Nerve Growth Factor down-regulates human monocyte response to Toll-like receptor ligands

Background Although it has been extensively described that a marked increase of NGF concentration characterizes synovial fluid and tissue obtained from patients with chronic arthritis (1), it is yet not clear how these changes in NGF levels affects immune cell activity during the inflammatory response. We focused our attention on monocytes that are key players in the initiation and perpetuation of inflammation and a critical link between innate and adaptive immunity. They express Toll like receptors (TLRs) whose signaling dysregulation contributes to the initiation and progression of autoimmune diseases, such as rheumatoid arthritis (2). Objective. In order to provide insight on the significance of high levels of NGF in inflamed joints we investigated NGF effects on cytokine production in human monocytes after pathogen-associated molecular patterns (PAMPs) activation of TLRs. Methods Using peripheral blood monocytes, purified by Percoll gradient, we investigated the NGF effects on human monocytes after TLR activation. To mimic the inflammatory condition in which there is a tissue increase in NGF concentration, monocytes were stimulated with TLR ligands and NGF was added either contemporary or at later times. The activation of signalling pathways known to control the synthesis of TNF-?, IL-6, IL-1?? IL-1Ra and IL-10, and the release of these cytokines were evaluated using real-time PCR, western blot and ELISA. Results The addition of NGF at the time of or after TLR stimulation with different ligands (LPS, PAM, LTA) induced in human monocytes a 50% reduction in IL-6, IL-1? and TNF-? production and a contemporary increase in IL-1Ra and IL-10 synthesis. NGF alone did not have any effect on cytokine production in the absence of TLR activation. We found that the activation of NGF signalling pathways affects TLR signalling. In fact, NGF addition interferes with the downstream pathway of TLR activation by reducing IkB phosphorylation and increasing AKT phosphorylation in a time-dependent manner. Conclusions Our results show that NGF influences TLR ligand activated pathways resulting in a down-regulation of inflammatory cytokine production and induction of anti-inflammatory mediators. These data suggest that NGF has a role in maintaining homeostasis of responses to PAMPs and that the well-known increase in NGF in chronic arthritis patients may represent a physiological mechanism modulating inflammatory response.