The Major Histocompatibility Complex in Old World Camelids

The current knowledge on the Major Histocompatibility Complex (MHC) in Old World Camelids is reviewed. The MHC region was mapped to the chromosome 20q12 in dromedary camels. Cytogenetic (FISH) and comparative whole genome sequence data analysis showed that the order of the three major sub-regions is Centromere - Class II – Class III – Class I. Exon 2 sequences encoding the antigen-binding sites of MHC class II molecules DRA, DRB, DQA and DQB showed unusually low extent of polymorphism in all three species of Old World camelids, Camelus bactrianus, Camelus dromedarius and Camelus ferus. On the other hand, the DRA locus is polymorphic in all three species. The DQB sequences showed a 12-bp long insertion compared to their counterpart in cattle. A high degree of sequence similarity and extensive allele sharing across the three species was observed in all loci studied. Data obtained from ancient DNA samples of C. dromedarius suggest that additional polymorphism might exist. Two MHC genes coding for other than antigen-presenting molecules were studied. Polymorphisms of potential functional importance were sought in the genomic sequences of the tumor necrosis factor alpha gene (TNFA) and of the MHC class I polypeptide-related sequence A (MICA). Despite high levels of conservation observed in other mammalian species, several single nucleotide polymorphisms were found in the TNFA sequences retrieved. Investigations on MICA polymorphism are currently underway. Evolutionary analyses showed complex relationships of camelid MHC class II genes with other mammalian families and suggested effects of positive selection on their exon 2 sequences.