Nematospiroides dubius: two H-2-linked genes influence levels of resistance to infection in mice

Evidence for genes controlling resistance to Heligmosomoides bakeri on mouse chromosome 1

Resistance to infections with Heligmosomoides bakeri is associated with a significant quantitative trait locus (QTL-Hbnr1) on mouse chromosome 1 (MMU1). We exploited recombinant mice, with a segment of MMU1 from susceptible C57Bl/10 mice introgressed onto MMU1 in intermediate responder NOD mice (strains 1094 and 6109). BALB/c (intermediate responder) and C57Bl/6 mice (poor responder) were included as control strains and strain 1098 (B10 alleles on MMU3) as NOD controls. BALB/c mice resisted infection rapidly and C57Bl/6 accumulated heavy worm burdens. Fecal egg counts dropped by weeks 10-11 in strain 1098, but strains 1094 and 6109 continued to produce eggs, harbouring more worms when autopsied (day 77). PubMed search identified 3 genes (Ctla4, Cd28, Icos) as associated with 'Heligmosomoides' in the B10 insert. Single nucleotide polymorphism (SNP) differences in Ctla4 could be responsible for regulatory changes in gene function, and a SNP within a splice site in Cd28 could have an impact on function, but no polymorphisms with predicted effects on function were found in Icos. Therefore, one or more genes encoded in the B10 insert into NOD mice contribute to the response phenotype, narrowing down the search for genes underlying the H. bakeri resistance QTL, and suggest Cd28 and Ctla4 as candidate genes.

Quantitative trait loci for resistance to Heligmosomoides bakeri and associated immunological and pathological traits in mice: comparison of loci on chromosomes 5, 8 and 11 in F2 and F6/7 inter-cross lines of mice

A comparison of F2 and F6/7 inter-cross lines of mice, derived from CBA and SWR parental strains, has provided strong evidence for several previously undetected quantitative trait loci (QTL) for resistance to Heligmosomoides bakeri. Five QTL affecting average faecal egg counts and/or worm burdens in week 6 were detected on mouse chromosomes 5 (Hbnr9 and Hbnr10), 8 (Hbnr11) and 11 (Hbnr13 and Hbnr14). Three QTL for faecal egg counts in weeks 4 and 6 were found on both chromosomes 5 (Hbnr9) and 11 (Hbnr13 and Hbnr14). Two QTL for the mucosal mast cell protease 1 (MCPT1) response were located on chromosomes 8 (Hbnr11) and 11 (Hbnr13), two for the IgG1 antibody response to adult worms on chromosomes 5 (Hbnr10) and 8 (Hbnr11), two for PCV in week 6 on chromosomes 5 (Hbnr9) and 11 (Hbnr13), and two for the granulomatous response on chromosome 8 (Hbnr12) and 11 (Hbnr15). Our data emphasize that the control of resistance to H. bakeri is multigenic, and regulated by genes within QTL regions that have a complex range of hierarchical relationships.

Heligmosomoides bakeri: a model for exploring the biology and genetics of resistance to chronic gastrointestinal nematode infections

The intestinal nematode Heligmosomoides bakeri has undergone 2 name changes during the last 4 decades. Originally, the name conferred on the organism in the early 20th century was Nematospiroides dubius, but this was dropped in favour of Heligmosomoides polygyrus, and then more recently H. bakeri, to distinguish it from a closely related parasite commonly found in wood mice in Europe. H. bakeri typically causes long-lasting infections in mice and in this respect it has been an invaluable laboratory model of chronic intestinal nematode infections. Resistance to H. bakeri is a dominant trait and is controlled by genes both within and outside the MHC. More recently, a significant QTL has been identified on chromosome 1, although the identity of the underlying genes is not yet known. Other QTL for resistance traits and for the accompanying immune responses were also defined, indicating that resistance to H. bakeri is a highly polygenic phenomenon. Hence marker-assisted breeding programmes aiming to improve resistance to GI nematodes in breeds of domestic livestock will need to be highly selective, focussing on genes that confer the greatest proportion of overall genetic resistance, whilst leaving livestock well-equipped genetically to cope with other types of pathogens and preserving important production traits.

High resolution mapping of chromosomal regions controlling resistance to gastrointestinal nematode infections in an advanced intercross line of mice

Fine mapping of quantitative trait loci (QTL) associated with resistance to the gastrointestinal parasite Heligmosomoides polygyrus was achieved on F(6)/F(7) offspring (1076 mice) from resistant (SWR) and susceptible (CBA) mouse strains by selective genotyping (top and bottom 20% selected on total worm count in week 6). Fecal egg counts were recorded at weeks 2, 4, and 6, and the average was also analyzed. Blood packed cell volume in weeks 3 and 6 and five immunological traits (mucosal mast cell protease 1, granuloma score, IgG1 against adult worm, IgG1, and IgE to L4 antigen) were also recorded. On Chromosome 1 single-trait analyses identified a QTL with effects on eight traits located at about 24 cM on the F(2) mouse genome database (MGD) linkage map, with a 95% confidence interval (CI) of 20-32 cM established from a multitrait analysis. On Chromosome 17 a QTL with effects on nine traits was located at about 18 cM on the MGD map (CI 17.9-18.4 cM). Strong candidate genes for the QTL position on Chromosome 1 include genes known to be involved in regulating immune responses and on Chromosome 17 genes within the MHC, notably the Class II molecules and tumor necrosis factor.

Genetic variation in resistance to repeated infections with Heligmosomoides polygyrus bakeri, in inbred mouse strains selected for the mouse genome project

Since the publication of the mouse genome, attention has focused on the strains that were selected for sequencing. In this paper we report the results of experiments that characterized the response to infection with the murine gastrointestinal nematode Heligmosomoides polygyrus of eight new strains (A/J, C57BL/6, C3H, DBA/2, BALB/c, NIH, SJL and 129/J), in addition to the well-characterized CBA (poor responder) and SWR (strong responder) as our controls. We employed the repeated infection protocol (consisting of 7 superimposed doses of 125L3 each administered at weekly intervals, faecal egg counts in weeks 2, 4 and 6 and assessment of worm burdens in week 6) that was used successfully to identify quantitative trait loci for genes involved in resistance to H. polygyrus. SWR, SJL and NIH mice performed indistinguishably and are confirmed as strong responder strains to H. polygyrus. CBA, C3H and A/J mice all tolerated heavy infections and are assessed as poor responders. In contrast, DBA/2, 129/J and BALB/c mice performed variably between experiments, some tolerating heavy worm burdens comparable to those in poor responders, and some showing evidence of resistance, although only in one experiment with female 129/J females and one with female BALB/c was the pattern and extent of worm loss much like that in SWR mice. Because the genetic relationships between six of the strains exploited in this study are now well-understood, our results should enable analysis through single nucleotide polymorphisms and thereby provide more insight into the role of the genes that control resistance to H. polygyrus.

The importance of immune gene variability (MHC) in evolutionary ecology and conservation

Genetic studies have typically inferred the effects of human impact by documenting patterns of genetic differentiation and levels of genetic diversity among potentially isolated populations using selective neutral markers such as mitochondrial control region sequences, microsatellites or single nucleotide polymorphism (SNPs). However, evolutionary relevant and adaptive processes within and between populations can only be reflected by coding genes. In vertebrates, growing evidence suggests that genetic diversity is particularly important at the level of the major histocompatibility complex (MHC). MHC variants influence many important biological traits, including immune recognition, susceptibility to infectious and autoimmune diseases, individual odours, mating preferences, kin recognition, cooperation and pregnancy outcome. These diverse functions and characteristics place genes of the MHC among the best candidates for studies of mechanisms and significance of molecular adaptation in vertebrates. MHC variability is believed to be maintained by pathogen-driven selection, mediated either through heterozygote advantage or frequency-dependent selection. Up to now, most of our knowledge has derived from studies in humans or from model organisms under experimental, laboratory conditions. Empirical support for selective mechanisms in free-ranging animal populations in their natural environment is rare. In this review, I first introduce general information about the structure and function of MHC genes, as well as current hypotheses and concepts concerning the role of selection in the maintenance of MHC polymorphism. The evolutionary forces acting on the genetic diversity in coding and non-coding markers are compared. Then, I summarise empirical support for the functional importance of MHC variability in parasite resistance with emphasis on the evidence derived from free-ranging animal populations investigated in their natural habitat. Finally, I discuss the importance of adaptive genetic variability with respect to human impact and conservation, and implications for future studies.

Chasing the genes that control resistance to gastrointestinal nematodes

The host-protective immune response to infection with gastrointestinal (GI) nematodes involves a range of interacting processes that begin with recognition of the parasite's antigens and culminate in an inflammatory reaction in the intestinal mucosa. Precisely which immune effectors are responsible for the loss of specific worms is still not known although many candidate effectors have been proposed. However, it is now clear that many different genes regulate the response and that differences between hosts (fast or strong versus slow or weak responses) can be explained by allelic variation in crucial genes associated with the gene cascade that accompanies the immune response and/or genes encoding constitutively expressed receptor/signalling molecules. Major histocompatibility complex (MHC) genes have been recognized for some time as decisive in controlling immunity, and evidence that non-MHC genes are equally, if not more important in this respect has also been available for two decades. Nevertheless, whilst the former have been mapped in mice, only two candidate loci have been proposed for non-MHC genes and relatively little is known about their roles. Now, with the availability of microsatellite markers, it is possible to exploit linkage mapping techniques to identify quantitative trait loci (QTL) responsible for resistance to GI nematodes. Four QTL for resistance to Heligmosomoides polygyrus, and additional QTL affecting faecal egg production by the worms and the accompanying immune responses, have been identified. Fine mapping and eventually the identification of the genes (and their alleles) underlying QTL for resistance/susceptibility will permit informed searches for homologues in domestic animals, and human beings, through comparative genomic maps. This information in turn will facilitate targeted breeding to improve resistance in domestic animals and, in human beings, focused application of treatment and control strategies for GI nematodes.

Mapping of chromosomal regions influencing immunological responses to gastrointestinal nematode infections in mice

This paper reports the results of a genome-wide search for quantitative trait loci (QTL) influencing immunological responses to infection with the gastro-intestinal nematode parasite Heligmosomoides polygyrus in an F2 population created by crossing the resistant SWR and the susceptible CBA inbred mouse strains. Following infections, intestinal granuloma score at post mortem, mucosal mast cell protease 1, and IgE and IgG1 titres were recorded. The susceptible CBA mice had significantly higher IgG1, but significantly lower IgE, mucosal mast cell protease 1 and granuloma scores than SWR mice. Significant QTL were mapped to chromosomes 4, 11, 13 and 17 for granuloma score; chromosomes 12 and 17 for IgE; chromosome 10, 17 and 18 for IgG1 and chromosomes 1, 9, 10, 11, 17 and 18 for mucosal mast cell protease 1. Chromosomes 10, 11, 17 and 18 had QTL affecting more than one trait, and these are most likely to represent single QTL with multiple effects rather than multiple QTL. Some of these QTL map to regions known to harbour genes responsible for the induction of immunological responses to intestinal worms.

Production of a recombinant version of a Heligmosomoides polygyrus antigen that is preferentially recognized by resistant mouse strains

Protective immunity to the mouse nematode parasite, Heligmosomoides polygyrus, has been characterized and found to be composed of the Th2 type. However, many inbred mouse strains cannot produce this protective immune response during a primary infection. A possible reason for this lack of protection in poor responding strains could be due to lack of recognition of specific protective antigens by these strains. Recently, evidence suggests that specific antigens exist that are only recognized by fast responding strains during a primary infection. Using monoclonal antibodies to screen an H. polygyrus cDNA library enabled the production of a recombinant protein, 3A4, which is antigenically similar to those found in the excretory/secretory antigens (E/S) of both L4 and adult parasites. Protein 3A4 shares approximately 70% sequence homology with an E/S protein that induces protection to Trichostrongylus colubriformis in guinea-pigs. Antibodies that bind to 3A4 are preferentially produced in SWR compared to BALB/c mice following immunization with L4 homogenate, although both strains of mice were able to produce comparable levels of specific antibodies after immunization with 3A4 protein. It is believed that 3A4 may have considerable importance in dissecting out the nature of the immune response to H. polygyrus infection, particularly in mouse strains of differing response phenotype.

The relationship between circulating and intestinal Heligmosomoides polygyrus-specific IgG1 and IgA and resistance to primary infection

Specific serum and intestinal immunoglobulin (Ig)G1 and IgA responses to Heligmosomoides polygyrus were measured in a panel of seven inbred mouse strains which exhibit 'rapid' (<6 weeks (SWRxSJL)F1), 'fast' (<8 weeks, SJL and SWR), 'intermediate' (10-20 weeks, NIH and BALB/c) or 'slow' (>25 weeks, C57BL/10 and CBA) resolution of primary infections. Mice with 'rapid', 'fast' or 'intermediate' response phenotypes produced greater serum and intestinal antibody responses than those with 'slow' phenotypes. The F1 hybrids ((SWRxSJL)F1) of two 'fast' responder strains showed the earliest antibody response with maximum titres evident within 6 weeks of infection. There was a negative correlation between the serum IgG1 responses and worm burdens in individual mice within a number of mouse strains, and also between serum IgG1 and IgA responses and worm burdens in the 'rapid' ((SWRxSJL)F1) responder strain. The presence of IgG1 in the gut was found to be due to local secretion rather than plasma leakage. Using Western immunoblotting, serum IgG1 from 'rapid' and 'fast' responder but not 'slow' responder mice was found to react with low molecular weight antigens (16-18 kDa) in adult worm excretory/secretory products.

H-2 genes and resistance to infection with Heligmosomoides polygyrus in selectively bred mice

Two lines of mice bred selectively for high resistance (RH) and susceptibility (SL) to reinfection with Heligmosomoides polygyrus demonstrated disparate levels of resistance to infection but did not differ in the frequency of H-2 antigens when assayed with antisera against antigens of 5 inbred H-2 haplotypes. The selected RH and SL mice were crossbred with, and backcrossed to, the inbred CBA mice. F1 mice from crosses between RH and CBA were as resistant to reinfection with H. polygyrus as their RH parents. F1 mice from crosses between SL and CBA were more resistant than either of their parents. BC1 mice were either positive or negative for H-2 antigens from RH and SL mice. BC1 mice that were positive for RH H-2 antigens were more resistant to infection than their negative littermates, but they were significantly more susceptible to infection with H. polygyrus than their F1 parents. These results demonstrated that genes within and mapped outside H-2 complex control the level of resistance to H. polygyrus in the selected mice and suggested that selective breeding of mice for resistance fixed the relevant genes in and outside the H-2 complex.

Genetic control of acquired resistance to Heligmosomoides polygyrus: overcoming genetically determined weak responder status by strategic immunization with ivermectin-abbreviated infections

The induction of acquired resistance to H. polygyrus, following treatment of mice by a 6 day immunizing infection abbreviated with the anthelmintic drug ivermectin (6d I-AI), was investigated. Four worms were sufficient to elicit > 80% protection against challenge and immunizing infections > 50 worms generated > 95% protection in female NIH mice. A few worms were recovered during the second week from immunized challenged mice but these were rapidly expelled from the gut lumen. Treatment with hydrocortisone from day 10 postinfection, permitted worm burdens to accumulate over the following 2 weeks. The 6d I-AI protocol enabled females of strains previously designated as weak responders to develop potent acquired resistance to challenge (CBA mice showed > 90% protection), although weak responder strain male mice were not significantly protected. Delaying treatment with ivermectin by as little as 24 h resulted in poorer expression of acquired resistance. A positive correlation between the increasing interval from infection to treatment with ivermectin and worm burdens after challenge, and the negative correlation with IgGI antibody responses after challenge indicated that the immunodepressive activities of 7 day and older worms down-regulated local intestinal immune responses. Mice characterized by weak responder phenotype were significantly more sensitive to downregulation than mouse strains showing strong responder phenotype. In consequence, optimal timing of treatment with anthelmintics during exposure to the immunizing infection, intending to minimize exposure to the immunodepressive stages of the parasite, is sufficient to overcome reported genetic constraints on the development of resistance in this system.

Genetic control of immunity to Heligmosomoides polygyrus: fixed H-2 E positive but not H-2 negative cells can present antigen to a parasite-specific T cell hybridoma

A number of T cell hybridomas were produced to adult worm homogenate (AWH) antigen of the nematode parasite Heligmosomoides polygyrus. All of the hybridomas were of the H-2d haplotype and could potentially accept antigen in the context of either the Ad or Ed, H-2 molecules. Three types of antigen presentation were observed, with some of the T cell hybridomas accepting antigen in the context of the E and some in the context of the A molecule. A third type of hybridoma responded to antigen presented by paraformaldehyde fixed APC, but only when APCs were E positive. These same hybridomas, were however, stimulated by AWH, when the antigen was presented by syngeneic but unfixed, E positive or E negative APC. Therefore these data indicate that certain H. polygyrus-specific T cell hybridomas can accept parasite antigen when presented in the context of either the H-2 A or E molecule, but the presentation of antigen by the two different MHC Class II molecules, can apparently utilize differing processing mechanisms.

Factors generating aggregation of Heligmosomoides polygyrus (Nematoda) in laboratory mice

The importance of host heterogeneity in generating aggregation was investigated using Heligmosomoides polygyrus (Nematoda) in laboratory mice. Parameters of infection were compared between inbred and outbred mice, between primary and challenge infection protocols, and between gavage and natural exposure protocols, to investigate the relative effects of innate resistance, acquired resistance and behaviour, respectively. Heterogeneity in acquired resistance was identified as the most consistent factor leading to variability and aggregation of H. polygyrus numbers in mice. This hypothesis was supported in two experiments where groups of mice did not develop resistance to challenge infection (use of certain inbred strains of mice and immunosuppression with corticosteroids in the drinking water) and where variability in worm numbers after the challenge infection was comparable with that after the primary infection. Heterogeneity in host behaviour, particularly in behaviours enhancing skin contact with larvae, also was associated with increased heterogeneity in worm burden, though not as consistently as heterogeneity in acquired resistance. Surprisingly, worm burdens were not more variable in outbred compared with inbred mice. Our data suggest that the relative contributions of innate resistance, acquired resistance and behaviour in generating variable worm burdens are likely to vary spatially and temporally.

Heligmosomoides polygyrus: A model for chronic gastrointestinal helminthiasis

Establishment of chronic infections and strain-dependent variation in resistance to challenge infections are well-known features of the relationship between mice and the intestinal nematode parasite Heligmosomoides polygyrus. Here, Fernando Monroy and Javier Enriquez examine host responses, immunogenic and nonimmunogenic antigens of the parasite, and parasite immune evasion strategies in this useful laboratory mouse model of nematode parasitism of mammals.

Immunological relationships during primary infection with Heligmosomoides polygyrus (Nematospiroides dubius): H-2 linked genes determine worm survival

The course of primary infection was studied in BALB and B10 H-2 congenic mouse strains. The duration of infection, as assessed with regular faecal egg counts and worm burdens, was shorter in mice carrying the H-2s, H-2d or H-2q haplotypes when compared to mice with H-2b. Strains with H-2k were intermediate. An experiment was carried out to test the hypothesis proposed by Wassom, Krco & David (1987) predicting that the progeny of I-E+ve mouse strains crossed with I-E-ve strains, would show susceptibility rather than resistance to infection. This hypothesis was not substantiated by our data and we conclude that it does not apply to primary infections with Heligmosomoides polygyrus. It is proposed that the gene products of at least two loci within the H-2 (associated with the H-2b and H-2k haplotypes) are crucial in determining the response phenotype of mice to primary infection with H. polygyrus. One allele, (associated with the H-2b haplotype) may be preferentially affected by parasite-mediated immunosuppression.

The influence of genes mapping within the major histocompatibility complex on resistance to Trichuris muris infections in mice

Two panels of H-2 recombinant strains of mice were used in an attempt to map the H-2-linked genes which control resistance to infection with Trichuris muris. Response phenotypes could be related to the presence of 'resistance' (q,b) or 'susceptibility' (k,d) alleles at I-A. The influence of these genes was modulated by other alleles, particularly q or d alleles, at the D end of the H-2. Absence of I-E molecules correlated with resistance to infection in some but not all strains studied. Thus the (B10.BR x B10.G) F1 strain which expressed I-Ek gene products was resistant to infection. A study of the time-course of infection in strains of mice expressing q alleles throughout the H-2 on 4 different genetic backgrounds (NIH, SWR, DBA and B10) revealed that most strains were resistant to infection. However, the DBA/1 strain exhibited differential responsiveness, 4 out of 6 individuals harbouring mature adult parasites on day 35 post-infection.