Joint report of the First International Workshop on Lymphocyte Alloantigens of the Horse held 24-29 October 1981

Linkage disequilibrium between the ELA and the A blood group systems in Standardbred horses

The linkage group formed by the ELA and A blood group system in horses was studied in American Standardbred horses. The distance between the ELA locus and the A blood group locus was measured as 1.61 centimorgans, observing only the haplotypes contributed by the sires. Strong linkage disequilibrium was found in pacing Standardbred horses for ELA-W1 with Aa, ELA-W5 with Ab and ELA-W10 with Ab. Linkage disequilibrium was apparent at both the population and family level. Among trotting Standardbred horses, linkage disequilibrium was found for ELA-W1 with Aa and for ELA-W10 with Ab. It was not possible to investigate linkage relationships in Thoroughbred horses because of the high frequency of Aa and low frequency of other A system markers.

Joint Report of the Fifth International Workshop on Lymphocyte Alloantigens of the Horse, Baton Rouge, Louisiana, 31 October-1 November 1987

Six laboratories participated in the Fifth International Workshop on Lymphocyte Alloantigens of the Horse, testing 132 alloantisera against lymphocytes of 880 horses chosen to represent different families and breeds. Most of the alloantisera were produced by lymphocyte immunization between horses matched at the ELA-A locus. All horses were also tested with antisera contributed to the workshop by participating laboratories which identified ELA specificities A1-A10 and W12-W21. Previously identified workshop specificities ELA-W14, W15 and W19 were accepted as products of the ELA-A locus based on family and population studies by the workshop. Their designations were changed to ELA-A14, ELA-A15 and ELA-A19, respectively. Two new specificities were identified, namely ELA-W22 (W22) and ELA-W23 (W23). Population and family studies indicated that W22 and W23 as well as W13 are products of an ELA locus other than ELA-A. The presence of these specificities was correlated with the presence of certain ELA-A locus specificities, e.g. W13 with A3, W22 with A2 and W23 with A5. However, the association was not complete and W13, W22 and W23 also segregated with other ELA-A specificities in some families. Evidence for recombination was found between the ELA-A locus and the locus or loci encoding these specificities resulting in seven recombinant haplotypes found among the data presented in this workshop. Further studies are required for definitive assignment of the specificities to a class I or class II locus.

Novel classical MHC class I alleles identified in horses by sequencing clones of reverse transcription-PCR products

Improved typing of horse classical MHC class I is required to more accurately define these molecules and to extend the number identified further than current serological assays. Defining classical MHC class I alleleic polymorphism is important in evaluating cytotoxic T lymphocyte (CTL) responses in horses. In this study, horse classical MHC class I genes were analyzed based on reverse transcription (RT)-PCR amplification of sequences encoding the polymorphic peptide binding region and the more conserved alpha 3, transmembrane and cytoplasmic regions followed by cloning and sequencing. Primer sets included a horse classical MHC class I-specific reverse primer and a forward primer conserved in all known horse MHC class I genes. Sequencing at least 25 clones containing MHC class I sequences from each of 13 horses identified 25 novel sequences and three others which had been described. Of these, nine alleles were identified from different horses or different RT-PCR and 19 putative alleles were identified in multiple clones from the same RT-PCR. The primer pairs did not amplify putative non-classical MHC class I genes as only classical MHC class I and related pseudogenes were found in 462 clones. This method also identified classical MHC class I alleles shared between horses by descent, and defined differences in alleles between horses varying in equine leukocyte antigen (ELA)-A haplotype as determined by serology. However, horses sharing ELA-A haplotypes defined by serotyping did not always share cDNA sequences, suggesting subhaplotypic variations within serologically defined ELA-A haplotypes. The 13 horses in this study had two to five classical MHC class I sequences, indicating that multiple loci code for these genes. Sequencing clones from RT-PCR with classical MHC class I-specific primers should be useful for selection of haplotype matched and mismatched horses for CTL studies, and provides sequence information needed to develop easier and more discriminating typing procedures.

Chromosomal localization of the major histocompatibility complex of the horse (ELA) by in situ hybridization

The first gene assignment to a horse chromosome is reported for equine leucocyte antigen (ELA), the major histocompatibility complex of the horse. A cloned DNA sequence derived from a class I gene of the porcine major histocompatibility complex was used as a probe for an in situ hybridization experiment. We present the regional localization of ELA, using this sequence, to equine chromosome 20q14-q22.

The ELY-1 locus controls a di-allelic alloantigenic system on equine lymphocytes

The ELY-1 locus controls the expression of a polymorphic cell surface antigen of equine lymphocytes which was detected using antibodies generated by alloimmunization with peripheral blood lymphocytes. The ELY-1 antigens were not detected on erythrocytes or platelets by absorption experiments. The two alleles, which have been designated ELY-1.1 and ELY-1.2, are expressed codominantly and appear to constitute a closed system at the population level. In family studies, the ELY-1 antigens segregated as products of an autosomal locus not linked to the major histocompatibility complex (MHC) of the horse. In the complement mediated lymphocyte microcytotoxicity test, antisera to the ELY-1 antigens selectively killed peripheral blood lymphocytes which did not express surface immunoglobulin. The ELY-1 antigens may be useful markers for equine T cells when assayed in this fashion. Three alloantisera were used in immune precipitation of iodinated and solubilized cell surface proteins from peripheral blood lymphocytes. Electrophoresis of the precipitates in sodium dodecyl sulphate (SDS)-polyacrylamide gels demonstrated strong bands in the Mr 180-190K range that were shared in the three different preparations. These results suggest that the ELY-1 allospecificities are expressed on an equine equivalent of the murine T200 molecule.

Equine T lymphocytes express MHC class II antigens

Six anti-HLA class II mouse monoclonal antibodies (mAbs) were used in conjunction with a rat monoclonal antibody raised against horse lymphocytes to define class II major histocompatibility complex (MHC) molecules in the horse. By utilizing an ELISA assay and complement dependent lymphocytotoxicity assay, five out of the six anti-HLA class II antibodies and the rat anti-horse monoclonal antibody were found to react with a high percentage of peripheral blood lymphocytes. Flow cytometry demonstrated a variable antigen density on peripheral blood lymphocytes and clear evidence for expression by lymphocytes that carried no detectable surface immunoglobulin. None of the antibodies reacted with equine platelets. The mAbs immunoprecipitated an antigenic complex of Mr 29,000-33,000 from horse lymphocytes. It appears that the distribution of MHC class II antigens in the horse is different from that in man but is similar to that in the dog, since MHC class II antigens are expressed on resting peripheral blood lymphocytes which lack membrane-bound immunoglobulins. Correlations between the distribution of MHC class II antigens on lymphocyte subpopulations and their role in immunological phenomena may contribute to our understanding of the functional properties of these molecules.