Characterization of monoclonal antibodies directed against the alpha-subunit of the human IgE high-affinity receptor

IgE regulates the expression of smMLCK in human airway smooth muscle cells

Previous studies have shown that enhanced accumulation of contractile proteins such as smooth muscle myosin light chain kinase (smMLCK) plays a major role in human airway smooth muscle cells (HASM) cell hypercontractility and hypertrophy. Furthermore, serum IgE levels play an important role in smooth muscle hyperreactivity. However, the effect of IgE on smMLCK expression has not been investigated. In this study, we demonstrate that IgE increases the expression of smMLCK at mRNA and protein levels. This effect was inhibited significantly with neutralizing abs directed against FcεRI but not with anti-FcεRII/CD23. Furthermore, Syk knock down and pharmacological inhibition of mitogen activated protein kinases (MAPK) (ERK1/2, p38, and JNK) and phosphatidylinositol 3-kinase (PI3K) significantly diminished the IgE-mediated upregulation of smMLCK expression in HASM cells. Taken together, our data suggest a role of IgE in regulating smMLCK in HASM cells. Therefore, targeting the FcεRI activation on HASM cells may offer a novel approach in controlling the bronchomotor tone in allergic asthma.

Accelerated dissociation of IgE-FcεRI complexes by disruptive inhibitors actively desensitizes allergic effector cells

BACKGROUND

The remarkably stable interaction of IgE with its high-affinity receptor FcεRI on basophils and mast cells is critical for the induction of allergic hypersensitivity reactions. Because of the exceptionally slow dissociation rate of IgE-FcεRI complexes, such allergic effector cells permanently display allergen-specific IgE on their surface and immediately respond to allergen challenge by releasing inflammatory mediators. We have recently described a novel macromolecular inhibitor that actively promotes the dissociation of IgE from FcεRI through a molecular mechanism termed facilitated dissociation.

OBJECTIVE

Here we assessed the therapeutic potential of this non-immunoglobulin-based IgE inhibitor E2_79, a designed ankyrin repeat protein (DARPin), as well as a novel engineered biparatopic DARPin bi53_79, and directly compared them with the established anti-IgE antibody omalizumab.

METHODS

IgE-FcεRI complex dissociation was analyzed in vitro by using recombinant proteins in ELISA and surface plasmon resonance, ex vivo by using human primary basophils with flow cytometry, and in vivo by using human FcεRI α-chain transgenic mice in a functional passive cutaneous anaphylaxis test.

RESULTS

We show that E2_79-mediated removal of IgE from primary human basophils fully abrogates IgE-dependent cell activation and release of proinflammatory mediators ex vivo. Furthermore, we report that omalizumab also accelerates the dissociation of IgE from FcεRI, although much less efficiently than E2_79. Using the biparatopic IgE targeting approach, we further improved the disruptive potency of E2_79 by approximately 100-fold and show that disruptive IgE inhibitors efficiently prevent passive cutaneous anaphylaxis in mice expressing the human FcεRI α-chain.

CONCLUSION

Our findings highlight the potential of such novel IgE inhibitors as important diagnostic and therapeutic tools for management of allergic diseases.

DARPins against a functional IgE epitope

The monoclonal anti-IgE antibody omalizumab (Xolair is mostly used for the treatment of severe allergic asthma. However, the requirement of high doses and suboptimal cost-effectiveness limits the use of the treatment. Here we propose to use a new drug format based on non-immunoglobulin structures, potentially offering increased clinical efficacy while being more cost-effective. For this purpose, DARPins™ (designed ankyrin repeat proteins) against the constant heavy chain region of IgE have been isolated. DARPins were binding to IgE with high specificity and affinities in the low nanomolar range. Selected DARPins antagonized the interaction between IgE and its high-affinity receptor in inhibition assays. Furthermore, anti-IgE DARPins were shown to inhibit proinflammatory mediator release from rat basophilic leukemia cells expressing human high-affinity IgE receptors with higher efficacy than the monoclonal anti-IgE antibody omalizumab. DARPins may thus represent promising future drug candidates for the treatment of allergy.

Proinflammatory and Th2 cytokines regulate the high affinity IgE receptor (FcepsilonRI) and IgE-dependant activation of human airway smooth muscle cells

Background

The high affinity IgE receptor (FcεRI) is a crucial structure for IgE-mediated allergic reactions. We have previously demonstrated that human airway smooth muscle (ASM) cells express the tetrameric (αβγ2) FcεRI, and its activation leads to marked transient increases in intracellular Ca²⁺ concentration, release of Th-2 cytokines and eotaxin-1/CCL11. Therefore, it was of utmost importance to delineate the factors regulating the expression of FcεRI in human (ASM) cells.

Methodology/Principal Findings

Incubation of human bronchial and tracheal smooth muscle (B/TSM) cells with TNF-α, IL-1β or IL-4 resulted in a significant increase in FcεRI-α chain mRNA expression (p<0.05); and TNF-α, IL-4 enhanced the FcεRI-α protein expression compared to the unstimulated control at 24, 72 hrs after stimulation. Interestingly, among all other cytokines, only TNF-α upregulated the FcεRI-γ mRNA expression. FcεRI-γ protein expression remained unchanged despite the nature of stimulation. Of note, as a functional consequence of FcεRI upregulation, TNF-α pre-sensitization of B/TSM potentially augmented the CC (eotaxin-1/CCL11 and RANTES/CCL5, but not TARC/CCL17) and CXC (IL-8/CXCL8, IP-10/CXCL10) chemokines release following IgE stimulation (p<0.05, n = 3). Furthermore, IgE sensitization of B/TSM cells significantly enhanced the transcription of selective CC and CXC chemokines at promoter level compared to control, which was abolished by Lentivirus-mediated silencing of Syk expression.

Conclusions/Significance

Our data depict a critical role of B/TSM in allergic airway inflammation via potentially novel mechanisms involving proinflammatory, Th2 cytokines and IgE/FcεRI complex.

Regulation of the high affinity IgE receptor (Fc epsilonRI) in human neutrophils: role of seasonal allergen exposure and Th-2 cytokines

The high affinity IgE receptor, FcεRI, plays a key role in the immunological pathways involved in allergic asthma. Previously we have demonstrated that human neutrophils isolated from allergic asthmatics express a functional FcεRI, and therefore it was of importance to examine the factors regulating its expression. In this study, we found that neutrophils from allergic asthmatics showed increased expression of FcεRI-α chain surface protein, total protein and mRNA compared with those from allergic non asthmatics and healthy donors (p<0.001). Interestingly, in neutrophils isolated from allergic asthmatics, FcεRI-α chain surface protein and mRNA expression were significantly greater during the pollen season than outside the pollen season (n = 9, P = 0.001), an effect which was not observed either in the allergic non asthmatic group or the healthy donors (p>0.05). Allergen exposure did not affect other surface markers of neutrophils such as CD16/FcγRIII or IL-17R. In contrast to stimulation with IgE, neutrophils incubated with TH2 cytokines IL-9, GM-CSF, and IL-4, showed enhanced FcεRI-α chain surface expression. In conclusion, these results suggest that enhanced FcεRI expression in human neutrophils from allergic asthmatics during the pollen season can make them more susceptible to the biological effects of IgE, providing a possible new mechanism by which neutrophils contribute to allergic asthma.

Mast cell-derived proteases control allergic inflammation through cleavage of IgE

BACKGROUND

Cross-linking of mast cell-bound IgE releases proinflammatory mediators, cytokines, and proteolytic enzymes and is a key event in allergic inflammation.

OBJECTIVE

We sought to study the effect of proteases released on effector cell activation on receptor-bound IgE and their possible role in the regulation of allergic inflammation.

METHODS

Using molar ratios of purified recombinant tryptase and human IgE, we studied whether tryptase can cleave IgE. Similar experiments were performed with mast cell lysates in the presence or absence of protease inhibitors. IgE cleavage products were detected in supernatants of allergen cross-linked, cultivated mast cells and in tissue fluids collected from patients' skin after IgE-mediated degranulation. The effects of protamine, an inhibitor of heparin-dependent proteases on IgE-mediated allergic in vivo skin inflammation in human subjects were studied.

RESULTS

We show that beta-tryptase, a major protease released during mast cell activation, cleaves IgE. IgE degradation products were detected in tryptase-containing tissue fluids collected from sites of allergic inflammation. The biologic significance of this mechanism is demonstrated by in vivo experiments showing that protease inhibition enhances allergic skin inflammation.

CONCLUSION

We suggest that IgE cleavage by effector cell proteases is a natural mechanism for controlling allergic inflammation.

A high-affinity natural autoantibody from human cord blood defines a physiologically relevant epitope on the FcepsilonRIalpha

Natural Abs represent the indigenous immune repertoire and are thus present at birth and persist throughout life. Previously, human autoantibodies to the alpha domain of the high-affinity IgE receptor (FcepsilonRIalpha) have been isolated from Ab libraries derived from normal donors and patients with chronic urticaria. To investigate whether these anti-FcepsilonRIalpha Abs are present in the germline repertoire, we constructed a phage Fab display library from human cord blood, which represents the naive immune repertoire before exposure to exogenous Ags. All isolated clones specific to the FcepsilonRIalpha had the same sequence. This single IgM Ab, named CBMalpha8, was strictly in germline configuration and had high affinity and functional in vitro anaphylactogenic activity. Inhibition experiments indicated an overlapping epitope on the FcepsilonRIalpha recognized by both CBMalpha8 and the previously isolated anti-FcepsilonRIalpha Abs from autoimmune and healthy donors. This common epitope on FcepsilonRIalpha coincides with the binding site for IgE. Affinity measurements demonstrated the presence of Abs showing CBMalpha8-like specificity, but with a significantly lower affinity in i.v. Ig, a therapeutic multidonor IgG preparation. We propose a hypothesis of escape mutants, whereby the resulting lower affinity IgG anti-FcepsilonRIalpha Abs are rendered less likely to compete with IgE for binding to FcepsilonRIalpha.

Expression of Fc(epsilon)-receptors by human acute myelogenous leukemia (AML) blasts: studies of high- and low- (CD23) affinity receptor expression and the effects of IgE-mediated receptor ligation on functional AML blast characteristics

Acute myelogenous leukemia (AML) blasts derived from 20 patients were examined for expression of high- (Fc(epsilon)RI) and low-affinity (Fc(epsilon)RII, CD23) IgE Fc(epsilon)-receptors. Fc(epsilon)RI expression was not detected for any patient. In contrast, expression of CD23 (at least 15% of the blasts stained positive) was detected for 6 out of the 20 patients. Acute lymphoblastic leukemia (ALL) blasts derived from 12 patients did not express CD23 (<1% positive cells for all patients). The functional effects of Fc(epsilon)R-receptor ligation were also examined for 20 patients, including the five patients with highest CD23 expression (30-55% positive cells) and five patients with verified low CD23 expression (<or=7% positive cells). The presence of IgE during in vitro culture altered the functional characteristics (spontaneous and cytokine-dependent proliferation, colony formation, cytokine secretion, or spontaneous in vitro apoptosis) of AML blasts for a subset of both CD23-positive patients and certain patients with very low CD23 expression. This last observation suggests that Fc(epsilon)R are either expressed at a very low level or receptors are expressed only by a minor cell subset for these patients. We conclude that functional Fc(epsilon)R can be expressed by human AML cells.

Inhibition of antigen-induced mediator release from IgE-sensitized cells by a monoclonal anti-Fc epsilon RI alpha-chain receptor antibody: implications for the involvement of the membrane-proximal alpha-chain region in Fc epsilon RI-mediated cell activation

The interaction between human IgE and its high affinity receptor, FcepsilonRI, is a critical event in mediating the allergic response. Aggregation of the alpha-chain of FcepsilonRI (FcepsilonRIalpha) occurs via cross-linking of receptor-bound IgE by Ag, resulting in cell activation and the release of mediators of hypersensitivity. Recently, we mapped the epitopes of two anti-FcepsilonRIalpha mAbs, 15/1 and 5H5F8. In contrast to 15/1, mAb 5H5F8 does not inhibit IgE binding to FcepsilonRIalpha. Here we demonstrate both 5H5F8 binding to FcepsilonRI(+) cells as well as a high level of IgE binding to 5H5F8-saturated cells. At the same time 5H5F8 strongly inhibits hexosaminidase release and Ca(2+) flux after Ag triggering from human IgE-sensitized RBL-2H3 cells stably transfected with human FcepsilonRIalpha. Further, 5H5F8 and its Fab inhibit sulfidoleukotriene and histamine release from primary human peripheral blood leukocytes, including cells bearing endogenous IGE: Furthermore, we confirm that 5H5F8 maps to a linear peptide sequence in close proximity to the cell membrane. Two chemically synthesized peptides containing the 5H5F8 epitope sequence PREKY were selected for detailed analysis of 5H5F8 and 5H5F8 Fab binding and were found to produce K(d) values of similar magnitude to that observed for binding to recombinant FcepsilonRIalpha. These peptides may prove useful as targets for the identification of antagonists of FcepsilonRIalpha-mediated biological activity. Moreover, our data indicate that FcepsilonRIalpha-mediated activation may involve a novel alpha-chain epitope in an early step of the cell-triggering pathway leading to cellular activation.

Enzymatic modification of heparan sulfate on a biochip promotes its interaction with antithrombin III

A heparan sulfate glycosaminoglycan chain, biotinylated at its reducing-end, was bound to a streptavidin-coated biochip. Surface plasmon resonance spectroscopy showed a low affinity interaction with antithrombin III (ATIII) when it was flowed over a surface containing heparan sulfate. ATIII bound tightly with high affinity when the same surface was enzymatically modified to using 3-O-sulfotransferase isoform 1 (3-OST-1) in the presence of 3'-phosphoadenosine 5'-phosphosulfate (PAPS). The 3-OST-1 enzyme is involved in heparan sulfate biosynthesis and introduces a critical 3-O-sulfo group into this glycosaminoglycan affording the appropriate pentasaccharide sequence capable of high affinity binding to ATIII. This experiment demonstrates the specific structural modification of a glycosaminoglycan bound to a biochip using a biosynthetic enzyme, suggesting a new approach to rapid screening glycosaminoglycan-protein interactions.

Molecular basis for nonanaphylactogenicity of a monoclonal anti-IgE antibody

IgE Abs mediate allergic responses by binding to specific high affinity receptors (FcepsilonRI) on mast cells and basophils. Therefore, the IgE/FcepsilonRI interaction is a target for clinical intervention in allergic disease. An anti-IgE mAb, termed BSW17, is nonanaphylactogenic, although recognizing IgE bound to FcepsilonRI, and interferes with binding of IgE to FcepsilonRI. Thus, BSW17 represents a candidate Ab for treatment of IgE-mediated disorders. By panning BSW17 against random peptide libraries displayed on phages, we defined mimotopes that mimic the conformational epitope recognized on human IgE. Two types of mimotopes, one within the Cepsilon3 and one within the Cepsilon4 domain, were identified, indicating that this mAb may recognize either a large conformational epitope or eventually two distinct epitopes on IgE. On the basis of alignments of the two mimotopes with the human IgE sequence, we postulate that binding of BSW17 to the Cepsilon3 region predominantly blocks binding of IgE to FcepsilonRI, leading to neutralization of IgE. Moreover, binding of BSW17 to the Cepsilon4 region may explain how BSW17 recognizes FcepsilonRI-bound IgE, and binding to this region may also interfere with degranulation of IgE sensitized cells (basophils and mast cells). As a practical application of these findings, mimotope peptides coupled to a carrier protein may be used for the development of a peptide-based anti-allergy vaccine by induction of anti-IgE Abs similar to the current approach of using humanized nonanaphylactogenic anti-IgE Abs as a passive vaccine.

Mimicry of human IgE epitopes by anti-idiotypic antibodies

According to Jerne's network hypothesis, the binding site of an anti-idiotypic antibody also represents the internal image of an epitope present on a foreign, or even a self antigen. In recent years, antigen mimicry has been defined at the molecular level for some xeno-antigens. However, until now there has been no demonstration of structural mimicry between a human anti-idiotypic antibody and a self structure. To address this question, we used human IgE as the self structure and a well-defined anti-human IgE mAb (BSW17). We describe the isolation of two anti- idiotypic antibodies specific for the anti-IgE antibody BSW17 from a non-immune human Fab phage display library. Interestingly, these two anti-idiotypic antibodies mimic the same molecular surface region as a previously described IgE peptide mimotope isolated by panning on BSW17, but they cover a much larger epitope on the IgE molecule. Accordingly, immunisation of rabbits with the two anti-idiotypic antibodies induced high-affinity antibodies with the same characteristics as BSW17. Thus, our data demonstrate that it is possible to isolate anti-idiotypic antibodies derived from the human genome without the need for hyperimmunization, and confirm Jerne's hypothesis that both foreign antigens and self structures can be mimicked by our own immunoglobulins.

The membrane-proximal part of FcepsilonRIalpha contributes to human IgE and antibody binding--implications for a general structural motif in Fc receptors

The high affinity receptor for human IgE (FcepsilonRI) on tissue mast cells and blood basophils is responsible for immediate hypersensitivity reactions. Binding of human IgE (hIgE) to FcepsilonRI has been shown to be mediated via three independent regions in the extracellular part of the alpha-subunit of FcepsilonRI (ecFcepsilonRIalpha). By site-directed mutagenesis we investigated the contribution of amino acids within the ecFcepsilonRIalpha FG loop (residues Lys154-Leu165) to binding to hIgE and two monoclonal anti-FcepsilonRIalpha antibodies (15/1, 5H5/F8). The mutated receptors were expressed and secreted from eukaryotic cells as amino-terminal fusion to HSA. We show that the proposed loop region contributes partly to hIgE binding and that the epitope of mAb 15/1, which inhibits hIgE/FcepsilonRIalpha interaction, maps to this region whereby a single W156A mutation results in complete loss of mAb 15/1 binding. In contrast, hIgE binding is not affected by the W156A mutation indicating that different amino acid residues within the loop are recognized by the mAbs 15/1 and hIgE. MAb 5H5/F8 does not recognize a receptor mutant truncated to Ile170. By screening a random dodecapeptide library displayed on bacterial flagella the epitope for mAb 5H5/F8 was mapped to P173REKY177 whereas one of the 15/1 binding clones displayed a peptide with an amino acid sequence homologous to Leu158-lle167. Based on the epitopes identified for the inhibitory mAb 15/1 and the non-inhibitory mAb 5H5F8 and on binding data obtained with polyclonal antisera raised against two ecFcepsilonRIalpha peptides, we propose a structural element in the membrane proximal part of ecFcepsilonRIalpha which forms a 3D structure which might facilitate specific and efficient attachment of hIgE.