In scleroderma (systemic sclerosis, SSc), an autoimmune disorder in which excessive extracellular matrix is deposited in skin and internal organs, one of the suggested contributory factors to the development of fibrosis is a decrease in collagenase activity that may be related to levels of serum tissue inhibitors of metalloproteinases 1 (TIMP-1). We recently reported that the serum TIMP-1 levels in SSc patients were elevated compared with normal controls. To determine the biologic significance of TIMP-1 in SSc, we compared the proliferative effects of TIMP-1 between normal and SSc fibroblasts. TIMP-1 showed significant mitogenic activity for both normal and SSc fibroblasts. The mitogenic responses to TIMP-1 (33-100 ng/ml) in SSc fibroblasts, however, were significantly greater than those in normal controls and were completely neutralized in the presence of anti-TIMP-1 IgG. Moreover, anti-TIMP-1 IgG partially but significantly blocked the basal mitogenic activities of SSc fibroblasts. SSc fibroblasts produced increased amounts of TIMP-1 relative to normal fibroblasts, as confirmed by western blotting, ELISA, and RT-PCR techniques. In contrast, transforming growth factor beta1 (TGF-beta1) upregulated TIMP-1 production in normal fibroblasts but not in SSc fibroblasts with elevated spontaneous secretion of TIMP-1. These observations suggest that TIMP-1 may play an important role as an autocrine growth factor in the fibrotic process in SSc.