Comparison of isolates and species of Toxocara and Toxascaris by biosynthetic labelling of somatic and ES proteins from infective larvae

Potential roles of Toxocara canis larval excretory secretory molecules in immunomodulation and immune evasion

Toxocara canis larvae invade various tissues of different vertebrate species without developing into adults in paratenic host. The long-term survival of the larvae despite exposure to the well-armed immune response is a notable achievement. The larvae modulate the immune response to help the survival of both the host and the larvae. They skew the immune response to type 2/regulatory phenotype. The outstanding ability of the larvae to modulate the host immune response and to evade the immune arms is attributed to the secretion of Toxocara excretory-secretory products (TESPs). TESPs are complex mixture of differing molecules. The present review deals with the molecular composition of the TESPs, their interaction with the host molecules, their effect on the innate immune response, the receptor recognition, the downstream signals the adaptive immunity and the repair of tissues. This review also addresses the role of TESPs molecules in the immune evasion strategy and the potential effect of the induced immunomodulation in some diseases. Identification of parasite components that influence the nematode-host interactions could enhance understanding the molecular basis of nematode pathogenicity. Furthermore, the identification of helminths molecules with immunomodulatory potential could be used in immunotherapies for some diseases.

Toxocara seroprevalence in Canada-Climate, environment and culture

Human infection with larvae of canine and feline roundworms belonging to the genus Toxocara can lead to devastating visceral, neural or ocular larvae migrans disease. However, such overt disease represents a fraction of cases. Far more common is covert toxocariasis, a less severe, but clinically symptomatic form of disease, and those who are exposed to infective larvae and seroconvert, but appear to be asymptomatic. Canada represents a unique epidemiological environment for Toxocara infection and exposure. Although the freezing conditions of the vast Arctic Tundra region of the North are thought unlikely to support the lifecycle of Toxocara spp., exposure and seroconversion does occur in people belonging to Inuit communities of this region. Further south, in the sub-Arctic of northern Quebec and Saskatchewan, there is a higher seroprevalence in many Canadian First Nations communities. The epidemiology of these infections is different to that seen in the non-Indigenous communities of the Humid Continental region. Poverty and climate play a major part in the risk of Toxocara seropositive status in Canada, but other factors such as unique cultural practices, population density of humans and reservoir hosts, and contact with wildlife are also factors in exposure and subsequent seroconversion in Canadian communities. This review discusses previous Toxocara seroprevalence studies performed in Canada, summarizes the data for domestic and wild animal reservoir hosts of Toxocara canis, Toxocara cati, Toxocara vitulorum and the closely related helminth, Toxascaris leonina. It also discusses how the unique and varied aspects of climate, culture and environment impacts human Toxocara exposure in Canada.

The serodiagnostic potential of recombinant proteins TES-30 and TES-120 in an indirect ELISA in the diagnosis of toxocariasis in cattle, horses, and sheep

Toxocariasis is a zoonotic disease that affects humans and animals alike. Although recombinant proteins are widely used for its diagnosis in humans, their performance in companion and production animals remains unknown. This study aimed to investigate the serodiagnostic potential of the recombinant proteins rTES–30 and rTES–120 from Toxocara canis in an indirect ELISA for cattle, horses, and sheep. Serum samples collected from the animals were tested with indirect ELISA and Western Blotting using T. canis TES–30 and TES–120 recombinant proteins produced in Escherichia coli, as well as native-TES. In the ELISA, rTES–30 showed high serodiagnostic potential in sheep and horses (92.6% and 85.2%, respectively), while the sensitivity of rTES–120 was higher in cattle and horses (97.2% and 92.6%, respectively). Furthermore, a highly positive association was observed between native and recombinant proteins in seropositive samples, while a moderately positive association was observed in seronegative samples, probably due to the lower specificity of native TES. In conclusion, our study indicates that the use of recombinant proteins in an indirect ELISA is an effective tool for the serodiagnosis of toxocariasis in animals, with the choice of protein being species-dependent.

Determination of IgG avidity in BALB/c mice experimentally infected with Toxocara canis

Toxocariasis is a zoonotic disease in that IgM titers can remain high for long periods making difficult to determine the stage of the disease. The aim of this study is to investigate the applicability of indirect ELISA, associated with urea, to discriminate between the acute and chronic toxocariasis. IgG avidity was evaluated in 25 BALB/c mice experimentally infected with 1000 Toxocara canis eggs. Blood samples were collected, and sera treated with 6 M urea and assayed by ELISA every two weeks. The percent IgG avidity was determined using the mean absorbance of sera treated with urea, divided by the mean absorbance of untreated sera. In the first 15 days post-inoculation, was observed a low percentage, between 7.25 and 27.5%, IgG avidity, characteristic of an acute infection. After 60 days of infection, all the mice showed between 31.4 and 58% IgG avidity, indicating a chronic infection.

An alternative method for producing Toxocara canis second stage larvae from a paratenic host (pigeon) for mRNA extraction purpose

Toxocara canis is a prevalent zoonotic parasite which can cause serious disease in puppies and humans. Excretory-secretory and coating antigens of the second stage larvae (L2) are the best targets for performing immunodiagnostic and also immunoprophylactic tests. Various hatching methods have been described to bring out L2 from the resistant infective egg shell; but these methods are difficult to do and have had different results when performed by different practitioners. In this study, second stage larvae were obtained from the viscera of pigeons (a paratenic host) which were infected with infective eggs. Infective Toxocara canis eggs were given to ten pigeons and live larvae were recovered from their excised livers and lungs by using the Baermann's apparatus in the next days. Two in vitro methods for larvae hatching were also performed including a so-called physiological hatching method according to Ponce-Macotela et al. (J Parasitol 175:382-385, 2010), and a mechanical hatching method according to Alcântara-Neves and Santos (J Exp Parasitol 119:349-351, 2008) and their results were compared with the in-vivo method. Results show that averagely 36.2 % of fed larvae recovered from livers and 0.15 % from lungs. Average larvae recovery in the first day after infection (24.2 %) was significantly lower than subsequent days (39 %). Maximum larvae recovered in day 3 (55 %). In-vitro methods we carried out did not have acceptable results and only a few larvae did hatch using these methods.

Toxocara canis: molecular basis of immune recognition and evasion

Toxocara canis has extraordinary abilities to survive for many years in the tissues of diverse vertebrate species, as well as to develop to maturity in the intestinal tract of its definitive canid host. Human disease is caused by larval stages invading musculature, brain and the eye, and immune mechanisms appear to be ineffective at eliminating the infection. Survival of T. canis larvae can be attributed to two molecular strategies evolved by the parasite. Firstly, it releases quantities of 'excretory-secretory' products which include lectins, mucins and enzymes that interact with and modulate host immunity. For example, one lectin (CTL-1) is very similar to mammalian lectins, required for tissue inflammation, suggesting that T. canis may interfere with leucocyte extravasation into infected sites. The second strategy is the elaboration of a specialised mucin-rich surface coat; this is loosely attached to the parasite epicuticle in a fashion that permits rapid escape when host antibodies and cells adhere, resulting in an inflammatory reaction around a newly vacated focus. The mucins have been characterised as bearing multiple glycan side-chains, consisting of a blood-group-like trisaccharide with one or two O-methylation modifications. Both the lectins and these trisaccharides are targeted by host antibodies, with anti-lectin antibodies showing particular diagnostic promise. Antibodies to the mono-methylated trisaccharide appear to be T. canis-specific, as this epitope is not found in the closely related Toxocara cati, but all other antigenic determinants are very similar between the two species. This distinction may be important in designing new and more accurate diagnostic tests. Further tools to control toxocariasis could also arise from understanding the molecular cues and steps involved in larval development. In vitro-cultivated larvae express high levels of four mRNAs that are translationally silenced, as the proteins they encode are not detectable in cultured larvae. However, these appear to be produced once the parasite has entered the mammalian host, as they are recognised by specific antibodies in infected patients. Elucidating the function of these genes, or analysing if micro-RNA translational silencing suppresses production of the proteins, may point towards new drug targets for tissue-phase parasites in humans.

Characterization of a Toxocara canis species-specific excretory-secretory antigen (TcES-57) and development of a double sandwich ELISA for diagnosis of visceral larva migrans

This study describes the isolation of a Toxocara canis species-specific excretory-secretory (ES) antigen and the development of an enzyme-linked immunosorbent assay (ELISA) based on this antigen. Analysis of the ES antigens of T. canis, Toxocara vitulorum, Ascaris lumbricoides and Necator americanus larval antigen was performed by SDS-PAGE followed by western blotting. A 57 kDa T. canis-specific antibody fraction (TcES-57) was identified by western blotting and labelling with anti-Toxocara antibodies (from experimental rabbits and human patients) and tracing with anti-human or anti-rabbit peroxidase conjugate. No protein fraction of 57 kDa was detected in ES or larval antigens collected from T. canis, T. vitulorum, A. lumbricoides and N. americanus. Using TcES-57, a specific antiserum was produced in rabbits and a double sandwich ELISA was developed. This test was validated using known seropositive sera from toxocariasis patients, sera from A. lumbricoides or N. americanus patients, and 50 serum samples from cats. These tests revealed that TcES-57 antigen is specific to T. canis infection and does not cross react with sera of other related infections. Thus, ELISA based on TcES-57 antigen was proven to be an effective tool in the diagnosis of toxocariasis and studies on the role of T. canis in the epidemiology of human toxocariasis.

O-methylated glycans from Toxocara are specific targets for antibody binding in human and animal infections

The parasitic helminth Toxocara canis is a widely distributed nematode of mammals. Larval parasites, which infect a wide range of hosts including mice and humans, export glycosylated macromolecules bearing novel methylated oligosaccharide structures, similar to the mammalian blood group antigen H but bearing one or two O-methylated substitutions on the terminal fucose and subterminal galactose residues. We have studied the reactivity of synthetic forms of these glycans to parasite-specific antibodies and mammalian immune system lectins. Murine antibodies, generated to T. canis infection, predominantly recognise the mono-O-methylated form with the beta-configuration of the GalNAc residue (MoMbeta), and antibodies are entirely IgM isotype. The mAb Tcn-2 reproduces this pattern, and shows little reactivity to either the alpha isomer (MoMalpha) or the di-O-methylated form (DiM). Antibodies generated to helminth infections other than T. canis were unreactive with the glycans, except antibodies to other members of the Toxocara genus. Hence, the carbohydrate structures represent immunogenic, genus-specific antigens. Antibodies from human toxocariasis patients are reactive with the same sugars, although preferentially towards DiM. Sera from unrelated helminth infections do not react, confirming the status of these structures as Toxocara-specific glycans. The human dendritic cell lectin, DC-SIGN, was found to bind both Toxocara excretory/secretory products and mammalian blood group antigen H3. However, DC-SIGN did not bind the synthetic glycans, indicating additional non-methylated carbohydrates may also play a role in the interaction between T. canis and its host.

A family of secreted mucins from the parasitic nematode Toxocara canis bears diverse mucin domains but shares similar flanking six-cysteine repeat motifs

Infective larvae of the parasitic nematode Toxocara canis secrete a family of mucin-like glycoproteins, which are implicated in parasite immune evasion. Analysis of T. canis expressed sequence tags identified a family of four mRNAs encoding distinct apomucins (Tc-muc-1-4), one of which had been previously identified in the TES-120 family of glycoproteins secreted by this parasite. The protein products of all four cDNAs contain signal peptides, a repetitive serine/threonine-rich tract, and varying numbers of 36-amino acid six-cysteine (SXC) domains. SXC domains are found in many nematode proteins and show similarity to cnidarian (sea anemone) toxins. Antibodies to the SXC domains of Tc-MUC-1 and Tc-MUC-3 recognize differently migrating members of TES-120. TES-120 proteins separated by chromatographic methods showed distinct amino acid composition, mass, and sequence information by both Edman degradation and matrix-assisted laser desorption ionization/time of flight mass spectrometry on peptide fragments. Tc-MUC-1, -2, and -3 were shown to be secreted mucins with real masses of 39.7, 47.8, and 45.0 kDa in contrast to their predicted peptide masses of 15.7, 16.2, and 26.0 kDa, respectively. The presence of SXC domains in all mucin products supports the suggestion that the SXC motif is required for mucin assembly or export. Homology modeling indicates that the six-cysteine domains of the T. canis mucins adopt a similar fold to the sea anemone potassium channel-blocking toxin BgK, forming three disulfide bonds within each subunit.

Toxocara canis: genes expressed by the arrested infective larval stage of a parasitic nematode

Toxocara canis is a widely distributed nematode parasite which reaches maturity in dogs. However, eggs voided by canid animals are infective to a very wide range of paratenic hosts including humans. In noncanid hosts, infective larvae emerge from the eggs and invade the soft tissues, often entering the brain and musculature. Such larvae may remain for many months or years in these tissues without further growth or differentiation, and yet appear to evade inflammatory reactions or other modes of immune attack. To understand the ability of T. canis larvae to survive in the immunocompetent host, we have undertaken a molecular analysis of the major genes expressed at this stage. By a combination of protein sequencing, gene identification, and expressed sequence tag (EST) analysis we have characterised a range of potentially important gene products from this parasite. Some of these are homologues of prominent mammalian proteins such as C-type lectins (represented by the secreted products TES-32 and TES-70), and mucins (TES-120), and additional products show strong similarities to known cysteine proteases, phosphatidylethanolamine-binding proteins and other ligands. A number of these proteins include a conspicuous 36-amino acid motif containing six cysteines. This domain (termed NC6 or SXC) appears to be an evolutionarily mobile module, which in T. canis is combined with a spectrum of diverse functional domains in different genes. In addition, we have identified a set of novel gene sequences that show no resemblance to any genes encoded by the free-living nematode C. elegans. Four of these are designated abundant novel transcripts, and collectively these account for nearly 20% of the cDNA isolated from the arrested infective stage. Such parasite-specific genes expressed at a high level by a stage that shows remarkable endurance may represent critical products necessary for the success of the parasitic mode of life.

A novel C-type lectin secreted by a tissue-dwelling parasitic nematode

Many parasitic nematodes live for surprisingly long periods in the tissues of their hosts, implying sophisticated mechanisms for evading the host immune system. The nematode Toxocara canis survives for years in mammalian tissues, and when cultivated in vitro, secretes antigens such as TES-32. From the peptide sequence, we cloned TES-32 cDNA, which encodes a 219 amino-acid protein that has a domain characteristic of host calcium-dependent (C-type) lectins, a family of proteins associated with immune defence. Homology modelling predicted that TES-32 bears remarkable structural similarity to mammalian immune-system lectins. Native TES-32 acted as a functional lectin in affinity chromatography. Unusually, it bound both mannose- and galactose-type monosaccharides, a pattern precluded in mammalian lectins by a constraining loop adjacent to the carbohydrate-binding site. In TES-32, this loop appeared to be less obtrusive, permitting a broader range of ligand binding. The similarity of TES-32 to host immune cell receptors suggests a hitherto unsuspected strategy for parasite immune evasion.

Production and characterisation of polyclonal antisera against feline IgE

Cats, naturally or experimentally infected with Toxocara cati were immunised with dinitrophenylated ascaris antigen (DNP-Asc). All cats developed immediate skin reactivity to DNP coupled to bovine serum albumin (DNP-BSA) and the sera of the nine cats had a heat labile homocytotropic antibody detectable by homologous Prausnitz-Küstner (PK) tests. Reagin-rich fractions were prepared from these sera and used for the preparation of polyclonal antisera in rabbits. Resultant antisera were passed through a immunoabsorbent column of Sepharose 4B coupled to heated normal cat serum. An immunoabsorbent column prepared with the resultant antisera removed the PK reactivity from the cat sera, and the activity was recovered following acid elution. The antiserum failed to detect any recognised immunoglobulin in cat sera, but precipitated with a heat labile protein with gamma-1 electrophoretic mobility in the sera of parasited cats. These findings support the contention that the antisera are specific for feline IgE.

Characterisation of Tc-cpl-1, a cathepsin L-like cysteine protease from Toxocara canis infective larvae

Cysteine proteases play vital biological roles in both intracellular and extracellular environments. A cysteine protease migrating at 30 kDa was identified in somatic extracts of Toxocara canis larvae (TEX), by its binding to the biotinylated inhibitor Phe-Ala-CH2F. TEX proteases readily cleaved the cathepsin L- and B-specific peptide substrate Z-Phe-Arg-AMC and to a lesser extent, the cathepsin B-specific peptide Z-Arg-Arg-AMC. Excretory/secretory (TES) products of T. canis larvae did not cleave either substrate. Partial sequence encoding the 5' end of a cysteine protease cDNA from infective T. canis larvae was then obtained from an expressed sequence tag (EST) project. The entire cDNA (termed Tc-cpl-1) was subsequently sequenced and found to encode a preproenzyme similar to cathepsin L-like proteases (identities between 36 and 69%), the closest homologues being two predicted proteins from Caenorhabditis elegans cosmids, a cathepsin L-like enzyme from Brugia pahangi and a range of parasite and plant papain-like proteases. Sequence alignment with homologues of known secondary structure indicated several charged residues in the S1 and S2 subsites involved in determining substrate specificity. Some of these are shared with human cathepsin B, including Glu 205 (papain numbering), known to permit cleavage of Arg-Arg peptide bonds. The recombinant protease (rTc-CPL-1) was expressed in bacteria for immunisation of mice and the subsequent antiserum shown to specifically react with the 30 kDa native protease in TEX. Sera from mice infected with the parasite also contained antibodies to rTc-CPL-1 as did sera from nine patients with proven toxocariasis; control sera did not. Larger scale studies are underway to investigate the efficacy of rTc-CPL-1 as a diagnostic antigen for human toxocariasis, the current test for which relies on whole excretory/secretory antigens of cultured parasites.

An abundant, trans-spliced mRNA from Toxocara canis infective larvae encodes a 26-kDa protein with homology to phosphatidylethanolamine-binding proteins

A full-length mRNA encoding a secreted 26-kDa antigen of infective larvae of the ascarid nematode parasite Toxocara canis has been identified. This was characterized as a 1,082-base pair clone highly abundant (0.8-1.9%) in cDNA prepared from infective stage larvae but absent from cDNA from adult male worms. Sequence analysis revealed an open reading frame corresponding to a hydrophilic 263-amino acid residue polypeptide with a 20-residue N-terminal signal peptide, indicating that it is secreted. The 5' end of the cDNA was isolated by polymerase chain reaction using a primer containing the nematode-spliced leader sequence, SL1, showing that the mRNA is trans-spliced. The molecular mass of the putative protein with the signal peptide removed is 26.01 kDa, and antibody to the recombinant protein expressed in bacterial vectors reacts with a similarly sized protein in T. canis excretory/secretory (TES) products. An identical sequence was obtained from a genomic clone isolated by expression screening with mouse antibody to TES. The 72 amino acid residues adjacent to the signal peptide form two homologous 36-residue motifs containing 6 cysteine residues; this motif is found also in the T. canis-secreted glycoprotein TES-120 and in genes of Caenorhabditis elegans. Sequence data base searches revealed significant similarity to 7 other sequences in a newly recognized gene family of phosphatidylethanolamine-binding proteins that includes yeast, Drosophila, rat, bovine, simian, and human genes and a representative from the filarial nematode Onchocerca volvulus. Assays with the T. canis recombinant 26-kDa protein expressed as a fusion with maltose-binding protein have confirmed phosphatidylethanolamine-binding specificity for this novel product.

Metabolic labelling of wild and laboratory subspecies of the trichostrongyle nematode Heligmosomoides polygyrus

The immunological, biochemical and taxonomic relationship between wild and laboratory subspecies of Heligmosomoides polygyrus was studied by metabolically labelling parasite proteins with [35S]-methionine. Much variability, both in content and synthesis of proteins was observed between the two subspecies. Laboratory female worms had a higher protein content and incorporated more radioactive label into somatic proteins than their wild counterparts. Incorporation of radioactivity into excretory/secretory (ES) proteins, predicted to contain important antigens, demonstrates a major reduction in synthesis of proteins with molecular weights 66, 55, 43, 41, 40, 39, 37, 28 and 16 kDa by laboratory females. These differences in protein synthesis might explain the differing infectivities of the two subspecies when passaged in inbred laboratory (Mus musculus) and wild (Apodemus sylvaticus) mice.

Epidemiology of Toxocara canis in red foxes (Vulpes vulpes) from urban areas of Bristol

A descriptive epidemiological survey was undertaken of the ascarid nematode Toxocara canis in 521 red foxes (Vulpes vulpes) during the period January 1986 to July 1990. Age-prevalence and age-intensity profiles show that worm burdens are significantly higher in cubs than in subadult or adult foxes and higher in subadult than in adult foxes. Seasonal variations in worm burdens occur, with the highest prevalences and intensities being found during the spring, when cubs are born, and in the summer months. Prevalences and intensities then decrease during the autumn and winter months in both subadult and adult foxes, but, during this period, prevalences are significantly higher in male than in female foxes. Variations in worm burdens in the fox population are likely to be related to the reproductive cycle of the fox, with a high proportion of cubs becoming infected in utero. The role of the fox in the transmission of T. canis in the urban environment is discussed.

Diagnosis of human toxocariasis by antigen capture enzyme linked immunosorbent assay

AIMS

To evaluate an antigen capture enzyme linked immunosorbent assay (ELISA) which detects a carbohydrate epitope on the excretory-secretory (ES) antigens of Toxocara canis in clinical practice.

METHODS

Serum specimens from healthy adults, patients with acute visceral larva migrans, ocular and inactive toxocariasis, and with other helminth infections were examined by two site antigen capture ELISA.

RESULTS

Over half of the patients (19/28) with acute toxocariasis had a positive result in contrast to a small proportion of those with inactive disease (1/10) or ocular infection (2/7). False positive reactions, however, were found in 25% of the patients with serologically confirmed schistosomiasis and filariasis.

CONCLUSIONS

This assay is useful in confirming the diagnosis of acute visceral larva migrans but could not be used alone in diagnosis because of false positive reactions in patients with other helminth infections.

Lectin binding to secretory structures, the cuticle and the surface coat of Toxocara canis infective larvae

Toxocara canis infective larvae are known to produce abundant glycosylated molecules which may be found associated with the surface or secreted into their environment. Using a range of fluorescein-conjugated and gold-conjugated lectins, the localization of particular carbohydrates was defined on the surface of live parasites, and internally at the ultrastructural level. Surface exposure of N-acetyl galactosamine and N-acetyl glucosamine was deduced by binding of FITC-conjugated Helix pomatia (HPA) and wheat-germ agglutinins (WGA). These sugars appear to be associated with a densely staining surface coat as conventional immuno-electron microscopy procedures dissipate this coat and reveal no surface binding site for these lectins. However, by using cryo-immuno-electron microscopical (C-IEM) techniques, the surface coat is retained and can be shown to bind WGA. The fluorescent lectins also revealed strong WGA binding to the secretory and amphidial pores, while the buccal opening and the cuticular alae bound HPA. Corresponding results were obtained at the ultra-structural level. Thus, HPA bound to the electron-dense area of the cuticle, areas of local cuticular thickening such as the alae and buccal labia, as well as to the oesophageal lumen. WGA also bound to the thickened cuticle of the alae and the buccal opening, but showed no reaction to either the electron-dense layer of the cuticle or the oesophageal lumen. Unlike HPA, WGA did bind specifically to the secretory column contents and the electron-dense regions of the lips associated with the chemosensory amphids. The compartmentalization of the sugars N-acetyl galactosamine and N-acetyl glucosamine, their sources and routes of surface expression and the possible association with the TES glycoprotein antigens are discussed.

Biosynthesis and glycosylation of serine/threonine-rich secreted proteins from Toxocara canis larvae

Toxocara canis infective stage larvae continually produce excretory-secretory (TES) glycoproteins in long-term in vitro culture. The kinetics of synthesis and secretion were studied by metabolic labelling with radioactive [35S]methionine, [14C]serine and [14C]threonine. Maximal incorporation rates required overnight pre-incubation of parasites in medium depleted of the appropriate amino acid. Larvae rapidly incorporated isotope into their somatic tissues, but there was a minimum delay of 10 h before secretion of labelled antigens. Labelling with [14C]serine and [14C]threonine demonstrated a relative abundance of these amino acids in the major surface/secreted glycoproteins of this nematode (TES-32 and 120). Pulse-chase experiments suggested that TES-120 may be derived from a 58 kDa precursor, reflecting extensive posttranslational glycosylation. Inhibition of N-glycosylation with tunicamycin and digestion with N-glycanase provided evidence of N-glycosylation in the lower molecular weight ES components (TES-32, 55 and 70). These agents had no effect on the higher molecular weight components (TES-120 and 400) implying that for these molecules glycosylation is predominantly O-linked. The largest ES component (TES-400) was unusual, in incorporating serine and threonine but not methionine, and by exhibiting increased apparent molecular weight following pronase digestion; it is suggested that this molecule is a proteoglycan.

Toxocara canis: monoclonal antibodies to carbohydrate epitopes of secreted (TES) antigens localize to different secretion-related structures in infective larvae

The major secreted glycoproteins of Toxocara canis larvae appear to be derived from two specialized organs within the nematode organism. Using immunogold electron microscopy, we have analyzed the binding patterns of a panel of monoclonal antibodies (Tcn-1 to Tcn-8) reactive with Toxocara excretory-secretory (TES) antigens. We find, first, that the esophageal gland and lumen are strongly reactive with monoclonals Tcn-4, Tcn-5, and Tcn-8, and because the posterior portion of the gut is closed, we hypothesize that products of this gland are released through the oral aperture. Second, a distinct anti-TES antibody (Tcn-2) localizes solely to the midbody secretory column, which opens onto the cuticle at a secretory pore. Thus, the secretory apparatus is probably functional in this stage of parasite as an important source of TES products. Only one monoclonal, Tcn-7, can bind to both esophageal and secretory structures. In addition, another antibody, Tcn-3, binds both to the epicuticle and to a TES antigen, but our data do not directly determine whether antigens located in the cuticle are subsequently released. Thus there are at least two, and possibly three, independent sources of TES antigens within Toxocara larvae.

Toxocara canis: a labile antigenic surface coat overlying the epicuticle of infective larvae

An electron-dense coat covering the surface of Toxocara canis infective-stage larvae is described. This coat readily binds to cationized ferritin and ruthenium red, indicating a net negative charge and mucopolysaccharide content, and can be visualized by immuno-electron microscopy only if cryosectioning is employed. Monoclonal antibodies reactive to the surface of live larvae bind the surface coat but not the underlying cuticle in ultrathin cryosections. The surface coat is dissipated on exposure to ethanol, explaining the lack of surface reactivity of conventionally prepared immunoelectron microscopy sections of T. canis. Differential ethanol extraction of surface-iodinated larvae demonstrates that the major component associated with the coat is TES-120, a 120-kDa glycoprotein previously identified by surface iodination, which is also a dominant secreted product. The surface-labeled TES-70 glycoprotein is linked with a more hydrophobic stratum at the surface, while a prominent 32-kDa glycoprotein, TES-32, is more strongly represented within the cuticle itself. Antibody binding to the coat under physiological conditions results in the loss of the surface coat, but this process is arrested at 4 degrees C. This result gives a physical basis to earlier observations on the shedding of surface-bound antibodies by this parasite. An extracuticular surface coat has been demonstrated on Toxocara larvae prior to hatching from the egg and during all stages of in vitro culture, suggesting that it may play a role both in protecting the parasite on hatching in the gastrointestinal tract and on subsequent tissue invasion in evading host immune responses directed at surface antigens.