Abstract
Genetic variation is associated with differences in disease resistance and susceptibility among individuals within a population. To date, molecular genetic analyses of host responses have relied on extraction of genomic DNA from whole blood or tissue samples. However, such samples are not routinely collected during large-scale field studies. We demonstrate that cell-free genomic DNA (cfDNA) may be extracted and amplified from archived plasma samples, allowing retrospective analysis of host genetic diversity. This technique was also applicable to archived serum samples up to 35 years old and to different ruminant species. As proof of concept, we used this cfDNA approach to genotype the major histocompatibility complex (MHC) class II DRB1 locus of 224 Merino sheep which had participated in field trials of a commercial Haemonchus contortus vaccine, Barbervax®, in Australia. This identified a total of 51 different DRB1 alleles and their relative frequencies. This is the first study to examine host MHC diversity using DNA extracted from archived plasma samples, an approach that may be applied to retrospective analyses of genetic diversity and responses to vaccination or infection across different species and populations.
Highlights
Previous molecular studies of the influence of genetic diversity on the response to vaccination or infection have relied on the extraction of genomic DNA from blood or tissue samples collected during field trials
While this study focussed on the major histocompatibility complex (MHC) class II DRB1 locus, the novel approach we describe using archived serum or plasma is applicable to retrospective studies of genetic diversity at other loci and to other infections or vaccinations
MHC genotype can impact on host responses to parasitic disease by influencing the range of antigenic peptides presented for recognition by the adaptive immune system
Summary
Previous molecular studies of the influence of genetic diversity on the response to vaccination or infection have relied on the extraction of genomic DNA from blood or tissue samples collected during field trials. Previous studies have identified associations between MHC diversity and resistance or susceptibility to gastrointestinal nematodes (GIN) infection in different sheep breeds [8–12]. MHC influence on the response to GIN vaccination has not been examined. Vaccination is highly effective under controlled challenge conditions [> 70% reduction in worm burden and > 90% reduction in faecal egg count (FEC)] [19–21]. Under field challenge during vaccine trials, there was considerable variation between vaccinated individuals in their level of protection, as measured by faecal egg count, blood haemoglobin concentration and anti-Barbervax® antibody titre [22]
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