Paul Hagan

Praziquantel: its use in control of schistosomiasis in sub-Saharan Africa and current research needs

Treatment with praziquantel (PZQ) has become virtually the sole basis of schistosomiasis control in sub-Saharan Africa and elsewhere, and the drug is reviewed here in the context of the increasing rate that it is being used for this purpose. Attention is drawn to our relative lack of knowledge about the mechanisms of action of PZQ at the molecular level, the need for more work to be done on schistosome isolates that have been collected recently from endemic areas rather than those maintained in laboratory conditions for long periods, and our reliance for experimental work mainly on Schistosoma mansoni, little work having been done on S. haematobium. There is no evidence that resistance to PZQ has been induced in African schistosomes as a result of its large-scale use on that continent to date, but there is also no assurance that PZQ and/or schistosomes are in any way unique and that resistant organisms will not be selected as a result of widespread drug usage. The failure of PZQ to produce complete cures in populations given a routine treatment should therefore solicit considerable concern. With few alternatives to PZQ currently available and/or on the horizon, methods to monitor drug-susceptibility in African schistosomes need to be devised and used to help ensure that this drug remains effective for as long a time as possible.

IgE and protective immunity to helminth infections.

It is now nearly 30 years since Bridget Ogilvie published her paper in Nature* on reaginic antibodies in helminthinfected animals. Not only was this the first description of such antibodies occurring naturally in animals as a result of infection, the data summarized showed a significant correlation with aspects of protective immunity in both trematode (Schistosoma) and nematode (Nippostrongylus) infections. Dr Ogilvie concluded her paper with the words: ‘it may well be that reagin-like antibodies are responsible for immunity to helminths in many species of animals.’ In the intervening years an enormous literature has accumulated on IgE and parasitic infections. Claims and counterclaims that IgE is or is not involved in protective responses have each been supported by experimental and field data, and there is still no consensus, despite the recent publication of a number of significant papers. It seems timely to publish two articles from parasite immunologists actively interested in IgE which summarize some of the arguments for and against a protective role for this intriguing isotype.

A comparison of humoral responses to Schistosoma haematobium in areas with low and high levels of infection

Antibody responses to Schistosoma haematobium of 280 Zimbabweans were studied in two areas of differing infection levels. 133 of the subjects came from a low infection area with a prevalence of 33.8% and geometric mean infection intensity of 0.8 eggs per 10 ml of urine, while 147 of the subjects came from a high infection area with a prevalence of 62.7% and geometric mean intensity of 3.2 eggs per 10 ml of urine. Both the age‐prevalence and age‐intensity curves exhibited a peak shift. IgA, IgE, IgG, IgG1, IgG2, IgG3, IgG4, and IgM antibody levels against soluble egg antigen (SEA) and whole worm homogenate (WWH) were assayed by ELISA. Females produced significantly more anti‐SEA IgG1, IgG4, IgM, anti‐WWH IgE and IgG1. People from the high infection area produced significantly more anti‐SEA IgE, IgG3 and anti‐WWH IgG3 and significantly lower levels of anti‐SEA IgA, IgM and anti‐WWH IgM when the effects of infection intensity, sex and age had been allowed for. The age profiles of anti‐SEA IgA, IgG and anti‐WWH IgA and IgM reflected current levels of infection while anti‐WWH IgG1, IgG2 and anti‐SEA IgG1 evolved more slowly with age, and anti‐WWH IgE rose with age in both areas. Some antibody responses, anti‐SEA/WWH IgM, anti‐SEA IgG1 and possibly anti‐SEA/WWH IgA showed different age profiles in the two areas, with changes in antibody levels occurring at a younger age in the high infection area suggesting that immune responses are evolving more rapidly in residents of this area. This result clearly demonstrates that antibody levels are not a function of age alone.

Leishmania mexicana and Leishmania major: Attenuation of Wild-Type Parasites and Vaccination with the Attenuated Lines

A method for attenuation of Leishmania species by culturing in vitro under gentamicin pressure has been used successfully with Leishmania mexicana, L. major, L. infantum, and L. donovani. The attenuated lines invaded but were unable to survive within bone marrow-derived macrophages in vitro, whereas wild-type parasites survived and multiplied. The attenuated lines of L. mexicana and L. major both failed to induce cutaneous lesions in the majority of BALB/c mice over a minimum 12-week observation period after subcutaneous injection of stationary phase parasites. The attenuated line of L. mexicana retained its properties in gentamicin-free medium over 40 subcultures. The attenuated lines of L. mexicana and L. major both induced significant protection in mice against challenge with wild-type parasites.

Exposure, infection and immune responses to Schistosoma haematobium in young children

Behavioural, parasitological and immunological data were obtained from 48 children up to 6 years old, resident in a Schistosoma haematobium endemic area in Zimbabwe. The children averaged more than 1 contact with infective water bodies every 3 days and all showed immunological evidence of exposure (an anti-cercarial and/or anti-egg antibody response). IgM was the dominant isotype and appeared in the youngest children, followed by IgA, IgE and IgG3. However, only 38 children showed evidence of infection (an anti-egg response or eggs in urine) and only 14 were excreting eggs. The best estimates from these data are that less than 1 in 100 contacts results in infection and less than 1 in 1000 result in egg output. This suggests that there may be substantial attrition of invading cercaria even in naïve individuals.

The importance of models of infection in the study of disease resistance

Models currently occupy the crucial first step in the research flow for the development of new drugs and vaccines. Some animal models are better at reflecting the host-pathogen interaction in humans than others; this depends on the pathogen and its host specificity. Data gathered from what are often poorly adapted models provide a mosaic of sometimes contradictory information, yet there is little incentive to better delineate the relevance of models or to exploit recent advances to develop improved ones. This review reports on three particularly intractable human pathogens - Mycobacterium, Plasmodium and Schistosoma - and reflects that the extent to which these model systems mimic infection and protection processes in humans might not be sufficiently well defined.

Eosinophilic interleukin 5 (IL‐5) transgenic mice: eosinophil activity and impaired clearance of Schistosoma mansoni

Eosinophilia is a feature common to many invasive helminth infections and eosinophils are often considered to be effector cells in immunity to helminths. This study examined the possible influence of constituitive eosinophilia on the clearance of Schistosoma mansoni infections in mice. Eosinophils from interleukin‐5 transgenic mice exhibit normal ultrastructure and function with regard to phagocytosis and killing of bacteria and responses to chemotactic stimuli. IL‐5 transgenics and non‐transgenic littermates were immunized once or four (hyperimmunization) times with irradiated cercariae of S. mansoni. Animals were challenged percutaneously with unirradiated cercariae one month after their last exposure to irradiated parasites. One month after challenge transgenic animals, whether unimmunized, vaccinated or hypervaccinated, carried significantly more liver‐stage parasites than non‐transgenic animals. These results suggest that although eosinophils from IL‐5 transgenic mice are functional for a number of key parameters, large numbers of eosinophils and/or high levels of IL‐5 may in some way impair clearance of S. mansoni. A re‐evaluation of the roles of eosinophils and IL‐5 in infections with this and other parasites may therefore be warranted.

Immune responses of IL-5 transgenic mice to parasites and aeroallergens

Eosinophils have long been thought to be effectors of immunity to helminths but have also been implicated in the pathogenesis of asthma. Patterns of cytokine production in the host may influence the pathogenesis of these diseases by regulating the activities of eosinophils and other components of the immune response. Mice which constitutively over-express IL-5 have profound and life-long eosinophilia in a restricted number of tissues. Although eosinophils from IL-5 transgenics are functionally competent for a number of parameters considered to be important in inflammation, untreated animals are overtly normal and free of disease. In addition, the responses of these animals when exposed to aeroallergens and helminths present a number of apparent paradoxes. Eosinophil accumulation in tissues adjacent to major airways is rapid and extensive in transgenics exposed to the aeroallergen, but even after treatment with antigen over many months these mice show no evidence of respiratory distress or pathology. Helminth-infected IL-5 transgenics and their non-transgenic littermates develop similar inflammatory responses at mucosal sites and are comparable for a number of T cell and antibody responses, but they differ considerably in their ability to clear some parasite species. The life-cycle of Nippostrongylus brasiliensis is significantly inhibited in IL-5 transgenics, but that of Toxocara canis is not. Our results also suggest that eosinophilia and/or over-expression of IL-5 may actually impair host resistance to Schistosoma mansoni and Trichinella spiralis. The pathogenesis of diseases in which eosinophils are involved may therefore be more complex than previously thought.

Schistosome Immunology: More Questions than Answers

The past 30 years have seen research on the immunology of schistosomiasis move to encompass studies of responses in naturally exposed human populations, in addition to the studies in animal model systems. While animal systems still retain an important place in research on the immunology of schistosomiasis, recent debate has centred on aspects of human immunological responses that may or may not be associated with resistance or susceptibility to infection. In this article, Paul Hagan, Patricia Ndhlovu and David Dunne take stock of the present state of knowledge and offer their views on prospects for the future.

Comparative proteomics profiling of a gentamicin-attenuated Leishmania infantum cell line identifies key changes in parasite thiol-redox metabolism

We have previously described an attenuated line of Leishmania infantum (H-line), selected by culturing promastigotes in vitro in the presence of gentamicin. To elucidate the molecular basis for this attenuation, we undertook a comparative proteomic analysis using multiplex 2-dimensional (2D) difference gel electrophoresis. Eighteen proteins that showed significant and reproducible changes in expression were identified. Many of these were components of the thiol-redox control system in Leishmania and this observation, validated by Western blot, prompted us to investigate the sensitivity of the attenuated line to oxidative stress. The attenuated line was found to be significantly more susceptible to hydrogen peroxide, a change which may explain the loss of virulence. In a direct assay of trypanothione-dependent peroxidase activity, hydrogen peroxide metabolism in the H-line was significantly lower than in wild type. Furthermore, trypanothione reductase activity was significantly lower in the H-line, suggesting that gentamicin selection may result in pleiotropic affects on thiol metabolism in Leishmania. A putative RNA-binding protein was very strongly up-regulated in the attenuated line, suggesting a possible target for gentamicin in Leishmania.