IL-21

IL-21 is a member of the common gamma chain (γc) family of cytokines and is expressed by multiple immune cell types, with activated CD4+ T cells, including T follicular helper (TFH) cells and natural killer (NK) T cells, being the major sources of this cytokine. The biological functions of IL-21 are mediated by binding to its corresponding receptor, IL-21R. IL-21R is expressed by a wide range of immune cells, including T and B cells, NK cells, DCs and macrophages as well as non-immune cells, including epithelial cells and keratinocytes. The ubiquitous expression of the IL-21R may explain the broad biological functions of IL-21 on the cells of hemopoietic and non-hemopoietic origins. The IL-21/IL-21R signaling activates the Janus kinase (JAK1/3)-signal transducer and activator of transcription (STAT) signaling pathway^[1][2].