Production of ISG-15,an Interferon-Inducible Protein,in Human Corneal Cells

ISG-15, a 15-kDa protein encoded by an interferon (IFN)-stimulated gene (ISG), was produced in human corneal cell cultures prepared from donor corneas in response to each of three major types of IFN, IFN-α, IFN-β, and IFN-γ. IFN-α and IFN-β induced more ISG-15 in the first 24 h of treatment than did IFN-γ. ISG-15 was detectable within the first 3 h of treatment with either type I IFN, and production peaked at 24 h, whereas IFN-γ did not induce detectable ISG-15 until 16 h and did not induce peak production until 48 h. Conjugates of ISG-15 to cellular proteins were detectable by Western blot beginning at 9 h after IFN-α or IFN-β treatment. ISG-15 persisted in IFN-treated cells for as long as 96 h. Free, unconjugated, ISG-15 was secreted by keratocytes into cell culture medium. The extent and kinetics of production, the conjugation of ISG-15 to cell proteins, and the secretion of ISG-15 in human fibroblast derived from corneas were IFN type dependent and dose dependent.