The major histocompatibility complex in swine

In swine, the major histocompatibility complex (Mhc) or swine leukocyte ant igen (SLA) is located on chromosome 7 and divided by the centromere. Thus, the telomeric class I and more centromeric class III regions are located on the p arm and the class II region is located on the q arm. The SLA region spans about 2 Mb, in which more than 70 genes have so far been characterize d. Despite its division by the centromere, the spatial relationships betwee n the genes in the class II and class III regions, and between the well-con served non-class I genes of the class I region, are similar to those found in the human HLA complex. On the other hand, no orthologous relationships h ave been found between the Mhc class I genes in man and swine. In swine, th e 12 SLA class I sequences constitute two distinct clusters. One cluster co mprises six classical class I-related sequences, while the other comprises five class I-distantly related sequences including two swine homologous gen es of the HLA Mhc class I chain-related gene (MIC) sequence family The numb er of functional SLA classical class I genes, as defined by serology, proba bly varies from one to four, depending on the haplotype. Some of the SLA cl ass I-distantly related sequences are clearly transcribed. As regards the S LA class II genes, some of them clearly code for at least one functional SW -DR and one SLA-DQ heterodimer product, but none code for any DP product. T he amino acid alignment of the variable domains of 33 SLA classical class I chains, and 62 DR beta and 20 DQ beta chains confirmed the exceptionally p olymorphic pattern of these polypeptides. Among the class II genes, the gen es are either monomorphic, like the DRA gene, or oligomorphic, like the DQA genes. In contrast, the DRB and DQB genes display considerable polymorphis m, which seems more marked in DRB than DQB genes.