ABSTRACT
The major histocompatibility complex (MHC) genes encode highly polymorphic cell-surface proteins of three classes. These antigens are markers of “self” and are important in regulation of cellular communication in the immune response. Restriction of cellular interactions by the MHC includes such phenomena as T-B cell cooperation in antibody production, generation of germinal centers, antigen presentation to T cells and T-cell cytotoxic reactions against virally infected cells. Via its role as a restriction element and via specific immune response genes, the MHC has profound effects on genetic control of immunoresponsiveness. The chicken MHC influences total immunoglobulin levels; antibody production to synthetic peptides, soluble antigens and cellular antigens; cell-mediated immunity; complement-mediated hemolytic activity; Chemotaxis and activity of macrophages; and the percentage of T cells of specific subsets. Allelic diversity of the MHC has also been associated with differences in various production traits such as mortality, egg production, body weight, and feed efficiency.
The B-G antigens (class IV) are expressed on erythrocytes and some other cell types. The B-F antigens (class I), expressed on all cell types, consist of 39- to 43-kDa glycoproteins of three extracellular domains and are associated with β2 microglobulin. The B-L (class II) antigens consist of one monomorphic α chain associating with polymorphic β chains and are primarily expressed on cells of the immune system. Recent molecular cloning of B-complex genes has helped establish a molecular map of the chicken MHC. There are several striking differences between chicken and mammalian MHC molecular genetic organization. The chicken B-F and B-Lβ genes are interspersed rather than in separate class I and II regions, the class III genes have not yet been mapped to the MHC, and the introns in chicken MHC genes are much smaller than introns in mammalian MHC genes. The telomeric end of the MHC is adjacent to the nucleolar organizing region on a microchromosome. There are several non-MHC genes interspersed in the region, including gene C8.4, which has characteristics of the immunoglobulin superfamily, and gene C12.3, which bears homology to GTP-binding proteins.
Because of the important role of the chicken MHC in health and fitness, and rapid advances in the molecular definition of this system, the chicken MHC should provide useful genetic markers for breeding populations and should be a good candidate for fruitful modifications by genetic engineering.
