Functional significance of nerve growth factor and its receptor (TrkA) in inflammatory arthritis

Abstract

We read the article by Barthel and colleagues [1] in this is sue of Arthritis Research & Th erapy with great interest. Th ey reported that neurotrophin receptors/ligands and specifi cally nerve growth factor (NGF) and NGF receptor (NGF-R) (TrkA and p75) are expressed in synovial fl uid (SF) cells and synovial tissue (ST) of patients with rheuma toid arthritis (RA) or spondyloarthritis (SpA). Th e authors also looked for the cellular source of NGF and demonstrated that the T cells and monocytes/macrophages derived from SF of patients with RA/SpA are enriched with NGF. Furthermore, they reported that STderived fi broblast-like synoviocytes (FLSs) did not produce NGF in vitro. In a recent publication, we observed similarly that NGF levels in SF were signifi cantly higher in patients with psoriatic arthritis (PsA) (365.5 ± 85.2 pg/ mL) or RA (120 ± 35 pg/mL) than in patients with osteoarthritis (OA) (30 ± 6 pg/mL) [2]. However, in regard to the source of NGF, we observed that FLSs produced a signifi cant amount of NGF. Here, we would like to share our observations about the NGF/TrkA system in human FLSs and its function. Synovial biopsies from patients with meniscal injury without any other joint diseases or PsA, OA, or RA were collected. FLSs were isolated and examined for NGF/ NGF-R expression in accordance with our standardized protocols [2]. Using a highly sensitive enzyme-linked immuno sorbent assay (ELISA) for human NGF (NGFEmax assay; Promega Corporation, Madison, WI, USA) and Hi-D fl uorescence-activated cell sorting analyses, we observed that under basal conditions FLSs from healthy individuals express low levels of NGF/TrkA. However, there was a marked upregulation of NGF and TrkA in these FLSs following stimulation with tumor necrosis factor-alpha and interleukin-1-beta (Table 1). A critical observation was that FLSs from patients with PsA or RA produced spontaneously higher levels of NGF and had increased expression of TrkA compared with FLSs of patients with OA (Table 1). Furthermore, we observed that NGF signifi cantly stimulated the proliferation of FLSs derived from PsA synovial tissue (Figure 1). Barthel and colleagues [1] cultured FLSs from only one patient with RA and one patient with SpA and noticed that FLSs did not produce NGF, but the authors do mention that they can’t rule out the production of NGF by FLS. We cultured FLSs from 15 subjects (Table 1), and it is also possible that the NGF ELISA kit that we used is more sensitive. A fully formed pannus is characterized by proliferation of FLSs, infl ammatory infi ltrates, and a marked angiogenesis. NGF and its receptor system are known to