Structure and Function of Fc ε Receptor II (Fc ε RII/CD23): A Point of Contact Between the Effector Phase of Allergy and B Cell Differentiation

Abstract

The Fc_ε receptor II (Fc_εRII/CD23) has been proposed to have multiple functions as a membrane‐bound or soluble molecule: a function in B cell growth and differentiation and a role in the effector phase of IgE‐mediated immunity. We recently demonstrated the presence of two forms of Fc_εRII (Fc_εRIIa and Fc_εRIIb) whose structures differ only at their N‐terminal cytoplasmic regions. The regulatory mechanisms of their expression strongly suggest that Fc_εRIIa and Fc_εRIIb function in B cells and in the effector cells of IgE‐mediated immunity, respectively. To elucidate the function of soluble Fc_εRII/CD23 (sFc_εRII) the recombinant soluble molecule was produced. This recombinant receptor could competitively block the IgE binding of eosinophils, monocytes and even basophils and could inhibit the IgE‐mediated function of effector cells such as monocytes. These findings suggested that sFc_εRII could competitively regulate the function of effector cells in IgE‐mediated immunity and that the recombinant sFc_εRII could be applied clinically for the control of allergic reactions. The expression of Fc_εRII on Fc_εRII‐negative B and T cell lines by cDNA transfection resulted in homocytic aggregation. The function of Fc_εRII on B cells as an adhesion molecule was also demonstrated.