Cutting edge: IL-7-independent regulation of IL-7 receptor alpha expression and memory CD8 T cell development

Collaboration between IL-7 and IL-15 enables adaptation of tissue-resident and circulating memory CD8+ T cells

Interleukin-7 (IL-7) is considered a critical regulator of memory CD8+ T cell homeostasis, but this is primarily based on analysis of circulating and not tissue-resident memory (TRM) subsets. Furthermore, the cell-intrinsic requirement for IL-7 signaling during memory homeostasis has not been directly tested. Using inducible deletion, we found that Il7ra loss had only a modest effect on persistence of circulating memory and TRM subsets and that IL-7Rα was primarily required for normal basal proliferation. Loss of IL-15 signaling imposed heightened IL-7Rα dependence on memory CD8+ T cells, including TRM populations previously described as IL-15-independent. In the absence of IL-15 signaling, IL-7Rα was upregulated, and loss of IL-7Rα signaling reduced proliferation in response to IL-15, suggesting cross-regulation in memory CD8+ T cells. Thus, across subsets and tissues, IL-7 and IL-15 act in concert to support memory CD8+ T cells, conferring resilience to altered availability of either cytokine.

The Role of Chemokine IL-7 in Tumor and Its Potential Antitumor Immunity

Interleukin-7 (IL-7) is a cytokine belonging to the chemokine family. It plays a key role in the differentiation, development, and maturation of T lymphocytes and B lymphocytes, which is pivotal to adaptive immunity. In addition to its role in lymphocyte development, recent studies have indicated the antitumor functions of IL-7 in the tumor microenvironment. In this review, we discuss the role of IL-7 in tumors and summarize its antitumor potential and clinical application in lymphoma, leukemia, breast cancer, colon cancer, and so on. Furthermore, the combinational strategies of IL-7 and other antitumor drugs have been also discussed.

Suppressor of cytokine signalling 3 is crucial for interleukin-7 receptor re-expression after T-cell activation and interleukin-7 dependent proliferation

SOCS3 is a crucial feedback inhibitor of several cytokine pathways with potential regulatory functions during T cell receptor activation. A role of SOCS3 in IL-7-dependent homeostatic mechanisms has been assumed but the underlying mechanisms remain unclear. We investigated the role of SOCS3 in IL-7 receptor α-chain (IL-7Rα) expression and IL-7 effects on activated human CD4⁺ T cells. SOCS3 expression modulation by lentiviral transduction combined with T cell phenotyping, receptor signalling analysis, and a novel competitive in vitro assay were applied. Time course analyses following T-cell activation showed IL-7Rα re-expression after initial down-regulation that was accompanied by increased SOCS3 expression starting on day 2. T cells with low SOCS3 expression (SOCS3_kd ) had decreased IL-7Rα levels due to impaired re-expression. SOCS3 mediated effects on IL-7Rα were not affected by recombinant IL-7 or blocking of IL-2. We found no evidence for SOCS3 effects on IL7RA transcriptional regulation. Functionally, SOCS3_kd T cells showed decreased IL-7-dependent proliferation as compared to vector control T cells under competitive in vitro conditions. This impaired IL-7 response of SOCS3_kd T cells was accompanied by decreased STAT5 phosphorylation late during IL-7 signalling. We identified a novel SOCS3 function in IL-7Rα regulation during T-cell activation with crucial implications for IL-7-dependent mechanisms.

Tissue-resident memory T cells and their biological characteristics in the recurrence of inflammatory skin disorders

The skin is the largest organ of the body. The establishment of immunological memory in the skin is a crucial component of the adaptive immune response. Once naive T cells are activated by antigen-presenting cells, a small fraction of them differentiate into precursor memory T cells. These precursor cells ultimately develop into several subsets of memory T cells, including central memory T (T_CM) cells, effector memory T (T_EM) cells, and tissue resident memory T (T_RM) cells. T_RM cells have a unique transcriptional profile, and their most striking characteristics are their long-term survival (longevity) and low migration in peripheral tissues, including the skin. Under physiological conditions, T_RM cells that reside in the skin can respond rapidly to pathogenic challenges. However, there is emerging evidence to support the vital role of T_RM cells in the recurrence of chronic inflammatory skin disorders, including psoriasis, vitiligo, and fixed drug eruption, under pathological or uncontrolled conditions. Clarifying and characterizing the mechanisms that are involved in skin T_RM cells will help provide promising strategies for reducing the frequency and magnitude of skin inflammation recurrence. Here, we discuss recent insights into the generation, homing, retention, and survival of T_RM cells and share our perspectives on the biological characteristics of T_RM cells in the recurrence of inflammatory skin disorders.

Cytokine-Mediated Regulation of CD8 T-Cell Responses During Acute and Chronic Viral Infection

The common γ-chain cytokines, interleukin (IL)-2, IL-7, and IL-15, regulate critical aspects of antiviral CD8 T-cell responses. During acute infections, IL-2 controls expansion and differentiation of antiviral CD8 T cells, whereas IL-7 and IL-15 are key cytokines to maintain memory CD8 T cells long term in an antigen-independent manner. On the other hand, during chronic infections, in which T-cell exhaustion is established, precise roles of these cytokines in regulation of antiviral CD8 T-cell responses are not well defined. Nonetheless, administration of IL-2, IL-7, or IL-15 can increase function of exhausted CD8 T cells, and thus can be an attractive therapeutic approach. A new subset of stem-cell-like CD8 T cells, which provides a proliferative burst after programmed cell death (PD)-1 therapy, has been recently described during chronic viral infection. Further understanding of cytokine-mediated regulation of this CD8 T-cell subset will improve cytokine therapies to treat chronic infections and cancer in combination with immune checkpoint inhibitors.

IL-2 and Beyond in Cancer Immunotherapy

The development of the T- and natural killer (NK) cell growth factor IL-2 has been a sentinel force ushering in the era of immunotherapy in cancer. With the advent of clinical grade recombinant IL-2 in the mid-1980s, oncologists could for the first time directly manipulate lymphocyte populations with systemic therapy. By itself, recombinant IL-2 can induce clinical responses in up to 15% of patients with metastatic cancer or renal cell carcinoma. When administered with adoptively transferred tumor-reactive lymphocytes, IL-2 promotes T cell engraftment and response rates of up to 50% in metastatic melanoma patients. Importantly, these IL-2-driven responses can yield complete and durable responses in a subset of patients. However, the use of IL-2 is limited by toxicity and concern of the expansion of T regulatory cells. To overcome these limitations and improve response rates, other T cell growth factors, including IL-15 and modified forms of IL-2, are in clinical development. Administering T cell growth factors in combination with other agents, such as immune checkpoint pathway inhibitors, may also improve efficacy. In this study, we review the development of T- and NK cell growth factors and highlight current combinatorial approaches based on these reagents.

Harnessing the IL-7/IL-7Rα axis to improve tumor immunotherapy

IL-7 and IL-15 are critical for supporting T cells transferred into a lymphopenic environment. As activated CD8(+) T cells downregulate IL-7Rα, it is thought IL-15 is more important. However, we find that CD8(+) T cells activated with IL-12 have elevated IL-7Rα and rely on IL-7 for persistence and antitumor immunity.

IL-7 induces clathrin-mediated endocytosis of CD127 and subsequent degradation by the proteasome in primary human CD8 T cells

Interleukin-7 (IL-7), a key immunoregulatory cytokine, plays an essential role in peripheral T-cell homeostasis and function. Signaling via the IL-7 receptor is tightly regulated and we and others have shown IL-7 provides negative feedback on its own signaling by downregulating expression of the IL-7 receptor alpha-chain (CD127) through both suppression of CD127 gene transcription and by internalization of existing CD127 proteins from the cell membrane. We show here for the first time in primary human CD8 T cells that upon stimulation with IL-7, CD127 is internalized through clathrin-coated pits, a process dependent on both lipid-raft formation and the activity of dynamin. As visualized by confocal microscopy, CD127 shows increased co-localization with clathrin within 5 min of IL-7 stimulation and within 15-30 min is seen in multiple intracellular punctae co-localizing with the early endosomal marker EEA1. By 2 h after addition of IL-7, CD127 staining associates with the late endosomal marker RAB7 and with the proteasomal 20S subunit. By inducing receptor internalization and translocation from early endosomes to the proteasome, IL-7 directly influences its receptor density on the cell surface and thus regulates the intensity of its own signaling cascades. Given the important role IL-7 plays in T-cell development, homeostasis and function, deciphering how expression of its receptor is controlled on the cell surface is essential in understanding how T-cell activity can be regulated in different microenvironments and in response to different pathogens.

Novel therapies for memory cells in autoimmune diseases

Autoimmune diseases are a major cause of morbidity, and their incidence and prevalence continue to rise. Treatments for these diseases are non-specific and result in significant adverse effects. Targeted therapies may help in improving the risk : benefit ratio associated with treatment. Immunological memory is an important feature of the vertebrate immune system that results in the production of cells that are long-lived and able to respond to antigens in a more robust manner. In the setting of autoimmunity this characteristic becomes detrimental due to the ongoing response to a self-antigen(s). These memory cells have been shown to play key roles in various autoimmune diseases such as type 1 diabetes, multiple sclerosis and psoriasis. Memory T cells and B cells can be identified based on various molecules expressed on their surface. Memory T cells can be divided into three main categories - central memory, effector memory and resident memory cells. These subsets have different proliferative potential and cytokine-producing abilities. Utilizing differentially expressed surface molecules or downstream signalling pathway proteins in these cells it is now possible to target memory cells while sparing naive cells. We will discuss the various available options for such a strategy and several potential strategies that may yield successful therapies in the future.

IL-12 is required for mTOR regulation of memory CTLs during viral infection

The induction of functional memory CTLs is a major goal of vaccination against intracellular pathogens. IL-12 is critical for the generation of memory CTLs, and inhibition of mTOR by rapamycin can effectively enhance the memory CTL response. Yet, the role of IL-12 in mTOR’s regulation of memory CTL is unknown. Here, we hypothesized that the immunostimulatory effects of mTOR on memory CTLs requires IL-12 signaling. Our results revealed that rapamycin increased the generation of memory CTLs in vaccinia virus infection, and this enhancement was dependent upon the IL-12 signal. Furthermore, IL-12 receptor deficiency diminished the secondary expansion of rapamycin-regulated memory, and resultant secondary memory CTLs were abolished. Rapamycin enhanced IL-12 signaling by up regulating IL-12 receptor β2 expression and STAT4 phosphorylation in CTLs during early infection. In addition, rapamycin continually suppressed T-bet expression in both WT and IL-12 receptor knockout CTLs. These results indicate an essential role for IL-12 in the regulation of memory CTLs by mTOR, and highlight the importance of considering the interplay between cytokines and adjuvants during vaccine design.

A direct and nonredundant role for thymic stromal lymphopoietin on antiviral CD8 T cell responses in the respiratory mucosa

Mucosally produced thymic stromal lymphopoietin (TSLP) regulates Th2 responses by signaling to dendritic cells and CD4 T cells. Activated CD8 T cells express the TSLP receptor (TSLPR), yet a direct role for TSLP in CD8 T cell immunity in the mucosa has not been described. Because TSLP shares signaling components with IL-7, a cytokine important for the development and survival of memory CD8 T cells in systemic infection models, we hypothesized that TSLP spatially and nonredundantly supports the development of these cells in the respiratory tract. In this study, we demonstrate that influenza infection induces the early expression of TSLP by lung epithelial cells with multiple consequences. The global loss of TSLP responsiveness in TSLPR(-/-) mice enhanced morbidity and delayed viral clearance. Using a competitive adoptive transfer system, we demonstrate that selective loss of TSLPR signaling on antiviral CD8 T cells decreases their accumulation specifically in the respiratory tract as early as day 8 after infection, primarily due to a proliferation deficiency. Importantly, the subsequent persistence of memory cells derived from this pool was also qualitatively and quantitatively affected. In this regard, the local support of antiviral CD8 T cells by TSLP is well suited to the mucosa, where responses must be tempered to prevent excessive inflammation. Taken together, these data suggest that TSLP uniquely participates in local immunity in the respiratory tract and modulation of TSLP levels may promote long-term CD8 T cell immunity in the mucosa when other prosurvival signals are limiting.

Anti-viral CD8 T cells and the cytokines that they love

Viral infections cause an immunological disequilibrium that provokes CD8 T cell responses. These cells play critical roles in purging acute infections, limiting persistent infections, and conferring life-long protective immunity. At every stage of the response anti-viral CD8 T cells are sensitive to signals from cytokines. Initially cytokines operate as immunological warning signs that inform of the presence of an infection, and also influence the developmental choices of the responding cells. Later during the course of the response other sets of cytokines support the survival and maintenance of the differentiated anti-viral CD8 T cells. Although many cytokines promote virus-specific CD8 T cells, other cytokines can suppress their activities and thus favor viral persistence. In this review we discuss how select cytokines act to regulate anti-viral CD8 T cells throughout the response and influence the outcome of viral infections.

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.

Transfer of CD8+ T cell memory using Bcl-2 as a marker

The processes that regulate T cell memory generation are important for therapeutic design and the immune response to disease. However, what allows a subset of effector T cells to survive the contraction period to become memory cells is incompletely understood. The Bcl-2 family is critical for T cell survival, and Bcl-2 has been proposed to be important for the survival of memory cells. However, previous studies have relied on double-knockout models, potentially skewing the role of Bcl-2, and the use of Bcl-2 as a marker in adoptive transfer experiments, a method required to confirm the memory potential of cell subsets, has not been possible because of the intracellular localization of the protein. In this study, we present a novel Bcl-2 reporter mouse model and, to our knowledge, show for the first time that a distinct subset of effector T cells, and also a subset within the CD127(hi)KLRG1(lo) memory precursor effector cell population, retains high Bcl-2 expression at the peak of the CD8(+) T cell response to Listeria monocytogenes. Furthermore, we show that Bcl-2 correlates with memory potential in adoptive transfer experiments using both total responding CD8(+) T cells and memory precursor effector cells. These results show that even within the memory precursor effector cell population, Bcl-2 confers a survival advantage in a subset of effector CD8(+) T cells that allows differentiation into memory cells and cement Bcl-2 as a critical factor for T cell memory.

CD8 lineage-specific regulation of interleukin-7 receptor expression by the transcriptional repressor Gfi1

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.

IL-7 signaling and CD127 receptor regulation in the control of T cell homeostasis

After their development in the thymus, mature T cells are maintained in the periphery by two sets of survival signals, namely TCR signals from contact with self-peptide/MHC ligands and the cytokine receptor signals from binding IL-7 and IL-15. These signals cooperate to maximize the utility of finite resources to support a diverse pool of mature T cells. It is becoming increasingly clear that multiple mechanisms exist to regulate expression of IL-7R at the transcriptional and post-translational levels. The interplay between TCR signals and IL-7R signals are also important in regulation of IL-7R expression. This review will focus on regulation of T cell homeostasis by IL-7R signaling, with an emphasis on the cross talk between signals from TCR and IL-7R.

Homeostatic maintenance of T cells and natural killer cells

Homeostasis in the immune system encompasses the mechanisms governing maintenance of a functional and diverse pool of lymphocytes, thus guaranteeing immunity to pathogens while remaining self-tolerant. Antigen-naïve T cells rely on survival signals through contact with self-peptide-loaded major histocompatibility complex (MHC) molecules plus interleukin (IL)-7. Conversely, antigen-experienced (memory) T cells are typically MHC-independent and they survive and undergo periodic homeostatic proliferation through contact with both IL-7 and IL-15. Also, non-conventional γδ T cells rely on a mix of IL-7 and IL-15 for their homeostasis, whereas natural killer cells are mainly dependent on contact with IL-15. Homeostasis of CD4(+) T regulatory cells is different in being chiefly regulated by contact with IL-2. Notably, increased levels of these cytokines cause expansion of responsive lymphocytes, such as found in lymphopenic hosts or following cytokine injection, whereas reduced cytokine levels cause a decline in cell numbers.

Expression and function of interleukin-7 in secondary and tertiary lymphoid organs

Interleukin-7 (IL-7) is known since many years as stromal-cell derived cytokine that plays a key role for the adaptive immune system. It promotes lymphocyte development in the bone marrow and thymus as well as naive and memory T cell homeostasis in the periphery. More recently, IL-7 reporter mice and other approaches have led to the further characterization of the various stromal cell sources of IL-7 in secondary lymphoid organs (SLO) and other tissues. We will review these advances along with a discussion of the regulation of IL-7 and its receptor, and compare the biological effects IL-7 has on adaptive as well as innate immune cells in SLO. Finally, we will review the role of IL-7 in development of SLO and tertiary lymphoid tissues that frequently are associated with sites of chronic inflammation.

T-cell biology in aging, with a focus on lung disease

T cells are essential for defending hosts against microorganisms and malignancy as well as for regulating the development of immune-mediated inflammatory diseases like autoimmunity. Alterations in T-cell immunity occur with aging, affecting the function and proportions of T-cell subsets. Probably, the most noticeable age-associated change in T-cell immunity is an alteration in the frequency of naive and memory CD4+ and CD8+ T cells. In fact, the frequency of naive CD4+ and CD8+ T cells decreases with aging, whereas the frequency of memory CD4+ and CD8+ T cells increases. Also, changes in T-cell proliferation, cytokine production, memory response, and cytotoxicity as well as in regulatory T-cell number and function have been reported with aging. Such alterations could contribute to the development of infections, malignancies, and inflammatory diseases that rise with aging. Of interest, T cells are closely involved in the development of inflammatory airway and lung diseases including asthma and chronic obstructive pulmonary disease, which are prevalent in the elderly people. In addition, T cells play a major role in defending host against influenza virus infection, a serious medical problem with high morbidity and mortality in the elderly people. Thus, it is conceivable that altered T-cell immunity may account in part for the development of such respiratory problems with aging. Here, we will review the recent advances in T-cell immunity and its alteration with aging and discuss the potential effects of such changes on the lung.

In vitro derivation of interferon-γ producing, IL-4 and IL-7 responsive memory-like CD4(+) T cells

CD4(+) memory is critical for successful protection against pathogenic challenge. As such, understanding the heterogeneity of cells that arise and survive after initial stimulation of naïve CD4(+) T cells will aid in the design of more successful vaccines. In previous studies, in vivo experimental systems have been extensively used to generate functional memory responses by lymphocytes. Here, we have attempted to develop an in vitro experimental system to generate memory CD4(+) T lymphocytes. CD4(+) T cells stimulated through the antigen receptor complex were examined for their memory-like characteristics after 3 weeks of cell culture. A subset of surviving cells expressed high levels of CD44 and low levels of CD45RB (CD44(hi)CD45(lo)), a phenotype that is similar to bonafide memory CD4(+) T cells. In vitro generated memory-like CD4(+) T cells secreted higher levels of IFN-γ, with rapid kinetics, upon re-stimulation than their naïve counterparts. In addition, these memory-like CD4(+) T cells did not produce either IL-2 or IL-4 but readily proliferated when cultured in the presence of IL-7 and IL-4. These observations suggest that CD4(+) cells surviving the expansion phase of immune response produce a Th1-signature cytokine and retain responsiveness to IL-4, a Th-2 cytokine, as well as to a well described survival factor, interleukin-7.

Regional and mucosal memory T cells

After infection, most antigen-specific memory T cells reside in nonlymphoid tissues. Tissue-specific programming during priming leads to directed migration of T cells to the appropriate tissue, which promotes the development of tissue-resident memory in organs such as intestinal mucosa and skin. Mechanisms that regulate the retention of tissue-resident memory T cells include transforming growth factor-β (TGF-β)-mediated induction of the E-cadherin receptor CD103 and downregulation of the chemokine receptor CCR7. These pathways enhance protection in internal organs, such as the nervous system, and in the barrier tissues--the mucosa and skin. Memory T cells that reside at these surfaces provide a first line of defense against subsequent infection, and defining the factors that regulate their development is critical to understanding organ-based immunity.

B cells help alloreactive T cells differentiate into memory T cells

B cells are recognized as effector cells in allograft rejection that are dependent upon T cell help to produce alloantibodies causing graft injury. It is not known if B cells can also help T cells differentiate into memory cells in the alloimmune response. We found that in B-cell-deficient hosts, differentiation of alloreactive T cells into effectors was intact whereas their development into memory T cells was impaired. To test if B cell help for T cells was required for their continued differentiation into memory T cells, activated T cells were sorted from alloimmunized mice and transferred either with or without B cells into naïve adoptive hosts. Activated T cells cotransferred with B cells gave rise to more memory T cells than those transferred without B cells and upon recall, mediated accelerated rejection of skin allografts. Cotransfer of B cells led to increased memory T cells by enhancing activated CD4 T-cell proliferation and activated CD8 T-cell survival. These results indicate that B cells help alloreactive T-cell differentiation, proliferation and survival to generate optimal numbers of functional memory T cells.

Nature and nurture: T-cell receptor-dependent and T-cell receptor-independent differentiation cues in the selection of the memory T-cell pool

The initiation of a T-cell response begins with the interaction of an individual T-cell clone with its cognate antigen presented by MHC. Although the strength of the T-cell receptor (TCR) -antigen-MHC (TCR-pMHC) interaction plays an important and obvious role in the recruitment of T cells into the immune response, evidence in recent years has suggested that the strength of this initial interaction can influence various other aspects of the fate of an individual T-cell clone and its daughter cells. In this review, we will describe differences in the way CD4(+) and CD8(+) T cells incorporate antigen-driven differentiation and survival signals during the response to acute infection. Furthermore, we will discuss increasing evidence that the quality and/or quantity of the initial TCR-pMHC interaction can drive the differentiation and long-term survival of T helper type 1 memory populations.

The early generation of a heterogeneous CD4+ T cell response to Leishmania major

CD4(+) T cells are an essential component of both the primary and secondary immune response against the intracellular protozoan parasite Leishmania major. Our laboratory has previously shown that CD62L(high) IL-7R(high) central memory T (T(CM)) cells mediate protective immunity following secondary challenge. To determine when T(CM) cells develop, we examined the phenotype of Leishmania-specific CD4(+) T cells in the first 2 wk following infection. As expected, we identified a population of CD4(+) T cells present in the draining lymph node with the characteristics of effector T cells. However, in addition, a second population phenotypically resembling T(CM) cells emerged coincident with the effector population. These T cells, expressing CD62L, CCR7, and IL-7R, failed to produce IFN-gamma, but had the capacity to give rise to IFN-gamma-producing effector cells. Our studies also demonstrated that the degree of proliferation and the timing of lymph node entry impact T(CM) cell development. The early generation of T(CM) cells following L. major infection indicates that T(CM) cells may not only control secondary infections, but may also contribute to the control of the primary infection.

Regulating functional cell fates in CD8 T cells

The attributes of specificity and memory enable CD8(+) T cells to provide long-lasting protection against a variety of challenges. Although, the importance of CD8(+) T cells for protection against intracellular infections and transformation is well-established, the functional type; effector phenotypes (Tc1, Tc2, Tc17 and/or Tcreg) and/or memory (effector or central), of CD8(+) T cells most desirable for tumor immunity is not established. To determine the tumor efficacy of various effector types and/or memory CD8 T cells, it is imperative to better understand intrinsic and extrinsic factors that regulate CD8(+) T cell differentiation and use this information to generate and test distinct functional cell types in tumor models. The focus of our laboratory investigations is to identify the extrinsic factors such as antigen strength, co-stimulatory molecules, cytokines, and small molecule modifiers that regulate intrinsic programs for various effector and/or memory cell fate in antigen specific CD8 T cells. The use of this information to generate immunity in murine tumor models has facilitated development of new adoptive cell transfer (ACT) as well as immunization strategies for cancer treatment.

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

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

Intrinsic and extrinsic control of effector T cell survival and memory T cell development

Following infection or vaccination T cells expand exponentially and differentiate into effector T cells in order to control infection and coordinate the multiple effector arms of the immune system. Soon after this expansion, the majority of antigen-specific T cells die to reattain homeostasis and a small pool of memory T cells forms to provide long-term immunity to subsequent re-infection. Our understanding of how this process is controlled has improved considerably over the recent years, but many questions remain outstanding. This review focuses on the recent advancements in this area with an emphasis on how the contraction of activated T cells is coordinately regulated by a combination of factors extrinsic and intrinsic to the activated T cells.

Once a killer, always a killer: from cytotoxic T cell to memory cell

The control of the differentiation pathways followed by responding CD8(+) T cells to produce protective memory cells has been intensely studied. Recent developments have identified heterogeneity at the effector cytotoxic T-lymphocyte level within which a bona fide memory cell precursor has emerged. The challenge now is to identify the cellular and molecular factors that control this developmental pathway. This review considers aspects of the regulation of the induction of effectors, the transition of effectors to memory cells, and the dynamics of the memory population.

Plasticity in programming of effector and memory CD8 T-cell formation

CD8(+) T cells (also called cytotoxic T lymphocytes) play a major role in protective immunity against many infectious pathogens and can eradicate malignant cells. The path from naive precursor to effector and memory CD8(+) T-cell development begins with interactions between matured antigen-bearing dendritic cells (DCs) and antigen-specific naive T-cell clonal precursors. By integrating differences in antigenic, costimulatory, and inflammatory signals, a developmental program is established that governs many key parameters associated with the ensuing response, including the extent and magnitude of clonal expansion, the functional capacities of the effector cells, and the size of the memory pool that survives after the contraction phase. In this review, we discuss the multitude of signals that drive effector and memory CD8(+) T-cell differentiation and how the differences in the nature of these signals contribute to the diversity of CD8(+) T-cell responses.

Memory CD8+ T cell differentiation

In response to infection or effective vaccination, naive antigen-specific CD8+ T cells undergo a dramatic highly orchestrated activation process. Initial encounter with an appropriately activated antigen-presenting cell leads to blastogenesis and an exponential increase in antigen-specific CD8+ T cell numbers. Simultaneously, a dynamic differentiation process occurs, resulting in formation of both primary effector and long-lived memory cells. Current findings have emphasized the heterogeneity of effector and memory cell populations with the description of multiple cellular subsets based on phenotype, function, and anatomic location. Yet, only recently have we begun to dissect the underlying factors mediating the temporal control of the development of distinct effector and memory CD8+ T cell sublineages. In this review we will focus on the requirements for mounting an effective CD8+ T cell response and highlight the elements regulating the differentiation of effector and memory subsets.

Regulation of memory T cells by γc cytokines

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

Adaptive immunity to hepatitis C virus

The precise role of adaptive immune responses in the clinical outcome of HCV infection is still only partially defined. Recent studies suggest that viral-host cell interactions during the acute phase of infection are essential for viral clearance or progression into chronic HCV infection. This review focuses on different aspects of the adaptive immune responses as determinants of the different outcomes of HCV infection, clearance or persistent infection, and outlines current concepts of HCV evasion strategies. Unravelling these important mechanisms of virus-host interaction will contribute to the development of novel strategies to prevent and control HCV infection.

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

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

Generation and maintenance of memory CD4(+) T Cells

In the course of an immune response to an infectious microbe, pathogen-specific naïve CD4(+) T cells proliferate extensively and differentiate into effector cells. Most of these cells die rapidly, but a small fraction of effector cells persist as memory cells to confer enhanced protection against the same pathogen. Recent advances indicate that strong TCR stimulation during the primary response is essential for the generation of long-lived memory CD4(+) T cells. Memory cells appear to be derived equally from all subsets of effector cells, and memory cells can also acquire additional functional capabilities during the secondary response. Resting memory CD4(+) cells are dependent on signals from contact with IL-7 and IL-15, but not MHC class II, for their survival and intermittent homeostatic proliferation.

Surviving the crash: transitioning from effector to memory CD8+ T cell

One outcome of infection is the formation of long-lived immunological memory, which provides durable protection from symptomatic re-infection. In response to infection or vaccination, T cells undergo dramatic proliferation and differentiate into effector T cells that mediate removal of the pathogen. Following pathogen clearance, the majority of effector cells die, restoring lymphocyte homeostasis. However, a small number of antigen-specific cells survive and seed the memory T cell population. Here, we focus on recent advances in identifying the key proteins and transcription factors that allow a portion of effector CD8(+) T cells to persist after contraction of the immune response, forming a memory cell population programmed for long-term self-renewal and survival. We also examine new findings addressing the role of environmental cues such as cytokines and co-stimulatory molecules in CD8(+) memory T cell formation and how the cell-extrinsic cues influence the molecular players of intracellular pathways important for memory formation.

A critical role for direct TLR2-MyD88 signaling in CD8 T-cell clonal expansion and memory formation following vaccinia viral infection

Recent advances have suggested a crucial role of the innate immunity in shaping adaptive immune responses. How activation of innate immunity promotes adaptive T-cell responses to pathogens in vivo is not fully understood. It has been thought that Toll-like receptor (TLR)-mediated control of adaptive T-cell responses is mainly achieved by the engagement of TLRs on antigen-presenting cells to promote their maturation and function. In this study, we showed that direct TLR2-myeloid differentiating factor 88 (MyD88) signaling in CD8 T cells was also required for their efficient clonal expansion by promoting the survival of activated T cells on vaccinia viral infection in vivo. Effector CD8 T cells that lacked direct TLR2-MyD88 signaling did not survive the contraction phase to differentiate into long-lived memory cells. Furthermore, we observed that direct TLR2 ligation on CD8 T cells promoted CD8 T-cell proliferation and survival in vitro in a manner dependent on the phosphatidylinositol 3-kinase (PI3K)-Akt pathway activation and that activation of Akt controlled memory cell formation in vivo. These results identify a critical role for intrinsic TLR2-MyD88 signaling and PI3K-Akt pathway activation in CD8 T-cell clonal expansion and memory formation in vivo and could lead to the development of new vaccine approaches.

Constitutive expression of IL-7 receptor alpha does not support increased expansion or prevent contraction of antigen-specific CD4 or CD8 T cells following Listeria monocytogenes infection

Expression of IL-7Ralpha (CD127) has been suggested as a major determinant in the survival of memory T cell precursors. We investigated whether constitutive expression of IL-7Ralpha on T cells increased expansion and/or decreased contraction of endogenous Ag-specific CD4 and CD8 T cells following infection with Listeria monocytogenes. The results indicate that constitutive expression of IL-7Ralpha alone was not enough to impart an expansion or survival advantage to CD8 T cells responding to infection, and did not increase memory CD8 T cell numbers over those observed in wild-type controls. Constitutive expression of IL-7Ralpha did allow for slightly prolonged expansion of Ag-specific CD4 T cells; however, it did not alter the contraction phase or protect against the waning of memory T cell numbers at later times after infection. Memory CD4 and CD8 T cells generated in IL-7Ralpha transgenic mice expanded similarly to wild-type T cells after secondary infection, and immunized IL-7Ralpha transgenic mice were fully protected against lethal bacterial challenge demonstrating that constitutive expression of IL-7Ralpha does not impair, or markedly improve memory/secondary effector T cell function. These results indicate that expression of IL-7Ralpha alone does not support increased survival of effector Ag-specific CD4 or CD8 T cells into the memory phase following bacterial infection.

IL-7 and IL-15 differentially regulate CD8+ T-cell subsets during contraction of the immune response

Although it is known that interleukin-7 (IL-7) and IL-15 influence the survival and turnover of CD8+ T cells, less is known about how these cytokines affect different subsets during the course of the immune response. We find that IL-7 and IL-15 differentially regulate CD8+ T-cell subsets defined by KLRG1 and CD127 expression during the contraction phase of the immune response. The provision of IL-15, or the related cytokine IL-2, during contraction led to the preferential accumulation of KLRG1(hi)CD127(lo) CD8+ T cells, whereas provision of IL-7 instead favored the accumulation of KLRG1(lo)CD127(hi) cells. While IL-7 and IL-15 both induced proliferation of KLRG1(lo) cells, KLRG1(hi) cells exhibited an extraordinarily high level of resistance to cytokine-driven proliferation in vivo despite their dramatic accumulation upon IL-15 administration. These results suggest that IL-15 and IL-2 greatly improve the survival of KLRG1(hi) CD8+ T cells, which are usually destined to perish during contraction, without inducing proliferation. As the availability of IL-15 and IL-2 is enhanced during periods of extended inflammation, our results suggest a mechanism in which a population of cytokine-dependent KLRG1(hi) CD8+ T cells is temporarily retained for improved immunity. Consideration of these findings may aid in the development of immunotherapeutic strategies against infectious disease and cancer.

Formation of IL-7Ralphahigh and IL-7Ralphalow CD8 T cells during infection is regulated by the opposing functions of GABPalpha and Gfi-1

IL-7 is essential for the survival of naive and memory T cells, and IL-7 receptor alpha-chain (IL-7Ralpha) expression is dynamically regulated in activated CD8 T cells during acute viral and bacterial infections. Most virus-specific CD8 T cells become IL-7Ralpha(low) and are relatively short-lived, but some escape IL-7Ralpha repression (referred to as IL-7Ralpha(high) memory precursor effector cells) and preferentially enter the memory CD8 T cell pool. How antiviral effector CD8 T cells regulate IL-7Ralpha expression in an "on and off" fashion remains to be characterized. During lymphocytic choriomeningitis virus infection, we found that opposing actions of the transcription factors GABPalpha (GA binding protein alpha) and Gfi-1 (growth factor independence 1) control IL-7Ralpha expression in effector CD8 T cells. Specifically, GABPalpha was required for IL-7Ralpha expression in memory precursor effector cells, and this correlated with hyperacetylation of the Il7ra promoter. In contrast, Gfi-1 was required for stable IL-7Ralpha repression in effector CD8 T cells and acted by antagonizing GABPalpha binding and recruiting histone deacetylase 1, which deacetylated the Il7ra promoter. Thus, Il7ra promoter acetylation and activity was dependent on the reciprocal binding of GABPalpha and Gfi-1, and these data provide a biochemical mechanism for the generation of stable IL-7Ralpha(high) and IL-7Ralpha(low) states in virus-specific effector CD8 T cells.

Differential regulation of human IL-7 receptor alpha expression by IL-7 and TCR signaling

IL-7Ralpha is essential for the development and homeostatic maintenance of mature T cells. Studies in humans and mice have shown that IL-7Ralpha expression is reduced by its cognate cytokine, IL-7, and Ag, suggesting that active regulation of IL-7 responsiveness is necessary to balance T cell numbers. We show that IL-7- or TCR/CD28-mediated signaling induced a rapid down-regulation of IL-7Ralpha expression on naive T cells on the mRNA and protein level, with a mild (10-fold) or strong (50-fold) gene suppression, respectively. In both situations, the down-regulation of IL-7Ralpha was blocked by cyclohexamide and actinomycin D, indicating the involvement of an active mechanism dependent on new transcription and protein synthesis. Upon IL-7 withdrawal, IL-7Ralpha mRNA and surface protein reappeared in a transcription-dependent manner within 7 h. Yet, IL-7Ralpha was hardly re-expressed during the same period after TCR/CD28-activation. Likewise, T cells that were activated through CMV in vivo did not re-express IL-7Ralpha after in vitro culture. Functionally, IL-7-induced down-regulation of IL-7Ralpha did not hinder the responsiveness of naive T cells to IL-7. Conversely, down-regulation of IL-7Ralpha on TCR/CD28-activated cells limited IL-7 responsiveness. Strikingly, ectopic expression of IL-7Ralpha cells on TCR/CD28-activated cells conferred a selective advantage in the response to IL-7. In conclusion, our data show that IL-7- and TCR/CD28-mediated signaling differentially regulate IL-7Ralpha expression on human T cells with a transient and chronic effect, respectively. The stringent and active regulation of IL-7Ralpha may constitute a homeostatic mechanism to curtail unwarranted T cell expansion.

Distinctive in vitro effects of T-cell growth cytokines on cytomegalovirus-stimulated T-cell responses of HIV-infected HAART recipients

Functional immune reconstitution is limited after HAART, maintaining the interest in adjunctive immune-modulators. We compared in vitro the effects of the gamma-chain T-cell growth cytokines IL-2, IL-4, IL-7 and IL-15 on cytomegalovirus-stimulated cell-mediated immunity. IL-2 and IL-15 increased cytomegalovirus-specific lymphocyte proliferation in HAART recipients, whereas IL-4 and IL-7 did not. The boosting effect of IL-2 and IL-15 on proliferation correlated with their ability to prevent late apoptosis. However, IL-2 increased the frequency of cells in early apoptosis, whereas IL-15 increased the frequency of fully viable cells. Both IL-2 and IL-15 increased cytomegalovirus-induced CD4+ and CD8+ T-cell proliferation and the synthesis of Th1 and pro-inflammatory cytokines and chemokines. However, only IL-2 increased the frequency of regulatory T cells and Th2 cytokine production, both of which have the potential to attenuate antiviral immune responses. Overall, compared to other gamma-chain cytokines, IL-15 had the most favorable profile for boosting antiviral cell-mediated immunity.

Antigen controls IL-7R alpha expression levels on CD8 T cells during full activation or tolerance induction

The high-affinity chain of the IL-7 receptor, IL-7Ralpha (CD127), is expressed by effector CD8 T cells that have the capacity to become memory cells. IL-7Ralpha expression is uniformly high on naive CD8 T cells, and the majority of these cells down-regulate expression upon antigenic challenge. At the peak of expansion, the fraction of effectors expressing high IL-7Ralpha varies depending on the response examined. The signals that a CD8 T cell receives during a response to Ag that lead to altered expression of IL-7Ralpha have not been fully defined. In vitro experiments demonstrated that Ag alone is sufficient to down-regulate IL-7Ralpha on all cells and most of the cells rapidly re-express the receptor upon removal from Ag. Expression was not altered by the B7.1 costimulatory ligand or when IL-12 was present to provide the signal needed for development of effector functions, indicating that TCR engagement is sufficient to regulate IL-7Ralpha expression. Consistent with this, in vivo priming with peptide Ag resulted in IL-7Ralpha expression that inversely correlated with Ag levels, and expression levels were not changed when IL-12 or adjuvant were administered with Ag. A large fraction of the cells present at the peak of expansion had re-expressed IL-7Ralpha, but most of these cells failed to survive; those that did survive expressed high IL-7Ralpha levels. Thus, Ag-dependent signals regulate IL-7Ralpha levels on responding CD8 T cells, and this occurs whether the responding cells become fully activated or are rendered tolerant by administration of peptide Ag alone.

Rapid culling of the CD4+ T cell repertoire in the transition from effector to memory

Requirements for CD4+ T cell memory differentiation were analyzed with adoptively transferred SMARTA T cell receptor (TCR) transgenic cells specific for alymphocytic choriomeningitis virus (LCMV) epitope. LCMV-induced effector and memory differentiation of SMARTA cells mimicked the endogenous CD4+ T cell response. In contrast, infection with a recombinant Listeria expressing the LCMV epitope, although resulting initially in massive SMARTA expansion, led to loss of effector function and rapid cell death characterized by high expression of the apoptosis regulator Bim. Defective memory differentiation was seen after stimulation of naive but not memory SMARTA cells, was independent of precursor frequency, and correlated with a lower TCR avidity compared to endogenous responders. In addition, long-lived endogenous CD4+ memory T cells skewed to a higher functional avidity over time. These results support a model in which CD4+ T cell memory differentiation and longevity depend on the strength of the TCR signal during the primary response.

Functional and genomic profiling of effector CD8 T cell subsets with distinct memory fates

An important question in memory development is understanding the differences between effector CD8 T cells that die versus effector cells that survive and give rise to memory cells. In this study, we provide a comprehensive phenotypic, functional, and genomic profiling of terminal effectors and memory precursors. Using killer cell lectin-like receptor G1 as a marker to distinguish these effector subsets, we found that despite their diverse cell fates, both subsets possessed remarkably similar gene expression profiles and functioned as equally potent killer cells. However, only the memory precursors were capable of making interleukin (IL) 2, thus defining a novel effector cell that was cytotoxic, expressed granzyme B, and produced inflammatory cytokines in addition to IL-2. This effector population then differentiated into long-lived protective memory T cells capable of self-renewal and rapid recall responses. Experiments to understand the signals that regulate the generation of terminal effectors versus memory precursors showed that cells that continued to receive antigenic stimulation during the later stages of infection were more likely to become terminal effectors. Importantly, curtailing antigenic stimulation toward the tail end of the acute infection enhanced the generation of memory cells. These studies support the decreasing potential model of memory differentiation and show that the duration of antigenic stimulation is a critical regulator of memory formation.

Non-redundant role for IL-7R signaling for the survival of CD8+ memory T cells

IL-7 and IL-15 are important cytokines for CD8 memory T cells. However, the extent that IL-7 is essential for CD8 T cell memory remains unclear because blocking IL-7 in vivo results in near complete inhibition of T cell development with the few mature T cells exhibiting functional abnormalities. To bypass this complication, CD8 memory development was examined utilizing a mouse model where transgenic IL-7Ralpha was selectively expressed in the thymus of IL-7Ralpha(-/-) mice. T cell development was corrected but the resulting peripheral T cells were essentially IL-7 non-responsive. Activation of IL-7R-defective OT-I CD8(+) T cells with OVA(257-264) and IL-2 readily yielded CTL. Upon further culture with IL-15, these CTL expressed phenotypic and functional properties of central memory-like cells. Thus, IL-7R-defective CD8(+) T cells do not exhibit intrinsic defects in effector or memory development. When IL-7R-defective OT-I CTL were adoptively transferred into normal or IL-15(-/-) recipient mice in a non-inflammatory setting, they converted into memory-like cells, but did not persist, which was even more striking in IL-15(-/-) recipients. This poor persistence was rescued after expression of transgenic Bcl-2 in IL-7R-defective OT-I T cells. Collectively, these data indicate that IL-7 is non-redundantly required for the survival of CD8 memory T cells.