Sheep lymphocyte antigens: a preliminary study

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.

Sheep histocompatibility antigens: a population level comparison between lymphocyte antigens previously defined in France, England and Scotland, and sheep red cell groups

A comparison test was performed to look for correlations between the three nomenclature systems for sheep histocompatibility antigens which have been previously described in France, England and Scotland. 187 French sheep from a wide variety of breeds were typed for lymphocyte antigens with antisera which detect the OLA, P and ED series of antigens; they were also tested against 387 uncharacterized French antisera. Six clusters of sera were found which showed correspondence between antigens of at least two of the three nomenclatures; five of these clusters gave high r values of 0.78-0.94. New antisera from French sheep were found which contributed to the above clusters but few additional clusters were noted. No correlation was found between any of the lymphocyte groups of antisera tested and the sheep red cell antigens which were also tested.

‘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.

Haplotype characterization of transcribed ovine major histocompatibility complex (MHC) class I genes

The ovine major histocompatibility complex (MHC) remains poorly characterized compared with those of other livestock species. Molecular genetic analysis of the bovine MHC has revealed considerable haplotype and allelic diversity that earlier serological analysis had not detected. To develop cellular and molecular tools to support development of vaccines against intracellular pathogens of sheep, we have undertaken a molecular genetic analysis of four distinct ovine MHC haplotypes carried by two heterozygous Blackface rams. We have identified 12 novel class I transcripts and used a class I sequence-specific genotyping system to assign each of these transcripts to individual haplotypes. Using a combination of phylogenetic analysis, haplotype and transcript expression data, we identified at least four distinct polymorphic class I MHC loci, three of which appear together in a number of combinations in individual haplotypes. The haplotypes were further characterized at the highly polymorphic Ovar-DRB1 locus, allowing selection of the progeny of the two founder rams for the establishment of an MHC-defined resource population.

Production and evaluation of alloantibodies against sheep MHC Class I antigens

In this work more than 600 sera obtained from three different sources (parous ewes and directed immunizations with whole blood or leucocytes) were tested for cytotoxic antilymphocyte antibodies. A high incidence of cytotoxic antibodies in pregnancy-stimulated animals was confirmed. For the sake of comparison, a comprehensive review of other studies in sheep and other species was performed. There are several different reasons that could explain the differences found between these studies, among them the time of sampling and the cytotoxic assay procedures. Moreover, antibodies were also found in non-pregnant females which may imply that pregnancy is not the only stimulus for antibody production and environmental factors, such as molecular mimicry between infectious agents and lymphocyte antigens, could be the reason for their appearance. In the case of alloimmunizations with leucocytes or whole blood the results were very close to those obtained in goats. Taking into account the Strength Index, the immunization sera were of higher quality. Our results support the methodology of Nesse and Larsen since one injection of whole blood of the lamb is an easy procedure that produces alloantisera with a high value for major histocompatibility complex Class I antigens typing.

Lymphocyte antigens in sheep: linkage to the MHC class II DRB1 gene

In this work a typing battery of sera was developed to test lymphocyte antigens in sheep. Eight antigens were detected in a Latxa sheep sample. The serological determination of these antigens is described. As some of the detected antigens segregated in close linkage with class II DRB1 SSCP patterns in two half-sib families, we can conclude that they are coded by genes located in the MHC. Gene frequencies were very similar in Latxa Mutur Gorria and Latxa Mutur Beltza, the two varieties of the Latxa breed. Although few animals were typed in the comparison with other typing sera, it seems that two of our sera clusters detect the same antigens as those detected by other research groups working in other breeds with their own typing batteries.

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.

Lack of evidence for complement-dependent cytotoxic antibodies to fetal paternally derived antigens in the marsupial Macropus eugenii (tammar wallaby)

A total of 241 serum samples from 145 parous tammar wallabies (Macropus eugenii) were screened for presence of antibodies to paternally derived antigens of the fetus. These samples were taken at different stages in late pregnancy after placental contact was intimate and after birth. Complement-dependent cytotoxicity tests were unable to detect any specific antibodies. It is concluded that the yolk sac placenta of M. eugenii does not allow intimate enough contact between fetal tissues and the maternal circulation to induce formation of cytotoxic antibodies by its mother. This is in contrast to eutherian mammals, in which such production of cytotoxic antibodies occurs frequently as a result of pregnancy. Together with other data it is suggested that the short implantation period in M. eugenii, which is common to all marsupials, has probably not evolved to prevent maternal immune attack upon the conceptus.

Biochemical and molecular analysis of sheep MHC class II molecules

A panel of monoclonal antibodies was used for structural and immunodepletion analysis of sheep MHC class II molecules. The results indicate the antibodies recognize molecules of molecular weight 32-34,000 (alpha chain) and 26-28,000 (beta chain). Immunodepletion analysis indicates that the antibodies may recognize up to four distinct class II molecules some of which are structurally distinguishable using SDS-PAGE. Southern blot analysis using HLA-D region DR, DQ, DP, DO and DZ cDNA probes showed that a number of the cDNA probes hybridized specifically to sheep DNA indicating the presence of closely related genes in sheep. Together the results suggest that the sheep MHC class II region contains distinct MHC class II genes similar to those found in man.

Analysis of the sheep MHC using HLA class I, II, and C4 cDNA probes

Four cDNA probes for the human major histocompatibility complex (MHC) were used to investigate the sheep MHC, in conjunction with serological typing for ovine lymphocyte antigen (OLA). Lymphocytes from a family (two parents and five offspring) of Romanov sheep were subjected to genomic DNA digestion by the restriction endonuclease Eco RI, followed by gel electrophoresis. A single Southern blot representing all seven individuals was then consecutively hybridized with the class I, alpha-DC, beta-DR, and C4 probes, which were originally designed to identify HLA class I, class II (DC and DR), and C4 products, respectively. Using each of the three class I/class II probes, several bands showing DNA polymorphism were detected. The segregation of these bands in the five offspring exactly paralleled the OLA haplotype segregation established by serological typing. A further eight individuals carrying haplotypes which were phenotypically identical to those in the above-mentioned family showed bands in the corresponding positions when tested with the same three probes. Using the C4 probe, no polymorphism was detected in these fifteen individuals.

A method of purifying sheep sIg+ lymphocytes as a tool for class II MHC antigen analysis

A method is described for the purification of sheep lymphocytes carrying class II MHC antigens. After incubation of purified blood lymphocytes on anti-IgM-coated petri dishes, the adherent fraction contained 95% sIg-positive cells determined by immunofluorescence. When tested with cross-reacting anti-class II (bovine and human) monoclonal antibodies, more than 95% of these cells were positive either by immunofluorescence or cytotoxicity. This technique will permit studies of the polymorphism of sheep class II antigens.

Lymphocyte alloantigens of the horse. II. Antibodies to ELA antigens produced during equine pregnancy

Evidence is presented for a reproducible maternal immune response to histocompatibility antigens during equine pregnancy. Mares were stimulated as a result of pregnancy to produce cytotoxic antibodies to paternal lymphocyte alloantigens. The majority of these antibodies were directed against antigens of the equine lymphocyte antigen (ELA) system, which is the major histocompatibility complex (MHC) of the horse. In 16 experimental pregnancies produced using 12 mares and 4 stallions which had been typed for ELA antigens, there was correlation between ELA incompatibility between sire and dam and the appearance of antibody in the serum of the dam as a result of pregnancy. In ELA incompatible pregnancies, antibody was first detected very early in pregnancy, usually by day 60 in a 336-day gestation. The appearance of antibody following shortly the development of the endometrial cups, which can be seen macroscopically for the first time between days 38 and 40. The strength (titre) of the cytotoxic antibody response appeared to vary with the paternal ELA antigens expressed by the fetus. Circulating antibody to 'third party' alloantigens generated during prior pregnancies did not alter the kinetics of the appearance of antibody in a current pregnancy. Evidence for an anamnestic response was obtained in second pregnancies in which the histocompatibility difference between mare and fetus was the same as in the first pregnancy. However, this accelerated response did not occur before the time of formation of the endometrial cups. Serum samples from approximately 90% of parturient mares on horse farms in New York State contained antibody reactive with paternal lymphocyte alloantigens.

Sheep lymphocyte antigens and scrapie

The frequencies of 15 lymphocyte antigens were determined in 2 experimental flocks of Herdwick sheep derived from the same foundation stock. The Blue Flock had been bred for resistance, and the Red flock for susceptibility, to a standard dose of the SSBP/1 source of scrapie injected subcutaneously. Although there were marked frequently differences between the 158 Red flock and 51 Blue flock sheep tested these differences could be attributed to genetic drift. Within the Red flock no significant frequency differences were observed between 16 sheep which developed natural scrapie and their healthy flockmates. All Red flock lambs born in 1980 were injected subcutaneously with SSBP/1 scrapie; there was no difference in antigen frequency between the group which subsequently developed experimental scrapie (30) and the group which did not (21). The segregation of antigen haplotypes in the Red flock pedigrees showed no evidence of MHC linkage with either natural or experimental scrapie.