The role of neurotrophins in inflammation and allergy.

Allergic inflammation is the result of a specific pattern of cellular and humoral responses leading to the activation of the innate and adaptive immune system which, in turn, results in physiological and structural changes affecting target tissues such as the airways and the skin. Eosinophils activation and production of soluble mediators such as IgE antibodies is a pivotal feature in the pathophysiology of allergic diseases. In the past few years, however, convincing evidence has shown that neurons and other neurosensory structures are not only a target of the inflammatory process but also participate in the regulation of immune responses by actively releasing soluble mediators. The main products of these activated sensory neurons are a family of protein growth factors called neurotrophins. They were first isolated in the central nervous system and identified as important factors for the survival and differentiation of neurons during fetal and post-natal development as well as neuronal maintenance later in life. Four members of this family have been identified and well defined: nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), and neurotrophin 4/5 (NT-4/5). Neurotrophins play a critical role in the bidirectional signaling mechanisms between immune cells and the neurosensory network structures in the airways and the skin. Pruritus and airway hyperresponsiveness (AHR), two major features of atopic dermatitis and asthma, respectively, are associated with the disruption of the neurosensory network activities. In this review we provide a comprehensive description of the neuroimmune interactions underlying the pathophysiological mechanisms of allergic and inflammatory diseases.