Sema Kurtulus

Interleukin 7 signaling in dendritic cells regulates the homeostatic proliferation and niche size of CD4 + T cells

Interleukin 7 (IL-7) and T cell antigen receptor signals have been proposed to be the main drivers of homeostatic T cell proliferation. However, it is not known why CD4+ T cells undergo less-efficient homeostatic proliferation than CD8+ T cells do. Here we show that systemic IL-7 concentrations increased during lymphopenia because of diminished use of IL-7 but that IL-7 signaling on IL-7 receptor-α–positive (IL-7Rα+) dendritic cells (DCs) in lymphopenic settings paradoxically diminished the homeostatic proliferation of CD4+ T cells. This effect was mediated at least in part by IL-7-mediated downregulation of the expression of major histocompatibility complex class II on IL-7Rα+ DCs. Our results indicate that IL-7Rα+ DCs are regulators of the peripheral CD4+ T cell niche and that IL-7 signals in DCs prevent uncontrolled CD4+ T cell population expansion in vivo.Interleukin 7 (IL-7) and T cell receptor (TCR) signals have been proposed to be the primary drivers of homeostatic T cell proliferation. However, it is not known why CD4+ T cells undergo less efficient homeostatic proliferation than CD8+ T cells. Here we showed that systemic IL-7 concentrations rise during lymphopenia due to diminished IL-7 utilization, but that IL-7 signaling on IL-7Rα+ dendritic cells (DCs) in lymphopenic settings paradoxically diminishes CD4+ T cell homeostatic proliferation. This effect is mediated, at least in part, by IL-7-mediated downregulation of MHC class II expression on IL-7Rα+ DCs. These results implicate IL-7Rα+ DCs as regulators of the peripheral CD4+ T cell niche, and indicate that IL-7 signals in DCs prevent uncontrolled CD4+ T cell expansion in vivo.

TIGIT predominantly regulates the immune response via regulatory T cells.

Coinhibitory receptors are critical for the maintenance of immune homeostasis. Upregulation of these receptors on effector T cells terminates T cell responses, while their expression on Tregs promotes their suppressor function. Understanding the function of coinhibitory receptors in effector T cells and Tregs is crucial, as therapies that target coinhibitory receptors are currently at the forefront of treatment strategies for cancer and other chronic diseases. T cell Ig and ITIM domain (TIGIT) is a recently identified coinhibitory receptor that is found on the surface of a variety of lymphoid cells, and its role in immune regulation is just beginning to be elucidated. We examined TIGIT-mediated immune regulation in different murine cancer models and determined that TIGIT marks the most dysfunctional subset of CD8+ T cells in tumor tissue as well as tumor-tissue Tregs with a highly active and suppressive phenotype. We demonstrated that TIGIT signaling in Tregs directs their phenotype and that TIGIT primarily suppresses antitumor immunity via Tregs and not CD8+ T cells. Moreover, TIGIT+ Tregs upregulated expression of the coinhibitory receptor TIM-3 in tumor tissue, and TIM-3 and TIGIT synergized to suppress antitumor immune responses. Our findings provide mechanistic insight into how TIGIT regulates immune responses in chronic disease settings.

STAT5 is critical to maintain effector CD8+ T cell responses.

During an immune response, most effector T cells die, whereas some are maintained and become memory T cells. Factors controlling the survival of effector CD4+ and CD8+ T cells remain unclear. In this study, we assessed the role of IL-7, IL-15, and their common signal transducer, STAT5, in maintaining effector CD4+ and CD8+ T cell responses. Following viral infection, IL-15 was required to maintain a subpopulation of effector CD8+ T cells expressing high levels of killer cell lectin-like receptor subfamily G, member 1 (KLRG1), and lower levels of CD127, whereas IL-7 and IL-15 acted together to maintain KLRG1lowCD127high CD8+ effector T cells. In contrast, effector CD4+ T cell numbers were not affected by the individual or combined loss of IL-15 and IL-7. Both IL-7 and IL-15 drove phosphorylation of STAT5 within effector CD4+ and CD8+ T cells. When STAT5 was deleted during the course of infection, both KLRG1highCD127low and KLRG1lowCD127high CD8+ T cells were lost, although effector CD4+ T cell populations were maintained. Furthermore, STAT5 was required to maintain expression of Bcl-2 in effector CD8+, but not CD4+, T cells. Finally, IL-7 and IL-15 required STAT5 to induce Bcl-2 expression and to maintain effector CD8+ T cells. Together, these data demonstrate that IL-7 and IL-15 signaling converge on STAT5 to maintain effector CD8+ T cell responses.

Conditional deletion of cytokine receptor chains reveals that IL-7 and IL-15 specify CD8 cytotoxic lineage fate in the thymus

Two cytokines that signal through the common gamma chain receptor, IL-7 and IL-15, are responsible for the differentiation of developing thymocytes into CD8+ cytotoxic lineage T cells in mice.

BCL-2 allows effector and memory CD8+ T cells to tolerate higher expression of BIM

As acute infections resolve, most effector CD8+ T cells die, whereas some persist and become memory T cells. Recent work showed that subsets of effector CD8+ T cells, identified by reciprocal expression of killer cell lectin-like receptor G1 (KLRG1) and CD127, have different lifespans. Similar to previous reports, we found that effector CD8+ T cells reported to have a longer lifespan (i.e., KLRG1lowCD127high) have increased levels of Bcl-2 compared with their shorter-lived KLRG1highCD127low counterparts. Surprisingly, we found that these effector KLRG1lowCD127high CD8+ T cells also had increased levels of Bim compared with KLRG1highCD127low cells. Similar effects were observed in memory cells, in which CD8+ central memory T cells expressed higher levels of Bim and Bcl-2 than did CD8+ effector memory T cells. Using both pharmacologic and genetic approaches, we found that survival of both subsets of effector and memory CD8+ T cells required Bcl-2 to combat the proapoptotic activity of Bim. Interestingly, inhibition or absence of Bcl-2 led to significantly decreased expression of Bim in surviving effector and memory T cells. In addition, manipulation of Bcl-2 levels by IL-7 or IL-15 also affected expression of Bim in effector CD8+ T cells. Finally, we found that Bim levels were significantly increased in effector CD8+ T cells lacking Bax and Bak. Together, these data indicate that cells having the highest levels of Bim are selected against during contraction of the response and that Bcl-2 determines the level of Bim that effector and memory T cells can tolerate.

TIGIT and CD96: new checkpoint receptor targets for cancer immunotherapy

While therapies targeting the co‐inhibitory or immune checkpoint receptors PD‐1 and CTLA‐4 have shown remarkable success in many cancers, not all patients benefit from these therapies. This has catalyzed enormous interest in the targeting of other immune checkpoint receptors. In this regard, TIGIT and CD96 have recently entered the limelight as novel immune checkpoint receptor targets. TIGIT and CD96 together with the co‐stimulatory receptor CD226 form a pathway that is analogous to the CD28/CTLA‐4 pathway, in which shared ligands and differential receptor:ligand affinities fine‐tune the immune response. Although the roles of TIGIT and CD96 as immune checkpoint receptors in T cell and natural killer cell biology are just beginning to be uncovered, accumulating data support the targeting of these receptors for improving anti‐tumor immune responses. A clear understanding of the immune cell populations regulated by TIGIT and CD96 is key to the design of immunotherapies that target these receptors in combination with other existing immune checkpoint blockade therapies.

Protecting and rescuing the effectors: roles of differentiation and survival in the control of memory T cell development

Vaccines, arguably the single most important intervention in improving human health, have exploited the phenomenon of immunological memory. The elicitation of memory T cells is often an essential part of successful long-lived protective immunity. Our understanding of T cell memory has been greatly aided by the development of TCR Tg mice and MHC tetrameric staining reagents that have allowed the precise tracking of antigen-specific T cell responses. Indeed, following acute infection or immunization, naïve T cells undergo a massive expansion culminating in the generation of a robust effector T cell population. This peak effector response is relatively short-lived and, while most effector T cells die by apoptosis, some remain and develop into memory cells. Although the molecular mechanisms underlying this cell fate decision remain incompletely defined, substantial progress has been made, particularly with regards to CD8+ T cells. For example, the effector CD8+ T cells generated during a response are heterogeneous, consisting of cells with more or less potential to develop into full-fledged memory cells. Development of CD8+ T cell memory is regulated by the transcriptional programs that control the differentiation and survival of effector T cells. While the type of antigenic stimulation and level of inflammation control effector CD8+ T cell differentiation, availability of cytokines and their ability to control expression and function of Bcl-2 family members governs their survival. These distinct differentiation and survival programs may allow for finer therapeutic intervention to control both the quality and quantity of CD8+ T cell memory. Effector to memory transition of CD4+ T cells is less well characterized than CD8+ T cells, emerging details will be discussed. This review will focus on the recent progress made in our understanding of the mechanisms underlying the development of T cell memory with an emphasis on factors controlling survival of effector T cells.

CONTRACTING THE ‘MUS CELLS’ – DOES DOWN-SIZING SUIT US FOR DIVING INTO THE MEMORY POOL ?

Summary:  Maintenance of T‐cell homeostasis is critical for normal functioning of the immune system. After thymocyte selection, T cells enter the peripheral lymphoid organs, where they are maintained as naive cells. Transient disruption of homeostasis occurs when naive T cells undergo antigen‐driven expansion and acquire effector functions. Effector T cells then either undergo apoptosis (i.e. contraction at the population level) or survive to become memory cells. This apoptotic process is crucial: it resets T‐cell homeostasis, promotes protective immunity, and limits autoimmunity. Although initial studies using in vitro models supported a role for death receptor signaling, more recent in vivo studies have implicated Bcl‐2 family members as being critical for the culling of T‐cell responses. While several Bcl‐2 family members likely contribute to T‐cell contraction, the pro‐apoptotic molecule Bim and its anti‐apoptotic antagonist Bcl‐2 are essential regulators of the process. This review discusses the progress made in our understanding of the mechanisms underlying contraction of T‐cell responses and how some cells avoid this cell death and become memory T cells.

Mechanisms of TIGIT-driven immune suppression in cancer

TIGIT is a co-inhibitory molecule that limits T cell proliferation and activation. TIGIT expression has been recently shown to identify a subset of regulatory T cells (Treg) that specifically suppresses Th1 and Th17 responses; however its role in tumor immunity has not been examined. Here, we determined whether TIGIT has a role in the suppression of anti-tumor immune responses. We found that TIGIT is highly up-regulated on Treg and CD8+ tumor-infiltrating lymphocytes (TILs) in multiple pre-clinical cancer models. Importantly, TIGIT expression is strongly associated with expression of other co-inhibitory molecules; PD-1, Tim-3 and Lag-3 and with production of IL-10 in Treg and CD8+ TILs. Moreover, TIGIT+ CD8+ TILs display an exhausted phenotype determined by decreased production of IL-2 and TNF-α. To understand whether TIGIT acts as a checkpoint in anti-tumor response, we monitored growth of implanted B16 melanoma in TIGIT-/- mice and found that absence of TIGIT significantly delayed tumor growth. As TIGIT is expressed on both T cells (Treg and CD8+) and NK cells in cancer, we addressed the role of TIGIT in these subsets in driving immune suppression. Our data indicate that TIGIT may play a dominant role in Treg in that deficiency of TIGIT in Treg alone results in better control of tumor growth and a heightened proliferative response in the draining lymph nodes and spleens of tumor-bearing mice. Finally, we show that a TIGIT blocking antibody can be used therapeutically to decrease tumor growth and that blockade of TIGIT synergizes with Tim-3 blockade to maximally decrease tumor growth. Our study is the first report showing that TIGIT acts as an immune checkpoint in cancer. Importantly, our data indicate that TIGIT and Tim-3 synergize to suppress anti-tumor responses and targeting these two molecules could provide a therapeutic effect on tumor growth.

CD8 Lineage-specific Regulation of Interleukin-7 Receptor Expression by the Transcriptional Repressor Gfi1

Background: Expression of the IL-7Rα gene is up-/down-regulated during T/B-lymphocyte development. Results: IL-7Rα gene transcription is repressed by the transcription factor Gfi1, specifically in CD8+ T-lymphocytes. Conclusion: Treatment by dexamethasone down-regulates Gfi1, which contributes to glucocorticoid receptor mediated up-regulation of IL-7R expression. Significance: The mechanism by which the IL-7R gene gets turned on and off during development is a critical issue in biology. Interleukin-7 receptor α (IL-7Rα) is essential for T cell survival and differentiation. Glucocorticoids are potent enhancers of IL-7Rα expression with diverse roles in T cell biology. Here we identify the transcriptional repressor, growth factor independent-1 (Gfi1), as a novel intermediary in glucocorticoid-induced IL-7Rα up-regulation. We found Gfi1 to be a major inhibitory target of dexamethasone by microarray expression profiling of 3B4.15 T-hybridoma cells. Concordantly, retroviral transduction of Gfi1 significantly blunted IL-7Rα up-regulation by dexamethasone. To further assess the role of Gfi1 in vivo, we generated bacterial artificial chromosome (BAC) transgenic mice, in which a modified Il7r locus expresses GFP to report Il7r gene transcription. By introducing this BAC reporter transgene into either Gfi1-deficient or Gfi1-transgenic mice, we document in vivo that IL-7Rα transcription is up-regulated in the absence of Gfi1 and down-regulated when Gfi1 is overexpressed. Strikingly, the in vivo regulatory role of Gfi1 was specific for CD8+, and not CD4+ T cells or immature thymocytes. These results identify Gfi1 as a specific transcriptional repressor of the Il7r gene in CD8 T lymphocytes in vivo.