No abstract

CD46 is a complement regulator with important immune-related roles. CD46 functions as a pathogen receptor and is a potent co-stimulator for the induction of interferon-γ (IFN-γ)-secreting T helper 1 (TH1) effector T cells and their subsequent switch into interleukin-10 (IL-10)-producing regulatory T cells. Here, we identify the Notch protein family member Jagged1 as a new physiological ligand for CD46. Further, CD46 regulates Notch receptors and ligands expression during T cell activation and disturbance of the CD46-Notch crosstalk impedes IFN-γ induction and IL-10 switching. Importantly, CD4+ T cells from CD46-deficient patients and patients with hypomorphic Jagged1 mutations (Alagille Syndrome) fail to mount appropriate TH1 responses in vitro and in vivo suggesting that CD46-Jagged1 crosstalk is responsible for the recurrent infections in subpopulations of these patients.

The low-affinity immunoglobulin E (IgE) receptor, CD23 (FcɛRII), binds both IgE and CD21 and, through these interactions, regulates the synthesis of IgE, the antibody isotype that mediates the allergic response. We have determined the three-dimensional structure of the C-type lectin domain of CD23 in solution by nuclear magnetic resonance spectroscopy. An analysis of concentration-dependent chemical shift perturbations have allowed us to identify the residues engaged in self-association to the trimeric state, whereas ligand-induced changes have defined the binding sites for IgE and CD21. The results further reveal that CD23 can bind both ligands simultaneously. Despite the C-type lectin domain structure, none of the interactions require calcium. We also find that IgE and CD23 can interact to form high molecular mass multimeric complexes. The interactions that we have described provide a solution to the paradox that CD23 is involved in both up- and down-regulation of IgE and provide a structural basis for the development of inhibitors of allergic disease.

Of all the antibody classes, IgE displays a uniquely slow dissociation rate from, and high affinity for, its cell surface receptor FcεRI. The structural basis for these key determinants of IgE's ability to mediate allergic hypersensitivity is now revealed by the 3.4Å resolution crystal structure of human IgE-Fc (consisting of the Cε2, Cε3 and Cε4 domains) bound to the extracellular domains of the FcεRI α-chain. Comparison with free IgE-Fc (reported here at 1.9Å) shows that the antibody, which has a compact, bent structure prior to receptor engagement, becomes even more acutely bent in the complex. Thermodynamic analysis indicates that the interaction is entropically driven, which explains how the non-contacting Cε2 domains, in place of the flexible hinge region of IgG antibodies, contribute together with the conformational changes to IgE's unique binding properties.The global incidence of allergic disease has increased markedly in recent years and continues to rise. Asthma currently affects 22.2 million people in North America and 5.4 million in the UK where the rate of incidence, especially in children, is among the highest in the world 1 . Anaphylactic reactions to foods such as nuts, virtually unknown thirty years ago, are now relatively common. The reason for this increase is debated, but these and other allergic conditions (rhinitis, eczema etc.) are all mediated by IgE, and the viability of AUTHOR CONTRIBUTIONS MDH and AMD carried out the crystallographic analysis of the complex, and BD the crystallographic analysis of IgE-Fc; MDH, JEN, JH, AJB and RJO produced the proteins; SCB and JMM carried out the thermodynamic analysis; EG contributed to the analysis of the conformational changes; HJG, AJB and BJS planned and directed the project; MDH, AMD, JMM and BJS wrote the paper.Accession codes. Atomic coordinates and structure factors have been deposited in the Protein Data Bank with entry codes 2WQR for IgE-Fc and 2Y7Q for the complex. [6][7][8][9] . Particular interest has centered on the role of the Cε2 domains, which were shown to contribute to the slow dissociation rate 10 , by comparing the binding kinetics of the complete IgE-Fc (dimer Cε2-Cε3-Cε4 domains) with a sub-fragment lacking the Cε2 domains, here referred to as Fcε3-4. The Cε2 domains have no counterpart in IgG, all four subclasses of which have in their place hinge regions of various lengths and degrees of flexibility. Europe PMC Funders GroupEarly fluorescence resonance energy transfer (FRET) studies of labelled chimeric IgE indicated a more compact and bent structure than the extended, flexible Y-shaped IgG 11 , and later solution scattering studies of IgE-Fc were consistent with such a structure 12 . The crystal structure of IgE-Fc revealed for the first time the extent and nature of this bend 13 , made possible by the presence of the Cε2 domains. Surprisingly the molecule was found to be so acutely and asymmetrically bent, with the Cε2 domain pair folded back across the Cε3 domains, that one Cε2 domain even contacted the Cε4 domai...