News and Publications

Structure and Function of the High-Affinity Receptor for IgE

M.W. Robertson, B. Albrecht, C. Dargan, J. de Guzman, R.A. Forcada-Lowrie

The high-affinity receptor for IgE (Fc/epsilonbdy/RI) is expressed by a number of hematopoietic cells, including mast cells and basophils. Activation of these cells occurs when the receptor binds to IgE-antigen complexes. Binding leads to a series of cellular events that culminate in the release of pharmacologically active substances that contribute to the clinical signs and symptoms of allergy. The interaction of cell-bound Fc/epsilonbdy/RI with circulating IgE is therefore of central importance to the molecular origin of atopic disorders.

REGULATION OF IGE SYNTHESIS

Soluble preparations of the -chain of Fc/epsilonbdy/RI, or a suitable analog, may have therapeutic potential for the treatment of atopic disease. We are examining ways to modulate the allergic response by using soluble high-affinity, high-avidity IgE-binding molecules. One line of investigation focuses on the capacity of soluble Fc/epsilonbdy/RI -chains to modulate IgE synthesis in primary B lymphocytes. Human peripheral blood leukocytes obtained from atopic donors are transferred into mice with severe combined immunodeficiency. High titers of IgE develop in the mice within 2--4 weeks after cell transfer, and these levels can be modulated by administration of a soluble IgE receptor construct. Additional studies are in progress to explore the mechanism of the receptor activity.

HUMAN ANTIBODIES TO THE IGE RECEPTOR

An additional goal in this laboratory is to develop an antibody-based strategy to block or attenuate the allergic response. From a large germ-line repertoire of human Fab fragments displayed on the surface of bacteriophage, we isolated a single Fab specific for the ectodomain of the Fc/epsilonbdy/RI -chain. This unique Fab binds to recombinant -chain with moderate affinity (10^-7 M); the interaction is characterized by a slow rate of dissociation (k_off = 3.5 x 10^-4). The Fab binds to the C-terminal (membrane-proximal) domain of the -chain ectodomain and to Fc/epsilonbdy/RI⁺-transfected CHO cells and appears to recognize an epitope distinct from the IgE-binding site.

CHARACTERIZATION OF IGG-FCGRIII INTERACTION

In other studies, we are characterizing the binding of recombinant low-affinity IgG receptor (FcRIII, CD16) to various human IgG subclasses. Binding of the Fc region of IgG antibodies to low-affinity Fc receptors triggers important effector functions in the immune system. To better understand the nature of the ligand-receptor interaction, we produced and purified recombinant FcRIII from both eukaryotic and prokaryotic cells. In collaboration with C. Sautès, Curie Institute, Paris, we used biosensor technology to determine the binding interaction of human IgG1 and IgG3 with immobilized CD16 (1.3 x 10⁶ M^-1 and 2.6 x 10⁵ M^-1, respectively). The affinity of human IgG3 for aglycosylated CD16 was significantly greater than that for fully glycosylated CD16.

PUBLICATIONS

Albrecht, B., Greist, R., Cox, J.P.L., Robertson, M.W. Selection and characterization of a human antibody fragment specific for the high-affinity IgE receptor. Mol. Immunol., in press.

Galon, J., Robertson, M.W., Galinha, A., Mazières, N., Spagnoli, R., Fridman, W.-H., Sautès, C. Affinity of the interaction between Fc gamma receptor type III and monomeric human IgG subclasses: Role of Fc gamma receptor type III glycosylation. Eur. J. Immunol. 27:1928, 1997.