Lipid contribution to the affinity of antigen association with specific antibodies conjugated to liposomes

Immunoliposomes, directed to clinically relevant cell-surface molecules with antibodies, antibody fragments or peptides, are used for site-specific diagnostic evaluation or delivery of therapeutic agents. We have developed intrinsically echogenic liposomes (ELIP) covalently linked to fibrin(ogen)-specific antibodies and Fab fragments for ultrasonic imaging of atherosclerotic plaques. In order to determine the effect of liposomal conjugation on the molecular dynamics of fibrinogen binding, we studied the thermodynamic characteristics of unconjugated and ELIP-conjugated antibody molecules. Utilizing radioimmunoassay and enzyme-linked immunosorbent assay protocols, binding affinities were derived from data obtained at three temperatures. The thermodynamic functions ΔH°, ΔG° and ΔS° were determined from van't Hoff plots and equations of state. The resultant functions indicated that both specific and nonspecific associations of antibody molecules with fibrinogen occurred through a variety of molecular interactions, including hydrophophic, ionic and hydrogen bonding mechanisms. ELIP conjugation of antibodies and Fab fragments introduced a characteristic change in both ΔH° and ΔS° of association, which corresponded to a variable contribution to binding by phospholipid gel-liquid crystal phase transitions. These observations suggest that a reciprocal energy transduction, affecting the strength of antibody-antigen binding, may be a singular characteristic of immunoliposomes, having utility for optimization and further development of the technology.