ELEVATED SERUM INTERLEUKIN-6 LEVELS ASSOCIATED WITH ACTIVE DISEASE INSYSTEMIC CONNECTIVE-TISSUE DISORDERS

Objective. It is well established that connective tissue diseases such as systemic lupus erythematosus (SLE) are associated with a weak or a bsent acute phase response, although elevated serum interleukin 6 leve ls have been described. In this study, we have sought to correlate ser um levels of IL-6 with standard laboratory avid clinical assessments o f disease activity in two connective tissue diseases, namely SLE and s ystemic sclerosis (SSc), and, for comparative purposes, rheumatoid art hritis (RA). Methods. Serum IL-6 levels were determined by bioassay an d also, in some sera by immunoradiometric assay. They were compared wi th two inflammatory parameters, serum C-reactive protein (CRP) and pla sma viscosity (PV) and with appropriate clinical measurements in the v arious patient groups, including BILAG in SLE, the skin score in SSC, and the Ritchie index in RA. Results. Serum IL-6 (SeIL-6) levels were elevated in active SLE, SSc, and RA. This was poorly correlated with t he acute phase response in SLE and SSc, but there was a strong relatio nship of SeIL-6 to disease activity in these conditions. In SLE, the B ILAG disease activity index correlated best with SeIL-6 levels while t here was only a weak relationship between CRP and IL-6, and no relatio nship between CRP and disease activity, In SSc there was a relationshi p disease activity to SeIL-6 but not between SeIL-6 and either CRP or PV. In a small RA group there was a much stronger relationship of SeIL -6 to CRP and PV, as has been previously described. Conclusion. The de termination of SeIL-6 may be a useful indicator of disease activity in those patients groups, including SLE and SSc, in which a normal acute phase response by the liver is often lacking. The mechanism underlyin g this hepatic impairment requires further investigation, but is clear ly not dire to a failure to generate the appropriate cytokine signal. Excessive local or systemic production of IL-6 in connective tissue di seases could play an important pathogenic role in these conditions, fo r example through stimulating autoantibody synthesis.