Objectives of the Presentation
Explain the function of the Major Histocompatibility Complex (MHC) in the dog
Explain why we are interested in the canine MHC
Describe the canine MHC
Characterization of the canine MHC
Extent of variation in the canine MHC
Disease associations
Overview of the Issue
The Major Histocompatibility Complex (MHC) gene products are critical for the regulation of the immune responses in all higher animals. They are fundamental for explaining variation in susceptibility to infections, immune response to vaccines, the development of auto-immunity and also for transplantation survival.
The canine MHC which is known as DLA (for Dog Leukocyte Antigen) has been extensively studied and shown to be very variable.1-3 Various DLA alleles and haplotypes have been shown to be associated with several immune mediated diseases4-9, severity of infection in Leishmaniasis10 and response to rabies vaccination.11
Hypothesis
Many human immune mediated diseases have been shown to have MHC associations. Dogs have similar spontaneously occurring diseases, and if such diseases are also MHC associated, this would provide good evidence that the canine diseases were homologues of the human diseases.
The Canine MHC
The canine MHC comprises three regions of genes which are called class I, class II and class III. The class I region contains several genes, one of which is very polymorphic. This gene is called DLA-88 and there are currently 51 official alleles, although we have identified at least another ten. The class II region has three polymorphic genes, called DLA-DRB1, DLA-DQA1 and DLA-DQB1. The class III region contains genes such as TNFalpha, and has been less well characterised.
DLA class II alleles can be identified using sequence based typing (SBT). Each locus is amplified using locus specific primers. The PCR product is sequenced and the homozygous or heterozygous sequence can be decoded using specialist software programs that compare the sequence to libraries of known alleles. Alleles can be assigned to haplotypes based on previously identified haplotypes. New haplotypes are identified in homozygous animals.
DLA Variation
We have now characterised over 7,000 dogs for their DLA class II alleles and haplotypes for three class II loci. These include dogs from over 150 different breeds with numbers per breed ranging from 1-500. We can see that most breeds have four to five major haplotypes with frequencies greater than 10%. Breeds with large populations, such as Labrador retrievers, tend to have more haplotypes, while breeds which have been through population bottlenecks, such as the Clumber spaniel have fewer haplotypes.
We have typed over 600 dogs from many multi-generation families, and shown that DLA alleles and haplotypes are inherited as expected.
DLA Disease Associations
Clear associations have been shown for several immune mediated diseases. These include diabetes,5,6 immune mediated haemolytic anaemia,4 hypothyroid disease,7,12 anal furunculosis8 and polyarthritis,9 It is clear that different breeds can have different DLA associations for the same disease.
Susceptibility to Leishmania infection has been shown to be associated with one DLA-DRB1 allele.10
We have also shown that there is variation of response to rabies vaccination that can be attributed to the breed of the dog,11 which is presumably genetic. Unpublished data indicates that there is an effect of DLA on this vaccine response.
Additional Detail
The Major Histocompatibility Complex (MHC) gene products are critical for the regulation of the immune responses in all higher animals. They provide the basis for self-recognition and are fundamental for explaining variation in susceptibility to infections, immune response to vaccines, the development of auto-immunity and also to allotransplantation and rejection. They are also involved in immune surveillance and tumor immunology. Both MHC class I and II cell surface molecules are responsible for presenting antigens to the T cell receptor. Class I molecules present endogenously derived antigen (e.g. viral) to CD8 positive T cells, while class II molecules present exogenously derived antigens (e.g. bacterial) to CD4 positive T cells.13Variability within the amino acid sequence of such MHC molecules influences the repertoire and affinity of peptide antigen binding and this is likely to influence the efficiency of acquired immunity to specific antigens. MHC sequence variability is thus under strong selective evolutionary pressure and consequently, in higher animals, these have become the most polymorphic genes observed at a population level.
When bottlenecks occur in the history of a population, such as those due to catastrophic events, or the establishment of new populations from a relatively small number of founders, major differences in the frequency distribution and extent of alleles can be seen between different populations. This is certainly the case in humans where HLA allele distribution can differ markedly between different populations across the world.14-17 Similarly differences are seen where closed selective line breeding has occurred, and this is the situation observed in a wide range of domestic dog breeds.1-3,18,19
The canine Major Histocompatibility Complex (MHC), which is known as DLA (for Dog Leukocyte Antigen) has been shown to be very variable.3 As more breeds have been studied, more alleles have been identified.1,2,19,20 The grey wolf is the ancestor of the domestic dog and we have also investigated the DLA profile of grey wolves from Alaska and Northern Canada.21
The dog population is composed of around 400 purebred breeds; each one can be considered to be a genetic isolate with unique characteristics resulting from continuous selection for specific attributes (e.g. short legs, floppy ears or curly coat), from genetic drift or inbreeding. Dogs tend to suffer from the same range of diseases as humans but the genetic complexity of these diseases within a breed is reduced as a consequence of the genetic drift and due to long-range linkage disequilibrium.
It is therefore easier to look for genetic markers for disease in dogs than humans, since fewer cases are needed to give the same power for a study.
Summary
The canine MHC is very variable with over 200 three locus class II haplotypes identified to date.
There is little intrabreed variation with most breeds having 4 or 5 frequent haplotypes
There is great interbreed variation
Some haplotypes are very common and are found in many breeds
Other haplotypes are found in few breeds
Certain haplotypes have been associated with diabetes, hypothyroid disease, immune mediated haemolytic anaemia, and anal furunculosis. There can be different associations in different breeds (similar to different HLA associations in different human ethnic groups)
References/Suggested Reading
1. Kennedy LJ, Barnes A, Happ GM, et al. Extensive interbreed, but minimal intrabreed, variation of DLA class II alleles and haplotypes in dogs. Tissue Antigens 2002; 59:194-204.
2. Kennedy LJ, Barnes A, Happ GM, et al. Evidence for extensive DLA polymorphism in different dog populations. Tissue Antigens 2002; 60:43-52.
3. Kennedy LJ, Carter SD, Barnes A, et al. Interbreed variation of DRB1, DQA1 alleles and haplotypes in the dog. Veterinary Immunology and Immunopathology 1999; 69:101-111.
4. Kennedy LJ, Barnes A, Ollier WER, et al. Association of a common DLA class II haplotype with canine primary immune-mediated haemolytic anaemia. Tissue Antigens 2006; 68:502-506.
5. Kennedy LJ, Davison LJ, Barnes A, et al. Susceptibility to canine diabetes mellitus is associated with MHC class II polymorphism. . BSAVA Annual Congress 2003 Scientific Proceedings 2003:593.
6. Kennedy LJ, Davison LJ, Barnes A, et al. Identification of susceptibility and protective Major Histocompatibility Complex (MHC) haplotypes in canine diabetes mellitus. Tissue Antigens 2006;68:467-476.
7. Kennedy LJ, Huson HJ, Leonard J, et al. Association of Hypothyroid disease in Doberman Pinscher dogs with a rare Major Histocompatibility Complex DLA class II haplotype. Tissue Antigens 2006; 67:53-56.
8. Kennedy LJ, O'Neill T, House A, et al. Risk of anal furunculosis in German Shepherd dogs is associated with the major histocompatibility complex. Tissue Antigens 2008;71:51-56
9. Ollier WER, Kennedy LJ, Thomson W, et al. Dog MHC alleles containing the "human RA shared epitope" confers susceptibility to canine rheumatoid arthritis. Immunogenetics 2001; 53:669-673.
10. Quinnell RJ, Kennedy LJ, Barnes A, et al. Susceptibility to visceral leishmaniasis in the domestic dog is associated with MHC class II polymorphism. Immunogenetics 2003; 55:23-28.
11. Kennedy LJ, Lunt M, Barnes A, et al. Factors influencing the antibody response of dogs vaccinated against rabies. Vaccine 2007;25:8500-8507.
12. Kennedy LJ, Quarmby S, Happ GM, et al. Association of canine Hypothyroidism with a common Major Histocompatibility Complex DLA class II allele. Tissue Antigens 2006; 68:82-86.
13. Janeway CA, Travers P. Immunobiology: The immune system in health and disease. 3rd Edition ed. London: Current Biology Ltd., 1997.
14. Bugawan TL, Mack SJ, Stoneking M, et al. HLA class I allele distributions in six Pacific/Asian populations: evidence of selection at the HLA-A locus. Tissue Antigens 1999;53:311-319.
15. Erlich HA, Mack SJ, Bergstrom T, et al. HLA class II alleles in Amerindian populations: implications for the evolution of HLA polymorphism and the colonization of the Americas. Hereditas 1997; 127:19-24.
16. Mack SJ, Bugawan TL, Moonsamy PV, et al. Evolution of Pacific/Asian populations inferred from HLA class II allele frequency distributions. Tissue Antigens 2000; 55:383-400.
17. Meyer D, Single RM, Mack SJ, et al. Signatures of demographic history and natural selection in the human major histocompatibility complex Loci. Genetics 2006; 173:2121-2142.
18. Angles JM, Kennedy LJ, Pedersen NC. Frequency and distribution of alleles of canine MHC-II DLA-DQB1, DLA-DQA1 and DLA-DRB1 in 25 representative American Kennel Club breeds. Tissue Antigens 2005; 66:173-184.
19. Kennedy LJ, Barnes A, Short AD, et al. Canine DLA diversity: 1. New alleles and haplotypes. Tissue Antigens 2007; 69 Suppl 1:272-288.
20. Masters D, Huson HJ, Happ GM, et al. New DLA class II alleles and haplotypes identified in an Alaskan Husky dog family. Tissue Antigens 2006; 68:98-99.
21. Kennedy LJ, Angles JM, Barnes A, et al. DLA-DRB1, DQA1, and DQB1 Alleles and Haplotypes in North American Gray Wolves. J Hered 2007; 98:491-499.