CHARACTERIZATION OF SOLUBLE SUBSTANCES IN THE PLASMA CARRYING HL-A ALLOANTIGENIC ACTIVITY AND HL-A COMMON ANTIGENIC ACTIVITY

Abstract

SUMMARY The soluble HL-A-active substances found in plasma have been investigated using a radioimmunoassay to determine their HL-A alloantigenic activities, such as HL-A1 and HL-A2, and the HL-A common activities that are characteristic of a number of the HL-A antigens. Plasma, which accounts for about 10% of a particular HL-A alloantigenic activity that can be measured in the whole blood, contains a series of soluble molecular fragments related to HL-A antigens of different sizes which appear to be degradation products of cell membranes. Gel filtration of plasma yields three fractions carrying HL-A alloantigenic or HL-A common activities or both. The molecules with a molecular size of about 2–8 × 10⁵ daltons appear to represent a definite portion of the cell membrane structure that carries the intact HL-A antigen. Molecules of 48,000 daltons appear to be identical to the molecular fragments that have been derived from cultured lymphoid cells by papain digestion of membrane fractions. Papain digestion of the first fraction yields fragments apparently identical to those in the second. The third fraction of 10,000 daltons contains a soluble component which carries no HL-A alloantigenic activity and only a part of the HL-A common activities present on the molecules of the other two fractions.