IgE based therapeutics are an emerging modality in cancer treatment

Abstract

Targeting human diseases using antibodies is a rapidly evolving area of research that could transform healthcare. Currently, various diseases, including cancer, are treated by immunotherapy using the IgG class of antibodies. Recently, efforts have been made to broaden the range of available treatments by utilizing the distinct features of other antibody classes. Notably, the IgE antibody has shown encouraging results in restricting tumor growth with the first therapeutic antibody MOv18 IgE showing promising results in clinical trials, with good tolerance and safety in patients with ovarian carcinoma. However, several challenges remain. This review discusses recent developments in the engineering of the IgE antibody for cancer therapeutics, focusing on improving manufacturability and reducing immunogenicity by modifying IgE glycosylation complexity. Additionally, bispecific formats of IgE are being established to engage multiple targets simultaneously. Building on the recent studies in these areas, the ongoing research on IgE therapeutics has provided potential opportunities for clinical development and as a therapy in the future. These outcomes and advancements, along with a better clinical safety profile, may increase the treatment options for cancer patients in the future together with existing and approved IgG therapies.

Introduction

Immunotherapy uses the body’s immune system to fight different diseases. To date, several immunotherapy modalities such as monoclonal antibodies, immunomodulators, cytokines, cancer vaccines, chimeric antigen receptor (CAR) T-cell therapy, oncolytic viruses, and checkpoint inhibitors, are used to manage and treat cancers [1]. Of these, antibody-based immunotherapy is the most widely used. This transformative approach stimulates or improves the immune system to find, attack, and destroy cancer cells in the body. In humans, humoral immunity is mediated by five different antibody classes, namely immunoglobulin A (IgA), immunoglobulin D (IgD), immunoglobulin E (IgE), immunoglobulin G (IgG), and immunoglobulin M (IgM), each displaying distinct properties and mode of action. Till recently, antibodies belonging to the IgG class have been predominantly used in immunotherapy. This is primarily because IgG is a well-characterized human antibody with extensive information available on its expression and biophysical characteristics. IgGs are found in a monomeric state with only one glycosylated site. In addition, they bind to neonatal Fc receptors (FcRn), are abundant in the serum, and play a crucial role in adaptive immunity [2]. For clinical application, Rituximab was the first IgG-based monoclonal antibody approved for the treatment of non-Hodgkin lymphoma and later autoimmune diseases [3].

Currently, over 100 IgG antibodies have been approved or are in clinical trials by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) (https://db.antibodysociety.org/db0/results_therapeutic). Of these, Pembrolizumab (Keytruda; Merck & Co.), which stimulates the body’s immune system to destroy cancer by blocking programmed cell death-1 (PD-1) expressed on active T cells, is the top-selling immunotherapeutic drug [4]. Nevertheless, there has been a growing interest in exploring different antibody isotypes for immunotherapy, particularly with IgA [5, 6], IgM [7, 8], and IgE [9] gaining attention.

This review focuses on the recent progress made towards developing IgE antibodies for immunotherapy and highlights the engineering strategies to enhance its manufacturability and efficacy in engaging multiple target antigens simultaneously.

Superior properties of IgE over IgG

IgE-based immunotherapy is a rapidly emerging area of research. Although IgE is classically associated with allergies and parasitic infections, certain properties make it attractive for treating cancers. For example, IgE shows higher binding affinity towards its cognate receptor, the “high-affinity” FcεRI, expressed on various types of immune cells such as mast cells, basophils, monocytes, macrophages, eosinophils, and dendritic cells (Fig. 1c) as compared to IgG’s affinity for FcγR receptor, thus leading to prolonged immune surveillance [10, 11]. The avidity of IgE towards its “low affinity” CD23 receptors, expressed on B cells and T cells (Fig. 1c) is compared to the affinity of IgG to its receptors. Tumor microenvironment (TME) contains pro-tumoral or tumor tolerant sub-types of effector cells such as monocytes, mast cells, dendritic cells, macrophages, and eosinophils which could be potentially activated by therapeutic IgE with higher efficacy [10]. Additionally, in contrast with IgG [12, 13], inhibitory receptors for IgE remain unknown. Table 1 summarizes the key differences between IgG and IgE antibodies.

Fig. 1.

Schematic representation of the IgE structure with the N-glycan sites shown as open circles. The conserved N275 residue is marked as red circle (a); Conserved N-glycan site of IgE (N275) and IgG (N297) is shown by an arrow (b); immune cell types expressing IgE receptors (c); and role of effector cells and IgE antibody in tumor destruction (d).

Table 1.

Table showing the key differences between IgE and IgG antibodies

	IgE antibody	IgG antibody
Subclasss	No sub class	IgG1, IgG2, IgG3,IgG4
Cognate Receptors	FcεRI, CD23	FcγRI, FcγRIIa, FcγRIIb, FcγRIIc, FcγRIIIa, FcγRIIIb
Inhibitory receptor	not known	FcγRIIb
Can cross placenta	No	Yes
Half-life	2–3 days (serum)	2–3 weeks (serum)
	2–3 weeks (tissue)	2–3 days (tissue)
Key effector cells	Macrophages	Macrophages
	Monocytes	Monocytes
	Dendritic cells	NK cells
	Eosinophils	Neutrophis
		Dendritic cells
Mechanism of action	Antibody dependent cellular cytotoxicity (ADCC)	Antibody dependent cellular cytotoxicity (ADCC)
	Antibody dependent cellular phagocytosis (ADCP)	Antibody dependent cellular phagocytosis (ADCP)
	Release of toxic mediators	Complement dependent cytotoxicity (CDC)
	Macrophage repolarisation	Opsonization
	Immune modulation	Immune modulation
	longer immune surveillance

Owing to the greater amount of serum IgG levels [14], higher doses of therapeutic IgG are used for treatment because of the increased competition between the therapeutic IgG and endogenous IgG for receptor binding [15, 16]. The advantage of IgE therapeutics lies in the low IgE serum levels which results in reduced competition between the endogenous and therapeutic IgE for receptor occupancy. Additionally, increased local IgE retention in the tissues can prolonged immunosurveillance [17]. With lower endogenous IgE levels in the blood IgE could facilitate treatment with lower therapeutic doses and less frequent administration compared to IgG therapy [9]. The ability of IgE antibody to mediate Antibody-Dependent Cellular Cytotoxicity (ADCC) and Antibody-Dependent Cellular Phagocytosis (ADCP) by activating various effector cells, such as monocytes, macrophages, and eosinophils, also enhances its therapeutic effects [18–21] (Fig. 1d).

Data from preclinical studies demonstrated the superiority of IgE over IgG when both antibodies were used to target the same antigen [22–24]. IgE-mediated tumor restriction has been shown for ovarian cancer and melanomas [24, 25].

Role of IgE glycosylation in manufacturability

Antibodies are glycoproteins with glycans attached to the asparagine (N) amino acid residue within the consensus sequence N-X (any amino acids except proline)-T (Threonine) in the heavy chain. The glycans are attached to the heavy chain during the post-translational modifications in the endoplasmic reticulum-Golgi network of the cell. The types of glycan and its composition are dependent on the choice of the expression host and culture conditions [26]. The glycosylation of therapeutic antibodies raises safety concerns such as immunogenicity [27, 28], can impact product consistency, influence solubility, stability, pharmacokinetics [29, 30] and effector functions [31]. Therefore, regulatory agencies like the Food and Drug administration (FDA) and the European Medicines Agency (EMA) adhere to stringent guidelines requiring thorough characterization and identification of glycans before an antibody is submitted for approval [32–34].

The IgG antibody is glycosylated at a single conserved asparagine residue (N297) in the Fc region of the heavy chain. Previous studies have elucidated the factors regulating glycosylation and have provided an extensive understanding of the structural and functional role of IgG glycans [35]. Empirical evidence has shown that each component of the complex glycan at the N297 position influences the structure and function of IgG. Specific glycan features such as galactosylation [36, 37], fucosylation [38], and sialylation [39, 40] significantly affect the functional activity of the IgG molecule. The glycan components, therefore, contribute to different IgG functions ranging from inhibitory and anti-inflammatory effects to complement activation and promotion of antibody-dependent cellular cytotoxicity (ADCC) [31, 41–44].

Although IgE antibodies are extensively glycosylated, the role of glycosylation in influencing the structure and function of IgE antibodies remained poorly understood until recently. Glycopeptide mass spectrometry analysis of a recombinant IgE expressed in HEK293T cells revealed N-glycosylation at seven specific asparagine residues (N21, N49, N99, N146, N252, N264, and N275) located in the heavy chain (Fig. 1a). The analysis showed the presence of complex glycans at all asparagine sites, except at the N275 position which predominantly contains oligomannosidic structures [45]. Notably, the N275 residue is conserved across all mammalian species with the IgE N275 residue homologous to the N297 residue in IgG heavy chain (Fig. 1b) [46, 47]. Furthermore, glycan analysis of IgE antibodies by liquid chromatography-electrospray ionization ion trap mass spectrometry (LC-ESI-IT-MS/MS) from the serum of healthy, myeloma, and hyperimmune donors revealed that the asparagine residue at N264 position remains unoccupied [48]. A similar observation was reported with glycan analysis of IgE antibody expressed in plants [49, 50].

A recent study systematically investigated the structural and functional roles of all seven N-glycosylation sites of IgE through amino acid substitutions, either individually or altogether [51]. This comprehensive analysis highlighted the essential role of the conserved asparagine residue (N275) in the stability and secretion of the recombinant IgE antibody. Substituting asparagine at the N275 position with aspartic acid (IgE’ N275D) and other glycosylation sites with glutamine significantly reduced the antibody yield while the presence of N275 residue alone (IgE’ N275) was not sufficient to restore antibody secretion and yield (Fig. 2). Interestingly, deglycosylated IgE glycovariant (IgE’ N275D) exhibited enhanced efficacy, challenging the notion that glycosylation is essential for IgE effector function (Fig. 2). This study also revealed the impact of glycosylation on thermostability with deglycosylated IgE antibody (IgE’ N275D) showing a reduced thermal tolerance [51].

Fig. 2.

Formats of therapeutic IgE molecules that can be adopted in the future clinical applications against cancers

Investigations into the functional role of IgE glycosylation are largely linked to its association with allergic conditions. However, the role of glycosylation in IgE-FcεRI receptor interaction is ambiguous. For example, amino acid substitution of IgE Fc region highlight the importance of N275 glycosylation in FcεRI receptor binding [46, 52]. However, another study suggested the dispensability of IgE glycosylation by enzymatic digestion of IgE Fc fragments in FcεRI binding [53]. Glycosylation of IgE might also contribute to its solubility as deglycosylated IgE-formed aggregates [53], possibly due to exposure of hydrophobic regions upon removal of the glycans.

A mutational analysis of all seven asparagine residues in the heavy chain of IgE demonstrated the importance of N-glycosylation in its interaction with FcεRI receptors as well as initiation of anaphylaxis [45]. The study showed that the substitution of the conserved asparagine residue with glutamine amino acid (N275Q) in the IgE heavy chain mitigated anaphylaxis, while the overall effector function of IgE depends on the glycans at both the N252 and N275 residues. Furthermore, the glycans in the CH1 domain of the Fab region of IgE antibody (N21, N49, N99) play a role in modulating interaction with the target. Overall, the study highlights the critical role of IgE glycosylation at N275 in allergic reactions [45]. Further investigations by the same group revealed elevated levels of sialic acid in IgE from individuals with peanut allergies, implicating sialic acid as a key regulator in allergic diseases [54]. For example, administration of asialylated IgE attenuated allergic reactions suggesting the crucial role of sialic acid in initiating anaphylaxis. They hypothesized that the reduced anaphylaxis may have been caused by exposure to an inhibitory glycan in asialylated IgE that attenuates FcεRI signaling [54]. However, a recent study by the same group reported that sialic acid on IgE molecule has a minimal impact on FcεRI interactions but does influence the magnitude of anaphylaxis [55]. In the future, a detailed functional analysis of the IgE site-specific sialic acid and other glycan components will be essential for interpreting the biology of anaphylactic reactions which will help design a better therapeutic molecule with diminished risk of anaphylaxis.

The half-life of IgE in serum is up to 2–3 days and 2–3 weeks in tissues [56, 57]. Interestingly, sialic acid was found to be crucial for increased half-life of IgE as removal of sialic acid reduces the half-life of the antibody in vivo [54, 58]. This observation aligns with the finding that allergic individuals have higher IgE levels in their serum for longer durations, correlating with increased sialic acid content in allergen-specific IgE [54]. Interestingly, a recent study demonstrated that the removal of sialic acid from a Chondroitin Sulfate Proteoglycan 4 IgE (CSPG4 IgE) antibody does not affect target binding, effector cell recognition, or degranulation. However, it increases the binding of CSPG4 IgE to low-affinity CD23 receptor as compared with FcεRI [59]. In addition to sialylation, the role of galactosylation in IgE function has been examined which shows an abundance of terminal galactose in nonatopic individuals, suggesting that the presence of galactose might contribute to reduced degranulation [54].

The development of IgE-based therapy is in its nascent stage owing largely to the knowledge gap in understanding the mechanistic role of IgE glycosylation in regulating its structure and function [60]. This provides an opportunity to explore the functional role of the IgE glycans and the associated amino acid residues which would not only provide valuable insights into the biology of this antibody but also help in the management of human conditions such as anaphylaxis. Future IgE therapeutics can be designed with or without glycans based on the therapeutic purpose and implications associated with the glycans.

Bispecific antibody

Bispecific antibodies are designed to target two different antigens simultaneously, and have been used to redirect T cells to tumor sites, and deliver payloads to target tumor sites [61]. Based on the therapeutic objectives and outcomes, bispecific antibodies are either designed with or without Fc fragment. Importantly, heterodimerization of two different heavy and light chains is necessary for the production of a bispecific antibody. This is challenging since the molecule is asymmetric due to the presence of at least two different variable regions. The expression of heavy and light chains for a bispecific antibody in a cell, in principle, will result in 16 combinations (10 different molecules) with only one being bispecific and the remaining ones being non-functional or monospecific [62]. Various engineering strategies have been implemented to increase heterodimer formation and reduce homodimer formation using the IgG platform [63, 64]. Bispecific antibodies are superior to conventional monoclonal antibodies offering an opportunity to investigate different antibody classes in this context. Indeed, nine bispecific antibodies of the IgG class were approved for cancer therapy between 2021 and 2023 [61] highlighting the rapid growth and advancements in this field.

In recent years, attempts have been made to generate bispecific IgE antibody (Fig. 2). Vukovic et al. provided a proof of concept for the development of a bispecific IgE antibody that binds to both target and effector cells without detectable homodimer formation [65]. This study demonstrated co-binding of a recombinant bispecific IgE antibody to the prostate-specific membrane antigen (PSMA) and epidermal growth factor receptor (EGFR) using knob-into-hole (KiH) strategy with Efab domain substitution [66] and leucine zipper (LZ)-mediated pairing. Thus, the bispecific IgE molecule shows dual specificity and is capable of eliciting Fc-mediated effector functions. Importantly, this bispecific IgE was designed with one arm containing a Fab domain and another arm containing a small domain (sD), which not only allows it to be distinguished from the monospecific IgE owing to its reduced size but also avoid light chain mispairings. Binding studies revealed comparable EC50 values for both bispecific IgEs targeting PSMA and EGFR stably expressed in CHO cells. In-vitro ADCC assays using human peripheral CD11b + cells revealed that the bispecific IgE antibody generated by the KiH strategy induced approximately 40% cell death which was significantly greater than that induced by the IgG counterpart (25% cell death) [65]. These findings were further validated by an in vivo target tumor killing assay using the bispecific KiH IgE, human CD11b + effector cells and Rag2−/−γc−/− immunodeficient mice. The results indicate effective targeted cell death with significant reduction in tumor to effector ratio [65]. Overall, this study highlights the preservation of dual antigen-binding properties and effector functions in bispecific IgE which results in superior cytotoxicity as compared to that of bispecific IgG [65].

While it provides a proof of concept about bispecific IgE antibodies with no detectable homodimer formation in this specific model, this observation may not hold true for other heterodimer IgE antibody formats harbouring similar KiH mutations in the heavy chain (manuscript in preparation). However, the development of a bispecific IgE represents a significant advancement in the development of IgE antibody for cancer therapy and opens new avenues for harnessing the superior properties of IgE antibodies with enhanced therapeutic outcomes by designing new heterodimer molecules. Exploring alternative heterodimerization strategies could provide additional insights into bispecific IgE engineering for customized therapeutic applications.

IgE, immune cells and solid cancers

Cancer is a complex and multifactorial disease. Different patients exhibit varying responses to antibody therapy due to multiple factors, such as genetic makeup, environmental influences, dietary choices, and, more importantly, the immunotype of the cancer [67–69]. The variability in the response presents a greater challenge for therapy than the development of resistance in tumors.

Cancer is broadly classified into three categories based on immune cell infiltration. The first is the tumor-inflamed condition, marked by the presence of T cells and myeloid lineage effector cells. The second category is the immune excluded condition, wherein T cells are present in the stromal region. The last category is the immune desert condition characterized by the absence of effector cells in the tumor. Given the complexity of cancer, multiple approaches are required to achieve consistent outcomes along with antibody therapy [70, 71]. Therefore, there is a need to understand the interactions among effector cells and tumor microenvironment (TME) in different cancer types while designing new effective therapies.

With the presence of IgE receptors on different types of immune cells, such as mast cells, monocytes, macrophages, basophils, dendritic cells, T cells, and B cells, it can be hypothesized that IgE-based treatments may be more effective in tumor-inflamed environments due to the presence of tumor-resident effector cells. This highlights the importance of designing treatment according to the complex interplay among effector cells, the TME, and the immune profile of solid cancer.

Mast cells, dendritic cells, eosinophils, monocytes, and macrophages are key immune cells that mediate the effector function of a therapeutic antibody (Fig. 1d). Mast cells can infiltrate the pre- or intra-tumoral region of the TME and engage in crosstalk with other immune cells [72–75]. Mast cells are commonly associated with IgE-mediated allergic reactions and express high-affinity FcεRI receptors on their surface. This might indicate the role of mast cells in enhancing the tumor-suppressive role of the IgE antibody, which warrants further investigation. Eosinophils together with monocytes, are involved in IgE-dependent tumor killing [76]. Interestingly, mast cells and eosinophils also display both pro-tumorigenic and anti-tumorigenic roles in different types of cancer [77, 78]. Therefore, a detailed investigation is needed to understand the role of eosinophils and mast cells in IgE-mediated tumor killing in different types of cancer. Monocytes differentiate into two different macrophage phenotypes, M1 and M2, with the M1 macrophages involved in tumor killing, and the M2 macrophages regulating the immune response. Anti-tumor IgE has been shown to re-educate tumor-associated macrophages (TAMs) to the M1 phenotype [79], thus enhancing cell death. Dendritic cells mediate tumor antigen presentation in presence of IgE antibody [80], priming cytotoxic T lymphocytes [81–83] thereby conferring robust anti-tumor immunity. Presently, among the immune cells, eosinophils, monocytes, and macrophages are involved in anti-tumor activity of IgE molecule (Fig. 1d) [21, 24, 76, 79].

IgE-based immunotherapy for solid tumors

The ability of IgE to prevent parasitic infections or mediate allergies can be exploited to inhibit tumors. The first report of a tumor suppressive role of IgE antibody was provided by Nagy et al. who demonstrated that mice injected with a lethal dose of H2712 mammary carcinoma exhibited tumor restriction when treated with IgE antibodies against anti-murine mammary tumor virus [84]. Additionally, epidemiological studies have suggested an inverse correlation between atopic patients and cancer, particularly ovarian cancer [85–87]. Since then, there have been significant advancements in the development of multiple engineered IgE antibodies for therapeutic purposes including tumor-associated antigens (Fig. 3). Particularly, two IgE antibodies, designed for cancer therapy, have shown promising preclinical results.

Fig. 3.

Timeline showing the progression of IgE therapeutic antibody from 1991 to 2024

MOv18 IgE

The first therapeutic monoclonal antibody of the IgE class approved for clinical trials is MOv18 IgE. It is a chimeric molecule consisting of a variable region derived from mice and a constant region of human origin. MOv18 IgE is engineered to target the folic acid receptor, a marker protein expressed abundantly in ovarian cancer cells [88]. The MOv18 IgE molecule has been characterized functionally, and its effectiveness has been validated through extensive in vitro and in vivo studies [25, 89].

Multiple studies have revealed a crucial role of MOv18 IgE in anti-tumor efficacy. For example, immunohistochemistry analysis revealed infiltration of monocytes in the stromal area of the tumor upon MOv18 IgE treatment [90]. Notably, when IGROV1 ovarian cancer cells and human monocytes were incubated with MOv18 IgE and MOv18 IgG antibodies in an ADCC assay, greater cytotoxicity was observed in presence of MOv18 IgE than MOv18 IgG [90]. Subsequent analysis showed the involvement of two different pathways in MOv18 IgE-dependent tumor killing, with monocytes engaging in ADCP and ADCC via CD23 and FcεRI receptors, respectively. This study also emphasized the involvement of eosinophils in ADCC activity [76].

Spicer et al. investigated the anaphylactic potential of MOv18 IgE. Their study demonstrated overall safety for most ovarian cancer patients administered with MOv18 IgE, except for one patient who was positive for basophil activation test (BAT) and experienced anaphylaxis upon the first exposure to MOv18 IgE treatment [9]. Thus, BAT became mandatory for screening patients before initiating therapy. This finding suggests that patients with a positive BAT are unsuitable for IgE-based therapeutics. Surprisingly, folic acid receptor alpha (FRα) or autoantibodies against FRα or galactose-α − 1,3-galactose (alpha-GAL) were absent in the BAT-positive patient. Few adverse skin effects, such as diffuse patchy macular erythematous rash accompanied by urticaria were predominant in cancer patients treated with MOv18 IgE. However, the study failed to establish a link between urticaria condition in the absence of anaphylaxis and anaphylactic reactions in patients with a positive basophil activation test (BAT). Although Phase 1 clinical trial results have been encouraging this might not be a suitable reference for superior clinical performance. However, MOv18 IgE which was approved for phase 1 clinical trial in 2016 has now entered phase 1b trial (NCT06547840) to evaluate dosage, safety, tolerability, and therapeutic outcomes. These findings will provide more insights into the safety profile of IgE antibodies as a therapeutic modality.

A recent study explored possible causes of transient urticaria observed in patients after MOv18 IgE treatment during a Phase 1 clinical trial [91]. These reactions were dose-dependent and were resolved within 24 h with subsequent infusions or prophylactic medication. The severity of the reactions was different among patients with only one patient showing signs of severe reaction [grade 3 per NCI Common terminology Criteria for Adverse events (CTCAE)] upon administration of an escalated dose [91]. Interestingly, blood samples of patients treated with MOv18 IgE with urticaria reactions had low circulating basophil levels and reduced basophil activation propensity ex vivo in response to immune stimulation. This data indicates non-involvement of basophils in urticarial reactions [91]. Furthermore, transcriptomic analysis of urticarial skin from the patient showing Grade 3 reaction upon MOv18 IgE treatment showed differential expression of urticaria-related genes but genes associated with allergic pathways remains largely unchanged. Additionally, patients who developed urticaria after MOv18 treatment did not express FRα in the skin and lacked cross-linking factors such as autoantibodies, anti-gal (galactose-alpha-1,3, galactose), or anti-MOv18 IgE antibodies. Notably pro-inflammatory cytokine pathways were activated, yet key allergic markers such as serum beta-tryptase levels and IL-4 remained unchanged in patients with or without urticaria. These results indicate that the urticarial reaction is unlikely to be allergic in nature and is not directly linked to MOv18 IgE treatment [91]. Similar reactions have been associated with infusion of therapeutic IgG antibodies [92, 93]. However, the underlying mechanism and potential involvement of immune cells other than basophils require further investigation.

Overall, the preclinical studies and phase 1 clinical trial of MOv18 IgE therapy is encouraging and offers an opportunity to develop therapeutic IgE molecules against various targets including solid tumors [9, 91]. However, detailed investigation is necessary in the future to address concerns associated with anaphylaxis.

CSPG4 IgE

Recently, a promising IgE antibody targeting chondroitin sulfate proteoglycan 4 (CSPG4) was generated for the treatment of melanomas [24] (Fig. 2). CSPG4 is overexpressed in melanomas and has been used as a target for the generation of therapeutic IgG monoclonal antibodies [94]. CSPG4 IgE is a chimeric antibody composed of the variable region derived from the mouse and the constant region of the human IgE [95]. This antibody showed tumor-suppressive functions by activating immune responses without triggering anaphylactic reactions. Similar to MOv18 IgE, monocytes and macrophages were involved in CSPG4-IgE-mediated tumor killing as revealed by transcriptomic analysis which showed the enrichment of cell type signature genes [24]. Interestingly, monocytes from both healthy and patient-derived samples showed up to 50% cytotoxicity against melanoma cells in this study. Compared with MOv18 IgE, antigen-specific crosslinking of CSPG4 IgE in monocytes results in an increased production of pro-inflammatory cytokines [18, 24]. Furthermore, the study demonstrated that the tumor suppressive function of CSPG4 IgE is mediated by activating the anti-parasitic response rather than the allergic reaction, thus highlighting the safety of this antibody in the treatment of melanoma [24].Overall, the functional characterization provides sufficient pre-clinical efficacy and safety data to support the development of CSPG4 IgE as a candidate molecule for cancer therapy against melanomas in the future.

In addition to MOv18 IgE and CSPG4 IgE, therapeutic IgEs have also been developed against various tumor-associated antigens including human epidermal growth factor receptor 2 (HER2) [22], epidermal growth factor receptor (EGFR) [96, 97], prostate-specific antigen (PSA) [98], Mucin 1 (MUC1) [99] and CD20, a marker of B cells [99].

In the future, investigations into the roles of immune cells such as B cells, mast cells, and dendritic cells, and their role in tumor suppression mediated by IgE therapeutics would be necessary. Furthermore, exploring treatment options in combination with therapeutic IgE antibodies may enhance its efficacy and broaden its applicability in treating solid cancers.

Conclusions

While most immunotherapies are IgG-based, there have been efforts to develop treatment regimens using other antibody classes, particularly IgE. Despite the superior properties of IgE over IgG for therapeutic purposes, safety remains a concern due to the role of IgE antibodies in anaphylaxis. However, data from the phase 1 clinical trial of MOv18 IgE and ex vivo studies with CSPG4 IgE do not show severe allergic reactions, except for one BAT positive patient experiencing allergic reactions when administered with MOv18 IgE. The clinical trial results indicated no increase in serum IL-4 in the patient, suggesting that the anti-tumor activity is driven by cell-mediated immunity rather than conventional Type I hypersensitivity [9]. The patient’s blood basophils were not activated by MOv18 IgE. Furthermore, in the presence of patient sera, MOv18 IgE does not cause degranulation in the rat basophilic leukemia cell line RBL-SX38 expressing human FcεRI [100]. Several studies have shown that MOv18 IgE triggers the release of pro-inflammatory cytokines such as MCP-1 and TNF-alpha and is associated with the activation of a cascade similar to that of a parasitic infection rather than an allergic response [9, 18, 21, 101]. Consistent with the previous findings for MOv18 IgE, ex vivo assays did not detect allergic reactions in the presence of patient serum and CSPG4 IgE. The lack of anaphylaxis is also explained by transcriptomic analysis, which showed no upregulation of IL-4 or IL-23 [24]. Transcriptomic analysis of the urticarial skin sample of patients treated with MOv18 IgE showing CTCAE grade 3 urticaria reactions further indicates the absence of involvement of allergic pathway in patients treated with MOv18 IgE [91]. Given that IgE is typically involved in allergic reactions, any IgE-based therapy should rigorously address safety issues in the clinical setting associated with anaphylaxis.

MOv18 IgE is currently undergoing a Phase 1b clinical trial (NCT06547840) for its safety and tolerability assessment and to determine the maximum tolerated dose (MTD) or maximum administered dose (MAD). It is expected that the results from this trial would be useful to understand IgE biology, its safety profile, and potential therapeutic applications. Currently, MOv18 IgE is produced in mouse myeloma Sp2/0 cells and carries non-human glycan structures, for example, α-Gal and Neu5Gc which could potentiate the risk of anaphylaxis. To further mitigate this risk and the safety of the patients, future IgE molecules for therapeutic applications could be produced in CHO or HEK cells, which will provide glycosylation patterns more consistent with human biology and therefore reduce the possibility of anaphylactic reactions.

The risk of anaphylaxis can also increase by cross linking of soluble antigens in the bloodstream. This can probably decrease by careful selection of the target epitope for the development of IgE therapeutics. For instance, a monovalent antigen, present in monomeric form in the bloodstream reduces the potential of cross-linking. Furthermore, skin prick test and BAT test should be considered essential preconditions for patient selection. Additionally, regular monitoring of autoantibodies against soluble antigens or receptors is recommended to ensure future effectiveness of IgE-based therapy. Given the availability of multiple IgG antibodies approved for therapy, IgE-based treatment strategies will remain a challenge unless the safety profile of the IgE antibodies is unequivocally established for clinical applications at least at par with the IgG antibodies. The association between IgE antibodies and tumors has been studied extensively. For example, epidemiological studies have revealed an inverse relationship between allergy and cancer [102]. IgE antibodies have been detected at tumor sites highlighting their role in tumor immunosurveillance [103, 104]. Additionally, one study has shown that IgE antibodies from pancreatic cancer patients elicit antibody-dependent cellular cytotoxicity against pancreatic cancer cells [105]. In addition, a comparative analysis of high-IgE KN1 mice, low-IgE ΔM1M2 mice, and WT BALB/c mice with different ε-B cell receptors, mimicking human conditions with high, low, and normal IgE levels, respectively, showed that the mice with high IgE levels exhibited better survival rates when challenged with tumors [106]. The same study reported that active and passive immunotherapy using anti-HER vaccination and anti-HER IgG antibodies did not prolong survival in high IgE KN1 mice suggesting the role of innate IgE against cancer [106]. Similarly, studies suggest that IgE deficiency may increase the risk of malignancy [107, 108]. Taken together, these findings suggest that IgE plays a role in natural immunosurveillance against cancers and can benefit and even confer anti-tumor immunity in patients with high IgE levels. In addition, the use of anti-tumor IgE presents a distinct advantage as IgE is less susceptible to the immunosuppressive signals prevalent in the tumor microenvironment. Future research directions aimed to leverage the tumor-associated IgE for triggering localized anaphylaxis is needed which could enhance therapeutic outcomes in patients. The extensive glycosylation of IgE molecule poses challenges vis-a-vis its manufacturability and potential immunogenicity. Engineering a deglycosylated IgE variant or an IgE molecule with reduced glycan complexity without altering the conserved N275 glycan site offers a promising strategy for creating more effective and stable IgE therapeutics (Fig. 2). This strategy will not only minimize challenges associated with production but also help in optimizing the therapeutic efficacy of the IgE antibodies. For example, reduced glycans or a deglycosylated IgE therapeutic antibody would reduce batch-to-batch heterogeneity and immunogenicity associated with the fully glycosylated antibody, thus reducing dosage and frequency of administration, potentially minimizing the adverse side effects. Indeed, a recent study shows the dispensability of glycosylation at the non-conserved sites for antibody function and stability [51] which might help in mitigating the current challenges associated with glycosylation. Given the effector function of IgE is mediated by immune cells such as mast cells, eosinophils, macrophages, and dendritic cells, its use in cancer therapy might pose challenges due to their pro- and anti-inflammatory roles in different types of cancer. Designing of IgE molecules for therapeutic applications is challenging due to limited number of investigators, insufficient clinical safety data and several unresolved aspects of IgE biology. IgE therapeutic research is in its early stage and IgE has shown promising results and preliminary evidence of anti-tumor activity in ovarian carcinoma patients. Confirmation of anti-tumor activity of MOv18 IgE in clinical studies might establish this antibody class as a potential option for next-generation immunotherapy. Future IgE formats might also exploit the bispecific design for dual targeting and combine with a deglycosylated version for better efficacy. Indeed, a bispecific IgE generated using the Knobs-into-holes (KiH) strategy induced efficient cell death in mice with cancer cells expressing the target antigens [65]. IgE-based therapy could also be used in combination with the existing IgG therapy, thus combining the strengths of both antibody classes. Indeed, innovative formats such as the hybrid IgE/IgG antibody (IgEG) platform for therapy is being developed by Epsilogen. IgEG antibody will have the potential to activate both IgG and IgE receptors, mediating complement-dependent cytotoxicity and extension of serum half-life compared to IgE by binding to neonatal Fc receptors (FcRn) (https://epsilogen.com/igeg-platform/). These innovative formats could open new avenues for advancements in the field. Taken together, the ongoing research on IgE therapeutics has provided potential opportunities for clinical development and as a therapy in the future. These outcomes and advancements, along with a better clinical safety profile may increase the treatment options for cancer patients in the future together with existing and approved IgG therapies.

Author contributions

SK: Conceptualization, literature collection, and writing – original draft preparation, SG: Conceptualization, writing – review & editing, AD: review & editing.

Funding

No Funders.

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No datasets were generated or analysed during the current study.

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Competing interests

The authors declare no competing interests.