Tissue patrol by resident memory CD8+ T cells in human skin

Biodegradable Silk Fibroin Matrices for Wound Closure in a Human 3D Ex Vivo Approach

In this study, the potential of silk fibroin biomaterials for enhancing wound healing is explored, focusing on their integration into a human 3D ex vivo wound model derived from abdominoplasties. For this purpose, cast silk fibroin membranes and electrospun nonwoven matrices from Bombyx mori silk cocoons were compared to untreated controls over 20 days. Keratinocyte behavior and wound healing were analyzed qualitatively and quantitatively by histomorphometric and immune histochemical methods (HE, Ki67, TUNEL). Findings reveal rapid keratinocyte proliferation on both silk fibroin membrane and nonwoven matrices, along with enhanced infiltration in the matrix, suggesting improved early wound closure. Silk fibroin membranes exhibited a significantly improved early regeneration, followed by nonwoven matrices (p < 0.05) compared to untreated wounds, resulting in the formation of multi-layered epidermal structures with complete regeneration. Overall, the materials demonstrated excellent biocompatibility, supporting cell activity with no signs of increased apoptosis or early degradation. These results underscore silk fibroin's potential in clinical wound care, particularly in tissue integration and re-epithelialization, offering valuable insights for advanced and-as a result of the electrospinning technique-individual wound care development. Furthermore, the use of an ex vivo wound model appears to be a viable option for pre-clinical testing.

Heterogeneity and plasticity of tissue-resident memory T cells in skin diseases and homeostasis: a review

Skin tissue-resident memory T (Trm) cells are produced by antigenic stimulation and remain in the skin for a long time without entering the peripheral circulation. In the healthy state Trm cells can play a patrolling and surveillance role, but in the disease state Trm cells differentiate into various phenotypes associated with different diseases, exhibit different localizations, and consequently have local protective or pathogenic roles, such as disease recurrence in vitiligo and maintenance of immune homeostasis in melanoma. The most common surface marker of Trm cells is CD69/CD103. However, the plasticity of tissue-resident memory T cells after colonization remains somewhat uncertain. This ambiguity is largely due to the variation in the functionality and ultimate destination of Trm cells produced from memory cells differentiated from diverse precursors. Notably, the presence of Trm cells is not stationary across numerous non-lymphoid tissues, most notably in the skin. These cells may reenter the blood and distant tissue sites during the recall response, revealing the recycling and migration potential of the Trm cell progeny. This review focuses on the origin and function of skin Trm cells, and provides new insights into the role of skin Trm cells in the treatment of autoimmune skin diseases, infectious skin diseases, and tumors.

Vitiligo: advances in pathophysiology research and treatment development

The autoimmune condition vitiligo, characterized by skin depigmentation, presents challenges for effective treatment design, with Janus kinase (JAK) inhibitors and other repurposed drugs offering a promising strategy for symptom management. This review explores advantages and shortcomings of current therapies, while presenting the urgent need for further innovative approaches. We emphasize the growing understanding of autoimmune involvement in vitiligo, highlighting several novel treatment avenues including relieving melanocyte stress, preventing dendritic cell activation, halting T cell migration, and suppressing inflammation and autoimmunity. Integrating psychodrama therapy to remediate stress alongside medical interventions marks a holistic approach to enhance patient well-being. The molecular underpinnings of vitiligo care are covered, emphasizing exciting advances revolutionizing vitiligo treatment and improving the quality of life for affected individuals.

Immune cell trafficking: a novel perspective on the gut-skin axis

Immune cell trafficking, an essential mechanism for maintaining immunological homeostasis and mounting effective responses to infections, operates under a stringent regulatory framework. Recent advances have shed light on the perturbation of cell migration patterns, highlighting how such disturbances can propagate inflammatory diseases from their origin to distal organs. This review collates and discusses current evidence that demonstrates atypical communication between the gut and skin, which are conventionally viewed as distinct immunological spheres, in the milieu of inflammation. We focus on the aberrant, reciprocal translocation of immune cells along the gut-skin axis as a pivotal factor linking intestinal and dermatological inflammatory conditions. Recognizing that the translation of these findings into clinical practices is nascent, we suggest that therapeutic strategies aimed at modulating the axis may offer substantial benefits in mitigating the widespread impact of inflammatory diseases.

CD8+ Tissue-Resident Memory T Cells: Versatile Guardians of the Tissue

Tissue-resident memory T (Trm) cells are a subset of T cells maintained throughout life within nonlymphoid tissues without significant contribution from circulating memory T cells. CD8+ Trm cells contribute to both tissue surveillance and direct elimination of pathogens through a variety of mechanisms. Reactivation of these Trm cells during infection drives systematic changes within the tissue, including altering the state of the epithelium, activating local immune cells, and contributing to the permissiveness of the tissue for circulating immune cell entry. Trm cells can be further classified by their functional outputs, which can be either subset- or tissue-specific, and include proliferation, tissue egress, and modulation of tissue physiology. These functional outputs of Trm cells are linked to the heterogeneity and plasticity of this population, and uncovering the unique responses of different Trm cell subsets and their role in immunity will allow us to modulate Trm cell responses for optimal control of disease.

Skin Homeostasis and Repair: A T Lymphocyte Perspective

Chronic, nonhealing wounds remain a clinical challenge and a significant burden for the healthcare system. Skin-resident and infiltrating T cells that recognize pathogens, microbiota, or self-antigens participate in wound healing. A precise balance between proinflammatory T cells and regulatory T cells is required for the stages of wound repair to proceed efficiently. When diseases such as diabetes disrupt the skin microenvironment, T cell activation and function are altered, and wound repair is hindered. Recent studies have used cutting-edge technology to further define the cellular makeup of the skin prior to and during tissue repair. In this review, we discuss key advances that highlight mechanisms used by T cell subsets to populate the epidermis and dermis, maintain skin homeostasis, and regulate wound repair. Advances in our understanding of how skin cells communicate in the skin pave the way for therapeutics that modulate regulatory versus effector functions to improve nonhealing wound treatment.

Skin wound healing effects of (+)-syringaresinol from ginseng berry

Background

Ginseng has been used as a traditional medicine and functional cosmetic ingredients for many years. Recent studies have focused on the potential biological effects of the ginseng berry and its ingredients. (+)-Syringaresinol (SYR) is enriched in ginseng berry and its beneficial effects on the skin have been recently reported. However, little is known about the its effects on the wound healing process of skin.

Methods

Here, we evaluated the skin wound healing effect of (+)-SYR using the human fibroblast Hs68 cell and ex vivo pig and human skin tissue model. Scratch wound test and hydrogen peroxide (HPO) induce chemical wound model were employed.

Results

(+)-SYR promoted the migration and proliferation of Hs68 cells without significant cytotoxicity at the tested concentrations. Especially, in ex vivo pig and human skin tissue, HPO-induced chemical wound was recovered almost completely by (+)-SYR. In line with the finding in Hs68, the protein expression levels of TGF-β and PCNA, a proliferation marker were increased, demonstrating the beneficial effects of (+)-SYR on skin wound repair.

Conclusion

Collectively, we demonstrated that (+)-SYR from ginseng berry, can enhance the wound healing effect by accelerating cell proliferation and skin regeneration, suggesting the potential utility of (+)-SYR for skin wound repair.

Brain resident memory T cells rapidly expand and initiate neuroinflammatory responses following CNS viral infection

The contribution of circulating verses tissue resident memory T cells (TRMs) to clinical neuropathology is an enduring question due to a lack of mechanistic insights. The prevailing view is TRMs are protective against pathogens in the brain. However, the extent to which antigen-specific TRMs induce neuropathology upon reactivation is understudied. Using the described phenotype of TRMs, we found that brains of naïve mice harbor populations of CD69+ CD103- T cells. Notably, numbers of CD69+ CD103- TRMs rapidly increase following neurological insults of various origins. This TRM expansion precedes infiltration of virus antigen-specific CD8 T cells and is due to proliferation of T cells within the brain. We next evaluated the capacity of antigen-specific TRMs in the brain to induce significant neuroinflammation post virus clearance, including infiltration of inflammatory myeloid cells, activation of T cells in the brain, microglial activation, and significant blood brain barrier disruption. These neuroinflammatory events were induced by TRMs, as depletion of peripheral T cells or blocking T cell trafficking using FTY720 did not change the neuroinflammatory course. Depletion of all CD8 T cells, however, completely abrogated the neuroinflammatory response. Reactivation of antigen-specific TRMs in the brain also induced profound lymphopenia within the blood compartment. We have therefore determined that antigen-specific TRMs can induce significant neuroinflammation, neuropathology, and peripheral immunosuppression. The use of cognate antigen to reactivate CD8 TRMs enables us to isolate the neuropathologic effects induced by this cell type independently of other branches of immunological memory, differentiating this work from studies employing whole pathogen re-challenge. This study also demonstrates the capacity for CD8 TRMs to contribute to pathology associated with neurodegenerative disorders and long-term complications associated with viral infections. Understanding functions of brain TRMs is crucial in investigating their role in neurodegenerative disorders including MS, CNS cancers, and long-term complications associated with viral infections including COVID-19.

Cutaneous immune-related adverse events to immune checkpoint inhibitors: from underlying immunological mechanisms to multi-omics prediction

The development of immune checkpoint inhibitors (ICIs) has dramatically altered the landscape of therapy for multiple malignancies, including urothelial carcinoma, non-small cell lung cancer, melanoma and gastric cancer. As part of their anti-tumor properties, ICIs can enhance susceptibility to inflammatory side effects known as immune-related adverse events (irAEs), in which the skin is one of the most commonly and rapidly affected organs. Although numerous questions still remain unanswered, multi-omics technologies have shed light into immunological mechanisms, as well as the correlation between ICI-induced activation of immune systems and the incidence of cirAE (cutaneous irAEs). Therefore, we reviewed integrated biological layers of omics studies combined with clinical data for the prediction biomarkers of cirAEs based on skin pathogenesis. Here, we provide an overview of a spectrum of dermatological irAEs, discuss the pathogenesis of this "off-tumor toxicity" during ICI treatment, and summarize recently investigated biomarkers that may have predictive value for cirAEs via multi-omics approach. Finally, we demonstrate the prognostic significance of cirAEs for immune checkpoint blockades.

FGL2-targeting T cells exhibit antitumor effects on glioblastoma and recruit tumor-specific brain-resident memory T cells

Although tissue-resident memory T (TRM) cells specific for previously encountered pathogens have been characterized, the induction and recruitment of brain TRM cells following immune therapy has not been observed in the context of glioblastoma. Here, we show that T cells expressing fibrinogen-like 2 (FGL2)-specific single-chain variable fragments (T-αFGL2) can induce tumor-specific CD8+ TRM cells that prevent glioblastoma recurrence. These CD8+ TRM cells display a highly expanded T cell receptor repertoire distinct from that found in peripheral tissue. When adoptively transferred to the brains of either immunocompetent or T cell-deficient naïve mice, these CD8+ TRM cells reject glioma cells. Mechanistically, T-αFGL2 cell treatment increased the number of CD69+CD8+ brain-resident memory T cells in tumor-bearing mice via a CXCL9/10 and CXCR3 chemokine axis. These findings suggest that tumor-specific brain-resident CD8+ TRM cells may have promising implications for the prevention of brain tumor recurrence.

Persistent virus-specific and clonally expanded antibody-secreting cells respond to induced self-antigen in the CNS

B cells contribute to the pathogenesis of both cellular- and humoral-mediated central nervous system (CNS) inflammatory diseases through a variety of mechanisms. In such conditions, B cells may enter the CNS parenchyma and contribute to local tissue destruction. It remains unexplored, however, how infection and autoimmunity drive transcriptional phenotypes, repertoire features, and antibody functionality. Here, we profiled B cells from the CNS of murine models of intracranial (i.c.) viral infections and autoimmunity. We identified a population of clonally expanded, antibody-secreting cells (ASCs) that had undergone class-switch recombination and extensive somatic hypermutation following i.c. infection with attenuated lymphocytic choriomeningitis virus (rLCMV). Recombinant expression and characterisation of these antibodies revealed specificity to viral antigens (LCMV glycoprotein GP), correlating with ASC persistence in the brain weeks after resolved infection. Furthermore, these virus-specific ASCs upregulated proliferation and expansion programs in response to the conditional and transient induction of the LCMV GP as a neo-self antigen by astrocytes. This class-switched, clonally expanded, and mutated population persisted and was even more pronounced when peripheral B cells were depleted prior to autoantigen induction in the CNS. In contrast, the most expanded B cell clones in mice with persistent expression of LCMV GP in the CNS did not exhibit neo-self antigen specificity, potentially a consequence of local tolerance induction. Finally, a comparable population of clonally expanded, class-switched, and proliferating ASCs was detected in the cerebrospinal fluid of relapsing multiple sclerosis (RMS) patients. Taken together, our findings support the existence of B cells that populate the CNS and are capable of responding to locally encountered autoantigens.

Decoding Roles of Exosomal lncRNAs in Tumor-Immune Regulation and Therapeutic Potential

Exosomes are nanovesicles secreted into biofluids by various cell types and have been implicated in different physiological and pathological processes. Interestingly, a plethora of studies emphasized the mediating role of exosomes in the bidirectional communication between donor and recipient cells. Among the various cargoes of exosomes, long non-coding RNAs (lncRNAs) have been identified as crucial regulators between cancer cells and immune cells in the tumor microenvironment (TME) that can interfere with innate and adaptive immune responses to affect the therapeutic efficiency. Recently, a few major studies have focused on the exosomal lncRNA-mediated interaction between cancer cells and immune cells infiltrated into TME. Nevertheless, a dearth of studies pertains to the immune regulating role of exosomal lncRNAs in cancer and is still in the early stages. Comprehensive mechanisms of exosomal lncRNAs in tumor immunity are not well understood. Herein, we provide an overview of the immunomodulatory function of exosomal lncRNAs in cancer and treatment resistance. In addition, we also summarize the potential therapeutic strategies toward exosomal lncRNAs in TME.

The Human Dermis as a Target of Nanoparticles for Treating Skin Conditions

Skin has a preventive role against any damage raised by harmful microorganisms and physical and chemical assaults from the external environment that could affect the body's internal organs. Dermis represents the main section of the skin, and its contribution to skin physiology is critical due to its diverse cellularity, vasculature, and release of molecular mediators involved in the extracellular matrix maintenance and modulation of the immune response. Skin structure and complexity limit the transport of substances, promoting the study of different types of nanoparticles that penetrate the skin layers under different mechanisms intended for skin illness treatments and dermo-cosmetic applications. In this work, we present a detailed morphological description of the dermis in terms of its structures and resident cells. Furthermore, we analyze the role of the dermis in regulating skin homeostasis and its alterations in pathophysiological conditions, highlighting its potential as a therapeutic target. Additionally, we describe the use of nanoparticles for skin illness treatments focused on dermis release and promote the use of metal-organic frameworks (MOFs) as an integrative strategy for skin treatments.

Keeping T cell memories in mind

The mammalian central nervous system (CNS) contains a vibrant community of resident adaptive immune cells at homeostasis. Among these are memory CD8+ and CD4+ T cells, which reside in the CNS in the settings of health, aging, and neurological disease. These T cells commonly exhibit a tissue-resident memory (TRM) phenotype, suggesting that they are antigen-experienced and remain separate from the circulation. Despite these characterizations, T cell surveillance of the CNS has only recently been studied through the lens of TRM immunology. In this Review, we outline emerging concepts of CNS TRM generation, localization, maintenance, function, and specificity. In this way, we hope to highlight roles of CNS TRM in health and disease to inform future studies of adaptive neuroimmunity.

New definitions of human lymphoid and follicular cell entities in lymphatic tissue by machine learning

Histological sections of the lymphatic system are usually the basis of static (2D) morphological investigations. Here, we performed a dynamic (4D) analysis of human reactive lymphoid tissue using confocal fluorescent laser microscopy in combination with machine learning. Based on tracks for T-cells (CD3), B-cells (CD20), follicular T-helper cells (PD1) and optical flow of follicular dendritic cells (CD35), we put forward the first quantitative analysis of movement-related and morphological parameters within human lymphoid tissue. We identified correlations of follicular dendritic cell movement and the behavior of lymphocytes in the microenvironment. In addition, we investigated the value of movement and/or morphological parameters for a precise definition of cell types (CD clusters). CD-clusters could be determined based on movement and/or morphology. Differentiating between CD3- and CD20 positive cells is most challenging and long term-movement characteristics are indispensable. We propose morphological and movement-related prototypes of cell entities applying machine learning models. Finally, we define beyond CD clusters new subgroups within lymphocyte entities based on long term movement characteristics. In conclusion, we showed that the combination of 4D imaging and machine learning is able to define characteristics of lymphocytes not visible in 2D histology.

Human T lymphocytes at tumor sites

CD4⁺ and CD8⁺ T lymphocytes mediate most of the adaptive immune response against tumors. Naïve T lymphocytes specific for tumor antigens are primed in lymph nodes by dendritic cells. Upon activation, antigen-specific T cells proliferate and differentiate into effector cells that migrate out of peripheral blood into tumor sites in an attempt to eliminate cancer cells. After accomplishing their function, most effector T cells die in the tissue, while a small fraction of antigen-specific T cells persist as long-lived memory cells, circulating between peripheral blood and lymphoid tissues, to generate enhanced immune responses when re-encountering the same antigen. A subset of memory T cells, called resident memory T (T_RM) cells, stably resides in non-lymphoid peripheral tissues and may provide rapid immunity independently of T cells recruited from blood. Being adapted to the tissue microenvironment, T_RM cells are potentially endowed with the best features to protect against the reemergence of cancer cells. However, when tumors give clinical manifestation, it means that tumor cells have evaded immune surveillance, including that of T_RM cells. Here, we review the current knowledge as to how T_RM cells are generated during an immune response and then maintained in non-lymphoid tissues. We then focus on what is known about the role of CD4⁺ and CD8⁺ T_RM cells in antitumor immunity and their possible contribution to the efficacy of immunotherapy. Finally, we highlight some open questions in the field and discuss how new technologies may help in addressing them.

(Not) Home alone: Antigen presenting cell - T Cell communication in barrier tissues

Priming of T cells by antigen presenting cells (APCs) is essential for T cell fate decisions, enabling T cells to migrate to specific tissues to exert their effector functions. Previously, these interactions were mainly explored using blood-derived cells or animal models. With great advances in single cell RNA-sequencing techniques enabling analysis of tissue-derived cells, it has become clear that subsets of APCs are responsible for priming and modulating heterogeneous T cell effector responses in different tissues. This composition of APCs and T cells in tissues is essential for maintaining homeostasis and is known to be skewed in infection and inflammation, leading to pathological T cell responses. This review highlights the commonalities and differences of T cell priming and subsequent effector function in multiple barrier tissues such as the skin, intestine and female reproductive tract. Further, we provide an overview of how this process is altered during tissue-specific infections which are known to cause chronic inflammation and how this knowledge could be harnessed to modify T cell responses in barrier tissue.

Isolation of Lymphocytes from Human Skin and Murine Tissues: A Rapid and Epitope-Preserving Approach

Tissue-resident immune cells have been shown to play an important role in skin health and disease. However, owing to limited access to human skin samples and time-consuming, technically demanding protocols, the characterization of tissue-derived cells remains challenging. For this reason, blood-derived leukocytes are frequently used as a surrogate specimen, although they do not necessarily reflect local immune responses in the skin. Therefore, we aimed to establish a rapid protocol to isolate a sufficient number of viable immune cells from 4-mm skin biopsies that can be directly used for a deeper characterization such as comprehensive phenotyping and functional studies of T cells. In this optimized protocol, only two enzymes, type IV collagenase and DNase I, were used to achieve both the highest possible cellular yield and marker preservation of leukocytes stained for multicolor flow cytometry. We further report that the optimized protocol may be used in the same manner for murine skin and mucosa. In summary, this study allows a rapid acquisition of lymphocytes from human or mouse skin suitable for comprehensive analysis of lymphocyte subpopulations, for disease surveillance, and for identification of potential therapeutic targets or other downstream applications.

Host-versus-commensal immune responses participate in the rejection of colonized solid organ transplants

Solid organ transplantation is the preferred treatment for end-stage organ failure. Although transplant recipients take life-long immunosuppressive drugs, a substantial percentage of them still reject their allografts. Strikingly, barrier organs colonized with microbiota have significantly shorter half-lives than non-barrier transplanted organs, even in immunosuppressed hosts. We previously demonstrated that skin allografts monocolonized with the common human commensal Staphylococcus epidermidis (S.epi) are rejected faster than germ-free (GF) allografts in mice because the presence of S.epi augments the effector alloimmune response locally in the graft. Here, we tested whether host immune responses against graft-resident commensal microbes, including S.epi, can damage colonized grafts independently from the alloresponse. Naive hosts mounted an anticommensal T cell response to colonized, but not GF, syngeneic skin grafts. Whereas naive antigraft commensal T cells modestly damaged colonized syngeneic skin grafts, hosts with prior anticommensal T cell memory mounted a post-transplant immune response against graft-resident commensals that significantly damaged colonized, syngeneic skin grafts. Importantly, allograft recipients harboring this host-versus-commensal immune response resisted immunosuppression. The dual effects of host-versus-commensal and host-versus-allograft responses may partially explain why colonized organs have poorer outcomes than sterile organs in the clinic.

Characterization of human epithelial resident memory regulatory T cells

Human resident memory regulatory T cells (Tregs) exist in the normal, noninflamed skin. Except one, all previous studies analyzed skin Tregs using full-thickness human skin. Considering that thick dermis contains more Tregs than thin epidermis, the current understanding of skin Tregs might be biased toward dermal Tregs. Therefore, we sought to determine the phenotype and function of human epidermal and epithelial Tregs. Human epidermis and epithelium were allowed to float on a medium without adding any exogenous cytokines and stimulations for two days and then emigrants from the explants were analyzed. Foxp3 was selectively expressed in CD4+CD103− T cells in the various human epithelia, as it is highly demethylated. CD4+CD103−Foxp3+ cells suppressed proliferation of other resident memory T cells. The generation and maintenance of epithelial Tregs were independent of hair density and Langerhans cells. Collectively, immune-suppressive CD4+CD103−Foxp3+ Tregs are present in the normal, noninflamed human epidermis and mucosal epithelia.

Phenotypic and Immunometabolic Aspects on Stem Cell Memory and Resident Memory CD8+ T Cells

The immune system, smartly and surprisingly, saves the exposure of a particular pathogen in its memory and reacts to the pathogen very rapidly, preventing serious diseases.

Immunologists have long been fascinated by understanding the ability to recall and respond faster and more vigorously to a pathogen, known as “memory”.

T-cell populations can be better described by using more sophisticated techniques to define phenotype, transcriptional and epigenetic signatures and metabolic pathways (single-cell resolution), which uncovered the heterogeneity of the memory T-compartment. Phenotype, effector functions, maintenance, and metabolic pathways help identify these different subsets. Here, we examine recent developments in the characterization of the heterogeneity of the memory T cell compartment. In particular, we focus on the emerging role of CD8+ TRM and TSCM cells, providing evidence on how their immunometabolism or modulation can play a vital role in their generation and maintenance in chronic conditions such as infections or autoimmune diseases.

Tissue-Resident Immune Cells in Humans

Tissue-resident immune cells span both myeloid and lymphoid cell lineages, have been found in multiple human tissues, and play integral roles at all stages of the immune response, from maintaining homeostasis to responding to infectious challenges to resolution of inflammation to tissue repair. In humans, studying immune cells and responses in tissues is challenging, although recent advances in sampling and high-dimensional profiling have provided new insights into the ontogeny, maintenance, and functional role of tissue-resident immune cells. Each tissue contains a specific complement of resident immune cells. Moreover, resident immune cells for each lineage share core properties, along with tissue-specific adaptations. Here we propose a five-point checklist for defining resident immune cell types in humans and describe the currently known features of resident immune cells, their mechanisms of development, and their putative functional roles within various human organs. We also consider these aspects of resident immune cells in the context of future studies and therapeutics.

Patho-immunological mechanisms of vitiligo: the role of the innate and adaptive immunities and environmental stress factors

Epidermal melanocyte loss in vitiligo, triggered by stresses ranging from trauma to emotional stress, chemical exposure or metabolite imbalance, to the unknown, can stimulate oxidative stress in pigment cells, which secrete damage-associated molecular patterns that then initiate innate immune responses. Antigen presentation to melanocytes leads to stimulation of autoreactive T-cell responses, with further targeting of pigment cells. Studies show a pathogenic basis for cellular stress, innate immune responses and adaptive immunity in vitiligo. Improved understanding of the aetiological mechanisms in vitiligo has already resulted in successful use of the Jak inhibitors in vitiligo. In this review, we outline the current understanding of the pathological mechanisms in vitiligo and locate loci to which therapeutic attack might be directed.

Building vs. Rebuilding Epidermis: Comparison Embryonic Development and Adult Wound Repair

Wound repair is essential to restore tissue function through the rebuilding of pre-existing structures. The repair process involves the re-formation of tissue, which was originally generated by embryonic development, with as similar a structure as possible. Therefore, these two processes share many similarities in terms of creating tissue architecture. However, fundamental differences still exist, such as differences in the cellular components, the status of neighboring tissues, and the surrounding environment. Recent advances in single-cell transcriptomics, in vivo lineage tracing, and intravital imaging revealed subpopulations, long-term cell fates, and dynamic cellular behaviors in live animals that were not detectable previously. This review highlights similarities and differences between adult wound repair and embryonic tissue development with a particular emphasis on the epidermis of the skin.

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.

CD8αα+T cells exert a pro-inflammatory role in patients with psoriasis

Background

Psoriasis is a common chronic inflammatory disease caused by excessive activation of CD4⁺T cells, including Th17, Th1 and Th22. The role of CD8⁺T cells in psoriasis pathogenesis remains poorly understood.

Aim

To identify the phenotype of CD8⁺T cells in patients with psoriasis and to investigate its role in the formation of lesions.

Methods

The phenotype of CD8⁺T cells in psoriatic lesions was detected by immunofluorescence staining. Flow cytometry was performed to detect their phenotype in peripheral blood. Thereafter, coculture of CD8αα⁺T cells with autogenous CD4⁺T cells was performed to investigate the function of CD8αα⁺T cells in patients with psoriasis. Finally, pro‐inflammatory factors produced by CD8αα⁺T cells were examined by immunofluorescence staining and flow cytometry.

Results

Compared to the CD8αβ⁺T cells, CD8αα⁺T cell infiltration in psoriatic lesions markedly increased. Moreover, epidermal CD8αα⁺T cells exhibited tissue‐resident memory T cells (T_RM) phenotypes and dermal CD8αα⁺T cells exhibited effector memory (T_EM) phenotypes in psoriatic lesions. Additionally, we found that CD8αα⁺T cells from patients with psoriasis did not express the markers of regulatory T cells and could promote the proliferation of CD4⁺T effector cells and produce interleukin‐17 and interferon‐γ.

Conclusions

Our findings demonstrate that CD8αα⁺T cells contribute to the pathogenesis of psoriasis by producing pro‐inflammatory factors.

Single-cell analysis of regions of interest (SCARI) using a photosensitive tag

The functional activity and differentiation potential of cells are determined by their interactions with surrounding cells. Approaches that allow unbiased characterization of cell states while at the same time providing spatial information are of major value to assess this environmental influence. However, most current techniques are hampered by a tradeoff between spatial resolution and cell profiling depth. Here, we develop a photocage-based technology that allows isolation and in-depth analysis of live cells from regions of interest in complex ex vivo systems, including primary human tissues. The use of a highly sensitive 4-nitrophenyl(benzofuran) cage coupled to a set of nanobodies allows high-resolution photo-uncaging of different cell types in areas of interest. Single-cell RNA-sequencing of spatially defined CD8+ T cells is used to exemplify the feasibility of identifying location-dependent cell states. The technology described here provides a valuable tool for the analysis of spatially defined cells in diverse biological systems, including clinical samples.

Diversity of T cells in the skin: Novel insights

T cells populate the skin to provide an effective immunosurveillance against external insults and to maintain tissue homeostasis. Most cutaneous T cells are αβ T cells, however, γδ T cells also exist although in much lower frequency. Different subsets of αβ T cells can be found in the skin, such as short-lived effector T cells, central memory T cells, effector memory T cells, and tissue-resident memory T cells. Their differential biology, function, and location provide an ample spectrum of immune responses in the skin. Foxp3+ memory regulatory T cells have a pivotal role in maintaining homeostasis in the skin and their dysregulation has been linked with different skin pathologies. The skin also contains populations of non-classical T cells, such as γδ T cells, NK T cells, and MR1-restricted T cells. Their role in skin homeostasis and response to pathogens has been well established in the past years, however, there is also growing evidence of their role in mediating allergic skin inflammation and promoting sensitization to allergens. In this review, we provide an updated overview on the different subsets of T cells that populate the skin with a specific focus on their role in allergic skin inflammation.

Skin Resident Memory T Cells May Play Critical Role in Delayed-Type Drug Hypersensitivity Reactions

Delayed-type drug hypersensitivity reactions (dtDHR) are immune-mediated reactions with skin and visceral manifestations ranging from mild to severe. Clinical care is negatively impacted by a limited understanding of disease pathogenesis. Though T cells are believed to orchestrate disease, the type of T cell and the location and mechanism of T cell activation remain unknown. Resident memory T cells (T_RM) are a unique T cell population potentially well situated to act as key mediators in disease pathogenesis, but significant obstacles to defining, identifying, and testing T_RM in dtDHR preclude definitive conclusions at this time. Deeper mechanistic interrogation to address these unanswered questions is necessary, as involvement of T_RM in disease has significant implications for prediction, diagnosis, and treatment of disease.

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.

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.

An ex vivo tumor fragment platform to dissect response to PD-1 blockade in cancer

Inhibitors of the PD-1-PD-L1 axis have been approved as therapy for many human cancers. In spite of the evidence for their widespread clinical activity, little is known about the immunological alterations that occur in human cancer tissue after PD-1 blockade. We developed and employed a patient-derived tumor fragment platform to dissect the early immunological response of human tumor tissue to ex vivo PD-1 blockade. We observed that the capacity of immune cells to be reactivated ex vivo was predictive of clinical response, and perturbation analyses identified tumor-resident T cells as a key component of this immunological response. In addition, through combined analysis of baseline properties and immune response capacity, we identified a new subgroup of infiltrated tumors that lacks the capacity to respond to PD-1 blockade. Finally, the baseline presence of tertiary lymphoid structures and their components correlated with the capacity of cancers to undergo intratumoral immune cell reactivation.

T-Cell Adhesion in Healthy and Inflamed Skin

The diverse populations of tissue-resident and transitory T cells present in the skin share a common functional need to enter, traverse, and interact with their environment. These processes are largely dependent on the regulated expression of adhesion molecules, such as selectins and integrins, which mediate bidirectional interactions between immune cells and skin stroma. Dysregulation and engagement of adhesion pathways contribute to ectopic T-cell activity in tissues, leading to the initiation and/or exacerbation of chronic inflammation. In this paper, we review how the molecular interactions supported by adhesion pathways contribute to T-cell dynamics and function in the skin. A comprehensive understanding of the molecular mechanisms underpinning T-cell adhesion in inflammatory skin disorders will facilitate the development of novel tissue-specific therapeutic strategies.

Epidermal T cell subsets-Effect of age and antigen exposure in humans and mice

BACKGROUND

Epidermal T cells play a central role in immune surveillance and in inflammatory skin diseases. Major differences in the epidermal T cell composition are found between adult humans and antigen-inexperienced laboratory mice. Whether this is due to inborn species differences, to different environmental exposures, or a combination of the two is a matter of debate.

OBJECTIVES

To investigate the role of age and exposure to antigens on epidermal T cell subsets in human and mouse skin.

METHODS

We isolated T cells from the epidermis from 19 infants and 26 adults, and determined the frequency of CD4+ and CD8+ αβ T cells and γδ T cells by flow cytometry. In addition, we determined the epidermal T cell composition in antigen-inexperienced and antigen-experienced mice.

RESULTS

We found that humans are born with very few epidermal T cells. The number increases and the composition changes with age. In antigen-inexperienced mice, the epidermal T cell composition is unaffected by age, but it is dramatically affected by antigen exposure.

CONCLUSION

Taken together, we show that antigen exposure, as opposed to age, is the major factor determining the composition of epidermal T cells, suggesting that the skin of antigen-experienced mice better reflects the immunological conditions in human skin.

Promises and challenges of adoptive T-cell therapies for solid tumours

Cancer is a leading cause of death worldwide and, despite new targeted therapies and immunotherapies, many patients with advanced-stage- or high-risk cancers still die, owing to metastatic disease. Adoptive T-cell therapy, involving the autologous or allogeneic transplant of tumour-infiltrating lymphocytes or genetically modified T cells expressing novel T-cell receptors or chimeric antigen receptors, has shown promise in the treatment of cancer patients, leading to durable responses and, in some cases, cure. Technological advances in genomics, computational biology, immunology and cell manufacturing have brought the aspiration of individualised therapies for cancer patients closer to reality. This new era of cell-based individualised therapeutics challenges the traditional standards of therapeutic interventions and provides opportunities for a paradigm shift in our approach to cancer therapy. Invited speakers at a 2020 symposium discussed three areas-cancer genomics, cancer immunology and cell-therapy manufacturing-that are essential to the effective translation of T-cell therapies in the treatment of solid malignancies. Key advances have been made in understanding genetic intratumour heterogeneity, and strategies to accurately identify neoantigens, overcome T-cell exhaustion and circumvent tumour immunosuppression after cell-therapy infusion are being developed. Advances are being made in cell-manufacturing approaches that have the potential to establish cell-therapies as credible therapeutic options. T-cell therapies face many challenges but hold great promise for improving clinical outcomes for patients with solid tumours.

Immunological memory in rheumatic inflammation - a roadblock to tolerance induction

Why do we still have no cure for chronic inflammatory diseases? One reason could be that current therapies are based on the assumption that chronic inflammation is driven by persistent 'acute' immune reactions. Here we discuss a paradigm shift by suggesting that beyond these reactions, chronic inflammation is driven by imprinted, pathogenic 'memory' cells of the immune system. This rationale is based on the observation that in patients with chronic inflammatory rheumatic diseases refractory to conventional immunosuppressive therapies, therapy-free remission can be achieved by resetting the immune system; that is, by ablating immune cells and regenerating the immune system from stem cells. The success of this approach identifies antigen-experienced and imprinted immune cells as essential and sufficient drivers of inflammation. The 'dark side' of immunological memory primarily involves memory plasma cells secreting pathogenic antibodies and memory T lymphocytes secreting pathogenic cytokines and chemokines, but can also involve cells of innate immunity. New therapeutic strategies should address the persistence of these memory cells. Selective targeting of pathogenic immune memory cells could be based on their specificity, which is challenging, or on their lifestyle, which differs from that of protective immune memory cells, in particular for pathogenic T lymphocytes. The adaptations of such pathogenic memory cells to chronic inflammation offers entirely new therapeutic options for their selective ablation and the regeneration of immunological tolerance.

Advances in the development of personalized neoantigen-based therapeutic cancer vaccines

Within the past decade, the field of immunotherapy has revolutionized the treatment of many cancers with the development and regulatory approval of various immune-checkpoint inhibitors and chimeric antigen receptor T cell therapies in diverse indications. Another promising approach to cancer immunotherapy involves the use of personalized vaccines designed to trigger de novo T cell responses against neoantigens, which are highly specific to tumours of individual patients, in order to amplify and broaden the endogenous repertoire of tumour-specific T cells. Results from initial clinical studies of personalized neoantigen-based vaccines, enabled by the availability of rapid and cost-effective sequencing and bioinformatics technologies, have demonstrated robust tumour-specific immunogenicity and preliminary evidence of antitumour activity in patients with melanoma and other cancers. Herein, we provide an overview of the complex process that is necessary to generate a personalized neoantigen vaccine, review the types of vaccine-induced T cells that are found within tumours and outline strategies to enhance the T cell responses. In addition, we discuss the current status of clinical studies testing personalized neoantigen vaccines in patients with cancer and considerations for future clinical investigation of this novel, individualized approach to immunotherapy.

Advances in the development of personalized neoantigen-based therapeutic cancer vaccines

Within the past decade, the field of immunotherapy has revolutionized the treatment of many cancers with the development and regulatory approval of various immune-checkpoint inhibitors and chimeric antigen receptor T cell therapies in diverse indications. Another promising approach to cancer immunotherapy involves the use of personalized vaccines designed to trigger de novo T cell responses against neoantigens, which are highly specific to tumours of individual patients, in order to amplify and broaden the endogenous repertoire of tumour-specific T cells. Results from initial clinical studies of personalized neoantigen-based vaccines, enabled by the availability of rapid and cost-effective sequencing and bioinformatics technologies, have demonstrated robust tumour-specific immunogenicity and preliminary evidence of antitumour activity in patients with melanoma and other cancers. Herein, we provide an overview of the complex process that is necessary to generate a personalized neoantigen vaccine, review the types of vaccine-induced T cells that are found within tumours and outline strategies to enhance the T cell responses. In addition, we discuss the current status of clinical studies testing personalized neoantigen vaccines in patients with cancer and considerations for future clinical investigation of this novel, individualized approach to immunotherapy.

Tissue-resident memory T cells in tumor immunity and immunotherapy

Tissue-resident memory T cells (T_RM) represent a heterogeneous T cell population with the functionality of both effector and memory T cells. T_RM express residence gene signatures. This feature allows them to traffic to, reside in, and potentially patrol peripheral tissues, thereby enforcing an efficient long-term immune-protective role. Recent studies have revealed T_RM involvement in tumor immune responses. T_RM tumor infiltration correlates with enhanced response to current immunotherapy and is often associated with favorable clinical outcome in patients with cancer. Thus, targeting T_RM may lead to enhanced cancer immunotherapy efficacy. Here, we review and discuss recent advances on the nature of T_RM in the context of tumor immunity and immunotherapy.

Organ-Specific Surveillance and Long-Term Residency Strategies Adapted by Tissue-Resident Memory CD8+ T Cells

Tissue-resident CD8⁺ T cells (CD8⁺ T_RM) populate lymphoid and non-lymphoid tissues after infections as first line of defense against re-emerging pathogens. To achieve host protection, CD8⁺ T_RM have developed surveillance strategies that combine dynamic interrogation of pMHC complexes on local stromal and hematopoietic cells with long-term residency. Factors mediating CD8⁺ T_RM residency include CD69, a surface receptor opposing the egress-promoting S1P1, CD49a, a collagen-binding integrin, and CD103, which binds E-cadherin on epithelial cells. Moreover, the topography of the tissues of residency may influence T_RM retention and surveillance strategies. Here, we provide a brief summary of these factors to examine how CD8⁺ T_RM reconcile constant migratory behavior with their long-term commitment to local microenvironments, with a focus on epithelial barrier organs and exocrine glands with mixed connective-epithelial tissue composition.

Pathophysiology of Skin Resident Memory T Cells

Tissue resident memory T (T_RM) cells reside in peripheral, non-lymphoid tissues such as the skin, where they act as alarm-sensor cells or cytotoxic cells. Physiologically, skin T_RM cells persist for a long term and can be reactivated upon reinfection with the same antigen, thus serving as peripheral sentinels in the immune surveillance network. CD8⁺CD69⁺CD103⁺ T_RM cells are the well-characterized subtype that develops in the epidermis. The local mediators such as interleukin (IL)-15 and transforming growth factor (TGF)-β are required for the formation of long-lived T_RM cell population in skin. Skin T_RM cells engage virus-infected cells, proliferate in situ in response to local antigens and do not migrate out of the epidermis. Secondary T_RM cell populations are derived from pre-existing T_RM cells and newly recruited T_RM precursors from the circulation. In addition to microbial pathogens, topical application of chemical allergen to skin causes delayed-type hypersensitivity and amplifies the number of antigen-specific CD8⁺ T_RM cells at challenged site. Skin T_RM cells are also involved in the pathological conditions, including vitiligo, psoriasis, fixed drug eruption and cutaneous T-cell lymphoma (CTCL). The functions of these T_RM cells seem to be different, depending on each pathology. Psoriasis plaques are seen in a recurrent manner especially at the originally affected sites. Upon stimulation of the skin of psoriasis patients, the CD8⁺CD103⁺CD49a^- T_RM cells in the epidermis seem to be reactivated and initiate IL-17A production. Meanwhile, autoreactive CD8⁺CD103⁺CD49a⁺ T_RM cells secreting interferon-γ are present in lesional vitiligo skin. Fixed drug eruption is another disease where skin T_RM cells evoke its characteristic clinical appearance upon administration of a causative drug. Intraepidermal CD8⁺ T_RM cells with an effector-memory phenotype resident in the skin lesions of fixed drug eruption play a major contributing role in the development of localized tissue damage. CTCL develops primarily in the skin by a clonal expansion of a transformed T_RM cells. CD8⁺ CTCL with the pagetoid epidermotropic histology is considered to originate from epidermal CD8⁺ T_RM cells. This review will discuss the current understanding of skin T_RM biology and their contribution to skin homeostasis and diseases.

Interplay between ESKAPE Pathogens and Immunity in Skin Infections: An Overview of the Major Determinants of Virulence and Antibiotic Resistance

The skin is the largest organ in the human body, acting as a physical and immunological barrier against pathogenic microorganisms. The cutaneous lesions constitute a gateway for microbial contamination that can lead to chronic wounds and other invasive infections. Chronic wounds are considered as serious public health problems due the related social, psychological and economic consequences. The group of bacteria known as ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter sp.) are among the most prevalent bacteria in cutaneous infections. These pathogens have a high level of incidence in hospital environments and several strains present phenotypes of multidrug resistance. In this review, we discuss some important aspects of skin immunology and the involvement of ESKAPE in wound infections. First, we introduce some fundamental aspects of skin physiology and immunology related to cutaneous infections. Following this, the major virulence factors involved in colonization and tissue damage are highlighted, as well as the most frequently detected antimicrobial resistance genes. ESKAPE pathogens express several virulence determinants that overcome the skin's physical and immunological barriers, enabling them to cause severe wound infections. The high ability these bacteria to acquire resistance is alarming, particularly in the hospital settings where immunocompromised individuals are exposed to these pathogens. Knowledge about the virulence and resistance markers of these species is important in order to develop new strategies to detect and treat their associated infections.

Skin T cells maintain their diversity and functionality in the elderly

Recent studies have highlighted that human resident memory T cells (T_RM) are functionally distinct from circulating T cells. Thus, it can be postulated that skin T cells age differently from blood-circulating T cells. We assessed T-cell density, diversity, and function in individuals of various ages to study the immunologic effects of aging on human skin from two different countries. No decline in the density of T cells was noted with advancing age, and the frequency of epidermal CD49a⁺ CD8 T_RM was increased in elderly individuals regardless of ethnicity. T-cell diversity and antipathogen responses were maintained in the skin of elderly individuals but declined in the blood. Our findings demonstrate that in elderly individuals, skin T cells maintain their density, diversity, and protective cytokine production despite the reduced T-cell diversity and function in blood. Skin resident T cells may represent a long-lived, highly protective reservoir of immunity in elderly people.

Salivary gland macrophages and tissue-resident CD8+ T cells cooperate for homeostatic organ surveillance

It is well established that tissue macrophages and tissue-resident memory CD8⁺ T cells (T_RM) play important roles for pathogen sensing and rapid protection of barrier tissues. In contrast, the mechanisms by which these two cell types cooperate for homeostatic organ surveillance after clearance of infections is poorly understood. Here, we used intravital imaging to show that T_RM dynamically followed tissue macrophage topology in noninflamed murine submandibular salivary glands (SMGs). Depletion of tissue macrophages interfered with SMG T_RM motility and caused a reduction of interepithelial T cell crossing. In the absence of macrophages, SMG T_RM failed to cluster in response to local inflammatory chemokines. A detailed analysis of the SMG microarchitecture uncovered discontinuous attachment of tissue macrophages to neighboring epithelial cells, with occasional macrophage protrusions bridging adjacent acini and ducts. When dissecting the molecular mechanisms that drive homeostatic SMG T_RM motility, we found that these cells exhibit a wide range of migration modes: In addition to chemokine- and adhesion receptor-driven motility, resting SMG T_RM displayed a remarkable capacity for autonomous motility in the absence of chemoattractants and adhesive ligands. Autonomous SMG T_RM motility was mediated by friction and insertion of protrusions into gaps offered by the surrounding microenvironment. In sum, SMG T_RM display a unique continuum of migration modes, which are supported in vivo by tissue macrophages to allow homeostatic patrolling of the complex SMG architecture.

Labeling and tracking of immune cells in ex vivo human skin

Human skin harbors various immune cells that are crucial for the control of injury and infection. However, the current understanding of immune cell function within viable human skin tissue is limited. We developed an ex vivo imaging approach in which fresh skin biopsies are mounted and then labeled with nanobodies or antibodies against cell surface markers on tissue-resident memory CD8⁺ T cells, other immune cells of interest, or extracellular tissue components. Subsequent longitudinal imaging allows one to describe the dynamic behavior of human skin-resident cells in situ. In addition, this strategy can be used to study immune cell function in murine skin. The ability to follow the spatiotemporal behavior of CD8⁺ T cells and other immune cells in skin, including their response to immune stimuli, provides a platform to investigate physiological immune cell behavior and immune cell behavior in skin diseases. The mounting, staining and imaging of skin samples requires ~1.5 d, and subsequent tracking analysis requires a minimum of 1 d. The optional production of fluorescently labeled nanobodies takes ~5 d.

Resident Memory T Cells in the Tumor Microenvironment

Tissue-resident memory T (T_RM) cells are strategically positioned within the epithelial layers of many tissues to provide enduring site-specific immunological memory. This unique T-cell lineage is endowed with the capacity to rapidly respond to tissue perturbations and has a well-documented role in eradicating pathogens upon reexposure. Emerging evidence has highlighted a key role for T_RM cells in cancer immunity. Single-cell approaches have identified T_RM cells among other CD8⁺ tumor-infiltrating lymphocyte (TIL) subsets, and their presence is a positive indicator of clinical outcome in cancer patients. Furthermore, recent preclinical studies have elegantly demonstrated that T_RM cells are a critical component of the antitumor immune response. Given their unique functional abilities, T_RM cells have emerged as a potential immunotherapeutic target. Here, we discuss T_RM cells in the framework of the cancer-immunity cycle and in the context of the T cell- and non-T cell-inflamed tumor microenvironments (TME). We highlight how their core features make T_RM cells uniquely suited to function within the metabolically demanding TME. Finally, we consider potential therapeutic avenues that target T_RM cells to augment the antitumor immune response.

Anti-viral protective capacity of tissue resident memory T cells

It has become increasingly clear that a subset of T cells which persist at diverse infection sites, known as tissue-resident memory T cells (T_RM), can mediate efficacious protective immunity against many types of viral infections. Recent studies have elucidated the mechanisms by which T_RM coordinate enhanced viral clearance in different sites through rapid production of effector cytokines and cytolytic mediators, in situ expansion, differentiation to circulating effector cells, and immune cell recruitment. This tissue-localized response also includes enhancement at the local lymphoid sites which contribute to fortifying T_RM-mediated protection. Understanding how these responses occur in a tissue-wide context will provide key insights for development of vaccines and therapeutics.

Divergence of Tissue-Memory T Cells: Distribution and Function-Based Classification

Tissue-resident memory T cells (Trm) comprise the majority of memory cells in nonlymphoid tissues and play a predominant role in immunity at barrier surfaces. A better understanding of Trm cell maintenance and function is essential for the development of vaccines that confer frontline protection. However, it is currently challenging to precisely distinguish Trm cells from other T cells, and this has led to confusion in the literature. Here we highlight gaps in our understanding of tissue memory and discuss recent advances in the classification of Trm cell subsets based on their distribution and functional characteristics.