Interleukin-7 (IL-7) was discovered in the year 1988 as a factor that enhanced the growth of murine B-cell precursors in bone marrow culture system (1). It was also known as lymphopoietin 1and pre-B cell factor. IL-7 plays an important role in the development of B and T cells in mouse and T cells in humans. It is also essential for mature and naive T-cell’s survival and proliferation (2,3,4,5). Human IL-7 gene maps to chromosome 8 and is about 72kb in length. The protein encoded by this gene is 177 amino acids in length with a molecular weight of 20 kDa. The active form of IL-7 in humans is a glycoprotein of 25 kDa (6, 7). In humans IL-7 has been shown to be produced from intestinal epithelial cells (8, 9), keratinocytes (10), hepatic tissues (11), peripheral blood dendritic cells (12), follicular dendritic cells (13, 14), endothelial cells, smooth muscle cells and fibroblasts (13).
The IL-7 receptor consists of IL-7 receptor alpha chain (IL-7Rα) and a common gamma chain (γc). The gamma chain is also shared by IL-2, IL-4, IL-9, IL-15 and IL-21 receptors. The signaling pathways activated upon IL-7 binding to the receptor complex are JAK-STAT, PI-3 kinase and Src kinase pathways (15). JAK3, a protein tyrosine kinase is constitutively associated with the carboxy-terminal region of the gamma chain. Studies in mice lacking JAK3 have shown that it is required for transducing γc dependent signals (16,17,18). Mutations in JAK3 and γc have been shown to be associated with the autosomal recessive form of T-B + SCID (19). JAK1, another protein tyrosine kinase is associated with IL-7Rα chain and is activated upon IL-7 binding. JAK1 deficient mice shows severely impaired thymic development and no hematopoietic colony formation in response to IL-7 (20). IL-7 would first bind to IL-7Rα and then associates with the gamma chain, bringing their intracellular domains bearing JAK1 and JAK3 together (15). JAK3 phosphorylates IL-7Rα chain creating docking sites for the transcription factors, STAT1, STAT3, and STAT5 (21, 22). JAK1 and JAK3 phosphorylate these STAT molecules and induces their dimerization and translocation to the nucleus where they activate specific target genes (15). PTK2B, a protein tyrosine kinase has been shown to be associated with JAK1 and plays an important role in the survival of thymocyte cell line. The enzymatic activity and its phosphorylation are highly induced by IL-7 (23).
PI-3 kinase pathway is also activated by IL-7 and this pathway is essential for the survival and proliferation of human T cell precursors (24). PI-3 kinase interacts with IL-7Rα upon IL-7 stimulation and activates its downstream target, AKT and its activation is mediated by γc (25). AKT in turn activates GSK3 beta and Bad, the death protein. Survival of pro T-cell survival by regulating Bad via PI3 kinase/AKT pathway is mediated by IL-7 (26). IL-7 also mediates the downregulation of cyclin-dependent kinase inhibitor 1B through the PI-3 kinase pathway and this effect is required for cell proliferation (27). IL-7 also induces the phosphorylation of a Src kinase family member, Fyn which is constitutively associated with IL7RA (28,29). In addition, IL-7 induces phosphorylation of MAPK family members including MAPK1 and MAPK3 (30,31,32).
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