Epidermis instructs skin homing receptor expression in human T cells

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.

Human Intestinal Dendritic Cells Can Overcome Retinoic Acid Signaling to Generate Proinflammatory CD4 T Cells with Both Gut and Skin Homing Properties

Retinoic acid, produced by intestinal dendritic cells (DCs), promotes T cell trafficking to the intestinal mucosa by upregulating α4β7 integrin and inhibiting the generation of cutaneous leukocyte Ag (CLA) required for skin entry. In the present study, we report that activation of human naive CD4 T cells in an APC-free system generates cells expressing α4β7 alone; in contrast, activation by intestinal DCs that produce retinoic acid and induce high levels of α4β7 also results in CLA expression, generating CLA+α4β7+ "dual tropic" cells, with both gut and skin trafficking potential, that also express high levels of α4β1 integrin. DC generation of CLA+α4β7+ T cells is associated with upregulation of FUT7, a fucosyltransferase involved in CLA generation; requires cell contact; and is enhanced by IL-12/IL-23. The blood CD4+ T cell population contains CLA+α4β7+ cells, which are significantly enriched for cells capable of IFN-γ, IL-17, and TNF-α production compared with conventional CLA-α4β7+ cells. Dual tropic lymphocytes are increased in intestinal tissue from patients with Crohn's disease, and single-cell RNA-sequencing analysis identifies a transcriptionally distinct cluster of FUT7-expressing cells present only in inflamed tissue; expression of genes associated with cell proliferation suggests that these cells are undergoing local activation. The expression of multiple trafficking molecules by CLA+α4β7+ T cells can enable their recruitment by alternative pathways to both skin and gut; they may contribute to both intestinal and cutaneous manifestations of inflammatory bowel disease.

Therapeutic targeting of regulatory T cells in cancer

The success of immunotherapy in oncology underscores the vital role of the immune system in cancer development. Regulatory T cells (Tregs) maintain a fine balance between autoimmunity and immune suppression. They have multiple roles in the tumor microenvironment (TME) but act particularly in suppressing T cell activation. This review focuses on the detrimental and sometimes beneficial roles of Tregs in tumors, our current understanding of recruitment and stabilization of Tregs within the TME, and current Treg-targeted therapeutics. Research identifying subpopulations of Tregs and their respective functions and interactions within the complex networks of the TME will be crucial to develop the next generation of immunotherapies. Through these advances, Treg-targeted immunotherapy could have important implications for the future of oncology.

T-cell subsets in the skin and their role in inflammatory skin disorders

T lymphocytes (T cells) are major players of the adaptive immune response. Naive T cells are primed in the presence of cytokines, leading to polarization into distinct T-cell subsets with specific functions. These subsets are classified based on their T-cell receptor profile, expression of transcription factors, surface cytokine and chemokine receptors, and their cytokine production, which together determine their specific function. This review provides an overview of the various T-cell subsets and their function in several inflammatory skin disorders ranging from allergic inflammation to skin tumors. Moreover, we highlight similarities of T-cell responses across different skin disorders, demonstrating the presence of similar and opposing functions for the different T-cell subsets. Finally, we discuss the effects of currently available and promising therapeutic approaches to harness T cells in inflammatory skin diseases for which efficacy next to unwanted side effects provide new insights into the pathophysiology of skin disorders.

Chemokine Receptor-Targeted Therapies: Special Case for CCR8

Immune checkpoint blockade inhibitors (CBIs) targeting cytotoxic T lymphocyte associated protein-4 (CTLA-4) and program death receptor-1 (PD-1) or its ligand-1 (PD-L1) have transformed the outlook of many patients with cancer. This remarkable progress has highlighted, from the translational point of view, the importance of immune cells in the control of tumor progression. There is still room for improvement, since current CBI therapies benefit a minority of patients. Moreover, interference with immune checkpoint receptors frequently causes immune related adverse events (irAEs) with life-threatening consequences in some of the patients. Immunosuppressive cells in the tumor microenvironment (TME), including intratumoral regulatory T (Treg) cells, tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), contribute to tumor progression and correlate with a negative disease outlook. Recent reports revealed the selective expression of the chemokine receptor CCR8 on tumor Treg cells, making CCR8 a promising target in translational research. In this review, I summarize our current knowledge about the cellular distribution and function of CCR8 in physiological and pathophysiological processes. The discussion includes an assessment of how the removal of CCR8-expressing cells might affect both anti-tumor immunity as well as immune homeostasis at remote sites. Based on these considerations, CCR8 appears to be a promising novel target to be considered in future translational research.

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

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

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.

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.

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.

Psoriasis-associated impairment of CCL27/CCR10-derived regulation leads to IL-17A/IL-22-producing skin T-cell overactivation

Psoriasis-associated suppression of the skin-specific chemokine/receptor CCL27/CCR10 axis leads to enhanced pathogenic IL-17A/IL-22-producing skin T cell activation and inflammation.

SLAMF7 and IL-6R define distinct cytotoxic versus helper memory CD8+ T cells

The prevailing ‘division of labor’ concept in cellular immunity is that CD8⁺ T cells primarily utilize cytotoxic functions to kill target cells, while CD4⁺ T cells exert helper/inducer functions. Multiple subsets of CD4⁺ memory T cells have been characterized by distinct chemokine receptor expression. Here, we demonstrate that analogous CD8⁺ memory T-cell subsets exist, characterized by identical chemokine receptor expression signatures and controlled by similar generic programs. Among them, Tc2, Tc17 and Tc22 cells, in contrast to Tc1 and Tc17 + 1 cells, express IL-6R but not SLAMF7, completely lack cytotoxicity and instead display helper functions including CD40L expression. CD8⁺ helper T cells exhibit a unique TCR repertoire, express genes related to skin resident memory T cells (T_RM) and are altered in the inflammatory skin disease psoriasis. Our findings reveal that the conventional view of CD4⁺ and CD8⁺ T cell capabilities and functions in human health and disease needs to be revised.

We classically consider the T cell compartment divided into cytotoxic CD8⁺ T cells and multiple, different helper CD4⁺ T cell subsets. Here the authors demonstrate that distinct memory CD8⁺ T cell subsets phenotypically inhabit CD4⁺ T cell like populations including some with helper-like characteristics.

Skin Resident γδ T Cell Function and Regulation in Wound Repair

The skin is a critical barrier that protects against damage and infection. Within the epidermis and dermis reside γδ T cells that play a variety of key roles in wound healing and tissue homeostasis. Skin-resident γδ T cells require T cell receptor (TCR) ligation, costimulation, and cytokine reception to mediate keratinocyte activity and inflammatory responses at the wound site for proper wound repair. While both epidermal and dermal γδ T cells regulate inflammatory responses in wound healing, the timing and factors produced are distinct. In the absence of growth factors, cytokines, and chemokines produced by γδ T cells, wound repair is negatively impacted. This disruption in γδ T cell function is apparent in metabolic diseases such as obesity and type 2 diabetes. This review provides the current state of knowledge on skin γδ T cell activation, regulation, and function in skin homeostasis and repair in mice and humans. As we uncover more about the complex roles played by γδ T cells in wound healing, novel targets can be discovered for future clinical therapies.

sLeX Expression Delineates Distinct Functional Subsets of Human Blood Central and Effector Memory T Cells

Sialyl Lewis X (sLeX) regulates T cell trafficking from the vasculature into skin and sites of inflammation, thereby playing a critical role in immunity. In healthy persons, only a small proportion of human blood T cells express sLeX, and their function is not fully defined. Using a combination of biochemical and functional studies, we find that human blood sLeX+CD4+T cells comprise a subpopulation expressing high levels of Th2 and Th17 cytokines, chemokine receptors CCR4 and CCR6, and the transcription factors GATA-3 and RORγT. Additionally, sLeX+CD4+T cells exclusively contain the regulatory T cell population (CD127lowCD25high and FOXP3+) and characteristically display immune-suppressive molecules, including the coinhibitor receptors PD-1 and CTLA-4. Among CD8+T cells, sLeX expression distinguishes a subset displaying low expression of cytotoxic effector molecules, perforin and granzyme β, with reduced degranulation and CD57 expression and, consistently, marginal cytolytic capacity after TCR engagement. Furthermore, sLeX+CD8+T cells present a pattern of features consistent with Th cell-like phenotype, including release of pertinent Tc2 cytokines and elevated expression of CD40L. Together, these findings reveal that sLeX display is associated with unique functional specialization of both CD4+ and CD8+T cells and indicate that circulating T cells that are primed to migrate to lesional sites at onset of inflammation are not poised for cytotoxic function.

A novel humanized mouse model to study the function of human cutaneous memory T cells in vivo in human skin

Human skin contains a population of memory T cells that supports tissue homeostasis and provides protective immunity. The study of human memory T cells is often restricted to in vitro studies and to human PBMC serving as primary cell source. Because the tissue environment impacts the phenotype and function of memory T cells, it is crucial to study these cells within their tissue. Here we utilized immunodeficient NOD-scid IL2rγ^null (NSG) mice that carried in vivo-generated engineered human skin (ES). ES was generated from human keratinocytes and fibroblasts and was initially devoid of skin-resident immune cells. Upon adoptive transfer of human PBMC, this reductionist system allowed us to study human T cell recruitment from a circulating pool of T cells into non-inflamed human skin in vivo. Circulating human memory T cells preferentially infiltrated ES and showed diverse functional profiles of T cells found in fresh human skin. The chemokine and cytokine microenvironment of ES closely resembled that of non-inflamed human skin. Upon entering the ES T cells assumed a resident memory T cell-like phenotype in the absence of infection, and a proportion of these cutaneous T cells can be locally activated upon injection of monocyte derived dendritic cells (moDCs) that presented Candida albicans. Interestingly, we found that CD69⁺ memory T cells produced higher levels of effector cytokines in response to Candida albicans, compared to CD69^- T cells. Overall, this model has broad utility in many areas of human skin immunology research, including the study of immune-mediated skin diseases.

A Review of Micronutrients and the Immune System-Working in Harmony to Reduce the Risk of Infection

Immune support by micronutrients is historically based on vitamin C deficiency and supplementation in scurvy in early times. It has since been established that the complex, integrated immune system needs multiple specific micronutrients, including vitamins A, D, C, E, B6, and B12, folate, zinc, iron, copper, and selenium, which play vital, often synergistic roles at every stage of the immune response. Adequate amounts are essential to ensure the proper function of physical barriers and immune cells; however, daily micronutrient intakes necessary to support immune function may be higher than current recommended dietary allowances. Certain populations have inadequate dietary micronutrient intakes, and situations with increased requirements (e.g., infection, stress, and pollution) further decrease stores within the body. Several micronutrients may be deficient, and even marginal deficiency may impair immunity. Although contradictory data exist, available evidence indicates that supplementation with multiple micronutrients with immune-supporting roles may modulate immune function and reduce the risk of infection. Micronutrients with the strongest evidence for immune support are vitamins C and D and zinc. Better design of human clinical studies addressing dosage and combinations of micronutrients in different populations are required to substantiate the benefits of micronutrient supplementation against infection.

Migration and Function of Memory CD8+ T Cells in Skin

CD8⁺ memory T cells provide anamnestic host defense against intracellular pathogens and cancer immunosurveillance but are also pathogenic in some autoimmune diseases. In mouse skin, there are two unique subsets of CD8⁺ memory T cells, resident memory cells that reside long-term in steady state skin and recirculating memory cells that are transient. They have distinct mechanisms of recruitment, development, and maintenance in response to skin-derived signals. In this review, we will focus on these mechanisms and the functional relationship of these two types of CD8⁺ memory cells with host defense and disease.

Skin barrier immunity and ageing

The skin is the outermost layer of the body with an extensive surface area of approximately 1·8 m² , and is the first line of defence against a multitude of external pathogens and environmental insults. The skin also has important homeostatic functions such as reducing water loss and contributing to thermoregulation of the body. The structure of the skin and its cellular composition work in harmony to prevent infections and to deal with physical and chemical challenges from the outside world. In this review, we discuss how the structural cells such as keratinocytes, fibroblasts and adipocytes contribute to barrier immunity. We also discuss specialized immune cells that are resident in steady-state skin including mononuclear phagocytes, such as Langerhans cells, dermal macrophages and dermal dendritic cells in addition to the resident memory T cells. Ageing results in an increased incidence of cancer and skin infections. As we age, the skin structure changes with thinning of the epidermis and dermis, increased water loss, and fragmentation of collagen and elastin. In addition, the skin immune composition is altered with reduced Langerhans cells, decreased antigen-specific immunity and increased regulatory populations such as Foxp3⁺ regulatory T cells. Together, these alterations result in decreased barrier immunity in the elderly, explaining in part their increased susceptiblity to cancer and infections.

Tissue-resident memory T cells and their biological characteristics in the recurrence of inflammatory skin disorders

The skin is the largest organ of the body. The establishment of immunological memory in the skin is a crucial component of the adaptive immune response. Once naive T cells are activated by antigen-presenting cells, a small fraction of them differentiate into precursor memory T cells. These precursor cells ultimately develop into several subsets of memory T cells, including central memory T (T_CM) cells, effector memory T (T_EM) cells, and tissue resident memory T (T_RM) cells. T_RM cells have a unique transcriptional profile, and their most striking characteristics are their long-term survival (longevity) and low migration in peripheral tissues, including the skin. Under physiological conditions, T_RM cells that reside in the skin can respond rapidly to pathogenic challenges. However, there is emerging evidence to support the vital role of T_RM cells in the recurrence of chronic inflammatory skin disorders, including psoriasis, vitiligo, and fixed drug eruption, under pathological or uncontrolled conditions. Clarifying and characterizing the mechanisms that are involved in skin T_RM cells will help provide promising strategies for reducing the frequency and magnitude of skin inflammation recurrence. Here, we discuss recent insights into the generation, homing, retention, and survival of T_RM cells and share our perspectives on the biological characteristics of T_RM cells in the recurrence of inflammatory skin disorders.

A Comprehensive Profile of Chemokine Gene Expression in the Tissues of the Female Reproductive Tract in Mice

Homeostatic leukocyte trafficking into and within the female reproductive tract (FRT) contributes to fertility and reproductive health. It is unclear how this process is regulated in the anatomically distinct reproductive tissues, or whether the genes involved are affected by cyclical changes in reproductive hormones. In tissues such as skin and intestine, mouse studies have defined evolutionarily conserved molecular mechanisms for tissue-specific homing, interstitial positioning, and leukocyte egress. Chemokine family members are invariably involved, with the chemokine expression profile of a tissue regulating leukocyte content. Reproductive tissues (ovary, vagina, cervix, uterine horn) of 8 week old virgin female C57BL/6 mice (n = 20) were collected, and expression of mRNA for leukocyte markers and chemokines conducted by qPCR. Lymphocytic and myeloid cell populations within the uterus, cervix, bone marrow and PALN from virgin C57BL/6 mice were determined by flow cytometric analysis. Variation in leukocyte content between reproductive tissues is evident, with the uterus and cervix containing complex mixtures of lymphocytes and myeloid cells. Twenty-six chemokine genes are expressed in the FRT, many by several component tissues, some preferentially by one. Most striking are Xcl1 and Ccl28, which are restricted to the uterus. Ccl20 and genes encoding CXCR2 ligands are primarily transcribed in cervix and vagina. Ovary shows the lowest expression of most chemokine genes, with the notable exception of Ccl21 and Ccl27. We also identify eight chemokines in the vagina whose expression fluctuates substantially across the oestrous cycle. These data reveal complex chemokine networks within the FRT, and provide a framework for future studies of homeostatic leukocyte trafficking into and within these tissues.Abbreviations: BM: bone marrow; DC: dendritic cell; DN: double negative; FRT: female reproductive tract; FSC: forward scatter; NK: natural killer; PALN: para-aortic lymph node; SSC: side scatter; Tregs: regulatory T cells.

T Cells Control Chemokine Secretion by Keratinocytes

The massive infiltration of lymphocytes into the skin is a hallmark of numerous human skin disorders. By co-culturing murine keratinocytes with splenic T cells we demonstrate here that T cells affect and control the synthesis and secretion of chemokines by keratinocytes. While pre-activated CD8⁺T cells induce the synthesis of CXCL9 and CXCL10 in keratinocytes and keep in check the synthesis of CXCL1, CXCL5, and CCL20, keratinocytes dampen the synthesis of CCL3 and CCL4 in pre-activated CD8⁺T cells. One key molecule is IFN-γ that is synthesized by CD8⁺T cells under the control of NFATc1 and NFATc2. CD8⁺T cells deficient for both NFAT factors are unable to induce CXCL9 and CXCL10 expression. In addition, CD8⁺T cells induced numerous type I IFN-inducible “defense genes” in keratinocytes encoding the PD1 and CD40 ligands, TNF-α and caspase-1. The enhanced expression of type I IFN-inducible genes resembles the gene expression pattern at the dermal/epidermal interface in lichen planus, an inflammatory T lymphocyte-driven skin disease, in which we detected the expression of CXCL10 in keratinocytes in close vicinity to the infiltration front of T cells. These data reflect the multifaceted interplay of lymphocytes with keratinocytes at the molecular level.

CXCR3 regulates CD4+ T cell cardiotropism in pressure overload-induced cardiac dysfunction

Heart failure (HF) is associated in humans and mice with increased circulating levels of CXCL9 and CXCL10, chemokine ligands of the CXCR3 receptor, predominantly expressed on CD4+ Th1 cells. Chemokine engagement of receptors is required for T cell integrin activation and recruitment to sites of inflammation. Th1 cells drive adverse cardiac remodeling in pressure overload-induced cardiac dysfunction, and mice lacking the integrin ligand ICAM-1 show defective T cell recruitment to the heart. Here, we show that CXCR3+ T cells infiltrate the heart in humans and mice with pressure overload-induced cardiac dysfunction. Genetic deletion of CXCR3 disrupts CD4+ T cell heart infiltration and prevents adverse cardiac remodeling. We demonstrate that cardiac fibroblasts and cardiac myeloid cells that include resident and infiltrated macrophages are the source of CXCL9 and CXCL10, which mechanistically promote Th1 cell adhesion to ICAM-1 under shear conditions in a CXCR3-dependent manner. To our knowledge, our findings identify a previously unrecognized role for CXCR3 in Th1 cell recruitment into the heart in pressure overload-induced cardiac dysfunction.

Transcriptional analysis distinguishes breast implant-associated anaplastic large cell lymphoma from other peripheral T-cell lymphomas

Breast implant-associated anaplastic large cell lymphoma is a new provisional entity in the revised World Health Organization classification of lymphoid malignancies, the pathogenesis and cell of origin of which are still unknown. We performed gene expression profiling of microdissected breast implant-associated anaplastic large cell lymphoma samples and compared their transcriptional profiles with those previously obtained from normal T-cells and other peripheral T-cell lymphomas and validated expression of selected markers by immunohistochemistry. Our results indicate that most breast implant-associated anaplastic large cell lymphomas exhibit an activated CD4+ memory T-cell phenotype, which is associated with CD25 and FoxP3 expression. Gene ontology analyses revealed upregulation of genes involved in cell motility programs (e.g., CCR6, MET, HGF, CXCL14) in breast implant-associated anaplastic large cell lymphomas compared to normal CD4+ T-cells and upregulation of genes involved in myeloid cell differentiation (e.g., PPARg, JAK2, SPI-1, GAB2) and viral gene transcription (e.g., RPS10, RPL17, RPS29, RPL18A) compared to other types of peripheral T-cell lymphomas. Gene set enrichment analyses also revealed shared features between the molecular profiles of breast implant-associated anaplastic large cell lymphomas and other types of anaplastic large cell lymphomas, including downregulation of T-cell receptor signaling and STAT3 activation. Our findings provide novel insights into the biology of this rare disease and further evidence that breast implant-associated anaplastic large cell lymphoma represents a distinct peripheral T-cell lymphoma entity.

Skin-homing CD8+ T cells preferentially express GPI-anchored peptidase inhibitor 16, an inhibitor of cathepsin K

This study sought to identify novel CD8⁺ T cell homing markers by studying acute graft versus host disease (aGvHD), typically involving increased T cell homing to the skin and gut. FACS-sorted skin-homing (CD8β⁺ /CLA⁺ ), gut-homing (CD8β⁺ /integrinβ7⁺ ), and reference (CD8β⁺ /CLA^- /integrinβ7^- ) T cells were compared in patients affected by cutaneous and/or gastrointestinal aGVHD. Microarray analysis, qPCR, and flow cytometry revealed increased expression of peptidase inhibitor 16 (PI16) in skin-homing CD8⁺ T cells. Robust association of PI16 with skin homing was confirmed in all types of aGvHD and in healthy controls, too. PI16 was not observed on CLA⁺ leukocytes other than T cells. Induction of PI16 expression on skin-homing T cells occurred independently of vitamin D3. Among skin-homing T cells, PI16 expression was most pronounced in memory-like CD45RO⁺ /CD127⁺ /CD25⁺ /CD69^- /granzyme B^- cells. PI16 was confined to the plasma membrane, was GPI-anchored, and was lost upon restimulation of memory CD8⁺ T cells. Loss of PI16 occurred by downregulation of PI16 transcription, and not by Phospholipase C (PLC)- or Angiotensin-converting enzyme (ACE)-mediated shedding, or by protein recycling. Inhibitor screening and pull-down experiments confirmed that PI16 inhibits cathepsin K, but may not bind to other skin proteases. These data link PI16 to skin-homing CD8⁺ T cells, and raise the possibility that PI16 may regulate cutaneous cathepsin K.

Peripheral Tissue Chemokines: Homeostatic Control of Immune Surveillance T Cells

Cellular immunity is governed by a complex network of migratory cues that enable appropriate immune cell responses in a timely and spatially controlled fashion. This review focuses on the chemokines and their receptors regulating the steady-state localisation of immune cells within healthy peripheral tissues. Steady-state immune cell traffic is not well understood but is thought to involve constitutive (homeostatic) chemokines. The recent discovery of tissue-resident memory T cells (T_RM cells) illustrates our need for understanding how chemokines control immune cell mobilisation and/or retention. These studies will be critical to unravel novel pathways for preserving tissue function (aging) and preventing tissue disease (vaccination).

Taking the lead - how keratinocytes orchestrate skin T cell immunity

The skin comprises a complex coordinated system of epithelial tissue cells and immune cells that ensure adequate immune reactions against trauma, toxins and pathogens, while maintaining tissue homeostasis. Keratinocytes form the outermost barrier of the skin, and sense changes in barrier integrity, intrusion of microbial components and stress molecules. Thus, they act as sentinels that continuously communicate the status of the organ to the cutaneous immune system. Upon damage the keratinocytes initiate a pro-inflammatory signaling cascade that leads to the activation of resident immune cells. Simultaneously, the tissue mediates and supports immune-suppressive functions to contain inflammation locally. After resolution of inflammation, the skin provides a niche for regulatory and effector memory T cells that can quickly respond to reoccurring antigens. In this review we discuss the central role of keratinocyte-derived signals in controlling cutaneous T cell immunity.

Short communication: Lymphocyte proliferative responses in cattle naturally infected with digital dermatitis consist of CD8+ and γδ-T cells but lack CD4+ T cells

Digital dermatitis is an infectious disease of cattle and the leading cause of lameness. This disease is complicated by the reoccurrence of the lesions and the observation of lesions on more than one limb at different time points, indicating infection may not result in a protective immune response. The objective of this study was to characterize the peripheral blood cellular response in naturally infected and naïve cattle to bacterial antigens derived from pathogens associated with digital dermatitis lesions. Peripheral blood mononuclear cells were isolated from dairy cattle identified as having active or chronic lesions during routine hoof-trimming. Following bacterial antigen stimulation, cells were analyzed for proliferation and phenotype by flow cytometry, and culture supernatants were analyzed for IFN-γ secretion. Digital-dermatitis-infected animals had greater serum antibody titers to treponemal antigens, higher percentages of proliferating CD8+, γδ-T cells, and B cells, and increased IFN-γ secretion in vitro when compared with responses of naïve animals. No increase in proliferation of CD4+ T cells was detected in infected or naïve cattle. Although CD8+ and γδ-T cell responses may be antigen specific, the memory nature or long-lived response is yet unknown. The lack of responsiveness of CD4+ memory cells to treponemal antigens could explain the high rate of reoccurrence of digital dermatitis in infected animals.

Human αβ and γδ T Cells in Skin Immunity and Disease

γδ T lymphocytes maintain skin homeostasis by balancing keratinocyte differentiation and proliferation with the destruction of infected or malignant cells. An imbalance in skin-resident T cell function can aggravate skin-related autoimmune diseases, impede tumor eradication, or disrupt proper wound healing. Much of the published work on human skin T cells attributes T cell function in the skin to αβ T cells, while γδ T cells are an often overlooked participant. This review details the roles played by both αβ and γδ T cells in healthy human skin and then focuses on their roles in skin diseases, such as psoriasis and alopecia areata. Understanding the contribution of skin-resident and skin-infiltrating T cell populations and cross-talk with other immune cells is leading to the development of novel therapeutics for patients. However, there is still much to be learned in order to effectively modulate T cell function and maintain healthy skin homeostasis.

Niches for the Long-Term Maintenance of Tissue-Resident Memory T Cells

Tissue-resident memory T cells (T_RM cells) are a population of immune cells that reside in the lymphoid and non-lymphoid organs without recirculation through the blood. These important cells occupy and utilize unique anatomical and physiological niches that are distinct from those for other memory T cell populations, such as central memory T cells in the secondary lymphoid organs and effector memory T cells that circulate through the tissues. CD8⁺ T_RM cells typically localize in the epithelial layers of barrier tissues where they are optimally positioned to act as sentinels to trigger antigen-specific protection against reinfection. CD4⁺ T_RM cells typically localize below the epithelial layers, such as below the basement membrane, and cluster in lymphoid structures designed to optimize interactions with antigen-presenting cells upon reinfection. A key feature of T_RM populations is their ability to be maintained in barrier tissues for prolonged periods of time. For example, skin CD8⁺ T_RM cells displace epidermal niches originally occupied by γδ T cells, thereby enabling their stable persistence for years. It is also clear that the long-term maintenance of T_RM cells in different microenvironments is dependent on multiple tissue-specific survival cues, although the specific details are poorly understood. However, not all T_RM persist over the long term. Recently, we identified a new spatial niche for the maintenance of CD8⁺ T_RM cells in the lung, which is created at the site of tissue regeneration after injury [termed repair-associated memory depots (RAMD)]. The short-lived nature of RAMD potentially explains the short lifespans of CD8⁺ T_RM cells in this particular tissue. Clearly, a better understanding of the niche-dependent maintenance of T_RM cells will be important for the development of vaccines designed to promote barrier immunity. In this review, we discuss recent advances in our understanding of the properties and nature of tissue-specific niches that maintain T_RM cells in different tissues.

A guide to chemokines and their receptors

The chemokines (or chemotactic cytokines) are a large family of small, secreted proteins that signal through cell surface G protein-coupled heptahelical chemokine receptors. They are best known for their ability to stimulate the migration of cells, most notably white blood cells (leukocytes). Consequently, chemokines play a central role in the development and homeostasis of the immune system, and are involved in all protective or destructive immune and inflammatory responses. Classically viewed as inducers of directed chemotactic migration, it is now clear that chemokines can stimulate a variety of other types of directed and undirected migratory behavior, such as haptotaxis, chemokinesis, and haptokinesis, in addition to inducing cell arrest or adhesion. However, chemokine receptors on leukocytes can do more than just direct migration, and these molecules can also be expressed on, and regulate the biology of, many nonleukocytic cell types. Chemokines are profoundly affected by post-translational modification, by interaction with the extracellular matrix (ECM), and by binding to heptahelical 'atypical' chemokine receptors that regulate chemokine localization and abundance. This guide gives a broad overview of the chemokine and chemokine receptor families; summarizes the complex physical interactions that occur in the chemokine network; and, using specific examples, discusses general principles of chemokine function, focusing particularly on their ability to direct leukocyte migration.

The lymphoid cell network in the skin

Cutaneous immunity represents a crucial component of the mammalian immune response. The presence of a large array of commensal microorganisms along with a myriad of environmental stresses necessitates constant immuno-surveillance of the tissue. To achieve a perfect balance between immune-tolerance and immune-activation, the skin harbors strategically localized immune cell populations that modulate these responses. To maintain homeostasis, innate and adaptive immune cells assimilate microenvironmental cues and coordinate cellular and molecular functions in a spatiotemporal manner. The role of lymphoid cells in cutaneous immunity is gaining much appreciation due to their important roles in regulating skin health and pathology. In this review, we aim to highlight the recent advances in the field of cutaneous lymphoid biology.

CCR8 Expression Defines Tissue-Resident Memory T Cells in Human Skin

Human skin harbors two major T cell compartments of equal size that are distinguished by expression of the chemokine receptor CCR8. In vitro studies have demonstrated that CCR8 expression is regulated by TCR engagement and the skin tissue microenvironment. To extend these observations, we examined the relationship between CCR8+ and CCR8- skin T cells in vivo. Phenotypic, functional, and transcriptomic analyses revealed that CCR8+ skin T cells bear all the hallmarks of resident memory T cells, including homeostatic proliferation in response to IL-7 and IL-15, surface expression of tissue localization (CD103) and retention (CD69) markers, low levels of inhibitory receptors (programmed cell death protein 1, Tim-3, LAG-3), and a lack of senescence markers (CD57, killer cell lectin-like receptor subfamily G member 1). In contrast, CCR8- skin T cells are heterogeneous and comprise variable numbers of exhausted (programmed cell death protein 1+), senescent (CD57+, killer cell lectin-like receptor subfamily G member 1+), and effector (T-bethi, Eomeshi) T cells. Importantly, conventional and high-throughput sequencing of expressed TCR β-chain (TRB) gene rearrangements showed that these CCR8-defined populations are clonotypically distinct, suggesting unique ontogenies in response to separate antigenic challenges and/or stimulatory conditions. Moreover, CCR8+ and CCR8- skin T cells were phenotypically stable in vitro and displayed similar levels of telomere erosion, further supporting the likelihood of a nonlinear differentiation pathway. On the basis of these results, we propose that long-lived memory T cells in human skin can be defined by the expression of CCR8.

Activation and trafficking of CD8+ T cells during viral skin infection: immunological lessons learned from vaccinia virus

Epicutaneous delivery of vaccinia virus (VacV) by scarification of the skin generates robust and durable protective immunity, which was ultimately responsible for eradicating smallpox from the human race. Therefore, infection of the skin with VacV is often used in experimental model systems to study the activation of adaptive immunity, as well as the development and functional features of immunological memory. Here, we describe recent advances using this viral infection to identify and characterize the mechanisms regulating the activation and trafficking of cytotoxic CD8+ T cells into the inflamed skin, the migratory features of CD8+ T cells within the skin microenvironment, and finally, their subsequent differentiation into tissue-resident memory cells.

B Cell Presentation of Chlamydia Antigen Selects Out Protective CD4γ13 T Cells: Implications for Genital Tract Tissue-Resident Memory Lymphocyte Clusters

Surveillance and defense of the enormous mucosal interface with the nonsterile world are critical to protecting the host from a wide range of pathogens. Chlamydia trachomatis is an intracellular bacterial pathogen that replicates almost exclusively in the epithelium of the reproductive tract. The fallopian tubes and vagina are poorly suited to surveillance and defense, with limited immune infrastructure positioned near the epithelium. However, a dynamic process during clearing primary infections leaves behind new lymphoid clusters immediately beneath the epithelium. These memory lymphocyte clusters (MLCs) harboring tissue-resident memory (Trm) T cells are presumed to play an important role in protection from subsequent infections. Histologically, human Chlamydia MLCs have prominent B cell populations. We investigated the status of genital tract B cells during C. muridarum infections and the nature of T cells recovered from immune mice using immune B cells as antigen-presenting cells (APCs). These studies revealed a genital tract plasma B cell population and a novel genital tract CD4 T cell subset producing both gamma interferon (IFN-γ) and interleukin-13 (IL-13). A panel of CD4 T cell clones and microarray analysis showed that the molecular fingerprint of CD4γ13 T cells includes a Trm-like transcriptome. Adoptive transfer of a Chlamydia-specific CD4γ13 T cell clone completely prevented oviduct immunopathology without accelerating bacterial clearance. Existence of a CD4γ13 T cell subset provides a plausible explanation for the observation that human peripheral blood mononuclear cell (PBMC) Chlamydia-specific IFN-γ and IL-13 responses predict resistance to reinfection.

Modulation of CD4+ T Helper Cell Memory Responses in the Human Skin

Immunological memory is defined as the capacity to mount faster and more effective immune responses against antigenic challenges that have been previously encountered by the host. CD4+ T helper (Th) cells play central roles in the establishment of immunological memory as they assist the functions of other leukocytes. Th cells express polarized cytokine profiles and distinct migratory and seeding capacities, but also retain a certain functional plasticity that allows them to modulate their proliferation, activity, and homing behaviour upon need. Thus, in healthy individuals, T cell immunomodulation fulfils the task of eliciting protective immune responses where they are needed. At times, however, Th plasticity can lead to collateral tissue damage and progression to autoimmune diseases or, conversely, incapacity to reject malignant tissues and clear chronic infections. Furthermore, common immune players and molecular pathways of diseases can lead to different outcomes in different individuals. A mechanistic understanding of those pathways is therefore crucial for developing precise and curative medical interventions. Here, I focus on the skin microenvironment and comprehensively describe some of the cellular and molecular determinants of CD4+ T cell memory responses in homeostatic and pathological conditions. In discussing the cellular network orchestrating cutaneous immunity, I comprehensively describe the bidirectional interaction of skin antigen-presenting cells and mononuclear phagocytes with Th17 lymphocytes, and examine how the outcome of this interaction is influenced by endogenous skin molecules, including sodium salts and neuropeptides.

The molecular cues regulating immune cell trafficking

Lymphocyte recirculation between the blood and the lymphoid/non-lymphoid tissues is an essential homeostatic mechanism that regulates humoral and cellular immune responses in vivo. This system promotes the encounter of naïve T and B cells with their specific cognate antigen presented by dendritic cells, and with the regulatory cells with which they need to interact to initiate, maintain, and terminate immune responses. The constitutive lymphocyte trafficking is mediated by particular types of blood vessels, including the high endothelial venules (HEVs) in lymph nodes and Peyer's patches, and the flat-walled venules in non-lymphoid tissues including the skin. The lymphocyte migration across HEVs involves tethering/rolling, arrest/firm adhesion/intraluminal crawling, and transendothelial migration. On the other hand, relatively little is known about how lymphocytes and other types of cells migrate across the venules of non-lymphoid tissues. Here we summarize recent findings about the molecular mechanisms that govern immune cell trafficking, including the roles of chemokines and lysophospholipids in regulating immune cell motility and endothelial permeability.

Clonal selection versus clonal cooperation: the integrated perception of immune objects

Analogies between the immune and nervous systems were first envisioned by the immunologist Niels Jerne who introduced the concepts of antigen "recognition" and immune "memory". However, since then, it appears that only the cognitive immunology paradigm proposed by Irun Cohen, attempted to further theorize the immune system functions through the prism of neurosciences. The present paper is aimed at revisiting this analogy-based reasoning. In particular, a parallel is drawn between the brain pathways of visual perception and the processes allowing the global perception of an "immune object". Thus, in the visual system, distinct features of a visual object (shape, color, motion) are perceived separately by distinct neuronal populations during a primary perception task. The output signals generated during this first step instruct then an integrated perception task performed by other neuronal networks. Such a higher order perception step is by essence a cooperative task that is mandatory for the global perception of visual objects. Based on a re-interpretation of recent experimental data, it is suggested that similar general principles drive the integrated perception of immune objects in secondary lymphoid organs (SLOs). In this scheme, the four main categories of signals characterizing an immune object (antigenic, contextual, temporal and localization signals) are first perceived separately by distinct networks of immunocompetent cells.  Then, in a multitude of SLO niches, the output signals generated during this primary perception step are integrated by TH-cells at the single cell level. This process eventually generates a multitude of T-cell and B-cell clones that perform, at the scale of SLOs, an integrated perception of immune objects. Overall, this new framework proposes that integrated immune perception and, consequently, integrated immune responses, rely essentially on clonal cooperation rather than clonal selection.

Tissue-Specific Induction of CCR6 and Nrp1 During Early CD4+ T Cell Differentiation

Upon differentiation, T cells acquire tissue-specific homing properties allowing efficient targeting of effector T cells into distinct inflamed organs. Priming of T cells within skin-draining, peripheral lymph nodes (pLNs) leads to the expression of E- and P-selectin ligands, which facilitate migration into inflamed skin, whereas activation within gut-draining, mesenteric LNs (mLNs) results in induction of chemokine receptor CCR9 and integrin α₄β₇, both required for migration of effector T cells into mucosal tissues. In addition to the local tissue microenvironment, both organ-specific dendritic cells and LN-resident stromal cells are critical factors to shape T cell migration properties. Here, we identify two additional homing-related molecules, CCR6 and Neuropilin-1 (Nrp1), upregulated in T cells early during differentiation solely in pLNs, but not mLNs. Surprisingly, intestinal inflammation resulted in an ameliorated induction of CCR6 and Nrp1 in pLNs, suggesting that a local inflammation within the gut can systemically alter T cell differentiation. Finally, transplantation of mLNs to a skin-draining environment revealed that LN stromal cells also contribute to efficient CCR6 induction in pLNs. Collectively, these findings identify further aspects of early T cell differentiation within skin-draining pLNs, which could be utilized to further develop tailored and highly specialized vaccination strategies.

Mechanisms of T cell organotropism

Protective immunity relies upon T cell differentiation and subsequent migration to target tissues. Similarly, immune homeostasis requires the localization of regulatory T cells (Tregs) to the sites where immunity takes place. While naïve T lymphocytes recirculate predominantly in secondary lymphoid tissue, primed T cells and activated Tregs must traffic to the antigen rich non-lymphoid tissue to exert effector and regulatory responses, respectively. Following priming in draining lymph nodes, T cells acquire the 'homing receptors' to facilitate their access to specific tissues and organs. An additional level of topographic specificity is provided by T cells receptor recognition of antigen displayed by the endothelium. Furthermore, co-stimulatory signals (such as those induced by CD28) have been shown not only to regulate T cell activation and differentiation, but also to orchestrate the anatomy of the ensuing T cell response. We here review the molecular mechanisms supporting trafficking of both effector and regulatory T cells to specific antigen-rich tissues.

Skin CD4(+) memory T cells exhibit combined cluster-mediated retention and equilibration with the circulation

Although memory T cells within barrier tissues can persist as permanent residents, at least some exchange with blood. The extent to which this occurs is unclear. Here we show that memory CD4⁺ T cells in mouse skin are in equilibrium with the circulation at steady state. These cells are dispersed throughout the inter-follicular regions of the dermis and form clusters with antigen presenting cells around hair follicles. After infection or administration of a contact sensitizing agent, there is a sustained increase in skin CD4⁺ T-cell content, which is confined to the clusters, with a concomitant CCL5-dependent increase in CD4⁺ T-cell recruitment. Skin CCL5 is derived from CD11b⁺ cells and CD8⁺ T cells, with the elimination of the latter decreasing CD4⁺ T-cell numbers. These results reveal a complex pattern of tissue-retention and equilibration for CD4⁺ memory T cells in skin, which is altered by infection and inflammation history.

Memory T cells are vital responders to skin inflammation, but cell localization and dynamics of exchange with the bloodstream are not clear. Here the authors use parabiosis and intravital microscopy to show that CD4⁺ memory T cells equilibrate with the circulation and cluster around hair follicles in response to CCL5-dependent responses to viral infection or contact sensitization.

Mucosal immunity and novel tuberculosis vaccine strategies: route of immunisation-determined T-cell homing to restricted lung mucosal compartments

Despite the use of bacille Calmette-Guérin (BCG) for almost a century, pulmonary tuberculosis (TB) continues to be a serious global health concern. Therefore, there has been a pressing need for the development of new booster vaccines to enhance existing BCG-induced immunity. Protection following mucosal intranasal immunisation with AdHu5Ag85A is associated with the localisation of antigen-specific T-cells to the lung airway. However, parenteral intramuscular immunisation is unable to provide protection despite the apparent presence of antigen-specific T-cells in the lung interstitium. Recent advances in intravascular staining have allowed us to reassess the previously established T-cell distribution profile and its relationship with the observed differential protection. Respiratory mucosal immunisation empowers T-cells to home to both the lung interstitium and the airway lumen, whereas intramuscular immunisation-activated T-cells are largely trapped within the pulmonary vasculature, unable to populate the lung interstitium and airway. Given the mounting evidence supporting the safety and enhanced efficacy of respiratory mucosal immunisation over the traditional parenteral immunisation route, a greater effort should be made to clinically develop respiratory mucosal-deliverable TB vaccines.

Selective programming of CCR10(+) innate lymphoid cells in skin-draining lymph nodes for cutaneous homeostatic regulation

Innate lymphoid cells (ILCs) 'preferentially' localize into barrier tissues, where they function in tissue protection but can also contribute to inflammatory diseases. The mechanisms that regulate the establishment of ILCs in barrier tissues are poorly understood. Here we found that under steady-state conditions, ILCs in skin-draining lymph nodes (sLNs) were continuously activated to acquire regulatory properties and high expression of the chemokine receptor CCR10 for localization into the skin. CCR10(+) ILCs promoted the homeostasis of skin-resident T cells and, reciprocally, their establishment in the skin required T cell-regulated homeostatic environments. CD207(+) dendritic cells expressing the transcription factor Foxn1 were required for the proper generation of CCR10(+) ILCs. These observations reveal mechanisms that underlie the specific programming and priming of skin-homing CCR10(+) ILCs in the sLNs.

HGF Guides T Cells into the Heart

Expression of distinct homing receptors guides adaptive immune cells to antigen-rich tissue. In this issue of Immunity, Komarowska et al. (2015) describe an autocrine loop that is initiated by cardiac-expressed hepatocyte growth factor to direct T cells into the heart during inflammation and cardiac transplant rejection.

Hepatocyte Growth Factor Receptor c-Met Instructs T Cell Cardiotropism and Promotes T Cell Migration to the Heart via Autocrine Chemokine Release

Effector-T-cell-mediated immunity depends on the efficient localization of antigen-primed lymphocytes to antigen-rich non-lymphoid tissue, which is facilitated by the expression of a unique set of “homing” receptors acquired by memory T cells. We report that engagement of the hepatocyte growth factor (HGF) receptor c-Met by heart-produced HGF during priming in the lymph nodes instructs T cell cardiotropism, which was associated with a specialized homing “signature” (c-Met⁺CCR4⁺CXCR3⁺). c-Met signals facilitated T cell recruitment to the heart via the chemokine receptor CCR5 by inducing autocrine CCR5 ligand release. c-Met triggering was sufficient to support cardiotropic T cell recirculation, while CCR4 and CXCR3 sustained recruitment during heart inflammation. Transient pharmacological blockade of c-Met during T cell priming led to enhanced survival of heart, but not skin, allografts associated with impaired localization of alloreactive T cells to heart grafts. These findings suggest c-Met as a target for development of organ-selective immunosuppressive therapies.

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HGF-induced c-Met signals during activation induce cardiotropic memory T cells

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Cardiotropic T cells express a specific molecular signature (c-Met⁺CCR4⁺CXCR3⁺)

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By inducing an autocrine chemokine loop, c-Met also promotes T cell recruitment

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Blockade of the HGF-c-Met axis prevents heart, but not skin, allograft rejection

Fine-Needle Aspiration Biopsy of the Lymph Node: A Novel Tool for the Monitoring of Immune Responses after Skin Antigen Delivery

Assessment of immune responses in lymph nodes (LNs) is routine in animals, but rarely done in humans. We have applied minimally invasive ultrasound-guided fine-needle aspiration of the LN to a before-and-after study of the immune response to intradermally delivered Ag in healthy volunteers (n = 25). By comparison with PBMCs from the same individual, LN cells (LNCs) were characterized by reduced numbers of effector memory cells, especially CD8(+) TEMRA cells (3.37 ± 1.93 in LNCs versus 22.53 ± 7.65 in PBMCs; p = 0.01) and a marked increased in CD69 expression (27.67 ± 7.49 versus 3.49 ± 2.62%, LNCs and PBMCs, respectively; p < 0.0001). At baseline, there was a striking absence of IFN-γ ELISPOT responses to recall Ags (purified protein derivative, Tetanus toxoid, or flu/EBV/CMV viral mix) in LN, despite strong responses in the peripheral blood. However, 48 h after tuberculin purified protein derivative administration in the ipsilateral forearm resulting in a positive skin reaction, a clear increase in IFN-γ ELISPOT counts was seen in the draining LN but not in PBMCs. This response was lost by 5 d. These data suggest that the low levels of effector memory cells in the LN may explain the low background of baseline ELISPOT responses in LNs as compared with PBMCs, and the appearance of a response after 48 h is likely to represent migration of effector memory cells from the skin to the LN. Hence, it appears that the combination of intradermal Ag administration and draining LN sampling can be used as a sensitive method to probe the effector memory T cell repertoire in the skin.