Efficiency of activation of complement by anti-hapten antibodies at the red cell surface: effect of patchy vs random distribution of hapten

Binding and activation of complement (C) by anti-hapten IgG and IgM antibodies (Abs) bound to a cell surface are dependent on the density and presumably on the distribution of cell-bound hapten. The purpose of this study was to find out if altering the distribution of the hapten on a red cell surface could modify the ability of anti-hapten IgG or IgM Ab to activate C. To test this we devised methods for comparing the C binding and activating efficiency of Abs bound to a hapten distributed randomly or in patches on cells. Random distribution was achieved by the covalent binding of methotrexate (MTX) directly to sheep red blood cells (E), while patchy distribution was achieved by the convalent binding to E of bovine serum albumin-MTX complexes. We considered bound albumin molecules as patches of MTX molecules. The results showed that, for IgG Ab, the number of hapten/E and the number of anti-hapten Ab molecules/cell required to generate one C-activating IgG complex were about an order of magnitude lower for hapten bound in patches than for randomly bound hapten. In contrast, IgM Ab bound to a hapten distributed in patches on an E surface lacked the ability to activate the lytic sequence of C, although maintaining a full ability to binding C1.