Blimp-1 transcription factor is required for the differentiation of effector CD8(+) T cells and memory responses

Use it or lose it: establishment and persistence of T cell memory

Pre-existing T cell memory provides substantial protection against viral, bacterial, and parasitic infections. The generation of protective T cell memory constitutes a primary goal for cell-mediated vaccines, thus understanding the mechanistic basis of memory development and maintenance are of major importance. The widely accepted idea that T cell memory pools are directly descended from the effector populations has been challenged by recent reports that provide evidence for the early establishment of T cell memory and suggest that the putative memory precursor T cells do not undergo full expansion to effector status. Moreover, it appears that once the memory T cells are established early in life, they can persist for the lifetime of an individual. This is in contrast to the reported waning of naïve T cell immunity with age. Thus, in the elderly, immune memory that was induced at an early age may be more robust than recently induced memory, despite the necessity for long persistence. The present review discusses the mechanisms underlying the early establishment of immunological memory and the subsequent persistence of memory T cell pools in animal models and humans.

FoxP3⁺ regulatory CD4 T cells control the generation of functional CD8 memory

During the primary immune response, CD8 memory emerges from an environment of strong immune activation. The FoxP3⁺ regulatory CD4 T-cell subset (Treg) is known as a key suppressive component of the immune system. Here we report that Tregs are required for the generation of functional CD8 memory. In the absence of Tregs during priming, the resulting memory cells proliferate poorly and fail to differentiate into functional cytotoxic secondary effectors following antigen reactivation. We find that the Tregs act early, during the expansion phase of primary CD8 effectors, by fine tuning interleukin-2 exposure of CD8 memory precursors. This crucial new role of Tregs has implications for optimal vaccine development.

The role of CD4⁺ T cells in the generation of memory CD8⁺ T cells is not fully understood. In this study, the exposure of CD8 memory precursors to interleukin-2 during early antigen priming is shown to be controlled by FoxP3⁺ regulatory CD4⁺ T cells, resulting in the production of functional memory cells.

Loss of E protein transcription factors E2A and HEB delays memory-precursor formation during the CD8+ T-cell immune response

The transcription factors E2A and HEB (members of the E protein family) have been shown to play essential roles in lymphocyte development, while their negative regulators, the Id proteins, have been implicated in both lymphocyte development and in the CD8(+) T-cell immune response. Here, we show that E proteins also influence CD8(+) T cells responding to infection. E protein expression was upregulated by CD8(+) T cells during the early stages of infection and increased E protein DNA-binding activity could be detected upon TCR stimulation. Deficiency in the E proteins, E2A and HEB, led to increased frequency of terminally differentiated effector KLRG1(hi) CD8(+) T cells in mice during infection, and decreased generation of longer-lived memory-precursor cells during the immune response. These data suggest a model whereby E protein transcription factor activity favors rapid memory-precursor T-cell formation while their negative regulators, Id2 and Id3, are both required for robust effector CD8(+) T-cell response during infection.

Contribution of pulmonary KLRG1(high) and KLRG1(low) CD8 T cells to effector and memory responses during influenza virus infection

In response to pathogen insult, CD8 T cells undergo expansion and a dynamic differentiation process into functionally different subpopulations. In this study, we show that during the effector response to influenza virus infection lung CD8 T cell subsets expressing killer cell lectin-like receptor G1 (KLRG1)(high) or KLRG1(low) had similar effector functions and immediate recall efficacy. The KLRG1 expression profile of lung CD8 T cells was not permanent after adoptive transfer and recall. Airway CD8 T cells exhibited a unique phenotype expressing low levels of KLRG1 together with high levels of markers of cellular activation. We investigated the functional characteristics of these cells by analyzing their capacity to survive and to respond to a secondary challenge outside of the airway environment. KLRG1(high) CD8 T cells isolated from the lung during the peak of the effector T cell response could survive for more than a month in the absence of cognate viral Ags after systemic adoptive transfer, and these "rested" CD8 T cells proliferated and participated in a recall response to influenza virus infection. These data highlight the unique phenotype and plasticity of effector CD8 T cell responses in the lung.

Transcriptional control of effector and memory CD8+ T cell differentiation

During an infection, T cells can differentiate into multiple types of effector and memory T cells, which help to mediate pathogen clearance and provide long-term protective immunity. These cells can vary in their phenotype, function and location, and in their long-term fate in terms of their ability to populate the memory T cell pool. Over the past decade, the signalling pathways and transcriptional programmes that regulate the formation of heterogeneous populations of effector and memory CD8(+) T cells have started to be characterized, and this Review discusses the major advances in these areas.

TCR signaling via Tec kinase ITK and interferon regulatory factor 4 (IRF4) regulates CD8+ T-cell differentiation

CD8(+) T-cell development in the thymus generates a predominant population of conventional naive cells, along with minor populations of "innate" T cells that resemble memory cells. Recent studies analyzing a variety of KO or knock-in mice have indicated that impairments in the T-cell receptor (TCR) signaling pathway produce increased numbers of innate CD8(+) T cells, characterized by their high expression of CD44, CD122, CXCR3, and the transcription factor, Eomesodermin (Eomes). One component of this altered development is a non-CD8(+) T cell-intrinsic role for IL-4. To determine whether reduced TCR signaling within the CD8(+) T cells might also contribute to this pathway, we investigated the role of the transcription factor, IFN regulatory factor 4 (IRF4). IRF4 is up-regulated following TCR stimulation in WT T cells; further, this up-regulation is impaired in T cells treated with a small-molecule inhibitor of the Tec family tyrosine kinase, IL-2 inducible T-cell kinase (ITK). In contrast to WT cells, activation of IRF4-deficient CD8(+) T cells leads to rapid and robust expression of Eomes, which is further enhanced by IL-4 stimulation. In addition, inhibition of ITK together with IL-4 increases Eomeso up-regulation. These data indicate that ITK signaling promotes IRF4 up-regulation following CD8(+) T-cell activation and that this signaling pathway normally suppresses Eomes expression, thereby regulating the differentiation pathway of CD8(+) T cells.

Molecular definition of the identity and activation of natural killer cells

Using whole-genome microarray data sets of the Immunological Genome Project, we demonstrate a closer transcriptional relationship between NK cells and T cells than between any other leukocytes, distinguished by their shared expression of genes encoding molecules with similar signaling functions. Whereas resting NK cells are known to share expression of a few genes with cytotoxic CD8(+) T cells, our transcriptome-wide analysis demonstrates that the commonalities extend to hundreds of genes, many encoding molecules with unknown functions. Resting NK cells demonstrate a 'preprimed' state compared with naive T cells, which allows NK cells to respond more rapidly to viral infection. Collectively, our data provide a global context for known and previously unknown molecular aspects of NK cell identity and function by delineating the genome-wide repertoire of gene expression of NK cells in various states.

Human lymphoma mutations reveal CARD11 as the switch between self-antigen-induced B cell death or proliferation and autoantibody production

CARD11 signaling determines whether antigen stimulation induces B cells to proliferate or die.

Self-tolerance and immunity are actively acquired in parallel through a poorly understood ability of antigen receptors to switch between signaling death or proliferation of antigen-binding lymphocytes in different contexts. It is not known whether this tolerance-immunity switch requires global rewiring of the signaling apparatus or if it can arise from a single molecular change. By introducing individual CARD11 mutations found in human lymphomas into antigen-activated mature B lymphocytes in mice, we find here that lymphoma-derived CARD11 mutations switch the effect of self-antigen from inducing B cell death into T cell–independent proliferation, Blimp1-mediated plasmablast differentiation, and autoantibody secretion. Our findings demonstrate that regulation of CARD11 signaling is a critical switch governing the decision between death and proliferation in antigen-stimulated mature B cells and that mutations in this switch represent a powerful initiator for aberrant B cell responses in vivo.

Cutting edge: generation of memory precursors and functional memory CD8+ T cells depends on T cell factor-1 and lymphoid enhancer-binding factor-1

T cell factor (TCF)-1 and lymphoid enhancer-binding factor (LEF)-1 transcription factors have redundant roles in promoting thymocyte maturation. TCF-1 has been recently shown to critically regulate memory CD8+ T cell differentiation and persistence. The complete spectra of regulatory roles for TCF-1 and LEF-1 in CD8+ T cell responses are yet unknown. We conditionally targeted LEF-1, and by combination with germline deletion of TCF-1, we found that loss of both factors completely abrogated the generation of KLR G1(lo)IL-7Rα+ memory precursors in effector CD8+ T cell populations in response to Listeria monocytogenes infection. Whereas CD8+ effectors deficient for TCF-1 and LEF-1 retained the capacity to express IFN-γ, granzyme B, and perforin, they were defective in TNF-α production. In the memory phase, the Ag-specific CD8+ T cells lacking TCF-1 and LEF-1 exhibited an effector phenotype and were severely impaired in secondary expansion upon rechallenge. Thus, TCF-1 and LEF-1 cooperatively regulate generation of memory precursors and protective memory CD8+ T cells.

Alternative splicing regulates Prdm1/Blimp-1 DNA binding activities and corepressor interactions

Prdm1/Blimp-1 is a master regulator of gene expression in diverse tissues of the developing embryo and adult organism. Its C-terminal zinc finger domain mediates nuclear import, DNA binding, and recruitment of the corepressors G9a and HDAC1/2. Alternatively spliced transcripts lacking exon 7 sequences encode a structurally divergent isoform (Blimp-1Δexon7) predicted to have distinct functions. Here we demonstrate that the short Blimp-1Δexon7 isoform lacks DNA binding activity and fails to bind G9a or HDAC1/2 but retains the ability to interact with PRMT5. To investigate functional roles of alternative splicing in vivo, we engineered novel mouse strains via embryonic stem (ES) cell technology. Like null mutants, embryos carrying a targeted deletion of exon 7 and exclusively expressing Blimp-1Δexon7 die at around embryonic day 10.5 (E10.5) due to placental defects. In heterozygous Δexon7 mice, there is no evidence of dominant-negative effects. Mice carrying a knock-in allele with an exon 6-exon 7 fusion express full-length Blimp-1 only, develop normally, are healthy and fertile as adults, and efficiently generate mature plasma cells. These findings strongly suggest that the short Blimp-1Δexon7 isoform is dispensable. We propose that developmentally regulated alternative splicing is influenced by chromatin structure at the locus and fine-tunes Blimp-1's functional capabilities.

The Prdm family: expanding roles in stem cells and development

Members of the Prdm family are characterized by an N-terminal PR domain that is related to the SET methyltransferase domain, and multiple zinc fingers that mediate sequence-specific DNA binding and protein-protein interactions. Prdm factors either act as direct histone methyltransferases or recruit a suite of histone-modifying enzymes to target promoters. In this way, they function in many developmental contexts to drive and maintain cell state transitions and to modify the activity of developmental signalling pathways. Here, we provide an overview of the structure and function of Prdm family members and discuss the roles played by these proteins in stem cells and throughout development.

Mouse Hobit is a homolog of the transcriptional repressor Blimp-1 that regulates NKT cell effector differentiation

The transcriptional repressor Blimp-1 mediates the terminal differentiation of many cell types, including T cells. Here we identified Hobit (Znf683) as a previously unrecognized homolog of Blimp-1 that was specifically expressed in mouse natural killer T cells (NKT cells). Through studies of Hobit-deficient mice, we found that Hobit was essential for the formation of mature thymic NKT cells. In the periphery, Hobit repressed the accumulation of interferon-γ (IFN-γ)-producing NK1.1(lo) NKT cells at steady state. After antigenic stimulation, Hobit repressed IFN-γ expression, whereas after innate stimulation, Hobit induced granzyme B expression. Thus, reminiscent of the function of Blimp-1 in other lymphocytes, Hobit controlled the maintenance of quiescent, fully differentiated NKT cells and regulated their immediate effector functions.

Regulating the immune system via IL-15 transpresentation

Transpresentation has emerged as an important mechanism mediating IL-15 responses in a subset of lymphocytes during the steady state. In transpresentation, cell surface IL-15, bound to IL-15Rα is delivered to opposing lymphocytes during a cell-cell interaction. The events most dependent on IL-15 include the development and homeostasis of memory CD8 T cells, Natural Killer cells, invariant Natural Killer T cells, and intraepithelial lymphocytes. As lymphocyte development and homeostasis involve multiple steps and mechanisms, IL-15 transpresentation can have diverse roles throughout. Moreover, distinct stages of lymphocyte differentiation require IL-15 transpresented by different cells, which include both hematopoietic and non-hematopoietic cell types. Herein, we will describe the points where IL-15 transpresentation impacts these processes, the specific cells thought to drive IL-15 responses, as well as their role in the course of development and homeostasis.

Activation of the Hippo pathway by CTLA-4 regulates the expression of Blimp-1 in the CD8+ T cell

During the primary response, the commitment of the CD8(+) T cell to Blimp-1 expression and the terminal differentiation that Blimp-1 induces must be timed so as not to impair the process of clonal expansion. We determined whether the Hippo pathway, which links cell-cell contact to differentiation in other cell lineages, controls Blimp-1 expression. Activating the CD8(+) T cell with antigen and IL-2 causes expression of the core Hippo pathway components, including the pivotal transcriptional cofactor Yap. Contact between activated CD8(+) T cells induces Hippo pathway-mediated Yap degradation and Blimp-1 expression; a Hippo-resistant, stable form of Yap suppresses Blimp-1 expression. Cytotoxic T lymphocyte antigen 4 (CTLA-4) and CD80 comprise the receptor-ligand pair that mediates contact-dependent Hippo pathway activation. In vivo, CD8(+) T cells expressing Hippo resistant-Yap or lacking CTLA-4 have diminished expression of the senescence marker, KLRG1, during a viral infection. The CTLA-4/Hippo pathway/Blimp-1 system may couple terminal differentiation of CD8(+) T cell with the magnitude of clonal expansion.

Changes in functional but not structural avidity during differentiation of CD8+ effector cells in vivo after virus infection

By the peak of the CD8(+) T cell response, the effector cell pool consists of a heterogeneous population of cells that includes both those with an increased propensity to become long-lived memory cells (memory precursor effector cells; MPEC) and those that are terminally differentiated cells (short-lived effector cells; SLEC). Numerous studies have established the critical role that functional avidity plays in determining the in vivo efficacy of CD8(+) effector cells. Currently, how functional avidity differs in MPEC versus SLEC and the evolution of this property within these two populations during the expansion and contraction of the response are unknown. The data presented in this study show that at the peak of the effector response generated after poxvirus infection, SLEC were of higher functional avidity than their MPEC counterpart. Over time, however, SLEC exhibited a decrease in peptide sensitivity. This is in contrast to MPEC, which showed a modest increase in peptide sensitivity as the response reached equilibrium. The decrease in functional avidity in SLEC was independent of CD8 modulation or the amount of Ag receptor expressed by the T cell. Instead, the loss in sensitivity was correlated with decreased expression and activation of ZAP70 and Lck, critical components of TCR membrane proximal signaling. These results highlight the potential contribution of avidity in the differentiation and evolution of the T cell effector response after viral infection.

Homing and adhesion patterns determine the cellular composition of the bone marrow plasma cell niche

According to commonly held concepts, plasma cell (PC) longevity in bone marrow (BM) depends upon their access to survival niches. These are thought to exist in nursery cell types, which support PCs by secreting PC survival factors. To better define PC survival niches and their functioning, we adoptively transferred traceable Blimp-1-(GFP) PCs into recipient mice lacking a proliferation-inducing ligand (APRIL), IL-6, or macrophage migration inhibitory factor. Transferred BMPCs were preferentially associated with Ly-6C(high) monocytes (normalized colocalization index: 9.84), eosinophils (4.29), and megakaryocytes (2.12). Although APRIL was essential for BMPC survival, PC recruitment into the proximity of nursery cells was unimpaired in APRIL-deficient mice, questioning the concept that the same factors account for attraction/retention of PCs as for their local survival. Rather, the order of colocalization with BMPCs (monocytes > eosinophils > megakaryocytes) reflected these cells' relative expression of CXCR4, VLA-4, and LFA-1, the homing and adhesion molecules that direct/retain PCs in the BM. This suggests a scenario wherein the cellular composition of the BMPC niche is defined by a common pattern of attraction/retention on CXCL12-abundant reticular docking cells. Thereby, PCs are directed to associate in a functional BM niche with hematopoietic CXCR4(+)VLA-4(+)LFA-1(+) nursery cells, which provide PC survival factors.

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.

CD8 T cells recruited early in mouse polyomavirus infection undergo exhaustion

Repetitive Ag encounter, coupled with dynamic changes in Ag density and inflammation, imparts phenotypic and functional heterogeneity to memory virus-specific CD8 T cells in persistently infected hosts. For herpesvirus infections, which cycle between latency and reactivation, recent studies demonstrate that virus-specific T cell memory is predominantly derived from naive precursors recruited during acute infection. Whether functional memory T cells to viruses that persist in a nonlatent, low-level infectious state (smoldering infection) originate from acute infection-recruited naive T cells is not known. Using mouse polyomavirus (MPyV) infection, we previously showed that virus-specific CD8 T cells in persistently infected mice are stably maintained and functionally competent; however, a sizeable fraction of these memory T cells are short-lived. Further, we found that naive anti-MPyV CD8 T cells are primed de novo during persistent infection and contribute to maintenance of the virus-specific CD8 T cell population and its phenotypic heterogeneity. Using a new MPyV-specific TCR-transgenic system, we now demonstrate that virus-specific CD8 T cells recruited during persistent infection possess multicytokine effector function, have strong replication potential, express a phenotype profile indicative of authentic memory capability, and are stably maintained. In contrast, CD8 T cells recruited early in MPyV infection express phenotypic and functional attributes of clonal exhaustion, including attrition from the memory pool. These findings indicate that naive virus-specific CD8 T cells recruited during persistent infection contribute to preservation of functional memory against a smoldering viral infection.

Role of interleukin 10 transcriptional regulation in inflammation and autoimmune disease

Interleukin 10 (IL-10) is a cytokine with potent anti-inflammatory properties that plays a central role in limiting host immune response to pathogens, thereby preventing damage to the host and maintaining normal tissue homeostasis. Dysregulation of IL-10 is associated with enhanced immunopathology in response to infection as well as increased risk for development of many autoimmune diseases. Thus a fundamental understanding of IL-10 gene expression is critical for our comprehension of disease progression and resolution of host inflammatory response. In this review, we discuss modes of regulation of IL-10 gene expression in immune effector cell types, including signal transduction, epigenetics, promoter architecture, and post-transcriptional regulation, and how aberrant regulation contributes to immunopathology and disease progression.

CD27 stimulation promotes the frequency of IL-7 receptor-expressing memory precursors and prevents IL-12-mediated loss of CD8(+) T cell memory in the absence of CD4(+) T cell help

Fully functional CD8(+) T cell memory is highly dependent upon CD4(+) T cell support. CD4(+) T cells play a critical role in inducing the expression of CD70, the ligand for CD27, on dendritic cells. In this study, we demonstrate that CD27 stimulation during primary CD8(+) T cell responses regulates the ability to mount secondary CD8(+) T cell responses. CD27 stimulation during vaccinia and dendritic cell immunization controls the expression of the IL-7R (CD127), which has been shown to be necessary for memory CD8(+) T cell survival. Furthermore, CD27 stimulation during primary CD8(+) T cell responses to vaccinia virus restrained the late expression on memory precursor cells of cytokine receptors that support terminal differentiation. The formation of CD8(+) T cell memory precursors and secondary CD8(+) T cell responses was restored in the absence of CD27 costimulation when endogenous IL-12 was not available. Similarly, the lesion in CD8(+) T cell memory that occurs in the absence of CD4(+) T cells did not occur in mice lacking IL-12. These data indicate that CD4(+) T cell help and, by extension, CD27 stimulation support CD8(+) T cell memory by modulating the expression of cytokine receptors that influence the differentiation and survival of memory CD8(+) T cells.

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.

Heterogeneity within T Cell Memory: Implications for Transplant Tolerance

Adaptive immunity in both mouse and man results in the generation of immunological memory. Memory T cells are both friend and foe to transplant recipients, as they are intimately involved and in many cases absolutely required for the maintenance of protective immunity in the face immunosuppression, yet from the evidence presented herein they clearly constitute a formidable barrier for the successful implementation of tolerance induction strategies in transplantation. This review describes the experimental evidence demonstrating the increased resistance of memory T cells to many distinct tolerance induction strategies, and outlines recent advances in our knowledge of the ways in which alloreactive memory T cells arise in previously untransplanted individuals. Understanding the impact of alloreactive memory T cell specificity, frequency, and quality might allow for better donor selection in order to minimize the donor-reactive memory T cell barrier in an individual transplant recipient, thus allowing stratification of relative risk of alloreactive memory T cell mediated rejection, and conversely increase the likelihood of successful establishment of tolerance. However, further research into the molecular and cellular pathways involved in alloreactive memory T cell-mediated rejection is required in order to design new strategies to overcome the memory T cell barrier, without critically impairing protective immunity.

The role of interleukin-2 during homeostasis and activation of the immune system

Interleukin-2 (IL-2) signals influence various lymphocyte subsets during differentiation, immune responses and homeostasis. As discussed in this Review, stimulation with IL-2 is crucial for the maintenance of regulatory T (T(Reg)) cells and for the differentiation of CD4(+) T cells into defined effector T cell subsets following antigen-mediated activation. For CD8(+) T cells, IL-2 signals optimize both effector T cell generation and differentiation into memory cells. IL-2 is presented in soluble form or bound to dendritic cells and the extracellular matrix. Use of IL-2 - either alone or in complex with particular neutralizing IL-2-specific antibodies - can amplify CD8(+) T cell responses or induce the expansion of the T(Reg) cell population, thus favouring either immune stimulation or suppression.

CD40-activated B cells can efficiently prime antigen-specific naïve CD8+ T cells to generate effector but not memory T cells

Background

The identification of the signals that should be provided by antigen-presenting cells (APCs) to induce a CD8⁺ T cell response in vivo is essential to improve vaccination strategies using antigen-loaded APCs. Although dendritic cells have been extensively studied, the ability of other APC types, such as B cells, to induce a CD8⁺ T cell response have not been thoroughly evaluated.

Methodology/Principal Findings

In this manuscript, we have characterized the ability of CD40-activated B cells, stimulated or not with Toll-like receptor (TLR) agonists (CpG or lipopolysaccharide) to induce the response of mouse naïve CD8⁺ T cells in vivo. Our results show that CD40-activated B cells can directly present antigen to naïve CD8⁺ T cells to induce the generation of potent effectors able to secrete cytokines, kill target cells and control a Listeria monocytogenes infection. However, CD40-activated B cell immunization did not lead to the proper formation of CD8⁺ memory T cells and further maturation of CD40-activated B cells with TLR agonists did not promote the development of CD8⁺ memory T cells. Our results also suggest that inefficient generation of CD8⁺ memory T cells with CD40-activated B cell immunization is a consequence of reduced Bcl-6 expression by effectors and enhanced contraction of the CD8⁺ T cell response.

Conclusions

Understanding why CD40-activated B cell immunization is defective for the generation of memory T cells and gaining new insights about signals that should be provided by APCs are key steps before translating the use of CD40-B cell for therapeutic vaccination.

A critical role for STAT3 transcription factor signaling in the development and maintenance of human T cell memory

STAT3 transcription factor signaling in specific T helper cell differentiation has been well described, although the broader roles for STAT3 in lymphocyte memory are less clear. Patients with autosomal-dominant hyper-IgE syndrome (AD-HIES) carry dominant-negative STAT3 mutations and are susceptible to a variety of bacterial and fungal infections. We found that AD-HIES patients have a cell-intrinsic defect in the number of central memory CD4(+) and CD8(+) T cells compared to healthy controls. Naive T cells from AD-HIES patients had lower expression of memory-related transcription factors BCL6 and SOCS3, a primary proliferation defect, and they failed to acquire central memory-like surface phenotypes in vitro. AD-HIES patients showed a decreased ability to control varicella zoster virus (VZV) and Epstein-Barr virus (EBV) latency, and T cell memory to both of these viruses was compromised. These data point to a specific role for STAT3 in human central memory T cell formation and in control of certain chronic viruses.

The effects of TLR activation on T-cell development and differentiation

Invading pathogens have unique molecular signatures that are recognized by Toll-like receptors (TLRs) resulting in either activation of antigen-presenting cells (APCs) and/or costimulation of T cells inducing both innate and adaptive immunity. TLRs are also involved in T-cell development and can reprogram Treg cells to become helper cells. T cells consist of various subsets, that is, Th1, Th2, Th17, T follicular helper (Tfh), cytotoxic T lymphocytes (CTLs), regulatory T cells (Treg) and these originate from thymic progenitor thymocytes. T-cell receptor (TCR) activation in distinct T-cell subsets with different TLRs results in differing outcomes, for example, activation of TLR4 expressed in T cells promotes suppressive function of regulatory T cells (Treg), while activation of TLR6 expressed in T cells abrogates Treg function. The current state of knowledge of regarding TLR-mediated T-cell development and differentiation is reviewed.

An interleukin-21-interleukin-10-STAT3 pathway is critical for functional maturation of memory CD8+ T cells

Memory CD8(+) T cells are critical for long-term immunity, but the genetic pathways governing their formation remain poorly defined. This study shows that the IL-10-IL-21-STAT3 pathway is critical for memory CD8(+) T cell development after acute LCMV infection. In the absence of either interleukin-10 (IL-10) and IL-21 or STAT3, virus-specific CD8(+) T cells retain terminal effector (TE) differentiation states and fail to mature into protective memory T cells that contain self-renewing central memory T cells. Expression of Eomes, BCL-6, Blimp-1, and SOCS3 was considerably reduced in STAT3-deficient memory CD8(+) T cells, and BCL-6- or SOCS3-deficient CD8(+) T cells also had perturbed memory cell development. Reduced SOCS3 expression rendered STAT3-deficient CD8(+) T cells hyperresponsive to IL-12, suggesting that the STAT3-SOCS3 pathway helps to insulate memory precursor cells from inflammatory cytokines that drive TE differentiation. Thus, memory CD8(+) T cell precursor maturation is an active process dependent on IL-10-IL-21-STAT3 signaling.

The transcriptional regulators Id2 and Id3 control the formation of distinct memory CD8+ T cell subsets

During infection, naive CD8(+) T cells differentiate into effector cells, which are armed to eliminate pathogens, and memory cells, which are poised to protect against reinfection. The transcriptional program that regulates terminal differentiation into short-lived effector-memory versus long-lived memory cells is not clearly defined. Through the use of mice expressing reporters for the DNA-binding inhibitors Id2 and Id3, we identified Id3(hi) precursors of long-lived memory cells before the peak of T cell population expansion or upregulation of cell-surface receptors that indicate memory potential. Deficiency in Id2 or Id3 resulted in loss of distinct CD8(+) effector and memory populations, which demonstrated unique roles for these inhibitors of E-protein transcription factors. Furthermore, cytokines altered the expression of Id2 and Id3 differently, which provides insight into how external cues influence gene expression.

Repression of the DNA-binding inhibitor Id3 by Blimp-1 limits the formation of memory CD8+ T cells

Blimp-1 is a transcriptional repressor that promotes the differentiation of CD8⁺ T cells into short-lived KLRG-1⁺ effector cells (SLEC), but how it operates remains poorly defined. Here we show that Blimp-1 binds and represses the Id3 promoter in SLEC. Repression of Id3 by Blimp-1 was dispensable for SLEC development but limited their capacity to persist as memory cells. Enforced expression of Id3 was sufficient to rescue SLEC survival and enhanced recall responses. Id3 function was mediated in part through inhibition of E2a transcriptional activity and induction of genes regulating genome stability. These findings identify a Blimp-1-Id3-E2a axis as a key molecular switch that determines whether effector CD8⁺ T cells are programmed to die or enter the memory pool.

CD8(+) T cells: foot soldiers of the immune system

Resting naive CD8(+) T cells have an astounding capacity to react to pathogens by massive expansion and differentiation into cytotoxic effector cells that migrate to all corners of the body to clear the infection. The initial interaction with antigen-presenting cells in the central lymphoid organs drives an orchestrated program of differentiation aimed at producing sufficient numbers of effectors to get the job done without resulting in clonal exhaustion. Interactions with antigen-presenting cells and other immune cells continue at the site of infection to regulate further on-site expansion and differentiation, all with the goal of protecting the host with minimal bystander tissue damage. Here we review recent advances in CD8(+) T cell recognition of antigen in lymphoid as well as in nonlymphoid tissues in the periphery, and how CD8(+) T cell expansion and differentiation are controlled in these contexts.

Osteopontin modulates the generation of memory CD8+ T cells during influenza virus infection

The adaptive immune system generates memory cells, which induce a rapid and robust immune response following secondary Ag encounter. Memory CD8(+) T cells are a critical component of protective immunity against infections and cancers. Therefore, understanding the mechanism whereby memory CD8(+) T cells are generated and maintained is important for inducing effective memory CD8(+) T cell response. Recent studies have demonstrated that the inflammatory cytokine IL-12 favors the generation of terminal effector CD8(+) T cells rather than memory precursor effector CD8(+) T cells by regulating the expression of the transcription factor T-bet. In this study, we report that the inflammatory cytokine osteopontin (Opn) modulates memory CD8(+) T cell generation during influenza virus infection. Although Opn wild-type and Opn knockout (KO) mice had similar numbers of virus-specific effector CD8(+) T cells, virus-specific effector CD8(+) T cells generated in Opn KO mice showed low levels of T-bet expression and an increased memory precursor cell population compared with cells generated in Opn wild-type mice. This resulted in the persistently increased number of memory CD8(+) T cells in Opn KO mice. Studies with bone marrow-derived dendritic cells demonstrated that Opn deficiency in bone marrow-derived dendritic cells results in low levels of IL-12 production in response to the stimulation with influenza virus. Thus, we hypothesize that Opn modulates the generation of memory precursor effector CD8(+) T cells by regulating cytokine milieu during the acute phase of virus infection. This finding may provide new insight into the role of Opn in adaptive immune response.

IL-33 synergizes with TCR and IL-12 signaling to promote the effector function of CD8+ T cells

The effector functions of CD8(+) T cells are influenced by tissue inflammatory microenvironments. IL-33, a member of the IL-1 family, acts as a danger signal after its release during cell necrosis. The IL-33/ST2 axis has been implicated in various Th2 responses. Its role in CD8(+) T-cell-mediated immune response is, however, not known. Here we find that type 1 cytotoxic T (Tc1) cells cultured in vitro unexpectedly express high levels of the IL-33 receptor ST2. Interestingly, the expression of ST2 in Tc1 cells is dependent on T-bet, a master Th1/Tc1 transcription factor. In addition, IL-33 enhances TCR-triggered IFN-γ production. IL-33 together with IL-12 can stimulate IFN-γ production in Tc1 cells. Moreover, IL-33 synergizes with IL-12 to promote CD8(+) T-cell effector function. The synergistic effect of IL-33 and IL-12 is partly mediated by Gadd45b. Together, these in vitro data establish a novel role of IL-33 in promoting effector type 1 adaptive immune responses.

The basis of distinctive IL-2- and IL-15-dependent signaling: weak CD122-dependent signaling favors CD8+ T central-memory cell survival but not T effector-memory cell development

Recent work suggests that IL-2 and IL-15 induce distinctive levels of signaling through common receptor subunits and that such varied signaling directs the fate of Ag-activated CD8(+) T cells. In this study, we directly examined proximal signaling by IL-2 and IL-15 and CD8(+) T cell primary and memory responses as a consequence of varied CD122-dependent signaling. Initially, IL-2 and IL-15 induced similar p-STAT5 and p-S6 activation, but these activities were only sustained by IL-2. Transient IL-15-dependent signaling is due to limited expression of IL-15Rα. To investigate the outcome of varied CD122 signaling for CD8(+) T cell responses in vivo, OT-I T cells were used from mouse models where CD122 signals were attenuated by mutations within the cytoplasmic tail of CD122 or intrinsic survival function was provided in the absence of CD122 expression by transgenic Bcl-2. In the absence of CD122 signaling, generally normal primary response occurred, but the primed CD8(+) T cells were not maintained. In marked contrast, weak CD122 signaling supported development and survival of T central-memory (T(CM)) but not T effector-memory (T(EM)) cells. Transgenic expression of Bcl-2 in CD122(-/-) CD8(+) T cells also supported the survival and persistence of T(CM) cells but did not rescue T(EM) development. These data indicate that weak CD122 signals readily support T(CM) development largely through providing survival signals. However, stronger signals, independent of Bcl-2, are required for T(EM) development. Our findings are consistent with a model whereby low, intermediate, and high CD122 signaling support T(CM) memory survival, T(EM) programming, and terminal T effector cell differentiation, respectively.

Increased numbers of preexisting memory CD8 T cells and decreased T-bet expression can restrain terminal differentiation of secondary effector and memory CD8 T cells

Memory CD8 T cells acquire effector memory cell properties after reinfection and may reach terminally differentiated, senescent states ("Hayflick limit") after multiple infections. The signals controlling this process are not well understood, but we found that the degree of secondary effector and memory CD8 T cell differentiation was intimately linked to the amount of T-bet expressed upon reactivation and preexisting memory CD8 T cell number (i.e., primary memory CD8 T cell precursor frequency) present during secondary infection. Compared with naive cells, memory CD8 T cells were predisposed toward terminal effector (TE) cell differentiation because they could immediately respond to IL-12 and induce T-bet, even in the absence of Ag. TE cell formation after secondary (2°) or tertiary infections was dependent on increased T-bet expression because T-bet(+/-) cells were resistant to these phenotypic changes. Larger numbers of preexisting memory CD8 T cells limited the duration of 2° infection and the amount of IL-12 produced, and consequently, this reduced T-bet expression and the proportion of 2° TE CD8 T cells that formed. Together, these data show that over repeated infections, memory CD8 T cell quality and proliferative fitness is not strictly determined by the number of serial encounters with Ag or cell divisions, but is a function of the CD8 T cell differentiation state, which is genetically controlled in a T-bet-dependent manner. This differentiation state can be modulated by preexisting memory CD8 T cell number and the intensity of inflammation during reinfection. These results have important implications for vaccinations involving prime-boost strategies.

ICOS receptor instructs T follicular helper cell versus effector cell differentiation via induction of the transcriptional repressor Bcl6

The nature of follicular helper CD4(+) T (Tfh) cell differentiation remains controversial, including the minimal signals required for Tfh cell differentiation and the time at which Tfh cell differentiation occurs. Here we determine that Tfh cell development initiates immediately during dendritic cell (DC) priming in vivo. We demonstrate that inducible costimulator (ICOS) provides a critical early signal to induce the transcription factor Bcl6, and Bcl6 then induces CXCR5, the canonical feature of Tfh cells. Strikingly, a bifurcation between Tfh and effector Th cells was measurable by the second cell division of CD4(+) T cells, at day 2 after an acute viral infection: IL2Rα(int) cells expressed Bcl6 and CXCR5 (Tfh cell program), whereas IL2Rα(hi) cells exhibited strong Blimp1 expression that repressed Bcl6 (effector Th cell program). Virtually complete polarization between Bcl6(+) Tfh cells and Blimp1(+) effector Th cell populations developed by 72 hr, even without B cells. Tfh cells were subsequently lost in the absence of B cells, demonstrating a B cell requirement for maintenance of Bcl6 and Tfh cell commitment via sequential ICOS signals.

Chemokine receptor CXCR3 facilitates CD8(+) T cell differentiation into short-lived effector cells leading to memory degeneration

Strength of inflammatory stimuli during the early expansion phase plays a crucial role in the effector versus memory cell fate decision of CD8(+) T cells. But it is not known how early lymphocyte distribution after infection has an impact on this process. We demonstrate that the chemokine receptor CXCR3 is involved in promoting CD8(+) T cell commitment to an effector fate rather than a memory fate by regulating T cell recruitment to an antigen/inflammation site. After systemic viral or bacterial infection, the contraction of CXCR3(-/-) antigen-specific CD8(+) T cells is significantly attenuated, resulting in massive accumulation of fully functional memory CD8(+) T cells. Early after infection, CXCR3(-/-) antigen-specific CD8(+) T cells fail to cluster at the marginal zone in the spleen where inflammatory cytokines such as IL-12 and IFN-α are abundant, thus receiving relatively weak inflammatory stimuli. Consequently, CXCR3(-/-) CD8(+) T cells exhibit transient expression of CD25 and preferentially differentiate into memory precursor effector cells as compared with wild-type CD8(+) T cells. This series of events has important implications for development of vaccination strategies to generate increased numbers of antigen-specific memory CD8(+) T cells via inhibition of CXCR3-mediated T cell migration to inflamed microenvironments.

Follicular helper CD4 T cells (TFH)

T cell help to B cells is a fundamental aspect of adaptive immunity and the generation of immunological memory. Follicular helper CD4 T (T(FH)) cells are the specialized providers of B cell help. T(FH) cells depend on expression of the master regulator transcription factor Bcl6. Distinguishing features of T(FH) cells are the expression of CXCR5, PD-1, SAP (SH2D1A), IL-21, and ICOS, among other molecules, and the absence of Blimp-1 (prdm1). T(FH) cells are important for the formation of germinal centers. Once germinal centers are formed, T(FH) cells are needed to maintain them and to regulate germinal center B cell differentiation into plasma cells and memory B cells. This review covers T(FH) differentiation, T(FH) functions, and human T(FH) cells, discussing recent progress and areas of uncertainty or disagreement in the literature, and it debates the developmental relationship between T(FH) cells and other CD4 T cell subsets (Th1, Th2, Th17, iTreg).

T cell exhaustion

T cell exhaustion is a state of T cell dysfunction that arises during many chronic infections and cancer. It is defined by poor effector function, sustained expression of inhibitory receptors and a transcriptional state distinct from that of functional effector or memory T cells. Exhaustion prevents optimal control of infection and tumors. Recently, a clearer picture of the functional and phenotypic profile of exhausted T cells has emerged and T cell exhaustion has been defined in many experimental and clinical settings. Although the pathways involved remain to be fully defined, advances in the molecular delineation of T cell exhaustion are clarifying the underlying causes of this state of differentiation and also suggest promising therapeutic opportunities.

Transcription factor T-bet represses expression of the inhibitory receptor PD-1 and sustains virus-specific CD8+ T cell responses during chronic infection

T cell exhaustion plays a major role in failure to control chronic infections. High expression of inhibitory receptors, including PD-1, and the inability to sustain functional T cell responses contribute to exhaustion. However, the transcriptional control of these processes remains unclear. Here we demonstrate that the transcription factor T-bet regulates CD8⁺ T cell exhaustion and inhibitory receptor expression. T-bet directly repressed Pdcd1 transcription and decreased the expression of other inhibitory receptors. While elevated T-bet promoted terminal differentiation following acute infection, high T-bet expression sustained exhausted CD8⁺ T cells and repressed inhibitory receptor expression during chronic viral infection. Persisting antigenic stimulation caused T-bet downregulation, which resulted in more severe exhaustion of CD8⁺ T cells. These observations suggest therapeutic opportunities involving increasing T-bet expression during chronic infection.

Immunity to seasonal and pandemic influenza A viruses

The introduction of a new influenza strain into human circulation leads to rapid global spread. This review summarizes innate and adaptive immunity to influenza viruses, with an emphasis on T-cell responses that provide cross-protection between distinct subtypes and strains. We discuss antigenic variation within T-cell immunogenic peptides and our understanding of pre-existing immunity towards the pandemic A(H1N1) 2009 strain.

Enhanced sensitivity to IL-2 signaling regulates the clinical responsiveness of IL-12-primed CD8(+) T cells in a melanoma model

The optimal expansion, trafficking, and function of adoptively transferred CD8(+) T cells are parameters that currently limit the effectiveness of antitumor immunity to established tumors. In this study, we addressed the mechanisms by which priming of self tumor-associated Ag-specific CD8(+) T cells influenced antitumor functionality in the presence of the inflammatory cytokine IL-12. In vitro priming of mouse tumor-specific CD8(+) T cells in the presence of IL-12 induced a diverse and rapid antitumor effector activity while still promoting the generation of memory cells. Importantly, IL-12-primed effector T cells dramatically reduced the growth of well-established s.c. tumors and significantly increased survival to highly immune resistant, established intracranial tumors. Control of tumor growth by CD8(+) T cells was dependent on IL-12-mediated upregulation of the high-affinity IL-2R (CD25) and a subsequent increase in the sensitivity to IL-2 stimulation. Finally, IL-12-primed human PBMCs generated tumor-specific T cells both phenotypically and functionally similar to IL-12-primed mouse tumor-specific T cells. These results highlight the ability of IL-12 to obviate the strict requirement for administering high levels of IL-2 during adoptive cell transfer-mediated antitumor responses. Furthermore, acquisition of a potent effector phenotype independent of cytokine support suggests that IL-12 could be added to adoptive cell transfer clinical strategies in cancer patients.

CD4+ memory T cell survival

Memory CD4+ T cells specific for a given antigen are generated during the primary response from the effector lymphoblast progeny of naïve precursors. How memory CD4+ T cells differentiate from the effector population is not understood but new tools to assess transcription factor and cytokine expression are allowing for a more careful assessment of this process. Here we review the factors that allow some effector CD4+ T cells to survive the contraction phase of the primary response and become memory cells, and consider whether parallels can be drawn between T and B cells.

The lineage-defining factors T-bet and Bcl-6 collaborate to regulate Th1 gene expression patterns

T-bet acts as a functional repressor in association with Bcl-6 to antagonize SOCS1, SOCS3, TCF-1, and late-stage IFN-γ to regulate Th1 development.

The T-box transcription factor T-bet is important for the differentiation of naive CD4⁺ T helper cells (Th cells) into the Th1 phenotype. Much is known about T-bet’s role as a transcriptional activator, but less is known about the mechanisms by which T-bet functionally represses alternative Th cell genetic programs. In this study, we first identify Socs1, Socs3, and Tcf7 (TCF-1) as gene targets that are negatively regulated by T-bet. Significantly, T-bet’s role in the repression of these genes is through a direct interaction with their promoters. Consistent with this, we identified two T-bet DNA-binding elements in the Socs1 promoter that are functionally used to down-regulate transcription in primary Th1 cells. Importantly, T-bet’s novel role in transcriptional repression is because of its ability to physically associate with, and functionally recruit, the transcriptional repressor Bcl-6 to a subset of promoters. Furthermore, T-bet functionally recruits Bcl-6 to the Ifng locus in late stages of Th1 differentiation to repress its activity, possibly to prevent the overproduction of IFN-γ, which could result in autoimmunity. Collectively, these data establish a novel mechanism for T-bet–mediated gene repression in which two lineage-defining transcription factors, one a classical activator and one a repressor, collaborate to promote and properly regulate Th1 development.

Asymmetric proteasome segregation as a mechanism for unequal partitioning of the transcription factor T-bet during T lymphocyte division

Polarized segregation of proteins in T cells is thought to play a role in diverse cellular functions including signal transduction, migration, and directed secretion of cytokines. Persistence of this polarization can result in asymmetric segregation of fate-determining proteins during cell division, which may enable a T cell to generate diverse progeny. Here, we provide evidence that a lineage-determining transcription factor, T-bet, underwent asymmetric organization in activated T cells preparing to divide and that it was unequally partitioned into the two daughter cells. This unequal acquisition of T-bet appeared to result from its asymmetric destruction during mitosis by virtue of concomitant asymmetric segregation of the proteasome. These results suggest a mechanism by which a cell may unequally localize cellular activities during division, thereby imparting disparity in the abundance of cell fate regulators in the daughter cells.

Meta-analysis identifies 29 additional ulcerative colitis risk loci, increasing the number of confirmed associations to 47

Genome-wide association studies and candidate gene studies in ulcerative colitis have identified 18 susceptibility loci. We conducted a meta-analysis of six ulcerative colitis genome-wide association study datasets, comprising 6,687 cases and 19,718 controls, and followed up the top association signals in 9,628 cases and 12,917 controls. We identified 29 additional risk loci (P < 5 × 10(-8)), increasing the number of ulcerative colitis-associated loci to 47. After annotating associated regions using GRAIL, expression quantitative trait loci data and correlations with non-synonymous SNPs, we identified many candidate genes that provide potentially important insights into disease pathogenesis, including IL1R2, IL8RA-IL8RB, IL7R, IL12B, DAP, PRDM1, JAK2, IRF5, GNA12 and LSP1. The total number of confirmed inflammatory bowel disease risk loci is now 99, including a minimum of 28 shared association signals between Crohn's disease and ulcerative colitis.

A role for Blimp1 in the transcriptional network controlling natural killer cell maturation

Natural killer (NK) cells are innate lymphocytes capable of immediate effector functions including cytokine production and cytotoxicity. Compared with B and T cells, the factors that control the peripheral maturation of NK cells are poorly understood. We show that Blimp1, a transcriptional repressor required for the differentiation of plasma cells and short-lived effector T cells, is expressed by NK cells throughout their development. Interleukin 15 (IL-15) is required for the early induction of Blimp1 in NK cells, with expression increasing in the most mature subsets of mouse and human NK cells. We show that Blimp1 is required for NK-cell maturation and homeostasis and for regulating their proliferative potential. It is also essential for high granzyme B expression, but not for most cytokine production and cytotoxicity. Surprisingly, interferon regulatory factor 4 (IRF4) and B-cell lymphoma 6 (Bcl6), 2 transcription factors crucial for the regulation of Blimp1 in B and T cells, are largely dispensable for Blimp1 expression in NK cells. T-bet deficiency, however, leads to attenuated Blimp1 expression. We have identified NK cells as the first hematopoietic cell type in which the IRF4-Blimp1-Bcl6 regulatory axis is not in operation, highlighting the distinct nature of the NK-cell gene-regulatory network.

CD4+ T cell help and innate-derived IL-27 induce Blimp-1-dependent IL-10 production by antiviral CTLs

Interleukin (IL)-10 is an important regulatory cytokine that can modulate excessive immune mediated injury. Several distinct cell types have been demonstrated to produce IL-10, including most recently CD8+ cytotoxic T lymphocytes (CTLs) responding to respiratory virus infection. Here we report that CD4+ T cell help in the form of IL-2 is required for IL-10 production by CTLs, but not for the induction of CTL effector cytokines. We show that IL-2 derived from CD4+ helper T cells cooperates with innate immune cell-derived IL-27 to amplify IL-10 production by CTLs through a Blimp-1-dependent mechanism. These findings reveal a previously unrecognized pathway that coordinates signals derived from innate and helper T cells to control the production of a regulatory cytokine by CTLs during acute viral infection.