The specificity of the antibody response to internal antigens of Ascaris: heterogeneity in infected humans, and MHC (H-2) control of the repertoire in mice

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.

CD4+ Th immunogenicity of the Ascaris spp. secreted products

Ascaris spp. is a major health problem of humans and animals alike, and understanding the immunogenicity of its antigens is required for developing urgently needed vaccines. The parasite-secreted products represent the most relevant, yet complex (>250 proteins) antigens of Ascaris spp. as defining the pathogen-host interplay. We applied an in vitro antigen processing system coupled to quantitative proteomics to identify potential CD4⁺ T_h cell epitopes in Ascaris-secreted products. This approach considerably restricts the theoretical list of epitopes using conventional CD4⁺ T_h cell epitope prediction tools. We demonstrate the specificity and utility of our approach on two sets of candidate lists, allowing us identifying hits excluded by either one or both computational methods. More importantly, one of the candidates identified experimentally, clearly demonstrates the presence of pathogen-reactive T cells in healthy human individuals against these antigens. Thus, our work pipeline identifies the first human T cell epitope against Ascaris spp. and represents an easily adaptable platform for characterization of complex antigens, in particular for those pathogens that are not easily amenable for in vivo experimental validation.

Genetic Variants in CHIA and CHI3L1 Are Associated with the IgE Response to the Ascaris Resistance Marker ABA-1 and the Birch Pollen Allergen Bet v 1

Helminth infections and allergic diseases are associated with IgE hyperresponsiveness but the genetics of this phenotype remain to be defined. Susceptibility to Ascaris lumbricoides infection and antibody levels to this helminth are associated with polymorphisms in locus 13q33-34. We aimed to explore this and other genomic regions to identify genetic variants associated with the IgE responsiveness in humans. Forty-eight subjects from Cartagena, Colombia, with extreme values of specific IgE to Ascaris and ABA-1, a resistance marker of this nematode, were selected for targeted resequencing. Burden analyses were done comparing extreme groups for IgE values. One-hundred one SNPs were genotyped in 1258 individuals of two well-characterized populations from Colombia and Sweden. Two low-frequency coding variants in the gene encoding the Acidic Mammalian Chitinase (CHIA rs79500525, rs139812869, tagged by rs10494133) were found enriched in high IgE responders to ABA-1 and confirmed by genetic association analyses. The SNP rs4950928 in the Chitinase 3 Like 1 gene (CHI3L1) was associated with high IgE to ABA-1 in Colombians and with high IgE to Bet v 1 in the Swedish population. CHIA rs10494133 and ABDH13 rs3783118 were associated with IgE responses to Ascaris. SNPs in the Tumor Necrosis Factor Superfamily Member 13b gene (TNFSF13B) encoding the cytokine B cell activating Factor were associated with high levels of total IgE in both populations. This is the first report on the association between low-frequency and common variants in the chitinases-related genes CHIA and CHI3L1 with the intensity of specific IgE to ABA-1 in a population naturally exposed to Ascaris and with Bet v 1 in a Swedish population. Our results add new information about the genetic influences of human IgE responsiveness; since the genes encode for enzymes involved in the immune response to parasitic infections, they could be helpful for understanding helminth immunity and allergic responses. We also confirmed that TNFSF13B has an important and conserved role in the regulation of total IgE levels, which supports potential evolutionary links between helminth immunity and allergic response.

Seropositivity for ascariosis and toxocariosis and cytokine expression among the indigenous people in the Venezuelan Delta region

The present study aimed at measuring seropositivities for infection by Ascaris suum and Toxocara canis using the excretory/secretory (E/S) antigens from Ascaris suum (AES) and Toxocara canis (TES) within an indigenous population. In addition, quantification of cytokine expressions in peripheral blood cells was determined. A total of 50 Warao indigenous were included; of which 43 were adults and seven children. In adults, 44.1% were seropositive for both parasites; whereas children had only seropositivity to one or the other helminth. For ascariosis, the percentage of AES seropositivity in adults and children was high; 23.3% and 57.1%, respectively. While that for toxocariosis, the percentage of TES seropositivity in adults and children was low; 9.3% and 14.3%, respectively. The percentage of seronegativity was comparable for AES and TES antigens in adults (27.9%) and children (28.6%). When positive sera were analyzed by Western blotting technique using AES antigens; three bands of 97.2, 193.6 and 200.2 kDas were mostly recognized. When the TES antigens were used, nine major bands were mostly identified; 47.4, 52.2, 84.9, 98.2, 119.1, 131.3, 175.6, 184.4 and 193.6 kDas. Stool examinations showed that Blastocystis hominis, Hymenolepis nana and Entamoeba coli were the most commonly observed intestinal parasites. Quantification of cytokines IFN-γ, IL-2, IL-6, TGF-β, TNF-α, IL-10 and IL-4 expressions showed that there was only a significant increased expression of IL-4 in indigenous with TES seropositivity (p < 0.002). Ascaris and Toxocara seropositivity was prevalent among Warao indigenous.

Maintaining microendemic primate species along an environmental gradient - parasites as drivers for species differentiation

Understanding the drivers of species adaptations to changing environments on the one hand and the limits for hybridization on the other hand is among the hottest questions in evolutionary biology. Parasites represent one of the major selective forces driving host evolution and at least those with free-living stages are at the same time dependent on the ecological conditions of their host's habitat. Local immunological adaptations of host species to varying parasite pressure are therefore expected and might represent the genetic basis for ecological speciation and the maintenance of recently diverged species. Madagascar provides one of the rare examples where two partially sympatric primate species (Microcebus griseorufus, M. murinus) and their hybrids, as well as an allopatric species (M. cf rufus) live in close proximity along a very steep environmental gradient ranging from southern dry spiny bush to gallery forest to evergreen eastern humid rain forest, thus mimicking the situation encountered during extensions and retreats of vegetation formations under changing climatic conditions. This system was used to study parasite infection and immune gene (MHC) adaptations to varying parasite pressure that might provide selective advantages to pure species over hybrids. Parasite burdens increased with increasing humidity. M. griseorufus, M. murinus, and their hybrids but not M. rufus shared the same MHC alleles, indicating either retention of ancestral polymorphism or recent gene flow. The hybrids had much higher prevalence of intestinal parasites than either of the parent species living under identical environmental conditions. The different representation of parasites can indicate a handicap for hybrids that maintains species identities.

The strength of the antibody response to the nematode Ascaris lumbricoides inversely correlates with levels of B-Cell Activating Factor (BAFF)

Background

B-Cell Activating Factor (BAFF) is a cytokine regulating antibody production. Polymorphisms in the gene encoding BAFF were associated with the antibody response to Ascaris but not to mite allergens. In the present study we evaluated the relationship between BAFF and specific antibodies against Ascaris and mites in 448 controls and 448 asthmatics. Soluble BAFF was measured by ELISA and BAFF mRNA by qPCR. Surface expression of BAFF and its receptor (BAFF-R) was analyzed by flow cytometry.

Results

Individuals with specific IgE levels to Ascaris >75th percentile had lower levels of soluble BAFF; those with specific IgG levels to Ascaris >75th percentile had reduced BAFF mRNA. Total IgE and specific IgE to mites were not related to BAFF levels. There were no differences in soluble BAFF or mRNA levels between asthmatics and controls. There was an inverse relationship between the cell-surface expression of BAFF-R on CD19⁺ B cells and BAFF levels at the transcriptional and protein level.

Conclusions

These findings suggest that differences in BAFF levels are related to the strength of the antibody response to Ascaris.

IgE cross-reactivity between Ascaris lumbricoides and mite allergens: possible influences on allergic sensitization and asthma

Nematode infections such as Ascariasis are important health problems in underdeveloped countries, most of them located in the tropics where environmental conditions also promote the perennial co-exposure to high concentrations of domestic mite allergens. Allergic diseases are common, and most of patients with asthma exhibit a predominant and strong IgE sensitization to mites. It is unknown whether co-exposure to Ascaris lumbricoides and the domestic mites Blomia tropicalis and Dermatophagoides pteronyssinus potentiates Th2 responses and IgE sensitization, thereby modifying the natural history of allergy. Recently, we obtained experimental evidence of a high cross-reactivity between the allergenic extracts of these invertebrates, involving well-known allergens such as tropomyosin and glutathione transferases. There is indirect evidence suggesting that the clinical impact of these findings may be important. In this review, we discuss the potential role of this cross-reactivity on several aspects of allergy in the tropics that have been a focus of a number of investigations, some of them with controversial results.

Functional associations of similar MHC alleles and shared parasite species in two sympatric lemurs

Several recent studies of animals in their natural surroundings found evidence for effects of certain major histocompatibility complex (MHC) immune gene alleles on the parasite load. However, in multi-infected individuals the particular selection pressure exerted by specific parasites has rarely been explored. In this study we took advantage of the parasitological and genetic data of two previously investigated Malagasy lemur species (Cheirogaleus medius and Microcebus murinus). We investigated whether the two sympatric and ecologically similar primates are infected by similar parasite species and explored if certain parasites are associated with particular MHC alleles. Our study revealed that most of the parasite egg morphotypes were found in both hosts. In each lemur species we identified one MHC allele which was positively associated with Ascaris-infection. Interestingly, these MHC alleles were very similar to each other but differed from all other investigated MHC alleles in an amino acid substitution in a putative functional important antigen binding site. Thus, our study gives first intriguing evidence for a direct connection between certain antigen binding sites of MHC molecules with a particular parasite in two wild primate populations. This may indicate that indeed certain parasites exert direct selective pressure on the MHC of wild living hosts.

Longitudinal clinical and serological survey of abdominal angiostrongyliasis in Guaporé, southern Brazil, from 1995 to 1999

Abdominal angiostrongyliasis is a zoonotic infection caused by Angiostrongylus costaricensis, a nematode with an intra-vascular location in the mesentery. Our objective was to address several aspects of the natural history of this parasitosis, in a longitudinal clinical and seroepidemiological study. A total of 179 individuals living in a rural area with active transmission in southern Brazil were followed for five years (1995-1999) resulting in yearly prevalence of 28.2%, 4.2%, 10%, 20.2% and 2.8% and incidences of 0%, 5.9%, 8% and 1.5%, respectively. Both men and woman were affected with higher frequencies at age 30-49 years. In 32 individuals serum samples were collected at all time points and IgG antibody reactivity detected by ELISA was variable and usually persisting not longer than one year. Some individual antibody patterns were suggestive of re-infection. There was no association with occurrence of abdominal pain or of other enteroparasites and there was no individual with a confirmed (histopathologic) diagnosis. Mollusks were found with infective third-stage larvae in some houses with an overall prevalence of 16% and a low parasitic burden. In conclusion, abdominal angiostrongyliasis in southern Brazil may be a frequent infection with low morbidity and a gradually decreasing serological reactivity.

Allergen-specific IgE and IgG4 are markers of resistance and susceptibility in a human intestinal nematode infection

IgG4 has been proposed to act as a 'blocking antibody' due to its ability to compete for the same epitopes as IgE thus preventing IgE-dependent allergic responses. IgG4 and IgE are both elevated in helminth infections and strong anti-parasite IgE responses are associated with resistance to infection. We wished to determine the relationship between anti-parasite IgG4 and IgE and Ascaris lumbricoides infection status. We examined anti-parasite responses, including antibody levels to recombinant Ascaris allergen-1A (rABA-1A), a target of serum IgE in endemic populations. Worm burden was indirectly estimated by measuring parasite egg output in a cross-sectional human population (N = 105). Levels of anti-parasite IgG4 and IgE in patients' plasma were quantified by immunoassay. Global anti-parasite antibody responses did not bear any significant relationships with intensity of Ascaris infection. Individuals who had detectable levels of IgE but not IgG4 to rABA-1A (11%) had lower average levels of infection compared with individuals who produced anti-rABA-1A IgG4 (40%) and sero-negative individuals (49%) (P = 0.008). The ratio of IgG4/IgE in rABA-1A responders positively correlated with intensity of infection (P < 0.025). IgG4 levels positively correlated with infection level in younger children (age 4-11) where average levels of infection were increasing (P = 0.038), whereas allergen specific IgE emerged as a correlate of immunity in older children and adults (age 12-36) where infection levels were decreasing (P = 0.048). Therefore, in a gastrointestinal helminth infection, differential regulation of anti-allergen antibody isotypes relate to infection level. Our results are consistent with the concept that IgG4 antibody can block IgE-mediated immunity and therefore allergic processes in humans.

Immunity, immunoregulation and the ecology of trichuriasis and ascariasis

Immune responses to human roundworm (Ascaris lumbricoides) and whipworm (Trichuris trichiura) and their role in controlling worm populations are reviewed. Recent immunoepidemiological data implicate T(H)2-mediated responses in limiting A. lumbricoides and T. trichiura populations. Reinfection studies further suggest that IL-5 cytokine responses are negatively associated with adult recruitment in T. trichiura but not A. lumbricoides and may therefore be involved in negative intraspecific and interspecific interactions mediated through the host immune system. The importance of inducible immunoregulatory networks in the ecology of the host-parasite relationship is considered, with particular regard to possible manipulative strategies by the parasites. This aspect of the worms' interaction with the host immune system is both poorly known and potentially central to an understanding of parasite population dynamics and the evolutionary pressures that have shaped present-day host-parasite associations. Some possible implications of worm-mediated immunomodulation for the occurrence of bystander infectious diseases in human populations and the management of de-worming programmes are also discussed.

Modulation of a heterologous immune response by the products of Ascaris suum

Helminth infections are among the most potent stimulators of Th2-type immune responses and have been widely demonstrated to modify responsiveness to both nonparasite antigens and other infectious agents in a nonspecific manner in infected animals. We investigated the immunomodulatory properties of pseudocoelomic body fluid from adult Ascaris suum gastrointestinal helminths (ABF) and its defined allergen (ABA-1) by examining their effects on the immune response to a heterologous antigen, ovalbumin. Our results indicate that ABF has potent immunomodulatory activity and that the effects observed are consistent with skewing towards a Th2-type response rather than induction of anergy. Our findings show that the immunomodulatory activities of ABF are associated with components other than the major constituent and putative allergen, ABA-1. Furthermore, the allergic responses to ABA-1 are not a result of an intrinsic allergenicity of the protein but are more a reflection of the wider induction of a Th2 response by the infection. Importantly, the induction of interleukin-10 by ABF also suggests that T regulatory cells may play a role in immunomodulation of immune responses by parasitic helminths.

Pharmacotherapy of ascariasis

Ascaris lumbricoides, the most frequent human intestinal nematode, is the causative agent of ascariasis, with an estimated worldwide prevalence of over one billion people, especially in moist tropical and subtropical regions, but also in cooler climates. Although characterised with low morbidity and mortality rates, the global prevalence of ascariasis still results in approximately 20,000 deaths annually, primarily as a consequence of intestinal obstruction. In humans, transmission usually occurs by hand-to-mouth route by way of contaminated agricultural products and food, or from dirty hands. Three phases of ascariasis may be present, namely, the pulmonary, intestinal and the complications stage. Although generally asymptomatic, heavy infestation may cause serious pulmonary disease, or partial or complete obstruction of biliary or intestinal tracts. Anthelminthic chemotherapy is required to eradicate the parasites and prevent potentially serious complications. Mebendazole, albendazole and pyrantel pamoate are the most widely used agents to treat ascariasis. Preventive chemotherapy delivered to communities in endemic regions may serve as an affordable and cost-effective strategy to reduce the prevalence and morbidity in endemic regions. Under unusual circumstances, Ascaris suum, the cause of helminthic infection in pigs, may also cause disease in humans.

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.

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.

Experimental Ascaris suum infection in the pig: protective memory response after three immunizations and effect of intestinal adult worm population

The protective immune response to larval migration in pigs, with or without adult intestinal worm populations, 10 weeks after 3 weekly Ascaris suum inoculations, was studied in 45 pigs. Controlled adult worm populations were achieved by oral transfer of 10 adult worms to previously immunized pigs after anthelmintic drenching. A significant reduction in larval recovery from lungs on day 7, and small intestine on day 14, was observed in immunized pigs compared with previously uninfected control pigs after challenge inoculation. The strong anamnestic response to larval migration was characterized by blood eosinophilia and specific immune responses measured by peripheral blood enzyme-linked immunospot and immunosorbent assays using larval excretory-secretory products and adult body fluid as well as Western blotting with a panel of stage-specific A. suum antigens. Immune detection of a previously unreported 10 kDa band, specific to the L2 larval stage and egg hatch fluid, emerged in all pigs after challenge, while the major adult body fluid constituent, ABA-1, remained unrecognized. No significant effect of an intestinal adult worm burden on the larval recovery after a challenge inoculation or on the immune response before or after challenge inoculation could be detected. These results indicate that a significant protective memory immune response to A. suum challenge inoculation can be induced in pigs, and that this protective immunity is not significantly modulated by the presence of adult parasites in the gut.

The recognition of molecules from fourth-stage larvae of Ostertagia circumcincta by IgA from infected sheep

The recognition of antigens from fourth-stage Ostertagia circumcincta by plasma IgA was studied in a group of 30, ten-month-old Scottish Blackface sheep which had been naturally, then deliberately infected. Western blotting revealed 49 bands that were recognized by antibody from one or more of the 30 sheep. There was a heterogeneous pattern of antigen recognition amongst the sheep. No individual recognized all the 49 bands and only one of the 49 bands was recognized by all sheep. Two antigens with approximate molecular weights 87,000 Da and 129,000 Da were significantly associated with a reduction in mean adult worm lengths. The observed variation in recognition of these two antigens on fourth-stage larval preparations accounted, in a statistical sense, for nearly 40% of the total variation in worm length. In addition to the variation in antibody mediated recognition of these two parasite molecules, three other components have been implicated in regulating worm length. They are a 37,000 Da band from adult worms, the amount of fourth-stage larval specific IgA in the abomasal mucosa and the density-dependent influence of adult worm burden. Together, these components and their interactions accounted for over 90% of the observed variation in worm length. These results indicate that the parasite-specific IgA response, or something extremely closely associated with it, is the major immunological mechanism controlling worm length.

Natural immunity to Ascaris lumbricoides associated with immunoglobulin E antibody to ABA-1 allergen and inflammation indicators in children

Children putatively immune to the large roundworm Ascaris lumbricoides were identified in an area of Nigeria where infection is hyperendemic. Immunity was associated with higher levels of serum ferritin, C-reactive protein, and eosinophil cationic protein, indicating ongoing acute phase or inflammatory processes. In contrast, children who were susceptible to the infection had little serological evidence of inflammation despite their high parasite burdens. Immunoglobulin G (IgG) antibody activity in all subclasses was present in high titer in most children but appeared to have no protective function. Despite exceptionally high total IgE levels, there was no evidence that atopic responses to local common allergens was associated with natural immunity to Ascaris. Among those individuals who produced IgG antibody to recombinant ABA-1 allergen of Ascaris, the naturally immune group had significantly more IgE antibody to the allergen than did those susceptible to the infection. IgE antibody responses in conjunction with innate inflammatory processes therefore appear to associate with natural immunity to ascariasis.

Experimental granulomatous vasculitis induced by sensitization with Ascaris suum antigen in mice

Experimental sensitization by repeated intramuscular injection of Ascaris suum antigen (Ag-As) supplemented with Freund's incomplete and complete adjuvants was carried out in 50 BALB/c CrSl c male mice (sensitized group) for 24 weeks, and the results were compared with those in a control group of 25 mice. At the injection sites of the sensitized group, granulomatous angiitis with eosinophil infiltration was observed in all mice, and fibrinoid angiitis in only four. By light and electron microscopic examinations pulmonary granulomatous vasculitis with a few eosinophils was observed at a high frequency (80%) after 12 experimental weeks. Immunohistochemical examination revealed pulmonary vascular and perivascular infiltration of L3/T4 (CD4) positive cells, B cells, IgG and C3 positive cells in addition to activated macrophages, Thy-1 T cells, IgE positive cells, and IgM positive cells after 12 experimental weeks. There were significant increases in the eosinophil cell count of the peripheral blood, the hemagglutination titers of the sheep erythrocytes, IgE and IgM antibodies to Ag-As by ELISA and Western blotting after 8 experimental weeks. After 12 experimental weeks the IgG antibody to the Ag-As was low, but it increased significantly, and the sera showed multiple precipitation lines to the Ag-As by the Ouchterlony method. In conclusion, the pulmonary granulomatous vasculitis in this study is considered to consist of allergic reactions of type IV and probably type III based on type I.

Fine specificity of the genetically controlled immune response to native and recombinant gp15/400 (polyprotein allergen) of Brugia malayi

Polyprotein allergens are a family of structurally homologous molecules from parasitic nematodes which induce specific immunoglobulin E in infected individuals. We show here that both H-2 and non-H-2 factors determine the ability of mice to generate T- and B-cell responses to the filarial polyprotein allergen (Brugia malayi gp15/400). Further, H-2 and non-H-2 genes can complement one another to overcome nonresponsiveness to this molecule. However, these genetic restrictions govern only responses to the native glycoprotein and all strains of mice respond equivalently when immunized with a recombinant polypeptide. Overlapping fragments of gp15/400 were constructed to compare the T-cell and antibody responses to native versus recombinant gp15/400 in responder (BALB/c H-2d) and nonresponder (B10.D2 H-2d, CBA H-2k, and BALB.K H-2k) strains. BALB/c mice generated T-cell responses to the same fragment (positions 89 to 133 and 1 to 21) whether immunized with native or recombinant material, although the antibody responses differed in fine specificity, H-2k mice, unresponsive to the native molecule, generated T cells responsive to the centrally located peptide (positions 57 to 100) only when immunized with the recombinant. Antibody responses in H-2k mice were directed at the peptide (positions 11 to 67) which is glycosylated in the native molecule. Our findings suggest that recognition of gp15/400 is affected by modifications that occur in the parasite but are absent when the molecule is produced in bacteria. This study provides a detailed evaluation of the immune response to an important nematode antigen as a start to the unraveling of the complex interaction of these multicellular parasites with mammalian hosts.

The effects of H2 and non-H2 genes on the survival of Onchocerca lienalis microfilariae in the mouse

The effect of H2 and non-H2 genes in a mouse model of protective immunity against Onchocerca lienalis microfilariae have been investigated. Non-H2 effects were determined using CBA, BALB/c, B10, SJL and TO strains. All were permissive for establishment of a primary infection with microfilariae, although significant differences in parasite recoveries were evident amongst the various strains. The effect of H2 genes upon a primary infection was investigated using H2 congenic B10 and BALB strains, B10, B10.S, B10.BR, B10.D2/n, BALB/c, BALB.B, and BALB.K. Significant H2 effects were seen among the relatively weak responder B10 strains, but were not present among the relatively strong responder BALB strains. These results support a dominant effect of non-H2 genes following primary exposure to microfilariae, and a 'fine tuning' effect of H2 genes that is apparent only in weaker responding strains. Upon reinfection of all the strains investigated, a gradation of protection was detected that appeared to be exclusively dependent upon H2.

The gp15/400 polyprotein antigen of Brugia malayi binds fatty acids and retinoids

Gp15/400 is a surface-proximal antigen of the filarial nematode Brugia malayi, produced as a large polyprotein precursor comprising an array of polypeptide units of approx. 14.5 kDa. Here we describe a biochemical function for gp15/400. A single 14.5-kDa unit of gp15/400 has been expressed in Escherichia coli, and found to dimerise spontaneously. This protein (designated P-RUNG) has high-affinity fatty acid and retinoid binding activity, suggesting that the parent polypeptide itself has these properties. Fluorescent fatty acid probes show significant enhancement of fluorescence intensity and shifts in emission wavelength in the presence of P-RUNG, which can be reversed by competing non-fluorescent fatty acids (oleic, palmitic, steric, arachidonic), retinoids (retinol and retinoic acid) and oleoyl Coenzyme A, but not by tryptophan, cholesterol, caproic acid, squalene, tocopherol, tocopherol acetate, succinyl CoA, 2-methylbutyric acid and 2-methylvaleric acid. Changes in intrinsic fluorescence of retinol or retinoic acid confirmed the retinoid binding function. The results of fluorescence titration experiments are consistent with stoichiometric binding to a single protein site per monomer unit with affinities (Kd) in the range 2 x 10(-6) M (for the fluorescent probe 11-((5-dansyl)amino)undecanoic acid) and 2 x 10(-7) M (for oleic acid). The extreme blue shift of the fluorescent fatty acid-protein complex suggests an unusually low polarity for the protein binding site. The intrinsic fluorescence of the single tryptophan residue of P-RUNG indicates that it also is deeply buried in a non-polar environment, but is probably not involved in ligand binding. Gp15/400, therefore, represents a new class of lipid binding protein which is possibly restricted to nematodes.

Immune responses to nematode exoantigens: sensitizing antibodies and basophil histamine release

High levels of IgE and eosinophilia are found in both allergy and helminth infections, but allergic symptoms are rare in naturally acquired helminth infections. The interrelation of specific IgE antibodies and in vitro basophil histamine release (HR) induced by exoantigens from the larval stages (L2/L3) of the nematodes Toxocara canis and Ascaris suum was examined in 148 patients visiting an outpatient clinic for parasitic diseases. The antigen sensitivity of the basophils was found to be dependent not only on the absolute amount of antigen-specific IgE present in patient plasma, but also on the ratio between specific and total IgE. Thus, large HR was observed in some patients in response to helminth antigens despite low levels of both specific and total IgE content in plasma. Patients with eosinophilia showed greater IgE-mediated HR than the other patients examined. In contrast, only five patients showed HR after challenge with anti-IgG4, despite the presence of high levels of antigen-specific IgG4 and IgG1 in all patients showing specific IgE antibodies.

Heterogeneity of IgG antibody responses to cloned Onchocerca volvulus antigens in microfiladermia positive individuals from Esmeraldas Province, Ecuador

The prevalence of IgG antibodies to three recombinant O. volvulus antigens, OvMBP/10, OvMBP/11 and OvMBP/29 was determined in a group of 94 microfilaria positive (mf+) individuals resident in the hyperendemic onchocercal area of Esmeraldas Province, Ecuador. Clone OvMBP/11 was the antigen most frequently recognized by patients sera followed by OvMBP/10 and OvMBP/29. When a cocktail of the three recombinant antigens was used the proportion of positive sera increased to 100%. Antibody responses to the fusion partner maltose binding protein (MBP) were low in comparison with those to the cloned antigens and no correlation of responses between individual antigens was observed. The relative level of antibody response to each of the clones in the cocktail varied between individuals. The distribution of IgG responses to OvMBP/11 was bimodal and those to OvMBP/29 and OvMBP/10 were positively and negatively skewed, respectively. When the three recombinant antigens were used in combination this variation was minimized and the pattern of responses showed a normal distribution as was also seen to crude O. volvulus antigen. The cocktail of recombinants thus offers excellent diagnostic sensitivity in combination with the parasite specificity demonstrated previously.

The behavior and pathogenicity of Toxacara canis larvae in mice of different strains

In the present study the behavior and pathogenicity of second-stage larvae of Toxocara canis were examined in different mouse strains with special emphasis on the major histocompatibility complex (MHC). Mice of the inbred strains BALB, C3H, C57BL, and DBA and the outbred strain NMRI were infected orally with 1000 second-stage larvae of T. canis. The clinical behavior of the animals; the numbers of larvae detected in the liver, lungs, brain, and musculature; the hematological and serological parameters; and histological sections were examined. In mice of the BALB strain, no death occurred during the entire period of the investigation and the pattern of body-weight development of infected and uninfected animals was almost identical. The highest larval counts in the brain of all strains were found in BALB mice. The percentage of eosinophils in the blood of BALB mice increased after the 8th week postinfection, whereas it decreased in the other strains. Histological and pathophysiological changes developed to a lesser extent in this strain than in the other strains. In mice of the strains C3H, C57BL, DBA, and NMRI, deaths occurred from the 4th week postinfection onward. The infected animals lost weight in comparison with the uninfected controls; the number of larvae found in the brains of infected mice of the above-mentioned strains were lower than those detected in the BALB strain. There is no evidence that mechanical damage caused by migrating larvae in the brain tissue is mainly responsible for symptoms of central nervous toxocariasis. Likewise, the assumption that the MHC is involved in the allergic-inflammatory response in the brain could not be proven: infected mice of the BALB and DBA strains reacted completely differently, although both are equipped with the same MHC haplotype.

The ABA-1 allergen of the nematode Ascaris suum: epitope stability, mass spectrometry, and N-terminal sequence comparison with its homologue in Toxocara canis

ABA-1 is a major allergen of nematode parasites of the genus Ascaris which includes the large roundworms of humans and pigs, A. lumbricoides and A. suum, respectively. The allergen was purified from A. suum by immunoaffinity chromatography for immunochemical examination. The IgE antibody repertoire is under MHC control in infected rodents and the IgE-binding epitopes were robust to treatment with heat or periodate, and electroblotting on nitrocellulose. This implies that the IgE epitopes comprise primary peptide sequence or an unusually stable secondary or tertiary structure. The molecular mass of ABA-1 is controversial, but mass spectrometry analysis indicated that there were five components of similar size, with the major species being 14,643.2 +/- 1.4 D. Finally, N-terminal sequence analysis of ABA-1 and TBA-1 (the homologue in the canine nematode infective to humans, Toxocara canis) revealed a high degree of similarity, and we have previous evidence that ABA-1 homologues are widespread amongst ascaridid parasites of humans. ABA-1 and its homologues might, therefore, be important to the immunopathology of many infections with nematode parasites, upon which the genetic constitution of the hosts will also have a bearing.

Comparison between the MHC-restricted antibody repertoire to Ascaris antigens in adjuvant-assisted immunization or infection

Genetic restrictions to the immune repertoire will be an important consideration in the development of anti-nematode vaccines. It has already been established that the major histocompatibility complex (MHC) limits responsiveness to nematode antigens in infection, but little is known of whether this also applies under other routes of sensitization, such as with adjuvants. The specificity of the antibody response was, therefore, compared in infection and adjuvant-assisted immunization using secreted and somatic antigens of Ascaris suum as a model system in mice and rats. The findings were, first, that the lack of responsiveness to certain antigens in infection was not circumvented by Freund's adjuvant-based immunization, despite the fact that the latter generally elicited higher levels of response. Secondly, that adjuvant-assisted immunization could elicit responses to parasite products which were not detectable in the context of infection. Conversely, some specificities were detectable in infection but absent under adjuvant immunization. Finally, immunization with a defined parasite allergen (ABA-1) in Freund's adjuvant did not provoke an IgE response which would be anticipated if the molecule were to have an intrinsic allergenic property. These results are likely to be of general importance to the application of subunit or recombinant vaccines against nematodiases and to the hypersensitivity reactions which vaccination might engender or recall.

H-2 (I-A) control of the antibody repertoire to secreted antigens of Trichinella spiralis in infection and its relevance to resistance and susceptibility

Humans infected with the parasitic nematode Trichinella spiralis vary in the specificity of their antibody responses to the antigens of the parasite. The possibility that such host variation in antigen recognition has a genetic basis was examined in infected inbred mice whose antigen recognition profiles were characterized by immunoprecipitation of biosynthetically labelled secreted materials of adult parasites and SDS-PAGE. The strains varied considerably in repertoire and none produced detectable antibody to all the potential antigens. Using a panel of H-2 congenic and recombinant strains it was established that the repertoire was determined by the major histocompatibility complex (MHC), the I-A region in particular. Other factors, such as level of infection and variation between individuals, affected antigen recognition profiles, but this was always within limits imposed by the MHC. Lastly, an attempt to correlate antibody repertoire with relative susceptibility or resistance to T. spiralis failed to reveal any clear association. This also applied to the AKR/J and AKR-Fv-1b strains, which are H-2-identical but differ in a non-MHC susceptibility locus. These findings would argue, therefore, that the I-A region controls the antibody repertoire in this nematode infection but that the repertoire overall has little influence on the efficiency with which the infections are controlled by the immune system. Should this also apply for other nematode infections, then antigen recognition profiles of infected individual humans and domestic animals might not, therefore, be useful indicators of relative resistance or susceptibility to infection.

MHC class II (I-A) region control of the IgE antibody repertoire to the ABA-1 allergen of the nematode Ascaris

ABA-1 is an approximately 14,000 molecular weight (MW) allergen which is among the most abundant proteins synthesized by the nematode parasite Ascaris. IgG and IgE responses to it are major histocompatibility complex (MHC)-restricted in rodents and have only been found to occur in rats of the RT1u haplotype and mice of the H-2s haplotype. Humans infected with the parasite vary substantially in their immune response to the allergen, but the genetic basis for this unknown. H-2 recombinant mice were used to identify the region within the MHC controlling antibody responses to the allergen. IgG antibody to immunoaffinity purified ABA-1 was assayed by radio-immunoassay and IgE by passive cutaneous anaphylaxis. This showed that the restriction element is the I-A molecule and that there was some evidence for I-E modulation of the level of response.

MHC (RT1) restriction of the antibody repertoire to infection with the nematode Nippostrongylus brasiliensis in the rat

It might be expected that infections with transmissible agents will elicit an immune response to all of their exoantigens and that immune response (Ir) gene control of responses to individual epitopes on a given parasite component would be obscured by reaction to the molecule as a whole. Humans infected with parasitic nematodes, however, mount antibody responses which are selective for certain parasite components. This was modelled in inbred rats infected with the parasitic nematode Nippostrongylus brasiliensis and their responses to secreted antigens analysed by immunoprecipitation and SDS-PAGE. No strain responded to all the potential antigens and only those of identical major histocompatibility complex (MHC) had similar recognition profiles. This MHC-restricted response applied to whole molecules synthesized by the parasite, rather than merely to epitopes thereon and is, therefore, contrary to expectation. Moreover, the response patterns of F1 hybrid animals were not merely summations of parental responses. This suggests defective antigen presentation of particular parasite components by certain MHC class II molecules and/or cross-tolerance with background gene products.