Bim/Bcl-2 balance is critical for maintaining naive and memory T cell homeostasis

The role of inflammation in the generation and maintenance of memory T cells

Following infection or vaccination, antigen-specific T cells undergo enormous expansion in numbers and differentiate into effector cells that control infection and modulate other aspects of innate and adaptive immunity. The effector T-cell expansion phase is followed by an abrupt period of contraction, during which 90-95% of antigen-specific T cells are eliminated. The surviving pool of T cells subsequently differentiates into long-lived memory populations that can persist for the life of the host and mediate enhanced protective immunity following pathogen re-infection. The generation and maintenance of memory T-cell populations are influenced by a multitude of factors, including inflammatory cytokines that can act on T cells at various points during their differentiation. Herein, we discuss our current understanding of how inflammation shapes not only the quantity and quality of memory T cells, but also the rate at which functional memory T-cell populations develop.

Naive T cell homeostasis: from awareness of space to a sense of place

The peripheral naive T cell pool is fairly stable in number, diversity and functional competence in the absence of vigorous immune responses. However, this apparent tranquility is not an intrinsic property of T cells but involves continuous tuning of the T cell pool composition by homeostatic signals. In the past decade, studies have revealed that naive T cells rely on combinatorial signals from self-peptide-MHC complexes and interleukin-7 for their physical and functional maintenance. Competition for these factors dictates T cell 'space'. In addition, recent studies show that these and other homeostatic factors are offered to T cells on stromal cell networks, which also serve to guide T cell trafficking in secondary lymphoid organs. Such findings suggest the importance of 'place' in the perception and integration of homeostatic cues for the maintenance and functional tuning of the naive T cell pool.

Mst1-FoxO signaling protects Naïve T lymphocytes from cellular oxidative stress in mice

Background

The Ste-20 family kinase Hippo restricts cell proliferation and promotes apoptosis for proper organ development in Drosophila. In C. elegans, Hippo homolog also regulates longevity. The mammalian Ste20-like protein kinase, Mst1, plays a role in apoptosis induced by various types of apoptotic stress. Mst1 also regulates peripheral naïve T cell trafficking and proliferation in mice. However, its functions in mammals are not fully understood.

Methodology/Principal Findings

Here, we report that the Mst1-FoxO signaling pathway plays a crucial role in survival, but not apoptosis, of naïve T cells. In Mst1^−/− mice, peripheral T cells showed impaired FoxO1/3 activation and decreased FoxO protein levels. Consistently, the FoxO targets, Sod2 and catalase, were significantly down-regulated in Mst1^−/− T cells, thereby resulting in elevated levels of intracellular reactive oxygen species (ROS) and induction of apoptosis. Expression of constitutively active FoxO3a restored Mst1^−/− T cell survival. Crossing Mst1 transgenic mice (Mst1 Tg) with Mst1^−/− mice reduced ROS levels and restored normal numbers of peripheral naïve T cells in Mst1 Tg;Mst1^−/− progeny. Interestingly, peripheral T cells from Mst1^−/− mice were hypersensitive to γ-irradiation and paraquat-induced oxidative stresses, whereas those from Mst1 Tg mice were resistant.

Conclusions/Significance

These data support the hypothesis that tolerance to increased levels of intracellular ROS provided by the Mst1-FoxOs signaling pathway is crucial for the maintenance of naïve T cell homeostasis in the periphery.

Interleukin-7 regulates Bim proapoptotic activity in peripheral T-cell survival

Interleukin-7 (IL-7) is critical for T-cell development and peripheral T-cell homeostasis. The survival of pro-T cells and mature T cells requires IL-7. The survival function of IL-7 is accomplished partly through induction of the antiapoptotic protein Bcl-2 and inhibition of proapoptotic proteins Bax and Bad. We show here that the proapoptotic protein Bim, a BH3-only protein belonging to the Bcl-2 family, also plays a role in peripheral T-cell survival. Deletion of Bim partially protected an IL-7-dependent T-cell line and peripheral T cells, especially cells with an effector memory phenotype, from IL-7 deprivation. However, T-cell development in the thymus was not restored in IL-7(-/-) Rag2(-/-) mice reconstituted with Bim(-/-) bone marrow. IL-7 withdrawal altered neither the intracellular location of Bim, which was constitutively mitochondrial, nor its association with Bcl-2; however, a reduction in its association with the prosurvival protein Mcl-1 was observed. IL-7 withdrawal did not increase Bim mRNA or protein expression but did induce changes in the isoelectric point of Bim(EL) and its reactivity with an antiphosphoserine antibody. Our findings suggest that the maintenance of peripheral T cells by IL-7 occurs partly through inhibition of Bim activity at the posttranslational level.

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.

Age-associated increase in lifespan of naive CD4 T cells contributes to T-cell homeostasis but facilitates development of functional defects

With age, T-cell generation from the thymus is much reduced, yet a substantial naïve T-cell pool is maintained even in aged animals, suggesting that naïve T cells either persist longer or turn over faster to maintain T-cell homeostasis. We found that with age, naïve CD4 T cells became progressively longer-lived. Their longer lifespan did not depend on recognition of self-peptide/class II. Newly generated naïve T cells derived from aged stem cells had a shorter lifespan, like that of young naïve T cells. Conversely, naïve CD4 T cells derived from middle-aged thymectomized mice were longer-lived in vivo, and their development of functional defects was accelerated. These observations suggest that naïve T cells develop their longer lifespan during their sojourn in the periphery. Increased longevity of naïve CD4 T cells correlated well with reduced expression of proapoptotic molecule Bim. We suggest that the intrinsic increase in longevity helps maintain naïve T-cell homeostasis but facilitates the development of functional defects in mice.

Generation of NO by bystander human CD8 T cells augments allogeneic responses by inhibiting cytokine deprivation-induced cell death

Nitric oxide (NO), generated by inducible NO synthase (iNOS) in bystander human CD8 T cells, augments the accumulation of allogeneically activated human CD8 T cells in vitro and in vivo. Here, we report that iNOS-derived NO does not affect T-cell proliferation but rather inhibits cell death of activated human CD8 T cells after activation by allogeneic endothelial cells in culture. Exogenous NO did not affect activation-induced cell death of human CD8 T cells but specifically reduced death of activated T cells due to cytokine deprivation. NO-mediated inhibition of T-cell death did not involve cGMP signaling, and NO did not affect the expression of Bcl-2-related proteins known to regulate cytokine deprivation-induced cell death. However, NO inhibited the activity of caspases activated as a consequence of cytokine deprivation in activated T cells. This protective effect correlated with S-nitrosylation of caspases and was phenocopied by z-VAD.fmk and z-LEHD.fmk, pharmacological inhibitors of caspases. In summary, our findings indicate that NO augments the accumulation of activated human T cells principally by inhibiting cytokine deprivation-induced cell death through S-nitrosylation of caspases.

Bim and Bmf in tissue homeostasis and malignant disease

Among all BH3-only proteins known to date, most information is available on the biological role and function of Bim (Bcl-2 interacting mediator of cell death)/BOD (Bcl-2 related ovarian death agonist), whereas little is still known about its closest relative, Bcl-2 modifying factor (Bmf). Although Bim has been implicated in the regulation of cell death induction in multiple cell types and tissues in response to a large number of stimuli, including growth factor or cytokine deprivation, calcium flux, ligation of antigen receptors on T and B cells, glucocorticoid or loss of adhesion, Bmf seems to play a more restricted role by supporting Bim in some of these cell death processes. This review aims to highlight similarities between Bim and Bmf function in apoptosis signaling and their role in normal development and disease.

IL-15 is critical for the maintenance and innate functions of self-specific CD8(+) T cells

IL-15 is a pleiotropic cytokine involved in host defense as well as autoimmunity. IL-15-deficient mice show a decrease of memory phenotype (MP) CD8(+) T cells, which develop naturally in naïve mice and whose origin is unclear. It has been shown that self-specific CD8(+) T cells developed in male H-Y antigen-specific TCR transgenic mice share many similarities with naturally occurring MP CD8(+) T cells in normal mice. In this study, we found that H-Y antigen-specific CD8(+) T cells in male but not female mice decreased when they were crossed with IL-15-deficient mice, mainly due to impaired peripheral maintenance. The self-specific TCR transgenic CD8(+) T cells developed in IL-15-deficient mice showed altered surface phenotypes and reduced effector functions ex vivo. Bystander activation of the self-specific CD8(+) T cells was induced in vivo during infection with Listeria monocytogenes, in which proliferation but not IFN-gamma production was IL-15-dependent. These results indicated important roles for IL-15 in the maintenance and functions of self-specific CD8(+) T cells, which may be included in the naturally occurring MP CD8(+) T-cell population in naïve normal mice and participate in innate host defense responses.

Opposing effects of TGF-beta and IL-15 cytokines control the number of short-lived effector CD8+ T cells

An effective immune response against infectious agents involves massive expansion of CD8(+) T cells. Once the infection is cleared, the majority of these effector cells die through unknown mechanisms. How is expansion controlled to maximize pathogen clearance and minimize immunopathology? We found, after Listeria infection, plasma transforming growth factor beta (TGF-beta) titers increased concomitant with the expansion of effector CD8(+) T cells. Blocking TGF-beta signaling did not affect effector function of CD8(+) T cells. However, TGF-beta controlled effector cell number by lowering Bcl-2 amounts and selectively promoting the apoptosis of short-lived effector cells. TGF-beta-mediated apoptosis of this effector subpopulation occurred during clonal expansion and contraction, whereas interleukin-15 (IL-15) promoted their survival only during contraction. We demonstrate that the number of effector CD8(+) T cells is tightly controlled by multiple extrinsic signals throughout effector differentiation; this plasticity should be exploited during vaccine design and immunotherapy against tumors and autoimmune diseases.

The Bcl-2 family antagonist ABT-737 significantly inhibits multiple animal models of autoimmunity

The Bcl-2 family of proteins plays a critical role in controlling immune responses by regulating the expansion and contraction of activated lymphocyte clones by apoptosis. ABT-737, which was originally developed for oncology, is a potent inhibitor of Bcl-2, Bcl-x(L), and Bcl-w protein function. There is evidence that Bcl-2-associated dysregulation of lymphocyte apoptosis may contribute to the pathogenesis of autoimmunity and lead to the development of autoimmune diseases. In this study, we report that ABT-737 treatment resulted in potent inhibition of lymphocyte proliferation as measured by in vitro mitogenic or ex vivo Ag-specific stimulation. More importantly, ABT-737 significantly reduced disease severity in tissue-specific and systemic animal models of autoimmunity. Bcl-2 family antagonism by ABT-737 was efficacious in treating animal models of arthritis and lupus. Our results suggest that treatment with a Bcl-2 family antagonist represents a novel and potentially attractive therapeutic approach for the clinical treatment of autoimmunity.

The presumed hyporesponsive behavior of rheumatoid arthritis T lymphocytes can be attributed to spontaneous ex vivo apoptosis rather than defects in T cell receptor signaling

Genetic associations and the clinical success of compounds targeting TCR costimulatory proteins suggest an active role for TCR signaling in the initiation and perpetuation of rheumatoid arthritis (RA). Paradoxically, T cells isolated from affected joints in RA show impaired proliferative and cytokine responses following stimulation with mitogens and recall Ags attributed in part to chronic T cell exposure to oxidative stress and inflammatory cytokines. Therefore, it is uncertain how local autoreactive TCR signaling contributes to pathology in established RA. Using single-cell analysis, we show that in contrast to results obtained in bulk culture assays, T cells from the synovial fluid of RA patients proliferate and produce cytokines (IL-2, TNF-alpha, and IFN-gamma) as efficiently, if not more so, than T cells isolated from healthy donors and RA patient peripheral blood following TCR/CD28 stimulation. RA synovial fluid T cell hyporesponsiveness observed in bulk cultures can be attributed to spontaneous apoptosis ex vivo, which is associated with altered ratios of proapoptotic Noxa and anti-apoptotic Mcl-1 expression. The absence of RA synovial T cell proliferation and cytokine production in situ, despite the capacity of these cells to support productive TCR signaling, suggests that T cells contribute to local pathology in established RA by TCR-independent mechanisms.

Diversity in CD8(+) T cell differentiation

CD8(+) T cells are key effector cells of the adaptive immune system, however their activity must be tightly regulated to allow pathogen clearance whilst preventing immunopathology and autoimmunity. In this review, we summarise the diversity of responses that CD8(+) T cells make to antigenic stimulation with a focus on how CD8(+) T cell responses are regulated to achieve different immune outcomes. In particular, we discuss phenotypic diversity during tolerance induction as well as signals that drive effector and memory cell differentiation in response to infection.

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.

Homeostasis of naive and memory T cells

The peripheral mature T cell pool is regulated by complex homeostatic mechanisms. Naive T cells are maintained by interleukin-7 (IL-7) and T cell receptor (TCR) signaling from contact with major histocompatibility complex (MHC), which sustain expression of antiapoptotic molecules and allow the cells to survive in interphase. Competition for these ligands declines when T cell numbers are reduced and causes residual naive T cells to proliferate and differentiate into memory-like cells. This memory cell population is thus heterogeneous and comprised of cells derived from responses to both foreign and self-antigens. Typical memory cells are kept alive and induced to divide intermittently by a mixture of IL-7 and IL-15. This review highlights recent advances in how naive and memory T cell homeostasis is regulated.

Foxo1 links homing and survival of naive T cells by regulating L-selectin, CCR7 and interleukin 7 receptor

Foxo transcription factors have a conserved role in the adaptation of cells and organisms to nutrient and growth factor availability. Here we show that Foxo1 has a crucial, nonredundant role in T cells. In naive T cells, Foxo1 controlled the expression of the adhesion molecule L-selectin, the chemokine receptor CCR7 and the transcription factor Klf2, and its deletion was sufficient to alter lymphocyte trafficking. Furthermore, Foxo1 deficiency resulted in a severe defect in interleukin 7 receptor alpha-chain (IL-7Ralpha) expression associated with its ability to bind an Il7r enhancer. Finally, growth factor withdrawal induced a Foxo1-dependent increase in Sell, Klf2 and Il7r expression. These data suggest that Foxo1 regulates the homeostasis and life span of naive T cells by sensing growth factor availability and regulating homing and survival signals.

TCR down-regulation controls virus-specific CD8+ T cell responses

The CD3gamma di-leucine-based motif plays a central role in TCR down-regulation. However, little is understood about the role of the CD3gamma di-leucine-based motif in physiological T cell responses. In this study, we show that the expansion in numbers of virus-specific CD8(+) T cells is impaired in mice with a mutated CD3gamma di-leucine-based motif. The CD3gamma mutation did not impair early TCR signaling, nor did it compromise recruitment or proliferation of virus-specific T cells, but it increased the apoptosis rate of the activated T cells by increasing down-regulation of the antiapoptotic molecule Bcl-2. This resulted in a 2-fold reduction in the clonal expansion of virus-specific CD8(+) T cells during the acute phase of vesicular stomatitis virus and lymphocytic choriomeningitis virus infections. These results identify an important role of CD3gamma-mediated TCR down-regulation in virus-specific CD8(+) T cell responses.

A novel role for CD4+ T cells in the control of cachexia

Cachexia is the dramatic weight loss and muscle atrophy seen in chronic disease states, including autoimmunity, cancer, and infection, and is often associated with lymphopenia. We have previously shown that CD4(+) T cells that express the lowest density of CD44 (CD4(+)CD44(v.low)) are significantly reduced in diabetic NOD mice that are cachexic compared with diabetic mice that are not cachexic. Using this model, and a model of cancer cachexia, we test the hypothesis that CD4(+)CD44(v.low) cells play an active role in protecting the host from cachexia. CD4(+)CD44(v.low) cells, but not CD4(+) cells depleted of CD44(v.low) cells, delay the onset of wasting when infused into either diabetic or prediabetic NOD recipients. However, no significant effect on the severity of diabetes was detected. In a model of cancer cachexia, they significantly reduce muscle atrophy, and inhibit muscle protein loss and DNA loss, even when given after the onset of cachexia. Protection from wasting and muscle atrophy by CD4(+)CD44(v.low) cells is associated with protection from lymphopenia. These data suggest, for the first time, a role for an immune cell subset in protection from cachexia, and further suggest that the mechanism of protection is independent of protection from autoimmunity.

The anti-apoptotic Bcl-2 family member Mcl-1 promotes T lymphocyte survival at multiple stages

T lymphocyte development and function are tightly regulated by the intrinsic death pathway through members of the Bcl-2 family. Genetic studies have demonstrated that the Bcl-2 family member Mcl-1 is an important anti-apoptotic protein in the development of multiple cell types including T lymphocytes. However, the expression pattern and anti-apoptotic roles of Mcl-1 in T lymphocytes at different developmental stages remain to be fully determined. In this study, we examined the expression pattern of Mcl-1 in different populations of T cells at the single-cell level and found that Mcl-1 protein is constitutively expressed in all T cell populations and up-regulated upon TCR stimulation. We then investigated the role of Mcl-1 in the survival of these different populations by conditionally deleting Mcl-1 at various T cell stages. Our results show that Mcl-1 is required for the survival of double-negative and single-positive thymocytes as well as naive and activated T cells. Furthermore, we demonstrate that Mcl-1 functions together with Bcl-xL to promote double-positive thymocyte survival. Thus, Mcl-1 is a critical anti-apoptotic factor for the survival of T cells at multiple stages in vivo.

Targeting the Bcl-2-regulated apoptosis pathway by BH3 mimetics: a breakthrough in anticancer therapy?

Induction of apoptosis in tumor cells by direct activation of the Bcl-2-regulated apoptosis pathway by small molecule drugs carries high hopes to overcome the shortcomings of current anticancer therapies. This novel therapy concept builds on emerging insights into how Bcl-2-like molecules maintain mitochondrial integrity and how pro-apoptotic BH3-only proteins lead to its disruption. Means to unleash the pro-apoptotic potential of BH3-only proteins in tumor cells, or to bypass the need for BH3-only proteins by directly blocking possible interactions of Bcl-2-like pro-survival molecules with Bax and/or Bak, constitute interesting options for the design of novel anticancer therapies. For the optimization and clinical implementation of these novel anticancer strategies, a detailed understanding of the role of individual BH3-only proteins in cell death signaling in healthy cells and during tumor suppression is required. In this review, we will touch on the latest findings on BH3-only protein function and attempts to define the molecular properties of the so-called 'BH3 mimetics,' a novel class of anticancer agents, able to prompt apoptosis in tumor cells, regardless of their p53 or Bcl-2 status.

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.

Shaping and reshaping CD8+ T-cell memory

The ability to develop and sustain populations of memory T cells after infection or immunization is a hallmark of the adaptive immune response and a basis for protective vaccination against infectious disease. Technical advances that allow direct ex vivo identification and characterization of antigen-specific CD8+ T cells at various stages of the response to infection or vaccination in mouse models have fuelled efforts to characterize the factors that control memory CD8+ T-cell generation. Here, we dissect the input signals that shape the characteristics of the memory CD8+ T-cell response and discuss how manipulation of these signals has the potential to reshape CD8+ T-cell memory and improve the efficacy of vaccination.

Proapoptotic Bcl-2 family member Bim promotes persistent infection and limits protective immunity

Following the peak of the T-cell response, most of the activated effector T cells die by apoptosis driven by the proapoptotic Bcl-2 family member Bim (Bcl-2-interacting mediator of death). Whether the absence of Bim-mediated T-cell apoptosis can affect protective immunity remains unclear. Here, we used a mouse model of Leishmania major infection, in which parasite persistence and protective immunity are controlled by an equilibrium reached between parasite-specific gamma interferon (IFN-gamma)-producing effector T cells and interleukin-10 (IL-10)-producing CD4+ CD25+ T regulatory cells. To further understand the role of Bim-mediated apoptosis in persistent infection and protective immunity, we infected Bim-/- mice with L. major. We found that the initial parasite growth and lesion development were similar in Bim-/- and wild-type mice after primary L. major infection. However, at later times after infection, Bim-/- mice had significantly increased L. major-specific CD4+ T-cell responses and were resistant to persistent infection. Interestingly, despite their resistance to primary L. major infection, Bim-/- mice displayed significantly enhanced protection against challenge with L. major. Increased resistance to challenge in Bim-/- mice was associated with a significant increase in the number of L. major-specific IFN-gamma-producing CD4+ T cells and a lack of IL-10 production at the challenge site. Taken together, these data suggest that Bim limits protective immunity and that the absence of Bim allows the host to bypass antigen persistence for maintenance of immunity against reinfection.