The role of neurotrophins in bronchial asthma

Allergic bronchial asthma is characterized by chronic inflammation of the airways, development of airway hyperreactivity and recurrent reversible airway obstruction. Target and effector cells responsible for airway hyperresponsiveness and airway obstruction include sensory and motor neurons as well as epithelial and smooth muscle cells. Although it is well established that the inflammatory process is controlled by T-helper (Th) 2 cells and the Th2-derived cytokines interleukin-4, airway hyperresponsiveness-5 and interleukin-13, the mechanisms by which immune cells interact with neurons, epithelial cells or smooth muscle cells still remain uncertain. Since there is growing evidence for extensive communication between neurons and immune cells, the mechanisms of this neuro-immune crosstalk in lung and airways of asthmatic patients are recently becoming the focus of asthma research. Neurotrophins represent candidate molecules regulating and controlling this crosstalk between the immune and peripheral nervous system. They are constitutively expressed by resident lung cells and produced in increasing concentrations by immune cells invading the airways under pathological conditions. They modify the functional activity of sensory and motor neurons, leading to enhanced and altered neuropeptide and tachykinin production. These effects are defined as "neuronal plasticity". The consequences are the development of "neurogenic inflammation" due to neuropeptide and tachykinin activities.