Allergen-specific T lymphocytes as targets for specific immunotherapy: striking at the roots of type I allergy

In the past decades allergic diseases have tremendously increased and hypersensitivity reactions represent a growing health concern in industrialized countries. Despite various effective therapeutic options for the treatment of allergic diseases, only specific immunotherapy (SIT) has been shown to have effects on the underlying immunological mechanisms, namely functional changes at the level of T helper (Th) lymphocytes. It was found that allergen-specific CD4+ Th2 lymphocytes play a key role in the pathophysiology of atopic diseases. During successful SIT, the Th2-dominated immune response is modified towards a Th1 response, leading to a decline in allergen-specific IgE levels in the long term. In order to improve the efficacy and safety of SIT, novel approaches were developed targeting allergen-specific Th2 lymphocytes since specific inactivation or modulation towards Th1 cells could interfere with the disease process. In view of this aspect, this review will basically focus on two new, promising approaches to improve SIT: (1) the use of hypoallergenic proteins characterized by reduced IgE-binding capacities but retained T lymphocyte-activating properties and (2) oligodeoxynucleotides containing CpG motifs as an example of adjuvants which foster Th1 immune responses. Both approaches promise to be capable of adjusting the pathological Th2 immune response.