Membrane-controlled depletion of complement activity by spin-label-specific IgM

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Abstract
Complement depletion mediated by high MW (Ig[immunoglobulin]M) rabbit antibodies specifically bound to spin-label lipid haptens dispersed in model membranes is controlled by various physical attributes of those membranes other than the total number of exposed determinants that they provide. Carrier lipids used at 32.degree. C were a fluid phosphatidyl-choline (PC), a solid PC and a cholesterol/PC mixture. The concentration of hapten in the plane of the membranes (2-dimensional concentration) was varied while the overall hapten molarity (3-dimensional concentration) was kept constant. Both specific binding and the efficiency of depletion by IgM are markedly enhanced by systematically decreasing the average distance between haptens (.infin. .fwdarw. 26 .ANG.). Heterogeneous distribution was more favorable than a random homogeneous distribution of the same number of haptens in the same total quantity of lipids. IgM efficiency is also markedly increased by the inclusion of cholesterol in PC membranes, an effect thought to result from enhanced projection of the determinant from the surface of the membrane and hence increased accessibility to the antibody-binding site. The efficiency of IgM was increased by using haptens dispersed in fluid rather than in solid PC membranes. IgM molecules probably must be bound to a critical multiple of antigenic determinants at a membrane surface to induce complement-mediated attack. Subtle variation of the physical state of membrane antigens can probably be the crucial factor in determining the outcome of this type of efferent immune response.