New Treatment Directions in Food Allergy

INTRODUCTION

There is general consensus that food allergies (FAs) are increasing in prevalence. However, because of the lack of a simple diagnostic test for food allergy and methodological differences between studies to determine prevalence of food allergy, true food allergy prevalence in the past or present is difficult to determine.¹ On the basis of a number of studies, FA is now estimated to affect about 5% of adults and 8% of children.² Increases in the rate of emergency department visits and hospital admissions due to food-induced anaphylaxis among children have been reported.³ These increases in rates of FA are worrisome and have intensified research into effective treatments for the disease. At the present time, there is no FDA-approved treatment for FA and the standard of care is individualized avoidance of the offending food allergens and treatment of symptoms on accidental ingestion.² Allergen avoidance is difficult to accomplish as many allergenic foods, such as milk, eggs, and peanuts are common ingredients in many foods. Accidental ingestion is common and one study reported that during a 5- and 10-year follow-up period, 58% and 75% individuals with peanut allergy, respectively, accidentally consume peanuts.⁴ Constant vigilance and monitoring of foods for potential allergens leads to stress and lowers quality of life. Localized reactions on accidental ingestion are treated with antihistamines or glucocorticoids while systemic reactions are treated with epinephrine.⁵ These pharmaceutical agents, however, are inadequate and offer only symptomatic relief and do not address the underlying immune disorder.

Food allergies are classified into IgE-mediated, non-IgE-mediated, and mixed (non-IgE and IgE-mediated). Although this classification is an oversimplification of the various pathologies underlying food allergies, it plays an essential part in diagnosis and treatment. IgE-mediated reactions are immediate (minutes to within 1–2 hours) and symptoms include urticaria, nausea, abdominal pain, respiratory symptoms, and potentially fatal systemic anaphylaxis. Non-IgE-mediated reactions are delayed and include food protein-induced allergic enterocolitis, food protein-induced allergic proctocolitis, and food protein-induced enteropathy. Eosinophilic gastrointestinal disorders constitute another group of FA disorders with chronic eosinophilic infiltration of the gastrointestinal wall.⁶ These were initially considered to be mixed (both IgE and non-IgE-mediated) but currently the role of IgE in these diseases is controversial.^6–8 This review focuses on current treatments that show promise in desensitizing individuals with IgE-mediated FA.

Immune Mechanism: Tolerance, IgE-mediated allergic sensitization and reaction, and desensitization with immunotherapy

A number of excellent reviews of the mechanisms underlying allergic tolerance, IgE-mediated allergic sensitization and reaction, and immunological changes with immunotherapy have been published.^2,6,9–12 These mechanisms are briefly discussed here.

Tolerance

Healthy immune tolerance to foods is an active process with T regulatory cells (Tregs) playing a central role in inhibiting mast cell degranulation and sustaining tolerance. Much of our understanding of responses to allergens in the gut is based on animal studies. It is now thought that tolerogenic responses primarily begin with the uptake of potential allergens at the gastrointestinal mucosa of the small intestine, which contains most of the gut-associated lymphoid tissue, or GALT. Uptake can occur through transcytosis, paracytosis, or endocytosis (via specialized microfold or M cells),¹³ or through capture by dendritic cells (CD103⁺) or macrophages (CX3CR1⁺), which are located in the lamina propria. CX3CR1⁺ cells can sample antigens present in the lumen of the gut by extending processes between intestinal epithelial cells. These cells do not migrate but are thought to transfer the antigens to CD103⁺ dendritic cells via a gap junction and promote tolerance through IL-10 production.¹⁴ CD103⁺ dendritic cells may capture antigens directly from the lumen by extending a process through a tight junction or through a transcellular pore in an M cell or may sample those antigens already translocated to the lamina propria. These dendritic cells then migrate to the draining lymph nodes where they induce differentiation of naïve T cells into Tregs and facilitate their homing to the gut by (1) production of transforming growth factor-β (TGF-β) and retinoic acid (2) Expression of indoleamine 2,3-dioxygenase (IDO), and (3) expression of CCR9 and α4β7 on Tregs. Although it is now clear that both Foxp3⁺ and Foxp3⁻ IL-10 and TGF-β producing Tregs are present in the gut; the role of the individual Tregs in promoting tolerance are poorly understood. Tregs that have honed to the gut may also promote B-cell production of non-inflammatory IgA^11,12 (Figure 1).

Figure 1.

Healthy immune tolerance to foods is an active process. Upregulation of T regulatory cells and inhibition of mast cell degranulation is key to maintaining tolerance.

Allergic sensitization and allergic response

IgE-mediated reactions include a sensitization phase, an activation phase, and an effector phase. Antigens are initially encountered at barrier surfaces, such as skin, gastrointestinal tract, or respiratory tract, and sensitization is thought to potentially occur through one or more of these routes. Increased allergen permeability caused by disruption of the epithelial barrier is believed to play an essential role in allergic sensitization. Loss of epithelial barrier integrity can be caused by injury or the presence of a loss-of-function mutation of the filaggrin gene, a key gene involved in skin barrier function. Studies have shown that infants who develop atopic dermatitis were significantly more likely to develop FA. Filaggrin mutations represent a significant risk factor for IgE-mediated peanut allergy¹⁵; however, not all children with a filaggrin mutation develop atopic dermatitis or FA and not all children with FA have atopic dermatitis. Increased allergen permeability leads to the production of pro-inflammatory epithelial-derived cytokines (IL-25, IL-33, and thymic stromal lymphopoietin (TSLP)).¹⁶ Some of the potential ways that these cytokines may drive Th2 responses and production of type 2 cytokines (IL-4, IL-5, IL-9, and IL-13) are via stimulation of innate lymphoid cells (ILC2s), upregulation of IL-9 producing mucosal mast cells (MMC9s),^17,18 and upregulation of OX40L on dendritic cells and driving differentiation of naïve CD4⁺ T cells to Th2 cells.⁶ Together, these type 2 cytokines promote tissue mast cells, basophils, and eosinophil accumulation, IgE class switching by B cells, and binding of allergen-specific IgE antibodies to FcεRI receptors on mast cells or basophils (Figure 2). At this stage, the individual is sensitized to the allergen. During the activation phase, further exposure to the specific allergens leads to cross linking of FcεRI bound IgE antibodies and subsequent degranulation of mast cells or basophils leading to the release of histamine and other inflammatory chemical mediators such as cytokines, interleukins, leukotrienes, and prostaglandins into the surrounding tissue. During the final effector phase, these pro-inflammatory mediators mediate allergic reactions via production of mucous, infiltration of eosinophils, vasodilation, smooth muscle contraction, and other effects.¹¹

Figure 2.

Upregulation of Th2 cytokines and IgE antibodies lead to mast cell degranulation and allergic response.

Desensitization with Immunotherapy

The mechanisms of desensitization with immunotherapy and the differences between desensitization and natural tolerance are not well understood. Immunotherapy has been shown to decrease specific IgE levels and diversity¹⁹ and basophil activation²⁰ and increase IgA²¹ and IgG4¹⁹ levels. IgG4 antibodies are thought to act as blocking antibodies by competing with IgE. Decreases in the number and function of Th2 cells through apoptosis and anergy and increases in Treg number and function (via hypomethylation of Foxp3)^22–25 has been demonstrated by a few studies. Increases in allergen-specific memory B cells (Bregs) have been observed, suggesting a potential role of these cells in tolerance.²⁶ Bregs have been implicated in the production of IgG4 and IL-10²⁷. Further research into the mechanism underlying oral immunotherapy (OIT), sublingual immunotherapy (SLIT), and epicutaneous immunotherapy (EPIT) are needed that can guide protocols towards safer and more efficacious treatments (Figure 3).

Figure 3.

The mechanism of desensitization with immunotherapy is still being investigated. IgE levels and basophil activation decrease while IgG4 levels and Foxp3 T regulatory cell function increase with immunotherapy.

Immunotherapy

Immunotherapy can increase threshold of reactivity (desensitize) to specific allergens in the majority of allergic individuals. However, the definition of successful desensitization can vary. It can either indicate a high level of desensitization as that required to enable individuals to consume normal amounts of the offending allergens or a lower level of desensitization as that required to prevent allergic reaction on accidental ingestion of allergenic foods.

One of the major drawbacks of immunotherapy is that, in the majority of individuals, desensitization is a temporary state and recurrence of allergenic sensitivity often occurs after discontinuation of therapy. To maintain desensitization, regular exposure to the offending allergen appears to be necessary in the majority of individuals. Desensitization differs from true tolerance, which is defined as a permanent immunological change, and can occur naturally in some individuals and perhaps with immunotherapy. At the present time, as biomarkers to determine tolerance are unavailable, focus has shifted on evaluating sustained unresponsiveness (SU), which is defined as the ability to successfully pass a food challenge after a period of allergen avoidance after cessation of active therapy. Long-term data are currently inadequate to determine whether SU (a few years and beyond) can be achieved and the immunological differences between SU, tolerance, and desensitization.

Other barriers to acceptance of immunotherapy into regular clinical practice include extended length of treatment (generally months to years), frequent clinic visits, and risk of adverse reactions during treatment. A recent systematic review and meta-analysis of immunotherapy trials concluded that although immunotherapy is effective in increasing threshold of reactivity to specific allergens, it is associated with a modest increase in serious systemic adverse reactions and a substantial increase in minor local adverse effects. Additionally, most immunotherapy protocols aim to desensitize individuals to only one offending allergen at a time, making it impractical for those with multiple food allergies. Optimization of immunotherapy protocols (route of delivery, dose, treatment frequency and duration, or use of allergen epitopes and adjunctive therapies) to address the above shortcomings of immunotherapy is an active area of research.

Clinical trials of OIT, SLIT, and EPIT for FA are currently being evaluated. Subcutaneous immunotherapy (SCIT) for food allergies was attempted in the 1990s and was shown to be efficacious; however, it was associated with high rates of adverse reactions²⁸, and studies on FA immunotherapy are now mostly limited to OIT, SLIT, or EPIT. However, there have been recent attempts to develop safer, chemically modified aluminum hydroxide adsorbed peanut extracts for SCIT administration and one formulation is currently being tested in clinical trials (ClinicalTrials.gov Identifier NCT02991885).

Oral Immunotherapy

OIT has mainly been conducted for milk, peanut, and egg allergy. Simultaneous OIT for multiple allergens (up to 5) has also been attempted and has shown to be feasible²⁹. Although protocols vary widely, in general, OIT consists of the following 4 phases (1) An initial dose escalation day (micrograms to milligrams) where the highest tolerated dose of allergen that can be safely consumed is determined, (2) a buildup phase of varying length when the allergen dose is gradually increased over time till a predetermined maintenance dose is reached, (3) a dose maintenance phase of variable length at the end of which an oral food challenge (OFC) is conducted to determine desensitization success or failure. To determined SU, many clinical trials now incorporate a 4th phase (allergen elimination phase) which includes varying periods of allergen withdrawal at the end of which SU is determined through OFCs. For peanut allergies, maintenance doses generally have varied between 300 mg to 4000 mg (equivalent to about 17 peanuts). High maintenance doses are associated with low compliance. A recent study by Vickery et al in infants 9–36 months indicates that a low 300 mg maintenance dose with peanut protein has similar efficacy and improved safety profile as a high 3000 mg dose³⁰. Adverse reactions are the primary concern with OIT and can occur during OFC, initial day 1 escalation, buildup phase, or during the maintenance phase. Serious anaphylaxis reactions are rare, but have been reported. Mild reactions (mainly abdominal) have been reported in 2–5% of doses with epinephrine/adrenaline being used in less than 1% of doses (but up to 24% of patients).³¹ A systematic review and meta-analysis showed that about 2.7% of patients undergoing OIT for FA developed new-onset eosinophilic esophagitis.³²

Novel methods of introducing allergens are being evaluated, such as the use of heated allergens (mainly eggs and milk) or standardized commercial oral formulations. Use of heated milk may reduce the rates of adverse effects with immunotherapy.³³ Some studies have indicated that the regular use of baked egg or milk in the diet can increase tolerance to similar non-baked foods in certain individuals. These individuals are thought to be sensitized to conformational epitopes that are denatured on heating and bind to IgE bound to mast cells or basophils less strongly that unheated epitopes.³⁴ However a recent systematic review does not support this hypothesis.³⁵ To facilitate a convenient and accurate way of delivering allergens, AR101, a peanut-derived pharmaceutical capsule has been developed by Aimmune Therapeutics (Brisbane, CA). It is currently in phase 3 immunotherapy trials and capsules are formulated to deliver between 0.5 milligrams up to 300 milligrams of peanut protein.³⁶

Sublingual Immunotherapy

In SLIT, allergens are applied under the tongue in the form of a tablet or drops on a daily basis for a period of years. Maintenance doses with SLIT are in the range of micrograms to milligrams and much lower than with OIT, which are in milligrams to gram quantities.³⁷ Another important difference between OIT and SLIT is that in SLIT allergens are delivered to the oral mucosa intact while in OIT, digested antigens are delivered to the gastric mucosa. Several SLIT clinical trials (kiwi, hazelnut, milk, peach, and peanut allergies) have shown promising results. Results from randomized controlled studies for milk and peanut allergy comparing safety and efficacy of SLIT and OIT found that SLIT has a better safety profile than OIT but OIT is more effective than SLIT in inducing desensitization. Side effects with SLIT are minimal and typically limited to oropharyngeal itching in less than 2% of doses.³¹ No severe anaphylactic reactions have been reported. SLIT efficacy is limited by the concentration of the allergen and the volume of liquid that can be administered sublingually. At the current time, it remains investigation therapy and there are no standardized SLIT formulations available.

A randomized study by Keet et al³⁸ evaluated the use of SLIT alone or SLIT followed by OIT in children. The protocol consisted of an initial SLIT escalation phase followed by continuation of SLIT escalation or initiation of OIT (high or low dose). OFC to 8g milk protein was conducted after 12 and 60 weeks of maintenance therapy. The study found that although the rates of reaction were the same, systemic reactions were more common during OIT than during SLIT and that SLIT followed by OIT is more effective than SLIT alone in achieving desensitization.

Epicutaneous immunotherapy

In EPIT, allergens are introduced epicutaneously through a patch applied to the back or upper arm.³⁹ Allergens applied to the patch are lower than that found in OIT and SLIT and are in the microgram range (about 100 to 500 micrograms of protein). Results of a recent multicenter, double-blind, randomized, placebo-controlled study of EPIT using a proprietary peanut patch (Viaskin patch, DBV Technologies, Montrouge, France) indicated increases in specific IgG4 levels and IgG4/IgE ratios and a trend toward reduced basophil activation and specific Th2 cytokines. Side effects were minimum and mainly consisted of mild skin irritation in the area of the skin where the patch was applied. No systemic reactions were reported. At 52 weeks, a 10-fold increase in the successfully consumed baseline dose, the primary endpoint, was significantly greater in the group treated with the 250 microgram Viaskin peanut patch (48.0%) than in the placebo group (12%). However, the differences between the above 2 groups was not significant when the additional criteria of a successfully consumed baseline dose of at least 1044 mg of protein was used. Furthermore, when the results were stratified by age, no treatment success was observed for those older than 11 years.^39,40 Phase 3 studies using Viaskin peanut patch are currently underway. EPIT for milk allergy using Viaskin milk and Viaskin egg are currently in phase 2 and preclinical trials, respectively.

Adjunctive therapies with OIT

Anti-IgE therapy

Omalizumab is an anti-IgE antibody that reduces circulating IgE concentrations⁴¹. Importantly, it does not bind to FcεRI bound IgE on mast cells or basophils and cannot induce IgE cross-linking or trigger anaphylactic reactions.⁴² It has been FDA-approved for treatment of allergic asthma and chronic urticaria. Omalizumab was first used by Nadeau et al as adjuvant therapy to OIT in 2011 in a pilot study of 11 milk-allergic individuals. The protocol consisted of 9 weeks of pretreatment with omalizumab, 7 weeks of omalizumab plus OIT therapy, followed by 8 weeks of maintenance OIT without omalizumab. A OFC was conducted at the end of the maintenance period. The results of the study demonstrated that adjunctive omalizumab with OIT was efficacious and might allow for faster desensitization. The frequency of adverse reactions was 1.6% of doses, and most symptoms were defined as mild. One subject dropped out of the study because of gastrointestinal symptoms and one subject showed rhinitis and generalized urticaria and responded to adrenaline at the time of OFC.⁴³

In addition to milk, clinical trials using adjunctive omalizumab with OIT have now been conducted for egg,⁴⁴ peanut,⁴⁵ and for those with multiple food allergies.⁴⁶ Three recent randomized double-blind, placebo-controlled trials of adjunctive omalizumab with OIT have now been published and results from these studies are promising. A study by Wood et al showed significant improvements in safety and time in achieving the maintenance dose in patients with milk allergy who received omalizumab during OIT but did not find increased success in desensitization or SU in patients.⁴⁷ In patients treated with omalizumab, percentages of doses per subject provoking symptoms, dose-related reactions requiring treatment, and doses required to achieve maintenance were reduced compared to the placebo-controlled group during OIT escalation. A multicenter trial for peanut allergy found a significant difference in efficacy between the active and placebo arms. Twelve weeks after stopping omalizumab, 76% of patients in the omalizumab arm passed the 4000 mg food challenge compared to 12.5% of patient in the placebo arm. Although the overall reaction rates were not significantly different in the omalizumab versus the placebo arm, there was a trend toward lower rates. Additionally, the omalizumab-treated subjects were exposed to much higher doses of peanut than the placebo group.⁴⁸ A phase 2 study of patients with multiple food allergies who were simultaneously treated with up to 5 allergens demonstrated that adjunctive omalizumab with OIT enables safe and rapid desensitization.⁴⁶

These studies indicate that adjunctive omalizumab with OIT expedites time to desensitization by allowing patients to start at a higher initial dose than OIT alone, reducing the number of doses required to reach maintenance dose, and that desensitization is maintained even after discontinuation of omalizumab. It also indicates that it is safe and feasible for use in patients with multiple allergies.

Probiotics

There is significant evidence implicating the role of the microbiome in allergic disease.^49,50 The introduction of fruits and vegetables containing fermentable fiber in early infancy has been shown to increase microbial diversity, short-chain fatty acid levels, promote epithelial integrity, and reduce penetration of intact food allergens.^51,52 A few OIT trials that have used adjunctive probiotics have shown promise for FA desensitization. In one study, peanut-allergic children were randomly given either a combination of the probiotic Lactobacillus rhamnosus with peanut protein, or a placebo, once daily for 18 months. Two to five weeks after the end of the trial, 82.1% of children who received OIT plus probiotics were deemed tolerant to peanuts compared with just 3.6% in the placebo group.⁵³ At 4-year follow-up, 70% of children who gained initial tolerance passed another challenge test suggesting that is effective at inducing SU.⁵⁴

Chinese Herbal Formula

FA Herbal Formula-2 (FAHF-2) is a Chinese 9-herb formula. In a murine model of multiple FA (peanut, egg, and codfish), FAHF-2 was efficacious and prevented anaphylactic reactions.⁵⁵ A double-blind, randomized, placebo-controlled phase I study of FAHF-2 was found to be safe and well tolerated with favorable in vitro immunomodulatory effects; however, no improvements in efficacy was observed.⁵⁶ The use of other Chinese herbal formulas in conjunction with multi-food OIT and anti-IgE are now being tested in clinical trials (ClinicalTrials.gov Identifier NCT02879006).

Other Therapies

Vaccines

A novel immunotherapeutic approach is to provide exposure to allergens via DNA vaccines. Lysosomal Associated Membrane Protein (LAMP) DNA plasmid vaccines are novel vaccines constructed to encode LAMP-1 along with allergenic sequences. In a murine model, CryJ1-and CryJ2-LAMP, which encode the major allergens found in Japanese Red Cedar (JRC), induced robust Th1-type immune responses.⁵⁷ A phase 1 studies of CryJ2-LAMP DNA vaccine indicated that the vaccine is safe and it may be immunologically effective in treating JRC induced allergy.⁵⁸ A DNA-LAMP vaccine, ASP0892, for treating peanut allergy has recently been developed by Astellas Pharma, Inc (Japan). Unlike conventional DNA vaccines that primarily elicit a cytotoxic T cell immune response, ASP0892, is designed to desensitize peanut-allergic individuals to the three major peanut allergens (Ara h1, h2, h3) and generate a Th1–mediated immune response. A phase 1 study (ClinicalTrials.gov Identifier NCT02851277) is now in progress to evaluate safety, tolerability, and immune response in adults with peanut allergies.

Biologics

In addition to omalizumab, which has shown promise as adjunctive therapy for food allergy, ANB020 (AnaptysBio, San Diego, CA), an anti-IL-33 antibody, is also being evaluated in an advanced phase 2 placebo-controlled clinical trial that is designed to determine safety, tolerability and activity in adult patients with peanut allergy (ClinicalTrials.gov Identifier NCT02920021). IL-33 is a pro-inflammatory cytokine that acts upstream of IgE and mediates B-class switching to IgE. Results of a phase I study (in healthy volunteers) indicate that ANB020 is well-tolerated in healthy volunteers and that a single dose is sufficient to suppress IL-33 function for approximately three months post-dosing.⁵⁹

A number of other biologics that target key molecules known to be involved in food allergy have been developed. A few of these drugs have been approved for the treatment of other allergic diseases or asthma. Anti-IL-5 (mepolizumab and reslizumab) and anti-IL-5R (benralizumab) antibodies have been approved by the US FDA for the treatment of asthma.⁶⁰ Dupilumab is an antibody directed against the IL-4Rα subunit of IL-4 and IL-13 receptors. It blocks the signaling pathways of IL-4 and IL-13, key cytokines that drive type 2 inflammatory response. It has been approved for the treatment of moderate-to-severe atopic dermatitis. ⁶¹ Other biologics are being tested in clinical trials for various allergic diseases and asthma. QAX576, an anti-IL-13 antibody, was found to be safe and efficacious for eosinophilic esophagitis in a preliminary study of 23 patients.⁶² Tezepelumab (AMG 157/MEDI-9929), an anti-TSLP antibody has been shown to lower rates of clinically significant asthma exacerbations.⁶³ Results obtained from clinical studies of MEDI-528, an anti-IL-9 antibody, has shown variable results, likely due to the heterogeneous nature of asthma in the populations studied.⁶⁴

CONCLUSION

Current immunotherapy studies are very encouraging and the lengthy treatment period and relatively high rates of adverse reactions are being addressed through the use of adjunctive therapy, such as anti-IgE antibodies, Chinese herbal therapy, and probiotics. With our increased understanding of the molecular mechanisms involved in FA and other atopic and immune diseases, we have made tremendous progress in the identification and development of other biologics. Besides anti-IL-33, which is currently being evaluated for FA, other biologics that alter immune response have been developed and are in varying stages of preclinical and clinical development or have been approved for specific diseases. There is a common underlying mechanism underlying atopic diseases and asthma and an understanding of the mechanisms underlying one disease can assist with our understanding and treatments of other immune diseases. For example, omalizumab was initially approved for asthma and was subsequently approved for chronic idiopathic urticaria. It has now been shown to be also effective for the treatment of food allergies. Research into identification of biomarkers for diagnosis and prognosis is ongoing and may soon assist with the identification of those best positioned to benefit from immunotherapy. The future looks promising. In the last decade, there has been tremendous progress in our understanding of the mechanism underlying FA and an approved treatment is likely within the next few years.

Abbreviations/Acronyms

FA

Food Allergy

TGF-β

Transforming Growth Factor-β

IDO

Indoleamine 2,3-dioxygenase

Tregs

T-Regulatory Cells

MMC9

IL-9-producing Mucosal Mast Cell

M cell

Microfold cell

TSLP

Thymic Stromal Lymphopoietin

Bregs

Allergen-specific Memory B Cells

OIT

Oral Immunotherapy

SLIT

Sublingual Immunotherapy

EPIT

Epicutaneous Immunotherapy

SU

Sustained Unresponsiveness

SCIT

Subcutaneous Immunotherapy

OFC

Oral Food Challenge

LAMP

Lysosomal Associated Membrane Protein