Dividing with IL-7

The cytokine interleukin (IL)-7 is well known to promote T cell survival and homeostatic proliferation of T cells, with the signals required for the latter being less well understood. A study by Li et al. (page 573) now shows that IL-7 induces T cell proliferation by triggering the degradation of the cell cycle inhibitor p27^kip1.

The surprising aspect of this study was not that a growth cytokine induces cell division by disposing of p27^kip1—this occurs in many cell types—but that T cells have a unique way of getting rid of this protein. In most cell types, growth signals trigger the phosphorylation of p27^kip1. Once phosphorylated, p27^kip1 becomes bound by a Skp2 (S phase kinase-associated protein 2)-containing ubiquitin ligase complex that marks the inhibitor for...