Paths to stemness: building the ultimate antitumour T cell

Abstract

Stem cells are defined by the ability to self-renew and to generate differentiated progeny, qualities that are maintained by evolutionarily conserved pathways that can lead to cancer when deregulated. There is now evidence that these stem cell-like attributes and signalling pathways are also shared among subsets of mature memory T lymphocytes. We discuss how using stem cell-like T cells can overcome the limitations of current adoptive T cell therapies, including inefficient T cell engraftment, persistence and ability to mediate prolonged immune attack. Conferring stemness to antitumour T cells might unleash the full potential of cellular therapies.

Be stirring as the time; be fire with fire; Threaten the threatener … seek the lion in his den. W. Shakespeare, The Life and Death of King John, c. 1595

Organismal homeostasis requires a precise balance between self-renewal and differentiation. Physiologically, the presence of limited numbers of stem cells in different tissues provides a hierarchical organization of cell types in which a few daughter cells retain a regenerative potential while most enter an irreversible process of differentiation that culminates in the generation of specialized cell types that are destined to die¹. The existence of stem cells has been documented in multiple tissues, including the haematopoietic² and central nervous³ systems, and the intestine⁴, skin⁵, cardiac muscle⁶ and lung⁷. Similar to their non-transformed counterparts, tumours contain heterogeneous cell populations at various stages of differentiation, suggesting that they might be sustained by relatively undifferentiated transformed progenitors, known as cancer stem cells (CSCs)^8,9.

Recently, the stem cell-like attributes of self-renewal and multipotency have been discovered in subsets of memory T lymphocytes. In this Review, we describe evidence supporting the existence and function of T memory stem cells (T_SCM). We further discuss how T helper (T_H) 17 cells and interleukin-17 (IL-17)-producing CD8⁺ T cells also exhibit stem cell-like behaviours. We high-light how signalling pathways that operate in embryonic stem cells, adult stem cells and CSCs, including WNT–β-catenin, SMAD, signal transducer and activator of transcription 3 (STAT3) and forkhead box O (FOXO) signalling, are active in subsets of T lymphocytes. Finally, we envision how triggering these pathways in tumour-reactive T cells or reprogramming terminally differentiated tumour-infiltrating lymphocytes (TILs) to confer stem cell-like properties might be used to augment immunotherapies against cancer and CSCs, which would be akin to fighting ‘fire with fire’.

The heterogeneity of memory T cells

Analogous to other organ systems, mature T cells are comprised of cells that are at various stages of differentiation, which are discernible by the expression of surface molecules, anatomic location and function^10–12 (FIG. 1). T cell subset diversity results from antigenic and environmental stimuli received during T cell priming and subsequent recall responses. In a primary immune response, antigen-specific naive T (T_N) cells encounter professional antigen-presenting cells (APCs) that have processed and presented tumour-associated antigens in the context of major histocompatibility complex molecules. Signals from multiple parameters, including the strength of T cell receptor (TCR), the balance of co-stimulatory and inhibitory molecules, and the quality of the inflammatory milieu, are integrated in responding T cells to initiate a programme of proliferation and differentiation, which culminates in the formation of effector T (T_EFF) cells¹³ (FIG. 1). Depending on the strength of signalling received^13–15, T cells differentiate into distinct subsets characterized by phenotypic and functional changes that are assessable through the use of polychromatic flow cytometry¹⁶ or, more recently, through mass cytometry¹⁷. Although the lineage relationship between T cell subsets remains controversial¹⁸ (BOX 1), T cells cluster in populations that can be arranged as a progressive continuum on the basis of phenotypic, functional and transcriptional attributes^17,19 (FIG. 1).

Figure 1 |. A model of progressive T cell differentiation.

During an immune response, naïve T (T_N) cells are primed by antigen-presenting cells (APCs). Depending on the strength and quality of stimulatory signals, proliferating T cells progress along a differentiation pathway that culminates in the generation of terminally differentiated short-lived effector T (T_EFF) cells. When antigenic and inflammatory stimuli cease, primed T cells become quiescent and enter into the memory stem cell (T_SCM), central memory (T_CM) cell or effector memory (T_EM) cell pools depending on the signal strength received. The phenotypic attributes, expression levels of key transcription factors and microRNAs (miRNAs), and the functional properties of naive and memory T cell subsets are illustrated as not expressed (–), low expression (+), intermediate expression (++) and high expression (+++). EOMES, eomesodermin; FOXP1, Forkhead box P1; ID, inhibitor of DNA-binding; IFNγ, interferon-γ; IL-2, interleukin-2; KLF, Kruppel-like factor; KLRG1, killer cell lectin-like receptor subfamily G, member 1; LEF1, lymphoid enhancer-binding factor 1; ND, not determined; PRDM1, PR domain-containing 1 with ZNF domain; TBX21, T-box 21; TCF7, T cell factor 7; ZEB2, zinc finger E-box binding homeobox 2.

Box 1 |. Models of effector and memory T cell lineage relationships.

There has been a long-standing controversy regarding how memory T cells form, and about their relationship with effector T cells (T_EFF). The precise understanding of this interrelationship is crucially important for developing immune strategies to enhance T cell responses against cancer. Thus far, three main models of memory formation have been proposed¹⁸.

• The linear differentiation model¹⁸⁷: in this model, the priming of naive T cells results in the generation of T_EFF cells that are destined either to die or to enter into the effector memory T (T_EM) cell pool. With time, T_EM cells can give rise to long-lived central memory T (T_CM) cells. Evidence in support of this model is derived from the observation that T_CM cells become the predominant persisting memory T cell subset following the transfer of a population highly enriched for T_EM cells.

• The bifuractive differentiation model^43,188: this model proposes that a primed naïve T cell can give rise to two daughter cells with alternative differentiation fates through asymmetric division. Evidence in support of this model is the finding that there is an unequal partitioning of key molecules and transcription factors that regulate effector differentiation at the first cell division following naive T cell priming.

• The progressive differentiation model (also known as the decreasing potential of memory development or the self-renewing effector model)¹⁹: this model proposes that, depending on the strength and quality of stimulatory signals received, naïve T cells are driven towards progressive stages of differentiation in the order T memory stem cell (T_SCM) to T_CM cell and to T_EM cell, as cells receive progressively greater signal strengths. This model culminates in the generation of short lived T_EFF cells, which are terminally differentiated. Evidence in support of this model include ex vivo phenotypic analyses of virus-specific T cells¹⁸⁹, measurement of telomere length^24,54, gene-expression profiling^19,190,191 and in vitro differentiation studies^19,54.

Progressive T cell differentiation.

T_N cells are conventionally defined by the co-expression of the RA isoform of the transmembrane phosphatase CD45, the lymph node homing molecules L-selectin (CD62L) and CCR7, and the co-stimulatory receptors CD27 and CD28 (REF. 20) (FIG. 1). These phenotypic attributes facilitate T cell entry into secondary lymphoid organs to probe APCs for cognate antigen and to respond to activating signals that give rise to more differentiated memory and effector progeny²¹. T cell activation results in the expression of the RNA-binding protein heterogeneous nuclear ribonucleoprotein L-like (HNRPLL), which regulates the alternative splicing of pre-mRNA encoding CD45 to form CD45RO, the prototypical antigen-experienced T cell marker^22,23 (FIG. 1). Among CD45RO-expressing T cells, two major subsets of memory T lymphocytes can be distinguished on the basis of CD62L and CCR7 expression²⁴. Similar to T_N cells, CD62L and CCR7 are maintained on central memory T (T_CM) cells, whereas these molecules are lost on more differentiated effector memory T (T_EM) cells (FIG. 1). Functionally, these phenotypic differences allow antigen-specific T_CM and T_EM cells to patrol central lymphoid organs and peripheral tissues, respectively^21,24. The co-stimulatory receptors CD27 and CD28 are also found on the majority of memory T cells; however, expression can be lost as cells become terminally differentiated by progressively acquiring inhibitory signalling molecules, suchas killer cell lectin-like receptor subfamily G, member 1 (KLRG1)^17,25 and through transition into senescence^17,26 (FIG. 1). In contrast to T_N cells, memory T cells are capable of rapidly releasing cytokines on restimulation²⁷. Although both subsets are capable of producing tumour necrosis factor-α (TNFα), T_CM cells more efficiently secrete IL-2, and T_EM cells have an increased capacity for interferon-γ (IFNγ) release and cytotoxicity^17,24 (FIG. 1). All antigen-experienced T cells upregulate the common IL-2 and IL-15β receptor (IL-2Rβ) — conferring the ability to undergo homeostatic proliferation in response to IL-15 (REFS 28,29) — and also display high amounts of CD95 (also known as FAS)³⁰, a receptor that provides either co-stimulatory or pro-apoptotic signals depending on the efficiency of CD95 signalling complex formation and on which particular intracellular signalling proteins are part of the complex³¹ (FIG. 1).

Recently, CD95 and IL-2Rβ have been found to be expressed in a subset of phenotypically naive-appearing T cells¹⁹. These cells were observed in viral and tumour-reactive T cell populations and, similar to conventional memory T cells, displayed a diluted content of TCR excision circles, possessed the ability to rapidly release cytokines on activation and proliferated in response to IL-15 (REF. 19). These cells, which are the least differentiated population of antigen-experienced T cells identified to date, were termed stem cell memory T (T_SCM) cells by virtue of their enhanced capacity to self-renew and their multipotent ability to generate all memory and effector T cell subsets¹⁹.

A model of T cell differentiation in which cells proceed from T^N cells to T_SCM, T_CM and T_EM cells is supported not only by progressive phenotypic and functional changes, but also by findings using whole-transcriptome analyses. These data revealed that two-thirds of differentially expressed genes are progressively upregulated or downregulated in the order T_N cells to T_SCM cells to T_CM cells and finally to T_EM cells¹⁹. The expression of genes that encode transcription factors that are associated with T_N cells, including the WNT–β-catenin signalling transducers T cell factor 7 (TCF7) and lymphoid enhancer-binding factor 1 (LEF1), multiple members of the Kruppel-like factor (KLF) family, Forkhead box P1 (FOXP1) and the inhibitor of DNA-binding 3 (ID3), is progressively lost as cells transition from T_SCM to T_CM and T_EM subsets¹⁹ (FIG. 1). Conversely, ID2, eomesodermin (EOMES), T-box 21 (TBX21; also known as T‑BET), PR domain-containing 1 with ZNF domain (PRDM1; also known as BLIMP‑1) and zinc finger E-box binding homeobox 2 (ZEB2), which each encodes key transcriptional regulators of effector differentiation are acquired in the order T_SCM cells to T_CM cells to T_EM cells¹⁹ (FIG. 1). These findings suggest that CD8⁺ T cell differentiation proceeds as a function of the graded expression of key naive or effector-associated transcription factors rather than being determined by the selective expression of subset-specific regulators.

Recently, small non-protein-coding RNAs with regulatory properties termed microRNAs (miRNAs) have been found to tune key aspects of both stem cell³² and mature T cell functions^33–35. Although a comprehensive profiling of miRNA expression across all naive, memory and effector T cell subsets has yet to be carried out, existing data demonstrate that subsets of miRNAs are reciprocally expressed in T_N cells, T_EM and T_EFF cells^36,37 (FIG. 1). Although little is known about the function of specific miRNAs in mature CD8⁺ T cells, miR-29, which is expressed in T_N cells, can limit effector functions through its ability to inhibit Eomes, Tbx21 and Ifng ^33,34, whereas miR-155, which is upregulated in T_EM cells, is linked to the development of inflammatory T_H1 and T_H17 subsets³⁵ and T cell-mediated graft versus host disease³⁸. These findings offer the possibility that graded changes in miRNA expression might influence T cell differentiation that is analogous to what was observed using gene expression profiling. Thus, although multiple models have been proposed to account for the formation of different T cell subsets¹⁸ (BOX 1), phenotypic, functional and molecular studies seem to be most consistent with a linear progressive model beginning with T_N cells and then proceeding in the order T_SCM cells, T_CM cells, T_EM cells, to ultimately terminate with T_EFF cells.

Stem cell-like features in memory T cells.

Similar to organ systems in which terminally differentiated cells are continually replaced by the progeny of less differentiated stem cells, it has been postulated that memory cells represent the stem cell-like cells of the adaptive immune system^39,40. Several defining attributes of stem cells are indeed also present in memory T and B cells, possibly as a function of a shared core set of genes that regulate stem cell-like behaviour⁴¹. Like stem cells, memory lymphocytes can self-renew throughout the lifetime of the host^39,40 and they exhibit multipotency, as shown by their ability to differentiate into both effector and memory populations^39,40. To confer diverse fates among daughter cells, stem cells undergo asymmetric cell division⁴². This process, which is also active in B and T lymphocytes during priming^43,44, has recently been proposed as a mechanism for the simultaneous generation of effector and memory daughter cells by memory T cells on secondary encounter with a pathogen⁴⁵. Commitment to an effector fate might result from the asymmetric segregation of IL-2Rα and T-BET, two crucial drivers of effector differentiation^46,47, in daughter cells⁴⁵. Finally, unlike most somatic cells, both stem cells and memory lymphocytes can activate telomerase to maintain telomere length and replicative potential^48,49.

Among memory T cells, T_CM cells were previously thought to represent the stem cell-like memory subset because of their enhanced capacity to undergo self-renewal and asymmetric division, as well as their higher replicative potential relative to T_EM cells, which are committed progenitor cells that are prone to terminal differentiation³⁹. The identification of T_SCM cells repositioned T_CM cells as a more committed cell population in the hierarchy of T cell potency and differentiation^19,50. The existence of T_SCM cells has been documented in mice, humans and non-human primates^19,51–53 (TABLE 1). This cell population is identifiable in multiple species by the expression of a core set of markers, including CD62L, CCR7, IL2Rβ, BCL-2 and the chemokine (C-X-C motif) receptor 3 (CXCR3)^19,51,52. Multiple lines of evidence indicate that T_SCM cells possess stem cell-like attributes to a greater extent than any other memory lymphocyte population. Experiments using carboxyfluorescein succinimidyl ester to track cell division have demonstrated that T_SCM cells regenerate themselves while giving rise to more differentiated progeny in both mice and humans^19,52. Although both T_CM and T_EM cells can also undergo self-renewal, the capacity to form diverse progeny is progressively restricted, so that only T_SCM cells are capable of generating all three memory subsets and T_EFF cells; T_CM cells can give rise to T_CM, T_EM and T_EFF cells; and T_EM cells can only produce themselves and T_EFF cells¹⁹. Thus, these data establish T_SCM cells at the apex of lineage potential among memory T cells (BOX 2). Consistent with this hierarchy, the proliferative and survival responses of memory T cell subsets to antigenic or homeostatic stimuli progressively decrease from T_SCM cells to T_CM cells and T_EM cells^19,52, possibly as a function of a stepwise loss of telomere length^24,54. Moreover, the refractoriness of T_SCM cells to undergo attrition in the absence of cognate antigen relative to other memory T cell subsets ensures a long-term reservoir of multipotent antigen-specific memory cells⁵³. Perhaps the most compelling evidence for T_SCM cell stemness comes from experiments in mice showing the ability of these cells to reconstitute the full diversity of the memory T cell compartment on serial transplantation⁵². Altogether, these findings support the conclusion that stem cell-like cells exist as part of the adaptive immune system in the form of memory T lymphocytes contained in a phenotypically naive-appearing T cell compartment.

Table 1 |.

Phenotypic and functional characteristics of T memory stem cells

Species	Phenotype	Anatomical location	Antigen specificity	Homeostasis	Evidence of stemness
Mouse	CD44−, CD62L+, CCR7+, SCA1+, IL-2RB+, BCL-2+ and CXCR3+ (REFS 51,52)	Not extensively characterized. Described in peripheral blood, spleen, lymph nodes and liver51	Alloantigens51	Enhanced proliferative response in lymphopaenic hosts. MHC class I independence52	Self-renewal and multipotency on serial transplantation52
Non-human primates	CD45RA+, CD62L+, CCR7+, CD27+ CD28+, IL-7RA+, CD95+, IL-2RB+, BCL-2+ and CXCR3+ (REF. 53)	Preferential homing to lymphoid tissues. Almost completely absent in the gut53	Simian immunodeficinecy virus53	In vivo and in vitro proliferative responses to IL-15 (REF. 53)	Long-term persistence in the absence of antigenic stimuli53
Human	CD45RA+, CD62L+, CCR7+, CD27+, CD28+, IL-7RA+, CD45RO−, CD95+, IL-2RB+, BCL-2+ and CXCR3+ (REF. 19)	Not extensively characterized. Described in peripheral and cord blood19	Influenza, CMV and MART1 (REF. 19)	Enhanced proliferative response in immunodeficient mice. In vitro proliferative responses to IL-15 (REF. 19)	Self-renewal and multipotency in in vitro assay. Enhanced engraftment in xenograft models19

CMV, cytomegalovirus; CXCR3, chemokine (C-X-C motif) receptor 3; IL, interleukin; MART1, melanoma antigen recognized by T-cells 1.

Box 2 |. A Waddington view of stem cell-like potential of CD4⁺ and CD8⁺ T cell subsets.

In 1957, Conrad Waddington, conceptualized an epigenetic landscape composed of ‘peaks’ and ‘valleys’ in which an undifferentiated, pluripotent cell residing at the peak of its potential can travel down various pathways of differentiation, like a ball placed precariously atop a hill¹⁹². Some 50 years later, it is now clear that underlying gene and microRNA expression dynamics are changes to the physical organization of chromatin through epigenetic modifications. Extrapolating from the Waddington model of cellular potential, T cells can be visualized as resting in valleys placed at different altitudes corresponding to T cell subsets with diverse differentiation potentials (see the figure). At the peak of the ‘hill’ a naive CD4⁺ T helper (T_H ) cell or CD8⁺ T cell exists that is capable of forming all T cell subsets within its respective lineage. As a cell moves down the hill, its potential to differentiate into other subsets becomes progressively restricted, culminating in a terminally differentiated cell that is destined to die. Cellular differentiation in the Waddington model generally proceeds unidirectionally from the least to the most differentiated cell. However, there is evidence to suggest that cells can dedifferentiate, under some circumstances, and reoccupy a vacant niche, such as that seen in the regeneration of diverse phenotypes like the reacquisition of stem cell antigen 1 in pro-erythrocytes⁵⁵ or the re-emergence of CD62L⁺ cells from effector memory T cells^140,187,193, although these processes are highly inefficient under physiological conditions.

Self-renewal pathways in stem cells and T cells

Stem cells are continuously maintained in a poised state between self-renewal and differentiation⁵⁵. What ultimately guides stem cells between these alternative fates are instructive and permissive signals that are provided by growth factors in the stem cell niche⁵⁶. Studies of embryonic stem cells (ESCs), tissue-specific adult stem cells and CSCs have revealed that a common set of cell-surface receptors and intracellular signal transduction pathways contribute to the regulation of this balance^8,57. Here, using ESCs as a prototypical model, we highlight how many of these same pathways are active in mature T cell subsets, promoting either self-renewal or T cell differentiation.

STAT3 and SMAD signalling.

It has long been recognized that ESCs can be maintained in an undifferentiated state in the presence of leukaemia inhibitory factor (LIF) or related cytokines acting through receptor complexes containing the GP130 signal transducer⁵⁸. The ability of LIF to promote self-renewal depends on the downstream activation of Janus kinase (JAK) and STAT3 (REFS 59,60) (FIG. 2). Recently, LIF–STAT3 signalling has been shown to induce KLF4 and KLF5 to reinforce the pluripotency network and to promote ESC self-renewal⁶¹. In addition to STAT3 activation, signalling through the LIF receptor–GP130 complex can also recruit the adaptor molecule protein tyrosine phosphatase, non-receptor type 11 (also known as SHP2) to activate the MAPK pathway, which delivers pro-differentiation rather than self-renewal cues^62,63 (FIG. 2). However, bone morphogenetic proteins (BMPs) can limit MAPK-driven differentiation and enhance ESC self-renewal by inducing the expression of both dual specificity phosphatase 9 (DUSP9) ⁶⁴, a MAPK phosphatase and ID proteins in a SMAD-dependent manner⁶⁵ (FIG. 2). ID proteins block ESC differentiation by antagonizing the transcriptional activity of E proteins as oversupply of transcription factor 3 (also known as E2A) abrogates the ability of ID proteins to sustain ESC maintenance⁶⁵. Recently, Yes-associated protein (YAP), a transcriptional coactivator that is negatively regulated by the AKT and Hippo pathways, has been found to promote ESC self-renewal by enhancing ID protein expression in response to BMP–SMAD signalling⁶⁶ and by inducing numerous pluripotent genes in response to LIF through its binding to the transcription factor TEA domain (TEAD)^67,68. These findings indicate that activation of the BMP–SMAD path-way is necessary to direct the response to LIF signalling from differentiation towards self-renewal, and that YAP is capable of potentiating both pathways.

Figure 2 |. Signalling pathways regulating self-renewal and differentiation shared between stem cells and T lymphocytes.

Self-renewal and differentiation are tightly balanced by opposing signals received from cell surface receptors. Self-renewal is promoted by WNT ligand binding to Frizzled–low-density lipoprotein receptor related protein 5 (LRP5) or LRP6 complexes or by ligand engagement of receptor complexes signalling through signal transducer and activator of transcription 3 (STAT3), including receptors containing the GP130 subunit or the interleukin-21 (IL-21) receptor. Activation of these signalling pathways leads to the transcription of target genes that favour self-renewal and that withhold differentiation, including STAT3 and Kruppel-like factor (KLF) family members and inhibitor of DNA binding (ID) proteins. Conversely, pro-mitotic cytokines such as IL-2 and growth factors can drive cellular differentiation by triggering the PI3K–AKT–mTOR pathway, as well as the RAS–RAF–MAPK pathway. The pro-differentiating influence of the RAS–RAF–MAPK pathway can be counteracted by SMAD signalling that is induced by transforming growth factor-β (TGFβ) or bone morphogenetic protein (BMP) family members through the induction of dual specificity phosphatase 9 (DUSP9) and the E protein regulators, ID molecules. Finally, activation of the Hippo pathways through a poorly characterized ligand–receptor interaction causes inactivation of Yes-associated protein (YAP), resulting in enhanced cellular differentiation. Between these self-renewal and pro-differentiation pathways exists a significant amount of crosstalk such that the net influence of each pathway is finely balanced and tuned. The dashed arrows indicate translocation into the nucleus. APC, adenomatous polyposis coli; CK1α, casein kinase 1, alpha 1; eIF4E, eukaryotic translation initiation factor 4E; FOXO, forkhead box O; GSK3β, glycogen synthase 3β; JAK, janus kinase; LATS, large tumour suppressor; LIF, leukaemia inhibitory factor; MOB, MOB kinase activator 1; MST, mammalian sterile-20-like kinases; p70S6K, p70 ribosomal protein S6 kinase 1; SAV1, salvador homologue 1; SHP2, SH2 domain-containing protein tyrosine phosphatase-2; TCF, T cell factor; TEAD, TEA domain family member; TSC, tuberous sclerosis.

Mature T lymphocytes can also receive environmental signals that trigger STAT3 activity. IL-21 has the unique ability among common γ-chain (γ_C) cytokines to sustain STAT3 activation⁶⁹ (FIG. 2). IL-21 has been shown to suppress the differentiation of CD8⁺ T cells into T_EFF cells, maintaining a T_SCM-like state that is associated with high proliferative potential and long-term T cell survival₇₀. Experiments in IL-21 or IL-21 receptor-deficient mice revealed that CD8⁺ T cells undergo greater exhaustion and fail to control viral replication compared with wild-type hosts^71,72. CD8⁺ T cells were impaired in IL-2 production, a cytokine that is released by less differentiated T cell subsets⁷³, and the depletion of the STAT3 signalling cytokine IL-10 in IL-21-deficient hosts promoted further accumulation of senescent KLRG1⁺ T cells⁷⁴.

IL-6 receptor-α (IL-6Rα) and the signal transducing chain GP130 are highly expressed in T_N and T_SCM cells, and they are progressively lost with T cell activation and differentiation, suggesting that IL-6-mediated activation of STAT3 might be implicated in the maintenance of less differentiated, multipotent T cells^19,75,76. This hypothesis is supported by the finding that IL-6Rα⁺ CD8⁺ T cells that were isolated at the peak of a primary immune response had increased long-term survival compared with IL-6Rα− T cells⁷⁷. IL-6Rα was not merely a marker of memory-forming potential, as activated T cells failed to generate physiological numbers of memory cells following adoptive transfer into IL-6-deficient mice⁷⁷. Taken together, these findings indicate that STAT3 signalling cytokines, including IL-6, IL-10 and IL-21, inhibit effector T cell differentiation and exhaustion while promoting long-term memory.

The role of STAT3 signalling in the formation and maintenance of CD8⁺ memory T cells has recently been investigated⁷⁴. STAT3-deleted T cells have a shortened lifespan, fail to form less differentiated T_CM cells and have reduced expression of the memory-associated transcription factor BCL-6 compared with wild-type cells⁷⁴. Furthermore, STAT3-deficient CD8⁺ T cells are less able to self-renewal and are impaired in their protective capacity against a secondary infection⁷⁴. These findings extend beyond mice, as human patients with autosomal-dominant hyper-IgE syndrome have a cell-intrinsic defect in T_CM formation and an impaired capacity to control intracellular viral infections⁷⁸. As observed in ESCs, STAT3 might limit cell differentiation by inducing KLF transcription factors that promote cell quiescence and the expression of lymphoid-homing molecules in mature T lymphocytes^79,80.

Analogous to ESC biology, ID proteins have now emerged as key regulators of CD8⁺ T cell memory formation. CD8⁺ T cells lacking Id3 failed to form physiological numbers of memory cells and enforced expression was sufficient to rescue long-term survival of terminally differentiated KLRG1⁺ T cells^81,82. As observed in ESCs, ID3 was found to mediate its effect by limiting the activity of E proteins. Indeed, deletion of E2A in CD8⁺ T cells augmented memory T cell formation in response to a viral infection, recapitulating the phenotype caused by enforced expression of ID3 (REFS 81,83).

Similar to ESC biology, YAP has recently been shown to prevent the acquisition of senescence in CD8⁺ T cells responding toviral infection⁸⁴. Activationof CD8⁺ T cells in the presence of the pro-differentiating cytokine IL-2 caused the induction of key components of the Hippo pathways, resulting in the degradation of YAP and in the gain of differentiation-associated molecules. Conversely, ectopic expression of a YAP isoform not susceptible to Hippo-mediated negative regulation suppressed the induction of the master of regulator of terminal differentiation BLIMP1, and favoured the maintenance of IL-7Rα⁺ and KLRG1− memory precursors. It remains to be determined whether YAP may also augment the expression of transcriptional regulators of stemness, including STAT3 and ID family members, in T cells as it does in ESCs.

Interestingly, STAT3 and SMAD signalling can be triggered by several type 17-polarizing cytokines, including IL-6, IL-21 and transforming growth factor-β (TGFβ), suggesting that these pathways might be activated in T_H 17 cells to regulate stem cell-like behaviour^85–87 (BOX 3). In summary, STAT3 signalling and ID proteins are active in mature T lymphocytes and can promote self-renewal and long-term survival.

Box 3 |. Stem cell-like qualities in CD4+ T cell subsets.

CD4⁺ T cells have crucial roles in coordinating immune responses to infectious diseases and cancer. Despite a wealth of knowledge relevant to CD4⁺ T cell biology and our increasing understanding and identification of new subsets, relatively little is known about memory CD4⁺ T cells and the ability of different CD4⁺ T helper (T_H) cell subsets to persist long term¹⁹⁴. The functional diversity and, in particular, the developmental plasticity of the T_H subsets has posed unique challenges in defining memory in this lineage and has sometimes led investigators to incorrect conclusions. For example, T_H cells releasing interleukin-17 (IL-17) (T_H17) have been purported as short-lived effector cells, as these cells display some phenotypic traits that are characteristic of terminally differentiated CD8⁺ T cells (that is, a lack of expression of CD62L and CD27), and IL-17A production by antigen-specific cells is extinguished over time during an infection¹⁹⁵. However, this interpretation is at odds with recent evidence indicating that congenically marked highly purified T_H17 cells exhibit not only superior recall and persistence relative to interferon-γ (IFNγ)-secreting T_H1 cells, but also mediate enhanced auto-immunity and antitumour immunity⁸⁵. Moreover, T_H17 cells were found to exhibit the stem cell-like attributes of multipotency (these cells could give rise to both IL-17- and IFNγ-secreting progeny), activation of stem cell-associated molecular pathways (these cells highly expressed β-catenin and Tcf7) and a shared gene expression signature with early memory CD8⁺ T cells⁸⁵. Similar findings have been described in human T_H17 cells⁸⁷. These findings place T_H17 cells at a higher cellular potential than T_H1 cells on a Waddington landscape (BOX 2). Recent findings demonstrating that T follicular helper (T_FH) cells can be recruited into other helper T subsets and can also give rise to memory cells¹⁹⁶, combined with the observation that T_FH cell formation is independent of T_H1, T_H2 and T_H17 development¹⁹⁷, place T_FH cells at the apex of cellular potential for antigen-experienced CD4⁺ T cells, paralleling memory stem cells among the CD8⁺ T cell lineage (BOX 2).

WNT–β-catenin signalling pathway.

Numerous studies have shown that activation of WNT–β-catenin signalling (FIG. 2) is causally associated with self-renewal in ESCs^88–94. The activity of this pathway is centred on β-catenin, which in the absence of WNT signalling is targeted for proteasome-dependent degradation by a destruction complex consisting of adenomatous polyposis coli, axin and the serine/threonine kinases casein kinase 1 and glycogen synthase kinase 3β (GSK3β)⁹⁵. On WNT ligation to the Frizzled receptor and low-density lipoprotein co-receptors, a signalling cascade is initiated that results in the disruption of the destruction complex, leading to the accumulation and nuclear translocation of β-catenin⁹⁵ (FIG. 2). Within the nucleus, β-catenin can interact with various DNA-binding partners, notably members of the TCF/LEF family, causing chromatin remodelling and modulation of transcription⁹⁵ (FIG. 2).

Analogous to ESCs, WNT reporter systems have shown that the WNT–β-catenin pathway is functionally active in mature T lymphocytes⁹⁶. The WNT signalling transducers TCF7 and LEF1 are highly expressed in T_N and T_SCM CD8⁺ T cells and are lost with reiterative stimulations and progressive differentiation, which suggests a possible role in maintaining T cells in a less differentiated state^19,52,75,97. Additionally, Tcf7 and β-catenin are abundantly expressed in T_H17 cells (BOX 3) and in IL-17-producing CD8⁺ T cells^85,87. Although the role of WNT–β-catenin signalling in IL-17-producing cells has yet to be defined, emerging evidence indicates that this pathway is critically important for the formation and long-term maintenance of memory CD8⁺ T cells^98,99. Similar to stem cell biology, WNT3A or inhibitors of GSK3β have been shown to inhibit the differentiation of T_N cells into T_EFF cells while promoting the generation of self-renewing T_SCM and T_CM cells^52,100,101. Consistent with these findings, enforced expression of a stabilized form of β-catenin inhibited T cell proliferation and the acquisition of effector functions¹⁰². Moreover, overexpression of TCF1 and stabilized β-catenin reduced the expansion of CD8⁺ T cells during the effector phase of the immune response and enhanced the generation of memory T cells in several infection models¹⁰³. Conversely, deletion of TCF1 enhanced effector differentiation, as shown by increased numbers of T cells expressing granzyme B and KLRG1 at the peak of the immune response, preventing the establishment of long-term T cell memory^104,105. Tcf1-deficient memory T cells were depleted of T_CM cells and were severely impaired in their ability to respond to pathogen rechallenge^104,105. Defective T cell memory responses could be rescuedby the p45 isoformof TCF1 but not by the p33 isoform, which lacks the catenin-binding domain, indicating that TCF1 activity was dependent on its ability to bind to β-catenin¹⁰⁴.

Recently, the finding that WNT–β-catenin signalling regulates CD8⁺ T cell memory has been called into question. GSK3β inhibitors were found to arrest effector differentiation even in T cells in which β-catenin was conditionally knocked out¹⁰⁶. Additionally, memory formation and function were not impaired in mice with a T cell-specific deletion of β-catenin relative to wild-type controls¹⁰⁷. Confounding the interpretation of these findings is the observation that WNT reporter activity is not extinguished by conditional deletion of β-catenin, suggesting that additional transducers of WNT signals compensate for β-catenin deficiency⁹⁶. Moreover, it should be noted that a 52 kDa truncated β-catenin protein containing at least three of the seven armadillo repeats that mediate interaction with TCF is generated after cremediated deletion of exon 2 to exon 6 of the Ctnnb1 locus, and this fragment might retain some functionality^96,108,109. Furthermore, γ-catenin, an armadillo repeat-containing homologue of β-catenin that is also regulated by the destruction complex, can promote TCF/LEF transcriptional activity in β-catenin-deficient cells¹¹⁰. In contrast to that observed in CD8⁺ T cells that are conditionally deficient in β-catenin¹⁰⁷, T cells lacking both β-catenin and γ-catenin were found to be severely compromised in mediating recall responses¹⁰⁴. In summary, extensive evidence indicates that WNT–β-catenin signalling inhibits cell differentiation while promoting stemness not only in stem cells but also in mature T lymphocytes.

PI3K–AKT–mTOR signalling pathway.

mTOR is a nutrient-sensitive kinase that regulates cell growth and metabolism. mTOR functions as a central metabolic node that integrates signals from multiple sources, including cytokines and growth factors via the PI3K–AKT pathway, as well as WNT ligands through GSK3β (FIG. 2). Although mTOR activity is essential for the survival and maintenance of pluripotency in ESCs¹¹¹, increased signalling through this pathway can also drive ESC differentiation by promoting protein translation via p70 ribosomal protein S6 kinase 1 (p70S6K)^112,113. Indeed, enhanced p70S6K activity by knockdown of tuberous sclerosis 2 (TSC2), a negative regulator of mTOR, or enforced expression of a constitutively active form of p70S6K impaired ESC self-renewal, indicating that protein synthesis is a major driver of ESC differentiation¹⁰⁰.

mTOR has now emerged as a crucial regulator of CD8⁺ T cell memory. Similar to ESCs, the activity of mTOR is tightly regulated in T cells to dictate cell fate decisions. For example, high doses of the mTOR inhibitor rapamycin can abrogate CD8⁺ T cell immune responses, whereas unrestrained mTOR activity by deletion of Tsc1 abrogates T_N cell quiescence, resulting in T cell effector differentiation and apoptotic cell death^114,115. Notably, modulation of mTOR activity with low doses of rapamycin has a profound quantitative and qualitative effect on memory responses, resulting in increased numbers of memory T cells, as well as the preferential formation of T_CM cells^114,116. Retroviral transduction of short hairpin RNAs targeting p70S6K and eukaryotic translation initiation factor 4E (eIF4E) recapitulated the immunostimulatory activity of rapamycin in memory formation¹¹⁴, demonstrating that restraint of mTOR-mediated protein translation can enhance self-renewal and can limit differentiation not only in ESCs but also in CD8⁺ T cells.

AKT, a serine/threonine-specific protein kinase the activity of which augments mTOR signalling, has also been found to control CD8⁺ T cell effector and memory differentiation^117–119. Sustained AKT function by expression of a constitutively active form of AKT induced terminal differentiation and loss of CD8⁺ memory T cells^117,119, whereas pharmacological blockade of AKT increased CD8⁺ memory T cell numbers by rescuing the survival of KLRG1⁺ short-lived T_EFF cells¹¹⁷. AKT activity enhanced effector differentiation by promoting mTOR signalling and inhibiting FOXO1 activity through cytosolic sequestration, resulting in augmented T-BET expression and IFNγ production, as well as repression of pro-memory factors, including KLF2, IL-7Rα and BCL-6 (REFS 117,118,120–124). Recently, FOXO1 has been shown to be essential for the maintenance of ESC pluripotency¹²⁵, again emphasizing the conserved nature of many of the signalling pathways that balance self-renewal and differentiation in both ESCs and mature T cells.

Conferring stemness to T cells for therapy

Immunotherapies based on the adoptive transfer of naturally occurring or genetically redirected tumour-reactive T cells represent the best evidence of the therapeutic power of T cells. Such approachs can mediate durable complete responses in a minority of patients with advanced haematological^126–129 and solid cancers^130,131. Why certain patients respond to T cell therapy while others do not remains poorly understood. Undoubtedly, multiple factors can influence the effect of T cell-based therapies, including tumour- and host-associated factors¹³²; however, the ability of T cells to engraft and persist long-term seems to be a prerequisite for success^{128,130,133,134}. T cell persistence has been highly correlated with tumour responses across multiple clinical trials and has been linked to intrinsic T cell properties that are reflective of their differentiation state and replicative history (FIG. 1). For example, longer telomere lengths^130,135, a short duration of ex vivo culture and a rapid expansion rate of T cells^136,137 are each significantly associated with tumour regression in patients. Additionally, the frequency of T_CM cells in the infusion product¹³⁸ and the expression of the co-stimulatory molecules CD28 and CD27 have been correlated with responses^130,135,139. Altogether, these data suggest that the transfer of less differentiated T cells conveys superior antitumour efficacy relative to terminally differentiated effector cells. Experiments in mice transferring defined T cell populations at all stages of differentiation have formally proved that infusion of less differentiated T cells results in greater expansion, persistence^140,141 and tumour destruction^{19,52,70,120,142–147}. Paralleling their engraftment and proliferative potentials, the ability of memory T cells to mediate tumour regression progressively decreases from T_SCM cells to T_CM cells and T_EM cells^{19,52,144,147} (BOX 2). Notably, the robust proliferative potential, long-term survival capacity and the ability to give rise to all memory and effector T cell subsets allows T_SCM cells to mediate highly effective tumour regression when limited numbers of cells are transferred, a condition in which other memory T cell subsets have little or no impact^19,52. Although tumour eradication is likely to involve multiple components of the innate and adaptive immune systems, maintaining a sustained immunological attack against tumour masses by transferring cells with stem cell-like properties might represent the most efficient approach for directly and indirectly destroying every cancer cell, including CSCs (FIG. 3). Thus, finding strategies that generate and expand T_SCM-like cells is pivotal to the development of the next generation of highly effective T cell-based immunotherapies.

Figure 3 |. Fighting fire with fire.

a | Current T cell-based immunotherapies predominantly transfer cells with effector memory (T_EM)-like phenotypic and functional characteristics. These cells have limited self-renewal capacity and are oligopotent. These cells can mediate tumour destruction but are handicapped to compete with expanding tumour masses (shown as purple tumour cells) that are sustained by the activity of self-renewing multipotent cancer stem cells (CSCs; shown as dark purple tumour cells). b | Future T cell-based immunotherapies might benefit from the transfer of T memory stem cells (T_SCM) that have enhanced self-renewal and the multipotent capacity to form all memory and effector subsets. These properties allow T_SCM cells to sustain a prolonged immune attack by giving rise to more differentiated, highly lytic effector T (T_EFF) and T_EM cells while maintaining a continuous supply of less differentiated T_SCM and central memory (T_CM) cells that can refresh the pool of cytotoxic T cells over time. In this manner, T_SCM cells might overtake the last tumour cell, including CSCs, and so cure the host.

Arresting T cell differentiation.

Current methods used to produce T cells for adoptive immunotherapy often rely on variations of a strategy developed more than 20 years ago^148,149, before the implication of T cell differentiation on in vivo tumour efficacy was fully appreciated¹⁵⁰. This approach is dependent on potent activating stimuli, including monoclonal antibodies to CD3, high concentrations of IL-2 and allogeneic feeder cells that allow for the generation of large numbers of tumour-reactive T cells but that inexorably drive T cells towards terminal differentiation and senescence. To limit the detrimental influence of ex vivo expansion on T cell differentiation, new methodologies have been explored, including the use of common γ_C cytokines other than IL-2, and small molecules that target key metabolic and developmental pathways¹⁵¹ (FIG. 4a).

Figure 4 |. Strategies that might be used to preserve or to confer stemness to T cells.

a | The process of arresting T cell development is shown. Differentiation of primed naive T (T_N) cells can be suppressed using cytokines, such as interleukin-21 (IL-21), or by using small molecules targeting key metabolic and developmental pathways. b | Two step reprogramming of terminally differentiated effector T (T_EFF) cells through an induced pluripotent stem (iPS) cell intermediate is shown. T_EFF cells are reprogrammed to generate iPS cells by ectopic co-expression of the Yamanaka factors, and OCT4, sex determining region Y (SRY) BOX 2 (SOX2) and Kruppel-like factor 4 (KLF4) with or without MYC or by forced expression of the microRNA (miRNA) cluster 302–367. iPS cells can be subsequently redifferentiated into T_N cells through the induction of NOTCH signalling. c | Direct reprogramming of T_EFF into T_N or memory stem (T_SCM) cells by enforced expression of T_N or T_SCM-associated transcription factors or miRNAs is shown. GSK3β, glycogen synthase 3β; T_CM, central memory; T_EM, effector memory.

IL-15 can sustain T cell proliferation without the robust pro-differentiating activity that characterizes IL-2. Although IL-2 promotes T cell differentiation into T_EFF and T_EM-like cells, priming of T cells in the presence of IL-15 results in the generation of T cells with the phenotypic, functional, metabolic and gene expression attributes found in naturally arising T_CM cells^{116,143,144,152–154}. Accordingly, tumour-reactive T cells mediated greater antitumour responses when generated in the presence of IL-15 than in the presence of IL-2 (REF. 144). More recently, several groups have evaluated the activity of another γ_C cytokine IL-21 on the expansion and differentiation of tumour-specific CD8⁺ T cells^70,155–157. IL-21 profoundly inhibits T cell differentiation, allowing for the generation of T_SCM-like T cells. In mouse T cells, the use of IL-21, in contrast to IL-2, caused a dose-dependent blockade of the acquisition of the antigen-experience marker CD44 and lytic capacity while preserving the expression of Tcf7, Lef1 and CD62L and while maintaining the ability to secrete IL-2 (REF. 70). Similarly, the expansion of human tumour-reactive CD45RA⁺ T cells in the presence of IL-21 prevented the loss of CD45RA, CD62L, CD28, CD27 and IL-7Rα and also retained the ability of the cell to release IL-2 (REFS 156,157). Most importantly, T cells primed and expanded in the presence of IL-21 exhibit enhanced antitumour activity compared with cells grown in other γ_C cytokines⁷⁰.

Emerging evidence indicates that the commitment of a cell between effector or memory fates is regulated by evolutionarily conserved metabolic and developmental pathways that integrate multiple signal inputs from cell surface receptors, including TCR, cytokine, co-stimulatory and growth factor receptors^{98,151,158–160}. Rational modulation of these pathways by small molecules provides an attractive means to alter T cell differentiation and to enhance the fitness of T cells for therapeutic use (FIG. 4a). As many of these molecules have already been approved for other purposes in patients, the use of these drugs to enhance T cell-based therapies can be rapidly incorporated into new clinical trials. For example, the mTOR inhibitor rapamycin, a drug that is currently used to facilitate solid organ and HSC transplantation¹⁶⁰, not only enhances the formation of CD8⁺ memory T cells but also augments their antitumour functions¹²⁰. Similarly, metformin, an AMPK agonist used to treat type 2 diabetes, improves T cell survival, recall responses and in vivo antitumour treatment¹¹⁶. Finally, inhibitors of GSK3β that are under clinical evaluation for Alzheimer’s disease and other neurodegenerative diseases¹⁶¹ can be repurposed to potentiate the WNT–β-catenin signalling pathway in T cells to generate self-renewing multipotent T_SCM-like cells^19,52. Although these reagents are effective at withholding T cell differentiation and potentiating in vivo antitumour functions, they also inhibit T cell proliferation. For this reason, the identification of molecules that uncouple the processes of cell expansion and differentiation is desirable. Recently, pharmacological inhibition of the AKT isoforms AKT1 and AKT2 has been shown to inhibit the acquisition of effector molecules and function while preserving a T_CM-like phenotype and migratory capacity without a detrimental effect on cell yield¹¹⁸. Thus, inhibitors of AKT might allow for the generation of large numbers of minimally differentiated tumour-reactive T cells for therapeutic purposes.

Reprogramming terminally differentiated T cells.

Chronic antigen stimulation in tumour-bearing hosts can drive tumour-specific T cells towards a state of terminal differentiation and exhaustion^162–164. Although TILs can be reactivated and expanded in vitro in the presence of immunostimulatory cytokines, these cells, although sometimes effective, are currently incapable of mediating durable complete responses in most patients¹³⁰. The successful derivation of pluripotent stem cells from mature fibroblasts by the ectopic co-expression of crucial ESC transcription factors^165,166 or miRNAs¹⁶⁷ has powerfully demonstrated how cell fates can be altered by the manipulation of a few key transcriptional regulators, paving the way for the possibility of reprogramming terminally differentiated TILs into highly effective, stem cell-like tumour-reactive T cells (FIG. 4b,c).

Since Yamanaka’s breakthrough study, numerous groups have shown that induced pluripotent stem (iPS) cells can be produced from different somatic cells, including T lymphocytes by enforced expression of the OCT4, SOX2, KLF4 and MYC transcription factors^168–171. Importantly, T cell-derived iPS cells maintain the rearranged variable (V), diversity (D) and joining regions (J) of the TCR chains, indicating that iPS cells generated from TILs could retain their antitumour reactivity. Recent insights into the nature of instructive signaling required for T cell development during thymopoiesis has led to the development of ex vivo methods that support the generation of T cells from ESC¹⁷², HSC^173–175 and iPS cells¹⁷⁶ providing the tools for re-differentiating TIL-derived iPS cells. Although conceptually attractive and theoretically feasible, this two-step reprogramming approach (FIG. 4b) is currently inefficient both in terms of the frequency of cells successfully reprogrammed and the duration necessary to achieve full reprogramming.

To overcome these limitations, a single approach to directly reprogramme terminally differentiated TILs into more naive, stem cell-like T cells might be possible. A number of reports have shown that direct reprogramming can be used to differentiate various mature cell types into alternative differentiated tissues such as neurons^177,178, cardiomyocytes¹⁷⁹, blood progenitors¹⁸⁰ and hepatocytes¹⁸¹ by enforced expression of tissue-specific transcription factors. Adapting this approach, ectopic expression of key T_N and T_SCM-associated transcription factors or miRNAs might result in the intra-lineage reprogramming of terminally differentiated T_EFF cells into less differentiated T cell subsets (FIG. 4c)

Concluding remarks

It is now clear that subsets of mature T cells exist that are endowed with the stem cell-like attributes of self-renewal, multipotency and the ability to undergo asymmetric division. Similar to conventional stem cells, evolutionarily conserved pathways regulating stemness are active in antigen-experienced T cells, especially T_SCM cells, T_H17 cells and IL-17-producing CD8⁺ T cells. As T cells transition through progressive stages of differentiation, they undergo a stepwise loss of stem cell-associated attributes, including proliferative potential, survival fitness and multipotency. Collectively, these functional changes result in cells that are therapeutically less efficacious upon adoptive transfer. Understanding the epigenetic, genetic and metabolic programmes that regulate T cell self-renewal and persistence provides the ability to pharmacologically or genetically confer stemness to tumour-specific T cells.

Building on recent technologies that have allowed the reprogramming of differentiated somatic cells into iPS cells or cell types of alternative lineages, it is now possible to envision dedifferentiating senescent tumour-reactive T cells from a cancer patient to generate antitumour T cells with improved fitness and therapeutic efficacy for adoptive T cell transfer. The application of regenerative medicine technology to T cell therapies for the treatment of cancer patients using iPS-derived or reprogrammed T cells has several advantages. Terminally ill patients with advanced cancer that is refractory to existing therapies have a favourable risk/benefit ratio profile for receiving reprogrammed cells that might have oncogenic potential¹⁸². Moreover, safety might be ensured by introducing suicide genes in reprogrammed cells to enable the elimination of infused cells if transformation or other adverse events occur^183,184. In addition, lymphocytes are motile circulating cells that are capable of autonomously finding their targets, so they do not suffer the ‘anatomical’ problems that plague regenerative medicine efforts in other organ systems, in which fine interactions between cell types are required for proper functioning¹⁸⁵. Finally, the routine use of immune ablation before adoptive immunotherapy¹⁵⁰ might avoid immune-mediated rejection of reprogrammed cells¹⁸⁶. In summary, preserving and regenerating stem cell-like qualities in T cells may finally enable cancer immunotherapists to fight ‘fire with fire’ with ever increasing effectiveness.

At a glance.

• T lymphocytes transition through progressive stages of differentiation that are characterized by a stepwise loss of functional and therapeutic potential.

• Subsets of mature T cells exhibit the stem cell-like attributes of self-renewal, multipotency and the ability to undergo asymmetric division.

• Evolutionarily conserved pathways regulating stemness are active in T cells, including T memory stem cells, T helper 17 cells and interleukin-17 (IL-17)-producing CD8⁺ T cells.

• Pharmacological and genetic induction of stem cell pathways can be used to generate tumour-specific T cells with stem cell-like properties.

• Reprogramming terminally differentiated tumour-reactive T cells to display naive or stem cell-like functionalities might be obtained through the expression of transcription factors or microRNAs that are associated with naive or T memory stem cells.

• Stem cell-like T cells possess enhanced capacities to engraft, persist and mediate prolonged immune attack against tumour masses that are sustained by long-lived cancer stem cells.

Glossary

Self-renewal

A biological process by which a cell gives rise to one or two daughter cells that have a developmental potential that is indistinguishable from that of the mother cell.

Multipotency

The potential for a cell to give rise to progeny with the capacity to form multiple, but not all possible, lineages.

Tumour-infiltrating lymphocytes (TILs).

The heterogeneous population of T cells found in a tumour bed. These cells are characterized by a diversity of phenotypes, antigen specificities, avidities and functional characteristics. They can be activated and expanded ex vivo and reinfused into a tumour-bearing host to mediate tumour regression.

Mass cytometry

Also known as cytometry by time-of-flight (CyTOF). A platform that couples flow cytometry with mass spectrometry. This technique enables the simultaneous evaluation of at least 45 simultaneous phenotypic and functional parameters on a single cell without the use of fluorescent agents or interference from spectral overlap.

Senescence

A biological process by which cells undergo growth arrest after extensive replication.

Homeostatic proliferation

A process of activation and proliferation of leukocytes in a lymphopaenic environment. T cell homeostatic proliferation is driven by T cell receptor interactions with self-peptide– MHC complexes and responsiveness to homeostatic cytokines such as interleukin-7 (IL-7), IL-15 and possibly IL-21.

TCR excision circles (TRECs).

Circular, stable extra-chromosomal DNA fragments that are generated during recombination of variable (V), diversity (D) and joining regions (J) of the T cell receptor. TRECs do not replicate with cellular proliferation and are thus diluted with every cell division, allowing the assessment of the replicative history of a T cell.

Asymmetric cell division

A conserved mechanism by which a cell divides into daughter cells of unequal size and cytoplasmic content, thus conferring differential developmental fates to progeny cells.

Telomere

The segment at the end of chromosomal arms consisting of a series of repeated DNA sequences (TTAGGG in all vertebrates) that regulates chromosomal replication at each cell division.

Stem cell niche

A specialized microenvironment containing stem cells that supports their maintenance and regulates their function.

Common γ-chain (γ_C).

A signalling subunit common to the receptors for interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15 and IL-21.

Exhaustion

A state of T cell dysfunction arising during reiterative antigen stimulations such as chronic infections and cancer. It is defined by poor effector function and proliferative response to antigenic stimuli, expression of inhibitory receptors and a transcriptional state that is distinct from that of functional effector or memory T cells.

Autosomal-dominant hyper-IgE syndrome (AD-HIES).

Also known as Job’s syndrome. A rare primary immunodeficiency characterized by recurrent skin abscesses, cyst-forming pneumonias and extreme increases of serum IgE levels. Most AD-HIES cases are caused by dominant-negative mutations in STAT3.

Epigenetic modifications

Heritable molecular alterations of the genome that do not involve changes to the nucleotide sequence that regulates gene or microRNA expression. They include DNA methylation, histone modifications and nucleosome positioning.

Induced pluripotent stem (iPS) cells

Pluripotent stem cells artificially derived from non-pluripotent cells, such as an adult somatic cell by forced expression of specific genes or microRNAs.

Suicide genes

Genes capable of selectively eliminating the cells into which they have been transduced following the administration of a drug.