Analysis of genes expressed at the infective larval stage validates utility of Litomosoides sigmodontis as a murine model for filarial vaccine development

Bibliometric analysis of parasite vaccine research from 1990 to 2019

Bibliometric and bibliographic analyses are popular tools for investigating publication metrics and thematic transitions in an expanding codex of biomedical literature. Bibliometric techniques have been employed in parasitology and vaccinology, with only a few malaria-specific literature analyses being reported specifically on parasite vaccines. The pursuit of parasite prophylactics is an important, global endeavour both medically and economically. As such, a comprehensive understanding of the research topics would be a valuable tool in assessing the current status and future directions of parasite vaccine development. Consequently, this study investigated parasite vaccinology from 1990 to 2019 by analysing literature exported from the Web of Science and Dimensions databases using two, commonly used, bibliometric programs: SciMAT and VOSviewer. The results of this study show the common, emerging, and transient themes within the discipline, and where the future lies as vaccine development moves further into the age of omics and informatics.

Involvement of cystatin C in immunity and apoptosis

As an abundantly expressed cysteine protease inhibitor widely distributed in the organisms, cystatin C is involved in various physiological processes. Due to its relatively small molecular weight and easy detection, cystatin C is commonly used as a measure for glomerular filtration rate. In pathological conditions, however, growing evidences suggest that cystatin C is associated with various immune responses against either exogenous or endogenous antigens, which ultimately result in inflammatory autoimmune diseases or tumor development if not properly controlled. Thus the fluctuation of cystatin C levels might have more clinical implications than a reflection of kidney functions. Here, we summarize the latest development of studies on the pathophysiological functions of cystatin C, with focus on its immune regulatory roles at both cellular and molecular levels including antigen presentation, secretion of cytokines, synthesis of nitric oxide, as well as apoptosis. Finally, we discuss the clinical implications and therapeutic potentials of what this predominantly expressed protease inhibitor can bring to us.

Ivermectin treatment of Loa loa hyper-microfilaraemic baboons (Papio anubis): Assessment of microfilarial load reduction, haematological and biochemical parameters and histopathological changes following treatment

BACKGROUND

Individuals with high intensity of Loa loa are at risk of developing serious adverse events (SAEs) post treatment with ivermectin. These SAEs have remained unclear and a programmatic impediment to the advancement of community directed treatment with ivermectin. The pathogenesis of these SAEs following ivermectin has never been investigated experimentally. The Loa/baboon (Papio anubis) model can be used to investigate the pathogenesis of Loa-associated encephalopathy following ivermectin treatment in humans.

METHODS

12 baboons with microfilarial loads > 8,000mf/mL of blood were randomised into four groups: Group 1 (control group receiving no drug), Group 2 receiving ivermectin (IVM) alone, Group 3 receiving ivermectin plus aspirin (IVM + ASA), and Group 4 receiving ivermectin plus prednisone (IVM + PSE). Blood samples collected before treatment and at Day 5, 7 or 10 post treatment, were analysed for parasitological, hematological and biochemical parameters using standard techniques. Clinical monitoring of animals for side effects took place every 6 hours post treatment until autopsy. At autopsy free fluids and a large number of standard organs were collected, examined and tissues fixed in 10% buffered formalin and processed for standard haematoxylin-eosin staining and specific immunocytochemical staining.

RESULTS

Mf counts dropped significantly (p<0.05) in all animals following ivermectin treatment with reductions as high as (89.9%) recorded; while no significant drop was observed in the control animals. Apart from haemoglobin (Hb) levels which recorded a significant (p = 0.028) drop post treatment, all other haematological and biochemical parameters did not show any significant changes (p>0.05). All animals became withdrawn 48 hours after IVM administration. All treated animals recorded clinical manifestations including rashes, itching, diarrhoea, conjunctival haemorrhages, lymph node enlargement, pinkish ears, swollen face and restlessness; one animal died 5 hours after IVM administration. Macroscopic changes in post-mortem tissues observed comprised haemorrhages in the brain, lungs, heart, which seen in all groups given ivermectin but not in the untreated animals. Microscopically, the major cellular changes seen, which were present in all the ivermectin treated animals included microfilariae in varying degrees of degeneration in small vessels. These were frequently associated with fibrin deposition, endothelial changes including damage to the integrity of the blood vessel and the presence of extravascular erythrocytes (haemorrhages). There was an increased presence of eosinophils and other chronic inflammatory types in certain tissues and organs, often in large numbers and associated with microfilarial destruction. Highly vascularized organs like the brain, heart, lungs and kidneys were observed to have more microfilariae in tissue sections. The number of mf seen in the brain and kidneys of animals administered IVM alone tripled that of control animals. Co-administration of IVM + PSE caused a greater increase in mf in the brain and kidneys while the reverse was noticed with the co-administration of IVM + ASA.

CONCLUSIONS

The treatment of Loa hyper-microfilaraemic individuals with ivermectin produces a clinical spectrum that parallels that seen in Loa hyper-microfilaraemic humans treated with ivermectin. The utilization of this experimental model can contribute to the improved management of the adverse responses in humans.

Epitope mapping of Brugia malayi ALT-2 and the development of a multi-epitope vaccine for lymphatic filariasis

Human lymphatic filariasis is a neglected tropical disease, causing permanent and long-term disability with severe immunopathology. Abundant larval transcript (ALT) plays a crucial role in parasite establishment in the host, due to its multi-faceted ability in host immune regulation. Although ALT protein is a key filarial target, its exact function is yet to be explored. Here, we report epitope mapping and a structural model of Brugia malayi ALT-2, leading to development of a multi-epitope vaccine. Structural analysis revealed that ALT represents unique parasitic defence proteins belonging to a toxin family that carries a 'knottin' fold. ALT-2 has been a favourite vaccine antigen and was protective in filarial models. Due to the immunological significance of ALT-2, we mapped B-cell epitopes systematically and identified two epitope clusters, 1-30 and 89-128. To explore the prophylactic potential of epitope clusters, a recombinant multi-epitopic gene comprising the epitopic domains was engineered and the protective efficacy of recombinant ALT epitope protein (AEP) was tested in the permissive model, Mastomys coucha. AEP elicited potent antibody responses with predominant IgG1 isotype and conferred significantly high protection (74.59%) compared to ALT-2 (61.95%). This proved that these epitopic domains are responsible for the protective efficacy of ALT-2 and engineering protective epitopes as a multi-epitope protein may be a novel vaccine strategy for complex parasitic infections.

Parasitological, Hematological and Biochemical Characteristics of a Model of Hyper-microfilariaemic Loiasis (Loa loa) in the Baboon (Papio anubis)

Background

Loiasis, a filarial infection caused by Loa loa usually thought to cause relatively minor morbidity, can cause serious and often fatal reactions in patients carrying very high levels of circulating Loa loa microfilariae (mf) following administration of microfilaricidal drugs. An experimental model of this condition would greatly aid the definition of the optimal management of this important clinical presentation.

Methodology/Principle Findings

Fifteen baboons (Papio anubis) were infected with 600 infective larvae (L3) isolated from Chrysops vector flies. Animals were observed for any clinical changes; blood samples were collected every 1–2 months for 22 months, and analysed for parasitological, hematological and biochemical profiles using standard techniques. All animals became patent but remained clinically normal throughout the study. The parasitological pre-patent period was between 4–8 months, with a majority (60%) of animals becoming patent by 5 months post infection (MPI); all animals were patent by 8 MPI. Microfilarial loads increased steadily in all animals and reached a peak at 18 MPI. By 10 MPI >70% of animals had mf >8,000 mf/mL, and at 18 MPI >70% of animals had mf >30,000mf/mL with 50% of these animals with mf >50,000mf/mL. Absolute eosinophil, creatinine, Ca²⁺ and K⁺ levels were generally above normal values (NV). Positive associations were seen between microfilariaemia and eosinophilia, Hb, Ca²⁺, and gamma-GT values, whilst significant negative associations were seen between microfilariaemia and potassium, glucose and mononuclear leukocyte levels.

Conclusions

Infection of splenectomised baboons with L. loa can induce levels of circulating microfilariae, and corresponding haematological profiles, which parallel those seen in those humans in danger of the severe post-microfilariacide clinical responses. Utilization of this experimental model could contribute to the improved management of the loiasis related adverse responses in humans.

Loiasis is a filarial infection of humans that, in addition to causing severe direct clinical effects, is of concern to the global community’s efforts to eliminate the important filarial diseases, onchocerciasis and lymphatic filariasis, through causing interruption to mass drug distribution activities. Hyper-microfilariaemia has been seen to be the characteristic parameter in patients suffering from post ivermectin encephalopathy, a condition which sometimes leads to death. Understanding and developing appropriate approaches to the treatment and prevention of these severe adverse reactions has been difficult due to the lack of suitable models. As primates can be infected with human L. loa, and can develop hyper-microfilariaemia, it is likely that they therefore can serve as suitable models for the investigation of this syndrome in humans. This current study shows that following splenectomy the circulating microfilarial loads are similar to those seen in humans, and that the clinical pathology profile following infection also appears to be similar. The consistent ability to induce microfilariae levels of above 30,000 mf/ml in more than 70% of the tested animals suggests that this is indeed a practical model for investigating the adverse events occurring in hyper-loiasis.

DNA vaccine encoding the moonlighting protein Onchocerca volvulus glyceraldehyde-3-phosphate dehydrogenase (Ov-GAPDH) leads to partial protection in a mouse model of human filariasis

River blindness, caused by the filarial parasite Onchocerca volvulus, is a major socio-economic and public health problem in Sub-Saharan Africa. In January 2015, The Onchocerciasis Vaccine for Africa (TOVA) Initiative has been launched with the aim of providing new tools to complement mass drug administration (MDA) of ivermectin, thereby promoting elimination of onchocerciasis in Africa. In this context we here present Onchocerca volvulus glyceraldehyde-3-phosphate dehydrogenase (Ov-GAPDH) as a possible DNA vaccine candidate. We report that in a laboratory model for filariasis, immunization with Ov-GAPDH led to a significant reduction of adult worm load and microfilaraemia in BALB/c mice after challenge infection with the filarial parasite Litomosoides sigmodontis. Mice were either vaccinated with Ov-GAPDH.DNA plasmid (Ov-pGAPDH.DNA) alone or in combination with recombinantly expressed Ov-GAPDH protein (Ov-rGAPDH). During the following challenge infection of immunized and control mice with L. sigmodontis, those formulations which included the DNA plasmid, led to a significant reduction of adult worm loads (up to 57% median reduction) and microfilaraemia (up to 94% reduction) in immunized animals. In a further experiment, immunization with a mixture of four overlapping, synthetic Ov-GAPDH peptides (Ov-GAPDHpept), with alum as adjuvant, did not significantly reduce worm loads. Our results indicate that DNA vaccination with Ov-GAPDH has protective potential against filarial challenge infection in the mouse model. This suggests a transfer of the approach into the cattle Onchocerca ochengi model, where it is possible to investigate the effects of this vaccination in the context of a natural host-parasite relationship.

Comparative analysis of the secretome from a model filarial nematode (Litomosoides sigmodontis) reveals maximal diversity in gravid female parasites

Filarial nematodes (superfamily Filarioidea) are responsible for an annual global health burden of ∼6.3 million disability-adjusted life-years, which represents the greatest single component of morbidity attributable to helminths affecting humans. No vaccine exists for the major filarial diseases, lymphatic filariasis and onchocerciasis; in part because research on protective immunity against filariae has been constrained by the inability of the human-parasitic species to complete their lifecycles in laboratory mice. However, the rodent filaria Litomosoides sigmodontis has become a popular experimental model, as BALB/c mice are fully permissive for its development and reproduction. Here, we provide a comprehensive analysis of excretory-secretory products from L. sigmodontis across five lifecycle stages and identifications of host proteins associated with first-stage larvae (microfilariae) in the blood. Applying intensity-based quantification, we determined the abundance of 302 unique excretory-secretory proteins, of which 64.6% were present in quantifiable amounts only from gravid adult female nematodes. This lifecycle stage, together with immature microfilariae, released four proteins that have not previously been evaluated as vaccine candidates: a predicted 28.5 kDa filaria-specific protein, a zonadhesin and SCO-spondin-like protein, a vitellogenin, and a protein containing six metridin-like ShK toxin domains. Female nematodes also released two proteins derived from the obligate Wolbachia symbiont. Notably, excretory-secretory products from all parasite stages contained several uncharacterized members of the transthyretin-like protein family. Furthermore, biotin labeling revealed that redox proteins and enzymes involved in purinergic signaling were enriched on the adult nematode cuticle. Comparison of the L. sigmodontis adult secretome with that of the human-infective filarial nematode Brugia malayi (reported previously in three independent published studies) identified differences that suggest a considerable underlying diversity of potential immunomodulators. The molecules identified in L. sigmodontis excretory-secretory products show promise not only for vaccination against filarial infections, but for the amelioration of allergy and autoimmune diseases.

Molecular characterization and evaluation of Onchocerca volvulus-secreted larval acidic protein 1 (SLAP1) as a putative vaccine candidate on endemic population of lymphatic filariasis

Filarial parasites infected nearly 160 million of the global population with onchocerciasis and lymphatic filariasis, and further, a billion of people are estimated to be at risk of infection, rendering them among the most prevalent infectious agents in the world today. Given the complexity of their life cycle and the immune evasion mechanisms of these organisms, development of a vaccine remains to be a long-term challenge. Though a number of immunodominant antigens have been characterized, the presence of homologous proteins in humans or the allelic variants are some of the major drawbacks. One of the extensively studied vaccine candidates is abundant larval transcripts (ALT) family of proteins for the following properties: highly regulated expression, abundance, excreted-secreted product of infective stage larvae, and essentially for parasite establishment and survival in the host. In the present study, stage-specific expression of secreted larval acidic protein 1 (SLAP1) was identified; an ALT orthologue from Onchocerca volvulus was cloned, expressed, and purified as a recombinant protein. Immunogenicity of OvSLAP1 was demonstrated with sera and peripheral blood mononuclear cells from endemic regions of Brugia malayi and Wuchereria bancrofti. OvSLAP1 antibodies were predominated by IgG1 and IgG2 in endemic normal (EN) and chronic pathology (CP) subjects. It has also induced marked cellular response as observed by lymphoproliferation assay. The study revealed that OvSLAP1 can segregate humoral (EN mean optical density (OD) = 0.87 ± 0.035, CP mean OD = 0.59 ± 0.029) and cellular (EN mean stimulation index (SI) = 5.87 ± 0.167, CP mean SI = 3.5 ± 0.134) immune responses between EN and CP individuals (P < 0.001), signifying its prophylactic ability and vitality for protection from filarial infections in endemic population.

Deletion of parasite immune modulatory sequences combined with immune activating signals enhances vaccine mediated protection against filarial nematodes

Background

Filarial nematodes are tissue-dwelling parasites that can be killed by Th2-driven immune effectors, but that have evolved to withstand immune attack and establish chronic infections by suppressing host immunity. As a consequence, the efficacy of a vaccine against filariasis may depend on its capacity to counter parasite-driven immunomodulation.

Methodology and Principal Findings

We immunised mice with DNA plasmids expressing functionally-inactivated forms of two immunomodulatory molecules expressed by the filarial parasite Litomosoides sigmodontis: the abundant larval transcript-1 (LsALT) and cysteine protease inhibitor-2 (LsCPI). The mutant proteins enhanced antibody and cytokine responses to live parasite challenge, and led to more leukocyte recruitment to the site of infection than their native forms. The immune response was further enhanced when the antigens were targeted to dendritic cells using a single chain Fv-αDEC205 antibody and co-administered with plasmids that enhance T helper 2 immunity (IL-4) and antigen-presenting cell recruitment (Flt3L, MIP-1α). Mice immunised simultaneously against the mutated forms of LsALT and LsCPI eliminated adult parasites faster and consistently reduced peripheral microfilaraemia. A multifactorial analysis of the immune response revealed that protection was strongly correlated with the production of parasite-specific IgG1 and with the numbers of leukocytes present at the site of infection.

Conclusions

We have developed a successful strategy for DNA vaccination against a nematode infection that specifically targets parasite-driven immunosuppression while simultaneously enhancing Th2 immune responses and parasite antigen presentation by dendritic cells.

Filarial infections are endemic in more that 80 countries, affecting over 120 million people and putting 1 billion more at risk. Antifilarial drugs must be administered regularly to infected people to control the disease, but they are contraindicated in under 6 year-olds and in pregnant women. Further, reports of drug resistance are now accumulating. A vaccine would therefore greatly help fight these diseases. Live attenuated L3 filariae larvae can evoke a protective immunity but their production is impractical and use in humans unacceptable while the efficacy of sub-unit vaccines has been poor. Filariae secrete proteins capable of suppressing their host's immune response, and have the potential to interfere with immunisation. We therefore decided to vaccinate hosts against secreted parasite products that modulate host immune responses rather than against structural components of the worms, and to boost the host's immune system by directly enhancing the uptake of parasite material by antigen presenting cells. This strategy generated substantial protection against both adult and offspring of a filarial parasite in mice. This provides a strong proof of principle for the anti-immunomodulatory approach we have developed.

Genomics and transcriptomics across the diversity of the Nematoda

The diversity of biology in nematodes is reflected in the diversity of their genomes. Parasitic species in particular have evolved mechanisms to invade and outwit their hosts, and these offer opportunities for the development of control measures. Genomic analyses can reveal the molecular underpinnings of phenotypes such as parasitism and thus, initiate and support research programmes that explore the manipulation of host and parasite physiologies to achieve favourable outcomes. Wide sampling across nematode diversity allows phylogenetically informed formulation of research hypotheses, identification of core features shared by all species or important evolutionary novelties present in isolated clades. Many nematode species have been investigated through the use of the expressed sequence tag approach, which samples from the transcribed genome. Gene catalogues generated in this way can be explored to reveal the patterns of expression associated with parasitism and candidates for testing as drug targets or vaccine components. Analysis environments, such as NEMBASE facilitate exploitation of these data. The development of new high-throughput DNA-sequencing technologies has facilitated transcriptomic and genomic approaches to parasite biology. Whole genome sequencing offers more complete catalogues of genes and assists a systems approach to phenotype dissection. These efforts are being coordinated through the 959 Nematode Genomes initiative.

Mitochondrial genes for heme-dependent respiratory chain complexes are up-regulated after depletion of Wolbachia from filarial nematodes

The filarial nematodes Brugia malayi, Wuchereria bancrofti and Onchocerca volvulus cause elephantiasis or dermatitis and blindness resulting in severe morbidity. Annually, 1.3 billion people are at risk of infection. Targeting the essential Wolbachia endobacteria of filarial nematodes with doxycycline has proven to be an effective therapy resulting in a block in embryogenesis, worm development and macrofilaricidal effects. However, doxycycline is contraindicated for a large portion of the at risk population. To identify new targets for anti-wolbachial therapy, understanding the molecular basis of the Wolbachia-filaria symbiosis is required. Using the B. malayi microarray we identified differentially expressed genes in the rodent filaria Litomosoides sigmodontis after depletion of Wolbachia which might have a role in symbiosis. The microarray data were filtered for regulated genes with a false discovery rate <5% and a > or = 2-fold-change. Most of the genes were differentially expressed at day 36 of tetracycline treatment, when 99.8% of Wolbachia were depleted. Several classes of genes were affected, including genes for translation, transcription, folding/sorting of proteins, motility, structure and metabolic and signalling pathways. Quantitative PCR validated 60% of the genes found to be regulated in the microarray. A nuclear encoded heme-binding protein of the globin family was up-regulated upon loss of Wolbachia. Interestingly, mitochondrial encoded subunits of respiratory chain complexes containing heme and riboflavin were also up-regulated. No change in the expression of these genes was seen in tetracycline treated Wolbachia-free Acanthocheilonema viteae. As Wolbachia synthesise heme and filaria do not, we hypothesise that without the endosymbionts no functional heme-containing enzymes can be formed, leading to loss of energy metabolism which then results in up-regulation of the mitochondrial encoded subunits in an attempt to correct the deviation from homeostasis. Our results support targeting the Wolbachia heme synthesis pathway for the discovery of new anti-filarial drugs.

Brugia malayi excreted/secreted proteins at the host/parasite interface: stage- and gender-specific proteomic profiling

Relatively little is known about the filarial proteins that interact with the human host. Although the filarial genome has recently been completed, protein profiles have been limited to only a few recombinants or purified proteins of interest. Here, we describe a large-scale proteomic analysis using microcapillary reverse-phase liquid chromatography-tandem-mass spectrometry to identify the excretory-secretory (ES) products of the L3, L3 to L4 molting ES, adult male, adult female, and microfilarial stages of the filarial parasite Brugia malayi. The analysis of the ES products from adult male, adult female, microfilariae (Mf), L3, and molting L3 larvae identified 852 proteins. Annotation suggests that the functional and component distribution was very similar across each of the stages studied; however, the Mf contributed a higher proportion to the total number of identified proteins than the other stages. Of the 852 proteins identified in the ES, only 229 had previous confirmatory expressed sequence tags (ESTs) in the available databases. Moreover, this analysis was able to confirm the presence of 274 “hypothetical” proteins inferred from gene prediction algorithms applied to the B. malayi (Bm) genome. Not surprisingly, the majority (160/274) of these “hypothetical” proteins were predicted to be secreted by Signal IP and/or SecretomeP 2.0 analysis. Of major interest is the abundance of previously characterized immunomodulatory proteins such as ES-62 (leucyl aminopeptidase), MIF-1, SERPIN, glutathione peroxidase, and galectin in the ES of microfilariae (and Mf-containing adult females) compared to the adult males. In addition, searching the ES protein spectra against the Wolbachia database resulted in the identification of 90 Wolbachia-specific proteins, most of which were metabolic enzymes that have not been shown to be immunogenic. This proteomic analysis extends our knowledge of the ES and provides insight into the host–parasite interaction.

Human lymphatic filariasis caused by the nematode parasites Brugia malayi and Wuchereria bancrofti are a major cause of concern in tropical countries. Studies over several decades have identified various proteins of these parasites that have highlighted their role in host–parasite interactions and possible chemotherapeutic and prophylactic interventions. The availability of the parasite genome facilitates the identification of all of the proteins of the parasite that could interact with the host. In this study, we have attempted to identify the excretory-secretory proteins of the various stages of the parasite that could be maintained in vitro for a limited period utilizing a high-throughput proteomics approach. We observe and report that the parasites expend resources to secrete out various molecules that they utilize to evade the host immune system and modulate its responses. Further, this study also provides information on the predicted hypothetical proteins to be bonafide proteins and thus a catalogue of the excretory-secretory proteins towards a better understanding of the host–parasite interactions.

Screening of different classes of proteases in microfilarial and adult stages of Setaria cervi

Many of the filarial proteases involved in critical physiological functions are expressed in stage-specific manner and belong to various mechanistic classes. Setaria cervi, a bovine filarial parasite express different classes of proteases. This parasite shows strong antigenic cross-reactivity with human filarial parasites Wuchereria bancrofti and Brugia malayi. Somatic extracts of S. cervi microfilariae (mf) and adult stages as well as their excretory-secretory (ES) products were screened for the presence of different classes of proteases using general (casein, bovine hemoglobin) and class specific substrates. Detergent-soluble extracts of male and female worms were also screened. Significant enzyme activity was detected in ES products both at pH 5.0 and 7.0 with casein. Cathepsin B-like activity was found to be much higher in membrane-bound extract than in the crude-soluble extract. However, it was also found to be actively secreted by both mf and adult worms. Cathepsin D-like activity assayed at pH 3.0 was very low both in somatic extract as well as in ES products. Collagenase activity at neutral pH showed higher levels, both in somatic extract and ES products. Cathepsin L-like activity was detected only in crude-soluble extract but was below detectable limit in ES products. Leucine aminopeptidase activity was significant both in crude-soluble extract and ES products. This study, thus, might be helpful for a better understanding of host-parasite interaction and identification of appropriate virulence factors that may be targeted as vaccine and/or drug targets against lymphatic filariasis.

Four abundant novel transcript genes from Toxocara canis with unrelated coding sequences share untranslated region tracts implicated in the control of gene expression

The Toxocara canis "abundant novel transcripts" (ant) are four highly expressed products, constituting >18% of ESTs from the infective stage of this widely prevalent nematode parasite. Using 5' RACE, we determined full-length sequences for each ant gene, between 1.8 and 2.8kb. The four genes (termed ant-3, -5, -30 and -34), share no coding sequence similarity, although their 3'UTRs (untranslated regions) are homologous. Predicted ANT-5 and ANT-30 proteins show distant similarity to RNA regulatory proteins, RNA-dependent RNA polymerase and DEAH-box helicase, respectively. Surprisingly, ant-3 appears to be bi-cistronic, encoding two ORFs (ANT-3.1 and -3.2), each with a predicted N-terminal signal sequence. Antibodies raised to recombinant proteins did not react with native parasite products, indicating that protein expression did not accord with transcript abundance. However, antibody reactivity to two gene products (ANT-3.1 and ANT-34) was present in patient sera, suggesting that these proteins are synthesized later in infection. To test whether 3'UTRs may regulate expression, the ant-34 3'UTR sequence was inserted adjacent to enhanced green fluorescent protein (EGFP) for transformation of Caenorhabditis elegans. The ant-34 3'UTR greatly reduced EGFP expression, inhibiting both transcription and translation. We identified a tract in this UTR with significant sequence complementarity to the C. elegans micro-RNA lin-4. While infective stage parasites stockpile high levels of the ant transcripts, we suggest that translation is repressed, possibly by a mechanism involving 3' UTR motifs shared by the four genes.

The use of Caenorhabditis elegans in parasitic nematode research

There is increasing interest in the use of the free-living nematode Caenorhabditis elegans as a tool for parasitic nematode research and there are now a number of compelling examples of its successful application. C. elegans has the potential to become a standard tool for molecular helminthology researchers, just as yeast is routinely used by molecular biologists to study vertebrate biology. However, in order to exploit C. elegans in a meaningful manner, we need a detailed understanding of the extent to which different aspects of C. elegans biology have been conserved with particular groups of parasitic nematodes. This review first considers the current state of knowledge regarding the conservation of genome organisation across the nematode phylum and then discusses some recent evolutionary development studies in free-living nematodes. The aim is to provide some important concepts that are relevant to the extrapolation of information from C. elegans to parasitic nematodes and also to the interpretation of experiments that use C. elegans as a surrogate expression system. In general, examples have been specifically chosen because they highlight the importance of careful experimentation and interpretation of data. Consequently, the focus is on the differences that have been found between nematode species rather than the similarities. Finally, there is a detailed discussion of the current status of C. elegans as a heterologous expression system to study parasite gene function and regulation using successful examples from the literature.

Of mice, cattle, and humans: the immunology and treatment of river blindness

River blindness is a seriously debilitating disease caused by the filarial parasite Onchocerca volvulus, which infects millions in Africa as well as in South and Central America. Research has been hampered by a lack of good animal models, as the parasite can only develop fully in humans and some primates. This review highlights the development of two animal model systems that have allowed significant advances in recent years and hold promise for the future. Experimental findings with Litomosoides sigmodontis in mice and Onchocerca ochengi in cattle are placed in the context of how these models can advance our ability to control the human disease.

Brugia malayi: Effects of radiation and culture on gene expression in infective larvae

Third-stage infective larvae (L3i) of Brugia malayi are developmentally arrested in mosquitoes but must quickly adapt to a new environment when they enter mammalian hosts to initiate infections. These changes can be studied by in vitro culture of L3 (L3c) under conditions that permit molting of L3-L4. Irradiated L3 (L3ir) have stunted growth and limited lifespan in mammalian hosts, and they induce high levels of immunity to challenge infections in animal models. This study explored differences in gene expression in L3i, L3c and L3ir by expressed sequence tag EST generation and qRT-PCR. 2506 ESTs generated from cDNA libraries constructed from L3i, L3c and L3ir were grouped into 1309 gene clusters. Despite extensive prior sampling from B. malayi (>22,000 ESTs in dbEST), 73% of the L3 clusters described here are novel. Sixty-three percentage of the clusters have homology to proteins from other species including 187 specific to nematodes and 141 that have to date only been described in non-nematode species. The transcript levels of 62 candidates for up- or down-regulation in L3i, L3c and L3ir based on EST frequencies were evaluated by qRT-PCR. Twenty-eight were confirmed to have > or = 3-fold differences in expression. Genes coding for proteins believed to be involved in establishment of infection, host adaptation and targets of protective immunity were confirmed to have higher expression in L3i than in L3c. Some of the genes that were down-regulated in L3c were highly expressed in L3ir. This study provides an improved description of the adaptations that accompany the transition from L3i to L3c and the special ability of L3ir to induce protective immunity.

Differential display of genes expressed in the filarial nematode Litomosoides sigmodontis reveals a putative phosphate permease up-regulated after depletion of Wolbachia endobacteria

Mutualist symbiotic Wolbachia endobacteria are found in most filarial nematodes. Wolbachia are essential for embryogenesis and for larval development into adults, and thus represent a new target for anti-filarial drug development. Tetracycline antibiotics deplete Wolbachia in animal model filaria Litomosoides sigmodontis and Brugia pahangi, as well as in the human parasites Brugia malayi, Onchocerca volvulus and Wuchereria bancrofti. Very little is known about the molecular details of the symbiotic interaction between Wolbachia and filarial nematodes. Nematode genes that respond to anti-Wolbachia antibiotic treatment may play important roles in the symbiosis. Differential display PCR was used to detect several candidate genes that are up-regulated after 3, 6, 15, 30 and 36 days of tetracycline treatment. One of these genes, Ls-ppe-1, was similar to a family of phosphate permeases, and had putative orthologues in O. volvulus and B. malayi. Ls-ppe-1 steady-state mRNA levels were elevated by day 3-6 of treatment, and remained elevated through to 70 days post-treatment. In Caenorhabditis elegans, the knockdown of a homologous phosphate permease results in embryonic lethality, with the production of degenerating embryos, a phenotype also seen in filarial nematodes after depletion of Wolbachia with tetracycline. The potential role of Ls-ppe-1 in the nematode-bacterial symbiosis is discussed.

Profiling of gender-regulated gene transcripts in the filarial nematode Brugia malayi by cDNA oligonucleotide array analysis

Microarray technology permits high-throughput comparisons of gene expression in different parasite stages or sexes and has been used widely. We report the first use of this technology for analysis of gene expression in filarial male and female worms. The slide array (comprised of 65-mer oligos representing 3569 EST clusters) was spotted with sequences selected from the extensive Brugia malayi EST database (). Arrays were hybridized with Cy dye labeled male and female cDNA. The experimental design included both biological and technical (dye-flip) replicates. The data were normalized for background and probe intensity, and the relative abundance of hybridized cDNA for each spot was determined. Genes showing two-fold or greater differences with P<0.05 were considered gender-regulated candidates. One thousand one hundred and seventy of 2443 clusters (48%) with signals above threshold in at least one sex were considered as gender-regulated gene candidates. This included 520 and 650 clusters up-regulated in male and female worms, respectively. Fifty of 53 (94%) gender-regulated candidate genes identified by microarray analysis were confirmed by real-time RT-PCR. Approximately 61% of gender-regulated genes had significant similarity to known genes in other organisms such as Caenorhabditis elegans. Many C. elegans homologues of these genes have been reported to have reproductive phenotypes (sterility or abnormal embryo development) by RNA interference. This study has provided the first broad view of gender-regulated gene expression in B. malayi; this should lead to improved understanding of reproduction in filarial nematodes. More generally, this approach holds great promise as a means of studying stage-specific or tissue-specific gene expression in parasitic nematodes.

RNA interference targeting cathepsin L and Z-like cysteine proteases of Onchocerca volvulus confirmed their essential function during L3 molting

We describe the successful use of RNA interference (RNAi) to investigate gene function in the human filarial parasite Onchocerca volvulus third-stage larvae (L3). We targeted two specific gene products, the O. volvulus cathepsin L (Ov-CPL) and cathepsin Z-like (Ov-CPZ) cysteine proteases, which were proposed to function during O. volvulus L3 molting. We show that fluorescent-labeled Cy3-dsRNA corresponding to cpl or cpz regions encoding the mature enzymes can enter the larvae. The molting rate of larvae treated overnight with 0.5 mg ml(-1) cpl was reduced by 92% and 86% in comparison to normal control worms. It appeared that although the larvae started the molting process the last stage of molting, ecdysis was inhibited. The effect was gene specific, as larvae that did not molt in the presence of cpl or cpz dsRNA expressed the other cysteine protease, CPZ and CPL, respectively. This was confirmed by immunoelectron microscopy using antibodies directed against each enzyme. Our present study validate conclusively that both enzymes are essential for the molting of O. volvulus L3 to fourth-stage larvae. We also confirmed that the activity of the enzymes is specific to the changes that occur during the molting process on days 1-3, when the separation between the cuticles is in progress. The development of RNAi in O. volvulus L3 could further help study many of the abundant L3 and molting L3 genes identified through the filarial genome project, many of which, although have no attributed function, were identified as vaccine candidates or potential drug targets.

Blood-feeding in the young adult filarial worms Litomosoides sigmodontis

In this study with the filarial model Litomosoides sigmodontis, we demonstrate that the worms ingest host red blood cells at a precise moment of their life-cycle, immediately after the fourth moult. The red blood cells (RBC) were identified microscopically in live worms immobilized in PBS at 4 degrees C, and their density assessed. Two hosts were used: Mongolian gerbils, where microfilaraemia is high, and susceptible BALB/c mice with lower microfilaraemia. Gerbils were studied at 12 time-points, between day 9 post-inoculation (the worms were young 4th stage larvae) and day 330 p.i. (worms were old adults). Only the very young adult filarial worms had red blood cells in their gut. Haematophagy was observed between days 25 and 56 p.i. and peaked between day 28 and day 30 p.i. in female worms. In males, haematophagy was less frequent and intense. Similar kinetics of haematophagy were found in BALB/c mice, but frequency and intensity tended to be lower. Haematophagy seems useful to optimize adult maturation. These observations suggest that haematophagy is an important step in the life-cycle of L. sigmodontis. This hitherto undescribed phenomenon might be characteristic of other filarial species including human parasites.

PartiGeneDB--collating partial genomes

Owing to the high costs involved, only 28 eukaryotic genomes have been fully sequenced to date. On the other hand, an increasing number of projects have been initiated to generate survey sequence data for a large number of other eukaryotic organisms. For the most part, these data are poorly organized and difficult to analyse. Here, we present PartiGeneDB (http://www.partigenedb.org), a publicly available database resource, which collates and processes these sequence datasets on a species-specific basis to form non-redundant sets of gene objects—which we term partial genomes. Users may query the database to identify particular genes of interest either on the basis of sequence similarity or via the use of simple text searches for specific patterns of BLAST annotation. Alternatively, users can examine entire partial genome datasets on the basis of relative expression of gene objects or by the use of an interactive Java-based tool (SimiTri), which displays sequence similarity relationships for a large number of sequence objects in a single graphic. PartiGeneDB facilitates regular incremental updates of new sequence datasets associated with both new and exisitng species. PartiGeneDB currently contains the assembled partial genomes derived from 1.83 million sequences associated with 247 different eukaryotes.

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.

Increased early local immune responses and altered worm development in high-dose infections of mice susceptible to the filaria Litomosoides sigmodontis

The relationship between the number of larvae inoculated and filarial infection outcome is an important fundamental and epidemiological issue. Our study was carried out with BALB/c mice infected with the filaria Litomosoides sigmodontis. For the first time, an immunological analysis of infection with various doses was studied in parallel with parasitological data. Mice were inoculated with 200, 60 or 25 infective larvae (third stage larvae, L3), and monitored over 80 days. At 60 h post-inoculation the immune response was stronger in the 200 L3 group than the 25 L3 group. Cells from lymph nodes draining the site of inoculation proliferated intensely and produced large amounts of IL-5 and IL-4. In the pleural cavity, leukocyte populations accumulated earlier and in larger quantities. IgG1, IL-4 and IL-10 serum concentrations were transiently higher. During the first 10 days the worm recovery rates were identical in all groups, but decreased thereafter in the 200 L3 group. In this group, the development of the worms was altered, with reduced lengths, diminished intra-uterine production of microfilariae and abnormalities of male copulatory organs. Whereas mice inoculated with 25 L3 became microfilaraemic, only one third reached patency in the 200 L3 group. However, detrimental effects of high numbers of worms are not seen in studies using different inoculation protocols. This suggests that the very early events determine subsequent immune response and infection outcome rather than competitive interactions between the worms.

A gene family of cathepsin L-like proteases of filarial nematodes are associated with larval molting and cuticle and eggshell remodeling

Cysteine proteinases are involved in a variety of important biological processes and have been implicated in molting and tissue remodeling in free living and parasitic nematodes. We show that in the lymphatic filarial nematode Brugia pahangi molting of third-stage larvae (L3) to fourth-stage larvae is dependent on the activity of a cathepsin L-like cysteine protease (CPL), which can be detected in the excretory/secretory (ES) products of molting L3. Directed cloning of a cysteine protease gene in B. pahangi and analysis of the expressed sequence tag (EST) and genomic sequences of the closely related human lymphatic filarial nematode Brugia malayi have identified a family of CPLs. One group of these enzymes, Bm-cpl-1, -4, -5 and Bp-cpl-4, is highly expressed in the B. malayi and B. pahangi infective L3 larvae. Immunolocalization indicates that the corresponding enzymes are synthesized and stored in granules of the glandular esophagus of L3 and released during the molting process. Functional analysis of these genes in Brugia and closely related CPL genes identified in the filarial nematode Onchocerca volvulus and the free living model nematode Caenorhabditis elegans indicate that these genes are also involved in cuticle and eggshell remodeling.

Signal sequence analysis of expressed sequence tags from the nematode Nippostrongylus brasiliensis and the evolution of secreted proteins in parasites

BACKGROUND

Parasitism is a highly successful mode of life and one that requires suites of gene adaptations to permit survival within a potentially hostile host. Among such adaptations is the secretion of proteins capable of modifying or manipulating the host environment. Nippostrongylus brasiliensis is a well-studied model nematode parasite of rodents, which secretes products known to modulate host immunity.

RESULTS

Taking a genomic approach to characterize potential secreted products, we analyzed expressed sequence tag (EST) sequences for putative amino-terminal secretory signals. We sequenced ESTs from a cDNA library constructed by oligo-capping to select full-length cDNAs, as well as from conventional cDNA libraries. SignalP analysis was applied to predicted open reading frames, to identify potential signal peptides and anchors. Among 1,234 ESTs, 197 (~16%) contain predicted 5' signal sequences, with 176 classified as conventional signal peptides and 21 as signal anchors. ESTs cluster into 742 distinct genes, of which 135 (18%) bear predicted signal-sequence coding regions. Comparisons of clusters with homologs from Caenorhabditis elegans and more distantly related organisms reveal that the majority (65% at P < e-10) of signal peptide-bearing sequences from N. brasiliensis show no similarity to previously reported genes, and less than 10% align to conserved genes recorded outside the phylum Nematoda. Of all novel sequences identified, 32% contained predicted signal peptides, whereas this was the case for only 3.4% of conserved genes with sequence homologies beyond the Nematoda.

CONCLUSIONS

These results indicate that secreted proteins may be undergoing accelerated evolution, either because of relaxed functional constraints, or in response to stronger selective pressure from host immunity.

Resistance and susceptibility to filarial infection with Litomosoides sigmodontis are associated with early differences in parasite development and in localized immune reactions

In order to understand natural resistance to filariasis, we compared Litomosoides sigmodontis primary infection of C57BL/6 mice, which eliminate the worms before patency, and BALB/c mice, in which worms complete their development and produce microfilariae. Our analysis over the first month of infection monitoredmigration of the infective larvae from the lymph nodes to the pleural cavity, where the worms settle. Although immune responses from the mouse strains differed from the outset, the duration of lymphatic migration (4 days) and filarial recovery rates were similar, thus confirming that the proportion of larvae that develop in the host species upon infection is not influenced by host genetic variability. The majority of worms reached the adult stage in both mouse strains; however, worm growth and molting were retarded in resistant C57BL/6 mice. Surprisingly, the only immune responses detected at 60 h postinfection occurred in the susceptible mice and only upon stimulation of cells from lymph nodes draining the inoculation site with infective larva extract: massive production of interleukin-6 (IL-6) and IL-5 (the latter cytokine was previously suspected to have an effect on L. sigmodontis growth). However, between days 10 and 30 postinfection, extraordinarily high levels of type 1 and type 2 cytokines and expansion of pleural leukocyte infiltration were seen in the resistant C57BL/6 mice, explaining the destruction of worms later. Our results suggest that events early in the infection determine susceptibility or resistance to subsequent microfilarial production and a parasite strategy to use specific immune responses to its own benefit.

Trehalose metabolism genes in Caenorhabditis elegans and filarial nematodes

The sugar trehalose is claimed to be important in the physiology of nematodes where it may function in sugar transport, energy storage and protection against environmental stresses. In this study we investigated the role of trehalose metabolism in nematodes, using Caenorhabditis elegans as a model, and also identified complementary DNA clones putatively encoding genes involved in trehalose pathways in filarial nematodes. In C. elegans two putative trehalose-6-phosphate synthase (tps) genes encode the enzymes that catalyse trehalose synthesis and five putative trehalase (tre) genes encode enzymes catalysing hydrolysis of the sugar. We showed by RT-PCR or Northern analysis that each of these genes is expressed as mRNA at all stages of the C. elegans life cycle. Database searches and sequencing of expressed sequence tag clones revealed that at least one tps gene and two tre genes are expressed in the filarial nematode Brugia malayi, while one tps gene and at least one tre gene were identified for Onchocerca volvulus. We used the feeding method of RNA interference in C. elegans to knock down temporarily the expression of each of the tps and tre genes. Semiquantitative RT-PCR analysis confirmed that expression of each gene was silenced by RNA interference. We did not observe an obvious phenotype for any of the genes silenced individually but gas-chromatographic analysis showed >90% decline in trehalose levels when both tps genes were targeted simultaneously. This decline in trehalose content did not affect viability or development of the nematodes.

Abundant larval transcript-1 and -2 genes from Brugia malayi: diversity of genomic environments but conservation of 5' promoter sequences functional in Caenorhabditis elegans

The genomic organisation of two abundant larval transcript (alt) genes from the filarial nematode Brugia malayi has been defined. The products of these genes are 78% identical in amino acid sequence, and are highly expressed in a stage-specific manner by mosquito-borne infective larvae. alt-1 is present as two near-identical copies organised in an inverted repeat of approximately 7.6 kb, occupying a total of 16 kb of the genome. alt-2 is a single-copy gene at a different locus to alt-1. The two alt-1 genes (alt-1.1 and -1.2) are 99.7% identical in coding sequence and 99.5% in intronic sequences. Both alt-1 and -2 contain 3 introns, and the third intron of alt-2 exhibits a size polymorphism evident in different individual parasites from the laboratory-maintained strain. Genomic sequence up- and down-stream from alt-1.1/1.2 (26 and 6 kb, respectively) and alt-2 (6 and 4 kb, respectively) show that neither gene is in a multiple array or an operon. Most notably, the neighbouring genes of alt-1 and -2 show no similarity to each other, or to the genes flanking the distant alt homologue in Caenorhabditis elegans. Despite this diversity in flanking genes, the 5' UTR tracts extending some 800 bp upstream of each B. malayi alt gene show a high degree of similarity (overall 59% identity with tracts of 77-86% identity). Surmising that this region may contain conserved promoter elements, constructs containing the B. malayi alt 5' UTR with or without coding sequence were made fused to beta-galactosidase reporter protein. These constructs were injected into the syncytical gonad of C. elegans and progeny stained for beta-gal expression. Our results show relatively strong expression in the gut cells of C. elegans for both alt-1 and -2 constructs, commencing in larval worms and continuing into adulthood. Moreover, expression was enhanced when constructs contained segments of alt-1 coding and intronic sequence in addition to the 5' UTR. We conclude that the high level of alt transcription in filarial L3s is not due to expression from a multi-copy gene family but to a set of strong promoter elements shared between the two alt genes.

The Brugia malayi genome project: expressed sequence tags and gene discovery

To advance and facilitate molecular studies of Brugia malayi, one of the causative agents of human lymphatic filariasis, an expressed sequence tag (EST)-based gene discovery programme has been carried out. Over 22,000 ESTs have been produced and deposited in the public databases by a consortium of laboratories from endemic and non-endemic countries. The ESTs have been analysed using custom informatic tools to reveal patterns of individual gene expression that may point to potential targets for future research on anti-filarial drugs and vaccines. Many genes first discovered as ESTs are now being analysed by researchers for immunodiagnostic, vaccine and drug target potential. Building on the success of the B. malayi EST programme, significant EST datasets are being generated for a number of other major parasites of humans and domesticated animals, and model parasitic species.

Caenorhabditis elegans: how good a model for veterinary parasites?

The organism about which most is known on a molecular level is a nematode, the free-living organism Caenorhabditis elegans. This organism has served as a reasonable model for the discovery of anthelmintic drugs and for research on the mechanism of action of anthelmintics. Useful information on mechanisms of anthelmintic resistance has also been obtained from studies on C. elegans. Unfortunately, there has not been a large-scale extension of genetic techniques developed in C. elegans to research on parasitic species of veterinary (or human) parasites. Much can be learned about the essentials of nematode biology by studying C. elegans, but discovering the basic biology of nematode parasitism can only be gained through comparative studies on multiple parasitic species.

Expression and immune recognition of Brugia malayi VAL-1, a homologue of vespid venom allergens and Ancylostoma secreted proteins

Several important nematode parasites have been found to express members of a gene family variously termed as venom allergen antigen homologue (vah) or Ancylostoma secreted protein (asp). In some cases these products are secreted by infective larval stages and have been suggested to be effective vaccine immunogens. We isolated the corresponding gene from the human filarial nematode, Brugia malayi, by first searching the expressed sequence tag (EST) dataset generated by the Filarial Genome Project and then using gene-specific nondegenerate primers matching the selected gene for PCR, from B. malayi cDNA libraries. We report here the full-length gene sequence, which we have designated as Bm-val-1, for vah/asp-like. The corresponding protein (Bm-VAL-1) contains 232 amino acids in a single homology unit, unlike products from some other species in which there is a tandem repeat. A putative signal sequence is present at the 5' end and there are two potential N-glycosylation sites. Murine antibodies to recombinant Bm-VAL-1 react with a 28 kDa protein in L3 extracts and recombinant Bm-VAL-1 is recognised by murine T cells primed with soluble L3 proteins. Of 82 ESTs corresponding to Bm-val-1, 72 are recorded from the infective larval (L3) stage. However, PCR on the first-strand cDNA from later mammalian stages revealed some expression at most subsequent time points. Over 95% (20/21) of microfilaraemic human filariasis patients are seropositive for antibodies to Bm-VAL-1, with particularly high levels of IgG3 and IgG4 isotypes. The IgG4 subclass may indicate stimulation by adult and/or microfilarial-derived immunogens. The association of Bm-VAL-1 with the infective stage and its recognition by humans exposed to filariasis suggests that further evaluation of this antigen as a vaccine candidate should be performed.

Antibody responses in onchocerciasis as a function of age and infection intensity

Onchocerciasis is caused by the filarial nematode Onchocerca volvulus and is a major public health problem in West and Central Africa. With only partial and long-term treatment currently available, there is a need to develop a suitable vaccine. We analysed the antibody response to infective L3 larvae because this stage is thought to be associated with host protective immunity. In addition, we have related our findings to the age, gender and current infection intensity of our participants: variables that may significantly influence antibody production. Interestingly, whilst 90% of our study group were seropositive for adult specific immunoglobulin (Ig)E, only 23% produced L3 specific IgE. This is in contrast to IgG4 where seropositivity was comparable at 96% and 92%, respectively. Furthermore, IgG levels were significantly affected by age and the intensity of infection but unaffected by host gender. This finding is independent for the IgG subclass (IgG1, IgG2, IgG3 and IgG4) and its specificity (L3 versus adult antigen). In summary, we show that L3 larvae induce little specific IgE and the antibody response shows a different isotype balance than that against adult antigens. Both host and parasite variables can influence antibody production in this disease.

Caenorhabditis elegans and the study of gene function in parasites

The free-living nematode Caenorhabditis elegans is a tractable experimental model system for the study of both vertebrate and invertebrate biology. Its most significant advantages are its simplicity, both in anatomy and in genomic organization, and the elaborate methods that have been developed to attribute function to previously uncharacterized genes. Importantly, > 40% of parasitic nematode genes exhibit high levels of homology to genes within the C. elegans genome. Studying such genes using the C. elegans model should yield new insights into key molecules and their possible implications in parasite survival, leading to the discovery of new drug targets and vaccine candidates.

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.

Bm-CPI-2, a cystatin homolog secreted by the filarial parasite Brugia malayi, inhibits class II MHC-restricted antigen processing

While interference with the class I MHC pathway by pathogen-encoded gene products, especially those of viruses, has been well documented, few examples of specific interference with the MHC class II pathway have been reported. Potential targets for such interference are the proteases that remove the invariant chain chaperone and generate antigenic peptides. Indeed, recent studies indicate that immature dendritic cells express cystatin C to modulate cysteine protease activity and the expression of class II MHC molecules [1]. Here, we show that Bm-CPI-2, a recently discovered cystatin homolog produced by the filarial nematode parasite Brugia malayi (W. F. Gregory et al., submitted), inhibits multiple cysteine protease activities found in the endosomes/lysosomes of human B lymphocyte lines. CPI-2 blocked the hydrolysis of synthetic substrates favored by two different families of lysosomal cysteine proteases and blocked the in vitro processing of the tetanus toxin antigen by purified lysosome fractions. Moreover, CPI-2 substantially inhibited the presentation of selected T cell epitopes from tetanus toxin by living antigen-presenting cells. Our studies provide the first example of a product from a eukaryotic parasite that can directly interfere with antigen presentation, which, in turn, may suggest how filarial parasites might inactivate the host immune response to a helminth invader.