The downregulation of IL-18R defines bona fide kidney-resident CD8+ T cells

T-bet deficiency and Hic1 induction override TGF-β-dependency in the formation of CD103+ intestine-resident memory CD8+ T cells

Transforming growth factor β (TGF-β) represents a well-established signal required for tissue-resident memory T cell (TRM) formation at intestinal surfaces, regulating the expression of a large collection of genes coordinately promoting intestinal TRM differentiation. The functional contribution from each TGF-β-controlled transcription factor is not entirely known. Here, we find that TGF-β-induced T-bet downregulation and Hic1 induction represent two critical events during intestinal TRM differentiation. Importantly, T-bet deficiency significantly rescues intestinal TRM formation in the absence of the TGF-β receptor. Hic1 induction further strengthens TRM maturation in the absence of TGF-β and T-bet. Our results reveal that provision of certain TGF-β-induced molecular events can partially replace TGF-β signaling to promote the establishment of intestinal TRMs, which allows the functional dissection of TGF-β-induced transcriptional targets and molecular mechanisms for TRM differentiation.

Insights into phenotypic and functional CD8+ TRM heterogeneity

Cytotoxic CD8+ T cells recognize and eliminate infected or cancerous cells. A subset of CD8+ memory T cells called tissue-resident memory T cells (TRM ) resides in peripheral tissues, monitors the periphery for pathogen invasion, and offers a rapid and potent first line of defense at potential sites of re-infection. TRM cells are found in almost all tissues and are transcriptionally and epigenetically distinct from circulating memory populations, which shows their ability to acclimate to the tissue environment to allow for long-term survival. Recent work and the broader availability of single-cell profiling have highlighted TRM heterogeneity among different tissues, as well as identified specialized subsets within individual tissues, that are time and infection dependent. TRM cell phenotypic and transcriptional heterogeneity has implications for understanding TRM function and longevity. This review aims to summarize and discuss the latest findings on CD8+ TRM heterogeneity using single-cell molecular profiling and explore the potential implications for immune protection and the design of immune therapies.

Lymphoid tissue residency: A key to understand Tcf-1+PD-1+ T cells

During chronic antigen exposure, a subset of exhausted CD8+ T cells differentiate into stem cell-like or progenitor-like T cells expressing both transcription factor Tcf-1 (T cell factor-1) and co-inhibitory receptor PD-1. These Tcf-1+ stem-like or progenitor exhausted T cells represent the key target for immunotherapies. Deeper understanding of the biology of Tcf-1+PD-1+ CD8+ T cells will lead to rational design of future immunotherapies. Here, we summarize recent findings about the migratory and resident behavior of Tcf-1+ T cells. Specifically, we will focus on TGF-β-dependent lymphoid tissue residency program of Tcf-1+ T cells, which may represent a key to understanding the differentiation and maintenance of Tcf-1+ stem-like CD8+ T cells during persistent antigen stimulation.

TGF-β-dependent lymphoid tissue residency of stem-like T cells limits response to tumor vaccine

TGF-β signaling is necessary for CD8+ T cell differentiation into tissue resident memory T cells (TRM). Although higher frequency of CD8+ TRM cells in the tumor microenvironment is associated with better prognosis, TGF-β-blockade typically improves rather than worsens outcomes. Here we show that in a mouse melanoma model, in the tumor-draining lymph nodes (TDLN) rather than in the tumors themselves, stem-like CD8+ T cells differentiate into TRMs in a TGF-β and tumor antigen dependent manner. Following vaccination against a melanoma-specific epitope, most tumour-specific CD8+ T cells are maintained in a stem-like state, but a proportion of cells lost TRM status and differentiate into CX3CR1+ effector CD8+ T cells in the TDLN, which are subsequently migrating into the tumours. Disruption of TGF-β signaling changes the dynamics of these developmental processes, with the net result of improving effector CD8+ T cell migration into the tumours. In summary, TDLN stem-like T cells transiently switch from a TGF-β-dependent TRM differentiation program to an anti-tumor migratory effector development upon vaccination, which transition can be facilitated by targeted TGF-β blockade.

TGF-β promotes stem-like T cells via enforcing their lymphoid tissue retention

Stem-like CD8+ T cells sustain the antigen-specific CD8+ T cell response during chronic antigen exposure. However, the signals that control the maintenance and differentiation of these cells are largely unknown. Here, we demonstrated that TGF-β was essential for the optimal maintenance of these cells and inhibited their differentiation into migratory effectors during chronic viral infection. Mechanistically, stem-like CD8+ T cells carried a unique expression pattern of α4 integrins (i.e., α4β1hi and α4β7lo) controlled by TGF-β. In the absence of TGF-β signaling, greatly enhanced expression of migration-related markers, including altered expression of α4 integrins, led to enhanced egress of stem-like CD8+ T cells into circulation accompanied by further differentiation into transitional states. Blocking α4 integrin significantly promoted their lymphoid tissue retention and therefore partially rescued the defective maintenance of Tcf-1+ subset in the absence of TGF-β signaling. Thus, TGF-β promotes the maintenance and inhibits the further differentiation of stem-like T cells at least partially via enforcing their lymphoid tissue residency.

Tissue-resident memory CD8+ T cells possess unique transcriptional, epigenetic and functional adaptations to different tissue environments

Tissue-resident memory T cells (T_RM cells) provide protective immunity, but the contributions of specific tissue environments to T_RM cell differentiation and homeostasis are not well understood. In the present study, the diversity of gene expression and genome accessibility by mouse CD8⁺ T_RM cells from distinct organs that responded to viral infection revealed both shared and tissue-specific transcriptional and epigenetic signatures. T_RM cells in the intestine and salivary glands expressed transforming growth factor (TGF)-β-induced genes and were maintained by ongoing TGF-β signaling, whereas those in the fat, kidney and liver were not. Constructing transcriptional-regulatory networks identified the transcriptional repressor Hic1 as a critical regulator of T_RM cell differentiation in the small intestine and showed that Hic1 overexpression enhanced T_RM cell differentiation and protection from infection. Provision of a framework for understanding how CD8⁺ T_RM cells adapt to distinct tissue environments, and identification of tissue-specific transcriptional regulators mediating these adaptations, inform strategies to boost protective memory responses at sites most vulnerable to infection.

Tissue-resident memory T cells in gastrointestinal cancer immunology and immunotherapy: ready for prime time?

Tissue-resident memory T (T_RM) cells have emerged as immune sentinels that patrol the tissue microenvironment and orchestrate localized antitumor immunity in various solid cancers. Recent studies have revealed that T_RM cells are key players in cancer immunosurveillance, and their involvement has been linked to favorable responses to immunotherapy as well as general better clinical outcome in cancer patients. In this review, we provide an overview of the major advances and recent findings regarding T_RM cells phenotype, transcriptional and epigenetic regulation in cancer with a special focus on gastrointestinal tumors. Finally, we highlight the exciting clinical implication of T_RM cells in these types of tumors.

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.

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.