Identification, synthesis and immunogenicity of cuticular collagens from the filarial nematodes Brugia malayi and Brugia pahangi

Onchocerca volvulus: molecular cloning, primary structure, and expression of a microfilarial surface-associated antigen

Although the filarial nematode parasite Onchocerca volvulus is an important human pathogen in large areas of Africa and Latin America, little is known of the molecular interactions that govern the clinical status of patients with this chronic, debilitating disease. As a step toward defining the parasite molecules important to the immunobiology of host-parasite interactions, we have identified and cloned a major surface-associated antigen expressed by O. volvulus microfilariae. Radiolabeling experiments demonstrated that O. volvulus microfilariae have a limited repertoire of peptides at the surface. Prominent among these labeled peptides is an 18-kDa component. Immunological cross-reactivity between a surface-associated component of Dirofilaria immitis microfilariae and the 18-kDa surface-associated molecule from O. volvulus was exploited in a strategy to clone this potentially important O. volvulus microfilarial antigen. The cross-reacting antibodies were used to immunoscreen O. volvulus cDNA expression libraries. One clone, M2f.e, contained an open reading frame of 495 bp encoding an 18.1-kDa protein (OVMS18). Antibodies produced against the expression product of M2f.e recognized an 18-kDa component in extracts of O. volvulus microfilariae and bound to the surface of intact O. volvulus and D. immitis microfilariae. Southern blot analyses showed that M2f.e-like sequences are present in the genomic DNA of a number of filarial nematode species, but not in DNAs from nonfilarial nematode species.

Surface associated glycoproteins from Toxocara canis larval parasites

The surface of infective larvae of Toxocara canis, the dog ascarid nematode, reveals relatively few exposed surface proteins which can be recovered in soluble form. The major components identified by surface labelling have molecular weights of 32, 55, 70 and 120 kilodaltons (kDa), and are all significantly glycosylated. All are recognised by the immune response in definitive (canine) and paratenic (murine or human) hosts. Expression of these antigens on the parasite surface begins after the larvae hatch from infective ova in vitro, and presumably in vivo. Each of these molecules may also be found in the set of secreted (ES) glycoconjugates released by larval parasites cultivated in vitro, and currently available biochemical and functional data on the surface/secreted ES glycoproteins are presented. Analysis with monoclonal antibodies (MAbs) confirms the identity of surface and ES molecules, and these MAbs show differing patterns of binding to the epicuticle, the cuticular matrix and to the oral orifice. Alternative mechanisms for antigen synthesis, insertion into the cuticle and export from the parasite are discussed.

Cuticular proteins of Brugia filarial parasites

Surface-labelling techniques have been used to delineate a number of constituent molecules of the cuticle in adult stage Brugia malayi and Brugia pahangi. These molecules can be separated by virtue of their physical properties, and localised either by sequential solubilisation of intact cuticles or immunoelectron microscopy with relevant antisera. The major structural components of the cuticular matrix consist of a set of collagenous proteins of diverse molecular weight ranging from 36 to 160 kDa, cross-linked by disulphide bonds and confined to the basal and inner cortical layers. Each stage of the parasite has a distinctive set of between 12 to 25 collagenous proteins whose synthesis is regulated temporally with respect to moulting. As in other nematodes, the outer cortex and epicuticle is composed of a cross-linked insoluble proteinaceous structure. Two non-structural and water-soluble proteins are also resolved by Iodogen-mediated labelling; a 15 kDa peptide which shows no evidence of glycosylation, and a major 29 kDa glycoprotein, which carries at least two N-linked oligosaccharide chains and which we have termed Gp29. The former protein can be detected in L3, L4 and adult B. malayi by surface labelling, whereas Gp29 appears to be restricted to L4 and adult worms. The possible significance of cuticular proteins as targets of immunity or causative agents of pathology is discussed.

Surface-associated antigens of second, third and fourth stage larvae of Dirofilaria immitis

The surface-associated molecules of the second (L2), third (L3) and fourth (L4) larval stages of Dirofilaria immitis were characterized employing radiolabeling techniques and SDS-PAGE analysis. Major labeled components of 35 kDa and 6 kDa were present in extracts from IODO-GEN-labeled L2 and L3 parasites. The results of lactoperoxidase-catalyzed reactions also demonstrated that L2 and L3 stages of D. immitis have a similar repertoire of surface-associated antigens. However, in contrast to the results obtained with IODO-GEN, lactoperoxidase reactions labeled components with apparent molecular weights of 66, 48, 25, 16.5 and 12 kDa. The similarities in the molecular weights of the L2 and L3 surface-associated components and the results of immunoprecipitation experiments which demonstrated that antibodies produced against the 35 kDa molecule from D. immitis L3s also recognize the 35 kDa component from L2 parasites suggest that synthesis of the molecules found at the surface of mature infective larvae begins as early as day 6 of development in the mosquito, D. immitis L4s emerged from the molting process with a repertoire of surface-associated antigens distinct from those found on L2s and L3s. IODO-GEN labeling of D. immitis L4s showed major surface-associated molecules with apparent molecular weights of 57, 40, 25, 12 and 10 kDa when analyzed under non-reducing conditions. In addition to molecules of 57, 40, 25, 12 and 10 kDa, extracts of D. immitis L4s labeled with lactoperoxidase contained additional major bands at 45, 43 and 8 kDa. Metabolic labeling experiments demonstrated a shift in the amount and complexity of the excretory/secretory products released by D. immitis during L3 to L4 development.

Haemonchus contortus: evidence that the 3A3 collagen gene is a member of an evolutionarily conserved family of nematode cuticle collagens

Rabbit antisera were raised against an 18 amino acid-long peptide that corresponds to the predicted sequence of the carboxy-terminal, nontriple helical region of the Haemonchus contortus 3A3 collagen gene. This sequence is highly conserved and diagnostic for members of the col-l collagen family, which includes the 3A3 gene. We find that these antisera react predominantly with multiple, high molecular weight (greater than 68 kDa) proteins on Western blots of whole worm extracts. The number and molecular weights of the reacting proteins vary depending upon the developmental stage of the worms analyzed. All of the reacting proteins are collagenase sensitive. The reacting collagens copurify with cuticles and are released from cuticles by reducing agents. In indirect immunofluorescence assays the antisera react only with the broken edges of isolated cuticles, suggesting that the antisera are reacting with an internal cuticle layer. This layer appears to be circular and to extend throughout the length of the worm. The antisera react on Western blots with multiple, high molecular weight collagens of eight other nematodes examined, representing two classes and several orders. These data provide additional support for the notion that the 3A3 collagen gene, and other members of the col-l collagen family, encode cuticle collagens. Collagens with this peptide sequence, presumably other members of the col-l collagen family, appear to be widely distributed in the phylum Nematoda.

Secreted antigens of filarial nematodes: a survey and characterization of in vitro excreted/secreted products of adult Brugia malayi

We report here a broad analysis of the excretory/secretory (E/S) products of adult Brugia malayi, collected by in-vitro cultivation of the parasite. Culture media and conditions were optimized, and non-essential amino acids were found to be crucial for efficient protein synthesis under cell- and serum-free culture conditions. A close correlation was found between total protein secretion, phosphorylcholine-bearing antigen release and lactate production on each day of culture, indicating that E/S molecules are actively secreted. Parasites cultured in vitro take 2-3 days to adjust to the new environment, and show peak levels of secretion at days 3 and 4. The active secretion of phosphorylcholine by the parasite therefore justifies the measurement of this molecule as an indication of active infection, possibly reflecting total worm burdens. By comparing metabolically labelled E/S from male and female worms, several molecules of low mol. wt, namely 10,000, 13,000, 14,000 and 22,000, together with high mol. wt components of above 12,000 were found to be female specific. Tracing the origin of the E/S products, several molecules were also found to be associated with the surface. Among these, there are at least two glycoproteins, 29,000 and 51,000 of which the 29,000 molecule is a major surface protein. The immunogenicity of the E/S was examined and antigenic cross-reactivity was found with sera from most filarial infections but not with non-filarial nematodiases such as hookworm or Trichinella. However, two molecules of low mol. wt, 15,000 and 19,000, were not recognized by anti-Onchocerca sera and appeared to be potential Brugia-specific diagnostic molecules. Possible functional roles of the adult E/S products were examined but we could find no evidence of protease activity in the E/S or glutathione S-transferase activity in either the E/S or in whole somatic extract.

Localization and immunogenicity of tubulin in the filarial nematodes Brugia malayi and B. pahangi

Tubulin was identified in the filarial nematodes Brugia malayi and B. pahangi by several approaches. Initially, a monoclonal antibody (6D8) was selected for its unusual binding to B. malayi microfilariae in indirect immunofluorescence assays: 6D8 showed granular, heterogeneously dispersed fluorescence on fixed parasites but did not bind to unfixed microfilariae. The microfilarial sheath did not bind 6D8, although it did bind fluoresceinated wheatgerm agglutinin. By Western blotting against microfilarial sonicate, 6D8 reacted with a 50,000-55,000 mol. wt protein, and also bound to purified chicken brain beta-tubulin. Additionally, this monoclonal antibody reacted with a recombinant fusion protein expressed by a clone (Bpa-7) originally isolated from an adult B. pahangi cDNA expression library by its reaction with chronic human filariasis serum. This clone encodes a small 40 amino acid C-terminal segment corresponding to residues 409-449 of beta-tubulin, and shows complete amino acid sequence homology with vertebrate beta-tubulin from 409 to 430 but 55% divergence (six amino acid substitutions, four insertions and one deletion) from human and chicken beta-tubulin over positions 431-449 at the C terminus. Antibody to both parasite and vertebrate (chicken) tubulin was found in filarial infection sera, with higher levels of autoreactive antibody apparent in amicrofilaraemic individuals. Immunogold electron microscopy was then used to localize beta-tubulin in B. malayi microfilariae and adult worms. Tubulin was shown not to be exposed on the microfilarial sheath or in the cuticle of either stage, but was found to be abundant in the somatic tissues. In microfilariae, 6D8 bound myofibril structures under the hypodermal layer, and also bound within cell nuclei. In the adult stage, tubulin was associated with muscle blocks, as well as the intestinal brush border and the embryonic uterine microfilariae.

Biochemical and immunochemical characterisation of a 20-kilodalton complex of surface-associated antigens from adult Onchocerca gutturosa filarial nematodes

Surface radioiodination of adult Onchocerca parasites reveals a restricted range of proteins associated with the cuticle. We present data to show that prominent among these is a complex of low-molecular-weight proteins which can be released in soluble form by homogenisation of surface-labelled Onchocerca gutturosa in phosphate-buffered saline (PBS). One of this groups of proteins, designated gp20, has a molecular mass of 20,000, is glycosylated with two N-linked carbohydrate side chains, and has a basic pI. Other PBS-soluble, 125I-labelled proteins of similar size appear not to be glycosylated. A distinct group of molecules are released only in the presence of reducing agents, and are likely to be cuticular collagens. The low-molecular-weight components are antigenic and cross-reactive with Onchocerca volvulus infection sera. Cross-reactions are also observed in immunoprecipitation experiments using sera from Brugia-immunised animals and infected humans. Comparative two-dimensional analyses of these immunoprecipitates reveal at least two Onchocerca specific components. As an alternative to radiolabelling and PBS homogenisation, incubation of worms in medium containing the reducing agent 2-mercaptoethanol resulted in a similar set of molecules being released into the medium. Since surface antigens of O. gutturosa from bovines and O. volvulus from humans appear similar in size and are antigenically cross-reactive, the more readily available parasite is being used to study further the properties of these molecules and to provide reagents for raising antisera reactive to the equivalent O. volvulus antigens.