Inhibitory checkpoint receptors control CD8+ resident memory T cells to prevent skin allergy

Tissue-resident memory T cells: Harnessing their properties against infection for cancer treatment

We have rapidly gained insights into the presence and function of T lymphocytes in non-lymphoid tissues, the tissue-resident memory T (TRM) cells. The central pillar of adaptive immunity has been expanded from classic central memory T cells giving rise to progeny upon reinfection and effector memory cells circulating through the blood and patrolling the tissues to include TRM cells that reside and migrate inside solid organs and tissues. Their development and maintenance have been studied in detail, providing exciting clues on how their unique properties used to fight infections may benefit therapies against solid tumors. We provide an overview of CD8 TRM cells and the properties that make them of interest for vaccination and cancer therapies.

Staphylococcus aureus-specific skin resident memory T cells protect against bacteria colonization but exacerbate atopic dermatitis-like flares in mice

BACKGROUND

The contribution of Staphylococcus aureus to the exacerbation of atopic dermatitis (AD) is widely documented, but its role as a primary trigger of AD skin symptoms remains poorly explored.

OBJECTIVES

This study sought to reappraise the main bacterial factors and underlying immune mechanisms by which S aureus triggers AD-like inflammation.

METHODS

This study capitalized on a preclinical model, in which different clinical isolates were applied in the absence of any prior experimental skin injury.

RESULTS

The development of S aureus-induced dermatitis depended on the nature of the S aureus strain, its viability, the concentration of the applied bacterial suspension, the production of secreted and nonsecreted factors, as well as the activation of accessory gene regulatory quorum sensing system. In addition, the rising dermatitis, which exhibited the well-documented AD cytokine signature, was significantly inhibited in inflammasome adaptor apoptosis-associated speck-like protein containing a CARD domain- and monocyte/macrophage-deficient animals, but not in T- and B-cell-deficient mice, suggesting a major role for the innate response in the induction of skin inflammation. However, bacterial exposure generated a robust adaptive immune response against S aureus, and an accumulation of S aureus-specific γδ and CD4+ tissue resident memory T cells at the site of previous dermatitis. The latter both contributed to worsen the flares of AD-like dermatitis on new bacteria exposures, but also, protected the mice from persistent bacterial colonization.

CONCLUSIONS

These data highlight the induction of unique AD-like inflammation, with the generation of proinflammatory but protective tissue resident memory T cells in a context of natural exposure to pathogenic S aureus strains.

Drug hypersensitivity reactions: review of the state of the science for prediction and diagnosis

Drug hypersensitivity reactions (DHRs) are a type of adverse drug reaction that can occur with different classes of drugs and affect multiple organ systems and patient populations. DHRs can be classified as allergic or non-allergic based on the cellular mechanisms involved. Whereas nonallergic reactions rely mainly on the innate immune system, allergic reactions involve the generation of an adaptive immune response. Consequently, drug allergies are DHRs for which an immunological mechanism, with antibody and/or T cell, is demonstrated. Despite decades of research, methods to predict the potential for a new chemical entity to cause DHRs or to correctly attribute DHRs to a specific mechanism and a specific molecule are not well-established. This review will focus on allergic reactions induced by systemically administered low-molecular weight drugs with an emphasis on drug- and patient-specific factors that could influence the development of DHRs. Strategies for predicting and diagnosing DHRs, including potential tools based on the current state of the science, will also be discussed.

CD69 Is Indispensable for Development of Functional Local Immune Memory in Murine Contact Hypersensitivity

Local immune memory develops at the site of antigen exposure and facilitates a rapid and strong local adaptive defense upon re-exposure. Resident memory T (TRM) cells play a role in local immune memory, and their cell-surface molecules CD69 and CD103 promote their tissue residency. However, the contribution of these molecules to skin immune memory remains unclear. In this study, by inducing contact hypersensitivity (CHS) in CD69KO (CD69-deficient) and CD103-deficient mice, where different degrees of TRM cell contribution are observed by repeated challenges on the right ear and a single challenge on the left ear, we found that the deficiency of CD69 but not CD103 leads to impaired CHS upon repeated antigen challenges, even although TRM cells-like CD8 T cells developed at the challenged site of CD69KO. CHS responses in both ears were diminished in CD69KO by FTY720 or CD8 neutralization, suggesting that CHS in CD69KO is ascribed to circulating CD8 T cells and that the developed TRM cell-like CD8 T cells do not behave as TRM cells. The infiltration of macrophages was reduced in the rechallenged site of CD69KO, along with the downregulation of Cxcl1 and Cxcl2. Thus, CD69 is considered essential for an effective recall response, involving the development of functional TRM cells and the recruitment of macrophages.

Resident memory T cells in nonlesional skin and healed lesions of patients with chronic inflammatory diseases: Appearances can be deceptive

Tissue-resident memory T (TRM) cells serve as a first line of defense in peripheral tissues to protect the organism against foreign pathogens. However, autoreactive TRM cells are increasingly implicated in autoimmunity, as evidenced in chronic autoimmune and inflammatory skin conditions. This highlights the need to characterize their phenotype and understand their role for the purpose of targeting them specifically without affecting local immunity. To date, the investigation of TRM cells in human skin diseases has focused mainly on lesional tissues of patients. Accumulating evidence suggests that self-reactive TRM cells are still present in clinically healed lesions of patients and play a role in disease flares, but TRM cells also populate skin that is apparently normal. This review discusses the ontogeny of TRM cells in the skin as well as recent insights regarding the presence of self-reactive TRM cells in both clinically healed skin and nonlesional skin of patients with autoimmune and inflammatory skin conditions, with a particular focus on psoriasis, atopic dermatitis, and vitiligo.

Tissue-Resident Memory T Cells in Allergy

Tissue-resident memory T (TRM) cells constitute a distinct subset within the memory T cell population, serving as the vanguard against invading pathogens and antigens in peripheral non-lymphoid tissues, including the respiratory tract, intestines, and skin. Notably, TRM cells adapt to the specific microenvironment of each tissue, predominantly maintaining a sessile state with distinctive phenotypic and functional attributes. Their role is to ensure continuous immunological surveillance and protection. Recent findings have highlighted the pivotal contribution of TRM cells to the modulation of adaptive immune responses in allergic disorders such as allergic rhinitis, asthma, and dermatitis. A comprehensive understanding of the involvement of TRM cells in allergic diseases bears profound implications for allergy prevention and treatment. This review comprehensively explores the phenotypic characteristics, developmental mechanisms, and functional roles of TRM cells, focusing on their intricate relationship with allergic diseases.

CD69 is a Promising Immunotherapy and Prognosis Prediction Target in Cancer

Immunotherapy utilizing T cells that attack tumors is a promising strategy for treatment, but immune suppressive T cell subsets, such as regulatory T cell (Treg), and immune checkpoint molecules, including programmed death-1 (PD-1), can suppress the intensity of a T cell immune reaction and thereby impair tumor clearance. Cluster of differentiation 69 (CD69), known as an early leukocyte activation marker, can be used as a measure or early marker of T cell activation. In recent years, the functions of CD69 in the regulation of Treg/Th17 (T helper cell 17) differentiation and in the tissue retention of T cells have attracted considerable interest. These functions are related to the role of CD69 in immune suppression in tumor environments (TME). In this review, we first summarized current perspectives in the biological function of CD69 and demonstrated that CD69 acts as a regulator of T cell activation, differentiation, retention, and exhaustion. Then, we discussed recent advances in understanding of CD69 deficiency and anti-CD69 antibody administration and shed light on the value of targeting on CD69 for cancer immunotherapy and prognosis prediction.

Topical corticosteroids inhibit allergic skin inflammation but are ineffective in impeding the formation and expansion of resident memory T cells

BACKGROUND

Tissue-resident memory T (TRM ) cells are detrimental in allergic contact dermatitis (ACD), in which they contribute to the chronicity and severity of the disease.

METHODS

We assessed the impact of a standard topical corticosteroid (TCS) treatment, triamcinolone acetonide (TA), on the formation, maintenance and reactivation of epidermal TRM cells in a preclinical model of ACD to 2,4-dinitrofluorobenzene. TA 0.01% was applied at different time points of ACD response and we monitored skin inflammation and tracked CD8+ CD69+ CD103+ TRM by flow cytometry and RNA sequencing.

RESULTS

The impact of TA on TRM formation depended on treatment regimen: (i) in a preventive mode, that is, in sensitized mice before challenge, TA transiently inhibited the infiltration of effector T cells and the accumulation of TRM upon hapten challenge. In contrast, (ii) in a curative mode, that is, at the peak of the ACD response, TA blocked skin inflammation but failed to prevent the formation of TRM . Finally, (iii) in a proactive mode, that is, on previous eczema lesions, TA had no effect on the survival of skin TRM , but transiently inhibited their reactivation program upon allergen reexposure. Indeed, specific TRM progressively regained proliferative functions upon TA discontinuation and expanded in the tissue, leading to exaggerated iterative responses. Interestingly, TRM re-expansion correlated with the decreased clearance of hapten moieties from the skin induced by repeated TA applications.

CONCLUSIONS

Our results demonstrate that TCS successfully treat ACD inflammation, but are mostly ineffective in impeding the formation and expansion of allergen-specific TRM , which certainly restricts the induction of lasting tolerance in patients with chronic dermatitis.

Resident cutaneous memory T cells: a clinical review of their role in chronic inflammatory dermatoses and potential as therapeutic targets

Resident memory T cells (T-RMs) remain in epithelial barrier tissues after antigen exposure and the initial effector phase. These T-RMs provide effective antimicrobial and anticancer immunity; however, pathogenic T-RMs have been shown to mediate various chronic inflammatory disorders in a variety of tissue types. In the skin, T-RMs are referred to as resident cutaneous memory T cells (cT-RMs). Understanding the mechanisms leading to the development and establishment of these cT-RMs populations may allow for targeted treatments that provide durable responses in chronic immune-mediated skin diseases, even after cessation. In this review, we summarize the evidence on cT-RMs as drivers of chronic inflammatory dermatoses, including psoriasis, vitiligo, atopic dermatitis, cutaneous lupus erythematosus and alopecia areata, among others. Data from in vitro, animal model and ex vivo human studies are presented, with a focus on the potential for cT-RMs to trigger acute disease flares, as well as recurrent disease, by establishing an immune 'memory' in the skin. Furthermore, the available data on the potential for existing and novel treatments to affect the development or survival of cT-RMs in the skin are synthesized. The data suggest a dynamic and rapidly growing area in the field of dermatology; however, we also discuss areas in need of greater research to allow for optimal treatment selection for long-term disease control.

Stem-like exhausted and memory CD8+ T cells in cancer

T cells can acquire a broad spectrum of differentiation states following activation. At the extreme ends of this continuum are short-lived cells equipped with effector machinery and more quiescent, long-lived cells with heightened proliferative potential and stem cell-like developmental plasticity. The latter encompass stem-like exhausted T cells and memory T cells, both of which have recently emerged as key determinants of cancer immunity and response to immunotherapy. Here, we discuss key similarities and differences in the regulation and function of stem-like exhausted CD8+ T cells and memory CD8+ T cells, and consider their context-specific contributions to protective immunity in diverse outcomes of cancer, including tumour escape, long-term control and eradication. Finally, we emphasize how recent advances in the understanding of the molecular regulation of stem-like exhausted T cells and memory T cells are being explored for clinical benefit in cancer immunotherapies such as checkpoint inhibition, adoptive cell therapy and vaccination.

Neutrophil recruitment by CD4 tissue-resident memory T cells induces chronic recurrent inflammation in atopic dermatitis

BACKGROUND

Atopic dermatitis (AD) is a chronic inflammatory skin disease that continues to impose significant physical, mental, and economic burdens on patients. Recent research has suggested the significant role of tissue-resident memory (TRM) cells in AD. However, the precise role and mechanisms of action of TRM cells in AD remain unclear. A deeper understanding of the involvement of TRM cells in AD will unveil promising pathways for future innovative therapeutic strategies.

METHODS

To investigate the involvement of TRM cells in AD, we used diverse mouse models and employed experimental techniques to manipulate cell formation and depletion. We assessed the inflammatory response by analyzing mouse ear phenotype, measuring ear thickness, and performing hematoxylin and eosin staining. Flow cytometry and immunofluorescence staining were used to identify different cell types and evaluate changes in cell quantity. Additionally, we used qPCR to analyze gene expression of relevant chemokines and cytokines.

RESULTS

Our study revealed the presence of TRM cells in the skin after exposure to calcipotriol. After a 24-h re-challenge, we observed substantial neutrophil infiltration into the previously irritated skin. Neutrophil depletion prior to re-challenge effectively prevented early flare-up responses during AD recurrence. Furthermore, we demonstrate that CD4+TRM cells upregulate expression of cytokines INF-γ and TNF-α, which may induce the expression of CXCL1, thereby recruiting neutrophils and contributing to the chronic recurrent inflammation observed in AD.

CONCLUSIONS

We have established a novel, chronic recurrent mouse model for investigating TRM cells in AD. Our findings demonstrate that CD4+TRM cells in the skin mediate early flare-up response during AD recurrence and influence the chronic recurrent inflammation of AD by recruiting neutrophils. Targeting CD4+TRM cells may represent a promising approach for the treatment of chronic recurrent inflammation in AD.

The junctional adhesion molecule-like protein (JAML) is important for the inflammatory response during contact hypersensitivity

BACKGROUND

The junctional adhesion molecule-like protein (JAML) plays important roles in wound healing and activation of epidermal γδ T cells in mice. Whether JAML plays a role in contact hypersensitivity (CHS), the animal model of allergic contact dermatitis (ACD), is not known.

METHODS

To examine the role of JAML in CHS, we used various mouse models of CHS in JAML knockout (KO) and wild-type (WT) mice. Furthermore, the expression of the JAML ligand coxsackievirus and adenovirus receptor (CXADR) on keratinocytes was accessed in vitro and in vivo.

RESULTS

JAML KO mice had a diminished inflammatory response during both the sensitization and elicitation phase of CHS and had reduced numbers of CD8+ and CD4+ T cells in the epidermis. Furthermore, interferon γ (IFNγ), interleukin 1β (IL-1β) and CXCL10 production were significantly reduced in JAML KO mice during the elicitation phase. We found that CD8+ T cells express JAML and that JAML is essential for rapid flare-up responses to contact allergens. Finally, we show that keratinocytes up-regulate the JAML ligand CXADR following exposure to contact allergens.

CONCLUSION

Our study is the first to show a central role of JAML in CHS and reveals a potential new target for the treatment of ACD in humans.

Resolved Psoriasis with Abundant Oleic Acid in Stratum Corneum Exhibits Lower T-Cell-Driven IL-17 Signature

Relapses of psoriasis involve T cells that stem and survive in the skin. Inherited from previous flares, the tissue-resident memory T cells are epidermal IL-17-producing CD8+ and IL-22-producing CD4+ T cells. Because the capacity of resident memory T cells to take in fatty acids is essential for their residence and function, the surface composition of fatty acids may affect underlying T-cell populations. In patients treated with biologics, we used gas chromatography/mass spectrometry to decipher the fatty acid composition in both resolved and nonlesional sites. Skin T cells were activated by OKT-3 in explants from the same body sites to perform bulk transcriptomic analysis (Nanostring). The fatty acid composition differed between skin from healthy donors and normal-looking skin of patients with psoriasis but not further between nonlesional and resolved skin. Patients in whom the resolved skin was rich in oleic acid had lower T-cell-driven IL-17 epidermal transcriptomic signature upon activation of T cells in skin explants. The skin lipid composition is linked with the functions of the underlying epidermal T cells. Testing the modulating effect of custom fatty acids on skin resident T cells could help with coming closer to disease oblivion in inflammatory skin diseases.

The therapeutic potential of PD-1/PD-L1 pathway on immune-related diseases: Based on the innate and adaptive immune components

Currently, immunotherapy targeting programmed cell death 1 (PD-1) or programmed death ligand 1 (PD-L1) has revolutionized the treatment strategy of human cancer patients. Meanwhile, PD-1/PD-L1 pathway has also been implicated in the pathogenesis of many immune-related diseases, such as autoimmune diseases, chronic infection diseases and adverse pregnancy outcomes, by regulating components of the innate and adaptive immune systems. Given the power of the new therapy, a better understanding of the regulatory effects of PD-1/PD-L1 pathway on innate and adaptive immune responses in immune-related diseases will facilitate the discovery of novel biomarkers and therapeutic drug targets. Targeting this pathway may successfully halt or potentially even reverse these pathological processes. In this review, we discuss recent major advances in PD-1/PD-L1 axis regulating innate and adaptive immune components in immune-related diseases. We reveal that the impact of PD-1/PD-L1 axis on the immune system is complex and manifold and multi-strategies on the targeted PD-1/PD-L1 axis are taken in the treatment of immune-related diseases. Consequently, targeting PD-1/PD-L1 pathway, alone or in combination with other treatments, may represent a novel strategy for future therapeutic intervention on immune-related diseases.

Immunomodulatory effects of nanoparticles on dendritic cells in a model of allergic contact dermatitis: importance of PD-L2 expression

Nanoparticle (NP) skin exposure is linked to an increased prevalence of allergic contact dermatitis. In our prior studies using the mouse contact hypersensitivity (CHS) model, we reported that silica 20 nm (SiO2) NPs suppressed the allergic response and titanium dioxide NPs doped with manganese (mTiO2) exacerbated it. In this work, we conducted in vitro experiments using bone marrow-derived dendritic cells (BMDCs) to study the combinatorial effect of the potent 2,4-dinitrofluorobenzene (DNFB) hapten sensitizer with SiO2 and mTiO2 NPs on BMDC cytotoxicity, cytokine secretion and phenotype using the B7 family ligands. Results show that DNFB and mTiO2 behave similarly and exhibit proinflammatory characteristics while SiO2 promotes a naive phenotype. We observe that the B7-H3 (CD276) ligand is only expressed on CD80 + (B7-1) BMDCs. Results from adoptive transfer CHS studies, combined with BMDC phenotype analysis, point to the importance of PD-L2 expression in modulating the adaptive immune response. This work identifies metrics that can be used to predict the effects of NPs on contact allergy and to guide efforts to engineer cell-based therapies to induce hapten specific immune tolerance.

The immune checkpoint protein PD-1: Its emerging regulatory role in memory T cells

Immunological memory helps the body rapidly develop immune defense when it re-encounters a bacterial or viral strain or encounters a similar mutation in healthy cells. The immune checkpoint molecule programmed cell death 1 (PD-1) influences memory T cell differentiation. However, the mechanism by which PD-1 regulates the development and maintenance of memory T cells and its impact on memory T cells function remain unclear. In this review, we first discuss the structure and function of PD-1 and then summarize the roles of PD-1 as a marker of tumor memory T cells and in tumor immunotherapy. We also discuss the potential mechanisms through which PD-1 regulates memory T cells development and maintenance during immune diseases such as viral infection-mediated diseases, psoriasis, and rheumatoid arthritis, and list the effects of PD-1 on memory T cells in pregnancy and their function in maternal-fetal immune balance. A complete understanding of how PD-1 influences the development, maintenance, and function of memory T cells will provide new insights into the prevention and treatment of immune-related diseases.

Tumor-resident memory T cells as a biomarker of the response to cancer immunotherapy

Tumor-infiltrating lymphocytes (TIL) often include a substantial subset of CD8+ tissue-resident memory T (TRM) cells enriched in tumor-specific T cells. These TRM cells play a major role in antitumor immune response. They are identified on the basis of their expression of the CD103 (αE(CD103)β7) and/or CD49a (α1(CD49a)β1) integrins, and the C-type lectin CD69, which are involved in tissue residency. TRM cells express several T-cell inhibitory receptors on their surface but they nevertheless react strongly to malignant cells, exerting a strong cytotoxic function, particularly in the context of blocking interactions of PD-1 with PD-L1 on target cells. These TRM cells form stable conjugates with autologous tumor cells and interact with dendritic cells and other T cells within the tumor microenvironment to orchestrate an optimal in situ T-cell response. There is growing evidence to indicate that TGF-β is essential for the formation and maintenance of TRM cells in the tumor, through the induction of CD103 expression on activated CD8+ T cells, and for the regulation of TRM effector functions through bidirectional integrin signaling. CD8+ TRM cells were initially described as a prognostic marker for survival in patients with various types of cancer, including ovarian, lung and breast cancers and melanoma. More recently, these tumor-resident CD8+ T cells have been shown to be a potent predictive biomarker of the response of cancer patients to immunotherapies, including therapeutic cancer vaccines and immune checkpoint blockade. In this review, we will highlight the major characteristics of tumor TRM cell populations and the possibilities for their exploitation in the design of more effective immunotherapy strategies for cancer.

VISTA expression and patient selection for immune-based anticancer therapy

V-domain Ig suppressor of T-cell activation (VISTA) is a B7 family member that plays key roles in maintaining T cell quiescence and regulation of myeloid cell populations, which together establish it as a novel immunotherapy target for solid tumors. Here we review the growing literature on VISTA expression in relation to various malignancies to better understand the role of VISTA and its interactions with both tumor cells and immune cells expressing other checkpoint molecules within the tumor microenvironment (TME). The biology of VISTA creates several mechanisms to maintain the TME, including supporting the function of myeloid-derived suppressor cells, regulating natural killer cell activation, supporting the survival of regulatory T cells, limiting antigen presentation on antigen-presenting cells and maintaining T cells in a quiescent state. Understanding these mechanisms is an important foundation of rational patient selection for anti-VISTA therapy. We provide a general framework to describe distinct patterns of VISTA expression in correlation with other known predictive immunotherapy biomarkers (programmed cell death ligand 1 and tumor-infiltrating lymphocytes) across solid tumors to facilitate investigation of the most efficacious TMEs for VISTA-targeted treatment as a single agent and/or in combination with anti-programmed death 1/anti-cytotoxic T lymphocyte antigen-4 therapies.

Tissue-resident, memory CD8+ T cells are effective in clearing intestinal Eimeria falciformis reinfection in mice

Eimeria, a cousin of malarial parasites, causes coccidiosis that results in huge losses in the poultry industry. Although live coccidiosis vaccines have been developed and used widely for the successful control of the disease, the mechanism underlying protective immunity remains largely unknown. Using Eimeria falciformis as a model parasite, we observed that tissue-resident memory CD8⁺ T (Trm) cells accumulated in cecal lamina propria following E. falciformis infection in mice, especially after reinfection. In convalescent mice challenged with a second infection, E. falciformis burden diminished within 48-72 h. Deep-sequencing revealed that CD8⁺ Trm cells were characterized by rapid up-regulation of effector genes encoding pro-inflammatory cytokines and cytotoxic effector molecules. While FTY720 (Fingolimod) treatment prevented the trafficking of CD8⁺ T cells in peripheral circulation and exacerbated primary E. falciformis infection, such treatment had no impact on the expansion of CD8⁺ Trm cells in convalescent mice receiving secondary infection. Adoptive transfer of cecal CD8⁺ Trm cells conferred immune protection in naïve mice, indicating that these cells provide direct and effective protection against infection. Overall, our findings not only explain a protective mechanism of live oocyst-based anti-Eimeria vaccines but also provide a valuable correlate for assessing vaccines against other protozoan diseases.

Tissue-resident memory T cells: The key frontier in local synovitis memory of rheumatoid arthritis

Rheumatoid arthritis (RA) is a highly disabling, systemic autoimmune disease. It presents a remarkable tendency to recur, which renders it almost impossible for patients to live without drugs. Under such circumstances, many patients have to suffer the pain of recurrent attacks as well as the side effects of long-term medication. Current therapies for RA are primarily systemic treatments without targeting the problem that RA is more likely to recur locally. Emerging studies suggest the existence of a mechanism mediating local memory during RA, which is closely related to the persistent residence of tissue-resident memory T cells (T_RM). T_RM, one of the memory T cell subsets, reside in tissues providing immediate immune protection but driving recurrent local inflammation on the other hand. The heterogeneity among synovial T_RM is unclear, with the dominated CD8⁺ T_RM observed in inflamed synovium of RA patients coming into focus. Besides local arthritis relapse, T_RM may also contribute to extra-articular organ involvement in RA due to their migration potential. Future integration of single-cell RNA sequencing (scRNA-seq) with spatial transcriptomics to explore the gene expression patterns of T_RM in both temporal dimension and spatial dimension may help us identify specific therapeutic targets. Targeting synovial T_RM to suppress local arthritis flares while using systemic therapies to prevent extra-articular organ involvement may provide a new perspective to address RA recurrence.

Accumulation of Cytotoxic Skin Resident Memory T Cells and Increased Expression of IL-15 in Lesional Skin of Polymorphic Light Eruption

Patients with polymorphic light eruption (PLE) develop lesions upon the first exposure to sun in spring/summer, but lesions usually subside during season due to the natural (or medical) photohardening. However, these lesions tend to reappear the following year and continue to do so in most patients, suggesting the presence of a disease memory. To study the potential role of skin resident memory T cells (Trm), we investigated the functional phenotype of Trm and the expression of IL-15 in PLE. IL-15 is known to drive Trm proliferation and survival. Multiplex immunofluorescence was used to quantify the expression of CD3, CD4, CD8, CD69, CD103, CD49a, CD11b, CD11c, CD68, granzyme B (GzmB), interferon-gamma (IFN-γ), and IL-15 in formalin-fixed, paraffin-embedded lesional skin samples from PLE patients and healthy skin from control subjects. Unlike the constitutive T cell population in healthy skin, a massive infiltration of T cells in the dermis and epidermis was observed in PLE, and the majority of these belonged to CD8⁺ T cells which express Trm markers (CD69, CD103, CD49a) and produced cytotoxic effector molecules GzmB and IFN-γ. Higher numbers of CD3⁺ T cells and CD11b⁺CD68⁺ macrophages produced IL-15 in the dermis as compared to healthy skin. The dominant accumulation of cytotoxic Trm cells and increased expression of IL-15 in lesional skin of PLE patients strongly indicates the potential role of skin Trm cells in the disease manifestation and recurrence.

Advances in the Pathomechanisms of Delayed Drug Hypersensitivity

Delayed drug hypersensitivity continues to contribute to major clinical problems worldwide. The clinical presentations of delayed drug hypersensitivity are diverse, ranging from mild skin rashes to life-threatening systemic reactions. The pathomechanism of delayed drug hypersensitivity involves human leukocyte antigens (HLA) presentation of drugs/metabolites to T cell receptors (TCR), resulting in T-cell activation. The pathogenesis of delayed drug hypersensitivity also has reactivation of the virus, and activation of many immune mediators. In this review, we discuss the immune pathogenesis, molecular interactions of HLA/drugs/TCR, and downstream signaling of cytotoxic proteins/cytokines/chemokines, as well as disease prevention and management for delayed drug hypersensitivity.

Cytokine-Mediated Crosstalk Between Keratinocytes and T Cells in Atopic Dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease characterized by barrier dysfunction, dysregulated immune response, and dysbiosis with increased Staphylococcus aureus colonization. Infiltration of various T helper cell subsets into lesional skin and subsequent cytokine release are a hallmark of AD. Release of cytokines by both T cells and keratinocytes plays a key role in skin inflammation and drives many AD features. This review aims to discuss cytokine-mediated crosstalk between T cells and keratinocytes in AD pathogenesis and the potential impact of virulence factors produced by Staphylococcus aureus on these interactions.

Effects of Fisetin on Allergic Contact Dermatitis via Regulating the Balance of Th17/Treg in Mice

Background. Allergic contact dermatitis (ACD) is a form of chronic cutaneous inflammatory disease of immunological origin that has adverse impacts on patient quality of life, underscoring the need for the development of safe and effective therapeutic agents to treat affected individuals. Fisetin is a Chinese herbal preparation that reportedly exhibits antitumor, antioxidant, antimicrobial, anticoagulatory, and antimalarial activity. In the current report, the immunomodulatory activity of fisetin was appraised by assessing its impact on balance between regulatory T (Treg) and Th17 cells in an ACD model. Methods. BALB/c mice ( $n=60$ ) were randomized into control, ACD model, CTX positive control (20 mg/kg), and fisetin treatment groups (three dose levels: 2, 4, or 8 mg/kg). ACD induction was achieved by sensitizing mice on the shaved ventral abdomen via the application of 5% DNFB (50 μL) on days 1 and 2, followed by rechallenge in the right ear with 5% DNFB (20 μL) on day 5. Beginning on day 1, immunized mice were intraperitoneally injected with the appropriate fisetin dose (in saline) once per day for 7 days. On day 7, ear swelling, transcription factor expression, Th17/Treg cell populations, and cytokine production were assessed in vivo. Results. Fisetin treatment significantly suppressed ear swelling and associated inflammatory cell infiltration, besides reducing the production of Th17 cytokines (IL-17, TNF-α, and IL-6) and the expression of the Th17 lineage transcription factor RORγt while simultaneously enhancing Treg-specific cytokine production (TGF-β and IL-10) and the expression of the Treg lineage transcription factor Foxp3, thereby restoring the Th17/Treg cell in ACD mice. Conclusions. These data indicate that fisetin exhibits immunomodulatory activity and can alter the Th17/Treg cell balance, highlighting its potential value as a treatment drug for ACD.

Novel insights into contact dermatitis

Contact dermatitis is a common disease, caused by repeated skin contact to contact allergens or irritants, resulting in allergic contact dermatitis (ACD) and/or irritant contact dermatitis. Attempts have been made to identify biomarkers to distinguish irritant and allergic patch test reactions, which could aid diagnosis. Some promising candidates have recently been identified, but verification and validation in clinical cases still need to be done. New causes of ACD are constantly recognized. In this review, 10 new contact allergens from recent years, were identified, several relating to anti-age products. Frequent allergens causing considerable morbidity in the population such as the preservative methylisothiazolinone (MI) have been regulated in EU. A significant drop in cases has been seen, while high rates are still occurring in other areas such as North America. Other frequent causes are fragrance allergens especially widely used terpenes and acrylates used in medical devises for control of diabetes. These represent unsolved problems. Recent advances in immunology have opened for a better understanding of the complexity of contact dermatitis, especially ACD. The disease may be more heterogenous that previous understood with several subtypes. With the rapidly evolving molecular understanding of the disease, the potential for development of new drugs for personalized treatment of contact dermatitis is considerable.

CD8+ tissue-resident memory T cells recruit neutrophils that are essential for flare-ups in contact dermatitis

BACKGROUND

Allergic contact dermatitis (ACD) is classically described as a delayed-type hypersensitivity reaction. However, patients often experience flare-ups characterized by itching erythema, edema, and often vesicles occurring within hours after re-exposure of previously sensitized skin to the specific contact allergen. Recent studies have indicated that skin-resident memory T (T_RM ) cells play a central role in ACD. However, the pathogenic role of T_RM cells in allergen-induced flare-ups is not known.

METHODS

By the use of various mouse models and cell depletion protocols, we investigated the role of epidermal T_RM cells in flare-up reactions to the experimental contact allergen 1-fluoro-2,4-dinitrobenzene. The inflammatory response was measured by changes in ear thickness, and the cellular composition in epidermis was determined by flow cytometry and confocal microscopy. Finally, adaptive transfer and inhibitors were used to determine the role of T_RM cells, neutrophils, and CXCL1/CXCL2 in the response.

RESULTS

We show that CD8⁺ T_RM cells initiate massive infiltration of neutrophils in the epidermis within 12 h after re-exposure to the contact allergen. Depletion of neutrophils before re-exposure to the allergen abrogated the flare-up reactions. Furthermore, we demonstrate that CD8⁺ T_RM cells mediate neutrophil recruitment by inducing CXCL1 and CXCL2 production in the skin, and that blockage of the C-X-C chemokine receptor type 1 and 2 inhibits flare-up reactions and neutrophil infiltration.

CONCLUSION

As the first, we show that epidermal CD8⁺ T_RM cells cause ACD flare-ups by rapid recruitment of neutrophils to the epidermis.

Low-dose intralesional injection of 5-fluorouracil and triamcinolone reduces tissue resident memory T cells in chronic eczema

BACKGROUND

Tissue resident memory T (T_RM) cells have been reported to play a significant role in the pathogenesis and relapse of chronic eczema.

AIM

To compare the efficacy and safety of the intralesional injection of 5-fluorouracil (5-FU) and triamcinolone (TA) with those associated with TA alone for the treatment of chronic eczema.

METHODS

A total of 168 patients were randomized to 5-FU+TA or TA groups and received a one-time intralesional injection of 5-FU+TA or TA only. Biopsies were collected before and 2 wk after treatment for evaluation of histopathological changes. All patients were followed up monthly for up to 1 year.

RESULTS

No serious adverse event was observed in either group. Although the mean atopic dermatitis severity index scores and effective rates were comparable between the two groups after 2 wk of treatment, the relapse rate was significantly lower in the 5-FU+TA group than in the TA group. Histological examination showed significantly fewer CD8⁺ and CD103⁺ T cells but not CD4⁺ T cells in the 5-FU+TA group.

CONCLUSION

One-time intralesional injection of 5-FU+TA is effective and safe for chronic eczema treatment and can further reduce the retention of T_RM cells in the lesional skin and the relapse rate of chronic eczema.

Unique molecular signatures typify skin inflammation induced by chemical allergens and irritants

BACKGROUND

Skin exposure to chemicals may induce an inflammatory disease known as contact dermatitis (CD). Distinguishing the allergic and irritant forms of CD often proves challenging in the clinic.

METHODS

To characterize the molecular signatures of chemical-induced skin inflammation, we conducted a comprehensive transcriptomic analysis on the skin lesions of 47 patients with positive patch tests to reference contact allergens and nonallergenic irritants.

RESULTS

A clear segregation was observed between allergen- and irritant-induced gene profiles. Distinct modules pertaining to the epidermal compartment, metabolism, and proliferation were induced by both contact allergens and irritants; whereas only contact allergens prompted strong activation of adaptive immunity, notably of cytotoxic T-cell responses. Our results also confirmed that: (a) unique pathways characterize allergen- and irritant-induced dermatitis; (b) the intensity of the clinical reaction correlates with the magnitude of immune activation. Finally, using a machine-learning approach, we identified and validated several minimal combinations of biomarkers to distinguish contact allergy from irritation.

CONCLUSION

These results highlight the value of molecular profiling of chemical-induced skin inflammation for improving the diagnosis of allergic versus irritant contact dermatitis.

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

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

Feeding of a fat-enriched diet causes the loss of resistance to contact hypersensitivity

BACKGROUND

Low-molecular weight chemicals or metal ions can cause allergic contact dermatitis, an inflammatory skin disease. Mice lacking Toll-like receptors 2 and 4 (TLR2/4 mice) are resistant to contact hypersensitivity (CHS). In the Western population obesity is increasing, which is known to have a proinflammatory impact.

OBJECTIVES

The aim of this study was to investigate the impact of a high-fat diet (HFD) on the sensitization and elicitation of CHS. We hypothesized that a proinflammatory micromilieu can be caused by an increase in adipose tissue, which might be sufficient to break the resistance of TLR2/4 mice.

METHODS

Four weeks prior to sensitization, wild-type (wt) or TLR2/4 mice were fed normal chow (NC), control diet (CD), or HFD. The effects on CHS and inflammation were analysed by measuring the ear swelling response, using flow cytometry and enzyme-linked immunosorbent assay.

RESULTS

The reaction of wt mice to 2,4,6-trinitro-1-chlorobenzene (TNCB) was increased by HFD. While NC-fed TLR2/4 mice were still resistant to CHS, HFD and, unexpectedly, CD feeding broke the resistance of TLR2/4 mice to TNCB.

CONCLUSIONS

These experiments suggest that the increased fat content or the different fatty acid composition of the diets increases inflammation and, therefore, the likelihood of developing CHS.

Expression of Tim-3 drives phenotypic and functional changes in Treg cells in secondary lymphoid organs and the tumor microenvironment

Regulatory T cells (Treg cells) are critical mediators of self-tolerance, but they can also limit effective anti-tumor immunity. Although under homeostasis a small fraction of Treg cells in lymphoid organs express the putative checkpoint molecule Tim-3, this protein is expressed by a much larger proportion of tumor-infiltrating Treg cells. Using a mouse model that drives cell-type-specific inducible Tim-3 expression, we show that expression of Tim-3 by Treg cells is sufficient to drive Treg cells to a more effector-like phenotype, resulting in increases in suppressive activity, effector T cell exhaustion, and tumor growth. We also show that T-reg-cell-specific inducible deletion of Tim-3 enhances anti-tumor immunity. Enhancement of Treg cell function by Tim-3 is strongly correlated with increased expression of interleukin-10 (IL-10) and a shift to a more glycolytic metabolic phenotype. Our data demonstrate that Tim-3+ Treg cells may be a relevant therapeutic target cell type for the treatment of cancer.

The Potential of Tissue-Resident Memory T Cells for Adoptive Immunotherapy against Cancer

Tissue-resident memory T cells (T_RM) comprise an important memory T cell subset that mediates local protection upon pathogen re-encounter. T_RM populations preferentially localize at entry sites of pathogens, including epithelia of the skin, lungs and intestine, but have also been observed in secondary lymphoid tissue, brain, liver and kidney. More recently, memory T cells characterized as T_RM have also been identified in tumors, including but not limited to melanoma, lung carcinoma, cervical carcinoma, gastric carcinoma and ovarian carcinoma. The presence of these memory T cells has been strongly associated with favorable clinical outcomes, which has generated an interest in targeting T_RM cells to improve immunotherapy of cancer patients. Nevertheless, intratumoral T_RM have also been found to express checkpoint inhibitory receptors, such as PD-1 and LAG-3. Triggering of such inhibitory receptors could induce dysfunction, often referred to as exhaustion, which may limit the effectiveness of T_RM in countering tumor growth. A better understanding of the differentiation and function of T_RM in tumor settings is crucial to deploy these memory T cells in future treatment options of cancer patients. The purpose of this review is to provide the current status of an important cancer immunotherapy known as TIL therapy, insight into the role of T_RM in the context of antitumor immunity, and the challenges and opportunities to exploit these cells for TIL therapy to ultimately improve cancer treatment.

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.

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

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Contact dermatitis

Contact dermatitis (CD) is among the most common inflammatory dermatological conditions and includes allergic CD, photoallergic CD, irritant CD, photoirritant CD (also called phototoxic CD) and protein CD. Occupational CD can be of any type and is the most prevalent occupational skin disease. Each CD type is characterized by different immunological mechanisms and/or requisite exposures. Clinical manifestations of CD vary widely and multiple subtypes may occur simultaneously. The diagnosis relies on clinical presentation, thorough exposure assessment and evaluation with techniques such as patch testing and skin-prick testing. Management is based on patient education, avoidance strategies of specific substances, and topical treatments; in severe or recalcitrant cases, which can negatively affect the quality of life of patients, systemic medications may be needed.

Tissue-Resident Memory T Cells in Antifungal Immunity

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

Resident Memory T Cells in Autoimmune Skin Diseases

Tissue resident memory T cells (TRM) are a critical component of the immune system, providing the body with an immediate and highly specific response against pathogens re-infecting peripheral tissues. More recently, however, it has been demonstrated that TRM cells also form during autoimmunity. TRM mediated autoimmune diseases are particularly destructive, because unlike foreign antigens, the self-antigens are never cleared, continuously activating self-reactive TRM T cells. In this article, we will focus on how TRMs mediate disease in autoimmune skin conditions, specifically vitiligo, psoriasis, cutaneous lupus erythematosus, alopecia areata and frontal fibrosing alopecia.

T-cell memory in tissues

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

Autoreactive T cells and their role in atopic dermatitis

Atopic dermatitis (AD) is an itchy, non-contagious relapsing and chronic inflammatory skin disease that usually develops in early childhood. This pathology is associated with food allergy, allergic asthma, allergic rhinitis and anaphylaxis which may persist in adulthood. The underlying mechanisms of AD (endotypes) are just beginning to be discovered and show a complex interaction of various pathways including skin barrier function and immune deviation. Immune reactions to self-proteins (autoantigens) of the skin have been identified in patients with inflammatory skin diseases, such as chronic spontaneous urticaria, connective tissue disease, pemphigus vulgaris and bullous pemphigoid. IgE antibodies and T cells directed against epitopes of the skin were observed in adult patients with severe and chronic AD as well. This was associated with disease severity and suggests a progression from allergic inflammation to severe autoimmune processes against the skin. IgE-mediated autoimmunity and self-reactive T cells might accelerate the ongoing skin inflammation or might contribute to the relapsing course of the disease. However, to date, the exact mechanisms of IgE-mediated autoimmunity and self-reactive T cells in the pathophysiology of AD are still unclear. The aim of this review is to evaluate the development of (autoreactive) T cells and their response to (auto)antigens, as well as the role of the peripheral tolerance in autoimmunity in the pathophysiology of AD, including the unmet needs and gaps.

Blocking TIM-3 in Treatment-refractory Advanced Solid Tumors: A Phase Ia/b Study of LY3321367 with or without an Anti-PD-L1 Antibody

PURPOSE

T-cell immunoglobulin and mucin-domain-containing molecule-3 (TIM-3) blunts anticancer immunity and mediates resistance to programmed death 1 (PD-1) and PD ligand 1 (PD-L1) inhibitors. We assessed a novel, first-in-class, TIM-3 mAb, LY3321367, alone or in combination with the anti-PD-L1 antibody, LY300054 in patients with advanced solid tumor.

PATIENTS AND METHODS

This open-label, multicenter, phase Ia/b study aimed to define the safety/tolerability and recommended phase II dose (RP2D) of LY3321367 with or without LY300054. Secondary objectives included pharmacokinetics/pharmacodynamics, immunogenicity, and efficacy. Biomarkers were assessed in exploratory analysis.

RESULTS

No dose-limiting toxicities were observed in the monotherapy (N = 30) or combination (N = 28) dose escalation. LY3321367 treatment-related adverse events (≥2 patients) included pruritus, rash, fatigue, anorexia, and infusion-related reactions. Dose-proportional increase in LY3321367 concentrations was not affected by either LY300054 or antidrug antibodies (observed in 50%-70% of patients). Pharmacokinetic/pharmacodynamic modeling indicated 100% target engagement at doses ≥600 mg. LY3321367 RP2D was 1,200 mg biweekly for four doses followed by 600 mg every 2 weeks thereafter. In the non-small cell lung cancer monotherapy expansion cohort, outcomes varied by prior anti-PD-1 therapy response status: anti-PD-1/L1 refractory patients [N = 23, objective response rate (ORR) 0%, disease control rate (DCR) 35%, progression-free survival (PFS) 1.9 months] versus anti-PD-1/L1 responders (N = 14, ORR 7%, DCR 50%, PFS 7.3 months). In combination expansion cohorts (N = 91), ORR and DCR were 4% and 42%; CD8 infiltration in paired biopsies increased in approximately half these patients.

CONCLUSIONS

LY3321367 exhibited acceptable safety profile with favorable pharmacokinetics/pharmacodynamics but only modest antitumor activity. The therapeutic relevance of TIM-3 blockade requires further investigation.

PD-1 Expression Defines Epidermal CD8+CD103+ T Cells Preferentially Producing IL-17A and Using Skewed TCR Repertoire in Psoriasis

In psoriasis, CD8⁺CD103⁺ memory T cells residing in the epidermis represent an effector population capable of maintaining the condition and driving a recurrence of the disease. Tissue-infiltrating CD8⁺ T cells expressing PD-1 are regarded as antigen-primed effector cells in others chronic inflammatory diseases. However, the expression and significance of PD-1 on skin-infiltrating CD8⁺ T cells in human psoriasis is not known. By analyzing skin-infiltrating T cells from human psoriasis, we found that active psoriatic epidermis contained PD-1 expressing CD8⁺CD103⁺ T cells that correlated with the disease severity and histopathology. PD-1⁺CD8⁺CD103⁺ T cells possessed a canonical psoriasis-specific resident memory phenotype with IL-23R expression and produced IL-17A, whereas PD-1^-CD8⁺CD103⁺ T cells preferentially produced IFN-γ. The diversity of skin-infiltrating T cells was dominated by CD4⁺ T cells, while CD8⁺ T cells, especially CD8⁺CD103⁺T cells, represented an oligoclonal population in active psoriasis. In addition, PD-1⁺CD8⁺CD103⁺T cells used different TCR Vβs from PD-1^-CD8⁺CD103⁺T cells counterpart. In the early resolved lesion, the composition and functional status of PD-1⁺CD8⁺CD103⁺T cells were markedly altered, while PD-1^-CD8⁺CD103⁺ T cells population was minimally changed. Collectively, PD-1 expression delineates a putative pathogenic subset of epidermal CD8⁺CD103⁺ T cells, which possibly play a role in psoriasis pathogenesis.

Discipline in Stages: Regulating CD8+ Resident Memory T Cells

Resident memory CD8⁺ T (T_RM) cells are a lymphocyte lineage distinct from circulating memory CD8⁺ T cells. T_RM lodge within peripheral tissues and secondary lymphoid organs where they provide rapid, local protection from pathogens and control tumor growth. However, dysregulation of CD8⁺ T_RM formation and/or activation may contribute to the pathogenesis of autoimmune diseases. Intrinsic mechanisms, including transcriptional networks and inhibitory checkpoint receptors control T_RM differentiation and response. Additionally, extrinsic stimuli such as cytokines, cognate antigen, fatty acids, and damage signals regulate T_RM formation, maintenance, and expansion. In this review, we will summarize knowledge of CD8⁺ T_RM generation and highlight mechanisms that regulate the persistence and responses of heterogeneous T_RM populations in different tissues and distinct microenvironments.

The Roles of Immunoregulatory Networks in Severe Drug Hypersensitivity

The immunomodulatory effects of regulatory T cells (Tregs) and co-signaling receptors have gained much attention, as they help balance immunogenic and immunotolerant responses that may be disrupted in autoimmune and infectious diseases. Drug hypersensitivity has a myriad of manifestations, which ranges from the mild maculopapular exanthema to the severe Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), and drug reaction with eosinophilia and systemic symptoms/drug-induced hypersensitivity syndrome (DRESS/DIHS). While studies have identified high-risk human leukocyte antigen (HLA) allotypes, the presence of the HLA allotype at risk is not sufficient to elicit drug hypersensitivity. Recent studies have suggested that insufficient regulation by Tregs may play a role in severe hypersensitivity reactions. Furthermore, immune checkpoint inhibitors, such as anti-CTLA-4 or anti-PD-1, in cancer treatment also induce hypersensitivity reactions including SJS/TEN and DRESS/DIHS. Taken together, mechanisms involving both Tregs as well as coinhibitory and costimulatory receptors may be crucial in the pathogenesis of drug hypersensitivity. In this review, we summarize the currently implicated roles of co-signaling receptors and Tregs in delayed-type drug hypersensitivity in the hope of identifying potential pharmacologic targets.

PD-L1 on mast cells suppresses effector CD8+ T-cell activation in the skin in murine contact hypersensitivity

BACKGROUND

The programmed cell death-1 (PD-1)/programmed death ligand 1 (PD-L1) pathway is known to inhibit the activation of effector CD8⁺ T cells. However, just how this regulatory pathway is involved in the pathophysiology of CD8⁺ T-cell-mediated inflammatory skin diseases remains unclear.

OBJECTIVE

Our aim was to elucidate the mechanisms by which the PD-1/PD-L1 pathway exerts its regulatory roles in CD8⁺ T-cell-mediated cutaneous immune responses.

METHODS

PD-L1-deficient (Pdl1^-/-) mice were used for the murine contact hypersensitivity model. Inflammatory responses such as IFN-γ production from CD8⁺ T cells in the skin was evaluated by flow cytometry.

RESULTS

Compared with wild-type mice, Pdl1^-/- mice exhibited exacerbated ear swelling and increased numbers of IFN-γ⁺ CD8⁺ T cells in the skin. Adoptive T-cell transfer experiments revealed the involvement of the PD-1/PD-L1 pathway in the elicitation phase of contact hypersensitivity. Bone marrow chimera experiments showed that PD-L1 on radioresistant cells was responsible for this regulatory pathway. Flow cytometric analysis revealed that among the radioresistant cells in the skin, PD-L1 was most highly expressed on mast cells (MCs) before and after elicitation. Administration of anti-PD-L1 blocking antibody during the elicitation phase significantly enhanced ear swelling responses and increased the number of IFN-γ⁺CD8⁺ T cells in the skin of wild-type mice, whereas no significant effects were observed in MC-deficient (WBB6F1/J-Kit^W/Kit^W-v/J and C57BL/6-KitW^-sh/W^-sh) mice. The high level of expression of PD-L1 on human skin MCs was confirmed by database analysis and immunohistochemical analysis.

CONCLUSION

PD-L1 on MCs negatively regulates CD8⁺ T-cell activation in the skin.

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.

Inactivation of the PD-1-Dependent Immunoregulation in Mice Exacerbates Contact Hypersensitivity Resembling Immune-Related Adverse Events

Blockade of PD-1, an indispensable physiological immunoregulatory mechanism, enhances immune activities and is widely used in the immunotherapy of cancer. This treatment often accompanies inflammatory complication called immune-related adverse events (irAE), most frequently in the skin. To analyze how skin inflammation develops by the blockade of PD-1-dependent immunoregulation, we studied the exacerbation of oxazolone-induced contact hypersensitivity by PD-L1 blockade. The inactivation of PD-1 signaling enhanced swelling of the skin with massive CD8⁺ T cell infiltration. Among PD-1-expressing cells, T cells were the predominant targets of anti-PD-L1 mAb treatment since PD-L1 blockade did not affect skin inflammation in RAG2^-/- mice. PD-L1 blockade during immunization with oxazolone significantly promoted the development of hapten-reactive T cells in the draining lymph nodes. The enhancement of local CD8⁺ T cell-dominant immune responses by PD-L1 blockade was correlated with the upregulation of CXCL9 and CXCL10. Challenges with a low dose of oxazolone did not demonstrate any significant dermatitis; however, the influence of PD-L1 blockade on T cell immunity was strong enough to cause the emergence of notable dermatitis in this suboptimal dosing, suggesting its relevance to dermal irAE development. In the low-dose setting, the blockade of CXCR3, receptor of CXCL9/10, prevented the induction of T cell-dominant inflammation by anti-PD-L1 mAb. This experimental approach reproduced CD8⁺ T cell-dominant form of cutaneous inflammation by the blockade of PD-L1 that has been observed in dermal irAE in human patients.

Functional Heterogeneity and Therapeutic Targeting of Tissue-Resident Memory T Cells

Tissue-resident memory T (T_RM) cells mediate potent local innate and adaptive immune responses and provide long-lasting protective immunity. T_RM cells localize to many different tissues, including barrier tissues, and play a crucial role in protection against infectious and malignant disease. The formation and maintenance of T_RM cells are influenced by numerous factors, including inflammation, antigen triggering, and tissue-specific cues. Emerging evidence suggests that these signals also contribute to heterogeneity within the T_RM cell compartment. Here, we review the phenotypic and functional heterogeneity of CD8⁺ T_RM cells at different tissue sites and the molecular determinants defining CD8⁺ T_RM cell subsets. We further discuss the possibilities of targeting the unique cell surface molecules, cytokine and chemokine receptors, transcription factors, and metabolic features of T_RM cells for therapeutic purposes. Their crucial role in immune protection and their location at the frontlines of the immune defense make T_RM cells attractive therapeutic targets. A better understanding of the possibilities to selectively modulate T_RM cell populations may thus improve vaccination and immunotherapeutic strategies employing these potent immune cells.

Competition for Active TGFβ Cytokine Allows for Selective Retention of Antigen-Specific Tissue- Resident Memory T Cells in the Epidermal Niche

Following antigen-driven expansion in lymph node, transforming growth factor-β (TGFβ) is required for differentiation of skin-recruited CD8+ T cell effectors into epidermal resident memory T (Trm) cells and their epidermal persistence. We found that the source of TGFβ -supporting Trm cells was autocrine. In addition, antigen-specific Trm cells that encountered cognate antigen in the skin, and bystander Trm cells that did not, both displayed long-term persistence in the epidermis under steady-state conditions. However, when the active-TGFβ was limited or when new T cell clones were recruited into the epidermis, antigen-specific Trm cells were more efficiently retained than bystander Trm cells. Genetically enforced TGFβR signaling allowed bystander Trm cells to persist in the epidermis as efficiently as antigen-specific Trm cells in both contexts. Thus, competition between T cells for active TGFβ represents an unappreciated selective pressure that promotes the accumulation and persistence of antigen-specific Trm cells in the epidermal niche.