Restricted dog leucocyte antigen (DLA) class II haplotypes and genotypes in Beagles

Beagles are commonly used in vaccine trials as part of the regulatory approval process. Genetic restriction within this breed and the impact this might have on vaccine responses are rarely considered. This study was designed to characterise diversity of dog leucocyte antigen (DLA) class II genes in a breeding colony of laboratory Beagles, whose offspring are used in vaccine studies. DLA haplotypes were determined by PCR and sequence-based typing from genomic DNA extracted from blood. Breeding colony Beagles had signiﬁcantly different DLA haplotype frequencies in comparison with pet Beagles and both groups showed limited DLA diversity. Restricted DLA class II genetic variability within Beagles might result in selective antigen presentation and vaccine responses that are not necessarily representative of those seen in other dog breeds. © 2014 The Authors. Published by Elsevier Ltd.

Vaccine efficacy is fundamental to ensuring protection of animals against infectious disease and in maintaining herd immunity.Beagles are used for vaccine trials, designed to provide evidence of efficacy for regulatory approval.However, dog breeds, including the Beagle, have been derived from at least two genetic bottlenecks (Lindblad-Toh et al., 2005), which might affect their immunogenetic diversity and vaccine responses.Vaccination induced antibody titres can vary with breed, although genetic factors have yet to be identified (Kennedy et al., 2007b).
Dog leucocyte antigen (DLA) class II genes encode major histocompatibility complex (MHC) molecules that are involved in antigen presentation to CD4 + T cells.In dogs, these consist of three, highly polymorphic, loci, DLA-DRB1, DLA-DQA1 and DLA-DQB1 (Wagner, 2003).There is a large degree of inter-breed variability, but often limited intra-breed diversity, of DLA haplotypes (Kennedy et al., 2002a).The aim of the present study was to characterise DLA haplotypes in laboratory Beagle dogs and to compare their DLA diversity with a pet Beagle population.
Residual blood samples were obtained from laboratory Beagles (n = 100) in a commercial breeding facility, that provided puppies for vaccine trials in Europe; samples were collected for another study (project number 114-C-E-01-10; date of Animal Care and Ethics Com-mittee approval 7 June 2010).Blood samples from pet Beagles (n = 47) were obtained from the Royal Veterinary College genetic archive, containing residual samples following completion of diagnostic testing with informed owner consent and with approval in 2004 from the institutional Ethics and Welfare Committee.Forty-eight pet Beagles, previously genotyped at the University of Manchester (date of Animal Care and Ethics Committee approval April 2004) were also analysed and included in the pet Beagle group.Information on pedigrees and relatedness of the dogs was not available.
These results show that the laboratory Beagles had a different DLA profile, compared with the pet Beagles.Founder effects, selective breeding within a closed gene pool and use of popular sires have had an impact on genetic diversity within pedigree breeds (Lindblad-Toh et al., 2005;Calboli et al., 2008).The difference between these populations could be the result of selective breeding practices, since the groups have been bred for different purposes and likely represent subpopulations from historical Beagle stock with distinct founders (Kennedy et al., 2002b).
Neither group represents the DLA diversity in the dog population as a whole, where some breeds (e.g.Jack Russell terrier) express a more varied DLA profile, whereas others (e.g.Rottweiler) are even more restricted (Kennedy et al., 2007a).The limited diversity of DLA types could influence vaccine responses in Beagles and a product tested in Beagles will not necessarily perform similarly in other dog breeds.
Both groups were relatively restricted in DLA haplotype diversity.Whilst this restriction is not as marked as for some breeds, such as the Rottweiler, where only four haplotypes have been identified (Kennedy et al., 2002a), it is likely to affect the repertoire of peptide epitopes presented to CD4 + T cells upon antigenic stimulation.Since some haplotypes are only represented in a small number of Beagles, there is a risk that their frequency may diminish in subsequent generations if breeding strategy is not considered within a closed colony.
There was a significant difference in DLA homozygosity between groups.Without pedigrees or breeding strategy information, we can only hypothesise that this high frequency of homozygosity within the pet Beagles might indicate a higher degree of inbreeding and/ or a potential dominant sire effect.Homozygosity appears to vary between dog breeds, depending on the size of the population base; a previous study found 28.4% homozygosity across breeds (Kennedy et al., 2002a).
Laboratory Beagles represent a different immunogenetic pool compared with pet Beagles.They may be a suitable choice for vaccine studies if genetic diversity and heterozygosity can be maintained by strategic breeding.However, laboratory Beagles do not express the full range of DLA haplotypes and thus are not representative of the diversity seen within the pet dog population as a whole.This poses potential problems for the use of any one breed in vaccine studies, since DLA type could influence responses to antigenic challenge.MHC class II heterozygosity in humans has been associated with enhanced resistance and more effective clearance of pathogens (Thursz et al., 1997;Carrington et al., 1999).Further studies are needed to investigate the impact of DLA genotype on susceptibility to infectious disease and response to vaccination in dogs.

Table 1
Frequencies of three DLA locus haplotypes in pet and laboratory Beagles.