Commensal-dendritic-cell interaction specifies a unique protective skin immune signature

Inflammatory memory sensitizes skin epithelial stem cells to tissue damage

The body’s first line of defense against environmental assaults, the skin barrier is maintained by epithelial stem cells (EpSCs). Despite EpSCs’ vulnerability to inflammatory pressures, neither the primary response nor its enduring consequences are understood. Here, we unearth a prolonged memory to acute inflammation that enables EpSCs to hasten barrier restoration following subsequent tissue damage. This functional adaptation does not require skin resident macrophages or T cells. Rather, EpSCs maintain chromosomal accessibility at key stress response genes that are activated by the primary stimulus. Upon a secondary challenge, genes governed by these domains are transcribed rapidly. Fueling this memory is Aim2, encoding an activator of the inflammasome. Absence of AIM2 or its downstream effectors, Caspase-1 and Interleukin-1β, erases EpSCs’ ability to recollect inflammation. While EpSCs benefit from inflammatory tuning by heightening their responsiveness to subsequent stressors, this enhanced sensitivity likely increases their susceptibility to autoimmune and hyperproliferative disorders, including cancer.

Skin Immune Landscape: Inside and Outside the Organism

The skin is an essential organ to the human body protecting it from external aggressions and pathogens. Over the years, the skin was proven to have a crucial immunological role, not only being a passive protective barrier but a network of effector cells and molecular mediators that constitute a highly sophisticated compound known as the “skin immune system” (SIS). Studies of skin immune sentinels provided essential insights of a complex and dynamic immunity, which was achieved through interaction between the external and internal cutaneous compartments. In fact, the skin surface is cohabited by microorganisms recognized as skin microbiota that live in complete harmony with the immune sentinels and contribute to the epithelial barrier reinforcement. However, under stress, the symbiotic relationship changes into a dysbiotic one resulting in skin disorders. Hence, the skin microbiota may have either positive or negative influence on the immune system. This review aims at providing basic background information on the cutaneous immune system from major cellular and molecular players and the impact of its microbiota on the well-coordinated immune responses in host defense.

Lactobacillus gasseri attenuates allergic airway inflammation through PPARγ activation in dendritic cells

Lactobacilli prevent the early development of allergic diseases in children and experimental asthma in mice. However, the detailed mechanism underlying this action remains unknown. We aimed to explore the activation pathway in the host by Lactobacillus and identify its immunomodulation mechanism in allergic asthma. Continuous administration of 10⁷ cfu, but not 10⁹ cfu, of L. gasseri for 4 weeks prevented Dermatophagoides pteronyssinus (Der p)-induced airway hypersensitivity and inflammation in a mouse model of allergic asthma. DNA microarray analysis of the mesenteric and lung draining lymph nodes revealed a significant decrease in inflammatory chemokines and increase in gene expression in carbohydrate and lipid metabolism, particularly of PPARγ, in 10⁷ cfu L. gasseri-administered mice compared with untreated mice. Compared with WT mice, Der p-sensitized PPARγ^L/+ mice showed increased airway hyperresponsiveness to methacholine, inflammatory cell infiltration, and inflammatory cytokine secretion in bronchoalveolar fluid. Moreover, the protective effects of L. gasseri were lost in Der p-induced airway inflammation in PPARγ^L/+ mice, and L. gasseri-induced PPARγ activation in BMDCs inhibited the development of allergic airway inflammation in both PPARγ WT and PPARγ^L/+ mice. L. gasseri may act via a novel PPARγ activation pathway in DCs to alleviate allergen-induced airway inflammation in allergic asthma.

KEY MESSAGES

L. gasseri prevents mite allergen (Der p)-induced airway inflammation. Prevention of airway inflammation occurs via activation of PPARγ in dendritic cells. L. gasseri administration does not reverse Der p-induced airway inflammation in PPARγ^(+/-) mice. L. gasseri-induced PPARγ activation inhibits development of airway inflammation in WT and PPARγ^(+/-) mice.

Combined Action of Human Commensal Bacteria and Amorphous Silica Nanoparticles on the Viability and Immune Responses of Dendritic Cells

Dendritic cells (DCs) regulate the host-microbe balance in the gut and skin, tissues likely exposed to nanoparticles (NPs) present in drugs, food, and cosmetics. We analyzed the viability and the activation of DCs incubated with extracellular media (EMs) obtained from cultures of commensal bacteria (Escherichia coli, Staphylococcus epidermidis) or pathogenic bacteria (Pseudomonas aeruginosa, Staphylococcus aureus) in the presence of amorphous silica nanoparticles (SiO₂ NPs). EMs and NPs synergistically increased the levels of cytotoxicity and cytokine production, with different nanoparticle dose-response characteristics being found, depending on the bacterial species. E. coli and S. epidermidis EMs plus NPs at nontoxic doses stimulated the secretion of interleukin-1β (IL-1β), IL-12, IL-10, and IL-6, while E. coli and S. epidermidis EMs plus NPs at toxic doses stimulated the secretion of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), IL-4, and IL-5. On the contrary, S. aureus and P. aeruginosa EMs induced cytokines only when they were combined with NPs at toxic concentrations. The induction of maturation markers (CD86, CD80, CD83, intercellular adhesion molecule 1, and major histocompatibility complex class II) by commensal bacteria but not by pathogenic ones was improved in the presence of noncytotoxic SiO₂ NP doses. DCs consistently supported the proliferation and differentiation of CD4⁺ and CD8⁺ T cells secreting IFN-γ and IL-17A. The synergistic induction of CD86 was due to nonprotein molecules present in the EMs from all bacteria tested. At variance with this finding, the synergistic induction of IL-1β was prevalently mediated by proteins in the case of E. coli EMs and by nonproteins in the case of S. epidermidis EMs. A bacterial costimulus did not act on DCs after adsorption on SiO₂ NPs but rather acted as an independent agonist. The inflammatory and immune actions of DCs stimulated by commensal bacterial agonists might be altered by the simultaneous exposure to engineered or environmental NPs.

Ontogeny and function of murine epidermal Langerhans cells

Langerhans cells (LCs) are epidermis-resident antigen-presenting cells that share a common ontogeny with macrophages but function as dendritic cells (DCs). Their development, recruitment and retention in the epidermis is orchestrated by interactions with keratinocytes through multiple mechanisms. LC and dermal DC subsets often show functional redundancy, but LCs are required for specific types of adaptive immune responses when antigen is concentrated in the epidermis. This Review will focus on those developmental and functional properties that are unique to LCs.

Role of the microbiota in skin immunity and atopic dermatitis

Atopic dermatitis (AD) is a chronic inflammatory skin disease that affects 15-20% of children and 2-5% of adults in industrialized countries. The pathogen Staphylococcus aureus selectively colonizes the lesional skin of AD patients while this bacterium is absent in the skin of the majority of healthy individuals. However, the role of S. aureus in the pathogenesis of AD remains poorly understood. In addition to S. aureus, recent studies show a contribution of the skin microbiota to the regulation of immune responses in the skin as well as to the development of inflammatory skin disease. This review summarizes current knowledge about the role of the microbiota in skin immune responses and the role of S. aureus virulent factors in the pathogenesis of AD.

Proteomic Profiling of Sweat Exosome Suggests its Involvement in Skin Immunity

Healthy human skin sustains an effective immune defense mechanism, formed by a complex physical and chemical epidermal barrier that coordinates with different cellular components of the skin immune system. However, the mechanism by which skin cells regulate local immune homeostasis in health and disease contexts is not well known. To investigate whether exosomes exist in sweat, sweat samples from healthy individuals were collected after aerobic exercise. Sweat exosome was isolated via differential ultracentrifugation, observed under transmission electron microscopy, measured by dynamic light scattering, and confirmed by immunoblot. Further, shotgun liquid chromatography (LC)-mass spectrometry (MS)/MS analysis was conducted to investigate the proteomic profiling of sweat exosome. Secreted exosome was detected in human sweat. A total of 1,062 proteins were identified in sweat exosome, including 997 different proteins compared with sweat proteomics and 896 unique proteins compared with urine, saliva, and plasma exosomes. Diverse antimicrobial peptides and immunological factors were found in sweat exosome, suggesting the involvement of exosome in skin immunity. This study provides direct evidence that secreted exosomes exist in human sweat. The proteomic profiling of sweat exosome provides insight into sweat features and the potential physiological significance of exosomes in immune homeostasis.

Tissue-Specific Immunity at the Oral Mucosal Barrier

The oral mucosal barrier is constantly exposed to a plethora of triggers requiring immune control, including a diverse commensal microbiome, ongoing damage from mastication, and dietary and airborne antigens. However, how these tissue-specific cues participate in the training of immune responsiveness at this site is minimally understood. Moreover, the mechanisms mediating homeostatic immunity at this interface are not yet fully defined. Here we present basic aspects of the oral mucosal barrier and discuss local cues that may modulate and train local immune responsiveness. We particularly focus on the immune cell network mediating immune surveillance at a specific oral barrier, the gingiva - a constantly stimulated and dynamic environment where homeostasis is often disrupted, resulting in the common inflammatory disease periodontitis.

Pathogenic Mechanisms and Host Interactions in Staphylococcus epidermidis Device-Related Infection

Staphylococcus epidermidis is a permanent member of the normal human microbiota, commonly found on skin and mucous membranes. By adhering to tissue surface moieties of the host via specific adhesins, S. epidermidis is capable of establishing a lifelong commensal relationship with humans that begins early in life. In its role as a commensal organism, S. epidermidis is thought to provide benefits to human host, including out-competing more virulent pathogens. However, largely due to its capacity to form biofilm on implanted foreign bodies, S. epidermidis has emerged as an important opportunistic pathogen in patients receiving medical devices. S. epidermidis causes approximately 20% of all orthopedic device-related infections (ODRIs), increasing up to 50% in late-developing infections. Despite this prevalence, it remains underrepresented in the scientific literature, in particular lagging behind the study of the S. aureus. This review aims to provide an overview of the interactions of S. epidermidis with the human host, both as a commensal and as a pathogen. The mechanisms retained by S. epidermidis that enable colonization of human skin as well as invasive infection, will be described, with a particular focus upon biofilm formation. The host immune responses to these infections are also described, including how S. epidermidis seems to trigger low levels of pro-inflammatory cytokines and high levels of interleukin-10, which may contribute to the sub-acute and persistent nature often associated with these infections. The adaptive immune response to S. epidermidis remains poorly described, and represents an area which may provide significant new discoveries in the coming years.

Severe Dermatitis Associated with Spontaneous Staphylococcus xylosus Infection in Rag-/-Tpl2-/- Mice

Staphylococcus xylosus is a commensal bacterium found on the skin and mucosal surfaces of SPF mice. S. xylosus is rarely pathogenic, most often causing skin lesions and dermatitis in immunocompromised mice, particularly those with impaired NADPH oxidase function. Here we report spontaneous infection with S. xylosus in Rag1-/-Tpl2-/- mice. Infection was characterized by the presence of alopecia, crusts, and scaly skin. S. xylosus was detected in the feces, skin, lymph nodes, and lungs of Rag1-/-Tpl2-/- mice and led to mortality or euthanasia due to humane endpoints. C57BL/6 mice were culture-positive for S. xylosus on the skin, and Rag1-/- and Tpl2-/- mice were culture-positive on the skin and occasionally in the feces. However, S. xylosus did not cause clinical symptoms in C57BL/6, Rag1-/-, or Tpl2-/- mice. Compared with those in Rag1-/- mice, relative concentrations of circulating monocytes, but not neutrophils or lymphocytes, were increased in Rag1-/-Tpl2-/- mice, consistent with their increased incidence of clinical symptoms. Overall, this case study suggests a novel role for Tpl2 in T-cell-independent host resistance to the otherwise commensal organism S. xylosus.

Topical ivermectin improves allergic skin inflammation

BACKGROUND

Ivermectin (IVM) is widely used in both human and veterinary medicine to treat parasitic infections. Recent reports have suggested that IVM could also have anti-inflammatory properties.

METHODS

Here, we investigated the activity of IVM in a murine model of atopic dermatitis (AD) induced by repeated exposure to the allergen Dermatophagoides farinae, and in standard cellular immunological assays.

RESULTS

Our results show that topical IVM improved allergic skin inflammation by reducing the priming and activation of allergen-specific T cells, as well as the production of inflammatory cytokines. While IVM had no major impact on the functions of dendritic cells in vivo and in vitro, IVM impaired T-cell activation, proliferation, and cytokine production following polyclonal and antigen-specific stimulation.

CONCLUSION

Altogether, our results show that IVM is endowed with topical anti-inflammatory properties that could have important applications for the treatment of T-cell-mediated skin inflammatory diseases.

The resident pathobiont Staphylococcus xylosus in Nfkbiz-deficient skin accelerates spontaneous skin inflammation

IκBζ, which is encoded by the Nfkbiz gene, is a member of the nuclear IκB family of proteins that act as transcriptional regulators via association with NF-κB. Nfkbiz-deficient (Nfkbiz ^-/-) mice develop spontaneous dermatitis; however, the underlying mechanism has yet to be elucidated. In our study, we found higher skin pathology scores and more serum IgE antibodies and trans-epidermal water loss in Nfkbiz ^-/- than in Nfkbiz-sufficient (Nfkbiz ^+/-) mice. There was also greater expansion of IFN-γ-, IL-17A-, and IL-22-secreting CD4⁺ T cells and of IL-17A-secreting γδ⁺ T cells in the skin of Nfkbiz ^-/- mice than in with Nfkbiz ^+/- mice. Pyrosequencing analysis showed decreased diversity of resident bacteria and markedly expanded Staphylococcus (S.) xylosus in the skin of Nfkbiz ^-/- mice. Oral administration of antibiotics including cephalexin and enrofloxacin ameliorated skin inflammation. Topical application of S. xylosus also resulted in the expansion of IL-17A-secreting CD4⁺ T cells along with high levels of pro-inflammatory cytokines and chemokines in the skin of Nfkbiz ^-/- mice. The expansion of commensal S. xylosus may be one cause of skin dysbiosis in Nfkbiz ^-/- mice and suggests that the Nfkbiz gene may play a regulatory role in the microbiota-skin immunity axis.

Gut Microbiota, Inflammation, and Colorectal Cancer

Colorectal cancer is the second-leading cause of cancer-related deaths in the United States and fourth-leading cause of cancer-related deaths worldwide. While cancer is largely considered to be a disease of genetic and environmental factors, increasing evidence has demonstrated a role for the microbiota (the microorganisms associated with the human body) in shaping inflammatory environments and promoting tumor growth and spread. Herein, we discuss both human data from meta'omics analyses and data from mechanistic studies in cell culture and animal models that support specific bacterial agents as potentiators of tumorigenesis-including Fusobacterium nucleatum, enterotoxigenic Bacteroides fragilis, and colibactin-producing Escherichia coli. Further, we consider how microbes can be used in diagnosing colorectal cancer and manipulating the tumor environment to encourage better patient outcomes in response to immunotherapy treatments.

Intradermal Immunization with rAAV1 Vector Induces Robust Memory CD8+ T Cell Responses Independently of Transgene Expression in DCs

Recombinant adeno-associated viral (rAAV) vectors exhibit interesting properties as vaccine carriers for their ability to induce long-lasting antibody responses. However, rAAV-based vaccines have been suggested to trigger functionally impaired long-term memory CD8⁺ T cell responses, in part due to poor dendritic cell (DC) transduction. Such results, albeit limited to intramuscular immunization, undermined the use of rAAV as vaccine vehicles against intracellular pathogens. We report here that intradermal immunization with a model rAAV2/1-based vaccine drives the development of bona fide long-term memory CD8⁺ T cell responses. The intradermal route of immunization and the presence of potent major histocompatibility complex (MHC) class II responses showed synergistic effects on the overall quantity and quality of systemic long-term effector memory transgene-specific CD8⁺ T cells being generated against the transgene. Of key interest, we found that the induction of memory cytotoxic T lymphocytes (CTLs) following intradermal immunization was solely dependent on the cross-presentation of skin-expressed transgene products, which appeared highly enhanced as compared to muscle-expressed transgene products. Overall our results highlight key tissue-specific differences in transgene presentation pathway requirements of importance for the design of rAAV-based T cell-inducing vaccines.

An Ocular Commensal Protects against Corneal Infection by Driving an Interleukin-17 Response from Mucosal γδ T Cells

Mucosal sites such as the intestine, oral cavity, nasopharynx, and vagina all have associated commensal flora. The surface of the eye is also a mucosal site, but proof of a living, resident ocular microbiome remains elusive. Here, we used a mouse model of ocular surface disease to reveal that commensals were present in the ocular mucosa and had functional immunological consequences. We isolated one such candidate commensal, Corynebacterium mastitidis, and showed that this organism elicited a commensal-specific interleukin-17 response from γδ T cells in the ocular mucosa that was central to local immunity. The commensal-specific response drove neutrophil recruitment and the release of antimicrobials into the tears and protected the eye from pathogenic Candida albicans or Pseudomonas aeruginosa infection. Our findings provide direct evidence that a resident commensal microbiome exists on the ocular surface and identify the cellular mechanisms underlying its effects on ocular immune homeostasis and host defense.

Secretory Leukoprotease Inhibitor (Slpi) Expression Is Required for Educating Murine Dendritic Cells Inflammatory Response Following Quercetin Exposure

Dendritic cells' (DCs) ability to present antigens and initiate the adaptive immune response confers them a pivotal role in immunological defense against hostile infection and, at the same time, immunological tolerance towards harmless components of the microbiota. Food products can modulate the inflammatory status of intestinal DCs. Among nutritionally-derived products, we investigated the ability of quercetin to suppress inflammatory cytokines secretion, antigen presentation, and DCs migration towards the draining lymph nodes. We recently identified the Slpi expression as a crucial checkpoint required for the quercetin-induced inflammatory suppression. Here we demonstrate that Slpi-KO DCs secrete a unique panel of cytokines and chemokines following quercetin exposure. In vivo, quercetin-enriched food is able to induce Slpi expression in the ileum, while little effects are detectable in the duodenum. Furthermore, Slpi expressing cells are more frequent at the tip compared to the base of the intestinal villi, suggesting that quercetin exposure could be more efficient for DCs projecting periscopes in the intestinal lumen. These data suggest that quercetin-enriched nutritional regimes may be efficient for suppressing inflammatory syndromes affecting the ileo-colonic tract.

Cutaneous Leishmaniasis Induces a Transmissible Dysbiotic Skin Microbiota that Promotes Skin Inflammation

Skin microbiota can impact allergic and autoimmune responses, wound healing, and anti-microbial defense. We investigated the role of skin microbiota in cutaneous leishmaniasis and found that human patients infected with Leishmania braziliensis develop dysbiotic skin microbiota, characterized by increases in the abundance of Staphylococcus and/or Streptococcus. Mice infected with L. major exhibit similar changes depending upon disease severity. Importantly, this dysbiosis is not limited to the lesion site, but is transmissible to normal skin distant from the infection site and to skin from co-housed naive mice. This observation allowed us to test whether a pre-existing dysbiotic skin microbiota influences disease, and we found that challenging dysbiotic naive mice with L. major or testing for contact hypersensitivity results in exacerbated skin inflammatory responses. These findings demonstrate that a dysbiotic skin microbiota is not only a consequence of tissue stress, but also enhances inflammation, which has implications for many inflammatory cutaneous diseases.

Unexplored diversity and strain-level structure of the skin microbiome associated with psoriasis

Psoriasis is an immune-mediated inflammatory skin disease that has been associated with cutaneous microbial dysbiosis by culture-dependent investigations and rRNA community profiling. We applied, for the first time, high-resolution shotgun metagenomics to characterise the microbiome of psoriatic and unaffected skin from 28 individuals. We demonstrate psoriatic ear sites have a decreased diversity and psoriasis is associated with an increase in Staphylococcus, but overall the microbiomes of psoriatic and unaffected sites display few discriminative features at the species level. Finer strain-level analysis reveals strain heterogeneity colonisation and functional variability providing the intriguing hypothesis of psoriatic niche-specific strain adaptation or selection. Furthermore, we accessed the poorly characterised, but abundant, clades with limited sequence information in public databases, including uncharacterised Malassezia spp. These results highlight the skins hidden diversity and suggests strain-level variations could be key determinants of the psoriatic microbiome. This illustrates the need for high-resolution analyses, particularly when identifying therapeutic targets. This work provides a baseline for microbiome studies in relation to the pathogenesis of psoriasis.

Keratinocytes as sensors and central players in the immune defense against Staphylococcus aureus in the skin

Healthy human skin provides an effective mechanical as well as immunologic barrier against pathogenic microorganisms with keratinocytes as the main cell type in the epidermis actively participating and orchestrating the innate immune response of the skin. As constituent of the outermost layer encountering potential pathogens they have to sense signals from the environment and must be able to initiate a differential immune response to harmless commensals and harmful pathogens. Staphylococci are among the most abundant colonizers of the skin: Whereas Staphylococcus epidermidis is part of the skin microbiota and ubiquitously colonizes human skin, Staphylococcus aureus is only rarely found on healthy human skin, but frequently colonizes the skin of atopic dermatitis (AD) patients. This review highlights recent advances in understanding how keratinocytes as sessile innate immune cells orchestrate an effective defense against S. aureus in healthy skin and the mechanisms leading to an impaired keratinocyte function in AD patients.

The Association of the Skin Microbiota With Health, Immunity, and Disease

The human skin is densely colonized by a highly diverse microbiota comprising all three domains of life. Long believed to represent mainly a source of infection, the human skin microbiota is nowadays well accepted as an important driver of human (skin) health and well-being. This microbiota is influenced by many host and environmental factors and interacts closely with the skin immune system. Although cause and effect are usually difficult to discriminate, changes in the skin microbiota clearly play a role in the pathobiology of many types of skin disease and cosmetic disorders. Consequently, treatment and prevention strategies have to respect this role, rendering pre- and probiotic and even transplantation therapies an additional option to the use of antibiotics.

Interaction of Candida Species with the Skin

The human skin is commonly colonized by diverse fungal species. Some Candida species, especially C. albicans, do not only reside on the skin surface as commensals, but also cause infections by growing into the colonized tissue. However, defense mechanisms at the skin barrier level are very efficient, involving residential non-immune and immune cells as well as immune cells specifically recruited to the site of infection. Therefore, the skin is an effective barrier against fungal infection. While most studies about commensal and pathogenic interaction of Candida species with host epithelia focus on the interaction with mucosal surfaces such as the vaginal and gastrointestinal epithelia, less is known about the mechanisms underlying Candida interaction with the skin. In this review, we focus on the ecology and molecular pathogenesis of Candida species on the skin and give an overview of defense mechanisms against C. albicans in this context. We also discuss new research avenues in dermal infection, including the involvement of neurons, fibroblasts, and commensal bacteria in both mouse and human model systems.

Homeostatic Immunity and the Microbiota

The microbiota plays a fundamental role in the induction, education, and function of the host immune system. In return, the host immune system has evolved multiple means by which to maintain its symbiotic relationship with the microbiota. The maintenance of this dialogue allows the induction of protective responses to pathogens and the utilization of regulatory pathways involved in the sustained tolerance to innocuous antigens. The ability of microbes to set the immunological tone of tissues, both locally and systemically, requires tonic sensing of microbes and complex feedback loops between innate and adaptive components of the immune system. Here we review the dominant cellular mediators of these interactions and discuss emerging themes associated with our current understanding of the homeostatic immunological dialogue between the host and its microbiota.

Antibiotics-induced gut microbiota dysbiosis promotes tumor initiation via affecting APC-Th1 development in mice

Gut microbiota is critical for maintaining body immune homeostasis and thus affects tumor growth and therapeutic efficiency. Here, we investigated the link between microbiota and tumorgenesis in a mice model of subcutaneous melanoma cell transplantation, and explored the underlying mechanism. We found disruption of gut microbiota by pretreating mice with antibiotics promote tumor growth and remodeling the immune compartment within the primary tumor. Indeed, gut microbial dysbiosis reduced the infiltrated mature antigen-presenting cells of tumor, together with lower levels of co-stimulators, such as CD80, CD86 and MHCII, as well as defective Th1 cytokines, including IFNγ, TNFα, IL12p40, and IL12p35. Meantime, splenic APCs displayed blunted ability in triggering T cell proliferation and IFNγ secretion. However, oral administration of LPS restored the immune surveillance effects and thus inhibited tumor growth in the antibiotics induced gut microbiota dysbiosis group. Taken together, these data highly supported that antibiotics induced gut microbiota dysbiosis promotes tumor initiation, while LPS supplementation would restore the effective immune surveillance and repress tumor initiation.

The microbiota of the respiratory tract: gatekeeper to respiratory health

The respiratory tract is a complex organ system that is responsible for the exchange of oxygen and carbon dioxide. The human respiratory tract spans from the nostrils to the lung alveoli and is inhabited by niche-specific communities of bacteria. The microbiota of the respiratory tract probably acts as a gatekeeper that provides resistance to colonization by respiratory pathogens. The respiratory microbiota might also be involved in the maturation and maintenance of homeostasis of respiratory physiology and immunity. The ecological and environmental factors that direct the development of microbial communities in the respiratory tract and how these communities affect respiratory health are the focus of current research. Concurrently, the functions of the microbiome of the upper and lower respiratory tract in the physiology of the human host are being studied in detail. In this Review, we will discuss the epidemiological, biological and functional evidence that support the physiological role of the respiratory microbiota in the maintenance of human health.

Establishing Tolerance to Commensal Skin Bacteria: Timing Is Everything

Commensal bacteria live intimately and in constant dialogue with skin immune cells. Regulating our immune response to these bacteria is critical for skin homeostasis. Using a new murine model to track Staphylococcus epidermidis-specific T cells, we found that colonization during neonatal but not adult life led to S.epidermidis-specific immune tolerance. This tolerance protected against skin inflammation and was mediated by a wave of regulatory T cells entering neonatal skin. These findings provide new insight into how we establish a healthy symbiosis with commensal microbes and highlight avenues for future research to identify novel therapies for inflammatory skin disease.

The Molecular Revolution in Cutaneous Biology: Investigating the Skin Microbiome

Building upon the knowledge garnered from investigations utilizing traditional culturing methods, advances in sequencing technologies have catalyzed a revolution in studying human-associated microbes: bacteria, fungi and viruses. Skin microbiome research in healthy individuals and patients with dermatologic disorders has provided insights into the complexity and biogeography of human skin microbes. The continual developments in sequencing and analyses will provide increasingly sophisticated tools to interrogate human-associated microbes, ultimately to improve our understanding of health and disease.

The Role of the Skin and Gut Microbiome in Psoriatic Disease

PURPOSE

To understand the changes in the microbiome in psoriatic disease, we conducted a systematic review of studies comparing the skin and gut microbiota in psoriatic individuals and healthy controls.

FINDINGS

Our review of studies pertaining to the cutaneous microbiome showed a trend towards an increased relative abundance of Streptococcus and a decreased level of Propionibacterium in psoriasis patients compared to controls. In the gut microbiome, the ratio of Firmicutes and Bacteroidetes was perturbed in psoriatic individuals compared to healthy controls. Actinobacteria was also relatively underrepresented in psoriasis patients relative to healthy individuals.

SUMMARY

Although the field of the psoriatic microbiome is relatively new, these first studies reveal interesting differences in microbiome composition that may be associated with the development of psoriatic comorbidities and serve as novel therapeutic targets.

Tyrosine Kinase SYK Licenses MyD88 Adaptor Protein to Instigate IL-1α-Mediated Inflammatory Disease

Mice carrying a hypomorphic point mutation in the Ptpn6 gene (Ptpn6^spin mice) develop an inflammatory skin disease that resembles neutrophilic dermatosis in humans. Here, we demonstrated that interleukin-1α (IL-1α) signaling through IL-1R and MyD88 in both stromal and immune cells drive inflammation in Ptpn6^spin mice. We further identified SYK as a critical kinase that phosphorylates MyD88, promoted MyD88-dependent signaling and mediates dermatosis in Ptpn6^spin mice. Our studies further demonstrated that SHP1 encoded by Ptpn6 binds and suppresses SYK activation to inhibit MyD88 phosphorylation. Downstream of SHP1 and SYK-dependent counterregulation of MyD88 tyrosine phosphorylation, we have demonstrated that the scaffolding function of receptor interacting protein kinase 1 (RIPK1) and tumor growth factor-β activated kinase 1 (TAK1)-mediating signaling were required to spur inflammatory disease. Overall, these studies identify SHP1 and SYK crosstalk as a critical regulator of MyD88 post-translational modifications and IL-1-driven inflammation.

Commensal Microbes and Hair Follicle Morphogenesis Coordinately Drive Treg Migration into Neonatal Skin

Regulatory T cells (Tregs) are required to establish immune tolerance to commensal microbes. Tregs accumulate abruptly in the skin during a defined window of postnatal tissue development. However, the mechanisms mediating Treg migration to neonatal skin are unknown. Here we show that hair follicle (HF) development facilitates the accumulation of Tregs in neonatal skin and that upon skin entry these cells localize to HFs, a primary reservoir for skin commensals. Further, germ-free neonates had reduced skin Tregs indicating that commensal microbes augment Treg accumulation. We identified Ccl20 as a HF-derived, microbiota-dependent chemokine and found its receptor, Ccr6, to be preferentially expressed by Tregs in neonatal skin. The Ccl20-Ccr6 pathway mediated Treg migration in vitro and in vivo. Thus, HF morphogenesis, commensal microbe colonization, and local chemokine production work in concert to recruit Tregs into neonatal skin, thereby establishing this tissue Treg niche early in life.

Host Response to Staphylococcus epidermidis Colonization and Infections

The majority of research in the Staphylococcus field has been dedicated to the understanding of Staphylococcus aureus infections. In contrast, there is limited information on infections by coagulase-negative Staphylococci (CoNS) and how the host responds to them. S. epidermidis, a member of the coagulase-negative Staphylococci, is an important commensal organism of the human skin and mucous membranes; and there is emerging evidence of its benefit for human health in fighting off harmful microorganisms. However, S. epidermidis can cause opportunistic infections, which include particularly biofilm-associated infections on indwelling medical devices. These often can disseminate into the bloodstream; and in fact, S. epidermidis is the most frequent cause of nosocomial sepsis. The increasing use of medical implants and the dramatic shift in the patient demographic population in recent years have contributed significantly to the rise of S. epidermidis infections. Furthermore, treatment has been complicated by the emergence of antibiotic-resistant strains. Today, S. epidermidis is a major nosocomial pathogen posing significant medical and economic burdens. In this review, we present the current understanding of mechanisms of host defense against the prototypical CoNS species S. epidermidis as a commensal of the skin and mucous membranes, and during biofilm-associated infection and sepsis.

Antimicrobials from human skin commensal bacteria protect against Staphylococcus aureus and are deficient in atopic dermatitis

The microbiome can promote or disrupt human health by influencing both adaptive and innate immune functions. We tested whether bacteria that normally reside on human skin participate in host defense by killing Staphylococcus aureus, a pathogen commonly found in patients with atopic dermatitis (AD) and an important factor that exacerbates this disease. High-throughput screening for antimicrobial activity against S. aureus was performed on isolates of coagulase-negative Staphylococcus (CoNS) collected from the skin of healthy and AD subjects. CoNS strains with antimicrobial activity were common on the normal population but rare on AD subjects. A low frequency of strains with antimicrobial activity correlated with colonization by S. aureus The antimicrobial activity was identified as previously unknown antimicrobial peptides (AMPs) produced by CoNS species including Staphylococcus epidermidis and Staphylococcus hominis These AMPs were strain-specific, highly potent, selectively killed S. aureus, and synergized with the human AMP LL-37. Application of these CoNS strains to mice confirmed their defense function in vivo relative to application of nonactive strains. Strikingly, reintroduction of antimicrobial CoNS strains to human subjects with AD decreased colonization by S. aureus These findings show how commensal skin bacteria protect against pathogens and demonstrate how dysbiosis of the skin microbiome can lead to disease.

CD49a Expression Defines Tissue-Resident CD8+ T Cells Poised for Cytotoxic Function in Human Skin

Tissue-resident memory T (Trm) cells form a heterogeneous population that provides localized protection against pathogens. Here, we identify CD49a as a marker that differentiates CD8⁺ Trm cells on a compartmental and functional basis. In human skin epithelia, CD8⁺CD49a⁺ Trm cells produced interferon-γ, whereas CD8⁺CD49a⁻ Trm cells produced interleukin-17 (IL-17). In addition, CD8⁺CD49a⁺ Trm cells from healthy skin rapidly induced the expression of the effector molecules perforin and granzyme B when stimulated with IL-15, thereby promoting a strong cytotoxic response. In skin from patients with vitiligo, where melanocytes are eradicated locally, CD8⁺CD49a⁺ Trm cells that constitutively expressed perforin and granzyme B accumulated both in the epidermis and dermis. Conversely, CD8⁺CD49a^– Trm cells from psoriasis lesions predominantly generated IL-17 responses that promote local inflammation in this skin disease. Overall, CD49a expression delineates CD8⁺ Trm cell specialization in human epithelial barriers and correlates with the effector cell balance found in distinct inflammatory skin diseases.

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CD49a expression marks CD8⁺ Trm cells poised for IFN-γ production in human skin

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IL-15 drives potent cytotoxic capacity in CD49a⁺ Trm cells

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IL-17 is preferentially produced by CD49a⁻ CD8⁺ Trm cells in the skin

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CD49a⁺ versus CD49^- Trm cell functional dichotomy is preserved in vitiligo and psoriasis

Selective Bactericidal Activity of Divalent Metal Salts of Lauric Acid

Bacteria play a crucial role in skin health. For example, Staphylococcus aureus and Propionibacterium acnes cause skin roughness and acne, whereas Staphylococcus epidermidis enhances innate barrier immunity. Therefore, controlling the bacterial flora is important in dermatology and cosmetic chemistry. In this study, the bactericidal activities of different metal salts of lauric acid were evaluated. The bactericidal behavior of the salts changed according to the type of metal ion. Specifically, the Mg-, Ca-, and Mn-containing salts effectively sterilized only S. aureus and P. acnes. Their Co, Ni, and Cu salts sterilized all bacteria, including S. epidermidis, whereas the Zn salt proved ineffective. The Cu salt displayed the strongest bactericidal activity. Spin-trapping, detected using electron spin resonance, showed that this salt catalyzed the generation of hydroxyl radicals, which can destroy bacterial cell membranes. These findings demonstrate that metal-ion selection is an important factor in the design of bactericidal agents for healthcare products.

Sebaceous Gland-Rich Skin Is Characterized by TSLP Expression and Distinct Immune Surveillance Which Is Disturbed in Rosacea

The microbial community exhibits remarkable diversity on topographically distinct skin regions, which may be accompanied by differences in skin immune characteristics. Our aim was to compare the immune milieu of healthy sebaceous gland-rich (SGR) and sebaceous gland-poor skin areas, and to analyze its changes in an inflammatory disease of SGR skin. For this purpose, immunohistochemical, immunocytochemical, and quantitative real-time PCR analyses of thymic stromal lymphopoietin (TSLP) and other cytokines, phenotypic immune cell markers and transcription factors were carried out in samples from sebaceous gland-poor, SGR skin and from papulopustular rosacea. TSLP mRNA and protein production was also studied in cultured keratinocytes. In SGR skin, higher TSLP expression, dendritic cell appearance without prominent activation, and T cell presence with IL-17/IL-10 cytokine milieu were detected compared with sebaceous gland-poor skin. Linoleic acid, a major sebum component, was found to induce TSLP expression dose-dependently in keratinocytes. In papulopustular rosacea, significantly decreased TSLP level and influx of inflammatory dendritic cells and T cells with IL-17/interferon-γ cytokine milieu were observed. According to our results, SGR skin is characterized by a distinct, noninflammatory immune surveillance, which may explain the preferred localization of inflammatory skin diseases, and can influence future barrier repair therapeutic concepts.

Expression and Purification of the Cas10-Csm Complex from Staphylococci

CRISPR-Cas (Clustered regularly interspaced short palindromic repeats-CRISPR-associated proteins) is a class of prokaryotic immune systems that degrade foreign nucleic acids in a sequence-specific manner. These systems rely upon ribonucleoprotein complexes composed of Cas nucleases and small CRISPR RNAs (crRNAs). Staphylococcus epidermidis and Staphylococcus aureus are bacterial residents on human skin that are also leading causes of antibiotic resistant infections (Lowy, 1998; National Nosocomial Infections Surveillance, 2004; Otto, 2009). Many staphylococci possess Type III-A CRISPR-Cas systems (Marraffini and Sontheimer, 2008; Cao et al., 2016), which have been shown to prevent plasmid transfer and protect against viral predators (Goldberg et al., 2014; Hatoum-Aslan et al., 2014; Samai et al., 2015) in these organisms. Thus, gaining a mechanistic understanding of these systems in the native staphylococcal background can lead to important insights into the factors that impact the evolution and survival of these pathogens. Type III-A CRISPR-Cas systems encode a five-subunit effector complex called Cas10-Csm (Hatoum-Aslan et al., 2013). Here, we describe a protocol for the expression and purification of Cas10-Csm from its native S. epidermidis background or a heterologous S. aureus background. The method consists of a two-step purification protocol involving Ni²⁺-affinity chromatography and a DNA affinity biotin pull-down, which together yield a pure preparation of the Cas10-Csm complex. This approach has been used previously to analyze the effects of mutations on Cas10-Csm complex integrity (Hatoum-Aslan et al., 2014), crRNA formation (Hatoum-Aslan et al., 2013), and to detect binding partners that directly interact with the core Cas10-Csm complex (Walker et al., 2016). Importantly, this approach can be easily adapted for use in other Staphylococcus species to probe and understand their native Type III-A CRISPR-Cas systems.

Hapten-Specific T Cell-Mediated Skin Inflammation: Flow Cytometry Analysis of Mouse Skin Inflammatory Infiltrate

Hapten-specific T cell-mediated skin inflammation also known as contact hypersensitivity (CHS) is characterized by a strong influx of CD8⁺ cytotoxic T cells within the skin upon reexposure of sensitized individuals to the same hapten. As many other leukocytes are also recruited during this elicitation phase, we attempted to revisit the skin infiltrate and characterize the inflammatory pattern. Recent improvement in the isolation in conventional as well as inflammatory dendritic cell and macrophage subsets from tissues and in the use of appropriate surface markers unraveling their heterogeneity should allow to determinate their specific functions in the CHS model. Here, we describe procedures to extract those cells from the skin and to analyze them by flow cytometry using a combination of appropriate surface markers allowing further transcriptomic analysis and functional assays.

Dendritic Cells in the Immune System—History, Lineages, Tissues, Tolerance, and Immunity

The aim of this review is to provide a coherent framework for understanding dendritic cells (DCs). It has seven sections. The introduction provides an overview of the immune system and essential concepts, particularly for the nonspecialist reader. Next, the “History” section outlines the early evolution of ideas about DCs and highlights some sources of confusion that still exist today. The “Lineages” section then focuses on five different populations of DCs: two subsets of “classical” DCs, plasmacytoid DCs, monocyte-derived DCs, and Langerhans cells. It highlights some cellular and molecular specializations of each, and also notes other DC subsets that have been proposed. The following “Tissues” section discusses the distribution and behavior of different DC subsets within nonlymphoid and secondary lymphoid tissues that are connected by DC migration pathways between them. In the “Tolerance” section, the role of DCs in central and peripheral tolerance is considered, including their ability to drive the differentiation of different populations of regulatory T cells. In contrast, the “Immunity” section considers the roles of DCs in sensing of infection and tissue damage, the initiation of primary responses, the T-cell effector phase, and the induction of immunological memory. The concluding section provides some speculative ideas about the evolution of DCs. It also revisits earlier concepts of generation of diversity and clonal selection in terms of DCs driving the evolution of T-cell responses. Throughout, this review highlights certain areas of uncertainty and suggests some avenues for future investigation.

Dendritic cell migration in health and disease

Dendritic cells (DCs) are potent and versatile antigen-presenting cells, and their ability to migrate is key for the initiation of protective pro-inflammatory as well as tolerogenic immune responses. Recent comprehensive studies have highlighted the importance of DC migration in the maintenance of immune surveillance and tissue homeostasis, and also in the pathogenesis of a range of diseases. In this Review, we summarize the anatomical, cellular and molecular factors that regulate the migration of different DC subsets in health and disease. In particular, we focus on new insights concerning the role of migratory DCs in the pathogenesis of diseases of the skin, intestine, lung, and brain, as well as in autoimmunity and atherosclerosis.

Dysbiotic Bacterial and Fungal Communities Not Restricted to Clinically Affected Skin Sites in Dandruff

Dandruff is a prevalent chronic inflammatory skin condition of the scalp that has been associated with Malassezia yeasts. However, the microbial role has not been elucidated yet, and the etiology of the disorder remains poorly understood. Using high-throughput 16S rDNA and ITS1 sequencing, we characterized cutaneous bacterial and fungal microbiotas from healthy and dandruff subjects, comparing scalp and forehead (lesional and non-lesional skin sites). Bacterial and fungal communities from dandruff analyzed at genus level differed in comparison with healthy ones, presenting higher diversity and greater intragroup variation. The microbial shift was observed also in non-lesional sites from dandruff subjects, suggesting that dandruff is related to a systemic process that is not restricted to the site exhibiting clinical symptoms. In contrast, Malassezia microbiota analyzed at species level did not differ according to health status. A 2-step OTU assignment using combined databases substantially increased fungal assigned sequences, and revealed the presence of highly prevalent uncharacterized Malassezia organisms (>37% of the reads). Although clinical symptoms of dandruff manifest locally, microbial dysbiosis beyond clinically affected skin sites suggests that subjects undergo systemic alterations, which could be considered for redefining therapeutic approaches.

CD301b+ dendritic cells stimulate tissue-resident memory CD8+ T cells to protect against genital HSV-2

Tissue-resident memory CD8+ T (CD8 T_RM) cells are an essential component of protective immune responses at barrier tissues, including the female genital tract. However, the mechanisms that lead to the initiation of CD8 T_RM-mediated protective immunity after viral infection are unclear. Here we report that CD8 T_RM cells established by ‘prime and pull' method confer protection against genital HSV-2 infection, and that IFN-γ produced by CD8 T_RM cells is required for this protection. Furthermore, we find that CD8 T_RM-cell restimulation depends on a population of CD301b⁺ antigen-presenting cells (APC) in the lamina propria. Elimination of MHC class I on CD301b⁺ dendritic cells abrogates protective immunity, suggesting the requirement for cognate antigen presentation to CD8 T_RM cells by CD301b⁺ dendritic cells. These results define the requirements for CD8 T_RM cells in protection against genital HSV-2 infection and identify the population of APC that are responsible for activating these cells.

Tissue-resident memory T cells are needed for optimal antiviral immunity at mucosal surfaces. Here the authors provide a mechanism for this protection, showing that vaginal CD301b+ DC-dependent IFN-γ production by CD8+ tissue-resident memory T cells, not circulating T cells, is central to HSV-2 resistance.

Aquaphilus dolomiae extract counteracts the effects of cutaneous S. aureus secretome isolated from atopic children on CD4+ T cell activation

CONTEXT

Skin microbiota takes part in the control of cutaneous inflammation. In skin diseases such as atopic dermatitis (AD) cutaneous dysbiosis and the emergence of Staphylococcus aureus contribute to the pathophysiology of the disease. New therapeutic approaches consist in topical application of natural products able to counteract S. aureus effects through activation of resident immune cells producing anti-inflammatory cytokines such as IL-10.

OBJECTIVE

This study investigates the potential immunosuppressive properties of Aquaphilus dolomiae (Neisseriaceae), a flagellated bacterium contained in Avène Thermal Spring Water used in hydrotherapy treatments of AD patients.

MATERIALS AND METHODS

An aqueous protein extract of Aquaphilus dolomiae (ADE, 60 μg/mL) was added to human monocyte-derived dendritic cells (moDC) for 24 h. Expression of HLA-DR, CD86 and CD83 was evaluated by flow cytometry and released cytokines (IL-10, IL-12) by cytometry bead array assay. The proliferation of allogeneic CFSE-labelled CD4⁺T cells stimulated with ADE-conditioned moDC and S. aureus secretome was analysed by flow cytometry.

RESULTS

MoDC exposed to ADE expressed lower levels of HLA-DR and CD86 than untreated cells, no CD83 and secreted barely detectable IL-12 but high amounts of IL-10 (N = 12, p < 0.0002). The proliferative effect of S. aureus secretome on CD4⁺T cells was reduced (p < 0.001) in the presence of ADE-moDC.

CONCLUSION

ADE counteracted the mitogenic effect of a S. aureus secretome on CD4⁺T cells. Owing to the role of S. aureus colonization in driving inflammation in AD the immunosuppressive property of the ADE might be useful to reduce disease severity.