Human blood lymphocytes were coated with increasing amounts of human kappa chain (2–85mug/107 cells) through the linking reagent CrCl3. These cells were then exposed to small unilamellar liposomes composed of egg phosphatidylcholine, cholesterol and phosphatidic acid (molar proportions 7:7:1) containing carboxyfluorescein and/or 111In-labelled bleomycin and bearing 131I-labelled affinity chromatography-purified or non-purified anti-(kappa-chain) immunoglobulin G (IgG) [see the preceding paper, Gregoriadis, Meehan & Mah (1981) Biochem. J.200, 203–210]. In some experiments liposomes contained [14C]phosphatidylcholine. (1) Lymphocytes (107) coated with 2–85mug of kappa chain and exposed to liposomes devoid of IgG or bearing non-purified anti-(kappa chain) IgG bound only a small proportion of the liposomal markers. Even with liposomes bearing the purified anti-(kappa chain) IgG, uptake of the labels improved only slightly for cells coated with up to 10mug of kappa chain. However, with higher concentrations of the antigen on the cell surface, binding was improved considerably to reach values of 31% (111In-labelled bleomycin) and 43% (131I-labelled IgG) of added liposomes for cells coated with 85mug of kappa chain. (2) Lymphocytes coated with kappa chain were exposed to liposomes bearing increasing amounts (0–180mug/0.9mg of egg phosphatidylcholine) of purified anti-(kappa chain) IgG. It was found that under the present conditions, binding of all three markers (111In-labelled bleomycin, 131I-labelled IgG and [14C]phosphatidylcholine) was directly proportional to the concentration of IgG on the liposomal surface. However, uptake values remained unchanged above 90mug of IgG. (3) Antibody-mediated uptake of liposomes by cells coated with the corresponding antigen without loss of their metabolic activities may provide a method of efficient targeting.

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