Interleukin 21 (IL-21) is a pleiotropic cytokine that is composed of four α-helical bundles and primarily produced by natural killer T (NKT) cells, T follicular helper (Tfh) cells, and Th17 cells (Spolski and Leonard 2008). IL-21 signals via receptor heterodimerization of IL-21 receptor and IL-2 receptor subunit gamma (IL-2RG or CD132), both of which have a common gamma-chain subunit and activate the JAK/STAT, MAPK, and PI3K pathways (Parrish-Novak et al.; Ozaki et al. 2000; Spolski and Leonard 2014). IL-21 has been shown to have a critical role in regulating immunoglobulin production and differentiation of the pro-inflammatory Th17 population of cells (Ozaki et al. 2002; Nurieva et al.). Additionally, IL-21 specifically sustains CD8+ T cell effector activity and provides a mechanism of CD4+ T cell help during chronic viral infection (Elsaesser et al.). IL-21 signaling was also found critical for the development of type 1 diabetes in non-obese diabetic (NOD) mice (Sutherland et al.) and control of T cell autoimmunity by regulatory B cells (Yoshizaki et al.).