Commensal-specific T cell plasticity promotes rapid tissue adaptation to injury

T cell immunity to commensal fungi

Fungi are an important part of the microbiota in healthy barrier tissues. Fungal dysbiosis in turn is associated with local and distal inflammatory diseases. Recent advances have shed light on the antigen-specific IL-17-dependent mechanisms that regulate fungal commensalism and prevent fungal overgrowth during homeostasis. Progress in our understanding of species-specific differences in fungus-host interactions provides new hypotheses of why Candida albicans-targeting T cells exceed those directed against other fungal species in the human T cell repertoire. Importantly, C. albicans-specific Th17 cells can also contribute to immune pathology in distant organs such as the lung via cross-reaction with heterologous antigens.

Developing Human Skin Contains Lymphocytes Demonstrating a Memory Signature

Lymphocytes in barrier tissues play critical roles in host defense and homeostasis. These cells take up residence in tissues during defined developmental windows, when they may demonstrate distinct phenotypes and functions. Here, we utilized mass and flow cytometry to elucidate early features of human skin immunity. Although most conventional αβ T (Tconv) cells in fetal skin have a naive, proliferative phenotype, a subset of CD4⁺ Tconv and CD8⁺ cells demonstrate memory-like features and a propensity for interferon (IFN)γ production. Skin regulatory T cells dynamically accumulate over the second trimester in temporal and regional association with hair follicle development. These fetal skin regulatory T cells (Tregs) demonstrate an effector memory phenotype while differing from their adult counterparts in expression of key effector molecules. Thus, we identify features of prenatal skin lymphocytes that may have key implications for understanding antigen and allergen encounters in utero and in infancy.

CyTOF reveals a complex lymphocyte landscape in developing human skin

Developing skin contains CD45RO⁺ conventional T cells with propensity to produce IFNγ

Regulatory T cells (Tregs) in skin before birth display effector memory properties

Skin Tregs increase in conjunction with initial hair follicle morphogenesis

Host-microbiota interactions in immune-mediated diseases

Host-microbiota interactions are fundamental for the development of the immune system. Drastic changes in modern environments and lifestyles have led to an imbalance of this evolutionarily ancient process, coinciding with a steep rise in immune-mediated diseases such as autoimmune, allergic and chronic inflammatory disorders. There is an urgent need to better understand these diseases in the context of mucosal and skin microbiota. This Review discusses the mechanisms of how the microbiota contributes to the predisposition, initiation and perpetuation of immune-mediated diseases in the context of a genetically prone host. It is timely owing to the wealth of new studies that recently contributed to this field, ranging from metagenomic studies in humans and mechanistic studies of host-microorganism interactions in gnotobiotic models and in vitro systems, to molecular mechanisms with broader implications across immune-mediated diseases. We focus on the general principles, such as breaches in immune tolerance and barriers, leading to the promotion of immune-mediated diseases by gut, oral and skin microbiota. Lastly, the therapeutic avenues that either target the microbiota, the barrier surfaces or the host immune system to restore tolerance and homeostasis will be explored.

Tissue regulatory T cells

Foxp3+  CD4+ regulatory T cells (Tregs) are an immune cell lineage endowed with immunosuppressive functionality in a wide array of contexts, including both anti-pathogenic and anti-self responses. In the past decades, our understanding of the functional diversity of circulating or lymphoid Tregs has grown exponentially. Only recently, the importance of Tregs residing within non-lymphoid tissues, such as visceral adipose tissue, muscle, skin and intestine, has been recognized. Not only are Tregs critical for influencing the kinetics and strength of immune responses, but the regulation of non-immune or parenchymal cells, also fall within the purview of tissue-resident or infiltrating Tregs. This review focuses on providing a systematic and comprehensive comparison of the molecular maintenance, local adaptation and functional specializations of Treg populations operating within different tissues.

A Complex Acetate-ment: Timing of Exposure Determines Memory T Cell Fate

In this issue of Cell Metabolism, Balmer et al. show that the timing and concentration of acetate exposure is critical to how it is metabolized by and affects the function of CD8 T cells. When abundantly present at the time of reactivation, acetate rewires CD8 T cell metabolism to suppress their reactivation and limit inflammation.

Skin Microbiota and its Interplay with Wound Healing

The skin microbiota is intimately coupled with cutaneous health and disease. Interactions between commensal microbiota and the multiple cell types involved in cutaneous wound healing regulate the immune response and promote barrier restoration. This dialog between host cells and the microbiome is dysregulated in chronic wounds. In this review, we first describe how advances in sequencing approaches and analysis have been used to study the chronic wound microbiota, and how these findings underscored the complexity of microbial communities and their association with clinical outcomes in patients with chronic wound disorders. We also discuss the mechanistic insights gathered from multiple animal models of polymicrobial wound infections. In addition to the well-described role of bacteria residing in polymicrobial biofilms, we also discuss the role of the intracellular bacterial niche in wound healing. We describe how, in contrast to pathogenic species capable of subverting skin immunity, commensals are essential for the regulation of the cutaneous immune system and provide protection from intracellular pathogens through modulation of the antimicrobial molecule, Perforin-2. Despite recent advances, more research is needed to shed light on host-microbiome crosstalk in both healing and nonhealing chronic wounds to appropriately guide therapeutic developments.

Microbes, helminths, and rheumatic diseases

There has been a progressive interest on modifications of the human defense system following insults occurring in the interface between our body and the external environment, as they may provoke or worsen disease states. Studies suggest that billions of germs, which compose the gut microbiota influence one's innate and adaptive immune responses at the intestinal level, but these microorganisms may also impact rheumatic diseases. The microbiota of the skin, respiratory, and urinary tracts may also be relevant in rheumatology. Evidence indicates that changes in the gut microbiome alter the pathogenesis of immune-mediated diseases such as rheumatoid arthritis and ankylosing spondylitis but also of other disorders like atherosclerosis and osteoarthritis. Therapeutic strategies to modify the microbiota, including probiotics and fecal microbiota transplantation, have been received with skepticism, which, in turn, has drawn attention back to previously developed interventions such as antibiotics. Helminths adapted to humans over the evolution process, but their role in disease modulation, particularly immune-mediated diseases, remains to be understood. The present review focuses on data concerning modifications of the immune system induced by interactions with microbes and pluricellular organisms, namely helminths, and their impact on rheumatic diseases. Practical aspects, including specific microbiota-targeted therapies, are also discussed.

The commensal skin microbiota triggers type I IFN-dependent innate repair responses in injured skin

Skin wounds heal by coordinated induction of inflammation and tissue repair, but the initiating events are poorly defined. Here we uncover a fundamental role of commensal skin microbiota in this process and show that it is mediated by the recruitment and the activation of type I interferon (IFN)-producing plasmacytoid DC (pDC). Commensal bacteria colonizing skin wounds trigger activation of neutrophils to express the chemokine CXCL10, which recruits pDC and acts as an antimicrobial protein to kill exposed microbiota, leading to the formation of CXCL10-bacterial DNA complexes. These complexes and not complexes with host-derived DNA activate pDC to produce type I IFNs, which accelerate wound closure by triggering skin inflammation and early T cell-independent wound repair responses, mediated by macrophages and fibroblasts that produce major growth factors required for healing. These findings identify a key function of commensal microbiota in driving a central innate wound healing response of the skin.

Potential of Skin Microbiome, Pro- and/or Pre-Biotics to Affect Local Cutaneous Responses to UV Exposure

The human skin hosts innumerable microorganisms and maintains homeostasis with the local immune system despite the challenges offered by environmental factors such as ultraviolet radiation (UVR). UVR causes cutaneous alterations such as acute (i.e., sunburn) and chronic inflammation, tanning, photoaging, skin cancer, and immune modulation. Phototherapy on the other hand is widely used to treat inflammatory skin diseases such as psoriasis, atopic dermatitis, polymorphic light eruption and graft-versus-host disease (GvHD), as well as neoplastic skin diseases such as cutaneous T cell lymphoma, among others. Previous work has addressed the use of pro- and pre-biotics to protect against UVR through anti-oxidative, anti-inflammatory, anti-aging, anti-carcinogenic and/or pro-and contra-melanogenic properties. Herein, we discuss and share perspectives of the potential benefits of novel treatment strategies using microbes and pro- and pre-biotics as modulators of the skin response to UVR, and how they could act both for protection against UVR-induced skin damage and as enhancers of the UVR-driven therapeutic effects on the skin.

The role of the changing human microbiome in the asthma pandemic

Asthma and allergy incidence continue to increase globally. We have made significant strides in treating disease, but it is becoming more apparent that we need to advance our knowledge into the origins of asthmatic disease. Much recent work has indicated that microbiome composition influences immune regulation and that multiple health care factors have driven a loss in microbiome diversity in modern human populations. Evidence is growing of microbiota-driven influences on immune development, asthma susceptibility, and asthma pathogenesis. The focus of this review is to highlight the strides the field has made in characterizing the constituents of the human gastrointestinal microbiota, such as Helicobacter pylori, other members of the neonatal intestinal microbiota, and microbial peptides and metabolites that influence host immunity and immune response to allergens. As we delve further into this field of research, the goal will be to find actionable and clinical interventions to identify at-risk populations earlier to prevent disease onset. Manipulation of the host microbial community during infancy might be an especially promising approach.

GAD-alum immunotherapy in type 1 diabetes expands bifunctional Th1/Th2 autoreactive CD4 T cells

AIMS/HYPOTHESIS

Antigen-specific therapy aims to modify inflammatory T cell responses in type 1 diabetes and restore immune tolerance. One strategy employs GAD65 conjugated to aluminium hydroxide (GAD-alum) to take advantage of the T helper (Th)2-biasing adjuvant properties of alum and thereby regulate pathological Th1 autoimmunity. We explored the cellular and molecular mechanism of GAD-alum action in the setting of a previously reported randomised placebo-controlled clinical trial conducted by Type 1 Diabetes TrialNet.

METHODS

In the clinical trial conducted by Type 1 Diabetes TrialNet, participants were immunised with 20 μg GAD-alum (twice or three times) or alum alone and peripheral blood mononuclear cell samples were banked at baseline and post treatment. In the present study, GAD-specific T cell responses were measured in these samples and GAD-specific T cell lines and clones were generated, which were then further characterised.

RESULTS

At day 91 post immunisation, we detected GAD-specific IL-13⁺ CD4 T cell responses significantly more frequently in participants immunised with GAD-alum (71% and 94% treated twice or three times, respectively) compared with those immunised with alum alone (38%; p = 0.003 and p = 0.0002, respectively) accompanied by high secreted levels of IL-13, IL-4 and IL-5, confirming a GAD-specific, GAD-alum-induced Th2 response. Of note, GAD-specific, IL-13⁺ CD4 T cells observed after immunisation co-secreted IFN-γ, displaying a bifunctional Th1/Th2 phenotype. Single-cell transcriptome analysis identified IL13 and IFNG expression in concert with the canonical Th2 and Th1 transcription factor genes GATA3 and TBX21, respectively. T cell receptor β-chain (TCRB) CDR3 regions of GAD-specific bifunctional T cells were identified in circulating naive and central memory CD4 T cell pools of non-immunised participants with new-onset type 1 diabetes and healthy individuals, suggesting the potential for bifunctional responses to be generated de novo by GAD-alum immunisation or via expansion from an existing public repertoire.

CONCLUSIONS/INTERPRETATION

GAD-alum immunisation activates and propagates GAD-specific CD4 T cells with a distinctive bifunctional phenotype, the functional analysis of which might be important in understanding therapeutic responses.

Single-cell transcriptional analyses of spasmolytic polypeptide-expressing metaplasia arising from acute drug injury and chronic inflammation in the stomach

OBJECTIVE

Spasmolytic polypeptide-expressing metaplasia (SPEM) is a regenerative lesion in the gastric mucosa and is a potential precursor to intestinal metaplasia/gastric adenocarcinoma in a chronic inflammatory setting. The goal of these studies was to define the transcriptional changes associated with SPEM at the individual cell level in response to acute drug injury and chronic inflammatory damage in the gastric mucosa.

DESIGN

Epithelial cells were isolated from the gastric corpus of healthy stomachs and stomachs with drug-induced and inflammation-induced SPEM lesions. Single cell RNA sequencing (scRNA-seq) was performed on tissue samples from each of these settings. The transcriptomes of individual epithelial cells from healthy, acutely damaged and chronically inflamed stomachs were analysed and compared.

RESULTS

scRNA-seq revealed a population Mucin 6 (Muc6)+gastric intrinsic factor (Gif)+ cells in healthy tissue, but these cells did not express transcripts associated with SPEM. Furthermore, analyses of SPEM cells from drug injured and chronically inflamed corpus yielded two major findings: (1) SPEM and neck cell hyperplasia/hypertrophy are nearly identical in the expression of SPEM-associated transcripts and (2) SPEM programmes induced by drug-mediated parietal cell ablation and chronic inflammation are nearly identical, although the induction of transcripts involved in immunomodulation was unique to SPEM cells in the chronic inflammatory setting.

CONCLUSIONS

These data necessitate an expansion of the definition of SPEM to include Tff2+Muc6+ cells that do not express mature chief cell transcripts such as Gif. Our data demonstrate that SPEM arises by a highly conserved cellular programme independent of aetiology and develops immunoregulatory capabilities in a setting of chronic inflammation.

A Role for MAIT Cells in Colorectal Cancer

MAIT cells are MR1-restricted T cells that are well-known for their anti-microbial properties, but they have recently been associated with different forms of cancer. Several studies have reported activated MAIT cells within the microenvironment of colorectal tumors, but there is conjecture about the nature of their response and whether they are contributing to anti-tumor immunity, or to the progression of the disease. We have reviewed the current state of knowledge about the role of MAIT cells in colorectal cancer, including their likely influence when activated and potential sources of stimulation in the tumor microenvironment. The prospects for MAIT cells being used in clinical settings as biomarkers or as targets of new immunotherapies designed to harness their function are discussed.

Type 1 Treg cells promote the generation of CD8+ tissue-resident memory T cells

Tissue-resident memory T (T_RM) cells, functionally distinct from circulating memory T cells, have a critical role in protective immunity in tissues, are more efficacious when elicited after vaccination and yield more effective antitumor immunity, yet the signals that direct development of T_RM cells are incompletely understood. Here we show that type 1 regulatory T (T_reg) cells, which express the transcription factor T-bet, promote the generation of CD8⁺ T_RM cells. The absence of T-bet-expressing type 1 T_reg cells reduces the presence of T_RM cells in multiple tissues and increases pathogen burden upon infectious challenge. Using infection models, we show that type 1 T_reg cells are specifically recruited to local inflammatory sites via the chemokine receptor CXCR3. Close proximity with effector CD8⁺ T cells and T_reg cell expression of integrin-β8 endows the bioavailability of transforming growth factor-β in the microenvironment, thereby promoting the generation of CD8⁺ T_RM cells.

Effector TH17 Cells Give Rise to Long-Lived TRM Cells that Are Essential for an Immediate Response against Bacterial Infection

Adaptive immunity provides life-long protection by generating central and effector memory T cells and the most recently described tissue resident memory T (T_RM) cells. However, the cellular origin of CD4 T_RM cells and their contribution to host defense remain elusive. Using IL-17A tracking-fate mouse models, we found that a significant fraction of lung CD4 T_RM cells derive from IL-17A-producing effector (T_H17) cells following immunization with heat-killed Klebsiella pneumonia (Kp). These exT_H17 T_RM cells are maintained in the lung by IL-7, produced by lymphatic endothelial cells. During a memory response, neither antibodies, γδ T cells, nor circulatory T cells are sufficient for the rapid host defense required to eliminate Kp. Conversely, using parabiosis and depletion studies, we demonstrated that exT_H17 T_RM cells play an important role in bacterial clearance. Thus, we delineate the origin and function of airway CD4 T_RM cells during bacterial infection, offering novel strategies for targeted vaccine design.

Resident Memory T Cells Escape 'Home Quarantine'

T resident memory (Trm) cells provide a first line of defense in non-lymphoid tissues. A new report in Nature Immunology by Fonseca et al. reveals that CD8⁺ Trm cells can give rise to circulating effector and memory T cells, but remain predisposed to migrate back into their tissue of origin.

Dendritic cells and the skin environment

The skin is inhabited by several immune cell populations that serve as a first line of defence against pathogen invasion. Amongst these populations are dendritic cells, which play an essential sentinel function by taking up antigen or infectious agents and transporting them to the lymph node for T cell recognition and the priming of immune responses. In this review, we briefly summarise recent advances showing how skin dendritic cells are connected to a network of epithelial and stromal cells, which provide structural support, growth factors, spatial cues, contact with the external environment and the skin microbiome, and favour interactions with other immune cells. We propose that this network creates a unique skin environment that may condition dendritic cell phenotype and function.

Regulatory T cells in skin injury: At the crossroads of tolerance and tissue repair

Skin injury is a highly inflammatory process that is carefully regulated to mitigate tissue damage and allow for proper barrier repair. Regulatory T cells (Tregs) are crucial coordinators of the immune response to injury in several organs. Here, we review the emerging role of Tregs in facilitating skin repair after injury. We focus on recently discovered interactions between lymphocytes and nonhematopoietic cells during wound healing and discuss how these interactions are regulated both by "classical" suppressive mechanisms of Tregs and by "nonclassical" reparative Treg functions.

Distribution and storage of inflammatory memory in barrier tissues

Memories of previous immune events enable barrier tissues to rapidly recall distinct environmental exposures. To effectively inform future responses, these past experiences can be stored in cell types that are long-term residents or essential constituents of tissues. There is an emerging understanding that, in addition to antigen-specific immune cells, diverse haematopoietic, stromal, parenchymal and neuronal cell types can store inflammatory memory. Here, we explore the impact of previous immune activity on various cell lineages with the goal of presenting a unified view of inflammatory memory to environmental exposures (such as allergens, antigens, noxious agents and microorganisms) at barrier tissues. We propose that inflammatory memory is distributed across diverse cell types and stored through shifts in cell states, and we provide a framework to guide future experiments. This distribution and storage may promote adaptation or maladaptation in homeostatic, maintenance and disease settings - especially if the distribution of memory favours cellular cooperation during storage or recall.

Impact of the microbiota on solid organ transplant rejection

PURPOSE OF REVIEW

The microbiota in mammalian hosts can affect maturation and function of the immune system and has been associated with health and disease. We will review new findings on how this dynamic environmental factor impacts alloimmunity and therapy in transplant hosts.

RECENT FINDINGS

The microbiota changes after transplantation and immunosuppressive therapy. New data indicate that different microbial community structures have distinct impact on graft outcome, from promoting, to inhibiting or being neutral to transplant survival. In addition, we will address reciprocal interactions between the microbiota and immunosuppressive drugs, as well as the suitability of the microbiota as a predictive biomarker and its utility as adjunct therapy in transplantation.

SUMMARY

Advances in microbiome sequencing and wider availability of gnotobiotic facilities are enabling mechanistic investigations into the commensal communities and pathways that modulate allograft outcome, responsiveness to immunosuppression and side effects of drugs. A better understanding of the functions of the microbiota may help mitigate drug toxicity, predict drug dosage and dampen alloimmunity in transplant patients.

Establishment and Stability of the Murine Oral Microbiome

Commensal microbiomes exert critical functions at barrier sites. In particular, establishment of the commensal microbiome after birth dictates immune functionality and tissue homeostasis at mucosal surfaces. To investigate the establishment and stability of the oral mucosal microbiome in mice, we evaluated oral microbiome communities shortly after birth, through adulthood, and up to 1 y of life in a controlled manner, using sequential oral samples from the same mice over time. We further evaluated transmissibility of oral microbiomes from parents and during cohousing experiments and evaluated susceptibility to oral inflammatory disease in mice harboring distinct microbiomes. Our work reveals basic principles in the establishment and stability of a health-associated oral microbiome after birth and provides insights that may be important for host-microbiome experimentation in animal models.

Meningeal Immunity and Its Function in Maintenance of the Central Nervous System in Health and Disease

Neuroimmunology, albeit a relatively established discipline, has recently sparked numerous exciting findings on microglia, the resident macrophages of the central nervous system (CNS). This review addresses meningeal immunity, a less-studied aspect of neuroimmune interactions. The meninges, a triple layer of membranes—the pia mater, arachnoid mater, and dura mater—surround the CNS, encompassing the cerebrospinal fluid produced by the choroid plexus epithelium. Unlike the adjacent brain parenchyma, the meninges contain a wide repertoire of immune cells. These constitute meningeal immunity, which is primarily concerned with immune surveillance of the CNS, and—according to recent evidence—also participates in postinjury CNS recovery, chronic neurodegenerative conditions, and even higher brain function. Meningeal immunity has recently come under the spotlight owing to the characterization of meningeal lymphatic vessels draining the CNS. Here, we review the current state of our understanding of meningeal immunity and its effects on healthy and diseased brains.

Organ-specific isoform selection of fatty acid–binding proteins in tissue-resident lymphocytes

Tissue-resident memory T (TRM) cells exist throughout the body, where they are poised to mediate local immune responses. Although studies have defined a common mechanism of residency independent of location, there is likely to be a level of specialization that adapts TRM cells to their given tissue of lodgment. It has been shown that TRM cells in the skin rely on the uptake of exogenous fatty acids for their survival and up-regulate fatty acid–binding protein 4 (FABP4) and FABP5 as part of their transcriptional program. However, FABPs exist as a larger family of isoforms, with different members selected in a tissue-specific fashion that is optimized for local fatty acid availability. Here, we show that although TRM cells in a range of tissue widely express FABPs, they are not restricted to FABP4 and FABP5. Instead, TRM cells show varying patterns of isoform usage that are determined by tissue-derived factors. These patterns are malleable because TRM cells relocated to different organs modify their FABP expression in line with their new location. As a consequence, these results argue for tissue-specific overlays to the TRM cell residency program, including FABP expression that is tailored to the particular tissue of TRM cell lodgment.

CD4+ teff cell heterogeneity: the perspective from single-cell transcriptomics

Single-cell transcriptomics (scRNAseq) holds the promise to generate definitive atlases of cell types. We review scRNAseq studies of conventional CD4+ αβ T cells performed in a variety of challenged contexts (infection, tumor, allergy) that aimed to parse the complexity and representativity of previously defined CD4+ T cell types, lineages, and cosmologies. With a few years' experience, the field has realized the difficulties and pitfalls of scRNAseq. With the very high-dimensionality of scRNAseq data, subset definitions based on low-dimensionality marker combinations tend to fade or blur: cell types prove more complex than expected; transcripts of key defining transcripts (cytokines, chemokines) are distributed as broad and partially overlapping continua; boundaries with innate lymphocytes are blurred. Tissue location and activation, either cytokine-driven or TCR-driven, determine Teff heterogeneity in sometimes unexpected ways. Emerging techniques for lineage and trajectory tracing, and RNA-protein connections, will further help define the space of differentiated CD4+ T cell heterogeneity.

Eavesdropping on the conversation between immune cells and the skin epithelium

The skin epithelium covers our body and serves as a vital interface with the external environment. Here, we review the context-specific interactions between immune cells and the epithelium that underlie barrier fitness and function. We highlight the mechanisms by which these two systems engage each other and how immune-epithelial interactions are tuned by microbial and inflammatory stimuli. Epithelial homeostasis relies on a delicate balance of immune surveillance and tolerance, breakdown of which results in disease. In addition to their canonical immune functions, resident and recruited immune cells also supply the epithelium with instructive signals to promote repair. Decoding the dialogue between immunity and the epithelium therefore has great potential for boosting barrier function or mitigating inflammatory epithelial diseases.

Unraveling Immune-Epithelial Interactions in Skin Homeostasis and Injury

The skin serves as a front line of defense against harmful environmental elements and thus is vital for organismal survival. This barrier is comprised of a water-tight epithelial structure reinforced by an arsenal of immune cells. The epithelial and immune components of the skin are interdependent and actively dialogue to maintain health and combat infectious, injurious, and noxious stimuli. Here, we discuss the molecular mediators of this crosstalk that establish tissue homeostasis and their dynamic adaptations to various stress conditions. In particular, we focus on immune-epithelial interactions in homeostatic tissue regeneration, during natural cycling of the hair follicle, and following skin injury. We also highlight the epithelial derived factors that orchestrate immunity. A comprehensive and mechanistic understanding of dynamic interactions between cutaneous immune cells and the epithelium can be leveraged to develop novel therapies to treat of range of skin diseases and boost skin health.

Loricrin: Past, Present, and Future

The terminal differentiation of the epidermis is a complex physiological process. During the past few decades, medical genetics has shown that defects in the stratum corneum (SC) permeability barrier cause a myriad of pathological conditions, ranging from common dry skin to lethal ichthyoses. Contrarily, molecular phylogenetics has revealed that amniotes have acquired a specialized form of cytoprotection cornification that provides mechanical resilience to the SC. This superior biochemical property, along with desiccation tolerance, is attributable to the proper formation of the macromolecular protein-lipid complex termed cornified cell envelopes (CE). Cornification largely depends on the peculiar biochemical and biophysical properties of loricrin, which is a major CE component. Despite its quantitative significance, loricrin knockout (LKO) mice have revealed it to be dispensable for the SC permeability barrier. Nevertheless, LKO mice have brought us valuable lessons. It is also becoming evident that absent loricrin affects skin homeostasis more profoundly in many more aspects than previously expected. Through an extensive review of aggregate evidence, we discuss herein the functional significance of the thiol-rich protein loricrin from a biochemical, genetic, pathological, metabolic, or immunological aspect with some theoretical and speculative perspectives.

Microbiome-skin-brain axis: A novel paradigm for cutaneous wounds

Chronic wounds cause a significant burden on society financially, medically, and psychologically. Unfortunately, patients with nonhealing wounds often suffer from comorbidities that further compound their disability. Given the high rate of depressive symptoms experienced by patients with chronic wounds, further studies are needed to investigate the potentially linked pathophysiological changes in wounds and depression in order to improve patient care. The English literature on wound healing, inflammatory and microbial changes in chronic wounds and depression, and antiinflammatory and probiotic therapy was reviewed on PubMed. Chronic wound conditions and depression were demonstrated to share common pathologic features of dysregulated inflammation and altered microbiome, indicating a possible relationship. Furthermore, alternative treatment strategies such as immune-targeted and probiotic therapy showed promising potential by addressing both pathophysiological pathways. However, many existing studies are limited to a small study population, a cross-sectional design that does not establish temporality, or a wide range of confounding variables in the context of a highly complex and multifactorial disease process. Therefore, additional preclinical studies in suitable wound models, as well as larger clinical cohort studies and trials are necessary to elucidate the relationship between wound microbiome, healing, and depression, and ultimately guide the most effective therapeutic and management plan for chronic wound patients.

CD8+ T-cell plasticity regulates vascular regeneration in type-2 diabetes

In this study, we observe that the ischemic tissues of type-2 diabetic (T2D) patients and mice have significantly more CD8⁺ T-cells than that of their normoglycemic counterparts, respectively. However, the role of CD8⁺ T-cells in the pathogenesis of diabetic vascular complication has been less studied. Methods: We employed loss-of-function studies in mouse models using the non-lytic anti-CD8 antibody that blocks tissue infiltration of CD8⁺ T-cells into the injured tissue. We also performed genome-wide, single-cell RNA-sequencing of CD8⁺ T-cells to uncover their role in the pathogenesis of diabetic vascular diseases. Results: The vascular density is negatively correlated with the number of CD8⁺ T-cells in the ischemic tissues of patients and mice after injury. CD8⁺ T-cells or their supernatant can directly impair human and murine angiogenesis. Compared to normoglycemic mice that can regenerate their blood vessels after injury, T2D mice fail in this regeneration. Treatment with the CD8 checkpoint blocking antibody increases the proliferation and function of endothelial cells in both Lepr^db/db mice and diet-induced diabetic Cdh5-Cre;Rosa-YFP lineage-tracing mice after ischemic injury. Furthermore, single-cell transcriptomic profiling reveals that CD8⁺ T-cells of T2D mice showed a de novo cell fate change from the angiogenic, tissue-resident memory cells towards the effector and effector memory cells after injury. Functional revascularization by CD8 checkpoint blockade is mediated through unleashing such a poised lineage commitment of CD8⁺ T-cells from T2D mice. Conclusion: Our results reveal that CD8⁺ T-cell plasticity regulates vascular regeneration; and give clinically relevant insights into the potential development of immunotherapy targeting vascular diseases associated with obesity and diabetes.

Women in immunology: 2020 and beyond

Women have been at the forefront of tremendous achievements in immunology in the past decade. However, disparities still exist, limiting upward potential and further advancements. As four NIH intramural women scientists who care deeply about scientific progress and the progress of women in our field, we review ongoing challenges and discuss potential approaches to help advance the promotion of women in the sciences.

On the cause and consequences of IgE to galactose-α-1,3-galactose: A report from the National Institute of Allergy and Infectious Diseases Workshop on Understanding IgE-Mediated Mammalian Meat Allergy

The mammalian meat allergy known as the "α-Gal syndrome" relates to IgE specific for galactose-α-1,3-galactose (α-Gal), an oligosaccharide that is present in cells and tissues of nonprimate mammals. The recognition of delayed reactions to food derived from mammals in patients with IgE to α-Gal and also the association with tick bites have been increasing worldwide. In 2018, the National Institute of Allergy and Infectious Diseases, Division of Allergy, Immunology and Transplantation, sponsored a workshop on this emerging tick-related disease. International experts from the fields of tick biology, allergy, immunology, infectious disease, and dermatology discussed the current state of our understanding of this emerging medical condition. The participants provided suggestions for specific research priorities and for the development of resources to advance our knowledge of the mechanisms, diagnosis, management, and prevention of this allergic disease. This publication is a summary of the workshop and the panel's recommendations are presented herein.

Regulatory T cells control the dynamic and site-specific polarization of total CD4 T cells following Salmonella infection

FoxP3⁺ regulatory T cells (Tregs) control inflammation and maintain mucosal homeostasis, but their functions during infection are poorly understood. Th1, Th2, and Th17 cells can be identified by master transcription factors (TFs) T-bet, GATA3, and RORγT; Tregs also express these TFs. While T-bet⁺ Tregs can selectively suppress Th1 cells, it is unclear whether distinct Treg populations can alter Th bias. To address this, we used Salmonella enterica serotype Typhimurium to induce nonlethal colitis. Following infection, we observed an early colonic Th17 response within total CD4 T cells, followed by a Th1 bias. The early Th17 response, which contains both Salmonella-specific and non-Salmonella-specific cells, parallels an increase in T-bet⁺ Tregs. Later, Th1 cells and RORγT⁺ Tregs dominate. This reciprocal dynamic may indicate that Tregs selectively suppress Th cells, shaping the immune response. Treg depletion 1-2 days post-infection shifted the early Th17 response to a Th1 bias; however, Treg depletion 6-7 days post-infection abrogated the Th1 bias. Thus, Tregs are necessary for the early Th17 response, and for a maximal Th1 response later. These data show that Tregs shape the overall tissue CD4 T cell response and highlight the potential for subpopulations of Tregs to be used in targeted therapeutic approaches.

Epithelial cells: liaisons of immunity

The surface and lining tissues of our body are exposed to the external environment, and as such these epithelial tissues must form structural barriers able to defend against microbes, environmental toxins, and mechanical stress. Their cells are equipped to detect a diverse array of surface perturbations, and then launch signaling relays to the immune system. The aim of these liaisons is to coordinate the requisite immune cell response needed to preserve and/or restore barrier integrity and defend the host. It has been recently appreciated that epithelial cells learn from these experiences. Following inflammatory exposure, long-lived stem cells within the tissue retain an epigenetic memory that endows them with heightened responsiveness to subsequent encounters with stress. Here, we review the recent literature on how epithelial cells sense signals from microbes, allergens, and injury at the tissue surface, and transmit this information to immune cells, while embedding a memory of the experience within their chromatin.

Metabolic Control of Treg Cell Stability, Plasticity, and Tissue-Specific Heterogeneity

Regulatory T (Treg) cells are crucial for peripheral immune tolerance and prevention of autoimmunity and tissue damage. Treg cells are inherently defined by the expression of the transcription factor Foxp3, which enforces lineage development and immune suppressive function of these cells. Under various conditions as observed in autoimmunity, cancer and non-lymphoid tissues, a proportion of Treg cells respond to specific environmental signals and display altered stability, plasticity and tissue-specific heterogeneity, which further shape their context-dependent suppressive functions. Recent studies have revealed that metabolic programs play pivotal roles in controlling these processes in Treg cells, thereby considerably expanding our understanding of Treg cell biology. Here we summarize these recent advances that highlight how cell-extrinsic factors, such as nutrients, vitamins and metabolites, and cell-intrinsic metabolic programs, orchestrate Treg cell stability, plasticity, and tissue-specific heterogeneity. Understanding metabolic regulation of Treg cells should provide new insight into immune homeostasis and disease, with important therapeutic implications for autoimmunity, cancer, and other immune-mediated disorders.

A three course menu for ILC and bystander T cell activation

The varied list of agonists that activate innate lymphoid cells (ILCs) continues to grow, but whether and how these signals interact is not well defined, especially in vivo. ILC subsets share master transcription factors, chromatin landscapes, and effector cytokines with their corresponding T helper (Th) cell subsets. Here we discuss how studies of these two cell types can inform each other. Specifically, we outline a framework in which ILC agonists are grouped by the transcription factors they activate. Optimal ILC activation requires at least three items from a 'menu' of non-redundant signals that collectively replicate the STAT and TCR signaling that drives effector Th cell function. This conceptual model may also apply to TCR-independent 'bystander' activation of Th cells.

Interferon target-gene expression and epigenomic signatures in health and disease

Multiple type I interferons and interferon-γ (IFN-γ) are expressed under physiological conditions and are increased by stress and infections, and in autoinflammatory and autoimmune diseases. Interferons activate the Jak-STAT signaling pathway and induce overlapping patterns of expression, called 'interferon signatures', of canonical interferon-stimulated genes (ISGs) encoding molecules important for antiviral responses, antigen presentation, autoimmunity and inflammation. It has now become clear that interferons also induce an 'interferon epigenomic signature' by activating latent enhancers and 'bookmarking' chromatin, thus reprogramming cell responses to environmental cues. The interferon epigenomic signature affects ISGs and other gene sets, including canonical targets of the transcription factor NF-κB that encode inflammatory molecules, and is involved in the priming of immune cells, tolerance and the training of innate immune memory. Here we review the mechanisms through which interferon signatures and interferon epigenomic signatures are generated, as well as the expression and functional consequences of these signatures in homeostasis and autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis and systemic sclerosis.

Gut Mycobiota in Immunity and Inflammatory Disease

The mammalian intestine is colonized by a wealth of microorganisms-including bacteria, viruses, protozoa, and fungi-that are all integrated into a functional trans-kingdom community. Characterization of the composition of the fungal community-the mycobiota-has advanced further than the much-needed mechanistic studies. Recent findings have revealed roles for the gut mycobiota in the regulation of host immunity and in the development and progression of human diseases of inflammatory origin. We review these findings here while placing them in the context of the current understanding of the pathways and cellular networks that induce local and systemic immune responses to fungi in the gastrointestinal tract. We discuss gaps in knowledge and argue for the importance of considering bacteria-fungal interactions as we aim to define the roles of mycobiota in immune homeostasis and immune-associated pathologies.

Keratinocyte-intrinsic MHCII expression controls microbiota-induced Th1 cell responses

The cross-talk between the microbiota and the immune system plays a fundamental role in the control of host physiology. However, the tissue-specific factors controlling this dialogue remain poorly understood. Here we demonstrate that T cell responses to commensal colonization are associated with the development of organized cellular clusters within the skin epithelium. These organized lymphocyte clusters are surrounded by keratinocytes expressing a discrete program associated with antigen presentation and antimicrobial defense. Notably, IL-22-mediated keratinocyte-intrinsic MHC class II expression was required for the selective accumulation of commensal-induced IFN-γ, but not IL-17A-producing CD4⁺ T cells within the skin. Taking these data together, this work uncovers an unexpected role for MHC class II expression by keratinocytes in the control of homeostatic type 1 responses to the microbiota. Our findings have important implications for the understanding of the tissue-specific rules governing the dialogue between a host and its microbiota.

Regulatory T Cell Development in the Thymus

Development of a comprehensive regulatory T (Treg) cell compartment in the thymus is required to maintain immune homeostasis and prevent autoimmunity. In this study, we review cellular and molecular determinants of Treg cell development in the thymus. We focus on the evidence for a self-antigen-focused Treg cell repertoire as well as the APCs responsible for presenting self-antigens to developing thymocytes. We also cover the contribution of different cytokines to thymic Treg development and the cellular populations that produce these cytokines. Finally, we update the originally proposed "two-step" model of thymic Treg differentiation by incorporating new evidence demonstrating that Treg cells develop from two Treg progenitor populations and discuss the functional importance of Treg cells generated via either progenitor pathway.

Demystifying the manipulation of host immunity, metabolism, and extraintestinal tumors by the gut microbiome

The trillions of microorganisms in the gut microbiome have attracted much attention recently owing to their sophisticated and widespread impacts on numerous aspects of host pathophysiology. Remarkable progress in large-scale sequencing and mass spectrometry has increased our understanding of the influence of the microbiome and/or its metabolites on the onset and progression of extraintestinal cancers and the efficacy of cancer immunotherapy. Given the plasticity in microbial composition and function, microbial-based therapeutic interventions, including dietary modulation, prebiotics, and probiotics, as well as fecal microbial transplantation, potentially permit the development of novel strategies for cancer therapy to improve clinical outcomes. Herein, we summarize the latest evidence on the involvement of the gut microbiome in host immunity and metabolism, the effects of the microbiome on extraintestinal cancers and the immune response, and strategies to modulate the gut microbiome, and we discuss ongoing studies and future areas of research that deserve focused research efforts.

Modulation of ERQC and ERAD: A Broad-Spectrum Spanner in the Works of Cancer Cells?

Endoplasmic reticulum glycoprotein folding quality control (ERQC) and ER-associated degradation (ERAD) preside over cellular glycoprotein secretion and maintain steady glycoproteostasis. When cells turn malignant, cancer cell plasticity is affected and supported either by point mutations, preferential isoform selection, altered expression levels, or shifts to conformational equilibria of a secreted glycoprotein. Such changes are crucial in mediating altered extracellular signalling, metabolic behavior, and adhesion properties of cancer cells. It is therefore conceivable that interference with ERQC and/or ERAD can be used to selectively damage cancers. Indeed, inhibitors of the late stages of ERAD are already in the clinic against cancers such as multiple myeloma. Here, we review recent advances in our understanding of the complex relationship between glycoproteostasis and cancer biology and discuss the potential of ERQC and ERAD modulators for the selective targeting of cancer cell plasticity.

TCR and Inflammatory Signals Tune Human MAIT Cells to Exert Specific Tissue Repair and Effector Functions

MAIT cells are an unconventional T cell population that can be activated through both TCR-dependent and TCR-independent mechanisms. Here, we examined the impact of combinations of TCR-dependent and TCR-independent signals in human CD8+ MAIT cells. TCR-independent activation of these MAIT cells from blood and gut was maximized by extending the panel of cytokines to include TNF-superfamily member TL1A. RNA-seq experiments revealed that TCR-dependent and TCR-independent signals drive MAIT cells to exert overlapping and specific effector functions, affecting both host defense and tissue homeostasis. Although TCR triggering alone is insufficient to drive sustained activation, TCR-triggered MAIT cells showed specific enrichment of tissue-repair functions at the gene and protein levels and in in vitro assays. Altogether, these data indicate the blend of TCR-dependent and TCR-independent signaling to CD8+ MAIT cells may play a role in controlling the balance between healthy and pathological processes of tissue inflammation and repair.

The γc Family of Cytokines: Basic Biology to Therapeutic Ramifications

The common cytokine receptor γ chain, γ_c, is a component of the receptors for interleukin-2 (IL-2), IL-4, IL-7, IL-9, IL-15, and IL-21. Mutation of the gene encoding γ_c results in X-linked severe combined immunodeficiency in humans, and γ_c family cytokines collectively regulate development, proliferation, survival, and differentiation of immune cells. Here, we review the basic biology of these cytokines, highlighting mechanisms of signaling and gene regulation that have provided insights for immunodeficiency, autoimmunity, allergic diseases, and cancer. Moreover, we discuss how studies of this family stimulated the development of JAK3 inhibitors and present an overview of current strategies targeting these pathways in the clinic, including novel antibodies, antagonists, and partial agonists. The diverse roles of these cytokines on a range of immune cells have important therapeutic implications.

How a farming environment protects from atopy

It is now well established that the exposure to certain environments such as farms has the potential to protect from the development of allergies later in life. This protection is achieved when repeated exposure to the farming environment occurs early in life, but persists when children spend sufficient amount of time in contact with livestock and hay, and drink unpasteurized milk. The capacity of farm dust to protect from allergy development lies, amongst others, in the microbe composition in the farm. These protective microbes release various metabolites and cell wall components that change farmers' home dust composition, when compared to urbanized home dust. Additionally, they can colonize various barrier sites (skin, lung, intestine) in farmers' children, leading to persistent changes in the way their immune system and their barrier cells respond to environmental allergens.