Physicochemical characterization and monoclonal and polyclonal antibody recognition of Baylisascaris procyonis larval excretory-secretory antigens

Baylisascaris procyonis larval excretory-secretory (ES) antigens consisted of complex glycoproteins ranging from 10 kDa to over 200 kDa as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis and lectin binding. Five monoclonal antibodies (Bapr1-Bapr5) produced against B. procyonis ES antigens were assayed by western blotting with larval ES antigens from B. procyonis, Baylisascaris melis, Baylisascaris transfuga, Ascaris suum, and Toxocara canis. Bapr1 and Bapr2 recognized periodate-sensitive epitopes on 14-kDa ES components of B. procyonis, B. melis, and B. transfuga, whereas Bapr4 and Bapr5 recognized periodate-resistant epitopes present on 55-kDa ES components of B. procyonis and B. melis. Bapr3 primarily recognized periodate-resistant epitopes on 33-45-kDa components of B. procyonis and B. melis ES. Heterologous rabbit antisera cross-reacted with many B. procyonis ES antigens on western blots, but recognition of the 33-45-kDa components was genus-specific. Normal human sera and T. canis-positive human sera also cross-reacted with many B. procyonis ES antigens, including those of 33-45 kDa. However, periodate oxidation markedly decreased cross-reactions and allowed for differential immunodiagnosis of B. procyonis versus T. canis. These studies demonstrated that antibody recognition of carbohydrate epitopes on ES components is an important cause of cross-reactions in antibody detection assays. Recognition of periodate-resistant (protein) epitopes on the 33-45-kDa B. procyonis ES components appears to be useful for genus-specific immunodiagnosis of larva migrans caused by Baylisascaris spp.     

Toxocara canis: monoclonal antibodies to larval excretory-secretory antigens that bind with genus and species specificity to the cuticular surface of infective larvae

When maintained in culture, the infective-stage larvae of Toxocara canis produce a group of excretory-secretory antigens. Monoclonal antibodies to these antigens have been produced and partially characterized. Hybridomas were made using spleens from mice that had been given 250 embryonated eggs of T. canis followed by immunization with excretory-secretory antigens. Monoclonal antibodies were first screened against excretory-secretory antigens using an indirect enzyme-linked immunosorbent assay. Those antibodies positive in this assay were then screened against the surfaces of formalin-fixed, infective-stage larvae using an indirect fluorescent antibody assay. The two monoclonal antibodies showing fluorescence were also tested against the surfaces of infective-stage larvae of Toxocara cati, Baylisascaris procyonis, Toxascaris leonina, Ascaris suum, a Porrocaecum sp., and Dirofilaria immitis. One of these two antibodies bound to the surface of T. canis and T. cati while the other bound only to the surface of T. canis; neither were reactive with the other ascaridoid larvae or the larvae of D. immitis. Enzyme-linked immunoelectrotransfer blotting techniques were used to demonstrate that the cross-reactive antibody recognized antigens with molecular weights of about 200 kDa while the more specific monoclonal antibody recognized antigens with approximate molecular weights of 80 kDa. The specificity of these two antibodies for T. canis and T. cati should prove helpful in the development of more specific assays for the diagnosis of visceral and ocular larva migrans.     

Biosynthetic labelling of the excretory and secretory antigens of Toxocara canis larvae

Toxocara canis larvae were cultured in vitro in medium containing [35S-]methionine for six days. The medium and the larval tissues were analysed for biosynthetically labelled polypeptides by sodium dodecyl sulphate polyacrylamide gel electrophoresis and autoradiography. Immunoprecipitates with positive and negative human antiserum were similarly analysed, using Staphylococcus aureus to absorb immunocomplexes. The larvae secrete biosynthetically labelled polypeptides into the medium, with three major polypeptides of molecular weights between 99 and 110 X 10(3) the major constituents. Both of these react strongly with human IgG in human positive sera. Many polypeptides become labelled in the larval tissue, but only one polypeptide with similar molecular weight to the ES antigens, strongly reacted with human IgG.     

Immunodiagnosis of parasitic zoonoses: purification of Toxocara canis antigens by affinity chromatography

A method of affinity chromatography developed for the purification of species-specific antigens from Toxocara canis adult worms is described. Immunochemical analyses by polyacrylamide gel electrophoresis, immunoelectrophoresis and immunodiffusion showed that ‘pure’ antigen contained fewer but more specific proteins than ‘crude’ antigen. Purified antigens and parasite sections from four parasite species (Toxocara canis, Dirofilaria immitis, Angiostrongylus cantonensis and Ascaris lumbricoides) were used in immunofluorescence tests to measure serum antibody levels in animals with natural or experimental T. canis infections and people with zoonotic toxocariasis. ‘Pure’ antigen showed higher specificity and sensitivity than ‘crude’ antigen in serological testing.     

Visceral larva migrans: examinations by means of enzyme-linked immunosorbent assay of human sera for antibodies to excretory-secretory antigens of the second-stage larvae of Toxocara canis

An enzyme-linked immunosorbent assay is described for the detection of serum antibodies to visceral larva migrans (Toxocariasis). Excretory-secretory antigens of the second-stage larvae of Toxocara canis were used as antigen to coat the polystyrene plates. With sera from patients high antibody titers were observed in both ocular and visceral disorders. Cross-reactions due to other parasitic infections could be excluded, including other migrating larval infections such as ascariasis, trichinellosis, strongyloidiasis, filariasis, and anisakiasis. In a small seroepidemiologic survey of healthy primary schoolchildren, a remarkably high percentage (7.1) reacted positively to this method. These children showed eosinophilia as compared to the seronegative group. The data were compared with those observed in other countries and the results prompt reconsideration of the significance of T. canis for public health.     

Characterization of nematode glycoproteins: the major O-glycans of Toxocara excretory-secretory antigens are O-methylated trisaccharides.

Toxocara excretory-secretory antigens (TES) were isolated from the culture media of T.canis and T.cati larvae and their O-glycan content was investigated using fast atom bombardment-mass spectrometry (FAB-MS), gas chromatography and electron impact mass spectrometry. The major oligosaccharides released by reductive elimination of T.canis TES glycoproteins were shown to be two, approximately equi-abundant, trisaccharides: 2-O-Me-Fucp(alpha 1----2)-4-O-Me-Galp(beta 1----3)GalNAcitol and 2-O-Me-Fucp(alpha 1----2)-Galp(beta 1----3)GalNAcitol. In contrast T.cati TES O-glycans are predominantly one component, shown by FAB-MS to be a di-O-methylated trisaccharide, which is probably identical to the di-O-methylated trisaccharide from T.canis. The O-methylated trisaccharides are strong candidates for the carbohydrate epitopes recognized by a panel of monoclonal antibodies which exhibit multiple reactivity against TES antigens. This study constitutes the first rigorous characterization of glycans from a parasitic nematode.     

The epidemiology of Toxocara canis

Bred as hunter, companion and pet, the dog has a long and honourable association with man. Yet the domestic dog can host a wide range of parasites - many of which can also infect humans. One of these, the ascarid nematode Toxocara canis (Fig. 1), is of particular interest because of retinal damage that may result from larvae becoming trapped in the eye. At the Hospital for Tropical Diseases in London, about 20-30 patients with toxocariasis are treated annually. Widespread fouling of public parks, playgrounds and pedestrian areas with dog faeces - especially in large cities - is well -recognized as one of the main sources of Toxocara infection. Yet as Stephen Gillespie discusses here, epidemiological indicators vary widely and the risk of infection is often treated too lightly.     

The use of 35S-labelled Toxocara canis to determine larval numbers in tissues.

The potential of using 35S-labelled larvae to determine the number of second-stage Toxocara canis larvae present in the tissues of infected animals was assessed. Infective larvae were labelled by in vitro culture with medium containing 35S-methionine. The amount of radiolabel attached to larvae decayed exponentially with time and had an in vitro mean half life of 3.54 +/- 0.65 days. The 'lost' radiolabel was incorporated into proteins which formed part of the worm's excretory/secretory products. The levels of radioactivity present in different organs of BALB/c mice, infected with 35S-labelled T. canis larvae, varied over the course of infection. Initially most of the radioactivity was present in liver, but over the course of infection 35S liver levels gradually decreased and brain levels increased. By day 14 post-infection the majority of the isotope was present in the brain (p < 0.01). Assessment of antibody levels on day 14 post-infection showed that infection with 35S labelled T. canis larvae induced the production of parasite-specific IgM, IgG and IgG1 antibodies.     

Toxocara canis and the allergic process

The protective effect of infectious agents against allergic reactions has been thoroughly investigated. Current studies have demonstrated the ability of some helminths to modulate the immune response of infected hosts. The objective of the present study was to investigate the relationship between Toxocara canis infection and the development of an allergic response in mice immunised with ovalbumin (OVA). We determined the total and differential blood and bronchoalveolar lavage fluid cells using BALB/c mice as a model. To this end, the levels of interleukin (IL)-4, IL-5 and IL-10 and anti-OVA-IgE were measured using an ELISA. The inflammatory process in the lungs was observed using histology slides stained with haematoxylin and eosin. The results showed an increase in the total number of leukocytes and eosinophils in the blood of infected and immunised animals at 18 days after infection. We observed a slight lymphocytic inflammatory infiltrate in the portal space in all infected mice. Anti-OVA-IgE levels were detected in smaller proportions in the plasma of immunised and infected mice compared with mice that were only infected. Therefore, we concluded that T. canis potentiates inflammation in the lungs in response to OVA, although anti-OVA-IgE levels suggest a potential reduction of the inflammatory process through this mechanism.     

Analysis of excretory-secretory and somatic antigens of Gastrothylax crumenifer

The excretory/secretory (ES) metabolic products released by Gastrothylax crumenifer (Trematoda: Digenea) during in vitro incubations and the somatic extract of the adult parasite were analysed using polyacrylamide gel electrophoresis (PAGE). Immunogenicity of ES and somatic extracts were evaluated by immunoblotting and ELISA using sera raised against ES and somatic antigens in rabbits. The electropherograms of ES and somatic extracts have been resolved into 38 and 41 polypeptides, respectively. The apparent molecular weights of these polypeptides range from < 29 to > 205 kDa. A total of 14 polypeptides were found to be common to both of the samples. The analysis of immunoblot results revealed 22 and 27 antigenic polypeptides in ES and somatic extracts respectively. Only 11 and 13 antigenic polypeptides were found specific to ES and tissue extract respectively. The molecular weights of these specific polypeptides were calculated to be < 14.4, 16, 20, 25, 33, 42, 119, 125 and > 205 kDa for metabolic products and < 14.4, 25, 30, 35, 78, 84 and > 205 kDa for the tissue extracts, respectively. Analysis of ELISA results revealed that a dilution of up to 1:3200 of the test sera could react with the ES product. Further, when the ES antigens were allowed to react with antisomatic extracts in hyperimmune sera the titre of IgG increased up to a dilution of 1:12800. The potential importance of these antigens in the immunodiagnosis of amphistomiasis is discussed.     

Analysis of excretory-secretory and somatic antigens of Gastrothylax crumenifer

The excretory/secretory (ES) metabolic products released by Gastrothylax crumenifer (Trematoda: Digenea) during in vitro incubations and the somatic extract of the adult parasite were analysed using polyacrylamide gel electro-phoresis (PAGE). Immunogenicity of ES and somatic extracts were evaluated by immunoblotting and ELISA using sera raised against ES and somatic antigens in rabbits. The electropherograms of ES and somatic extracts have been resolved into 38 and 41 polypeptides, respectively. The apparent molecular weights of these polypeptides range from <29 to > 205 kDa. A total of 14 polypeptides were found to be common to both of the samples. The analysis of immunoblot results revealed 22 and 27 antigenic polypeptides in ES and somatic extracts respectively. Only 11 and 13 antigenic polypeptides were found specific to ES and tissue extract respectively. The molecular weights of these specific polypeptides were calculated to be <14.4, 16, 20, 25, 33, 42, 119, 125 and > 205 kDa for metabolic products and <14.4, 25, 30, 35, 78, 84 and > 205 kDa for the tissue extracts, respectively. Analysis of ELISA results revealed that a dilution of up to 1:3200 of the test sera could react with the ES product. Further, when the ES antigens were allowed to react with antisomatic extracts in hyperimmune sera the titre of IgG increased up to a dilution of 1:12800. The potential importance of these antigens in the immunodiagnosis of amphistomiasis is discussed.     

Shared carbohydrate epitopes on distinct surface and secreted antigens of the parasitic nematode Toxocara canis

The nematode parasite Toxocara canis is found in all dog populations and poses a poorly defined health hazard to humans. We have studied excretory-secretory antigen (ES) and surface antigens of the infective larval stage which is tissue-invasive in mammalian hosts. Antigens were probed with a panel of eight monoclonal antibodies raised in mice to whole ES. Six of eight antibodies reacted with periodate-sensitive carbohydrate epitopes on ES molecules, and the remaining two (Tcn-3 and Tcn-6) recognized either peptide or periodate-resistant sugar determinants. By immunoprecipitation and immunoblotting, the anti-carbohydrate monoclonals each reacted with several distinct ES molecules, known from previously published work to possess contrasting biochemical properties. Tcn-3 and -6 were directed predominantly against 32,000 and 120,000 m.w. molecules, respectively. Iodinated surface antigens of similar m.w. were precipitated by each antibody after detergent solubilization, but only two clones (Tcn-2 and -8) were able to bind exposed sites on the epicuticle of intact Toxocara larvae. Significantly, these antibodies do not bind to newly hatched larvae, and their target antigens are poorly expressed until the second day of in vitro cultivation. The specificities of the monoclonals were further studied by cold antibody inhibition of radiolabeled monoclonal binding, and by a matrix of two-site binding assays. These data show that Tcn-2, -4, -5, and -8 recognize a related group of repetitive carbohydrate epitopes, whereas Tcn-1, -6, and -7 bind discrete determinants on the same molecules. These studies are being continued to define further the structure of antigenic Toxocara carbohydrates and to compare the diagnostic utility of carbohydrate and peptide antigens.     

Follow-up of antibody avidity in BALB/c mice infected with Toxocara canis

In human Toxocara canis infection, an association has been shown between high IgG avidity in the chronic phase and low IgG avidity in recently acquired toxocarosis. The evolution of the antibody response in terms of avidity has been carried out through a T. canis infection in BALB/c mice. Infection with T. canis embryonated eggs (EE) was carried out with single doses (SD) of 6, 12, 50, 100, 200 or 1000 EE/mouse and with multiple doses (MD) of 200 and 1000 EE. Specific antibodies against T. canis (IgM+G, IgG, IgG1 and IgM) were detected by ELISA and Western Blot (WB) techniques in the presence and absence of urea. With the ELISA method, an increase in the avidity index (AI) of around 50% was detected from days 40-80 p.i. to the end of the study, with all the doses studied. The WB method showed the presence of high avidity antibodies bound to 100 kDa and 75 kDa T. canis proteins in all the cases when the IgM+G and the IgG1 antibodies were investigated. Antibodies of variable avidity were observed in those sera that recognized the group of low molecular weight proteins, between 37 kDa and 25 kDa.     

Do Toxocara canis larval antigens used in enzyme-linked immunosorbent assay for visceral larva migrans cross-react with AB isohemagglutinins and give false positive results?

A variety of helminth parasites have A and B blood group antigens on their surface. These antigens may cross-react with elevated concentrations of A and B isohemagglutinins in some patients and give false-positive results in the serologic diagnosis of visceral larva migrans caused by T. canis. To clarify this point, serum from patients with visceral larva migrans and elevated T. canis antibody titers as determined by ELISA were absorbed with AB blood cells and retested by ELISA without a demonstrable decline in T. canis antibody titers. Similarly, absorption with T. canis embryonated egg antigens of serum containing elevated levels of anti-A or anti-B isohemagglutinins failed to decrease the isohemagglutinin titer. This indicates that the ELISA using T. canis embryonated egg antigen does not give false positive results with sera containing high concentrations of anti-A or anti-B isohemagglutinins.