OBJECTIVE To determine the effects of interleukin-4 (IL-4) on IL-1 induction of collagenase (matrix metalloproteinase 1 [MMP-1]) and stromelysin-1 (MMP-3) in human synovial fibroblasts. METHODS Northern blot analysis was performed to determine the effects of IL-4 on IL-1 induction of MMP messenger RNA (mRNA). MMP protein levels were determined by enzyme-linked immunosorbent assay, and prostaglandin E2 (PGE2) levels were measured by enzyme immunoassay. Run-on transcription assays and transient transfection experiments were performed to determine whether the effects of IL-4 occur at the level of transcription. Activator protein 1 (AP-1) binding was assessed by electrophoretic mobility shift assay. RESULTS Northern blot analysis revealed that coincubation of synovial fibroblasts with IL-1 and IL-4 resulted in a significant decrease in both collagenase and stromelysin mRNA levels compared with IL-1 alone, with a concomitant decrease in MMP protein levels. This inhibition is dose dependent, with an IC50 (50% inhibition concentration) for both MMPs of approximately 0.3 ng of IL-4 per ml, and is at least somewhat selective, since IL-1 induction of c-fos mRNA is not affected. Nuclear run-on experiments and transient transfection studies demonstrated that the suppression of IL-1-induced collagenase and stromelysin expression by IL-4 occurs at least in part at the transcriptional level, and that binding of transcription factor AP-1 is not affected. Although IL-1-induced levels of PGE2 are reduced by IL-4, exogenous addition of PGE2 does not abrogate the inhibitory effects of IL-4 on MMP expression. CONCLUSIONS IL-4 inhibits IL-1 induction of both collagenase and stromelysin, as well as PGE2 production, in human synovial fibroblasts. The inhibition occurs at least in part at the level of transcription, does not affect binding of transcription factor AP-1, and appears to involve a mechanism that is independent of the ability of IL-4 to inhibit production of PGE2.