Two Novel IL-1 Family Members, IL-1δ and IL-1ε, Function as an Antagonist and Agonist of NF-κB Activation Through the Orphan IL-1 Receptor-Related Protein 2

摘要

IL-1 is of utmost importance in the host response to immunological challenges. We identified and functionally characterized two novel IL-1 ligands termed IL-1δ and IL-1ε. Northern blot analyses show that these IL-1s are highly abundant in embryonic tissue and tissues containing epithelial cells (i.e., skin, lung, and stomach). In extension, quantitative real-time PCR revealed that of human skin-derived cells, only keratinocytes but not fibroblasts, endothelial cells, or melanocytes express IL-1δ and ε. Levels of keratinocyte IL-1δ are ～10-fold higher than those of IL-1ε. In vitro stimulation of keratinocytes with IL-1β/TNF-α significantly up-regulates the expression of IL-1ε mRNA, and to a lesser extent of IL-1δ mRNA. In NF-κB-luciferase reporter assays, we demonstrated that IL-1δ and ε proteins do not initiate a functional response via classical IL-1R pairs, which confer responsiveness to IL-1α and β or IL-18. However, IL-1ε activates NF-κB through the orphan IL-1R-related protein 2 (IL-1Rrp2), whereas IL-1δ, which shows striking homology to IL-1 receptor antagonist, specifically and potently inhibits this IL-1ε response. In lesional psoriasis skin, characterized by chronic cutaneous inflammation, the mRNA expression of both IL-1 ligands as well as IL-1Rrp2 are increased relative to normal healthy skin. In total, IL-1δ and ε and IL-1Rrp2 may constitute an independent signaling system, analogous to IL-1αβ/receptor agonist and IL-1R1, that is present in epithelial barriers of our body and takes part in local inflammatory responses.