Peptide - Based Immunotherapy for Food Allergy
4. Current Experimental Models of PIT in 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
4.1. Cow ’ s Milk Allergy Model
Cow ’ s milk allergy is mainly reported as the most prevalent food allergy in infants and young children, and it affects 2 – 3% of the general population (Skripak et al. 2007 ). First, it is important that a distinction be made between the concept of “ hypoal- lergenicity ” and “ tolerogenicity. ” Hypoallergenicity following section how milk allergy has been the
ongoing focus of peptide - based approaches using enzymatic hydrolyzed formulas, while peanut and egg allergies have been the object of very recent PIT investigations using the mouse model of food allergy. Other allergen models, such as wheat and beef proteins, have been the focus of more scarce investigations.
Figure 8.4. Potential mechanisms underlying T - cell epitope - based immunotherapy. Upon peptide administration, exposure to multiple allergen T - cell epitopes may lead to activation of a population of FOXP3 - and TGF - β - expressing cells, which are likely to be represented by CD4 + CD25 + regulatory T cells and Th3 cells, respectively. TGF - β was documented to be capable of inducing FOXP3 expression in peripheral tissues and stimulating production of allergen - specifi c IgA at mucosal surfaces.
As a result of the functional counter - inhibition between Th1 - and Th2 - biased responses, a Th1 - cytokine profi le may predomi- nate, leading to the production of allergen - specifi c IgG in line with the “ blocking IgG ” theory. APC = antigen presenting cells; CD = cluster of differentiation; FOXP3 = Forkhead box protein 3; IFN = interferon; IL = interleukin; Ig = immunoglobulin;
Th = T helper cells; TGF - β = transforming growth factor - β .
derived from the peptic digestion of peanut allergens, which contained T - cell epitopes but no IgE - binding epitopes, could preferentially induce production of IFN - γ (Th1 - cytokine) in PBMC cul- tures, in a concentration - dependent manner (Hong et al. 1999 ). Additionally, a recent review reported unpublished results demonstrating that the repeated subcutaneous administration of 30 overlapping pep- tides (3 doses/week for 4 weeks), covering the entire sequence of the major peanut allergen Ara h 2, led to a signifi cant reduction of specifi c IgE and hista- mine levels in peanut - sensitized C3H/HeJ mice, compared to placebo - treated mice. These changes were accompanied by lower anaphylaxis scores and increased production of IFN - γ by splenocytes cul- tured in vitro with Ara h 2 (Sicherer and Sampson 2007 ). However, to the best of our knowledge, no further investigations were reported.
4.3. Egg Allergy Model
Egg allergy was documented as the second - most - common food allergy in children in the United States (Eggesb ø et al. 2001 ; Heine et al. 2006 ; Sampson 2004 ). In European and Asian industrial- ized countries, egg allergy has recently been reported as the most prevalent food hypersensitivity in the pediatric population, exceeding that of cow ’ s milk allergy (Crespo et al. 1995 ; Imai and Iikura 2003 ; Teuber et al. 2006 ; Han et al. 2004 ; Gustafsson et al. 2003 ; Chiang et al. 2007 ).
In vivo studies using BALB/c mice were recently carried out in the laboratory and provided supportive evidence to the therapeutic potential of a peptide - based approach using epitope - containing peptides.
Investigations of the immunomodulatory properties of a preparation of egg white hydrolyzate led to identifi cation of immunodominant B - and T - cell epitope sequences of OVA. Using a BALB/c model of orally induced egg allergy, the oral administration of the egg hydrolyzate was shown to decrease the level of specifi c IgE and histamine titers. Spleen cell cultures revealed inhibitory effects on both allergen - induced Th1 and Th2 responses, while gene expression analyses indicated the induction of regulatory mechanisms involving FOXP3 and refers to a situation where the food material is made
into a preparation lacking IgE - binding activity and aiming at preventing an allergic response. On the other hand, the notion of “ tolerogenicity ” refers to the active induction of immune tolerance for the allergen prior to any exposure (prophylactic effect) or the capacity to downregulate allergic responses in already sensitized individuals (therapeutic effect).
There are only a few examples of hypoallergenic foods that have been brought to the market, and enzymatically hydrolyzed milk proteins for infant formulations is probably the best - known example (Hays and Wood 2005 ). In spite of a number of randomized clinical trials, the prophylactic value of milk hydrolyzed formulas remains controversial (Osborn and Sinn 2006 ); and no therapeutic value has been attributed to any of them. A few years ago, interesting studies using murine models reported the isolation of so - designated “ tolerogenic ” peptides released from the tryptic digestion of β - lactoglobulin, a major milk allergen. Using a rat model, it was shown that a partially hydrolyzed formula adminis- tered before and during sensitization could induce a suppressive effect on the production of specifi c IgE, mast cell sensitization, and allergen - induced lym- phocyte proliferation (Fritsch é et al. 1997 ). Further studies by the same authors identifi ed several tolero- genic peptides with sizes ranging from 8 to 23 amino acids, establishing a fi rst step toward the understand- ing of the correlation between peptide structure and tolerogenicity (Pecquet et al. 2000 ). Whether such tolerogenic peptides may be used to induce immune tolerance in a therapeutic manner remains to be investigated, and translation of these fi ndings into human applications demands further investigation.
4.2. Peanut Allergy Model
Responsible for the most clinically severe form of food allergy, the peanut has been and continues to be the focus of intensive research. Various immu- notherapeutic approaches were investigated using peanut allergy as a model, and multiple reviews have focused on the disease (Li 2005 ; de Leon et al.
2007 ). Of the reports investigating peptide - based approaches, one in vitro study showed that peptides
treatment of allergic disorders. It is thus interesting to present some of the key results obtained. Several inhalant allergens have, indeed, been the object of PIT investigations: the major cat allergen Fel d 1, the bee venom allergen Api m 1 (Fellrath et al. 2003 ; Tarzi et al. 2006 ), and more recently the ragweed allergen Amb a 1 (Larch é 2007 ) and the dog aller- gen Can f 1 (Virtanen 2006 ). The most promising fi ndings were obtained with cat allergens, as evi- denced by studies completed in clinical settings (Larch é 2007 ). Administration of peptides contain- ing T - cell epitopes led to increased pulmonary functions in cat allergic patients and allowed exposure to cats with reduced symptoms (Maguire et al. 1999 ). In a double - blind placebo - controlled study, production of IL - 10 was increased in PBMC obtained from patients with cat allergic asthma after treatment with Fel d 1 peptides (Oldfi eld et al.
2002 ). Reduction in allergen - specifi c proliferative response as well as in the production of Th1 - biased and Th2 - biased cytokines was also observed. Co - culture experiments using PBMCs led to the isola- tion of CD4 + cells capable of actively suppressing allergen - specifi c proliferative responses of CD4 − cells obtained prior to peptide treatment (Verhoef et al. 2005 ). In the context of food allergy, it is reason- able to expect that similar phenomena may occur.
However, they may also differ on the basis of the differences between the sensitization mechanisms occurring at respiratory vs. intestinal mucosa.
5. Translation of PIT into Human