Interleukin-21 in immune and allergic diseases.

Interleukin-21 (IL-21), a cytokine produced by various subsets of activated CD4+ T cells, plays a major role in the control of innate and adaptive immune responses. IL-21 biological activity is mediated by binding of the cytokine to a heterodimeric receptor, composed of a specific subunit, termed IL-21 receptor (IL-21R), and the common γ-chain, that is shared with IL-2, IL-4, IL-7, IL-9 and IL-15 receptors. IL-21 stimulates the proliferation of CD4+ and CD8+ T lymphocytes and regulates the profile of cytokines secreted by these cells, drives the differentiation of B cells into memory cells and Ig-secreting plasma cells, and enhances the activity of natural killer cells. IL-21 controls also the activity of non-immune cells, such as epithelial cells and stromal cells. The demonstration that IL-21 is involved in the immune responses occurring in chronic inflammatory and allergic diseases suggests that either disrupting or enhancing IL-21 signalling may be useful in specific clinical settings.