Studies of the Mechanism of Binding of Chemically Modified Cytophilic Antibodies to Macrophages

Chemical modification of lysine or tryptophan groups of rabbit γG antibody led to marked reduction in its cytophilic binding ability. The procedures used, carbamylation, amidination and benzylation, were highly selective in their effect on the various properties of antibody. Thus, although they led to loss of complement-fixing ability and diminution in the rate of immune aggregation, they had no significant effect on the primary union of antibody with antigen or on the ability of antibody to mediate anaphylaxis in the guinea pig. This selectivity suggested that the modifications led to specific alteration of Fc binding sites rather than nonspecific alteration of the molecule as a whole. The fact that these chemical alterations affect both cytophilic binding and complement fixation does not imply a functional relationship between the two. Cytophilic binding did occur in the absence of complement. Moreover, the various treated antibodies had dissociation of these activities; concentrations of conjugated antibody sufficient for cytophilic rosette formation were not capable of fixing enough complement for detectable erythrocyte lysis. In this regard, differences in the ability of untreated antibody to mediate rosette formation and complement fixation in the direct and indirect procedures for cytophilic binding suggest that the mechanisms of complement-antibody and macrophage-antibody interaction are distinct, even though both reactions can be a property of the same antibody molecule.