Binding site for IgE of the human lymphocyte low-affinity Fc epsilon receptor (Fc epsilon RII/CD23) is confined to the domain homologous with animal lectins.

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Abstract
The lymphocyte low-affinity receptor for IgE (Fc.epsilon.RII) is involved in two seemingly unrelated processes: (i) promotion of general B-cell growth and (ii) isotype-specific IgE synthesis. To characterize domains of Fc.epsilon. RII important for effector function, we have expressed Fc.epsilon.RII mutants in mammalian cells. The results show that the IgE-binding region of Fc.epsilon.RII corresponds almost exactly to a domain of 123 amino acid residues homologus with the carbohydrate-binding domain of C-type animal lectins. With the recent demonstration that Fc.epsilon.RII binds to IgE independently of any lectin-like activity [Vercelli, D., Helm, B., Marsh, P., Padlan, E., Geha, R. S. and Gould, H. (1989) Nature (London) 338, 649-651], it is now clear that, in this case, the lectin module has evolved to interact with a protein rather than a carbohydrate moiety. The epitopes of several independent monoclonal antibodies that inhibit the binding of IgE to Fc.epsilon.RII are clustered within the lectin-like domain. Some of these antibodies are also known to suppress, isotype-specifically, the interleukin 4-promoted IgE synthesis from peripheral blood mononuclear cells or the spontaneous synthesis of IgE by B cells isolated from atopic donors. The epitope of MHM6, an anti-FeRII monoclonal antibody delivering an epitope-restricted growth-promoting effect on B cells, is also located within the lectin-like domain. Thus, the lectin module of Fc.epsilon.RII not only acts as a carbohydrate-independent, isotype-specific Fc receptor but may also participate in the general regulation of B-cell growth.