Dynamics and function of Langerhans cells in vivo: dermal dendritic cells colonize lymph node areas distinct from slower migrating Langerhans cells

Mucosal Langerhans Cells Promote Differentiation of Th17 Cells in a Murine Model of Periodontitis but Are Not Required for Porphyromonas gingivalis-Driven Alveolar Bone Destruction

Periodontitis is a chronic oral inflammatory disease affecting one in five individuals that can lead to tooth loss. CD4(+) Th cells activated by a microbial biofilm are thought to contribute to the destruction of alveolar bone surrounding teeth by influencing osteoclastogenesis through IL-17A and receptor activator for NF-κB ligand effects. The relative roles of mucosal Ag presentation cells in directing Th cell immune responses against oral pathogens and their contribution to destruction of alveolar bone remain unknown. We tested the contribution of mucosal Langerhans cells (LCs) to alveolar bone homeostasis in mice following oral colonization with a well-characterized human periodontal pathogen, Porphyromonas gingivalis We found that oral mucosal LCs did not protect from or exacerbate crestal alveolar bone destruction but were responsible for promoting differentiation of Th17 cells specific to P. gingivalis. In mice lacking LCs the Th17 response was suppressed and a Th1 response predominated. Bypassing LCs with systemic immunization of P. gingivalis resulted in a predominantly P. gingivalis-specific Th1 response regardless of whether LCs were present. Interestingly, we find that in vivo clonal expansion of P. gingivalis-specific Th cells and induced regulatory T cells does not depend on mucosal LCs. Furthermore, destruction of crestal alveolar bone induced by P. gingivalis colonization occurred regardless of the presence of mucosal LCs or P. gingivalis-specific Th17 cells. Our data indicate that both LCs and Th17 cells are redundant in contributing to alveolar bone destruction in a murine model of periodontitis.

Novel insights into cutaneous immune systems revealed by in vivo imaging

In vivo imaging is a novel experimental approach for biological research. Multiphoton microscopy (MPM), a type of fluorescence microscopy, is a new tool for in vivo imaging analysis. MPM allows observation of both tissue structures and cell behaviors or cell-cell interactions in living animals in real time. Skin is an ideal tissue for MPM analysis as it is directly accessible to the microscope. In the skin, immune cells cooperate to maintain skin homeostasis or to exert immune responses against foreign antigens. In vivo imaging by MPM analysis provides precise information on cell dynamics in the skin, and has significantly expanded our knowledge of the cutaneous immune system. In this review, we will discuss recent insights related to the mechanisms of allergic skin inflammation that have been revealed by MPM analysis.

Autoinflammatory Skin Disorders: The Inflammasomme in Focus

Autoinflammatory skin disorders are a group of heterogeneous diseases that include diseases such as cryopyrin-associated periodic syndrome (CAPS) and familial Mediterranean fever (FMF). Therapeutic strategies targeting IL-1 cytokines have proved helpful in ameliorating some of these diseases. While inflammasomes are the major regulators of IL-1 cytokines, inflammasome-independent complexes can also process IL-1 cytokines. Herein, we focus on recent advances in our understanding of how IL-1 cytokines, stemming from inflammasome-dependent and -independent pathways are involved in the regulation of skin conditions. Importantly, we discuss several mouse models of skin inflammation generated to help elucidate the basic cellular and molecular effects and modulation of IL-1 in the skin. Such models offer perspectives on how these signaling pathways could be targeted to improve therapeutic approaches in the treatment of these rare and debilitating inflammatory skin disorders.

Endogenous and tumour-derived microRNAs regulate cross-presentation in dendritic cells and consequently cytotoxic T cell function

Dendritic cells (DCs) are potent antigen presenting cells (APCs). They are also specialized in the induction of cytotoxic T lymphocyte mediated responses against extracellular antigens, including tumour-specific antigens, by presenting peptide-Major Histocompatibility Complex (MHC) I complexes to naïve CD8⁺ T cells in lymphoid tissues, a process called cross-presentation. Emerging evidence suggests that the efficiency of cross-presentation can be influenced by a unique set of microRNAs (miRNAs). Some are differentially expressed in the course of morphological and functional development of DCs while tumorigenic miRNAs (onco-miRs) can be delivered to and inserted into DCs via exosomes. The latter reprogram the miRNA repertoire of DCs, transforming them from effective APCs to negative modulators of immunity, ultimately aiding cancers to evade host immunity. On the other hand, endogenous microRNAs can influence cross-presentation either positively or negatively. In this review, we discuss the possible mechanisms by which specific miRNAs influence cross-presentation as well as the viability of manipulating the expression of miRNAs that regulate DC cross-presentation as a potential cancer immunotherapy intervention.

EpCAM Expressed by Murine Epidermal Langerhans Cells Modulates Immunization to an Epicutaneously Applied Protein Antigen

Langerhans cells (LCs) induce type 2 antibodies reactive with protein antigens that are applied to murine skin in the absence of adjuvant after extending their dendrites through tight junctions to acquire antigens and migrating to regional lymph nodes. In response to contact sensitizers, epithelial cell adhesion molecule (EpCAM) on LCs promotes LC dendrite mobility and LC migration. In epithelial cells, EpCAM regulates expression and distribution of selected tight junctions-associated claudins. To determine if EpCAM regulates claudins in LC and immune responses to externally applied proteins, we studied conditional knockout mice with EpCAM-deficient LCs. Although LC claudin-1 levels were dramatically reduced in the absence of EpCAM, conditional knockout mice with EpCAM-deficient LCs and control LC dendrites docked with epidermal tight junctions with equal efficiencies and ingested surface proteins. Topical immunization of conditional knockout mice with EpCAM-deficient LCs with ovalbumin led to increased induction of type 2 Ova-specific antibodies and enhanced proliferation of ovalbumin-reactive T cells associated with increased accumulation of LCs in lymph nodes. These results suggest that, in the absence of strong adjuvants, EpCAM-deficient LCs exhibit increased migration to regional lymph nodes. EpCAM appears to differentially regulate LC mobility/migration in the setting of limited inflammation as compared with the intense inflammation triggered by contact sensitizers.

Clec4A4 is a regulatory receptor for dendritic cells that impairs inflammation and T-cell immunity

Dendritic cells (DCs) comprise several subsets that are critically involved in the initiation and regulation of immunity. Clec4A4/DC immunoreceptor 2 (DCIR2) is a C-type lectin receptor (CLR) exclusively expressed on CD8α⁻ conventional DCs (cDCs). However, how Clec4A4 controls immune responses through regulation of the function of CD8α⁻ cDCs remains unclear. Here we show that Clec4A4 is a regulatory receptor for the activation of CD8α⁻ cDCs that impairs inflammation and T-cell immunity. Clec4a4^−/−CD8α⁻ cDCs show enhanced cytokine production and T-cell priming following Toll-like receptor (TLR)-mediated activation. Furthermore, Clec4a4^−/− mice exhibit TLR-mediated hyperinflammation. On antigenic immunization, Clec4a4^−/− mice show not only augmented T-cell responses but also progressive autoimmune pathogenesis. Conversely, Clec4a4^−/− mice exhibit resistance to microbial infection, accompanied by enhanced T-cell responses against microbes. Thus, our findings highlight roles of Clec4A4 in regulation of the function of CD8α⁻ cDCs for control of the magnitude and quality of immune response.

Clec4A4 is a C-type lectin receptor highly expressed by CD8α⁻ dendritic cells. Here the authors show that its loss of function results in enhanced T cell responses and exacerbated autoimmunity, implicating Clec4A4 in limiting activation of the CD8α⁻ dendritic cells.

Atopic dermatitis induces the expansion of thymus-derived regulatory T cells exhibiting a Th2-like phenotype in mice

Atopic dermatitis (AD) is a widespread inflammatory skin disease with an early onset, characterized by pruritus, eczematous lesions and skin dryness. This chronic relapsing disease is believed to be primarily a result of a defective epidermal barrier function associated with genetic susceptibility, immune hyper-responsiveness of the skin and environmental factors. Although the important role of abnormal immune reactivity in the pathogenesis of AD is widely accepted, the role of regulatory T cells (Tregs) remains elusive. We found that the Treg population is expanded in a mouse model of AD, i.e. mice topically treated with vitamin D3 (VitD). Moreover, mice with AD-like symptoms exhibit increased inducible T-cell costimulator (ICOS)-, cytotoxic T-lymphocyte antigen-4 (CTLA-4)- and Glycoprotein-A repetitions predominant receptor (GARP)-expressing Tregs in skin-draining lymph nodes. Importantly, the differentiation of Tregs into thymus-derived Tregs is favoured in our mouse model of AD. Emigrated skin-derived dendritic cells are required for Treg induction and Langerhans cells are responsible for the biased expansion of thymus-derived Tregs . Intriguingly, thymus-derived Tregs isolated from mice with AD-like symptoms exhibit a Th2 cytokine profile. Thus, AD might favour the expansion of pathogenic Tregs able to produce Th2 cytokines and to promote the disease instead of alleviating symptoms.

Beyond antigens and adjuvants: formulating future vaccines

The need to optimize vaccine potency while minimizing toxicity in healthy recipients has motivated studies of the formulation of vaccines to control how, when, and where antigens and adjuvants encounter immune cells and other cells/tissues following administration. An effective subunit vaccine must traffic to lymph nodes (LNs), activate both the innate and adaptive arms of the immune system, and persist for a sufficient time to promote a mature immune response. Here, we review approaches to tailor these three aspects of vaccine function through optimized formulations. Traditional vaccine adjuvants activate innate immune cells, promote cell-mediated transport of antigen to lymphoid tissues, and promote antigen retention in LNs. Recent studies using nanoparticles and other lymphatic-targeting strategies suggest that direct targeting of antigens and adjuvant compounds to LNs can also enhance vaccine potency without sacrificing safety. The use of formulations to regulate biodistribution and promote antigen and inflammatory cue co-uptake in immune cells may be important for next-generation molecular adjuvants. Finally, strategies to program vaccine kinetics through novel formulation and delivery strategies provide another means to enhance immune responses independent of the choice of adjuvant. These technologies offer the prospect of enhanced efficacy while maintaining high safety profiles necessary for successful vaccines.

Microbiota-Dependent Sequelae of Acute Infection Compromise Tissue-Specific Immunity

Infections have been proposed as initiating factors for inflammatory disorders; however, identifying associations between defined infectious agents and the initiation of chronic disease has remained elusive. Here, we report that a single acute infection can have dramatic and long-term consequences for tissue-specific immunity. Following clearance of Yersinia pseudotuberculosis, sustained inflammation and associated lymphatic leakage in the mesenteric adipose tissue deviates migratory dendritic cells to the adipose compartment, thereby preventing their accumulation in the mesenteric lymph node. As a consequence, canonical mucosal immune functions, including tolerance and protective immunity, are persistently compromised. Post-resolution of infection, signals derived from the microbiota maintain inflammatory mesentery remodeling and consequently, transient ablation of the microbiota restores mucosal immunity. Our results indicate that persistent disruption of communication between tissues and the immune system following clearance of an acute infection represents an inflection point beyond which tissue homeostasis and immunity is compromised for the long-term. VIDEO ABSTRACT.

Induction of broad immunity by thermostabilised vaccines incorporated in dissolvable microneedles using novel fabrication methods

Dissolvable microneedle (DMN) patches for immunization have multiple benefits, including vaccine stability and ease-of-use. However, conventional DMN fabrication methods have several drawbacks. Here we describe a novel, microfluidic, drop dispensing-based dissolvable microneedle production method that overcomes these issues. Uniquely, heterogeneous arrays, consisting of microneedles of diverse composition, can be easily produced on the same patch. Robustness of the process was demonstrated by incorporating and stabilizing adenovirus and MVA vaccines. Clinically-available trivalent inactivated influenza vaccine (TIV) in DMN patches is fully stable for greater than 6months at 40°C. Immunization using low dose TIV-loaded DMN patches induced significantly higher antibody responses compared to intramuscular-based immunization in mice. TIV-loaded patches also induced a broader, heterosubtypic neutralizing antibody response. By addressing issues that will be faced in large-scale fill-finish DMN fabrication processes and demonstrating superior thermostable characteristics and immunogenicity, this study progresses the translation of this microneedle platform to eventual clinical deployment.

Imaging of the cross-presenting dendritic cell subsets in the skin-draining lymph node

Dendritic cells (DCs) are antigen-presenting cells specialized for activating T cells to elicit effector T-cell functions. Cross-presenting DCs are a DC subset capable of presenting antigens to CD8(+) T cells and play critical roles in cytotoxic T-cell-mediated immune responses to microorganisms and cancer. Although their importance is known, the spatiotemporal dynamics of cross-presenting DCs in vivo are incompletely understood. Here, we study the T-cell zone in skin-draining lymph nodes (SDLNs) and find it is compartmentalized into regions for CD8(+) T-cell activation by cross-presenting DCs that express the chemokine (C motif) receptor 1 gene, Xcr1 and for CD4(+) T-cell activation by CD11b(+) DCs. Xcr1-expressing DCs in the SDLNs are composed of two different populations: migratory (CD103(hi)) DCs, which immigrate from the skin, and resident (CD8α(hi)) DCs, which develop in the nodes. To characterize the dynamic interactions of these distinct DC populations with CD8(+) T cells during their activation in vivo, we developed a photoconvertible reporter mouse strain, which permits us to distinctively visualize the migratory and resident subsets of Xcr1-expressing DCs. After leaving the skin, migratory DCs infiltrated to the deep T-cell zone of the SDLNs over 3 d, which corresponded to their half-life in the SDLNs. Intravital two-photon imaging showed that after soluble antigen immunization, the newly arriving migratory DCs more efficiently form sustained conjugates with antigen-specific CD8(+) T cells than other Xcr1-expressing DCs in the SDLNs. These results offer in vivo evidence for differential contributions of migratory and resident cross-presenting DCs to CD8(+) T-cell activation.

Analysis of DC Functions Using CD205-DTR Knock-In Mice

Dendritic cells (DCs) are essential antigen-presenting cells (APCs) that consist of heterogeneous subsets, mainly classified as conventional DCs (cDCs) and plasmacytoid DCs (pDCs). CD205, an endocytic type I C-type lectin-like molecule that belongs to the mannose receptor family, is mainly expressed on CD8α(+) cDCs. However, it is unclear how CD205(+) cDCs control immune responses in vivo. To evaluate the contribution of CD205(+) cDCs to the immune system, we engineered knock-in (KI) mice that express the diphtheria toxin receptor (DTR) under the control of the Cd205 gene, which allows the selective conditional ablation of CD205(+) cDCs in vivo. Conditional ablation of CD205(+) cDCs impaired the antigen-specific priming of CD8(+) T cells to generate cytotoxic T lymphocytes (CTLs) mediated through cross presentation of soluble antigen. Upon microbial infection, CD205(+) cDCs contributed to the cross priming of CD8(+) T cells for generating antibacterial CTLs to efficiently eliminate pathogens. Here, we provide a protocol for the generation of bone marrow WT/CD205-DT chimeric mice, depletion of CD205(+) DCs and assessment of depletion efficiency, and protocols for in vivo cross presentation assay, CTL generation assay, and antibacterial immunity assay.

Ontogeny of Tumor-associated Macrophages and Its Implication in Cancer Regulation

Macrophages are innate immune cells with evolutionarily conserved functions in tissue maintenance and host defense. As such, macrophages are among the first hematopoietic cells that seed developing tissues, and respond to inflammatory insults by in situ proliferation or de novo differentiation from monocytes. Recent studies have revealed that monocyte-derived tumor-induced macrophages represent a major tumor-associated macrophage population, which can further expand following their differentiation in tumors. Compared to tissue-resident tumor-associated macrophages, these newly differentiated cells are phenotypically distinct, and likely play a unique role in tissue dysregulation and immune modulation in cancer. These findings imply that tumor growth elicits a specific innate immune response. In this review, we explore the different routes of macrophage seeding and maintenance in tissues during steady state and inflammation and how these principles underlie the responses observed during tumor development. In addition, we highlight the relationship between the origin and function of macrophages in different settings and how this knowledge may be used to create new opportunities for cancer immunotherapy.

In Vivo Imaging of Cutaneous DCs in Mice

Varieties of cells orchestrate immune responses. To capture such dynamic phenomena, intravital imaging is an important technique, and it may provide substantial information that is not available using conventional histological analyses. Multiphoton microscopy enables the direct, three-dimensional, and minimally invasive imaging of biological samples with high spatiotemporal resolution, and it has now become the leading method for in vivo imaging studies. Here we describe a basic method for in vivo imaging of dendritic cells (DCs) in the mouse ear skin using multiphoton microscopy.

In Vivo Ablation of a Dendritic Cell Subset Expressing the Chemokine Receptor XCR1

Dendritic cells (DCs) are one of the key populations controlling immune responses. To establish a cell depletion system in vivo, human diphtheria toxin (DT) receptor (DTR) is transduced to the mice in which DTR is expressed under the control of a specific promoter. In these mice, DTR-expressing cells are inducibly depleted after DT injection. Using this system, analysis of mouse models in which DTR was expressed under the CD11c promoter has contributed to our knowledge of DC biology by depleting CD11c(+) cells. Other mouse models to inducibly eliminate specific DC subsets upon DT treatment have been also generated. Here, we describe a new mouse model in which the XCR1(+) DC subset is inducibly and transiently depleted in vivo.

Melanoma growth effects on molecular clearance from tumors and biodistribution into systemic tissues versus draining lymph nodes

Factors produced within or administered directly into the tumor interstitium, such as cytokines, chemokines, proteases, exosomes, microvesicles, or therapeutic agents, play important and multifaceted roles in the regulation of malignant disease progression. Their bioavailability to mediate signaling in distributed tissues outside of the tumor microenvironment, however, has not been well described. We therefore sought to elucidate the relative extent to which factors from within the primary tumor disseminate to systemic tissues as well as how these distribution profiles are influenced by both hydrodynamic size and the remodeling tumor vasculature. To accomplish this goal, we intratumorally co-infused into the dermal lesions of B16F10 melanoma-bearing mice at prescribed times post tumor implantation a near infrared fluorescent tracer panel ranging from 5 to 500nm in hydrodynamic diameter and compared the in vivo clearance and biodistribution profiles to that of naïve animals. Our results indicate that tumor growth reduces tumor-draining lymph node accumulation and alters the distribution of tumor-derived factors amongst systemic tissues. Despite these changes, previously developed principles of size-dependent lymph node drug targeting are conserved in melanomas, suggesting their applicability to sentinel lymph node-targeted drug delivery. Tumor progression was also found to result in a significant increase in the hydrodynamic size of factors originating from the tumor that accumulated within systemic tissues. This suggests that tumor vascular remodeling may redirect the organism-wide signaling activity of tumor-derived factors and may negatively contribute to disease progression by altering the bioavailability of molecules important to the regulation of pre-metastatic niche formation and the induction of anti-tumor immunity.

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.

XCR1+ dendritic cells promote memory CD8+ T cell recall upon secondary infections with Listeria monocytogenes or certain viruses

Alexandre et al. demonstrate the XCR1⁺ DCs are instrumental in memory CD8⁺ T cell responses to Listeria, VSV or vaccinia virus infection, but not CMV. Depending on the infection, robust memory CTL responses require cytokine- and chemokine-dependent cross-talk between XCR1⁺ DCs and NK cells or other IFN-γ–producing lymphocytes.

Naive CD8⁺ T cell priming during tumor development or many primary infections requires cross-presentation by XCR1⁺ dendritic cells (DCs). Memory CD8⁺ T lymphocytes (mCTLs) harbor a lower activation threshold as compared with naive cells. However, whether their recall responses depend on XCR1⁺ DCs is unknown. By using a new mouse model allowing fluorescent tracking and conditional depletion of XCR1⁺ DCs, we demonstrate a differential requirement of these cells for mCTL recall during secondary infections by different pathogens. XCR1⁺ DCs were instrumental to promote this function upon secondary challenges with Listeria monocytogenes, vesicular stomatitis virus, or Vaccinia virus, but dispensable in the case of mouse cytomegalovirus. We deciphered how XCR1⁺ DCs promote mCTL recall upon secondary infections with Listeria. By visualizing for the first time the in vivo choreography of XCR1⁺ DCs, NK cells and mCTLs during secondary immune responses, and by neutralizing in vivo candidate molecules, we demonstrate that, very early after infection, mCTLs are activated, and attracted in a CXCR3-dependent manner, by NK cell–boosted, IL-12–, and CXCL9-producing XCR1⁺ DCs. Hence, depending on the infectious agent, strong recall of mCTLs during secondary challenges can require cytokine- and chemokine-dependent cross-talk with XCR1⁺ DCs and NK cells.

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

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

The PDK1-Rsk Signaling Pathway Controls Langerhans Cell Proliferation and Patterning

Langerhans cells (LC), the dendritic cells of the epidermis, are distributed in a distinctive regularly spaced array. In the mouse, the LC array is established in the first few days of life from proliferating local precursors, but the regulating signaling pathways are not fully understood. We found that mice lacking the kinase phosphoinositide-dependent kinase 1 selectively lack LC. Deletion of the phosphoinositide-dependent kinase 1 target kinases, ribosomal S6 kinase 1 (Rsk1) and Rsk2, produced a striking perturbation in the LC network: LC density was reduced 2-fold, but LC size was increased by the same magnitude. Reduced LC numbers in Rsk1/2(-/-) mice was not due to accelerated emigration from the skin but rather to reduced proliferation at least in adults. Rsk1/2 were required for normal LC patterning in neonates, but not when LC were ablated in adults and replaced by bone marrow-derived cells. Increased LC size was an intrinsic response to reduced LC numbers, reversible on LC emigration, and could be observed in wild type epidermis where LC size also correlated inversely with LC density. Our results identify a key signaling pathway needed to establish a normal LC network and suggest that LC might maintain epidermal surveillance by increasing their "footprint" when their numbers are limited.

HIV-1 border patrols: Langerhans cells control antiviral responses and viral transmission

Langerhans cells (LCs) reside in the mucosal epithelia and are refractory to HIV-1 infection; HIV-1 capture by C-type lectin receptor langerin and subsequent targeting to Birbeck granules prevents infection. Furthermore, LCs restrict transmission of CXCR4-using HIV-1 variants, which underscores the role of immature LCs as gatekeepers in the selection of HIV-1 variants. Interaction of langerin on LCs with hyaluronic acid on dendritic cells facilitates cross-presentation of HIV-1 to CD8+ T cells. Activation of LCs upon inflammation bypasses the langerin-dependent barrier, which favors cross-presentation and increases susceptibility of LCs to HIV-1 infection. These recent developments not only highlight the plasticity of LCs but also define an important role for LC-dendritic cell crosstalk at the periphery in directing adaptive immune responses to viruses.

Functional Specialization of Skin Dendritic Cell Subsets in Regulating T Cell Responses

Dendritic cells (DC) are a heterogeneous family of professional antigen-presenting cells classically recognized as most potent inducers of adaptive immune responses. In this respect, Langerhans cells have long been considered to be prototypic immunogenic DC in the skin. More recently this view has considerably changed. The generation of in vivo cell ablation and lineage tracing models revealed the complexity of the skin DC network and, in particular, established the existence of a number of phenotypically distinct Langerin⁺ and negative DC populations in the dermis. Moreover, by now we appreciate that DC also exert important regulatory functions and are required for the maintenance of tolerance toward harmless foreign and self-antigens. This review summarizes our current understanding of the skin-resident DC system in the mouse and discusses emerging concepts on the functional specialization of the different skin DC subsets in regulating T cell responses. Special consideration is given to antigen cross-presentation as well as immune reactions toward contact sensitizers, cutaneous pathogens, and tumors. These studies form the basis for the manipulation of the human counterparts of the murine DC subsets to promote immunity or tolerance for the treatment of human disease.

Sampling of Candida albicans and Candida tropicalis by Langerin-positive dendritic cells in mouse Peyer's patches

Members of the Candida genus, including C. albicans and C. tropicalis are opportunistic fungal pathogens that are increasingly associated with gastrointestinal infections and inflammatory bowel diseases. In healthy populations, however, C. albicans and C. tropicalis are considered benign members of the mycobiome, and are presumably kept in check by the mucosal immune system. In this study, we demonstrate in mice that C. albicans and C. tropicalis are sampled by Peyer's patch (PP) dendritic cells (DCs). Uptake into gut-associated lymphoid tissues occurred rapidly and was at least partly M cell-dependent. C. albicans and C. tropicalis preferentially localized in (and persisted within) a recently identified sub- population of Peyer's patch DCs distinguished by their expression of the C-type lectin receptor, Langerin. This study is the first to identify a subset of PP DCs capable of sampling Candida species.

Topical steroid therapy induces pro-tolerogenic changes in Langerhans cells in human skin

We have investigated the efficacy of conditioning skin Langerhans cells (LCs) with agents to promote tolerance and reduce inflammation, with the goal of improving the outcomes of antigen-specific immunotherapy. Topical treatments were assessed ex vivo, using excised human breast skin maintained in organ bath cultures, and in vivo in healthy volunteers by analysing skin biopsies and epidermal blister roof samples. Following topical treatment with a corticosteroid, tumour necrosis factor-α levels were reduced in skin biopsy studies and blister fluid samples. Blister fluid concentrations of monocyte chemoattractant protein-1, macrophage inflammatory proteins -1α and 1β and interferon-γ inducible protein-10 were also reduced, while preserving levels of interleukin-1α (IL-1α), IL-6, IL-8 and IL-10. Steroid pre-treatment of the skin reduced the ability of LCs to induce proliferation, while supernatants showed an increase in the IL-10/interferon-γ ratio. Phenotypic changes following topical steroid treatment were also observed, including reduced expression of CD83 and CD86 in blister-derived LCs, but preservation of the tolerogenic signalling molecules immunoglobulin-like transcript 3 and programmed death-1. Reduced expression of HLA-DR, CD80 and CD86 were also apparent in LCs derived from excised human skin. Topical therapy with a vitamin D analogue (calcipotriol) and steroid, calcipotriol alone or vitamin A elicited no significant changes in the parameters studied. These experiments suggest that pre-conditioning the skin with topical corticosteroid can modulate LCs by blunting their pro-inflammatory signals and potentially enhancing tolerance. We suggest that such modulation before antigen-specific immunotherapy might provide an inexpensive and safe adjunct to current approaches to treat autoimmune diseases.

Spatiotemporally Distinct Interactions with Dendritic Cell Subsets Facilitates CD4+ and CD8+ T Cell Activation to Localized Viral Infection

The dynamics of when and where CD4(+) T cells provide help for CD8(+) T cell priming and which dendritic cells (DCs) activate CD4(+) T cells in vivo after localized infection are poorly understood. By using a cutaneous herpes simplex virus infection model combined with intravital 2-photon imaging of the draining lymph node (LN) to concurrently visualize pathogen-specific CD4(+) and CD8(+) T cells, we found that early priming of CD4(+) T cells involved clustering with migratory skin DCs. CD8(+) T cells did not interact with migratory DCs and their activation was delayed, requiring later clustering interactions with LN-resident XCR1(+) DCs. CD4(+) T cells interacted with these late CD8(+) T cell clusters on resident XCR1(+) DCs. Together, these data reveal asynchronous T cell activation by distinct DC subsets and highlight the key role of XCR1(+) DCs as the central platform for cytotoxic T lymphocyte activation and the delivery of CD4(+) T cell help.

Robust Anti-viral Immunity Requires Multiple Distinct T Cell-Dendritic Cell Interactions

Host defense against viruses and intracellular parasites depends on effector CD8(+) T cells, whose optimal clonal expansion, differentiation, and memory properties require signals from CD4(+) T cells. Here, we addressed the role of dendritic cell (DC) subsets in initial activation of the two T cell types and their co-operation. Surprisingly, initial priming of CD4(+) and CD8(+) T cells was spatially segregated within the lymph node and occurred on different DCs with temporally distinct patterns of antigen presentation via MHCI versus MHCII molecules. DCs that co-present antigen via both MHC molecules were detected at a later stage; these XCR1(+) DCs are the critical platform involved in CD4(+) T cell augmentation of CD8(+) T cell responses. These findings delineate the complex choreography of cellular interactions underlying effective cell-mediated anti-viral responses, with implications for basic DC subset biology, as well as for translational application to the development of vaccines that evoke optimal T cell immunity.

Phenotypic and functional analysis of CD1a+ dendritic cells from cats chronically infected with feline immunodeficiency virus

Numerous studies suggest dendritic cell (DC) dysfunction is central to the dysregulated immune response during HIV infection; however, in vivo studies are lacking. In the present study we used feline immunodeficiency virus (FIV) infection of cats as a model for HIV-1 infection to assess the maturation and function of dendritic cells, in vivo and in vitro. We compared CD1a+ DC migration, surface phenotype, endocytosis, mixed leukocyte reaction (MLR) and regulatory T cell (Treg) phenotype induction by CD1a+ cells isolated from lymph nodes of FIV-infected and control cats. Results showed that resident CD1a+ DC in lymph nodes of chronically FIV-infected cats are phenotypically mature, can stimulate normal primary T cell proliferation, override Treg suppression and do not skew toward Treg induction. In contrast, FIV infection had deleterious effects on antigen presentation and migratory capacity of CD1a+ cells in tissues.

Novel full-spectral flow cytometry with multiple spectrally-adjacent fluorescent proteins and fluorochromes and visualization of in vivo cellular movement

Flow cytometric analysis with multicolor fluoroprobes is an essential method for detecting biological signatures of cells. Here, we present a new full-spectral flow cytometer (spectral-FCM). Unlike conventional flow cytometer, this spectral-FCM acquires the emitted fluorescence for all probes across the full-spectrum from each cell with 32 channels sequential PMT unit after dispersion with prism, and extracts the signals of each fluoroprobe based on the spectral shape of each fluoroprobe using unique algorithm in high speed, high sensitive, accurate, automatic and real-time. The spectral-FCM detects the continuous changes in emission spectra from green to red of the photoconvertible protein, KikGR with high-spectral resolution and separates spectrally-adjacent fluoroprobes, such as FITC (Emission peak (Em) 519 nm) and EGFP (Em 507 nm). Moreover, the spectral-FCM can measure and subtract autofluorescence of each cell providing increased signal-to-noise ratios and improved resolution of dim samples, which leads to a transformative technology for investigation of single cell state and function. These advances make it possible to perform 11-color fluorescence analysis to visualize movement of multilinage immune cells by using KikGR-expressing mice. Thus, the novel spectral flow cytometry improves the combinational use of spectrally-adjacent various FPs and multicolor fluorochromes in metabolically active cell for the investigation of not only the immune system but also other research and clinical fields of use.

Cross-Presentation of Cell-Associated Antigens by MHC Class I in Dendritic Cell Subsets

Dendritic cells (DCs) have the unique ability to pick up dead cells carrying antigens in tissue and migrate to the lymph nodes where they can cross-present cell-associated antigens by MHC class I to CD8⁺ T cells. There is strong in vivo evidence that the mouse XCR1⁺ DCs subset acts as a key player in this process. The intracellular processes underlying cross-presentation remain controversial and several pathways have been proposed. Indeed, a wide number of studies have addressed the cellular process of cross-presentation in vitro using a variety of sources of antigen and antigen-presenting cells. Here, we review the in vivo and in vitro evidence supporting the current mechanistic models and disscuss their physiological relevance to the cross-presentation of cell-associated antigens by DCs subsets.

Langerhans Cell-Dendritic Cell Cross-Talk via Langerin and Hyaluronic Acid Mediates Antigen Transfer and Cross-Presentation of HIV-1

Human epidermal and mucosal Langerhans cells (LCs) express the C-type lectin receptor langerin that functions as a pattern recognition receptor. LCs are among the first immune cells to interact with HIV-1 during sexual transmission. In this study, we demonstrate that langerin not only functions as a pattern recognition receptor but also as an adhesion receptor mediating clustering between LCs and dendritic cells (DCs). Langerin recognized hyaluronic acid on DCs and removal of these carbohydrate structures partially abrogated LC-DC clustering. Because LCs did not cross-present HIV-1-derived Ags to CD8(+) T cells in a cross-presentation model, we investigated whether LCs were able to transfer Ags to DCs. LC-DC clustering led to maturation of DCs and facilitated Ag transfer of HIV-1 to DCs, which subsequently induced activation of CD8(+) cells. The rapid transfer of Ags to DCs, in contrast to productive infection of LCs, suggests that this might be an important mechanism for induction of anti-HIV-1 CD8(+) T cells. Induction of the enzyme hyaluronidase-2 by DC maturation allowed degradation of hyaluronic acid and abrogated LC-DC interactions. Thus, we have identified an important function of langerin in mediating LC-DC clustering, which allows Ag transfer to induce CTL responses to HIV-1. Furthermore, we showed this interaction is mediated by hyaluronidase-2 upregulation after DC maturation. These data underscore the importance of LCs and DCs in orchestrating adaptive immunity to HIV-1. Novel strategies might be developed to harness this mechanism for vaccination.

Specific roles for dendritic cell subsets during initiation and progression of psoriasis

Several subtypes of APCs are found in psoriasis patients, but their involvement in disease pathogenesis is poorly understood. Here, we investigated the contribution of Langerhans cells (LCs) and plasmacytoid DCs (pDCs) in psoriasis. In human psoriatic lesions and in a psoriasis mouse model (DKO* mice), LCs are severely reduced, whereas pDCs are increased. Depletion of pDCs in DKO* mice prior to psoriasis induction resulted in a milder phenotype, whereas depletion during active disease had no effect. In contrast, while depletion of Langerin-expressing APCs before disease onset had no effect, depletion from diseased mice aggravated psoriasis symptoms. Disease aggravation was due to the absence of LCs, but not other Langerin-expressing APCs. LCs derived from DKO* mice produced increased IL-10 levels, suggesting an immunosuppressive function. Moreover, IL-23 production was high in psoriatic mice and further increased in the absence of LCs. Conversely, pDC depletion resulted in reduced IL-23 production, and therapeutic inhibition of IL-23R signaling ameliorated disease symptoms. Therefore, LCs have an anti-inflammatory role during active psoriatic disease, while pDCs exert an instigatory function during disease initiation.

Activated NKT Cells Can Condition Different Splenic Dendritic Cell Subsets To Respond More Effectively to TLR Engagement and Enhance Cross-Priming

The function of dendritic cells (DCs) can be modulated through multiple signals, including recognition of pathogen-associated molecular patterns, as well as signals provided by rapidly activated leukocytes in the local environment, such as innate-like T cells. In this article, we addressed the possibility that the roles of different murine DC subsets in cross-priming CD8(+) T cells can change with the nature and timing of activatory stimuli. We show that CD8α(+) DCs play a critical role in cross-priming CD8(+) T cell responses to circulating proteins that enter the spleen in close temporal association with ligands for TLRs and/or compounds that activate NKT cells. However, if NKT cells are activated first, then CD8α(-) DCs become conditioned to respond more vigorously to TLR ligation, and if triggered directly, these cells can also contribute to priming of CD8(+) T cell responses. In fact, the initial activation of NKT cells can condition multiple DC subsets to respond more effectively to TLR ligation, with plasmacytoid DCs making more IFN-α and both CD8α(+) and CD8α(-) DCs manufacturing more IL-12. These results suggest that different DC subsets can contribute to T cell priming if provided appropriately phased activatory stimuli, an observation that could be factored into the design of more effective vaccines.

Langerhans Cells Maintain Local Tissue Tolerance in a Model of Systemic Autoimmune Disease

Systemic autoimmune diseases such as lupus affect multiple organs, usually in a diverse fashion where only certain organs are affected in individual patients. It is unclear whether the "local" immune cells play a role in regulating tissue specificity in relation to disease heterogeneity in systemic autoimmune diseases. In this study, we used skin as a model to determine the role of tissue-resident dendritic cells (DCs) in local and systemic involvement within a systemic lupus disease model. Skin-resident DCs, namely, Langerhans cells (LCs), have been implicated in regulating tolerance or autoimmunity using elegant transgenic models, however, their role in local versus systemic immune regulation is unknown. We demonstrate that although lymphocytes from skin-draining lymph nodes of autoimmune-prone MRL/MpJ-Fas(lpr/lp) (r) (MRL-lpr) mice react spontaneously to a physiological skin self-Ag desmoglein-3, epicutaneous applications of desmoglein-3 induced tolerance that is dependent on LCs. Inducible ablation of LCs in adult preclinical MRL-lpr and MRL/MpJ-Fas(+/+) mice resulted in increased autoantibodies against skin Ags and markedly accelerated lupus dermatitis with increased local macrophage infiltration, but had no effect on systemic autoantibodies such as anti-dsDNA Abs or disease in other organs such as kidneys, lung, and liver. Furthermore, skin-draining lymph nodes of LC-ablated MRL-lpr mice had significantly fewer CD4(+) T cells producing anti-inflammatory cytokine IL-10 than LC-intact controls. These results indicate that a skin-resident DC population regulates local tolerance in systemic lupus and emphasize the importance of the local immune milieu in preventing tissue-specific autoimmunity, yet have no effect on systemic autoimmunity.

Immune Reactions against Gene Gun Vaccines Are Differentially Modulated by Distinct Dendritic Cell Subsets in the Skin

The skin accommodates multiple dendritic cell (DC) subsets with remarkable functional diversity. Immune reactions are initiated and modulated by the triggering of DC by pathogen-associated or endogenous danger signals. In contrast to these processes, the influence of intrinsic features of protein antigens on the strength and type of immune responses is much less understood. Therefore, we investigated the involvement of distinct DC subsets in immune reactions against two structurally different model antigens, E. coli beta-galactosidase (betaGal) and chicken ovalbumin (OVA) under otherwise identical conditions. After epicutaneous administration of the respective DNA vaccines with a gene gun, wild type mice induced robust immune responses against both antigens. However, ablation of langerin+ DC almost abolished IgG1 and cytotoxic T lymphocytes against betaGal but enhanced T cell and antibody responses against OVA. We identified epidermal Langerhans cells (LC) as the subset responsible for the suppression of anti-OVA reactions and found regulatory T cells critically involved in this process. In contrast, reactions against betaGal were not affected by the selective elimination of LC, indicating that this antigen required a different langerin+ DC subset. The opposing findings obtained with OVA and betaGal vaccines were not due to immune-modulating activities of either the plasmid DNA or the antigen gene products, nor did the differential cellular localization, size or dose of the two proteins account for the opposite effects. Thus, skin-borne protein antigens may be differentially handled by distinct DC subsets, and, in this way, intrinsic features of the antigen can participate in immune modulation.

Murine Langerin+ dermal dendritic cells prime CD8+ T cells while Langerhans cells induce cross-tolerance

Skin dendritic cells (DCs) control the immunogenicity of cutaneously administered vaccines. Antigens targeted to DCs via the C-type lectin Langerin/CD207 are cross-presented to CD8⁺ T cells in vivo. We investigated the relative roles of Langerhans cells (LCs) and Langerin⁺ dermal DCs (dDCs) in different vaccination settings. Poly(I:C) and anti-CD40 agonist antibody promoted cytotoxic responses upon intradermal immunization with ovalbumin (OVA)-coupled anti-Langerin antibodies (Langerin/OVA). This correlated with CD70 upregulation in Langerin⁺ dDCs, but not LCs. In chimeric mice where Langerin targeting was restricted to dDCs, CD8⁺ T-cell memory was enhanced. Conversely, providing Langerin/OVA exclusively to LCs failed to prime cytotoxicity, despite initial antigen cross-presentation to CD8⁺ T cells. Langerin/OVA combined with imiquimod could not prime CD8⁺ T cells and resulted in poor cytotoxicity in subsequent responses. This tolerance induction required targeting and maturation of LCs. Altogether, Langerin⁺ dDCs prime long-lasting cytotoxic responses, while cross-presentation by LCs negatively influences CD8⁺ T-cell priming. Moreover, this highlights that DCs exposed to TLR agonists can still induce tolerance and supports the existence of qualitatively different DC maturation programs.

Langerhans cell homeostasis and activation is altered in hyperplastic human papillomavirus type 16 E7 expressing epidermis

It has previously been shown that expression of human papillomavirus type 16 (HPV) E7 in epidermis causes hyperplasia and chronic inflammation, characteristics of pre-malignant lesions. Importantly, E7-expressing epidermis is strongly immune suppressed and is not rejected when transplanted onto immune competent mice. Professional antigen presenting cells are considered essential for initiation of the adaptive immune response that results in graft rejection. Langerhans cells (LC) are the only antigen presenting cells located in normal epidermis and altered phenotype and function of these cells may contribute to the immune suppressive microenvironment. Here, we show that LC are atypically activated as a direct result of E7 expression in the epidermis, and independent of the presence of lymphocytes. The number of LC was significantly increased and the LC are functionally impaired, both in migration and in antigen uptake. However when the LC were extracted from K14E7 skin and matured in vitro they were functionally competent to present and cross-present antigen, and to activate T cells. The ability of the LC to present and cross-present antigen following maturation supports retention of full functional capacity when removed from the hyperplastic skin microenvironment. As such, opportunities are afforded for the development of therapies to restore normal LC function in hyperplastic skin.

Understanding high endothelial venules: Lessons for cancer immunology

High endothelial venules (HEVs) are blood vessels especially adapted for lymphocyte trafficking which are normally found in secondary lymphoid organs such as lymph nodes (LN) and Peyer's patches. It has long been known that HEVs develop in non-lymphoid organs during chronic inflammation driven by autoimmunity, infection or allografts. More recently, HEVs have been observed in solid, vascularized tumors and their presence correlated with reduced tumor size and improved patient outcome. It is proposed that newly formed HEV promote antitumor immunity by recruiting naive lymphocytes into the tumor, thus allowing the local generation of cancerous tissue-destroying lymphocytes. Understanding how HEVs develop and function are therefore important to unravel their role in human cancers. In LN, HEVs develop during embryonic and early post-natal life and are actively maintained by the LN microenvironment. Systemic blockade of lymphotoxin-β receptor leads to HEV de-differentiation, but the LN components that induce HEV differentiation have remained elusive. Recent elegant studies using gene-targeted mice have demonstrated clearly that triggering the lymphotoxin-β receptor in endothelial cells (EC) induces the differentiation of HEV and that CD11c⁺ dendritic cells play a crucial role in this process. It will be important to determine whether lymphotoxin-β receptor-dependent signaling in EC drives the development of HEV during tumorigenesis and which cells have HEV-inducer properties. This may reveal therapeutic approaches to promote HEV neogenesis and determine the impact of newly formed HEV on tumor immunity.

Laser-assisted intradermal delivery of adjuvant-free vaccines targeting XCR1+ dendritic cells induces potent antitumoral responses

The development of vaccines inducing efficient CD8(+) T cell responses is the focus of intense research. Dendritic cells (DCs) expressing the XCR1 chemokine receptor, also known as CD103(+) or CD8α(+) DCs, excel in the presentation of extracellular Ags to CD8(+) T cells. Because of its high numbers of DCs, including XCR1(+) DCs, the skin dermis is an attractive site for vaccine administration. By creating laser-generated micropores through the epidermis, we targeted a model protein Ag fused to XCL1, the ligand of XCR1, to dermal XCR1(+) DCs and induced Ag-specific CD8(+) and CD4(+) T cell responses. Efficient immunization required the emigration of XCR1(+) dermal DCs to draining lymph nodes and occurred irrespective of TLR signaling. Moreover, a single intradermal immunization protected mice against melanoma tumor growth in prophylactic and therapeutic settings, in the absence of exogenous adjuvant. The mild inflammatory milieu created in the dermis by skin laser microporation itself most likely favored the development of potent T cell responses in the absence of exogenous adjuvants. The existence of functionally equivalent XCR1(+) dermal DCs in humans should permit the translation of laser-assisted intradermal delivery of a tumor-specific vaccine targeting XCR1(+) DCs to human cancer immunotherapy. Moreover, considering that the use of adjuvants in vaccines is often associated with safety issues, the possibility of inducing protective responses against melanoma tumor growth independently of the administration of exogenous adjuvants should facilitate the development of safer vaccines.

Cervical Lymph Nodes as a Selective Niche for Brucella during Oral Infections

Cervical lymph nodes (CLN) are the first lymph nodes encountered by material taking the oral route. To study their role in orally acquired infections, we analyzed 307 patients of up to 14 years treated in the university clinic of Skopje, Macedonia, for brucellosis, a zoonotic bacterial disease frequently acquired by ingestion of contaminated dairy products. From these children, 36% had lymphadenopathy. Among orally infected children, lymphadenopathy with CLN being the only lymph nodes affected was significantly more frequent as compared to those infected by contact with animals (83% vs. 63%), suggesting a possible involvement of CLN during orally acquired human brucellosis. Using a murine model where bacteria are delivered into the oral cavity, we show that Brucella quickly and selectively colonize the CLN where they proliferate and persist over long periods of time for up to 50 days post-infection. A similar efficient though less specific drainage to CLN was found for Brucella, Salmonella typhimurium and fluorescent microspheres delivered by gavage, a pathway likely representing a mixed infection mode of intragastric and oral infection, suggesting a central pathway of drained material. Microspheres as well as bacteria drained to CLN predominately reside in cells expressing CD68 and no or low levels of CD11c. Even though no systemic response could be detected, Brucella induced a locally restricted inflammatory reaction with increased expression levels of interferon γ, interleukin (IL)-6, IL-12, granzyme B and a delayed induction of Nos2. Inflammation led to pronounced lymphadenopathy, infiltration of macrophages/monocytes expressing high levels of major histocompatibility complex II and to formation of epitheloid granulomas. Together, these results highlight the role of CLN in oral infections as both, an initial and efficient trap for bacterial invaders and as possible reservoir for chronic pathogens. They likewise cast a new light on the significance of oral routes for means of vaccination.

Allergen immunotherapy, routes of administration and cytokine networks: an update

Allergen immunotherapy is a disease-modifying therapy, effective for the treatment of allergic rhinitis, allergic asthma, conjunctivitis or stinging insect allergy. Allergen immunotherapy involves the administration of increasing doses of allergens with the aim of ameliorating the allergic response. Although precise underlying mechanisms of the induction of immune tolerance remain unclear, immunotherapy has been associated with the induction of distinct subsets of Tregs that eventually lead to peripheral tolerance by inducing a deviation from Th2 to Th1 immune responses. This review focuses on the current knowledge of the mechanisms of immunotherapy in relationship to different routes of administration and also provides a unifying view.