The eye is considered an immunologically privileged organ and is separated from the rest of the body by blood-ocular barriers. Part of the blood-retina barrier consists of the retinal pigment epithelium (RPE). In addition to the physical barrier which the monolayer of RPE cells forms, these cells contribute to ocular immune privilege by producing anti-inflammatory molecules that down-regulate potential damaging immune reactions. In this study the mRNA expression of IL-1 receptor antagonist (IL-1ra) by RPE cells was studied in 15 donor-derived cell lines. Expression of both the intracellular and secreted IL-1ra was detected in unstimulated and IL-1beta- or phorbol 12-myristate 13-acetate-exposed RPE. Analysis of IL-1ra protein in RPE cell lysates and cell culture supernatants indicated that these cells produce mainly intracellular IL-1ra. No correlation between IL-1ra expression levels and the IL-1ra gene polymorphism could be detected. In addition to the two known intracellular IL-1ra variants (intracellular IL-1ra type I and type II) evidence is provided for the expression of a hitherto unknown splice variant of the IL-1ra mRNA by RPE cells. Expression was not confined to RPE cells and could also be detected in cultured human fibroblasts and macrophages. This variant, which we have tentatively named intracellular IL-1ra type III, encodes a C-terminally truncated protein of only 27 amino acids.