Covalent coupling of palmitate to wheat germ agglutinin and its incorporation into liposomes

In order to study cell to cell interaction a model system was designed utilizing liposomes containing a membrane-bound protein which binds to many cell surfaces. Specifically, wheat germ agglutinin (WGA), a plant lectin which binds specifically to N-acetylglucosamine and N-acetylneuraminic acid, was covalently modified with a fatty acid by a reaction with the N-hydroxysuccinimide ester of palmitic acid. The palmitoyl-WGA was subsequently incorporated into liposomes with a detergent removal procedure.

Liposome bound WGA could agglutinate chick erythrocytes at a concentration 8-fold lower than the native WGA. Liposome binding to mouse spleen cells was observed only when palmitoyl-WGA was incorporated; protein-free liposomes did not bind to spleen cells. These results indicate WGA remained active after derivatization and incorporation into liposomes. These observations are explained on the basis of a multivalent binding theory.

The second part of the project involves an in vitro system for the induction of cytotoxic T-lymphocytes (CTL) using liposomes containing trinitrophenol (TNP) haptens. CTL could be obtained by stimulating immune or non-immune mouse T-lymphocytes with hapten modified syngeneic cells. The effector cells could only lyse hapten-modified, syngeneic target cells, but not allogeneic target cells whether haptenated or not. Thus, a H-2 restricted, hapten-specific CTL response of mouse T-lymphocytes was established in vitro. Liposomes containing syngeneic plasma membrane proteins as a source of H-2 antigen and a lipid hapten, trinitrophenyl-caproyl-phosphatidylethanolamine (TNP-CAP-PE), did not induce any CTL. Attempts to replace the plasma membrane proteins with palmitoyl-WGA as a non-specific binder in liposomes also failed to induce a CTL response. These results are inconsistent with the "dual recognition" model of H-2 restriction in CTL.