Biologic and Chemical Characterization of HLA Antigens in Human Serum¹

HLA antigens of both the A and B loci were shown to be associated with the high density lipoprotein fraction of serum prepared by ultracentrifugal flotation. HLA-A9 antigens were purified 100-fold with essentially complete recovery by a simple procedure of high density lipoprotein preparation involving precipitation with polyanions and ultracentrifugal flotation. The purified lipid-associated antigen was immunogenic since it elicited the formation of cytotoxic xenoantibodies in rabbits. Serum HLA-A9 antigens were found by immunoprecipitation and gel electrophoresis to consist of a 45,000 m.w. heavy chain associated with β₂-microglobulin. The size of the HLA-lipid complex (<190,000 m.w.) and of the HLA-deoxycholate complex (<102,000 m.w.) suggests that HLA antigens are shed into plasma as a complex of a single HLA molecule and a single β₂-microglobulin chain, associated with boundary lipid.