Conformational changes in IgE contribute to its uniquely slow dissociation rate from receptor FcɛRI

Among antibody classes, IgE has a uniquely slow dissociation rate from, and high affinity for, its cell surface receptor FcɛRI. We show the structural basis for these key determinants of the ability of IgE to mediate allergic hypersensitivity through the 3.4-Å-resolution crystal structure of human IgE-Fc (consisting of the Cɛ2, Cɛ3 and Cɛ4 domains) bound to the extracellular domains of the FcɛRI α chain. Comparison with the structure of free IgE-Fc (reported here at a resolution of 1.9 Å) shows that the antibody, which has a compact, bent structure before receptor engagement, becomes even more acutely bent in the complex. Thermodynamic analysis indicates that the interaction is entropically driven, which explains how the noncontacting Cɛ2 domains, in place of the flexible hinge region of IgG antibodies, contribute together with the conformational changes to the unique binding properties of IgE.

Filamentous bacteria masquerading as fungi: a diagnostic pitfall in direct smear interpretation with report of two cases

Bacteria, particularly Gram-negative bacilli, can develop abnormal morphology after the administration of subinhibitory concentrations of antibacterial agents. Filamentation is a common response in which bacteria replicate but incompletely divide, leading to long slender chains that resemble fungal hyphae. Pathologists are frequently consulted to examine direct smears of body fluids, which often contain microorganisms. Antibiotic-related filamentous morphology may resemble fungal hyphae and this potential misinterpretation can lead to inappropriate treatment for presumed fungal infections. Two cases are described in which direct smears of body fluids were examined by on-call pathology residents who misinterpreted filamentous bacteria as fungal organisms, with one case leading to the initiation of antifungal medication. Although well-established within the field of microbiology, many residents and practising pathologists are less familiar with antibiotic-related bacterial morphology, as it may not be routinely encountered. It is important for pathologists to be aware of this phenomenon in order to avoid misinterpretation.

Cushing's syndrome and nocardiosis associated with a pulmonary carcinoid tumor: report of a case and review of the literature

Ectopic hormone production is an uncommon complication of neoplastic lung disease. Rarely, patients may present with signs and symptoms of systemic endocrine dysfunction related to a hormone-secreting tumor. Bronchopulmonary carcinoids are the most common neoplasm implicated in ectopic ACTH-dependent Cushing's syndrome. Persistent hypercortisolism, such as that which occurs in Cushing's syndrome, causes immunosuppression and makes patients vulnerable to opportunistic infections. We present a case of a 42-year-old woman diagnosed with ACTH-dependent Cushing's syndrome which was originally thought to stem from a pituitary lesion as interpreted on magnetic resonance imaging. Her symptoms persisted after undergoing hypophysectomy, and further work-up involving a fine needle aspiration of the left lung revealed an ACTH-producing carcinoid tumor. Before treatment could be administered, the patient developed several new suspicious nodules in the left lung that were shown by fine needle aspiration to be infectious in nature. A Gram stain revealed numerous Gram positive branching organisms, and culture of the specimen grew Nocardia asteroides. Her pulmonary infection was treated with antibiotics and she underwent successful ablation of the carcinoid tumor.

Mutations in an avian IgY-Fc fragment reveal the locations of monocyte Fc receptor binding sites

The avian IgY antibody isotype shares a common ancestor with both mammalian IgG and IgE and so provides a means to study the evolution of their structural and functional specialisations. Although both IgG and IgE bind to their leukocyte Fc receptors with 1:1 stoichiometry, IgY binds to CHIR-AB1, a receptor expressed in avian monocytes, with 2:1 stoichiometry. The mutagenesis data reported here explain the structural basis for this difference, mapping the CHIR-AB1 binding site to the Cupsilon3/Cupsilon4 interface and not the N-terminal region of Cupsilon3 where, at equivalent locations, the IgG and IgE leukocyte Fc receptor binding sites lie. This finding, together with the phylogenetic relationship of the antibodies and their receptors, indicates that a substantial shift in the nature of Fc receptor binding occurred during the evolution of mammalian IgG and IgE.

Total synthesis of (±)-aspercyclide A and its C19 methyl ether

The total syntheses of (±)-aspercyclide A (1) and its C19 methyl ether derivative (15a) are described. ELISA studies show that both compounds display comparable antagonist activity against the IgE–FcεRI protein–protein interaction.

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.

The crystal structure of an avian IgY-Fc fragment reveals conservation with both mammalian IgG and IgE

Avian IgY is closely related to an ancestor of both mammalian IgG and IgE and thus provides insights into the evolution of antibody structure and function. A recombinant fragment of IgY-Fc consisting of a dimer of the Cupsilon3 and Cupsilon4 domains, Fcupsilon3-4, was expressed and crystallized and its X-ray structure determined to 1.75 A resolution. Fcupsilon3-4 is the only nonmammalian Fc fragment structure determined to date and provides the first structural evidence for an ancient origin of antibody architecture. The Fcupsilon3-4 structure reveals features common to both IgE-Fc and IgG-Fc, and the implications for IgY binding to its receptor are discussed.

Structure and substrate specificity of acetyltransferase ACIAD1637 from Acinetobacter baylyi ADP1

Gene ACIAD1637 from Acinetobacter baylyi ADP1 encodes a 182 amino acid putative antibiotic resistance protein. The structure of this protein (termed acepita) has been solved in space group P(2) to 2.35 A resolution. Acepita belongs to the GCN5-related N-acetyltransferase (GNAT) family, and contains the four sequence motifs conserved among family members. The structure of acepita is compared with that of pita, its homologue from Pseudomonas aeruginosa. Acepita has a similar substrate profile to pita and performs a similar function.

Attenuation of IgE affinity for FcepsilonRI radically reduces the allergic response in vitro and in vivo

The high affinity of IgE for its receptor, FcepsilonRI (K(a) approximately 10(10) M(-1)), is responsible for the persistence of mast cell sensitization. Cross-linking of FcepsilonRI-bound IgE by multivalent allergen leads to cellular activation and release of pro-inflammatory mediators responsible for the symptoms of allergic disease. We previously demonstrated that limiting the IgE-FcepsilonRI interaction to just one of the two Cepsilon3 domains in IgE-Fc, which together constitute the high affinity binding site, results in 1000-fold reduced affinity. Such attenuation, effected by a small molecule binding to part of the IgE:FcepsilonRI interface or a distant allosteric site, rather than complete blocking of the interaction, may represent a viable approach to the treatment of allergic disease. However, the degree to which the interaction would need to be disrupted is unclear, because the importance of high affinity for immediate hypersensitivity has never been investigated. We have incorporated into human IgE a mutation, R334S, previously characterized in IgE-Fc, which reduces its affinity for FcepsilonRI approximately 50-fold. We have compared the ability of wild type and R334S IgE to stimulate allergen-induced mast cell activation in vitro and in vivo. We confirmed the expected difference in affinity between wild type and mutant IgE for FcepsilonRI (approximately 50-fold) and found that, in vitro, mast cell degranulation was reduced proportionately. The effect in vivo was also marked, with a 75% reduction in the passive cutaneous anaphylaxis response. We have therefore demonstrated that the high affinity of IgE for FcepsilonRI is critical to the allergic response, and that even moderate attenuation of this affinity has a substantial effect in vivo.

Structure of a putative acetyltransferase (PA1377) from Pseudomonas aeruginosa

Gene PA1377 from Pseudomonas aeruginosa encodes a 177-amino-acid conserved hypothetical protein of unknown function. The structure of this protein (termed pitax) has been solved in space group I222 to 2.25 A resolution. Pitax belongs to the GCN5-related N-acetyltransferase family and contains all four sequence motifs conserved among family members. The beta-strand structure in one of these motifs (motif A) is disrupted, which is believed to affect binding of the substrate that accepts the acetyl group from acetyl-CoA.

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.

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.

The allergic march from Staphylococcus aureus superantigens to immunoglobulin E

Staphylococcus aureus is a commensal bacterium in the respiratory tract mucosa of most people and infects the skin of atopic dermatitis patients. This might imply a symbiotic relationship between host and bacterium or a standoff between bacterial infection and the host immune system. But superantigens produced by S. aureus in these locations are of particular interest because they are strongly implicated in the pathogenesis of allergic disorders and airway disease. They appear to act locally in these conditions by stimulating polyclonal T cell and B cell proliferation and driving somatic hypermutation, class switching to immunoglobulin (Ig) E and the production of allergen-specific IgE in mucosal B cells. IgE antibodies directed against the superantigens ('superallergens') themselves engender chronic inflammation and the persistent sensitization to conventional allergens of mast cells and antigen-presenting cells in mucosal tissues in atopic dermatitis, rhinitis and asthma. Moreover, S. aureus superantigens inhibit the activity of T regulatory cells that normally control inflammation, and generate a state of steroid resistance that confounds treatment of allergic disorders and airway disease.

Crystal structure of a human autoimmune complex between IgM rheumatoid factor RF61 and IgG1 Fc reveals a novel epitope and evidence for affinity maturation

Rheumatoid factors (RF) are autoantibodies that recognize epitopes in the Fc region of immunoglobulin (Ig) G and that correlate with the clinical severity of rheumatoid arthritis (RA). Here we report the X-ray crystallographic structure, at 3 A resolution, of a complex between the Fc region of human IgG1 and the Fab fragment of a monoclonal IgM RF (RF61), derived from an RA patient and with a relatively high affinity for IgG Fc. In the complex, two Fab fragments bind to each Fc at epitopes close to the C terminus, and each epitope comprises residues from both Cgamma3 domains. A central role in the unusually hydrophilic epitope is played by the side-chain of Arg355, accounting for the subclass specificity of RF61, which recognizes IgG1,-2, and -3 in preference to IgG4, in which the corresponding residue is Gln355. Compared with a previously determined complex of a lower affinity RF (RF-AN) bound to IgG4 Fc, in which only residues at the very edge of the antibody combining site were involved in binding, the epitope bound by RF61 is centered in classic fashion on the axis of the V(H):V(L) beta-barrel. The complementarity determining region-H3 loop plays a key role, forming a pocket in which Arg355 is bound by two salt-bridges. The antibody contacts also involve two somatically mutated V(H) residues, reinforcing the suggestion of a process of antigen-driven maturation and selection for IgG Fc during the generation of this RF autoantibody.

Three-colour flow cytometric method to measure antibody-dependent tumour cell killing by cytotoxicity and phagocytosis

We have developed a three-colour flow cytometric method to assay the contributions of cytotoxicity and phagocytosis to antibody-dependent cell-mediated tumour cell killing. In this assay, tumour target cells are pre-labelled with CFSE, and mixed with effector cells and a tumour antigen-specific monoclonal antibody. After incubation of the cells with the antibody, effector cells are labelled with PE and dead cells with PI. Using flow cytometry, dead and phagocytosed tumour cells can be quickly and easily counted and the numbers summed to determine the total number of killed cells. One can thereby measure the phagocytic aspect of antibody-dependent cell-mediated tumour cell killing, otherwise only revealed by microscopic examination. The failure to detect phagocytosed, in addition to live and dead target cells, by standard assays may result in an underestimation of tumour cell killing and hence the potential of an antibody for immunotherapy of cancer. We illustrate the new method by analysing human monocyte-mediated cytotoxic and phagocytic cell killing of IGROV1 ovarian tumour cells by the ovarian tumour antigen-specific anti-folate binding protein monoclonal antibody, MOv18 IgE.

Asp-120 locates Zn2 for optimal metallo-beta-lactamase activity

Metallo-beta-lactamases are zinc-dependent hydrolases that inactivate beta-lactam antibiotics, rendering bacteria resistant to them. Asp-120 is fully conserved in all metallo-beta-lactamases and is central to catalysis. Several roles have been proposed for Asp-120, but so far there is no agreed consensus. We generated four site-specifically substituted variants of the enzyme BcII from Bacillus cereus as follows: D120N, D120E, D120Q, and D120S. Replacement of Asp-120 by other residues with very different metal ligating capabilities severely impairs the lactamase activity without abolishing metal binding to the mutated site. A kinetic study of these mutants indicates that Asp-120 is not the proton donor, nor does it play an essential role in nucleophilic activation. Spectroscopic and crystallographic analysis of D120S BcII, the least active mutant bearing the weakest metal ligand in the series, reveals that this enzyme is able to accommodate a dinuclear center and that perturbations in the active site are limited to the Zn2 site. It is proposed that the role of Asp-120 is to act as a strong Zn2 ligand, locating this ion optimally for substrate binding, stabilization of the development of a partial negative charge in the beta-lactam nitrogen, and protonation of this atom by a zinc-bound water molecule.

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.

Structural insights into autoreactive determinants in thyroid peroxidase composed of discontinuous and multiple key contact amino acid residues contributing to epitopes recognized by patients' autoantibodies

Thyroid peroxidase (TPO) is a major autoantigen of thyroid autoimmune disease, and the autoantibodies that are produced recognize two immunodominant regions (IDR) of the molecule, termed IDR-A and -B. Based upon our structural model of the TPO ectodomain, we recently identified R225 and K627 as key residues in IDR-A and -B, respectively. We report here on rational mutagenic investigations to identify additional residues surrounding R225 and K627 that affect the binding of recombinant human Fabs (rhFabs) specific for each IDR. Two residues R646 and D707 were identified from the model as promising surface-exposed amino acids adjacent to R225. Similarly, residues E604, D620, D624, and D630 were identified in the vicinity of K627. These residues were substituted in different combinations of single, double, and multiple mutations, and stably expressed in Chinese hamster ovary cells. By fluorescence-activated cell sorting and capture ELISA, we found that R225A, R646A, and D707N specifically led to the loss of binding of IDR-A rhFabs, whereas E604A, D620R, K627G, and D630N specifically abrogated the binding of IDR-B rhFabs. Further supportive evidence of the importance of these residues for the IDR epitopes was obtained with patients' sera. We conclude that R646 and D707 together with R225 constitute a functional epitope within IDR-A, and that residues E604, D620, and D630, together with K627, constitute a functional epitope within IDR-B. This identification of key residues within the autoreactive epitopes will help in understanding the structural basis for the breakdown of immune tolerance to TPO in thyroid autoimmune disease.

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.

An attempt to define allergen-specific molecular surface features: a bioinformatic approach

Allergens are proteins that elicit T helper lymphocyte type 2 (Th2) responses culminating in IgE antibody production and allergic disease. However, we have no answer to the fundamental question of why certain proteins are allergens, while others are not. We hypothesized that analysis of the surface of diverse allergens may reveal common structural features which might enable them to be recognized as Th2-inducing antigens by cells of the innate immune system. We have therefore used the ConSurf server to search for allergen-specific motifs. This has enabled us to identify residue conservation patterns in the homologues of Ara t 8 (plant profilin), Act c 1 (actinidin), Bet v 1 (plant pathogenesis-related protein) and Ves v 5 (venom allergen). The results demonstrate the presence of allergen-specific patches consisting of an unusually high proportion of surface-exposed hydrophobic residues. The patches that have been identified may represent molecular patterns recognizable by cells of the innate immune system.

Biased use of VH5 IgE-positive B cells in the nasal mucosa in allergic rhinitis

BACKGROUND

IgE antibody-producing B cells are enriched in the nasal mucosa in patients with allergic rhinitis because of local class switching to IgE. The expressed IgE VH genes also undergo somatic hypermutation in situ to generate clonal families. The antigenic driving force behind these events is unknown.

OBJECTIVE

To examine the possible involvement of a superantigen in allergic rhinitis, we compared the variable (VH) gene use and patterns of somatic mutation in the expressed IgE heavy-chain genes in nasal biopsy specimens and blood from allergic patients and the IgA VH use in the same biopsy specimens and also those from nonallergic controls.

METHODS

We extracted mRNA from the nasal biopsy specimens of 13 patients and 4 nonallergic control subjects and PBMCs from 7 allergic patients. IgE and IgA VH regions were RT-PCR amplified, and the DNA sequences were compared with those of control subjects. We constructed a molecular model of VH5 to locate amino acids of interest.

RESULTS

We observed a significantly increased frequency of IgE and IgA VH5 transcripts in the nasal mucosa of the allergic patients compared with the normal PBMC repertoire. Within IgE and IgA VH5 sequences in the nasal mucosa, the distribution of replacement amino acids was skewed toward the immunoglobulin framework regions. Three of 4 nonintrinsic hotspots of mutation identified in the VH5 sequences were in framework region 1. The hotspots and a conserved VH5-specific framework residue form a tight cluster on the surface of VH5.

CONCLUSION

Our results provide evidence for the activity of a superantigen in the nasal mucosa in patients with allergic rhinitis.

Homology modelling of transferrin-binding protein A from Neisseria meningitidis

Neisseria meningitidis, a causative agent of bacterial meningitis, obtains transferrin-bound iron by expressing two outer membrane located transferrin-binding proteins, TbpA and TbpB. TbpA is thought to be an integral outer membrane pore that facilitates iron uptake. Evidence suggests that TbpA is a useful antigen for inclusion in a vaccine effective against meningococcal disease, hence the identification of regions involved in ligand binding is of paramount importance to design strategies to block uptake of iron. The protein shares sequence and functional similarities to the Escherichia coli siderophore receptors FepA and FhuA, whose structures have been determined. These receptors are composed of two domains, a 22-stranded beta-barrel and an N-terminal plug region that sits within the barrel and occludes the transmembrane pore. A three-dimensional TbpA model was constructed using FepA and FhuA structural templates, hydrophobicity analysis and homology modelling. TbpA was found to possess a similar architecture to the siderophore receptors. In addition to providing insights into the highly immunogenic nature of TbpA and allowing the prediction of potentially important ligand-binding epitopes, the model also reveals a narrow channel through its entire length. The relevance of this channel and the spatial arrangement of external loops, to the mechanism of iron translocation employed by TbpA is discussed.