Interleukin-1 receptor antagonist production by human keratinocytes

Human keratinocytes produce biologically active pro-IL-1 alpha and inactive pro-IL-1 beta with most protein remaining intracellular. IL-1 receptor antagonist (IL-1ra) is a newly described member of the IL-1 family that is secreted by stimulated monocytes and binds competitively to IL-1 receptors without stimulating target cells. We examined the characteristics of IL-1ra production by cultured human keratinocytes. By ELISA, keratinocyte lysates contained 390 ng IL-1ra/mg total protein with little IL-1ra detected in supernatants. In contrast, monocytes produced 297 ng IL-1ra/mg total protein during 24 h of culture on adherent IgG with about half of the IL-1ra detected in supernatants. By Western blot analysis, keratinocyte IL-1ra was approximately 20 kD in size and was slightly larger than recombinant monocyte IL-1ra. In contrast to monocytes, human keratinocyte IL-1ra was not secreted in 22-25-kD molecular weight glycosylated forms. Affinity-purified keratinocyte IL-1ra exhibited identical biologic activity to recombinant monocyte IL-1ra, each inhibiting IL-1-dependent augmentation of murine thymocyte proliferation to the same degree per amount of protein. An IL-1ra mRNA of 1.8 kb was detected by Northern blot analysis in RNA extracted from keratinocytes. In order to determine the effect of differentiation on IL-1 and IL-1ra production, human keratinocytes were cultured for 72 h in low (0.03 mM), medium (0.15 mM), or high (1.0 mM)-calcium concentrations. The absolute amounts of IL-1ra increased twofold and the ratio of IL-1ra to IL-1 alpha in keratinocyte lysates increased from approximately 12:1 to 25:1 during differentiation. These results indicate that keratinocytes constitutively produce large amounts of a biologically active intracellular variant of IL-1ra that increase with differentiation. IL-1ra released during keratinocyte damage may be important in modifying the inflammatory effects of IL-1 alpha in human skin.