Intracellular Major Histocompatibility Complex Class II and C-X-C Motif Chemokine Ligand 10-Expressing Neutrophils Indicate the State of Anti-Tumor Activity Induced by Bacillus Calmette-Guérin.

(1) Background: Inflammatory responses induce the formation of both anti-tumor and pro-tumor neutrophils known as myeloid-derived suppressor cells (MDSCs). Intermittent intravesical infusion of Bacillus Calmette-Guérin (BCG) is an established cancer immunotherapy for non-muscle-invasive bladder cancer (NMIBC). However, the types of neutrophils induced via the inflammatory response to both tumor-bearing and BCG remain unclear. (2) Methods: We therefore analyzed neutrophil dynamics in the peripheral blood and urine of patients with NMIBC who received BCG therapy. Further, we analyzed the effects of BCG in a mouse intraperitoneal tumor model. (3) Results: BCG therapy induced the formation of CXCL10 and MHC class II-positive neutrophils in the urine of patients with NMIBC but did not induce MDSC formation. CXCL10- and MHC class II-expressing neutrophils were detected in peritoneal exudate cells formed after BCG administration. Partial neutrophil depletion using an anti-Ly6G antibody suppressed the upregulation of CXCL10 and MHC class II in neutrophils and reversed the anti-tumor activity of BCG in mouse models. (4) Conclusions: These results indicated that intracellular MHC class II- and CXCL10-expressing neutrophils indicate the state of anti-tumor activity induced via BCG. The status of neutrophils in mixed inflammation of immunosuppressive and anti-tumor responses may therefore be useful for evaluating immunological systemic conditions.

GPI-80 Augments NF-κB Activation in Tumor Cells.

Recent studies have discovered a relationship between glycosylphosphatidylinositol (GPI)-anchored protein 80 (GPI-80)/VNN2 (80 kDa GPI-anchored protein) and malignant tumors. GPI-80 is known to regulate neutrophil adhesion; however, the action of GPI-80 on tumors is still obscure. In this study, although the expression of GPI-80 mRNA was detectable in several tumor cell lines, the levels of GPI-80 protein were significantly lower than that in neutrophils. To clarify the function of GPI-80 in tumor cells, GPI-80-expressing cells and GPI-80/VNN2 gene-deleted cells were established using PC3 prostate cancer cells. In GPI-80-expressing cells, GPI-80 was mainly detected in vesicles. Furthermore, soluble GPI-80 in the conditioned medium was associated with the exosome marker CD63 and was also detected in the plasma obtained from prostate cancer patients. Unexpectedly, cell adhesion and migration of GPI-80-expressing PC3 cells were not modulated by anti-GPI-80 antibody treatment. However, similar to the GPI-80 family molecule, VNN1, the pantetheinase activity and oxidative state were augmented in GPI-80-expressing cells. GPI-80-expressing cells facilitated non-adhesive proliferation, slow cell proliferation, NF-κB activation and IL-1ß production. These phenomena are known to be induced by physiological elevation of the oxidative state. Thus, these observations indicated that GPI-80 affects various tumor responses related to oxidation.

Anti-Inflammatory Effect of Muscle-Derived Interleukin-6 and Its Involvement in Lipid Metabolism.

Interleukin (IL)-6 has been studied since its discovery for its role in health and diseases. It is one of the most important pro-inflammatory cytokines. IL-6 was reported as an exacerbating factor in coronavirus disease. In recent years, it has become clear that the function of muscle-derived IL-6 is different from what has been reported so far. Exercise is accompanied by skeletal muscle contraction, during which, several bioactive substances, collectively named myokines, are secreted from the muscles. Many reports have shown that IL-6 is the most abundant myokine. Interestingly, it was indicated that IL-6 plays opposing roles as a myokine and as a pro-inflammatory cytokine. In this review, we discuss why IL-6 has different functions, the signaling mode of hyper-IL-6 via soluble IL-6 receptor (sIL-6R), and the involvement of soluble glycoprotein 130 in the suppressive effect of hyper-IL-6. Furthermore, the involvement of a disintegrin and metalloprotease family molecules in the secretion of sIL-6R is described. One of the functions of muscle-derived IL-6 is lipid metabolism in the liver. However, the differences between the functions of IL-6 as a pro-inflammatory cytokine and the functions of muscle-derived IL-6 are unclear. Although the involvement of myokines in lipid metabolism in adipocytes was previously discussed, little is known about the direct relationship between nonalcoholic fatty liver disease and muscle-derived IL-6. This review is the first to discuss the relationship between the function of IL-6 in diseases and the function of muscle-derived IL-6, focusing on IL-6 signaling and lipid metabolism in the liver.

Physiological heating augments the anti-inflammatory reactions during granulocyte/monocyte apheresis: A in vitro study.

Granulocyte and monocyte adsorptive apheresis (GMA), an effective therapy for inflammatory disorders, exerts an anti-inflammatory influence by utilizing the biological reaction between blood and cellulose acetate (CA) beads, which are the carriers of the GMA column. Although the biological reaction has an optimum temperature, blood contacts the CA beads below body temperature as GMA is performed in an extracorporeal circulation system. We investigated various soluble factors in blood treated with CA beads at 25°C and 37°C. Here, the optimal temperature for IL-1 receptor antagonist (IL-1ra) release induced by CA beads was 37°C, and IL-6 production from monocytic cells was inhibited by the addition of plasma prepared from the CA bead-treated blood at 37°C, rather than at 25°C. These results indicated that physiological heating of the apheresis carrier augmented the anti-inflammatory reaction in vitro. Thus, heating during GMA may be a new approach for augmenting clinical efficacy.

Analysis of intestinal immunity and flora in a collagen-induced mouse arthritis model: differences during arthritis progression.

Intestinal immunity and flora are reported to be associated with the onset of rheumatoid arthritis. However, differences in the intestinal immunity and flora dynamics between the initial peak and relapse of arthritis have not been investigated. Here we analyzed the lymphocyte populations in different lymphoid tissues, the IgA in feces, and the intestinal flora at the initial peak and the relapse phase of arthritis in a collagen-induced arthritis (CIA) mouse model. In this model compared with the control group, the percentage of RORγt+CD4+ T cells in the mesenteric lymph nodes (mLN) was increased at the initial peak but decreased at the relapse stage of arthritis, and the opposite changes were observed in the spleen. The percentage of Foxp3+CD4+ T cells was unchanged at the initial peak in both tissues but increased only in the mLN at the relapse stage. The IgA in feces increased with the progression of arthritis, and bacterial analysis revealed that some specific bacterial families were changed at the peak and relapse stages of arthritis. Finally, the immune dynamics under different arthritic conditions were examined by integrating these factors using principal component analysis (PCA). PCA showed that the immunological and intestinal flora profiles were different between the initial peak and the relapse of the arthritis. Our findings suggest that the intestinal immunity and the environment change drastically with the progress of arthritis.

GPI-80 as a Useful Index for Myeloid Cell Heterogeneity and a Potential Prognostic Biomarker for Metastatic Renal Cell Carcinoma.

Myeloid-derived suppressor cells (MDSCs), which include neutrophilic MDSCs and monocytic MDSCs, exhibit high immunosuppressive activity. Glycosylphosphatidylinositol-anchored 80 kD protein (GPI-80) is selectively expressed on mature neutrophils in healthy individuals. Increased GPI-80 expression on monocytes and variations in GPI-80 expression on neutrophils indicate the appearance of MDSCs in the peripheral blood of cancer patients. However, it is still unclear whether GPI-80 expression on myeloid cells, neutrophilic MDSCs and monocytic MDSCs, is correlated with the clinical outcomes of patients with cancer. In this study, we investigated the characteristics of myeloid cells expressing GPI-80 and the implication of GPI-80 expression in the clinical outcomes of patients with metastatic renal cell carcinoma (mRCC), in which primary renal cell carcinoma spreads from the kidney to other organs. The study included 20 patients with mRCC (a mean age of 66.0 years) and 16 healthy volunteers (a mean age of 47.8 years). To determine the heterogeneity of myeloid cells in peripheral blood samples, we performed the three-dimensional principal component analysis using the combination of GPI-80, CD16, and latency-associated peptide-1 (LAP), derived from the N-terminal region of transforming growth factor-ß1 precursor. The results showed that myeloid cells in mRCC patients were widely distributed and clearly distinguishable from those in the healthy volunteers. The survival analysis revealed that GPI-80 expression on neutrophils and monocytes was correlated with poor prognostic outcomes of patients with mRCC. In conclusion, the expression of GPI-80 on myeloid cells, a useful index for the heterogeneity of MDSCs, serves as a potential prognostic biomarker for mRCC.

Interleukin-12p40 variant form reduces Interleukin-12p80 secretion.

IL-12 is a key cytokine for the promotion of CD4+ T cells differentiation to type 1 helper T cells. IL-12 is a heterodimer (IL-12p70) consisting of p40 and p35 subunits, and is mainly secreted from activated antigen-presenting cells, such as macrophages and dendritic cells (DCs). In this study, we found that activated mouse bone marrow-derived DCs (BMDCs) produced a p40 splice variant form mRNA in addition to the conventional p40 mRNA. This p40 variant mRNA was produced by alternative splicing in exon 5, and possessed a premature stop codon. As a result, the p40 variant protein contained 157 amino acids of the N-terminal part of p40 and an additional 10 novel amino acids. When the p40 variant was expressed in HEK-293T cells, it was not secreted from the cells. To investigate the function of the p40 variant, it was co-expressed with p40 and/or p35. The p40 variant did not affect the secretion of IL-12p40 or IL-12p70, or the function of the secreted p70. In contrast, the secretion of IL-12p80, a homodimeric IL-12 with two p40 subunits, was significantly decreased when the p40 variant was expressed. This new splicing variant p40 may act to fine-tune the function of IL-12p80.

IL-21 Enhances the Development of Colitis-Associated Colon Cancer: Possible Involvement of Activation-Induced Cytidine Deaminase Expression.

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.

Systemic neutrophil migration and rapid consumption of neutrophils in the spleen.

The systemic migration of neutrophils is not fully understood. In this study, we purified neutrophils from rat peripheral blood and labeled them with [51Cr] sodium chromate. The labeled cells were injected into the tail veins of rats, and were traced. Neutrophils were rapidly trapped in the liver and the spleen within 6 h. The migration ratios of neutrophils in the lung and the gut were lower compared with those in the liver and the spleen. Interestingly, migrated cells into the spleen were rapidly phagocytosed by monocytes/macrophages. Therefore, accumulation of intact neutrophils in the spleen may be difficult to measure.

Adsorption of Soluble Immunoglobulin-Type Adhesion Molecules to Cellulose Acetate Beads.

Circulating levels of soluble intercellular adhesion molecule-1 (sICAM-1) and vascular adhesion molecule-1 (sVCAM-1) are elevated in patients with inflammatory bowel disease. Cellulose acetate (CA) beads are used as carriers for granulocyte and monocyte (GM) adsorptive apheresis (GMA). We investigated the effect of CA beads on sICAM-1 and sVCAM-1 plasma concentrations in vitro. Because GM adsorption to CA beads increased with a rise in the incubation temperature in our previous study, peripheral blood was incubated with and without CA beads at 5, 25, 37, or 43 °C and plasma sICAM-1 and sVCAM-1 was measured. The sICAM-1 and sVCAM-1 concentrations in samples incubated with CA beads were significantly lower than those without CA beads at all four temperatures. However, no significant differences were observed both sICAM-1 and sVCAM-1 plasma levels at the four different temperatures after incubation with CA beads. These results suggest that independent of incubation temperature, sICAM-1 and sVCAM-1 are likely to adsorb CA beads. These molecules may be a new index for predicting the therapeutic effects of GMA.

Augmentation of the expression of the eotaxin receptor on duodenal neutrophils by IL-21.

Inflammation can occur via different mechanisms, such as via acute and chronic responses, on numerous occasions and function accordingly through various roles. There are more than five subsets of neutrophils; neutrophilic heterogeneity is modulated by the inflammatory condition. To understand the characteristics of inflammation, identification of atypical neutrophils is important. In this study, we found that the expression of eotaxin receptor (CD193) on atypical neutrophils in the duodenum is augmented in IL-21 isoform transgenic (Tg) mice. In a series of studies, we have established a Tg mouse strain to further investigate the functions of IL-21 in vivo. Interestingly, Tg mice immunized with ovalbumin (OVA) were more sensitive to OVA-induced systemic anaphylaxis as compared with wild type mice with duodenal and splenic gross congestion. Further analysis conducted in the duodenum of Tg mice revealed that only the number of neutrophils migrating into the duodenum was significantly increased prior to immunization. Previous studies have shown that the gastrointestinal compartment and the spleen constantly produce eotaxin, which regulates basal levels of tissue eosinophils. Therefore, we analyzed CD193 expression on neutrophils and eosinophils. As expected, its expression by duodenal neutrophils was upregulated in Tg mice. Furthermore, the addition of IL-21 into bone marrow cell culture increased the number of CD193+ neutrophils, which easily migrated into the duodenum. These observations suggested that CD193+ neutrophils increase in number under inflammatory conditions due to chronic IL-21 production.

Protein Tyrosine Phosphatase SHP-2 Is Involved in the Interleukin-21-Induced Activation of Extracellular Signal-Regulated Kinase 1/2.

The cytokine interleukin-21 (IL-21) is mainly produced from activated CD4+ T cells and natural killer T (NKT) cells. IL-21 enhances the proliferation and differentiation of T cells and B cells and also increases cytotoxicity of CD8+ T cells and NK cells through the IL-21 receptor and its downstream signaling molecules such as signal transducers and activator of transcription 3 (STAT3) and extracellular signal-regulated kinase 1/2 (ERK1/2). SH2 domain-containing tyrosine phosphatase (SHP-2) is ubiquitously expressed, including hematopoietic cells. SHP-2 has been implicated in the regulation of IL-6 and IL-3 signaling, but its function in IL-21 signaling has not been investigated. Therefore, we studied the role of SHP-2 in IL-21 signaling by SHP-2 overexpression and knockdown experiments. For the SHP-2 overexpression, we used 293T human embryonic kidney cells, in which the IL-21 receptor system were easily reconstituted and high amounts of exogenous SHP-2 were expressed by vector transfection. In 293T cells, overexpressed SHP-2 caused the increase in the degree of the IL-21-induced ERK1/2 activation. Subsequently, SHP-2 knockdown experiments were performed in the mouse pro-B cell line, BAF21RWT-1, which constitutively expresses human IL-21 receptor and proliferates in an IL-21-dependent manner. SHP-2 knockdown reduced the degree of the IL-21-induced ERK1/2 activation and suppressed cell proliferation. These results suggest that SHP-2 may augment the ERK1/2 activity and cell proliferation activity in IL-21 signaling. We propose that SHP-2 is involved in the IL-21-mediated ERK1/2 activation and cell proliferation.

IL-21 Attenuates FITC-Induced Contact Hypersensitivity Response via Regulation of Dendritic Cell Function.

IL-21 is mainly produced by activated CD4+ T cells and is involved in the activation of immune cells such as T cells and macrophages. In contrast, IL-21 suppresses dendritic cell maturation. We studied the effect of IL-21 in a mouse model of FITC-induced contact hypersensitivity using IL-21 isoform transgenic (IL-21iso-Tg) mice. Tissue inflammation at 24 hours after elicitation in IL-21iso-Tg mice was significantly weaker than that in wild-type mice. In agreement with tissue inflammation, recruitment of CD4+ and CD8+ T cells, neutrophils, and macrophages into the inflamed tissue was decreased in IL-21iso-Tg mice. In addition, both mRNA expression and protein production of inflammatory cytokines were lower in IL-21iso-Tg mice. In the skin, T cells were activated at inducible skin-associated lymphoid tissue, which is likely a gut-associated lymphoid tissue. The mRNA level of CXCL2, an essential chemokine for inducible skin-associated lymphoid tissue formation, was significantly lower in IL-21iso-Tg mice, and histological analysis showed that dendritic cell clustering, a preliminary step in inducible skin-associated lymphoid tissue formation, was impaired. Our study showed that IL-21 down-regulated inducible skin-associated lymphoid tissue formation and reduced contact hypersensitivity response.

Effect of Temperature on Granulocyte and Monocyte Adsorption to Cellulose Acetate Beads.

Granulocyte and monocyte (GM) adsorptive apheresis (GMA) is an effective therapy for inflammatory disorders including inflammatory bowel disease (IBD). During GMA, the blood of a patient with IBD passes through a column to contact cellulose acetate (CA) beads at a temperature below body temperature, likely close to room temperature. Here we investigated the effect of temperature on GM adsorption to CA beads in vitro. We incubated peripheral blood with and without CA beads at 5°C, 25°C, 37°C, and 43°C and calculated the ratios of adsorbed GMs. The ratios of adsorbed GMs increased as the temperature was raised. Additionally, we measured complement activation fragment concentrations. C3a and C5a concentrations also increased as the temperature was raised, and C5a concentrations had a positive correlation with the ratios of adsorbed GMs. These results suggest that warming the column during GMA might increase GM adsorption to CA beads, thereby enhancing the clinical efficacy of GMA.

Apurinic/Apyrimidinic Endonuclease 1/Redox Factor-1 (Ape1/Ref-1) Modulates Antigen Presenting Cell-mediated T Helper Cell Type 1 Responses.

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.

Selective induction of anti-inflammatory monocyte-platelet aggregates in a model of pulsatile blood flow at low shear rates.

In patients with cardiovascular abnormalities or immunological disorders, an increased number of circulating leukocyte-platelet aggregates is observed. Leukocyte-platelet aggregates play an essential role in linking the hemostatic and immune systems. High shear stress and pro-coagulant and pro-inflammatory stimulants are known to activate platelets and promote the formation of aggregates. Pulsatile blood flow under low shear stress can also induce platelet activation in comparatively mild conditions. However, the effect of such events on leukocyte-platelet aggregates has not yet been investigated. To determine whether low shear stress affects the formation of aggregates, we established a simple "inverting rotation" method of inducing periodic changes in the direction of blood flow in combination with low shear stress. We demonstrated that after the inverting rotation treatment for 10-20 min more than 70% of monocytes selectively aggregated with platelets. The formation of monocyte-platelet complexes was inhibited by an anti-CD162 (PSGL-1) monoclonal antibody or a Ca(2+) chelator. The phagocytic activity of monocytes was augmented by inverting rotation, whereas phagocytosis mediated by granulocytes remained unaffected. Interestingly, the formation of monocyte-platelet complexes suppressed the production of pro-inflammatory cytokines such as interleukin (IL)-1ß. At the same time, monocyte-platelet complexes augmented the expression of the anti-inflammatory cytokine IL-10. Our results suggest that platelet-bound monocytes show an anti-inflammatory phenotype under low shear stress conditions. Thus, our method provided new insights into the mechanisms of monocyte-platelet aggregate formation and regulation.

Human peripheral neutrophils express functional IL-21 receptors.

Neutrophils play key roles in the inflammatory response. The IL-21 cytokine and receptor system is known to be involved in various inflammatory diseases. However, the direct action of IL-21 on neutrophils has not been reported. Here, we show that human neutrophils in peripheral blood express functional IL-21 receptors (IL-21Rs). Expression of the IL-21Rα chain (IL-21Rα) was reduced following various treatments to remove red blood cells, including hypotonic shock, ammonium chloride-mediated lysis, and Percoll density centrifugation. Thus, we utilized whole blood flow cytometric assays to investigate the neutrophil responses to IL-21. IL-21 upregulated the surface expression of CD11b and CD16 on neutrophils and augmented the neutrophils' phagocytic ability. Our data indicated that IL-21 has the potential to enhance the neutrophil functions.

Role of interleukin-21 isoform in dextran sulfate sodium (DSS)-induced colitis.

Interleukin-21 (IL-21) is overproduced in human intestines affected by inflammatory bowel disease (IBD) and in the gut of mice with DSS-induced colitis. IL-21-deficient mice are largely protected against DSS-induced colitis, indicating that IL-21 plays a key role in the development of IBD. We previously identified a novel IL-21 isoform named IL-21iso. In this study, we found that in addition to the conventional IL-21, IL-21iso mRNA was also expressed in the colon with DSS-induced colitis. To investigate whether IL-21iso plays a role in DSS-induced colitis, we established transgenic mice (mIL-21iso-Tg mice) that expressed mouse IL-21iso under the control of the lck proximal promoter. Although mIL-21iso-Tg mice did not have any gross physical abnormalities, their peripheral lymphocytes counts were higher than those in wild-type littermates. Notably, their CD8(+) T cell and CD4(+) effector memory T-cell populations were elevated. DSS-induced colitis was far more severe in the mIL-21iso-Tg mice than in wild-type mice, and was accompanied by a marked loss of body weight and by colon inflammation with increased cellular infiltration. In DSS-treated mice, colon tissues from mIL-21iso-Tg mice had significantly higher gene activation levels for cytokines such as IL-17A, TNF-α, IL-6, IL-10, and IL-4, and for transcription factors such as T-bet, GATA-3, RORγt, and Foxp3, than were found in wild-type mice. These results indicate that besides IL-21, IL-21iso may be another regulator of gut inflammation.

IL-21 isoform is a membrane-bound ligand and activates directly interacted cells.

IL-21 is a pleiotropic cytokine that regulates the function of T cells, B cells, natural killer cells, and myeloid cells. We previously identified an IL-21 isoform, IL-21iso, in humans and mice, and found that IL-21iso was secreted in much smaller amounts than conventional IL-21. In this study, we determined that secreted IL-21iso also has less signaling activity than IL-21. However, the amounts of intracellular IL-21 or IL-21iso, and the level of STAT3 phosphorylation induced by the two IL-21 forms, were similar. IL-21-sensitive reporter cells co-cultured with cells producing IL-21iso showed STAT3 activation, apoptosis, and proliferation. However, when IL-21iso-producing cells were cultured in a transwell chamber, which prevented direct contact with the IL-21-sensitive cells, no IL-21iso-induced signaling was observed. Though IL-21iso is secreted in smaller amounts and has less potent signaling activity than IL-21, IL-21iso acts both on IL-21iso-bearing cells and other IL-21-sensitive cells through direct interactions probably without being secreted. Thus, IL-21iso's regulation of immune cells may be limited to the immediate proximity around the IL-21iso-producing cells, in regions such as immune organs or inflammation sites.

Apurinic/apyrimidinic endonuclease1/redox factor-1 (Ape1/Ref-1) is essential for IL-21-induced signal transduction through ERK1/2 pathway.

IL-21 is a pleiotropic cytokine that regulates T-cell and B-cell differentiation, NK-cell activation, and dendritic cell functions. IL-21 activates the JAK-STAT, ERK, and PI3K pathways. We report here that Ape1/Ref-1 has an essential role in IL-21-induced cell growth signal transduction. Overexpression of Ape1/Ref-1 enhances IL-21-induced cell proliferation, but it is suppressed by overexpressing an N-terminal deletion mutant of Ape1/Ref-1 that lacks the redox domain. Furthermore, knockdown of the Ape1/Ref-1 mRNA dramatically compromises IL-21-induced ERK1/2 activation and cell proliferation with increasing cell death. These impaired activities are recovered by the re-expression of Ape1/Ref-1 in the knockdown cells. Our findings are the first demonstration that Ape1/Ref-1 is an indispensable molecule for the IL-21-mediated signal transduction through ERK1/2 activation.