Coupling of ligands to liposomes independently of solute entrapment: observations on the formed vesicles.

Bovine serum albumin (BSA), employed as a model ligand, was covalently linked (about 16% of the amount used) to small unilamellar vesicles (SUV) composed of phospholipid, cholesterol and N-(p-aminophenyl)stearylamide (APSA) (molar ratios 1:1:0.05). SUV with bound BSA were then used to generate dehydration-rehydration vesicles (DRV) in the presence of tetanus toxoid and/or carboxyfluorescein (CF). Nearly all of the SUV-bound BSA (about 15% of the original amount) was recovered in the multilamellar DRV formed, with a considerable proportion (42-62%) of the ligand becoming available on the outer bilayers. This apparent spatial reorientation of BSA within DRV also caused the entrapped toxoid to shift to some extent to the liposomal surface. There was no significant difference in the z average mean size between DRV with and without coupled BSA (543 and 555 nm diameter, respectively). Percent number diameter distribution data revealed that 71.2 (BSA-free) and 76.4% (BSA-containing DRV) of the vesicles had diameters of about 300-440 and 330-420 nm, respectively. However, in terms of percent mass diameter distribution, 69.5% (BSA-free) and 65.2% (BSA-containing DRV) of the mass was in vesicles with corresponding ranges of diameter of 1381-2975 and 1086-2840 nm. Vesicle size heterogeneity in both preparations was confirmed by freeze-fracture electron microscopy which also indicated that structures with or without bound BSA, were mostly vesicular of the multilamellar type. Judging from CF latency values, ligand-bearing DRV were stable on incubation with blood plasma at 37 degrees C for 24 h. Stability was, however, reduced significantly when the amount of ligand bound was excessive. The present approach allows for the coupling of ligands to and the entrapment of antigens and other labile solutes in liposomes independently, thus avoiding potential damage of such solutes by the coupling reagents.