Liposome--lymphocyte interaction: saturable sites for transfer and intracellular release of liposome contents.

The water-soluble dye 6-carboxyfluorescein was trapped in the internal aqueous compartments of small sonicated dioleoyl lecithin vesicles and used to assess the kinetics of transfer of vesicle contents to human lymphocytes. By using flow microfluorometry, the initial rate of dye transfer to the cells was measured as a function of the concentration of vesicles in the external medium. The rate of transfer consists of at least two components, one of which saturates at high vesicle concentration and the other of which does not saturate in the range of concentrations explored. The saturable component was competitively inhibited by vesicles not containing dye. Both the saturable and nonsaturable components of transfer were inhibited by fetal calf serum or bovine serum albumin but neither component was affected by bovine IgG, choline chloride, or heparin. Pretreatment of the lymphocytes with trypsin or Pronase had no effect on either component. The saturable component can be interpreted in terms of a two-step process in which vesicles bind reversely to sites on the cell surface, and dye is then transferred into the cell from the vesicle-site complex.