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Crystallographic and solution studies have shown that IgE molecules are acutely bent in their Fc region. Crystal structures reveal the Cɛ2 domain pair folded back onto the Cɛ3-Cɛ4 domains, but is the molecule exclusively bent or can the Cɛ2 domains adopt extended conformations and even 'flip' from one side of the molecule to the other? We report the crystal structure of IgE-Fc captured in a fully extended, symmetrical conformation and show by molecular dynamics, calorimetry, stopped-flow kinetic, surface plasmon resonance (SPR) and Forster resonance energy transfer (FRET) analyses that the antibody can indeed adopt such extended conformations in solution. This diversity of conformational states available to IgE-Fc offers a new perspective on IgE function in allergen recognition, as part of the B-cell receptor and as a therapeutic target in allergic disease.

Phosphoinositide 3-kinases (PI3K) are key signaling enzymes regulating cellular survival, development, and function. Expression of the PI3K δ isoform is largely restricted to leukocytes and it plays a key role in immune cell development and function. Seletalisib is a novel small-molecule inhibitor of PI3K δ that was evaluated in biochemical assays, cellular assays of adaptive and innate immunity, and an in vivo rat model of inflammation. Our findings show that seletalisib is a potent, ATP-competitive, and selective PI3K δ inhibitor able to block protein kinase B (AKT) phosphorylation following activation of the B-cell receptor in a B-cell line. Moreover, seletalisib inhibited N -formyl peptide–stimulated but not phorbol myristate acetate–stimulated superoxide release from human neutrophils, consistent with a PI3K δ -specific activity. No indications of cytotoxicity were observed in peripheral blood mononuclear cells (PBMCs) or other cell types treated with seletalisib. Findings from cellular assays of adaptive immunity demonstrated that seletalisib blocks human T-cell production of several cytokines from activated T-cells. Additionally, seletalisib inhibited B-cell proliferation and cytokine release. In human whole blood assays, seletalisib inhibited CD69 expression upon B-cell activation and anti-IgE-mediated basophil degranulation. Seletalisib showed dose-dependent inhibition in an in vivo rat model of anti-CD3-antibody-induced interleukin 2 release. Collectively, these data characterize seletalisib as a selective PI3K δ inhibitor and potential therapeutic candidate for the treatment of B-cell malignancies and autoimmune diseases driven by dysregulated proinflammatory cytokine secretion.