Biological properties of a recombinant human immunoglobulin epsilon-chain fragment

Heat denaturation of the antibody, a multi-domain protein

The antibody is one of the most well-studied multi-domain proteins because of its abundance and physiological importance. In this article, we describe the effect of the complex, multi-domain structure of the antibody on its denaturation by heat. Natural antibodies are composed of 6 to 70 immunoglobulin fold domains, and are irreversibly denatured at high temperatures. Although the separated single immunoglobulin fold domain can be refolded after heat denaturation, denaturation of pairs of such domains is irreversible. Each antibody subclass exhibits a distinct heat tolerance, and IgE is especially known to be heat-labile. IgE starts unfolding at a lower temperature compared to other antibodies, because of the low stability of its CH3 domain. Each immunoglobulin domain starts unfolding at different temperatures. For instance, the CH3 domain of IgG unfolds at a higher temperature than its CH2 domain. Thus, the antibody has a mixture of folded and unfolded structures at a certain temperature. Co-existence of these folded and unfolded domains in a single polypeptide chain may increase the tendency to aggregate which causes the inactivation of the antibody.

IgE antibodies, FcεRIα, and IgE-mediated local anaphylaxis can limit snake venom toxicity

BACKGROUND

Type 2 cytokine-related immune responses associated with development of antigen-specific IgE antibodies can contribute to pathology in patients with allergic diseases and to fatal anaphylaxis. However, recent findings in mice indicate that IgE also can enhance defense against honeybee venom.

OBJECTIVE

We tested whether IgE antibodies, IgE-dependent effector mechanisms, and a local anaphylactic reaction to an unrelated antigen can enhance defense against Russell viper venom (RVV) and determined whether such responses can be influenced by immunization protocol or mouse strain.

METHODS

We compared the resistance of RVV-immunized wild-type, IgE-deficient, and Fcer1a-deficient mice after injection of a potentially lethal dose of RVV.

RESULTS

A single prior exposure to RVV enhanced the ability of wild-type mice, but not mice lacking IgE or functional FcεRI, to survive challenge with a potentially lethal amount of RVV. Moreover, IgE-dependent local passive cutaneous anaphylaxis in response to challenge with an antigen not naturally present in RVV significantly enhanced resistance to the venom. Finally, we observed different effects on resistance to RVV or honeybee venom in BALB/c versus C57BL/6 mice that had received a second exposure to that venom before challenge with a high dose of that venom.

CONCLUSION

These observations illustrate the potential benefit of IgE-dependent effector mechanisms in acquired host defense against venoms. The extent to which type 2 immune responses against venoms can decrease pathology associated with envenomation seems to be influenced by the type of venom, the frequency of venom exposure, and the genetic background of the host.

A beneficial role for immunoglobulin E in host defense against honeybee venom

Allergies are widely considered to be misdirected type 2 immune responses, in which immunoglobulin E (IgE) antibodies are produced against any of a broad range of seemingly harmless antigens. However, components of insect venoms also can sensitize individuals to develop severe IgE-associated allergic reactions, including fatal anaphylaxis, upon subsequent venom exposure. We found that mice injected with amounts of honeybee venom similar to that which could be delivered in one or two stings developed a specific type 2 immune response that increased their resistance to subsequent challenge with potentially lethal amounts of the venom. Our data indicate that IgE antibodies and the high affinity IgE receptor, FcεRI, were essential for such acquired resistance to honeybee venom. The evidence that IgE-dependent immune responses against venom can enhance survival in mice supports the hypothesis that IgE, which also contributes to allergic disorders, has an important function in protection of the host against noxious substances.

L-selectin expression on neutrophils from allergic patients

BACKGROUND

L-selectin (CD62L) is an adhesion molecule involved in leucocyte attachment to endothelium at sites of inflammation, and it has been demonstrated that L-selectin is rapidly shed after neutrophil activation. Recently, it has been reported that there is increasing evidence of neutrophil participation in asthma and the allergic process.

OBJECTIVE

The present study was designed to determine whether an IgE-dependent mechanism can modulate L-selectin expression on the surface of neutrophils. Moreover, we analyse the potential implication of intracellular signal-transduction pathways and whether specific immunotherapy (IT), glucocorticoids and antihistamines might regulate this process.

METHODS

Peripheral blood neutrophils from three groups of donors (asthmatic group without IT treatment, IT-treated asthmatic group and healthy group) were used. Cells were challenged in vitro with the specific allergen that produced clinical symptoms in asthmatic patients and also with the allergen to which the patients were not sensitive. Neutrophils from healthy donors were also challenged with allergens. Expression of CD62L on the neutrophil surface was analysed by flow cytometry, and soluble CD62L (sCD62L) in culture supernatant by ELISA. In an attempt to discover which IgE receptor is involved, we also challenged the neutrophils with monoclonal antibody to FcepsilonRI, FcepsilonRII (CD23) and galectin-3 receptors.

RESULTS

When neutrophils from allergic patients were challenged with specific allergens that produce clinical allergy symptoms, L-selectin was down-regulated from the surface of those cells, accompanied by a concomitant up-regulation of soluble L-selectin in the supernatant. The challenge with antibodies against FCepsilonRI, FCepsilonRII (CD23) and galectin-3, induces down-modulation of L-selectin on the surface of the neutrophils in all three cases. Calphostin C, wortmannin and manoalide attenuated CD62L down-regulation, suggesting the potential implication of protein kinase C, phosphatidylinositol 3-kinase and phospholipase A(2) in the process. IT and glucocorticoids modulated allergen-dependent CD62L down-regulation, whereas antihistamines (terfenadine, loratadine and cetirizine) or nedocromil sodium did not affect the shedding of L-selectin.

CONCLUSIONS

We present evidence that the neutrophil surface expression of CD62L can be modulated by an allergen-dependent mechanism. The modulation of CD62L expression can be induced through the three receptors of IgE. This process can be affected by IT.

IgE-Dependent Activation of T cells by Allergen in Atopic Dermatitis: Pathophysiologic Relevance

The importance of interactions between allergen and IgE in allergen-mediated activation of T lymphocytes from patients with atopic dermatitis (AD) is unclear. A role for this interaction is implied by past evidence for IgE-facilitated presentation of allergen to T cells, but this phenomenon has only been demonstrated in specific in vitro systems biased to maximize the effect. It is therefore not known whether the process is relevant in patients. We now show that the responses to allergen of unmodified peripheral blood mononuclear cells (PBMC) from individual AD patients are significantly greater in the presence of fresh, unheated, IgE-containing autologous serum than the same serum heated under IgE-denaturing conditions or specifically depleted of IgE by immunoprecipitation. In six independent experiments, 59%-67% of the maximal in vitro PBMC response to allergen was found to be dependent upon the presence of IgE in autologous serum used at 5% final concentration. These data provide the first evidence that sufficient amounts of allergen-specific IgE and allergen-reactive T cells occur concomitantly in the blood of individual AD patients to allow IgE-enhanced T cell responses to allergen. We conclude that IgE-enhanced T cell responses are pathophysiologically relevant and a therapeutic target in AD.

The crystal structure of IgE Fc reveals an asymmetrically bent conformation

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

Recognition of pathogenically relevant house dust mite hypersensitivity in adults with atopic dermatitis: a new approach?

BACKGROUND

The pathogenic importance of the ubiquitous house dust mite, Dermatophagoides pteronyssinus (Dp), in atopic dermatitis is unclear.

OBJECTIVE

We aimed to explore the relevance of Dp hypersensitivity in adult patients with atopic dermatitis by using an in vivo topical challenge method and in vitro assays for T-cell reactivity.

METHODS

Dp and control skin prick test solutions were applied to the cubital fossae of 20 patients twice daily for 4 days; the severity of dermatitis and pruritus in the challenge sites were determined before and after testing. The same solutions were used in PBMC proliferation assays that included 10% fresh, autologous serum, the latter aimed at maximizing IgE facilitated allergen presentation.

RESULTS

Although most patients had markedly elevated Dp-specific serum IgE levels, only 6 of 20 patients developed increases in cubital fossa dermatitis severity and pruritus scores that were greater at sites of application of Dp solution than at control sites. In addition, PBMC proliferation in response to Dp solution in the presence of autologous serum was significantly greater in the in vivo challenge-positive patients than in those who did not respond to challenge. A subgroup of patients (7/20) also developed transient but pronounced contact urticaria at sites of Dp application.

CONCLUSION

These findings suggest that hypersensitivity to Dp might be clinically relevant in approximately one third of the adult atopic dermatitis population studied. They also point to methods of identifying patients who might respond to house dust avoidance measures.

The novel human IgE epsilon heavy chain, epsilon tailpiece, is present in plasma as part of a covalent complex

Several splice variants of the secreted human epsilon heavy chain have previously been identified by reverse transcription-PCR. The heavy chain of one isoform, IgE tailpiece, differs from the originally identified IgE, IgE classic, by the replacement of the 2 carboxy-terminal amino acids by 8 novel amino acids including a carboxy-terminal cysteine residue. Recombinant human epsilon tailpiece and epsilon classic heavy chains were expressed and secreted as H2L2 monomers in Sp2/0 murine myeloma cells. We have investigated the in vitro function and in vivo occurrence of epsilon tailpiece heavy chains using receptor binding assays, granule release assays, flow cytometry, half-life studies, immunoprecipitation, SDS-PAGE, two-dimensional SDS-PAGE, and Western blotting. IgE tailpiece and IgE classic exhibited similar in vivo half-lives in BALB/c mice, bound the human high- and low-affinity IgE receptors with similar affinities and triggered equivalent levels of high affinity IgE receptor induced degranulation. In humans, IgE classic is present as a 190 kD circulating protein in vivo. In contrast, we found that in plasma epsilon tailpiece was primarily present as part of covalent complexes of approximately 300 and 338 kD. Dissociation of the complexes revealed that two species of epsilon tailpiece heavy chains were present therein and surprisingly, these in vivo derived epsilon tailpiece heavy chains were approximately 5 and 10 kD smaller than the recombinant expressed epsilon tailpiece or epsilon classic heavy chains. These results show that epsilon tailpiece is present in novel covalent complexes in humans.

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.

The glycosylation and structure of human serum IgA1, Fab, and Fc regions and the role of N-glycosylation on Fcα receptor interactions

The human serum immunoglobulins IgG and IgA1 are produced in bone marrow and both interact with specific cellular receptors that mediate biological events. In contrast to IgA1, the glycosylation of IgG has been well characterized, and its interaction with various Fc receptors (Fc Rs) has been well studied. In this paper, we have analyzed the glycosylation of IgA1 and IgA1 Fab and Fc as well as three recombinant IgA1 molecules, including two N-glycosylation mutants. Amino acid sequencing data of the IgA1 Fc O-glycosylated hinge region indicated that O-glycans are located at Thr228, Ser230, and Ser232, while O-glycan sites at Thr225 and Thr236 are partially occupied. Over 90% of the N-glycans in IgA1 were sialylated, in contrast to IgG, where < 10% contain sialic acid. This paper contains the first report of Fab glycosylation in IgA1, and (in contrast to IgG Fab, which contains only N-linked glycans) both N- and O-linked oligosaccharides were identified. Analysis of the N-glycans attached to recombinant IgA1 indicated that the Cα 2 N-glycosylation site contained mostly biantennary glycans, while the tailpiece site, absent in IgG, contained mostly triantennary structures. Further analysis of these data suggested that processing at one Fc N-glycosylation site affects the other. Neutrophil Fcα R binding studies, using recombinant IgA1, indicated that neither the tailpiece region nor the N-glycans in the C alpha 2 domain contribute to IgA1-neutrophil Fcα R binding. This contrasts with IgG, where removal of the Fc N-glycans reduces binding to the Fcγ R. The primary sequence and disulfide bond pattern of IgA1, together with the crystal structures of IgG1 Fc and mouse IgA Fab and the glycan sequencing data, were used to generate a molecular model of IgA1. As a consequence of both the primary sequence and S-S bond pattern, the N-glycans in IgA1 Fc are not confined within the inter-α-chain space. The accessibility of the Cα 2 N-glycans provides an explanation for the increased sialylation and galactosylation of IgA1 Fc over that of IgG Fc N-glycans, which are confined in the space between the two Cγ 2 domains. This also suggests why in contrast to IgG Fc, the IgA1 N-glycans are not undergalactosylated in rheumatoid arthritis.

Engineering of Fc(1) and Fc(3) from human immunoglobulin G to analyse subclass specificity for staphylococcal protein A

A system for production of recombinant Fc fragments of human IgG in Escherichia coli has been developed to allow for structural and functional studies of human Fc. The genes for the Fc fragments of human IgG subclasses 1 and 3, designated Fc(1) and Fc(3), were cloned from a human spleen cDNA library. The interactions to Staphylococcal protein A (SpA), a bacterial Fc receptor, that interacts with human IgG-Fc(1), but not with human IgG-Fc(3), were analyzed. To corroborate the involvement of amino acid residues in Fc, responsible for these differences in binding, two Fc variants were constructed; Fc(1(3)) and Fc(3(1)), each containing an isotypic dipeptide substitution. Production levels in E. coli of 1-10 mg/l of secreted Fc proteins, covalently linked as dimers, were routinely obtained. SpA-binding analyses of all four Fc variants using biosensor technology, showed that Fc(1) and Fc(3(1)) interact with SpA, while Fc(3) and Fc(1(3)) lack detectable SpA binding. The rendered SpA binding of the Fc variant Fc(3(1)), is concluded to result from the introduced dipeptide substitution (R435H, F436Y). The results demonstrate that the Fc expression system efficiently can be used in Fc engineering.

Targeted elimination of cells expressing the high-affinity receptor for IgE (Fc epsilon RI) by a Pseudomonas exotoxin-based chimeric protein

The interaction between IgE and its high-affinity receptor Fc epsilon RI found on mast cells and basophils is the primary effector pathway in allergic response. To achieve a targeted elimination of cells expressing Fc epsilon RI receptors, we constructed a chimeric protein in which a Fc fragment of mouse IgE is attached to a truncated form of Pseudomonas exotoxin (PE). To prepare the targeting moiety, we used a DNA sequence corresponding to amino acids 301-437, representing 30 residues of domain 2 and domain 3 of the mouse IgE constant region. This sequence was fused at the 5' of a cDNA encoding PE40, a truncated form of PE lacking the cell binding domain. The chimeric protein, termed Fc(2'-3)-PE40, was expressed in Escherichia coli and partially purified. The protein is highly cytotoxic to mouse mast cell lines and bone marrow-derived primary mast cells. This cytotoxicity is specific, as it could be blocked upon addition of whole IgE. Moreover, the protein had no effect on other cell lines of hemopoietic origin. The Fc(2'-3)-PE40 chimeric protein offers a new approach to the treatment of allergic disorders.

Polymorphism of IgE gene in chronic urticaria

Our aim was to determine whether heterogeneity of the IgE (C epsilon) gene could be demonstrated in patients with chronic urticaria (CU). We performed Southern blots on DNA extracted from peripheral blood leucocytes of 20 patients with CU and 20 normal controls. Using a human C epsilon gene probe containing four exons of the constant segment of the IgE heavy chain, we showed the presence of a restriction fragment length polymorphism of the C epsilon gene segment in four of 20 patients with CU, but in none of 20 normal subjects. Family studies of two propositi revealed the presence of this C epsilon gene polymorphism in other family members. Our data show that a proportion of patients with CU have a polymorphism of the constant segment of the C epsilon gene. Further studies of this polymorphic gene fragment indicated that it was derived from duplication of the 3rd and 4th exons of functional C epsilon gene and was very likely to be located close to this gene. It raises the possibility that polymorphism of the functional C epsilon gene may affect expression of this gene. This could possibly lead to dysfunctional IgE-receptor interaction with consequent alteration in mediator release.

Anti-human IgE monoclonal antibodies recognizing epitopes related to the binding sites of high and low affinity IgE receptors

Anti-human IgE monoclonal antibodies (mAbs) were produced and eight clones recognizing epitopes on native IgE were selected. Epitopes were mapped by a competitive inhibition enzyme-linked immunosorbent assay, Western blotting and a multi-pin peptide technology. Four sites (one each in the C epsilon 1, C epsilon 2, C epsilon 2/C epsilon 3 junction and C epsilon 3) were recognized by the mAbs. The relationship between the four epitopes and the binding sites of high and low affinity IgE receptors (Fc epsilon RI and Fc epsilon RII, respectively) was studied using a monovalent Fab fragment of each mAb as a binding inhibitor. The IgE-Fc epsilon RII binding was clearly inhibited by the mAb recognizing the C epsilon 2/C epsilon 3 junction, suggesting that Fc epsilon RII binds to a rather limited area around the C epsilon 2/C epsilon 3 junction. The IgE-Fc epsilon RI binding, on the other hand, was scarcely inhibited by any single mAb. However, the binding was inhibited when the epitope in C epsilon 2 was blocked simultaneously with that at the C epsilon 2/C epsilon 3 junction or with that in C epsilon 3, indicating that these three distinct epitopes are related to the Fc epsilon RI binding sites. When these three epitopes were shown in the stereograph of human IgE, the Fc epsilon RI binding area was spread largely on the groove side between C epsilon 2 and C epsilon 3 domains. These results suggest that Fc epsilon RI acquires the high affinity through multiple bindings.

Antibody engineering using Escherichia coli as host

The expression of immunoglobulin fragments with antigen binding activities in E. coli is now routinely possible. Using such expression systems, Fv, Fab, and scFv fragments and single VH domains can be produced as secreted proteins in yields of the order of milligrams per liter. Moreover, expression systems are being rapidly developed for the production of antibody scFv or Fab fragments by repertoire cloning followed by selection. Diverse repertoires of genes encoding VH and VL domains can be isolated by the PCR and cloned for expression using these systems, which allow the selection of recombinants that produce fragments with the desired antigen binding specificities. This technology is rapidly evolving and, coupled with the development of systems for the random mutagenesis and selection of higher-affinity antibody fragments, could, in the longer term, provide an alternative rapid route to hybridoma technology.

Human IgE mediates stimulus secretion coupling in rat basophilic leukemia cells transfected with the alpha chain of the human high-affinity receptor

Cell surface expression of the alpha chain of the human (h) Fc epsilon receptor type 1 (Fc epsilon R1) can be observed following stable transfection of RBL-2H3 cells with the h alpha chain of the Fc epsilon R1 complex. Association and dissociation constants for hIgE are similar to those previously reported for the ligand and its receptor. The demonstration of mediator release following the cross-linking of hFc epsilon R1 alpha by hIgE and antigen or anti-hFc epsilon R1 alpha antibody suggests that this system will assist the identification of structural motifs and specific amino acids that are involved in the generation of the signal(s) that will initiate and/or propagate the secretion of mediators from mast cells and basophils.

Blocking of passive sensitization of human mast cells and basophil granulocytes with IgE antibodies by a recombinant human epsilon-chain fragment of 76 amino acids

The recombinant peptide corresponding to residues 301-376 at the junction of constant regions 2 and 3 of the human IgE epsilon chain blocked the in vivo passive sensitization of human skin mast cells and in vitro sensitization of human basophil granulocytes with human IgE antibodies. An injection of the recombinant peptide or E myeloma protein into normal skin sites 1 hr before sensitization with an allergic serum blocked passive sensitization. In this system, approximately 10-fold higher molar concentration of the recombinant peptide than E myeloma protein was required for 50% inhibition of Prausnitz-Küstner reactions. When the mononuclear cells of two normal individuals were preincubated with the recombinant peptide or E myeloma protein for 15 min before passive sensitization with the same allergic serum and the cells were challenged with an optimal concentration of an antigen, approximately 11- to 13-fold higher concentration of the recombinant peptide than E myeloma protein was required for 50% inhibition of antigen-induced histamine release. Further studies with several recombinant peptides indicated that amino acid resides 363-376 in the Fc epsilon-chain fragment are not essential for binding of the peptide to Fc epsilon-chain receptor I.

The mast cell binding site on human immunoglobulin E

Antibodies of the immunoglobulin E isotype sensitize mast cells and basophils for antigen-induced mediator release by binding through the Fc portion to a high-affinity receptor (Fc epsilon R1, Ka = 10(9)M-1) on the cell surface causing the clinical manifestations of type I hypersensitivity. As the amino acid sequence of the human epsilon chain is now known, attempts have been made to map the Fc epsilon R1 binding site on IgE to a fragment smaller than Fc epsilon using proteolytic cleavage products, none of which proved to be active. Cleavage between the C epsilon 2 and C epsilon 3 domains released two inactive fragments, suggesting that the junction between these segments could be important in receptor binding. This region is protected against protease digestion in the rat IgE complex with the receptor of rat basophilic leukaemia cells. Here we report the mapping of the mast cell receptor binding site on human IgE to a sequence of 76 amino acids at the C epsilon 2/C epsilon 3 junction. Recombinant peptides containing this sequence inhibit passive sensitization of skin mast cells in vivo and sensitize mast cells to degranulation by anti-IgE in vitro almost as efficiently as a myeloma IgE. Fragments containing the separate domains are inactive. Additional sequences are required for rapid assembly of fragments into disulphide-linked dimers, suggesting that a single chain can form the active site. In a three-dimensional model of the human Fc epsilon, the two identical segments are far apart. Each folds to generate a cleft between the C epsilon 2 and C epsilon 3 domains on the surface of the Fc epsilon. The docking of IgE on to mast cells could take place within this cleft.

Purification and characterization of a recombinant human IgE Fc epsilon fragment lacking the C4 domain

Complementary DNA of human IgE Fc epsilon fragment (residues 226-480) lacking the C epsilon 4 domain was expressed in Escherichia coli and the product was purified by immunoaffinity chromatography on a monoclonal antibody (E12 0.02)-Affi-Gel 10 column. About 1.8 mg of an apparent dimer and 5.9 mg of a monomer were obtained from 65 g E. coli cells with 9.3% recovery. The purified products were found to lack more than half of the COOH-terminal portion of the C epsilon 3 domain. The apparent dimer showed high immunological specific activity (3.6 x 10(6) U/mg protein) comparable to that of natural human IgE when measured by commercial human IgE determination kits.