Tissue-Resident Memory T Cells in Cancer Immunosurveillance

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Review

Tissue-Resident Memory T Cells in Cancer Immunosurveillance Simone L. Park,1 Thomas Gebhardt,1 and Laura K. Mackay1,* Following their activation and expansion in response to foreign threats, many T cells are retained in peripheral tissues without recirculating in the blood. These tissue-resident CD8+ memory T (TRM) cells patrol barrier surfaces and nonlymphoid organs, where their critical role in protecting against viral and bacterial infections is well established. Recent evidence suggests that TRM cells also play a vital part in preventing the development and spread of solid tumors. Here, we discuss the emerging role of TRM cells in anticancer immunity. We highlight defining features of tumor-localizing TRM cells, examine the mechanisms through which they have recently been shown to suppress cancer growth, and explore their potential as future targets of cancer immunotherapy.

Highlights TRM cells are non-recirculating immune cells that reside in peripheral tissues where they can protect against local infections and cancer. CD69+CD103+ TRM-like cells accumulate in various human solid cancers where they have been associated with improved disease outcomes and patient survival. Vaccine-generated TRM cells can protect against tumor challenge independently of TCIRC cells.

Local and Regional Cancer Immunosurveillance: A Critical Role for Tissue-Resident Memory T (TRM) Cells The immune system can suppress cancer development and progression by engaging in a process termed ‘cancer immunosurveillance’ (Figure 1, Box 1). Although many innate and adaptive immune cells can contribute to tumor surveillance, CD8+ T cells play a particularly important role [1] and are often considered the major targets of highly successful immune checkpoint (IC) immunotherapies (see Glossary) that have revolutionized the course of cancer treatments [2]. The ability of CD8+ T cells to mediate cancer protection depends on their capacity to access solid tumors and persist within the immunosuppressive tumor microenvironment. The vast majority of human cancers arise in epithelial or other peripheral tissues [3] but only certain subsets of CD8+ T cells are capable of entering these compartments in the absence of overt inflammation [4]. Tissue-resident memory T (TRM) cells are a population of non-recirculating CD8+ T cells that reside permanently within peripheral tissues, conveniently positioned to mediate regional tumor surveillance. Although TRM cells are best recognized for their ability to protect against local viral and bacterial infections [5], accumulating evidence now indicates that these cells also play an integral role in inhibiting solid cancer growth. Targeting TRM cell responses might therefore represent a novel avenue to improve the treatment of solid cancers. In this review, we explore recent evidence linking TRM cells to improved cancer prognosis and protection. We highlight key phenotypical and functional properties of tumor TRM cells and provide insight into how they might be implicated in cancer immunotherapy.

TRM cells may mediate tumor protection by promoting tumor-immune equilibrium through the secretion of cytokines and/ or via CD103-enhanced tumor cell killing. TRM cells express inhibitory checkpoint molecules and may serve as potential targets for cancer immunotherapy.

Defining Features of TRM Cells Phenotypic Hallmarks of TRM Cells The hallmark feature of TRM cells is a lack of equilibration with the circulating memory T (TCIRC) cell pool [5]. Parabiosis, organ transplantation, and targeted antibody-mediated T cell depletion experiments in mice have proven the existence of non-recirculating CD8+ TRM cells in diverse tissues, including the skin, lung, gut, liver, and reproductive tract, as well as in lymphoid organs [5]. Equivalent peripherally localized T cells that escape antibody-mediated depletion or persist following organ transplantation have also been observed in humans [6]. TRM cells differ phenotypically and functionally from TCIRC cells, including classically defined effector memory T (TEM) and

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Department of Microbiology & Immunology at the Peter Doherty Institute for Infection and Immunity, The University of Melbourne, Melbourne, Australia

*Correspondence: [email protected] (L.K. Mackay).

Trends in Immunology, August 2019, Vol. 40, No. 8 https://doi.org/10.1016/j.it.2019.06.002 Crown Copyright © 2019 Published by Elsevier Ltd. All rights reserved.

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Figure 1. T Cells Can Engage in Cancer Immunosurveillance and Immunoediting. Various immune cells, such as T cells and NK cells (pink, orange, green cells), can survey tissues for the presence of malignant cells (dark purple cells). Following the detection of abnormal cells, immune cells may completely eliminate them from the host (‘elimination’). Alternatively, immune cells may suppress the growth and spread of cancerous cells without completely removing them, such that both the cancer and immune cells are maintained in a state of cancer-immune equilibrium (‘equilibrium’). Immune cell effector functions leading to elimination or suppression of susceptible cancer cells may create immune pressure that promotes selection of mutated cancer cells with less immunogenic features that are not easily recognized by the immune system (‘immunoediting’). Eventual overgrowth of these cells may lead to cancer escape and tumor outgrowth.

central memory T (TCM) cells that can temporarily transit through peripheral tissues, but ultimately return to secondary lymphoid organs (SLOs) and blood [4] (Figure 2). Although populations of CD4+ TRM cells have been identified in several healthy tissues [7] and potentially some human tumors [8], these cells are less well characterized than CD8+ TRM cells; only the CD8+ TRM subset has been associated with improved solid cancer prognosis at this time. As such, here, we only focus on the role of CD8+ TRM cells in cancer surveillance. Traditionally, CD8+ TRM cells in mice and humans have been identified by their coexpression of the surface molecules CD69 and CD103, which are usually absent on TCIRC cells [9,10]. CD69 is an early activation marker that is widely expressed by TRM cells in most tissues and can be induced by antigen recognition or cytokine signaling [11,12]. CD69 is upregulated prior to CD103 during TRM cell development in mice [13] and serves to promote early peripheral retention of differentiating TRM cell precursors by antagonizing the function of the tissue egress facilitator S1pr1 [14]. In mice and humans, CD103 (αe integrin) is expressed by most CD8+ TRM cells found in epithelial tissue compartments, including the skin epidermis and intestinal epithelium [9,15], where it binds to E-cadherin to promote peripheral maintenance [13]. However, parabiosis and antibody depletion experiments in mice have revealed that non-recirculating CD69+ TRM cells lacking CD103 expression can be found in several tissues, including the intestinal lamina propria [16], female reproductive tract [17], SLOs [18], and liver [19] following

Box 1. The Cancer Immunosurveillance Hypothesis The mechanisms by which the immune system regulates and shapes cancer progression can be divided into three distinct phases of tumor-immune interaction: elimination, equilibrium, and escape [143]. The elimination phase encompasses initial detection of nascent cells by immune cells that actively eradicate and may completely extinguish malignant targets [1]. When immune cells fail to completely remove transformed cells, they may instead induce a phase of ‘equilibrium’ during which tumor growth is actively suppressed by the counteraction of nascent cell division with immune-induced apoptosis or by the induction of cancer cell senescence [144,145]. During this period of cellular competition, tumors can be subjected to a process of ‘immunoediting’, whereby fitter transformed variants with lower immunogenicity are selected in the face of persistent immune pressure. Consequently, tumors are able to ‘escape’ immunosurveillance by acquiring mutations that facilitate immune evasion. Although innate immune cells can engage in cancer immunoediting, adaptive immune cells, including CD8+ T cells, are critically important for maintaining tumor equilibrium [1].

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Glossary Adoptive cell therapy (ACT): transfer of preactivated and/or tumor-specific immune cells into a patient to treat cancer. CD39: ectonucleoside triphosphate diphosphohydrolase-1 (ENTPD1); cell-surface molecule that hydrolyzes extracellular ATP; usually marks activated or exhausted T cells. Central memory T (TCM) cells: subset of memory T cells that migrate through blood and secondary lymphoid organs; characterized by heightened expression of L-selectin (CD62L) and CCR7. Chimeric antigen receptor (CAR) T cell: T cell engineered to express a modified T cell receptor (TCR) specific for a protein expressed by cancer cells that differs from that recognized by their endogenous TCR. Circulating memory T (TCIRC) cells: memory T cells that continuously migrate through blood and secondary lymphoid organs; includes both effector and central memory T cell subsets. Effector memory T (TEM) cells: subset of memory T cells that migrate through peripheral tissues and the blood; characterized by low expression of both CD62L and CCR7. Elimination: process through which immune cells completely kill and eradicate cancerous cells. Epithelial-to-mesenchymal transition: process during which epithelial or cancerous cells lose cell–cell adhesion properties and acquire heightened invasive and metastatic potential. Equilibrium: process through which immune cells suppress cancer growth without completely removing tumor cells from the body. Exhaustion: state of T cell dysfunction characterized by a loss of effector function resulting from unrelenting antigen stimulation and/or chronic infection. FTY720: sphingosine 1-phosphate receptor antagonist that enforces lymphocyte retention within lymphoid organs by blocking their egress. Hobit: DNA-binding transcription factor encoded by the Znf638 gene; a master regulator of tissue residency in both innate and adaptive lymphocytes. Immune checkpoint (IC) immunotherapies: drug treatments (usually antibody therapies, e.g., anti-PD-1, anti-PD-L1, anti-CTLA-4) designed to activate or enhance anticancer immune responses.

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bacterial or viral infections or after vaccination, inferring that CD69 expression may be sufficient to denote tissue residency in some settings. Nevertheless, CD69+CD103– CD8+ T cells have been shown to recirculate in the kidney, liver, skin, thymus, and lymph nodes of mice under certain circumstances [17,20–22], emphasizing the unreliability of CD69 as a global marker of tissue residency. In addition, non-recirculating CD69–CD103–CD8+ T cells have been shown to exist in various murine tissues after lymphocytic choriomeningitis virus (LCMV) infection [17]. CD69+CD103+CD8+ TRM cells typically express the collagen-binding molecule Very Late Antigen-1 (CD49a) and produce Th1-type cytokines, including interferon gamma (IFN-γ), tumor necrosis factor (TNF), and IL-2 upon stimulation [10,23], but recent studies have demonstrated that CD69+CD103+CD49a–CD8+ TRM cells expressing Th17- or Th2-type cytokines can also develop in mouse and human skin [24–26]. With these discoveries, we are now beginning to appreciate that even populations of TRM cells residing in the same tissue can be highly heterogeneous with respect to phenotype and function. Development and Maintenance of TRM Cells CD8+ TRM cells develop from effector-like T cells that enter peripheral tissues early after activation, where they are exposed to local microenvironmental cues directing commitment to a resident fate [13,27,28]. In mice, TRM cells from diverse tissues share the expression of a common transcriptional signature, paralleling that of innate resident cells but distinct from TCIRC cells [13,29]. This includes upregulation of genes that promote tissue retention (e.g., Cd69, Itgae, Itga1) and the residency-defining transcription factor (TF) Hobit, as well as downregulation of tissue-egress genes (e.g., S1pr1, Klf2) [13,21]. For the most part, putative CD69+CD103+/– TRM cells found in human tissues acquire a similar transcriptional profile to those isolated from mice [30], although human TRM cells may possess unique developmental requirements; for instance, Hobit expression in human CD8+CD69+ tissue T cells can be lower than in blood-tropic CD8+ T cells in both healthy lungs and chronically infected liver tissue [30–32]. Murine TRM cells have been shown to rely on various factors, including transforming growth factor β (TGFβ) and IL-15, for their development and maintenance [5,33], although such requirements might not be universal between all organs, species, or infection contexts [13,20,28,34]. Furthermore, local antigen recognition appears to be required for CD8+ TRM cell formation in the murine nervous system or lung following viral infection [e.g., herpes simplex virus (HSV)] but can be dispensable for the development of these cells in most other tissue sites, including the skin, gut, salivary glands, and liver [5,27,35]. In the steady state, most TRM cells can be maintained independently of TCIRC cell replenishment via slow and ongoing homeostatic proliferation [10,36], except in the mouse lung, where continual recruitment of circulating T cell precursors appears to be required to maintain TRM cell numbers in some but not all studies utilizing influenza infection [37,38]. Following reactivation, TRM cells in mouse skin or the female reproductive tract can proliferate locally without being displaced [39,40] and appear to be able to migrate from their original tissue of residence to participate in systemic immune responses and establish secondary TRM cell populations in other tissue sites, including SLOs [22]. Functions of TRM Cells Until recently, the protective functions of TRM cells had only been examined in the context of infection. TRM cells can provide superior protection against several peripheral viral and bacterial infections in mice, including skin HSV or vaccinia virus (VV) infection, LCMV infection, and oral Listeria monocytogenes infection, compared with TCIRC cells [10,27,41–43], in a manner that depends on their local density within tissues [39]. Upon restimulation, TRM cells can coordinate antipathogen immunity by rapidly secreting cytokines such as IFNγ, TNF, or IL-2 that summon and activate downstream innate and adaptive immune cells, including TCIRC cells [23,44,45]. However, in mice, TRM cells can also protect against HSV, VV, and LCMV infections in the absence of TCIRC cells [20,39,41,46].

Immunoediting: evolutionary process comprising elimination, equilibrium, and escape phases through which immune cells select for less immunogenic tumor cell clones that arise during tumor progression. Immunological synapse: point of contact between a lymphocyte and an antigen-presenting or target cell. Neoantigen: newly generated tumor-specific antigen that arises through mutations occurring during cancer growth. Parabiosis: experimental process during which the circulatory systems of two animals are joined to investigate migratory properties of immune cells. Programmed cell death protein 1 (PD-1): cell-surface immune checkpoint molecule encoded by the Pdcd1 gene that negatively regulates T cell activation and function. T cell factor 1 (TCF1): DNA-binding transcription factor encoded by the Tcf7 gene, involved in T cell differentiation and expressed by proliferative or stem-like subsets of tumor-infiltrating T cells. T cell immunoglobulin and mucin-domain containing-3 (TIM-3): cell-surface immune checkpoint molecule encoded by the Havcr2 gene and associated with T cell dysfunction. Th1: T helper 1; CD4+ T cells or immune response involving effector molecules canonically tailored to combat intracellular pathogens or cancerous cells, including IFN-γ and TNF. Th2: T helper 2; CD4+ T cells or immune response involving effector molecules canonically tailored to combat parasites; may be associated with allergy, including IL-4, IL-5, and IL-13. Th17: T helper 17; CD4+ T cells or immune response involving effector molecules canonically tailored to combat extracellular bacteria and that may be associated with autoimmune diseases, including IL-17. Tissue-resident memory T (TRM) cells: subset of memory T cells that reside permanently within peripheral or nonlymphoid tissues, without recirculating in blood. Tumor microenvironment: space and factors within and surrounding cancer cells, including stromal and immune cells that infiltrate a tumor and the molecules they produce. Very Late Antigen-1 (CD49a): cellsurface molecule that binds collagen IV and may be involved in T cell migration, cell adhesion, and cytotoxic activity.

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Recent work has revealed additional roles for TRM cells in orchestrating autoimmunity. Specifically, CD103+CD69+CD49a– CD8+ TRM cells producing IFNγ are abundant in patient vitiligo skin lesions [24], and studies in vitiligo mouse models indicate that TRM cells may be integral to maintaining skin disease [47,48], in collaboration with TCIRC cells [49]. Conversely, CD103+CD69+CD49a– TRM cells producing IL-17 are enriched within human psoriatic skin lesions [24,50] relative to healthy skin sites, and resident T cells present in lesional skin are sufficient to induce skin inflammation and autoimmune reactions following engraftment in mice [51]. CD103+ TRM cells may also be associated with nonskin autoimmune disorders and inflammation, including type 1 diabetes as they have been shown to accumulate in human pancreatic islets [52] and central nervous system pathology in a mouse model of multiple sclerosis [53]. They might also play a role in driving inflammatory conditions such as colitis [54]. In other settings, however, commensal-specific skin CD8+ TRM cells can uphold tissue homeostasis by accelerating wound healing following tissue injury [55]. Notably, we are now also beginning to appreciate an additional emerging role for TRM cells in anticancer immunity.

TRM Cells Protect against Cancer Initial clues that TRM cells might contribute to tumor surveillance were reflected in the observation that anticancer immune responses are frequently compartmentalized. For instance, the frequency of TCIRC cells in the blood of melanoma patients rarely correlates with disease progression [56–58] and tumor infiltrating lymphocytes (TIL) can accumulate in patient melanoma lesions in the absence of detectable TCIRC responses [59,60]. Furthermore, the heterogeneity and composition of TIL infiltrates can vary dramatically between individual tumor lesions in any given patient [61–63]. Over the past decade, TILs that express the TRM cell markers CD69 and CD103 have been identified in many human solid cancers, including melanoma [61,64,65], lung [66,67], bladder [68,69], ovarian [70–72], cervical [73], breast [74,75], and colorectal tumors [76]; these TILs tend to accumulate in epithelial tumor regions or at the tumor border [68,73,76–79] and are often associated with improved patient outcomes [64,72,75,78,80]. Of relevance to our discussion, these tumor-localizing TRM cells might better predict survival than total CD8+ TILs [64,75] and often exhibit enhanced cytotoxic potential and effector functions compared with CD103– TILs [66–68,73,75]. This suggests that TRM cells might potentially mediate superior tumor surveillance relative to circulating tumor-specific T cells. Recent studies in mice have affirmed a critical role for TRM cells in mediating antitumor immunity. Mice lacking molecules that facilitate TRM cell generation, such as CD69 (Cd69–/–animals) or CD103 (Itgae–/– animals) [47,77], or in which the TRM-associated molecule CD49a has been blocked using antibodies [81], are more susceptible to transplantable melanoma challenge than wild type (WT) or untreated mice, respectively. TRM cells pregenerated by immunization with VV [82] or DNA vaccines encoding tumor-derived or model neoantigens [83,84] can protect mice against challenge with transplantable melanoma or head and neck tumors. In addition, autoimmune T cells specific for the melanin-derived protein gp100, elicited by melanoma implantation with subsequent surgical tumor excision (and responsible for inducing vitiligo [47]), can protect against rechallenge with the same transplantable melanoma cells in mice. In these settings, TRM cells can provide some protection against tumor development independently of TCIRC cells, but tumor control is often enhanced via collaboration of both T cell subsets [82,84]. In one study, VV-generated TCIRC cells could be reactivated in SLOs following melanoma challenge to provide comparable tumor protection to that offered by TRM cells [82]. Given that solid tumors most commonly arise in epithelial tissue compartments in the absence of pre-existing antitumor immunity [3], our group developed an epicutaneous model of melanoma that targets tumor growth to the outermost layers of mouse skin. In this model, antitumor TRM cells generated both during tumorigenesis or prior to tumor challenge were able to suppress cancer growth, despite antibody-mediated depletion of TCIRC cells [77]. Similarly, non-recirculating innate-like T cells 738

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Vitiligo: autoimmune disease in which immune cells target and kill melanocytes, leading to depigmentation of the skin.

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that phenotypically parallel conventional CD8+ TRM cells and depend on many of the same survival factors could accumulate in mouse oncogenic epithelial-derived PyMT mammary cancers during carcinogenesis and were critical in inhibiting tumor progression [85]. Together with the observation that CD103+CD8+ TRM cells correlate with improved survival in many types of human cancers, including melanoma, breast, lung, and ovarian tumors [64,72,75,78,80], these findings support a key role for TRM cells in natural cancer surveillance and, presumably, tumor protection.

TRM Cells in the Tumor Microenvironment Features of Tumor-Localizing TRM Cells CD69+CD103+ TILs in human cancers can display several transcriptional similarities with bona fide TRM cells found in mice, including downregulation of molecules controlling tissue exit (e.g., Klf2, S1pr1, Ccr7) [66,67,75,80,86] and upregulation of residency-promoting TFs, including Hobit [80,86]. In-depth transcriptional profiling of CD8+ TILs suggests that distinct subsets of TRM cells with heterogeneous phenotypic or functional properties can coexist in the same tumor [75,80,86–89]. At least two or three unique clusters of CD103+CD8+ TRM-like cells can be identified in human lung, colon, and breast tumors, where they may differentially express molecules associated with T cell activation or exhaustion, such as Pdcd1 [programmed cell death protein 1 (PD-1)], Havcr2 [T cell immunoglobulin and mucin-domain containing-3 (TIM-3)] and Entpd1 (CD39) [80,86,87], as well as proliferation markers [75]. These findings raise the possibility that tumor-associated CD103+CD8+ TRM cells might become dysfunctional under certain circumstances. In support of this, TRM cells isolated from healthy human lung tissue may be more functional with respect to cytokine production than those isolated from lung tumors [87], and TRM-like cells that share fewer transcriptional features with exhausted T (TEX) cells might better predict overall survival in human lung cancer than CD103-expressing T cells with a higher exhaustion ‘score’ [80]. However, tumor-associated CD103+CD8+ T cells typically display heightened functionality with an increased ability to produce IFN-γ when compared with alternate CD103– exhausted TIL subsets [87]. They also retain their capacity to protect against HSV viral

Tissues Lymphoid organs TEM TCM

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Figure 2. Subsets of Memory CD8 T Cells in Mice. The memory CD8 T cell compartment is comprised of both circulating and resident subsets. Circulating memory CD8+ T cells (TCIRC) include central memory T cells (TCM) that generally recirculate through blood and secondary lymphoid organs such as the spleen and lymph nodes, and effector memory T cells (TEM) that enter and migrate through peripheral nonlymphoid tissues as well as blood and lymphoid organs. Tissue-resident memory CD8+ T cells (TRM) reside permanently in peripheral organs (often within epithelial tissue compartments) in disconnect from the blood circulation.

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infection and proliferate in mice despite constitutive expression of multiple inhibitory checkpoint (IC) molecules associated with T cell dysfunction [39,90]; this suggests that they might be more resistant to exhaustion than TCIRC cells. In fact, CD103+CD8+ TRM cells in human melanoma, lung, breast, and cervical tumor tissue are often characterized by heightened expression of IC molecules, including PD-1, TIM-3, and Lymphocyte-activation gene 3 (LAG-3), in comparison with CD103– TIL subsets, but maintain their ability to produce cytotoxic molecules and effector cytokines [61,66,67,73,75]. CD103+CD8+ T cells found in tumors are likely heterogeneous with respect to antigen specificity, encompassing both tumor-specific TRM cells and non-tumor-specific bystander T cells. These bystander TRM cells may have accumulated prior to tissue transformation in response to commensal microbes [26] or following viral infection [88,91], or may be antecedent TRM cells generated in response to now defunct tumor antigens that have since been lost from the tumor microenvironment following immunoediting or during cancer progression [92–94]. In the human lung, colorectal, melanoma, and ovarian cancers, tumor-specific TRM cells may be distinguished from these bystander T cells by the expression of the activation marker CD39, in addition to CD103; accordingly, tumor-reactive clones have been shown to be enriched within a subpopulation of T cells characterized by the expression of both of these markers [88,89]. Although CD69 and CD103 demarcate TRM cells in multiple healthy tissues under homeostatic conditions, the extent to which these molecules can reliably identify bona fide TRM cells in tumors is less clear. TIL CD69 expression may reflect recent cognate antigen stimulation [11] or oxygen deprivation in the solid tumor microenvironment [95] rather than tissue residency. Similarly, CD103 expression can be rapidly induced on activated human blood CD8+ T cells that recognize antigen in the presence of TGFβ [89,96], which is typically abundant in the tumor microenvironment of many human solid cancers [97]. Comparing TIL transcriptional profiles with the core gene signature characterizing TRM cells in healthy tissues [13] may identify putative TRM cells more dependably, particularly if TILs are found to downregulate tissue egress regulators, but this approach is also complicated by the fact that many of the genes comprising the TRM gene signature can overlap with those expressed by recently activated (e.g., Cd69, Nr4a1) or TEX cells (e.g., Pdcd1, Havcr2) in mice and humans [98,99]. As such, expression of these factors by T cells in the tumor microenvironment may not have the same implications as in resting tissues following the resolution of infection. Further work will be required to accurately delineate properties of tumor-infiltrating TRM cells that reliably differentiate them from acutely or chronically stimulated recirculating T cells. Regulation of TRM Cells in Tumors The number and frequency of TRM cells residing in solid tumors can vary widely between patients presenting with the same cancer [64,88] and between lesions from the same patient [61]. Factors licensing tumor TRM cell development in some contexts but not others are currently unclear. At the simplest level, tumor TRM cell differentiation will require efficient T cell priming and the generation of memory precursor-like cells that can seed the TRM pool [13], as well as upregulation of migratory molecules such as the chemokine receptors CXCR3 and CXCR6, or tissue-specific homing molecules that may direct their recruitment into peripheral tissues and tumors [13,19,100,101]. Local antigen recognition does not appear to be essential for tumor TRM cell differentiation as CD8+CD103+ TRM cells can form in murine epicutaneous melanomas and human lung and colorectal cancers that are devoid of their cognate antigen [77,88]; subsequent TRM cell differentiation might therefore hinge on the availability of microenvironmental cues necessary for their development. Heightened expression of the TRM cell-promoting cytokine IL-15 has been associated with increased CD8+CD103+ T cell accumulation in melanoma [64] and improved survival in melanoma and colorectal cancer patients [64,102]. Although TGFβ can promote accumulation of CD8+CD103+ TRM cells in human lung tumor tissues [103], it can have pleiotropic effects as a potent negative regulator of cancer immunosurveillance (reviewed in [104]). Of note, TGFβ can promote a pattern of T cell ‘exclusion’ in mouse and human 740

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colorectal and urothelial tumor tissues [105,106] that mirrors CD103+CD8+ TRM cell accumulation at the invasive tumor margin of mouse and human melanomas [77]. Paradoxically, inhibition of TGFβ signaling in mouse models of colorectal or mammary cancer can catalyze tumor rejection by stimulating translocation of CD8+ T cells from the tumor border to the tumor mass [105,106]. These findings suggest that the clinical benefit of TGFβ and TRM cells in tumors may be contextually dependent.

Mechanisms of TRM Cell Cancer Immunosurveillance In theory, CD8+ T cells could enact tumor suppression by producing an arsenal of effector molecules, including cytotoxic mediators such as granzyme B and perforin and noncytolytic factors such as IFN-γ and TNF [1], which can be produced by TRM cells in the context of infection [23,44,107,108]. In some circumstances, TRM cells might inhibit tumor growth by direct killing in a process of elimination. Supporting this notion, TRM cells isolated from various patient tumor samples express high amounts of granzyme B and perforin compared with alternate TIL subsets [66,67,69,75] and can kill autologous tumor cells in vitro [66] (Figure 3, Key Figure). Expression of

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Potential Mechanisms of Tissue-Resident Memory T (TRM) Cell-Mediated Tumor Control

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Figure 3. TRM cells can suppress the growth of peripherally localized solid tumors independently of circulating CD8+ memory T (TCIRC) cells. CD8+ TRM cells can contribute to promoting a state of tumor-immune equilibrium, whereby they prevent tumor outgrowth without completely eliminating cancerous cells. Induction of equilibrium may depend on the production of noncytolytic effector molecules, such as tumor necrosis factor (TNF) and possibly interferon gamma (IFN-γ). Whether TRM cells can also induce tumor elimination by completely eradicating malignant cells in vivo remains to be determined. However, CD8+ TRM cells from tumors exhibit heightened expression of cytotoxic mediators, including granzyme B (gzmB) and perforin. Expression of CD103 by human TRM cells can also enhance direct killing of autologous tumor cell targets by CD103+CD8+ TRM-like cells in vitro.

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CD103 by TRM cells isolated from human lung tumors can facilitate killing of human lung cancer cell targets in vitro by enhancing binding to E-cadherin and stabilizing the immunological synapse to induce targeted membrane polarization of antitumor effector molecules such as granzyme B and IFN-γ [109–112]. The functional relevance of this mechanism in other settings likely depends on tumor cell expression of E-cadherin, which is often lost as cancers undergo the epithelial-to-mesenchymal transition [113]. Moreover, direct killing of tumor cells by TRM cells has yet to be demonstrated in vivo. Efficient target cell killing typically requires high densities of CD8+ effector T cells [114] that may not easily be achieved by TRM cells, given that their lateral motility within epithelial and peripheral tissue compartments is often restricted [115,116]. Our group recently demonstrated that CD8+ TRM cells can induce a durable cancer-immune equilibrium in mouse skin by maintaining melanoma cells in a dormant, but viable state [77] (Figure 3). In this setting, tumor suppression appeared to be more dependent on the production of the effector molecule TNF than on cytotoxic mediator perforin, or on IFN-γ [77]. This noncytolytic mode of immune protection echoes the control of various latent or persistent viruses, including HSV, Epstein Barr virus, and cytomegalovirus, or acute infection with norovirus, where TRM-mediated immunity does not necessarily result in complete virus eradication [117–120]. Moreover, this type of immunity is powerful enough to prevent symptomatic disease in these settings without causing overt tissue pathology [117–120] and may suggest a specialized function of TRM cells that allows them to act as ‘controllers’ rather than ‘killers’; however, this will require further investigation. In theory, the discovery that TRM cells can promote cancer-immune equilibrium provides a possible explanation as to how occult tumors can be controlled for up to decades in human peripheral tissue compartments [121] that are not typically patrolled by TCIRC cells in the absence of overt inflammation [15]. Whether TRM cells residing in tissues other than skin epithelium have a similar function in promoting tumor-immune equilibrium is unclear. Although ongoing TRM-mediated surveillance is sufficient to achieve prolonged tumor protection against melanoma, colorectal cancer, and head and neck cancers in mice [47,77,82–84], extended maintenance of tumor equilibrium at the expense of elimination might facilitate eventual cancer escape via immunoediting resulting from ongoing immune cell-mediated selection of less immunogenic tumor cell clones (Box 1). It is tempting to speculate that artificially enhancing TRM cell density or activity using immunotherapies might permit total tumor elimination, although this remains to be tested.

TRM Cells in Cancer Immunotherapy TRM Cells in IC Blockade Therapy Both tumor-specific and steady-state TRM cells express a range of IC molecules, such as PD-1, TIM-3, and LAG-3, even in the absence of antigen stimulation in humans and mice [39,61,67,73, 75,78], raising the possibility that tumor-localized TRM cells might serve as prominent targets of IC inhibitor therapies. In line with this, blockade of IC molecules such as PD-1 and TIM-3 in vitro can enhance cytokine production or killing by CD8+ TRM cells isolated from lung tumors and healthy tissues [66,122,123]; this suggests that these IC molecules might serve to restrain TRM cell activity. Further support for this stemmed from the observation that melanoma patients receiving IC blockade treatments regularly experience adverse skin reactions that are associated with TRM cell dysregulation, such as vitiligo [24,48,124]. Recently, blockade of PD-1 and TIM-3 was shown to exacerbate skin inflammation induced by skin TRM cells in a mouse model of contact hypersensitivity relative to controls [123]. These observations suggested that at least TRM cell effector functions might be enhanced by IC blockade therapy, but whether TRM cells might also proliferate in response to treatment is currently unknown. T cells expressing high amounts of T cell factor 1 (TCF1) and low amounts of IC molecules, including TIM-3, have been shown to expand most readily following IC blockade therapy during chronic LCMV infection in mice and in 742

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multiple mouse tumor models, including melanoma and colon carcinoma [125–130]. Conversely, TRM cells in mice and solid cancer patients express low amounts of TCF1 [29,75] and can constitutively express TIM-3 [39,61,66], meaning they are phenotypically dissimilar to canonical proliferative TIL. The frequency of TRM cells or expression of TRM signature genes in melanoma may be increased following IC blockade [64,75], but it is unclear whether TRM cell accumulation in this context represents intratumoral proliferation of TRM cells or induction of de novo TRM cell responses via naïve or TCIRC cell activation. Studies in models of B16 melanoma or CT26 colon carcinoma in mice have shown that although pre-existing T cells in tumors were sufficient to induce cancer regression or protection against tumors following IC blockade when T cell infiltration was blocked using FTY720 [131–134], recruitment of circulating T cells was found to be required in other mouse models, such as PyMT or 4T1 breast cancer, or MC38 colon carcinoma [135, 136]. It is reasonable to speculate that heightened expression of TRM-associated genes in melanoma tumors prior to treatment might better predict patient responses to IC blockade [64,75]. However, further investigation will be necessary to characterize the nature of TRM cell responses to IC blockade in vivo and to determine their therapeutic relevance. TRM Cells in Tumor Vaccination and Adoptive Cell Therapy Augmenting TRM cell formation via tumor vaccination or adoptive cell therapy (ACT) might also serve as an avenue to improve solid cancer treatment. Mucosal tumor vaccination protocols that encourage peripheral CD49+CD103+ TRM cell formation have been shown to provide enhanced protection against subsequent challenge with head and neck or melanoma tumors when compared with systemic vaccination strategies in mice [84,137,138]. Moreover, subcutaneous peptide immunization of mice bearing established melanoma tumors generates intratumoral CD49+CD103+ TRM cells that exhibit enhanced effector functions, including elevated IFN-γ and granzyme B production, in comparison with other TIL populations and blockade of either CD49a or CD103 in this context interferes with tumor control [81]. Similarly, vaccination of human melanoma patients with melanocyte-derived peptide via the subcutaneous route generates tumorlocalizing CD49a+CD103+ TRM cells, and expression of CD49a on circulating melanoma-specific CD8+ T cells in these patients correlates with improved survival [81]. These findings suggest that designing vaccination protocols that efficiently induce TRM cells might promote tumor rejection, although this will require robust testing. Repeated systemic boosting of pre-existing TCIRC cells with heterologous viruses expressing cognate antigen can also drive widespread TRM cell accumulation in tissues, including the skin, salivary gland, and female reproductive tract in mice [34,139], and this approach might also enhance protective TRM cell formation in tumors. Separately, one study using transplantable and genetic models of melanoma in mice suggests that peptide injection targeting virus-specific, tumor-associated bystander TRM cells could be an effective means to trigger tumor regression, particularly in combination with IC blockade [91]. The efficacy of ACT depends on the ability of T cells to migrate to and persist and function within established tumors [140]. Activated T cells infiltrating murine tumors following adoptive transfer rapidly acquire expression of 70%–80% of the core transcriptional signature typifying TRM cells from steady-state peripheral tissues [141,142], and overexpression of the residency-promoting TF Runx3 in preactivated T cells prior to ACT could increase TIL accumulation and augment cancer suppression in mice bearing subcutaneous melanomas [142]. Editing similar TRM-fate promoting genes in tumor-specific or chimeric antigen receptor (CAR) T cells prior to adoptive transfer might improve the efficacy of ACT therapies for the treatment of solid human cancers by potentially enhancing TRM cell over TCIRC differentiation. However, work in models of melanoma, colorectal, and head and neck cancer indicate that mice harboring both tumor-specific TRM and TCIRC cells are usually better protected from tumor challenge than those possessing only one of these subsets [77,82–84]. This ultimately suggests that immunotherapy approaches

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generating both TRM and TCIRC cells working in concert, rather than TRM cells alone, are likely to be the most effective in potentiating antitumor treatments.

Outstanding Questions

Concluding Remarks

Do tumor TRM cells exhibit similar developmental requirements to TRM cells found in the tissue of tumor origin? Or, do tumor-associated TRM cells possess unique requirements for their differentiation and survival?

It is becoming increasingly clear that TRM cells play an integral role in tumor surveillance in both animal models and human cancers. The observations that TRM cell accumulation is associated with improved patient prognoses and can be used to predict the likelihood of success of IC blockade renders these cells a promising candidate clinical biomarker, pending further testing. In addition, elevated expression of IC molecules and long-term persistence within tumors make TRM cells ideal candidates for immunotherapeutic targeting. However, capitalizing on TRM cells to improve cancer treatments will require a more comprehensive understanding of the factors that identify resident TILs and regulate their development, as well as the mechanisms through which they might protect against various cancers (see Outstanding Questions). Furthermore, TRM cells found in the same tumor may be phenotypically and functionally heterogeneous, raising the possibility that certain subpopulations of TRM cells might be more protective than others. While they share unifying commonalities, TRM cells in different healthy tissues diverge with respect to their developmental requirements and transcriptional circuitry. Whether tumor TRM cells possess unique metabolic requirements and cytokine dependencies or align with those characterizing TRM cells from the tissue of cancer origin is currently uncertain. Although TRM cells possess the cytotoxic machinery necessary to kill tumor cells and can orchestrate melanoma-immune equilibrium in mouse skin, the extent to which they engage either process to protect against diverse solid human cancers remains unclear. Understanding the properties of tumor TRM cells in multiple cancer settings may reveal novel pathways through which these cells might be manipulated for therapeutic gain. Acknowledgments S.L.P. is supported by a Cancer Council Victoria Postdoctoral Fellowship. T.G. and L.K.M. are Senior Medical Research Fellows supported by the Sylvia and Charles Viertel Charitable Foundation.

References 1. 2. 3.

4. 5.

6. 7.

8.

9.

10.

11.

744

Vesely, M.D. et al. (2011) Natural innate and adaptive immunity to cancer. Annu. Rev. Immunol. 29, 235–271 Ribas, A. and Wolchok, J.D. (2018) Cancer immunotherapy using checkpoint blockade. Science 359, 1350–1355 Ferlay, J. et al. (2015) Cancer incidence and mortality worldwide: sources, methods and major patterns in GLOBOCAN 2012. Int. J. Cancer 136, E359–E386 Jameson, S.C. and Masopust, D. (2018) Understanding subset diversity in T cell memory. Immunity 48, 214–226 Mueller, S.N. and Mackay, L.K. (2015) Tissue-resident memory T cells: local specialists in immune defence. Nat. Rev. Immunol. 16, 79 Kumar, B.V. et al. (2018) Human T cell development, localization, and function throughout life. Immunity 48, 202–213 Schreiner, D. and King, C.G. (2018) CD4+ memory T cells at home in the tissue: mechanisms for health and disease. Front. Immunol. 9, 2394 Oja, A.E. et al. (2018) Functional heterogeneity of CD4+ tumorinfiltrating lymphocytes with a resident memory phenotype in NSCLC. Front. Immunol. 9, 2654 Masopust, D. et al. (2010) Dynamic T cell migration program provides resident memory within intestinal epithelium. J. Exp. Med. 207, 553–564 Gebhardt, T. et al. (2009) Memory T cells in nonlymphoid tissue that provide enhanced local immunity during infection with herpes simplex virus. Nat. Immunol. 10, 524–530 Cibrián, D. and Sánchez-Madrid, F. (2017) CD69: from activation marker to metabolic gatekeeper. Eur. J. Immunol. 47, 946–953

Trends in Immunology, August 2019, Vol. 40, No. 8

12.

13.

14.

15. 16.

17. 18.

19.

20.

21.

Shiow, L.R. et al. (2006) CD69 acts downstream of interferonα/β to inhibit S1P1 and lymphocyte egress from lymphoid organs. Nature 440, 540–544 Mackay, L.K. et al. (2013) The developmental pathway for CD103+CD8+ tissue-resident memory T cells of skin. Nat. Immunol. 14, 1294–1301 Mackay, L.K. et al. (2015) Cutting edge: CD69 interference with sphingosine-1-phosphate receptor function regulates peripheral T cell retention. J. Immunol. 194, 2059–2063 Gebhardt, T. et al. (2011) Different patterns of peripheral migration by memory CD4+ and CD8+ T cells. Nature 477, 216–219 Bergsbaken, T. and Bevan, M.J. (2015) Proinflammatory microenvironments within the intestine regulate the differentiation of tissue-resident CD8+ T cells responding to infection. Nat. Immunol. 16, 406–414 Steinert, E. et al. (2015) Quantifying memory CD8+ T cells reveals regionalization of immunosurveillance. Cell 161, 737–749 Schenkel, J.M. et al. (2014) Cutting edge: resident memory CD8 T cells occupy frontline niches in secondary lymphoid organs. J. Immunol. 192, 2961–2964 Fernandez-Ruiz, D. et al. (2016) Liver-resident memory CD8+ T cells form a front-line defense against malaria liver-stage infection. Immunity 45, 889–902 Mackay, L.K. et al. (2015) T-box transcription factors combine with the cytokines TGF-β and IL-15 to control tissue-resident memory T cell fate. Immunity 43, 1101–1111 Park, S. et al. (2016) Distinct recirculation potential of CD69+ CD103− and CD103+ thymic memory CD8+ T cells. Immunol. Cell Biol. 94, 975–980

What novel surface markers or gene signatures best distinguish tumor TRM cells from other TIL subsets?

What factors dictate whether tumor T cells become TRM cells or TEX cells? Can tumor TRM cells become dysregulated in the tumor microenvironment and develop into TEX cells? How long-lived are TRM cells in tumors? Are they maintained as a self-sufficient population, or are tumor-associated TRM cell populations replenished by TCIRC cells over time? Can tumor-associated TRM cells drive complete cancer elimination in addition to promoting tumor-immune equilibrium? Are tumor TRM cells direct targets of IC blockade therapies? Can we enhance tumor TRM cell generation to augment immunotherapy?

Trends in Immunology

22.

23.

24.

25.

26.

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

40.

41.

42.

43.

44. 45.

Beura, L.K. et al. (2018) T cells in nonlymphoid tissues give rise to lymph-node-resident memory T cells. Immunity 48, 327–338 Schenkel, J. et al. (2014) T cell memory. Resident memory CD8 T cells trigger protective innate and adaptive immune responses. Science 346, 98–101 Cheuk, S. et al. (2017) CD49a expression defines tissueresident CD8+ T cells poised for cytotoxic function in human skin. Immunity 46, 287–300 Harrison, O.J. et al. (2019) Commensal-specific T cell plasticity promotes rapid tissue adaptation to injury. Science 363, eaat6280 Naik, S. et al. (2015) Commensal-dendritic-cell interaction specifies a unique protective skin immune signature. Nature 520, 104–108 Mackay, L.K. et al. (2012) Long-lived epithelial immunity by tissue-resident memory T (TRM) cells in the absence of persisting local antigen presentation. Proc. Natl. Acad. Sci. U. S. A. 109, 7037–7042 Herndler-Brandstetter, D. et al. (2018) KLRG1 + effector CD8 + T cells lose KLRG1, differentiate into all memory T cell lineages, and convey enhanced protective immunity. Immunity 48, 716–729 Mackay, L.K. et al. (2016) Hobit and Blimp1 instruct a universal transcriptional program of tissue residency in lymphocytes. Science 352, 459–463 Kumar, B.V. et al. (2017) Human tissue-resident memory T cells are defined by core transcriptional and functional signatures in lymphoid and mucosal sites. Cell Rep. 20, 2921–2934 Pallett, L.J. et al. (2017) IL-2high tissue-resident T cells in the human liver: sentinels for hepatotropic infection. J. Exp. Med. 214, 1567–1580 Stelma, F. et al. (2017) Human intrahepatic CD69+ CD8+ T cells have a tissue resident memory T cell phenotype with reduced cytolytic capacity. Sci. Rep. 7, 6172 Mohammed, J. et al. (2016) Stromal cells control the epithelial residence of DCs and memory T cells by regulated activation of TGF-β. Nat. Immunol. 17, 414 Schenkel, J.M. et al. (2016) IL-15–independent maintenance of tissue-resident and boosted effector memory CD8 T cells. J. Immunol. 196, 3920–3926 Casey, K.A. et al. (2012) Antigen-independent differentiation and maintenance of effector-like resident memory T cells in tissues. J. Immunol. 188, 4866–4875 Lian, C.G. et al. (2014) Biomarker evaluation of face transplant rejection: association of donor T cells with target cell injury. Mod. Pathol. 27, 788 Slütter, B. et al. (2017) Dynamics of influenza-induced lungresident memory T cells underlie waning heterosubtypic immunity. Sci. Immunol. 2, eeag2031 Takamura, S. et al. (2016) Specific niches for lung-resident memory CD8+ T cells at the site of tissue regeneration enable CD69-independent maintenance. J. Exp. Med. 213, 3057 Park, S.L. et al. (2018) Local proliferation maintains a stable pool of tissue-resident memory T cells after antiviral recall responses. Nat. Immunol. 19, 183–191 Beura, L.K. et al. (2018) Intravital mucosal imaging of CD8+ resident memory T cells shows tissue-autonomous recall responses that amplify secondary memory. Nat. Immunol. 19, 173 Jiang, X. et al. (2012) Skin infection generates non-migratory memory CD8+ TRM cells providing global skin immunity. Nature 483, 227–231 Hofmann, M. and Pircher, H. (2011) E-cadherin promotes accumulation of a unique memory CD8 T-cell population in murine salivary glands. Proc. Natl. Acad. Sci. U. S. A. 108, 16741–16746 Sheridan, B.S. et al. (2014) Oral infection drives a distinct population of intestinal resident memory CD8 + T cells with enhanced protective function. Immunity 40, 747–757 Schenkel, J.M. et al. (2013) Sensing and alarm function of resident memory CD8+ T cells. Nat. Immunol. 14, 509–513 Ariotti, S. et al. (2014) Skin-resident memory CD8+ T cells trigger a state of tissue-wide pathogen alert. Science 346, 101–105

46. 47.

48.

49.

50.

51.

52.

53.

54.

55.

56.

57.

58.

59.

60.

61.

62.

63.

64.

65.

66.

67.

68.

69.

Hofmann, M. et al. (2013) Thymus-resident memory CD8+ T cells mediate local immunity. Eur. J. Immunol. 43, 2295–2304 Malik, B.T. et al. (2017) Resident memory T cells in the skin mediate durable immunity to melanoma. Sci. Immunol. 2, eaam6346 Richmond, J.M. et al. (2018) Antibody blockade of IL-15 signaling has the potential to durably reverse vitiligo. Sci. Transl. Med. 10, eaam7710 Richmond, J.M. et al. (2019) Resident memory and recirculating memory T cells cooperate to maintain disease in a mouse model of vitiligo. J. Investig. Dermatol. 139, 769–778 Cheuk, S. et al. (2014) Epidermal Th22 and Tc17 cells form a localized disease memory in clinically healed psoriasis. J. Immunol. 192, 3111–3120 Boyman, O. et al. (2004) Spontaneous development of psoriasis in a new animal model shows an essential role for resident T cells and tumor necrosis factor-α. J. Exp. Med. 199, 731–736 Kuric, E. et al. (2017) Demonstration of tissue resident memory CD8 T cells in insulitic lesions in adult patients with recentonset type 1 diabetes. Am. J. Pathol. 187, 581–588 Sasaki, K. et al. (2014) Relapsing–remitting central nervous system autoimmunity mediated by GFAP-specific CD8 T cells. J. Immunol. 192, 3029–3042 Zundler, S. et al. (2019) Hobit- and Blimp-1-driven CD4+ tissue-resident memory T cells control chronic intestinal inflammation. Nat. Immunol. 20, 288–300 Linehan, J.L. et al. (2018) Non-classical immunity controls microbiota impact on skin immunity and tissue repair. Cell 172, 784–796 Lee, P.P. et al. (1999) Characterization of circulating T cells specific for tumor-associated antigens in melanoma patients. Nat. Med. 5, 677 Appay, V. et al. (2006) New generation vaccine induces effective melanoma-specific CD8+ T cells in the circulation but not in the tumor site. J. Immunol. 177, 1670–1678 Haanen, J. et al. (2006) Melanoma-specific tumor-infiltrating lymphocytes but not circulating melanoma-specific T cells may predict survival in resected advanced-stage melanoma patients. Cancer Immunol. Immunother. 55, 451–458 Corbière, V. et al. (2011) Antigen spreading contributes to MAGE vaccination-induced regression of melanoma metastases. Cancer Res. 71, 1253–1262 Ferradini, L. et al. (1993) Analysis of T cell receptor variability in tumor-infiltrating lymphocytes from a human regressive melanoma. Evidence for in situ T cell clonal expansion. J. Clin. Investig. 91, 1183 Boddupalli, C.S. et al. (2016) Interlesional diversity of T cell receptors in melanoma with immune checkpoints enriched in tissue-resident memory T cells. JCI Insight 1, e88955 Zhao, F. et al. (2016) Melanoma lesions independently acquire T-cell resistance during metastatic latency. Cancer Res. 4347–4358 Jiménez-Sánchez, A. et al. (2017) Heterogeneous tumorimmune microenvironments among differentially growing metastases in an ovarian cancer patient. Cell 170, 927–938 Edwards, J. et al. (2018) CD103+ tumor-resident CD8+ T cells are associated with improved survival in immunotherapy-naïve melanoma patients and expand significantly during anti–PD-1 treatment. Clin. Cancer Res. 24, 3036–3045 Salerno, E.P. et al. (2014) T cells in the human metastatic melanoma microenvironment express site-specific homing receptors and retention integrins. Int. J. Cancer 134, 563–574 Djenidi, F. et al. (2015) CD8+CD103+ tumor–infiltrating lymphocytes are tumor-specific tissue-resident memory T cells and a prognostic factor for survival in lung cancer patients. J. Immunol. 194, 3475–3486 Ganesan, A.-P. et al. (2017) Tissue-resident memory features are linked to the magnitude of cytotoxic T cell responses in human lung cancer. Nat. Immunol. 18, 940–950 Wang, B. et al. (2015) CD103+ tumor infiltrating lymphocytes predict a favorable prognosis in urothelial cell carcinoma of the bladder. J. Urol. 194, 556–562 Hartana, C.A. et al. (2018) Tissue-resident memory T cells are epigenetically cytotoxic with signs of exhaustion in human urinary bladder cancer. Clin. Exp. Immunol. 194, 39–53

Trends in Immunology, August 2019, Vol. 40, No. 8

745

Trends in Immunology

70.

71.

72.

73.

74. 75.

76.

77.

78.

79.

80.

81.

82.

83.

84. 85.

86. 87.

88.

89.

90.

91.

92. 93.

94.

746

Webb, J.R. et al. (2010) Profound elevation of CD8+ T cells expressing the intraepithelial lymphocyte marker CD103 (aEb7 Integrin) in high-grade serous ovarian cancer. Gynecol. Oncol. 118, 228–236 Webb, J.R. et al. (2014) Tumor-infiltrating lymphocytes expressing the tissue resident memory marker CD103 are associated with increased survival in high-grade serous ovarian cancer. Clin. Cancer Res. 20, 434–444 Bösmüller, H.-C. et al. (2016) Combined immunoscore of CD103 and CD3 identifies long-term survivors in high-grade serous ovarian cancer. Int. J. Gynecol. Cancer 26, 671–679 Komdeur, F.L. et al. (2017) CD103+ tumor-infiltrating lymphocytes are tumor-reactive intraepithelial CD8+ T cells associated with prognostic benefit and therapy response in cervical cancer. Oncoimmunology 6, 6290–6297 Wang, Z.-Q. et al. (2016) CD103 and intratumoral immune response in breast cancer. Clin. Cancer Res. 22, 6290–6297 Savas, P. et al. (2018) Single-cell profiling of breast cancer T cells reveals a tissue-resident memory subset associated with improved prognosis. Nat. Med. 24, 986–993 Quinn, E. et al. (2003) CD103+ intraepithelial lymphocytes—a unique population in microsatellite unstable sporadic colorectal cancer. Eur. J. Cancer 39, 469–475 Park, S.L. et al. (2019) Tissue-resident memory CD8+ T cells promote melanoma–immune equilibrium in skin. Nature 565, 366–371 Workel, H.H. et al. (2016) CD103 defines intraepithelial CD8+ PD1+ tumour-infiltrating lymphocytes of prognostic significance in endometrial adenocarcinoma. Eur. J. Cancer 60, 1–11 Webb, J.R. et al. (2015) PD-1 and CD103 are widely coexpressed on prognostically favorable intraepithelial CD8 T cells in human ovarian cancer. Cancer Immunol. Res. 3, 926–935 Guo, X. et al. (2018) Global characterization of T cells in nonsmall-cell lung cancer by single-cell sequencing. Nat. Med. 24, 978–985 Murray, T. et al. (2016) Very late antigen-1 marks functional tumor-resident CD8 T cells and correlates with survival of melanoma patients. Front. Immunol. 7, 573 Enamorado, M. et al. (2017) Enhanced anti-tumour immunity requires the interplay between resident and circulating memory CD8+ T cells. Nat. Commun. 8, 16073 Gálvez-Cancino, F. et al. (2018) Vaccination-induced skinresident memory CD8+ T cells mediate strong protection against cutaneous melanoma. Oncoimmunology 7, e1442163 Nizard, M. et al. (2017) Induction of resident memory T cells enhances the efficacy of cancer vaccine. Nat. Commun. 8, 15221 Dadi, S. et al. (2016) Cancer immunosurveillance by tissue-resident innate lymphoid cells and innate-like T cells. Cell 164, 365–377 Zhang, L. et al. (2018) Lineage tracking reveals dynamic relationships of T cells in colorectal cancer. Nature 564, 268–272 Bengsch, B. et al. (2018) Epigenomic-guided mass cytometry profiling reveals disease-specific features of exhausted CD8+ T cells. Immunity 48, 1029–1045 Simoni, Y. et al. (2018) Bystander CD8+ T cells are abundant and phenotypically distinct in human tumour infiltrates. Nature 557, 575–579 Duhen, T. et al. (2018) Co-expression of CD39 and CD103 identifies tumor-reactive CD8 T cells in human solid tumors. Nat. Commun. 9, 2724 Mackay, L.K. et al. (2012) Maintenance of T cell function in the face of chronic antigen stimulation and repeated reactivation for a latent virus infection. J. Immunol. 188, 2173–2178 Rosato, P.C. et al. (2019) Virus-specific memory T cells populate tumors and can be repurposed for tumor immunotherapy. Nat. Commun. 10, 567 Verdegaal, E.M.E. et al. (2016) Neoantigen landscape dynamics during human melanoma–T cell interactions. Nature 536, 91 Matsushita, H. et al. (2012) Cancer exome analysis reveals a T-cell-dependent mechanism of cancer immunoediting. Nature 482, 400–404 Anagnostou, V. et al. (2017) Evolution of neoantigen landscape during immune checkpoint blockade in non-small cell lung cancer. Cancer Discov. 7, 264–276

Trends in Immunology, August 2019, Vol. 40, No. 8

95.

96.

97. 98.

99. 100.

101.

102.

103.

104.

105.

106.

107.

108.

109.

110.

111.

112.

113. 114.

115.

116.

117. 118.

Labiano, S. et al. (2017) CD69 is a direct HIF-1α target gene in hypoxia as a mechanism enhancing expression on tumorinfiltrating T lymphocytes. Oncoimmunology 6, e1283468 Floc’h, A.L. et al. (2007) αEβ7 integrin interaction with Ecadherin promotes antitumor CTL activity by triggering lytic granule polarization and exocytosis. J. Exp. Med. 204, 559–570 Pickup, M. et al. (2013) The roles of TGFβ in the tumour microenvironment. Nat. Rev. Cancer 13, 788–799 Ashouri, J.F. and Weiss, A. (2017) Endogenous Nur77 is a specific indicator of antigen receptor signaling in human T and B cells. J. Immunol. 198, 657–668 McLane, L. et al. (2019) CD8 T cell exhaustion during chronic viral infection and cancer. Annu. Rev. Immunol. 37, 457–495 Zaid, A. et al. (2017) Chemokine receptor–dependent control of skin tissue–resident memory T cell formation. J. Immunol. 199, 2451–2459 Masopust, D. and Schenkel, J.M. (2013) The integration of T cell migration, differentiation and function. Nat. Rev. Immunol. 13, 309 Mlecnik, B. et al. (2014) Functional network pipeline reveals genetic determinants associated with in situ lymphocyte proliferation and survival of cancer patients. Sci. Transl. Med. 6, 228ra37 Boutet, M. et al. (2016) TGF-β signaling intersects with CD103 integrin signaling to promote T lymphocyte accumulation and antitumor activity in the lung tumor microenvironment. Cancer Res. 76, 1757–1769 Yang, L. et al. (2010) TGF-β and immune cells: an important regulatory axis in the tumor microenvironment and progression. Trends Immunol. 31, 220–227 Mariathasan, S. et al. (2018) TGFβ attenuates tumour response to PD-L1 blockade by contributing to exclusion of T cells. Nature 554, 544–548 Tauriello, D.V. et al. (2018) TGFβ drives immune evasion in genetically reconstituted colon cancer metastasis. Nature 554, 538–543 Steinbach, K. et al. (2016) Brain-resident memory T cells represent an autonomous cytotoxic barrier to viral infection. J. Exp. Med. 213, 1571–1587 McMaster, S.R. et al. (2015) Airway-resident memory CD8 T cells provide antigen-specific protection against respiratory virus challenge through rapid IFN-γ production. J. Immunol. 195, 203–209 Floc’h, A.L. et al. (2011) Minimal engagement of CD103 on cytotoxic T lymphocytes with an E-cadherin-Fc molecule triggers lytic granule polarization via a phospholipase Cγ–dependent pathway. Cancer Res. 71, 328–338 Franciszkiewicz, K. et al. (2009) Intratumoral induction of CD103 triggers tumor-specific CTL function and CCR5-dependent T-cell retention. Cancer Res. 69, 6249–6255 Franciszkiewicz, K. et al. (2013) CD103 or LFA-1 engagement at the immune synapse between cytotoxic T cells and tumor cells promotes maturation and regulates T-cell effector functions. Cancer Res. 73, 617–628 Gauthier, L. et al. (2017) Paxillin binding to the cytoplasmic domain of CD103 promotes cell adhesion and effector functions for CD8+ resident memory T cells in tumors. Cancer Res. 77, 7072–7082 Lamouille, S. et al. (2014) Molecular mechanisms of epithelial– mesenchymal transition. Nat. Rev. Mol. Cell Biol. 15, 178–196 Halle, S. et al. (2016) In vivo killing capacity of cytotoxic T cells is limited and involves dynamic interactions and T cell cooperativity. Immunity 44, 233–245 Ariotti, S. et al. (2012) Tissue-resident memory CD8+ T cells continuously patrol skin epithelia to quickly recognize local antigen. Proc. Natl. Acad. Sci. U. S. A. 109, 19739–19744 Zaid, A. et al. (2014) Persistence of skin-resident memory T cells within an epidermal niche. Proc. Natl. Acad. Sci. U. S. A. 111, 5307–5312 Zhu, J. et al. (2013) Immune surveillance by CD8+ skin-resident T cells in human herpes virus infection. Nature 497, 494–497 Zhu, J. et al. (2007) Virus-specific CD8+ T cells accumulate near sensory nerve endings in genital skin during subclinical HSV-2 reactivation. J. Exp. Med. 204, 595–603

Trends in Immunology

119. Gordon, C.L. et al. (2017) Tissue reservoirs of antiviral T cell immunity in persistent human CMV infection. J. Exp. Med. 214, 651 120. Tomov, V.T. et al. (2017) Differentiation and protective capacity of virus-specific CD8+ T cells suggest murine norovirus persistence in an immune-privileged enteric niche. Immunity 47, 723–738 121. Manjili, M.H. (2017) Tumor dormancy and relapse: from a natural byproduct of evolution to a disease state. Cancer Res. 77, 2564–2569 122. Abdelsamed, H.A. et al. (2017) Maintenance of PD-1 on brainresident memory CD8 T cells is antigen independent. Immunol. Cell Biol. 95, 953–959 123. Gamradt, P. et al. (2019) Inhibitory checkpoint receptors control CD8+ resident memory T cells to prevent skin allergy. J. Allergy Clin. Immunol. 143, 2147–2157 124. Byrne, E.H. and Fisher, D.E. (2017) Immune and molecular correlates in melanoma treated with immune checkpoint blockade. Cancer 123, 2143–2153 125. Utzschneider, D.T. et al. (2016) T cell factor 1-expressing memory-like CD8+ T cells sustain the immune response to chronic viral infections. Immunity 45, 415–427 126. Miller, B.C. et al. (2019) Subsets of exhausted CD8+ T cells differentially mediate tumor control and respond to checkpoint blockade. Nat. Immunol. 20, 326–336 127. He, R. et al. (2016) Follicular CXCR5-expressing CD8+ T cells curtail chronic viral infection. Nature 537, 412 128. Im, S.J. et al. (2016) Defining CD8+ T cells that provide the proliferative burst after PD-1 therapy. Nature 537, 417–421 129. Siddiqui, I. et al. (2019) Intratumoral Tcf1+ PD-1+ CD8+ T cells with stem-like properties promote tumor control in response to vaccination and checkpoint blockade immunotherapy. Immunity 50, 195–211 130. Kurtulus, S. et al. (2019) Checkpoint blockade immunotherapy induces dynamic changes in PD-1− CD8+ tumor-infiltrating T cells. Immunity 50, 181–194 131. Horton, B.L. et al. (2018) Intratumoral CD8+ T-cell apoptosis is a major component of T-cell dysfunction and impedes antitumor immunity. Cancer Immunol. Res. 6, 14–24 132. Crittenden, M.R. et al. (2018) Tumor cure by radiation therapy and checkpoint inhibitors depends on pre-existing immunity. Sci. Rep. 8, 7012

133. Williams, J.B. et al. (2017) The EGR2 targets LAG-3 and 4-1BB describe and regulate dysfunctional antigen-specific CD8+ T cells in the tumor microenvironment. J. Exp. Med. 214, 381–400 134. Spranger, S. et al. (2014) Mechanism of tumor rejection with doublets of CTLA-4, PD-1/PD-L1, or IDO blockade involves restored IL-2 production and proliferation of CD8 (+) T cells directly within the tumor microenvironment. J. Immunother. Cancer 2, 3 135. Tang, H. et al. (2016) Facilitating T cell infiltration in tumor microenvironment overcomes resistance to PD-L1 blockade. Cancer Cell 29, 285–296 136. Spitzer, M.H. et al. (2017) Systemic immunity is required for effective cancer immunotherapy. Cell 168, 487–502 137. Sandoval, F. et al. (2013) Mucosal imprinting of vaccine-induced CD8+ T cells is crucial to inhibit the growth of mucosal tumors. Sci. Transl. Med. 5, 172ra20 138. Liu, L. et al. (2010) Epidermal injury and infection during poxvirus immunization is crucial for the generation of highly protective T cell-mediated immunity. Nat. Med. 16, 224–227 139. Davies, B. et al. (2017) Cutting edge: tissue-resident memory T cells generated by multiple immunizations or localized deposition provide enhanced immunity. J. Immunol. 198, 2233–2237 140. Sadelain, M. et al. (2017) Therapeutic T cell engineering. Nature 545, 423–431 141. Doedens, A.L. et al. (2016) Molecular programming of tumorinfiltrating CD8+ T cells and IL15 resistance. Cancer Immunol. Res. 2, 799–811 142. Milner, J.J. et al. (2017) Runx3 programs CD8+ T cell residency in non-lymphoid tissues and tumours. Nature 552, 253–257 143. Schreiber, R.D. et al. (2011) Cancer immunoediting: integrating immunity’s roles in cancer suppression and promotion. Science 331, 1565–1570 144. Koebel, C.M. et al. (2007) Adaptive immunity maintains occult cancer in an equilibrium state. Nature 450, 903–907 145. Müller-Hermelink, N. et al. (2008) TNFR1 signaling and IFN-γ signaling determine whether T cells induce tumor dormancy or promote multistage carcinogenesis. Cancer Cell 13, 507–518

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