B iological effects of cytokines are in part determined by their interactions in the regulation of cytokine production. This study analyzes the effects of leukemia inhibitory factor (LIF) on cytokine expression in different cell lineages. Recombinant human LIF increases levels of IL-1 beta, IL-6, and IL-8 mRNA in human articular chondrocytes as demonstrated by Northern blotting. These cytokine mRNAs are detectable as early as 1.3 h after stimulation and reach their maximum after 5 h. The LIF effects are dose dependent and of similar magnitude to those of IL-1. By metabolic labeling and immunoprecipitation it is shown that LIF induces synthesis and secretion of IL-6. IL-6 bioactivity in conditioned media, as measured by the B9 hybridoma proliferation assay, is increased by LIF. Effects of LIF on cytokine expression are not confined to connective tissue cells. By PCR it is shown that human blood monocytes express IL-6 mRNA after stimulation with LIF. An increase in IL-6 mRNA levels is detectable 2 h after stimulation, and this starts to decline by 5 h. The response is of shorter duration as compared with IL-1 beta. In addition to increased mRNA expression, LIF also stimulates release of biologically active IL-6 from blood monocytes. In synoviocytes and neuronal as well as epithelial cell lines, LIF increases IL-1 beta and IL-6 gene expression. In summary, LIF induces cytokine expression in a wide variety of tissues. These results suggest that through the induction of cytokines, LIF can modulate inflammation, immune responses, and connective tissue metabolism, and act as a pathogenetic mediator in different disease states.