4 DISCUSSION
AIT is the only disease modifying treatment to treat the cause of allergy. Several immune cells and mediators contribute to various degrees to the severity of the allergic reaction. Mast cells and basophils are effector cells of the early phase that are involved in tissue damage, itching and swelling. However, ablating those cells may be technically difficult and also be potentially dangerous, as they are also involved in the fighting against tick and helminth infections, detoxification of arthropod and reptile venoms, and, as recently shown, preserving cardiac function after myocardial infarction43-45. B cells expressing as well as IgE and Th2 cells specific for the respective allergen are therefore potentially better targets. However, allergen-specific lymphocytes do not have a unique characteristic to specifically delete them other than the T cell receptor which is notoriously difficult to target. AIT focusses on rendering the body more tolerant to the allergen by changing the immune environment and induction of IgG antibodies25,46,47. The high potential of IgG antibodies to reduce the severity of allergic reactions has been shown in mouse models30,40,32 and more recently in a clinical study demonstrating that it is possible to treat cat allergy with monoclonal anti-Fel d 1 IgG antibodies34. In addition, induction of allergen-specific IgG4 or allergen-specific IgE/IgG4 ratio are considered to be the best correlate of AIT efficacy. However, whether IgG4 is more potent than other IgG subclasses or merely happens to be the preferred IgG subclass induced by classical AIT remis still a matter of debate. In fact, induction of IgG4 during classical AIT may actually reflects to some degree natural allergen exposure as seen in bee keepers who are not allergic to bee venom but have high serum levels of specific IgG448,49. Indeed, inducing B cell responses in the absence of innate stimuli, such as toll-like receptor ligands, may preferentially drive IgG4 responses12,36.
Nevertheless, the role and importance of the induced IgG subclasses during AIT may have important consequences, since use of modern vaccination regimens such as inclusion of stronger adjuvants or formulation with virus-like particles (VLPs) may favour induction of IgG1 rather than IgG430. Indeed, a clinical study to treat house dust mite with Der p 1 coupled to a VLP induced strong IgG1 instead of IgG4 responses26. Another important aspect of the safety of AIT is the availability, standardization, and formulation of allergens. Given the fact that allergens coupled to VLPs do not activate basophils50, but induce strong IgG responses in compared to other approaches, may render AITs safer and more efficient in future; however, the ability of IgG1 to block the allergic reaction remains an important caveat for such new therapies.
To investigate whether IgG4 was more potent than other IgG subclasses at blocking basophil activation, we expressed 3 different monoclonal antibodies recognizing distinct epitopes on the allergen Fel d 1 in a human IgG1 and IgG4 format40. We compared 1) the ability to neutralize the allergen and block basophil activation, 2) to inhibit basophil activation via engagement of FcγRIIβ, and 3) the ability of the IgG1 and IgG4 subclasses to bind to FcγRIIβ. In all 3 types of assays, IgG1 and IgG4 antibodies showed similar efficacy at blocking basophil activation and engaging FcγRIIβ. Hence, IgG4 does not have preferable characteristics for the treatment of allergy. This may indicate that induction of IgG4 is not a pre-requisite for efficient therapy but that amounts and affinities of total IgG may be more important.