Characterization of the MHC class II region in cattle. The number of DQ genes varies between haplotypes

The organization of the major histocompatibility complex (MHC) class II region in cattle was investigated by Southern blot analysis using human probes corresponding to DO, DP, DQ, and DR genes. Exon-specific probes were also employed to facilitate the assessment of the number of different bovine class II genes. The results indicated the presence of single DOand DRgenes, at least three DRgenes, while the number of DQ genes was found to vary between MHC haplotypes. Four DQ haplotypes, DQ¹¹ to DQ²⁴, possessed a single DQ and a single DQgene whereas both these genes were duplicated in eight other haplotypes, DQ³⁵ to DQ⁹¹². No firm evidence for the presence of bovine DP genes was obtained. The same human probes were also used to investigate the genetic polymorphism of bovine class II genes. DQ DQ, DR DR, and DO restriction fragment length polymorphisms (RFLPs) were resolved and in particular the DQ restriction fragment patterns were highly polymorphic. Comparison of the present result with the current knowledge of the class II region in other mammalian species suggested that the DO, DP, DQ, DR, and DZ subdivision of the class II region was established already in the ancestor of mammals. The DP genes appear to be the least conserved class II genes among mammalian species and may have been lost in cattle. The degree of polymorphism of different class II genes, as revealed by RFLP analyses, shows striking similarities between species.