Binding studies of human IgG proteins to murine P388D1 cells indicated that they bind to an apparently homogeneous Fc receptor population. The association constant was 0.89 × 106 M-1 at 22°C and was comparable to the binding affinities of homologous murine IgG2a and IgG2b. The number of receptor sites was found to be approximately 6 × 105/cell. Fcγ1 and Fcγ3 fragments bound with an affinity comparable to that of the parent proteins. The P388D1 receptors could discriminate between the human IgG subclasses; the relative cytophilic activity was IgG3 > IgG1 > IgG4 and IgG2 was devoid of binding activity. Fragments corresponding to the Cγ2 and Cγ3 domains of human IgG1 were both unable to bind to the P388D1 receptors either alone or in equimolar combination. This suggests that the cytophilic site may be formed cooperatively by interaction between the two domains. The integrity of the hinge region appeared to be essential for full expression of cytophilic activity since reduction of the hinge-region disulfides in both human IgG1 and its Fc fragment markedly decreased their binding affinity. In addition, a mutant IgG1 molecule lacking the hinge region was significantly less cytophilic than its normal counterpart.

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This work was supported by Grant MT 4259 from the Medical Research Council of Canada.

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