Canine Leishmaniosis: immunogenetics of response to infection and vaccination

BACKGROUND Canine leishmaniosis is caused by the protozoan parasite Leishmania infantum that is transmitted by a phlebotomine sand-fly vector. Visceral leishmaniosis is a zoonotic disease, for which dogs are considered the chief reservoir for transmission to humans. Leishmaniosis is an emerging disease in the UK and there is a risk that it might become epidemic and endemic, should future climate conditions support the life-cycle of a suitable vector. The course of L. infantum infection in dogs is determined by a complex interaction between host and pathogen. Both innate and adaptive immune systems engage with the parasite and the subsequent response is diverse within the dog population. A spectrum of clinical outcomes exists post-infection where some animals become sub-clinically infected, whereas others develop severe disease with high morbidity and mortality. Development of cell-mediated immunity is essential for control of the parasite burden. The absence of an adequate T cell response, with production of high levels of antibody and/or regulatory cytokines (e.g. IL-10), typically leads to appearance of severe clinical disease. Immune response genes, particularly those encoding the Dog Leukocyte Antigen (DLA) are highly polymorphic. This allows diversity of immune responses within a population, an evolutionary advantage when dealing with mutating pathogens. Selective breeding of dogs has resulted in considerable interbreed but often minimal intra-breed variability in both innate and adaptive immune response genes, which is likely to impact on immune responses to infection and vaccination. Thus, individuals and some breeds may carry alleles that are detrimental in this particular infection. Most vaccine trials are undertaken in Beagle dogs that are somewhat restricted in the DLA alleles they express. This is likely to influence the results of vaccine trials, in which these animals are used. AIMS AND OBJECTIVES The aim of this project is to test the HYPOTHESES that canine immune response genes are associated with susceptibility and resistance to Leishmania infection and that these genes also determine the response to vaccination. Two populations will be studied; the first will consist of dogs recruited from endemic areas, that are exposed to natural infection and whose disease and immunological status have been established, the second will consist of laboratory Beagles that are being used for leishmania vaccine studies by the industrial partner. The hypotheses will be tested by pursuing the following objectives: 1) Single nucleotide polymorphisms (SNPs) have previously been characterised for a variety of canine innate immune response genes (TLRs, TNF-alpha, IL-12beta, IL-18) and adaptive immune response genes (DLA, CTLA4, IFN-gamma, IL-4, IL-10). A case : control association study will be performed using clinically affected cases and asymptomatic infected controls to identify susceptibility / resistance alleles and haplotypes. 2) Beagles will be genotyped for innate and adaptive immune response genes to establish the degree of restriction within this inbred population compared to the general dog population. Breeding pairs will be selected based on maintaining diversity and heterozygosity in litters to be used for vaccine trials. Puppies will be genotyped prior to entry into vaccine trials to ensure matching between vaccinated and control groups. 3) Following completion of vaccine trials, genetic factors associated with immune responses to vaccination will be investigated. This will aim to identify specific loci/genes that are associated with variability in the cell-mediated IFN-gamma response (low to high; increasing resistance) and antibody response (low to high; increasing susceptibility). Quantitative Trait Loci (QTL) mapping will be undertaken, using the Illumina Canine SNP20 BeadChip system to identify loci and associated genes that are associated with variability in immune responses.