NGF and Immune Regulation

For some time after its discovery, the action of Nerve Growth Factor (NGF) was considered restricted to the nervous system. Instead, a variety of experimental data indicate that NGF can influence the activity of both the nervous and immune systems. This should not be surprising since these two systems are responsible both for maintaining homeostasis and for adapting the body to the environment. To orchestrate strictly integrated responses they need to have close anatomical connections and to share common chemical signals and specific receptors. The well-known effects of NGF on peripheral neuron survival and maintenance and dynamic control by NGF of innervation and neuropeptide synthesis, together with its direct effects on immune cell functions, indicate that NGF has a key role in the complex network of bi-directional signals between the nervous and immune systems. NGF receptors are expressed in immune organs and cell populations, allowing NGF to modulate cell differentiation and regulate immune response. NGF concentrations in tissues change during inflammation, and inflammatory mediators induce NGF synthesis in a variety of cell types. As a growing number of studies has shown, an enhanced production of NGF characterizes inflamed tissues of patients with inflammatory and autoimmune diseases. Unfortunately, although the dynamic regulation of NGF synthesis seems to be a common feature of chronic inflammatory diseases, the reasons why NGF concentrations are enhanced and how this can affect inflammatory responses and the course of the diseases are far from being understood.