Transient enhanced IL-2R signaling early during priming rapidly amplifies development of functional CD8+ T effector-memory cells

BCL6 promotes a stem-like CD8+ T cell program in cancer via antagonizing BLIMP1

Overcoming CD8+ T cell exhaustion is critical in cancer immunotherapy. Recently, an intratumor stem/progenitor-like CD8+ T cell (Tprog cell) population that mediates the persistence of antitumor responses has been defined, which can further develop into a terminally differentiated CD8+ T cell (Tterm cell) subpopulation with potent cytotoxic functions. Tprog cells are the main responders to immune checkpoint blockade therapies, yet how extrinsic signals via transcription factors control Tprog cell generation and persistence in tumors is unclear. Here, we found that BCL6 inhibits tumor-specific Tterm cell generation from Tprog cell downstream of TCF1. We show that Bcl6 deficiency reduced the persistence of Tprog cells, without affecting their generation, thus abrogating long-term tumor control. High-level BCL6 expression was observed in tumor-specific T cells in draining lymph nodes (LNs) and was associated with T cell exhaustion. This was observed in TOX+TCF1+ Tprog cells in both LNs and tumors. BCL6 expression in CD8+ T cells was up-regulated by TGF-β-SMAD2 signaling but down-regulated by the IL-2-STAT5 pathway. Mechanistically, BCL6 transcriptionally repressed the expression of Tterm cell-associated genes and induced those of Tprog cell-related genes, in a manner antagonistic to BLIMP1. Prdm1 deficiency also promoted the Tprog cell program and greatly improved the efficacy of anti-PD-1 therapy. Thus, we identified the TGF-β-BCL6 and IL-2-BLIMP1 antagonistic pathways in regulation of antitumor CD8+ T cells, which may benefit the development of long-lasting and effective cancer immunotherapy.

IL2RA Methylation and Gene Expression in Relation to the Multiple Sclerosis-Associated Gene Variant rs2104286 and Soluble IL-2Rα in CD8+ T Cells

CD8⁺ T cells are involved in the pathogenesis of multiple sclerosis (MS). The interleukin-2 receptor α (IL-2Rα) is important for CD8⁺ T cell function, and single nucleotide polymorphisms (SNPs) in the IL2RA gene encoding IL-2Rα increase the risk of MS. Therefore, in isolated CD8⁺ T cells we investigated IL2RA gene methylation and gene expression in relation to the MS-associated IL2RA SNP rs2104286 and soluble IL-2Rα (sIL-2Rα). We have identified allele specific methylation of the CpG-site located in intron 1 that is perturbed by the rs2104286 SNP in CD8⁺ T cells from genotype-selected healthy subjects (HS). However, methylation of selected CpG-sites in the promotor or 5'UTR region of the IL2RA gene was neither associated with the rs2104286 SNP nor significantly correlated with IL2RA gene expression in HS. In CD8⁺ T cells from HS, we explored expression of immune relevant genes but observed only few associations with the rs2104286 SNP. However, we found that sIL-2Rα correlated negatively with expression of 55 immune relevant genes, including the IL-7 receptor gene, with Spearman's rho between -0.49 and -0.32. Additionally, in HS by use of flow cytometry we observed that the IL-7 receptor on naïve CD8⁺ T cells correlated negatively with sIL-2Rα and was downregulated in carriers of the rs2104286 MS-associated risk genotype. Collectively, our study of resting CD8⁺ T cells indicates that the rs2104286 SNP has a minor effect and sIL-2Rα may negatively regulate the CD8⁺ T cell response.

The functions of IL-23 and IL-2 on driving autoimmune effector T-helper 17 cells into the memory pool in dry eye disease

Long-lived memory T-helper 17 (Th17) cells actively mediate the chronic inflammation in autoimmune disorders, including dry eye disease (DED). The mechanisms responsible for the maintenance and reactivation of these cells in autoimmunity have been subject of investigation. However, the process through which memory Th17 are generated from their effector precursors remains to be elucidated. Herein, using our murine model of DED, we detect a linear transition from effector-to-memory Th17 cells during the abatement phase of acute inflammation, which is accompanied by persistently high levels of IL-23 and diminished levels of IL-2. In addition, in vitro culture of effector Th17 cells derived from the DED animals with IL-23, but not IL-2, leads to significant generation of memory Th17 cells, along with upregulated expression levels of IL-7R and IL-15R by these cells. Furthermore, supplementation of IL-2 abolishes and blockade of IL-2 enhances IL-23-induced generation of memory Th17 cells in vitro. Finally, in vivo blockade of IL-23 signaling during the contraction phase of primary response inhibits the generation of memory Th17 cells from their effector precursors. Together, our data demonstrate a new dichotomy between IL-23 and IL-2 in driving effector Th17 cells into the memory pool in autoimmune-mediated ocular surface inflammation.

Sustained IL-2R signaling of limited duration by high-dose mIL-2/mCD25 fusion protein amplifies tumor-reactive CD8+ T cells to enhance antitumor immunity

High-dose IL-2 induces cancer regression but its therapeutic use is limited due to high toxicities resulting from its broad cell targeting. In one strategy to overcome this limitation, IL-2 has been modified to selectively target the intermediate affinity IL-2R that broadly activates memory-phenotypic CD8⁺ T and NK cells, while minimizing Treg-associated tolerance. In this study, we modeled an alternative strategy to amplify tumor antigen-specific TCR transgenic CD8⁺ T cells through limited application of a long-acting IL-2 fusion protein, mIL-2/mCD25, which selectively targets the high-affinity IL-2R. Here, mice were vaccinated with a tumor antigen and high-dose mIL-2/mCD25 was applied to coincide with the induction of the high affinity IL-2R on tumor-specific T cells. A single high dose of mIL-2/mCD25, but not an equivalent amount of IL-2, amplified the frequency and function of tumor-reactive CD8⁺ T effector (Teff) and memory cells. These mIL-2/mCD25-dependent effects relied on distinctive requirements for TLR signals during priming of CD8⁺ tumor-specific T cells. The mIL-2/mCD25-amplified tumor-reactive effector and memory T cells supported long-lasting antitumor responses to B16-F10 melanoma. This regimen only transiently increased Tregs, yielding a favorable Teff-Treg ratio within the tumor microenvironment. Notably, mIL-2/mCD25 did not increase non-tumor-specific Teff or NK cells within tumors, further substantiating the specificity of mIL-2/mCD25 for tumor antigen-activated T cells. Thus, the selectivity and persistence of mIL-2/mCD25 in conjunction with a tumor vaccine supports antitumor immunity through a mechanism that is distinct from recombinant IL-2 or IL-2-based biologics that target the intermediate affinity IL-2R.

Antigen-reactive regulatory T cells can be expanded in vitro with monocytes and anti-CD28 and anti-CD154 antibodies

BACKGROUND

In recent years, therapies with CD4⁺CD25^highFoxP3⁺ regulatory T cells (Tregs) have been successfully tested in many clinical trials. The important issue regarding the use of this treatment in autoimmune conditions remains the specificity toward particular antigen, as because of epitope spread, there are usually multiple causative autoantigens to be regulated in such conditions.

METHODS

Here we show a method of generation of Tregs enriched with antigen-reactive clones that potentially covers the majority of such autoantigens. In our research, Tregs were expanded with anti-CD28 and anti-CD154 antibodies and autologous monocytes and loaded with a model peptide, such as whole insulin or insulin β chain peptide 9-23. The cells were then sorted into cells recognizing the presented antigen. The reactivity was verified with functional assays in which Tregs suppressed proliferation or interferon gamma production of autologous effector T cells (polyclonal and antigen-specific) used as responders challenged with the model peptide. Finally, we analyzed clonotype distribution and TRAV gene usage in the specific Tregs.

RESULTS

Altogether, the applied technique had a good yield and allowed us to obtain a Treg product enriched with a specific subset, as confirmed in the functional tests. The product consisted of many clones; nevertheless, the content of these clones was different from that found in polyclonal or unspecific Tregs.

CONCLUSIONS

The presented technique might be used to generate populations of Tregs enriched with cells reactive to any given peptide, which can be used as a cellular therapy medicinal product in antigen-targeted therapies.

Regulatory T Cells Restrain Interleukin-2- and Blimp-1-Dependent Acquisition of Cytotoxic Function by CD4+ T Cells

In addition to helper and regulatory potential, CD4+ T cells also acquire cytotoxic activity marked by granzyme B (GzmB) expression and the ability to promote rejection of established tumors. Here, we examined the molecular and cellular mechanisms underpinning the differentiation of cytotoxic CD4+ T cells following immunotherapy. CD4+ transfer into lymphodepleted animals or regulatory T (Treg) cell depletion promoted GzmB expression by tumor-infiltrating CD4+, and this was prevented by interleukin-2 (IL-2) neutralization. Transcriptional analysis revealed a polyfunctional helper and cytotoxic phenotype characterized by the expression of the transcription factors T-bet and Blimp-1. While T-bet ablation restricted interferon-γ (IFN-γ) production, loss of Blimp-1 prevented GzmB expression in response to IL-2, suggesting two independent programs required for polyfunctionality of tumor-reactive CD4+ T cells. Our findings underscore the role of Treg cells, IL-2, and Blimp-1 in controlling the differentiation of cytotoxic CD4+ T cells and offer a pathway to enhancement of anti-tumor activity through their manipulation.

Relationship between Multiple Sclerosis-Associated IL2RA Risk Allele Variants and Circulating T Cell Phenotypes in Healthy Genotype-Selected Controls

Single nucleotide polymorphisms (SNPs) in or near the IL2RA gene, that encodes the interleukin-2 (IL-2) receptor α (CD25), are associated with increased risk of immune-mediated diseases including multiple sclerosis (MS). We investigated how the MS-associated IL2RA SNPs rs2104286 and rs11256593 are associated with CD25 expression on T cells ex vivo by multiparameter flow cytometry in paired genotype-selected healthy controls. We observed that MS-associated IL2RA SNPs rs2104286 and rs11256593 are associated with expression of CD25 in CD4⁺ but not CD8⁺ T cells. In CD4⁺ T cells, carriers of the risk genotype had a reduced frequency of CD25⁺ T_FH1 cells (p = 0.001) and an increased frequency of CD25⁺ recent thymic emigrant cells (p = 0.006). Furthermore, carriers of the risk genotype had a reduced surface expression of CD25 in post-thymic expanded CD4⁺ T cells (CD31⁻CD45RA⁺), CD39⁺ T_Reg cells and in several non-follicular memory subsets. Our study found novel associations of MS-associated IL2RA SNPs on expression of CD25 in CD4⁺ T cell subsets. Insight into the associations of MS-associated IL2RA SNPs, as these new findings provide, offers a better understanding of CD25 variation in the immune system and can lead to new insights into how MS-associated SNPs contribute to development of MS.

Autism genes and the leukocyte transcriptome in autistic toddlers relate to pathogen interactomes, infection and the immune system. A role for excess neurotrophic sAPPα and reduced antimicrobial Aβ

Prenatal and early childhood infections have been implicated in autism. Many autism susceptibility genes (206 Autworks genes) are localised in the immune system and are related to immune/infection pathways. They are enriched in the host/pathogen interactomes of 18 separate microbes (bacteria/viruses and fungi) and to the genes regulated by bacterial toxins, mycotoxins and Toll-like receptor ligands. This enrichment was also observed for misregulated genes from a microarray study of leukocytes from autistic toddlers. The upregulated genes from this leukocyte study also matched the expression profiles in response to numerous infectious agents from the Broad Institute molecular signatures database. They also matched genes related to sudden infant death syndrome and autism comorbid conditions (autoimmune disease, systemic lupus erythematosus, diabetes, epilepsy and cardiomyopathy) as well as to estrogen and thyrotropin responses and to those upregulated by different types of stressors including oxidative stress, hypoxia, endoplasmic reticulum stress, ultraviolet radiation or 2,4-dinitrofluorobenzene, a hapten used to develop allergic skin reactions in animal models. The oxidative/integrated stress response is also upregulated in the autism brain and may contribute to myelination problems. There was also a marked similarity between the expression signatures of autism and Alzheimer's disease, and 44 shared autism/Alzheimer's disease genes are almost exclusively expressed in the blood-brain barrier. However, in contrast to Alzheimer's disease, levels of the antimicrobial peptide beta-amyloid are decreased and the levels of the neurotrophic/myelinotrophic soluble APP alpha are increased in autism, together with an increased activity of α-secretase. sAPPα induces an increase in glutamatergic and a decrease in GABA-ergic synapses creating and excitatory/inhibitory imbalance that has also been observed in autism. A literature survey showed that multiple autism genes converge on APP processing and that many are able to increase sAPPalpha at the expense of beta-amyloid production. A genetically programmed tilt of this axis towards an overproduction of neurotrophic/gliotrophic sAPPalpha and underproduction of antimicrobial beta-amyloid may explain the brain overgrowth and myelination dysfunction, as well as the involvement of pathogens in autism.

CD122 signaling in CD8+ memory T cells drives costimulation-independent rejection

Interrupting T cell costimulatory signals as a strategy to control undesired immune responses, such as occur in autoimmunity or transplantation, has the potential to alleviate many of the unwanted side effects associated with current immunosuppressive therapies. Belatacept, a high-affinity version of CTLA4-Ig that blocks ligand ligation to CD28, has been approved for use in kidney transplant recipients. Despite the long-term benefits associated with its use, such as improved renal function and lower cardiovascular risk, a subset of patients treated with belatacept experience elevated rates of acute T cell-mediated rejection, tempering enthusiasm for its use. Here we demonstrate that costimulation-independent T cell alloreactivity relies on signaling through CD122, the shared IL-2 and IL-15 receptor β-chain. Combined costimulatory and CD122 blockade improved survival of transplanted tissue in mice and nonhuman primates by controlling proliferation and effector function of CD8+ T cells. The high-affinity IL-2 receptor was dispensable for memory CD8+ T cell responses, whereas signaling through CD122 as a component of the high-affinity IL-15 receptor was critical for costimulation-independent memory CD8+ T cell recall, distinguishing specific roles for IL-2 and IL-15 in T cell activation. These studies outline a novel approach for clinical optimization of costimulatory blockade strategies in transplantation by targeting CD122.

Dok-1 and Dok-2 Regulate the Formation of Memory CD8+ T Cells

Diverse signals received by CD8⁺ T cells are integrated to achieve the required magnitude of cell expansion and the appropriate balance of effector/memory CD8⁺ T cell generation. Notably, the strength and nature of TCR signaling influence the differentiation and functional capacity of effector and memory CD8⁺ T cells. Dok-1 and Dok-2, the two members of the Dok family expressed in T cells, negatively regulate TCR signaling in vitro. However, the role of Dok proteins in modulating T cell function in vivo has not yet studied. We studied the function of Dok-1 and Dok-2 proteins in the regulation of the CD8⁺ T cell response to vaccinia virus infection. Comparison of responses to vaccinia virus expressing OVA peptide SIINFEKL by wild-type and Dok-1/2^-/- CD8⁺ OT-I cells showed that the absence of Dok-1 and Dok-2 slightly reduced the magnitude of virus-specific effector CD8⁺ T cell expansion. This was not due to reduced proliferation or enhanced apoptosis of effector CD8⁺ T cells. Dok-1/2-deficient effector CD8⁺ T cells showed increased cell surface TCR expression following virus infection in vivo and increased expression of granzyme B and TNF upon stimulation with peptide Ag ex vivo. Finally, Dok-1/2-deficient effector CD8⁺ T had a severe defect in survival that resulted in impaired generation of memory CD8⁺ T cells. These results reveal the critical involvement of Dok-1 and Dok-2 in a negative-feedback loop that prevents overactivation of CD8⁺ T cells and promotes memory formation.

Manipulating Memory CD8 T Cell Numbers by Timed Enhancement of IL-2 Signals

As a result of the growing burden of tumors and chronic infections, manipulating CD8 T cell responses for clinical use has become an important goal for immunologists. In this article, we show that dendritic cell (DC) immunization coupled with relatively early (days 1-3) or late (days 4-6) administration of enhanced IL-2 signals increase peak effector CD8 T cell numbers, but only early IL-2 signals enhance memory numbers. IL-2 signals delivered at relatively late time points drive terminal differentiation and marked Bim-mediated contraction and do not increase memory T cell numbers. In contrast, early IL-2 signals induce effector cell metabolic profiles that are more conducive to memory formation. Of note, downregulation of CD80 and CD86 was observed on DCs in vivo following early IL-2 treatment. Mechanistically, early IL-2 treatment enhanced CTLA-4 expression on regulatory T cells, and CTLA-4 blockade alongside IL-2 treatment in vivo prevented the decrease in CD80 and CD86, supporting a cell-extrinsic role for CTLA-4 in downregulating B7 ligand expression on DCs. Finally, DC immunization followed by early IL-2 treatment and anti-CTLA-4 blockade resulted in lower memory CD8 T cell numbers compared with the DC+early IL-2 treatment group. These data suggest that curtailed signaling through the B7-CD28 costimulatory axis during CD8 T cell activation limits terminal differentiation and preserves memory CD8 T cell formation; thus, it should be considered in future T cell-vaccination strategies.

Antibodies to Interleukin-2 Elicit Selective T Cell Subset Potentiation through Distinct Conformational Mechanisms

Interleukin-2 (IL-2) is a pleiotropic cytokine that regulates immune cell homeostasis and has been used to treat a range of disorders including cancer and autoimmune disease. IL-2 signals via interleukin-2 receptor-β (IL-2Rβ):IL-2Rγ heterodimers on cells expressing high (regulatory T cells, Treg) or low (effector cells) amounts of IL-2Rα (CD25). When complexed with IL-2, certain anti-cytokine antibodies preferentially stimulate expansion of Treg (JES6-1) or effector (S4B6) cells, offering a strategy for targeted disease therapy. We found that JES6-1 sterically blocked the IL-2:IL-2Rβ and IL-2:IL-2Rγ interactions, but also allosterically lowered the IL-2:IL-2Rα affinity through a "triggered exchange" mechanism favoring IL-2Rα(hi) Treg cells, creating a positive feedback loop for IL-2Rα(hi) cell activation. Conversely, S4B6 sterically blocked the IL-2:IL-2Rα interaction, while also conformationally stabilizing the IL-2:IL-2Rβ interaction, thus stimulating all IL-2-responsive immune cells, particularly IL-2Rβ(hi) effector cells. These insights provide a molecular blueprint for engineering selectively potentiating therapeutic antibodies.

An Excess of the Proinflammatory Cytokines IFN-γ and IL-12 Impairs the Development of the Memory CD8+ T Cell Response to Chlamydia trachomatis

The obligate intracellular bacterium Chlamydia trachomatis is the most common cause of bacterial sexually transmitted disease in the United States and the leading cause of preventable blindness worldwide. Transfer of cultured Chlamydia-specific CD8(+) T cells or vaccination with recombinant virus expressing an MHC I-restricted Chlamydia Ag confers protection, yet surprisingly a protective CD8(+) T cell response is not stimulated following natural infection. In this study, we demonstrate that the presence of excess IL-12 and IFN-γ contributes to poor memory CD8(+) T cell development during C. trachomatis infection of mice. IL-12 is required for CD8(+) T cell expansion but drives effector CD8(+) T cells into a short-lived fate, whereas IFN-γ signaling impairs the development of effector memory cells. We show that transient blockade of IL-12 and IFN-γ during priming promotes the development of memory precursor effector CD8(+) T cells and increases the number of memory T cells that participate in the recall protection against subsequent infection. Overall, this study identifies key factors shaping memory development of Chlamydia-specific CD8(+) T cells that will inform future vaccine development against this and other pathogens.

Challenges and developing solutions for increasing the benefits of IL-2 treatment in tumor therapy

Interleukin-2 (IL-2) is a cytokine with pleiotropic effects on the immune system. Systemic IL-2 treatment has produced durable responses in melanoma and renal cancer patients, but unfortunately this is effective only in a fraction of patients. Moreover, IL-2 treatment also engenders serious side effects, which limit its clinical utility. It is now appreciated that IL-2 not only stimulates NK and effector T cells but also has a critical role in the generation and maintenance of regulatory T cells, which act to dampen immune responses. Thus, successful immunotherapy of cancers using IL-2 has to address two fundamentally important issues: (1) how to limit side effects yet be active where it is needed, and (2) how to preferentially activate effector T cells while limiting the stimulation of Tregs. Strategies are now being developed to address these critical obstacles that may lead to a renaissance of IL-2 therapy.

The allure and peril of hematopoietic stem cell transplantation: overcoming immune challenges to improve success

Since its inception in the mid-twentieth century, the complication limiting the application and utility of allogeneic hematopoietic stem cell transplantation (allo-HSCT) to treat patients with hematopoietic cancer is the development of graft-versus-host disease (GVHD). Ironically, GVHD is induced by the cells (T lymphocytes) transplanted for the purpose of eliminating the malignancy. Damage ensuing to multiple tissues, e.g., skin, GI, liver, and others including the eye, provides the challenge of regulating systemic and organ-specific GVH responses. Because the immune system is also targeted by GVHD, this both: (a) impairs reconstitution of immunity post-transplant resulting in patient susceptibility to lethal infection and (b) markedly diminishes the individual's capacity to generate anti-cancer immunity--the raison d'etre for undergoing allo-HSCT. We hypothesize that deleting alloreactive T cells ex vivo using a new strategy involving antigen stimulation and alkylation will prevent systemic GVHD thereby providing a platform for the generation of anti-tumor immunity. Relapse also remains the major complication following autologous HSCT (auto-HSCT). While GVHD does not complicate auto-HSCT, its absence removes significant grant anti-tumor responses (GVL) and raises the challenge of generating rapid and effective anti-tumor immunity early post-transplant prior to immune reconstitution. We hypothesize that effective vaccine usage to stimulate tumor-specific T cells followed by their amplification using targeted IL-2 can be effective in both the autologous and allogeneic HSCT setting. Lastly, our findings support the notion that the ocular compartment can be locally targeted to regulate visual complications of GVHD which may involve both alloreactive and self-reactive (i.e., autoimmune) responses.

Heat shock protein vaccination and directed IL-2 therapy amplify tumor immunity rapidly following bone marrow transplantation in mice

Tumor relapse is the primary cause of mortality in patients with hematologic cancers following autologous hematopoietic stem cell transplantation (HSCT). Vaccination early after HSCT can exploit both the state of lymphopenia and minimal residual disease for generating antitumor immunity. Here, multiple vaccinations using lymphoma cells engineered to secrete heat shock protein fusion gp96-Ig within 2 weeks of T cell-replete syngeneic HSCT led to cross-presentation and increased survival of lymphoma-bearing mice. To enhance vaccine efficacy, interleukin (IL)-2 was directed to predominantly memory phenotype CD8(+) T lymphocytes and natural killer (NK) cells via administration bound to anti-IL-2 monoclonal antibody clone S4B6 (IL-2S4B6). Combination therapy with gp96-Ig vaccination and coordinated infusions of IL-2S4B6 resulted in marked prolongation of survival, which directly correlated with ~500% increase in effector CD8(+) T-cell numbers. Notably, this dual regimen elicited large increases in both donor CD8(+) T and NK cells, but not CD4(+) T lymphocytes; the former 2 populations are essential for both vaccine efficacy and protection against opportunistic infections after HSCT. Indeed, IL-2S4B6-treated HSCT recipients infected with Listeria monocytogenes exhibited decreased bacterial levels. These preclinical studies validate a new strategy particularly well suited to the post-HSCT environment, which may augment adaptive and innate immune function in patients with malignant disease receiving autologous HSCT.

Interleukin-21 is a critical regulator of CD4 and CD8 T cell survival during priming under Interleukin-2 deprivation conditions

Optimal T cell activation and expansion require binding of the common gamma-chain (γc) cytokine Interleukin-2 (IL-2) to its cognate receptor that in turn engages a γc/Janus tyrosine kinase (Jak)3 signaling pathway. Because of its restricted expression by antigen-activated T cells and its obligatory role in promoting their survival and proliferation, IL-2 has been considered as a selective therapeutic target for preventing T cell mediated diseases. However, in order to further explore IL-2 targeted therapy, it is critical to precisely understand its role during early events of T cell activation. In this study, we delineate the role of IL-2 and other γc cytokines in promoting the survival of CD4 and CD8 T cells during early phases of priming. Under IL-2 inhibitory conditions (by neutralizing anti-IL-2 mAbs), the survival of activated CD8⁺ T cells was reduced, whereas CD4⁺ T cells remained much more resistant. These results correlated with reduced Bcl-2 expression, and mitochondrial membrane potential in CD8⁺ T cells in comparison to CD4⁺ T cells. However, using transwell co-culture assays we have found that CD4⁺ T cells could rescue the survival of CD8⁺ T cells even under IL-2 deprived conditions via secretion of soluble factors. A cytokine screen performed on CD8⁺ T cells cultured alone revealed that IL-21, another γc cytokine, was capable of rescuing their survival under IL-2 deprivation. Indeed, blocking the IL-21 signaling pathway along with IL-2 neutralization resulted in significantly reduced survival of both CD4⁺ and CD8⁺ T cells. Taken together, we have shown that under IL-2 deprivation conditions, IL-21 may act as the major survival factor promoting T cell immune responses. Thus, investigation of IL-2 targeted therapies may need to be revisited to consider blockade of the IL-21 signaling pathways as an adjunct to provide more effective control of T cell immune responses.