Concepts and strategies for anti-parasite immunoprophylaxis and therapy

Rodent Models for the Study of Soil-Transmitted Helminths: A Proteomics Approach

Soil-transmitted helminths (STH) affect hundreds of millions worldwide and are some of the most important neglected tropical diseases in terms of morbidity. Due to the difficulty in studying STH human infections, rodent models have become increasingly used, mainly because of their similarities in life cycle. Ascaris suum and Trichuris muris have been proven appropriate and low maintenance models for the study of ascariasis and trichuriasis. In the case of hookworms, despite most of the murine models do not fully reproduce the life cycle of Necator americanus, their proteomic similarity makes them highly suitable for the development of novel vaccine candidates and for the study of hookworm biological features. Furthermore, these models have been helpful in elucidating some basic aspects of our immune system, and are currently being used by numerous researchers to develop novel molecules with immunomodulatory proteins. Herein we review the similarities in the proteomic composition between Nippostrongylus brasiliensis, Heligmosomoides polygyrus bakeri and Trichuris muris and their respective human counterpart with a focus on the vaccine candidates and immunomodulatory proteins being currently studied.

Variation in resistance to haemonchosis: selection of female sheep resistant to Haemonchus contortus

Seventy female lambs (6–7 months old) which were exposed to natural infections of Haemonchus contortus were designated as responders or non-responders on the basis of 10 weekly cumulative faecal egg counts. Selected responder and non-responder lambs were treated with ivermectin, housed separately and 6 weeks post-housing, seven lambs from each group were given a trickle infection of Haemonchus contortus at 1000 L3 daily for 5 days per week up to 2 weeks and examined weekly for 10 weeks after first infection. Analysis of data revealed significantly lower mean faecal egg counts and non-significantly less weight loss in responder than non-responder lambs. Mean values of haemoglobin, packed cell volume, total serum protein and peripheral eosinophil counts were significantly higher in responders than non-responders. In contrast, serum pepsinogen concentration was significantly less in responders than in non-responders. At 10 weeks post-infection, there were fewer pathological lesions and significantly lower worm burdens in responders than in non-responders. These results demonstrate a distinct resistance in responders to Haemonchus contortus infection.

Effect of oxygen radicals and differential expression of catalase and superoxide dismutase in adult Heligmosomoides polygyrus during primary infections in mice with differing response phenotypes

The ability of oxygen radicals to kill Heligmosomoides polygyrus adult worms was examined by assessing parasite survival following incubation with hydrogen peroxide and acetaldehyde/xanthine oxidase, generators of H2O2 and H2O2/O2(-), respectively. H. polygyrus worms could tolerate levels of < 0.25 mM hydrogen peroxide and < 0.5 mM/20 mU acetaldehyde/xanthine oxidase for 20 h, but, at higher concentrations, marked sex-dependent susceptibility was observed, with males being more sensitive to H2O2 and O2(-) than female worms. The ability to evade free radical-mediated damage was also evaluated by measuring superoxide dismutase (SOD) and catalase levels in worms isolated at different time points from four strains of mice with differing resistance phenotypes. Levels of both catalase and SOD in female worms isolated from 'rapid'[(SWRxSJL)F1], 'fast' (SWR) or 'intermediate' (BALB/c), but not 'slow' (C57BL/10), responder mice showed a strain-dependent increase with time. Moreover, male worms were rejected faster than female worms in the 'rapid', 'fast' and 'intermediate' responder strains of mice. The results suggest that host-derived free radicals can damage adult worms and that female worms can increase production of their scavenging enzymes in response to the immune onslaught that eventually leads to worm expulsion in mice with 'fast', 'rapid' or 'intermediate' response phenotypes.

Juvenile Fasciola hepatica are resistant to killing in vitro by free radicals compared with larvae of Schistosoma mansoni

Free radicals have previously been shown to kill the immature stages of the trematode, Schistosoma mansoni but their effect on newly excysted juvenile (NEJ) flukes of Fasciola hepatica has not been established. Using acetaldehyde and xanthine oxidase to chemically generate reactive oxygen intermediates (ROI), up to 61% of NEJ were killed but only when exposed to high levels of ROI. At low concentrations of acetaldehyde and xanthine oxidase as sources of reactive oxygen intermediates, only 6-29% of NEJ were killed compared with 70-92% of schistosomula. Incubation with lipopolysaccharide (LPS)-stimulated rat peritoneal lavage cells (PLCs) killed only 7-15% of NEJ whereas 78-87% of schistosomula were killed under the same conditions by a mechanism dependent on the production of reactive nitrogen intermediates. Relative to immature and adult parasites, NEJ expressed 2.5-20-fold lower levels of superoxide dismutase and glutathione S-transferase but no catalase activity was detected. Incubation of NEJ with inhibitors of peroxidases and glutathione metabolism increased the mean killing of NEJ by LPS-stimulated rat PLCs to 40-75%. These results demonstrate that, in comparison to schistosomula of S. mansoni, NEJ of F. hepatica are relatively resistant to killing by free radicals and this resistance could, in part, be due to the activity of oxidant scavenger enzymes of NEJ.

Polymorphic DNA markers in the genome of parasitic nematodes

Polymorphic molecular markers are being identified to characterize the genomes of parasitic nematodes. The aim is to construct a map with markers evenly spread over the six chromosomes. With such a map, regions can be identified that are under selection pressure when attempts are being made to eradicate worms, be it by drugs, vaccines or genetic resistance in the sheep. Several types of markers have been developed, microsatellites, transposon-associated markers, amplified fragment length polymorphism (AFLP) and expressed sequence tag (EST) markers. Linkage groups can be constructed using several genetic crosses between inbred and drug resistant strains. EST markers will be especially important for comparative mapping with the genome of Caenorhabditis elegans, and therefore localization of the linkage group on a chromosome. It will then be possible to identify functional genes close to markers that have changed allele frequencies under selection pressure and identify the mechanisms of resistance to parasite control.

Vaccination against worm parasites of animals

The 1990s have seen the culmination of decades of painstaking research with the registration and launch of Tickgard (Hoechst), a recombinant vaccine against Boophilus microplus, and the provisional registration of a Taenia ovis vaccine. Research continues to hold promise for immunological control of Echinococcus, Fasciola, Haemonchus, Trichostrongylus and Ostertagia. Blood-sucking parasites (e.g. ticks and H. contortus) are susceptible to control by vaccines containing 'novel' or 'concealed' antigens where serum antibodies in blood meals attack targets in the gut. Antibodies also provide protection in taeniid models, whereas the protective response to be sought in Fasciola remains unclear. More problematic are formulations and delivery strategies to induce expulsion of gastrointestinal nematodes, using vaccines containing recombinant 'conventional' antigens. The use of computer models to simulate vaccine efficacy in worm control and challenges to the concept of 'hypo-responsiveness' of young lambs will encourage cautious optimism and lively debate as to the prospects for integrated worm control using parasite vaccines. This review covers the aspirations, current success and problems faced by researchers in the parasite arena.

Cytokines as adjuvants for ruminant vaccines

Successful vaccination against any potential pathogen is critically dependent on inducing an appropriate immune response. The pivotal role of cytokines in the immune response to vaccination suggests that non-protective responses or responses that exacerbate disease subsequent to infectious challenge may be the result of limiting or preferential production of one or a number of these mediators. This suggests that the use of recombinant cytokines as vaccine adjuvants may offer a mechanism whereby the magnitude and phenotype of the immune response to vaccination can be specifically modified. In mice, recombinant cytokines have been used extensively as therapeutics, while studies describing vaccine adjuvant activity are more limited. Recombinant (r) interleukin (IL)-1, IL-2 and interferon (IFN) gamma have been used primarily to enhance humoral responses with enhanced protection assessed where appropriate. Cytokine adjuvant studies in ruminants have been restricted to recombinant ovine (rov) and bovine (rbov) IL-1 and IL-2. In sheep, their application has been optimised with respect to dose, route of delivery and formulation, for induction of humoral and cell mediated immunity (DTH and cytotoxicity) to the model protein antigen (Ag) avidin. The level of adjuvant activity of IL-1 in particular compares favourably to that of a variety of other traditional and new chemical adjuvants and detailed analysis has indicated no adverse local or systemic side-effects. Recent studies in our laboratory demonstrating the effectiveness of rovIL-1 as an adjuvant in single and multi-component bacterial toxoid vaccines, and studies from other laboratories demonstrating the application of rbovIL-1 as an adjuvant for the response in cattle to live attenuated viral vaccines, suggest that rIL-1 may become the adjuvant of choice for diseases where protection is mediated by high levels of circulating antibody (Ab). With respect to helminth parasites, IL-1 may prove useful as a component of vaccines based on "hidden Ags" which rely on induction of Ab. Based on analysis in mouse models of helminth infection, other cytokines such as IL-4 and IL-10 may be appropriate for vaccines based on induction of mechanisms involved in natural immunity.

The effect of BCG, zymosan and Coxiella burnetti extract on Eimeria infections

Infection of animals with species of Eimeria induces a hyper-reactivity to endotoxin as manifest by a greatly increased capacity of infected animals to produce TNF in response to LPS in vivo compared with uninfected animals. This finding indicates priming for hyperactivation of macrophages by Eimeria infection and raises the possibility that non-specific triggering of macrophages by agents such as Bacille Calmette-Guerin (BCG), zymosan or Coxiella burnetti extract may be a simple means of control for coccidiosis. However, all of these agents enhanced oocyst excretion in mice, rats or chickens infected with Eimeria vermiformis, Eimeria nieschulzi or Eimeria tenella, respectively, without affecting the patent period.

Effect of Mycobacterium phlei on the development of immunity to Babesia bigemina

The immunomodulatory role of Mycobacterium phlei against intracellular blood protozoan Babesia bigemina was demonstrated following experimental immunisation and challenge in bovine calves. A lysate of erythrocytes infected (6 x 10(9)) with B. bigemina was used as a source of dead antigen either with Freund's complete adjuvant (FCA) or with a trypsinised culture of M. phlei as a non-specific immunomodulation (NSI) agent with appropriate controls. Following virulent challenge with B. bigemina infected erythrocytes (1 x 10(7)), the NSI printed calves showed 100% protection, while the dead antigen alone with FCA afforded 75% protection. The protective status of the immunising regimes was studied by clinicopathological parameters and assessment of humoral and cell-mediated immune responses. The role of babesial dead antigen and the effects of M. phlei on the development of immunity to B. bigemina is discussed.

Cytokines and immunological control of Eimeria spp

Protozoan parasites belonging to the genus Eimeria cause considerable losses in livestock production in which stocking densities are high or environments restricted. The ability of hosts to mount immunological responses which limit parasite reproduction vary according to the particular species of Eimeria. Typically though, immune responses restrict parasite reproduction during primary infection and limit, if not prevent, subsequent infections. Although mechanisms of immunity are unknown, host immune responses have been exploited in the development of a method to control coccidiosis-immunisation with attenuated strains of Eimeria. Limitations of this control method, predominantly the cost of producing the attenuated parasites, necessitates identification of protective immune responses to facilitate selection of antigens for use in non-living vaccines. As in immune responses to many other parasitic infections of the gastrointestinal tract, the role of antibodies is at best minor, whereas T-cells are crucial. Numerous studies have shown that the intestinal mucosal T-cell population is dynamic; the number and phenotype of T-cells changes in response to Eimeria-infection. Specific changes in the intestinal T-cell population have not, however, been correlated with limitation of parasite reproduction. Experiments involving adoptive transfer of T-cell sub-populations and in vivo depletion of specific T-cells have shown that CD4+ T-cells and to a lesser extent CD8+ T-cells are important in immune responses which limit primary infection. In contrast, CD8+ T-cells are more important in subsequent infections with CD4+ T-cells having a lesser role. The effects of T-cells on Eimeria are partially mediated by the cytokines they release. Most attention has concentrated on interferon-gamma (IFN-gamma) and tumour necrosis factor-alpha (TNF-alpha) because these cytokines have been shown to limit other protozoan infections. IFN-gamma is produced in Eimeria-infected hosts but evidence that it is present at the site of infection is limited. Intestinal levels of IFN-gamma increase earlier in response to primary Eimeria-infection in mice which are relatively resistant, than in mice which are relatively susceptible. Neutralisation of endogenously produced IFN-gamma has shown that this cytokine limits oocyst production in either primary or secondary infections depending on the species of Eimeria. Production of TNF-alpha is also increased in infected hosts. In comparison with relatively susceptible mice, TNF-alpha is produced earlier and to a greater extent in the intestines of relatively resistant mice. Unexpectedly, injections of TNF-alpha into infected mice increased oocyst production. It remains to be determined whether the effects of endogenous TNF-alpha are the same as those of exogenous TNF-alpha. Mechanisms by which IFN-gamma and TNF-alpha modulate parasite reproduction have not been identified. A number of lines of experimentation have suggested that it is unlikely that IFN-gamma limits parasite reproduction through induction of the synthesis of reactive oxygen or reactive nitrogen intermediates, since both of these reactive intermediates have the capacity to exacerbate Eimeria-infection.

[Towards a vaccination against malaria]

For 20 years, the prospect of anti-malarial vaccination has aroused many hopes, but in the end, it has mostly given rise to doubts and disappointment. If most attempts have been to no avail, this is because the issue at stake is amazingly difficult. Besides the very complex antigenic structure of the protozoa Plasmodium, there is first the existence of at least three different targets during the plasmodial cycle, then the necessity of appropriate adjuvants and, most of all, the imperfection of our experimental models. Recently, Pattaroyo and the various groups who worked with him have eventually met success with vaccine trials in man: they used a synthetic antigene, SPf66, on volunteers in South America, then on a larger population sample in East Africa. The results are still quite modest: people are protected against the malarial disease but not against the parasitemia and only in approximately 40% of cases. Nevertheless, these results have the merit of representing the first successful anti-malarial vaccination in man. Although great advances are still needed, a decisive step forward has been taken. Other types of vaccine will soon be tested by other groups (anti-gametocyte vaccines) and prospects of significant improvements are offered by the technique of DNA-vaccines. If it is now certain that one or several vaccines will be available in a near future, no one is able to set the time delay necessary to reach this stage. In any case, hoping that this type of vaccine will eradicate the disease is not realistic since a disease as complex as malaria, in terms of epidemiology, cannot be eliminated by only one method.(ABSTRACT TRUNCATED AT 250 WORDS)

Attempted immunisation of sheep against Fasciola hepatica using gamma-irradiated metacercariae

The potential of gamma-irradiated Fasciola hepatica metacercariae to vaccinate sheep against fascioliasis was examined. The effect of the size of the inocula of irradiated metacercariae and the level of gamma-irradiation on the recovery of non-irradiated fluke was assessed following homologous challenge. Groups of Merino wethers were vaccinated with a single infection of either 500 or 2000 metacercariae, previously exposed to either 30, 100 or 400 Gy of gamma-irradiation. No significant reduction of fluke burdens were observed in any group, although a nonsignificant 20% reduction was observed in sheep vaccinated with 2000 metacercariae irradiated with 100 Gy. A second trial was conducted in which groups of sheep were vaccinated with 2 doses, given 4 weeks apart, of 2000 metacercariae, previously irradiated at either 70, 100 or 150 Gy. In both trials parasite viability was severely affected by doses of gamma-irradiation of 30 Gy or greater and no mature flukes were recovered from control sheep given metacercariae attenuated with 70 Gy or greater. A strong humoral immune response to somatic F. hepatica antigens was observed in all sheep. Only sera from sheep receiving 70 Gy irradiated metacercariae recognised the 2 candidate liver fluke vaccine molecules, F. hepatica glutathione S-transferase and cathepsin-L proteases. No reduction was observed in either the number of flukes or the production of fluke eggs in any vaccinated group. Vaccination appeared to affect the development of the challenge fluke population, resulting in reduced hepatic damage during migration, as measured by levels of serum glutamate dehydrogenase, and an increase in mean fluke weight.

Immunisation of calves with Ostertagia ostertagi fourth stage larval antigens failed to protect calves from infection

Excretory-secretory and somatic antigens of Ostertagia ostertagi fourth stage larvae, emulsified with complete Freund's adjuvant, were intraperitoneally administered to calves on three occasions. Two weeks after the last immunisation all calves were infected with a single dose of 130,000 O. ostertagi third stage larvae. All animals were necropsied 25 days after infection. The immunisation procedure resulted in IgG1 and IgG2 antibodies, however no protective immunity was induced as O. ostertagi worm burdens and worm length were similar in all groups.

Differential expression of glutathione S-transferase (GST) by adult Heligmosomoides polygyrus during primary infection in fast and slow responding hosts

Glutathione S-transferase (GST) specific enzymatic activity, assayed with the model substrate 1-chloro-2,4-dinitrobenzene, was 45% higher in adult Heligmosomoides polygyrus passaged through a slow responder mouse strain, C57/BL10 compared to worms passaged through a fast-responder strain (SWR x SJL) F1. Western analysis using polyclonal antisera raised to purified H. polygyrus GSTs did not appear to positively correlate the expression of GST protein with functional enzymatic activity. However, western blotting did indicate a sex-linked expression pattern of GST protein, with male worms expressing a higher ratio of the 24 kDa to the 23 kDa GST family than female worms.

Parasites, animal production and sustainable development

Ecologically sustainable development is aimed at reducing environmental degradation while enabling economic development with equity between the developed and developing worlds and between generations. Parasite control in livestock can both contribute to, and take advantage of, sustainable agriculture. This will tend towards less intensive, lower input, diversified crop and animal production with less risk of parasite-induced losses and greater opportunities for integrated control including the exploitation of grazing management. Chemotherapy will continue to play a part but the most serious problem is resistance in the target species. Except for a few isolated issues, currently used parasiticides are relatively minor contaminants of the food supply or the environment. Nevertheless, the compounds of the future will need to be narrow-spectrum, non-persistent and rapidly degraded, with convenience in the hands of the user reduced in importance. Environmentally friendly alternatives to chemotherapy, including genetic resistance of hosts, vaccines, and biological control, show considerable promise and must be pursued. Sustainable systems pose optimisation problems and more attention must be given to systems research, models and products to aid decisions. If governments are serious about sustainable development, greater support will be needed for longer-term patient, multi-disciplinary research.

Vaccination against animal parasites

A decade of molecular parasitology is beginning to bear fruit, with the appearance of several new, highly effective, practical vaccines against parasitic diseases. Recombinant antigen vaccines have been developed against cestode, nematode, trematode, protozoan and arthropod parasites. Greatest progress has been made with veterinary vaccines, where the ability to test numerous vaccine formulations in challenge trials has allowed more rapid identification of host-protective antigens than is possible with many medically important parasites. Several quite different approaches to vaccine development have been successful. The traditional approach using live, attenuated parasites continues to provide effective vaccines against several protozoan and nematode parasites. Recombinant DNA technology, monoclonal antibody technology, protein chemistry and immunochemistry have played critical roles in the outstanding success which has been achieved over the last 5 years in the development of defined-antigen vaccines. Two approaches have been successful in research towards defined antigen vaccines against parasites: (1) the 'natural antigen' approach where immune responses are stimulated to parasite molecules which are normally antigenic, and possibly host-protective, in infected hosts; (2) the 'naive antigen' approach where parasite molecules which are not antigenic, or of very low antigenicity, in infected hosts are used to raise immune responses capable of killing the parasite. This review examines the successful approaches taken towards the development of effective anti-parasite vaccines and the vaccines which have been produced to date.

Molecular aspects of drug resistance in parasitic helminths

Parasitic helminths (worms) cause serious infectious diseases in humans and animals. As control of these infections relies mostly on chemotherapeutics, the anthelmintics, resistance has developed against most of these drugs in several parasite species. These resistant parasites are being used to elucidate the molecular mechanisms of drug action.