Structural and dynamical aspects of membrane immunochemistry using model membranes

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Abstract
Three different phospholipid haptens were synthesized, in which the haptenic group is the paramagnetic nitroxide (spin-label) group. These lipid haptens differ from one another in the length and composition of the molecular chain linking the 2,2,6,6-tetramethylpiperidinyl-N-oxy moiety to the phosphodiester group of the lipid. These lipid haptens were incorporated at low molar concentrations (0.01-0.5 mol %) in liposomes containing various proportions of cholesterol and dipalmitoylphosphatidylcholine (DPPC). A study was made of specific antinitroxide Ig[immunoglobulin]G (and Fab) binding to these liposomes, and the fixation of complement. For lipid haptens whose possible extension above the bilayer plane is limited (e.g., .apprx. 10-20 .ANG.), antibody binding and complement fixation depend strongly on the hapten structure and host lipid composition, because of steric limitations on the accessibility of lipid haptens to the binding sites in the protein. Complement fixation by specific IgG antibodies directed against the nitroxide group as part of a lipid hapten depends strongly on the lateral mobility of the lipid hapten when its molar concentration in the plane of the membrane is of the order of 0.1 mol % or less. This conclusion probably applies to many lipid haptens, and possibly other membrane components. The inclusion of cholesterol in lipid membranes has at least 2 distinct effects on complement fixation involving lipid haptens. Through a steric effect on bilayer structure (probably involving lateral molecular ordering) cholesterol in phosphatidylcholine bilayers can enhance hapten exposure to antibody binding sites, enhance antibody binding and thereby enhance complement fixation. Cholesterol probably affects complement fixation at low hapten concentrations through a modification of membrane fluidity.

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