The structural basis of human IgE-Fc receptor interactions

Fc Fragment of IgE Receptor Ig (FCER1G) acts as a key gene involved in cancer immune infiltration and tumour microenvironment

Although recent studies have revealed the relationship between Fc Fragment of IgE Receptor Ig (FCER1G) and human tumours, there is still a lack of a more comprehensive pan-cancer analysis of FCER1G as an immune-related gene. In this study, we investigated the expression pattern and prognostic value of FCER1G based on multiple databases. Subsequently, we further explored the role of FCER1G in tumour proliferation and metastasis, as well as its genomic alterations and DNA methylation levels, we next assessed the association between FCER1G and the immune infiltrating cells of the tumour microenvironment in different cancers and verified it by immunohistochemical staining. The correlation between FCER1G and immune checkpoint genes expression and its predictive power in the immune checkpoint blockade treatment cohorts were used to evaluate the importance of FCER1G in immunotherapy. Enrichment analysis of FCER1G-associated partners was also performed. In addition, we substantiated the expression of FCER1G in specific cell types of different tumours using single-cell RNA sequencing data from different databases. Our research results showed that FCER1G is up-regulated in most tumour. Positive associations were found between FCER1G expression and tumour prognosis, proliferation, and metastasis, we also found that FCER1G is closely related to the tumour microenvironment and tumour immunity. Moreover, FCER1G-associated partners were enriched in pathways associated with neutrophils activation. Finally, we confirmed that FCER1G was mainly expressed in monocyte/macrophages of the tumour microenvironment. In conclusion, our findings provided a comprehensive understanding of FCER1G in oncogenesis and tumour immunology among various tumours and demonstrated its potential value in prognosis prediction and tumour immunotherapy.

Development of small molecules to target the IgE:FcεRI protein-protein interaction in allergies

The protein-protein interaction (PPI) between IgE and its high-affinity receptor (FcεRI) is a key component of the allergic response. Inhibiting the IgE:FcεRI PPI is an attractive strategy for therapeutic intervention and the development of allergy treatments. This PPI has been validated as a viable target by the monoclonal anti-IgE antibody omalizumab (Xolair(®)), which has demonstrated clinical efficacy when prescribed to treat moderate-to-severe asthma and hay fever, but small molecules would be a more convenient form of treatment. Cyclic peptides, small proteins and a natural product have all been developed to target the IgE:FcεRI PPI, and these will be discussed in this review. Targeting the IgE:FcεRI complex with small molecules presents various challenges, some of which are inherent in all PPI targets but some of which are unique to this system, which presents great opportunities for the development of new therapeutics for the treatment of allergies.

The high affinity IgE receptor (FcεRI) expression and function in airway smooth muscle

The airway smooth muscle (ASM) is no longer considered as merely a contractile apparatus and passive recipient of growth factors, neurotransmitters and inflammatory mediators signal but a critical player in the perpetuation and modulation of airway inflammation and remodeling. In recent years, a molecular link between ASM and IgE has been established through Fc epsilon receptors (FcεRs) in modulating the phenotype and function of these cells. Particularly, the expression of high affinity IgE receptor (FcεRI) has been noted in primary human ASM cells in vitro and in vivo within bronchial biopsies of allergic asthmatic subjects. The activation of FcεRI on ASM cells suggests a critical yet almost completely ignored network which may modulate ASM cell function in allergic asthma. This review is intended to provide a historical perspective of IgE effects on ASM and highlights the recent updates in the expression and function of FcεRI, and to present future perspectives of activation of this pathway in ASM cells.

Synthesis and incorporation into cyclic peptides of tolan amino acids and their hydrogenated congeners: construction of an array of A-B-loop mimetics of the Cε3 domain of human IgE

The disruption of the human immunolobulin E-high affinity receptor I (IgE-FcεRI) protein-protein interaction (PPI) is a validated strategy for the development of anti asthma therapeutics. Here, we describe the synthesis of an array of conformationally constrained cyclic peptides based on an epitope of the A-B loop within the Cε3 domain of IgE. The peptides contain various tolan (i.e., 1,2-biarylethyne) amino acids and their fully and partially hydrogenated congeners as conformational constraints. Modest antagonist activity (IC(50) ∼660 μM) is displayed by the peptide containing a 2,2'-tolan, which is the one predicted by molecular modeling to best mimic the conformation of the native A-B loop epitope in IgE.

IgE modulates neutrophil survival in asthma: role of mitochondrial pathway

The high-affinity IgE receptor (FcepsilonRI) has recently been reported to be expressed by neutrophils in atopic asthmatic individuals, leading to speculations that IgE could influence biological functions of these cells. In this study, we demonstrate that monomeric human IgE delayed spontaneous apoptosis of primary human neutrophils from atopic asthmatics in vitro. This effect was not dependent on FcepsilonRI cross-linking or autocrine release of soluble mediators; however, it was associated with increased expression of the antiapoptotic myeloid cell leukemia-1 protein, retention of the proapoptotic molecule Bax in the cytoplasm, decreased release of Smac from mitochondria, and reduced caspase-3 activity. Taken together, our results indicate that in vitro IgE can delay programmed cell death of neutrophils from allergic asthmatics and this may possibly contribute to neutrophilic inflammation in atopic asthma.

The Importance of Lys-352 of Human Immunoglobulin E in FcϵRII/CD23 Recognition

The interaction of immunoglobulin E (IgE) with its low affinity receptor (FcepsilonRII/CD23) plays a central role in the initiation and regulation of type I hypersensitivity responses. We have previously identified the importance of amino acid residues in the A-B loop of the Cepsilon3 domain of human IgE and implicated a region close to the glycosylation site at asparagine 371 as contributing to IgE-CD23 interaction. These residues were now targeted by site-directed mutagenesis. The IgE-CD23 interaction was assessed by semiquantitative flow cytometry. Replacement of the entire Cepsilon3 A-B loop (residues 341-356) with the homologous rat IgE sequence resulted in complete loss of human CD23 recognition, as did replacement of residues 346-353, indicating that class-specific effector residue(s) are contained within these eight amino acids. Lysine 352 within the A-B loop was identified as contributing directly to human CD23 interaction. Mutation to the rodent homologue glycine or glutamate resulted in a significant reduction in binding compared with native IgE, whereas conservative substitution with arginine effected a small, but statistically significant, enhancement of CD23 binding. Mutation of the Cepsilon3 glycosylation site at asparagine 371 to threonine or glutamine did not significantly affect CD23 recognition. Our results yield new insights into the structural basis of the hIgE-CD23 interaction and hold promise for the rational design of drugs that can manipulate IgE-mediated regulation of the allergic response.

Human antibody–Fc receptor interactions illuminated by crystal structures

Immunoglobulins couple the recognition of invading pathogens with the triggering of potent effector mechanisms for pathogen elimination. Different immunoglobulin classes trigger different effector mechanisms through interaction of immunoglobulin Fc regions with specific Fc receptors (FcRs) on immune cells. Here, we review the structural information that is emerging on three human immunoglobulin classes and their FcRs. New insights are provided, including an understanding of the antibody conformational adjustments that are required to bring effector cell and target cell membranes sufficiently close for efficient killing and signal transduction to occur. The results might also open up new possibilities for the design of therapeutic antibodies.

Solid-phase synthesis of an A-B loop mimetic of the Cepsilon3 domain of human IgE: macrocyclization by Sonogashira coupling

The solid-phase synthesis of a cyclic peptide containing the 21-residue epitope found in the A-B loop of the Cepsilon3 domain of human immunoglobulin E has been carried out. The key macrocyclization step to form the 65-membered ring is achieved in approximately 15% yield via an "on-resin" Sonogashira coupling reaction which concomitantly installs a diphenylacetylene amino acid conformational constraint within the loop.

Expression of a 4-(hydroxy-3-nitro-phenyl) acetyl (NP) specific equi-murine IgE antibody that mediates histamine release in vitro and a type I skin reaction in vivo

Due to characteristic clinical signs, immunoglobulins of isotype E (IgE) are believed to be involved in several allergic diseases of the horse. To date, closer investigations have been hampered by the fact that neither purified equine IgE nor anti-equine IgE monoclonal antibodies were available for IgE isotype determination. As an approach to solve this problem, we constructed a stable cell line (EqE6) that expresses recombinant equi-murine IgE specific for 4-(hydroxy-3-nitro-phenyl) acetyl (NP). Biochemical analysis of the purified protein revealed a highly glycosilated IgE monomer of approximately 230,000 Da. The biological ability of the NP-IgE to mediate histamine release after crosslinking with antigen was demonstrated in vitro using equine blood leucocytes. In vivo, the intradermal application of NP-IgE followed by antigen crosslinking induced a type I hypersensitivity skin reaction in horses. Both results indicate that the recombinant NP-IgE contains an intact and functional Fc(epsilon) RI binding site and mediates effector functions in equine basophils and cutaneous mast cells. This equi-murine IgE can be used for the production of IgE-specific polyclonal and monoclonal antibodies. In addition, the NP specificity allows the antigen-specific activation of equine Fc(epsilon)-receptor-expressing cells, such as mast cells and basophils. This property could be used to investigate IgE-mediated mechanisms for a better understanding of equine type I allergic diseases.

Production of humanized antibody against human high-affinity IgE receptor in a serum-free culture of CHO cells, and purification of the Fab fragments

We describe the preparation of Fab fragments of a humanized anti-human high-affinity IgE receptor (Fc epsilonRIalpha) antibody potentially useful for treatment of IgE-mediated allergic diseases. IgE-binding capacities of sixteen combinations of light and heavy chains of four recombinant anti-Fc epsilonRIalpha antibodies, chimeric CRA2, humanized CRA2, chimeric CRA4, and humanized CRA4, were compared. A combination in which both chains were of humanized CRA2 had the highest activity. Stable transfectant clones of four kinds of host cells expressing recombinant antibodies were established. CHO-K1 cells were the most productive. Serum-free media suitable for culture of the stable CHO-transfectant clones were screened. The concentration of the humanized CRA2, which the most productive clone secreted into the chosen serum-free medium, was approximately 100 microg/ml. A procedure for the purification of the antibody, papain-digestion, and purification of Fab fragments was established. The highly purified humanized Fab fragments are suitable for use to examine their in vivo activity and immunogenicity in primates.