Interleukin 10 (IL-10), initially designated cytokine synthesis inhibitory factor (CSIF), was originally identified as a product of mouse T helper 2 (Th2) cells that inhibited the cytokine production by Th1 cells. It is a pleiotropic cytokine that regulates the immune and inflammatory responses of hematopoietic cells (1, 2). IL-10 has immunosuppressive activities and has been shown to inhibit the effector functions of monocyte/macrophage and CD4+ T cells. Conversely, IL-10 has immunostimulatory activities and can induce the proliferation and cytotoxic activity of CD8+ T cells and NK cells. IL-10 also regulates the growth and differentiation of B cells, mast cells, dendritic cells and neutrophils (1). The biological activities of IL-10 is mediated by the heteromeric IL-10 receptor complex, which is composed of the ligand-binding IL-10R alpha and the accessory IL-10R beta subunits. Both subunits belong to the class II cytokine receptor family. IL-10R beta is also utilized as a subunit in the heterodimer receptor complex for IL-22, IL-28 and IL-29. Besides IL-10, five novel cytokines (IL-19, -20, -22, -24, and -26) that share structural and limited sequence homology with IL-10 have been identified. These proteins constitute the IL-10 cytokine family (3).
Equine IL-10 cDNA encodes a 178 amino acid residue (aa) precursor protein with an 18 aa signal peptide and 160 aa mature protein that contains two potential N-linked glycosylation sites. Analogous to human IL-10, equine IL-10 likely exists as nondisulfide-linked homodimers. Equine IL-10 shares 71% and 78% aa sequence homology with mouse and human IL-10, respectively.
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