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.

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.

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.

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.

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.