Antibody-mediated targeting of liposomes. Binding to lymphocytes does not ensure incorporation of vesicle contents into the cells

Small sonicated lipid vesicles containing the water-soluble fluorescent dye 6-carboxyfluorescein were formed from dioleoyl phosphatidylcholine and the antigenic lipid N-dinitrophenylaminocaproyl phosphatidylethanolamine. When these vesicles were incubated with trinitrophenyl-modified human lymphocytes and divalent anti-trinitrophenyl antibody, the antibody bound 5000 to 15 000 vesicles to each cell. Binding was detected by fluorescence microscopy and quantitated by fluorometry and flow microfluorometry. Binding was three times greater with F(ab′)₂ fragments than with the whole antibody and, as expected, was almost absent with the monovalent F(ab′) fragments. It was also absent or greatly reduced, (i) with control immunoglobulin G, (ii) in the presence of excess soluble trinitrophenyl hapten, or (iii) if hapten was omitted from either cells or vesicles. It was unaffected by sodium azide and 2-deoxy-d-glucose but was markedly decreased at 3°C. It was not reversed by incubation at 3°C with excess trinitrophenyl lysine. Self-quenching of the fluorescence of 6-carboxyfluorescein was used to distinguish between release of vesicle contents into the cells and simple binding of intact vesicles (Weinstein, J.N., Yoshikami, S., Henkart, P., Blumenthal, R. and Hagins, W.A. (1977) Science 195, 489-491). Antibody-mediated binding led to little or no increase over spontaneous background levels in the amount of vesicle contents released into the lymphocytes.