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

Tissue-resident memory T cells: decoding intra-organ diversity with a gut perspective

Tissue-resident memory T cells (TRM) serve as the frontline of host defense, playing a critical role in protection against invading pathogens. This emphasizes their role in providing rapid on-site immune responses across various organs. The physiological significance of TRM is not just confined to infection control; accumulating evidence has revealed that TRM also determine the pathology of diseases such as autoimmune disorders, inflammatory bowel disease, and cancer. Intensive studies on the origin, mechanisms of formation and maintenance, and physiological significance of TRM have elucidated the transcriptional and functional diversity of these cells, which are often affected by local cues associated with their presence. These were further confirmed by the recent remarkable advancements of next-generation sequencing and single-cell technologies, which allow the transcriptional and phenotypic characterization of each TRM subset induced in different microenvironments. This review first overviews the current knowledge of the cell fate, molecular features, transcriptional and metabolic regulation, and biological importance of TRM in health and disease. Finally, this article presents a variety of recent studies on disease-associated TRM, particularly focusing and elaborating on the TRM in the gut, which constitute the largest and most intricate immune network in the body, and their pathological relevance to gut inflammation in humans.

The role of tissue-resident memory T cells as mediators for response and toxicity in immunotherapy-treated melanoma-two sides of the same coin?

Tissue-resident memory T cells (TRM cells) have become an interesting subject of study for antitumor immunity in melanoma and other solid tumors. In the initial phases of antitumor immunity, they maintain an immune equilibrium and protect against challenges with tumor cells and the formation of primary melanomas. In metastatic settings, they are a prime target cell population for immune checkpoint inhibition (ICI) because they highly express inhibitory checkpoint molecules such as PD-1, CTLA-4, or LAG-3. Once melanoma patients are treated with ICI, TRM cells residing in the tumor are reactivated and expand. Tumor killing is achieved by secreting effector molecules such as IFN-γ. However, off-target effects are also observed. Immune-related adverse events, such as those affecting barrier organs like the skin, can be mediated by ICI-induced TRM cells. Therefore, a detailed understanding of this memory T-cell type is obligatory to better guide and improve immunotherapy regimens.

PD-1 defines a distinct, functional, tissue-adapted state in Vδ1+ T cells with implications for cancer immunotherapy

Checkpoint inhibition (CPI), particularly that targeting the inhibitory coreceptor programmed cell death protein 1 (PD-1), has transformed oncology. Although CPI can derepress cancer (neo)antigen-specific αβ T cells that ordinarily show PD-1-dependent exhaustion, it can also be efficacious against cancers evading αβ T cell recognition. In such settings, γδ T cells have been implicated, but the functional relevance of PD-1 expression by these cells is unclear. Here we demonstrate that intratumoral TRDV1 transcripts (encoding the TCRδ chain of Vδ1+ γδ T cells) predict anti-PD-1 CPI response in patients with melanoma, particularly those harboring below average neoantigens. Moreover, using a protocol yielding substantial numbers of tissue-derived Vδ1+ cells, we show that PD-1+Vδ1+ cells display a transcriptomic program similar to, but distinct from, the canonical exhaustion program of colocated PD-1+CD8+ αβ T cells. In particular, PD-1+Vδ1+ cells retained effector responses to TCR signaling that were inhibitable by PD-1 engagement and derepressed by CPI.

Merkel cell polyomavirus-specific and CD39+CLA+ CD8 T cells as blood-based predictive biomarkers for PD-1 blockade in Merkel cell carcinoma

Merkel cell carcinoma is a skin cancer often driven by Merkel cell polyomavirus (MCPyV) with high rates of response to anti-PD-1 therapy despite low mutational burden. MCPyV-specific CD8 T cells are implicated in anti-PD-1-associated immune responses and provide a means to directly study tumor-specific T cell responses to treatment. Using mass cytometry and combinatorial tetramer staining, we find that baseline frequencies of blood MCPyV-specific cells correlated with response and survival. Frequencies of these cells decrease markedly during response to therapy. Phenotypes of MCPyV-specific CD8 T cells have distinct expression patterns of CD39, cutaneous lymphocyte-associated antigen (CLA), and CD103. Correspondingly, overall bulk CD39+CLA+ CD8 T cell frequencies in blood correlate with MCPyV-specific cell frequencies and similarly predicted favorable clinical outcomes. Conversely, frequencies of CD39+CD103+ CD8 T cells are associated with tumor burden and worse outcomes. These cell subsets can be useful as biomarkers and to isolate blood-derived tumor-specific T cells.

Coordinated Immune Cell Networks in the Bone Marrow Microenvironment Define the Graft versus Leukemia Response with Adoptive Cellular Therapy

Understanding how intra-tumoral immune populations coordinate to generate anti-tumor responses following therapy can guide precise treatment prioritization. We performed systematic dissection of an established adoptive cellular therapy, donor lymphocyte infusion (DLI), by analyzing 348,905 single-cell transcriptomes from 74 longitudinal bone-marrow samples of 25 patients with relapsed myeloid leukemia; a subset was evaluated by protein-based spatial analysis. In acute myelogenous leukemia (AML) responders, diverse immune cell types within the bone-marrow microenvironment (BME) were predicted to interact with a clonally expanded population of ZNF683 + GZMB + CD8+ cytotoxic T lymphocytes (CTLs) which demonstrated in vitro specificity for autologous leukemia. This population, originating predominantly from the DLI product, expanded concurrently with NK and B cells. AML nonresponder BME revealed a paucity of crosstalk and elevated TIGIT expression in CD8+ CTLs. Our study highlights recipient BME differences as a key determinant of effective anti-leukemia response and opens new opportunities to modulate cell-based leukemia-directed therapy.

Tissue-Resident Memory T Cell: Ontogenetic Cellular Mechanism and Clinical Translation

Mounting evidence has indicated the essential role of tissue-resident memory T (TRM) cells for frontline protection against viral infection and for cancer immune surveillance (Mueller SN, Mackay LK. Tissue-resident memory T cells: local specialists in immune defense. Nat Rev Immunol 2016, 16, 79-89. doi:10.1038/nri.2015.3.). TRM cells are transcriptionally, phenotypically, and functionally distinct from circulating memory T (Tcirm) cells. It is necessary to understand the unique ontogenetic mechanism, migratory regulation, and biological function of TRM cells. In this review, we discuss recent insights into cellular mechanisms and discrete responsiveness in different tissue microenvironments underlying TRM cell development. We also emphasize the translational potential of TRM cells by focusing on their establishment in association with improved protection in mucosal tissues against various types of diseases and effective strategies for eliciting TRM cells in both pre-clinical and clinical studies.

Egress of resident memory T cells from tissue with neoadjuvant immunotherapy: Implications for systemic anti-tumor immunity

INTRODUCTION

Resident memory T (TRM) cells are embedded in peripheral tissue and capable of acting as sentinels that can respond quickly to repeat pathogen exposure as part of an endogenous anti-microbial immune response. Recent evidence suggests that chronic antigen exposure and other microenvironment cues may promote the development of TRM cells within solid tumors as well, and that this TRM phenotype can sequester tumor-specific T cells into tumors and out of circulation resulting in limited systemic antitumor immunity. Here, we perform a review of the published English literature and describe tissue-specific mediators of TRM cell differentiation in states of infection and malignancy with special focus on the role of TGF-β and how targeting TGF-β signaling could be used as a therapeutical approach to promote tumor systemic immunity.

DISCUSSION

The presence of TRM cells with antigen specificity to neoepitopes in tumors associates with positive clinical prognosis and greater responsiveness to immunotherapy. Recent evidence indicates that solid tumors may act as reservoirs for tumor specific TRM cells and limit their circulation - possibly resulting in impaired systemic antitumor immunity. TRM cells utilize specific mechanisms to egress from peripheral tissues into circulation and other peripheral sites, and emerging evidence indicates that immunotherapeutic approaches may initiate these processes and increase systemic antitumor immunity.

CONCLUSIONS

Reversing tumor sequestration of tumor-specific T cells prior to surgical removal or radiation of tumor may increase systemic antitumor immunity. This finding may underlie the improved recurrence free survival observed with neoadjuvant immunotherapy in clinical trials.

Rethinking IL-1 Antagonism in Respiratory Viral Infections: A Role for IL-1 Signaling in the Development of Antiviral T Cell Immunity

IL-1R integrates signals from IL-1α and IL-1β, and it is widely expressed across tissues and immune cell types. While the expression pattern and function of IL-1R within the innate immune system is well studied, its role in adaptive immunity, particularly within the CD8 T cell compartment, remains underexplored. Here, we show that CD8 T cells dynamically upregulate IL-1R1 levels during priming by APCs, which correlates with their proliferation status and the acquisition of an effector phenotype. Notably, this IL-1 sensitivity persists in memory CD8 T cells of both mice and humans, influencing effector cytokine production upon TCR reactivation. Furthermore, our study highlights that antiviral effector and tissue-resident CD8 T cell responses against influenza A virus infection become impaired in the absence of IL-1 signaling. Altogether, these data support the exploitation of IL-1 activity in the context of T cell vaccination strategies and warrant consideration of the impact of clinical IL-1 inhibition on the rollout of T cell immunity.

Single-cell profiling of muscle-infiltrating T cells in idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIM) are rare autoimmune systemic diseases characterized by muscle weakness and the presence of muscle-infiltrating T cells. IIM represent a clinical challenge due to heterogeneity of symptoms and variability of response to immunosuppressive treatment. Here, we performed in-depth single-cell sequencing on muscle-infiltrating T cells and peripheral blood memory T cells in six patients with recently diagnosed IIM. We identified tissue resident memory T-cell (TRM ) signatures including the expression of HOBIT, XCL1 and CXCR6 in the muscle biopsies of all patients with IIM. Clonally expanded T-cell clones were mainly found among cytotoxic and TRM implying their role in the disease pathogenesis. Finally, identical expanded T-cell clones persisting at follow-up in the muscle tissue of two patients suggest their involvement in disease chronicity. Our study reveals a muscle tissue resident memory T-cell signature in patients with IIM and a transcriptomic map to identify novel therapeutic targets in IIM.

Resident memory T cell development is associated with AP-1 transcription factor upregulation across anatomical niches

Tissue-resident memory T (T RM ) cells play a central role in immune responses to pathogens across all barrier tissues after infection. However, the underlying mechanisms that drive T RM differentiation and priming for their recall effector function remains unclear. In this study, we leveraged both newly generated and publicly available single-cell RNA-sequencing (scRNAseq) data generated across 10 developmental time points to define features of CD8 T RM across both skin and small-intestine intraepithelial lymphocytes (siIEL). We employed linear modeling to capture temporally-associated gene programs that increase their expression levels in T cell subsets transitioning from an effector to a memory T cell state. In addition to capturing tissue-specific gene programs, we defined a consensus T RM signature of 60 genes across skin and siIEL that can effectively distinguish T RM from circulating T cell populations, providing a more specific T RM signature than what was previously generated by comparing bulk T RM to naïve or non-tissue resident memory populations. This updated T RM signature included the AP-1 transcription factor family members Fos, Fosb and Fosl2 . Moreover, ATACseq analysis detected an enrichment of AP-1-specific motifs at open chromatin sites in mature T RM . CyCIF tissue imaging detected nuclear co-localization of AP-1 members Fosb and Junb in resting CD8 T RM >100 days post-infection. Taken together, these results reveal a critical role of AP-1 transcription factor members in T RM biology and suggests a novel mechanism for rapid reactivation of resting T RM in tissue upon antigen encounter.

Preexisting tumor-resident T cells with cytotoxic potential associate with response to neoadjuvant anti-PD-1 in head and neck cancer

About 50% of patients with locally advanced head and neck squamous cell carcinoma (HNSCC) experience recurrences after definitive therapy. The presurgical administration of anti-programmed cell death protein 1 (PD-1) immunotherapy results in substantial pathologic tumor responses (pTR) within the tumor microenvironment (TME). However, the mechanisms underlying the dynamics of antitumor T cells upon neoadjuvant PD-1 blockade remain unresolved, and approaches to increase pathologic responses are lacking. In a phase 2 trial (NCT02296684), we observed that 45% of patients treated with two doses of neoadjuvant pembrolizumab experienced marked pTRs (≥50%). Single-cell analysis of 17,158 CD8+ T cells from 14 tumor biopsies, including 6 matched pre-post neoadjuvant treatment, revealed that responding tumors had clonally expanded putative tumor-specific exhausted CD8+ tumor-infiltrating lymphocytes (TILs) with a tissue-resident memory program, characterized by high cytotoxic potential (CTX+) and ZNF683 expression, within the baseline TME. Pathologic responses after 5 weeks of PD-1 blockade were consistent with activation of preexisting CTX+ZNF683+CD8+ TILs, paralleling loss of viable tumor and associated tumor antigens. Response was associated with high numbers of CD103+PD-1+CD8+ T cells infiltrating pretreatment lesions, whereas revival of nonexhausted persisting clones and clonal replacement were modest. By contrast, nonresponder baseline TME exhibited a relative absence of ZNF683+CTX+ TILs and subsequent accumulation of highly exhausted clones. In HNSCC, revival of preexisting ZNF683+CTX+ TILs is a major mechanism of response in the immediate postneoadjuvant setting.

Homeostatic PD-1 signaling restrains EOMES-dependent oligoclonal expansion of liver-resident CD8 T cells

The co-inhibitory programmed death (PD)-1 signaling pathway plays a major role in the context of tumor-specific T cell responses. Conversely, it also contributes to the maintenance of peripheral tolerance, as patients receiving anti-PD-1 treatment are prone to developing immune-related adverse events. Yet, the physiological role of the PD-1/PDL-1 axis in T cell homeostasis is still poorly understood. Herein, we show that under steady-state conditions, the absence of PD-1 signaling led to a preferential expansion of CD8+ T cells in the liver. These cells exhibit an oligoclonal T cell receptor (TCR) repertoire and a terminally differentiated exhaustion profile. The transcription factor EOMES is required for the clonal expansion and acquisition of this differentiation program. Finally, single-cell transcriptomics coupled with TCR repertoire analysis support the notion that these cells arise locally from liver-resident memory CD8+ T cells. Overall, we show a role for PD-1 signaling in liver memory T cell homeostasis.

Tissue-resident memory T cells and lung immunopathology

Rapid reaction to microbes invading mucosal tissues is key to protect the host against disease. Respiratory tissue-resident memory T (TRM ) cells provide superior immunity against pathogen infection and/or re-infection, due to their presence at the site of pathogen entry. However, there has been emerging evidence that exuberant TRM -cell responses contribute to the development of various chronic respiratory conditions including pulmonary sequelae post-acute viral infections. In this review, we have described the characteristics of respiratory TRM cells and processes underlying their development and maintenance. We have reviewed TRM -cell protective functions against various respiratory pathogens as well as their pathological activities in chronic lung conditions including post-viral pulmonary sequelae. Furthermore, we have discussed potential mechanisms regulating the pathological activity of TRM cells and proposed therapeutic strategies to alleviate TRM -cell-mediated lung immunopathology. We hope that this review provides insights toward the development of future vaccines or interventions that can harness the superior protective abilities of TRM cells, while minimizing the potential for immunopathology, a particularly important topic in the era of coronavirus disease 2019 (COVID-19) pandemic.

Functional heterogeneity of human skin-resident memory T cells in health and disease

The human skin is populated by a diverse pool of memory T cells, which can act rapidly in response to pathogens and cancer antigens. Tissue-resident memory T cells (TRM ) have been implicated in range of allergic, autoimmune and inflammatory skin diseases. Clonal expansion of cells with TRM properties is also known to contribute to cutaneous T-cell lymphoma. Here, we review the heterogeneous phenotypes, transcriptional programs, and effector functions of skin TRM . We summarize recent studies on TRM formation, longevity, plasticity, and retrograde migration and contextualize the findings to skin TRM and their role in maintaining skin homeostasis and altered functions in skin disease.

Cord blood-derived Vδ2+ and Vδ2- T cells acquire differential cell state compositions upon in vitro expansion

Human cord blood-derived γδ T cells (CBγδ) display a highly diverse TCRγδ repertoire and have a unique subtype composition different from fetal or adult peripheral blood counterparts. We expanded CBγδ in vitro using an irradiated Epstein-Barr virus-transformed feeder cell-based modified rapid expansion protocol (REP). Single-cell RNA sequencing tracked progressive differentiation of naïve CBγδ into cells expressing neoantigen-reactive tumor-infiltrating lymphocyte as well as tissue-resident memory precursor-like and antigen-presenting cell-like gene signatures. TCRγδ clonal tracing revealed a bias toward cytotoxic effector differentiation in a much larger proportion of Vδ2- clones compared to Vδ2+ clones, resulting in the former being more cytotoxic at the population level. These clonotype-specific differentiation dynamics were not restricted to REP and were recapitulated upon secondary nonviral antigen stimulations. Thus, our data showed intrinsic cellular differences between major subtypes of human γδ T cells already in operation at early postnatal stage and highlighted key areas of consideration in optimizing cell manufacturing processes.

Dynamics and specificities of T cells in cancer immunotherapy

Recent advances in cancer immunotherapy - ranging from immune-checkpoint blockade therapy to adoptive cellular therapy and vaccines - have revolutionized cancer treatment paradigms, yet the variability in clinical responses to these agents has motivated intense interest in understanding how the T cell landscape evolves with respect to response to immune intervention. Over the past decade, the advent of multidimensional single-cell technologies has provided the unprecedented ability to dissect the constellation of cell states of lymphocytes within a tumour microenvironment. In particular, the rapidly expanding capacity to definitively link intratumoural phenotypes with the antigen specificity of T cells provided by T cell receptors (TCRs) has now made it possible to focus on investigating the properties of T cells with tumour-specific reactivity. Moreover, the assessment of TCR clonality has enabled a molecular approach to track the trajectories, clonal dynamics and phenotypic changes of antitumour T cells over the course of immunotherapeutic intervention. Here, we review the current knowledge on the cellular states and antigen specificities of antitumour T cells and examine how fine characterization of T cell dynamics in patients has provided meaningful insights into the mechanisms underlying effective cancer immunotherapy. We highlight those T cell subsets associated with productive T cell responses and discuss how diverse immunotherapies might leverage the pre-existing tumour-reactive T cell pool or instruct de novo generation of antitumour specificities. Future studies aimed at elucidating the factors associated with the elicitation of productive antitumour T cell immunity are anticipated to instruct the design of more efficacious treatment strategies.

Fibrinogen-like protein 1 promotes liver-resident memory T-cell exhaustion in hepatocellular carcinoma

Background and aims

The key role of tissue-resident memory T (TRM) cells in the immune regulation of hepatocellular carcinoma (HCC) has been investigated and reported, but the regulatory mechanism of tumor microenvironment on TRM cells is still unclear. Lymphocyte activating gene 3 (LAG-3) is a promising next-generation immune checkpoint that is continuously expressed due to persistent antigen exposure in the tumor microenvironment. Fibrinogen-like protein 1 (FGL1) is a classical ligand of LAG-3 and can promote T cell exhaustion in tumors. Here, we excavated the effect of FGL1-LAG3 regulatory axis on TRM cells in HCC.

Methods

The function and phenotype of intrahepatic CD8+ TRM cells in 35 HCC patients were analyzed using multicolor flow cytometry. Using a tissue microarray of 80 HCC patients, we performed the prognosis analysis. Moreover, we investigated the suppressive effect of FGL1 on CD8+ TRM cells both in in vitro induction model and in vivo orthotopic HCC mouse model.

Results

There was an increase in LAG3 expression in CD8+ TRM cells in end-stage HCC; moreover, FGL1 levels were negatively correlated with CD103 expression and related to poor outcomes in HCC. Patients with high CD8+ TRM cell proportions have better outcomes, and FGL1-LAG3 binding could lead to the exhaustion of CD8+ TRM cells in tumors, indicating its potential as a target for immune checkpoint therapy of HCC. Increased FGL1 expression in HCC may result in CD8+ TRM cell exhaustion, causing tumor immune escape.

Conclusions

We identified CD8+TRM cells as a potential immunotherapeutic target and reported the effect of FGL1-LAG3 binding on CD8+ TRM cell function in HCC.

Homologous recombination deficiency in triple-negative breast cancer: Multi-scale transcriptomics reveals distinct tumor microenvironments and limitations in predicting immunotherapy response

BACKGROUND

Triple-negative breast cancer (TNBC) is the most aggressive subtype of breast cancer and has the highest proportion of homologous recombination deficiency (HRD). HRD has been considered a biomarker of response to immune checkpoint inhibitors (ICIs), but the reality is more complicated. A comprehensive comparison of the tumor microenvironment (TME) in HRD and non-HRD TNBC samples may be helpful.

METHODS

Datasets from single-cell, spatial, and bulk RNA-sequencing were collected to explore the role of HRD in the development of TME at multiple scales. Based on the findings in the TME, machine learning algorithms were used to construct a response prediction model in eleven ICI therapy cohorts.

RESULTS

A more exhausted phenotype of T cells and a more tolerogenic phenotype of dendritic cells were found in the non-HRD group. HRD reprograms the predominant phenotype of cancer-associated fibroblasts (CAFs) from myofibroblastic CAFs to inflammatory-like CAFs. As interactions between myofibroblastic CAFs and other cells, DPP4-chemokines associated with reduced immune cell recruitment were unique in the non-HRD group. The prediction model based on DPP4-related genes had acceptable performance in predicting response, prognosis, and immune cell content. Higher HRD scores in bulk RNA-sequencing samples indicated more activated immune cell function, but not higher immune cell content, which may be affected by factors such as antigen-presenting capacity.

CONCLUSIONS

Based on multi-scale transcriptomics, our findings comprehensively reveal differences in the TME between HRD and non-HRD samples. Combining HRD with the prediction model or other methods for assessing immune cell content, may better predict response to ICIs in TNBC.

Sepsis-induced changes in differentiation, maintenance, and function of memory CD8 T cell subsets

Formation of long-lasting memory lymphocytes is one of the foundational characteristics of adaptive immunity and the basis of many vaccination strategies. Following the rapid expansion and contraction of effector CD8 T cells, the surviving antigen (Ag)-specific cells give rise to the memory CD8 T cells that persist for a long time and are phenotypically and functionally distinct from their naïve counterparts. Significant heterogeneity exists within the memory CD8 T cell pool, as different subsets display distinct tissue localization preferences, cytotoxic ability, and proliferative capacity, but all memory CD8 T cells are equipped to mount an enhanced immune response upon Ag re-encounter. Memory CD8 T cells demonstrate numerical stability under homeostatic conditions, but sepsis causes a significant decline in the number of memory CD8 T cells and diminishes their Ag-dependent and -independent functions. Sepsis also rewires the transcriptional profile of memory CD8 T cells, which profoundly impacts memory CD8 T cell differentiation and, ultimately, the protective capacity of memory CD8 T cells upon subsequent stimulation. This review delves into different aspects of memory CD8 T cell subsets as well as the immediate and long-term impact of sepsis on memory CD8 T cell biology.

Small intestine and colon tissue-resident memory CD8+ T cells exhibit molecular heterogeneity and differential dependence on Eomes

Tissue-resident memory CD8+ T (TRM) cells are a subset of memory T cells that play a critical role in limiting early pathogen spread and controlling infection. TRM cells exhibit differences across tissues, but their potential heterogeneity among distinct anatomic compartments within the small intestine and colon has not been well recognized. Here, by analyzing TRM cells from the lamina propria and epithelial compartments of the small intestine and colon, we showed that intestinal TRM cells exhibited distinctive patterns of cytokine and granzyme expression along with substantial transcriptional, epigenetic, and functional heterogeneity. The T-box transcription factor Eomes, which represses TRM cell formation in some tissues, exhibited unexpected context-specific regulatory roles in supporting the maintenance of established TRM cells in the small intestine, but not in the colon. Taken together, these data provide previously unappreciated insights into the heterogeneity and differential requirements for the formation vs. maintenance of intestinal TRM cells.

Microenvironment-driven metabolic adaptations guiding CD8+ T cell anti-tumor immunity

The metabolic stress occurring in the tumor microenvironment (TME) hampers T cell anti-tumor immunity by disturbing T cell metabolic and epigenetic programs. Recent studies are making headway toward identifying strategies to unleash T cell activities by targeting T cell metabolism. Furthermore, efforts have been made to improve the efficacy of immune checkpoint blockade and adoptive cell transfer therapies. However, distinct treatment outcomes across different cancers raise the question of whether our understanding of the features of CD8⁺ T cells within the TME are universal, regardless of their tissue of origin. Here, we review the common and distinct environmental factors affecting CD8⁺ T cells across tumors. Moreover, we discuss how distinct tissue-specific niches are interpreted by CD8⁺ T cells based on studies on tissue-resident memory T (Trm) cells and how these insights can pave the way for a better understanding of the metabolic regulation of CD8⁺ T cell differentiation and anti-tumor immunity.

Unique properties of tissue-resident memory T cells in the lungs: implications for COVID-19 and other respiratory diseases

Tissue-resident memory T (T_RM) cells were originally identified as a tissue-sequestered population of memory T cells that show lifelong persistence in non-lymphoid organs. That definition has slowly evolved with the documentation of T_RM cells having variable terms of tissue residency combined with a capacity to return to the wider circulation. Nonetheless, reductionist experiments have identified an archetypical population of T_RM cells showing intrinsic permanent residency in a wide range of non-lymphoid organs, with one notable exception: the lungs. Despite the fact that memory T cells generated during a respiratory infection are maintained in the circulation, local T_RM cell numbers in the lung decline concomitantly with a decay in T cell-mediated protection. This Perspective describes the mechanisms that underpin long-term T cell lodgement in non-lymphoid tissues and explains why residency is transient for select T_RM cell subsets. In doing so, it highlights the unusual nature of memory T cell egress from the lungs and speculates on the broader disease implications of this process, especially during infection with SARS-CoV-2.

Secondary infections rejuvenate the intestinal CD103 + tissue-resident memory T cell pool

Resident T lymphocytes (T RM ) protect tissues during pathogen reexposure. Although T RM phenotype and restricted migratory pattern are established, we have a limited understanding of their response kinetics, stability, and turnover during reinfections. Such characterizations have been restricted by the absence of in vivo fate-mapping systems. We generated two mouse models, one to stably mark CD103 + T cells (a marker of T RM cells) and the other to specifically deplete CD103 − T cells. Using these models, we observed that intestinal CD103 + T cells became activated during viral or bacterial reinfection, remained organ-confined, and retained their original phenotype but failed to reexpand. Instead, the population was largely rejuvenated by CD103 + T cells formed de novo during reinfections. This pattern remained unchanged upon deletion of antigen-specific circulating T cells, indicating that the lack of expansion was not due to competition with circulating subsets. Thus, although intestinal CD103 + resident T cells survived long term without antigen, they lacked the ability of classical memory T cells to reexpand. This indicated that CD103 + T cell populations could not autonomously maintain themselves. Instead, their numbers were sustained during reinfection via de novo formation from CD103 − precursors. Moreover, in contrast to CD103 - cells, which require antigen plus inflammation for their activation, CD103 + T RM became fully activated follwing exposure to inflammation alone. Together, our data indicate that primary CD103 + resident memory T cells lack secondary expansion potential and require CD103 − precursors for their long-term maintenance.

A molecular signature of lung-resident CD8+ T cells elicited by subunit vaccination

Natural infection as well as vaccination with live or attenuated viruses elicit tissue resident, CD8⁺ memory T cell (Trm) response. Trm cells so elicited act quickly upon reencounter with the priming agent to protect the host. These Trm cells express a unique molecular signature driven by the master regulators-Runx3 and Hobit. We previously reported that intranasal instillation of a subunit vaccine in a prime boost vaccination regimen installed quick-acting, CD8⁺ Trm cells in the lungs that protected against lethal vaccinia virus challenge. It remains unexplored whether CD8⁺ Trm responses so elicited are driven by a similar molecular signature as those elicited by microbes in a real infection or by live, attenuated pathogens in conventional vaccination. We found that distinct molecular signatures distinguished subunit vaccine-elicited lung interstitial CD8⁺ Trm cells from subunit vaccine-elicited CD8⁺ effector memory and splenic memory T cells. Nonetheless, the transcriptome signature of subunit vaccine elicited CD8⁺ Trm resembled those elicited by virus infection or vaccination. Clues to the basis of tissue residence and function of vaccine specific CD8⁺ Trm cells were found in transcripts that code for chemokines and chemokine receptors, purinergic receptors, and adhesins when compared to CD8⁺ effector and splenic memory T cells. Our findings inform the utility of protein-based subunit vaccination for installing CD8⁺ Trm cells in the lungs to protect against respiratory infectious diseases that plague humankind.

Regulation of CD4 T Cell Responses by the Transcription Factor Eomesodermin

Central to the impacts of CD4 T cells, both positive in settings of infectious disease and cancer and negative in the settings of autoimmunity and allergy, is their ability to differentiate into distinct effector subsets with specialized functions. The programming required to support such responses is largely dictated by lineage-specifying transcription factors, often called ‘master regulators’. However, it is increasingly clear that many aspects of CD4 T cell immunobiology that can determine the outcomes of disease states involve a broader transcriptional network. Eomesodermin (Eomes) is emerging as an important member of this class of transcription factors. While best studied in CD8 T cells and NK cells, an increasing body of work has focused on impacts of Eomes expression in CD4 T cell responses in an array of different settings. Here, we focus on the varied impacts reported in these studies that, together, indicate the potential of targeting Eomes expression in CD4 T cells as a strategy to improve a variety of clinical outcomes.

CTLs Get SMAD When Pathogens Tell Them Where to Go

Vaccines protect against infections by eliciting both Ab and T cell responses. Because the immunity wanes as protective epitopes get modified by accruing mutations, developing strategies for immunization against new variants is a major priority for vaccine development. CTLs eliminate cells that support viral replication and provide protection against new variants by targeting epitopes from internal viral proteins. This form of protection has received limited attention during vaccine development, partly because reliable methods for directing pathogen-specific memory CD8 T cells to vulnerable tissues are currently unavailable. In this review we examine how recent studies expand our knowledge of mechanisms that contribute to the functional diversity of CTLs as they respond to infection. We discuss the role of TGF-β and the SMAD signaling cascade during genetic programming of pathogen-specific CTLs and the pathways that promote formation of a newly identified subset of terminally differentiated memory CD8 T cells that localize in the vasculature.

Tissue-resident memory T cells in chronic liver diseases: Phenotype, development and function

Tissue-resident memory (T_RM) T cells are a unique subset of memory T cells that are critical for the first line of defense against pathogens or antigens in peripheral non-lymphoid tissues such as liver, gut, and skin. Generally, T_RM cells are well adapted to the local environment in a tissue-specific manner and typically do not circulate but persist in tissues, distinguishing them from other memory T cell lineages. There is strong evidence that liver T_RM cells provide a robust adaptive immune response to potential threats. Indeed, the potent effector function of hepatic T_RM cells makes it essential for chronic liver diseases, including viral and parasite infection, autoimmune liver diseases (AILD), nonalcoholic fatty liver disease (NAFLD), hepatocellular carcinoma (HCC) and liver transplantation. Manipulation of hepatic T_RM cells might provide novel promising strategies for precision immunotherapy of chronic liver diseases. Here, we provide insights into the phenotype of hepatic T_RM cells through surface markers, transcriptional profiles and effector functions, discuss the development of hepatic T_RM cells in terms of cellular origin and factors affecting their development, analyze the role of hepatic T_RM cells in chronic liver diseases, as well as share our perspectives on the current status of hepatic T_RM cell research.

The evolving role of tissue-resident memory T cells in infections and cancer

Resident memory T cells (TRM) form a distinct type of T memory cells that stably resides in tissues. TRM form an integral part of the immune sensing network and have the ability to control local immune homeostasis and participate in immune responses mediated by pathogens, cancer, and possibly autoantigens during autoimmunity. TRM express residence gene signatures, functional properties of both memory and effector cells, and remarkable plasticity. TRM have a well-established role in pathogen immunity, whereas their role in antitumor immune responses and immunotherapy is currently evolving. As TRM form the most abundant T memory cell population in nonlymphoid tissues, they are attractive targets for therapeutic exploitation. Here, we provide a concise review of the development and physiological role of CD8+ TRM, their involvement in diseases, and their potential therapeutic exploitation.

SMAD4 and TGFβ are architects of inverse genetic programs during fate-determination of antiviral CTLs

Transforming growth factor β (TGFβ) is an important differentiation factor for cytotoxic T lymphocytes (CTLs) and alters the expression levels of several of homing receptors during infection. SMAD4 is part of the canonical signaling network used by members of the transforming growth factor family. For this study, genetically modified mice were used to determine how SMAD4 and TGFβ receptor II (TGFβRII) participate in transcriptional programming of pathogen-specific CTLs. We show that these molecules are essential components of opposing signaling mechanisms, and cooperatively regulate a collection of genes that determine whether specialized populations of pathogen-specific CTLs circulate around the body, or settle in peripheral tissues. TGFβ uses a canonical SMAD-dependent signaling pathway to downregulate Eomesodermin (EOMES), KLRG1, and CD62L, while CD103 is induced. Conversely, in vivo and in vitro data show that EOMES, KLRG1, CX₃CR1, and CD62L are positively regulated via SMAD4, while CD103 and Hobit are downregulated. Intravascular staining also shows that signaling via SMAD4 promotes formation of long-lived terminally differentiated CTLs that localize in the vasculature. Our data show that inflammatory molecules play a key role in lineage determination of pathogen-specific CTLs, and use SMAD-dependent signaling to alter the expression levels of multiple homing receptors and transcription factors with known functions during memory formation.

An Overview of Tissue-Resident Memory T Cells in the Intestine: From Physiological Functions to Pathological Mechanisms

The human intestine contains a complex network of innate and adaptive immune cells that provide protective immunity. The dysfunction of this network may cause various chronic diseases. A large number of T cells in the human intestine have been identified as tissue-resident memory T cells (T_RM). T_RM are present in the peripheral tissues, and they do not recirculate through the blood. It is known that T_RM provide rapid immune responses at the frontline of pathogen invasion. Recent evidence also suggests that these cells play a role in tumor surveillance and the pathogenesis of autoimmune diseases. In this review, we discuss the general features of intestinal T_RM together with their role in intestinal infection, colorectal cancer (CRC), and inflammatory bowel disease (IBD).

Distinct Effector Programs of Brain-Homing CD8+ T Cells in Multiple Sclerosis

The effector programs of CD8⁺ memory T cells are influenced by the transcription factors RUNX3, EOMES and T-bet. How these factors define brain-homing CD8⁺ memory T cells in multiple sclerosis (MS) remains unknown. To address this, we analyzed blood, CSF and brain tissues from MS patients for the impact of differential RUNX3, EOMES and T-bet expression on CD8⁺ T cell effector phenotypes. The frequencies of RUNX3- and EOMES-, but not T-bet-expressing CD8⁺ memory T cells were reduced in the blood of treatment-naïve MS patients as compared to healthy controls. Such reductions were not seen in MS patients treated with natalizumab (anti-VLA-4 Ab). We found an additional loss of T-bet in RUNX3-expressing cells, which was associated with the presence of MS risk SNP rs6672420 (RUNX3). RUNX3⁺EOMES⁺T-bet⁻ CD8⁺ memory T cells were enriched for the brain residency-associated markers CCR5, granzyme K, CD20 and CD69 and selectively dominated the MS CSF. In MS brain tissues, T-bet coexpression was recovered in CD20^dim and CD69⁺ CD8⁺ T cells, and was accompanied by increased coproduction of granzyme K and B. These results indicate that coexpression of RUNX3 and EOMES, but not T-bet, defines CD8⁺ memory T cells with a pre-existing brain residency-associated phenotype such that they are prone to enter the CNS in MS.

Ovarian cancer immunogenicity is governed by a narrow subset of progenitor tissue-resident memory T cells

Despite repeated associations between T cell infiltration and outcome, human ovarian cancer remains poorly responsive to immunotherapy. We report that the hallmarks of tumor recognition in ovarian cancer-infiltrating T cells are primarily restricted to tissue-resident memory (TRM) cells. Single-cell RNA/TCR/ATAC sequencing of 83,454 CD3+CD8+CD103+CD69+ TRM cells and immunohistochemistry of 122 high-grade serous ovarian cancers shows that only progenitor (TCF1low) tissue-resident T cells (TRMstem cells), but not recirculating TCF1+ T cells, predict ovarian cancer outcome. TRMstem cells arise from transitional recirculating T cells, which depends on antigen affinity/persistence, resulting in oligoclonal, trogocytic, effector lymphocytes that eventually become exhausted. Therefore, ovarian cancer is indeed an immunogenic disease, but that depends on ∼13% of CD8+ tumor-infiltrating T cells (∼3% of CD8+ clonotypes), which are primed against high-affinity antigens and maintain waves of effector TRM-like cells. Our results define the signature of relevant tumor-reactive T cells in human ovarian cancer, which could be applicable to other tumors with unideal mutational burden.

Two sides of the same coin: Protective versus pathogenic CD4+ resident memory T cells

The ability of the adaptive immune system to form memory is key to providing protection against secondary infections. Resident memory T cells (T_RM) are specialized T cell populations that reside within tissue sites where they await reencounter with their cognate antigen. T_RM are distinct from circulating memory cells, including central and effector memory T cells, both functionally and transcriptionally. Since the discovery of T_RM, most research has focused on CD8⁺ T_RM, despite that CD4⁺ T_RM are also abundant in most tissues. In the past few years, more evidence has emerged that CD4⁺ T_RM can contribute both protective and pathogenic roles in disease. A complexity inherent to the CD4⁺ T_RM field is the ability of CD4⁺ T cells to polarize into a multitude of distinct subsets and recognize not only viruses and intracellular bacteria but also extracellular bacteria, fungi, and parasites. In this review, we outline the key features of CD4⁺ T_RM in health and disease, including their contributions to protection against SARS-CoV-2 and potential contributions to immunopathology associated with COVID-19.

Hobit and Blimp-1 regulate TRM abundance after LCMV infection by suppressing tissue exit pathways of TRM precursors

Tissue‐resident memory T cells (Trm) are retained in peripheral tissues after infection for enhanced protection against secondary encounter with the same pathogen. We have previously shown that the transcription factor Hobit and its homolog Blimp‐1 drive Trm development after viral infection, but how and when these transcription factors mediate Trm formation remains poorly understood. In particular, the major impact of Blimp‐1 in regulating several aspects of effector T‐cell differentiation impairs study of its specific role in Trm development. Here, we used the restricted expression of Hobit in the Trm lineage to develop mice with a conditional deletion of Blimp‐1 in Trm, allowing us to specifically investigate the role of both transcription factors in Trm differentiation. We found that Hobit and Blimp‐1 were required for the upregulation of CD69 and suppression of CCR7 and S1PR1 on virus‐specific Trm precursors after LCMV infection, underlining a role in their retention within tissues. The early impact of Hobit and Blimp‐1 favored Trm formation and prevented the development of circulating memory T cells. Thus, our findings highlight a role of Hobit and Blimp‐1 at the branching point of circulating and resident memory lineages by suppressing tissue egress of Trm precursors early during infection.

Tissue-resident memory T cells in the urogenital tract

Our understanding of T cell memory responses changed drastically with the discovery that specialized T cell memory populations reside within peripheral tissues at key pathogen entry sites. These tissue-resident memory T (T_RM) cells can respond promptly to an infection without the need for migration, proliferation or differentiation. This rapid and local deployment of effector functions maximizes the ability of T_RM cells to eliminate pathogens. T_RM cells do not circulate through peripheral tissues but instead form isolated populations in the skin, gut, liver, kidneys, the reproductive tract and other organs. This long-term retention in the periphery might allow T_RM cells to fully adapt to the local conditions of their environment and mount customized responses to counter infection and tumour growth in a tissue-specific manner. In the urogenital tract, T_RM cells must adapt to a unique microenvironment to confer protection against potential threats, including cancer and infection, while preventing the onset of auto-inflammatory disease. In this Review, we discuss insights into the diversification of T_RM cells from other memory T cell lineages, the adaptations of T_RM cells to their local environment, and their enhanced capacity to counter infection and tumour growth compared with other memory T cell populations, especially in the urogenital tract.

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.

Malignant and Benign T Cells Constituting Cutaneous T-Cell Lymphoma

Cutaneous T-cell lymphoma (CTCL) is a heterogeneous group of non-Hodgkin lymphoma, including various clinical manifestations, such as mycosis fungoides (MF) and Sézary syndrome (SS). CTCL mostly develops from CD4 T cells with the skin-tropic memory phenotype. Malignant T cells in MF lesions show the phenotype of skin resident memory T cells (T_RM), which reside in the peripheral tissues for long periods and do not recirculate. On the other hand, malignant T cells in SS represent the phenotype of central memory T cells (T_CM), which are characterized by recirculation to and from the blood and lymphoid tissues. The kinetics and the functional characteristics of malignant cells in CTCL are still unclear due, in part, to the fact that both the malignant cells and the T cells exerting anti-tumor activity possess the same characteristics as T cells. Capturing the features of both the malignant and the benign T cells is necessary for understanding the pathogenesis of CTCL and would lead to new therapeutic strategies specifically targeting the skin malignant T cells or benign T cells.

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.