Cytokines: IL-21 joins the γc-dependent network?

Cytokines are essential for the regulation of immune function, serving as an important means of communication between various hematopoietic cells and influencing lymphocyte development. In recent months, new cytokines have been discovered faster than their names can be agreed on. Unfortunately, this has led to confusion in the literature, as two new cytokines that are clearly distinct molecular entities were both initially designated interleukin-21 (IL-21). To remedy the situation, the molecule described by Parrish-Novak et al. [1] has been formally designated IL-21, while the IL-9-induced factor, IL-TIF [2] has provisionally been renamed as IL-22 [3]. The identification of IL-21 and its receptor [1,4] seems to be of particular significance because these proteins are most related to the cytokines and receptor chains that use the common cytokine receptor γ chain, γc (reviewed in [5,6]; Figure 1). Also, the expression and function of this cytokine and its receptor suggest that IL-21 is an exciting new player in lymphoid differentiation that may provide new insights into outstanding questions regarding the immunological phenotype caused by γc deficiency.