Antigens in phenotypes of Heligmosomoides polygyrus raised selectively from different strains of mice

Immunity to the model intestinal helminth parasite Heligmosomoides polygyrus

Heligmosomoides polygyrus is a natural intestinal parasite of mice, which offers an excellent model of the immunology of gastrointestinal helminth infections of humans and livestock. It is able to establish long-term chronic infections in many strains of mice, exerting potent immunomodulatory effects that dampen both protective immunity and bystander reactions to allergens and autoantigens. Immunity to the parasite develops naturally in some mouse strains and can be induced in others through immunization; while the mechanisms of protective immunity are not yet fully defined, both antibodies and a host cellular component are required, with strongest evidence for a role of alternatively activated macrophages. We discuss the balance between resistance and susceptibility in this model system and highlight new themes in innate and adaptive immunity, immunomodulation, and regulation of responsiveness in helminth infection.

Variation and immunity to intestinal worms

Genetically determined variation in host capacity to express resistance to a given parasite plays a major role in determining the outcome of infection. It can be assumed that the same is true of variation in parasites, but very much less is known of its influence on the host-parasite relationship. Phenotypic and genotypic variation within species of intestinal worms is now well documented, detailed studies having been made of parasites such as Ascaris in humans and trichostrongyles in domestic animals. However, the extent to which this variation affects the course of infection or the host immune response in these hosts is limited. Of the nematodes used as experimental models in laboratory rodents, detailed data on phenotypic or genotypic variation are limited to Strongyloides and Trichinella. Parasite variation is known to be subject to host-mediated selection, the emergence of anthelmintic resistance being a good example. Repeated passage has been used to select lines of parasite that survive in abnormal hosts or which show adaptation to host immunity. Experimental studies with Trichinella genotypes in mice have demonstrated the extent to which parasite variation influences the nature and degree of the host's immune and inflammatory responses, the complex interplay between immunogenicity and pathogenicity influencing both partners in the relationship. Recent studies with isolates of Trichuris muris have shown how parasite variation influences the capacity of mice to express the T helper cell responses necessary for resistance. Molecular differences between T. muris isolates have been shown in their excreted/secreted products as well as at the level of their DNA. Knowledge of the functional consequences of parasite variation will add to our understanding of host-parasite evolution as well as providing a rational basis for predicting the outcome of controls strategies that rest on the improvement of host resistance through vaccination or selective breeding.

Immunity, antigenic heterogeneity, and aggregation of helminth parasites

Empirical studies of helminth parasites reveal that the distribution of parasite burdens in their host populations is highly aggregated. This aggregation is fundamental to the ecology and epidemiology of helminth parasites. Results from a stochastic model predict that aggregation of helminth parasites is inversely related to the intensity of host immunity. Aggregation also decreases with antigenic heterogeneity and increases with heterogeneity in transmissibility among parasite strains. It is also found that the degree of aggregation is greater when immunity affects parasite fecundity than when immunity acts on host susceptibility. Potential relevance of this result for assessing the influence of vaccines that target either host susceptibility or parasite fecundity on the level of aggregation and consequent effects on drug resistance and disease prevalence are discussed.

The maintenance of sex in parasites

The maintenance of sex is an unresolved paradox in evolutionary biology, given the inherent twofold fitness advantage for asexuals. Parasitic helminths offer a unique opportunity to address this enigma. Parasites that can create novel antigenic strains are able to escape pre-existing host immunity. Viruses produce diversity through mutation with rapid clonal proliferation. The long generation times of helminth parasites prevent them from adopting this strategy. Instead, we argue that sexual reproduction enables parasitic helminths to rapidly generate strain diversity. We use both a stochastic, individual-based model and a simple analytical model to assess the selective value of sexual versus asexual reproduction in helminth parasites. We demonstrate that sexual reproduction can more easily produce and maintain strain diversity than asexual reproduction for long-lived parasites. We also show that sexual parasite populations are resistant to invasion by rare asexual mutants. These results are robust to high levels of cross-immunity between strains. We suggest that the enhancement of strain diversity, despite stochastic extinction of strains, may be critical to the evolutionary success of sex in long-lived parasites.

The effects of mating probability on the population genetics of nematodes

We review how constraints on the mating probability of female worms by segregation within individual host guts form a critical element in the parasite population structure of nematodes. We consider the effects of these constraints on the population genetics of nematodes under various assumptions regarding worm competition as reflected in the relationship between the abundance of a certain genotype within the gut and the probability of reproductive success. The consequences for the emergence of resistance to drugs and immunotherapy and implications for host-parasite coevolution are discussed. We also review evidence for genetic heterogeneity in parasite populations as a necessary prerequisite for the applicability of mating probability models designed to assess the population genetics of nematodes.

Helminth immunogenetics: Why bother?

The importance of host genotype as a determinant of protective responses against helminth infection is well established. In contrast, there have been relatively few investigations of the role of helminth genotype, despite the importance accorded to the genetics of other disease-causing organisms. Here, Andrew Read and Mark Viney discuss the reasons for this oversight. They argue that it is not for any compelling empirical reason: there is at least as much evidence that worm genetics affects host protective responsiveness as there is that it does not.