Expanding the Anti-Phl p 7 Antibody Toolkit: An Anti-Idiotype Nanobody Inhibitor

We have previously produced a toolkit of antibodies, comprising recombinant human antibodies of all but one of the human isotypes, directed against the polcalcin family antigen Phl p 7. In this work, we complete the toolkit of human antibody isotypes with the IgD version of the anti-Phl p 7 monoclonal antibody. We also raised a set of nanobodies against the IgD anti-Phl p 7 antibody and identify and characterize one paratope-specific nanobody. This nanobody also binds to the IgE isotype of this antibody, which shares the same idiotype, and orthosterically inhibits the interaction with Phl p 7. The 2.1 Å resolution X-ray crystal structure of the nanobody in complex with the IgD Fab is described.

Crystal structures of the human IgD Fab reveal insights into CH1 domain diversity

Antibodies of the IgD isotype remain the least well characterized of the mammalian immunoglobulin isotypes. Here we report three-dimensional structures for the Fab region of IgD, based on four different crystal structures, at resolutions of 1.45-2.75 Å. These IgD Fab crystals provide the first high-resolution views of the unique Cδ1 domain. Structural comparisons identify regions of conformational diversity within the Cδ1 domain, as well as among the homologous domains of Cα1, Cγ1 and Cμ1. The IgD Fab structure also possesses a unique conformation of the upper hinge region, which may contribute to the overall disposition of the very long linker sequence between the Fab and Fc regions found in human IgD. Structural similarities observed between IgD and IgG, and differences with IgA and IgM, are consistent with predicted evolutionary relationships for the mammalian antibody isotypes.

Nucleolin acts as the receptor for C1QTNF4 and supports C1QTNF4-mediated innate immunity modulation

The C1q and TNF related 4 (C1QTNF4) protein is a structurally unique member of the C1QTNF family, a family of secreted proteins that have structural homology with both complement C1q and the tumor necrosis factor superfamily. C1QTNF4 has been linked to the autoimmune disease systemic lupus erythematosus through genetic studies; however, its role in immunity and inflammation remains poorly defined and a cell surface receptor of C1QTNF4 has yet to be identified. Here we report identification of nucleolin as a cell surface receptor of C1QTNF4 using mass spectrometric analysis. Additionally, we present evidence that the interaction between C1QTNF4 and nucleolin is mediated by the second C1q-like domain of C1QTNF4 and the C terminus of nucleolin. We show that monocytes and B cells are target cells of C1QTNF4 and observe extensive binding to dead cells. Imaging flow cytometry experiments in monocytes show that C1QTNF4 becomes actively internalized upon cell binding. Our results suggest that nucleolin may serve as a docking molecule for C1QTNF4 and act in a context-dependent manner through coreceptors. Taken together, these findings further our understanding of C1QTNF4's function in the healthy immune system and how dysfunction may contribute to the development of systemic lupus erythematosus.

Structure-functional changes in eNAMPT at high concentrations mediate mouse and human beta cell dysfunction in type 2 diabetes

AIMS/HYPOTHESIS

Progressive decline in functional beta cell mass is central to the development of type 2 diabetes. Elevated serum levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT) are associated with beta cell failure in type 2 diabetes and eNAMPT immuno-neutralisation improves glucose tolerance in mouse models of diabetes. Despite this, the effects of eNAMPT on functional beta cell mass are poorly elucidated, with some studies having separately reported beta cell-protective effects of eNAMPT. eNAMPT exists in structurally and functionally distinct monomeric and dimeric forms. Dimerisation is essential for the NAD-biosynthetic capacity of NAMPT. Monomeric eNAMPT does not possess NAD-biosynthetic capacity and may exert distinct NAD-independent effects. This study aimed to fully characterise the structure-functional effects of eNAMPT on pancreatic beta cell functional mass and to relate these to beta cell failure in type 2 diabetes.

METHODS

CD-1 mice and serum from obese humans who were without diabetes, with impaired fasting glucose (IFG) or with type 2 diabetes (from the Body Fat, Surgery and Hormone [BodyFatS&H] study) or with or at risk of developing type 2 diabetes (from the VaSera trial) were used in this study. We generated recombinant wild-type and monomeric eNAMPT to explore the effects of eNAMPT on functional beta cell mass in isolated mouse and human islets. Beta cell function was determined by static and dynamic insulin secretion and intracellular calcium microfluorimetry. NAD-biosynthetic capacity of eNAMPT was assessed by colorimetric and fluorescent assays and by native mass spectrometry. Islet cell number was determined by immunohistochemical staining for insulin, glucagon and somatostatin, with islet apoptosis determined by caspase 3/7 activity. Markers of inflammation and beta cell identity were determined by quantitative reverse transcription PCR. Total, monomeric and dimeric eNAMPT and nicotinamide mononucleotide (NMN) were evaluated by ELISA, western blot and fluorometric assay using serum from non-diabetic, glucose intolerant and type 2 diabetic individuals.

RESULTS

eNAMPT exerts bimodal and concentration- and structure-functional-dependent effects on beta cell functional mass. At low physiological concentrations (~1 ng/ml), as seen in serum from humans without diabetes, eNAMPT enhances beta cell function through NAD-dependent mechanisms, consistent with eNAMPT being present as a dimer. However, as eNAMPT concentrations rise to ~5 ng/ml, as in type 2 diabetes, eNAMPT begins to adopt a monomeric form and mediates beta cell dysfunction, reduced beta cell identity and number, increased alpha cell number and increased apoptosis, through NAD-independent proinflammatory mechanisms.

CONCLUSIONS/INTERPRETATION

We have characterised a novel mechanism of beta cell dysfunction in type 2 diabetes. At low physiological levels, eNAMPT exists in dimer form and maintains beta cell function and identity through NAD-dependent mechanisms. However, as eNAMPT levels rise, as in type 2 diabetes, structure-functional changes occur resulting in marked elevation of monomeric eNAMPT, which induces a diabetic phenotype in pancreatic islets. Strategies to selectively target monomeric eNAMPT could represent promising therapeutic strategies for the treatment of type 2 diabetes.

Structural basis for selective inhibition of immunoglobulin E-receptor interactions by an anti-IgE antibody

Immunoglobulin E (IgE) antibodies play a central role in the allergic response: interaction with FcεRI on mast cells and basophils leads to immediate hypersensitivity reactions upon allergen challenge, while interaction with CD23/FcεRII, expressed on a variety of cells, regulates IgE synthesis among other activities. The receptor-binding IgE-Fc region has recently been found to display remarkable flexibility, from acutely bent to extended conformations, with allosteric communication between the distant FcεRI and CD23 binding sites. We report the structure of an anti-IgE antibody Fab (8D6) bound to IgE-Fc through a mixed protein-carbohydrate epitope, revealing further flexibility and a novel extended conformation with potential relevance to that of membrane-bound IgE in the B cell receptor for antigen. Unlike the earlier, clinically approved anti-IgE antibody omalizumab, 8D6 inhibits binding to FcεRI but not CD23; the structure reveals how this discrimination is achieved through both orthosteric and allosteric mechanisms, supporting therapeutic strategies that retain the benefits of CD23 binding.

A fluorescent biosensor reveals conformational changes in human immunoglobulin E Fc: implications for mechanisms of receptor binding, inhibition, and allergen recognition

IgE binding to its high affinity receptor FcεRI on mast cells and basophils is a key step in the mechanism of allergic disease and a target for therapeutic intervention. Early indications that IgE adopts a bent structure in solution have been confirmed by recent x-ray crystallographic studies of IgEFc, which further showed that the bend, contrary to expectation, is enhanced in the crystal structure of the complex with receptor. To investigate the structure of IgEFc and its conformational changes that accompany receptor binding in solution, we created a Förster resonance energy transfer (FRET) biosensor using biologically encoded fluorescent proteins fused to the N- and C-terminal IgEFc domains (Cε2 and Cε4, respectively) together with the theoretical basis for quantitating its behavior. This revealed not only that the IgEFc exists in a bent conformation in solution but also that the bend is indeed enhanced upon FcεRI binding. No change in the degree of bending was seen upon binding to the B cell receptor for IgE, CD23 (FcεRII), but in contrast, binding of the anti-IgE therapeutic antibody omalizumab decreases the extent of the bend, implying a conformational change that opposes FcεRI engagement. HomoFRET measurements further revealed that the (Cε2)(2) and (Cε4)(2) domain pairs behave as rigid units flanking the conformational change in the Cε3 domains. Finally, modeling of the accessible conformations of the two Fab arms in FcεRI-bound IgE revealed a mutual exclusion not seen in IgG and Fab orientations relative to the membrane that may predispose receptor-bound IgE to cross-linking by allergens.

Monitoring the systemic human memory B cell compartment of melanoma patients for anti-tumor IgG antibodies

Melanoma, a potentially lethal skin cancer, is widely thought to be immunogenic in nature. While there has been much focus on T cell-mediated immune responses, limited knowledge exists on the role of mature B cells. We describe an approach, including a cell-based ELISA, to evaluate mature IgG antibody responses to melanoma from human peripheral blood B cells. We observed a significant increase in antibody responses from melanoma patients (n = 10) to primary and metastatic melanoma cells compared to healthy volunteers (n = 10) (P<0.0001). Interestingly, we detected a significant reduction in antibody responses to melanoma with advancing disease stage in our patient cohort (n = 21) (P<0.0001). Overall, 28% of melanoma patient-derived B cell cultures (n = 1,800) compared to 2% of cultures from healthy controls (n = 600) produced antibodies that recognized melanoma cells. Lastly, a patient-derived melanoma-specific monoclonal antibody was selected for further study. This antibody effectively killed melanoma cells in vitro via antibody-mediated cellular cytotoxicity. These data demonstrate the presence of a mature systemic B cell response in melanoma patients, which is reduced with disease progression, adding to previous reports of tumor-reactive antibodies in patient sera, and suggesting the merit of future work to elucidate the clinical relevance of activating humoral immune responses to cancer.

Heterodimerization of the human RNase P/MRP subunits Rpp20 and Rpp25 is a prerequisite for interaction with the P3 arm of RNase MRP RNA

Rpp20 and Rpp25 are two key subunits of the human endoribonucleases RNase P and MRP. Formation of an Rpp20–Rpp25 complex is critical for enzyme function and sub-cellular localization. We present the first detailed in vitro analysis of their conformational properties, and a biochemical and biophysical characterization of their mutual interaction and RNA recognition. This study specifically examines the role of the Rpp20/Rpp25 association in the formation of the ribonucleoprotein complex. The interaction of the individual subunits with the P3 arm of the RNase MRP RNA is revealed to be negligible whereas the 1:1 Rpp20:Rpp25 complex binds to the same target with an affinity of the order of nM. These results unambiguously demonstrate that Rpp20 and Rpp25 interact with the P3 RNA as a heterodimer, which is formed prior to RNA binding. This creates a platform for the design of future experiments aimed at a better understanding of the function and organization of RNase P and MRP. Finally, analyses of interactions with deletion mutant proteins constructed with successively shorter N- and C-terminal sequences indicate that the Alba-type core domain of both Rpp20 and Rpp25 contains most of the determinants for mutual association and P3 RNA recognition.

A monomeric chicken IgY receptor binds IgY with 2:1 stoichiometry

IgY is the principal serum antibody in birds and reptiles, and an IgY-like molecule was the evolutionary precursor of both mammalian IgG and IgE. A receptor for IgY on chicken monocytes, chicken leukocyte receptor AB1 (CHIR-AB1), lies in the avian leukocyte receptor cluster rather than the classical Fc receptor cluster where the genes for mammalian IgE and IgG receptors are found. IgG and IgE receptors bind to the lower hinge region of their respective antibodies with 1:1 stoichiometry, whereas the myeloid receptor for IgA, FcalphaRI, and the IgG homeostasis receptor, FcRn, which are found in the mammalian leukocyte receptor cluster, bind with 2:1 stoichiometry between the heavy chain constant domains 2 and 3 of each heavy chain. In this paper, the extracellular domain of CHIR-AB1 was expressed in a soluble form and shown to be a monomer that binds to IgY-Fc with 2:1 stoichiometry. The two binding sites have similar affinities: K(a)(1) = 7.22 +/- 0.22 x 10(5) m(-1) and K(a)(2) = 3.63 +/- 1.03 x 10(6) m(-1) (comparable with the values reported for IgA binding to its receptor). The affinity constants for IgY and IgY-Fc binding to immobilized CHIR-AB1 are 9.07 +/- 0.07 x 10(7) and 6.11 +/- 0.02 x 10(8) m(-1), respectively, in agreement with values obtained for IgY binding to chicken monocyte cells and comparable with reported values for human IgA binding to neutrophils. Although the binding site for CHIR-AB1 on IgY is not known, the data reported here with a monomeric receptor binding to IgY at two sites with low affinity suggest an IgA-like interaction.

Role of IgE receptors in IgE antibody-dependent cytotoxicity and phagocytosis of ovarian tumor cells by human monocytic cells

Antibodies directed against tumor-associated antigens are emerging as effective treatments for a number of cancers, although the mechanism(s) of action for some are unclear and still under investigation. We have previously examined a chimeric IgE antibody (MOv18 IgE), against the ovarian tumor-specific antigen, folate binding protein (FBP), and showed that it can direct human PBMC to kill ovarian cancer cells. We have developed a three-color flow cytometric assay to investigate the mechanism by which IgE receptors on U937 monocytes target and kill ovarian tumor cells. U937 monocytes express three IgE receptors, the high-affinity receptor, FcepsilonRI, the low-affinity receptor, CD23, and galectin-3, and mediate tumor cell killing in vitro by two mechanisms, cytotoxicity, and phagocytosis. Our results suggest that CD23 mediates phagocytosis, which is enhanced by upregulation of CD23 on U937 cells with IL-4, whereas FcepsilonRI mediates cytotoxicity. We show that effector : tumor cell bridging is associated with both activities. Galectin-3 does not appear to be involved in tumor cell killing. U937 cells and IgE exerted ovarian tumor cell killing in vivo in our xenograft model in nude mice. Harnessing IgE receptors to target tumor cells suggests the potential of tumor-specific IgE antibodies to activate effector cells in immunotherapy of ovarian cancer.

IgE-antibody-dependent immunotherapy of solid tumors: cytotoxic and phagocytic mechanisms of eradication of ovarian cancer cells

Abs have a paramount place in the treatment of certain, mainly lymphoid, malignancies, although tumors of nonhemopoietic origin have proved more refractory ones. We have previously shown that the efficacy of immunotherapy of solid tumors, in particular ovarian carcinoma, may be improved by the use of IgE Abs in place of the conventional IgG. An IgE Ab (MOv18 IgE) against an ovarian-tumor-specific Ag (folate binding protein), in combination with human PBMC, introduced into ovarian cancer xenograft-bearing mice, greatly exceeded the analogous IgG1 in promoting survival. In this study, we analyzed the mechanisms by which MOv18 IgE may exert its antitumor activities. Monocytes were essential IgE receptor-expressing effector cells that mediated the enhanced survival of tumor-bearing mice by MOv18 IgE and human PBMC. Monocytes mediated MOv18 IgE-dependent ovarian tumor cell killing in vitro by two distinct pathways, cytotoxicity and phagocytosis, acting respectively through the IgE receptors FcepsilonRI and CD23. We also show that human eosinophils were potent effector cells in MOv18 IgE Ab-dependent ovarian tumor cell cytotoxicity in vitro. These results demonstrate that IgE Abs can engage cell surface IgE receptors and activate effector cells against ovarian tumor cells. Our findings offer a framework for an improved immunotherapeutic strategy for combating solid tumors.

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.

l-Methionine sulfoximine, but not phosphinothricin, is a substrate for an acetyltransferase (gene PA4866) from Pseudomonas aeruginosa: structural and functional studies

The gene PA4866 from Pseudomonas aeruginosa is documented in the Pseudomonas genome database as encoding a 172 amino acid hypothetical acetyltransferase. We and others have described the 3D structure of this protein (termed pita) [Davies et al. (2005) Proteins: Struct., Funct., Bioinf. 61, 677-679; Nocek et al., unpublished results], and structures have also been reported for homologues from Agrobacterium tumefaciens (Rajashankar et al., unpublished results) and Bacillus subtilis [Badger et al. (2005) Proteins: Struct., Funct., Bioinf. 60, 787-796]. Pita homologues are found in a large number of bacterial genomes, and while the majority of these have been assigned putative phosphinothricin acetyltransferase activity, their true function is unknown. In this paper we report that pita has no activity toward phosphinothricin. Instead, we demonstrate that pita acts as an acetyltransferase using the glutamate analogues l-methionine sulfoximine and l-methionine sulfone as substrates, with Km(app) values of 1.3 +/- 0.21 and 1.3 +/- 0.13 mM and kcat(app) values of 505 +/- 43 and 610 +/- 23 s-1 for l-methionine sulfoximine and l-methionine sulfone, respectively. A high-resolution (1.55 A) crystal structure of pita in complex with one of these substrates (l-methionine sulfoximine) has been solved, revealing the mode of its interaction with the enzyme. Comparison with the apoenzyme structure has also revealed how certain active site residues undergo a conformational change upon substrate binding. To investigate the role of pita in P. aeruginosa, a mutant strain, Depp4, in which pita was inactivated through an in-frame deletion, was constructed by allelic exchange. Growth of strain Depp4 in the absence of glutamine was inhibited by l-methionine sulfoximine, suggesting a role for pita in protecting glutamine synthetase from inhibition.

Biophysical characterisation reveals structural disorder in the nucleolar protein, Dribble

Dribble (DBE) is an essential protein in Drosophila that belongs to the evolutionarily conserved Krr1p protein family. Proteins in this family are localised in the cell nucleolus and are important for the processing of ribosomal RNAs. However, little is known about their structural and biophysical properties. We have expressed and purified full-length DBE protein from Escherichia coli. Consistent with the native role of DBE in RNA processing, recombinant DBE was shown to bind RNA homo-polymers in vitro. By bioinformatics, size-exclusion chromatography, equilibrium sedimentation analysis, controlled proteolysis, and a variety of spectroscopic techniques, we have found that DBE is a monomeric protein in solution containing both alpha- and beta-structures. Moreover, the structure of DBE is expanded and significantly disordered (approximately 45% disordered). Natively disordered proteins are thought to provide a disproportionately large surface area and structural plasticity for nucleic acid binding. We therefore propose that the presence of structural disorder is an important feature of DBE that facilitates the protein to interact with RNAs in the nucleolus.

The structure of human CD23 and its interactions with IgE and CD21

The low-affinity immunoglobulin E (IgE) receptor, CD23 (FcepsilonRII), 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.

Disulfide linkage controls the affinity and stoichiometry of IgE Fcepsilon3-4 binding to FcepsilonRI

IgE antibodies cause long-term sensitization of tissue mast cells and blood basophils toward allergen-induced cross-linking and triggering of allergic inflammation. This persistence of IgE binding is due to its uniquely high affinity for the receptor FcepsilonRI and in particular its slow rate of dissociation once bound. The binding interface consists of two subsites, one contributed by each Cepsilon3 domain of IgE Fc in a 1:1 complex. We have investigated the contributions of Cepsilon3 disulfide linkage and glycosylation to the kinetics and affinity of binding of an Fc subfragment (Fcepsilon3-4) to a soluble receptor fragment (sFcepsilonRIalpha). In contrast to IgG Fc where deglycosylation abrogates receptor binding activity, the removal of the N-linked carbohydrate at Asn-394 in Fcepsilon3-4 only reduces binding affinity by a factor of 4, principally because of a faster off-rate. Removal of the inter-heavy chain disulfide bond unexpectedly resulted in a fragment with a much faster off-rate and the potential to form a complex with a 2:1 stoichiometry (sFcepsilonRIalpha:Fcepsilon3-4). This permitted the determination of the affinity of a single, natively folded Cepsilon3 domain for the first time. The low affinity Ka approximately 10(5)-10(6) m-1, similar to that determined previously for an isolated and partially folded Cepsilon3 domain, demonstrates that substantial reduction in affinity can be achieved by preventing the engagement of one of the two Cepsilon3 domains. Recent structural data indicate that conformational change in IgE is required to allow both Cepsilon3 domains to bind, and thus an allosteric inhibitor that prevents access to the second Cepsilon3 has the potential to reduce the ability of IgE to sensitize allergic effector cells.

The biology of IGE and the basis of allergic disease

Allergic individuals exposed to minute quantities of allergen experience an immediate response. Immediate hypersensitivity reflects the permanent sensitization of mucosal mast cells by allergen-specific IgE antibodies bound to their high-affinity receptors (FcepsilonRI). A combination of factors contributes to such long-lasting sensitization of the mast cells. They include the homing of mast cells to mucosal tissues, the local synthesis of IgE, the induction of FcepsilonRI expression on mast cells by IgE, the consequent downregulation of FcgammaR (through an insufficiency of the common gamma-chains), and the exceptionally slow dissociation of IgE from FcepsilonRI. To understand the mechanism of the immediate hypersensitivity phenomenon, we need explanations of why IgE antibodies are synthesized in preference to IgG in mucosal tissues and why the IgE is so tenaciously retained on mast cell-surface receptors. There is now compelling evidence that the microenvironment of mucosal tissues of allergic disease favors class switching to IgE; and the exceptionally high affinity of IgE for FcepsilonRI can now be interpreted in terms of the recently determined crystal structures of IgE-FcepsilonRI and IgG-FcgammaR complexes. The rate of local IgE synthesis can easily compensate for the rate of the antibody dissociation from its receptors on mucosal mast cells. Effective mechanisms ensure that allergic reactions are confined to mucosal tissues, thereby minimizing the risk of systemic anaphylaxis.

The crystal structure of IgE Fc reveals an asymmetrically bent conformation

The distinguishing structural feature of immunoglobulin E (IgE), the antibody responsible for allergic hypersensitivity, is the C epsilon 2 domain pair that replaces the hinge region of IgG. The crystal structure of the IgE Fc (constant fragment) at a 2.6-A resolution has revealed these domains. They display a distinctive, disulfide-linked Ig domain interface and are folded back asymmetrically onto the C epsilon 3 and C epsilon 4 domains, which causes an acute bend in the IgE molecule. The structure implies that a substantial conformational change involving C epsilon 2 must accompany binding to the mast cell receptor Fc epsilon RI. This may be the basis of the exceptionally slow dissociation rate of the IgE-Fc epsilon RI complex and, thus, of the ability of IgE to cause persistent allergic sensitization of mast cells.

Mutagenesis within human FcepsilonRIalpha differentially affects human and murine IgE binding

Soluble fragments of the alpha-chain of FcepsilonRI, the high-affinity receptor for IgE, compete with membrane-bound receptors for IgE and may thus provide a means to combat allergic responses. Mutagenesis within FcepsilonRIalpha is used in this study, in conjunction with the crystal structure of the FcepsilonRIalpha/IgE complex, to define the relative importance of specific residues within human FcepsilonRIalpha for IgE binding. We have also compared the effects of these mutants on binding to both human and mouse IgE, with a view to evaluating the mouse as an appropriate model for the analysis of future agents designed to mimic the human FcepsilonRIalpha and attenuate allergic disease. Three residues within the C-C' region of the FcepsilonRIalpha2 domain and two residues within the alpha2 proximal loops of the alpha1 domain were selected for mutagenesis and tested in binding assays with human and mouse IgE. All three alpha2 mutations (K117D, W130A, and Y131A) reduced the affinity of human IgE binding to different extents, but K117D had a far more pronounced effect on mouse IgE binding, and although Y131A had little effect, W130A modestly enhanced binding to mouse IgE. The mutations in alpha1 (R15A and F17A) diminished binding to both human and mouse IgE, with these effects most likely caused by disruption of the alpha1/alpha2 interface. Our results demonstrate that the effects of mutations in human FcepsilonRIalpha on mouse IgE binding, and hence the inhibitory properties of human receptor-based peptides assayed in rodent models of allergy, may not necessarily reflect their activity in a human IgE-based system.

Regulation of IgE production requires oligomerization of CD23

Here we describe the production of a rabbit polyclonal Ab (RAS1) raised against the stalk of murine CD23. RAS1 inhibits release of CD23 from the surface of both M12 and B cells resulting in an increase of CD23 on the cell surface. Despite this increase, these cells are unable to bind IgE as determined by FACS. CD23 has previously been shown to bind IgE with both a high (4-10 x 10(7) M(-1)) and low (4-10 x 10(6) M(-1)) affinity. Closer examination by direct binding of (125)I-IgE revealed that RAS1 blocks high affinity binding while having no effect on low affinity binding. These data support the model proposing that oligomers of CD23 mediate high affinity IgE binding. These experiments suggest that RAS1 binding to cell surface CD23 results in a shift from oligomers to monomers, which, according to the model, only bind IgE with low affinity. These experiments also suggest that high affinity binding of IgE is required for IgE regulation by CD23 and is demonstrated by the fact that treatment of Ag/Alum-immunized mice treated with RAS1 results in a significant increase in IgE production similar to the levels seen in CD23-deficient mice. These mice also had significantly decreased levels of serum soluble CD23 and Ag-specific IgG1. RAS1 had no effect on IgE or Ag-specific IgG1 production in CD23-deficient mice.

The structure of the IgE Cepsilon2 domain and its role in stabilizing the complex with its high-affinity receptor FcepsilonRIalpha

The stability of the complex between IgE and its high-affinity receptor, FcepsilonRI, on mast cells is a critical factor in the allergic response. The long half-life of the complex of IgE bound to this receptor in situ ( approximately 2 weeks, compared with only hours for the comparable IgG complex) contributes to the permanent sensitization of these cells and, hence, to the immediate response to allergens. Here we show that the second constant domain of IgE, Cepsilon2, which takes the place of the flexible hinge in IgG, contributes to this long half-life. When the Cepsilon2 domain is deleted from the IgE Fc fragment, leaving only the Cepsilon3 and Cepsilon4 domains (Cepsilon3-4 fragment), the rate of dissociation from the receptor is increased by greater than 1 order of magnitude. We report the structure of the Cepsilon2 domain by heteronuclear NMR spectroscopy and show by chemical shift perturbation that it interacts with FcepsilonRIalpha. By sedimentation equilibrium we show that the Cepsilon2 domain binds to the Cepsilon3-4 fragment of IgE. These interactions of Cepsilon2 with both FcepsilonRIalpha and Cepsilon3-4 provide a structural explanation for the exceptionally slow dissociation of the IgE-FcepsilonRIalpha complex.

IgE isotype determination: epsilon-germline gene transcription, DNA recombination and B-cell differentiation

Immunoglobulin class switching is the process which determines whether a B-cell secretes antibodies of the IgM, IgG, IgA or IgE class (or isotype). IgE is the antibody that mediates the allergic response by sensitising mast cells to allergens at the mucosal barrier. Class switching proceeds by three successive steps, culminating in the synthesis and secretion of antibody: these are germline gene transcription, DNA recombination and B-cell differentiation. We review here the present state of knowledge concerning the mechanisms involved in each of these steps, with particular reference to IgE. Intervention in the mechanisms that specify the selection of IgE may offer a means to combat allergy.

Inhibition of IgE-receptor interactions

Immunoglobulin E plays a central role in allergic disease and, as our understanding of the network of interactions between IgE and its receptors improves, new opportunities for therapeutic intervention emerge. IgE binding to its 'high-affinity' receptor, Fc epsilon RI, first identified on mast cells and now known to be expressed on a variety of other cell types, is the best characterised interaction, and has attracted most attention. The 'low affinity' receptor, Fc epsilon RII/CD23, first found on B-cells, appears to be part of a more complex network that has yet to be fully elucidated. Two recent advances concerning the IgE-Fc epsilon RI interaction are noteworthy. The first is the development of a monoclonal anti-IgE antibody, now in advanced clinical trials, which inhibits this interaction and certainly proves the viability of this approach. The second is the publication of the crystal structure of the complex between IgE and Fc epsilon RI, which opens the way for the first structure-based design of small molecule inhibitors.

Cleavage of the low-affinity receptor for human IgE (CD23) by a mite cysteine protease: nature of the cleaved fragment in relation to the structure and function of CD23

Der p I, a cysteine protease representing a major allergen of the house dust mite Dermatophagoides pteronyssinus, has recently been shown to cleave CD23 from the surface of cultured human B cells (RPMI 8866 B cell line). We have now undertaken a detailed investigation of CD23 cleavage by Der p I. We demonstrate that Der p I cleaves CD23 at two sites (Ser155-Ser156 and Glu298-Ser299) to produce a 17-kDa fragment containing the lectin domain and only part of the C-terminal tail. No such effect was demonstrable with mouse CD23, a finding which was anticipated based on its lack of the cleavage sites identified on human CD23. Based on the cleavage pattern and the model of CD23, we propose a sequence of events leading to the liberation of the 17-kDa soluble CD23 fragment. The biological significance of such cleavage is underlined by the demonstration that Der p I-treated B lymphocytes lose their ability to bind IgE, and that the 17-kDa fragment (amino acids 156-298) contains the minimum structural requirement (amino acids 156-288) for binding to both IgE and CD21.

Interaction of the low-affinity receptor CD23/Fc epsilonRII lectin domain with the Fc epsilon3-4 fragment of human immunoglobulin E

CD23/Fc epsilonRII, the low-affinity receptor for IgE, is a multifunctional protein of importance in blood cell development and the immune system. We have studied the interaction of CD23 with IgE in solution using hydrodynamic methods applied to recombinant fragments of both ligands: sCD23, corresponding to the soluble lectin domain of CD23, and Fc epsilon3-4, a dimer of the C epsilon3-C epsilon4 sequence of IgE. The hydrodynamic, spectroscopic, and biological properties of these fragments suggest that they have a fully native structure. Sedimentation equilibrium studies on mixtures of sCD23 and Fc epsilon3-4 indicate that IgE has two binding sites for CD23, each characterized by affinities of approximately 10(5) M(-1). Analysis of the sedimentation as a function of temperature allows conclusions to be drawn about the thermodynamics of binding at the two sites. Binding at the first site is characterized by large changes in enthalpy (delta H(degree)To = -2.1 +/- 3.3 kcal mol(-1)) and heat capacity (delta Cp(degree) = -320 +/- 320 cal mol(-1) K(-1)), whereas binding at the second site is characterized by small changes in enthalpy (delta H(degree)To = 0.1 +/- 5.6 kcal mol(-1)) and heat capacity (delta Cp(degree) = -140 +/- 550 cal mol(-1) K(-1)). In native CD23, there are two or three lectin domains, associated through an alpha-helical coiled-coil stalk. The predicted structure of the CD23 oligomers and symmetry considerations rule out the possibility of two lectin domains from one oligomer binding to identical sites in IgE. The notion of two types of interaction in the 2:1 complex between CD23 and IgE is consistent with the thermodynamic data presented.

Automated hydrodynamic modelling of a complex between a human IgE fragment (Fc epsilon 3-4) and the IgE high affinity receptor Fc epsilon RI alpha-chain

The binding of IgE to its high affinity receptor Fc epsilon RI plays an important role in the allergic response. The interaction between soluble Fc epsilon RI alpha-chain (sFc epsilon RI alpha) and Fc epsilon 3-4, a fragment of IgE consisting of the C epsilon 3 and C epsilon 4 heavy chain constant domains, has been studied using analytical ultracentrifugation (Keown et al. this volume). Here we describe the development of a simple automated hydrodynamic modelling technique and its application to this interaction. This procedure utilises sphere models of the two molecules and performs an automated systematic translational search of sFc epsilon RI alpha relative to Fc epsilon 3-4. The result of this is the generation of 40,359 individual models of how the receptor can be placed relative to Fc epsilon 3-4. These are then assessed for consistency by comparing the sedimentation coefficients generated for the models to the experimentally determined sedimentation coefficients, and are displayed graphically to show allowed and disallowed complexes. From this analysis, it is clear that the complex between sFc epsilon RI alpha and Fc epsilon 3-4 is compact, with the most elongated models being excluded. In addition, sFc epsilon RI alpha appears not to interact with the C-terminal end of Fc epsilon 3-4, and probably binds either to the sides or face, observations which are consistent with other experimental data on the Fc epsilon RI alpha/IgE interaction. Automated hydrodynamic modelling also has the potential to be used for other interactions, providing a simple way of looking at a large number of models, and making rigorous studies of interacting components more feasible.

CD23/Fc epsilon RII and its soluble fragments can form oligomers on the cell surface and in solution

Human CD23 (also known as Fc epsilon RII) is a 45,000 MW glycoprotein with homology to C-type animal lectins. It is involved in B-cell differentiation and IgE regulation, and is naturally cleaved to give soluble products of 37,000, 33,000, 29,000, 25,000 and 16,000 MW. Previous work has suggested that the region between the transmembrane sequence and the extracellular lectin head is capable of forming an alpha-helical coiled coil, one of the main consequences of which would be formation of dimers or trimers. Here we present protein-protein cross-linking data showing that CD23 forms trimers on the cell surface and hexamers in solution, and we use several different fragments to determine the regions of the protein involved in this self-association. The region of the putative coiled coil is indeed responsible for trimerization, with additional interactions between the lectin heads resulting in the formation of hexamers observed in solution.