Recombinant CD16A-Ig forms a homodimer and cross-blocks the ligand binding functions of neutrophil and monocyte Fcgamma receptors.

Abstract

Receptors for the Fc region of IgG (FcgammaR) are expressed as membrane-bound and soluble forms in inflammatory cells. These receptors mediate a variety of immunoregulatory functions. FcgammaR-bearing neutrophils and macrophages play a major role in mediating immune complex induced inflammatory diseases and antibody-mediated tissue injury in autoimmune diseases. To delineate the biological role of the soluble FcgammaR, a recombinant soluble CD16A (CD16A-Ig) was expressed and characterized. CD16A-Ig is secreted as a disulfide-linked homodimer of 70kDa. The purified CD16A-Ig bound to human IgG1 and IgG3 immobilized onto ELISA plates and on IgG1- and IgG3-coated erythrocytes. Saturation binding studies and Scatchard plot analysis showed that immune complex bound to the purified CD16A-Ig with high avidity. Moreover, CD16A-Ig competitively blocked the binding of cell surface-expressed CD16A-CHO cells to IgG-coated plates. The dimeric CD16A-Ig also efficiently cross-blocked the binding of IgG-coated target cells to other FcgammaR expressed on neutrophils and monocytes. These results demonstrate that CD16A-Ig forms a high avidity dimer and is capable of blocking cell-cell interactions mediated by inflammatory cells expressing FcgammaR and IgG-coated target cells. These findings suggest that the dimeric FcgammaR molecules could be used therapeutically for the intervention of immune complex-mediated inflammatory disease.