T-box Transcription Factors Combine with the Cytokines TGF-β and IL-15 to Control Tissue-Resident Memory T Cell Fate

Sphingosine 1-phosphate receptor 5 (S1PR5) regulates the peripheral retention of tissue-resident lymphocytes

Tissue-resident memory T (TRM) cells provide long-lasting immune protection. One of the key events controlling TRM cell development is the local retention of TRM cell precursors coupled to downregulation of molecules necessary for tissue exit. Sphingosine-1-phosphate receptor 5 (S1PR5) is a migratory receptor with an uncharted function in T cells. Here, we show that S1PR5 plays a critical role in T cell infiltration and emigration from peripheral organs, as well as being specifically downregulated in TRM cells. Consequentially, TRM cell development was selectively impaired upon ectopic expression of S1pr5, whereas loss of S1pr5 enhanced skin TRM cell formation by promoting peripheral T cell sequestration. Importantly, we found that T-bet and ZEB2 were required for S1pr5 induction and that local TGF-β signaling was necessary to promote coordinated Tbx21, Zeb2, and S1pr5 downregulation. Moreover, S1PR5-mediated control of tissue residency was conserved across innate and adaptive immune compartments. Together, these results identify the T-bet-ZEB2-S1PR5 axis as a previously unappreciated mechanism modulating the generation of tissue-resident lymphocytes.

Vitiligo: An Autoimmune Skin Disease and its Immunomodulatory Therapeutic Intervention

Vitiligo is a chronic autoimmune depigmenting skin disorder characterized by patches of the skin losing functional melanocytes. Multiple combinatorial factors are involved in disease development, among which immune T cells play a prominent role. The immune cells implicated in melanocyte destruction through adaptive immunity include CD8⁺ cytotoxic T cells and regulatory T cells, and aberrantly activated skin-resident memory T cells also play a role in melanocyte destruction. Over the past several years, major progress in understanding vitiligo pathogenesis has led to the development of targeted therapies. Janus kinase (JAK) inhibitors, which share the similar mechanism that autoactivates CD8⁺ T cells in chronic inflammatory diseases, have been reported to have therapeutic significance in vitiligo. Recently, immunomodulatory therapeutic interventions in vitiligo have been emerging. Mesenchymal stem cells (MSCs) regulate cytokine secretion and the balance of T-cell subsets, which makes them a promising cell-based treatment option for autoimmune diseases. The induction of MSC-mediated immunomodulation is complicated and occurs by contact-dependent mechanisms and soluble extracellular vesicle (EV) mediators. EVs released from MSCs contain various growth factors and cytokines with anti-inflammatory effects in the skin immune response. Here, we summarize and discuss the progress to date in targeted therapies that immunomodulate the niche environment of vitiligo, from the clinical trial of JAK inhibitors to the potential of MSCs and MSC-EVs. The available information was collected to highlight the need for further research into the treatment of vitiligo.

Tissue resident memory T cells in the respiratory tract

Owing to their capacity to rapidly spread across the population, airborne pathogens represent a significant risk to global health. Indeed, several of the past major global pandemics have been instigated by respiratory pathogens. A greater understanding of the immune cells tasked with protecting the airways from infection will allow for the development of strategies that curb the spread and impact of these airborne diseases. A specific subset of memory T-cell resident in both the upper and lower respiratory tract, termed tissue-resident memory (Trm), have been shown to play an instrumental role in local immune responses against a wide breadth of both viral and bacterial infections. In this review, we discuss factors that influence respiratory tract Trm development, longevity, and immune surveillance and explore vaccination regimes that harness these cells, such approaches represent exciting new strategies that may be utilized to tackle the next global pandemic.

The precursors of CD8+ tissue resident memory T cells: from lymphoid organs to infected tissues

CD8⁺ tissue resident memory T cells (T_RM cells) are essential for immune defence against pathogens and malignancies, and the molecular processes that lead to T_RM cell formation are therefore of substantial biomedical interest. Prior work has demonstrated that signals present in the inflamed tissue micro-environment can promote the differentiation of memory precursor cells into mature T_RM cells, and it was therefore long assumed that T_RM cell formation adheres to a 'local divergence' model, in which T_RM cell lineage decisions are exclusively made within the tissue. However, a growing body of work provides evidence for a 'systemic divergence' model, in which circulating T cells already become preconditioned to preferentially give rise to the T_RM cell lineage, resulting in the generation of a pool of T_RM cell-poised T cells within the lymphoid compartment. Here, we review the emerging evidence that supports the existence of such a population of circulating T_RM cell progenitors, discuss current insights into their formation and highlight open questions in the field.

Tissue-resident immunity in the lung: a first-line defense at the environmental interface

The lung is a vital organ that incessantly faces external environmental challenges. Its homeostasis and unimpeded vital function are ensured by the respiratory epithelium working hand in hand with an intricate fine-tuned tissue-resident immune cell network. Lung tissue-resident immune cells span across the innate and adaptive immunity and protect from infectious agents but can also prove to be pathogenic if dysregulated. Here, we review the innate and adaptive immune cell subtypes comprising lung-resident immunity and discuss their ontogeny and role in distinct respiratory diseases. An improved understanding of the role of lung-resident immunity and how its function is dysregulated under pathological conditions can shed light on the pathogenesis of respiratory diseases.

Tissue-resident memory T cells in the kidney

The identification of tissue-resident memory T cells (T_RM cells) has significantly improved our understanding of immunity. In the last decade, studies have demonstrated that T_RM cells are induced after an acute T-cell response, remain in peripheral organs for several years, and contribute to both an efficient host defense and autoimmune disease. T_RM cells are found in the kidneys of healthy individuals and patients with various kidney diseases. A better understanding of these cells and their therapeutic targeting might provide new treatment options for infections, autoimmune diseases, graft rejection, and cancer. In this review, we address the definition, phenotype, and developmental mechanisms of T_RM cells. Then, we further discuss the current understanding of T_RM cells in kidney diseases, such as infection, autoimmune disease, cancer, and graft rejection after transplantation.

The fellowship of regulatory and tissue-resident memory cells

T cells located in non-lymphoid tissues have come to prominence in recent years. CD8+ tissue-resident memory (Trm) cells are important for tissue immune surveillance, provide an important line of defence against invading pathogens and show promise in cancer therapies. These cells differ in phenotype from other memory populations, are adapted to the tissue they home to where they found their cognate antigen and have different metabolic requirements for survival and activation. CD4+ Foxp3+ regulatory T (Treg) cells also consist of specialised populations, found in non-lymphoid tissues, with distinct transcriptional programmes. These cells have equally adapted to function in the tissue they made their home. Both Trm and Treg cells have functions beyond immune defence, involving tissue homeostasis, repair and turnover. They are part of a multicellular communication network. Intriguingly, occupying the same niche, Treg cells are important in the establishment of Trm cells, which may have implications to harness the immune surveillance and tissue homeostasis properties of Trm cells for future therapies.

Overcoming TGFβ-mediated immune evasion in cancer

Transforming growth factor-β (TGFβ) signalling controls multiple cell fate decisions during development and tissue homeostasis; hence, dysregulation of this pathway can drive several diseases, including cancer. Here we discuss the influence that TGFβ exerts on the composition and behaviour of different cell populations present in the tumour immune microenvironment, and the context-dependent functions of this cytokine in suppressing or promoting cancer. During homeostasis, TGFβ controls inflammatory responses triggered by exposure to the outside milieu in barrier tissues. Lack of TGFβ exacerbates inflammation, leading to tissue damage and cellular transformation. In contrast, as tumours progress, they leverage TGFβ to drive an unrestrained wound-healing programme in cancer-associated fibroblasts, as well as to suppress the adaptive immune system and the innate immune system. In consonance with this key role in reprogramming the tumour microenvironment, emerging data demonstrate that TGFβ-inhibitory therapies can restore cancer immunity. Indeed, this approach can synergize with other immunotherapies - including immune checkpoint blockade - to unleash robust antitumour immune responses in preclinical cancer models. Despite initial challenges in clinical translation, these findings have sparked the development of multiple therapeutic strategies that inhibit the TGFβ pathway, many of which are currently in clinical evaluation.

Epigenetics and CD8+ T cell memory

Following antigen recognition, CD8⁺ T lymphocytes can follow different patterns of differentiation, with the generation of different subsets characterized by distinct phenotypes, functions, and migration properties. The changes of transcription factors activity and chromatin structure dynamics drive the functional differentiation and phenotypic heterogeneity of these T cell subsets, which include short-lived effectors, long-term survival of memory, and also dysfunctional exhausted T cells. Recent progress in the field has shed light on the key contribution of chromatin organization to control the T cell fate specification. In fact, the understanding of these processes has important implications for the development of new immunotherapy protocols and to design new vaccination strategies. Here, we review the current understanding of the contribution of chromatin architecture and transcription factor activity orchestrating the gene expression programs guiding the CD8⁺ T cell subset commitment. We will focus on epigenetic changes, acting sequentially or in combination, which control the transcriptional programs governing T cell plasticity, stability, and memory. New molecular insights into the mechanisms of maintenance of cellular memory and identity, favoring or impeding the reprogramming, will be discussed in the context of T cell memory differentiation in infection and cancer.

Hobit confers tissue-dependent programs to type 1 innate lymphoid cells

Identification of type 1 innate lymphoid cells (ILC1s) has been problematic. The transcription factor Hobit encoded by Zfp683 has been proposed as a major driver of ILC1 programs. Using Zfp683 reporter mice, we showed that correlation of Hobit expression with ILC1s is tissue- and context-dependent. In liver and intestinal mucosa, Zfp683 expression correlated well with ILC1s; in salivary glands, Zfp683 was coexpressed with the natural killer (NK) master transcription factors Eomes and TCF1 in a unique cell population, which we call ILC1-like NK cells; during viral infection, Zfp683 was induced in conventional NK cells of spleen and liver. The impact of Zfp683 deletion on ILC1s and NK cells was also multifaceted, including a marked decrease in granzyme- and interferon-gamma (IFNγ)-producing ILC1s in the liver, slightly fewer ILC1s and more Eomes+ TCF1+ ILC1-like NK cells in salivary glands, and only reduced production of granzyme B by ILC1 in the intestinal mucosa. NK cell-mediated control of viral infection was unaffected. We conclude that Hobit has two major impacts on ILC1s: It sustains liver ILC1 numbers, while promoting ILC1 functional maturation in other tissues by controlling TCF1, Eomes, and granzyme expression.

Epigenetics and tissue immunity-Translating environmental cues into functional adaptations

There is an increasing appreciation that many innate and adaptive immune cell subsets permanently reside within non-lymphoid organs, playing a critical role in tissue homeostasis and defense. The best characterized are macrophages and tissue-resident T lymphocytes that work in concert with organ structural cells to generate appropriate immune responses and are functionally shaped by organ-specific environmental cues. The interaction of tissue epithelial, endothelial and stromal cells is also required to attract, differentiate, polarize and maintain organ immune cells in their tissue niche. All of these processes require dynamic regulation of cellular transcriptional programmes, with epigenetic mechanisms playing a critical role, including DNA methylation and post-translational histone modifications. A failure to appropriately regulate immune cell transcription inevitably results in inadequate or inappropriate immune responses and organ pathology. Here, with a focus on the mammalian kidney, an organ which generates differing regional environmental cues (including hypersalinity and hypoxia) due to its physiological functions, we will review the basic concepts of tissue immunity, discuss the technologies available to profile epigenetic modifications in tissue immune cells, including those that enable single-cell profiling, and consider how these mechanisms influence the development, phenotype, activation and function of different tissue immune cell subsets, as well as the immunological function of structural cells.

Interrelation Between Fibroblasts and T Cells in Fibrosing Interstitial Lung Diseases

Interstitial lung diseases (ILDs) are a heterogeneous group of diseases characterized by varying degrees of inflammation and fibrosis of the pulmonary interstitium. The interrelations between multiple immune cells and stromal cells participate in the pathogenesis of ILDs. While fibroblasts contribute to the development of ILDs through secreting extracellular matrix and proinflammatory cytokines upon activation, T cells are major mediators of adaptive immunity, as well as inflammation and autoimmune tissue destruction in the lung of ILDs patients. Fibroblasts play important roles in modulating T cell recruitment, differentiation and function and conversely, T cells can balance fibrotic sequelae with protective immunity in the lung. A more precise understanding of the interrelation between fibroblasts and T cells will enable a better future therapeutic design by targeting this interrelationship. Here we highlight recent work on the interactions between fibroblasts and T cells in ILDs, and consider the implications of these interactions in the future development of therapies for ILDs.

Limited Predictive or Prognostic Role of Tumor-Infiltrating Tissue-Resident Memory CD8 T Cells in Patients with Hepatocellular Carcinoma Receiving Immunotherapy

Simple Summary

Total tumor-infiltrating CD8 T cells inconsistently correlate with the efficacy of immune checkpoint blockade (ICB) in hepatocellular carcinoma. Tumor-infiltrating CD8 tissue-resident memory T cells (T_RM) are considered a surrogate of tumor-specific T cells and correlated better with survival in patients with melanoma, non–small-cell lung cancer, head and neck cancer or bladder cancer who received ICB. However, in this study, compared with total tumor-infiltrating CD8 T cells, tumor-infiltrating CD8 T_RM cells failed to provide additional advantages in predicting the efficacy of ICB-based immunotherapy in patients with hepatocellular carcinoma.

Abstract

Purpose: Tumor-infiltrating tissue-resident memory CD8 T cells (CD8 T_RM; CD103+ CD8+) are considered tumor-specific and may correlate better with the tumor response to immune checkpoint blockade (ICB). This study evaluated the association of tumor-infiltrating CD8 T_RM and their subsets with the efficacy of immunotherapy in patients with advanced hepatocellular carcinoma (HCC). Experimental Design: Consecutive HCC patients who received ICB in prospective trials were analyzed. Formalin-fixed paraffin-embedded tumor sections were stained for DAPI, CD8, CD103, CD39, programmed cell death-1 (PD-1), and programmed cell death ligand 1 (PD-L1) using a multiplex immunohistochemical method. The densities of CD8 T cells, CD8 T_RM, and CD39+ or PD-L1+ subsets of CD8 T_RM were correlated with tumor response and overall survival (OS). Results: A total of 73 patients were identified, and 48 patients with adequate pretreatment tumor specimens and complete follow-up were analyzed. A median of 32.7% (range: 0–92.6%) of tumor-infiltrating CD8 T cells were T_RM. In subset analyses, 66.6% ± 34.2%, 69.8% ± 33.4%, and 0% of CD8 T_RM cells coexpressed CD39, PD-L1, and PD-1, respectively. The objective response rates for CD8 T cell-high, CD8 T_RM-high, CD39+ CD8 T_RM-high, and PD-L1+ CD8 T_RM-high groups were 41.7%, 37.5%, 37.5%, and 29.2%, respectively. Patients with CD8 T cell-high, but not those with CD8 T_RM-high, CD39+ CD8 T_RM-high, or PD-L1+ CD8 T_RM-high, tumors, had significantly prolonged OS (p = 0.0429). Conclusions: Compared with total tumor-infiltrating CD8 T cells, tumor-infiltrating CD8 T_RM or their subsets failed to provide additional advantages in predicting the efficacy of immunotherapy for HCC.

CXCR6 by increasing retention of memory CD8+ T cells in the ovarian tumor microenvironment promotes immunosurveillance and control of ovarian cancer

Purpose

Resident memory CD8 T cells, owing to their ability to reside and persist in peripheral tissues, impart adaptive sentinel activity and amplify local immune response, and have beneficial implications for tumor surveillance and control. The current study aimed to clarify the less known chemotactic mechanisms that govern the localization, retention, and residency of memory CD8 T cells in the ovarian tumor microenvironment.

Experimental design

RNA and protein expressions of chemokine receptors in CD8⁺ resident memory T cells in human ovarian tumor-infiltrating CD8⁺ T cells and their association with survival were analyzed. The role of CXCR6 on antitumor T cells was investigated using prophylactic vaccine models in murine ovarian cancer.

Results

Chemokine receptor profiling of CD8⁺CD103⁺ resident memory tumor-infiltrating lymphocytes in patients with ovarian cancer revealed high expression of CXCR6. Analysis of The Cancer Genome Atlas (TCGA) (ovarian cancer database revealed CXCR6 to be associated with CD103 and increased patient survival. Functional studies in mouse models of ovarian cancer revealed that CXCR6 is a marker of resident, but not circulatory, tumor-specific memory CD8⁺ T cells. CXCR6-deficient tumor-specific CD8⁺ T cells showed reduced retention in tumor tissues, leading to diminished resident memory responses and poor control of ovarian cancer.

Conclusions

CXCR6, by promoting retention in tumor tissues, serves a critical role in resident memory T cell-mediated immunosurveillance and control of ovarian cancer. Future studies warrant exploiting CXCR6 to promote resident memory responses in cancers.

The Inhibitory Receptor GPR56 (Adgrg1) Is Specifically Expressed by Tissue-Resident Memory T Cells in Mice But Dispensable for Their Differentiation and Function In Vivo

Tissue-resident memory T (TRM) cells with potent antiviral and antibacterial functions protect the epithelial and mucosal surfaces of our bodies against infection with pathogens. The strong proinflammatory activities of TRM cells suggest requirement for expression of inhibitory molecules to restrain these memory T cells under steady state conditions. We previously identified the adhesion G protein-coupled receptor GPR56 as an inhibitory receptor of human cytotoxic lymphocytes that regulates their cytotoxic effector functions. Here, we explored the expression pattern, expression regulation, and function of GPR56 on pathogen-specific CD8+ T cells using two infection models. We observed that GPR56 is expressed on TRM cells during acute infection and is upregulated by the TRM cell-inducing cytokine TGF-β and the TRM cell-associated transcription factor Hobit. However, GPR56 appeared dispensable for CD8+ T-cell differentiation and function upon acute infection with LCMV as well as Listeria monocytogenes. Thus, TRM cells specifically acquire the inhibitory receptor GPR56, but the impact of this receptor on TRM cells after acute infection does not appear essential to regulate effector functions of TRM cells.

Decoding Tissue-Residency: Programming and Potential of Frontline Memory T Cells

Memory T-cell responses are partitioned between the blood, secondary lymphoid organs, and nonlymphoid tissues. Tissue-resident memory T (Trm) cells are a population of immune cells that remain permanently in tissues without recirculating in blood. These nonrecirculating cells serve as a principal node in the anamnestic response to invading pathogens and developing malignancies. Here, we contemplate how T-cell tissue residency is defined and shapes protective immunity in the steady state and in the context of disease. We review the properties and heterogeneity of Trm cells, highlight the critical roles these cells play in maintaining tissue homeostasis and eliciting immune pathology, and explore how they might be exploited to treat disease.

The SKI proto-oncogene restrains the resident CD103+CD8+ T cell response in viral clearance

Acute viral infection causes illness and death. In addition, an infection often results in increased susceptibility to a secondary infection, but the mechanisms behind this susceptibility are poorly understood. Since its initial identification as a marker for resident memory CD8⁺ T cells in barrier tissues, the function and regulation of CD103 integrin (encoded by ITGAE gene) have been extensively investigated. Nonetheless, the function and regulation of the resident CD103⁺CD8⁺ T cell response to acute viral infection remain unclear. Although TGFβ signaling is essential for CD103 expression, the precise molecular mechanism behind this regulation is elusive. Here, we reveal a TGFβ-SKI-Smad4 pathway that critically and specifically directs resident CD103⁺CD8⁺ T cell generation for protective immunity against primary and secondary viral infection. We found that resident CD103⁺CD8⁺ T cells are abundant in both lymphoid and nonlymphoid tissues from uninfected mice. CD103 acts as a costimulation signal to produce an optimal antigenic CD8⁺ T cell response to acute viral infection. There is a reduction in resident CD103⁺CD8⁺ T cells following primary infection that results in increased susceptibility of the host to secondary infection. Intriguingly, CD103 expression inversely and specifically correlates with SKI proto-oncogene (SKI) expression but not R-Smad2/3 activation. Ectopic expression of SKI restricts CD103 expression in CD8⁺ T cells in vitro and in vivo to hamper viral clearance. Mechanistically, SKI is recruited to the Itgae loci to directly suppress CD103 transcription by regulating histone acetylation in a Smad4-dependent manner. Our study therefore reveals that resident CD103⁺CD8⁺ T cells dictate protective immunity during primary and secondary infection. Interfering with SKI function may amplify the resident CD103⁺CD8⁺ T cell response to promote protective immunity.

Multipeptide stimulated PBMCs generate TEM/TCM for adoptive cell therapy in multiple myeloma

Multiple Myeloma (MM) patients suffer disease relapse due to the development of therapeutic resistance. Increasing evidence suggests that immunotherapeutic strategies can provide durable responses. Here we evaluate the possibility of adoptive cell transfer (ACT) by generating ex vivo T cells from peripheral blood mononuclear cells (PBMCs) isolated from MM patients by employing our previously devised protocols. We designed peptides from antigens (Ags) including cancer testis antigens (CTAs) that are over expressed in MM. We exposed PBMCs from different healthy donors (HDs) to single peptides. We observed reproducible Ag-specific cluster of differentiation 4⁺ (CD4⁺) and CD8⁺ T cell responses on exposure of PBMCs to different single peptide sequences. These peptide sequences were used to compile four different peptide cocktails. Naïve T cells from PBMCs from MM patients or HDs recognized the cognate Ag in all four peptide cocktails, leading to generation of multiclonal Ag-specific CD4⁺ and CD8⁺ effector and central memory T (T_EM and T_CM, respectively) cells which produced interferon-gamma (IFN-γ), granzyme B and perforin on secondary restimulation. Furthermore, this study demonstrated that immune cells from MM patients are capable of switching metabolic programs to induce effector and memory responses. Multiple peptides and cocktails were identified that induce IFN-γ⁺, T1-type, metabolically active T cells, thereby paving the way for feasibility testing of ACT in phase I clinical trials.

Tissue-specific differentiation of CD8+ resident memory T cells

CD8⁺ tissue-resident memory T (T_RM) cells play crucial roles in defense against infections and cancer and have been implicated in autoimmune diseases such as psoriasis. In mice and humans, they exist in all nonlymphoid organs and share key characteristics across all tissues, including downregulation of tissue egress and lymph node homing pathways. However, recent studies demonstrate considerable heterogeneity across T_RM cells lodged in different tissues - linked to the activity of tissue-specific molecules, including chemokines, cytokines, and transcription factors. Current work indicates that transforming growth factor (TGF)-β plays a major role in generating T_RM heterogeneity at phenotypic and functional levels. Here, we review common and unique features of T_RM cells in different tissues and discuss putative strategies aimed at harnessing T_RM cells for site-specific protection against infectious and malignant diseases.

CD8+ and CD4+ T Cells Infiltrate into the Brain during Plasmodium berghei ANKA Infection and Form Long-Term Resident Memory

In the Plasmodium berghei ANKA mouse model of malaria, accumulation of CD8⁺ T cells and infected RBCs in the brain promotes the development of experimental cerebral malaria (ECM). In this study, we used malaria-specific transgenic CD4⁺ and CD8⁺ T cells to track evolution of T cell immunity during the acute and memory phases of P. berghei ANKA infection. Using a combination of techniques, including intravital multiphoton and confocal microscopy and flow cytometric analysis, we showed that, shortly before onset of ECM, both CD4⁺ and CD8⁺ T cell populations exit the spleen and begin infiltrating the brain blood vessels. Although dominated by CD8⁺ T cells, a proportion of both T cell subsets enter the brain parenchyma, where they are largely associated with blood vessels. Intravital imaging shows these cells moving freely within the brain parenchyma. Near the onset of ECM, leakage of RBCs into areas of the brain can be seen, implicating severe damage. If mice are cured before ECM onset, brain infiltration by T cells still occurs, but ECM is prevented, allowing development of long-term resident memory T cell populations within the brain. This study shows that infiltration of malaria-specific T cells into the brain parenchyma is associated with cerebral immunopathology and the formation of brain-resident memory T cells. The consequences of these resident memory populations is unclear but raises concerns about pathology upon secondary infection.

Autoimmunity in dry eye disease - An updated review of evidence on effector and memory Th17 cells in disease pathogenicity

The classic Th1/Th2 dogma has been significantly reshaped since the subsequent introduction of several new T helper cell subsets, among which the most intensively investigated during the last decade is the Th17 lineage that demonstrates critical pathogenic roles in autoimmunity and chronic inflammation - including the highly prevalent dry eye disease. In this review, we summarize current concepts of Th17-mediated disruption of ocular surface immune homeostasis that leads to autoimmune inflammatory dry eye disease, by discussing the induction, activation, differentiation, migration, and function of effector Th17 cells in disease development, highlighting the phenotypic and functional plasticity of Th17 lineage throughout the disease initiation, perpetuation and sustention. Furthermore, we emphasize the most recent advance in Th17 memory formation and function in the chronic course of dry eye disease, a major area to be better understood for facilitating the development of effective treatments in a broader field of autoimmune diseases that usually present a chronic course with recurrent episodes of flare in the target tissues or organs.

Co-Ordination of Mucosal B Cell and CD8 T Cell Memory by Tissue-Resident CD4 Helper T Cells

Adaptive cellular immunity plays a major role in clearing microbial invasion of mucosal tissues in mammals. Following the clearance of primary pathogens, memory lymphocytes are established both systemically and locally at pathogen entry sites. Recently, resident memory CD8 T and B cells (T_RM and B_RM respectively), which are parked mainly in non-lymphoid mucosal tissues, were characterized and demonstrated to be essential for protection against secondary microbial invasion. Here we reviewed the current understanding of the cellular and molecular cues regulating CD8 T_RM and B_RM development, maintenance and function. We focused particularly on elucidating the role of a novel tissue-resident helper T (T_RH) cell population in assisting T_RM and B_RM responses in the respiratory mucosa following viral infection. Finally, we argue that the promotion of T_RH responses by future mucosal vaccines would be key to the development of successful universal influenza or coronavirus vaccines, providing long-lasting immunity against a broad spectrum of viral strains.

Signatures and Specificity of Tissue-Resident Lymphocytes Identified in Human Renal Peritumor and Tumor Tissue

BACKGROUND

Tissue-resident memory T (T_RM) cells are known to be important for the first line of defense in mucosa-associated tissues. However, the composition, localization, effector function, and specificity of T_RM cells in the human kidney and their relevance for renal pathology have not been investigated.

METHODS

Lymphocytes derived from blood, renal peritumor samples, and tumor samples were phenotypically and functionally assessed by applying flow cytometry and highly advanced histology (multi-epitope ligand cartography) methods.

RESULTS

CD69⁺CD103⁺CD8⁺ T_RM cells in kidneys display an inflammatory profile reflected by enhanced IL-2, IL-17, and TNFα production, and their frequencies correlate with increasing age and kidney function. We further identified mucosa-associated invariant T and CD56^dim and CD56^bright natural killer cells likewise expressing CD69 and CD103, the latter significantly enriched in renal tumor tissues. CD8⁺ T_RM cell frequencies were not elevated in kidney tumor tissue, but they coexpressed PD-1 and TOX and produced granzyme B. Tumor-derived CD8⁺ T_RM cells from patients with metastases were functionally impaired. Both CD69⁺CD103^-CD4⁺ and CD69⁺CD103^-CD8⁺ T_RM cells form distinct clusters in tumor tissues in proximity to antigen-presenting cells. Finally, EBV, CMV, BKV, and influenza antigen-specific CD8⁺ T cells were enriched in the effector memory T cell population in the kidney.

CONCLUSIONS

Our data provide an extensive overview of T_RM cells' phenotypes and functions in the human kidney for the first time, pointing toward their potential relevance in kidney transplantation and kidney disease.

Integrin-αV-mediated activation of TGF-β regulates anti-tumour CD8 T cell immunity and response to PD-1 blockade

TGF-β is secreted in the tumour microenvironment in a latent, inactive form bound to latency associated protein and activated by the integrin α_V subunit. The activation of latent TGF-β by cancer-cell-expressed α_V re-shapes the tumour microenvironment, and this could affect patient responses to PD-1-targeting therapy. Here we show, using multiplex immunofluorescence staining in cohorts of anti-PD-1 and anti-PD-L1-treated lung cancer patients, that decreased expression of cancer cell α_V is associated with improved immunotherapy-related, progression-free survival, as well as with an increased density of CD8⁺CD103⁺ tumour-infiltrating lymphocytes. Mechanistically, tumour α_V regulates CD8 T cell recruitment, induces CD103 expression on activated CD8⁺ T cells and promotes their differentiation to granzyme B-producing CD103⁺CD69⁺ resident memory T cells via autocrine TGF-β signalling. Thus, our work provides the underlying principle of targeting cancer cell α_V for more efficient PD-1 checkpoint blockade therapy.

Response to PD-1 checkpoint blockade is unpredictable in lung cancer patients. Here authors show in human lung and mouse tumour models that low or absent α_V integrin expression leads to better tumour growth control by anti-PD-1 via reduced TGF-β activation and hence increased infiltration of anti-tumour CD8⁺ T cells.

Clone Wars: Survival of the Fittest CD8+ Trm Cells

In this issue of Immunity, Hirai et al. illuminate competition for the cytokine TGFβ as a key regulator of CD8⁺ tissue-resident memory T cell persistence and occupancy in the skin epidermal niche.

TGF-β and Eomes control the homeostasis of CD8+ regulatory T cells

In addition to Foxp3+ CD4+ regulatory T cells (CD4+ T reg cells), Foxp3- CD8+ regulatory T cells (CD8+ T reg cells) are critical to maintain immune tolerance. However, the molecular programs that specifically control CD8+ but not CD4+ T reg cells are largely unknown. Here, we demonstrate that simultaneous disruption of both TGF-β receptor and transcription factor Eomesodermin (Eomes) in T cells results in lethal autoimmunity due to a specific defect in CD8+ but not CD4+ T reg cells. Further, TGF-β signal maintains the regulatory identity, while Eomes controls the follicular location of CD8+ T reg cells. Both TGF-β signal and Eomes coordinate to promote the homeostasis of CD8+ T reg cells. Together, we have identified a unique molecular program designed for CD8+ T reg cells.

Skin-Resident Memory T Cells: Pathogenesis and Implication for the Treatment of Psoriasis

Tissue-resident memory T cells (T_RM) stay in the peripheral tissues for long periods of time, do not recirculate, and provide the first line of adaptive immune response in the residing tissues. Although T_RM originate from circulating T cells, T_RM are physiologically distinct from circulating T cells with the expression of tissue-residency markers, such as CD69 and CD103, and the characteristic profile of transcription factors. Besides defense against pathogens, the functional skew of skin T_RM is indicated in chronic skin inflammatory diseases. In psoriasis, IL-17A-producing CD8⁺ T_RM are regarded as one of the pathogenic populations in skin. Although no licensed drugs that directly and specifically inhibit the activity of skin T_RM are available to date, psoriatic skin T_RM are affected in the current treatments of psoriasis. Targeting skin T_RM or using T_RM as a potential index for disease severity can be an attractive strategy in psoriasis.

Discrete tissue microenvironments instruct diversity in resident memory T cell function and plasticity

Tissue-resident memory T (T_RM) cells are non-recirculating cells that exist throughout the body. Although T_RM cells in various organs rely on common transcriptional networks to establish tissue residency, location-specific factors adapt these cells to their tissue of lodgment. Here we analyze T_RM cell heterogeneity between organs and find that the different environments in which these cells differentiate dictate T_RM cell function, durability and malleability. We find that unequal responsiveness to TGFβ is a major driver of this diversity. Notably, dampened TGFβ signaling results in CD103^- T_RM cells with increased proliferative potential, enhanced function and reduced longevity compared with their TGFβ-responsive CD103⁺ T_RM counterparts. Furthermore, whereas CD103^- T_RM cells readily modified their phenotype upon relocation, CD103⁺ T_RM cells were comparatively resistant to transdifferentiation. Thus, despite common requirements for T_RM cell development, tissue adaptation of these cells confers discrete functional properties such that T_RM cells exist along a spectrum of differentiation potential that is governed by their local tissue microenvironment.

Hobit identifies tissue-resident memory T cell precursors that are regulated by Eomes

Tissue-resident memory CD8⁺ T cells (T_RM) constitute a noncirculating memory T cell subset that provides early protection against reinfection. However, how T_RM arise from antigen-triggered T cells has remained unclear. Exploiting the T_RM-restricted expression of Hobit, we used T_RM reporter/deleter mice to study T_RM differentiation. We found that Hobit was up-regulated in a subset of LCMV-specific CD8⁺ T cells located within peripheral tissues during the effector phase of the immune response. These Hobit⁺ effector T cells were identified as T_RM precursors, given that their depletion substantially decreased T_RM development but not the formation of circulating memory T cells. Adoptive transfer experiments of Hobit⁺ effector T cells corroborated their biased contribution to the T_RM lineage. Transcriptional profiling of Hobit⁺ effector T cells underlined the early establishment of T_RM properties including down-regulation of tissue exit receptors and up-regulation of T_RM-associated molecules. We identified Eomes as a key factor instructing the early bifurcation of circulating and resident lineages. These findings establish that commitment of T_RM occurs early in antigen-driven T cell differentiation and reveal the molecular mechanisms underlying this differentiation pathway.

Tissue-Resident Memory T Cells in Skin Diseases: A Systematic Review

In health, the non-recirculating nature and long-term persistence of tissue-resident memory T cells (TRMs) in tissues protects against invading pathogens. In disease, pathogenic TRMs contribute to the recurring traits of many skin diseases. We aimed to conduct a systematic literature review on the current understanding of the role of TRMs in skin diseases and identify gaps as well as future research paths. EMBASE, PubMed, SCOPUS, Web of Science, Clinicaltrials.gov and WHO Trials Registry were searched systematically for relevant studies from their inception to October 2020. Included studies were reviewed independently by two authors. This study was conducted in accordance with the PRISMA-S guidelines. This protocol was registered with the PROSPERO database (ref: CRD42020206416). We identified 96 studies meeting the inclusion criteria. TRMs have mostly been investigated in murine skin and in relation to infectious skin diseases. Pathogenic TRMs have been characterized in various skin diseases including psoriasis, vitiligo and cutaneous T-cell lymphoma. Studies are needed to discover biomarkers that may delineate TRMs poised for pathogenic activity in skin diseases and establish to which extent TRMs are contingent on the local skin microenvironment. Additionally, future studies may investigate the effects of current treatments on the persistence of pathogenic TRMs in human skin.

Distinct populations of antigen-specific tissue-resident CD8+ T cells in human cervix mucosa

The ectocervix is part of the lower female reproductive tract (FRT), which is susceptible to sexually transmitted infections (STIs). Comprehensive knowledge of the phenotypes and T cell receptor (TCR) repertoire of tissue-resident memory T cells (TRMs) in the human FRT is lacking. We took single-cell RNA-Seq approaches to simultaneously define gene expression and TCR clonotypes of the human ectocervix. There were significantly more CD8+ than CD4+ T cells. Unsupervised clustering and trajectory analysis identified distinct populations of CD8+ T cells with IFNGhiGZMBloCD69hiCD103lo or IFNGloGZMBhiCD69medCD103hi phenotypes. Little overlap was seen between their TCR repertoires. Immunofluorescence staining showed that CD103+CD8+ TRMs were preferentially localized in the epithelium, whereas CD69+CD8+ TRMs were distributed evenly in the epithelium and stroma. Ex vivo assays indicated that up to 14% of cervical CD8+ TRM clonotypes were HSV-2 reactive in HSV-2-seropositive persons, reflecting physiologically relevant localization. Our studies identified subgroups of CD8+ TRMs in the human ectocervix that exhibited distinct expression of antiviral defense and tissue residency markers, anatomic locations, and TCR repertoires that target anatomically relevant viral antigens. Optimization of the location, number, and function of FRT TRMs is an important approach for improving host defenses to STIs.

Motility Matters: How CD8+ T-Cell Trafficking Influences Effector and Memory Cell Differentiation

Immunological memory is a hallmark of adaptive immunity that confers long-lasting protection from reinfections. Memory CD8⁺ T cells provide protection by actively scanning for their cognate antigen and migrating into inflamed tissues. Trafficking patterns of CD8⁺ T cells are also a major determinant of cell fate outcomes during differentiation into effector and memory cell states. CD8⁺ T-cell trafficking must therefore be dynamically and tightly regulated to ensure that CD8⁺ T cells arrive at the correct locations and differentiate to acquire appropriate effector functions. This review aims to discuss the importance of CD8⁺ T-cell trafficking patterns in regulating effector and memory differentiation, maintenance, and reactivation.

Formation of Tissue-Resident CD8+ T-Cell Memory

Resident memory CD8⁺ T (Trm) cells permanently reside in nonlymphoid tissues where they act as a first line of defense against recurrent pathogens. How and when antigen-inexperienced CD8⁺ T cells differentiate into Trm has been a topic of major interest, as knowledge on how to steer this process may be exploited in the development of vaccines and anticancer therapies. Here, we first review the current understanding of the early signals that CD8⁺ T cells receive before they have entered the tissue and that govern their capacity to develop into tissue-resident memory T cells. Subsequently, we discuss the tissue-derived factors that promote Trm maturation in situ. Combined, these data sketch a model in which a subset of responding T cells develops a heightened capacity to respond to local cues present in the tissue microenvironment, which thereby imprints their ability to contribute to the tissue-resident memory CD8⁺ T-cell pool that provide local control against pathogens.

The Clinical Significance of Hepatic CD69+ CD103+ CD8+ Resident-Memory T Cells in Autoimmune Hepatitis

BACKGROUND AND AIMS

The diverse inflammatory response found in the liver of patients with autoimmune hepatitis (AIH) is well established, but identification of potentially pathogenic subpopulations has proven enigmatic.

APPROACH AND RESULTS

We report herein that CD69⁺ CD103⁺ CD8⁺ tissue-resident memory T cells (T_RM ) are significantly increased in the liver of patients with AIH compared to chronic hepatitis B, NAFLD, and healthy control tissues. In addition, there was a significant statistical correlation between elevation of CD8⁺ T_RM cells and AIH disease severity. Indeed, in patients with successful responses to immunosuppression, the frequencies of such hepatic CD8⁺ T_RM cells decreased significantly. CD69⁺ CD8⁺ and CD69⁺ CD103⁺ CD8⁺ T cells, also known as CD8⁺ T_RM cells, reflect tissue residency and are well known to provide intense immune antigenic responses. Hence, it was particularly interesting that patients with AIH also manifest an elevated expression of IL-15 and TGF-β on inflammatory cells, and extensive hepatic expression of E-cadherin; these factors likely contribute to the development and localization of CD8⁺ T_RM cells. Based on these data and, in particular, the relationships between disease severity and CD8⁺ T_RM cells, we studied the mechanisms involved with glucocorticoid (GC) modulation of CD8⁺ T_RM cell expansion. Our data reflect that GCs in vitro inhibit the expansion of CD8⁺ T_RM cells induced by IL-15 and TGF-β and with direct down-regulation of the nuclear factor Blimp1 of CD8⁺ T_RM cells.

CONCLUSIONS

Our data suggest that CD8⁺ T_RM cells play a critical role in the pathogenesis of AIH, and GCs attenuate hepatic inflammation through direct inhibition of CD8⁺ T_RM cell expansion.

Role of tissue-resident memory T cells in the pathophysiology of allergic contact dermatitis

PURPOSE OF REVIEW

We bring updated knowledge on tissue-resident memory T cells (TRM), underlining their major role in the recurrence and the severity of allergic contact dermatitis (ACD).

RECENT FINDINGS

ACD is a frequently encountered skin disease. It is defined as a delayed-type hypersensitivity reaction initiated by the recruitment of antigen-specific T cells into the skin of sensitized patients. ACD lesions tend to develop on already-exposed areas and worsen over time. That clinical observation has raised questions on the contribution of TRM to ACD recurrence and severity. TRM are memory T cells that persist in peripheral tissues, such as the skin, without recirculating through the blood. These cells provide effective immune memory against pathogens, but they may also participate in the development or exacerbation of numerous inflammatory diseases, including skin allergies. Recent works have demonstrated a major role for TRM in ACD pathophysiology.

SUMMARY

In ACD, TRM accumulate preferentially at the allergen contact site during the sensitization phase. Thereafter, these cells cause a rapid and intense response to any new allergen exposure. They also play a key role in flare-ups of ACD and the chronicity and severity of the disease. These aspects suggest that TRM may have an interest as therapeutic targets.

Tissue-Resident T Cells in Chronic Relapsing-Remitting Intestinal Disorders

Tissue-resident memory T (TRM) cells critically contribute to the rapid immunoprotection and efficient immunosurveillance against pathogens, particularly in barrier tissues, but also during anti-tumor responses. However, the involvement of TRM cells also in the induction and exacerbation of immunopathologies, notably in chronically relapsing auto-inflammatory disorders, is becoming increasingly recognized as a critical factor. Thus, TRM cells may also represent an attractive target in the management of chronic (auto-) inflammatory disorders, including multiple sclerosis, rheumatoid arthritis, celiac disease and inflammatory bowel diseases. In this review, we focus on current concepts of TRM cell biology, particularly in the intestine, and discuss recent findings on their involvement in chronic relapsing-remitting inflammatory disorders. Potential therapeutic strategies to interfere with these TRM cell-mediated immunopathologies are discussed.

Spatiotemporal single-cell profiling reveals that invasive and tissue-resident memory donor CD8+ T cells drive gastrointestinal acute graft-versus-host disease

Organ infiltration by donor T cells is critical to the development of acute graft-versus-host disease (aGVHD) in recipients after allogeneic hematopoietic stem cell transplant (allo-HCT). However, deconvoluting the transcriptional programs of newly recruited donor T cells from those of tissue-resident T cells in aGVHD target organs remains a challenge. Here, we combined the serial intravascular staining technique with single-cell RNA sequencing to dissect the tightly connected processes by which donor T cells initially infiltrate tissues and then establish a pathogenic tissue residency program in a rhesus macaque allo-HCT model that develops aGVHD. Our results enabled creation of a spatiotemporal map of the transcriptional programs controlling donor CD8⁺ T cell infiltration into the primary aGVHD target organ, the gastrointestinal (GI) tract. We identified the large and small intestines as the only two sites demonstrating allo-specific, rather than lymphodepletion-driven, T cell infiltration. GI-infiltrating donor CD8⁺ T cells demonstrated a highly activated, cytotoxic phenotype while simultaneously developing a canonical tissue-resident memory T cell (T_RM) transcriptional signature driven by interleukin-15 (IL-15)/IL-21 signaling. We found expression of a cluster of genes directly associated with tissue invasiveness, including those encoding adhesion molecules (ITGB2), specific chemokines (CCL3 and CCL4L1) and chemokine receptors (CD74), as well as multiple cytoskeletal proteins. This tissue invasion transcriptional signature was validated by its ability to discriminate the CD8⁺ T cell transcriptome of patients with GI aGVHD from those of GVHD-free patients. These results provide insights into the mechanisms controlling tissue occupancy of target organs by pathogenic donor CD8⁺ T_RM cells during aGVHD in primate transplant recipients.

CCR2 Regulates Vaccine-Induced Mucosal T-Cell Memory to Influenza A Virus

Elicitation of lung tissue-resident memory CD8 T cells (T_RMs) is a goal of T cell-based vaccines against respiratory viral pathogens, such as influenza A virus (IAV). C-C chemokine receptor type 2 (CCR2)-dependent monocyte trafficking plays an essential role in the establishment of CD8 T_RMs in lungs of IAV-infected mice. Here, we used a combination adjuvant-based subunit vaccine strategy that evokes multifaceted (T_C1/T_C17/T_H1/T_H17) IAV nucleoprotein-specific lung T_RMs to determine whether CCR2 and monocyte infiltration are essential for vaccine-induced T_RM development and protective immunity to IAV in lungs. Following intranasal vaccination, neutrophils, monocytes, conventional dendritic cells (DCs), and monocyte-derived dendritic cells internalized and processed vaccine antigen in lungs. We found that basic leucine zipper ATF-like transcription factor 3 (BATF3)-dependent DCs were essential for eliciting T cell responses, but CCR2 deficiency enhanced the differentiation of CD127^hi, KLRG-1^lo, OX40^+ve CD62L^+ve, and mucosally imprinted CD69^+ve CD103^+ve effector and memory CD8 T cells in lungs and airways of vaccinated mice. Mechanistically, increased development of lung T_RMs induced by CCR2 deficiency was linked to dampened expression of T-bet but not altered TCF-1 levels or T cell receptor signaling in CD8 T cells. T1/T17 functional programming, parenchymal localization of CD8/CD4 effector and memory T cells, recall T cell responses, and protective immunity to a lethal IAV infection were unaffected in CCR2-deficient mice. Taken together, we identified a negative regulatory role for CCR2 and monocyte trafficking in mucosal imprinting and differentiation of vaccine-induced T_RMs. Mechanistic insights from this study may aid the development of T-cell-based vaccines against respiratory viral pathogens, including IAV and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). IMPORTANCE While antibody-based immunity to influenza A virus (IAV) is type and subtype specific, lung- and airway-resident memory T cells that recognize conserved epitopes in the internal viral proteins are known to provide heterosubtypic immunity. Hence, broadly protective IAV vaccines need to elicit robust T cell memory in the respiratory tract. We have developed a combination adjuvant-based IAV nucleoprotein vaccine that elicits strong CD4 and CD8 T cell memory in lungs and protects against H1N1 and H5N1 strains of IAV. In this study, we examined the mechanisms that control vaccine-induced protective memory T cells in the respiratory tract. We found that trafficking of monocytes into lungs might limit the development of antiviral lung-resident memory T cells following intranasal vaccination. These findings suggest that strategies that limit monocyte infiltration can potentiate vaccine-induced frontline T-cell immunity to respiratory viruses, such as IAV and SARS-CoV-2.

Inflammatory signals are sufficient to elicit TOX expression in mouse and human CD8+ T cells

T cell receptor (TCR) stimulation leads to the expression of the transcription factor thymocyte selection–associated high-mobility group box (TOX). Prolonged TCR signaling, such as encountered during chronic infections or in tumors, leads to sustained TOX expression, which is required for the induction of a state of exhaustion or dysfunction. Although CD8⁺ memory T (Tmem) cells in mice typically do not express TOX at steady state, some human Tmem cells express TOX but appear fully functional. This seeming discrepancy between mouse and human T cells has led to the speculation that TOX is differentially regulated between these species, which could complicate the interpretation of preclinical mouse model studies. We report here that, similar to TCR-mediated signals, inflammatory cytokines are also sufficient to increase TOX expression in human and mouse Tmem cells. Thus, TOX expression is controlled by the environment, which provides an explanation for the different TOX expression patterns encountered in T cells isolated from specific pathogen–free laboratory mice versus humans. Finally, we report that TOX is not necessary for cytokine-driven expression of programmed cell death 1. Overall, our data highlight that the mechanisms regulating TOX expression are conserved across species and indicate that TOX expression reflects a T cell’s activation state and does not necessarily correlate with T cell dysfunction.

The Natural History of T Cell Metabolism

The cells of the immune system, particularly the T lymphocytes, have two main features that distinguish them from the cells of other tissues. They proliferate after activation and have the ability to move in tissues and organs. These characteristics compel them to develop metabolic plasticity in order to fulfil their immune function. This review focuses on the different known mechanisms that allow T cells to adapt their metabolism to the real-life circumstances they operate in, whether it is to exit quiescence, to differentiate into effector cells, or to participate in immune memory formation. Some of the metabolic adaptations to environmental variations that T cells are likely to undergo in their immune monitoring function are also discussed.

Interplay of Inflammatory, Antigen and Tissue-Derived Signals in the Development of Resident CD8 Memory T Cells

CD8 positive, tissue resident memory T cells (TRM) are a specialized subset of CD8 memory T cells that surveil tissues and provide critical first-line protection against tumors and pathogen re-infection. Recently, much effort has been dedicated to understanding the function, phenotype and development of TRM. A myriad of signals is involved in the development and maintenance of resident memory T cells in tissue. Much of the initial research focused on the roles tissue-derived signals play in the development of TRM, including TGFß and IL-33 which are critical for the upregulation of CD69 and CD103. However, more recent data suggest further roles for antigenic and pro-inflammatory cytokines. This review will focus on the interplay of pro-inflammatory, tissue and antigenic signals in the establishment of resident memory T cells.

Cervicovaginal Tissue Residence Confers a Distinct Differentiation Program upon Memory CD8 T Cells

Tissue-resident memory CD8 T cells (CD8 T_RM) are critical for maintaining barrier immunity. CD8 T_RM have been mainly studied in the skin, lung and gut, with recent studies suggesting that the signals that control tissue residence and phenotype are highly tissue dependent. We examined the T cell compartment in healthy human cervicovaginal tissue (CVT) and found that most CD8 T cells were granzyme B⁺ and TCF-1^- To address if this phenotype is driven by CVT tissue residence, we used a mouse model to control for environmental factors. Using localized and systemic infection models, we found that CD8 T_RM in the mouse CVT gradually acquired a granzyme B⁺, TCF-1^- phenotype as seen in human CVT. In contrast to CD8 T_RM in the gut, these CD8 T_RM were not stably maintained regardless of the initial infection route, which led to reductions in local immunity. Our data show that residence in the CVT is sufficient to progressively shape the size and function of its CD8 T_RM compartment.

Tissue-Resident Memory T Cells in Antifungal Immunity

Fungi are an integral part of the mammalian microbiota colonizing most if not all mucosal surfaces and the skin. Maintaining stable colonization on these surfaces is critical for preventing fungal dysbiosis and infection, which in some cases can lead to life threatening consequences. The epithelial barriers are protected by T cells and additional controlling immune mechanisms. Noncirculating memory T cells that reside stably in barrier tissues play an important role for host protection from commensals and recurrent pathogens due to their fast response and local activity, which provides them a strategic advantage. So far, only a few specific examples of tissue resident memory T cells (TRMs) that act against fungi have been reported. This review provides an overview of the characteristics and functional attributes of TRMs that have been established based on human and mouse studies with various microbes. It highlights what is currently known about fungi specific TRMs mediating immunosurveillance, how they have been targeted in preclinical vaccination approaches and how they can promote immunopathology, if not controlled. A better appreciation of the host protective and damaging roles of TRMs might accelerate the development of novel tissue specific preventive strategies against fungal infections and fungi-driven immunopathologies.

Hypoxia acts as an environmental cue for the human tissue-resident memory T cell differentiation program

Tissue-resident memory T cells (T_RM) provide frontline defense against infectious diseases and contribute to antitumor immunity; however, aside from the necessity of TGF-β, knowledge regarding T_RM-inductive cues remains incomplete, particularly for human cells. Oxygen tension is an environmental cue that distinguishes peripheral tissues from the circulation, and here, we demonstrate that differentiation of human CD8⁺ T cells in the presence of hypoxia and TGF-β1 led to the development of a T_RM phenotype, characterized by a greater than 5-fold increase in CD69⁺CD103⁺ cells expressing human T_RM hallmarks and enrichment for endogenous human T_RM gene signatures, including increased adhesion molecule expression and decreased expression of genes involved in recirculation. Hypoxia and TGF-β1 synergized to produce a significantly larger population of T_RM phenotype cells than either condition alone, and comparison of these cells from the individual and combination conditions revealed distinct phenotypic and transcriptional profiles, indicating a programming response to milieu rather than a mere expansion. Our findings identify a likely previously unreported cue for the T_RM differentiation program and can enable facile generation of human T_RM phenotype cells in vitro for basic studies and translational applications such as adoptive cellular therapy.

Dynamic changes to tissue-resident immunity after MHC-matched and MHC-mismatched solid organ transplantation

The heterogeneous pool of tissue-resident lymphocytes in solid organs mediates infection responses and supports tissue integrity and repair. Their vital functions in normal physiology suggest an important role in solid organ transplantation; however, their detailed examination in this context has not been performed. Here, we report the fate of multiple lymphocyte subsets, including T, B, and innate lymphoid cells, after murine liver and heart transplantation. In major histocompatibility complex (MHC)-matched transplantation, donor lymphocytes are retained in liver grafts and peripheral lymphoid organs of heart and liver transplant recipients. In MHC-mismatched transplantation, increased infiltration of the graft by recipient cells and depletion of donor lymphocytes occur, which can be prevented by removal of recipient T and B cells. Recipient lymphocytes fail to recreate the native organs' phenotypically diverse tissue-resident lymphocyte composition, even in MHC-matched models. These post-transplant changes may leave grafts vulnerable to infection and impair long-term graft function.

T-cell memory in tissues

Immunological memory equips our immune system to respond faster and more effectively against reinfections. This acquired immunity was originally attributed to long-lived, memory T and B cells with body wide access to peripheral and secondary lymphoid tissues. In recent years, it has been realized that both innate and adaptive immunity to a large degree depends on resident immune cells that act locally in barrier tissues including tissue-resident memory T cells (Trm). Here, we will discuss the phenotype of these Trm in mice and humans, the tissues and niches that support them, and their function, plasticity, and transcriptional control. Their unique properties enable Trm to achieve long-lived immunological memory that can be deposited in nearly every organ in response to acute and persistent infection, and in response to cancer. However, Trm may also induce substantial immunopathology in allergic and autoimmune disease if their actions remain unchecked. Therefore, inhibitory and activating stimuli appear to balance the actions of Trm to ensure rapid proinflammatory responses upon infection and to prevent damage to host tissues under steady state conditions.

TGF-β: Many Paths to CD103+ CD8 T Cell Residency

CD8 tissue-resident memory T (T_RM) cells primarily reside in nonlymphoid tissues without recirculating and provide front-line protective immunity against infections and cancers. CD8 T_RM cells can be generally divided into CD69⁺ CD103⁻ T_RM cells (referred to as CD103⁻ T_RM cells) and CD69⁺ CD103⁺ T_RM cells (referred to as CD103⁺ T_RM cells). TGF-β plays a critical role in the development and maintenance of CD103⁺ CD8 T_RM cells. In this review, we summarize the current understanding of tissue-specific activation of TGF-β mediated by integrins and how it contributes to CD103⁺ CD8 T_RM cell development and maintenance. Furthermore, we discuss the underlying mechanisms utilized by TGF-β to regulate the development and maintenance of CD103⁺ CD8 T_RM cells. Overall, this review highlights the importance of TGF-β in regulating this unique subset of memory CD8 T cells that may shed light on improving vaccine design to target this population.

The Role of CD4+ Resident Memory T Cells in Local Immunity in the Mucosal Tissue - Protection Versus Pathology

Memory T cells are crucial for both local and systemic protection against pathogens over a long period of time. Three major subsets of memory T cells; effector memory T (T_EM) cells, central memory T (T_CM) cells, and tissue-resident memory T (T_RM) cells have been identified. The most recently identified subset, T_RM cells, is characterized by the expression of the C-type lectin CD69 and/or the integrin CD103. T_RM cells persist locally at sites of mucosal tissue, such as the lung, where they provide frontline defense against various pathogens. Importantly, however, T_RM cells are also involved in shaping the pathology of inflammatory diseases. A number of pioneering studies revealed important roles of CD8⁺ T_RM cells, particularly those in the local control of viral infection. However, the protective function and pathogenic role of CD4⁺ T_RM cells that reside within the mucosal tissue remain largely unknown. In this review, we discuss the ambivalent feature of CD4⁺ T_RM cells in the protective and pathological immune responses. We also review the transcriptional and epigenetic characteristics of CD4⁺ T_RM cells in the lung that have been elucidated by recent technical approaches. A better understanding of the function of CD4⁺ T_RM cells is crucial for the development of both effective vaccination against pathogens and new therapeutic strategies for intractable inflammatory diseases, such as inflammatory bowel diseases and chronic allergic diseases.

Intratumoral CD103+ CD8+ T cells predict response to PD-L1 blockade

BACKGROUND

CD8+ tissue-resident memory T (T_RM) cells, marked by CD103 (ITGAE) expression, are thought to actively suppress cancer progression, leading to the hypothesis that their presence in tumors may predict response to immunotherapy.

METHODS

Here, we test this by combining high-dimensional single-cell modalities with bulk tumor transcriptomics from 1868 patients enrolled in lung and bladder cancer clinical trials of atezolizumab (anti-programmed cell death ligand 1 (PD-L1)).

RESULTS

ITGAE was identified as the most significantly upregulated gene in inflamed tumors. Tumor CD103+ CD8+ T_RM cells exhibited a complex phenotype defined by the expression of checkpoint regulators, cytotoxic proteins, and increased clonal expansion.

CONCLUSIONS

Our analyses indeed demonstrate that the presence of CD103+ CD8+ T_RM cells, quantified by tracking intratumoral CD103 expression, can predict treatment outcome, suggesting that patients who respond to PD-1/PD-L1 blockade are those who exhibit an ongoing antitumor T-cell response.