Peptide - Based Immunotherapy for Food Allergy
3. Rationale, Strategies, and Potential Mechanisms of PIT
effects. For this reason, new approaches are being investigated. In line with the scope of this book, the utilization of peptides will be the main topic described in this section. The concept behind pep- tide - based immunotherapy is not new and stems from the development of peptide - based vaccines designed to elicit a protective immune response toward pathogens such as viruses, bacteria, and parasites. Such vaccines are designed to incorporate immunoreactive regions derived from the protein native structure and give rise to a specifi c immune response toward the intact protein. In the context of food allergy, the terminology associated with a good therapeutic prognosis remains controversial and is often referred to as “ desensitization ” or “ tolerance ” induction (Niggemann et al. 2006 ). A recent mono- graph emphasized that discrimination should be made between true “ tolerance, ” which refers to situ- ations where the food may be ingested without trig- gering any allergy symptoms even during elimination periods, and “ desensitization, ” which refers to cases where the allergen is ingested without symptoms during treatment but requires a maintenance regimen (Nowak - Wegrzyn and Sicherer 2008 ). Within this conceptual framework, a PIT - based approach aims at designing so - called “ tolerogenic ” peptides (Pecquet et al. 2000 ), that is, peptides capable of inducing a status of immune tolerance toward the allergen(s) of interest. Specifi c to food allergy, the rationale of PIT relies on the use of nonanaphylactic (synthetic or not) peptides, unable to crosslink IgE molecules (Ferreira et al. 2004 ). The reduced ability of peptides to react with IgE will prevent not only mediator release from mast cells and baso- phils (Figure 8.1 ) but also the uptake on IgE via Fc epsilon receptors of antigen presenting cells, which may contribute to an amplifi cation of the allergic response.
3.1.2. Strategies Potential PIT - based strategies for food allergy are presented in the following para- graphs based on the source of peptides (i.e., synthe- tisized vs. enzymatically obtained peptides) and on the nature of the immunogenic sequences contained in the preparation (i.e., T - cell epitopes vs. B - cell epitopes).
solution, particularly in young children, as it may lead to malnutrition (Sampson 1999 ). Additionally, recent studies have suggested that an elimination diet may not represent the safest alternative for aller- gic patients and may in fact have a deleterious impact by lowering the patient reactivity threshold (Morisset et al. 2007 ; Rolinck - Werninghaus et al.
2005 ). Current pharmacological treatments mainly induce symptomatic relief through the use of agents such as antihistamines, steroids, and, in severe cases, epinephrine (Thompson and Chandra 2002 ).
2.6. Overview of Novel Immunotherapeutic Strategies
The current lack of effi cient treatments for food allergy has prompted the development of novel pre- ventive and therapeutic approaches that can provide safer alternatives for combatting food allergies. This chapter focuses on the key fi ndings brought by pep- tide - based approaches. The interested reader is referred to a number of recent reviews focusing on the various immunotherapeutic strategies currently investigated for the treatment and prevention of IgE - mediated food allergy (Sicherer and Sampson 2008 ; Nowak - Wegrzyn and Sicherer 2008 ), as are listed in Table 8.1 . Immunotherapeutic approaches can be divided into specifi c and nonspecifi c approaches.
Specifi c immunotherapy (SIT) targets the individual allergens responsible for the patient ’ s disease, whereas nonspecifi c immunotherapy aims at modu- lating the immune system in an allergen - indepen- dent manner (Mine and Yang 2008 ). Specifi c immunotherapies can be further divided into those using native forms of food allergens and those using recombinant or altered forms of allergens.
3. Rationale, Strategies, and Potential
Table 8.1. Potential immunotherapeutic approaches for IgE - mediated food allergy. Adapted from Sicherer and Sampson 2008 ; Nowak - Wegrzyn and Sicherer 2008 .
Immunotherapeutic Approaches Rationale Current Status/Comments
Specifi c immunotherapy with native forms of food allergens
Subcutaneous injection Antigen presentation in nonmucosal sites favors Th1 - biased response
No active development due to safety concerns; i.e., risks of severe side effects Oral administration Antigen presentation at mucosal sites
leads to desensitization and/or immune tolerance
Effi ciency documented by nonrandomized clinical trials and case reports only.
Highly sensitive and anaphylactic patients remain excluded due to safety concerns.
Sublingual administration Induction of “ tolerogenic ” antigen presenting cells leads to immune tolerance.
Preliminary data, including randomized DBPC trials, indicate promising results, but protocol optimization is still required.
Specifi c immunotherapy with altered forms of food allergens
Recombinant mutant proteins Targeted mutations of IgE - binding sites
Effi ciency documented in mouse models.
Human studies envisaged Peptide - based immunotherapy Short peptides lack crosslinking
activity but maintain T - cell activation potential.
Experimental studies using murine models expected to shed light on underlying mechanisms
Conjugation with ISS sequences Enhance Th1 response by activating innate immune receptors
Isolated reports documented effi ciency using murine models. No recent development reported in food allergy Plasmid DNA - encoded allergens Endogenous production of allergens
may favor tolerance.
No active development in food allergy Chemically or physically modifi ed
allergens
Hypoallergenic preparation reduces risks of inducing severe side effects.
Applications likely to exist in the context of food industry
Nonspecifi c immunotherapy (immunomodulatory treatments)
Anti - IgE antibodies Inactivate and bind IgE. Recommended use in combination with specifi c forms of immunotherapy
Well investigated in the context of peanut allergy e.g., FDA - approved drug Omalizumab (trade name Xolair).
Cytokine/anticytokine agents Blocks infl ammatory signals Promising results obtained with murine studies. Anti - IL5 therapy being
investigated for eosinophilic eosophagitis Chinese herbal medicine Immunosuppressive mechanisms
remain to be elucidated.
Promising results obtained. Safety studies in human under way. Identifi cation of active component ongoing
Probiotics Stems from the conceptual framework
of the “ hygiene hypothesis ”
Immunomodulatory effects still under investigation. Elucidation of underlying mechanisms under way; e.g., innate receptors may be involved DBPC, Double - Blind Placebo Controlled.
3.1.2.1. Use of Overlapping Synthetic Peptides Under experimental conditions, a library of overlap- ping synthetic peptides covering the entire sequence of the native protein is often used as a preliminary experiment to the identifi cation of T - cell epitope regions. In a therapeutic context, the main advantage of using a series of overlapping peptides resides in the fact that the mapping or mutation of the epitope
regions is no longer required (Nowak - Wegrzyn and Sicherer 2008 ). This approach has been documented in murine models of peanut allergy, as will be described in section 4 .
3.1.2.2. Food - Based Hydrolyzates Together with synthetic peptide libraries, the use of food - based hydrolyzates represents a more random and
3.1.2.4. T - cell Epitope - Based Immunotherapy The epitope mapping trend was initiated in the early 1980s with the identifi cation of T - cell epitopes in parasitic and viral proteins and stands as a milestone in the development of vaccines (De Groot and Moise 2007 ). Within the same conceptual framework, T - cell epitope - containing peptides were put forward as an effi cient approach toward the induction of tolerance in food allergy. By virtue of its central role in the pathoetiology of food allergy, the T - cell com- partment represents a prime target for therapeutic interventions. For a preparation to be useful in immunotherapy, it is suggested that immunodomi- nant T - cell epitopes be present because of the ben- efi cial effects mediated by T lymphocytes (Zeiler and Virtanen 2008 ). The rationale behind this strategy relies on the use of peptides lacking the secondary or tertiary structure responsible for the crosslinking of IgE — similar to the use of food - based hydrolyzates — but the main requirement is that peptides be capable of maintaining their T - cell activation potential. Studies carried out in animal models of respiratory allergies as well as in clinical settings have provided proof - of - principle demon- strating the induction of hyporesponsiveness to the allergen (Larch é 2005 ). A clear association with the production of regulatory cytokine IL - 10 was high- lighted in these studies. We have investigated in our laboratory the feasibility of such an approach using a BALB/c mouse model of egg allergy, as presented in section 4 .
3.2. Advantages of
Peptide - Based Immunotherapy
Peptide - based immunotherapy stands as an appeal- ing approach based on the criterion that the manu- facturing of nonsynthetic (e.g., food hydrolyzates) or synthetic peptides has today become a low - cost routine exercise (Thomas 2004 ). Standardization of synthesis methods, characterization of peptides, and analysis for purity are currently carried out using well - established analytical techniques, for example, liquid chromatography and mass spec- trometry. This presents signifi cant benefi ts com- pared to other approaches such as the diligent task probably less specifi c approach compared to inter-
ventions based on T - or B - cell epitopes. This approach has been particularly investigated in a prophylactic context and relies on the notion that peptides obtained from the enzymatic hydrolysis of native protein lack secondary structures — by virtue of their sizes — a feature that is necessary for the process of allergen sensitization and/or the crosslinking of IgE at the surface of mast cell and basophils. The nature of their immunomodulatory properties (e.g., peptide structures) remains, however, to be elucidated. The approach has been particularly researched in the context of cow ’ s milk allergy, one of the most prevalent forms of food allergy in infants and young children. The preven- tive use of milk - based infant hydrolyzed formula is discussed in section 4 .
3.1.2.3. B - cell Epitope - Based Immunotherapy (IgG - Binding) The approach has been docu- mented by a few reports using respiratory allergens represented by grass and birch pollen allergens in mice models (Vrtala et al. 2004 ; Focke et al. 2001, 2004 ). The authors tested the prophylactic effects of synthetic peptides (25 – 32 amino acids) rich in sol- vent - exposed amino acids but lacking both second- ary structure and T - cell activating potential. The study hypothesized that such peptides might favor production of allergen - specifi c IgG to the detriment of specifi c IgE, in line with the “ blocking IgG ” theory. This theory is based on the belief that aller- gen - specifi c IgG can exert protective effects through mechanisms involving either (1) the direct inhibi- tion of IgE - mediated release of infl ammatory medi- ators from mast cells and basophils or (2) the inhibition of IgE - facilitated antigen presentation to T cells (Wachholz and Durham 2004 ; Larch é and Wraith 2005 ). This latter mechanism relies on the presence of specifi c - IgE bound to high - (Fc ε RI) or low - affi nity IgE receptors (Fc ε RII, CD23) present at the surface of antigen - presenting cells, which can effi ciently capture the allergen and promote its presentation to T cells (Wilcock et al.
2006 ; van Neerven et al. 2006 ). No such approach has yet been documented with models of food allergy.
(Kern et al. 1998 ; Larch é and Wraith 2005 ; Hochweller et al. 2006 ). In humans, it has been suggested that this phenomenon could involve the activity of chaperone molecules such as HLA - DM (Santambrogio et al. 1999 ). Once complexed with MHC molecules, how T - cell determinants induce regulatory mechanisms is poorly understood. It has been proposed that peptides may bind to MHC mol- ecules without providing the co - stimulatory signals (e.g., CD80/CD86 family) required for activation and maturation of APCs (Hochweller et al. 2006 ), an activation state coined “ tolerogenic. ” As a result, presentation of peptide - MHC complexes by “ tolero- genic ” APC may yield to hyporesponsive allergen - specifi c T cells and to the generation of regulatory T cells.
In the context of food allergy, PIT - based strate- gies aim at the modulation of CD4+ T cells and, more particularly, at the immune balance existing between Th1, Th2, and Treg cells (Figure 8.2 ). Of the different PIT - based strategies, T - cell epitope - PIT represents a more targeted approach and receives, therefore, greater attention in current research. In clinical trials with cat allergic patients, T - cell epitope - based IT were documented to acti- vate regulatory events involving regulatory cyto- kines IL - 10, TGF - β , and FOXP3 - expressing cells.
Based on knowledge provided by current literature, we schematically represented the potential mecha- nisms underlying PIT based on the use of T - cell epitope peptides (Figure 8.4 ). Further investigation will be required to shed light on the intricacies of these complex events.
4. Current Experimental Models of PIT