The Anti-Inflammatory Action of Nerve Growth Factor Is Mediated by Trka Signalling: Modification in Trka Expression in Juvenile Idiopathic Arthritis Mononuclear Cells Create An Unbalance Between Inflammatory and Anti-Inflammatory Pathways?

Background/Purpose: Nerve Growth Factor (NGF) production in the organism is a strictly regulated process. Inflammation increases NGF expression and indeed high NGF levels are a feature of synovial fluid and tissue in patients with chronic arthritis. Although all mononuclear cells express NGF receptors, it is, however, unclear what are the biological effects of these high NGF levels on immune cell activity. In order to evaluate the ability of mononuclear cells to respond to NGF in vivo, we analyzed the expression of TrkA and p75-NTR in mononuclear cells of patients with juvenile idiopathic arthritis (JIA). For better understanding the possible effect of NGF during the inflammatory response we studied how intracellular signalling pathways induced by NGF affect TLR downstream signalling in monocytes, which are key players in inflammation and a critical link between innate and adaptive immunity. Methods: Mononuclear cells were isolated from synovial fluid and peripheral blood samples of 25 children with JIA and age matched controls by Ficoll-Hypaque density centrifugation. Healthy donor monocytes, purified using Percoll discontinuous density gradients, were stimulated with TLR ligands with or without NGF addition. The expression of TrkA and p75NTR and the effects of TrkA activation or inhibition on TLR signalling and cytokine release were evaluated using real-time PCR, western blot and ELISA. Results: We found that mononuclear cells of JIA patients from both peripheral blood and synovial fluid are characterized by a significant decrease in TrkA expression, while p75-NTR expression remains stable. In normal monocytes, phosphorylation of TrkA after NGF binding activated intracellular pathways that interfered with TLR signalling. Indeed NGF addition increased Akt phosphorylation, inactivated GSK3 and reduced IkB phosphorylation. Blocking TrkA enhanced inflammatory cytokine production, while reducing IL-10 synthesis and resulted in a greater activation of the NF-kB pathway and in inhibition of the PI3K pathway. Conclusion: Our results imply that NGF is a relevant component of the endogenous mechanisms limiting excessive inflammatory response. Indeed, through TrkA NGF potentiates anti-inflammatory responses usually activated after TLR stimulation. The well-known increase in NGF in chronic arthritis patients may be de facto not efficacious in controlling inflammation because of the decrease of TrkA expression observed in JIA patients. This downregulation of TrkA expression may influence the balance between inflammatory and anti-inflammatory pathways.