Cellular Distribution, Purification, and Molecular Nature of Human la Antigens

Human Ia antigens were extensively purified (1390‐fold increase in specific activity) in 32% yield from BRI8 cells, a lymphoblastoid B‐cell line. Purification was monitored by using allogeneic antisera arising by foetal‐maternal stimulation. The product, a glycoprotein fraction, contained the la antigens, the HLA‐A and ‐B antigens, and a glycoprotein of unknown function. The glycoptotein fraction was composed of four glycosylated polypeptides with molecular weights of 43,000, 39,000, 33,000, and 28,000, and β₂‐microglobulin; no polypeptide was linked to another by disulphide bridges. The A and B antigens only were absorbed by antibody against β₂‐microglobulin. The Ia antigens comprised one each of the 33,000 and 28,000 molecular weight glycosylated polypeptides noncovalently linked together. Thus, only these chains were absorbed by xenogeneic anti‐Ia antisera and were cross‐linked by dimethyl‐3–3′‐dithiobispropionimidate dihydrochloride. The dimeric molecule bound deoxycholate (0.26 g/g of protein) and, when solubilized in deoxycholate, has a molecular weight of 77,000. The Ia allo‐ and xeno‐antigenic activities were labile to heating and proteolysis and are probably determined by the polypeptide structure. Xenogeneic specific anti‐Ia antisera were raised in rabbits and mice by immunizing with the glycoprotein fraction. These antisera reacted with B lymphocytes and monocytes but not T lymphocytes and fibroblasts. Their Fab fragments blocked the cytotoxicity of the allogeneic antisera for B lymphocytes and were potent inhibitors of the mixed lymphocyte reaction.