Neuropeptides and interleukin-6 in human joint inflammation. Relationship between intraarticular substance P and interleukin-6 concentrations

Plasma and synovial fluid concentrations of interleukin-6 (IL-6), using an enzyme-linked immunosorbent assay, as well as immunoreactive levels of calcitonin gene-related peptide (CGRP), substance P and vasoactive intestinal peptide (VIP) were measured in 18 patients with rheumatoid arthritis and 20 with osteoarthritis of the knee. The concentrations of IL-6 were elevated in both plasma and synovial fluids from patients with rheumatoid arthritis whereas higher levels of substance P-, CGRP- and VIP-like immunoreactivities were found in the synovial fluid, but not in plasma, from patients with rheumatoid arthritis when compared with those in osteoarthritis. Furthermore, IL-6 and substance P levels in synovial fluid were significantly correlated both in rheumatoid arthritis and osteoarthritis patients. Our data seem to support the idea of an important role shared by neuropeptides and IL-6 in the pathogenesis of human inflammatory joint disease.