The PCR typing of MHC-DRB genes in the sheep using primers for an intronic microsatellite: application to nematode parasite resistance.

The strong association between polymorphisms in an intronic microsatellite and the coding sequences for (BoLA)-DRB3 genes, previously described for demonstrating alleles of class II major histocompatibility complex (MHC) in the cow, was examined in sheep to see if similar polymorphisms could be demonstrated in the DRB region of the MHC. The bovine primes LA53 and LA54, previously used to amplify the bovine DRB3 microsatellites, were used with DNA from Australian sheep, eight DRB alleles were identified by length polymorphisms of polymerase chain reaction (PCR) products amplified from the DRB microsatellite region. Incomplete amplification of both alleles was sometimes found for sheep DNA samples using bovine primers, so a modified primer (LA53b) was used, and found to amplify the microsatellite next to intron 2 of the MHC more reliably than the LA53 primer. Two additional primers (LA31 and LA32), used in amplification of the exon 2 region of bovine DRB3, were used in the sheep, and the PCR products were analysed by single-stranded conformation polymorphism (SSCP). These primers successfully amplified the variable region of the ovine DRB region coded by exon 2, and the SSCP technique demonstrated polymorphisms with sheep DNA. Family studies demonstrated the segregation of alleles, by amplification both of intronic microsatellites and of the exon 2 variable region. Close correspondence was found between the two regions for several alleles, suggesting that the intronic microsatellites were closely linked to DRB-variable region alleles. Three families of Merino sheep with different antibody responses to intestinal nematode parasites were examined. The sire group with the highest antibody levels possessed two microsatellite alleles of closely similar length (alleles 3 and 4) inherited from the sire and present in high frequency in the lambs. In contrast, the other two sires did not possess these two alleles and the alleles were in low frequency in their progeny. Further studies are required in unrelated sheep to confirm whether these two alleles are associated with resistance to nematode parasites.