Studies on Biosynthesis, Assembly and Expression of Human Major Transplantation Antigens

Abstract

Biosynthesis and regulation of expression of transplantation antigens as detected by a monoclonal antibody to HLA-A,B,C antigens (human leukocytic antigen) and a polyclonal antiserum to .beta.2-microglobulin [.beta.2-MG] were investigated using radioactive amino acids and sugars to label human lymphoid cells. Synthesis of HLA H chains and .beta.2-MG was unbalanced, the latter being in excess and secreted to the extracellular medium. In [Burkitt''s lymphoma] DAUDI cells, which are defective in .beta.2-MG, no HLA-A,B,C could be detected intracellularly even in the presence of added .beta.2-MG. Treatment of [lymphoblastoid] BRI-8 cells with tunicamycin, an antibiotic which inhibits glycosylation of polypeptides, almost had no effect on the levels of .beta.2-MG, while it markedly decreased that of HLA H chains, on the cell surface and intracellularly. Glycosylation of the HLA H chains appeared to be an essential requirement for normal expression of HLA-A,B,C antigens. The translation in vitro in a messenger-dependent reticulocyte system with total polysomes obtained from BRI-8 cells showed that .beta.2-MG was synthesized as a precursor. This larger polypeptide was converted into mature .beta.2-MG when protein synthesis was performed with microsomes instead of polysomes.