IgE, IgE Receptors and Anti-IgE Biologics: Protein Structures and Mechanisms of Action

The evolution of IgE in mammals added an extra layer of immune protection at body surfaces to provide a rapid and local response against antigens from the environment. The IgE immune response employs potent expulsive and inflammatory forces against local antigen provocation, at the risk of damaging host tissues and causing allergic disease. Two well-known IgE receptors, the high-affinity FcεRI and low-affinity CD23, mediate the activities of IgE. Unlike other known antibody receptors, CD23 also regulates IgE expression, maintaining IgE homeostasis. This mechanism evolved by adapting the function of the complement receptor CD21. Recent insights into the dynamic character of IgE structure, its resultant capacity for allosteric modulation, and the potential for ligand-induced dissociation have revealed previously unappreciated mechanisms for regulation of IgE and IgE complexes. We describe recent research, highlighting structural studies of the IgE network of proteins to analyze the uniquely versatile activities of IgE and anti-IgE biologics.

Ontogeny of human IgE-expressing B cells and plasma cells

Background

IgE‐expressing (IgE⁺) plasma cells (PCs) provide a continuous source of allergen‐specific IgE that is central to allergic responses. The extreme sparsity of IgE⁺ cells in vivo has confined their study almost entirely to mouse models.

Objective

To characterize the development pathway of human IgE⁺PCs and to determine the ontogeny of human IgE⁺PCs.

Methods

To generate human IgE⁺ cells, we cultured tonsil B cells with IL‐4 and anti‐CD40. Using FACS and RT‐PCR, we examined the phenotype of generated IgE⁺ cells, the capacity of tonsil B‐cell subsets to generate IgE⁺PCs and the class switching pathways involved.

Results

We have identified three phenotypic stages of IgE⁺PC development pathway, namely (i) IgE⁺germinal centre (GC)‐like B cells, (ii) IgE⁺PC‐like ‘plasmablasts’ and (iii) IgE⁺PCs. The same phenotypic stages were also observed for IgG1⁺ cells. Total tonsil B cells give rise to IgE⁺PCs by direct and sequential switching, whereas the isolated GC B‐cell fraction, the main source of IgE⁺PCs, generates IgE⁺PCs by sequential switching. PC differentiation of IgE⁺ cells is accompanied by the down‐regulation of surface expression of the short form of membrane IgE (mIgE_S), which is homologous to mouse mIgE, and the up‐regulation of the long form of mIgE (mIgE_L), which is associated with an enhanced B‐cell survival and expressed in humans, but not in mice.

Conclusion

Generation of IgE⁺PCs from tonsil GC B cells occurs mainly via sequential switching from IgG. The mIgE_L/mIgE_S ratio may be implicated in survival of IgE⁺ B cells during PC differentiation and allergic disease.

Intrinsic properties of germinal center-derived B cells promote their enhanced class switching to IgE

Background

Research on the origins and development of human IgE‐expressing (IgE⁺) cells is required for understanding the pathogenesis of allergy and asthma. These studies have been thwarted by the rarity of IgE⁺ cells in vivo and the low frequency of class switch recombination (CSR) to IgE ex vivo. To determine the main source of IgE⁺ cells, we investigated the relation between the phenotypic composition of tonsil B cells and the CSR to IgE ex vivo.

Methods

Human tonsil B cells were analyzed by flow cytometry (FACS) and cultured with IL‐4 and anti‐CD40 to induce CSR to IgE. Naïve, germinal center (GC), early GC (eGC), and memory tonsil B cells were isolated by FACS, and their capacities for IL‐4 and anti‐CD40 signaling, cell proliferation, and de novo class switching to IgE were analyzed by RT‐PCR and FACS.

Results

B cells from different tonsils exhibited varying capacities for CSR to IgE ex vivo. This was correlated with the percentage of eGC B cells in the tonsil at the outset of the culture. Despite relatively poor cell viability, eGC and GC B‐cell cultures produced the highest yields of IgE⁺ cells compared to naïve and memory B‐cell cultures. The main factors accounting for this result were the strength of IL‐4R and CD40 signaling and relative rates of cell proliferation.

Conclusions

This study shows that the maturation state of tonsil B cells determines their capacity to undergo class switching to IgE ex vivo, with the GC‐derived B cells yielding the highest percentage of IgE⁺ cells.

Validating the GTP-cyclohydrolase 1-feedback regulatory complex as a therapeutic target using biophysical and in vivo approaches

BACKGROUND AND PURPOSE

6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4 ) is an essential cofactor for nitric oxide biosynthesis. Substantial clinical evidence indicates that intravenous BH4 restores vascular function in patients. Unfortunately, oral BH4 has limited efficacy. Therefore, orally bioavailable pharmacological activators of endogenous BH4 biosynthesis hold significant therapeutic potential. GTP-cyclohydrolase 1 (GCH1), the rate limiting enzyme in BH4 synthesis, forms a protein complex with GCH1 feedback regulatory protein (GFRP). This complex is subject to allosteric feed-forward activation by L-phenylalanine (L-phe). We investigated the effects of L-phe on the biophysical interactions of GCH1 and GFRP and its potential to alter BH4 levels in vivo.

EXPERIMENTAL APPROACH

Detailed characterization of GCH1-GFRP protein-protein interactions were performed using surface plasmon resonance (SPR) with or without L-phe. Effects on systemic and vascular BH4 biosynthesis in vivo were investigated following L-phe treatment (100 mg·kg(-1) , p.o.).

KEY RESULTS

GCH1 and GFRP proteins interacted in the absence of known ligands or substrate but the presence of L-phe doubled maximal binding and enhanced binding affinity eightfold. Furthermore, the complex displayed very slow association and dissociation rates. In vivo, L-phe challenge induced a sustained elevation of aortic BH4 , an effect absent in GCH1(fl/fl)-Tie2Cre mice.

CONCLUSIONS AND IMPLICATIONS

Biophysical data indicate that GCH1 and GFRP are constitutively bound. In vivo, data demonstrated that L-phe elevated vascular BH4 in an endothelial GCH1 dependent manner. Pharmacological agents which mimic the allosteric effects of L-phe on the GCH1-GFRP complex have the potential to elevate endothelial BH4 biosynthesis for numerous cardiovascular disorders.

Inhibition of allergen-dependent IgE activity by antibodies of the same specificity but different class

IgG4 purified from patients undergoing specific allergen immunotherapy inhibits the activities of the serum IgE in in vitro assays and is thought to reduce the symptoms of the disease. However, it is not known whether this is related to an intrinsic property of this subclass or only the allergen specificity. We tested the hypothesis that allergen specificity is the critical determinant for this activity using a panel of antibodies with identical specificity but different subclasses. The different antibodies were all able to inhibit the activity of IgE to the same extent. We demonstrate that specificity is the dominant factor determining the ability of an antibody to block allergen-dependent IgE activity.

Human IgE against the major allergen Bet v 1--defining an epitope with limited cross-reactivity between different PR-10 family proteins

Background

The interaction between IgE and allergen is a key event at the initiation of an allergic response, and its characteristics have substantial effects on the clinical manifestation. Despite this, the molecular details of the interaction between human IgE and the major birch allergen Bet v 1, one of the most potent tree allergens, still remain poorly investigated.

Objective

To isolate Bet v 1-specific human monoclonal IgE and characterize their interaction with the allergen.

Methods

Recombinant human IgE were isolated from a combinatorial antibody fragment library and their interaction with Bet v 1 assessed using various immunological assays. The structure of one such IgE in the single-chain fragment variable format was determined using X-ray crystallography.

Results

We present four novel Bet v 1-specific IgE, for one of which we solve the structure, all with their genetic origin in the IGHV5 germline gene, and demonstrate that they target two non-overlapping epitopes on the surface of Bet v 1, thereby fulfilling the basic criteria for FcεRI cross-linkage. We further define these epitopes and for one epitope pinpoint single amino acid residues important for the interaction with human IgE. This provides a potential explanation, at the molecular level, for the differences in recognition of isoforms of Bet v 1 and other allergens in the PR-10 protein family displayed by IgE targeting this epitope. Finally, we present the first high-resolution structure of a human allergen-specific IgE fragment in the single-chain fragment variable (scFv) format.

Conclusions and Clinical Relevance

We here display the usefulness of allergen-specific human monoclonal IgE as a tool in studies of the crucial molecular interaction taking place at the initiation of an allergic response. Such studies may aid us in development of better diagnostic tools and guide us in the development of new therapeutic compounds.

Evaluation of conformational epitopes on thyroid peroxidase by antipeptide antibody binding and mutagenesis

Autoantibodies to thyroid peroxidase (TPO) recognize predominantly conformational epitopes, which are restricted to two distinct determinants, termed immunodominant domain region (IDR) A and B. These dominant determinants reside in the region with structural homology to myeloperoxidase (MPO)-like domain and may extend into the adjacent complement control protein (CCP) domain. We have explored the location of these determinants on the MPO-like domain of the structural model of TPO, by identifying exposed hydrophilic loops that are potential candidates for the autoantigenic sites, generating rabbit antipeptide antisera, and competing with well characterized murine monoclonal antibodies (mabs) specific for these two IDRs. We recently defined the location of IDR-B, and here report our findings on the location of IDR-A and its relationship to IDR-B, defined with a new panel of 15 antipeptide antisera. Moreover, in combination with single amino acid replacements by in vitro mutagenesis, we have defined the limits of the IDR-B region on the TPO model. The combination of antisera to peptides P12 (aa 549-563), P14 (aa 599-617) and P18 (aa 210-225) inhibited the binding of the mab specific for IDR-A (mab 2) by 75%. The same combination inhibited the binding of autoantibodies to native TPO from 67 to 94% (mean 81.5%) at autoantibody levels of 5 IU. Fabs prepared from the antipeptide IgG and pooled in this combination were also effective in competition assays, thus defining the epitopes more precisely. IDR-A was found to lie immediately adjacent to IDR-B and thus the two immunodominant epitopes form an extended patch on the surface of TPO. Finally, by single amino acid mutagenesis, we show that IDR-B extends to residue N642, thus further localizing the boundary of this autoantigenic region on the structural model.

Adenovirus type 9 fiber knob binds to the coxsackie B virus-adenovirus receptor (CAR) with lower affinity than fiber knobs of other CAR-binding adenovirus serotypes

The coxsackie B virus and adenovirus (Ad) receptor (CAR) functions as an attachment receptor for multiple Ad serotypes. Here we show that the Ad serotype 9 (Ad9) fiber knob binds to CAR with much reduced affinity compared to the binding by Ad5 and Ad12 fiber knobs as well as the knob of the long fiber of Ad41 (Ad41L). Substitution of Asp222 in Ad9 fiber knob with a lysine that is conserved in Ad5, Ad12, and Ad41L substantially improved Ad9 fiber knob binding to CAR, while the corresponding substitution in Ad5 (Lys442Asp) significantly reduced Ad5 binding. The presence of an aspartic acid residue in Ad9 therefore accounts, at least in part, for the reduced CAR binding affinity of the Ad9 fiber knob. Site-directed mutagenesis of CAR revealed that CAR residues Leu73 and Lys121 and/or Lys123 are critical contact residues, with Tyr80 and Tyr83 being peripherally involved in the binding interaction with the Ad5, Ad9, Ad12, and Ad41L fiber knobs. The overall affinities and the association and dissociation rate constants for wild-type CAR as well as Tyr80 and Tyr83 CAR mutants differed between the serotypes, indicating that their binding modes, although similar, are not identical.

Complex between Peptostreptococcus magnus protein L and a human antibody reveals structural convergence in the interaction modes of Fab binding proteins

BACKGROUND

Peptostreptococcus magnus protein L (PpL) is a multidomain, bacterial surface protein whose presence correlates with virulence. It consists of up to five homologous immunoglobulin binding domains that interact with the variable (VL) regions of kappa light chains found on two thirds of mammalian antibodies.

RESULTS

We refined the crystal structure of the complex between a human antibody Fab fragment (2A2) and a single PpL domain (61 residues) to 2.7 A. The asymmetric unit contains two Fab molecules sandwiching a single PpL domain, which contacts similar VL framework regions of two light chains via independent interfaces. The residues contacted on VL are remote from the hypervariable loops. One PpL-Vkappa interface agrees with previous biochemical data, while the second is novel. Site-directed mutagenesis and analytical-centrifugation studies suggest that the two PpL binding sites have markedly different affinities for VL. The PpL residues in both interactions are well conserved among different Peptostreptococcus magnus strains. The Fab contact positions identified in the complex explain the high specificity of PpL for antibodies with kappa rather than lambda chains.

CONCLUSIONS

The PpL-Fab complex shows the first interaction of a bacterial virulence factor with a Fab light chain outside the conventional combining site. Structural comparison with two other bacterial proteins interacting with the Fab heavy chain shows that PpL, structurally homologous to streptococcal SpG domains, shares with the latter a similar binding mode. These two bacterial surface proteins interact with their respective immunoglobulin regions through a similar beta zipper interaction.

Endocytosis and recycling of the complex between CD23 and HLA-DR in human B cells

The presentation of extremely low doses of antigen to T cells is enhanced by immunoglobulin E (IgE)-dependent antigen focusing to CD23, the low-affinity receptor for IgE, expressed on activated B cells. CD23 contains a C-type lectin domain in its extracellular sequence and a targeting signal for coated pits, required for endocytosis, in its cytoplasmic sequence. CD23 is non-covalently associated with the major histocompatibility complex class II antigen, human leucocyte antigen HLA-DR, on the surface of human B cells, but the fate of this complex following endocytosis is unknown. To answer this question we have labelled these proteins on the surface of RPMI 8866 B cells and traced their route through the cytoplasm. Endocytosis mediated by anti-CD23 antibodies (BU38 and MHM6) led to the loss of CD23 from the cells. Endocytosis mediated by an antibody to HLA-DR (CR3/43) or an antigen-IgE complex (NP-BSA-anti-NP IgE), however, led to recycling of the HLA-DR-CD23 complex to the cell surface on a time scale (3-6 hr) consistent with the recycling of HLA-DR in antigen presentation. Along the latter pathway CD23 label was observed in cytoplasmic organelles that resembled the 'compartments for peptide loading' or 'class II vesicles' described by previous authors. Two features of the recycling process may contribute to the efficiency of antigen presentation. Peptide exchange may be facilitated by the proximity of HLA-DR and antigen in peptide loading compartments of the endosomal network. The return of CD23 with HLA-DR to the cell surface may then help to stabilize specific B-cell-T-cell interactions, contributing to T-cell activation.

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.

Human thyroid peroxidase: mapping of autoantibodies, conformational epitopes to the enzyme surface

The enzyme, thyroid peroxidase (TPO), is a dominant antigen in thyroid autoimmune diseases. Autoantibodies recognised two major dominant conformational epitopes termed A and B. The epitopes have been defined by mAbs, but the amino acid residues which constitute these determinants remain unknown. Using a model of TPO, built from the structure of myeloperoxidase (MPO), we have synthesised peptides corresponding to exposed loops and generated rabbit antibodies to the peptides. Antisera to peptide sequence 599-617 (peptide 14) representing a highly protrusive loop on the TPO, showed the highest inhibition in 65 sera from patients positive with anti-TPO antibodies. The inhibition was by 15-80% (mean 41%), and no other antibody showed any inhibition. Binding of hFabs to the B determinant on TPO was inhibited by anti-peptide 14 antibodies more then 85%, but not Fabs to the A determinant. In conclusion, the peptide 14 defines a sequence taking part in building up the B major conformational epitope. None of generated anti-peptide antibodies alone inhibited the binding of human Fabs to the A epitope, however a combination of four anti-peptide antibodies (P1, P12, P14 and P18) inhibits Fabs binding to the A determinant by more then 60% and autoantibodies binding from 65% to 94%. Combination of antibodies reacting with peptides outside the surface defined by those four antipeptide antibodies did not give any inhibition of Fabs to TPO. The inhibition of Fabs and auto Abs to TPO by this combination of anti-peptide Abs is the result of steric hindrance as none of these Abs individually inhibited auto Abs' or Fabs' binding to TPO. The four peptides define an area on the enzyme surface where the A and B major conformational epitopes are localised.

Specificity and binding kinetics of murine lupus anti-DNA monoclonal antibodies implicate different stimuli for their production

The origin and relative biological importance of the many different DNA-reactive antibodies that appear in systemic lupus erythematosus are not well understood. A detailed analysis of their fine specificity and binding characteristics with DNA is a necessary step in understanding their biology. We have examined here two monoclonal antibodies (mAb) IV-228 and V-88 that are, respectively, characteristic of antibodies, which bind exclusively to single-stranded (ss) DNA and to both double-stranded (ds) DNA and ssDNA. By surface plasmon resonance (SPR) on BIAcore, we characterized the kinetics of binding of each antibody to synthetic ss and ds oligonucleotides. Antibody V-88 and IV-228 showed different patterns of reactivity for both ss and ds oligonucleotides, characterized by distinctly different kinetic parameters. Analysis of their binding kinetics indicates the importance of base composition in defining DNA epitopes, and shows that some epitopes, such as that recognized by mAb V-88, are expressed on dsDNA and ssDNA, whereas others, as recognized by IV-228, are not. The base preferences of V-88 for ds GC-rich structures over AT-rich, and of IV-228 for ss T-rich structures, also reveal distinct differences between these antibodies. We conclude that the different binding properties of the antibodies will relate to their biological activities. The base preferences of the antibodies suggest that they might be induced by different immunological stimuli, such as those that could be provided by the various DNA fragments and structures released during programmed cell death.

Molecular model of a lattice of signalling proteins involved in bacterial chemotaxis

Coliform bacteria detect chemical attractants by means of a membrane-associated cluster of receptors and signalling molecules. We have used recently determined molecular structures, in conjunction with plastic models generated by three-dimensional printer technology, to predict how the proteins of the complex are arranged in relation to the plasma membrane. The proposed structure is a regular two-dimensional lattice in which the cytoplasmic ends of chemotactic-receptor dimers are inserted into a hexagonal array of CheA and CheW molecules. This structure creates separate compartments for adaptation and downstream signalling, and indicates a possible basis for the spread of activity within the cluster.

Conformation of the isolated cepsilon3 domain of IgE and its complex with the high-affinity receptor, FcepsilonRI

Immunoglobulin E (IgE) exhibits a uniquely high affinity for its receptor, FcepsilonRI, on the surface of mast cells and basophils. Previous work has implicated the third domain of the constant region of the epsilon-heavy chain (Cepsilon3) in binding to FcepsilonRI, but the smallest fragment of IgE that is known to bind with full affinity is a covalent dimer of the Cepsilon3 and Cepsilon4 domains. We have expressed the isolated Cepsilon3 in Escherichia coli, measured its affinity for FcepsilonRI, and examined its conformation alone and in the complex with FcepsilonRI. Sedimentation equilibrium in the analytical centrifuge reveals that this product is a monomer. The kinetics of binding to an immobilized fragment of the FcepsilonRI alpha-chain, measured by surface plasmon resonance, yields an affinity constant K(a) = 5 x 10(6) M(-)(1), as compared with 4 x 10(9) M(-)(1) for IgE. The circular dichroism spectrum and measurements of fluorescence as a function of the concentration of a denaturant do not reveal any recognizable secondary structure or hydrophobic core. On binding to the FcepsilonRI alpha-chain fragment, there is no change in the circular dichroism spectrum, indicating that the conformation of Cepsilon3 is unchanged in the complex. Thus the isolated Cepsilon3 domain is sufficient for binding to FcepsilonRI, but with lower affinity than IgE. This may be due to the loss of its native immunoglobulin domain structure or to the requirement for two Cepsilon3 domains to constitute the complete binding site for FcepsilonRI or to a combination of these factors.

Identification of an immunodominant region recognized by human autoantibodies in a three-dimensional model of thyroid peroxidase

Autoimmune thyroid diseases (AITD) are characterized by the presence of autoantibodies to thyroid peroxidase (TPO). This response is dominated by autoantibodies to two conformational determinants, termed A and B, that have been defined by monoclonal antibodies but whose structures and location within TPO are unknown. We have modeled the three-dimensional structure of the extracellular region of TPO, raised antisera to prominent surface structures, and identified an epitope that we show to be a critical part of the B determinant. Antibodies to this epitope inhibit the binding to TPO of human autoantibodies in virtually all serum samples from 65 patients with AITD that were tested. This first description of a model of the three-dimensional structure and location of a major autoantigenic determinant within the TPO molecule may provide structural clues for identifying causative agents or developing novel therapeutic strategies.

Crystal structure of a Staphylococcus aureus protein A domain complexed with the Fab fragment of a human IgM antibody: structural basis for recognition of B-cell receptors and superantigen activity

Staphylococcus aureus produces a virulence factor, protein A (SpA), that contains five homologous Ig-binding domains. The interactions of SpA with the Fab region of membrane-anchored Igs can stimulate a large fraction of B cells, contributing to lymphocyte clonal selection. To understand the molecular basis for this activity, we have solved the crystal structure of the complex between domain D of SpA and the Fab fragment of a human IgM antibody to 2.7-A resolution. In the complex, helices II and III of domain D interact with the variable region of the Fab heavy chain (V(H)) through framework residues, without the involvement of the hypervariable regions implicated in antigen recognition. The contact residues are highly conserved in human V(H)3 antibodies but not in other families. The contact residues from domain D also are conserved among all SpA Ig-binding domains, suggesting that each could bind in a similar manner. Features of this interaction parallel those reported for staphylococcal enterotoxins that are superantigens for many T cells. The structural homology between Ig V(H) regions and the T-cell receptor V(beta) regions facilitates their comparison, and both types of interactions involve lymphocyte receptor surface remote from the antigen binding site. However, T-cell superantigens reportedly interact through hydrogen bonds with T-cell receptor V(beta) backbone atoms in a primary sequence-independent manner, whereas SpA relies on a sequence-restricted conformational binding with residue side chains, suggesting that this common bacterial pathogen has adopted distinct molecular recognition strategies for affecting large sets of B and T lymphocytes.

Domain one of the high affinity IgE receptor, FcepsilonRI, regulates binding to IgE through its interface with domain two

The high affinity receptor for IgE, FcepsilonRI, binds IgE through the second Ig-like domain of the alpha subunit. The role of the first Ig-like domain is not well understood, but it is required for optimal binding of IgE to FcepsilonRI, either through a minor contact interaction or in a supporting structural capacity. The results reported here demonstrate that domain one of FcepsilonRI plays a major structural role supporting the presentation of the ligand-binding site, by interactions generated within the interdomain interface. Analysis of a series of chimeric receptors and point mutants indicated that specific residues within the A' strand of domain one are crucial to the maintenance of the interdomain interface, and IgE binding. Mutation of the Arg(15) and Phe(17) residues caused loss in ligand binding, and utilizing a homology model of FcepsilonRI-alpha based on the solved structure of FcgammaRIIa, it appears likely that this decrease is brought about by collapse of the interface and consequently the IgE-binding site. In addition discrepancies in results of previous studies using chimeric IgE receptors comprising FcepsilonRIalpha with either FcgammaRIIa or FcgammaRIIIA can be explained by the presence or absence of Arg(15) and its influence on the IgE-binding site. The data presented here suggest that the second domain of FcepsilonRI-alpha is the only domain involved in direct contact with the IgE ligand and that domain one has a structural function of great importance in maintaining the integrity of the interdomain interface and, through it, the ligand-binding site.

Crystal structure of the anti-(carcinoembryonic antigen) single-chain Fv antibody MFE-23 and a model for antigen binding based on intermolecular contacts

MFE-23 is the first single-chain Fv antibody molecule to be used in patients and is used to target colorectal cancer through its high affinity for carcinoembryonic antigen (CEA), a cell-surface member of the immunoglobulin superfamily. MFE-23 contains an N-terminal variable heavy-chain domain joined by a (Gly(4)Ser)(3) linker to a variable light-chain (V(L)) domain (kappa chain) with an 11-residue C-terminal Myc-tag. Its crystal structure was determined at 2.4 A resolution by molecular replacement with an R(cryst) of 19.0%. Five of the six antigen-binding loops, L1, L2, L3, H1 and H2, conformed to known canonical structures. The sixth loop, H3, displayed a unique structure, with a beta-hairpin loop and a bifurcated apex characterized by a buried Thr residue. In the crystal lattice, two MFE-23 molecules were associated back-to-back in a manner not seen before. The antigen-binding site displayed a large acidic region located mainly within the H2 loop and a large hydrophobic region within the H3 loop. Even though this structure is unliganded within the crystal, there is an unusually large region of contact between the H1, H2 and H3 loops and the beta-sheet of the V(L) domain of an adjacent molecule (strands DEBA) as a result of intermolecular packing. These interactions exhibited remarkably high surface and electrostatic complementarity. Of seven MFE-23 residues predicted to make contact with antigen, five participated in these lattice contacts, and this model for antigen binding is consistent with previously reported site-specific mutagenesis of MFE-23 and its effect on CEA binding.

Identification of contact residues and definition of the CAR-binding site of adenovirus type 5 fiber protein

The binding of adenovirus (Ad) fiber knob to its cellular receptor, the coxsackievirus and Ad receptor (CAR), promotes virus attachment to cells and is a major determinant of Ad tropism. Analysis of the kinetics of binding of Ad type 5 (Ad5) fiber knob to the soluble extracellular domains of CAR together (sCAR) and each immunoglobulin (Ig) domain (IgV and IgC2) independently by surface plasmon resonance demonstrated that the IgV domain is necessary and sufficient for binding, and no additional membrane components are required to confer high-affinity binding to Ad5 fiber knob. Four Ad5 fiber knob mutations, Ser408Glu and Pro409Lys in the AB loop, Tyr477Ala in the DG loop, and Leu485Lys in beta strand F, effectively abolished high-affinity binding to CAR, while Ala406Lys and Arg412Asp in the AB loop and Arg481Glu in beta strand E significantly reduced the level of binding. Circular dichroism spectroscopy showed that these mutations do not disorder the secondary structure of the protein, implicating Ser408, Pro409, Tyr477, and Leu485 as contact residues, with Ala406, Arg412, and Arg481 being peripherally or indirectly involved in CAR binding. The critical residues have exposed side chains that form a patch on the surface, which thus defines the high-affinity interface for CAR. Additional site-directed mutagenesis of Ad5 fiber knob suggests that the binding site does not extend to the adjacent subunit or toward the edge of the R sheet. These findings have implications for our understanding of the biology of Ad infection, the development of novel Ad vectors for targeted gene therapy, and the construction of peptide inhibitors of Ad infection.

Cross-reactivity of antiidiotypic antibodies with DNA in systemic lupus erythematosus

OBJECTIVE

To assess the functional relationship between antibodies reactive with DNA and antibodies reactive with the idiotypes (idiopeptides) of anti-DNA antibodies that are associated with systemic lupus erythematosus (SLE) in mice.

METHODS

Antiidiotypic antibodies that appeared spontaneously in lupus mice, and others that were induced by immunization of normal, non-lupus mice, were analyzed for their reactivity by a range of direct binding, competition enzyme-linked immunosorbent assay (ELISA), and surface plasmon resonance (SPR) methods. Their reactions were assessed against synthetic peptides representing sequences of the V(H) region of anti-DNA monoclonal antibody (mAb) V-88, against the native mAb itself, and against mammalian DNA.

RESULTS

In lupus mice, only sera with the highest reactivity against double-stranded DNA (dsDNA) also reacted with idiopeptides in ELISA, and this showed a strong statistical correlation. However, there was no significant relationship between antiidiotypic antibodies and anti-single-stranded DNA antibodies. Immunization of (BALB/c x NZW)F1 mice with idiopeptides p64 (V(H) residues 64-80) or p92 (V(H) residues 92-105) induced antibodies that reacted not only against the respective peptides, but also against the native parent anti-DNA mAb V-88. Furthermore, the immune antiidiopeptide antibodies cross-reacted with dsDNA. Competition SPR experiments with the BIAcore system supported this observation. The binding reaction of V(H) peptide p64 (representing the CDR-H2/FR-H3 region of V-88) with antiidiopeptide antibodies was inhibited by dsDNA.

CONCLUSION

This study identified a unique set of autoantibodies in SLE. They react with both autoantibody idiotopes and with dsDNA, thus having a dual specificity for 2 autoantigens. Because these antiidiotope antibodies arise naturally during the development of lupus disease, and because they bind also to dsDNA, this provides a mechanism whereby the production of anti-dsDNA antibodies is stimulated. These idiotopes on autoantibodies in lupus act as natural mimotopes for inducing anti-dsDNA antibodies, which, due to their dual specificity, may significantly contribute to the pathology of nephritis in SLE.

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.

Mutations in the DG loop of adenovirus type 5 fiber knob protein abolish high-affinity binding to its cellular receptor CAR

The amino acid residues in adenovirus type 5 (Ad5) fiber that interact with its cellular receptor, the coxsackie B virus and Ad receptor (CAR), have not been defined. To investigate this, multiple mutations were constructed in the region between residues 479 and 497 in Ad5 fiber (beta-strands E and F and the adjacent region of the DG loop). The effects of these mutations on binding to CAR were determined by use of cell-binding competition experiments, surface plasmon resonance, and direct binding studies. The mutation effects on the overall folding and secondary structure of the protein were assessed by circular dichroism (CD) spectroscopy. Deletions of two consecutive amino acids between residues 485 and 493 abolished high-affinity binding to CAR; the CD spectra indicated that although there was no disruption of the overall folding and secondary structure of the protein, local conformational changes did occur. Moreover, single site mutations in this region of residues with exposed, surface-accessible side chains, such as Thr492, Asn493, and Val495, had no effect on receptor binding, which demonstrates that these residues are not in contact with CAR themselves. This implies the involvement of residues in neighboring loop regions. Replacement of the segment containing the two very short beta-strands E and F and the turn between them (residues 479 to 486) with the corresponding sequence from Ad3 (betaEFAd3-->5 mutation) resulted in the loss of receptor binding. The identical CD spectra for betaEFAd3-->5 and wild-type proteins suggest that these substitutions caused no conformational rearrangement and that the loss of binding may thus be due to the substitution of one or more critical contact residues. These findings have implications for our understanding of the interaction of Ad5 fiber with CAR and for the construction of targeted recombinant Ad5 vectors for gene therapy purposes.

Distinct immunological and biochemical properties of thyroid peroxidase purified from human thyroid glands and recombinant protein produced in insect cells

The biosynthesis of thyroid hormone from thyroglobulin is catalysed by thyroid peroxidase (TPO), an integral membrane protein. TPO is also a major autoantigen in autoimmune thyroid disease and autoantibodies to TPO are markers for disease activity. Large quantities of purified TPO are essential for elucidating its structure and understanding its role in disease activity. We describe the high yield purification of full-length recombinant human TPO from baculovirus infected insect cells and compare it to purified native TPO from human thyroid glands. In contrast to native human TPO, the human TPO produced in insect cells as a recombinant protein was insoluble and resistant to solubilisation in detergents. Reversible substitution of lysine residues with citraconic anhydride led to increased solubility of the recombinant TPO, allowing high-yield purification by monoclonal antibody chromatography. The purified enzyme preparation was shown to be TPO by its reactivity with monoclonal and polyclonal antibodies by enzyme linked immunosorbent assay and Western blotting. Both the human and recombinant purified TPO preparations also react with sera from patients with autoimmune thyroid disease, although the binding of conformational dependent autoantibodies was considerably lower to the recombinant TPO than to the native TPO. This suggests that the recombinant TPO may differ in some aspects of its tertiary structure. The purified recombinant TPO was devoid of enzyme activity, in contrast to the enzymatically active, purified human TPO preparations. Both preparations contained comparable amounts of haem (R(z)=0.269), but a shift in the Soret band of recombinant TPO (402 nm) from that of natural TPO (409 nm) indicates that the lack of enzymatic activity of the recombinant enzyme may be due to changes in the protein backbone surrounding the haem. Both the purified native and recombinant TPO, under non-denaturing conditions, show evidence of high molecular mass oligomers, although the latter preparation is prone to a greater degree of aggregation. In conclusion, our studies indicate that recombinant TPO generated in insect cells is conformationally distinct from the native TPO, is insoluble and enzymatically inactive, consistent with the difficulties associated with its purification and crystallisation.

Upregulation of FcepsilonRI on human basophils by IgE antibody is mediated by interaction of IgE with FcepsilonRI

BACKGROUND

IgE is now known to upregulate the expression of FcepsilonRI on human basophils. It is not known which receptor on basophils mediates this process of upregulation.

OBJECTIVE

We sought to determine whether galectin-3, FcepsilonRII (CD23), or FcepsilonRI were involved in the upregulation of FcepsilonRI by IgE.

METHODS

The role of galectin-3 was examined by measuring the influence of alpha-lactose on upregulation. Basophils were examined for expression of FcepsilonRII (CD23) by flow cytometry and messenger (m)RNA expression. Functional discrimination between binding to FcepsilonRII or FcepsilonRI was examined through the use of mutant IgE-Fc fragments or anti-FcepsilonRII antibody.

RESULTS

Upregulation of FcepsilonRI on basophils in the presence of IgE was not altered by coincubation with alpha-lactose, eliminating a role for galectin-3. Basophils were not found to express FcepsilonRII, as determined by flow cytometry with enriched basophil preparations or RT-PCR with highly purified basophil preparations. A mutant of the Fc fragment of IgE (IgE-Fc), which binds to FcepsilonRI with a greater than 10-fold lower affinity than IgE or wild-type IgE-Fc but exhibits no change in affinity for FcepsilonRII, allowed us to distinguish between the functions of the two Fc receptors. The mutant (R334S; Henry et al 1997) was required at about 30-fold higher concentration than the wild-type IgE-Fc for the same stimulation of FcepsilonRI expression on basophils, thus excluding a role for FcepsilonRII in the response. In addition, treatment of basophils with anti-FcepsilonRII antibody (MHM6), which is known to be competitive with IgE, had no effect on the expression of FcepsilonRI or the ability of IgE to upregulate expression of FcepsilonRI.

CONCLUSION

Collectively, these data indicate that IgE interacts with FcepsilonRI to upregulate its expression on human basophils.

Evidence that cysteine-166 is the active-site nucleophile of Pseudomonas aeruginosa amidase: crystallization and preliminary X-ray diffraction analysis of the enzyme

Wild-type and site-specific mutants C166S and C166A (Cys-166-->Ser and Cys-166-->Ala respectively) of the amidase (acylamide amidohydrolase, EC 3.5.1.4) from Pseudomonas aeruginosa were expressed in Escherichia coli by using the vector pKK223-3. Both mutant proteins were catalytically inactive but showed complete cross-reactivity with polyclonal antiserum raised against the wild-type enzyme, as well as CD spectra identical with that of the wild-type enzyme, which were indicative of correct folding. Cys-166 is therefore implicated as the active-site nucleophile. Titration of free thiol groups with 5,5'-dithiobis-(2-nitrobenzoic acid) indicated that Cys-166 is not a rapidly reacting residue. Crystals of both wild-type and C166S amidase grew with identical, rhombohedral morphology; X-ray diffraction analysis established the unit cell dimensions (a=b=c=84 A; alpha=beta=gamma=75 degrees) and space group (R3 or R32). These results imply a quaternary structure of six subunits, with most probably 32 symmetry; the existence of a hexameric structure was supported by molecular mass determinations based on gel filtration and electrophoretic mobility.

Interactions between a single immunoglobulin-binding domain of protein L from Peptostreptococcus magnus and a human kappa light chain

The placement of a tryptophan residue into a single Ig-binding-domain of protein L from Peptostreptococcus magnus has been used to examine the binding interactions between the binding domain and kappa light chains (kappa-chains). The fluorescence intensity of the mutant domain increases on the formation of a complex with kappa-chains. This has been used to determine the Kd of the complex under a range of conditions by using both pre-equilibrium and equilibrium methods. The Kd values determined for the complex with kappa-chains at a number of different pH values are very close to those obtained with the wild-type domain, indicating that the mutation has not substantially affected its binding properties. Examination of the reaction between the mutant domain and kappa-chains by stopped-flow fluorescence shows that complex formation takes place by two discrete, sequential processes. A fast bimolecular reaction, with a rate constant of 8.3x10(5) M-1. s-1 (at pH8.0 and 25 degrees C), is followed by a slow unimolecular process with a rate (1.45 s-1) that is independent of the concentration of the reactants. This suggests that a conformational change occurs after the initial encounter complex is formed. The dissociation of the complex at equilibrium occurs in a single process of rate 0.095 s-1 at pH8.0 and 25 degrees C. Stopped-flow CD studies show that a slow decrease in ellipticity at 275 nm occurs with a rate of 1.3 s-1 when wild-type protein binds to kappa-chains, suggesting that the conformational transition might involve a change in environment around one or more tyrosine residues.

Molecular modeling of an anti-DNA autoantibody (V-88) and mapping of its V region epitopes recognized by heterologous and autoimmune antibodies

Anti-DNA autoantibodies are a characteristic feature of human systemic lupus erythematosus (SLE) and lupus diseases in the mouse. V-88 is an IgG1/kappa ssDNA-binding Ab, derived from a lupus mouse, that bears a cross-species, cross-reactive Id (CRI) that has been implicated in the pathogenesis of both human and murine disease. A linear epitope map of V-88 has been determined with anti-idiotypic antisera obtained from rabbits, and candidate sequences for the idiotopes of the CRI have been proposed. We now report the modeling of the three-dimensional structure of the V regions of Ab V-88, to map the location of these idiotopes. The V region framework structure was derived from those of crystallographically determined Ab structures, and the complementarity determining region (CDR) structures were based upon the set of canonical structures adopted by these loop regions in Abs of known structure. One of the idiotopes is an extensive, highly accessible epitope consisting of framework regions spatially adjacent to CDR2 in the heavy chain. Epitopes recognized by an anti-idiotypic rabbit antiserum were compared with those recognized by autoimmune sera from SLE-prone mice, and common features were identified. By analogy with the crystal structure of an anti-DNA Ab BV04-01 complexed with a trinucleotide, the modeled structure also suggests a mode of binding of ssDNA to V-88. The location of the candidate CRI, although within the framework region of VH, is such that it could influence Ag specificity.

Crystal structure of the zinc-dependent beta-lactamase from Bacillus cereus at 1.9 A resolution: binuclear active site with features of a mononuclear enzyme

The structure of the zinc-dependent beta-lactamase II from Bacillus cereus has been determined at 1.9 A resolution in a crystal form with two molecules in the asymmetric unit and 400 waters (space group P3121; Rcryst = 20.8%). The active site contains two zinc ions: Zn1 is tightly coordinated by His86, His88, and His149, while Zn2 is loosely coordinated by Asp90, Cys168, and His210. A water molecule (W1) lies between the two zinc ions but is significantly closer to Zn1 and at a distance of only 1.9 A is effectively a hydroxide moiety and a potential, preactivated nucleophile. In fact, Asp90 bridges W1 to Zn2, and its location is thus distinct from that of the bridging water molecules in the binuclear zinc peptidases or other binuclear zinc hydrolases. Modeling of penicillin, cephalosporin, and carbapenem binding shows that all are readily accommodated within the shallow active site cleft of the enzyme, and the Zn1-bound hydroxide is ideally located for nucleophilic attack at the beta-lactam carbonyl. This enzyme also functions with only one zinc ion present. The Zn1-Zn2 distances differ in the two independent molecules in the crystal (3.9 and 4.4 A), yet the Zn1-W1 distances are both 1.9 A, arguing against involvement of Zn2 in W1 activation. The role of Zn2 is unclear, but the B. cereus enzyme may be an evolutionary intermediate between the mono- and bizinc metallo-beta-lactamases. The broad specificity of this enzyme, together with the increasing prevalence of zinc-dependent metallo-beta-lactamases, poses a real clinical threat, and this structure provides a basis for understanding its mechanism and designing inhibitors.

Thermodynamics of the interaction of human immunoglobulin E with its high-affinity receptor Fc epsilon RI

We have employed isothermal titration calorimetry (ITC) and circular dichroism (CD) spectroscopy to characterize the binding of soluble fragments of IgE (IgE-Fc and Fc epsilon 3-4) to a soluble fragment of the high-affinity receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha). The thermodynamic parameters for the interaction of IgE-Fc and Fc epsilon 3-4 with sFc epsilon RI alpha, determined using ITC, confirm the earlier conclusion that the C epsilon 2 domain is not involved in the interaction and that the stoichiometry of both complexes is 1:1. For both IgE-Fc and Fc epsilon 3-4, the value of Delta H degrees is -36.9 +/- 4.6 kcal mol-1 at 37.3 degreesC and Delta Cp degrees is -820 +/- 120 cal mol-1 K-1. The temperature at which DeltaS degrees is zero is 284 +/- 1 K, indicating that the entropy contribution to the thermodynamics of association is unfavorable at physiological temperature. Of particular interest is the large value of Delta Cp degrees. The large surface area of IgE and Fc epsilon RI alpha that is implicated in complex formation from previous mutagenesis studies on the two proteins may account in part for the magnitude of Delta Cp degrees. Additional contributions may arise from hydration within the binding site and changes in tertiary structure of the individual components of the complex. However, the CD spectra of IgE, IgE-Fc, and Fc epsilon 3-4 complexes with sFc epsilon RI alpha are merely the sum of the spectra of their individual components, indicating that the secondary structure of the immunoglobulin domain folds are preserved on complex formation. Thus, any change in tertiary structure must be limited to the relative disposition of the immunoglobulin domains C epsilon 3 and C epsilon 4 in IgE and the two immunoglobulin-like domains in the alpha-chain of Fc epsilon RI.

The structure of a human rheumatoid factor bound to IgG Fc

This is the first crystal structure analysis of a complex between an autoantibody and its autoantigen, and it reveals a mode of interaction never before seen in an antibody-antigen complex. Not only are there relatively few antibody contact residues, contributing perhaps to its very low affinity, but these residues are to be found on only one side of the potential combining site surface. Indeed, so many CDR residues are not involved in Fc binding, including those in the central region of the combining site, that it is easy to envisage that this RF may have another, entirely different, specificity. The antibody may therefore have originated in response to another, as yet unidentified, antigen, and the reactivity with IgG Fc may be an unfortunate cross-reactivity. Certainly some of the CDR residues which do interact with IgG Fc are germline encoded, but significantly one of only two residues in the light chain, Pro56, which makes many contacts with Fc, is a somatic mutation. Since this mutation would appear to make a significant contribution to the binding affinity, it is therefore evidence for an antigen driven response to the IgG Fc in the generation of this autoantibody. The Fc epitope recognised by RF-AN is strikingly similar to the binding sites for the bacterial binding proteins A and G, but the significance of this is not clear. What is clear however is that the epitope does not include any part of the Fc carbohydrate residues, although the structure of the complex does reveal that there is an alteration in the carbohydrate conformation when the galactose residues are absent. Loss of the interaction between the terminal galactose residue on the alpha (1-6) linked branch and the C gamma 2 domain appears to allow the carbohydrate chains to become mobile, at the same time exposing a predominantly hydrophobic patch on the C gamma 2 surface. Accessibility to either the agalactosyl carbohydrate chains or the newly exposed residues may account for the enhanced reactivity for G0-IgG that has been reported for certain RFs, and such an epitope need not be very different to that recognised by RF-AN. In order to understand more completely the effect of the presence or absence of the terminal galactose residue, the fully galactosylated glycoform of Fc must be studied for comparison; this work is underway. It is also important now to study a RF which is known to sense this difference in oligosaccharide composition, and also to study RFs of higher affinity, of the IgG class, and from the synovium. RF-AN was the first RF to be immortalised as a cell line, and in many ways it is a typical RF (in terms of specificity, relationship to germline sequence and affinity), but we must now establish whether the novel structural features revealed in this analysis are indeed typical of other RFs. Only when comparisons can be made between RFs of different origin and with contrasting functional properties will we begin to understand what constitutes a pathogenic RF, and the mechanism by which such auto-reactive antibodies are generated.

Identification of contact residues in the IgE binding site of human FcepsilonRIalpha

The high-affinity receptor for immunoglobulin E (IgE), FcepsilonRI, is an alphabetagamma2 tetramer found on mast cells, basophils, and several other types of immune effector cells. The interaction of IgE with the alpha-subunit of FcepsilonRI is central to the pathogenesis of allergy. Detailed knowledge of the mode of interaction of FcepsilonRI with IgE may facilitate the development of inhibitors for general use in the treatment of allergic disease. To this end we have performed site-directed mutagenesis on a soluble form of the FcepsilonRI alpha-chain (sFcepsilonRIalpha). The effects of four mutations in the second immunoglobulin-like domain of sFcepsilonRIalpha upon the kinetics of binding to IgE and fragments of IgE have been analyzed using surface plasmon resonance. As described in the preceding paper of this issue [Henry, A. J., et al. (1997) Biochemistry 36, 15568-15578], biphasic binding kinetics was observed. Two of the mutations had significant effects on binding: K117D reduced the affinity of sFcepsilonRIalpha for IgE by a factor of 30, while D159K increased the affinity for IgE by a factor of 7, both principally through changes in the rates of dissociation of the slower phase of the interaction. Circular dichroism spectra of sFcepsilonRIalpha incorporating either of these mutations were indistinguishable from those of wild-type sFcepsilonRIalpha, demonstrating that the native conformation had not been disrupted. Our results, together with those from site-directed mutagenesis on fragments of IgE presented in the accompanying paper, define the contact surfaces in the IgE:sFcepsilonRIalpha complex.

Participation of the N-terminal region of Cepsilon3 in the binding of human IgE to its high-affinity receptor FcepsilonRI

The binding of immunoglobulin E (IgE) to its high-affinity receptor (FcepsilonRI) expressed on mast cells and basophils is central to the development of an allergic reaction. Previous studies have implicated the third constant domain of IgE-Fc (Cepsilon3) as the site of the interaction with FcepsilonRI. We have prepared a series of site-directed mutants of human IgE-Fc, particularly focusing on the N-terminal "linker" region and AB loop of Cepsilon3. The kinetics of binding IgE and its Fc fragments to the immobilized receptor were determined by surface plasmon resonance (SPR), and two phases of binding were observed. We identified one mutation in the N-terminal linker region, R334S, that has a dramatic effect on binding. R334S lowers the affinity of IgE-Fc for FcepsilonRI by 120-fold, principally through an increase in the dissociation rate of the slower phase of the interaction. This mutation has a similar effect in Fcepsilon3-4, a truncated form of IgE-Fc which lacks the Cepsilon2 domain pair, and thus it does not exert its effect through altering the quaternary structure of IgE-Fc, firmly implicating Arg334 as a contact residue in the complex. However R334S has no effect on the binding of FcepsilonRII (CD23), the low-affinity receptor for IgE, demonstrating the structural integrity of the mutated IgE-Fc. Circular dichroism spectroscopy and thermal stability studies further indicate that the R334S mutation does not disorder or destabilize the structure of IgE-Fc or Fcepsilon3-4. These results demonstrate the importance of the N-terminal linker region of Cepsilon3 in the interaction of IgE with FcepsilonRI.

Human thyroid peroxidase (TPO) isoforms, TPO-1 and TPO-2: analysis of protein expression in Graves' thyroid tissue

Thyroid peroxidase (TPO) is the key enzyme involved in the biosynthesis of thyroid hormones and is an important autoantigen in autoimmune thyroid disease. Different messenger RNA species coding for TPO are present in thyroid tissue, including the species coding for a 933-amino acid protein (termed TPO-1) and a second in which exon 10 is deleted and which is 57 residues shorter (termed TPO-2). However, it is not known whether the smaller, TPO-2 isoform is expressed as a protein in thyroid cells. In SDS-PAGE under reducing conditions, TPO appears in the thyroid microsome and purified protein preparations as a closely migrating double band of approximately 105 (larger form) and 100 kilodaltons (smaller form). We investigated the presence of the isoform TPO-2 polypeptide in Graves' thyroid tissue using rabbit antisera to three different synthetic peptides from exon 10 (specific for TPO-1) and a polyclonal rabbit and monoclonal anti-TPO antibody (both of which are specific for the two forms of TPO). The larger and smaller forms of TPO were purified by electroelution after gel electrophoresis of highly purified natural TPO from Graves' thyroid microsomes. Both of the purified forms of TPO react with all three anti-exon 10 peptide antibodies, the polyclonal anti-TPO and the monoclonal antibody anti-TPO. This shows that both forms of TPO contain exon 10-encoded polypeptide of TPO-1. Interestingly, the proportion of the larger and smaller forms of TPO varied in different Graves' thyroid microsome preparations. To investigate the presence of the smaller TPO-2 isoform in the purified natural TPO preparation, affinity depletion of TPO-1 using the anti-exon 10 peptide antibodies was carried out. The binding of anti-exon 10 peptide antibodies to the immunodepleted TPO-1 fraction was considerably diminished in comparison to binding of polyclonal anti-TPO, suggesting the presence of small amounts (< 10%) of TPO-2 expressed as a protein in thyroid cells. Our results extend previous observations by showing that the alternatively spliced form of TPO, in which exon 10 is excised, is expressed at low levels in Graves' thyroid tissue. Furthermore, we confirm that both the larger and smaller forms of TPO observed on gel electrophoresis contain TPO-1, suggesting that the difference is caused by posttranslational modifications. The presence of small amounts of TPO-2 in Graves' thyroid glands argues for its role in thyroid function, which remains to be clarified.

Peptide analogs that inhibit IgE-Fc epsilon RI alpha interactions ameliorate the development of lethal graft-versus-host disease

Significant increases in serum levels of IgE have often been observed in allogeneic bone marrow transplantation patients and have generally been thought to be diagnostic of graft-versus-host disease (GVHD), rather than an agent involved in the pathogenesis of the disease. Experimental murine GVHD models have also indicated associations of hyper-IgE activity, yet the role of IgE in GVHD pathogenesis has never been tested directly. In the current study, we have tried to address this issue by using recently developed peptide analog antagonists for the interaction of IgE with the Fc epsilon RI receptor, which is necessary for triggering mast cells and other cell types when cross-linked by antigens. A synthetic cyclized 13-amino acid peptide was previously designed from the modeled C-C' loop region of the Fc epsilon RI alpha-chain and was found to act as a competitive inhibitor of IgE-Fc epsilon RI alpha binding. The peptide was generated in two forms, a cyclic L-(L-IgEtide) and retro D-amino acid composition (rDIgEtide), the latter to increase resistance to protease degradation for in vivo applications. These two inhibitor peptides were then used to test the hypothesis that IgE could be involved in the pathogenesis of acute GVHD, in the B10.D2-->DBA/2 (900 cGy) strain combination, with GVHD directed to minor histocompatibility antigens. Both peptides demonstrated significant inhibition of the development of lethal GVHD, supporting the involvement of IgE at some level of disease pathogenesis.

Purification and crystallisation of the autoantigen thyroid peroxidase from human Graves' thyroid tissue

Milligram quantities of the human membrane autoantigen thyroid peroxidase (TPO) have been purified to a high degree of homogeneity by a combination of detergent solubilisation, monoclonal antibody affinity, and ion exchange chromatography, from pooled Graves' disease thyroid glands. The purified TPO of greater than 90% purity was enzymatically active as judged by its ability to oxidise guaiacol. Crystals of TPO have been grown from solutions of the protein solubilised in sodium deoxycholate, in the presence of ammonium sulphate. The crystals exhibited birefringence under polarised light, indicative of molecular order. Crystallisation of this large, membrane autoantigen represents the first step in delineating the complete three-dimensional structure of a human autoantigen involved in destructive thyroiditis.

Structure of human IgM rheumatoid factor Fab bound to its autoantigen IgG Fc reveals a novel topology of antibody-antigen interaction

Rheumatoid factors are the characteristic autoantibodies of rheumatoid arthritis, which bind to the Fc regions of IgG molecules. Here we report the crystal structure of the Fab fragment of a patient-derived IgM rheumatoid factor (RF-AN) complexed with human IgG4 Fc, at 3.2 A resolution. This is the first structure of an autoantibody-autoantigen complex. The epitope recognised in IgG Fc includes the C gamma 2/C gamma 3 cleft region, and overlaps the binding sites of bacterial Fc-binding proteins. The antibody residues involved in autorecognition are all located at the edge of the conventional combining site surface, leaving much of the latter available, potentially, for recognition of a different antigen. Since an important contact residue is somatic mutation, the structure implicates antigen-driven selection, following somatic mutation of germline genes, in the production of pathogenic rheumatoid factors.

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.

Basis of the 1:1 stoichiometry of the high affinity receptor Fc epsilon RI-IgE complex

A soluble fragment of the high-affinity IgE receptor Fc epsilon RI alpha-chain (sFc epsilon RI alpha) binds to the Fc fragment of IgE (IgE-Fc) as a 1:1 complex. IgE-Fc consists of a dimer of the C epsilon 2, C epsilon 3 and C epsilon 4 domains of the epsilon-heavy chain of IgE. This region of IgE has been modelled on the crystal structure of the Fc region of IgG1, which exhibits twofold rotational symmetry. This implies that IgE should be divalent with respect to its ligands. X-ray scattering studies reveal however that the twofold rotational symmetry of IgE-Fc is perturbed by a bend in the linker region between the C epsilon 2 and C epsilon 3 domains. The 1:1 stoichiometry could then arise from the conformational asymmetry or from steric occlusion of one of the sites by the overhanging C epsilon 2 domains. To test this hypothesis we have expressed a recombinant epsilon-chain fragment containing C epsilon 3 and C epsilon 4. This product, Fc epsilon 3-4, is secreted from cells as a disulphide linked dimer and binds with higher affinity than either IgE or IgE-Fc to cell surface Fc epsilon RI. Titration experiments, together with molecular mass measurements of the Fc epsilon 3-4/sFc epsilon RI alpha complex, reveal that Fc epsilon 3-4 binds only a single receptor molecule. This excludes the possibility that steric hindrance by C epsilon 2 accounts for the unexpected stoichiometry.

Crystallization of a complex between the Fab fragment of a human immunoglobulin M (IgM) rheumatoid factor (RF-AN) and the Fc fragment of human IgG4

Rheumatoid factors (RF) are the characteristic autoantibodies found in patients with rheumatoid arthritis. They recognize epitopes in the Fc region of immunoglobulin G (IgG) and are often of the IgM isotype. In order to analyse the nature of RF-Fc interactions, we have crystallized a complex between the Fab fragment of a human monoclonal IgM rheumatoid factor (RF-AN) and the Fc fragment of human IgG4. The stoichiometry of the complex within the crystals was found to be 2:1 Fab:Fc. The crystals diffracted X-rays to 0.3 nm resolution, and the space group was C2, with cell dimensions a = 16.03 nm, b = 8.19 nm, c = 6.42 nm, beta = 98.3 degrees. We have also determined the sequence of the variable region of the RF-AN light chain, not hitherto reported. This belongs to the V lambda III-a subgroup and is closely related to the germline gene Humlv318, from which it differs in three amino acid residues. This is the first reported crystallized complex between a human autoantibody and its autoantigen.

Structure based design and characterization of peptides that inhibit IgE binding to its high-affinity receptor

We have designed synthetic peptide inhibitors of the interaction between IgE and its high affinity receptor, Fc epsilon RI. The structure of the second domain of CD2 was used as a modelling template for the second alpha-chain domain of Fc epsilon RI, the C-C' loop of which has been implicated in the interaction with IgE. An L-amino acid peptide and a retro-enantiomeric D-amino acid peptide were designed to mimic the conformation of the C-C' region. Both peptides were cyclized by disulphide bond formation between terminal cysteine residues, and show mirror image symmetry by circular dichroism analysis. The C-C' peptide mimics act as competitive inhibitors of IgE binding. The cyclic L- and retro D-peptides exhibited KDs of approximately 3 microM and 11 microM, respectively, for IgE. Further, the peptides inhibit IgE-mediated mast cell degranulation, an in vitro model of an allergic response.