Nerve growth factor and asthma.

An increasing body of evidence shows that nerve growth factor (NGF) exerts biological activity not only on the central and peripheral nervous system, but also on the immune system thereby influencing allergic diseases and asthma. (1) NGF circulating levels are increased in patients with allergic diseases and asthma, and are related to the severity of the inflammatory process and disease. In vernal keratoconjunctivitis, NGF plasma levels correlate with the number of mast cells infiltrating the conjunctiva, and NGF mRNA is increased in nasal mucosal scrapings of patients with allergic rhinitis who have high levels of NGF in serum and nasal fluids; NGF is further increased in nasal fluids after specific allergen challenge. (2) NGF is produced and released by several modulatory and effector cells of allergic inflammation and asthma, for example T-helper 2 lymphocytes, mast cells and eosinophils. (3) NGF receptors are expressed on the conjunctival epithelium of patients with allergic conjunctivitis and the number of NGF-receptor positive cells is increased in the conjunctiva of these patients. Indeed, local administration of NGF induces fibroblast activation and healing processes of human corneal ulcers, which suggests that NGF plays a role in tissue remodelling processes occurring in asthma. (4) NGF increases airway hyperreactivity to histamine in an animal model of asthma, while anti-NGF treatment reduces airway hyperreactivity induced by ovalbumin topical challenge in the sensitized mouse.