Background: To characterize the role of intestinal epithelial cells in mucosal host defense, we have examined constitutive cytokine expression and regulated expression of interleukin (IL)-8 by human colonic epithelial cells. Methods: Cytokine expression by the human colonic epithelial cell lines, T₈₄, Caco-2, SW620, and HT29 was assessed by using polymerase chain reaction amplification of reverse-transcribed RNA. Regulated IL-8 expression was analyzed by nuclear run-off assays, Northern blot analysis, and enzymelinked immunosorbent assay. Results: The cell lines constitutively expressed messenger RNA (mRNA) for IL-8 and transforming growth factor β1. In addition, some cell lines expressed mRNA for IL-1α, IL-1β, IL-10 and tumor necrosis factor a (TNFα). None of the cell lines expressed mRNA for IL-2, IL-4, IL-5, IL-6, or interferon gamma. Cell lines secreted IL-8 either constitutively or after stimulation with the physiological agonists TNFα, IL-1β, or lipopolysaccharide. Increased IL-8 secretion after TNFα stimulation of T₈₄ cells was accompanied by increased IL-8 mRNA levels and an increased transcription rate of the IL-8 gene. IL-8 was preferentially secreted at the basolateral surface of polarized T₈₄ cells. In further studies, freshly isolated human colon epithelial cells also secreted IL-8. Conclusions: These results support the notion of bidirectional communication between intestinal epithelial cells and mucosal immune and inflammatory cells.