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

Abstract

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.