No abstract

Reduction of the single disulfide bond between heavy chains in the hinge region of rabbit IgG antibody causes destabilization of the CH2 region of the molecule. Our studies indicate that reduced antibody molecules undergo a large change in quaternary structure in the CH2 region upon aggregation with a small bivalent hapten. The conformational change was observed both in hydrodynamic studies and by electron microscopy. Immunoglobulins trigger the classical pathway of complement fixation by activation of the first component of the serum complement cascade, Cl; the immunoglobulins interact directly with C1 by binding to the structural component of the C1 complex, Clq. Still, the mechanism of activation is not well understood. While both monomeric and aggregated IgG have been shown to be capable of binding Clq (1, 2), monomeric antibodies (with a few notable exceptions) have been found to be very poor activators of complement (3, 4).Formation of immune complexes greatly enhances the ability of antibodies to fix complement. Aggregation of monomeric IgG by treatment with antigen, heat, or crosslinking reagents has been found to convert the antibodies to potent activators of complement (5, 6). Other studies, however, have shown the requirements for activation to be more complex in that some critical elements of native antibody structure must be preserved in the aggregate. Mild reduction of immunoglobulins has been reported to greatly reduce the complement-fixing ability of the antibody without affecting antigen binding (7,8). Investigations of the effects of mild reduction have demonstrated that the hinge region disulfide of rabbit IgG is preferentially cleaved under these conditions (9). The importance of the hinge region in the mechanism of complement activation has been further suggested by the localization of the site of Clq binding within the CH2 region of the immunoglobulin molecule, at or near the hinge region (10, 11). However, cleavage of this disulfide has been reported to have no effect on the tertiary structure of the antibody or on its ability to bind Clq (12-16).Crystallographic analyses of human IgG have recently shown the CH2 region to be atypical in its lack of contacts between the individual CH2 domains (17, 18). Fc fragments of immunoglobulins have been shown to be capable of binding complement but, surprisingly, their activity seemed to be independent of the integrity of the hinge disulfide (19,20). The Fc fragment of human IgG4 has also been found to activate complement while the whole antibody was inactive, suggesting that the Fab fragments can somehow interact with CH2 region and might play a role in the complement activation (19).These results seem to indicate that the complement fixation depends upon the quaternary structure of the CH2 region, and that the structure of the CH2 region is modulated in aggregates by the presence of the Fab arms of the antibody and also by the hinge disulfide bond between heavy chains.We have examined the effects of reduction upon small, well-defined antib...

No abstract

No abstract

No abstract