Liposomes in immunology: Evidence that their adjuvant effect results from surface exposition of the antigens

The immune responses against human serum albumin (HSA) and bovine gamma globulin (BGG) were studied in rabbits after intravenous injections of various preparations of these antigens. Antigens were injected free in saline, coated on “empty” liposomes or both coated on liposomes, and entrapped in their inner compartments. The earlier established adjuvant effect of the liposomes was confirmed for both antigens. Although the amount of antigen entrapped in the liposomes was much higher than the amount coated on their outer surfaces, liposomes containing the antigen both in their inner compartments and on their outer surface showed no stronger adjuvant effect than “empty” liposomes coated with the antigen only. The results support the hypothesis that the adjuvant effect of liposomes is mediated by antigens exposed on the outer surfaces of the liposomes. Suggestions are made for the use of liposomes as a practical immunoadjuvant with definite advantages over many other adjuvants.