Enhanced expression of interleukin 6 in rat and murine arthritis models

Interleukin 6 (IL-6) is a multifunctional cytokine and plays an important role in host defense mechanisms. Enhanced production of IL-6 has been reported in polyclonal B-cell abnormalities and autoimmune diseases such as rheumatoid arthritis (RA). To investigate the role of IL-6 inflammatory joint diseases, serum IL-6 levels of three animal models of RA, namely type II collagen (CII)-induced murine, rat arthritis and adjuvant-induced rat arthritis, were monitored. In these models, serum IL-6 increased with the development of arthritis. Serum IL-6 was not elevated by immunization with a non-arthritogenic immunogen such as bovine type I collagen (CI) and bovine serum albumin (BSA) to DBA/1J mice. The serum IL-6 level was correlated well with the severity of adjuvant-induced arthritis. The elevated IL-6 in sera may be associated with the overproduction of IL-6 at the arthritis paws, because higher IL-6 activity was detected in the homogenates of arthritic paws as compared with the control paws. Synovial fibroblasts were isolated from the arthritis knee joints of DBA/1J mice. These cells expressed type I interleukin 1 (IL-1) receptor constitutively and produced large amounts of IL-6 in response to IL-1 in vitro. Enhanced production of IL-1 was also detected at the arthritis paws. These results suggest that the elevated IL-6 in sera may be associated with the overproduced IL-6 in response to the increased IL-1 at the arthritic joints. Serum IL-6 may be a useful parameter for monitoring disease activity.