Patching it together: epicutaneous vaccination with heat‐labile Escherichia coli toxin against birch pollen allergy

In immunotherapy, a person’s immune system is utilized to induce or suppress specific immune responses that are needed for the prevention or treatment of disease. This requires the use of vaccines (biological preparations of disease-causing agents) that elicit a coordinated response by the adaptive and innate immune systems, with professional antigen-presenting cells such as dendritic cells playing a crucial role. Successful immunotherapy results in the induction of broad and specific immune responses involving B and/or T cells depending on vaccine design (1). Vaccine development is considered among the greatest achievements in the history of medicine (2); they can be applied in the treatment of allergic and autoimmune diseases, organ transplantation, chronic infection, and even cancer (3, 4). Allergen-specific immunotherapy (AIT) entails administration of increasing doses of allergens in an attempt to restore tolerance and reduce allergic symptoms, once a patient has identified the root cause(s) of their allergy. The allergen can be given intramuscularly, subcutaneously, sublingually, orally, or intralymphatically. The doses of allergen are increased as the patient’s tolerance grows over time (5). AIT has been used for IgE-mediated allergic diseases since the early 1900s (3, 6–11), although it remains fraught with problems (3, 6–11). For example, it is difficult to produce a strictly regulated composition for the allergenic compounds and adjuvants used, or to establish consensus in regard to the doses, intervals, and length of application. The danger of anaphylaxis occurring during administration of AIT also remains high. When successful however, AIT treats and prevents development of allergic asthma, rhinitis, and venom-induced anaphylaxis and was demonstrated to prevent sensitization with new allergens for up to 12 years (3, 12). Therefore, the quest to develop safer and more effective AIT continues today. Induction of allergen tolerance requires a decrease in IgE and increase in allergen-specific IgG levels (13). In this issue of Allergy, Cabauatan et al. describe birch pollen (rBet v 1)specific AIT in a guinea pig model, using a patch delivery system (originally developed for epicutaneous vaccination against traveler’s diarrhea). They chose guinea pigs to work with because their skin has similar features to human skin and the patch delivery system of the same size is used for humans (14). To enhance IgG levels, Cabauatan and colleagues applied heat-labile Escherichia coli (E. coli) toxin (LT) as adjuvant together with the rBet v 1 birch allergen in the skin patch and compared the effects with a traditional alum-adsorbed rBet v 1 vaccine given subcutaneously. Patch vaccination with a high allergen dose induced relevant allergen-specific IgG responses (similar to that of the subcutaneous immunization) but only when the patch contained LT. These responses were detected on day 43 after only three vaccinations, even though outbred guinea pigs might be poor responders for the rBet v 1 allergen. During successful allergen-specific immunotherapy, changes in IgG and in particular in IgG4 can result in a 10to 100-fold reduction in the allergen-specific IgE/IgG4 ratio (15). IgG4 is a nonproinflammatory immunoglobulin that is uniquely capable to form bispecific and functionally monovalent antibodies through Fab arm exchange (16), has low affinity for activating Fcc receptors, and cannot activate complement. Allergen-specific IgG4 protects against allergic responses by competitively sequestering the allergen from IgE binding (13, 17, 18). It is significant therefore that the LT adjuvant in this paper was essential for the induction of allergen-specific IgG. According to the authors, in an unpublished clinical study epicutaneous application of a high dose of rBet v 1 (without adjuvant) only induced T-cell responses but no relevant allergen-specific IgG. The authors here demonstrate that sera derived from the guinea pigs with robust rBet v 1specific IgG responses inhibited the binding of allergic patients’ IgE to the rBet v 1 allergen. This elegant result indicated that patch vaccination using LT as adjuvant can induce allergen-specific blocking IgG. By changing the ratio of allergen-specific IgE and IgG, effective AIT should desensitize the high-affinity IgE-receptor (FceRI)-bearing mast cells and basophils, decrease the numbers and activity of eosinophils and basophils in the circulation and mast cells in mucosal allergic tissues, and induce regulatory Tand B-cell activation (reviewed in (4)). To assess the clinical significance of the findings of Cabauatan and colleagues, it would be important to verify whether allergen-specific patch vaccination with LT would induce these changes. The heat-labile enterotoxigenic E. coli enterotoxin (LT) contains an ‘A’ and a pentameric ‘B’ subunit (AB5). The highly enterotoxic A subunit has ADP-ribosyltransferase activity. It activates the Gsa component of adenylate cyclase leading to elevation of intracellular cAMP, activation of cAMP-dependent PKA, and inhibition of NF-jB-dependent transcription of proinflammatory cytokines (such as TNF-a). The A subunit is delivered into the cell by the nontoxic B pentamer subunit. Given the toxic nature of the A subunit, for vaccination purposes only the engineered B subunit and various mutant derivatives are used. The main receptors for LT are the ubiquitous gangliosides, but it also binds TLR-2