**4. Targeting T cells in allergen-specific immunotherapy**

Allergy and asthma are debilitating diseases that are most commonly treated using pharmacotherapy which are designed to improve the symptoms but not the cause of disease. To date, the only disease-modifying therapy available is allergen-specific immunotherapy (AIT). First administered over a century ago [47], AIT has been widely demonstrated to be a clinically effective treatment, inducing immunological tolerance and improvement of clinical symptoms beyond the time of treatment [48]. Despite its favorable duration of efficacy, a considerable effort is invested to improve current AIT protocols. Allergen-specific immunotherapy with whole extract can be associated with IgE-mediated adverse reactions that result from the patient's allergen-specific IgE molecules being cross-linked by the allergen present in the extract used for treatment, triggering degranulation and immediate-type reactions. The occurrence of such adverse events and the need for extended treatment periods that last several years can have a negative impact on treatment compliance. For this reason, researchers have strived to find a treatment that targets T cells and circumvents potential IgE reactivity. Removal of IgE epitopes, thereby eliminating the risk of IgE cross-linking, is one obvious approach. There are a variety of methods to achieve this goal, some of which have been evaluated in clinical trials.

#### **4.1. Peptide immunotherapy**

One extensively pursued approach for AIT focused on T cells while omitting IgE epitopes is called peptide immunotherapy, where instead of using whole allergen extract, allergic patients are treated with a mixture of short, synthetic peptides that constitute the major T cell epitopes of the allergen the patient is allergic to. The clinical efficacy of peptide immunotherapy has been demonstrated in several Phase IIb double-blind, placebo-controlled trials [49, 50]. A significant reduction in symptoms, measured as the total rhinoconjunctivitis symptom score (TRSS), was observed following the administration of only eight intradermal injections of the peptide formulation. In this study, TRSS levels remained suppressed both at the 1- and 2-year follow-up time point [51]. The immunological mechanisms by which peptide immunotherapy induces tolerance are not yet fully understood. However, studies have reported a downregulation of pathological type 2 cell responses and a concomitant increase in regulatory signals, such as the production of IL-10 in the periphery. Further, significant increases in IFNg-producing Th1 cells and CD25+ cells have been reported. The induction of IgG4-blocking antibodies, which are believed to contribute to clinical efficacy by occupying the allergen-binding sites, thereby preventing IgE-allergen binding, is a hallmark event during conventional AIT with allergen extract. Interestingly, increased levels of IgG4 are rarely observed, probably due to the lack of conformational B cell epitopes decreasing the likelihood of B cell stimulation and resulting IgG production. Therefore, though modulatory events on the cellular level appear to be broadly similar to those believed to occur during extract-based AIT, humoral responses may be more distinct. Although peptide immunotherapy has been shown to be clinically effective, it is also associated with challenges that need to be addressed. The route of administration has been debated, and the clinical effects seem to be very sensitive to dosing. Lower doses may not induce tolerance due to lack of potency for induction of regulatory T cells, while too high dose may stimulate and expand pathogenic Th2 cells. The selection of peptides is also a factor of consideration. Typically, mixtures used for peptide immunotherapy include between 5 and 10 peptides. However, epitope specificities can be very heterologous in a given population, and therefore the selection may not be straightforward. The consideration of these factors and others make the development of peptide immunotherapy challenging at times.

#### **4.2. Fragmented allergens**

Another approach of AIT that was designed to target T cells while bypassing IgE binding to avoid IgE-mediated side effects is the generation of fragmented allergens. This approach was tested using the major birch pollen allergen, Bet v 1, as a model. The fragmentation of the allergen involved its division into non-IgE-binding fragments, which retain their T cell reactivity. Birch pollen allergic patients were then vaccinated with these hypoallergenic derivatives in a double-blind, placebo-controlled study. This vaccination was found to reduce cutaneous sensitivity, improve symptoms, and significantly reduce rises in birch-specific IgE levels during season in the active group compared to placebo [52]. However, immunological mechanisms and long-term efficacy were not evaluated.
