- Email: [email protected]
Back to the Future: Effector Fate during T Cell Exhaustion
Immunity
Previews Back to the Future: Effector Fate during T Cell Exhaustion Veit R. Buchholz1,* and Dirk H. Busch1,2,*
€t Mu €nchen (TUM), Munich, Germany for Medical Microbiology, Immunology and Hygiene, Technische Universita Center for Infection Research (DZIF), Partner site Munich, Germany *Correspondence: [email protected] (V.R.B.), [email protected] (D.H.B.) https://doi.org/10.1016/j.immuni.2019.11.007 1Institute
2German
Exhausted CD8+ T cells adopt a functionally attenuated state but still confer a certain degree of pathogen control. Chen et al. (2019), Hudson et al. (2019), and Zander et al. (2019) assign the lasting maintenance of this restrained pathogen control to an equilibrium of effector-like, transitory, terminal, and memory-like exhausted T cells. Over the past decade, it has become clear that developmental decisions taken during the first few days of clonal expansion critically determine the fate of CD8+ T cells throughout the contraction and memory phases of adaptive immune responses to acutely resolving infections. Early on, activated CD8+ T cells segregate into T central memory precursors—that later home to secondary lymphoid organs and are capable of self-renewal—and more differentiated T cells that lack these qualities (Buchholz et al., 2013). Both subsets can be delineated after only three cell divisions by high versus low expression of the transcription factor T cell factor 1 (TCF-1) (Lin et al., 2016). Moreover, development of optimal CD8+ T cell memory depends on CD4+ T cell help. Such help has recently been shown to also support the generation of highly cytolytic terminal T effector (Teff) cells, which have reached an end stage of T cell differentiation and are characterized by expression of chemokine receptor CX3CR1 and Killer cell lectin-like receptor G1 (KLRG1) (Ahrends et al., 2017). During the first 2 weeks of infection with chronically persistent pathogens such as lymphocytic choriomeningitis virus clone 13 (Cl13), CD8+ T cells adopt and later maintain a state of limited functionality. This so called T cell ‘‘exhaustion’’ is characterized by reduced secretion of effector cytokines and sustained expression of inhibitory receptors such as PD-1. It, thereby, prevents excessive tissue damage but still enables some degree of pathogen control. Despite this distinctiveness, many of the general developmental rules identified for classical CD8+ T cell responses seem to
also apply in the altered context of T cell exhaustion. For example, it has been known for quite some time that infection with Cl13 will take a distinct course in the presence or absence of CD4+ T cell help. In the presence of help, fully functional CD8+ T cells reappear, and protracted Cl13 infection resolves after 2–3 months. In the absence of help, however, infection remains chronic, viral titers remain high, and CD8+ T cells stay permanently exhausted (Fuller et al., 2004). In addition, it has been found that exhausted T cell populations originate from KLRG1 memory precursors and not from KLRG1+ Teff cells and that maintenance of exhausted immune responses depends on a subset of TCF-1+ T memory-like exhausted (Tmex) cells (Im et al., 2016; Utzschneider et al., 2016). Recently, a subdivision into TCF-1-expressing and non-expressing cells has been identified via single-cell RNA sequencing (scRNA-seq) already at day 4.5 post infection with Cl13 (Yao et al., 2019). Interestingly, TCF-1-positive transcriptional clusters, generated by Cl13 infection versus acutely resolving LCMV Armstrong infection, largely overlap at this early time point and clearly segregate only by day 7 post infection. During the same time frame, T cells responding to Cl13 specifically upregulate the transcription factor TOX, which has been found to transcriptionally and epigenetically program CD8+ T cell exhaustion (Alfei et al., 2019; Khan et al., 2019; Yao et al., 2019). Trajectory inference at day 7 further suggests that TCF-1+ T cells serve as precursors of TCF-1 T exhausted cells (Tex) (Yao et al., 2019).
970 Immunity 51, December 17, 2019 ª 2019 Elsevier Inc.
Three papers published in the November and December issues of Immunity now shed further light on these processes and the lineage relationships, cellular interactions, and cytokine cues that modulate them. The Wherry laboratory used scRNAseq, genetic ablation, and fate mapping via adoptive transfer of sorted T cell subsets to study the regulation of effector versus exhausted T cell development during the early phase of Cl13 infection (Chen et al., 2019). By combining scRNA-seq data derived from naive T cells (Tn) and antigen-specific T cells found at day 8 post infection with Cl13, the authors identified a branching trajectory that emerged from Tn cells and separated into TCF-1+ and TCF-1– T cells. Among the latter, the authors found both KLRG1+PD-1 Tefflike cells that retained relatively high functionality, as well as KLRG1 PD-1+ Tex cells. Importantly, a substantial number of TCF-1+ T cells were situated at the bifurcation of the branching trajectory, consistent with a precursor-progeny relationship of TCF-1+ with TCF-1 T cells. Based on further experiments, using TCF-1-deficient mice and overexpression of TCF-1 p45 and p33 isoforms, Chen et al. (2019) suggest that absence of this transcription factor preferentially enhances the development of Teff-like over Tex cells. However, lack of TCF-1 has previously been found to enhance both development of terminal Tex and Teff cells in the context of established chronic and acute infection, respectively. Thus, TCF-1 may have to be considered rather as a general blocker of terminal differentiation and less as a binary switch between Tex and Teff cell development. Due to the
Immunity
Previews TCF-1CX3CR1hi KLRG1+ PD1CD101-
Functional Exhausted Terminal Stem-like
Teff Tn
TCF-1 CX3CR1KLRG1PD1+/CD101-
PD-L1 blockade
TO X
Tmex TCF-1+ CX3CR1KLRG1PD1+ CD101-
Teff -like
?
Ttrans
Tmp +
TCF-1CX3CR1hi KLRG1+ PD1CD101-
TCF-1CX3CR1int KLRG1PD1+ CD101-
?
T cell help IL-21
Tex TCF-1CX3CR1KLRG1PD1+ CD101+
Figure 1. Proposed Developmental Trajectory of CD8+ T Cell Subsets during the Early and Later Phases of Chronic Infection After activation of naive T cells (Tn), proliferating T cells segregate into TCF-1+ self-renewing (curved arrow) T memory precursor (Tmp) cells and TCF-1 CX3CR1high KLRG1+ T effector (Teff) cells. Tmp cells then undergo TOX-driven epigenetic changes into TCF-1+ PD1+ self-renewing T memory-like exhausted (Tmex) cells, which give rise to TCF-1 PD1+ CX3CR1int T transitory exhausted (Ttrans) cells that continue to differentiate into TCF-1 PD1+ CD101+ terminal T exhausted (Tex) cells in absence of CD4+ T cell help. PD-L1 blockade enhances proliferation and differentiation of Tmex cells into downstream subsets, but only IL-21 mediated CD4+ T cell help enables differentiation into fully functional TCF-1 CX3CR1high KLRG1+ terminal T effector-like (Teff-like) cells.
TOX-induced transcriptional and epigenetic changes that the TCF-1+ precursor population undergoes during the first 2 weeks of chronic infection (Alfei et al., 2019; Khan et al., 2019; Yao et al., 2019), the apparent role of TCF-1 could change over time from a blocker of Teff to a blocker of Tex cell differentiation (see Figure 1). Interestingly, work coming from the Ahmed and Cui laboratories now demonstrates that, during established chronic Cl13 infection, Teff-like and terminal Tex subsets continue to exist in parallel (Hudson et al., 2019; Zander et al., 2019). Both groups found that Teff-like cells were characterized by expression of the chemokine receptor CX3CR1, enhanced cytotoxicity, heightened production of effector cytokines, and increased cell-cycle activity when compared to Tmex and Tex subsets. By diphtheria-toxin-mediated depletion of CX3CR1-expressing cells, the authors of both studies showed that CX3CR1+ T cells contributed significantly to viral control. In addition, Zander et al. (2019) elegantly revealed that generation and/or maintenance of Teff-like cells was dependent on CD4+ T cell help pro-
vided via interleukin-21 (IL-21). However, there are also a few interesting differences in the findings of both studies, which may be related to the applied CD4-replete (Zander et al., 2019) versus CD4-depleted (Hudson et al., 2019) models of Cl13 infection. Around day 30 post infection and by using scRNA-seq and trajectory inference combined with sorting of phenotypically delineated subsets, Zander et al. (2019) identified a branching trajectory that originated from Tmex cells and divided into a CX3CR1+ KLRG1+ Teff-like and a CX3CR1 KLRG1 Tex cell branch. Hudson et al. (2019) instead found a linear developmental order in which Tmex cells generated CD101 TIM-3+CX3CR1+ transitory T cells, which then gave rise to CD101+TIM-3+CX3CR1 terminal Tex cells. Importantly, while these transitory T cells shared CX3CR1 expression with Teff-like cells in the CD4-replete infection setting, KLRG1 was generally absent during the later stages of Cl13 infection under CD4-depleted conditions (Chen et al., 2019). Interestingly, Zander et al. (2019) also identified a minor cluster in their scRNA-seq data, in which KLRG1 was
absent and CX3CR1 showed intermediate expression levels. T cells from this cluster were found mainly within the proximal, but not the distal, parts of both Tex and Teff-like branches, suggesting a transitory nature of this cluster. As stated above, chronicity of Cl13 infection is stably maintained in the absence of but eventually resolves in the presence of CD4+ T cell help. We speculate that Zander et al. (2019) observed indications of this resolution of infection, signified by a branching development of CX3CR1+KLRG1 transitory T cells into both CX3CR1 KLRG1 terminal Tex and CX3CR1+KLRG1+ Teff-like cells. Instead, Hudson et al. (2019) likely studied a situation of stable maintenance of exhaustion with unidirectional progression of transitory T cells into the terminal Tex compartment. Upon anti-PD-L1 checkpoint blockade, these authors found a selective increase in the frequency of the transitory T cell subset. However, Zander et al. (2019) showed that generation of terminal Teff-like cells was not preferentially enhanced by checkpoint inhibition alone. Instead, they found that CD4+ T cellderived IL-21 was required to redirect the described differentiation streams toward terminal Teff-like cells (see Figure 1). Many questions still remain with respect to how IL-21 and potential other factors orchestrate this transcriptional redirection and how these mechanisms can be harnessed therapeutically. In addition, it remains to be explored which subset of CD8+ T cells is subjected to IL-21-based reprogramming and when during infection this reprogramming occurs. Potentially, a careful look at the ‘‘past’’—i.e., at the rules and regulations governing classical immunological memory—will help to solve these emerging questions. Taken together, the three highlighted manuscripts show that long-term CD8+ T cell exhaustion and fate decisions taken early during classical T cell responses appear more and more interconnected. Or using the words of Marty McFly, who, before returning ‘‘back to the future’’ in his DeLorean time machine, comes to a stunning realization at his parents’ prom: ‘‘Wait, you don’t understand. If you don’t play, there’s no music. If there’s no music, they don’t dance. If they don’t dance, they don’t kiss and fall in love and I’m history.’’ Immunity 51, December 17, 2019 971
Immunity
Previews REFERENCES
Exhausted CD8 T Cell-Fate Decision. Immunity 51, 840–855.
Ahrends, T., Spanjaard, A., Pilzecker, B., Ba˛ba1a, N., Bovens, A., Xiao, Y., Jacobs, H., and Borst, J. (2017). CD4+ T Cell Help Confers a Cytotoxic T Cell Effector Program Including Coinhibitory Receptor Downregulation and Increased Tissue Invasiveness. Immunity 47, 848–861.e5.
Fuller, M.J., Khanolkar, A., Tebo, A.E., and Zajac, A.J. (2004). Maintenance, loss, and resurgence of T cell responses during acute, protracted, and chronic viral infections. J. Immunol. 172, 4204–4214.
Alfei, F., Kanev, K., Hofmann, M., Wu, M., Ghoneim, H.E., Roelli, P., Utzschneider, D.T., von Hoesslin, M., Cullen, J.G., Fan, Y., et al. (2019). TOX reinforces the phenotype and longevity of exhausted T cells in chronic viral infection. Nature 571, 265–269.
Hudson, W.H., Gensheimer, J., Hashimoto, M., Wieland, A., Valanparambil, R.M., Li, P., Lin, J.X., Konieczny, B.T., Im, S.J., Freeman, G.J., et al. (2019). Proliferating Transitory T Cells with an Effector-like Transcriptional Signature Emerge from PD-1+ Stem-like CD8+ Cells during Chronic Infection. Immunity 51, this issue, 1043–1058.
Buchholz, V.R., Flossdorf, M., Hensel, I., €f, P., Kretschmer, L., Weissbrich, B., Gra Verschoor, A., Schiemann, M., Ho¨fer, T., and Busch, D.H. (2013). Disparate individual fates compose robust CD8+ T cell immunity. Science 340, 630–635.
Im, S.J., Hashimoto, M., Gerner, M.Y., Lee, J., Kissick, H.T., Burger, M.C., Shan, Q., Hale, J.S., Lee, J., Nasti, T.H., et al. (2016). Defining CD8+ T cells that provide the proliferative burst after PD-1 therapy. Nature 537, 417–421.
Chen, Z., Ji, Z., Ngiow, S.F., Manne, S., Cai, Z., Huang, A.C., Johnson, J., Staupe, R.P., Bengsch, B., Xu, C., et al. (2019). TCF-1-Centered Transcriptional Network Drives an Effector versus
Khan, O., Giles, J.R., McDonald, S., Manne, S., Ngiow, S.F., Patel, K.P., Werner, M.T., Huang, A.C., Alexander, K.A., Wu, J.E., et al. (2019). TOX transcriptionally and epigenetically programs CD8+ T cell exhaustion. Nature 571, 211–218.
Lin, W.W., Nish, S.A., Yen, B., Chen, Y.-H., Adams, W.C., Kratchmarov, R., Rothman, N.J., Bhandoola, A., Xue, H.-H., and Reiner, S.L. (2016). CD8+ T Lymphocyte Self-Renewal during Effector Cell Determination. Cell Rep. 17, 1773–1782. Utzschneider, D.T., Charmoy, M., Chennupati, V., Pousse, L., Ferreira, D.P., Calderon-Copete, S., Danilo, M., Alfei, F., Hofmann, M., Wieland, D., et al. (2016). T Cell Factor 1-Expressing Memory-like CD8(+) T Cells Sustain the Immune Response to Chronic Viral Infections. Immunity 45, 415–427. Yao, C., Sun, H.-W., Lacey, N.E., Ji, Y., Moseman, E.A., Shih, H.-Y., Heuston, E.F., Kirby, M., Anderson, S., Cheng, J., et al. (2019). Single-cell RNA-seq reveals TOX as a key regulator of CD8+ T cell persistence in chronic infection. Nat. Immunol. 20, 890–901. Zander, R., Schauder, D., Xin, G., Nguyen, C., Wu, X., Zajac, A., and Cui, W. (2019). CD4+ Help Is Required for the Formation of a Cytoloytic CD8+ T Cell Subset that Protects against Chronic Infection and Cancer. Immunity 51, this issue, 1028–1042.
Dimers Aren’t Forever: CD80 Breaks up with PD-L1 David M. Sansom1,* and Lucy S.K. Walker1,* 1UCL Institute of Immunity and Transplantation, Royal Free Hospital, London NW3 2PF, UK *Correspondence: [email protected] (D.M.S.), [email protected] (L.S.K.W.) https://doi.org/10.1016/j.immuni.2019.11.011
Targeting the CTLA-4 and PD-1 ‘‘checkpoints’’ is an effective treatment for a number of cancers. In this issue of Immunity, Hui et al. reveal that interaction between a CTLA-4 ligand, CD80, and its counterpart in the PD-1 pathway, PD-L1, affects both PD-1 and CTLA-4 function, raising new questions about the biological effects of using checkpoint inhibitors alone and in combination.
T cell immune checkpoints are regulatory pathways that limit the activity of T cells, often preventing autoimmune consequences. The CTLA-4 pathway was originally identified as an essential checkpoint from studies of Ctla-4 / mice that suffer from fatal perinatal autoimmunity. Similarly, PD-1-deficient mice also have a propensity for autoimmunity, albeit with somewhat milder effects compared to CTLA-4. The appreciation that such inhibitory pathways could be manipulated to enhance T cell responses to cancer ultimately led to the 2018 Nobel Prize for physiology and medicine to Allison and Honjo. Surprisingly, despite successful therapeutic manipulation, the interactions between these pathways and the molecu-
lar controls that dictate their function are still rather poorly understood. In this issue of Immunity, Zhao et al. (2019) provide new insights into this important issue. While CTLA-4 and PD-1 pathways are both important negative regulators of T cell responses, there are some clear differences between them. CTLA-4 is part of a complex system involving two ligands— CD80 and CD86—that are shared with a second receptor, CD28. Whereas CTLA4 is inhibitory to T cell responses, CD28 is an activating receptor, which, alongside the T cell receptor, provides costimulatory signals that are required for effective T cell and B cell immunity. Indeed, evidence suggests that the role of CTLA-4 is to directly oppose the activating function of
972 Immunity 51, December 17, 2019 ª 2019 Elsevier Inc.
CD28 and that loss of control of this pathway results in activation of self-reactive T cells. One mechanism by which CTLA-4 can affect such control is the physical removal of their shared ligands from antigen-presenting cells (APCs) in a process known as transendocytosis (Qureshi et al., 2011). Since CTLA-4 is highly expressed on regulatory T cells (Tregs) and is required for their function (Wing et al., 2008), a plausible model is that Treg-expressed CTLA-4 regulates CD80 and CD86 levels on APCs, thereby limiting T cell costimulation in the steady state. In contrast to CTLA-4, PD-1 appears to act as a more straightforward inhibitory receptor, acting in a cell-intrinsic manner to inhibit the cells that express it.
Previews Back to the Future: Effector Fate during T Cell Exhaustion Veit R. Buchholz1,* and Dirk H. Busch1,2,*
€t Mu €nchen (TUM), Munich, Germany for Medical Microbiology, Immunology and Hygiene, Technische Universita Center for Infection Research (DZIF), Partner site Munich, Germany *Correspondence: [email protected] (V.R.B.), [email protected] (D.H.B.) https://doi.org/10.1016/j.immuni.2019.11.007 1Institute
2German
Exhausted CD8+ T cells adopt a functionally attenuated state but still confer a certain degree of pathogen control. Chen et al. (2019), Hudson et al. (2019), and Zander et al. (2019) assign the lasting maintenance of this restrained pathogen control to an equilibrium of effector-like, transitory, terminal, and memory-like exhausted T cells. Over the past decade, it has become clear that developmental decisions taken during the first few days of clonal expansion critically determine the fate of CD8+ T cells throughout the contraction and memory phases of adaptive immune responses to acutely resolving infections. Early on, activated CD8+ T cells segregate into T central memory precursors—that later home to secondary lymphoid organs and are capable of self-renewal—and more differentiated T cells that lack these qualities (Buchholz et al., 2013). Both subsets can be delineated after only three cell divisions by high versus low expression of the transcription factor T cell factor 1 (TCF-1) (Lin et al., 2016). Moreover, development of optimal CD8+ T cell memory depends on CD4+ T cell help. Such help has recently been shown to also support the generation of highly cytolytic terminal T effector (Teff) cells, which have reached an end stage of T cell differentiation and are characterized by expression of chemokine receptor CX3CR1 and Killer cell lectin-like receptor G1 (KLRG1) (Ahrends et al., 2017). During the first 2 weeks of infection with chronically persistent pathogens such as lymphocytic choriomeningitis virus clone 13 (Cl13), CD8+ T cells adopt and later maintain a state of limited functionality. This so called T cell ‘‘exhaustion’’ is characterized by reduced secretion of effector cytokines and sustained expression of inhibitory receptors such as PD-1. It, thereby, prevents excessive tissue damage but still enables some degree of pathogen control. Despite this distinctiveness, many of the general developmental rules identified for classical CD8+ T cell responses seem to
also apply in the altered context of T cell exhaustion. For example, it has been known for quite some time that infection with Cl13 will take a distinct course in the presence or absence of CD4+ T cell help. In the presence of help, fully functional CD8+ T cells reappear, and protracted Cl13 infection resolves after 2–3 months. In the absence of help, however, infection remains chronic, viral titers remain high, and CD8+ T cells stay permanently exhausted (Fuller et al., 2004). In addition, it has been found that exhausted T cell populations originate from KLRG1 memory precursors and not from KLRG1+ Teff cells and that maintenance of exhausted immune responses depends on a subset of TCF-1+ T memory-like exhausted (Tmex) cells (Im et al., 2016; Utzschneider et al., 2016). Recently, a subdivision into TCF-1-expressing and non-expressing cells has been identified via single-cell RNA sequencing (scRNA-seq) already at day 4.5 post infection with Cl13 (Yao et al., 2019). Interestingly, TCF-1-positive transcriptional clusters, generated by Cl13 infection versus acutely resolving LCMV Armstrong infection, largely overlap at this early time point and clearly segregate only by day 7 post infection. During the same time frame, T cells responding to Cl13 specifically upregulate the transcription factor TOX, which has been found to transcriptionally and epigenetically program CD8+ T cell exhaustion (Alfei et al., 2019; Khan et al., 2019; Yao et al., 2019). Trajectory inference at day 7 further suggests that TCF-1+ T cells serve as precursors of TCF-1 T exhausted cells (Tex) (Yao et al., 2019).
970 Immunity 51, December 17, 2019 ª 2019 Elsevier Inc.
Three papers published in the November and December issues of Immunity now shed further light on these processes and the lineage relationships, cellular interactions, and cytokine cues that modulate them. The Wherry laboratory used scRNAseq, genetic ablation, and fate mapping via adoptive transfer of sorted T cell subsets to study the regulation of effector versus exhausted T cell development during the early phase of Cl13 infection (Chen et al., 2019). By combining scRNA-seq data derived from naive T cells (Tn) and antigen-specific T cells found at day 8 post infection with Cl13, the authors identified a branching trajectory that emerged from Tn cells and separated into TCF-1+ and TCF-1– T cells. Among the latter, the authors found both KLRG1+PD-1 Tefflike cells that retained relatively high functionality, as well as KLRG1 PD-1+ Tex cells. Importantly, a substantial number of TCF-1+ T cells were situated at the bifurcation of the branching trajectory, consistent with a precursor-progeny relationship of TCF-1+ with TCF-1 T cells. Based on further experiments, using TCF-1-deficient mice and overexpression of TCF-1 p45 and p33 isoforms, Chen et al. (2019) suggest that absence of this transcription factor preferentially enhances the development of Teff-like over Tex cells. However, lack of TCF-1 has previously been found to enhance both development of terminal Tex and Teff cells in the context of established chronic and acute infection, respectively. Thus, TCF-1 may have to be considered rather as a general blocker of terminal differentiation and less as a binary switch between Tex and Teff cell development. Due to the
Immunity
Previews TCF-1CX3CR1hi KLRG1+ PD1CD101-
Functional Exhausted Terminal Stem-like
Teff Tn
TCF-1 CX3CR1KLRG1PD1+/CD101-
PD-L1 blockade
TO X
Tmex TCF-1+ CX3CR1KLRG1PD1+ CD101-
Teff -like
?
Ttrans
Tmp +
TCF-1CX3CR1hi KLRG1+ PD1CD101-
TCF-1CX3CR1int KLRG1PD1+ CD101-
?
T cell help IL-21
Tex TCF-1CX3CR1KLRG1PD1+ CD101+
Figure 1. Proposed Developmental Trajectory of CD8+ T Cell Subsets during the Early and Later Phases of Chronic Infection After activation of naive T cells (Tn), proliferating T cells segregate into TCF-1+ self-renewing (curved arrow) T memory precursor (Tmp) cells and TCF-1 CX3CR1high KLRG1+ T effector (Teff) cells. Tmp cells then undergo TOX-driven epigenetic changes into TCF-1+ PD1+ self-renewing T memory-like exhausted (Tmex) cells, which give rise to TCF-1 PD1+ CX3CR1int T transitory exhausted (Ttrans) cells that continue to differentiate into TCF-1 PD1+ CD101+ terminal T exhausted (Tex) cells in absence of CD4+ T cell help. PD-L1 blockade enhances proliferation and differentiation of Tmex cells into downstream subsets, but only IL-21 mediated CD4+ T cell help enables differentiation into fully functional TCF-1 CX3CR1high KLRG1+ terminal T effector-like (Teff-like) cells.
TOX-induced transcriptional and epigenetic changes that the TCF-1+ precursor population undergoes during the first 2 weeks of chronic infection (Alfei et al., 2019; Khan et al., 2019; Yao et al., 2019), the apparent role of TCF-1 could change over time from a blocker of Teff to a blocker of Tex cell differentiation (see Figure 1). Interestingly, work coming from the Ahmed and Cui laboratories now demonstrates that, during established chronic Cl13 infection, Teff-like and terminal Tex subsets continue to exist in parallel (Hudson et al., 2019; Zander et al., 2019). Both groups found that Teff-like cells were characterized by expression of the chemokine receptor CX3CR1, enhanced cytotoxicity, heightened production of effector cytokines, and increased cell-cycle activity when compared to Tmex and Tex subsets. By diphtheria-toxin-mediated depletion of CX3CR1-expressing cells, the authors of both studies showed that CX3CR1+ T cells contributed significantly to viral control. In addition, Zander et al. (2019) elegantly revealed that generation and/or maintenance of Teff-like cells was dependent on CD4+ T cell help pro-
vided via interleukin-21 (IL-21). However, there are also a few interesting differences in the findings of both studies, which may be related to the applied CD4-replete (Zander et al., 2019) versus CD4-depleted (Hudson et al., 2019) models of Cl13 infection. Around day 30 post infection and by using scRNA-seq and trajectory inference combined with sorting of phenotypically delineated subsets, Zander et al. (2019) identified a branching trajectory that originated from Tmex cells and divided into a CX3CR1+ KLRG1+ Teff-like and a CX3CR1 KLRG1 Tex cell branch. Hudson et al. (2019) instead found a linear developmental order in which Tmex cells generated CD101 TIM-3+CX3CR1+ transitory T cells, which then gave rise to CD101+TIM-3+CX3CR1 terminal Tex cells. Importantly, while these transitory T cells shared CX3CR1 expression with Teff-like cells in the CD4-replete infection setting, KLRG1 was generally absent during the later stages of Cl13 infection under CD4-depleted conditions (Chen et al., 2019). Interestingly, Zander et al. (2019) also identified a minor cluster in their scRNA-seq data, in which KLRG1 was
absent and CX3CR1 showed intermediate expression levels. T cells from this cluster were found mainly within the proximal, but not the distal, parts of both Tex and Teff-like branches, suggesting a transitory nature of this cluster. As stated above, chronicity of Cl13 infection is stably maintained in the absence of but eventually resolves in the presence of CD4+ T cell help. We speculate that Zander et al. (2019) observed indications of this resolution of infection, signified by a branching development of CX3CR1+KLRG1 transitory T cells into both CX3CR1 KLRG1 terminal Tex and CX3CR1+KLRG1+ Teff-like cells. Instead, Hudson et al. (2019) likely studied a situation of stable maintenance of exhaustion with unidirectional progression of transitory T cells into the terminal Tex compartment. Upon anti-PD-L1 checkpoint blockade, these authors found a selective increase in the frequency of the transitory T cell subset. However, Zander et al. (2019) showed that generation of terminal Teff-like cells was not preferentially enhanced by checkpoint inhibition alone. Instead, they found that CD4+ T cellderived IL-21 was required to redirect the described differentiation streams toward terminal Teff-like cells (see Figure 1). Many questions still remain with respect to how IL-21 and potential other factors orchestrate this transcriptional redirection and how these mechanisms can be harnessed therapeutically. In addition, it remains to be explored which subset of CD8+ T cells is subjected to IL-21-based reprogramming and when during infection this reprogramming occurs. Potentially, a careful look at the ‘‘past’’—i.e., at the rules and regulations governing classical immunological memory—will help to solve these emerging questions. Taken together, the three highlighted manuscripts show that long-term CD8+ T cell exhaustion and fate decisions taken early during classical T cell responses appear more and more interconnected. Or using the words of Marty McFly, who, before returning ‘‘back to the future’’ in his DeLorean time machine, comes to a stunning realization at his parents’ prom: ‘‘Wait, you don’t understand. If you don’t play, there’s no music. If there’s no music, they don’t dance. If they don’t dance, they don’t kiss and fall in love and I’m history.’’ Immunity 51, December 17, 2019 971
Immunity
Previews REFERENCES
Exhausted CD8 T Cell-Fate Decision. Immunity 51, 840–855.
Ahrends, T., Spanjaard, A., Pilzecker, B., Ba˛ba1a, N., Bovens, A., Xiao, Y., Jacobs, H., and Borst, J. (2017). CD4+ T Cell Help Confers a Cytotoxic T Cell Effector Program Including Coinhibitory Receptor Downregulation and Increased Tissue Invasiveness. Immunity 47, 848–861.e5.
Fuller, M.J., Khanolkar, A., Tebo, A.E., and Zajac, A.J. (2004). Maintenance, loss, and resurgence of T cell responses during acute, protracted, and chronic viral infections. J. Immunol. 172, 4204–4214.
Alfei, F., Kanev, K., Hofmann, M., Wu, M., Ghoneim, H.E., Roelli, P., Utzschneider, D.T., von Hoesslin, M., Cullen, J.G., Fan, Y., et al. (2019). TOX reinforces the phenotype and longevity of exhausted T cells in chronic viral infection. Nature 571, 265–269.
Hudson, W.H., Gensheimer, J., Hashimoto, M., Wieland, A., Valanparambil, R.M., Li, P., Lin, J.X., Konieczny, B.T., Im, S.J., Freeman, G.J., et al. (2019). Proliferating Transitory T Cells with an Effector-like Transcriptional Signature Emerge from PD-1+ Stem-like CD8+ Cells during Chronic Infection. Immunity 51, this issue, 1043–1058.
Buchholz, V.R., Flossdorf, M., Hensel, I., €f, P., Kretschmer, L., Weissbrich, B., Gra Verschoor, A., Schiemann, M., Ho¨fer, T., and Busch, D.H. (2013). Disparate individual fates compose robust CD8+ T cell immunity. Science 340, 630–635.
Im, S.J., Hashimoto, M., Gerner, M.Y., Lee, J., Kissick, H.T., Burger, M.C., Shan, Q., Hale, J.S., Lee, J., Nasti, T.H., et al. (2016). Defining CD8+ T cells that provide the proliferative burst after PD-1 therapy. Nature 537, 417–421.
Chen, Z., Ji, Z., Ngiow, S.F., Manne, S., Cai, Z., Huang, A.C., Johnson, J., Staupe, R.P., Bengsch, B., Xu, C., et al. (2019). TCF-1-Centered Transcriptional Network Drives an Effector versus
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Dimers Aren’t Forever: CD80 Breaks up with PD-L1 David M. Sansom1,* and Lucy S.K. Walker1,* 1UCL Institute of Immunity and Transplantation, Royal Free Hospital, London NW3 2PF, UK *Correspondence: [email protected] (D.M.S.), [email protected] (L.S.K.W.) https://doi.org/10.1016/j.immuni.2019.11.011
Targeting the CTLA-4 and PD-1 ‘‘checkpoints’’ is an effective treatment for a number of cancers. In this issue of Immunity, Hui et al. reveal that interaction between a CTLA-4 ligand, CD80, and its counterpart in the PD-1 pathway, PD-L1, affects both PD-1 and CTLA-4 function, raising new questions about the biological effects of using checkpoint inhibitors alone and in combination.
T cell immune checkpoints are regulatory pathways that limit the activity of T cells, often preventing autoimmune consequences. The CTLA-4 pathway was originally identified as an essential checkpoint from studies of Ctla-4 / mice that suffer from fatal perinatal autoimmunity. Similarly, PD-1-deficient mice also have a propensity for autoimmunity, albeit with somewhat milder effects compared to CTLA-4. The appreciation that such inhibitory pathways could be manipulated to enhance T cell responses to cancer ultimately led to the 2018 Nobel Prize for physiology and medicine to Allison and Honjo. Surprisingly, despite successful therapeutic manipulation, the interactions between these pathways and the molecu-
lar controls that dictate their function are still rather poorly understood. In this issue of Immunity, Zhao et al. (2019) provide new insights into this important issue. While CTLA-4 and PD-1 pathways are both important negative regulators of T cell responses, there are some clear differences between them. CTLA-4 is part of a complex system involving two ligands— CD80 and CD86—that are shared with a second receptor, CD28. Whereas CTLA4 is inhibitory to T cell responses, CD28 is an activating receptor, which, alongside the T cell receptor, provides costimulatory signals that are required for effective T cell and B cell immunity. Indeed, evidence suggests that the role of CTLA-4 is to directly oppose the activating function of
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CD28 and that loss of control of this pathway results in activation of self-reactive T cells. One mechanism by which CTLA-4 can affect such control is the physical removal of their shared ligands from antigen-presenting cells (APCs) in a process known as transendocytosis (Qureshi et al., 2011). Since CTLA-4 is highly expressed on regulatory T cells (Tregs) and is required for their function (Wing et al., 2008), a plausible model is that Treg-expressed CTLA-4 regulates CD80 and CD86 levels on APCs, thereby limiting T cell costimulation in the steady state. In contrast to CTLA-4, PD-1 appears to act as a more straightforward inhibitory receptor, acting in a cell-intrinsic manner to inhibit the cells that express it.