Site-specific activation of the alternative pathway of complement. Synthesis of a hybrid molecule consisting of antibody and cobra venom factor.

Cobra venom factor (CoF) is an analog of the activated third component of human complement (C3b). Unlike C3b, CoF is completely resistant to inactivation by factors H and I, the endogenous control proteins of the alternative pathway of complement. This property makes CoF a useful reagent when unrestricted activation of complement is desired. CoF was covalently conjugated to the IgG fraction of rabbit antihuman erythrocyte antiserum, and the ability of the hybrid molecules to initiate activation of the alternative pathway at a specific site was tested. The hybrid molecules were heterogeneous polymers (CoFn-IgGn, where n is an integer greater than 1 for at least one of the components), and both components retained their individual functional activities. The alternative-pathway C3/C5 convertase complex was formed by adding purified factors B and D to the hybrid in the presence of magnesium. This complex (antibody-CoFBb) was able to bind to erythrocytes and activate both C3 and C5. Activation of C5 initiated formation of the potentially cytolytic membrane attack complex of complement on the surface of the cell. When used in combination with rabbit serum, the hybrid molecule was a potent mediator of complement-induced hemolysis. The decay kinetics of the hybrid C3/C5 convertase complex when bound to normal human erythrocytes were not first order, but 40% of the activity remained after 3.5 h at 37 degrees C. Conjugation of CoF to specific antibodies will permit investigation of the consequences of alternative-pathway activation at selective sites. These hybrids may also have therapeutic potential.