Interleukin 21 and its receptor are involved in NK cell expansion and regulation of lymphocyte function

Interleukin-21 signaling: functions in cancer and autoimmunity

Interleukin-21 (IL-21) is a cytokine with structural and sequence homology to IL-2 and IL-15, yet possesses several biological properties distinct from these cytokines. IL-21 is produced mainly by activated CD4(+) T cells and natural killer T cells and mediates its activity by binding to the IL-21 receptor (IL-21R), consisting of an IL-21-specific alpha chain (IL-21Ralpha; JAK/STAT) that heterodimerizes with the common gamma chain (CD132). Intracellular signaling occurs through the Janus-activated kinase/signal transducer and activator of transcription pathways. Physiologic expression of IL-21R is restricted to lymphoid tissues and peripheral blood mononuclear cells; however, other tissues such as epithelium, synovium, or transformed cells can acquire expression of both components of IL-21R heterodimer. IL-21 has complex activities on a wide variety of cell types, leading to enhancement of adaptive T-cell immunity, antibody production, activation of natural killer cell subtypes, and opposition to suppressive effects mediated by regulatory T cells. Functionally, these activities promote immune responses and point to a physiologic role of IL-21 in autoimmunity and immune enhancement. Therapeutic manipulation of IL-21 activity may allow improved immunotherapy for cancer as well as insights into autoimmune disease. Recently conducted phase 1 trials in metastatic melanoma and renal cell carcinoma have shown that recombinant IL-21 has a favorable safety profile and support its continued investigation as a potential anticancer drug.

IL-21 is expressed in Hodgkin lymphoma and activates STAT5: evidence that activated STAT5 is required for Hodgkin lymphomagenesis

Classical Hodgkin lymphoma (HL) is a malignant disorder characterized by the presence of neoplastic mononucleated Hodgkin and multinucleated Reed-Sternberg cells. Here, we show that both the interleukin (IL)-21 receptor as well as IL-21 are expressed by HL cells. IL-21 activates signal transducer of activation and transcription 3 (STAT3) and STAT5 in HL cell lines and activated human B cells. Ectopic expression of constitutively active STAT5 in primary human B cells resulted in immortalized B cells that have lost the B-cell phenotype and strongly resembled HL cells, which could partially be rescued by ectopic expression of the B cell-determining transcription factor E47. Data from experiments using reporter assays and overexpression of constitutively active IKK2 support the hypothesis that the STAT5 and nuclear factor-kappaB (NF-kappaB) pathways collaborate in HL genesis.

Measurement of interleukin-21

This unit describes three procedures for measurement of interleukin-21 (IL-21). The first employs the use of an antibody sandwich ELISA. An alternative procedure measures proliferative responses of T cells to a combination of IL-21 and IL-15 using CFSE. Finally, a method to assess IL-21-induced tyrosine phosphorylation of Stat3 in splenic CD8(+) T cells using a flow cytometry-based analysis is described.

Gene therapy of inherited immunodeficiencies

BACKGROUND

Primary immunodeficiencies (PID) are a group of inherited diseases that affect the development or activity of the immune system. In severe cases allogeneic haematopoietic stem cell transplantation has proved to be a successful curative modality but it is limited by toxicity and reduced efficacy in mismatched donor settings.

OBJECTIVE

Gene therapy for PID has been developed as an alternative strategy and has entered the clinical arena. In this review we discuss the outcomes of recent gene therapy trials and some of the problems that remain to be tackled.

METHODS

Results from clinical trials for X-linked severe combined immunodeficiency (SCID-X1), adenosine deaminase deficient SCID (ADA-SCID), and X-linked chronic granulomatous disease (X-CGD) are discussed. In addition, other conditions are highlighted such as the Wiskott Aldrich Syndrome (WAS) for which gene therapy has shown considerable promise in preclinical studies, and are currently being translated into novel clinical approaches.

RESULTS/CONCLUSION

Whilst these encouraging results demonstrate that gene therapy can be used successfully to treat monogenic PID, the occurrence of vector-related side effects has highlighted the need for accurate assessment of the associated risks and a requirement for improvements in vector design.

Disruption of the gamma c cytokine network in T cells during HIV infection

The common gamma chain (gammac)-sharing cytokines (IL's-2, 4, 7, 9, 15, and 21) play a vital role in the survival, proliferation, differentiation and function of T lymphocytes. As such, disruption of their signaling pathways would be expected to have severe consequences on the integrity of the immune system. Indeed, it appears that the signaling network of these cytokines is both disrupted and exploited by HIV at various stages of infection. IL-2 secretion and signaling downstream of its receptor are impaired in T cells from chronically-infected HIV+ patients. Elevated plasma IL-7 levels and decreased IL-7Ralpha expression in patient T cells results in significantly decreased responsiveness to this critical cytokine. Interestingly, IL-2 and IL-15 are also able to render CD4+ T cells permissive to HIV infection through their influence on the activity of the APOBEC3G deaminase enzyme. Herein, we describe the current state of knowledge on how the gammac cytokine network is affected during HIV infection, with a focus on how this impairs CD4+ and CD8+ T cell function while also benefiting the virus itself. We also address the use of cytokines as adjuncts to highly active antiretroviral therapy to bolster immune reconstitution in infected patients.

Release from regulatory T cell-mediated suppression during the onset of tissue-specific autoimmunity is associated with elevated IL-21

The activity of regulatory T cells (Treg) is widely accepted to play a central role in preventing pathogenic immune responses against self-Ags. However, it is not clear why such regulation breaks down during the onset of autoimmunity. We have studied self-Ag-specific Treg during the induction of spontaneous diabetes. Our data reveal a shift in the balance between regulatory and pathogenic islet-reactive T cells in the pancreas-draining lymph nodes during disease onset. Treg function was not compromised during disease initiation, but instead conventional T cells showed reduced susceptibility to Treg-mediated suppression. Release from Treg suppression was associated with elevated levels of IL-21 in vivo, and provision of this cytokine abrogated Treg suppression in vitro and in vivo. These data suggest that immunological protection of a peripheral tissue by Treg can be subverted by IL-21, suggesting new strategies for intervention in autoimmunity.

Follicular associated T cells and their B-cell helper qualities

The immune system utilizes sophisticated cellular surveillance mechanisms to maintain the integrity of the multicellular host. Adaptive immunosurveillance in particular constitutes a powerful branch of the immune system that houses the capacity to mount exquisitely specific responses against a diverse array of foreign antigens. Central to the development of adaptive immunity is the activation of T and B cells. Upon antigen engagement, T and B cells have been observed to undergo striking changes in their migratory status and distribution within secondary lymphoid organs, a phenomenon that is to a large extent controlled through their altered responsiveness to homeostatic T- and B-zone chemokines. Changes in their chemokine receptor expression and/or sensitivity to their respective ligands assist in bringing rare antigen-specific T and B lymphocytes, dendritic cells and CD4+CD3(-) accessory cells together. Cognate interaction between these cells at the T-B junction can support the generation of extrafollicular foci of antibody producing plasma cells and the formation of germinal centers. Such T-dependent antibody responses are highly dependent on the functional properties and activity of a specialized subset of CXCR5+ICOS+ CD4 T cells referred to as T follicular helper cells (T FH). This review presents an overview of some of the defining characteristics of this subset of T-helper cells and the chemokine receptors and their ligands that help dictate the migratory activity of T(FH) cells within secondary lymphoid organs.

The biological functions of T helper 17 cell effector cytokines in inflammation

T helper 17 (Th17) cells belong to a recently identified T helper subset, in addition to the traditional Th1 and Th2 subsets. These cells are characterized as preferential producers of interleukin-17A (IL-17A), IL-17F, IL-21, and IL-22. Th17 cells and their effector cytokines mediate host defensive mechanisms to various infections, especially extracellular bacteria infections, and are involved in the pathogenesis of many autoimmune diseases. The receptors for IL-17 and IL-22 are broadly expressed on various epithelial tissues. The effector cytokines of Th17 cells, therefore, mediate the crucial crosstalk between immune system and tissues, and play indispensable roles in tissue immunity.

STAT3-mediated up-regulation of BLIMP1 Is coordinated with BCL6 down-regulation to control human plasma cell differentiation

STAT family members have been implicated in regulating the balance between B cell lymphoma (BCL)6 and B lymphocyte induced maturation protein (BLIMP)1 to control plasma cell differentiation. We previously showed that STAT5 induces BCL6 to block plasma cell differentiation and extend the life span of human B cells. The heterogeneity in STAT activation by cytokines and their effects on B cell differentiation prompted us to investigate the effect of STAT3 activation in plasma cell differentiation. First stimulation with IL-21, which promotes plasma cell differentiation, induced robust and prolonged STAT3 activation in primary human B cells. We then investigated effects of direct STAT3 activation on regulation of plasma cell genes, cellular phenotype, and Ig production. Activation of a tamoxifen-regulated STAT3-estrogen receptor fusion protein triggered BLIMP1 mRNA and protein up-regulation, plasma cell phenotypic features, and Ig secretion. When STAT3 was activated by IL-21 in B cells ectopically expressing BCL6, BLIMP1 was up-regulated, but only partial plasma cell differentiation was achieved. Lastly, through coexpression of BCL6 and STAT3-ER, we verified that STAT3 activation functionally mimicked IL-21 treatment and that STAT3-mediated BLIMP1 up-regulation occurred despite high BCL6 expression levels indicating that BCL6 is not the dominant repressor of BLIMP1. Thus, up-regulation of BLIMP1 alone is not sufficient for differentiation of primary human B cells into plasma cells; concomitant down-regulation of BCL6 is absolutely required for completion of the plasma cell differentiation program.

Genetic association of interleukin-21 polymorphisms with systemic lupus erythematosus

OBJECTIVE

The aetiology of systemic lupus erythematosus (SLE) is incompletely understood. Both genetic and environmental factors are implicated in the pathogenesis of the disease. Herein, we describe genetic association between SLE and polymorphisms in the interleukin (IL)-21 gene. The reported effect of IL-21 on B-cell differentiation into plasma cells and its effect on dendritic cell maturation and T-cell responses make IL-21 an attractive candidate gene for SLE.

METHODS

Three single nucleotide polymorphisms (SNPs) in the IL-21 gene were genotyped in a total of 2636 individuals (1318 cases and 1318 controls matched for age, sex and race). Population-based case-control association analyses were performed.

RESULTS

We found a genetic association with SLE and two SNPs located within the IL-21 gene (rs907715: chi(2) = 11.55, p<0.001; rs2221903: chi(2) = 5.49, p = 0.019). Furthermore, genotypes homozygous for the risk alleles were more frequent than genotypes homozygous for the non-risk alleles in European-American patients as compared to controls (rs907715 (GG versus AA): odds ratio (OR) = 1.66, p = 0.0049; rs2221903 (GG versus AA): OR = 1.60, p = 0.025).

CONCLUSION

Our findings indicate that IL-21 polymorphism is a candidate association with SLE. The functional effects of this association, when revealed, might improve our understanding of the disease and provide new therapeutic targets.

Phase I study of recombinant interleukin-21 in patients with metastatic melanoma and renal cell carcinoma

PURPOSE

A phase I study of patients with metastatic malignant melanoma (MM) and renal cell carcinoma (RCC) evaluated the safety and maximum tolerated dose (MTD), pharmacokinetics, pharmacodynamics, and preliminary antitumor activity of recombinant human interleukin-21 (rIL-21).

PATIENTS AND METHODS

Patients who had one or fewer prior systemic treatments for metastatic MM or RCC were treated with rIL-21 administered for two 5-day cycles on days 1 through 5 and 15 through 19 of a treatment course; rIL-21 was administered by rapid intravenous infusion in an outpatient setting. Cohorts of patients received doses ranging from 3 to 100 microg/kg/dose, and an expanded cohort was treated at the MTD. Patients with stable disease (SD) or better could receive additional treatment cycles.

RESULTS

Forty-three patients were treated (24 MM; 19 RCC), including 28 in the expanded cohort. Dose-limiting toxicities consisted primarily of transient grade 3 laboratory abnormalities. The MTD was estimated to be 30 microg/kg. The most common adverse events included flu-like symptoms, pruritus, and rash. Twelve patients received up to five additional two-cycle courses of treatment without cumulative toxicity, except for one patient with reversible grade 4 hepatotoxicity. Serum concentrations of rIL-21 increased in a dose-proportional manner. Dose-dependent increases in soluble CD25 reflected lymphocyte activation. Antitumor activity was observed in both MM (one complete response and 11 SD) and RCC (four partial responses, 13 SD).

CONCLUSION

Outpatient therapy with rIL-21 at 30 microg/kg was well tolerated, had dose-dependent pharmacokinetics and pharmacodynamics, and was associated with antitumor activity in patients with MM and RCC.

The proinflammatory cytokines IL-2, IL-15 and IL-21 modulate the repertoire of mature human natural killer cell receptors

Natural killer (NK) cells play a crucial role in the immune response to micro-organisms and tumours. Recent evidence suggests that NK cells also regulate the adaptive T-cell response and that it might be possible to exploit this ability to eliminate autoreactive T cells in autoimmune disease and alloreactive T cells in transplantation. Mature NK cells consist of a highly diverse population of cells that expresses different receptors to facilitate recognition of diseased cells and possibly pathogens themselves. Ex vivo culture of NK cells with cytokines such as IL-2 and IL-15 is an approach that permits significant expansion of the NK cell subpopulations, which are likely to have potent antitumour, antiviral, or immunomodulatory effects in autoimmunity. Our data indicate that the addition of IL-21 has a synergistic effect by increasing the numbers of NK cells on a large scale. IL-2 and IL-15 may induce the expression of killer cell immunoglobulin-like receptors (KIRs) in KIR-negative populations, the c-lectin receptor NKG2D and the natural cytotoxic receptor NKp44. The addition of IL-21 to IL-15 or IL-2 can modify the pattern of the KIR receptors and inhibit NKp44 expression by reducing the expression of the adaptor DAP-12. IL-21 also preserved the production of interferon-γ and enhanced the cytotoxic properties of NK cells. Our findings indicate that the proinflammatory cytokines IL-2, IL-15 and IL-21 can modify the peripheral repertoire of NK cells. These properties may be used to endow subpopulations of NK cells with specific phenotypes, which may be used in ex vivo cellular immunotherapy strategies.

Cytokine-mediated regulation of human B cell differentiation into Ig-secreting cells: predominant role of IL-21 produced by CXCR5+ T follicular helper cells

Differentiation of B cells into Ig-secreting cells (ISC) is critical for the generation of protective humoral immune responses. Because of the important role played by secreted Ig in host protection against infection, it is necessary to identify molecules that control B cell differentiation. Recently, IL-21 was reported to generate ISC from activated human B cells. In this study, we examined the effects of IL-21 on the differentiation of all human mature B cell subsets--neonatal, transitional, naive, germinal center, IgM-memory, and isotype-switched memory cells--into ISC and compared its efficacy to that of IL-10, a well-known mediator of human B cell differentiation. IL-21 rapidly induced the generation of ISC and the secretion of vast quantities IgM, IgG and IgA from all of these B cell subsets. Its effect exceeded that of IL-10 by up to 100-fold, highlighting the potency of IL-21 as a B cell differentiation factor. Strikingly, IL-4 suppressed the stimulatory effects of IL-21 on naive B cells by reducing the expression of B-lymphocyte induced maturation protein-1 (Blimp-1). In contrast, memory B cells were resistant to the inhibitory effects of IL-4. Finally, the ability of human tonsillar CD4+CXCR5+CCR7- T follicular helper (TFH) cells, known to be a rich source of IL-21, to induce the differentiation of autologous B cells into ISC was mediated by the production of IL-21. These findings suggest that IL-21 produced by TFH cells during the primary as well as the subsequent responses to T cell-dependent Ag makes a major contribution to eliciting and maintaining long-lived humoral immunity.

IL-21 induces inhibitor of differentiation 2 and leads to complete abrogation of anaphylaxis in mice

IL-21 exerts pleiotrophic immunomodulatory activities on a variety of target cells including B cells that undergo class switch recombination (CSR) to IgE. In this study, we examined whether IgE-mediated systemic anaphylaxis was controlled by in vivo administration of IL-21 using the peanut allergy model in mice and investigated the molecular mechanisms underlying the IL-21-induced regulation of IgE. The anaphylactic reaction was completely abolished by the administration of recombinant mouse IL-21 or an IL-21 expression plasmid in terms of the change of body temperature and anaphylactic symptoms. The recombinant mouse IL-21 treatment remarkably suppressed IgE CSR in splenic B cells, resulting in significant decrease in serum concentrations of total as well as allergen-specific IgE. In the meanwhile, IL-21 provoked B cells in normal as well as allergic mice to express the inhibitor of differentiation 2 (Id2) gene that was shown to be crucially involved in the regulation of the activation-induced cytidine deaminase and IgE CSR. Moreover, mice genetically deficient for Id2 were completely unsusceptible to IL-21-induced prevention of IgE CSR and anaphylaxis. The present study strongly suggests that IL-21 is capable of regulating systemic allergic reactions by inducing the transcriptional regulator Id2, and the cytokine may be useful for clinical intervention for allergic diseases including anaphylaxis.

IL-21 comes of age as a regulator of effector T cells in the gut

In healthy individuals, antigens from the gut lumen are tolerated by the mucosal immune system. However, a loss of tolerance toward the bacterial microflora probably causes inflammatory bowel disease (IBD), Crohn's disease (CD) and ulcerative colitis. The abnormal activation of the immune system in the gut of IBD patients is characterized by a cascade of cellular events orchestrated by cytokine cross talk between immune and non-immune cells. Interleukin (IL)-21, the newest member of the common gamma-chain-dependent cytokine family, is a key component of the inflammatory cascade in the gut. It is highly expressed in CD and sustains the ongoing T helper type 1 (Th1)-mediated immune response. IL-21 is essential for the differentiation of Th17 cells. IL-21 is also involved in recruiting T cells to the inflamed gut and eliciting the secretion of matrix-degrading enzymes by gut fibroblasts. Overall, there is now sufficient evidence to suggest that targeting IL-21 will be of therapeutic benefit in IBD.

Interleukin-21 stimulates antigen uptake, protease activity, survival and induction of CD4+ T cell proliferation by murine macrophages

Interleukin (IL)-21 is a T cell-derived cytokine which uses a heterodimeric receptor, composed of the common gamma-chain (CD132) and an IL-21Ralpha-chain. IL-21 activates lymphoid T and B cells, modulates antibody production but also suppresses maturation of myeloid dendritic cells; however, its role in the differentiation and function of other myeloid cells remains less clear. In this study we analysed IL-21/IL-21Ralpha effects on macrophage (MPhi) differentiation and function. MPhi could be generated readily from bone marrow with MPhi-colony-stimulating factor in the presence of IL-21 (designated IL-21MPhi) or from IL-21Ralpha-/- mice. IL-21Ralpha-/- mice had normal MPhi numbers, suggesting a non-essential role of both IL-21 and the IL-21Ralpha for MPhi generation. We could demonstrate that mature MPhi express the IL-21Ralpha and the common gamma-chain. However, short-term IL-21 stimulation did not enhance MPhi proliferation but induced anti-apoptotic cell-cycle regulators p21(waf1)/p27(Kip1) and expression of suppressors of cytokine signalling (SOCS)2/SOCS3. Moreover, IL-21 enhanced phagocytosis by MPhi via IL-21Ralpha signalling and supports protease activity and matrix metalloproteinase 12 expression. Stimulating MPhi with IL-21 enhanced their capacity to induce antigen-specific CD4+ T cell proliferation in dependence from the IL-21Ralpha, which was not the case for CD8+ T cells. Taken together, IL-21 plays a previously unrecognized role in modulating innate and acquired effector mechanisms of murine MPhi by linking these different functions to support CD4+ T cell-mediated immune responses.

IL-21 induces in vivo immune activation of NK cells and CD8(+) T cells in patients with metastatic melanoma and renal cell carcinoma

PURPOSE

Human interleukin-21 (IL-21) is a class I cytokine previously reported in clinical studies on immune responsive cancers. Here we report the effects of systemic IL-21 therapy on the immune system in two phase 1 trials with this novel cytokine.

EXPERIMENTAL DESIGN

Recombinant IL-21 was administered by intravenous bolus injection at dose levels from 1 to 100 microg/kg using two planned treatment regimens: thrice weekly for 6 weeks (3/week); or once daily for five consecutive days followed by nine dose-free days (5 + 9). The following biomarkers were studied in peripheral blood mononuclear cells (PBMC) during treatment: phosphorylation of STAT3, alterations in the composition of leukocyte subsets, ex vivo cytotoxicity, expression of effector molecules in enriched CD8(+) T cells and CD56(+) NK cells by quantitative RT-PCR, and gene array profiling of CD8(+) T cells.

RESULTS

Effects of IL-21 were observed at all dose levels. In the 5 + 9 regimen IL-21 induced a dose dependent decrease in circulating NK cells and T cells followed by a return to baseline in resting periods. In both CD8(+) T cells and CD56(+) NK cells we found up-regulation of perforin and granzyme B mRNA. In addition, full transcriptome analysis of CD8(+) T cells displayed changes in several transcripts associated with increased cell cycle progression, cellular motility, and immune activation. Finally, cytotoxicity assays showed that IL-21 enhanced the ability of NK cells to kill sensitive targets ex vivo.

CONCLUSIONS

IL-21 was biologically active at all dose levels administered with evidence of in vivo NK cell and CD8(+) T cell activation.

IL-2 and IL-21 confer opposing differentiation programs to CD8+ T cells for adoptive immunotherapy

IL-2 and IL-21 are closely related cytokines that might have arisen by gene duplication. Both cytokines promote the function of effector CD8(+) T cells, but their distinct effects on antigen-driven differentiation of naive CD8(+) T cells into effector CD8(+) T cells are not clearly understood. We found that antigen-induced expression of Eomesodermin (Eomes) and maturation of naive CD8(+) T cells into granzyme B- and CD44-expressing effector CD8(+) T cells was enhanced by IL-2, but, unexpectedly, suppressed by IL-21. Furthermore, IL-21 repressed expression of IL-2Ra and inhibited IL-2-mediated acquisition of a cytolytic CD8(+) T-cell phenotype. Despite its inhibitory effects, IL-21 did not induce anergy, but instead potently enhanced the capacity of cells to mediate tumor regression upon adoptive transfer. In contrast, IL-2 impaired the subsequent antitumor function of transferred cells. Gene expression studies revealed a distinct IL-21 program that was characterized phenotypically by increased expression of L-selectin and functionally by enhanced antitumor immunity that was not reversed by secondary in vitro stimulation with antigen and IL-2. Thus, the efficacy of CD8(+) T cells for adoptive immunotherapy can be influenced by opposing differentiation programs conferred by IL-2 and IL-21, a finding with important implications for the development of cellular cancer therapies.

Autocrine regulation of IL-21 production in human T lymphocytes

IL-21 has pathologic function in immune-inflammatory diseases. IL-21 mediates its functions through a heterodimeric receptor, composed of a specific subunit, termed IL-21R, and the common gamma-chain. IL-21 is mostly produced by CD4(+) T cells, but molecular mechanisms that regulate IL-21 synthesis are not fully understood. The fact that CD4(+) T cells express high levels of IL-21R and are capable of functionally responding to IL-21 raises the possibility that IL-21 may regulate its own production. We here show that IL-21 enhances IL-21 RNA and protein expression in human peripheral blood CD3(+) T cells in a dose- and time-dependent fashion. Additionally, both IL-7 and IL-15, but not IL-4, induce IL-21, thus suggesting that common gamma-chain signals are not sufficient to promote IL-21 synthesis. Analysis of molecular mechanisms underlying IL-21 induction reveals that IL-21 activates Stat3 and enhances its recruitment to IL-21 gene promoter. Pharmacologic inhibition and knockdown of Stat3 by small interference RNA largely prevent IL-21 induction in IL-21-treated cells. Consistently, IL-21 is inducible in T cells by IL-6, another cytokine that activates Stat3. Finally, we show that IL-21 positively regulates its own expression in human intestinal CD3(+) lamina propria lymphocytes, and blockade of endogenous IL-21 in cultures of CD3(+) lamina propria lymphocytes isolated from patients with Crohn's disease, a chronic inflammatory bowel disease characterized by high IL-21, down-regulates Stat3 activation and IL-21 expression. These data suggest the existence of a positive autocrine loop that could help to amplify and stabilize IL-21-driven, T cell-mediated responses.

Interleukin-21 regulates expression of key Epstein-Barr virus oncoproteins, EBNA2 and LMP1, in infected human B cells

Epstein-Barr virus (EBV) persists for the life of the host by accessing the long-lived memory B cell pool. It has been proposed that EBV uses different combinations of viral proteins, known as latency types, to drive infected B cells to make the transition from resting B cells to memory cells. This process is normally antigen-driven. A major unresolved question is what factors coordinate expression of EBV latency proteins. We have recently described novel type III latency EBV+ B cell lines (OCI-BCLs) that were induced to differentiate into late plasmablasts/early plasma cells in culture with interleukin-21 (IL-21), mimicking normal B cell development. The objective of this study was to determine whether IL-21-mediated signals also regulate the expression of key EBV latent proteins during this window of development. Here we show that IL-21-reduced gene and protein expression of growth-transforming EBV nuclear antigen 2 (EBNA2) in OCI-BCLs. By contrast, the expression of CD40-like, latent membrane protein 1 (LMP1) strongly increased in these cells suggesting an EBNA2-independent mode of regulation. Same results were also observed in Burkitt's lymphoma line Jijoye and B95-8 transformed lymphoblastoid cell lines. The effect of IL-21 on EBNA2 and LMP1 expression was attenuated by a pharmacological JAK inhibitor indicating involvement of JAK/STAT signalling in this process. Our study also shows that IL-21 induced transcription of ebna1 from the viral Q promoter (Qp).

IL-21 synergizes with IL-7 to augment expansion and anti-tumor function of cytotoxic T cells

IL-21, a recently identified member of the common gamma-chain (gammac) receptor cytokine family, has been shown to be an important regulator of both innate and adaptive immune responses. In this study, we investigated whether IL-21 could synergize with another gammac cytokine, IL-7, to induce enhanced proliferation and effector function of tumor antigen-specific CD8(+) T cells. Our results showed that IL-21 could significantly augment the IL-7-induced expansion of cytotoxic T cells, possibly by preventing the cytokine-induced down-regulation of the IL-7Ralpha (CD127) on antigen-stimulated T cells. IL-21 also greatly enhanced the production of T(h)1 and inflammatory cytokines by activated T cells, including IFN-gamma, IL-2, tumor necrosis factor-alpha, granulocyte macrophage colony-stimulating factor, IL-1beta and IL-6. Finally, the addition of IL-21 to IL-7-cultured CTLs resulted in a considerably higher level of cytolytic activity, as measured by specific killing of tumor cells or antigen-pulsed target cells. These results suggest that IL-21 may play a cooperative role with IL-7 in modulating primary CD8(+) T-cell responses and may have important implications for immunotherapy of cancer.

IL-21 mediates apoptosis through up-regulation of the BH3 family member BIM and enhances both direct and antibody-dependent cellular cytotoxicity in primary chronic lymphocytic leukemia cells in vitro

Interleukin-21 (IL-21) is a recently identified gamma-chain receptor cytokine family member that promotes B-cell apoptosis as well as activation of innate immune system. Based on this, we hypothesized that IL-21 might enhance the apoptosis induced by fludarabine and rituximab and also play a role in augmenting immune-mediated clearance of the chronic lymphocytic leukemia (CLL) cells. Our studies demonstrate that the majority of CLL patients have surface IL-21 receptor-alpha, and its expression correlates with apoptosis, tyrosine phosphorylation of STAT1, and up-regulation of the proapoptotic BH3 domain protein BIM. IL-21-induced BIM up-regulation is critical for apoptosis because inhibition of BIM expression using small interfering RNA prevented IL-21-induced apoptosis. IL-21 treatment of CLL cells but not normal T cells with fludarabine or rituximab additively enhanced the direct cytotoxic effect of these therapies. In addition to its proapoptotic effect, IL-21 promoted STAT1 and STAT5 phosphorylation in natural killer cells with concurrent enhanced antibody-dependent cellular cytotoxicity against rituximab-coated CLL cells in vitro. These data provide justification for combination studies of IL-21 with fludarabine and rituximab in CLL and suggest that BIM up-regulation might serve as relevant pharmacodynamic end point to measure biologic effect of this cytokine in vivo.

Essential role of IL-21 in B cell activation, expansion, and plasma cell generation during CD4+ T cell-B cell collaboration

During T cell-B cell collaboration, plasma cell (PC) differentiation and Ig production are known to require T cell-derived soluble factors. However, the exact nature of the cytokines produced by activated T cells that costimulate PC differentiation is not clear. Previously, we reported that costimulation of purified human B cells with IL-21 and anti-CD40 resulted in efficient PC differentiation. In this study, we addressed whether de novo production of IL-21 was involved in direct T cell-induced B cell activation, proliferation, and PC differentiation. We found that activated human peripheral blood CD4(+) T cells expressed mRNA for a number of cytokines, including IL-21, which was confirmed at the protein level. Using a panel of reagents that specifically neutralize cytokine activity, we addressed which cytokines are essential for B cell activation and PC differentiation induced by anti-CD3-activated T cells. Strikingly, neutralization of IL-21 with an IL-21R fusion protein (IL-21R-Fc) significantly inhibited T cell-induced B cell activation, proliferation, PC differentiation, and Ig production. Inhibition of PC differentiation was observed even when the addition of IL-21R-Fc was delayed until after initial B cell activation and expansion had occurred. Importantly, IL-21 was found to be involved in PC differentiation from both naive and memory B cells. Finally, IL-21R-Fc did not inhibit anti-CD3-induced CD4(+) T cell activation, but rather directly blocked T cell-induced B cell activation and PC differentiation. These data are the first to document that B cell activation, expansion, and PC differentiation induced by direct interaction of B cells with activated T cells requires IL-21.

Involvement of mature donor T cells in the NK cell reconstitution after haploidentical hematopoietic stem-cell transplantation

We previously demonstrated that natural killer (NK) cells generated after haploidentical stem-cell transplantation (SCT) are blocked at an immature state characterized by phenotypic features and impaired functioning and that this may affect transplantation outcome. We hypothesize that the absence of mature donor T cells in the graft may affect NK cell differentiation. NK cells from 21 transplant recipients who underwent either partial (pTCD; n=11) or extensive (eTCD; n=10) T-cell depletion were compared with NK cells from their healthy donors. We report that despite the strong graft-versus-host disease (GvHD) reaction, pTCD patients, with T cells present during SCT, had a better clinical outcome than patients with eTCD transplants. In addition, the frequency of CD3- CD56(bright) and NKG2A+ NK cells was much lower in pTCD than in eTCD patients after transplantation, and the level of cytotoxicity against primary haplo-mismatched blasts was significantly more pronounced after pTCD than eTCD transplants. These finding strongly suggest that mature donor T cells in the graft may play a key role in NK cell differentiation in vivo, after haploidentical SCT.

IL-21 receptor expression determines the temporal phases of experimental autoimmune encephalomyelitis

The IL-21 receptor (IL-21R) consists of a unique subunit and a common gamma chain (gamma(c)) that is shared with other cytokines including IL-2, IL-4, IL-7, and IL-15. The interaction between IL-21 and IL-21R results in significant effects on both innate and adaptive immune responses. In this study we examined the influence of IL-21R deficiency (IL-21R(-/-)) on the development of experimental autoimmune encephalomyelitis (EAE), an animal model of human multiple sclerosis (MS). IL-21R(-/-) mice developed EAE earlier and more severe neurological impairment than control mice, yet those mice could effectively recover from neurological deficits. The impact on EAE initiation by IL-21R deficiency was associated with a defect of CD4(+)CD25(+) T regulatory (Treg) cells and a down-regulated expression of Foxp3. The recovery from IL-21R(-/-) EAE was correlated with an expansion of Treg cells as well as an organ-specific redistribution of NK cells. These results suggest that a temporal influence of IL-21 on the activity of immunoregulatory circuits can be important in the modulation of the course of the autoimmune disease.

Interleukin-21 activates human natural killer cells and modulates their surface receptor expression

Interleukin (IL)-21 is a novel cytokine that has been shown to enhance proliferation and activation of CD8+ T cells, enhance natural killer (NK) cell activity and costimulate anti-CD40-driven B-cell proliferation in mice. Several studies have furthermore demonstrated antitumour effects of IL-21 administration in mouse models. In this study we have investigated how IL-21 affects the survival and cytotoxicity of human NK cells and modulates their expression of surface receptors and of the effector molecules granzyme B and perforin. In contrast to murine NK cells, where IL-21 alone cannot sustain survival, IL-21 and IL-2 were equally efficient in sustaining survival of human NK cells. In the absence of other cytokines, IL-21 had little effect on expression of a panel of surface receptors on human NK cells. However, IL-21 synergized with IL-2 to up-regulate several surface receptors, including NKG2A, CD25, CD86 and CD69. The CD25+ CD86+ NK cells were CD56(bright) and were large and granular. Expression of the effector molecules perforin and granzyme A and B was up-regulated by IL-21 at both mRNA and protein levels. Furthermore, IL-21 increased the cytotoxicity of NK cells against K562 target cells. These findings suggest that IL-21 modulates NK cell activity through induction of intracellular effector molecules as well as modulation of surface receptor expression.

IL-21 induces apoptosis of antigen-specific CD8+ T lymphocytes

IL-21, a member of the common gamma-chain family of cytokines, has pleiotropic effects on T, B, and NK cells. We found that IL-21 and the prototype common gamma-chain cytokine IL-2 can stimulate proliferation and cytokine secretion by Ag-specific rhesus monkey CD8+ T cells. However, unique among the members of this family of cytokines, we found that IL-21 drives these cells to apoptosis by down-regulation of Bcl-2. These findings suggest that IL-21 may play an important role in the contraction of CD8+ T cell responses.

Immunotherapy of cancer by IL-12-based cytokine combinations

Cancer is a multi-faceted disease comprising complex interactions between neoplastic and normal cells. Over the past decade, there has been considerable progress in defining the molecular, cellular and environmental contributions to the pathophysiology of tumor development. Despite these advances, the conventional treatment of patients still generally involves surgery, radiotherapy and/or chemotherapy, and the clinical outcome for many of these efforts remains unsatisfactory. Recent studies have highlighted the feasibility of using immunotherapeutic approaches that seek to enhance host immune responses to developing tumors. These strategies include immunomodulatory cytokines, with TNF-alpha, type I or type II IFNs, IL-2, IL-12, IL-15 and IL-18 being among the most potent inducers of anti-tumor activity in a variety of preclinical studies. More recently, some exciting new cytokines have been characterized, such as IL-21, IL-23, IL-27 and their immunomodulatory and antitumor effects in vitro and in vivo suggest that they may have considerable promise for future immunotherapy protocols. The promise of cytokine therapy does indeed derive from the identification of these novel cytokines but even more fundamentally, the field is greatly benefiting from the ever-expanding amount of preclinical data that convincingly demonstrate synergistic and/or novel biologic effects, which may be achieved through the use of several combinations of cytokines with complementary immune-stimulating capabilities. One cytokine in particular, IL-12, holds considerable promise by virtue of the fact that it plays a central role in regulating both innate and adaptive immune responses, can by itself induce potent anticancer effects, and synergizes with several other cytokines for increased immunoregulatory and antitumor activities. This review discusses the antitumor activity of IL-12, with a special emphasis on its ability to synergize with other cytokines for enhancement of immune effector cell populations and regulation of host-tumor cell interactions and the overall tumor microenvironment.

New players in the cytokine orchestra of inflammatory bowel disease

In both Crohn's disease (CD) and ulcerative colitis, the pathologic process is almost certainly driven by an aberrant local immune response directed against normal components of the bacterial microflora. Mucosal immune cells interact with nonimmune cells such as epithelial cells and fibroblasts to promote tissue damage; cytokines are essential mediators of this cross talk. Accumulating evidence now suggests that interleukin-21 (IL-21), the newest member of the common gamma-chain-dependent cytokine family, is a key component of the inflammatory cascade. IL-21 is highly produced by activated CD4+ lymphocytes in the inflamed gut of patients with CD, where it contributes to sustaining the ongoing Th1 inflammation. IL-21 also increases the secretion of extracellular matrix-degrading enzymes by fibroblasts and of MIP-3alpha by epithelial cells. Two other cytokines, IL-27 and IL-32, may also be important in the inflammatory pathways that operate in IBD.

The activation of natural killer cell effector functions by cetuximab-coated, epidermal growth factor receptor positive tumor cells is enhanced by cytokines

PURPOSE

Natural killer (NK) cells express an activating Fc receptor (FcgammaRIIIa) that mediates antibody-dependent cellular cytotoxicity (ADCC) and production of immune modulatory cytokines in response to antibody-coated targets. Cetuximab is a therapeutic monoclonal antibody directed against the HER1 antigen. We hypothesized that the NK cell response to cetuximab-coated tumor cells could be enhanced by the administration of NK cell-stimulatory cytokines.

EXPERIMENTAL DESIGN

Human NK cells stimulated with cetuximab-coated tumor cells and interleukin-2 (IL-2), IL-12, or IL-21 were assessed for ADCC and secretion of IFN-gamma and T cell-recruiting chemokines. IL-21 and cetuximab were given to nude mice bearing HER1-positive xenografts.

RESULTS

Stimulation of human NK cells with cetuximab-coated tumor cells and IL-2, IL-12, or IL-21 resulted in 3-fold to 10-fold higher IFN-gamma production than was observed with either agent alone. NK cell-derived IFN-gamma significantly enhanced monocyte ADCC against cetuximab-coated tumor cells. Costimulated NK cells also secreted elevated levels of chemokines (IL-8, macrophage inflammatory protein-1alpha, and RANTES) that could direct the migration of naive and activated T cells. IL-2, IL-12, and IL-21 enhanced NK cell ADCC against tumor cells treated with cetuximab. The combination of cetuximab, trastuzumab (an anti-HER2 monoclonal antibody), and IL-21 mediated greater NK cell cytokine secretion and ADCC than any agent alone. Furthermore, administration of IL-21 enhanced the effects of cetuximab in a murine tumor model.

CONCLUSIONS

These results show that cetuximab-mediated NK cell activity can be significantly enhanced in the presence of NK cell-stimulatory cytokines. These factors, therefore, may be effective adjuvants to administer, in combination with cetuximab, to patients with HER1-positive malignancies.

The opposite effects of IL-15 and IL-21 on CLL B cells correlate with differential activation of the JAK/STAT and ERK1/2 pathways

The clonal expansion of chronic lymphocytic leukemia (CLL) cells requires the interaction with the microenvironment and is under the control of several cytokines. Here, we investigated the effect of IL-15 and IL-21, which are closely related to IL-2 and share the usage of the common gamma chain and of its JAK3-associated pathway. We found remarkable differences in the signal transduction pathways activated by these cytokines, which determined different responses in CLL cells. IL-15 caused cell proliferation and prevented apoptosis induced by surface IgM cross-linking. These effects were more evident in cells stimulated via surface CD40, which exhibited increased cell expression of IL-15Ralpha chain and, in some of the cases, also of IL-2Rbeta. IL-21 failed to induce CLL cell proliferation and instead promoted apoptosis. Following cell exposure to IL-15, phosphorylation of STAT5 was predominantly observed, whereas, following stimulation with IL-21, there was predominant STAT1 and STAT3 activation. Moreover, IL-15 but not IL-21 caused an increased phosphorylation of Shc and ERK1/2. Pharmacological inhibition of JAK3 or of MEK, which phosphorylates ERK1/2, efficiently blocked IL-15-induced CLL cell proliferation and the antiapoptotic effect of this cytokine. The knowledge of the signaling pathways regulating CLL cell survival and proliferation may provide new molecular targets for therapeutic intervention.

Human natural killer cells in health and disease

Natural killer (NK) cells are an essential component of the innate immune system and play a critical role in tumor immune surveillance. NK cells express their own repertoire of receptors (NKRs) that bind to major histocompatibility class I or class I-like molecules. The balance of signals from stimulation or inhibition of NKRs determines the ability of NK cells to lyse specific targets. In haploidentical stem cell transplantation with purified stem cells, NK cell alloreactivity (killer immunoglobulin-like receptor [KIR] mismatch) has been demonstrated to reduce the risk of relapse in acute myeloid leukemia. There is a need for adequately powered prospective randomized studies to determine the usefulness of NK cells as adoptive immunotherapy, optimal NK cell doses and timing of administration. Further studies are required to determine optimal selection of donors and recipients, both on NKR matching/mismatching, undergoing haploidentical and unrelated hematopoetic stem cell transplantation.

IL-21 mediated Foxp3 suppression leads to enhanced generation of antigen-specific CD8+ cytotoxic T lymphocytes

Efforts to reproducibly isolate tumor antigen-specific T cells from patients would be facilitated by removing immunoregulatory barriers. Using a human model for eliciting T-cell responses to tumor-associated antigens, we develop a novel strategy that eliminates nearly all Foxp3-expressing cells through the combination of CD25 depletion and IL-21 treatment resulting in a more than 150-fold decrease in Foxp3(+) cells to virtually undetectable levels and a more than 200-fold increase in antigen-specific cytotoxic T lymphocytes (CTLs). The extent of Foxp3 elimination and degree of expansion of antigen-specific CTLs shown in this study have not previously been achievable and are unique to IL-21. We demonstrate for the first time a possible mechanism for IL-21-mediated expansion of antigen-specific CTLs that involves suppression of Foxp3-expressing cells and reversal of inhibition to tumor-associated antigen-specific CTL generation in vitro. Taken together, the combination of CD25 depletion and IL-21 exposure, by releasing regulatory constraints, leads to markedly enhanced CTL induction and represents a robust strategy for the ex vivo generation of antigen-specific T cells for adoptive cellular therapy.

Role of IL-21 in immune-regulation and tumor immunotherapy

IL-21, the most recently discovered member of the IL-2 cytokine family, is an attractive subject for research due to its involvement in experimental models of autoimmunity, its ability to down-regulate IgE production, and its anti-tumor properties. Its interest for cancer immunotherapy stems from its physiological immune-enhancing functions. These include regulation of T, B and NK cell proliferation, survival, differentiation, and effector functions. IL-21's functional activities partially overlap those of IL-2. Both cytokines display similar structural features and use the common gamma-chain receptor and its downstream signaling pathways. Besides its activities on normal lymphoid cells, IL-21 is an in vitro growth factor for myeloma and acute-T cell leukemia cells, whereas it induces the apoptosis of B-CLL (chronic lymphocytic leukemia) cells. These findings indicate that the IL-21/IL-21R system exerts opposite functions in different lymphoid neoplasias, and suggest its employment in B-CLL therapy. Since IL-2, but not IL-21, is specifically required for the development of regulatory T (Treg) cell immune-suppressive functions, IL-21 may be a new tool for cancer immunotherapy. It is, in fact, a powerful anti-tumor agent in a variety of murine experimental tumor models through its activation of specific or innate immune responses against neoplastic cells. The preliminary data from phase-I clinical studies suggest that the use of IL-21 is feasible and may result in immune-enhancing effects.

Induction of protective immune responses against NXS2 neuroblastoma challenge in mice by immunotherapy with GD2 mimotope vaccine and IL-15 and IL-21 gene delivery

The GD2 ganglioside expressed on neuroectodermal tumor cells is weakly immunogenic in tumor-bearing patients and induces predominantly IgM antibody responses in the immunized host. Using a syngeneic mouse challenge model with GD2-expressing NXS2 neuroblastoma, we investigated novel strategies for augmenting the effector function of GD2-specific antibody responses induced by a mimotope vaccine. We demonstrated that immunization of A/J mice with DNA vaccine expressing the 47-LDA mimotope of GD2 in combination with IL-15 and IL-21 genes enhanced the induction of GD2 cross-reactive IgG2 antibody responses that exhibited cytolytic activity against NXS2 cells. The combined immunization regimen delivered 1 day after tumor challenge inhibited subcutaneous (s.c.) growth of NXS2 neuroblastoma in A/J mice. The vaccine efficacy was reduced after depletion of NK cells as well as CD4(+) and CD8(+) T lymphocytes suggesting involvement of innate and adaptive immune responses in mediating the antitumor activity in vivo. CD8(+) T cells isolated from the immunized and cured mice were cytotoxic against syngeneic neuroblastoma cells but not against allogeneic EL4 lymphoma, and exhibited antitumor activity after adoptive transfer in NXS2-challenged mice. We also demonstrated that coimmunization of NXS2-challenged mice with the IL-15 and IL-21 gene combination resulted in enhanced CD8(+) T cell function that was partially independent of CD4(+) T cell help in inhibiting tumor growth. This study is the first demonstration that the mimotope vaccine of a weakly immunogenic carbohydrate antigen in combination with plasmid-derived IL-15 and IL-21 cytokines induces both innate and adaptive arms of the immune system leading to the generation of effective protection against neuroblastoma challenge.

Interleukin 21 therapy increases the density of tumor infiltrating CD8+ T cells and inhibits the growth of syngeneic tumors

Interleukin (IL)-21 is a recently discovered cytokine in early clinical development, which has shown anti-tumor activity in various animal models. In the present study, we examine the anti-tumor activity of IL-21 protein therapy in two syngeneic tumor models and its effect on the density of tumor infiltrating T cells. We treated mice bearing established subcutaneous B16 melanomas or RenCa renal cell carcinomas with intraperitoneal (i.p.) or subcutaneous (s.c.) IL-21 protein therapy and subsequently scored the densities of tumor infiltrating CD4(+) and CD8(+) T cells by immunohistochemistry. Whereas both routes of IL-21 administration significantly inhibited growth of small, established RenCa and B16 tumors, only s.c. therapy significantly inhibited the growth of large, established tumors. We found a greater bioavailability and significant drainage of IL-21 to regional lymph nodes following s.c. administration, which could account for the apparent increase in anti-tumor activity. Specific depletion of CD8(+) T cells with monoclonal antibodies completely abrogated the anti-tumor activity, whereas NK1.1(+) cell depletion did not affect tumor growth. In accordance, both routes of IL-21 administration significantly increased the density of tumor infiltrating CD8(+) T cells in both B16 and RenCa tumors; and in the RenCa model s.c. administration of IL-21 led to a significantly higher density of tumor infiltrating CD8(+) T cells compared to i.p. administration. The densities of CD4(+) T cells were unchanged following IL-21 treatments. Taken together, these data demonstrate that IL-21 protein has anti-tumor activity in established syngeneic tumors, and we show that IL-21 therapy markedly increases the density of tumor infiltrating CD8(+) T cells.

IL-21 enhances antitumor responses without stimulating proliferation of malignant T cells of patients with Sézary syndrome

IL-21, a common gamma-chain cytokine secreted by activated CD4+ T cells, influences both humoral and cell-mediated immune responses through the regulation of T, B, dendritic, and natural killer (NK) cells. Sézary syndrome is an advanced form of cutaneous T-cell lymphoma, a clonally derived malignancy of CD4+ T cells that is characterized by profound defects in host cellular immune function. As a modulator of both innate and adaptive immune responses, IL-21 could play an important role in augmenting cell-mediated immunity in these patients. Normal donor and Sézary syndrome patient peripheral blood mononuclear cells were cultured with IL-21 and tested for CD8+ T- and NK-cell activation, NK-cell cytotoxicity, and tumor cell proliferation and apoptosis. IL-21 resulted in a modest increase in CD8+ T- and NK-cell activation, associated with a marked increase in cytolytic activity against both K562 and malignant CD4+ T-cell targets. Although IL-21 failed to demonstrate pro-apoptotic effects on the malignant CD4+ T cells, it is noteworthy that it had no demonstrable proliferative effects on these cells. Thus, IL-21 may play an important role in enhancing the host immune response of Sézary syndrome patients through the increased cytolytic activity of T and NK cells.

Thymic stromal lymphopoietin augments the cytolytic activity of the human natural killer cell line NK-92

Culturing the human natural killer cell line NK-92 for 24 h in the presence of thymic stromal lymphopoietin (TSLP) potentiated its cytotoxic capacity against the erythroleukemia cell line K562. Longer incubation times did not augment the NK activity any further. No synergistic effects with respect to either proliferation or cytotoxicity were observed when TSLP was mixed with suboptimal concentrations of IL-2. FACS analysis of the NK-92 cells indicated expression of TSLPR but not the other component of the TSLP receptor complex, namely IL-7Ralpha. Some of the surface molecules known to be involved in NK cell-mediated cytotoxicity were also monitored. None of the receptors analyzed altered their expression to any major extent upon culture in TSLP or IL-2. However, a limited number of NK-92 cells were observed that had a rather low CD94/NKG2A expression, which increased upon stimulation with TSLP or IL-2.

Human CD4+ central and effector memory T cells produce IL-21: effect on cytokine-driven proliferation of CD4+ T cell subsets

IL-21 regulates certain functions of T cells, B cells, NK cells and dendritic cells. Although activated CD4(+) T cells produce IL-21, data identifying the specific CD4(+) T cell subsets that produce IL-21 are conflicting. In a previous study, mouse IL-21 message was detected in T(H)2, whereas human IL-21 (hIL-21) message was found in both T(H)1 and follicular helper T cells. To identify the IL-21-secreting cell populations in human, we established a hybridoma cell line producing an anti-hIL-21 mAb. Intracellular hIL-21-staining experiments showed that hIL-21 was mainly expressed in activated CD4(+) central memory T cells and in activated CD4(+) effector memory T cells, but not in activated CD4(+) naive T cells. Moreover, IL-21 was produced upon activation by some IFN-gamma-producing T(H)1-polarized cells and some IL-17-producing T(H)17-polarized cells, but not by IL-4-producing T(H)2-polarized cells. These results suggest that specific CD4(+) T cell populations produce IL-21. In the functional analysis, we found that IL-21 significantly enhanced the cytokine-driven proliferation of CD4(+) helper T cells synergistically with IL-7 and IL-15 without T cell activation stimuli. Taken together, IL-21 produced from CD4(+) memory T cells may have a supportive role in the maintenance of CD4(+) T cell subsets.

The Existence of Multiple Conformers of Interleukin-21 Directs Engineering of a Superpotent Analogue

The high resolution three-dimensional structure of human interleukin (hIL)-21 has been resolved by heteronuclear NMR spectroscopy. Overall, the hIL-21 structure is dominated by a well defined central four-helical bundle, arranged in an up-up-down-down topology, as observed for other cytokines. A segment of the hIL-21 molecule that includes the third helical segment, helix C, is observed to exist in two distinct and interchangeable states. In one conformer, the helix C segment is presented in a regular, alpha-helical conformation, whereas in the other conformer, this segment is largely disordered. A structure-based sequence alignment of hIL-21 with receptor complexes of the related cytokines, interleukin-2 and -4, implied that this particular segment is involved in receptor binding. An hIL-21 analog was designed to stabilize the region around helix C through the introduction of a segment grafted from hIL-4. This novel hIL-21 analog was demonstrated to exhibit a 10-fold increase in potency in a cellular assay.

Cloning and characterization of an isoform of interleukin-21

Interleukin-21 (IL-21) has pleiotropic functions on the cells, which play roles in both innate and acquired immunity, such as T cells, B cells, natural killer (NK) cells and dendritic cells. In this study we identified a novel isoform of IL-21, IL-21iso in human and mouse. IL-21iso might be an alternative splicing variant form and the C-terminal region of predicted IL-21iso amino acid sequences were different from original IL-21 in both human and mouse. In spite of the differences in C-terminal amino acid sequences, both human IL-21 and IL-21iso showed comparable proliferative effect on anti-CD40 Ab-activated primary B cells, anti-CD3 Ab-activated primary T cells and human NK cell line, NK0, and upregulated IFN-gamma production from NK0. Furthermore IL-21 and IL-21iso similarly activated STAT1 and STAT3. IL-21iso mRNA was expressed in activated T cells as well as IL-21 mRNA. However, cycloheximide treatment partially blocked the upregulation of IL-21iso mRNA in activated T cells while little affected the IL-21 mRNA expression suggesting that de novo protein synthesis is required for the full expression of IL-21iso transcript. We also show that the secretion efficiency of hIL-21iso is much lower than that of hIL-21. These results may suggest there are some different regulatory mechanisms to produce IL-21 or IL-21iso in transcriptional and secretory steps.

Interleukin-21 differentially affects human natural killer cell subsets

Interleukin-21 (IL-21) is a cytokine with pleiotropic effects on various cell types including dendritic cells, B cells, T cells and natural killer (NK) cells. To evaluate if IL-21 affects human NK cell subpopulations in a similar fashion, functional studies were performed on CD56(dim) and CD56(bright) NK cells, both bearing IL-21 receptors at identical densities. Stimulation with IL-21 strongly induced proliferation of CD56(bright) NK cells and cytotoxicity against K562 target cells was preferentially augmented in CD56(dim) NK cells. In contrast, stimulation with IL-2 and IL-21 alone or in combination failed to induce interferon-gamma and tumour necrosis factor-alpha production in the two NK cell subsets. Intracellular analysis of signal transducer and activator of transcription (STAT) proteins revealed that IL-21 by itself induces phosphorylation of STAT1 and STAT3 in CD56(dim) NK cells, and to an even higher degree in CD56(bright) NK cells. In this CD56(bright) NK cell population alone, IL-2 weakly phosphorylated STAT1 and STAT3, which was further increased when cells were treated with the combination of both cytokines. In contrast, STAT5 was strongly phosphorylated only in CD56(bright) NK cells by low-dose IL-2, while IL-21 did not affect STAT5 at all. In summary, we present data indicating that the NK-cell-directed cytokines IL-2 and IL-21 not only affect functions in NK cell subpopulations differently but can also act additively.

Cellular immune therapy for chronic lymphocytic leukemia

Although chemotherapy can induce complete responses in patients with chronic lymphocytic leukemia (CLL), it is not considered curative. Treated patients generally develop recurrent disease requiring additional therapy, which can cause worsening immune dysfunction, myelosuppression, and selection for chemotherapy-resistant leukemia-cell subclones. Cellular immune therapy promises to mitigate these complications and potentially provide for curative treatment. Most experience with this is in the use of allogeneic hematopoietic stem-cell transplantation (allo-HSCT), in which graft-versus-leukemia (GVL) effects can be observed and shown responsible for long-term disease-free survival. However, use of allo-HSCT for CLL is limited because of the lack of suitable donors and the treatment-related morbidity/mortality for elderly patients, who constitute the majority at risk for developing this disease. The GVL effect, however, suggests there are specific CLL-associated antigens that could be targeted in autologous cellular immune therapy. Effective strategies for this will have to overcome the disease-related acquired immune deficiency and the capacity of the leukemia-cell to induce T-cell tolerance, thereby compromising the activity of even conventional vaccines in patients with this disease. We will discuss the different strategies being developed to overcome these limitations that might provide for effective cellular immune therapy of CLL.

CD56 marks an effector T cell subset in the human intestine

T cells are key mediators of intestinal immunity, and specific T cell subsets can have differing immunoregulatory roles in animal models of mucosal inflammation. In this study, we describe human CD56+ T cells as a morphologically distinct population expressing a mature, nonproliferative phenotype that is frequent in the gut. Enhanced potential for IFN-gamma and TNF synthesis suggested a proinflammatory function, and we directly demonstrate effector function mediated by direct T-T interaction with responder cells in vitro. CD56+ T cells from peripheral blood responded to the gut-related CD2 signal, and were necessary for effective CD2-mediated proliferation of peripheral blood CD56- T cells. Our findings associate CD56+ T cells with the intestinal immune compartment and suggest a putative effector function in human mucosal immunity.