IL-7 was initially described as a stromal derived factor which is capable of inducing the growth of pre-B cells in vitro. IL-7 acts on a variety of cells through its receptor (IL-7R), a heterodimer consisting of IL-7Ralpha (CD127) and a common gammac chain (CD132) shared by other cytokine (IL-2, IL-4, IL-9, IL-15, and IL-21) receptors. In addition, IL-7Ralpha is shared with TSLP. The generation of IL-7-deficient and IL-7Ralpha-deficient mice and monoclonal antibody blocking experiments confirmed the requirement of IL-7 for B-cell development in mice. Nevertheless, mutations in the alpha chain of the IL-7 receptor in patients with severe combined immunodeficiency (SCID) confirmed that IL-7 is indispensable for T-cell development in humans. However, the presence of B cells in these individuals suggests important differences between the role of IL-7 in murine and human lymphocyte development. Thus, although human B-cell development does not appear to require IL-7, immature human B cells do proliferate in response to IL-7. Nevertheless, most recent information suggests that IL-7 dependence in human lymphopoiesis increases during the progression of ontogeny in cord blood and bone marrow. IL-7 can be associated to hepatocyte grow factor (HGFbeta) to form a hybrid cytokine (IL-7/HGFbeta), which induces greater proliferation of CFU-S, SLPs, and pre–pro-B cells than does native IL-7. The hybrid cytokine signals through both IL-7R (IL-7Ralpha plus gammac) and c-Met. IL-7 has a thymic antiapoptotic effect and induces the expression of antiapoptotic proteins Bcl-2 and Mcl-1 and the inhibition of proapoptotic proteins Bax and Bad. In addition, IL-7 is a key regulator of glucose uptake in T lymphocytes. TGF-beta has been shown to down-regulate IL-7 mRNA and protein secretion from human bone marrow stromal cells. In addition, TGF-beta inhibits IL-7-induced proliferation of pre-B cells.
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