Instruct: Given a scientific claim, retrieve passages that support or refute the claim.
Query: CD28 initiates tonic signaling in conventional T cells, which causes an exhaustion phenotype and limited efficiency.
Chimeric antigen receptors (CARs) targeting CD19 have mediated dramatic antitumor responses in hematologic malignancies, but tumor regression has rarely occurred using CARs targeting other antigens. It remains unknown whether the impressive effects of CD19 CARs relate to greater susceptibility of hematologic malignancies to CAR therapies, or superior functionality of the CD19 CAR itself. We show that tonic CAR CD3-ζ phosphorylation, triggered by antigen-independent clustering of CAR single-chain variable fragments, can induce early exhaustion of CAR T cells that limits antitumor efficacy. Such activation is present to varying degrees in all CARs studied, except the highly effective CD19 CAR. We further determine that CD28 costimulation augments, whereas 4-1BB costimulation reduces, exhaustion induced by persistent CAR signaling. Our results provide biological explanations for the antitumor effects of CD19 CARs and for the observations that CD19 CAR T cells incorporating the 4-1BB costimulatory domain are more persistent than those incorporating CD28 in clinical trials.
Thymic-derived natural T regulatory cells (Tregs) are characterized by functional and phenotypic heterogeneity. Recently, a small fraction of peripheral Tregs has been shown to express Klrg1, but it remains unclear as to what extent Klrg1 defines a unique Treg subset. In this study, we show that Klrg1(+) Tregs represent a terminally differentiated Treg subset derived from Klrg1(-) Tregs. This subset is a recent Ag-responsive and highly activated short-lived Treg population that expresses enhanced levels of Treg suppressive molecules and that preferentially resides within mucosal tissues. The development of Klrg1(+) Tregs also requires extensive IL-2R signaling. This activity represents a distinct function for IL-2, independent from its contribution to Treg homeostasis and competitive fitness. These and other properties are analogous to terminally differentiated short-lived CD8(+) T effector cells. Our findings suggest that an important pathway driving Ag-activated conventional T lymphocytes also operates for Tregs.
Sustained signaling from the T cell receptor (TCR) and costimulatory molecules is thought necessary for generating high numbers of effector T cells. Here, we show that Survivin is controlled in peripheral T cells by OX40 cosignaling via sustained PI3k and PKB activation. Survivin is induced by OX40 independent of mitotic progression in late G1, and blocking Survivin suppresses S-phase transition and division of T cells and leads to apoptosis. Moreover, Survivin expression alone is sufficient to restore proliferation and to antagonize apoptosis in costimulation-deficient T cells and can rescue T cell expansion in vivo. Survivin allows effector T cells to accumulate in large numbers, but Bcl-2 family proteins are required for T cell survival after the phase of active division. Thus, sustained Survivin expression from costimulatory signaling maintains T cell division over time and regulates the extent of clonal expansion.
The transcription factor Kruppel-like factor 2 (KLF2) was proposed to regulate genes involved in cell cycle entry and T cell trafficking; however, the physiological role of its expression in postactivated T cells is not well defined. Previous studies suggested that the cytokines IL-2 and IL-15 differentially regulate KLF2 re-expression in postactivation T cells and that these cytokines also influence effector versus memory T cell differentiation. Using conditional and inducible KLF2-knockout model systems, we tested the specific role of KLF2 expression in activated CD8(+) T cells cultured with these cytokines. KLF2 was required for effective transcription of sphingosine-1-phosphate receptor-1 (S1P(1)) and CD62L in postactivation T cells. However, although different cytokines dramatically altered the expression of cell-cycle-related genes, endogenous KLF2 had a minimal impact. Correspondingly, KLF2-deficient T cells showed dysregulated trafficking but not altered proliferative characteristics following in vivo responses to Ag. Thus, our data help to define KLF2-dependent and -independent aspects of activated CD8(+) T cell differentiation and argue against a physiological role in cell cycle regulation.
Immunization results in the differentiation of CD8+ T cells, such that they acquire effector abilities and convert into a memory pool. Priming of T cells takes place via an immunological synapse formed with an antigen-presenting cell (APC). By disrupting synaptic stability at different times, we found that the differentiation of CD8+ T cells required cell interactions beyond those made with APCs. We identified a critical differentiation period that required interactions between primed T cells. We found that T cell–T cell synapses had a major role in the generation of protective CD8+ T cell memory. T cell–T cell synapses allowed T cells to polarize critical secretion of interferon-γ (IFN-γ) toward each other. Collective activation and homotypic clustering drove cytokine sharing and acted as regulatory stimuli for T cell differentiation.
Newly activated CD8(+) T cells reprogram their metabolism to meet the extraordinary biosynthetic demands of clonal expansion; however, the signals that mediate metabolic reprogramming remain poorly defined. Here we demonstrate an essential role for sterol regulatory element-binding proteins (SREBPs) in the acquisition of effector-cell metabolism. Without SREBP signaling, CD8(+) T cells were unable to blast, which resulted in attenuated clonal expansion during viral infection. Mechanistic studies indicated that SREBPs were essential for meeting the heightened lipid requirements of membrane synthesis during blastogenesis. SREBPs were dispensable for homeostatic proliferation, which indicated a context-specific requirement for SREBPs in effector responses. Our studies provide insights into the molecular signals that underlie the metabolic reprogramming of CD8(+) T cells during the transition from quiescence to activation.
CD4(+) T follicular helper (Tfh) cells provide the required signals to B cells for germinal center reactions that are necessary for long-lived antibody responses. However, it remains unclear whether there are CD4(+) memory T cells committed to the Tfh cell lineage after antigen clearance. By using adoptive transfer of antigen-specific memory CD4(+) T cell subpopulations in the lymphocytic choriomeningitis virus infection model, we found that there are distinct memory CD4(+) T cell populations with commitment to either Tfh- or Th1-cell lineages. Our conclusions are based on gene expression profiles, epigenetic studies, and phenotypic and functional analyses. Our findings indicate that CD4(+) memory T cells "remember" their previous effector lineage after antigen clearance, being poised to reacquire their lineage-specific effector functions upon antigen reencounter. These findings have important implications for rational vaccine design, where improving the generation and engagement of memory Tfh cells could be used to enhance vaccine-induced protective immunity.
The memory T cell pool functions as a dynamic repository of antigen-experienced T lymphocytes that accumulate over the lifetime of the individual. Recent studies indicate that memory T lymphocytes contain distinct populations of central memory (TCM) and effector memory (TEM) cells characterized by distinct homing capacity and effector function. This review addresses the heterogeneity of TCM and TEM, their differentiation stages, and the current models for their generation and maintenance in humans and mice.
CD4 memory T cells surviving in the absence of MHC class II contact lose their characteristic memory function. To investigate the mechanisms underlying the impaired function of memory T cells in the absence of MHC class II molecules, we analyzed gene expression profiles of resting memory T cells isolated from MHC class II-competent or -deficient hosts. The analysis focused on five transcripts related to T cell activation, metabolism, and survival that are underexpressed in resting memory T cells from MHC class II-deficient hosts compared with MHC class II-competent hosts. CD4 memory cells isolated from MHC class II-deficient hosts display alterations in their degree of differentiation as well as metabolic activity, and these changes are already manifest in the effector phase despite the presence of Ag-expressing dendritic cells. Our data suggest that the absence of interactions with noncognate MHC class II molecules compromises the progressive accumulation of signals that ensure optimal survival and fitness to sustain the metabolic activity of activated T cells and shape the functional capacity of the future memory compartment. Signals via AKT coordinate survival and metabolic pathways and may be one of the crucial events linking interaction with MHC class II molecules to the successful generation of a long-lived functional memory CD4 T cell population.
CD8(+) T cells can respond to unrelated infections in an Ag-independent manner. This rapid innate-like immune response allows Ag-experienced T cells to alert other immune cell types to pathogenic intruders. In this study, we show that murine CD8(+) T cells can sense TLR2 and TLR7 ligands, resulting in rapid production of IFN-γ but not of TNF-α and IL-2. Importantly, Ag-experienced T cells activated by TLR ligands produce sufficient IFN-γ to augment the activation of macrophages. In contrast to Ag-specific reactivation, TLR-dependent production of IFN-γ by CD8(+) T cells relies exclusively on newly synthesized transcripts without inducing mRNA stability. Furthermore, transcription of IFN-γ upon TLR triggering depends on the activation of PI3K and serine-threonine kinase Akt, and protein synthesis relies on the activation of the mechanistic target of rapamycin. We next investigated which energy source drives the TLR-induced production of IFN-γ. Although Ag-specific cytokine production requires a glycolytic switch for optimal cytokine release, glucose availability does not alter the rate of IFN-γ production upon TLR-mediated activation. Rather, mitochondrial respiration provides sufficient energy for TLR-induced IFN-γ production. To our knowledge, this is the first report describing that TLR-mediated bystander activation elicits a helper phenotype of CD8(+) T cells. It induces a short boost of IFN-γ production that leads to a significant but limited activation of Ag-experienced CD8(+) T cells. This activation suffices to prime macrophages but keeps T cell responses limited to unrelated infections.
Cytotoxic T cell (CTL) activation by antigen requires the specific detection of peptide–major histo-compatibility class I (pMHC) molecules on the target-cell surface by the T cell receptor (TCR). We examined the effect of mutations in the antigen-binding site of a Kb-restricted TCR on T cell activation, antigen binding and dissociation from antigen. These parameters were also examined for variants derived from a Kd-restricted peptide that was recognized by a CTL clone. Using these two independent systems, we show that T cell activation can be impaired by mutations that either decrease or increase the binding half-life of the TCR-pMHC interaction. Our data indicate that efficient T cell activation occurs within an optimal dwell-time range of TCR-pMHC interaction. This restricted dwell-time range is consistent with the exclusion of either extremely low or high affinity T cells from the expanded population during immune responses.
Hyperactivation of T cells, particularly of CD8(+) T cells, is a hallmark of chronic HIV 1 (HIV-1) infection. Little is known about the antigenic specificities and the mechanisms by which HIV-1 causes activation of CD8(+) T cells during chronic infection. We report that CD8(+) T cells were activated during in vivo HIV-1 replication irrespective of their Ag specificity. Cytokines present during untreated HIV-1 infection, most prominently IL-15, triggered proliferation and expression of activation markers in CD8(+) T cells, but not CD4(+) T cells, in the absence of TCR stimulation. Moreover, LPS or HIV-1-activated dendritic cells (DCs) stimulated CD8(+) T cells in an IL-15-dependent but Ag-independent manner, and IL-15 expression was highly increased in DCs isolated from viremic HIV-1 patients, suggesting that CD8(+) T cells are activated by inflammatory cytokines in untreated HIV-1 patients independent of Ag specificity. This finding contrasts with CD4(+) T cells whose in vivo activation seems biased toward specificities for persistent Ags. These observations explain the higher abundance of activated CD8(+) T cells compared with CD4(+) T cells in untreated HIV-1 infection.
Activation of self-reactive T cells and their trafficking to target tissues leads to autoimmune organ destruction. Mice lacking the co-inhibitory receptor cytotoxic T lymphocyte antigen-4 (CTLA-4) develop fatal autoimmunity characterized by lymphocytic infiltration into nonlymphoid tissues. Here, we demonstrate that the CD28 co-stimulatory pathway regulates the trafficking of self-reactive Ctla4(-/-) T cells to tissues. Concurrent ablation of the CD28-activated Tec family kinase ITK does not block spontaneous T cell activation but instead causes self-reactive Ctla4(-/-) T cells to accumulate in secondary lymphoid organs. Despite excessive spontaneous T cell activation and proliferation in lymphoid organs, Itk(-/-); Ctla4(-/-) mice are otherwise healthy, mount antiviral immune responses and exhibit a long lifespan. We propose that ITK specifically licenses autoreactive T cells to enter tissues to mount destructive immune responses. Notably, ITK inhibitors mimic the null mutant phenotype and also prevent pancreatic islet infiltration by diabetogenic T cells in mouse models of type 1 diabetes, highlighting their potential utility for the treatment of human autoimmune disorders.
The activation of T cells through interaction of their T-cell receptors with antigenic peptide bound to major histocompatibility complex (MHC) on the surface of antigen presenting cells (APCs) is a crucial step in adaptive immunity. Here we use three-dimensional fluorescence microscopy to visualize individual peptide–I-Ek class II MHC complexes labelled with the phycobiliprotein phycoerythrin in an effort to characterize T-cell sensitivity and the requirements for forming an immunological synapse in single cells. We show that T cells expressing the CD4 antigen respond with transient calcium signalling to even a single agonist peptide–MHC ligand, and that the organization of molecules in the contact zone of the T cell and APC takes on the characteristics of an immunological synapse when only about ten agonists are present. This sensitivity is highly dependant on CD4, because blocking this molecule with antibodies renders T cells unable to detect less than about 30 ligands.
Interleukin(IL)-2 and inflammation regulate effector and memory cytolytic T-lymphocyte (CTL) generation during infection. We demonstrate a complex interplay between IL-2 and inflammatory signals during CTL differentiation. IL-2 stimulation induced the transcription factor eomesodermin (Eomes), upregulated perforin (Prf1) transcription, and repressed re-expression of memory CTL markers Bcl6 and IL-7Ralpha. Binding of Eomes and STAT5 to Prf1 cis-regulatory regions correlated with transcriptional initiation (increased recruitment of RNA polymerase II to the Prf1 promoter). Inflammation (CpG, IL-12) enhanced expression of IL-2Ralpha and the transcription factor T-bet, but countered late Eomes and perforin induction while preventing IL-7Ralpha repression by IL-2. After infection of mice with lymphocytic choriomeningitis virus, IL-2Ralpha-deficient effector CD8(+) T cells expressed more Bcl6 but less perforin and granzyme B, formed fewer KLRG-1(+) and T-bet-expressing CTL, and killed poorly. Thus, inflammation influences both effector and memory CTL differentiation, whereas persistent IL-2 stimulation promotes effector at the expense of memory CTL development.
T cell receptor (TCR) signaling is initiated and sustained in microclusters; however, it's not known whether signaling also occurs in the TCR-rich central supramolecular activation cluster (cSMAC). We showed that the cSMAC formed by fusion of microclusters contained more CD45 than microclusters and is a site enriched in lysobisphosphatidic acid, a lipid involved in sorting ubiquitinated membrane proteins for degradation. Calcium signaling via TCR was blocked within 2 min by anti-MHCp treatment and 1 min by latrunculin-A treatment. TCR-MHCp interactions in the cSMAC survived these perturbations for 10 min and hence were not sufficient to sustain signaling. TCR microclusters were also resistant to disruption by anti-MHCp and latrunculin-A treatments. We propose that TCR signaling is sustained by stabilized microclusters and is terminated in the cSMAC, a structure from which TCR are sorted for degradation. Our studies reveal a role for F-actin in TCR signaling beyond microcluster formation.
During ageing thymic function declines and is unable to meet the demand for peripheral T helper (Th) cell replenishment. Therefore, population maintenance of naive Th cells must be at least partly peripherally based. Such peripheral postthymic expansion of recent thymic emigrants (RTEs) during ageing consequently should lead to loss or dilution of T cell receptor excision circles (TRECs) from a subset of naive T cells. We have identified two subsets of naive Th cells in human adult peripheral blood characterized by a striking unequal content of TRECs, indicating different peripheral proliferative histories. TRECs are highly enriched in peripheral naive CD45RA+ Th cells coexpressing CD31 compared with peripheral naive CD45RA+ Th cells lacking CD31 expression, in which TRECs can hardly be detected. Furthermore we show that CD31−CD45RA+ Th cells account for increasing percentages of the naive peripheral Th cell pool during ageing but retain phenotypic and functional features of naive Th cells. As CD31 is lost upon T cell receptor (TCR) engagement in vitro, we hypothesize that TCR triggering is a prerequisite for homeostatically driven peripheral postthymic expansion of human naive RTEs. We describe here the identification of peripherally expanded naive Th cells in human adult blood characterized by the loss of CD31 expression and a highly reduced TREC content.
T cell activation is predicated on the interaction between the T cell receptor and peptide-major histocompatibility (pMHC) ligands. The factors that determine the stimulatory potency of a pMHC molecule remain unclear. We describe results showing that a peptide exhibiting many hallmarks of a weak agonist stimulates T cells to proliferate more than the wild-type agonist ligand. An in silico approach suggested that the inability to form the central supramolecular activation cluster (cSMAC) could underlie the increased proliferation. This conclusion was supported by experiments that showed that enhancing cSMAC formation reduced stimulatory capacity of the weak peptide. Our studies highlight the fact that a complex interplay of factors determines the quality of a T cell antigen.
Evidence has accumulated that cytokines have a fundamental role in the differentiation of memory T cells. Here, we follow the CD8+ T cell from initial activation to memory-cell generation, indicating the checkpoints at which cytokines determine the fate of the T cell. Members of the common cytokine-receptor γ-chain (γc)-cytokine family — in particular, interleukin-7 (IL-7) and IL-15 — act at each stage of the immune response to promote proliferation and survival. In this manner, a stable and protective, long-lived memory CD8+ T-cell pool can be propagated and maintained.
Regulatory T (Treg) cells play a pivotal role in suppressing self-harmful T cell responses, but how Treg cells mediate suppression to maintain immune homeostasis and limit responses during inflammation is unclear. Here we show that effector Treg cells express high amounts of the integrin αvβ8, which enables them to activate latent transforming growth factor-β (TGF-β). Treg-cell-specific deletion of integrin αvβ8 did not result in a spontaneous inflammatory phenotype, suggesting that this pathway is not important in Treg-cell-mediated maintenance of immune homeostasis. However, Treg cells lacking expression of integrin αvβ8 were unable to suppress pathogenic T cell responses during active inflammation. Thus, our results identify a mechanism by which Treg cells suppress exuberant immune responses, highlighting a key role for effector Treg-cell-mediated activation of latent TGF-β in suppression of self-harmful T cell responses during active inflammation.
Successful cancer treatment requires understanding host immune response against tumor cells. PD-1 belongs to the CD28 superfamily of receptors that work as “checkpoints” of immune activation. PD-1 maintains immune self-tolerance to prevent autoimmunity and controls T-cell reaction during infection to prevent excessive tissue damage. Tumor cells that arise from normal tissue acquire mutations that can be targeted by lymphocytes. Accumulating lines of evidence suggest that tumor cells evade host immune attack by expressing physiological PD-1 ligands and stimulating PD-1 on the lymphocytes. Based on this idea, researchers have successfully demonstrated that systemic administration of monoclonal antibodies that inhibit the binding of PD-1 to the ligands reactivated T cells and augmented the anti-cancer immune response. In this review, I summarize the basics of T-cell biology and its regulation by PD-1 and discuss the current understanding and questions about this multifaceted molecule.
Although memory T cells respond more vigorously to stimulation and they are more sensitive to low doses of antigen than naive T cells, the molecular basis of this increased sensitivity remains unclear. We have previously shown that the T cell receptor (TCR) exists as different-sized oligomers on the surface of resting T cells and that large oligomers are preferentially activated in response to low antigen doses. Through biochemistry and electron microscopy, we now showed that previously stimulated and memory T cells have more and larger TCR oligomers at the cell surface than their naive counterparts. Reconstitution of cells and mice with a point mutant of the CD3ζ subunit, which impairs TCR oligomer formation, demonstrated that the increased size of TCR oligomers was directly responsible for the increased sensitivity of antigen-experienced T cells. Thus, we propose that an "avidity maturation" mechanism underlies T cell antigenic memory.
The reorientation of the T cell microtubule-organizing center (MTOC) toward the antigen-presenting cell enables the directional secretion of cytokines and lytic factors. By single-cell photoactivation of the T cell antigen receptor, we show that MTOC polarization is driven by localized accumulation of diacylglycerol (DAG). MTOC reorientation was closely preceded first by production of DAG and then by recruitment of the microtubule motor protein dynein. Blocking DAG production or disrupting the localization of DAG impaired MTOC recruitment. Localized DAG accumulation was also required for cytotoxic T cell–mediated killing. Furthermore, photoactivation of DAG itself was sufficient to induce transient polarization. Our data identify a DAG-dependent pathway that signals through dynein to control microtubule polarity in T cells.
CD4+ T cells can differentiate into multiple effector subsets, but the potential roles of these subsets in anti-tumor immunity have not been fully explored. Seeking to study the impact of CD4+ T cell polarization on tumor rejection in a model mimicking human disease, we generated a new MHC class II-restricted, T-cell receptor (TCR) transgenic mouse model in which CD4+ T cells recognize a novel epitope in tyrosinase-related protein 1 (TRP-1), an antigen expressed by normal melanocytes and B16 murine melanoma. Cells could be robustly polarized into Th0, Th1, and Th17 subtypes in vitro, as evidenced by cytokine, chemokine, and adhesion molecule profiles and by surface markers, suggesting the potential for differential effector function in vivo. Contrary to the current view that Th1 cells are most important in tumor rejection, we found that Th17-polarized cells better mediated destruction of advanced B16 melanoma. Their therapeutic effect was critically dependent on interferon-gamma (IFN-gamma) production, whereas depletion of interleukin (IL)-17A and IL-23 had little impact. Taken together, these data indicate that the appropriate in vitro polarization of effector CD4+ T cells is decisive for successful tumor eradication. This principle should be considered in designing clinical trials involving adoptive transfer-based immunotherapy of human malignancies.
Follicular T helper (TFH) cells orchestrate the germinal center (GC) reaction locally. Local mechanisms regulating their dynamics and helper functions are not well defined. Here we found that GC-expressed ephrin B1 (EFNB1) repulsively inhibited T cell to B cell adhesion and GC TFH retention by signaling through TFH-expressed EPHB6 receptor. At the same time, EFNB1 promoted interleukin-21 production from GC TFH cells by signaling predominantly through EPHB4. Consequently, EFNB1-null GCs were associated with defective production of plasma cells despite harboring excessive TFH cells. In a competitive GC reaction, EFNB1-deficient B cells more efficiently interacted with TFH cells and produced more bone-marrow plasma cells, likely as a result of gaining more contact-dependent help. Our results reveal a contact-dependent repulsive guidance system that controls GC TFH dynamics and effector functions locally.