Increased content of bombesin/GRP in human synovial fluid in early arthritis: different pattern compared with substance P.

OBJECTIVE Bombesin (BN) and the mammalian homologue gastrin-releasing peptide (GRP) are known trophic factors, neurotransmitters and paracrine hormones. BN/GRP has not previously been demonstrated in synovial fluid. In this study, the amounts of BN/GRP and substance P (SP) present in synovial fluid from the knee joints of patients with rheumatoid arthritis (RA) and of healthy controls were measured. METHODS Synovial fluid from the knee joint was collected from patients with either longstanding RA (n = 32) or early arthritis (symptoms for < 12 months; n = 9) and from control subjects, i.e., individuals without known joint disease (n = 10). These samples were analyzed using radioimmunoassays. RESULTS Levels of BN/GRP-like peptide were below the assay detection limits in synovial fluid from controls. Detectable levels of immunoreactive BN/GRP were present in the majority of patients with either longstanding RA or early arthritis. The levels were significantly higher in the synovial fluid from patients classified as having early arthritis compared with those with longstanding RA (p < 0.05). There was a strong correlation between BN/GRP levels and the number of leukocytes in the synovial fluid in the patients with early arthritis. The levels of SP-like peptide in the patients, whether with early arthritis or longstanding RA, were significantly elevated compared with controls. However, there was no difference in the levels between these two patient groups. CONCLUSIONS These observations show that BN/GRP-like peptide is present in the synovial fluid of joints affected by arthritis and that the pattern of BN/GRP increase differs from that of SP. It appears as if the presence of BN/GRP is particularly related to the early processes of joint involvement. These observations are of interest because BN/GRP has well-known trophic and paracrine effects and chondrocytes have recently been shown to produce neuropeptides such as BN/GRP.