Immunospecific targeting of liposomes to cells: a novel and efficient method for covalent attachment of Fab' fragments via disulfide bonds.

An efficient method for covalently cross-linking 50K Fab' antibody fragments to the surface of lipid vesicles is reported. Coupling up to 600 microgram of Fab'/mumol of phospholipid (about 6000 Fab' molecules per 0.2-micrometer vesicle) is achieved via a disulfide interchange reaction between the thiol group exposed on each Fab' fragment and a pyridyldithio-derivative of phosphatidylethanolamine present in low concentration in the membranes of preformed large unilamellar vesicles. The coupling reaction is efficient, proceeds rapidly under mild conditions, and yields well-defined products. Each vesicle-linked Fab' fragment retains its original antigenic specificity and full capacity to bind antigen. We have used Fab' fragments, coupled to vesicles by this method, to achieve immunospecific targeting of liposomes to cells in vitro. Vesicles bearing antihuman erythrocyte Fab' fragments bind quantitatively to human erythrocytes (at multiplicities up to 5000 0.2-micrometer vesicles per cell) while essentially no binding is observed to sheep or ox red blood cells. Vesicle-cell binding is stable over a pH range from 6 to 8 and is virtually unaffected by the presence of human serum (50%). Cell-bound vesicles retain their aqueous contents and can be eluted intact from cells by treatment with reducing agents (dithiothreitol or mercaptoethanol) at alkaline pH.