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

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Abstract
Biosynthesis and regulation of expression of transplantation antigens as detected by a monoclonal antibody to HLA‐A,B,C antigens (human leucocytic antigen) and a polyclonal antiserum to β2‐micro‐globulin have been investigated using radioactive amino acids and sugars to label human lymphoid cells. We found unbalanced synthesis of HLA heavy chains and β2‐microglobulin, the latter being in excess and secreted to the extracellular medium. In DAUDI cells, which are defective in β2‐microglobulin, no HLA‐A,B,C could be detected intracellularly even in the presence of added β2‐microglobulin. Treatment of BRI‐8 cells with tunicamycin, an antibiotic which inhibits glycosylation of polypeptides, almost had no effect on the levels of β2‐microglobulin, while it markedly decreased that of HLA heavy chains, both on the cell surface and intracellularly. Glycosylation of the HLA heavy chains appeared to be an essential requirement for the 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 β2‐microglobulin was synthesized as a precursor. This larger polypeptide was converted into mature β2‐microglobulin when protein synthesis was performed with microsomes instead of polysomes.