Interleukin (IL)-7 is a secreted soluble globular protein 25 kDa in size encoded by the IL7 gene. It was first identified as a growth factor for murine B cell precursors found in the bone marrow. Other researchers found that mRNA encoding IL-7 was enriched in the thymus, the lymphoid organ in which T cells are made. Addition of recombinant IL-7 to in vitro cultures was found to promote survival and growth of thymocytes, the cells from the thymus that ultimately develop into mature T lymphocytes. These early experiments suggested that IL-7 was important for development of normal T lymphocytes in the thymus and B lymphocytes in bone marrow. The generation of genetically modified “knockout” mice, in which the IL-7 gene was specifically disrupted, ultimately confirmed these views (von Freeden-Jeffry et al. 1995). While these IL-7-deficient mice were grossly normal in appearance, the suggested importance of IL-7 for lymphocyte development was...
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