Novel approaches and perspectives in allergen immunotherapy

Allergies have become a public health concern of pandemic proportions that affect >150 million Europeans. More alarming, their prevalence and impact are on the rise. It has been predicted that within the next few decades, up to half of the European population may at some point in their lives experience some type of allergy 1. Allergen immunotherapy (AIT) is the only currently available medical intervention that can limit the natural course of the disease 2. Years of preclinical research, clinical trials, systematic reviews and meta-analyses have convincingly shown that AIT can achieve significant reduction of symptoms of patients, improving the allergic individuals’ quality of life, changing the course of the disease and reducing the long-term costs and burden of allergies 2-5. These effects of AIT are of utmost importance from the public health point of view.

Nevertheless, despite these advances during the past century (Fig. 1) AIT usage varies across the globe with market penetration rates (the fraction of possible sales achieved) from <1% in emerging markets in Asia to 20% in the United States 6-8, so there is clearly room for improved allergy management and new innovation in the field to unleash this potential. Within this context, leading researchers from academia and industry discussed the latest advances in the field at the first Aarhus Immunotherapy Symposium on 2 December 2015. This review summarizes the main findings from the symposium on the current understanding of allergic disease mechanism, new emerging technologies, development of diagnostic and therapy monitoring technologies and disease management programmes including prevention.

Tolerance induction and prevention in allergy – a public health issue

The prevalence of allergic diseases has increased in many industrialized and urbanized countries during the last 50 years. Although the origin of allergy remains unresolved, increasing evidence indicates that modern living in an urban environment is deprived of environmental protective factors that are fundamental for normal tolerance induction 9. The concept of induction of immune tolerance has become a prime target for prevention and treatment strategies for many chronic inflammatory diseases such as allergy, asthma and autoimmunity in which dysregulation of the immune system plays an essential role.

There are few nationwide, comprehensive public health programmes on allergic disorders with defined goals and systematic follow-up 10. One practical example is the Finnish Allergy Programme 2008–2018. It is based on the idea that the allergy epidemic in modern societies is caused by inadequately developed or broken tolerance 11. Whilst allergies have traditionally been associated with industrial countries, they are also endemic in the developing world. Recent reports indicate that the prevalence of allergic disease in the Asia–Pacific region has reached the highest levels in 50 years 12, a trend also observed in tropical South-East Asia 13, 14. Although a similar set of allergens to those eliciting responses from populations in western regions has been proposed, the course, specificity and complexity of the allergic response in tropical countries may differ substantially from that in temperate zones. In a recent study of two independent cohorts of ethnic Chinese living in Singapore, the allergic response in a tropical urban environment was dominated by house dust mites 15. This monospecific IgE sensitization translates into increased prevalence of allergic airway diseases, which now impact a large proportion of the population in Singapore. It presents a unique opportunity to treat and manage these patients with AIT.

As the first allergic sensitization is a strong predictor for ensuing sensitizations, it should be treated with (i) AIT in addition to (ii) individual guided self-management, (iii) symptomatic treatment and possibly with (iv) avoidance of allergens that clearly worsen the symptoms. The WHO AIT guidelines from 1998 state that AIT acts globally on IgE-mediated inflammation in various organs 2. Allergen immunotherapy could be administered in primary health care setting with enough know-how according to the national AIT guidelines based on international guidelines, as well as student and occupational health care where feasible 16. Subcutaneous immunotherapy (SCIT) has proven value but requires specialist supervision due to associated risks of severe allergic reactions. The sublingual route is an effective and safer alternative suitable for daily self-administration.

From the public health point of view, an important fact is that allergic rhinitis is a risk factor for asthma. Allergen immunotherapy is considered to prevent asthma in patients treated with AIT for allergic rhinitis in clinical trials 17 as well as in real-world settings 18. Confounding by indication cannot be excluded but would lead to an underestimation of the true preventive effects of AIT.

The Grazax Asthma Prevention (GAP) trial, investigating the preventive effect on asthma development of grass AIT tablet in children aged from 5 to 12 years with grass pollen-induced allergic rhinitis (AR), is the first double-blind, placebo-controlled randomized trial to assess the preventive effects of AIT 19. Although the trial did not achieve its primary endpoint of preventing asthma, the asthma diagnosis criteria used in the time to onset analysis were rather rigid and dependent on objective tests of reversibility. Even so children treated with the grass tablet AIT had a significantly reduced risk of experiencing asthma symptoms and a reduced use of asthma medication not only during the 3 years of AIT but also for 2 years after discontinuation. They also had a beneficial effect on their allergic nasal and eye symptoms throughout the 5 year study period 20. Thus, the results of the GAP trial confirmed the disease-modifying effect of grass tablet AIT on grass pollen-induced asthma and rhinoconjunctivitis in children.

Early introduction of allergenic food has been shown to be good primary prevention of peanut allergy 21, which may face difficulties when introduced in primary care 22. Despite the lack of effect in the intention-to-treat analysis and reluctance to introduce solids as early as at 3 months of age, the reduction in prevalence by 2/3 in the per-protocol analysis when ingesting 2 g of peanut or egg protein is encouraging. Oral immunotherapy (OIT) of IgE-mediated food allergy has been shown to desensitize individuals at risk of or having experienced severe allergic reactions against peanut 23, egg 24, 25 and cow's milk 26. However, these oral desensitization protocols were experimental, had low success rates, carried significant risks of inducing anaphylaxis and, in contrast to immunotherapy for inhalant allergens, have not been shown to induce long-term tolerance after discontinuation. Although the data are encouraging, current guidelines confine OIT for food to research protocols in the hands of trained allergy specialists and are not recommended for routine clinical practice.

Allergen immunotherapy has not yet received adequate attention from European institutions, and thus far, too many allergic patients in the general population remain unaware of the benefits of AIT 3. It is time to re-evaluate the allergy paradigm and implement new kinds of actions as allergic individuals are becoming a significant minority of Western populations, and their number is increasing worldwide. National and regional action plans, such as the Finnish Allergy Programme 2008–2018 11, are needed to meet this challenge.

Evaluation of efficacy in AIT trials

To advocate that AIT resolves the unmet need of allergy in public health, good, standardized assessment of clinical efficacy is mandatory. Throughout the last decade, increasing emphasis has been put on clear guidance in standardization of clinical trials in the field of AIT 27, 28. Besides these academic positions, the European Medicines Agency (EMA) 29 and the Center for Drug Evaluation and Research of the US Food and Drug Administration 30 have published regulatory guidelines which outline standards for the clinical development and documentation of new products in clinical trials. The relevant EMA guideline 29 requires that the primary endpoint in AIT trials has to ‘reflect both, symptom severity as well as the intake of rescue medication’ and, moreover, specifies several secondary outcome parameters. However, it also highlights that at present ‘no validated symptom score exists’ and ‘different approaches to combine symptom score and intake of rescue medication are possible’. Recently, a Task Force initiative from the European Academy of Allergy and Clinical Immunology (EAACI) recommended a standard for the primary endpoint for future randomized controlled trials in AIT for allergic rhinoconjunctivitis, the ‘combined symptom and medication score’ (CSMS; Table 1 31). Besides these parameters, several secondary endpoints such as Health-Related Quality of Life Questionnaires, visual analog scales or ‘Global Assessments’ are also used in AIT trials 32, 33. Among these secondary endpoints, allergen provocation tests (conjunctival, bronchial or nasal provocations) can be performed that directly measure the allergen sensitivity (and possible changes throughout the course of AIT) in the allergic target organ 29. Therefore, the EMA also recommends provocation tests to be used as primary endpoints in dose-finding trials of AIT 29 (EMA register of clinical trials at www.clinicaltrialsregister.eu). For pivotal phase III trials, these tests ‘can give additional information but are no surrogate markers and cannot replace the measurement of clinical symptoms’ 29. Exposure in an allergen exposure chamber (AEC), through standardized protocols under controlled environmental conditions (temperature, humidity), is an attractive alternative. Their technical validation underlines their principal advantage compared to other challenge methods 34. These models have been used to demonstrate the proof of concept, onset of action and magnitude of clinical effects 35-37. A clear unmet need in the future is a thorough technical standardization and (clinical) validation within and between different AEC models 31, 34.

Mechanisms of AIT

Long-term clinical improvement associates with humoral and cellular modifications to the allergen-specific immune response 38. Allergen immunotherapy may reduce the risk of progression from allergic rhinitis to asthma 17. There are some data to suggest that AIT may reduce the risk of developing new sensitizations to allergens, although further studies are needed to clarify this 39, 40. A greater understanding of the underlying mechanisms (Fig. 2) following the allergen-specific interventions of SCIT and sublingual immunotherapy (SLIT) is important for better understanding of the disease, for the development of predictive biomarkers and to assist the rational design and testing of novel more effective and safer immunotherapy strategies.

Allergic rhinitis is characterized by IgE synthesis, mast cell activation and tissue eosinophilia, events under the regulation of Th2 cytokines that are produced preferentially by CD4⁺ helper Th2 cells but also by mast cells, basophils and innate lymphoid cells (ILC2s). Subcutaneous immunotherapy is associated with a decrease in effector cells in target organs, transient increases in allergen-specific IgE followed by blunting of seasonal increases in IgE and a marked increase in IgG, particularly IgG₄ 41 (Fig. 2). For example, grass pollen SCIT resulted in a decrease in numbers of mast cells 42 and eosinophils 43 in the skin at sites of suppressed allergen-induced late cutaneous responses. Subcutaneous immunotherapy suppressed late nasal responses that paralleled decreases in local c-kit⁺ mast cells 44, eosinophils 45 and basophils 46 in the nasal mucosa. Recent studies have shown suppression of basophil activation 47, 48 and a decrease in circulating ILC2s 49.

Akdis et al. 50 first introduced the concept of early involvement of regulatory T (T reg) cells. Bee venom SCIT resulted in increased IL-10 production from both blood T and B cells. T reg involvement was confirmed by flow cytometry following grass and house dust mite SCIT 51, 52. Local induction of T regs following grass SCIT was shown by increases in IL-10⁺ 53 and TGF-beta⁺ T cells 54 in the nasal mucosa that accompanied increases in serum IgG₄ and IgA (in keeping with their known properties in promoting preferential switching of B cells in favour of IgG₄ and IgA) and an increase in local nasal FOXP3⁺CD25⁺, IL-10-producing T cells 55 by immunofluorescence histology.

The mechanism of SLIT has been shown to be broadly similar to SCIT; in particular, SLIT also induces IL-10 production by T cells 56. Nasal and conjunctival eosinophils and adhesion molecules decrease following mite SLIT 57. Serum allergen-IgG increased after birch SLIT. Higher local numbers of CD4⁺FOXP3⁺ and CD25⁺FOXP3⁺ T cells were found in the sublingual mucosa and increases in peripheral allergen-specific IgG (IgG₁ and IgG₄) and IgA₂ following grass SLIT 58, 59. Blood Th1 cells increased and blood Th2 cells decreased following birch SLIT 60. Peripheral CD25⁺FOXP3⁺, presumed T reg cells, increased after SLIT 61, findings confirmed in relation to grass SLIT 62. Two recent studies showed that birch 63 and mite 64 SLIT resulted in early increases in T regs at 4–6 weeks and delayed increases in allergen-specific Th1 cells at 12 months. The mechanism of SCIT and SLIT may differ in terms of kinetics, quality and quantity of circulating antibody. Transient early increases in specific IgE are greater after SLIT compared to SCIT, whereas both inhibit seasonal increases in IgE to the same degree. Conversely, immunoreactive IgG levels are approximately 10-fold less for SLIT compared to SCIT 59, 65, whereas both are accompanied by equivalent increases in serum IgG-associated IgE-blocking activity. This disparity suggests that IgG antibodies produced by SLIT may be more functionally active, with greater avidity or affinity for allergen. Although speculative, this could possibly occur as a consequence of more efficient antigen processing and/or T–B cell cooperation in the local environment of the sublingual mucosa and draining cervical lymph glands 66.

The LEAP and LEAP ON studies have brought major advance to our understanding of primary prevention of food allergy, as the number of high risk individuals regularly exposed to allergen acquired fivefold less food allergy than control individuals who avoided allergen, as suggested by international guidelines 21.

The demands to OIT are high as consumption of allergen may result in severe symptoms 67. Adverse side-effects during conventional OIT can be severe, and only half of the enrolled patients reach maintenance dose 23, 68. Half again could be maintained on maintenance dose for planned duration of treatment, and between a quarter and half of these patients had sustained unresponsiveness or developed persistent tolerance 24, 67. To improve this performance, treatment is attempted with hypoallergic allergens 25 or under a blanket of anti-IgE treatment to limit side-effects 26. Persistent biomarker changes associated with successful primary prevention and OIT are a reduction of skin prick test (SPT) response and improvement in basophil reactivity and sensitivity 67, 69.

Biomarkers for diagnosis and monitoring in AIT trials

Patients with an indication for AIT respond with individual side-effect profiles, degree of symptom relief and persistence of treatment effect (Table 2) 70-74. This may be due to the diversity of their pattern of IgE sensitization (i.e. involvement of seasonal vs perennial allergens, mono- vs polysensitization), varying genetic susceptibility and comorbidities. Identification of biomarkers that predict benefit of AIT for the individual is therefore of high priority. According to the EMA guideline, biomarkers can be analysed in dose-finding studies but need to be validated 29.

Knowledge of mechanisms underlying AIT has been translated into developing biomarkers beyond specific IgE to predict efficacy and side-effects, and to monitor clinical response to and persistence of immunotherapy. Basophil sensitivity 48 and IgG-associated serum inhibitory activity for IgE-facilitated binding of allergen–IgE complexes to B cells (IgE-FAB) correlated closely with clinical response to immunotherapy 75. The latter assay has been standardized 76 and translated into a cell-free solid phase ELISA-based assay 77. Allergen-stimulated basophil activation has been validated 73.

Novel approaches for AIT

A better understanding of mechanisms has been translated into novel immunotherapy approaches. Targeting IgE by anti-IgE antibody in combination with AIT reduced the incidence of side-effects during immunotherapy and resulted in prolonged suppression of IgE-FAB for longer than the half-life of anti-IgE, suggesting this strategy may have a durable effect 91. The combination of anti-IL-4 with AIT was effective in suppressing circulating Th2 cells and allergen-induced late responses although it had no obvious advantages over allergen extract alone 92. Targeting immune deviation using the TLR4 agonist monophosphoryl lipid A in combination with AIT was effective with four preseasonal injections without an increase in side-effects 93. The use of Bacterial DNA oligonucleotides rich in CpG sequences, covalently linked to the major ragweed allergen Amb a 1, was effective, possibly by inducing T regs and/or immune deviation although this has not been pursued 94. Further strategies such as targeting epithelial cytokines thymic stromal lymphopoietin and IL-33 in combination with AIT or possibly the combined use of probiotics with AIT are yet to be tested. Strategies to reduce allergic adverse events in AIT include the use of engineered recombinant hypoallergenic molecules, allergen multimers and allergen fragments including peptides. These approaches rely on alterations to the structure of the allergen protein reducing the ability to cross-link IgE whilst maintaining the ability to target allergen-specific T cells and induce immune modulation. The following emerging approaches aim at optimizing delivery and immunization schemes towards shorter course therapy to improve compliance by applying targeted delivery to immune compartments or peptide-based technologies.

Conclusion

Allergic diseases are underdiagnosed and undertreated. Allergen immunotherapy has not yet received adequate attention from European (public) institutions, and many allergic patients remain unaware of the potential benefit of AIT. Areas for improvement and innovation are found through a better understanding of the mode of action and dose effects, which will lead to improved delivery and immunization schedules. More effective preparations with faster onset and reduced doses are likely to improve compliance. A previous trial of AIT for asthma prevention had methodological limitations, so the recent results from the randomized, double-blind placebo-controlled GAP trial are encouraging. With improvement in grading of evidence, standardizing clinical outcomes, we can expect better studies in general in this area. Identification of biomarkers for improved patient selection and monitoring will increase the value and usefulness of AIT.

In summary, it is time to re-evaluate the allergy paradigm and implement new treatment approaches with a special focus on AIT as allergic individuals are becoming a significant minority of Western and global populations. Innovation at all levels and national and regional action plans, such as the Finnish Allergy Programme, are needed to meet the challenge. This first interdisciplinary Aarhus Immunotherapy Symposium brought together specialists from different disciplines to document exciting developments in the field of AIT and provided hope for improvement of this therapeutic option for allergic patients in the future.

Author contributions

EV, OP, PM, JMS, SHS, LOC, KS, ML and SRD contributed specific sections of the manuscript. PS and HJH wrote introduction and conclusion. All authors reviewed the final document.

Conflicts of interest

Drs. Hoffmann, Schmid and Cardell have nothing to disclose. Dr. Valovirta reports grants and personal fees from ALK-Abello, personal fees from TEVA and Stallergenes-Greer and personal fees and other from MEDA, outside the submitted work. Dr. Pfaar reports grants and personal fees from ALK Abelló, Allergopharma, Stallergenes-Greer, Allergy Therapeutics/Bencard Allergie gmbH, Anergis S.A., Lofarma, Biotech Tools S.A., Laboratorios LETI/LETI Pharma, Novartis Pharma; HAL Allergy Holding B.V./HAL-Allergie gmbH, grants from Biomay, Nuvo and Circassia; and personal fees from MEDA Pharma, Sanofi Aventis, Mobile Chamber Experts (a GA2LEN Partner) and Pohl-Boskamp, outside the submitted work. Dr Durham has received research funding from ALK Denmark, Merck USA and Regeneron USA and consulting fees from Merck, Stallergenes, Allergy Therapeutics, Circassia, Anergis, Biomay and Boehringer Ingelheim. Dr. Skaarup reports fees from UsABCD for lectures on clinical ultrasound. Dr Simonsen is an employee of Anergis SA. Dr. Larché reports grants and personal fees from Adiga Life Sciences and grants, personal fees and other from Circassia Ltd/Circassia Pharmaceuticals plc, during the conduct of the study; personal fees from UCB, Sanofi, Aravax Pty, Brandon Capital Ventures and Air Canada, outside the submitted work; in addition, Dr. Larché is an inventor on 17 patent families (pending or issued) held by Circassia Ltd. Dr. Moingeon and Dr. Sørensen are employees of Stallergenes-Greer.