Recombinant human IL-11 (rhIL-11) is an anti-inflammatory cytokine that can reduce the production of inflammatory mediators such as TNF-alpha, IL-1beta, IL-12, IL-6, and nitric oxide. Inhibition of proinflammatory cytokine production from activated macrophages was associated with a reduction in the levels of LPS-induced TNF-alpha, IL-1beta, IL-6, and IL-12 p40 mRNA. Analysis of rhIL-11 effects on transcription factors that activate proinflammatory cytokines demonstrated that the level of LPS-induced NF-kappaB binding activity in the nucleus of rhIL-11-treated peritoneal macrophages was significantly reduced. The block to NF-kappaB nuclear translocation correlated with the ability of rhIL-11 to maintain or increase protein levels of the inhibitors of NF-kappaB, IkappaB-alpha, and IkappaB-beta following LPS treatment. Furthermore, rhIL-11-treatment of LPS macrophages resulted in significant elevation of IkappaB-alpha and IkappaB-beta mRNA levels. These results suggest that the anti-inflammatory activity of rhIL-11 is mediated in part by inhibition of NF-kappaB-dependent transcriptional activation. Furthermore, these studies demonstrate for the first time the regulation of IkappaB-beta by an anti-inflammatory cytokine. Given the finding that inappropriate activation of NF-kappaB contributes to multiple inflammatory conditions, the ability of rhIL-11 to inhibit the binding activity of this pleiotropic transcription factor indicates that rhIL-11 has therapeutic potential in a wide range of diseases.