1268 Immunocytochemical Detection of Interleukin

The term IL-1 has been used to describe a family of monocyte derived polypeptides that have immunomodulatory and proinflammatory biological properties (1, 2). While most studies have focused on IL-1 activity that is found in monocyte/macrophage culture supernatants, early studies showed that high amounts of activity could also be recovered from cell lysates. These studies suggested that IL-1 may accumulate within mononuclear phagocytes in a precursor form before release into the surrounding medium (3-5). Recently it has been shown that there are at least two distinct human IL-1 molecules termed ILla and IL-13. Separate mRNAs for these IL-1 species each code for a precursor of ~31 kD, which is subsequently processed by still undefined mechanisms to a "mature" molecule of ~17.5 kD (6). The absence of a typical hydrophobic leader sequence in either of the IL-1 molecules identified to date suggests that they may not be typical secretory proteins. A 33-kD precursor for murine IL-1, which is ~62% homologous to human ILla (6), has been documented at the protein level in cell lysates of peritoneal macrophages, as well as in lysates of the murine macrophage line P388D~ (7). Comparable studies have not yet been reported for human monocytes and macrophages. The successful development of highly specific heterologous antisera (8) for the species of human IL-1 having a pI of 6.8 (IL-18) (6, 9) has allowed us to identify its precursor in the cytoplasm of activated human monocytes. In the present report, the intracellular accumulation of IL-1 is documented using indirect immunofluorescence performed on fixed and permeabilized cells, and immunoblot analysis of cell lysates. These studies show that it is possible to identify IL-1producing cells at the light microscopic level and suggest that anti-IL-1 antibodies may be useful for the in situ localization of IL-1 production in inflammed tissue.