Overexpression of IL-21 promotes massive CD8+ memory T cell accumulation

Interleukin-21 is critically required in autoimmune and allogeneic responses to islet tissue in murine models

OBJECTIVE

Type 1 diabetes is an incurable chronic autoimmune disease. Although transplantation of pancreatic islets may serve as a surrogate source of insulin, recipients are subjected to a life of immunosuppression. Interleukin (IL)-21 is necessary for type 1 diabetes in NOD mice. We examined the efficacy of an IL-21-targeted therapy on prevention of diabetes in NOD mice, in combination with syngeneic islet transplantation. In addition, we assessed the role of IL-21 responsiveness in islet allograft rejection in mouse animal models.

RESEARCH DESIGN AND METHODS

NOD mice were treated with IL-21R/Fc, an IL-21-neutralizing chimeric protein. This procedure was combined with syngeneic islet transplantation to treat diabetic NOD mice. Survival of allogeneic islet grafts in IL-21R-deficient mice was also assessed.

RESULTS

Evidence is provided that IL-21 is continually required by the autoimmune infiltrate, such that insulitis was reduced and reversed and diabetes inhibited by neutralization of IL-21 at a late preclinical stage. Recovery from autoimmune diabetes was achieved by combining neutralization of IL-21 with islet transplantation. Furthermore, IL-21-responsiveness by CD8+ T-cells was sufficient to mediate islet allograft rejection.

CONCLUSIONS

Neutralization of IL-21 in NOD mice can inhibit diabetes, and when paired with islet transplantation, this therapeutic approach restored normoglycemia. The influence of IL-21 on a graft-mounted immune response was robust, since the absence of IL-21 signaling prevented islet allograft rejection. These findings suggest that therapeutic manipulation of IL-21 may serve as a suitable treatment for patients with type 1 diabetes.

Interleukin-21 and cellular activation concurrently induce potent cytotoxic function and promote antiviral activity in human CD8 T cells

Infection with human immunodeficiency virus (HIV)-1 induces a progressive deterioration of the immune system that ultimately leads to acquired immune deficiency syndrome (AIDS). Murine models indicate that the common γ-chain (γ(c))-sharing cytokine interleukin (IL)-21 and its receptor (IL-21R) play a crucial role in maintaining polyfunctional T cell responses during chronic viral infections. Therefore, we analyzed the ability of this cytokine to modulate the properties of human CD8 T cells in comparison with other γ(c)-sharing cytokines (IL-2, IL-7, and IL-15). CD8 T cells from healthy volunteers were stimulated in vitro via T cell receptor signals to mimic the heightened status of immune activation of HIV-infected patients. The administration of IL-21 upregulated cytotoxic effector function and the expression of the costimulatory molecule CD28. Notably, this outcome was not accompanied by increased cellular proliferation or activation. Moreover, IL-21 promoted antiviral activity while not inducing HIV-1 replication in vitro. Thus, IL-21 may be a favorable molecule for immunotherapy and a suitable vaccine adjuvant in HIV-infected individuals.

Interleukin-21: a multifunctional regulator of immunity to infections

Interleukin-21 (IL-21) is a cytokine that has broad effects on both innate and adaptive immune responses. The roles of IL-21 in determining immunity to infections are currently being defined, and notably, it has been shown that IL-21 is most critical for sustaining T cell responses during chronic viral infections. This article discusses our current understanding of the immunobiology of IL-21, as well as its known and potential roles in influencing immunity to infections.

Critical role for IL-21 in both primary and memory anti-viral CD8+ T-cell responses

While it is well established that CD8(+) T cells generated in the absence of CD4(+) T cells mediate defective recall responses, the mechanism by which CD4(+) T cells confer help in the generation of CD8(+) T-cell responses remains poorly understood. To determine whether CD4(+) T-cell-derived IL-21 is an important regulator of CD8(+) T-cell responses in help-dependent and -independent viral infections, we examined these responses in the IL-21Rα(-/-) mouse model. We show that IL-21 has a role in primary CD8(+) T-cell responses and in recall CD8(+) T-cell responses in help-dependent viral infections. This effect is due to a direct action of IL-21 in enhancing the proliferation of virus-specific CD8(+) T cells and reducing their TRAIL expression. These findings indicate that IL-21 is an important mediator of CD4(+) T-cell help to CD8(+) T cells.

Generating diversity: transcriptional regulation of effector and memory CD8 T-cell differentiation

SUMMARY

In response to acute infections or vaccines, naive antigen-specific CD8(+) T cells proliferate and differentiate into effector cytotoxic lymphocytes that acquire the ability to kill infected cells. While the majority of differentiated effector cells die after pathogen clearance, a small number evade terminal differentiation, downregulate active effector functions, and survive as long-lived, self-renewing memory T cells. Our understanding of how effector CD8(+) T cells adopt these different cell fates has grown greatly in recent years. In this review, we discuss the transcriptional regulators that are known to support general effector differentiation, terminal effector differentiation, and memory cell formation. We propose that the diversity of activated CD8(+) T-cell differentiation states is achieved via gradients of activity or expression of transcriptional regulators that are regulated by the level of inflammation and antigenic signaling the T cells experience during infection.

The IL-2 defect in systemic lupus erythematosus disease has an expansive effect on host immunity

IL-2 production is decreased in systemic lupus erythematosus (SLE) patients and affects T cell function and other aspects of host immunity. Transcription factors regulating IL-2 production behave aberrantly in SLE T cells. In addition to IL-2 dysregulation, other IL-2 family members (IL-15 and IL-21) are abnormally expressed in SLE. Decreased IL-2 production in SLE patients leads to many immune defects such as decreased T_reg production, decreased activation-induced cell death (AICD), and decreased cytotoxicity. IL-2 deficiency results in systemic dysregulation of host immune responses in patients suffering from SLE disease.

Absence of both IL-7 and IL-15 severely impairs the development of CD8 T cell response against Toxoplasma gondii

CD8⁺ T cells play an essential role in the protection against both acute as well as chronic Toxoplasma gondii infection. Although the role of IL-15 has been reported to be important for the development of long-term CD8⁺ T cell immunity against the pathogen, the simultaneous roles played by both IL-15 and related γ-chain family cytokine IL-7 in the generation of this response during acute phase of infection has not been described. We demonstrate that while lack of IL-7 or IL-15 alone has minimal impact on splenic CD8⁺ T cell maturation or effector function development during acute Toxoplasmosis, absence of both IL-7 and IL-15 only in the context of infection severely down-regulates the development of a potent CD8⁺ T cell response. This impairment is characterized by reduction in CD44 expression, IFN-γ production, proliferation and cytotoxicity. However, attenuated maturation and decreased effector functions in these mice are essentially downstream consequences of reduced number of antigen-specific CD8⁺ T cells. Interestingly, the absence of both cytokines did not impair initial CD8⁺ T cell generation but affected their survival and differentiation into memory phenotype IL-7Rα^hi cells. Significantly lack of both cytokines severely affected expression of Bcl-2, an anti-apoptotic protein, but minimally affected proliferation. The overarching role played by these cytokines in eliciting a potent CD8⁺ T cell immunity against T. gondii infection is further evidenced by poor survival and high parasite burden in anti IL-7 treated IL-15^−/− mice. These studies demonstrate that the two cytokines, IL-7 and IL-15, are exclusively important for the development of protective CD8⁺ T cell immune response against T. gondii. To the best of our knowledge this synergism between IL-7 and IL-15 in generating an optimal CD8⁺ T cell immunity against intracellular parasite or any other infectious disease model has not been previously reported.

Shaping successful and unsuccessful CD8 T cell responses following infection

CD8 T cells play a vital role in the immunological protection against intracellular pathogens. Ideally, robust effector responses are induced, which eradicate the pathogen, and durable memory CD8 T cells are also established, which help confer protection against subsequent reinfection. The quality and magnitude of these responses is dictated by multiple factors, including their initial interactions with professional antigen-presenting cells, as well as the cytokine milieu and availability of CD4 T cell help. These factors set the transcriptional landscape of the responding T cells, which in turn influences their phenotypic and functional attributes as well as ultimate fate. Under certain conditions, such as during chronic infections, the development of these usually successful responses becomes subverted. Here we discuss advances in our understanding of the cellular and molecular determinants of T cell quality, and the formation of effector, memory, and exhausted CD8 T cells, during acute and chronic infections.

IL-7 and IL-21 are superior to IL-2 and IL-15 in promoting human T cell-mediated rejection of systemic lymphoma in immunodeficient mice

The gamma(c)-cytokines are critical regulators of immunity and possess both overlapping and distinctive functions. However, comparative studies of their pleiotropic effects on human T cell-mediated tumor rejection are lacking. In a xenogeneic adoptive transfer model, we have compared the therapeutic potency of CD19-specific human primary T cells that constitutively express interleukin-2 (IL-2), IL-7, IL-15, or IL-21. We demonstrate that each cytokine enhanced the eradication of systemic CD19(+) B-cell malignancies in nonobese diabetic/severe combined immunodeficient (NOD/SCID)/gamma(c)(null) mice with markedly different efficacies and through singularly distinct mechanisms. IL-7- and IL-21-transduced T cells were most efficacious in vivo, although their effector functions were not as enhanced as IL-2- and IL-15-transduced T cells. IL-7 best sustained in vitro T-cell accumulation in response to repeated antigenic stimulation, but did not promote long-term T-cell persistence in vivo. Both IL-15 and IL-21 overexpression supported long-term T-cell persistence in treated mice, however, the memory T cells found 100 days after adoptive transfer were phenotypically dissimilar, resembling central memory and effector memory T cells, respectively. These results support the use of gamma(c)-cytokines in cancer immunotherapy, and establish that there exists more than 1 human T-cell memory phenotype associated with long-term tumor immunity.

Editorial: memory CD8 T cells now join "Club 21"

In this volume, Nguyen and Weng [( 1) ] show an intriguing study in which they observed an antagonizing effect by IL-21 on the IL-7- or IL-15-driven expansion of CD28(-) nonfunctional memory CD8 T cells and thereby, proposed a new role for IL-21 to keep the memory pool fresh and competent. I will summarize previous studies and discuss the recent advancement in this context.

Endogenous IL-21 restricts CD8+ T cell expansion and is not required for tumor immunity

IL-21 has antitumor activity through actions on NK cells and CD8(+) T cells, and is currently in clinical development for the treatment of cancer. However, no studies have addressed the role of endogenous IL-21 in tumor immunity. In this study, we have studied both primary and secondary immune responses in IL-21(-/-) and IL-21R(-/-) mice against several experimental tumors. We found intact immune surveillance toward methylcholanthrene-induced sarcomas in IL-21(-/-) and IL-21R(-/-) mice compared with wild-type mice and B16 melanomas showed equal growth kinetics and development of lung metastases. IL-21R(-/-) mice showed competent NK cell-mediated rejection of NKG2D ligand (Rae1beta) expressing H-2b(-) RMAS lymphomas and sustained transition to CD8(+) T cell-dependent memory against H-2b(+) RMA lymphomas. alpha-Galactosylceramide stimulation showed equal expansion and activation of NKT and NK cells and mounted a powerful antitumor response in the absence of IL-21 signaling, despite reduced expression of granzyme B in NKT, NK, and CD8(+) T cells. Surprisingly, host IL-21 significantly restricted the expansion of Ag-specific CD8(+) T cells and inhibited primary CD8(+) T cell immunity against OVA-expressing EG7 lymphomas, as well as the secondary expansion of memory CD8(+) T cells. However, host IL-21 did not alter the growth of less immunogenic MC38 colon carcinomas with dim OVA expression. Overall, our results show that endogenous IL-21/IL-21R is not required for NK, NKT, and CD8(+) T cell-mediated tumor immunity, but restricts Ag-specific CD8(+) T cell expansion and rejection of immunogenic tumors, indicating novel immunosuppressive actions of this cytokine.

Regulation of memory T cells by γc cytokines

T cells rely on a duality of TCR and gammac cytokine signals for development, activation and peripheral T cell homeostasis. Previous data had suggested that the requirements for CD4 and CD8 memory T cell regulation were qualitatively distinct, but emerging data has shown that the requirements for true antigen specific memory T cells are very similar between these two cell types. This review will focus on contributions made by members of the gammac cytokine family (IL-2, IL-4, IL-7, IL-15 and IL-21) to homeostasis of naïve, memory phenotype and antigen experienced memory T cells.

New insights into the regulation of T cells by gamma(c) family cytokines

Common cytokine receptor gamma-chain (gamma(c)) family cytokines have crucial roles in the development, proliferation, survival and differentiation of multiple cell lineages of both the innate and adaptive immune systems. In this Review, we focus on our current understanding of the distinct and overlapping effects of interleukin-2 (IL-2), IL-7, IL-9, IL-15 and IL-21, as well as the IL-7-related cytokine thymic stromal lymphopoietin (TSLP), on the survival and proliferation of conventional alphabeta T cells, gammadelta T cells and regulatory T cells. This knowledge potentially allows for the therapeutic manipulation of immune responses for the treatment of cancer, autoimmunity, allergic diseases and immunodeficiency, as well as for vaccine development.

Reducing the activity and secretion of microbial antioxidants enhances the immunogenicity of BCG

BACKGROUND

In early clinical studies, the live tuberculosis vaccine Mycobacterium bovis BCG exhibited 80% protective efficacy against pulmonary tuberculosis (TB). Although BCG still exhibits reliable protection against TB meningitis and miliary TB in early childhood it has become less reliable in protecting against pulmonary TB. During decades of in vitro cultivation BCG not only lost some genes due to deletions of regions of the chromosome but also underwent gene duplication and other mutations resulting in increased antioxidant production.

METHODOLOGY/PRINCIPAL FINDINGS

To determine whether microbial antioxidants influence vaccine immunogenicity, we eliminated duplicated alleles encoding the oxidative stress sigma factor SigH in BCG Tice and reduced the activity and secretion of iron co-factored superoxide dismutase. We then used assays of gene expression and flow cytometry with intracellular cytokine staining to compare BCG-specific immune responses in mice after vaccination with BCG Tice or the modified BCG vaccine. Compared to BCG, the modified vaccine induced greater IL-12p40, RANTES, and IL-21 mRNA in the spleens of mice at three days post-immunization, more cytokine-producing CD8+ lymphocytes at the peak of the primary immune response, and more IL-2-producing CD4+ lymphocytes during the memory phase. The modified vaccine also induced stronger secondary CD4+ lymphocyte responses and greater clearance of challenge bacilli.

CONCLUSIONS/SIGNIFICANCE

We conclude that antioxidants produced by BCG suppress host immune responses. These findings challenge the hypothesis that the failure of extensively cultivated BCG vaccines to prevent pulmonary tuberculosis is due to over-attenuation and suggest instead a new model in which BCG evolved to produce more immunity-suppressing antioxidants. By targeting these antioxidants it may be possible to restore BCG's ability to protect against pulmonary TB.

Malaria antigen-mediated enhancement of interleukin-21 responses of peripheral blood mononuclear cells in African adults

We recently showed that IL-21 is associated with high level of anti-EBA-175 IgG1 and IgG3. Here we have investigated the ability of two malarial antigens, Glutamate-rich protein and merozoite surface protein 3 to induce IL-21 production from PBMCs from malaria-exposed and non-exposed donors. We found that malaria-exposed donors produced significantly more IL-21 compared to non-exposed donors. These data suggest that IL-21 could be involved in the acquisition of immunity to malaria.

Interleukin-10/interleukin-5 responses at birth predict risk for respiratory infections in children with atopic family history

RATIONALE

Respiratory infections in early life are associated with risk for wheezing bronchiolitis, especially in children at high risk of atopy. The underlying mechanisms are unknown, but are suspected to involve imbalance(s) in host defense responses against pathogens stemming from functional immaturity of the immune system in this age group.

OBJECTIVES

To assess the contribution of eosinophil-trophic IL-5, and the potent antiinflammatory cytokine IL-10, to risk for infection in early life.

MEASUREMENTS AND MAIN RESULTS

We prospectively monitored a cohort of 198 high-risk children to age 5 years, recording every acute respiratory infection episode and classifying them by severity. We measured cord blood T-cell capacity to produce IL-10 and IL-5, and related these functions to subsequent infection history. IL-10 and IL-5 were associated, respectively, with resistance versus susceptibility to infections. The greatest contrasting effects of these two cytokines were seen when they were considered in combination by generating IL-10/IL-5 response ratios for each subject. The low IL-10/high IL-5 T-cell response phenotype was strongly associated with susceptibility to all grades of acute respiratory infection, relative to the more resistant high IL-10/low IL-5 phenotype.

CONCLUSIONS

Excessive production of IL-5 by T cells at birth is associated with heightened risk for subsequent severe respiratory infections, and this risk is attenuated by concomitant IL-10 production. The underlying mechanisms may involve IL-10-mediated feedback inhibition of IL-5-dependent eosinophil-induced inflammation, which is a common feature of host antiviral responses in early life.

T cell homeostasis

The pool of mature T cells comprises a heterogeneous mixture of naive and memory CD4(+) and CD8(+) cells. These cells are long lived at a population level but differ markedly in their relative rates of turnover and survival. Here, we review how contact with exogenous stimuli, notably self MHC ligands and various gamma(c) cytokines, plays a decisive role in controlling normal T cell homeostasis.