Antibody-Mediated Targeting of Liposomes
We have studied a number of permutations on the use of antibody and antigen for targeting liposomes in vitro. Our first strategy was to make liposomes with lipid bearing the dinitrophenyl (DNP) hapten. These liposomes bound specifically to cells in three experimental configurations: (i) with TNP-modified human peripheral blood lymphocytes as targets and sheep IgG anti-TNP as cross-linking agent; (ii) with murine myeloma MOPC 315 cells (which bear an IgA with high affinity for nitrophenyl haptens) as target, using endogeneous surface immunoglobulin on the cells as a point of attachment; (iii) with Fc-receptor bearing cells as targets and rabbit anti-TNP as an opsonizing agent. We monitored the interactions by encapsulating carboxyfluorescein and/or methotrexate in the liposomes, and in some experiments by incorporating 14C-dipalmitoyl phosphatidylcholine or a fluorescent marker in the lipid. In each study large numbers of liposomes could be bound specifically to the target cells, but they were internalized in significant numbers only in (iii) when cells capable of Fc-mediated endocytosis were used. The most striking internalization was found with the mouse macrophage line P388D1. Encapsulated methotrexate had a several-fold greater effect on the metabolism of P388D1 (as assayed by 3H-deoxyuridine uptake) than did an equivalent amount of methotrexate free in solution. This finding indicated that an appropriately chosen drug can escape the phagosomal system to exert its effect in the cytoplasm.
Our second principal effort has been to couple immunoglobulin to liposomal phosphatidylethanolamine covalently, or else through covalently coupled Staphylococcus aureus protein A. Coupling is achieved with the heterobifunctional agent N-succinimidyl 3-(2-pyridyldithio) propionate (SPDP). This method of coupling results in minimal aggregation and little leakage of vesicle contents. Liposomes bearing covalently coupled monoclonal antibody bind with high specificity to cells with the corresponding determinants.
KeywordsP388 Cell RAJI Cell Vesicle Content Covalent Coupling Murine Tumor Cell
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