Soluble CD23 controls IgE synthesis and homeostasis in human B cells.

CD23, the low-affinity receptor for IgE, exists in membrane and soluble forms. Soluble CD23 (sCD23) fragments are released from membrane (m)CD23 by the endogenous metalloprotease a disintegrin and metalloprotease 10. When purified tonsil B cells are incubated with IL-4 and anti-CD40 to induce class switching to IgE in vitro, mCD23 is upregulated, and sCD23 accumulates in the medium prior to IgE synthesis. We have uncoupled the effects of mCD23 cleavage and accumulation of sCD23 on IgE synthesis in this system. We show that small interfering RNA inhibition of CD23 synthesis or inhibition of mCD23 cleavage by an a disintegrin and metalloprotease 10 inhibitor, GI254023X, suppresses IL-4 and anti-CD40-stimulated IgE synthesis. Addition of a recombinant trimeric sCD23 enhances IgE synthesis in this system. This occurs even when endogenous mCD23 is protected from cleavage by GI254023X, indicating that IgE synthesis is positively controlled by sCD23. We show that recombinant trimeric sCD23 binds to cells coexpressing mIgE and mCD21 and caps these proteins on the B cell membrane. Upregulation of IgE by sCD23 occurs after class-switch recombination, and its effects are isotype-specific. These results suggest that mIgE and mCD21 cooperate in the sCD23-mediated positive regulation of IgE synthesis on cells committed to IgE synthesis. Feedback regulation may occur when the concentration of secreted IgE becomes great enough to allow binding to mCD23, thus preventing further release of sCD23. We interpret these results with the aid of a model for the upregulation of IgE by sCD23.

Soluble CD23 monomers inhibit and oligomers stimulate IGE synthesis in human B cells.

The low affinity IgE receptor, CD23, is implicated in IgE regulation and the pathogenesis of allergic disease. CD23 is a type II integral membrane protein, comprising a lectin "head," N-terminal "stalk," and C-terminal "tail" in the extracellular sequence. Endogenous proteases cleave CD23 in the stalk and the tail to release soluble fragments that either stimulate or inhibit IgE synthesis in human B cells. The molecular basis of these paradoxical activities is not understood. We have characterized three fragments of CD23, monomeric derCD23, monomeric exCD23, and oligomeric lzCD23. We show that the monomers inhibit and the oligomer stimulates IgE synthesis in human B cells after heavy chain switching to IgE. CD23 fragments could be targets for therapeutic intervention in allergic disease.

High incidence of spontaneous disease in an HLA-DR15 and TCR transgenic multiple sclerosis model.

Multiple sclerosis (MS) is thought to involve CD4 T cell recognition of self myelin, many studies focusing on a pathogenic role for anti-myelin, HLA-DR15-restricted T cells. In experimental allergic encephalomyelitis, it is known which epitopes trigger disease and that disease is associated with determinant spread of T cell reactivity. Characterization of these events in human MS is critical for the development of peptide immunotherapies, but it has been difficult to define the role of determinant spread or define which epitopes might be involved. In this study, we report humanized transgenic mice, strongly expressing HLA-DR15 with an MS-derived TCR; even on a RAG-2 wild-type background, mice spontaneously develop paralysis. Disease, involving demyelination and axonal degeneration, correlates with inter- and intramolecular spread of the T cell response to HLA-DR15-restricted epitopes of myelin basic protein, myelin oligodendrocyte glycoprotein, and alphaB-crystallin. Spread is reproducible and progressive, with two of the epitopes commonly described in responses of HLA-DR15 patients. The fact that this pattern is reiterated as a consequence of CNS tissue damage in mice demonstrates the value of the transgenic model in supplying an in vivo disease context for the human responses. This model, encompassing pathologically relevant, spontaneous disease with the presentation of myelin epitopes in the context of HLA-DR15, should offer new insights and predictions about T cell responses during MS as well as a more stringent test bed for immunotherapies.