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Studies on the Fc Receptor of the Murine Macrophage-Like Cell Line P388D₁

II. Binding of Human IgG Subclass Proteins and Their Proteolytic Fragments¹

From the Department of Biochemistry, University of Toronto, Toronto, Canada M5S 1A8, and Laboratory of Immunology, Toronto Western Hospital, Toronto, Canada M5T 2S8

Abstract

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 x 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 x 10⁵/cell. Fc1 and Fc3 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 C2 and C3 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.

Footnotes

¹ This work was supported by Grant MT 4259 from the Medical Research Council of Canada.

² Supported by the National Research Council of Canada under the terms of a Canada-France Exchange Agreement.

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