metabolomics of parasitic nematodes are still in their infancy, and there are major gaps in our knowledge and understanding of the molecular biology of parasitic nematodes. The information on signalling molecules, molecular pathways and microRNAs (miRNAs) that are known to be involved in developmental processes in C. elegans and the availability of some molecular resources (draft genomes, transcriptomes and some proteomes) for selected parasitic nematodes provide a basis to start exploring the developmental biology of parasitic nematodes. Indeed, some studies have identified molecules and pathways that might associate with developmental processes in related, parasitic nematodes, such as Haemonchus contortus (barber's pole worm). However, detailed information is often scant and 'omics resources are limited, preventing a proper integration of 'omic data sets and comprehensive analyses. Moreover, little is known about the functional roles of pheromones, hormones, signalling pathways and post-transcriptional/post-translational regulations in the development of key parasitic nematodes throughout their entire life cycles. Although C. elegans is an excellent model to assist molecular studies of parasitic nematodes, its use is limited when it comes to explorations of processes that are specific to parasitism within host animals. A deep understanding of parasitic nematodes, such as H. contortus, requires substantially enhanced resources and the use of integrative 'omics approaches for analyses. The improved genome and well-established in vitro larval culture system for H. contortus provide unprecedented opportunities for comprehensive studies of the transcriptomes (mRNA and miRNA), proteomes (somatic, excretory/secretory and phosphorylated proteins) and lipidomes (e.g., polar and neutral lipids) of this nematode. Such resources should enable in-depth explorations of its developmental biology at a level, not previously possible. The main aims of this review are (i) to provide a background on the development of nematodes, with a particular emphasis on the molecular aspects involved in the dauer formation and exit in C. elegans; (ii) to critically appraise the current state of knowledge of the developmental biology of parasitic nematodes and identify key knowledge gaps; (iii) to cover salient aspects of H. contortus, with a focus on the recent advances in genomics, transcriptomics, proteomics and lipidomics as well as in vitro culturing systems; (iv) to review recent advances in our knowledge and understanding of the molecular and developmental biology of H. contortus using an integrative multiomics approach, and discuss the implications of this approach for detailed explorations of signalling molecules, molecular processes and pathways likely associated with nematode development, adaptation and parasitism, and for the identification of novel intervention targets against these pathogens. Clearly, the multiomics approach established recently is readily applicable to exploring a wide range of interesting and socioeconomically significant parasitic worms (including also trematodes and cestodes) at the molecular level, and to elucidate host-parasite interactions and disease processes.

A TGF-β type II receptor that associates with developmental transition in Haemonchus contortus in vitro

Background

The TGF-β signalling pathway plays a key role in regulating dauer formation in the free-living nematode Caenorhabditis elegans, and previous work has shown that TGF-β receptors are involved in parasitic nematodes. Here, we explored the structure and function of a TGF-β type II receptor homologue in the TGF-β signalling pathway in Haemonchus contortus, a highly pathogenic, haematophagous parasitic nematode.

Methodology/Principal findings

Amino acid sequence and phylogenetic analyses revealed that the protein, called Hc-TGFBR2 (encoded by the gene Hc-tgfbr2), is a member of TGF-β type II receptor family and contains conserved functional domains, both in the extracellular region containing cysteine residues that form a characteristic feature (CXCX₄C) of TGF-β type II receptor and in the intracellular regions containing a serine/threonine kinase domain. The Hc-tgfbr2 gene was transcribed in all key developmental stages of H. contortus, with particularly high levels in the infective third-stage larvae (L3s) and male adults. Immunohistochemical results revealed that Hc-TGFBR2 was expressed in the intestine, ovary and eggs within the uterus of female adults, and also in the testes of male adults of H. contortus. Double-stranded RNA interference (RNAi) in this nematode by soaking induced a marked decrease in transcription of Hc-tgfbr2 and in development from the exsheathed L3 to the fourth-stage larva (L4) in vitro.

Conclusions/Significance

These results indicate that Hc-TGFBR2 plays an important role in governing developmental processes in H. contortus via the TGF-β signalling pathway, particularly in the transition from the free-living to the parasitic stages.

Haemonchus contortus is a gastrointestinal parasitic nematode that causes major economic losses in small ruminants. Here, we investigated the structure and function of a TGF-β type II receptor homologue (Hc-TGFBR2) and its role in regulating H. contortus development. The results showed that the Hc-tgfbr2 gene was transcribed in all developmental stages of H. contortus, with the highest level in L3s and male adults; the encoded protein Hc-TGFBR2 was expressed in the intestine and gonads of adult stages of this nematode. The transcriptional abundance of Hc-tgfbr2 decreased significantly following knockdown by RNA interference in xL3s of H. contortus, which also caused a marked reduction in the number of xL3s developing to L4s in vitro. These findings reveal that the TGF-β type II receptor (Hc-TGFBR2) associates with development of H. contortus, particularly in its transition from the free-living to the parasitic stage.

Novel strategies in immunotherapy for allergic diseases

Conventional immunotherapy (IT) for optimal control of respiratory and food allergies has been fraught with concerns of efficacy, safety, and tolerability. The development of adjuvants to conventional IT has potentially increased the effectiveness and safety of allergen IT, which may translate into improved clinical outcomes and sustained unresponsiveness even after cessation of therapy. Novel strategies incorporating the successful use of adjuvants such as allergoids, immunostimulatory DNA sequences, monoclonal antibodies, carriers, recombinant proteins, and probiotics have now been described in clinical and murine studies. Future approaches may include fungal compounds, parasitic molecules, vitamin D, and traditional Chinese herbs. More robust comparative clinical trials are needed to evaluate the safety, clinical efficacy, and cost effectiveness of various adjuvants in order to determine ideal candidates in disease-specific and allergen-specific models. Other suggested approaches to further optimize outcomes of IT include early introduction of IT during an optimal window period. Alternative routes of administration of IT to optimize delivery and yet minimize potential side effects require further evaluation for safety and efficacy before they can be recommended.

Effect of Transforming Growth Factor-β upon Taenia solium and Taenia crassiceps Cysticerci

Taeniids exhibit a great adaptive plasticity, which facilitates their establishment, growth, and reproduction in a hostile inflammatory microenvironment. Transforming Growth Factor-β (TGFβ), a highly pleiotropic cytokine, plays a critical role in vertebrate morphogenesis, cell differentiation, reproduction, and immune suppression. TGFβ is secreted by host cells in sites lodging parasites. The role of TGFβ in the outcome of T. solium and T. crassiceps cysticercosis is herein explored. Homologues of the TGFβ family receptors (TsRI and TsRII) and several members of the TGFβ downstream signal transduction pathway were found in T. solium genome, and the expression of Type-I and -II TGFβ receptors was confirmed by RT-PCR. Antibodies against TGFβ family receptors recognized cysticercal proteins of the expected molecular weight as determined by Western blot, and different structures in the parasite external tegument. In vitro, TGFβ promoted the growth and reproduction of T. crassiceps cysticerci and the survival of T. solium cysticerci. High TGFβ levels were found in cerebrospinal fluid from untreated neurocysticercotic patients who eventually failed to respond to the treatment (P = 0.03) pointing to the involvement of TGFβ in parasite survival. These results indicate the relevance of TGFβ in the infection outcome by promoting cysticercus growth and treatment resistance.

Signaling in Parasitic Nematodes: Physicochemical Communication Between Host and Parasite and Endogenous Molecular Transduction Pathways Governing Worm Development and Survival

Signaling or communication between host and parasite may occur over relatively long ranges to enable host finding and acquisition by infective parasitic nematode larvae. Innate behaviors in infective larvae transmitted from the soil that enhance the likelihood of host contact, such as negative geotaxis and hypermotility, are likely mediated by mechanoreception and neuromuscular signaling. Host cues such as vibration of the substratum, elevated temperature, exhaled CO2, and other volatile odorants are perceived by mechanosensory and chemosensory neurons of the amphidial complex. Beyond this, the molecular systems that transduce these external cues within the worm are unknown at this time. Overall, the signal transduction mechanisms that regulate switching between dauer and continuous reproductive development in Caenorhabditis elegans, and doubtless other free-living nematodes, have provided a useful framework for testing hypotheses about how the morphogenesis and development of infective parasitic nematode larvae and the lifespan of adult parasites are regulated. In C. elegans, four major signal transduction pathways, G protein-coupled receptor signaling, insulin/insulin-like growth factor signaling, TGFβ-like signaling and steroid-nuclear hormone receptor signaling govern the switch between dauer and continuous development and regulate adult lifespan. Parasitic nematodes appear to have conserved the functions of G-protein-coupled signaling, insulin-like signaling and steroid-nuclear hormone receptor signaling to regulate larval development before and during the infective process. By contrast, TGFβ-like signaling appears to have been adapted for some other function, perhaps modulation of the host immune response. Of the three signal transduction pathways that appear to regulate development in parasitic nematodes, steroid-nuclear hormone signaling is the most straightforward to manipulate with administered small molecules and may form the basis of new chemotherapeutic strategies. Signaling between parasites and their hosts' immune systems also occurs and serves to modulate these responses to allow chronic infection and down regulate acute inflammatory responses. Knowledge of the precise nature of this signaling may form the basis of immunological interventions to protect against parasitism or related lesions and to alleviate inflammatory diseases of various etiologies.

TGF-β in tolerance, development and regulation of immunity

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The broader superfamily of TGF-β-like proteins is reviewed, and signaling pathways summarised.

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The role of TGF-β in the immune tolerance and control of infectious disease is discussed.

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The superfamily member AMH is involved in embryonic sexual differentiation.

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Helminth parasites appear to exploit the TGF-β pathway to suppress host immunity.

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TGF-β homologues and mimics from parasites offer a new route for therapeutic tolerance induction.

The TGF-β superfamily is an ancient metazoan protein class which cuts across cell and tissue differentiation, developmental biology and immunology. Its many members are regulated at multiple levels from intricate control of gene transcription, post-translational processing and activation, and signaling through overlapping receptor structures and downstream intracellular messengers. We have been interested in TGF-β homologues firstly as key players in the induction of immunological tolerance, the topic so closely associated with Ray Owen. Secondly, our interests in how parasites may manipulate the immune system of their host has also brought us to study the TGF-β pathway in infections with longlived, essentially tolerogenic, helminth parasites. Finally, within the spectrum of mammalian TGF-β proteins is an exquisitely tightly-regulated gene, anti-Müllerian hormone (AMH), whose role in sex determination underpins the phenotype of freemartin calves that formed the focus of Ray’s seminal work on immunological tolerance.

Transcriptome profiles of the protoscoleces of Echinococcus granulosus reveal that excretory-secretory products are essential to metabolic adaptation

Background

Cystic hydatid disease (CHD) is caused by the larval stages of the cestode and affects humans and domestic animals worldwide. Protoscoleces (PSCs) are one component of the larval stages that can interact with both definitive and intermediate hosts. Previous genomic and transcriptomic data have provided an overall snapshot of the genomics of the growth and development of this parasite. However, our understanding of how PSCs subvert the immune response of hosts and maintains metabolic adaptation remains unclear. In this study, we used Roche 454 sequencing technology and in silico secretome analysis to explore the transcriptome profiles of the PSCs from E. granulosus and elucidate the potential functions of the excretory-secretory proteins (ESPs) released by the parasite.

Methodology/Principal Findings

A large number of nonredundant sequences as unigenes were generated (26,514), of which 22,910 (86.4%) were mapped to the newly published E. granulosus genome and 17,705 (66.8%) were distributed within the coding sequence (CDS) regions. Of the 2,280 ESPs predicted from the transcriptome, 138 ESPs were inferred to be involved in the metabolism of carbohydrates, while 124 ESPs were inferred to be involved in the metabolism of protein. Eleven ESPs were identified as intracellular enzymes that regulate glycolysis/gluconeogenesis (GL/GN) pathways, while a further 44 antigenic proteins, 25 molecular chaperones and four proteases were highly represented. Many proteins were also found to be significantly enriched in development-related signaling pathways, such as the TGF-β receptor pathways and insulin pathways.

Conclusions/Significance

This study provides valuable information on the metabolic adaptation of parasites to their hosts that can be used to aid the development of novel intervention targets for hydatid treatment and control.

The successful infection establishment of parasites depends on their ability to combat their host's immune system while maintaining metabolic adaptation to their hosts. The mechanisms of these processes are not well understood. We used the protoscoleces (PSCs) of E. granulosus as a model system to study this complex host-parasite interaction by investigating the role of excretory-secretory proteins (ESPs) in the physiological adaptation of the parasite. Using Roche 454 sequencing technology and in silico secretome analysis, we predicted 2280 ESPs and analyzed their biological functions. Our analysis of the bioinformatic data suggested that ESPs are integral to the metabolism of carbohydrates and proteins within the parasite and/or hosts. We also found that ESPs are involved in mediating the immune responses of hosts and function within key development-related signaling pathways. We found 11 intracellular enzymes, 25 molecular chaperones and four proteases that were highly represented in the ESPs, in addition to 44 antigenic proteins that showed promise as candidates for vaccine or serodiagnostic development purposes. These findings provide valuable information on the mechanisms of metabolic adaptation in parasites that will aid the development of novel hydatid treatment and control targets.

An analysis of the transcriptome of Teladorsagia circumcincta: its biological and biotechnological implications

BACKGROUND

Teladorsagia circumcincta (order Strongylida) is an economically important parasitic nematode of small ruminants (including sheep and goats) in temperate climatic regions of the world. Improved insights into the molecular biology of this parasite could underpin alternative methods required to control this and related parasites, in order to circumvent major problems associated with anthelmintic resistance. The aims of the present study were to define the transcriptome of the adult stage of T. circumcincta and to infer the main pathways linked to molecules known to be expressed in this nematode. Since sheep develop acquired immunity against T. circumcincta, there is some potential for the development of a vaccine against this parasite. Hence, we infer excretory/secretory molecules for T. circumcincta as possible immunogens and vaccine candidates.

RESULTS

A total of 407,357 ESTs were assembled yielding 39,852 putative gene sequences. Conceptual translation predicted 24,013 proteins, which were then subjected to detailed annotation which included pathway mapping of predicted proteins (including 112 excreted/secreted [ES] and 226 transmembrane peptides), domain analysis and GO annotation was carried out using InterProScan along with BLAST2GO. Further analysis was carried out for secretory signal peptides using SignalP and non-classical sec pathway using SecretomeP tools. For ES proteins, key pathways, including Fc epsilon RI, T cell receptor, and chemokine signalling as well as leukocyte transendothelial migration were inferred to be linked to immune responses, along with other pathways related to neurodegenerative diseases and infectious diseases, which warrant detailed future studies. KAAS could identify new and updated pathways like phagosome and protein processing in endoplasmic reticulum. Domain analysis for the assembled dataset revealed families of serine, cysteine and proteinase inhibitors which might represent targets for parasite intervention. InterProScan could identify GO terms pertaining to the extracellular region. Some of the important domain families identified included the SCP-like extracellular proteins which belong to the pathogenesis-related proteins (PRPs) superfamily along with C-type lectin, saposin-like proteins. The 'extracellular region' that corresponds to allergen V5/Tpx-1 related, considered important in parasite-host interactions, was also identified. Six cysteine motif (SXC1) proteins, transthyretin proteins, C-type lectins, activation-associated secreted proteins (ASPs), which could represent potential candidates for developing novel anthelmintics or vaccines were few other important findings. Of these, SXC1, protein kinase domain-containing protein, trypsin family protein, trypsin-like protease family member (TRY-1), putative major allergen and putative lipid binding protein were identified which have not been reported in the published T. circumcincta proteomics analysis. Detailed analysis of 6,058 raw EST sequences from dbEST revealed 315 putatively secreted proteins. Amongst them, C-type single domain activation associated secreted protein ASP3 precursor, activation-associated secreted proteins (ASP-like protein), cathepsin B-like cysteine protease, cathepsin L cysteine protease, cysteine protease, TransThyretin-Related and Venom-Allergen-like proteins were the key findings.

CONCLUSIONS

We have annotated a large dataset ESTs of T. circumcincta and undertaken detailed comparative bioinformatics analyses. The results provide a comprehensive insight into the molecular biology of this parasite and disease manifestation which provides potential focal point for future research. We identified a number of pathways responsible for immune response. This type of large-scale computational scanning could be coupled with proteomic and metabolomic studies of this parasite leading to novel therapeutic intervention and disease control strategies. We have also successfully affirmed the use of bioinformatics tools, for the study of ESTs, which could now serve as a benchmark for the development of new computational EST analysis pipelines.

Microbe-dendritic cell dialog controls regulatory T-cell fate

Each microenvironment is controlled by a specific set of regulatory elements that have to be finely and constantly tuned to maintain local homeostasis. These environments could be site specific, such as the gut environment, or induced by chronic exposure to microbes. Various populations of dendritic cells are central to the orchestration of this control. In this review, we discuss some new findings associating dendritic cells from defined compartments with the induction and control of regulatory T cells in the context of exposure to both commensal and pathogenic microbes.

daf-7-related TGF-beta homologues from Trichostrongyloid nematodes show contrasting life-cycle expression patterns

The transforming growth factor-beta (TGF-beta) gene family regulates critical processes in animal development, and plays a crucial role in regulating the mammalian immune response. We aimed to identify TGF-beta homologues from 2 laboratory model nematodes (Heligmosomoides polygyrus and Nippostrongylus brasiliensis) and 2 major parasites of ruminant livestock (Haemonchus contortus and Teladorsagia circumcincta). Parasite cDNA was used as a template for gene-specific PCR and RACE. Homologues of the TGH-2 subfamily were isolated, and found to differ in length (301, 152, 349 and 305 amino acids respectively), with variably truncated N-terminal pre-proteins. All contained conserved C-terminal active domains (>85% identical over 115 amino acids) containing 9 cysteine residues, as in C. elegans DAF-7, Brugia malayi TGH-2 and mammalian TGF-beta. Surprisingly, only the H. contortus homologue retained a conventional signal sequence, absent from shorter proteins of other species. RT-PCR assays of transcription showed that in H. contortus and N. brasiliensis expression was maximal in the infective larval stage, and very low in adult worms. In contrast, in H. polygyrus and T. circumcincta, tgh-2 transcription is higher in adults than infective larvae. The molecular evolution of this gene family in parasitic nematodes has diversified the pre-protein and life-cycle expression patterns of TGF-beta homologues while conserving the structure of the active domain.

Macrophage migration inhibitory factor homologs of anisakis simplex suppress Th2 response in allergic airway inflammation model via CD4+CD25+Foxp3+ T cell recruitment

We have cloned the macrophage migration inhibitory factor (MIF)-like protein (Anisakis simplex (As)-MIF) from larvae of the whale worm (Anisakis simplex third-stage larvae). Asthma was induced in the mice using OVA/alum, with or without various concentrations of rAs-MIF treatment before OVA/alum challenge. Treatment with rAs-MIF coupled with OVA/alum during the challenge period induced a complete inhibition of eosinophilia and goblet cell hyperplasia within the lung and profoundly ameliorated the development of lung hyperreactivity. Also, rAs-MIF was shown to reduce profoundly the quantity of Th2-related cytokines (IL-4, IL-5, and IL-13) in the bronchial alveolar lavage fluid and allergen-specific IgG2a in sera. IL-10 and TGF-beta levels in the bronchoalveolar lavage fluid of the rAs-MIF-treated group were significantly higher than in the other groups. Additionally, CD4(+)CD25(+)Foxp3(+) T cells (regulatory T) were recruited to the spleen and lungs of the rAs-MIF-treated mice, but this recruitment was inhibited by anti-rAs-MIF Ab.

Regulatory T cells in the control of host-microorganism interactions (*)

Each microenvironment requires a specific set of regulatory elements that are finely and constantly tuned to maintain local homeostasis. Various populations of regulatory T cells contribute to the maintenance of this equilibrium and establishment of controlled immune responses. In particular, regulatory T cells limit the magnitude of effector responses, which may result in failure to adequately control infection. However, regulatory T cells also help limit collateral tissue damage caused by vigorous antimicrobial immune responses against pathogenic microbes as well as commensals. In this review, we describe various situations in which the balance between regulatory T cells and effector immune functions influence the outcome of host-microorganism coexistence and discuss current hypotheses and points of polemic associated with the origin, target, and antigen specificity of both endogenous and induced regulatory T cells during these interactions.

Tuning microenvironments: induction of regulatory T cells by dendritic cells

The body requires the generation of regulatory T (Treg) cells to preserve its integrity. Each microenvironment is controlled by a specific set of regulatory elements that have to be finefrly and constantly tuned to maintain local homeostasis. These environments could be site specific, such as the gut environment, or induced by chronic exposure to microbes or tumors. Various populations of dendritic cells (DCs) are central to the orchestration of this control. In this review, we will discuss some new findings associating DCs from defined compartments with the induction of antigen-specific Treg cells.

Stage-specific gene expression in Teladorsagia circumcincta (Nematoda: Strongylida) infective larvae and early parasitic stages

Suppression subtractive hybridisation was used to enrich genes expressed in a stage-specific manner in infective, exsheathed L3s (xL3) versus early L4s of the ovine nematode, Teladorsagia circumcincta prior to gene expression profiling by microarray. The 769 cDNA sequences obtained from the xL3-enriched library contained 361 unique sequences, with 292 expressed sequence tags (ESTs) being represented once ("singletons") and 69 sequences which were represented more than once (overlapping and non-overlapping "contigs"). The L4-enriched EST dataset contained 472 unique sequences, with 314 singletons and 158 contigs. Of these 833 sequences, 85% of the xL3 sequences and 86% of the L4 sequences exhibited homology to known genes or ESTs derived from other species of nematode. Quantitative differential expression (P<0.05) was demonstrated for 563 (68%) of the ESTs by microarray. Within the L3-specific dataset, more than 30% of the transcripts represented the enzyme, guanosine-5'-triphosphate (GTP)-cyclohydrolase, which is the first and rate-limiting enzyme of the tetrahydrobiopterin synthesis pathway and may be involved in critical elements of larval development. In L4s, proteolytic enzymes were highly up-regulated, as were collagens and a number of previously characterised secretory proteins, reflecting the rapid growth of these larvae in abomasal glands. Nucleotide sequence data reported in this paper are available in the EMBL, GenBank and DDJB databases under accession numbers AM 743198-AM 744942.

Genetic regulation of arrested development in nematodes: are age-1 and daf-gene orthologs present in Dictyocaulus viviparus?

In opposite to the free-living soil nematode Caenorhabditis elegans, the genetic regulation of hypobiosis or inhibited or arrested development in parasitic nematodes is completely unknown. In C. elegans, the daf-genes or the age-1 gene are of major importance in signaling pathways regulating arrested development. To investigate if orthologs of these genes are present in the bovine lungworm Dictyocaulus viviparus, a PCR analysis with gene-specific primer combinations was performed. No orthologs of the age-1 or daf-genes could be identified in D. viviparus. The possible differences in the role of the daf-genes concerning arrested development in parasitic and free-living nematodes will be discussed.

Growth factor receptors in helminth parasites: Signalling and host-parasite relationships

Parasitic helminths remain major pathogens of both humans and animals throughout the world. The success of helminth infections depends on the capacity of the parasite to counteract host immune responses but also to exploit host-derived signal molecules for its development. Recent progress has been made in the characterization of growth factor receptors of various nematode and flatworm parasites with the demonstration that transforming growth factor beta (TGF-beta), epidermal growth factor (EGF) and insulin receptor signalling pathways are conserved in helminth parasites and potentially implicated in the host-parasite molecular dialogue and parasite development.

Identification of a DAF-7 ortholog from the hookworm Ancylostoma caninum

Infective hookworm L3 encounter a host specific signal during invasion that re-activates suspended developmental pathways. Response to this cue is critical for the successful infection and completion of the life cycle in the host. In the free-living nematode Caenorhabditis elegans, recovery from the developmentally arrested dauer stage in response to environmental cues is analogous to the resumption of development in invading hookworm L3. Transforming growth factor beta (TGF-beta) and insulin-like signalling pathways mediate dauer formation and recovery. An insulin-like signalling pathway mediates L3 activation in hookworms. To determine the role of TGF-beta signalling in hookworm infection, an ortholog of the C. elegans TGF-beta signalling molecule daf-7 was cloned and characterised. Sequence from a hookworm expressed sequence tag was used to design specific primers for PCR amplification of Ac-daf-7 from Ancylostoma caninum infective L3 cDNA. Amplicons from the 5' and 3' ends were cloned, sequenced, and combined to create a full-length composite Ac-daf-7 cDNA sequence. The 1,634 nucleotide cDNA encoded a 355 amino acid open reading frame with significant homology to Ce-DAF-7 and other TGF-beta signalling molecules. The deduced amino acid sequence contained seven conserved cysteines characteristic of TGF-beta family members, as well as two additional conserved cysteines found in members of the TGF-beta/activin subfamily. Ac-DAF-7 contains a characteristic C-terminal ligand domain that is predicted to be released from a propeptide by proteolytic cleavage at a tetrabasic cleavage site. Ac-daf-7 mRNA was strongly detected by reverse transcriptase PCR in L3 and serum stimulated L3 cDNA, and weakly in cDNA from L1 and adult life cycle stages. Antiserum against Escherichia coli expressed recombinant Ac-DAF-7 detected the mature protein in L3 and adult soluble extracts, but not in excretory/secretory products from serum stimulated L3 or adults. Increased expression in arrested L3 stages suggests that Ac-daf-7 is important for developmental arrest.

Expression of TGF-?-like molecules in the life cycle of Schistosoma japonicum

The transforming growth factor beta (TGF-beta) family controls an extremely wide range of biological activities, such as the growth and differentiation of cells, and immunological events against infectious agents. Although TGF-beta homologs appear to be widely present in metazoan animals, studies of parasite-derived molecules are relatively few. Using antibodies against anti-mouse TGF-beta1, -beta2, and -beta3, we show the expression of TGF-beta-like molecules in Schistosoma japonicum cercariae, schistosomula, eggs and adult worms. Intense immunoreactivity was found on the surface of free-living cercarial bodies. In transverse sections of cercariae, the molecules were localized in the tegument and subtegumental cells, and the number and distribution of producing cells significantly differed with each antibody. In the skin-migrating stage, the expression in the tegumental surface gradually decreased and became almost negative within 48 h of exposure. In adult worms and eggs, the reactivity was found in subtegumental cells and in cells of a tubular structure, respectively. In western blot analysis, the detection of conventional TGF-beta molecules failed. The expression of TGF-beta-like molecules was distinctly regulated at each developmental stage.

Helminth parasites--masters of regulation

Immune regulation by parasites is a global concept that includes suppression, diversion, and conversion of the host immune response to the benefit of the pathogen. While many microparasites escape immune attack by antigenic variation or sequestration in specialized niches, helminths appear to thrive in exposed extracellular locations, such as the lymphatics, bloodstream, or gastrointestinal tract. We review here the multiple layers of immunoregulation that have now been discovered in helminth infection and discuss both the cellular and the molecular interactions involved. Key events among the host cell population are dominance of the T-helper 2 cell (Th2) phenotype and the selective loss of effector activity, against a background of regulatory T cells, alternatively activated macrophages, and Th2-inducing dendritic cells. Increasingly, there is evidence of important effects on other innate cell types, particularly mast cells and eosinophils. The sum effect of these changes to host reactivity is to create an anti-inflammatory environment, which is most favorable to parasite survival. We hypothesize therefore that parasites have evolved specific molecular strategies to induce this conducive landscape, and we review the foremost candidate immunomodulators released by helminths, including cytokine homologs, protease inhibitors, and an intriguing set of novel products implicated in immune suppression.

Activation of transforming growth factor β by Trypanosoma cruzi

The anti-inflammatory cytokine, transforming growth factor beta (TGFbeta), plays an important role in Chagas disease, which is caused by the protozoan parasite Trypanosoma cruzi. In the current study, we show that the addition of an anti-TGFbeta antibody inhibited T. cruzi infection of cardiomyocytes, demonstrating the requirement for active endogenous TGFbeta. As TGFbeta is synthesized as a biologically inactive precursor, which is proteolytically processed to yield a mature, active homodimer, we hypothesized that T. cruzi could activate latent TGFbeta. To test this, we added recombinant latent TGFbeta to a TGFbeta-responsive reporter cell line in the presence of T. cruzi. We observed that T. cruzi was able to activate latent recombinant TGFbeta in this cellular model. We then investigated the ability of T. cruzi to activate latent TGFbetain vitro. We found that live T. cruzi, or cytosolic extracts of T. cruzi, activated latent TGFbeta in a dose- and temperature-dependent manner. The agent involved in TGFbeta activation was shown to be thermolabile and hydrophobic. Taken together, our studies demonstrate that T. cruzi directly activates latent TGFbeta. This activation is required for parasite entry into the mammalian cells and is likely to play an important role in modulating the outcome of T. cruzi infection.

Identification and characterisation of two distinct Smad proteins from the fox-tapeworm Echinococcus multilocularis

Members of the transforming growth factor-beta (TGF-beta) family of cytokines and their corresponding receptors regulate cellular key processes such as proliferation and differentiation, and could be involved in communication mechanisms between parasitic helminths and their hosts. A pivotal role in intracellular TGF-beta signalling is played by Smad factors which directly transmit incoming signals from the cell surface receptors to the nucleus. In this study, we have identified and characterised two novel members of the Smad family, EmSmadA and EmSmadB, which are expressed by the human parasite Echinococcus multilocularis. Based on amino acid sequence comparisons, both echinococcal Smad homologues could be classified as members of the R-Smad subfamily. EmSmadB showed a typical domain structure consisting of conserved MH1 and MH2 domains separated by a proline-rich linker region. EmSmadA, on the other hand, lacked an MH1 region and merely contained an MH2 domain, a feature which has so far not been described for R-Smads. Based on the structures of the corresponding chromosomal loci and on sequence features of the conserved L3 loop regions, EmSmadA and EmSmadB are most likely involved in the transmission of TGF-beta- and bone morphogenetic protein (BMP) signals, respectively. Yeast two-hybrid analyses revealed that both Echinococcus Smads are capable of homo- and heterodimer formations. However, while the formation of homodimers for EmSmadB required previous activation of the protein at the C-terminal SSVS motif, EmSmadA homodimers were already formed in the basal state of the factor. Upon expression of the Echinococcus Smads in human cells, EmSmadA, but not EmSmadB, was phosphorylated by the human TGF-beta type I receptor. Furthermore, both factors functionally interacted with human BMP receptors. By reverse transcriptase-PCR experiments, the encoding genes, emsmadA and emsmadB, were shown to be expressed in the larval stages metacestode and protoscolex during an infection of the intermediate host. Taken together, our data suggest an involvement of EmSmadA and EmSmadB in echinococcal developmental processes during natural infections and provide a solid basis for further investigations on TGF-beta signalling mechanisms in cestodes.

Molecular biology of reproduction and development in parasitic nematodes: progress and opportunities

Molecular biological research on the development and reproduction of parasites is of major significance for many fundamental and applied areas of medical and veterinary parasitology. Together with knowledge of parasite biology and epidemiology, the application of molecular tools and technologies provides unique opportunities for elucidating developmental and reproductive processes in helminths. This article focuses specifically on recent progress in studying the molecular mechanisms of development, sexual differentiation and reproduction in parasitic nematodes of socio-economic importance and comparative analyses, where appropriate, with the free-living nematode Caenorhabditis elegans. It also describes the implications of such work for understanding reproduction, tissue migration, hypobiosis, signal transduction and host-parasite interactions at the molecular level, and for seeking new means of parasite intervention.

Activation of transforming growth factor beta by malaria parasite-derived metalloproteinases and a thrombospondin-like molecule

Much of the pathology of malaria is mediated by inflammatory cytokines (such as interleukin 12, interferon γ, and tumor necrosis factor α), which are part of the immune response that kills the parasite. The antiinflammatory cytokine transforming growth factor (TGF)-β plays a crucial role in preventing the severe pathology of malaria in mice and TGF-β production is associated with reduced risk of clinical malaria in humans. Here we show that serum-free preparations of Plasmodium falciparum, Plasmodium yoelii 17XL, and Plasmodium berghei schizont-infected erythrocytes, but not equivalent preparations of uninfected erythrocytes, are directly able to activate latent TGF-β (LatTGF-β) in vitro. Antibodies to thrombospondin (TSP) and to a P. falciparum TSP-related adhesive protein (PfTRAP), and synthetic peptides from PfTRAP and P. berghei TRAP that represent homologues of TGF-β binding motifs of TSP, all inhibit malaria-mediated TGF-β activation. Importantly, TRAP-deficient P. berghei parasites are less able to activate LatTGF-β than wild-type parasites and their replication is attenuated in vitro. We show that activation of TGF-β by malaria parasites is a two step process involving TSP-like molecules and metalloproteinase activity. Activation of LatTGF-β represents a novel mechanism for direct modulation of the host response by malaria parasites.

Developmental plasticity in schistosomes and other helminths

Developmental plasticity in helminth life cycles serves, in most cases, to increase the probability of transmission between hosts, suggesting that the necessity to achieve transmission is a prominent selective pressure in the evolution of this phenomenon. Some evidence suggests that digenean trematodes from the genus Schistosoma are also capable of limited developmental responses to host factors. Here we review the currently available data on this phenomenon and attempt to draw comparisons with similar processes in the life cycles of other helminths. At present the biological significance of developmental responses by schistosomes under laboratory conditions remains unclear. Further work is needed to determine whether developmental plasticity plays any role in increasing the probability of schistosome transmission and life cycle propagation under adverse conditions, as it does in other helminth life cycles.

Immune regulation by helminth parasites: cellular and molecular mechanisms

Immunology was founded by studying the body's response to infectious microorganisms, and yet microbial prokaryotes only tell half the story of the immune system. Eukaryotic pathogens--protozoa, helminths, fungi and ectoparasites--have all been powerful selective forces for immune evolution. Often, as with lethal protozoal parasites, the focus has been on acute infections and the inflammatory responses they evoke. Long-lived parasites such as the helminths, however, are more remarkable for their ability to downregulate host immunity, protecting themselves from elimination and minimizing severe pathology in the host.

Transforming growth factor-beta and insulin-like signalling pathways in parasitic helminths

The signal transduction pathways involved in regulating developmental arrest in the free-living nematode, Caenorhabditis elegans, are fairly well characterised. However, much less is known about how these processes may influence the developmental timing and maturation in helminth parasites. Here, we provide an overview of two signalling pathways implicated in the regulation of dauer larva formation in C. elegans, the insulin-like signalling pathway and the transforming growth factor-beta pathway, and explore what is known about these signalling pathways in a variety of parasitic helminths. Understanding the differences about how these pathways are affected by environmental cues in free-living versus parasitic species of helminths may provide insights into novel mechanisms for the control or prevention of helminth-induced disease.

Human transforming growth factor-beta activates a receptor serine/threonine kinase from the intravascular parasite Schistosoma mansoni

The biology of the helminth parasite Schistosoma mansoni is closely integrated with that of its mammalian host. SmRK1, a divergent type I transforming growth factor-beta (TGF-beta) receptor of unknown ligand specificity, was previously identified as a candidate for a receptor that allows schistosomes to respond to host-derived growth factors. The TGF-beta family includes activin, bone morphogenetic proteins (BMPs), and TGF-beta, all of which can play crucial roles in metazoan development. The downstream signaling protein of receptors that respond to TGF-beta and activin is Smad2, whereas the receptors that respond to BMPs signal via Smad1. When a constitutively active mutant of SmRK1 was overexpressed with either schistosome Smad1 (SmSmad1) or SmSmad2, a receptor-dependent modulation of SmSmad phosphorylation and luciferase reporter activity occurred only with SmSmad2. To evaluate potential ligand activators of SmRK1, a chimeric receptor containing the extracellular domain of SmRK1 joined to the intracellular domain of the human type I TGF-beta receptor was used. The chimeric receptor bound radiolabeled TGF-beta and could activate a luciferase reporter gene in response to both TGF-beta 1 and TGF-beta 3 but not BMP7. Confirmatory results were obtained using full-length SmRK1. These experiments implicate TGF-beta as a ligand for SmRK1 and as a potential host-derived regulator of parasite growth and development.

In vitro reactivation of Ancylostoma caninum tissue-arrested third-stage larvae by transforming growth factor-beta

Developmental arrest in Ancylostoma caninum is associated with preparasitic, free-living third-stage (L3) larvae, as well as anthelmintic-resilient hypobiotic L3 larvae within the tissues of an infected dog. With the tissue-arrested larvae, pregnancy and, more specifically, the hormonal effects of estrogen and prolactin mediate reactivation resulting in transmammary transmission of infection to nursing puppies. Estrogen and prolactin have been shown to be critically involved in upregulation of transforming growth factor (TGF)-beta2 during pregnancy, and studies on the soil nematode Caenorhabditis elegans further implicate TGF-beta and insulin-like signaling pathways with larval arrest and reactivation. In this report, an in vitro assay was used to show that neither estrogen, prolactin, nor insulin had a direct effect on the feeding/reactivation response of tissue-arrested larvae; however, TGF-beta isoforms 1 and 2 both had significant stimulatory effects that were comparable to the effects of dog serum. The stimulatory effects of serum could be blocked by preincubation with anti-TGF-beta antibodies. Taken together, the results support the hypothesis that during pregnancy, host-derived TGF-beta can signal a parasite-encoded receptor to trigger the reactivation of tissue-arrested larvae. TGF-beta had no effect on preparasitic larvae, suggesting that different signals may be involved in reactivation of the 2 different arrested forms of A. caninum L3 larvae.

Immune evasion genes from filarial nematodes

Helminth parasites have large genomes (approximately 10(8) bp) which are likely to encode a spectrum of products able to block or divert the host immune response. We have employed three parallel approaches to identify the first generation of 'immune evasion genes' from parasites such as the filarial nematode Brugia malayi. The first strategy is a conventional route to characterise prominent surface or secreted antigens. In this way we have identified a 15-kDa protein, which is located on the surface of both L3 and adult B. malayi, and secreted by these parasites in vitro, as a member of the cystatin (cysteine protease inhibitor) family. This product, Bm-CPI-2, blocks conventional cysteine proteases such as papain, but also the aspariginyl endopeptidase involved in the Class II antigen processing pathway in human B cells. In parallel, we identified the major T cell-stimulating antigen from the microfilarial stage as a serpin (serine protease inhibitor), Bm-SPN-2. Microfilariae secrete this product which blocks two key proteases of the neutrophil, a key mediator of inflammation and innate immunity. The second route involves a priori hypotheses that helminth parasites encode homologues of mammalian cytokines such as TGF-beta which are members of broad, ancient metazoan gene families. We have identified two TGF-beta homologues in B. malayi, and shown that one form (Bm-TGH-2) is both secreted by adult parasites in vitro and able to bind to host TGF-beta receptors. Likewise, B. malayi expresses homologues of mammalian MIF, which are remarkably similar in both structure and function to the host protein, even though amino acid identity is only 28%. Finally, we deployed a third method of selecting critical genes, using an expression-based criterion to select abundant mRNAs taken from key points in parasite life histories. By this means, we have shown that the major transcript present in mosquito-borne infective larvae, Bm-ALT, is a credible vaccine candidate for use against lymphatic filariasis, while a second abundantly-expressed gene, Bm-VAL-1, is similar to a likely vaccine antigen being developed against hookworm parasites.

Immunological genomics of Brugia malayi: filarial genes implicated in immune evasion and protective immunity

Filarial nematodes are metazoan parasites with genome sizes of> 100 million base pairs, probably encoding 15 000-20 000 genes. Within this considerable gene complement, it seems likely that filariae have evolved a spectrum of immune evasion products which underpin their ability to live for many years within the human host. Moreover, no suitable vaccine currently exists for human filarial diseases, and few markers have yet been established for diagnostic use. In this review, we bring together biochemical and immunological data on prominent filarial proteins with the exciting new information provided by the Filarial Genome Project's expressed sequence tag (EST) database. In this discussion, we focus on those genes with the highest immunological profile, such as inhibitors of host enzymes, cytokine homologues and stage-specific surface proteins, as well as products associated with the mosquito-borne infective larva which offer the best opportunity for an anti-filarial vaccine. These gene products provide a fascinating glimpse of the molecular repertoire which helminth parasites have evolved to manipulate and evade the mammalian immune response.

Identification of tgh-2, a filarial nematode homolog of Caenorhabditis elegans daf-7 and human transforming growth factor beta, expressed in microfilarial and adult stages of Brugia malayi

A novel member of the transforming growth factor beta (TGF-beta) family has been identified in the filarial nematode parasite Brugia malayi by searching the recently developed Expressed Sequence Tag (EST) database produced by the Filarial Genome Project. Designated tgh-2, this new gene shows most similarity to a key product regulating dauer larva formation in Caenorhabditis elegans (DAF-7) and to the human down-modulatory cytokine TGF-beta. Homology to DAF-7 extends throughout the length of the 349-amino-acid (aa) protein, which is divided into an N-terminal 237 aa, including a putative signal sequence, a 4-aa basic cleavage site, and a 108-aa C-terminal active domain. Similarity to human TGF-beta is restricted to the C-terminal domain, over which there is a 32% identity between TGH-2 and TGF-beta1, including every cysteine residue. Expression of tgh-2 mRNA has been measured over the filarial life cycle. It is maximal in the microfilarial stage, with lower levels of activity around the time of molting within the mammal, but continues to be expressed by mature adult male and female parasites. Expression in both the microfilaria, which is in a state of arrested development, and the adult, which is terminally differentiated, indicates that tgh-2 may play a role other than purely developmental. This is consistent with our observation that TGH-2 is secreted by adult worms in vitro. Recombinant TGH-2 expressed in baculovirus shows a low level of binding to TGF-beta-receptor bearing mink lung epithelial cells (MELCs), which is partially inhibited (16 to 39%) with human TGF-beta, and activates plasminogen activator inhibitor-1 transcription in MELCs, a marker for TGF-beta-mediated transduction. Further tests will be required to establish whether the major role of B. malayi TGH-2 (Bm-TGH-2) is to modulate the host immune response via the TGF-beta pathway.

Filarial nematode parasites secrete a homologue of the human cytokine macrophage migration inhibitory factor

Filarial nematode parasites establish long-term chronic infections in the context of an antiparasite immunity that is strongly biased toward a Th2 response. The mechanisms that lead to this Th2 bias toward filarial antigens are not clear, but one possibility is that the parasites produce molecules that have the capacity to proactively modify their immunological environment. Here we report that filarial parasites of humans secrete a homologue of the human proinflammatory cytokine macrophage migration inhibitory factor (MIF) that has the capability of modifying the activity of human monocytes/macrophages. A cDNA clone isolated from a Brugia malayi infective-stage larva expression library encoded a 12.5-kDa protein product (Bm-MIF) with 42% identity to human and murine MIF. MIF homologues were also found to be expressed in the related filarial species Wuchereria bancrofti and Onchocerca volvulus. Bm-mif was transcribed by adult and larval parasites, and the protein product was found in somatic extracts and in the parasite's excretory-secretory products. Immunohistocytochemistry revealed that Bm-MIF was localized to cells of the hypodermis/lateral chord, the uterine wall, and larvae developing in utero. Unexpectedly, the activities of recombinant Bm-MIF and human MIF on human monocytes/macrophages were found to be similar. When placed with monocytes/macrophages in a cell migration assay, Bm-MIF inhibited random migration. When placed away from cells, Bm-MIF induced an increase in monocyte/macrophage migration that was specifically inhibited by neutralizing anti-Bm-MIF antibodies. Bm-MIF is the first demonstration that helminth parasites produce cytokine homologues that have the potential to modify host immune responses to promote parasite survival.

A novel member of the transforming growth factor-beta (TGF-beta) superfamily from the filarial nematodes Brugia malayi and B. pahangi

Transforming growth factor-beta (TGF-beta) superfamily genes encode products controlling pattern formation, cell differentiation, and immune-mediated inflammation. Members of this superfamily are known in multicellular organisms from mammals to the model nematode Caenorhabditis elegans. Using PCR with oligonucleotides complementary to highly conserved motifs in the TGF-beta superfamily, we first isolated a genomic clone from the filarial nematode Brugia malayi. This gene, termed Bm-tgh-1 (TGF-beta homolog-1), spans 2.5 kb of genomic DNA and contains seven exons. Transcripts of this gene are poorly represented in cDNA libraries, but a full-length cDNA was isolated by RACE from B. pahangi (Bp-tgh-1). The tgh-1 genes from the two species are >98% identical at the nucleotide and amino acid levels, differing at 18/1576 base pairs and 5/428 amino acids; all nonsynonymous substitutions are in the long N-terminal propeptide. They show a high level of similarity throughout all seven exons to a C. elegans gene on cosmid T25F10. Homology to other members of the TGF-beta superfamily is restricted to the C-terminal domain which contains the mature active protein. Key features shared with other members of the superfamily include the tetrabasic proteolytic cleavage site to release an active C-terminal peptide, seven cysteines arrayed in identical fashion, and conserved sequence motifs. tgh-1 is most similar to the BMP-1 subfamily involved in developmental signaling in nematodes, insects, and vertebrates. RT-PCR on first-strand cDNA from both Brugia species, with primers specific to the 3' end, showed that tgh-1 is not expressed in the microfilarial stage, but is detectable in the mosquito-derived infective larvae and is maximal in maturing parasites around the time of molting in the mammalian host. Adult parasites show a relatively low level of expression. The identification of tgh-1, and its preferential expression in developing parasites, suggests that it may be involved in key developmental events in the complex filarial life cycle.