Improved FcγRIIb targeting functionally translates into enhanced inhibition of basophil activation.

BACKGROUND: Mast cells and basophils express the high-affinity IgE receptor, FcεRI, as well as the low-affinity IgG receptor, FcγRIIb. While FcεRI is responsible for IgE-dependent degranulation upon coaggregation with allergens, FcγRIIb has been shown to downregulate degranulation through cross-linking with FcεRI. A previously developed fusion protein consisting of an anti-IgE DARPin linked to the human IgG1-Fc part (DE53-Fc) has been shown to simultaneously target FcεRI and FcγRIIb with low affinity and to thereby prevent basophil activation. The affinity of a ligand for its receptor is known to be critical for the functional consequences of the binding. So we generated two mutated DE53-Fc molecules with either an improved (DE53-Fc mut+) or a reduced (DE53-Fc mut-) binding to FcγRIIb and assessed their potential to inhibit IgE-dependent basophil activation. METHODS: DE53-Fc was modified by introducing single site-directed point mutations in the Fc part. The mutated constructs were used to assess kinetic parameters as well as the inhibitory capacity on basophil activation and the production of leukotriene C4 (LTC4) and IL-13. RESULTS: DE53-Fc mut+ showed increased affinity for FcγRIIb as well as an enhanced potential to inhibit IgG1 binding to FcγRIIb, resulting in improved efficacy in functional assays. Furthermore, DE53-Fc mut+ decreased de novo-synthesized LTC4 as well as the cytokine IL-13, suggesting that it might be an inhibitor of the allergic late-phase reaction. CONCLUSION: Our data suggest that improved binding to FcγRIIb at constant low-affinity binding to IgE leads to more efficient coaggregation of FcεRI-FcγRIIb and results in the enhanced inhibition of basophil activation.

IL-3 but not monomeric IgE regulates FcεRI levels and cell survival in primary human basophils.

Binding of allergen-specific IgE to its primary receptor FcεRI on basophils and mast cells represents a central event in the development of allergic diseases. The high-affinity interaction between IgE and FcεRI results in permanent sensitization of these allergic effector cells and critically regulates their release of pro-inflammatory mediators upon IgE cross-linking by allergens. In addition, binding of monomeric IgE has been reported to actively regulate FcεRI surface levels and promote survival of mast cells in the absence of allergen through the induction of autocrine cytokine secretion including interleukin-3 (IL-3). As basophils and mast cells share many biological commonalities we sought to assess the role of monomeric IgE binding and IL-3 signaling in FcεRI regulation and cell survival of primary human basophils. FcεRI cell surface levels and survival of isolated blood basophils were assessed upon addition of monomeric IgE or physiologic removal of endogenous cell-bound IgE with a disruptive IgE inhibitor by flow cytometry. We further determined basophil cell numbers in both low and high serum IgE blood donors and mice that are either sufficient or deficient for FcεRI. Ultimately, we investigated the effect of IL-3 on basophil surface FcεRI levels by protein and gene expression analysis. Surface levels of FcεRI were passively stabilized but not actively upregulated in the presence of monomeric IgE. In contrast to previous observations with mast cells, monomeric IgE binding did not enhance basophil survival. Interestingly, we found that IL-3 transcriptionally regulates surface levels of FcεRI in human primary basophils. Our data suggest that IL-3 but not monomeric IgE regulates FcεRI expression and cell survival in primary human basophils. Thus, blocking of IL-3 signaling in allergic effector cells might represent an interesting approach to diminish surface FcεRI levels and to prevent prolonged cell survival in allergic inflammation.

High affinity targeting of CD23 inhibits IgE synthesis in human B cells.

The low-affinity IgE receptor FcϵRII (CD23) is part of the regulatory system controlling IgE synthesis in human B cells and exists in membrane and soluble forms. Binding of IgE to CD23 has been described to have stabilizing effects and to prevent cleavage of CD23. Previous experiments using anti-CD23 antibodies reduced IgE synthesis but were difficult to interpret as the antibody Fc part might also mediate feedback mechanisms. The purpose of this study was to investigate the regulatory role of CD23, by using designed ankyrin repeat proteins (DARPins) that specifically recognize CD23. Anti-CD23 DARPins were isolated by ribosome display and were produced as monovalent and bivalent constructs. Affinities to CD23 were measured by surface plasmon resonance. IgE synthesis and up-regulation of CD23 in human peripheral B cells were induced by IL-4 and anti-CD40 antibody. We assessed CD23 expression and its stabilization by FACS and used an ELISA for detecting soluble CD23. IgE synthesis was measured by ELISA and real-time PCR. Surface plasmon resonance revealed affinities of the DARPins to CD23 in the pico-molar range. Anti-CD23 DARPins strongly inhibited binding of IgE to CD23 and share thus a similar binding epitope as IgE. The DARPins stabilized membrane CD23 and reduced IgE synthesis in an isotype specific manner. Furthermore, the anti-CD23 DARPins decreased IgE transcript through inhibition of mature Cϵ RNA synthesis suggesting a posttranscriptional control mechanism. This study demonstrates that targeting CD23 alone is sufficient to inhibit IgE synthesis and suggests that a negative signaling occurs directly through the CD23 molecule.