Although human epidermis contains levels of interleukin-1 (IL-1) up to 100 times higher than other tissues, the role of this cytokine in epidermal biology is unknown. Here, we show that interleukin-1 regulates the expression of mRNAs for two proteins associated with the differentiated phenotype of human keratinocytes, cellular retinoic acid-binding protein type II (CRABP II) and small, proline rich protein 1 (SPRR1). The ability of IL-1 to induce these transcripts correlates directly with

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... nocyte expression of the IL-1 receptor type I (IL-1 RI) during differentiation and inversely with the expression of the type II IL-1 receptor (IL-1 RII), shown in other cell types to be a nonfunctional, decoy receptor. Furthermore, addition to keratinocyte cultures of an IL-1 RI-blocking, but not an IL-1 RII-blocking, antibody reduces the levels of CRABP II and SPRR1 mRNAs in these cells. These data suggest that epidermal IL-1 functions to promote keratinocyte differentiation and that a change in the IL-1 receptor profile of these cells initiates this IL-1 response through a relative enhanced expression of functional IL-1 receptors.