An ovine lymphocyte antigen marker for acquired resistance to Trichostrongylus colubriformis

Genomic Regions Associated with Sheep Resistance to Gastrointestinal Nematodes

Genetic markers for sheep resistance to gastrointestinal parasites have long been sought by the livestock industry as a way to select more resistant individuals and to help farmers reduce parasite transmission by identifying and removing high egg shedders from the flock. Polymorphisms related to the major histocompatibility complex and interferon (IFN)-γ genes have been the most frequently reported markers associated with infection. Recently, a new picture is emerging from genome-wide studies, showing that not only immune mechanisms are important determinants of host resistance but that gastrointestinal mucus production and hemostasis pathways may also play a role.

Association between major histocompatibility complex microsatellites, fecal egg count, blood packed cell volume and blood eosinophilia in Pelibuey sheep infected with Haemonchus contortus

The objective of this study was to assess the correlation among traits associated with resistance or susceptibility to Haemonchus contortus infestation and to evaluate the participation of the ovine major histocompatibility complex (MHC) in Pelibuey sheep, a prevalent breed in tropical and sub-tropical regions in Mexico and elsewhere. Association among the fecal egg count (FEC), blood packed cell volume (PCV), antibody (AB) levels, serum proteins (SP) and blood eosinophil count (EOS) was assessed in 52 lambs experimentally infected with H. contortus, and the participation of the MHC was evaluated using polymorphisms in three microsatellites, located at the class I (OMHC1) and class II (OLADRB1, OLADRB2) regions of the MHC. Spearman correlation analysis among the traits showed a negative association (P<0.01) between FEC and PCV (-0.35), EOS (-0.50), SP (-0.30) and AB (-0.57), and a positive correlation of antibodies with EOS (0.50). The homozygotes for the OMHC1-188 and OLADRB2-282 alleles were associated with a reduction in FEC (-813 and -551, respectively). Conversely, the OMHC1-200 and OMHC1-206 alleles were associated with an increase in FEC (1704 and 1008, respectively). Furthermore, the OLADRB1-482 allele was associated with an increase of 163 EOS by allele copy, while the OMHC1-200 allele showed a reduction of 95 EOS in homozygotes. The associations among microsatellite MHC loci and the remaining variables were not significant. These results reinforce the evidence that MHC polymorphisms have an important role in parasite resistance or susceptibility in Pelibuey sheep and could be used as genetic markers to assist selection and improve parasite resistance to H. contortus.

Association of microsatellite polymorphisms with immune responses to a killed Mycobacterium avium subsp. paratuberculosis vaccine in Merino sheep

AIM

To study the association of polymorphisms at five microsatellite loci with immune responses to a killed Mycobacterium avium subsp. paratuberculosis (Map) vaccine.

METHODS

Merino sheep (504 vaccinates and 430 unvaccinated controls) from a long-term Johne's vaccine trial undertaken on three different properties in the Central Tablelands of New South Wales, Australia, were genotyped for five microsatellite markers located in three immunologically significant chromosome regions. The marker loci included three from the major histocompatibility complex (MHC), namely DYMS1, OLADRB and SMHCC1; and one each from the solute carrier family 11 member 1 (SLC11A1), OVINRA1, and the interferon-γ (IFN-γ), o(IFN)-γ, gene regions. Associations between immune responses and genetic polymorphisms at the marker loci were examined by analysing both allelic and genotypic effects.

RESULTS

The o(IFN)-γ locus had only two alleles, whereas the other four loci exhibited extensive polymorphism, with the number of alleles ranging from 10 (OVINRA1) to 21 (DYMS1), resulting in 30-92 genotypes per locus. Heterozygosities varied between 37% (o(IFN)-γ) and 87% (SMHCC1), while information on polymorphic contents ranged from 0.31 (o(IFN)-γ) to 0.87 (DYMS1). Each of the three properties exhibited unique allelic and genotypic frequencies. Analysis of immune response data revealed strong antibody and IFN-γ responses as early as 2 months post-vaccination. Immune responses in control animals on all three properties remained consistently low, except for slightly elevated IFN-γ responses at a few time-points on two properties, concomitant with exposure to natural infection. Genotype-phenotype association analyses revealed a number of marker genotypes/alleles to be significantly associated with antibody and IFN-γ responses. However, the effects of only five genotypes (one each at DYMS1, OLADRB, SMHCC1, OVINRA1 and o(IFN)-γ) and three alleles (one each at o(IFN)-γ, DYMS1and OLADRB) on IFN-γ responses were consistent across the three properties.

CONCLUSION

Considering the significance of IFN-γ responses in protection against Map, it is possible that the genotypes/alleles identified might have a role in protective immune responses to natural Map infections, and further studies are warranted to confirm this.

Ovine lymphocyte antigens: a comparison of Australian and European antisera

Lymphocyte alloantigens were determined in 183 Australian merino sheep, using antisera from Australian and European laboratories. The study had two aims: (1) to compare reagents characterized in the different laboratories and to correlate antigen definition; and (2) to define lymphocyte antigens for use as genetic markers in two flocks of sheep which had been selectively bred for resistance or susceptibility to the intestinal parasite Trichostrongylus colubriformis, in order to extend a previous study which had indicated linkage between resistance to the parasite and the sheep major histocompatibility system. Good or excellent correspondence was found between four Australian and four European specificities and it was concluded that continued international collaboration would allow definition of markers for future genetic or disease susceptibility studies.

‘Ovar-Mhc‘— Ovine major histocompatibility complex: Role in genetic resistance to diseases

Research on the structure of the ovine major histocompatibility complex (MHC), Ovar-Mhc, and its association with resistance to various diseases in sheep has received increasing attention during recent years. The term 'resistance' is used to denote the capacity of an animal to defend itself against disease or to withstand the effects of a harmful environmental agent. The Ovar-Mhc is poorly characterised when compared to MHCs of other domestic animals. However, its basic structure is similar to that of other animals, comprising Class I, II and III regions. Products of the Class I and II genes, the histocompatibility molecules, are of paramount importance as these present antigens to T-lymphocytes, thereby eliciting immune responses. Several studies have been conducted in sheep on the involvement of MHC genes/antigens in genetic resistance to diseases, the majority being concerned with gastrointestinal nematodes. Studies on resistance to footrot, Johne's disease and bovine leukaemia virus (BLV)-induced leukaemogenesis have also been reported. Genes of all three regions were implicated in the disease association studies. In addition to disease resistance, Ovar-Mhc genes have been found to be associated with traits such as marbling and birthweight. The use of genetic markers from within the Ovar-Mhc may be useful, via marker-assisted selection, for increasing resistance to various diseases provided they do not impact negatively on other economically-important traits. This review summarises current knowledge of the role of Ovar-Mhc in genetic resistance to diseases in sheep.

Microarray analysis of selection lines from outbred populations to identify genes involved with nematode parasite resistance in sheep

Gastrointestinal nematodes infect sheep grazing contaminated pastures. Traditionally, these have been controlled with anthelmintic drenching. The selection of animals resistant to nematodes is an alternative to complete reliance on drugs, but the genetic basis of host resistance is poorly understood. Using a 10,204 bovine cDNA microarray, we have examined differences in gene expression between genetically resistant and susceptible lambs previously field challenged with larval nematodes. Northern blot analysis for a selection of genes validated the data obtained from the microarrays. The results identified over one hundred genes that were differentially expressed based on conservative criteria. The microarray results were further analyzed to identify promoter motifs common to the differentially expressed genes. Motifs identified in upregulated gene promoters were primarily restricted to those promoters; however, motifs identified in downregulated gene promoters were also found in the promoters of upregulated genes but not in the promoters of genes whose expression was unaltered. Protein Annotators’ Assistant was used for lexical analysis of the differentially expressed genes, and Gene Ontology was used to look for metabolic and cell signaling pathways associated with parasite resistance. Two pathways represented by genes differentially expressed in resistant animals were those involved with the development of an acquired immune response and those related to the structure of the intestine smooth muscle. Genes involved in these processes appear from our analysis to be key genetic determinants of parasite resistance.

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.

The use of genetically resistant sheep to control nematode parasitism

Research in the last 20 years has firmly established that it is possible to exploit genetic variation in resistance to the nematode parasites of sheep by selection. Selected sheep are more resistant to infection and commercial application of this research is under way in both Australia and in New Zealand. Not all the consequences of these breeding programs have been established, particularly long-term consequences for productivity and the interaction with other control measures, but the technology is available in the public domain with no commercial restrictions. Faecal worm egg count remains the most effective way of selecting sheep although many alternatives, such as DNA markers, host antibody and parasite antigen assays are being developed for use as selection criteria. Strategic nutritional supplementation is an immediate candidate for inclusion in worm control programs and although nematode vaccines remain elusive, it is likely that some will be field-tested in the next few years. For both of these approaches, nutritional and immunological, it is critical that the response of selected genotypes are assessed under commercial conditions and as components of worm control programs. There is evidence to support the possibility that selected sheep will be more responsive to vaccination while the long-term interaction between the effect of strategic nutritional supplements and resistant genotypes needs to be investigated.

An ovine major histocompatibility complex DRB1 allele is associated with low faecal egg counts following natural, predominantly Ostertagia circumcincta infection

Infection with Ostertagia circumcincta is a major constraint on sheep production in temperate areas of the world. A potential control strategy is the use of genetically resistant sheep. Therefore we examined the association between MHC-DRB1 alleles and faecal egg counts following natural, predominately O. circumcincta infection in a flock of Scottish Blackface sheep. Nineteen DRB1 alleles were identified by a combination of variation in the length of simple repetitive sequences within the intron between exons 2 and 3 and hybridisation of selected oligonucleotides to polymorphisms within exon 2. Faecal samples were taken from 200 lambs from one to six months of age at intervals of 4 weeks. Genetic effects were strongest at 6 months of age. Least-squares analysis indicated that substitution of the most common allele (I) by allele G2 would result in a 58-fold reduction in faecal egg counts in 6-month-old lambs and a 22-fold reduction in 5-month-old lambs. These results suggest that the major histocompatibility complex plays an important role in the development of resistance to O. circumcincta.

The immunogenetics of resistance to Trichostrongylus colubriformis and Haemonchus contortus parasites in sheep

Three possible immunogenetic markers for resistance to intestinal parasites in sheep have been studied. Allotypes of the major histocompatibility complex (MHC) of the sheep have been investigated as markers, using serological typing or gene probes, for associations between allotypes and resistance to parasites in selected high responder and low responder lines of sheep. Only the serologically-determined class I ovine leucocyte antigen (OLA) types SY 1a and SY 1b have been found to be consistently associated with increased resistance to Trichostrongylus colubriformis, but this association has not extended to the immunologically distinct Haemonchus contortus parasite. Gene probes of the sheep DRB, DQB and DQA MHC class II loci have detected animals with increased susceptibility to T. colubriformis. Eosinophilia was investigated as a marker and found to be associated with increased resistance to parasites in lines of Australian Merinos and New Zealand Romneys selected for resistance on the basis of low faecal egg count. Blood eosinophilia was distinct from eosinophil infiltration of the gut which was poorly associated with resistance. The mechanism of parasite resistance appeared to involve the release of vasoactive amines and leukotrienes into intestinal mucus, since the selected high responder sheep to T. colubriformis and H. contortus had significantly increased amounts of these agents in their gut mucus, compared with selected low responder or random-bred sheep. Antibodies to T. colubriformis and H. contortus have also been used as markers to select high responder sire groups of lambs in contact with the parasites, for the first time, at weaning. This assay had the advantage of detecting distinct antigens for the two parasites, which would allow resistance to the species of parasite to be selected in the lambs. Vaccines have been developed against H. contortus using 'novel' gut antigens from the parasite, but variable responsiveness of the host sheep seemed to result in varying degrees of protection which were stimulated by these vaccines.

Genetic resistance to parasitic disease: particularly of resistance in ruminants to gastrointestinal nematodes

There is substantial variation among individuals in susceptibility to a wide variety of parasitic diseases and part of this variation in susceptibility is due to genetic factors. The challenge now is to determine the best methods of using the variation to improve our understanding of parasitic infection and to reduce the ravages of parasitic disease. Scientific and commercial applications will depend upon the type of genetic variation. Variation among breeds can be easily exploited by a policy of breed substitution. Variation within a breed can be exploited by selective breeding to improve resistance to infection or to disease, but more work is needed to develop selection indices which are acceptable to livestock breeders. Identifying genes which contribute to the variation in resistance provides a better understanding of the mechanisms of resistance but more work is needed to determine if such genes, alone or in combination, account for a sufficient proportion of the variation in resistance to allow marker assisted selection. A comparison of responses in susceptible and resistant stock provides a powerful tool to distinguish among protective, irrelevant and pathological responses. These themes have been illustrated by three studies of gastrointestinal nematode infections in ruminants.

A search for associations between major histocompatibility complex restriction fragment length polymorphism bands and resistance to Haemonchus contortus infection in sheep

Polymorphic bands were detected within the DQB and DRB regions of the ovine major histocompatibility complex by probing TaqI digested DNA from three large sheep half-sib families derived from a highly resistant ram. All animals were phenotypically assessed for Haemonchus contortus resistance by faecal egg counts and associations with RFLP bands and haplotypes were estimated using mixed model, best linear unbiased prediction statistical methods. Although the highly resistant sire was homozygous at the MHC, no significant associations were found between any band or haplotype and faecal egg count.

Host genes, parasites and parasitic infections

Resistance to infection of mammalian hosts by parasites is under genetic control at many different levels: between species, between races, breeds and lines of single species and between individuals. These genetic effects have been described in many host-parasite systems. Here we review the interaction between three elements: host genes, parasites and the environment in which parasitic infections develop. Already livestock industries exploit genetic variation between breeds, particularly for the control of trypanosomiasis and tick infestation in cattle. In most populations, and to many diseases, resistance is heritable and selective breeding for resistance in commercial livestock species has been successful experimentally. Attempts at utilizing genetic variation are placed in the broad context of the coevolution of host and parasite, the limited knowledge we have of the mode of action of resistance genes and our ability to use genetic information to predict resistance to parasites.

Relationship between eosinophilia and responsiveness to infection with Trichostrongylus colubriformis in sheep

Lambs with genetically determined increased immunological responsiveness to Trichostrongylus colubriformis (high responders) had more eosinophils in cutaneous reactions to the mitogen phytohaemagglutinin (PHA) both before and during infection compared with those bred for susceptibility (low responders). In contrast, eosinophil numbers in both blood and cutaneous reactions elicited by third-stage T. colubriformis larval antigen were similar in high and low responders before infection. Following vaccination and challenge, high responders had elevated eosinophil numbers in blood and antigen-stimulated skin. In unselected sheep, although eosinophil numbers in cutaneous reactions to PHA were related to responsiveness to a challenge infection with T. colubriformis, there was a closer relationship between blood eosinophil numbers and responsiveness. Infection with T. colubriformis increased eosinophil numbers in cutaneous reactions to PHA and appeared to augment the difference between eosinophil counts in high and low responder sheep. Measurement of the ability to produce eosinophil activating factors, or for eosinophils to respond to such factors might therefore be useful in identifying individual sheep with increased responsiveness to T. colubriformis infection.

Breeding cattle and sheep for resistance to gastrointestinal nematodes

Gastrointestinal nematodes are an important cause of reduced production of meat, milk and wool in domestic livestock. It is generally believed that problems caused by these parasites have increased owing to the intensification of animal husbandry(1-3) of resistance to anthelmintics, current research is focussed on alternative control strategies that do not rely on anthelmintics. Here, Bram Kloosterman, Henk Parmentier and Harm Ploeger review work on the genetic resistance of domestic ruminants to these nematodes and discuss the practicality of breeding programmes.

The expression of bovine lymphocyte antigen and response of Hereford cattle to vaccination against Boophilus microplus

The influence of the bovine lymphocyte antigen MB5 on the immune response of Hereford cattle to antigens of the tick (Boophilus microplus) was examined. Groups of cattle having or lacking the antigen MB5 (MB5+ and MB5-, respectively) were vaccinated with either soluble extracts of the gut membrane of adult ticks and adjuvant or injected with adjuvant alone. Cattle vaccinated with soluble gut membrane were significantly protected from two challenges with ticks. Protection against ticks was not significantly affected by the presence or absence of the lymphocyte antigen MB5. Similarly, no differences occurred in antibody levels (measured by enzyme-linked immunosorbent assay) and cellular responses (measured by lymphocyte blastogenesis assay) in MB5+ and MB5- groups to gut antigen.

No association between the ovine leucocyte antigen (OLA) system in the Australian merino and susceptibility to Haemonchus contortus infection

A genetic analysis has been made of the Ovine Leucocyte Antigenic (OLA) system in Australian merinos. The animals consisted of sires, dams and their progeny. The typing data were consistent with previous findings of a high degree of polymorphism. At least two closely linked loci with several alleles at each are necessary to explain the data. No evidence was found for an association between OLA types and three measures of susceptibility to infection by the blood-sucking parasite Haemonchus contortus. Attention is drawn to the utility of half-sib data for analysis of the genetic control of resistance to parasites in sheep and other animals with a similar breeding structure.

A positive association between resistance to ovine footrot and particular lymphocyte antigen types

The distribution of 12 Class I ovine lymphocyte antigens (OLA) was examined in 4 flocks of sheep vaccinated against and/or challenged with Bacteroides nodosus, the transmitting agent of footrot. In a flock of 47 Corriedales in New Zealand, which had been specially bred for resistance to footrot, a higher frequency (70.2%) of OLA type SY6 was found compared with 42.9% in 49 unselected Corriedale sheep (P = 0.001). The serum antibody response of 12 selected Corriedale ewes was compared with that of 12 unselected ewes of the same age after vaccination with a multivalent footrot vaccine and the selected ewes had significantly (P = 0.01) higher agglutinin titres than the unselected ewes, 7 weeks after vaccination. In 3 trials involving 108, 120 and 135 Australian Merinos in Victoria, SYlb was associated with a reduction in the number of feet affected with severe footrot (P = 0.05, P = 0.01, P = 0.02) and in 2 of the trials there was a relationship between SY6 and high vaccinal agglutinin titres. This SY6 effect was evident in the first trial 31 days after primary vaccination (P = 0.05) and again 20 days later after secondary vaccination (P = 0.01). In the second trial, when the sheep were vaccinated 49 days after challenge, an association was again found between SY6 and high agglutinin titres (P = 0.05) after primary but not after secondary vaccination. Exposure of 157 vaccinated Merino rams to B. nodosus during a footrot outbreak in New South Wales also showed an association between low infection and SY6 and SYlb.(ABSTRACT TRUNCATED AT 250 WORDS)