Calprotectin is an antimicrobial complex composed of the S100A8 and S100A9 protein family subunits. Contributing to innate immunity, calprotectin expression is increased by interleukin-1 alpha (IL-1 alpha), which modulates keratinocyte differentiation. Keratinocyte growth factor (KGF) is produced by mesenchymal cells and has a mitogenic activity for epithelial cells. In this study, we investigated the effect of KGF on calprotectin expression in keratinocytes and modulation by IL-1 alpha. Human keratinocytes were cultured with KGF in the presence or absence of a KGF receptor (KGFR) inhibitor or mitogen-activated protein kinase (MAPK) inhibitors. Calprotectin (S100A8/S100A9) expression was determined by northern blotting and enzyme-linked immunosorbent assay, respectively, whereas MAPK phosphorylation was analyzed by western blot analysis. KGF significantly decreased the expression of S100A8/S100A9-specific mRNAs and calprotectin protein. In the presence of KGF, KGFR inhibitor or extracellular-regulated kinase inhibitor restored KGF-downregulated expression of S100A8/S100A9. KGF increased IL-1 alpha expression in keratinocytes, whereas IL-1 alpha increased KGF expression in fibroblasts. Cocultured fibroblast and keratinocytes showed lower S100A8/S100A9 mRNA expression than keratinocytes alone in the presence or absence of IL-1 alpha or KGF. These results suggest that fibroblast-derived KGF reduces or restricts calprotectin expression in keratinocytes, which supports our hypothesis that calprotectin expression in keratinocytes is modulated by factors associated with epithelial mesenchymal interactions. Immunology and Cell Biology (2010) 88, 328-333; doi: 10.1038/icb.2009.104; published online 12 January 2010