Mature rat Interleukin-10 (IL-10), also referred to as cytokine synthesis inhibitory factor (CSIF), is an ~18 kDa protein of 160 amino acids (aa) after cleavage of an 18 aa signal peptide. There are two potential N-linked glycosylation sites and five cysteine residues in the mature molecule. Four of its five cysteine residues are involved in the formation of two intrachain disulfide linkages that are essential for activity. Based on human and murine studies, biologically active rat interleukin-10 is likely to be a nondisulfide-linked homodimer in solution. IL-10 has pleiotropic effects in immunoregulation and inflammation. It is produced primarily by monocytes and to a lesser extent by lymphocytes. IL-10 down-regulates the expression of Th1 cytokines, MHC class II Ags, and costimulatory molecules on macrophages. It also enhances B cell survival, proliferation, and antibody production. This cytokine can block NF-κB activity, and is involved in the regulation of the JAK-STAT signaling pathway. Rat IL-10 shares 85% aa sequence identity with mouse IL-10 and 74% aa sequence identity with human IL-10. Rat and human IL-10 are both active on mouse cells, however; mouse IL-10 is not active on human cells