Characterisation of an immunodominant glycoprotein antigen of Onchocerca volvulus with homologues in other filarial nematodes and Caenorhabditis elegans

Function of lipid binding proteins of parasitic helminths: still a long road

Infections with parasitic helminths cause severe debilitating and sometimes lethal diseases in humans and domestic animals on a global scale. Unable to synthesize de novo their own fatty acids and sterols, helminth parasites (nematodes, trematodes, cestodes) rely on their hosts for their supply. These organisms produce and secrete a wide range of lipid binding proteins that are, in most cases, structurally different from the ones found in their hosts, placing them as possible novel therapeutic targets. In this sense, a lot of effort has been made towards the structure determination of these proteins, but their precise function is still unknown. In this review, we aim to present the current knowledge on the functions of LBPs present in parasitic helminths as well as novel members of this highly heterogeneous group of proteins.

Fatty acid and retinol-binding protein: A novel antigen for immunodiagnosis of human strongyloidiasis

The tenacious human parasitic helminth Strongyloides stercoralis is a significant health problem worldwide. The current lack of a definitive diagnostic laboratory test to rule out this infection necessitates designing more specific diagnostic methods. Fatty acid and retinol-binding protein (FAR) plays a crucial role in the development and reproduction of nematodes. We generated a recombinant form of this protein and determined its applicability for immunodiagnosis of S. stercoralis. The L3 form of S. stercoralis was harvested and used for RNA extraction and cDNA synthesis. The coding sequence of S. stercoralis FAR (SsFAR) was cloned into pET28a(+) vector, expressed in E. coli BL21 and purified. ELISA and immunoblotting were employed to determine the specificity and sensitivity of rSsFAR using a set of defined sera. In addition, we analyzed the phylogenetic relationship of SsFAR with different FAR sequences from other nematodes. The cloned SsFAR had an open reading frame of 447 bp encoding 147 amino acids, with a deduced molecular mass of 19 kD. The SsFAR amino acid sequence was 93% identical to FAR of S. ratti. For differential immunodiagnosis of strongyloidiasis, rSsFAR exhibited 100% sensitivity and 97% specificity. However, cross-reactivity with FAR proteins of other parasites, namely Toxocara canis and Echinococcus granulosus, was noted. Our results provide a novel approach for immunodiagnosis of S. stercoralis infections using rSsFAR with reliable sensitivity and specificity.

Ligand binding properties of two Brugia malayi fatty acid and retinol (FAR) binding proteins and their vaccine efficacies against challenge infection in gerbils

Parasitic nematodes produce an unusual class of fatty acid and retinol (FAR)-binding proteins that may scavenge host fatty acids and retinoids. Two FARs from Brugia malayi (Bm-FAR-1 and Bm-FAR-2) were expressed as recombinant proteins, and their ligand binding, structural characteristics, and immunogenicities examined. Circular dichroism showed that rBm-FAR-1 and rBm-FAR-2 are similarly rich in α-helix structure. Unexpectedly, however, their lipid binding activities were found to be readily differentiated. Both FARs bound retinol and cis-parinaric acid similarly, but, while rBm-FAR-1 induced a dramatic increase in fluorescence emission and blue shift in peak emission by the fluorophore-tagged fatty acid (dansyl-undecanoic acid), rBm-FAR-2 did not. Recombinant forms of the related proteins from Onchocerca volvulus, rOv-FAR-1 and rOv-FAR-2, were found to be similarly distinguishable. This is the first FAR-2 protein from parasitic nematodes that is being characterized. The relative protein abundance of Bm-FAR-1 was higher than Bm-FAR-2 in the lysates of different developmental stages of B. malayi. Both FAR proteins were targets of strong IgG1, IgG3 and IgE antibody in infected individuals and individuals who were classified as endemic normal or putatively immune. In a B. malayi infection model in gerbils, immunization with rBm-FAR-1 and rBm-FAR-2 formulated in a water-in-oil-emulsion (®Montanide-720) or alum elicited high titers of antigen-specific IgG, but only gerbils immunized with rBm-FAR-1 formulated with the former produced a statistically significant reduction in adult worms (68%) following challenge with B. malayi infective larvae. These results suggest that FAR proteins may play important roles in the survival of filarial nematodes in the host, and represent potential candidates for vaccine development against lymphatic filariasis and related filarial infections.

Characterization of Three Novel Fatty Acid- and Retinoid-Binding Protein Genes (Ha-far-1, Ha-far-2 and Hf-far-1) from the Cereal Cyst Nematodes Heterodera avenae and H. filipjevi

Heterodera avenae and H. filipjevi are major parasites of wheat, reducing production worldwide. Both are sedentary endoparasitic nematodes, and their development and parasitism depend strongly on nutrients obtained from hosts. Secreted fatty acid- and retinol-binding (FAR) proteins are nematode-specific lipid carrier proteins used for nutrient acquisition as well as suppression of plant defenses. In this study, we obtained three novel FAR genes Ha-far-1 (KU877266), Ha-far-2 (KU877267), Hf-far-1 (KU877268). Ha-far-1 and Ha-far-2 were cloned from H. avenae, encoding proteins of 191 and 280 amino acids with molecular masses about 17 and 30 kDa, respectively and sequence identity of 28%. Protein Blast in NCBI revealed that Ha-FAR-1 sequence is 78% similar to the Gp-FAR-1 protein from Globodera pallida, while Ha-FAR-2 is 30% similar to Rs-FAR-1 from Radopholus similis. Only one FAR protein Hf-FAR-1was identified in H. filipjevi; it had 96% sequence identity to Ha-FAR-1. The three proteins are alpha-helix-rich and contain the conserved domain of Gp-FAR-1, but Ha-FAR-2 had a remarkable peptide at the C-terminus which was random-coil-rich. Both Ha-FAR-1 and Hf-FAR-1 had casein kinase II phosphorylation sites, while Ha-FAR-2 had predicted N-glycosylation sites. Phylogenetic analysis showed that the three proteins clustered together, though Ha-FAR-1 and Hf-FAR-1 adjoined each other in a plant-parasitic nematode branch, but Ha-FAR-2 was distinct from the other proteins in the group. Fluorescence-based ligand binding analysis showed the three FAR proteins bound to a fluorescent fatty acid derivative and retinol and with dissociation constants similar to FARs from other species, though Ha-FAR-2 binding ability was weaker than that of the two others. In situ hybridization detected mRNAs of Ha-far-1 and Ha-far-2 in the hypodermis. The qRT-PCR results showed that the Ha-far-1and Ha-far-2 were expressed in all developmental stages; Ha-far-1 expressed 70 times more than Ha-far-2 in all stages. The highest expression level of Ha-far-1 was observed in fourth-stage juvenile (J4), whereas the highest expression level of Ha-far-2 occurred in second-stage juvenile (J2). In conclusion, we have identified two novel far genes from H. avenae and one from H. filipjevi and have provided further indication that nematode far genes are present in a variety of nematode species, where the FAR proteins share similar basic structure, expression pattern and biochemical activities.

The unusual lipid binding proteins of parasitic helminths and their potential roles in parasitism and as therapeutic targets

In this review paper we aim at presenting the current knowledge on structural aspects of soluble lipid binding proteins (LBPs) found in parasitic helminths and to discuss their potential role as novel drug targets. Helminth parasites produce and secrete a great variety of LBPs that may participate in the acquisition of nutrients from their host, such as fatty acids and cholesterol. It is also postulated that LBPs might interfere in the regulation of the host׳s immune response by sequestering lipidic intermediates or delivering bioactive lipids. A detailed comprehension of the structure of these proteins, as well as their interactions with ligands and membranes, is important to understand host-parasite relationships that they may mediate. This information could also contribute to determining the role that these proteins may play in the biology of parasitic helminths and how they modulate the immune systems of their hosts, and also towards the development of new therapeutics and prevention of the diseases caused by these highly pathogenic parasites.

Vaccines to combat river blindness: expression, selection and formulation of vaccines against infection with Onchocerca volvulus in a mouse model

Human onchocerciasis is a neglected tropical disease caused by Onchocerca volvulus and an important cause of blindness and chronic disability in the developing world. Although mass drug administration of ivermectin has had a profound effect on control of the disease, additional tools are critically needed including the need for a vaccine against onchocerciasis. The objectives of the present study were to: (i) select antigens with known vaccine pedigrees as components of a vaccine; (ii) produce the selected vaccine antigens under controlled conditions, using two expression systems and in one laboratory and (iii) evaluate their vaccine efficacy using a single immunisation protocol in mice. In addition, we tested the hypothesis that joining protective antigens as a fusion protein or in combination, into a multivalent vaccine, would improve the ability of the vaccine to induce protective immunity. Out of eight vaccine candidates tested in this study, Ov-103, Ov-RAL-2 and Ov-CPI-2M were shown to reproducibly induce protective immunity when administered individually, as fusion proteins or in combination. Although there was no increase in the level of protective immunity induced by combining the antigens into one vaccine, these antigens remain strong candidates for inclusion in a vaccine to control onchocerciasis in humans.

Molecular characterization and functions of fatty acid and retinoid binding protein gene (Ab-far-1) in Aphelenchoides besseyi

Rice white tip nematode, Aphelenchoides besseyi, is a kind of plant parasitic nematodes that cause serious losses in rice and many other crops. Fatty acid and retinoid binding protein (FAR) is a specific protein in nematodes and is related to development, reproduction, infection to the host, and disruption of plant defense reactions, so the inhibition of FAR function is the potential approach to control A. besseyi. The full-length of Ab-far-1 cDNA is 805 bp, including 546 bp of ORF that encodes 181 amino acids. Software analysis revealed that the Ab-FAR-1 was rich in α-helix structure, contained a predicted consensus casein kinase II phosphorylation site and a hydrophobic secretory signal peptide, but did not have glycosylation sites. The Ab-FAR-1 had 52% homology to Gp-FAR-1 protein. The Ab-FAR-1 and Gp-FAR-1 were grouped in the same branch according to the phylogenetic tree. Fluorescence-based ligand binding analysis confirmed that the recombinant Ab-FAR-1 (rAb-FAR-1) bound with the fluorescent analogues 11-((5-dimethylaminonaphthalene-1-sulfonyl) amino) undecannoic acid (DAUDA), cis-parinaric acid and retinol, but the oleic acid would compete with the binding site. Quantitative PCR (qPCR) was used to assess the expression level of Ab-far-1 at different development stages of A. besseyi, the highest expression was found in the females, followed by eggs, juveniles and males. Using in situ hybridization technique, Ab-far-1 mRNA was present in the hypodermis of juveniles and adults, the ovaries of females and the testes of males. When A. besseyi was treated with Ab-far-1 dsRNA for 48 h, the silencing efficiency of Ab-far-1 was the best and the number of nematodes on the carrot was the least. Thus FAR plays important roles in the development and reproduction of nematodes. This study is useful and helpful to figure out a new way to control the plant parasitic nematodes.

A four-antigen mixture for rapid assessment of Onchocerca volvulus infection

Background

Onchocerciasis, an infection caused by the filarial nematode Onchocerca volvulus, is a major public health concern. Given the debilitating symptoms associated with onchocerciasis and concerns about recrudescence in areas of previous onchocerciasis control, more efficient tools are needed for diagnosis and monitoring of control measures. We investigated whether luciferase immunoprecipitation systems (LIPS) may be used as a more rapid, specific, and standardized diagnostic assay for Onchocerca volvulus infection.

Methods

Four recombinantly produced Onchocerca volvulus antigens (Ov-FAR-1, Ov-API-1, Ov-MSA-1 and Ov-CPI-1) were tested by LIPS on a large cohort of blinded sera comprised of both uninfected controls and patients with a proven parasitic infection including Onchocerca volvulus (Ov), Wuchereria bancrofti (Wb), Loa loa (Ll), Strongyloides stercoralis (Ss), and with other potentially cross-reactive infections. In addition to testing all four Ov antigens separately, a mixture that tested all four antigens simultaneously was evaluated in the standard 2-hour incubation format as well as in a 15-minute rapid LIPS format.

Findings

Antibody responses to the four different Ov antigens allowed for unequivocal differentiation between Ov-infected and uninfected control sera with 100% sensitivity and 100% specificity. Analysis of the antibody titers to each of these four antigens in individual Ov-infected sera revealed that they were markedly different and did not correlate (r_S = –0.11 to 0.58; P = 0.001 to 0.89) to each other. Compared to Ov-infected sera, patients infected with Wb, Ll, Ss, and other conditions had markedly lower geometric mean antibody titers to each of the Ov 4 antigens (P<0.0002 for each antigen). The simplified method of using a mixture of the 4 Ov antigens simultaneously in the standard format or a quick 15-minute format (QLIPS) showed 100% sensitivity and 100% specificity in distinguishing the Ov-infected sera from the uninfected control sera. Finally, the QLIPS format had the best performance with 100% sensitivity and specificity values of 76%, 84% and 93% for distinguishing Ov from Wb, Ll and Ss-infected sera.

Conclusions

The multi-antigen LIPS assay can be used as a rapid, high throughput, and specific tool to not only to diagnose individual Ov infections but also as a sensitive and potentially point-of-care method for early detection of recrudescent infections in areas under control and for mapping new areas of transmission of Ov infection.

Caused by the filarial parasite Onchocerca volvulus, onchocerciasis is a neglected tropical disease associated with blindness and severe dermatitis. Available diagnostic methods are either invasive, require hours or days to perform, and/or need sophisticated equipment to be conducted. Thus, there is an urgent need for simple and rapid technologies for the specific diagnosis of Onchocerca volvulus infection. Here we investigated whether luciferase immunoprecipitation systems (LIPS) can produce a more rapid and specific test for diagnosis of O. volvulus infection. Using modified versions of previously identified Onchocerca-specific antigens, LIPS tests detected antibodies to all four O. volvulus antigens and easily distinguished the O. volvulus-infected samples from uninfected samples. We also tested these four different antigens in a simpler format as a combined mixture and distinguished 100% of the confirmed cases from the uninfected controls. A rapid 15-minute version of this mixture test (QLIPS) also allowed distinction of 100% of the cases from those uninfected and performed even better in identifying Onchocerca from other cross-reactive parasitic infections. This study suggests that this rapid LIPS test (QLIPS) has the potential to be used in point-of-care detection of onchocerciasis and thereby may provide a new tool for diagnosis and the monitoring of transmission control measures.

Identification and location of albumin-like antigens in third-stage larva of W. bancrofti, in adult forms of Litomosoides chagasfilhoi and in the free-living nematode Caenorhabditis elegans

Antigens resembling those of host proteins have been identified on the surface of several filarial parasites, such as immunoglobulins and serum albumins. The origin of albumin-like antigens on filarial parasites remains unclear. Several authors suggested that they have been adsorbed, or that they were metabolic waste products from nutritional utilization of human albumin, or perhaps a contamination with human products. This study searched for human albumin-like antigens by Western blot and ultrastructural analyses on filarial parasites, third stage of W. bancrofti and adult females of Litomosoides chagasfilhoi, and on the free-living Caenorhabditis elegans nematode. Our results showed approximately 67kDa proteins recognized by anti-human albumin antibodies on extracts and excretory-secretory (ES) products of the third-stage W. bancrofti. Similar albumin-like proteins were also detected on the filarial parasite L. chagasfilhoi and on C. elegans extracts. The immunocytochemistry analysis showed human albumin-like antigens on similar tissues of these nematodes. These results provide evidence that these proteins have antigenic similarity and similar distribution in nematodes tissues. Our observations suggest that albumin-like antigens presented on filarial parasites are not acquired from the host, but rather are shared antigenic determinants found even in the third-stage larvae recovered from the invertebrate host.

Antibody detection tests for Onchocerca volvulus: comparison of the sensitivity of a cocktail of recombinant antigens used in the indirect enzyme-linked immunosorbent assay with a rapid-format antibody card test

There is a need for a specific, sensitive, and practical diagnostic test to monitor onchocerciasis elimination campaigns. In April 2001, an enzyme-linked immunosorbent assay (ELISA) using 3 recombinant antigens and a rapid-format antibody card test (immunochromatographic test; ICT) using an individual antigen were compared in a Mexican population with onchocerciasis. The sensitivity of the ELISA and ICT was 97% and 86%, respectively.

The FAR protein family of the nematode Caenorhabditis elegans. Differential lipid binding properties, structural characteristics, and developmental regulation

Parasitic nematodes of humans and plants secrete a structurally novel type of fatty acid- and retinol-binding protein, FAR, into the tissues they occupy. These proteins may interfere with intercellular lipid signaling to manipulate the defense reactions of the host or acquire essential lipids for the parasites. The genome of the nematode Caenorhabditis elegans encodes eight FAR-like proteins (Ce-FAR-1 to -8). These fall into three discrete groups as indicated by phylogenetic sequence comparisons and intron positions, the proteins from parasitic nematodes falling into group A. Recombinant Ce-FAR-1 to -7 were produced in Escherichia coli and tested for lipid binding in fluorescence-based assays. Ce-FAR-1 to -6 bound DAUDA (11-((5-dimethylaminonaphthalene-1-sulfonyl)amino)undecanoic acid), cis-parinaric acid, and retinol with dissociation constants in the micromolar range, whereas Ce-FAR-7 bound the latter two lipids relatively poorly. Each protein produced a characteristic shift in peak fluorescence emission of DAUDA, and one (Ce-FAR-5) produced a shift greater than has been observed previously for any lipid-binding protein. Selected Ce-FAR proteins were analyzed by circular dichroism (CD) and differential scanning calorimetry, were found to be helix-rich, and exhibited high thermal stability (transition midpoint, 82.7 degrees C). CD and secondary structure predictions, however, both indicated that Ce-FAR-7 possesses substantially less helix than the other FAR proteins. The genes encoding the Ce-FAR proteins were found to be transcribed differentially through the life cycle of C. elegans, such that Ce-far-4 was transcribed at highest levels in the fourth larval stage, and Ce-far-3 and -7 predominated in males.

The FAR proteins of filarial nematodes: secretion, glycosylation and lipid binding characteristics

The FAR proteins of nematodes are small ( approximately 20 kDa), helix-rich, fatty acid and retinol-binding (FAR) proteins that appear to be confined to nematodes. We have carried out a comparative sequence and biochemical analysis of selected FAR proteins often species of filarial parasites (from the genera Onchocerca, Brugia, Wuchereria, Loa, Acanthocheilonema and Litomosoides). The sequences fall into two main groups corresponding broadly to the onchocercal and lymphatic filariasis parasites, and only those with unsheathed microfilariae were found to produce glycosylated FAR proteins. The proteins were released into culture medium by all the species and developmental stages investigated. Recombinant forms of two of these proteins (Ov-FAR-1 from O. volvulus and Bm-FAR-1 from B. malayi) were compared for ligand binding in fluorescence-based assays. Both were found to bind all-trans-retinol, (dansylamino) undecanoic acid (DAUDA), and oleic acid by competition. Both produced an identical, and dramatic, blue-shift in the fluorescence emission of DAUDA (from 541 to approximately 483 nm), indicative of similarity in the binding site environments of the two proteins. These findings indicate that there is strong conservation of the biochemical activities of the FAR proteins between the different parasite species, although they appear to have different post-translational modifications which may relate to the biology of the larvae.

River blindness: a role for parasite retinoid-binding proteins in the generation of pathology?

A new family of fatty acid- and retinoid-binding proteins has recently been identified in nematodes. These are apparently nematode specific and have very different structures and binding characteristics to their mammalian counterparts. Retinoids have important roles in vision, tissue differentiation and repair, and can profoundly affect collagen synthesis. Binding proteins released by a parasite might therefore play a part in the generation of the skin and eye pathology seen in river blindness. They might also be involved in the formation of the subcutaneous nodules induced by this parasite.

A surface-associated retinol- and fatty acid-binding protein (Gp-FAR-1) from the potato cyst nematode Globodera pallida: lipid binding activities, structural analysis and expression pattern

Parasitic nematodes produce at least two structurally novel classes of small helix-rich retinol- and fatty-acid-binding proteins that have no counterparts in their plant or animal hosts and thus represent potential targets for new nematicides. Here we describe a protein (Gp-FAR-1) from the plant-parasitic nematode Globodera pallida, which is a member of the nematode-specific fatty-acid- and retinol-binding (FAR) family of proteins but localizes to the surface of this species, placing it in a strategic position for interaction with the host. Recombinant Gp-FAR-1 was found to bind retinol, cis-parinaric acid and the fluorophore-tagged lipids 11-(dansylamino)undecanoic acid and dansyl-D,L-alpha-amino-octanoic acid. The fluorescence emission characteristics of the dansylated analogues indicated that the entire ligand enters the binding cavity. Fluorescence competition experiments showed that Gp-FAR-1 binds fatty acids in the range C(11) to C(24), with optimal binding at C(15). Intrinsic fluorescence analysis of a mutant protein into which a tryptophan residue had been inserted supported computer-based predictions of the position of this residue at the protein's interior and possibly also at the binding site. Of direct relevance to plant defence systems was the observation that Gp-FAR-1 binds two lipids (linolenic and linoleic acids) that are precursors of plant defence compounds and the jasmonic acid signalling pathway. Moreover, Gp-FAR-1 was found to inhibit the lipoxygenase-mediated modification of these substrates in vitro. Thus not only does Gp-FAR-1 function as a broad-spectrum retinol- and fatty-acid-binding protein, the results are consistent with the idea that Gp-FAR-1 is involved in the evasion of primary host plant defence systems.

Purification of a diagnostic, secreted cysteine protease-like protein from the hookworm Ancylostoma caninum

The enteric infection of humans with the canine hookworm Ancylostoma caninum varies in its clinical presentation, ranging from asymptomatic to eosinophilic gastroenteritis requiring surgical intervention. Infections are not patent, but can be diagnosed immunologically by detecting antibodies to an immunodominant secreted hookworm protein termed Ac68. To characterise Ac68, we purified the native protein from A. caninum excretory/secretory products using size exclusion followed by anion exchange chromatography. The epitopes in the purified protein recognised by human infection sera were shown to be proteins and not carbohydrates. The N-terminal amino acid sequence of the purified Ac68 was determined and six of the 11 residues obtained were shared with a previously characterised cysteine protease of A. caninum, AcCP1.

Use of the recombinant Onchocerca volvulus protein Ov20/OvS1 for the immunodiagnostic differentiation between onchocerciasis and mansonelliasis and for the characterization of hyperreactive onchocerciasis (sowda)

The protein Ov20/OvS1 was used as antigen in ELISA and Western blot in order to differentiate onchocerciasis from African mansonelliasis and to characterize the hyperreactive form of Onchocerca volvulus infection (sowda). The specificity of the IgG4 Western blot was 98% for the differentiation between persons with onchocerciasis and Mansonella microfilariae (mf) carriers (125 persons with M. perstans and 92 with M. streptocerca), whereas the IgG4 ELISA showed a specificity of 81% in 137 M. perstans mf carriers and 85% in 94 M. streptocerca mf carriers. The sensitivity of Ov20/OvS1 in identifying onchocerciasis using the IgG4 ELISA was 75% for 103 O. volvulus mf carriers with the generalized and 89% for 44 patients with the sowda form of onchocerciasis. IgE antibodies against OvS1 were found in 95% of 39 patients with hyperreactive onchocerciasis but only in 15% of 47 persons with the generalized form. Thus, Ov20/-OvS1 appears a promising candidate antigen for the diagnosis of onchocerciasis and in particular for the detection of the sowda type of disease.

The nematode polyprotein allergens/antigens

Interest in the nematode polyprotein allergens/antigens (NPAs) originally arose because they were often found to be immunodominant antigens of nematode parasites, and in some cases also potent allergens. Quite separately, they attracted attention as the 'ladder' proteins found close to the surface of filarial nematodes. Screening of cDNA expression libraries with antibody from infected humans or domestic animals continues to reveal more NPAs. The search for the biochemical function(s) of NPAs was originally hampered by the lack of amino acid sequence similarities between NPAs and proteins of known function, but much is now known about their biochemical activity, the highly unusual means of their biosynthesis and their gene structure. Here, Malcolm Kennedy provides an update on these intriguing proteins.

Humoral responses to a secretory Onchocerca volvulus protein: differences in the pattern of antibody isotypes to recombinant Ov20/OvS1 in generalized and hyperreactive onchocerciasis

The Onchocerca volvulus secretory protein Ov20/OvS1 represents a dominant antigen expressed in the infective larvae, microfilariae and adult stages of the parasite. The humoral responses to this protein have not yet been analysed in the polar clinical and immunological forms of onchocerciasis. Analysis by ELISA of class and subclass antibodies to Ov20/OvS1 in persons with the generalized or the hyperreactive form of onchocerciasis revealed similar strong responses of IgG1, IgG4 and IgM antibody levels in both forms of onchocerciasis and significant differences were observed in the IgE and IgA antibody classes. Computation of the ratios of antibodies showed that persons with the generalized form exhibited significantly higher ratios of IgG4 to IgG1, IgG4 to IgE, and IgM to IgE than patients with the hyperreactive form. To investigate the isotype recognition of antigenic sites on Ov20/OvS1 protein, three recombinantly expressed fragments (F1-3) of Ov20/OvS1 were probed using sera which strongly reacted with intact recombinant Ov20/OvS1. Epitope(s) on F1 comprising amino acid residues 1-63 were significantly recognized by IgG1 and IgE, while IgM recognized epitopes on all three fragments. The strongest reaction of IgM occurred with epitope(s) formed by residues 108-171 (F3). In contrast, IgG4 type antibodies were not reactive with either of the three OvS1 fragments, but they reacted with intact Ov20/OvS1 protein. Generalized onchocerciasis, unable to eliminate microfilariae, and hyperreactive onchocerciasis, with a high potency to eliminate or to reduce parasite loads, can be distinguished by a distinct pattern of isotype responses to Ov20/OvS1.

The polyprotein lipid binding proteins of nematodes

The nematode polyprotein allergens/antigens (NPAs) are specific to nematodes, and are synthesised as tandemly repetitive polypeptides comprising 10 or more repeated units. The polyproteins are post-translationally cleaved at consensus sites to yield multiple copies of the approximately 15-kDa NPA units. These units can be highly diverse in their amino acid sequences, but absolutely conserved signature amino acid positions are identifiable. NPA units are helix-rich and possibly fold as four helix bundle proteins. The NPA units have relatively non-specific lipid binding activities, binding fatty acids and retinoids, with dissociation constants similar to those of lipid transport proteins of vertebrates. Fluorescence-based analysis has indicated that, like most lipid transport proteins, the ligand is taken into the binding site in its entirety, but the binding site environment is unusual. NPAs are synthesised in the gut of nematodes, and presumably act to distribute small lipids from the gut, via the pseudocoelomic fluid, to consuming tissues (muscles, gonads, etc.). In some species, one of the units has a histidine-rich extension peptide which binds haems and certain divalent metal ions. NPAs appear to be released by parasitic nematodes, and may thereby be involved in modification of the local inflammatory and immunological environment of the tissues they inhabit by delivering or sequestering pharmacologically active lipids - they are known to bind arachidonic acids and some of its metabolites, lysophospholipids, and retinoids. NPAs are the only known lipid binding protein made as polyproteins, and are exceptions to the rule that repetitive polyproteins are only produced by cells undergoing programmed cell death and producing specialist products.

Secretion of a novel class of iFABPs in nematodes: coordinate use of the Ascaris/Caenorhabditis model systems

A novel fatty acid binding protein, As-p18, is secreted into both the perivitelline and perienteric fluids of the parasitic nematode, Ascaris suum, and at least eight potential homologues of As-p18 have been identified in the Caenorhabditis elegans genome. The products of the three most closely related homologues are fatty acid binding proteins (LBP-1, LBP-2 and LBP-3) which contain putative secretory signals. Phylogenetic analysis revealed that these secreted fatty acid binding proteins comprise a distinct gene class within the fatty acid binding protein family and are possibly unique to nematodes. To examine the potential sites of As-p18 secretion, the expression of the putative promoters of the C. elegans homologues was examined with GFP reporter constructs. The developmental expression of lbp-1 was identical to that of As-p18 and consistent with the secretion of LBP-1 from the hypodermis to the perivitelline fluid. The expression patterns of lbp-2 and lbp-3 were consistent with the secretion of LBP-2 and LBP-3 from muscle into the perienteric fluid later in development. These studies demonstrate that at least some perivitelline fluid proteins appear to be secreted from the hypodermis prior to the formation of the cuticle and, perhaps more importantly, that this coordinate C. elegans/A. suum approach may be potentially useful for examining a number of key physiological processes in parasitic nematodes.

The secretory Onchocerca volvulus protein OvS1/Ov20 exhibits the capacity to compete with serum albumin for the host's long-chain fatty acids

The mechanism by which filarial parasites derive fatty acids bound to the host's carrier protein is poorly understood. The capacity of a secretory protein of Onchocerca volvulus (OvS1/Ov20) to compete with serum albumin for arachidonic and other fatty acids was investigated in this study. Binding affinities of the two proteins for the long-chain fatty acids were determined using displacement assays. The fluorescent probes used included 11-((5-dimethylaminonaphthalene-1-sulfonyl)amino) undecanoic acid (DAUDA) and cis-parinaric acid. OvS1 protein bound arachidonic acid with an affinity five-fold greater than the affinity exhibited by serum albumin. Oleic acid was bound by the parasite protein with an affinity two-fold greater than the affinity shown by serum albumin. Furthermore, the affinities exhibited by OvS1 protein in binding arachidonic and linoleic acid were about two times higher than the affinity for oleic acid. The results suggest that the OvS1 protein has the capacity to compete with the main host's fatty acid carrier protein for the long-chain fatty acids, in particular arachidonic acid, the precursor for eicosanoids.

Comparative analysis of glycosylated and nonglycosylated filarial homologues of the 20-kilodalton retinol binding protein from Onchocerca volvulus (Ov20)

Ov20 is a structurally novel 20-kDa retinol binding protein secreted by Onchocerca volvulus. Immunological and biological investigation of this protein has been hampered by the inability to maintain O. volvulus in a laboratory setting. In an effort to find a system more amenable to laboratory investigation, we have cloned, sequenced, and expressed cDNA encoding homologues of Ov20 from two closely related filarial species, Brugia malayi (Bm20) and Acanthocheilonema viteae (Av20). Sequence comparisons have highlighted differences in glycosylation of the homologues. We present here an analysis of mouse immune responses to Ov20, Bm20, and Av20. The results suggest a strong genetic restriction in response to native Bm20 that is overcome when recombinant, nonnative material is used. Reactivity of human filarial sera to the three recombinant proteins confirmed previous specificity studies with Ov20 but highlighted important differences in the reactivity patterns of the O. volvulus and B. malayi homologues that may be due to differences in glycosylation patterns. Ov20 is a dominant antigen in infected individuals, while Bm20 is not. The availability of the B. malayi homologue enabled us to use defined murine reagents and inbred strains for genetic analysis of responsiveness in a way that is not possible for Ov20. However, the close sequence similarity between Ov20 and Av20 suggests that the A. viteae model may be more suited to the investigation of the biological functions of Ov20.

Comparison of serological and parasitological assessments of Onchocerca volvulus transmission after 7 years of mass ivermectin treatment in Mexico

OBJECTIVE AND METHOD

To compare the utility of an ELISA using 3 recombinant antigens with that of the skin biopsy to estimate incidence of infections in a sentinel cohort of individuals living in an endemic community in southern Mexico during a set of 11 subsequent ivermectin treatments.

RESULTS

The apparent community prevalence of infection and microfilarial skin infection before and after 11 treatments with ivermectin plus nodulectomy were 78% and 13%, and 0.68 mf/mg and 0.04 mf/mg, respectively, as measured by skin biopsy. Of a group of 286 individuals participating in all surveys, a sentinel cohort of 42 mf and serologically negative individuals had been followed since 1994. The annual percentage of individuals becoming positive in this cohort was 24% (10/42), 28% (9/33), 0%, and 4.3% (1/23) in 1995, 1996, 1997 and 1998, respectively. Likewise, the incidence in children 5 years and under (n = 13) within this sentinel cohort was 15% (2/13), 18% (2/11), 0% and 11% (1/9), respectively. All individuals became positive to both tests simultaneously, indicating that seroconversion assessed infection incidence as accurately as skin biopsy in the sentinel group.

CONCLUSION

Incidence monitoring of a sentinel cohort provides an estimation of the parasite transmission in the community; it is less costly than massive sampling, and a finger prick blood test might be more acceptable in some communities.

Onchocerciasis hyperendemic in the Unturán Mountains: the value of recombinant antigens in describing a new transmission area in southern Venezuela

A recently described hyperendemic onchocerciasis area, located in the Unturán Mountains (between the Siapa and Orinoco basins) of southern Venezuela was studied using a cocktail of 3 low molecular weight onchocercal recombinant antigens (OvMBP/10, OvMBP/11, and OvMBP/29). The resulting seroepidemiological data were compared with those from a hypoendemic community (Altamira) situated in the northern coastal mountain range. Parasitological (skin biopsy) and serological (enzyme-linked immunosorbent assay, ELISA) methods for the specific diagnosis of Onchocerca volvulus in these 2 very different endemic areas were, respectively, 88% and 96% sensitive in Unturán, and 57% and 91% sensitive in Altamira. The mean microfilarial load, the mean optical density (OD), and the seropositivity rates all increased significantly with age in both communities. The serological variables (mean OD and prevalence of anti-O. volvulus antibodies) were both significantly higher in Unturán than in Altamira for children and young adults (aged < 25 years), although above this age no differences between communities were detected. Seroprevalence had already reached 50% in the under 15 year-olds examined at Unturán but was just 5% at Altamira for the same age-class. The prevalence of specific antibodies (mainly a marker of exposure to risk of infection) exceeded 85% in the remaining age-categories at the hyperendemic area. This is in agreement with the high community microfilarial load recorded in Unturán (> 20 mf/mg) and the presence of sclerosing keratitis and hanging groin, suggesting that onchocerciasis is a public health problem in this community. The ELISA test used here, based on a cocktail of 3 low molecular weight onchocercal recombinant antigens, appears, therefore, to constitute a practical tool for the description of endemicity levels in remote areas, particularly given the fact that finger-prick blood samples are routinely taken from children in the Upper Orinoco region for surveys of malaria incidence. Such studies could aid in defining the true extent of the Amazon focus (still unknown) and providing priority indicators for the selection of communities where onchocerciasis control programmes should be implemented.

Caenorhabditis elegans is a nematode

Caenorhabditis elegans is a rhabditid nematode. What relevance does this have for the interpretation of the complete genome sequence, and how will it affect the exploitation of the sequence for scientific and social ends? Nematodes are only distantly related to humans and other animal groups; will this limit the universality of the C. elegans story? Many nematodes are parasites; can knowledge of the C. elegans sequence aid in the prevention and treatment of disease?

The Ov20 protein of the parasitic nematode Onchocerca volvulus. A structurally novel class of small helix-rich retinol-binding proteins

Ov20 is a major antigen of the parasitic nematode Onchocerca volvulus, the causative agent of river blindness in humans, and the protein is secreted into the tissue occupied by the parasite. DNA encoding Ov20 was isolated, and the protein was expressed in Escherichia coli. Fluorescence-based ligand binding assays show that the protein contains a high affinity binding site for retinol, fluorescent fatty acids (11-((5-dimethylaminonaphthalene-1-sulfonyl)amino)undecanoic acid, dansyl-DL-alpha-aminocaprylic acid, and parinaric acid) and, by competition, oleic and arachidonic acids, but not cholesterol. The fluorescence emission of dansylated fatty acids is significantly blue-shifted upon binding in comparison to similarly sized beta-sheet-rich mammalian retinol- and fatty acid-binding proteins. Secondary structure prediction algorithms indicate that a alpha-helix predominates in Ov20, possibly in a coiled coil motif, with no evidence of beta structures, and this was confirmed by circular dichroism. The protein is highly stable in solution, requiring temperatures in excess of 90 degrees C or high denaturant concentrations for unfolding. Ov20 therefore represents a novel class of small retinol-binding protein, which appears to be confined to nematodes. The retinol binding activity of Ov20 could possibly contribute to the eye defects associated with onchocerciasis and, because there is no counterpart in mammals, represents a strategic target for chemotherapy.

Differential immune response to Onchocerca volvulus: IgG4 antibody responses differ in onchocerciasis patients from Guatemala and Ghana

Geographical differences exist in the clinical features of onchocerciasis in Central America and West Africa, which could be due in part from variations in the antigenic composition of the infecting organism. In an attempt to address this question, adult female worms of Onchocerca volvulus derived from nodules of patients from Guatemala and Ghana were compared in terms of polypeptide composition and the IgG4 antibody responses induced in patients. It was shown that a Tris-buffer soluble extract from the worms obtained in the two regions differ in polypeptide composition. Furthermore, the diagnostic polypeptides were found to be in the 30 kDa region but the recognition of these antigens was less intense and less frequently observed in the sera of microfilaria (mf) positive patients from Ghana than equivalent age and sex matched patients from Guatemala.