THE LOW AFFINITY Fc RECEPTOR FOR IgE (FCєR2)

Abstract

Fc receptors for IgE (FcєR) may be classified into 2 groups by their affinity for IgE - the high affinity FcєR, also known as FcєRl, that is present on mast cells and basophils; and the low affinity FcєR, also known as FcєR2, and found on lymphocytes, monocytes (Mꬹ), eosinophils and platelets. These receptors are structurally distinct and differ in function. The existence of the FcєR2 was discovered only 15 years ago. Since then, its binding properties, structure and function has been extensively studied.

In this thesis, 3 aspects of the FcєR2 were studied - (1) the antigenic relationship between FcєR2 expressed on B-lymphocytes, Mꬹ and eosinophils; (2) the factors responsible for the expression of FcєR2 on normal human Mꬹ; and (3) the intracellular processing of FcєER2.

At the time that this research work was being performed (1986/7), cDNAs encoding human FcєR2 from a B cell line were cloned (Kikutani et al 1986b, Ikuta et al 1987, Ludin et al 1987) and the molecular structure of the FcєR2 more clearly defined. In contrast to other Fc receptors, the FcєR2 has not evolved as a member of the immunoglobulin supergene family but has substantial homology with several animal lectins. It was also shown that the FcєR2 cloned from the B lymphocyte cell line was identical to the previously defined B cell specific, CD23 (Yukawa et al 1987, Bonnefoy et al 1987). Prior to this, CD23 has been described as a B cell-restricted antigen that is expressed on B cell activation. In view of these data, it was therefore important to investigate the antigenic relationship between FcєR2 on the other cell types (Mꬹ and eosinophils) which expressed the FcєR2 and determine its relationship with the "B cell specific" antigen CD23. The results of this work reported in Chapter 3, indicate that FcєR2 expressed in B cells and Mꬹ are closely related if not identical to the B cell specific activation antigen CD23. In contrast, "hypodense" eosinophils were found to specifically bind IgE but did not express the FcєR2/CD23 antigen, suggesting that FcєR2 expressed by eosinophils is antigenically distinct.

Since the Mꬹ, like the B cell, was found to express FcєR2/CD23, the factor(s) responsible for receptor expression and modulation were the next aspects of the FcєER2/CD23 evaluated in this thesis (Chapter 4). It had been shown that interleukin-4 (IL-4) induces FcєR2/CD23 expression on normal human B-lymphocytes, while gamma-interferon (ꭇIFN) inhibits its expression (Kikutani et al 1986; Defrance et al 1987). Since the signals responsible for induction of FcєER2 expression in Mꬹ had not been clearly defined, the ability of a variety of lymphokines (IL-1, IL-2, IL-3, IL-4, IL-5, granulocyte macrophage colony stimulating factor [GMCSF) and (ꭇIFN) to induce the expression of FcєR2 on normal human peripheral blood Mt was studied. The results indicate that resting Mt do not express FcєR2, and like B-lymphocytes its expression may be induced by incubation with ꭇIL-4. This effect was shown to be specific as none of the other IL's could induce its expression either alone or in combination. In contrast to B cells, however, ꭇIFN was not able to inhibit the IL-4-induced expression of FcєR2 on Mꬹ.

In the final chapter of this thesis, the intracellular processing of the FcєER2 was evaluated. The FcєR2/CD23 is cleaved at the plasma membrane to shed a 25-30 kD soluble fragment (Thorley-Lawson et al 1986, Nakajima et al 1987b). In view of reports that this soluble fragment may play both a potential role as a growth factor (Swendemann and Thorley Lawson 1987), as well as a regulatory role in the modulation of IL-4 induced IgE synthesis (Pene et al 1988a), particular emphasis was made in this study on the evaluation of potential sources of the sFcєR2. In addition, this receptor was also evaluated for receptor-mediated endocytosis, since the FcєR2 shares sequence homology with the asialoglycoprotein receptor (Kikutani et al 1986a) , which is a receptor that recycles rapidly between endosomes and the plasma membrane. The results from pulse-chase metabolic labelling experiments indicate that intracellular cleavage of newly synthesised FcєR2 provides an additional pathway for the generation of the sFcєR2, demonstrating that proteolytic processing of the receptor can also occur inside the cell. Experiments directed towards the deaonatration of receptor mediated endocytosis of the FcєR2 failed to demonstrate this phenomenon.