Interleukin (IL)-21 is a cytokine that affects the differentiation and function of lymphoid and myeloid cells and regulates both innate and adaptive immune responses. In addition to regulating the immune response to tumor and viral infections, IL-21 also has a profound effect on the development of autoimmune and inflammatory diseases. IL-21 is produced mainly from CD4⁺ T cells—in particular, follicular helper T (Tfh) cells—which have a great influence on the regulation of antibody production. It is also an important cytokine for the activation of CD8⁺ T cells, and its role in recovering the function of CD8⁺ T cells exhausted by chronic microbial infections and cancer has been clarified. Thus, IL-21 plays an extremely important role in viral infections, especially chronic viral infections. In this review, I will introduce the findings to date on how IL-21 is involved in some typical viral infections and the potential of treating viral diseases with IL-21.

IL-22 is a cytokine that plays a pivotal role in regulating tissue homeostasis at barrier surfaces and is produced by activated CD4⁺ Th cells. Currently, the molecular mechanisms regulating Il22 gene expression are still unclear. In this study, we have identified a crucial cis-regulatory element located 32 kb upstream of the mouse Il22 promoter, termed conserved noncoding sequence (CNS)–32. We demonstrated that CNS-32 acts as an enhancer in reporter assays and contains binding motifs for Runt-related transcription factor (Runx)1 and retinoic acid–related orphan receptor γt (RORγt). Mutation of these motifs significantly abrogated the reporter activity, suggesting a role for both factors in the control of enhancer-mediated Il22 expression. Runx1 and RORγt occupancy and elevated histone H4 acetylation at CNS-32 were evident, as naive T cells differentiated into IL-22–producing Th22 cells. Overexpression of Runx1 promoted IL-22 production by inducing RORγt and IL-23 receptor, all critical to Th22 cell induction. Although Runx1 alone enhanced IL-22 production in Th22 cells, it was further enhanced in the presence of RORγt. Conversely, short hairpin RNA–mediated knockdown of core-binding factor β, a cofactor essential for Runx1 activity, was effective in limiting IL-22 production. Collectively, our results suggest that IL-22 production is controlled by a regulatory circuit in which Runx1 induces RORγt and then partners with RORγt to direct Il22 expression through their targeting of the Il22 enhancer. Key Points CNS-32 is a distal enhancer element to upregulate Il22 expression. CNS-32 interacts with the Il22 promoter via chromatin looping. Runx1 and RORγt cooperatively upregulate Il22 expression through binding to CNS-32.

Inflammatory bowel diseases are known to be the origin of colitis-associated colon cancer (CAC). We previously reported that dextran sulfate sodium (DSS)–induced colitis is exacerbated in mouse-IL-21-isoform transgenic (Tg) mice. In this study, we assessed the CAC development induced by azoxymethane (AOM) and DSS in our Tg mice. AOM-DSS–induced tumor development was dramatically increased in the Tg mice compared with wild-type mice. IL-21 is known to enhance activation-induced cytidine deaminase (AID) expression in B cells and induce Ab class switching. In contrast, the AID expression in cells other than B cells initiates tumor development in many tissues. Therefore, we investigated whether IL-21 induces the AID expression in the large intestinal epithelial cells (IECs) during CAC development. AID gene and protein expression was increased in the IECs of AOM-DSS– or DSS-treated Tg mice compared with those of wild-type mice. Furthermore, we confirmed IL-21 induced AID gene expression in the purified IECs ex vivo. The present study also showed IL-21R gene expression in unstimulated wild-type mouse IECs, and this gene expression was augmented by TNF-α stimulation. The IL-21R expression and IL-21–induced AID gene activation were further confirmed in the Colon-38 cell line. Taken together, IL-21 may be involved in increasing the risk of CAC by enhancing the AID expression in IECs.

Apurinic/apyrimidinic endonuclease 1/redox factor-1 (Ape1/Ref-1) is a multifunctional protein possessing DNA repair, redox control, and transcriptional regulatory activities. Although Ape1/Ref-1 plays multiple roles in the immune system, its functions in helper T (Th) cell activation and differentiation are largely unknown. In this study, the function of Ape1/Ref-1 in Th cell activation was analyzed using an Ape1/Ref-1 redox-specific inhibitor, E3330. When splenocytes from OT-II mice, which are ovalbumin (OVA)-specific T-cell receptor transgenic mice, were activated with OVA in the presence of E3330, the induction of IFN-γ-producing OT-II T cells was significantly increased. In contrast, E3330 did not enhance IFN-γ production from plate-bound anti-CD3 antibody-stimulated CD4⁺ T cells in the absence of antigen presenting cells (APCs). Furthermore, E3330-pretreated and OVA-pulsed APCs also enhanced the IFN-γ production from OT-II T cells. These results suggested that E3330 enhances Th1 responses by modifying APC function. E3330 did not alter the surface expression of MHC-II or the co-stimulatory molecules CD80 and CD86 on APCs. On the other hand, E3330 up-regulated the IL-12 p35 and p40 gene expression, and IL-12 surface retention, but decreased the IL-12 secretion from Toll-like receptor (TLR) ligand-stimulated APCs. These results were confirmed with Ape1/Ref-1 knockdown experiments. Taken together, our findings indicated that the suppression of Ape1/Ref-1 redox function leads to an increased cell surface retention of IL-12 and enhances Th1 responses. This is the first study to demonstrate that Ape1/Ref-1 modulates the IL-12 production and secretion from APCs and controls Th1 immune responses.