Histocompatibility Antigens of Human Plasma

SUMMARY Assays for histocompatibility antigens HL-A2 and HL-A7 were performed on lipoprotein fractions isolated from normal plasma by hydroxylapatite chromatography and by density gradient ultracentrifugation. Both antigens were recovered solely in fractions containing lipoproteins of the HDL-3 subclass having α electrophoretic mobility. Soluble HL-A2 antigens were found to constitute approximately 14% of total HL-A2 activity in normal human blood, while 73, 7, and 6% of the total was bound to platelets, granulocytes, and lymphocytes, respectively. When platelets from HL-A2-positive donors were incubated with gentle agitation in plasma from HL-A2-negative subjects, HDL-3 lipoproteins having HL-A2 activity were recovered in the plasma, apparently having been released from platelet membranes. These data indicate that soluble histocompatibility antigens of human plasma float in the ultracentrifuge and elute from hydroxylapatite with HDL-3 lipoproteins. They may be synthesized originally as cell membrane components and then released into the general circulation. The possible implications of these findings for transplantation immunity are discussed.