Common antigenic structures of HL-A antigens. IV. HL-A common portion fragment isolated from spent culture medium of human lymphoid cell lines.

Spent culture media of all the human cell lines tested have been found to contain the antigenic activity present on the 11,000-Dalton HL-A common portion fragment of the HL-A antigen molecule that appears to be a characteristic, invariant portion of HL-A antigen molecules. From the culture medium of one of these lines, RPMI 1788, a lymphoid cell line, the substance carrying HL-A common activity was isolated, which was shown to be identical to the HL-A common portion fragment with respect to molecular size, electrophoretic mobility, isoelectric focusing patterns, and certain antigenic characteristics. By an isolation procedure involving differential ultrafiltration, gel filtration, and column electrophoresis, 8 litres of the culture medium yielded 1.5–2.0 A units of the substance representing 15–20 per cent of the HL-A common antigenic activity originally present. A single protein band with a R of 0.47 was obtained by disc electrophoresis. The molecular size was shown to be about 11,000 Daltons by gel filtration and by sodium dodecyl sulphate—acrylamide gel electrophoresis. Upon isoelectric focusing two bands were obtained which corresponded exactly to those obtained with HL-A common portion fragment prepared from papain-solubilized HL-A antigen preparations by acid dissociation. The isoelectric point of the major band was 5.0. The reactions of this substance with rabbit antisera against human lymphoid cell membrane and against the substance were essentially identical to the reactions of HL-A common portion fragment with these same antisera.