Molecular genetics of the HLA complex.

The Human Leukocyte Antigen (HLA) system can be defined as a set of glycoproteins which take up peptides intracellulary and become ligands for immune receptors once they are expressed on the cell membrane. Functional HLA molecules are trimers consisting of a heavy chain and a light chain which bind a peptide. The production of HLA molecules in the endoplasmic reticulum, their assembly and their trafficking to the cell membrane have been studied in great detail. A salient feature of HLA molecules is their polymorphism which is essentially a tremendous amount of variation in the amino acid sequence of mainly the heavy chain between different HLA types. Polymorphic chains of HLA molecules are encoded in the human major histocompatibility complex (MHC) on the short arm of chromosome 6. A map of the nucleotide sequence of the human MHC has been established. The polymorphism of HLA molecules, i.e. the HLA type, is intimately associated with the nature of the peptides bound. In principle, HLA molecules can bind a variety of different peptides. However, peptides with distinct biochemical features are preferentially bound according to the HLA-type. HLA-typing is applied clinically in transplantation and disease association. Recently, recombinant HLA molecules have been used as a tool to define the T cell receptor repertoire.