The skin immune atlas: three-dimensional analysis of cutaneous leukocyte subsets by multiphoton microscopy

Neutrophil Recruitment: From Model Systems to Tissue-Specific Patterns

Neutrophil recruitment is not only vital for host defense, but also relevant in pathological inflammatory reactions, such as sepsis. Model systems have been established to examine different steps of the leukocyte recruitment cascade in vivo and in vitro under inflammatory conditions. Recently, tissue-specific recruitment patterns have come into focus, requiring modification of formerly generalized assumptions. Here, we summarize existing models of neutrophil recruitment and highlight recent discoveries in organ-specific recruitment patterns. New techniques show that previously stated assumptions of integrin activation and tissue invasion may need revision. Similarly, neutrophil recruitment to specific organs can rely on different organ properties, adhesion molecules, and chemokines. To advance our understanding of neutrophil recruitment, organ-specific intravital microscopy methods are needed.

Controlled Ultrasound Erosion for Transdermal Delivery and Hepatitis B Immunization

Although ultrasound is effective for transdermal delivery, it remains difficult to control the position, shape and size of localized skin transport regions. We developed an ultrasound erosion protocol to generate a single-site, circular delivery region with controlled size at the center of patched skin. We found that (i) shorter ultrasound pulses (25 cycles) with higher pulse repetition frequency (4 kHz) and higher peak negative pressure (17.0 MPa) resulted in larger (0.995 mm²) and deeper (∼300 µm) skin delivery regions with a higher success rate (94.44%); and (ii) temperature elevation of the skin increased with ultrasound exposure time, with a 30-s safety threshold. Furthermore, we found that hair follicles decreased the delivery controllability of ultrasound erosion. Therefore, we selected the skin of the hind legs of mice without dense hair follicles to deliver more than 1 μL of vaccine solution and successfully elicit immune responses against hepatitis B surface antigen.

Intravital imaging of host-parasite interactions in skin and adipose tissues

Intravital microscopy allows the visualisation of how pathogens interact with host cells and tissues in living animals in real time. This method has enabled key advances in our understanding of host-parasite interactions under physiological conditions. A combination of genetics, microscopy techniques, and image analysis have recently facilitated the understanding of biological phenomena in living animals at cellular and subcellular resolution. In this review, we summarise findings achieved by intravital microscopy of the skin and adipose tissues upon infection with various parasites, and we present a view into possible future applications of this method.

Intravital imaging of skin infections

Intravital imaging of cutaneous immune responses has revealed intricate links between the skin's structural properties, the immune cells that reside therein, and the carefully orchestrated migratory dynamics that enable rapid sensing and subsequent elimination of skin pathogens. In particular, the development of 2-photon intravital microscopy (2P-IVM), which enables the excitation of fluorescent molecules within deep tissue with minimal light scattering and tissue damage, has proven an invaluable tool in the characterization of different cell subset's roles in skin infection. The ability to visualize cells, tissue structures, pathogens and track migratory dynamics at designated times following infection, or during inflammatory responses has been crucial in defining how immune responses in the skin are coordinated, either locally or in concert with circulating immune cells. Skin pathogens affect millions of people worldwide, and skin infections leading to cutaneous pathology have a considerable impact on the quality of life and longevity of people affected. In contrast, pathogens that infect the skin to later cause systemic illness, such as malaria parasites and a variety of arthropod-borne viruses, or infection in distant anatomical sites are a significant cause of morbidity and mortality worldwide. Here, we review recent advances and seminal studies that employed intravital imaging to characterize key immune response mechanisms in the context of viral, bacterial and parasitic skin infections, and provide insights on skin pathogens of global significance that would benefit from such investigative approaches.

A low inflammatory, Langerhans cell-targeted microprojection patch to deliver ovalbumin to the epidermis of mouse skin

In a low inflammatory skin environment, Langerhans cells (LCs) - but not dermal dendritic cells (dDCs) - contribute to the pivotal process of tolerance induction. Thus LCs are a target for specific-tolerance therapies. LCs reside just below the stratum corneum, within the skin's viable epidermis. One way to precisely deliver immunotherapies to LCs while remaining minimally invasive is with a skin delivery device such as a microprojection arrays (MPA). Today's MPAs currently achieve rapid delivery (e.g. within minutes of application), but are focussed primarily at delivery of therapeutics to the dermis, deeper within the skin. Indeed, no MPA currently delivers specifically to the epidermal LCs of mouse skin. Without any convenient, pre-clinical device available, advancement of LC-targeted therapies has been limited. In this study, we designed and tested a novel MPA that delivers ovalbumin to the mouse epidermis (eMPA) while maintaining a low, local inflammatory response (as defined by low erythema after 24 h). In comparison to available dermal-targeted MPAs (dMPA), only eMPAs with larger projection tip surface areas achieved shallow epidermal penetration at a low application energy. The eMPA characterised here induced significantly less erythema after 24 h (p = 0.0004), less epidermal swelling after 72 h (p < 0.0001) and 52% less epidermal cell death than the dMPA. Despite these differences in skin inflammation, the eMPA and dMPA promoted similar levels of LC migration out of the skin. However, only the eMPA promoted LCs to migrate with a low MHC II expression and in the absence of dDC migration. Implementing this more mouse-appropriate and low-inflammatory eMPA device to deliver potential immunotherapeutics could improve the practicality and cell-specific targeting of such therapeutics in the pre-clinical stage. Leading to more opportunities for LC-targeted therapeutics such as for allergy immunotherapy and asthma.

Imaging of mast cells

Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. The distribution, morphology and biochemical composition of mast cells has been studied in detail in vitro and on tissue sections both at the light microscopic and ultrastructural level. More recently, the development of fluorescent reporter strains and intravital imaging modalities has enabled first glimpses of the real-time behavior of mast cells in situ. In this review, we describe commonly used imaging approaches to study mast cells in cell culture as well as within normal and diseased tissues. We further describe the interrogation of mast cell function via imaging by providing a detailed description of mast cell-nerve plexus interactions in the intestinal tract. Together, visualizing mast cells has expanded our view of these cells in health and disease.

Cutaneous immune responses mediated by dendritic cells and mast cells

In the skin, complex cellular networks maintain barrier function and immune homeostasis. Tightly regulated multicellular cascades are required to initiate innate and adaptive immune responses. Innate immune cells, particularly DCs and mast cells, are central to these networks. Early studies evaluated the function of these cells in isolation, but recent studies clearly demonstrate that cutaneous DCs (dermal DCs and Langerhans cells) physically interact with neighboring cells and are receptive to activation signals from surrounding cells, such as mast cells. These interactions amplify immune activation. In this review, we discuss the known functions of cutaneous DC populations and mast cells and recent studies highlighting their roles within cellular networks that determine cutaneous immune responses.

IL-23- and Imiquimod-Induced Models of Experimental Psoriasis in Mice

Genome-wide association studies have found that polymorphisms in genes for IL-23 and its receptor are important in psoriasis, and blocking IL-23 is an effective therapy in the disease. The use of Aldara^™ , a cream that contains the TLR7 and TLR8 agonist imiquimod (IMQ), was found to exacerbate psoriasis in some patients with pre-existing disease. Intradermal injections of IL-23 and topical application of Aldara/IMQ induce skin inflammation in mice with features similar to psoriasis-including epidermal hyperplasia and accumulation of inflammatory cells in epidermis and dermis-which is mediated by IL-17A, IL-22, and other factors implicated in the human disease. Consequently, these models can be used in preclinical studies to investigate the molecular and cellular pathogenesis of psoriasis, as well as in the evaluation of potential therapies. This article provides detailed methodologies for creating and evaluating the IL-23- and Aldara/IMQ-induced mouse models of psoriasis. The article also provides a protocol for analyzing skin leukocytes by flow cytometry. © 2019 by John Wiley & Sons, Inc.

Imaging of Inflammatory Responses in the Mouse Ear Skin

The skin is one of the most physiologically important organs where the organism comes into contact with the external environment and is often a site where pathogen entry first occurs. Thus, a better understanding of the specialized cellular behavior of the immune system in the skin may be important for the improved treatment of diseases. Here, we describe in detail a procedure to image the dorsal mouse ear skin, using a customized ear stage and its associated coverslip holder, with an upright multiphoton microscope. As a demonstrative example, we describe the specific protocol for visualizing robust neutrophil trafficking in albino lysozyme-EGFP mice in response to zymosan particles. Instructive sections are provided for the mouse ear preparation, intradermal delivery of zymosan, design and use of the custom ear stage, as well as a solution for the uninterrupted live imaging of mice during prolonged sessions within a dark box. The mouse ear is easily accessible for imaging, and unlike most other organs, does not require any invasive surgery to be performed.

Immunohistochemical Localization of Fibrinogen C Domain Containing 1 on Epithelial and Mucosal Surfaces in Human Tissues

Fibrinogen C domain containing 1 (FIBCD1) is a transmembrane receptor that binds chitin and other acetylated compounds with high affinity. FIBCD1 has previously been shown to be present in the epithelium of the gastrointestinal tract. In the present study, we performed a detailed analysis of normally structured human tissues for the expression of FIBCD1 by quantitative PCR and immunohistochemistry. We find that FIBCD1 is expressed in epithelial cells derived from all three germ layers. Endodermal-derived epithelial cells throughout the gastrointestinal tract and the respiratory system showed high expression of FIBCD1 and also mesodermal-derived cells in the genitourinary system and ectodermal-derived epidermis and sebaceous glands cells expressed FIBCD1. In some columnar epithelial cells, for example, in the salivary gland and gall bladder, the FIBCD1 expression was clearly polarized with strong apical reaction, while other columnar cells, for example, in small and large intestine and in bronchi, the staining was equally strong apically and basolaterally. In keratinocytes in skin, tongue, and oral cavity, the FIBCD1 staining was granular. This expression pattern together with the known binding properties supports that FIBCD1 plays a role in innate immunity in the skin and at mucosal surfaces.

Unveiling and Characterizing Early Bilateral Interactions between Biofilm and the Mouse Innate Immune System

A very substantial progress has been made in our understanding of infectious diseases caused by invasive bacteria. Under their planktonic forms, bacteria transiently reside in the otherwise sterile mammal body tissues, as the physiological inflammation insures both their clearance and repair of any tissue damage. Yet, the bacteria prone to experience planktonic to biofilm developmental transition still need to be studied. Of note, sessile bacteria not only persist but also concur preventing the effectors and regulators of the physiological inflammation to operate. Thus, it is urgent to design biologically sound experimental approaches aimed to extract, at the earliest stage, immune signatures of mono-bacteria planktonic to biofilm developmental transition in vivo and ex vivo. Indeed, the transition is often the first event to which succeeds the “chronicization” process whereby classical bacteria-targeting therapies are no more efficacious. An in vivo model of micro-injection of Staphylococcus aureus planktonic or biofilm cells in the ear pinna dermis of laboratory transgenic mice with fluorescent immune cells is proposed. It allows visualizing, in real time, the range of the early interactions between the S. aureus and myeloid cell subsets- the resident macrophages and dendritic cells, the recruited neutrophil granulocytes/polymorphonuclear neutrophils, monocytes otherwise known to differentiate as macrophages or dendritic cells. One main objective is to extract contrasting immune signatures of the modulation of the physiological inflammation with respect to the two bacterial lifestyles.

Sustained accumulation of antigen-presenting cells after infection promotes local T-cell immunity

Antigen-presenting cells (APC), such as dendritic cells (DC) and macrophages, are critical for T-cell-mediated immunity. Although it is established that memory T cells accumulate and persist in peripheral tissues after the resolution of infection, whether this is also the case for APC remains unclear. Here, we report that CCR2-dependent cells infiltrate skin during acute infection with herpes simplex virus (HSV)-1 and subsequently give rise to localized populations of DCs and macrophages. These APC are found at elevated numbers at sites of resolved infection or inflammation compared with unaffected regions of skin. Importantly, this local accumulation of APC is sustained for prolonged periods of time and has important functional consequences, as it promotes interferon-γ responses by virus-specific CD4⁺ T cells upon localized challenge infection with HSV-1. Thus, our results highlight how infection history determines long-term changes in immune cell composition in skin and how different types of immune cells accumulate, persist and co-operate to provide optimal immunity at this critical barrier site.

Neutrophils are dispensable in the modulation of T cell immunity against cutaneous HSV-1 infection

Neutrophils rapidly infiltrate sites of inflammation during peripheral infection or tissue injury. In addition to their well described roles as pro-inflammatory phagocytes responsible for pathogen clearance, recent studies have demonstrated a broader functional repertoire including mediating crosstalk between innate and adaptive arms of the immune system. Specifically, neutrophils have been proposed to mediate antigen transport to lymph nodes (LN) to modulate T cell priming and to influence T cell migration to infected tissues. Using a mouse model of cutaneous herpes simplex virus type 1 (HSV-1) infection we explored potential contributions of neutrophils toward anti-viral immunity. While a transient, early influx of neutrophils was triggered by dermal scarification, we did not detect migration of neutrophils from the skin to LN. Furthermore, despite recruitment of neutrophils into LN from the blood, priming and expansion of CD4⁺ and CD8⁺ T cells was unaffected following neutrophil depletion. Finally, we found that neutrophils were dispensable for the migration of effector T cells into infected skin. Our study suggests that the immunomodulatory roles of neutrophils toward adaptive immunity may be context-dependent, and are likely determined by the type of pathogen and anatomical site of infection.

Inducing Ischemia-reperfusion Injury in the Mouse Ear Skin for Intravital Multiphoton Imaging of Immune Responses

Ischemia-reperfusion injury (IRI) occurs when there is transient hypoxia due to the obstruction of blood flow (ischemia) followed by a subsequent re-oxygenation of the tissues (reperfusion). In the skin, ischemia-reperfusion (IR) is the main contributing factor to the pathophysiology of pressure ulcers. While the cascade of events leading up to the inflammatory response has been well studied, the spatial and temporal responses of the different subsets of immune cells to an IR injury are not well understood. Existing models of IR using the clamping technique on the skin flank are highly invasive and unsuitable for studying immune responses to injury, while similar non-invasive magnet clamping studies in the skin flank are less-than-ideal for intravital imaging studies. In this protocol, we describe a robust model of non-invasive IR developed on mouse ear skin, where we aim to visualize in real-time the cellular response of immune cells after reperfusion via multiphoton intravital imaging (MP-IVM).

T cells mediate autoantibody-induced cutaneous inflammation and blistering in epidermolysis bullosa acquisita

T cells are key players in autoimmune diseases by supporting the production of autoantibodies. However, their contribution to the effector phase of antibody-mediated autoimmune dermatoses, i.e., tissue injury and inflammation of the skin, has not been investigated. In this paper, we demonstrate that T cells amplify the development of autoantibody-induced tissue injury in a prototypical, organ-specific autoimmune disease, namely epidermolysis bullosa acquisita (EBA) – characterized and caused by autoantibodies targeting type VII collagen. Specifically, we show that immune complex (IC)-induced inflammation depends on the presence of T cells – a process facilitated by T cell receptor (TCR)γδ and NKT cells. Because tissue damage in IC-induced inflammation is neutrophil-dependent, we further analyze the interplay between T cells and neutrophils in an experimental model of EBA. We demonstrate that T cells not only enhance neutrophil recruitment into the site of inflammation but also interact with neutrophils in lymphatic organs. Collectively, this study shows that T cells amplify the effector phase of antibody-induced tissue inflammation.

Total-Body Irradiation Exacerbates Dissemination of Cutaneous Candida Albicans Infection

Exposure to radiation, particularly a large or total-body dose, weakens the immune system through loss of bone marrow precursor cells, as well as diminished populations of circulating and tissue-resident immune cells. One such population is the skin-resident immune cells. Changes in the skin environment can be of particular importance as the skin is also host to a number of commensal organisms, including Candida albicans , a species of fungus that causes opportunistic infections in immunocompromised patients. In a previous study, we found that a 6 Gy sublethal dose of radiation in mice caused a reduction of cutaneous dendritic cells, indicating that the skin may have a poorer response to infection after irradiation. In this study, the same 6 Gy sublethal radiation dose led to a weakened response to a C. ablicans cutaneous infection, which resulted in systemic dissemination from the ear skin to the kidneys. However, this impaired response was mitigated through the use of interleukin-12 (IL-12) administered to the skin after irradiation. Concomitantly with this loss of local control of infection, we also observed a reduction of CD4⁺ and CD8⁺ T cells in the skin, as well as the reduced expression of IFN-γ, CXCL9 and IL-9, which influence T-cell infiltration and function in infected skin. These changes suggest a mechanism by which an impaired immune environment in the skin after a sublethal dose of radiation increases susceptibility to an opportunistic fungal infection. Thus, in the event of radiation exposure, it is important to include antifungal agents, or possibly IL-12, in the treatment regimen, particularly if wounds are involved that result in loss of the skin's physical barrier function.

Gene Expression Architecture of Mouse Dorsal and Tail Skin Reveals Functional Differences in Inflammation and Cancer

Inherited germline polymorphisms can cause gene expression levels in normal tissues to differ substantially between individuals. We present an analysis of the genetic architecture of normal adult skin from 470 genetically unique mice, demonstrating the effect of germline variants, skin tissue location, and perturbation by exogenous inflammation or tumorigenesis on gene signaling pathways. Gene networks related to specific cell types and signaling pathways, including sonic hedgehog (Shh), Wnt, Lgr family stem cell markers, and keratins, differed at these tissue sites, suggesting mechanisms for the differential susceptibility of dorsal and tail skin to development of skin diseases and tumorigenesis. The Pten tumor suppressor gene network is rewired in premalignant tumors compared to normal tissue, but this response to perturbation is lost during malignant progression. We present a software package for expression quantitative trait loci (eQTL) network analysis and demonstrate how network analysis of whole tissues provides insights into interactions between cell compartments and signaling molecules.

The Regulation of Immunological Processes by Peripheral Neurons in Homeostasis and Disease

The nervous system and the immune system are the principal sensory interfaces between the internal and external environment. They are responsible for recognizing, integrating, and responding to varied stimuli, and have the capacity to form memories of these encounters leading to learned or 'adaptive' future responses. We review current understanding of the cross-regulation between these systems. The autonomic and somatosensory nervous systems regulate both the development and deployment of immune cells, with broad functions that impact on hematopoiesis as well as on priming, migration, and cytokine production. In turn, specific immune cell subsets contribute to homeostatic neural circuits such as those controlling metabolism, hypertension, and the inflammatory reflex. We examine the contribution of the somatosensory system to autoimmune, autoinflammatory, allergic, and infectious processes in barrier tissues and, in this context, discuss opportunities for therapeutic manipulation of neuro-immune interactions.

Microbial pathogenesis revealed by intravital microscopy: pros, cons and cautions

Intravital multiphoton imaging allows visualization of infections and pathogenic mechanisms within intact organs in their physiological context. Today, most organs of mice and rats are applicable to in vivo or ex vivo imaging, opening completely new avenues for many researchers. Advances in fluorescent labeling of pathogens and infected cells, as well as improved small animal models for human pathogens, led to the increased application of in vivo imaging in infectious diseases research in recent years. Here, we review the latest literature on intravital or ex vivo imaging of viral and bacterial infections and critically discuss requirements, benefits and drawbacks of applied animal models, labeling strategies, and imaged organs.

Imaging CD4 T Cell Interstitial Migration in the Inflamed Dermis

The ability of CD4 T cells to carry out effector functions is dependent upon the rapid and efficient migration of these cells in inflamed peripheral tissues through an as-yet undefined mechanism. The application of multiphoton microscopy to the study of the immune system provides a tool to measure the dynamics of immune responses within intact tissues. Here we present a protocol for non-invasive intravital multiphoton imaging of CD4 T cells in the inflamed mouse ear dermis. Use of a custom imaging platform and a venous catheter allows for the visualization of CD4 T cell dynamics in the dermal interstitium, with the ability to interrogate these cells in real-time via the addition of blocking antibodies to key molecular components involved in motility. This system provides advantages over both in vitro models and surgically invasive imaging procedures. Understanding the pathways used by CD4 T cells for motility may ultimately provide insight into the basic function of CD4 T cells as well as the pathogenesis of both autoimmune diseases and pathology from chronic infections.

The site of tumor development determines immunogenicity via temporal mobilization of antigen-laden dendritic cells in draining lymph nodes

The elimination of solid tumors largely depends on effective T-cell priming by dendritic cells (DCs). For decades, studies focusing on antitumoral immune responses have been performed with tumors transplanted subcutaneously (s.c.). These studies however do not take into account the heterogeneous tissue distribution and functionality of the different DC subsets. Given the crucial role of DCs in inducing protective immune response, we postulated that the anatomic location of tumor development may greatly impact tumor immunogenicity. We therefore implanted tumor cells either in the DC-rich dermis environment or in the s.c. tissue that mainly contains macrophages and monocytes. We showed that intradermal (i.d.), but not s.c. tumors are rapidly rejected in a T-cell-dependent manner and induce protective T-cell responses. The rejection of i.d. tumors correlates with rapid recruitment of dermal DCs presenting the tumor antigen to both CD4 and CD8 T cells in the draining lymph nodes (dLNs). The same DC subsets were mobilized upon s.c. tumor transplantation but with delayed kinetics. Altogether, our results show that the anatomical site of tumor development influences tumor immunogenicity, notably by controlling the kinetics of DC mobilization in the draining LNs.

Mast cells have no impact on cutaneous leishmaniasis severity and related Th2 differentiation in resistant and susceptible mice

The genus leishmania comprises different protozoan parasites which are causative agents of muco-cutaneous and systemic, potentially lethal diseases. After infection with the species Leishmania major, resistant mice expand Th1 cells which stimulate macrophages for Leishmania destruction. In contrast, susceptible mice generate Th2 cells which deactivate macrophages, leading to systemic spread of the pathogens. Th-cell differentiation is determined within the first days, and Th2 cell differentiation requires IL-4, whereby the initial IL-4 source is often unknown. Mast cells are potential sources of IL-4, and hence their role in murine leishmaniasis has previously been studied in mast cell-deficient Kit mutant mice, although these mice display immunological phenotypes beyond mast cell deficiency. We therefore readdressed this question by infecting Kit-independent mast cell-deficient mice that are Th1 (C57BL/6 Cpa(Cre) ) or Th2 (BALB/c Cpa(Cre) ) prone with L. major. Using different parasite doses and intra- or subcutaneous infection routes, the results demonstrate no role of mast cells on lesion size development, parasite load, immune cell phenotypes expanding in draining lymph nodes, and cytokine production during murine cutaneous leishmaniasis. Thus, other cell types such as ILCs or T cells have to be considered as primary source of Th2-driving IL-4.

The role of tissue resident cells in neutrophil recruitment

Neutrophils are first responders of the immune system, rapidly migrating into affected tissues in response to injury or infection. To effectively call in this first line of defense, strategically placed cells within the vasculature and tissue respond to noxious stimuli by sending out coordinated signals that recruit neutrophils. Regulation of organ-specific neutrophil entry occurs at two levels. First, the vasculature supplying the organ provides cues for neutrophil egress out of the bloodstream in a manner dependent upon its unique cellular composition and architectural features. Second, resident immune cells and stromal cells within the organ send coordinated signals that guide neutrophils to their final destination. Here, we review recent findings that highlight the importance of these tissue-specific responses in the regulation of neutrophil recruitment and the initiation and resolution of inflammation.

Dermal-resident versus recruited γδ T cell response to cutaneous vaccinia virus infection

The study of T cell immunity at barrier surfaces has largely focused on T cells bearing the αβ TCR. However, T cells that express the γδ TCR are disproportionately represented in peripheral tissues of mice and humans, suggesting they too may play an important role responding to external stimuli. In this article, we report that, in a murine model of cutaneous infection with vaccinia virus, dermal γδ T cell numbers increased 10-fold in the infected ear and resulted in a novel γδ T cell population not found in naive skin. Circulating γδ T cells were specifically recruited to the site of inflammation and differentially contributed to dermal populations based on their CD27 expression. Recruited γδ T cells, the majority of which were CD27(+), were granzyme B(+) and made up about half of the dermal population at the peak of the response. In contrast, recruited and resident γδ T cell populations that made IL-17 were CD27(-). Using a double-chimera model that can discriminate between the resident dermal and recruited γδ T cell populations, we demonstrated their divergent functions and contributions to early stages of tissue inflammation. Specifically, the loss of the perinatal thymus-derived resident dermal population resulted in decreased cellularity and collateral damage in the tissue during viral infection. These findings have important implications for our understanding of immune coordination at barrier surfaces and the contribution of innate-like lymphocytes on the front lines of immune defense.