Interleukin 10 (IL-10), initially designated cytokine synthesis inhibitor, is a pleiotropic cytokine that inhibits the production of a number of cytokines (including IL-1, GM-CSF, TNF, IL-6, IL-8, IL-12 and IFN-γ) by activated Th1 cells, NK cells, and monocytes/macrophages.

Mouse IL-10 is produced by a wide variety of cells, including activated Th2 cells, fetal thymocytes, monocytes/macrophages, keratinocytes, B cells, and glial cells (1-3). Mouse IL-10 cDNA encodes a 178 amino acid residue precursor protein with a hydrophobic signal peptide that is cleaved to generate the 160 amino acid residue mature protein (4). At the amino acid sequence level, there is approximately 73% identity between mouse and rat or mouse and human IL-10. Although human IL-10 is active on mouse cells, mouse IL-10 does not show species cross-reactivity on human cells (1). Herpes viruses, including EBV and equine herpes virus type 2, have been shown to encode viral homologues of IL-10 that exhibit some of the activities of IL-10 on both mouse and human cells (5,6).

IL-10 has been shown to inhibit macrophage cytotoxic activity and to stimulate the proliferation and differentiation of B cells, mast cells, and thymic T cells. IL-10 is a potent modulator of monocyte/macrophage function. IL-10 also enhances the release of soluble TNF receptors and inhibits the expression of surface ICAM-1 and B7 (7, 8). Finally, IL-10 has been reported to suppress the synthesis of superoxide anion (9) plus reactive oxygen intermediates (ROI), and either inhibit or facilitate nitric oxide synthesis, depending on the time of exposure to activated macrophages (10). IL-10 has marked effects on B cells. IL-10 also has documented activity on endothelial cells, where it mimics IL-4 (11), and on thymocytes (12) and mast cells, where it acts as a growth costimulator (13, 14).