Organisation and diversity of the class II DM region of the chicken MHC.

In mammals, the DM molecules are encoded by the major histocompatibility complex (MHC) and execute key functions in the class II antigen presentation pathway. Here, we characterised three DM genes in the MHC B region of the chicken (Gallus gallus): B-DMA, B-DMB1 and B-DMB2. They encode one class II DM α chain and two β chains, exhibiting motifs of chicken class II molecules as well as specificities of mammal DM proteins. We also studied the expression pattern of those three chicken B-DM genes; they are expressed in immune related tissues. Thus we provide the comprehensive description of the genomic sequence of a class II α gene in the chicken and a valuable description of DM genes in a non-mammalian vertebrate, reinforcing the hypothesis of the existence of DM genes in the primordial MHC, as suggested by previous studies in mammals. We were also able to reconstruct 124 haplotypes corresponding to the 8.8 kb B-DM region, in accordance with the 212 SNPs identified in 146 individuals representing a wide range of experimental, commercial, and local breeds from Europe, Asia and Africa, and three wild species of fowl. We also discovered a repeat inside the B-DMA second intron, making possible the design and the typing of a new marker for the chicken MHC, linked to the class II region. Therefore this study not only describes three DM genes in the chicken, it also provides an overview of MHC diversity in the chicken.