Signal integration by Akt regulates CD8 T cell effector and memory differentiation

Engineering CAR-T Cells for Improved Function Against Solid Tumors

Genetic engineering T cells to create clinically applied chimeric antigen receptor (CAR) T cells has led to improved patient outcomes for some forms of hematopoietic malignancies. While this has inspired the biomedical community to develop similar strategies to treat solid tumor patients, challenges such as the immunosuppressive character of the tumor microenvironment, CAR-T cell persistence and trafficking to the tumor seem to limit CAR-T cell efficacy in solid cancers. This review provides an overview of mechanisms that tumors exploit to evade eradication by CAR-T cells as well as emerging approaches that incorporate genetic engineering technologies to improve CAR-T cell activity against solid tumors.

B cell adaptor for PI3-kinase (BCAP) modulates CD8+ effector and memory T cell differentiation

Singh et al. show that expression of B cell adaptor for PI3-kinase (BCAP) is induced upon T cell activation and that this helps control effector and memory CD8⁺ T cell differentiation.

CD8⁺ T cells respond to signals via the T cell receptor (TCR), costimulatory molecules, and immunoregulatory cytokines by developing into diverse populations of effector and memory cells. The relative strength of phosphoinositide 3-kinase (PI3K) signaling early in the T cell response can dramatically influence downstream effector and memory T cell differentiation. We show that initial PI3K signaling during T cell activation results in up-regulation of the signaling scaffold B cell adaptor for PI3K (BCAP), which further potentiates PI3K signaling and promotes the accumulation of CD8⁺ T cells with a terminally differentiated effector phenotype. Accordingly, BCAP-deficient CD8⁺ T cells have attenuated clonal expansion and altered effector and memory T cell development following infection with Listeria monocytogenes. Thus, induction of BCAP serves as a positive feedback circuit to enhance PI3K signaling in activated CD8⁺ T cells, thereby acting as a molecular checkpoint regulating effector and memory T cell development.

Paths to expansion: Differential requirements of IRF4 in CD8+ T-cell expansion driven by antigen and homeostatic cytokines

Interferon regulatory factor 4 (IRF4) regulates the clonal expansion and metabolic activity of activated T cells, but the precise context and mechanisms of its function in these processes are unclear. In this issue of the European Journal of Immunology, Miyakoda et al. [Eur. J. Immunol. 2018. 48: 1319-1328] show that IRF4 is required for activation and expansion of naïve and memory CD8+ T cells driven by T-cell receptor (TCR) signaling, but dispensable for memory CD8+ T-cell maintenance and homeostatic proliferation driven by homeostatic cytokines. The authors show that the function of IRF4 in CD8+ T-cell expansion is partially dependent upon activation of the PI3K/AKT pathway through direct or indirect attenuation of PTEN expression. These data shed light upon the differential intracellular pathways required for naïve and memory T cells to respond to self-antigens and/or homeostatic cytokines, and highlight the potential translational relevance of these findings in the context of immune reconstitution such as following allogeneic stem cell transplantation.

Review and Meta-Analyses of TAAR1 Expression in the Immune System and Cancers

Since its discovery in 2001, the major focus of TAAR1 research has been on its role in monoaminergic regulation, drug-induced reward and psychiatric conditions. More recently, TAAR1 expression and functionality in immune system regulation and immune cell activation has become a topic of emerging interest. Here, we review the immunologically-relevant TAAR1 literature and incorporate open-source expression and cancer survival data meta-analyses. We provide strong evidence for TAAR1 expression in the immune system and cancers revealed through NCBI GEO datamining and discuss its regulation in a spectrum of immune cell types as well as in numerous cancers. We discuss connections and logical directions for further study of TAAR1 in immunological function, and its potential role as a mediator or modulator of immune dysregulation, immunological effects of psychostimulant drugs of abuse, and cancer progression.

CD57 identifies T cells with functional senescence before terminal differentiation and relative telomere shortening in patients with activated PI3 kinase delta syndrome

Premature T-cell immunosenescence with CD57⁺ CD8⁺ T-cell accumulation has been linked to immunodeficiency and autoimmunity in primary immunodeficiencies including activated PI3 kinase delta syndrome (APDS). To address whether CD57 marks the typical senescent T-cell population seen in adult individuals or identifies a distinct population in APDS, we compared CD57⁺ CD8⁺ T cells from mostly pediatric APDS patients to those of healthy adults with similarly prominent senescent T cells. CD57⁺ CD8⁺ T cells from APDS patients were less differentiated with more CD27⁺ CD28⁺ effector memory T cells showing increased PD1 and Eomesodermin expression. In addition, transition of naïve to CD57⁺ CD8⁺ T cells was not associated with the characteristic telomere shortening. Nevertheless, they showed the increased interferon-gamma secretion, enhanced degranulation and reduced in vitro proliferation typical of senescent CD57⁺ CD8⁺ T cells. Thus, hyperactive PI3 kinase signaling favors premature accumulation of a CD57⁺ CD8⁺ T-cell population, which shows most functional features of typical senescent T cells, but is different in terms of differentiation and relative telomere shortening. Initial observations indicate that this specific differentiation state may offer the opportunity to revert premature T-cell immunosenescence and its potential contribution to inflammation and immunodeficiency in APDS.

PI3K orchestration of the in vivo persistence of chimeric antigen receptor-modified T cells

In vivo persistence of chimeric antigen receptor (CAR)-modified T cells correlates with therapeutic efficacy, yet CAR-specific factors that support persistence are not well resolved. Using a CD33-specific CAR in an acute myeloid leukemia (AML) model, we show how CAR expression alters T cell differentiation in a ligand independent manner. Ex vivo expanded CAR-T cells demonstrated decreased naïve and stem memory populations and increased effector subsets relative to vector-transduced control cells. This was associated with reduced in vivo persistence. Decreased persistence was not due to specificity or tumor presence, but to pre-transfer tonic signaling through the CAR CD3ζ ITAMs. We identified activation of the PI3K pathway in CD33 CAR-T cells as responsible. Treatment with a PI3K inhibitor modulated the differentiation program of CAR-T cells, preserved a less differentiated state without affecting T cell expansion, and improved in vivo persistence and reduced tumor burden. These results resolve mechanisms by which tonic signaling of CAR-T cells modulates their fate, and identifies a novel pharmacologic approach to enhance the durability of CAR-T cells for immunotherapy.

FOXOs in cancer immunity: Knowns and unknowns

In the tumor microenvironment (TME), cancer cells, stromal cells, and immune cells, along with their extracellular factors, have profound effects on either promoting or repressing anti-cancer immunity. Accumulating evidence has shown the paradoxical intrinsic role of the Forkhead box O (FOXO) family of transcription factors in cancer, which can act as a tumor repressor while also maintaining cancer stem cells. FOXOs also regulate cancer immunity. FOXOs promote antitumor activity through negatively regulating the expression of immunosuppressive proteins, such as programmed death 1 ligand 1 (PD-L1), and vascular endothelial growth factor (VEGF) in tumor cells or stromal cells, which can shape an immunotolerant state in the TME. FOXOs also intrinsically control the anti-tumor immune response as well as the homeostasis and development of immune cells, including T cells, B cells, natural killer (NK) cells, macrophages, and dendritic cells. As a cancer repressor, reviving the activity of Foxo1 forces tumor-infiltrating activated regulatory T (T_reg) cells to egress from tumor tissues. As a promoter of cancer development, Foxo3 and Foxo1 negatively regulate cytotoxicity of both CD8⁺ T cells and NK cells against tumor cells. In this review, we focus on the complex role of FOXOs in regulating cancer immunity due to the various roles that they play in cancer cells, stromal cells, and immune cells. We also speculate on some possible additional roles of FOXOs in cancer immunity based on findings regarding FOXOs in non-cancer settings, such as infectious disease.

CAR-T cells and combination therapies: What's next in the immunotherapy revolution?

Cancer immunotherapies are dramatically reshaping the clinical management of oncologic patients. For many of these therapies, the guidelines for administration, monitoring, and management of associated toxicities are still being established. This is especially relevant for adoptively transferred, genetically-modified T cells, which have unique pharmacokinetic properties, due to their ability to replicate and persist long-term, following a single administration. Furthermore, in the case of CAR-T cells, the use of synthetic immune receptors may impact signaling pathways involved in T cell function and survival in unexpected ways. We, herein, comment on the most salient aspects of CAR-T cell design and clinical experience in the treatment of solid tumors. In addition, we discuss different possible scenarios for combinations of CAR-T cells and other treatment modalities, with a special emphasis on kinase inhibitors, elaborating on the strategies to maximize synergism. Finally, we discuss some of the technologies that are available to explore the molecular events governing the success of these therapies. The young fields of synthetic and systems biology are likely to be major players in the advancement of CAR-T cell therapies, providing the tools and the knowledge to engineer patients' T lymphocytes into intelligent cancer-fighting micromachines.

Adoptive transfer of murine T cells expressing a chimeric-PD1-Dap10 receptor as an immunotherapy for lymphoma

Adoptive transfer of T cells is a promising cancer therapy and expression of chimeric antigen receptors can enhance tumour recognition and T-cell effector functions. The programmed death protein 1 (PD1) receptor is a prospective target for a chimeric antigen receptor because PD1 ligands are expressed on many cancer types, including lymphoma. Therefore, we developed a murine chimeric PD1 receptor (chPD1) consisting of the PD1 extracellular domain fused to the cytoplasmic domain of CD3ζ. Additionally, chimeric antigen receptor therapies use various co-stimulatory domains to enhance efficacy. Hence, the inclusion of a Dap10 or CD28 co-stimulatory domain in the chPD1 receptor was compared to determine which domain induced optimal anti-tumour immunity in a mouse model of lymphoma. The chPD1 T cells secreted pro-inflammatory cytokines and lysed RMA lymphoma cells. Adoptive transfer of chPD1 T cells significantly reduced established tumours and led to tumour-free survival in lymphoma-bearing mice. When comparing chPD1 receptors containing a Dap10 or CD28 domain, both receptors induced secretion of pro-inflammatory cytokines; however, chPD1-CD28 T cells also secreted anti-inflammatory cytokines whereas chPD1-Dap10 T cells did not. Additionally, chPD1-Dap10 induced a central memory T-cell phenotype compared with chPD1-CD28, which induced an effector memory phenotype. The chPD1-Dap10 T cells also had enhanced in vivo persistence and anti-tumour efficacy compared with chPD1-CD28 T cells. Therefore, adoptive transfer of chPD1 T cells could be a novel therapy for lymphoma and inclusion of the Dap10 co-stimulatory domain in chimeric antigen receptors may induce a preferential cytokine profile and T-cell differentiation phenotype for anti-tumour therapies.

IL-21 Augments Rapamycin in Expansion of Alpha Fetoprotein Antigen Specific Stem-Cell-like Memory T Cells in vitro

INTRODUCTION

Alloreactive tumor specific T cells are important arsenals of the adaptive immune system in the fight against tumors. However, stem cell-like memory T cells (Tscm) provide the key to effective elimination of tumor cells. Methods for generating these T cell subsets already exist. However, they could be made more efficient. Further, they are expensive and unattainable to the resource poor laboratories. In this regard, we are hereby describing a novel in vitro allogeneic co-culture method for raising allo-restricted tumor specific Tscm cells that we developed.

METHODS

We started by obtaining PBLs that screened negative for HLA-A2 molecules from healthy donors followed by co-culture with T2/AFP cells to generate AFP peptide specific tumor-reactive T cells. Controls, IL-21 and/or rapamycin were applied to samples in 24 well plates. Samples were harvested and stained with anti-human CD3, CD8, CD44, CD62L, and HLA-A2/AFP dimer followed by flow cytometry analysis. Cell viability was measured by Trypan blue exclusion assay. One Way ANOVA and independent t test were used to compare the mean differences among and between groups where P values less than 0.05 were considered significant.

RESULTS

Our results show that rapamycin arrests the differentiation of, and expands AFP specific Tscm cells. Further, the expansion of Tscm cells is augmented in the presence of IL-21.

CONCLUSION

IL-21 and Rapamycin can be used concurrently to raise and maintain antigen specific Tscm cells in vitro for purposes of augmenting immunotherapy strategies against cancers.

Enhanced Therapeutic Efficacy and Memory of Tumor-Specific CD8 T Cells by Ex Vivo PI3K-δ Inhibition

Inhibition of specific Akt isoforms in CD8⁺ T cells promotes favored differentiation into memory versus effector cells, the former of which are superior in mediating antitumor immunity. In this study, we investigated the role of upstream PI3K isoforms in CD8⁺ T-cell differentiation and assessed the potential use of PI3K isoform-specific inhibitors to favorably condition CD8⁺ T cells for adoptive cell therapy. The phenotype and proliferative ability of tumor antigen-specific CD8+ T cells was assessed in the presence of PI3K-α, -β, or -δ inhibitors. Inhibition of PI3K-δ, but not PI3K-α or PI3K-β, delayed terminal differentiation of CD8⁺ T cells and maintained the memory phenotype, thus enhancing their proliferative ability and survival while maintaining their cytokine and granzyme B production ability. This effect was preserved in vivo after ex vivo PI3K-δ inhibition in CD8⁺ T cells destined for adoptive transfer, enhancing their survival and also the antitumor therapeutic activity of a tumor-specific peptide vaccine. Our results outline a mechanism by which inhibitions of a single PI3K isoform can enhance the proliferative potential, function, and survival of CD8⁺ T cells, with potential clinical implications for adoptive cell transfer and vaccine-based immunotherapies. Cancer Res; 77(15); 4135-45. ©2017 AACR.

mTORC1 regulates mannose-6-phosphate receptor transport and T-cell vulnerability to regulatory T cells by controlling kinesin KIF13A

Mannose-6-phosphate receptor (M6PR) that facilitates cellular uptake of M6P-bearing proteins, including serine-protease granzyme-B (Gzm-B) has an important role in T-cell activation, migration and contraction. However, molecular mechanisms controlling M6PR expression in T cells remain poorly understood. Here, we show that M6PR expression on T cells is distinctively controlled by two common γ-chain cytokines interleukin-2 (IL-2) and IL-7, and the differential M6PR expression is not caused by an altered synthesis of M6PR protein, but is a result of distinct regulation of kinesin-3 motor-protein KIF13A that transport M6PR onto cell surfaces. Using signaling pathway-specific inhibitors, we determine that IL-2 and IL-7 distinctly regulate KIF13A and β1-adaptin and cell-surface M6PR by controlling a kinase mammalian target of rapamycin complex-1 (mTORC1). Inflammatory cytokine IL-2 and prosurvival cytokine IL-7 induce strong and weak activation of mTORC1, leading to up- and downregulation of motor-protein KIF13A and KIF13A-motorized M6PR on T cells, and formation of IL-2 and IL-7 effectors with M6PR^high and M6PR^low cell-surface expression, respectively. Inhibition of mTORC1 by rapamycin reduces T-cell expression of KIF13A and cell-surface M6PR, and increases T-cell survival in Listeria monocytogenes-infected mice. Using regulatory T (T_reg)-cell-enriched mouse tumor model, we determine that M6PR^high IL-2 effectors but not M6PR^low IL-7 effectors adoptively transferred into tumors are vulnerable to T_reg Gzm-B-mediated cell apoptosis. Inhibition of mTORC1 or small interfering RNA-mediated knockdown of KIF13A or M6PR renders IL-2 effectors refractory to T_reg Gzm-B lethal hit. Overall, our data offer novel mechanistic insights into T-cell M6PR regulation, and T_reg-resistant/T_reg-susceptible phenomenon. Furthermore, regulation of T-cell fate vis-à-vis T_reg suppression via the mTORC1-KIF13A-M6PR axis provides a proof of concept for therapeutic strategies to target cancer, infectious and autoimmune diseases.

Ex vivo Akt inhibition promotes the generation of potent CD19CAR T cells for adoptive immunotherapy

Background

Insufficient persistence and effector function of chimeric antigen receptor (CAR)-redirected T cells have been challenging issues for adoptive T cell therapy. Generating potent CAR T cells is of increasing importance in the field. Studies have demonstrated the importance of the Akt pathway in the regulation of T cell differentiation and memory formation. We now investigate whether inhibition of Akt signaling during ex vivo expansion of CAR T cells can promote the generation of CAR T cells with enhanced antitumor activity following adoptive therapy in a murine leukemia xenograft model.

Methods

Various T cell subsets including CD8+ T cells, bulk T cells, central memory T cells and naïve/memory T cells were isolated from PBMC of healthy donors, activated with CD3/CD28 beads, and transduced with a lentiviral vector encoding a second-generation CD19CAR containing a CD28 co-stimulatory domain. The transduced CD19CAR T cells were expanded in the presence of IL-2 (50U/mL) and Akt inhibitor (Akti) (1 μM) that were supplemented every other day. Proliferative/expansion potential, phenotypical characteristics and functionality of the propagated CD19CAR T cells were analyzed in vitro and in vivo after 17-21 day ex vivo expansion. Anti-tumor activity was evaluated after adoptive transfer of the CD19CAR T cells into CD19+ tumor-bearing immunodeficient mice. Tumor signals were monitored with biophotonic imaging, and survival rates were analyzed by the end of the experiments.

Results

We found that Akt inhibition did not compromise CD19CAR T cell proliferation and expansion in vitro, independent of the T cell subsets, as comparable CD19CAR T cell expansion was observed after culturing in the presence or absence of Akt inhibitor. Functionally, Akt inhibition did not dampen cell-mediated effector function, while Th1 cytokine production increased. With respect to phenotype, Akti-treated CD19CAR T cells expressed higher levels of CD62L and CD28 as compared to untreated CD19CAR T cells. Once adoptively transferred into CD19+ tumor-bearing mice, Akti treated CD19CAR T cells exhibited more antitumor activity than did untreated CD19CAR T cells.

Conclusions

Inhibition of Akt signaling during ex vivo priming and expansion gives rise to CD19CAR T cell populations that display comparatively higher antitumor activity.

Electronic supplementary material

The online version of this article (doi:10.1186/s40425-017-0227-4) contains supplementary material, which is available to authorized users.

Tethered IL-15 augments antitumor activity and promotes a stem-cell memory subset in tumor-specific T cells

Adoptive immunotherapy retargeting T cells to CD19 via a chimeric antigen receptor (CAR) is an investigational treatment capable of inducing complete tumor regression of B-cell malignancies when there is sustained survival of infused cells. T-memory stem cells (T_SCM) retain superior potential for long-lived persistence, but challenges exist in manufacturing this T-cell subset because they are rare among circulating lymphocytes. We report a clinically relevant approach to generating CAR⁺ T cells with preserved T_SCM potential using the Sleeping Beauty platform. Because IL-15 is fundamental to T-cell memory, we incorporated its costimulatory properties by coexpressing CAR with a membrane-bound chimeric IL-15 (mbIL15). The mbIL15-CAR T cells signaled through signal transducer and activator of transcription 5 to yield improved T-cell persistence independent of CAR signaling, without apparent autonomous growth or transformation, and achieved potent rejection of CD19⁺ leukemia. Long-lived T cells were CD45RO^negCCR7⁺CD95⁺, phenotypically most similar to T_SCM, and possessed a memory-like transcriptional profile. Overall, these results demonstrate that CAR⁺ T cells can develop long-term persistence with a memory stem-cell phenotype sustained by signaling through mbIL15. This observation warrants evaluation in clinical trials.

AKT-STAT3 Pathway as a Downstream Target of EGFR Signaling to Regulate PD-L1 Expression on NSCLC cells

While cancer development and progression can be controlled by cytotoxic T cells, it is also known that tumor-specific CD8+T cells become functionally impaired by acquiring a group of inhibitory receptors known as immune checkpoints. Amongst those, programmed death-1 (PD-1) is one of the most recognized negative regulators of T cell function. In non-small lung cancers (NSCLCs), the aberrant activation of epidermal growth factor receptor (EGFR) is known to induce PD-L1 expression and further the treatment with gefitinib, a tyrosine kinase inhibitor (TKI) for EGFR, decrease the expression of PD-L1 on NSCLC. Given the acquired resistance to gefitinib treatment frequently observed by developing secondary-site mutations limiting its efficacy, it is important to understand the downstream mechanism of activated-EGFR signaling for regulating PD-L1 in NSCLC. In this study, we demonstrated that AKT-STAT3 pathway could be a potential target for regulating the surface expression of PD-L1 on NSCLCs with aberrant EGFR activity and, further, the inhibition of AKT or STAT3 activity could down-regulate the expression of PD-L1 even in gefitinib-resistant NSCLCs. These results highlight an importance of AKT-STAT3 pathway as a promising target for potentiating anti-tumor immune responses by regulating PD-L1 expression on cancer cells with aberrant EGFR activity.

BACH2 regulates CD8(+) T cell differentiation by controlling access of AP-1 factors to enhancers

T cell antigen receptor (TCR) signaling drives distinct responses depending on the differentiation state and context of CD8(+) T cells. We hypothesized that access of signal-dependent transcription factors (TFs) to enhancers is dynamically regulated to shape transcriptional responses to TCR signaling. We found that the TF BACH2 restrains terminal differentiation to enable generation of long-lived memory cells and protective immunity after viral infection. BACH2 was recruited to enhancers, where it limited expression of TCR-driven genes by attenuating the availability of activator protein-1 (AP-1) sites to Jun family signal-dependent TFs. In naive cells, this prevented TCR-driven induction of genes associated with terminal differentiation. Upon effector differentiation, reduced expression of BACH2 and its phosphorylation enabled unrestrained induction of TCR-driven effector programs.

Mcl-1 regulates effector and memory CD8 T-cell differentiation during acute viral infection

Mcl-1, an anti-apoptotic member of Bcl-2 family maintains cell viability during clonal expansion of CD8 T cells, but the cell intrinsic role of Mcl-1 in contraction of effectors or the number of memory CD8 T cells is unknown. Mcl-1 levels decline during the contraction phase but rebound to high levels in memory CD8 T cells. Therefore, by overexpressing Mcl-1 in CD8 T cells we asked whether limiting levels of Mcl-1 promote contraction of effectors and constrain CD8 T-cell memory. Mcl-1 overexpression failed to affect CD8 T-cell expansion, contraction or the magnitude of CD8 T-cell memory. Strikingly, high Mcl-1 levels enhanced mTOR phosphorylation and augmented the differentiation of terminal effector cells and effector memory CD8 T cells to the detriment of poly-cytokine-producing central memory CD8 T cells. Taken together, these findings provided unexpected insights into the role of Mcl-1 in the differentiation of effector and memory CD8 T cells.

Polyfunctional Melan-A-specific tumor-reactive CD8(+) T cells elicited by dacarbazine treatment before peptide-vaccination depends on AKT activation sustained by ICOS

The identification of activation pathways linked to antitumor T-cell polyfunctionality in long surviving patients is of great relevance in the new era of immunotherapy. We have recently reported that dacarbazine (DTIC) injected one day before peptide-vaccination plus IFN-α improves the antitumor lytic activity and enlarges the repertoire of Melan-A-specific T-cell clones, as compared with vaccination alone, impacting the overall survival of melanoma patients. To identify the mechanisms responsible for this improvement of the immune response, we have analyzed the endogenous and treatment-induced antigen (Ag)-specific response in a panel of Melan-A-specific CD8⁺ T-cell clones in terms of differentiation phenotype, inhibitory receptor profile, polyfunctionality and AKT activation. Here, we show that Melan-A-specific CD8⁺ T cells isolated from patients treated with chemoimmunotherapy possess a late differentiated phenotype as defined by the absence of CD28 and CD27 co-stimulatory molecules and high levels of LAG-3, TIM-3 and PD-1 inhibitory receptors. Nevertheless, they show higher proliferative potential and an improved antitumor polyfunctional effector profile in terms of co-production of TNF-α, IFNγ and Granzyme-B (GrB) compared with cells derived from patients treated with vaccination alone. Polyfunctionality is dependent on an active AKT signaling related to the engagement of the co-stimulatory molecule ICOS. We suggest that this phenotypic and functional signature is dictated by a fine-tuned balance between TCR triggering, AKT activation, co-stimulatory and inhibitory signals induced by chemoimmunotherapy and may be associated with antitumor T cells able to protect patients from tumor recurrence.

Mammalian Target of Rapamycin Complex 2 Controls CD8 T Cell Memory Differentiation in a Foxo1-Dependent Manner

Upon infection, antigen-specific naive CD8 T cells are activated and differentiate into short-lived effector cells (SLECs) and memory precursor cells (MPECs). The underlying signaling pathways remain largely unresolved. We show that Rictor, the core component of mammalian target of rapamycin complex 2 (mTORC2), regulates SLEC and MPEC commitment. Rictor deficiency favors memory formation and increases IL-2 secretion capacity without dampening effector functions. Moreover, mTORC2-deficient memory T cells mount more potent recall responses. Enhanced memory formation in the absence of mTORC2 was associated with Eomes and Tcf-1 upregulation, repression of T-bet, enhanced mitochondrial spare respiratory capacity, and fatty acid oxidation. This transcriptional and metabolic reprogramming is mainly driven by nuclear stabilization of Foxo1. Silencing of Foxo1 reversed the increased MPEC differentiation and IL-2 production and led to an impaired recall response of Rictor KO memory T cells. Therefore, mTORC2 is a critical regulator of CD8 T cell differentiation and may be an important target for immunotherapy interventions.

Natural killer group 2D and CD28 receptors differentially activate mammalian/mechanistic target of rapamycin to alter murine effector CD8+ T-cell differentiation

Memory CD8+ T cells are an essential component of anti-tumour and anti-viral immunity. Activation of the mammalian/mechanistic target of rapamycin (mTOR) pathway has been implicated in regulating the differentiation of effector and memory T cells. However, the mechanisms that control mTOR activity during immunity to tumours and infections are not well known. Activation of co-stimulatory receptors, including CD28 and natural killer group 2D (NKG2D), activate phosphatidylinositol-3 kinase and subsequently may activate the mTOR pathway in CD8+ T cells. This study compared the activation of the mTOR signalling pathway after co-stimulation through CD28 or NKG2D receptors in murine effector CD8+ T cells. Compared with CD28 co-stimulation, activation through CD3 and NKG2D receptors had weaker activation of mTORc1, as shown by decreased phosphorylation of mTORc1 targets S6K1, ribosomal protein S6 and eukaryotic initiation factor 4E binding protein 1. NKG2D co-stimulation also showed increased gene expression of tuberous sclerosis protein 2, a negative regulator of mTORc1, whereas CD28 co-stimulation increased gene expression of Ras homologue enriched in brain, an activator of mTORc1, and hypoxia-inducible factor-1α and vascular endothelial growth factor-α, pro-angiogenic factors downstream of mTORc1. Strong mTORc1 activation in CD28-co-stimulated cells also increased expression of transcription factors that support effector cell differentiation, namely T-bet, B lymphocyte-induced maturation protein (BLIMP-1), interferon regulatory factor 4, and inhibitor of DNA binding 2, whereas low levels of mTORc1 activation allowed for the expression of Eomes, B-cell lymphoma 6 (BCL6), and inhibitor of DNA binding 3 during NKG2D stimulation, and increased expression of memory markers CD62 ligand and CD127. These data show that compared with CD28, co-stimulation through the NKG2D receptor leads to the differential activation of the mTOR signalling pathway and potentially supports memory CD8+ T-cell differentiation.

Enhanced cytotoxic T-cell function and inhibition of tumor progression by Mst1 deficiency

Mammalian ste-20 like kinase Mst1 plays important roles during apoptosis, proliferation, cell polarity, and migration. Here, we report a novel role of Mst1 for cytotoxic T-cell responses and tumor suppression. The defect of Mst1 caused decreased levels of FoxO, and promoted cytotoxicity in vitro. Mst1(-/-) cytotoxic T cells also exhibited enhanced T-bet expression that was associated with elevated expression levels of IFNγ and granzyme B. Moreover, Mst1(-/-) cytotoxic T cells suppressed tumor growth in vivo. The data suggest that Mst1 inhibits cytotoxicity via T-bet suppression by FoxO1 and FoxO3a. Thus, Mst1 is a potential therapeutic target for tumor immunotherapy.

TCR Signaling in T Cell Memory

T cell memory plays a critical role in our protection against pathogens and tumors. The antigen and its interaction with the T cell receptor (TCR) is one of the initiating elements that shape T cell memory together with inflammation and costimulation. Over the last decade, several transcription factors and signaling pathways that support memory programing have been identified. However, how TCR signals regulate them is still poorly understood. Recent studies have shown that the biochemical rules that govern T cell memory, strikingly, change depending on the TCR signal strength. Furthermore, TCR signal strength regulates the input of cytokine signaling, including pro-inflammatory cytokines. These highlight how tailoring antigenic signals can improve immune therapeutics. In this review, we focus on how TCR signaling regulates T cell memory and how the quantity and quality of TCR–peptide–MHC interactions impact the multiple fates a T cell can adopt in the memory pool.

Asymmetric PI3K Signaling Driving Developmental and Regenerative Cell Fate Bifurcation

Metazoan sibling cells often diverge in activity and identity, suggesting links between growth signals and cell fate. We show that unequal transduction of nutrient-sensitive PI3K/AKT/mTOR signaling during cell division bifurcates transcriptional networks and fates of kindred cells. A sibling B lymphocyte with stronger signaling, indexed by FoxO1 inactivation and IRF4 induction, undergoes PI3K-driven Pax5 repression and plasma cell determination, while its sibling with weaker PI3K activity renews a memory or germinal center B cell fate. PI3K-driven effector T cell determination silences TCF1 in one sibling cell, while its PI3K-attenuated sibling self-renews in tandem. Prior to bifurcations achieving irreversible plasma or effector cell fate determination, asymmetric signaling during initial divisions specifies a more proliferative, differentiation-prone lymphocyte in tandem with a more quiescent memory cell sibling. By triggering cell division but transmitting unequal intensity between sibling cells, nutrient-sensitive signaling may be a frequent arbiter of cell fate bifurcations during development and repair.

IL-2 and IL-15 regulate CD8+ memory T-cell differentiation but are dispensable for protective recall responses

The ability to mount effective secondary responses is a cardinal feature of memory CD8(+) T cells. An understanding of the factors that regulate the generation and recall capacities of memory T cells remains to be ascertained. Several cues indicate that two highly related cytokines, IL-2 and IL-15, share redundant functions in this process. To establish their combined roles in memory CD8(+) T-cell development, maintenance, and secondary responses, we compared the outcome of adoptively transferred IL2Rβ(+/-) or IL2Rβ(-/-) CD8(+) T cells after an acute viral infection in mice. Our results demonstrate that both IL-2 and IL-15 signals condition the differentiation of primary and secondary short-lived effector cells by altering the transcriptional network governing lineage choices. These two cytokines also regulate the homeostasis of the memory T-cell pool, with effector memory CD8(+) T cells being the most sensitive to these two interleukins. Noticeably, the inability to respond to both cytokines limits the proliferation and survival of primary and secondary effectors cells, whereas it does not preclude potent cytotoxic functions and viral control either initially or upon rechallenge. Globally, these results indicate that lack of IL-2 and IL-15 signaling modulates the CD8(+) T-cell differentiation program but does not impede adequate effector functions.

CD73-mediated adenosine production promotes stem cell-like properties in mouse Tc17 cells

The CD73 ectonucleotidase catalyses the hydrolysis of AMP to adenosine, an immunosuppressive molecule. Recent evidence has demonstrated that this ectonucleotidase is up-regulated in T helper type 17 cells when generated in the presence of transforming growth factor-β (TGF-β), and hence CD73 expression is related to the acquisition of immunosuppressive potential by these cells. TGF-β is also able to induce CD73 expression in CD8(+) T cells but the function of this ectonucleotidase in CD8(+) T cells is still unknown. Here, we show that Tc17 cells present high levels of the CD73 ectonucleotidase and produce adenosine; however, they do not suppress the proliferation of CD4(+) T cells. Interestingly, we report that adenosine signalling through A2A receptor favours interleukin-17 production and the expression of stem cell-associated transcription factors such as tcf-7 and lef-1 but restrains the acquisition of Tc1-related effector molecules such as interferon-γ and Granzyme B by Tc17 cells. Within the tumour microenvironment, CD73 is highly expressed in CD62L(+) CD127(+) CD8(+) T cells (memory T cells) and is down-regulated in GZMB(+) KLRG1(+) CD8(+) T cells (terminally differentiated T cells), demonstrating that CD73 is expressed in memory/naive cells and is down-regulated during differentiation. These data reveal a novel function of CD73 ectonucleotidase in arresting CD8(+) T-cell differentiation and support the idea that CD73-driven adenosine production by Tc17 cells may promote stem cell-like properties in Tc17 cells.

CD8+ T cell exhaustion, suppressed gamma interferon production, and delayed memory response induced by chronic Brucella melitensis infection

Brucella melitensis is a well-adapted zoonotic pathogen considered a scourge of mankind since recorded history. In some cases, initial infection leads to chronic and reactivating brucellosis, incurring significant morbidity and economic loss. The mechanism by which B. melitensis subverts adaptive immunological memory is poorly understood. Previous work has shown that Brucella-specific CD8(+) T cells express gamma interferon (IFN-γ) and can transition to long-lived memory cells but are not polyfunctional. In this study, chronic infection of mice with B. melitensis led to CD8(+) T cell exhaustion, manifested by programmed cell death 1 (PD-1) and lymphocyte activation gene 3 (LAG-3) expression and a lack of IFN-γ production. The B. melitensis-specific CD8(+) T cells that produced IFN-γ expressed less IFN-γ per cell than did CD8(+) cells from uninfected mice. Both memory precursor (CD8(+) LFA1(HI) CD127(HI) KLRG1(LO)) and long-lived memory (CD8(+) CD27(HI) CD127(HI) KLRG1(LO)) cells were identified during chronic infection. Interestingly, after adoptive transfer, mice receiving cells from chronically infected animals were able to contain infection more rapidly than recipients of cells from acutely infected or uninfected donors, although the proportions of exhausted CD8(+) T cells increased after adoptive transfer in both challenged and unchallenged recipients. CD8(+) T cells of challenged recipients initially retained the stunted IFN-γ production found prior to transfer, and cells from acutely infected mice were never seen to transition to either memory subset at all time points tested, up to 30 days post-primary infection, suggesting a delay in the generation of memory. Here we have identified defects in Brucella-responsive CD8(+) T cells that allow chronic persistence of infection.

Intrinsic MyD88-Akt1-mTOR Signaling Coordinates Disparate Tc17 and Tc1 Responses during Vaccine Immunity against Fungal Pneumonia

Fungal infections have skyrocketed in immune-compromised patients lacking CD4⁺ T cells, underscoring the need for vaccine prevention. An understanding of the elements that promote vaccine immunity in this setting is essential. We previously demonstrated that vaccine-induced IL-17A⁺ CD8⁺ T cells (Tc17) are required for resistance against lethal fungal pneumonia in CD4⁺ T cell-deficient hosts, whereas the individual type I cytokines IFN-γ, TNF-α and GM-CSF, are dispensable. Here, we report that T cell-intrinsic MyD88 signals are crucial for these Tc17 cell responses and vaccine immunity against lethal fungal pneumonia in mice. In contrast, IFN-γ⁺ CD8⁺ cell (Tc1) responses are largely normal in the absence of intrinsic MyD88 signaling in CD8⁺ T cells. The poor accumulation of MyD88-deficient Tc17 cells was not linked to an early onset of contraction, nor to accelerated cell death or diminished expression of anti-apoptotic molecules Bcl-2 or Bcl-xL. Instead, intrinsic MyD88 was required to sustain the proliferation of Tc17 cells through the activation of mTOR via Akt1. Moreover, intrinsic IL-1R and TLR2, but not IL-18R, were required for MyD88 dependent Tc17 responses. Our data identify unappreciated targets for augmenting adaptive immunity against fungi. Our findings have implications for designing fungal vaccines and immune-based therapies in immune-compromised patients.

Patients with AIDS, cancer or immune suppressive treatments are vulnerable to infection with invasive fungi. We have found that even when helper CD4 T cells are profoundly reduced in a mouse model that mimics this defect in AIDS, other remaining T cells are capable of mounting vaccine immunity against a deadly fungal infection, and they do so by producing the powerful, soluble product, IL-17. It has been widely believed that the activation and instruction of such cells, called Tc17 cells, is governed by another population of immune cells in the body, but we have found here that pathways within these Tc17 cells themselves mediate their activation and ability to produce the IL-17 needed for resistance to infection. We have also identified elements of the circuitry controlling this pathway—elements called MyD88, Akt1 and mTOR—and found that they control the production of IL-17 and not other products such as IFN-γ often produced by these cells. Further, we determined that this circuitry controls the development of Tc17 cells by regulating their ability to divide and expand. Thus, in a mouse model of vaccination against lethal fungal pneumonia caused by Blastomyces dermatitidis, we uncovered an important cellular arsenal that can be recruited to bolster resistance against a fungal infection, and identified novel ways in which the cells develop and expand into potent killers of fungi.

Development and Function of Protective and Pathologic Memory CD4 T Cells

Immunological memory is one of the defining features of the adaptive immune system. As key orchestrators and mediators of immunity, CD4 T cells are central to the vast majority of adaptive immune responses. Generated following an immune response, memory CD4 T cells retain pertinent information about their activation environment enabling them to make rapid effector responses upon reactivation. These responses can either benefit the host by hastening the control of pathogens or cause damaging immunopathology. Here, we will discuss the diversity of the memory CD4 T cell pool, the signals that influence the transition of activated T cells into that pool, and highlight how activation requirements differ between naïve and memory CD4 T cells. A greater understanding of these factors has the potential to aid the design of more effective vaccines and to improve regulation of pathologic CD4 T cells, such as in the context of autoimmunity and allergy.

Over-expressing Akt in T cells to resist tumor immunosuppression and increase anti-tumor activity

BACKGROUND

Tumor employs various means to escape immunosurveillance and inhibit immune attack, and strategies have been developed to counteract the inhibitory signals. However, due to the complex suppressive mechanisms in the tumor microenvironment, blocking one or a few inhibitory signals has only limited effects on therapeutic efficacy. Instead of targeting tumor immunosuppression, we considered from another point of view, and hypothesized that manipulating T cells to make them resist any known or unknown suppressive mechanism may be more effective for cancer treatment.

METHODS

We used OT-1 cells transduced with retroviruses encoding Akt and human peripheral blood lymphocytes (PBLs) transduced with retroviruses encoding both Akt and a chimeric antigen receptor (CAR) specific for tumor antigen EpCAM to examine the effect of over-expressing Akt on tumor specific T cells in tumor environment.

RESULTS

We show that Akt activity of T cells in the tumor environment was inhibited, and over-expressing Akt in OT-1 cells increased the cytokine production and cell proliferation in the presence of B16-OVA tumor cells. What's more, adoptive transfer of OT-1 cells over-expressing Akt inhibited B16-OVA tumor growth and prolonged mouse survival. To examine if over-expressing Akt could increase the anti-tumor activity of T cells in human cancer, PBLs co-expressing EpCAM specific CAR and Akt were cultured with EpCAM-expressing human prostate cancer cells PC3M, and less inhibition on cell proliferation and less apoptosis were observed. In addition, adoptive transfer of PC3M specific T cells over-expressing Akt resulted in more dramatic tumor inhibitory effects in PC3M bearing NOD/SCID mice.

CONCLUSIONS

These data indicates that over-expressing Akt in tumor specific T cells increases T cell proliferation and activity in the tumor environment, and enhances anti-tumor effects of adoptively transferred T cells. Our study provides a new strategy to improve the efficacy of adoptive T cell therapy, and serves as an important foundation for clinical translation.

CD73 and CD39 ectonucleotidases in T cell differentiation: Beyond immunosuppression

Extracellular ATP is a danger signal released by dying and damaged cells, and it functions as an immunostimulatory signal that promotes inflammation. However, extracellular adenosine acts as an immunoregulatory signal that modulates the function of several cellular components of the adaptive and innate immune response. Consequently, the balance between ATP and adenosine concentration is crucial in immune homeostasis. CD39 and CD73 are two ectonucleotidases that cooperate in the generation of extracellular adenosine through ATP hydrolysis, thus tilting the balance towards immunosuppressive microenvironments. Extracellular adenosine can prevent activation, proliferation, cytokine production and cytotoxicity in T cells through the stimulation of the A2A receptor; however, recent evidence has shown that adenosine may also affect other processes in T-cell biology. In this review, we discuss evidence that supports a role of CD73 and CD39 ectonucleotidases in controlling naive T-cell homeostasis and memory cell survival through adenosine production. Finally, we propose a novel hypothesis of a possible role of these ectonucleotidases and autocrine adenosine signaling in controlling T-cell differentiation.

Persistent Antigen and Prolonged AKT-mTORC1 Activation Underlie Memory CD8 T Cell Impairment in the Absence of CD4 T Cells

Recall responses by memory CD8 T cells are impaired in the absence of CD4 T cells. Although several mechanisms have been proposed, the molecular basis is still largely unknown. Using a local influenza virus infection in the respiratory tract and the lung of CD4(-/-) mice, we show that memory CD8 T cell impairment is limited to the lungs and the lung-draining lymph nodes, where viral Ags are unusually persistent and abundant in these mice. Persistent Ag exposure results in prolonged activation of the AKT-mTORC1 pathway in Ag-specific CD8 T cells, favoring their development into effector memory T cells at the expense of central memory T cells, and inhibition of mTORC1 by rapamycin largely corrects the impairment by promoting central memory T cell development. The findings suggest that the prolonged AKT-mTORC1 activation driven by persistent Ag is a critical mechanism underlying the impaired memory CD8 T cell development and responses in the absence of CD4 T cells.

The Notch signaling pathway controls short-lived effector CD8+ T cell differentiation but is dispensable for memory generation

Following an infection, naive CD8(+) T cells expand and differentiate into two main populations of effectors: short-lived effector cells (SLECs) and memory precursor effector cells (MPECs). There is limited understanding of the molecular mechanism and cellular processes governing this cell fate. Notch is a key regulator of cell fate decision relevant in many immunological pathways. In this study, we add to the role of Notch in cell fate decision and demonstrate that the Notch signaling pathway controls the MPEC/SLEC differentiation choice following both Listeria infection and dendritic cell immunization of mice. Although fewer SLECs were generated, Notch deficiency did not alter the rate of memory CD8(+) T cell generation. Moreover, we reveal that the Notch signaling pathway plays a context-dependent role for optimal cytokine production by effector CD8(+) T cells. Together, our results unravel critical functions for the Notch signaling pathway during effector CD8(+) T cell differentiation.

The interface between transcriptional and epigenetic control of effector and memory CD8⁺ T-cell differentiation

Immunity to many intracellular pathogens requires the proliferation, differentiation, and function of CD8(+) cytotoxic T lymphocytes (CTLs). While the majority of effector CTLs die upon clearance of the pathogen, a small proportion of them survive to become long-lived memory CTLs. Memory CTLs can provide protective immunity against re-exposure to the same pathogen and are the principle motivation behind T-cell- based vaccine design. While a large body of cellular immunologic research has proven invaluable to define effector and memory CTLs by their different phenotypes and functions, an emerging focus in the field has been to understand how environmental cues regulate CTL differentiation on a genomic level. Genome-wide studies to profile transcriptional and epigenetic changes during infection have revealed that dynamic changes in DNA methylation patterns and histone modifications accompany transcriptional signatures that define and regulate CTL differentiation states. In this review, we emphasize the importance of epigenetic regulation of CD8(+) T-cell differentiation and the likely role that transcription factors play in this process.

CD8+ T Cell Fate and Function Influenced by Antigen-Specific Virus-Like Nanoparticles Co-Expressing Membrane Tethered IL-2

A variety of adjuvants fostering humoral immunity are known as of today. However, there is a lack of adjuvants or adjuvant strategies, which directly target T cellular effector functions and memory. We here determined whether systemically toxic cytokines such as IL-2 can be restricted to the site of antigen presentation and used as ‘natural adjuvants’. Therefore, we devised antigen-presenting virus-like nanoparticles (VNP) co-expressing IL-2 attached to different membrane-anchors and assessed their potency to modulate CD8⁺ T cell responses in vitro and in vivo. Efficient targeting of IL-2 to lipid rafts and ultimately VNP was achieved by fusing IL-2 at its C-terminus to a minimal glycosylphosphatidylinositol (GPI)-anchor acceptor sequence. To identify optimal membrane-anchor dimensions we inserted one (1Ig), two (2Ig) or four (4Ig) immunoglobulin(Ig)-like domains of CD16b between IL-2 and the minimal GPI-anchor acceptor sequence of CD16b (GPI). We found that the 2IgGPI version was superior to all other evaluated IL-2 variants (IL-2v) in terms of its i) degree of targeting to lipid rafts and to the VNP surface, ii) biological activity, iii) co-stimulation of cognate T cells in the absence of bystander activation and iv) potency to induce differentiation and acquisition of CD8⁺ T cell effector functions in vitro and in vivo. In contrast, the GPI version rather favored memory precursor cell formation. These results exemplify novel beneficial features of membrane-bound IL-2, which in addition to its mere T cell stimulatory capacity include the induction of differential effector and memory functions in CD8⁺ T lymphocytes.

Lenalidomide Polarizes Th1-specific Anti-tumor Immune Response and Expands XBP1 Antigen-Specific Central Memory CD3+CD8+ T cells against Various Solid Tumors

Introduction

Effective combination immunotherapeutic strategies may be required to enhance effector cells’ anti-tumor activities and improve clinical outcomes.

Methods

XBP1 antigen-specific cytotoxic T lymphocytes (XBP1-CTL) generated using immunogenic heteroclitic XBP1 US184-192 (YISPWILAV) and XBP1 SP367-375 (YLFPQLISV) peptides or various solid tumor cells over-expressing XBP1 target antigen were evaluated, either alone or in combination with lenalidomide, for phenotype and immune functional activity.

Results

Lenalidomide treatment of XBP1-CTL increased the proportion of CD45RO⁺ memory CD3+CD8+ T cells, but not the total CD3⁺CD8⁺ T cells. Lenalidomide upregulated critical T cell activation markers and costimulatory molecules (CD28, CD38, CD40L, CD69, ICOS), especially within the central memory CTL subset of XBP1-CTL, while decreasing TCRαβ and T cell checkpoint blockade (CTLA-4, PD-1). Lenalidomide increased the anti-tumor activities of XBP1-CTL memory subsets, which were associated with expression of Th1 transcriptional regulators (T-bet, Eomes) and Akt activation, thereby resulting in enhanced IFN-γ production, granzyme B upregulation and specific CD28/CD38-positive and CTLA-4/PD-1-negative cell proliferation.

Conclusions

These studies suggest the potential benefit of lenalidomide treatment to boost anti-tumor activities of XBP1-specific CTL against a variety of solid tumors and enhance response to an XBP1-directing cancer vaccine regime.

Survival of effector CD8+ T cells during influenza infection is dependent on autophagy

The activation and expansion of effector CD8(+) T cells are essential for controlling viral infections and tumor surveillance. During an immune response, T cells encounter extrinsic and intrinsic factors, including oxidative stress, nutrient availability, and inflammation, that can modulate their capacity to activate, proliferate, and survive. The dependency of T cells on autophagy for in vitro and in vivo activation, expansion, and memory remains unclear. Moreover, the specific signals and mechanisms that activate autophagy in T effector cells and their survival are not known. In this study, we generated a novel inducible autophagy knockout mouse to study T cell effector responses during the course of a virus infection. In response to influenza infection, Atg5(-/-) CD8(+) T cells had a decreased capacity to reach the peak effector response and were unable to maintain cell viability during the effector phase. As a consequence of Atg5 deletion and the impairment in effector-to-memory cell survival, mice fail to mount a memory response following a secondary challenge. We found that Atg5(-/-) effector CD8(+) T cells upregulated p53, a transcriptional state that was concomitant with widespread hypoxia in lymphoid tissues of infected mice. The onset of p53 activation was concurrent with higher levels of reactive oxygen species (ROS) that resulted in ROS-dependent apoptotic cell death, a fate that could be rescued by treating with the ROS scavenger N-acetylcysteine. Collectively, these results demonstrate that effector CD8(+) T cells require autophagy to suppress cell death and maintain survival in response to a viral infection.

Lipid and Protein Co-Regulation of PI3K Effectors Akt and Itk in Lymphocytes

The phosphoinositide 3-kinase (PI 3-kinase, PI3K) pathway transduces signals critical for lymphocyte function. PI3K generates the phospholipid PIP₃ at the plasma membrane to recruit proteins that contain pleckstrin homology (PH) domains – a conserved domain found in hundreds of mammalian proteins. PH domain–PIP₃ interactions allow for rapid signal propagation and confer a spatial component to these signals. The kinases Akt and Itk are key PI3K effectors that bind PIP₃ via their PH domains and mediate vital processes – such as survival, activation, and differentiation – in lymphocytes. Here, we review the roles and regulation of PI3K signaling in lymphocytes with a specific emphasis on Akt and Itk. We also discuss these and other PH domain-containing proteins as they relate more broadly to immune cell function. Finally, we highlight the emerging view of PH domains as multifunctional protein domains that often bind both lipid and protein substrates to exert their effects.

Akt1 and -2 inhibition diminishes terminal differentiation and enhances central memory CD8+ T-cell proliferation and survival

The CD8⁺ T-cell response comprises terminally differentiated effector cells and antigen-experienced memory T cells. The latter encompass central (T_CM) and effector (T_EM) memory cells. T_CM cells are superior in their protection against viral and bacterial challenges and mediation of antitumor immunity due to their higher proliferative ability upon antigen re-encounter. Defining a mechanism to enhance T_CM cells and delay terminal differentiation of CD8⁺ T cells is crucial for cancer immune therapy, as it can promote a better tumor immune response. The differentiation of CD8⁺ memory T cells is thought to be coordinated by the phosphoinositide 3-kinase (PI3K)/Akt pathway. We, therefore, investigated the role of Akt isoforms in the differentiation and proliferation of memory CD8⁺ T cells. We found that Akt1 and Akt2, but not Akt3, drive the terminal differentiation of CD8⁺ T cells, and their inhibition enhances the therapeutically superior T_CM phenotype. Furthermore, the inhibition of Akt1 and Akt2, but not Akt 3, delays CD8⁺ T-cell exhaustion and preserves naïve and T_CM CD8⁺ T cells, thus enhancing their proliferative ability and survival and prolonging their cytokine and Granzyme B production ability. Here, we define a mechanism in which proliferative potential, function, and survival of CD8⁺ T cells are enhanced by maintaining a reservoir of T_CM and naïve cells using only Akt1 and Akt2 inhibition. Therefore, our findings strongly suggest the utility of using Akt1 and Akt2 inhibitors to modulate CD8⁺ T cells, both for adoptive cell transfer and vaccine-based cancer immune therapies.

Akt inhibition enhances expansion of potent tumor-specific lymphocytes with memory cell characteristics

Adoptive cell therapy (ACT) using autologous tumor-infiltrating lymphocytes (TIL) results in complete regression of advanced cancer in some patients, but the efficacy of this potentially curative therapy may be limited by poor persistence of TIL after adoptive transfer. Pharmacologic inhibition of the serine/threonine kinase Akt has recently been shown to promote immunologic memory in virus-specific murine models, but whether this approach enhances features of memory (e.g., long-term persistence) in TIL that are characteristically exhausted and senescent is not established. Here, we show that pharmacologic inhibition of Akt enables expansion of TIL with the transcriptional, metabolic, and functional properties characteristic of memory T cells. Consequently, Akt inhibition results in enhanced persistence of TIL after adoptive transfer into an immunodeficient animal model and augments antitumor immunity of CD8 T cells in a mouse model of cell-based immunotherapy. Pharmacologic inhibition of Akt represents a novel immunometabolomic approach to enhance the persistence of antitumor T cells and improve the efficacy of cell-based immunotherapy for metastatic cancer.

The transcription factor FoxO1 sustains expression of the inhibitory receptor PD-1 and survival of antiviral CD8(+) T cells during chronic infection

Protein kinase B (also known as AKT) and the mechanistic target of rapamycin (mTOR) are central regulators of T cell differentiation, proliferation, metabolism, and survival. Here, we show that during chronic murine lymphocytic choriomeningitis virus infection, activation of AKT and mTOR are impaired in antiviral cytotoxic T lymphocytes (CTLs), resulting in enhanced activity of the transcription factor FoxO1. Blockade of inhibitory receptor programmed cell death protein 1 (PD-1) in vivo increased mTOR activity in virus-specific CTLs, and its therapeutic effects were abrogated by the mTOR inhibitor rapamycin. FoxO1 functioned as a transcriptional activator of PD-1 that promoted the differentiation of terminally exhausted CTLs. Importantly, FoxO1-null CTLs failed to persist and control chronic viral infection. Collectively, this study shows that CTLs adapt to persistent infection through a positive feedback pathway (PD-1?FoxO1?PD-1) that functions to both desensitize virus-specific CTLs to antigen and support their survival during chronic viral infection.

Autophagy is a critical regulator of memory CD8(+) T cell formation

During infection, CD8⁺ T cells initially expand then contract, leaving a small memory pool providing long lasting immunity. While it has been described that CD8⁺ T cell memory formation becomes defective in old age, the cellular mechanism is largely unknown. Autophagy is a major cellular lysosomal degradation pathway of bulk material, and levels are known to fall with age. In this study, we describe a novel role for autophagy in CD8⁺ T cell memory formation. Mice lacking the autophagy gene Atg7 in T cells failed to establish CD8⁺ T cell memory to influenza and MCMV infection. Interestingly, autophagy levels were diminished in CD8⁺ T cells from aged mice. We could rejuvenate CD8⁺ T cell responses in elderly mice in an autophagy dependent manner using the compound spermidine. This study reveals a cell intrinsic explanation for poor CD8⁺ T cell memory in the elderly and potentially offers novel immune modulators to improve aged immunity.

DOI:http://dx.doi.org/10.7554/eLife.03706.001

In the face of an infection, the immune system mounts an aggressive response by producing many copies of killer immune cells called CD8⁺ T cells that recognize and destroy any cells infected with the invading pathogen. The number of killer cells produced depends on the extent of the infection. Once the infection has been brought under control, most of the CD8⁺ T cells die off. The small numbers that are retained—called memory cells—‘remember’ the pathogen, so that if it invades the body again, they can help the immune system to respond more quickly and effectively.

Memory cells are also critical to the effectiveness of vaccines, many of which introduce a dead or weakened pathogen into the body. This does not cause an infection, but does allow the immune system to create memory cells that are able to fend off the same pathogen in the future. However, vaccines only work in individuals that are able to produce and maintain memory cells, which many older people are less able to do.

An important system that maintains cells, called autophagy, destroys and removes the ‘junk’ and toxic by products that all cells accumulate over time as a result of normal cell functions. Without autophagy, cells become less able to produce energy and they may die. Puleston et al. show that autophagy begins to fail in old mice, which prevents the formation of a proper memory response. In addition, mice that lack an important gene needed for autophagy are unable to produce memory cells after being infected with viruses such as influenza.

Puleston et al. found that boosting autophagy in older mice using a chemical called spermidine—which is also found naturally in many tissues—helped to restore the mice's ability to create and maintain memory cells. Spermidine-treated mice developed a stronger immunity to influenza after vaccination compared with other mice of a similar age. Further research is required to better understand how spermidine works to see if it could be developed into a drug that safely boosts the immune system of humans.

DOI:http://dx.doi.org/10.7554/eLife.03706.002

A central role for Notch in effector CD8(+) T cell differentiation

Activated CD8⁺ T cells choose between terminal effector cell (TEC) or memory precursor cell (MPC) fates. We show that Notch controls this choice. Notch promoted differentiation of immediately protective TECs and was correspondingly required for clearance of an acute influenza virus infection. Notch activated a major portion of the TEC-specific gene expression program and suppressed the MPC-specific program. Expression of Notch receptors was induced on naïve CD8⁺ T cells by inflammatory mediators and interleukin 2 (IL-2) via mTOR and T-bet dependent pathways. These pathways were subsequently amplified downstream of Notch, creating a positive feedback loop. Notch thus functions as a central hub where information from different sources converges to match effector T cell differentiation to the demands of the infection.