Interleukin 15

Interleukin-15 (IL-15) is a protein that in humans is encoded by the IL15 gene. IL-15 is an inflammatory cytokine with structural similarity to Interleukin-2 (IL-2). Like IL-2, IL-15 binds to and signals through a complex composed of IL-2/IL-15 receptor beta chain (CD122) and the common gamma chain (gamma-C, CD132). IL-15 is secreted by mononuclear phagocytes (and some other cells) following infection by virus(es). This cytokine induces the proliferation of natural killer cells, i.e. cells of the innate immune system whose principal role is to kill virally infected cells. IL-15 was discovered in 1994 by two different laboratories, and characterized as T cell growth factor. Together with Interleukin-2 (IL-2), Interleukin-4 (IL-4), Interleukin-7 (IL-7), Interleukin-9 (IL-9), granulocyte colony-stimulating factor (G-CSF), and granulocyte-macrophage colony-stimulating factor (GM-CSF), IL-15 belongs to the four α-helix bundle family of cytokines. IL-15 is constitutively expressed by a large number of cell types and tissues, including monocytes, macrophages, dendritic cells (DC), keratinocytes, fibroblasts, myocyte and nerve cells. As a pleiotropic cytokine, it plays an important role in innate and adaptive immunity. IL-15 is 14–15 kDa glycoprotein encoded by the 34 kb region of chromosome 4q31 in humans, and at the central region of chromosome 8 in mice. The human IL-15 gene comprises nine exons (1–8 and 4A) and eight introns, four of which (exons 5 through 8) code for the mature protein (Figure 1). Two alternatively spliced transcript variants of this gene encoding the same protein have been reported. The originally identified isoform, with long signal peptide of 48 amino acids (IL-15 LSP) consisted of a 316 bp 5'-untranslated region (UTR), 486 bp coding sequence and the C-terminus 400 bp 3'-UTR region. The other isoform (IL-15 SSP) has a short signal peptide of 21 amino acids encoded by exons 4A and 5. Both isoforms shared 11 amino acids between signal sequences of the N-terminus.