Human orbital fibroblasts from patients with severe thyroid-associated ophthalmopathy are particularly susceptible to the actions of a variety of proinflammatory molecules. In this study, we demonstrate that the inductions of prostaglandin endoperoxide H synthase-2 (PGHS-2), interleukin (IL)-1, and IL-1 by leukoregulin, a product of activated T lymphocytes, are far more robust in orbital fibroblasts than those observed in dermal fibroblasts. These actions of leukoregulin are mediated through an intermediate induction of IL-1. In contrast, leukoregulin also induces IL-1-receptor antagonist (IL-1ra) expression in orbital fibroblasts, but this induction is considerably greater in dermal fibroblasts (2.3- vs. 8.5-fold). Interrupting the effects of IL-1, either with a neutralizing antibody or with exogenous IL-1ra, can block the induction of PGHS-2 by leukoregulin. Leukoregulin increases PGHS-2 gene transcription in orbital fibroblasts but exerts the major effect on cyclooxygenase expression by enhancing the stability of mature PGHS-2 mRNA. The cytokine triggers nuclear translocation of nuclear factor-B (NF-B) p50/p50 homodimers and p50/p65 heterodimers, and an inhibitor of this transcriptional factor, pyrrolidinedithiocarbamate, can attenuate the PGHS-2 induction. Thus differential inducibility of the members of the IL-1 family of genes in orbital fibroblasts would appear to underlie, at least in part, the differences in PGHS-2 induction observed in orbital and dermal fibroblasts. NF-B plays an important role in mediating the effects of leukoregulin on PGHS-2 expression.