Presentation of exogenous protein antigens by dendritic cells to T cell clones. Intact protein is presented best by immature, epidermal Langerhans cells

Dendritic cells in liver transplantation immune response

Dendritic cells (DCs) are the most powerful antigen presenting cells (APCs), they are considered one of the key regulatory factors in the liver immune system. There is currently much interest in modulating DC function to improve transplant immune response. In liver transplantation, DCs participate in both the promotion and inhibition of the alloreponse by adopting different phenotypes and function. Thus, in this review, we discussed the origin, maturation, migration and pathological effects of several DC subsets, including the conventional DC (cDC), plasmacytoid DC (pDC) and monocyte-derived DC (Mo-DC) in liver transplantation, and we summarized the roles of these DC subsets in liver transplant rejection and tolerance. In addition, we also outlined the latest progress in DC-based related treatment regimens. Overall, our discussion provides a beneficial resource for better understanding the biology of DCs and their manipulation to improve the immune adaptability of patients in transplant status.

Tissue-specific abundance of interferon-gamma drives regulatory T cells to restrain DC1-mediated priming of cytotoxic T cells against lung cancer

Local environmental factors influence CD8+ T cell priming in lymph nodes (LNs). Here, we sought to understand how factors unique to the tumor-draining mediastinal LN (mLN) impact CD8+ T cell responses toward lung cancer. Type 1 conventional dendritic cells (DC1s) showed a mLN-specific failure to induce robust cytotoxic T cells responses. Using regulatory T (Treg) cell depletion strategies, we found that Treg cells suppressed DC1s in a spatially coordinated manner within tissue-specific microniches within the mLN. Treg cell suppression required MHC II-dependent contact between DC1s and Treg cells. Elevated levels of IFN-γ drove differentiation Treg cells into Th1-like effector Treg cells in the mLN. In patients with cancer, Treg cell Th1 polarization, but not CD8+/Treg cell ratios, correlated with poor responses to checkpoint blockade immunotherapy. Thus, IFN-γ in the mLN skews Treg cells to be Th1-like effector Treg cells, driving their close interaction with DC1s and subsequent suppression of cytotoxic T cell responses.

Stem Cells in the Tumor Immune Microenvironment -Part of the Cure or Part of the Disease? Ontogeny and Dichotomy of Stem and Immune Cells has Led to better Understanding

Stem cells are at the basis of tissue homeostasis, hematopoiesis and various regenerative processes. Epigenetic changes in their somatically imprinted genes, prolonged exposure to mutagens/carcinogens or alteration of their niche can lead to the development of an enabling environment for tumor growth and progression. The involvement of stem cells in both health and disease becomes even more compelling with ontogeny as embryonic and extraembryonic stem cells which persist into adulthood in well established and specific niche may have distinct implications in tumorigenesis. Immune surveillance plays an important role in this interplay since the response of immune cells toward the oncogenic process can range from reactivity to placidity and even complicity, being orchestrated by intercellular molecular dialogues with the other key players of the tumor microenvironment. With the current understanding that every developing and adult tissue contains inherent stem and progenitor cells, in this manuscript we review the most relevant interactions carried out between the stem cells, tumor cells and immune cells in a bottom-up incursion through the tumor microenvironment beginning from the perivascular niche and going through the tumoral parenchyma and the related stroma. With the exploitation of various factors that influence the behavior of immune effectors toward stem cells and other resting cells in their niche, new therapeutic strategies to tackle the polarization of immune effectors toward a more immunogenic phenotype may arise.

Innate control of adaptive immunity and adaptive instruction of innate immunity: bi-directional flow of information

The ability of the innate and adaptive immune systems to communicate with each other is central to protective immune responses and maintenance of host health. Myeloid cells of the innate immune system are able to sense microbial ligands, perturbations in cellular homeostasis, and virulence factors, thereby allowing them to relay distinct pathogen-specific information to naïve T cells in the form of pathogen-derived peptides and a unique cytokine milieu. Once primed, effector T helper cells produce lineage-defining cytokines to help combat the original pathogen, and a subset of these cells persist as memory or effector-memory populations. These memory T cells then play a dual role in host protection by not only responding rapidly to reinfection, but by also directly instructing myeloid cells to express licensing cytokines. This means there is a bi-directional flow of information first from the innate to the adaptive immune system, and then from the adaptive back to innate immune system. Here, we focus on how signals, first from pathogens and then from primed effector and memory T cells, are integrated by myeloid cells and its consequences for protective immunity or systemic inflammation.

Arboviruses: How Saliva Impacts the Journey from Vector to Host

Arthropod-borne viruses, referred to collectively as arboviruses, infect millions of people worldwide each year and have the potential to cause severe disease. They are predominately transmitted to humans through blood-feeding behavior of three main groups of biting arthropods: ticks, mosquitoes, and sandflies. The pathogens harbored by these blood-feeding arthropods (BFA) are transferred to animal hosts through deposition of virus-rich saliva into the skin. Sometimes these infections become systemic and can lead to neuro-invasion and life-threatening viral encephalitis. Factors intrinsic to the arboviral vectors can greatly influence the pathogenicity and virulence of infections, with mounting evidence that BFA saliva and salivary proteins can shift the trajectory of viral infection in the host. This review provides an overview of arbovirus infection and ways in which vectors influence viral pathogenesis. In particular, we focus on how saliva and salivary gland extracts from the three dominant arbovirus vectors impact the trajectory of the cellular immune response to arbovirus infection in the skin.

The discovery of dendritic cells

An excerpt from Ralph Steinman’s Harvey Lecture describing the discovery of dendritic cells.

Proteomic analysis of psoriatic skin lesions in a Chinese population

Psoriasis is a chronic skin disorder with undefined pathogenesis. Several biomarkers for this disease have been identified by proteomic analysis. We explored the whole-proteomic changes in 45 pairs of psoriatic and adjacent noninvolved skin tissues in a Chinese population. A total of 3686 proteins were identified, of which 3008 were quantified. A total of 102 and 124 proteins were upregulated and downregulated in lesional skin, respectively. SART1 (P = 3.55 × 10^-5) and GLTP (P = 1.54 × 10^-3) were the most significantly down- and upregulated proteins. Nearly 90% of these differentially regulated proteins exhibited the same expression trends as those in an online RNA sequencing dataset for psoriasis; 19 differentially regulated proteins exhibited a negative relationship with DNA methylation data for psoriatic lesions. The differentially expressed proteins were enriched in ribosomes, antigen processing and presentation, immune response, and IL-17 signalling pathways. This study identified multiple differentially regulated proteins in psoriatic lesions, which suggested that changes in the proteome play important regulatory roles in psoriasis-associated processes. SIGNIFICANCE: Proteomic analysis was performed in 45 pairs of psoriatic and adjacent noninvolved skin tissues in a Chinese population. More than 3000 proteins were quantified, of which 226 were differentially expressed in psoriatic skin tissues. These proteins were mainly enriched in the immune response, antigen processing and presentation and IL-17 signalling pathways, which have been reported to be associated with the pathogenesis of psoriasis.

Cytokine network analysis of immune responses before and after autologous dendritic cell and tumor cell vaccine immunotherapies in a randomized trial

Background

In a randomized phase II trial conducted in patients with metastatic melanoma, patient-specific autologous dendritic cell vaccines (DCV) were associated with longer survival than autologous tumor cell vaccines (TCV). Both vaccines presented antigens from cell-renewing autologous tumor cells. The current analysis was performed to better understand the immune responses induced by these vaccines, and their association with survival.

Methods

110 proteomic markers were measured at a week-0 baseline, 1 week before the first of 3 weekly vaccine injections, and at week-4, 1 week after the third injection. Data was presented as a deviation from normal controls. A two-component principal component (PC) statistical analysis and discriminant analysis were performed on this data set for all patients and for each treatment cohort.

Results

At baseline PC-1 contained 64.4% of the variance and included the majority of cytokines associated with Th1 and Th2 responses, which positively correlated with beta-2-microglobulin (B2M), programmed death protein-1 (PD-1) and transforming growth factor beta (TGFβ1). Results were similar at baseline for both treatment cohorts. After three injections, DCV-treated patients showed correlative grouping among Th1/Th17 cytokines on PC-1, with an inverse correlation with B2M, FAS, and IL-18, and correlations among immunoglobulins in PC-2. TCV-treated patients showed a positive correlation on PC-1 among most of the cytokines and tumor markers B2M and FAS receptor. There were also correlative changes of IL12p40 with both Th1 and Th2 cytokines and TGFβ1. Discriminant analysis provided additional evidence that DCV was associated with innate, Th1/Th17, and Th2 responses while TCV was only associated with innate and Th2 responses.

Conclusions

These analyses confirm that DCV induced a different immune response than that induced by TCV, and these immune responses were associated with improved survival.

Trial registration Clinical trials.gov NCT004936930 retrospectively registered 28 July 2009

The Dynamics of the Skin's Immune System

The skin is a complex organ that has devised numerous strategies, such as physical, chemical, and microbiological barriers, to protect the host from external insults. In addition, the skin contains an intricate network of immune cells resident to the tissue, crucial for host defense as well as tissue homeostasis. In the event of an insult, the skin-resident immune cells are crucial not only for prevention of infection but also for tissue reconstruction. Deregulation of immune responses often leads to impaired healing and poor tissue restoration and function. In this review, we will discuss the defensive components of the skin and focus on the function of skin-resident immune cells in homeostasis and their role in wound healing.

Human Dendritic Cells: Ontogeny and Their Subsets in Health and Disease

Dendritic cells (DCs) are a type of cells derived from bone marrow that represent 1% or less of the total hematopoietic cells of any lymphoid organ or of the total cell count of the blood or epithelia. Dendritic cells comprise a heterogeneous population of cells localized in different tissues where they act as sentinels continuously capturing antigens to present them to T cells. Dendritic cells are uniquely capable of attracting and activating naïve CD4⁺ and CD8⁺ T cells to initiate and modulate primary immune responses. They have the ability to coordinate tolerance or immunity depending on their activation status, which is why they are also considered as the orchestrating cells of the immune response. The purpose of this review is to provide a general overview of the current knowledge on ontogeny and subsets of human dendritic cells as well as their function and different biological roles.

Engineered hybrid spider silk particles as delivery system for peptide vaccines

The generation of strong T-cell immunity is one of the main challenges for the development of successful vaccines against cancer and major infectious diseases. Here we have engineered spider silk particles as delivery system for a peptide-based vaccination that leads to effective priming of cytotoxic T-cells. The recombinant spider silk protein eADF4(C16) was fused to the antigenic peptide from ovalbumin, either without linker or with a cathepsin cleavable peptide linker. Particles prepared from the hybrid proteins were taken up by dendritic cells, which are essential for T-cell priming, and successfully activated cytotoxic T-cells, without signs of immunotoxicity or unspecific immunostimulatory activity. Upon subcutaneous injection in mice, the particles were taken up by dendritic cells and accumulated in the lymph nodes, where immune responses are generated. Particles from hybrid proteins containing a cathepsin-cleavable linker induced a strong antigen-specific proliferation of cytotoxic T-cells in vivo, even in the absence of a vaccine adjuvant. We thus demonstrate the efficacy of a new vaccine strategy using a protein-based all-in-one vaccination system, where spider silk particles serve as carriers with an incorporated peptide antigen. Our study further suggests that engineered spider silk-based vaccines are extremely stable, easy to manufacture, and readily customizable.

Suppression of the CD8 T cell response by human papillomavirus type 16 E7 occurs in Langerhans cell-depleted mice

Human papillomavirus (HPV) is an epitheliotropic virus that is the primary causal agent for cervical cancer. Langerhans cells (LC) are skin antigen presenting cells that are reduced in number in HPV-infected skin. The aim of this study was to understand the immune-modulatory effects of HPV16 E7 on LC and on the CD8 T cell response to a skin-expressed antigen. To test this, HPV16 E7 was expressed in mouse skin keratinocytes with the model antigen ovalbumin (Ova). Similar to what is observed in HPV-infected human skin, LC numbers were significantly reduced in E7-expressing mouse skin. This shows that expression of the E7 protein alone is sufficient to mediate LC depletion. Expression of E7 with Ova in keratinocytes strongly suppressed the Ova-specific CD8⁺ T cell response in the skin draining lymph node. When tested in LC-ablated mice, the CD8 T cell response to skin-expressed Ova in control mice was not affected, nor was the T cell response to Ova restored in E7-expressing skin. These data indicate a role for E7 in regulation of LC homeostasis in the skin and in suppression of antigen specific CD8 T cell expansion, but suggest that these two effects occur independent of each other.

Direct Delivery of Antigens to Dendritic Cells via Antibodies Specific for Endocytic Receptors as a Promising Strategy for Future Therapies

Dendritic cells (DCs) are the most potent professional antigen presenting cells and are therefore indispensable for the control of immunity. The technique of antibody mediated antigen targeting to DC subsets has been the basis of intense research for more than a decade. Many murine studies have utilized this approach of antigen delivery to various kinds of endocytic receptors of DCs both in vitro and in vivo. Today, it is widely accepted that different DC subsets are important for the induction of select immune responses. Nevertheless, many questions still remain to be answered, such as the actual influence of the targeted receptor on the initiation of the immune response to the delivered antigen. Further efforts to better understand the induction of antigen-specific immune responses will support the transfer of this knowledge into novel treatment strategies for human diseases. In this review, we will discuss the state-of-the-art aspects of the basic principles of antibody mediated antigen targeting approaches. A table will also provide a broad overview of the latest studies using antigen targeting including addressed DC subset, targeted receptors, outcome, and applied coupling techniques.

Dendritic Cells in Oncolytic Virus-Based Anti-Cancer Therapy

Dendritic cells (DCs) are specialized antigen-presenting cells that have a notable role in the initiation and regulation of innate and adaptive immune responses. In the context of cancer, appropriately activated DCs can induce anti-tumor immunity by activating innate immune cells and tumor-specific lymphocytes that target cancer cells. However, the tumor microenvironment (TME) imposes different mechanisms that facilitate the impairment of DC functions, such as inefficient antigen presentation or polarization into immunosuppressive DCs. These tumor-associated DCs thus fail to initiate tumor-specific immunity, and indirectly support tumor progression. Hence, there is increasing interest in identifying interventions that can overturn DC impairment within the TME. Many reports thus far have studied oncolytic viruses (OVs), viruses that preferentially target and kill cancer cells, for their capacity to enhance DC-mediated anti-tumor effects. Herein, we describe the general characteristics of DCs, focusing on their role in innate and adaptive immunity in the context of the TME. We also examine how DC-OV interaction affects DC recruitment, OV delivery, and anti-tumor immunity activation. Understanding these roles of DCs in the TME and OV infection is critical in devising strategies to further harness the anti-tumor effects of both DCs and OVs, ultimately enhancing the efficacy of OV-based oncotherapy.

Increased number of Langerhans cells in the epidermis of diabetic foot ulcers correlates with healing outcome

Langerhans cells (LCs) are a specialized subset of epidermal dendritic cells. They represent one of the first cells of immunologic barrier and play an important role during the inflammatory phase of acute wound healing. Despite considerable progress in our understanding of the immunopathology of diabetes mellitus and its associated comorbidities such as diabetic foot ulcers (DFUs), considerable gaps in our knowledge exist. In this study, we utilized the human ex vivo wound model and confirmed the increased epidermal LCs at wound edges during early phases of wound healing. Next, we aimed to determine differences in quantity of LCs between normal human and diabetic foot skin and to learn if the presence of LCs correlates with the healing outcome in DFUs. We utilized immunofluorescence to detect CD207+ LCs in specimens from normal and diabetic foot skin and DFU wound edges. Specimens from DFUs were collected at the initial visit and 4 weeks later at the time when the healing outcome was determined. DFUs that decreased in size by >50 % were considered to be healing, while DFUs with a size reduction of <50 % were considered non-healing. Quantitative assessment of LCs showed a higher number of LCs in healing when compared to non-healing DFU's. Our findings provide evidence that LCs are present in higher number in diabetic feet than normal foot skin. Healing DFUs show a higher number of LCs compared to non-healing DFUs. These findings indicate that the epidermal immune barrier plays an important role in the DFU healing outcome and may offer new therapeutic avenues targeting LC in non-healing DFUs.

Efficacy of imiquimod-based transcutaneous immunization using a nano-dispersed emulsion gel formulation

Background

Transcutaneous immunization (TCI) approaches utilize skin associated lymphatic tissues to elicit specific immune responses. In this context, the imidazoquinoline derivative imiquimod formulated in Aldara applied onto intact skin together with a cytotoxic T lymphocyte (CTL) epitope induces potent CTL responses. However, the feasibility and efficacy of the commercial imiquimod formulation Aldara is limited by its physicochemical properties as well as its immunogenicity.

Methodology/Principal Findings

To overcome these obstacles, we developed an imiquimod-containing emulsion gel (IMI-Gel) and characterized it in comparison to Aldara for rheological properties and in vitro mouse skin permeation in a Franz diffusion cell system. Imiquimod was readily released from Aldara, while IMI-Gel showed markedly decreased drug release. Nevertheless, comparing vaccination potency of Aldara or IMI-Gel-based TCI in C57BL/6 mice against the model cytotoxic T-lymphocyte epitope SIINFEKL, we found that IMI-Gel was equally effective in terms of the frequency of peptide-specific T-cells and in vivo cytolytic activity. Importantly, transcutaneous delivery of IMI-Gel for vaccination was clearly superior to the subcutaneous or oral route of administration. Finally, IMI-Gel based TCI was at least equally effective compared to Aldara-based TCI in rejection of established SIINFEKL-expressing E.G7 tumors in a therapeutic setup indicated by enhanced tumor rejection and survival.

Conclusion/Significance

In summary, we developed a novel imiquimod formulation with feasible pharmaceutical properties and immunological efficacy that fosters the rational design of a next generation transcutaneous vaccination platform suitable for the treatment of cancer or persistent virus infections.

Modulation of dendritic cell immunobiology via inhibition of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase

The maturation status of dendritic cells determines whether interacting T cells are activated or if they become tolerant. Previously we could induce T cell tolerance by applying a 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase inhibitor (HMGCRI) atorvastatin, which also modulates MHC class II expression and has therapeutic potential in autoimmune disease. Here, we aimed at elucidating the impact of this therapeutic strategy on T cell differentiation as a consequence of alterations in dendritic cell function. We investigated the effect of HMGCRI during differentiation of peripheral human monocytes and murine bone marrow precursors to immature DC in vitro and assessed their phenotype. To examine the stimulatory and tolerogenic capacity of these modulated immature dendritic cells, we measured proliferation and suppressive function of CD4+ T cells after stimulation with the modulated immature dendritic cells. We found that an HMGCRI, atorvastatin, prevents dendrite formation during the generation of immature dendritic cells. The modulated immature dendritic cells had a diminished capacity to take up and present antigen as well as to induce an immune response. Of note, the consequence was an increased capacity to differentiate naïve T cells towards a suppressor phenotype that is less sensitive to proinflammatory stimuli and can effectively inhibit the proliferation of T effector cells in vitro. Thus, manipulation of antigen-presenting cells by HMGCRI contributes to an attenuated immune response as shown by promotion of T cells with suppressive capacities.

Development and function of dendritic cell subsets

Classical dendritic cells (cDCs) form a critical interface between innate and adaptive immunity. As myeloid immune cell sentinels, cDCs are specialized in the sensing of pathogen challenges and cancer. They translate the latter for T cells into peptide form. Moreover, cDCs provide additional critical information on the original antigen context to trigger a diverse spectrum of appropriate protective responses. Here we review recent progress in our understanding of cDC subsets in mice. We will discuss cDC subset ontogeny and transcription factor dependencies, as well as emerging functional specializations within the cDC compartment in lymphoid and nonlymphoid tissues.

Regulatory dendritic cells for immunotherapy in immunologic diseases

We recognize well the abilities of dendritic cells to activate effector T cell (Teff cell) responses to an array of antigens and think of these cells in this context as pre-eminent antigen-presenting cells, but dendritic cells are also critical to the induction of immunologic tolerance. Herein, we review our knowledge on the different kinds of tolerogenic or regulatory dendritic cells that are present or can be induced in experimental settings and humans, how they operate, and the diseases in which they are effective, from allergic to autoimmune diseases and transplant tolerance. The primary conclusions that arise from these cumulative studies clearly indicate that the agent(s) used to induce the tolerogenic phenotype and the status of the dendritic cell at the time of induction influence not only the phenotype of the dendritic cell, but also that of the regulatory T cell responses that they in turn mobilize. For example, while many, if not most, types of induced regulatory dendritic cells lead CD4⁺ naïve or Teff cells to adopt a CD25⁺Foxp3⁺ Treg phenotype, exposure of Langerhans cells or dermal dendritic cells to vitamin D leads in one case to the downstream induction of CD25⁺Foxp3⁺ regulatory T cell responses, while in the other to Foxp3⁻ type 1 regulatory T cells (Tr1) responses. Similarly, exposure of human immature versus semi-mature dendritic cells to IL-10 leads to distinct regulatory T cell outcomes. Thus, it should be possible to shape our dendritic cell immunotherapy approaches for selective induction of different types of T cell tolerance or to simultaneously induce multiple types of regulatory T cell responses. This may prove to be an important option as we target diseases in different anatomic compartments or with divergent pathologies in the clinic. Finally, we provide an overview of the use and potential use of these cells clinically, highlighting their potential as tools in an array of settings.

Dendritic cells primed with protein-protein fusion adjuvant

To develop efficient T cell priming cancer vaccines, various recombinant fusion proteins have been developed by fusing a tumor antigen with a protein capable of stimulating or targeting dendritic cells (DC), the most important antigen-presenting cells for inducing CD8(+) cytotoxic T lymphocytes (CTL) which can efficiently kill tumor cells expressing the tumor antigen. The DC-stimulating or DC-targeting proteins, including granulocyte/macrophage colony-stimulating factor (GM-CSF), anti-DEC-205 monoclonal antibodies, flagellin, and heat shock proteins (HSP), function as promising intermolecular adjuvants. Herein, we describe in vitro assays on human DC pulsed with HSP fusion proteins, which might be useful in preclinical studies for the screening and assessment of candidate cancer vaccines.

Allergen immunotherapy: routes, safety, efficacy, and mode of action

Allergic rhinitis, allergic conjunctivitis, and allergic asthma have been steadily increasing in prevalence in recent years. These allergic diseases have a major impact on quality of life and are a major economic burden in the US. Although allergen avoidance and pharmacotherapy are currently the mainstays of therapy, they are not always successful in treating patients' symptoms effectively. If a patient fails allergen avoidance and medical therapy, immunotherapy may be indicated. Furthermore, immunotherapy is the only therapy that may change the course of the disease and induce long-term remission. Though subcutaneous administration has been the standard route for immunotherapy for many decades, there are several other routes of administration that have been and are currently being studied. The goal of utilizing alternative routes of immunotherapy is to improve safety without decreasing the efficacy of treatment. This paper will review the novel routes of immunotherapy, including sublingual, oral, local nasal, epicutaneous, and intralymphatic.

Immunological cells and functions in Gaucher disease

The macrophage (MΦ) has been the focus of causality, research, and therapy of Gaucher disease, but recent evidence casts doubt its solitary role in the disease pathogenesis. The excess of glucosylceramide (GC) in such cells accounts for some of the disease manifestations. Evidence of increased expression of C-C and C-X-C chemokines (i.e., CCL2,CXCL1, CXCL8) in Gaucher disease could be critical for monocyte transformation to inflammatory subsets of macrophages and dendritic cells (DC) as well as neutrophil (PMNs) recruitment to visceral organs. These immune responses could be essential for activation of T- and B-cell subsets, and the induction of numerous cytokines and chemokines that participate in the initiation and propagation of the molecular pathogenesis of Gaucher disease. The association of Gaucher disease with a variety of cellular and humoral immune responses is reviewed here to provide a potential foundation for expanding the complex pathophysiology of Gaucher disease.

Antigen presentation by Langerhans cells

Langerhans cells and other skin-resident dendritic cells (DC) are required for the development of cutaneous adaptive immune responses. In vivo experiments using mice with selective DC-subset deficiencies and ex vivo experiments using isolated DC suggests that each subset makes a unique contribution to the adaptive response. This review focuses on the functional outcome of antigen presentation by Langerhans cells. Special attention is given to their ability to promote CD4 T cell differentiation in a variety of inflammatory contexts and whether this subset has the capacity to cross-prime CD8 T cells.

Re(de)fining the dendritic cell lineage

Dendritic cells (DCs) are essential mediators of innate and adaptive immune responses. Study of these critical cells has been complicated by their similarity to other hematopoietic lineages, particularly monocytes and macrophages. Progress has been made in three critical areas of DC biology: the characterization of lineage-restricted progenitors in the bone marrow, the identification of cytokines and transcription factors required during differentiation, and the development of genetic tools for the visualization and depletion of DCs in vivo. Collectively, these advances have clarified the nature of the DC lineage and have provided novel insights into their function during health and disease.

Dendritic cell science: more than 40 years of history

Over 40 years of research into the field of DCs has revolutionized our understanding into the activation and regulation of the immune system. This minireview discusses the major breakthroughs in DC science that have paved the way to the 2011 Nobel Prize in Physiology-Medicine awarded to Bruce A. Beutler and Jules A. Hoffmann (for their discoveries in innate immune recognition) and Ralph M. Steinman (for his discovery of the DC).

Polymyxin B inadequately quenches the effects of contaminating lipopolysaccharide on murine dendritic cells

Dendritic cell (DC) activation is commonly used as a measure of the immunomodulatory potential of candidate exogenous and endogenous molecules. Residual lipopolysaccharide (LPS) contamination is a recurring theme and the potency of LPS is not always fully appreciated. To address this, polymyxin B (PmB) is often used to neutralise contaminating LPS. However, the limited capacity of this antibiotic to successfully block these effects is neglected. Therefore, this study aimed to determine the minimum LPS concentration required to induce murine bone marrow-derived dendritic cell (BMDC) maturation and cytokine secretion and to assess the ability of PmB to inhibit these processes. LPS concentrations as low as 10 pg/ml and 20 pg/ml induced secretion of interleukin (IL)-6 and tumor necrosis factor (TNF)-α respectively, while a concentration of 50 pg/ml promoted secretion of IL-12p40. A much higher threshold exists for IL-12p70 as an LPS concentration of 500 pg/ml was required to induce secretion of this cytokine. The efficacy of PmB varied substantially for different cytokines but this antibiotic was particularly limited in its ability to inhibit LPS-induced secretion of IL-6 and TNF-α. Furthermore, an LPS concentration of 50 pg/ml was sufficient to promote DC expression of costimulatory molecules and PmB was limited in its capacity to reverse this process when LPS concentrations of greater than 20 ng/ml were used. There is a common perception that LPS is heat resistant. However, heat treatment attenuated the ability of low concentrations of LPS to induce secretion of IL-6 and IL-12p40 by BMDCs, thus suggesting that heat-inactivation of protein preparations is also an ineffective control for discounting potential LPS contamination. Finally, LPS concentrations of less than 10 pg/ml were incapable of promoting secretion of IL-6 independently but could synergise with heat-labile enterotoxin (LT) to promote IL-6, indicating that reducing contaminating endotoxin concentrations to low pg/ml concentrations is essential to avoid misleading conclusions regarding candidate immunomodulators.

Microneedle-mediated vaccine delivery: harnessing cutaneous immunobiology to improve efficacy

INTRODUCTION

Breaching the skin's stratum corneum barrier raises the possibility of the administration of vaccines, gene vectors, antibodies and even nanoparticles, all of which have at least their initial effect on populations of skin cells.

AREAS COVERED

Intradermal vaccine delivery holds enormous potential for improved therapeutic outcomes for patients, particularly those in the developing world. Various vaccine-delivery strategies have been employed, which are discussed in this review. The importance of cutaneous immunobiology on the effect produced by microneedle-mediated intradermal vaccination is also discussed.

EXPERT OPINION

Microneedle-mediated vaccines hold enormous potential for patient benefit. However, in order for microneedle vaccine strategies to fulfill their potential, the proportion of an immune response that is due to the local action of delivered vaccines on skin antigen-presenting cells, and what is due to a systemic effect from vaccines reaching the systemic circulation, must be determined. Moreover, industry will need to invest significantly in new equipment and instrumentation in order to mass-produce microneedle vaccines consistently. Finally, microneedles will need to demonstrate consistent dose delivery across patient groups and match this to reliable immune responses before they will replace tried-and-tested needle-and-syringe-based approaches.

CD70-CD27 interaction augments CD8+ T-cell activation by human epidermal Langerhans cells

Human cutaneous dendritic cells (DCs) from epidermal and dermal compartments exhibit functional differences in their induction of CD4+ T-cell and humoral immune responses; however, differences in the regulation of memory CD8+ T-cell responses by human skin DCs remain poorly characterized. We tested the capacity of human Langerhans cells (LCs) and dermal dendritic cells (DDCs) to induce antigen-specific cytokine production and proliferation of memory CD8+ cells. Although tumor necrosis factor-α-matured human DCs from both epidermal and dermal compartments showed efficient potential to activate CD8+ cells, LCs were constitutively more efficient than DDCs in cross-presenting CD8+ epitopes, as well as direct presentation of viral antigen to Epstein-Barr virus-specific CD8+ T cells. LCs showed greater expression of CD70, and blockade of CD70-CD27 signaling demonstrated that superiority of CD8+ activation by epidermal LC is CD70 dependent. This CD70-related activation of CD8+ cells by LCs denotes a central role of LCs in CD8+ immunity in skin, and suggests that regulation of LC CD70 expression is important in enhancing immunity against cutaneous epithelial pathogens and cancer.

Glioblastoma-induced inhibition of Langerhans cell differentiation from CD34(+) precursors is mediated by IL-6 but unaffected by JAK2/STAT3 inhibition

AIMS

Langerhans cell (LC) infiltration has been observed in glioblastoma, but the glioblastoma microenvironment may be conditioned to resist antitumor immune responses. As little is known about how glioblastoma may affect dendritic cell differentiation, here we set out to delineate the effects of glioblastoma-derived soluble factors on LC differentiation.

METHODS

CD34(+) precursor cells of the human myeloid cell line MUTZ-3 were differentiated into LC in the presence of conditioned media of the human glioblastoma cell lines U251 or U373 and phenotypically and functionally characterized.

RESULTS

Glioblastoma-conditioned media inhibited LC differentiation, resulting in functional impairment, as determined by allogeneic mixed leukocyte reactivity, and induction of STAT3 activation. IL-6 blockade completely abrogated these glioblastoma-induced immunosuppressive effects and reduced STAT3 phosphorylation. However, neither addition of JSI-124 (cucurbitacin-I; a JAK2/STAT3 inhibitor), nor of GW5074 (a Raf-1 inhibitor), both of which interfere with signaling pathways reported to act downstream of the IL-6 receptor, prevented the observed inhibitory effects on LC differentiation.

CONCLUSION

Glioblastoma-derived IL-6 is responsible for the observed suppression of LC differentiation from CD34(+) precursors but appears to exert this effect in a STAT3 and Raf-1 independent fashion.

An appreciation of Ralph Marvin Steinman (1943–2011)

Ralph Steinman, an editor at the Journal of Experimental Medicine since 1978, shared the 2011 Nobel Prize in Physiology or Medicine for his discovery of dendritic cells (DCs) and their role in immunity. Ralph never knew. He died of pancreatic cancer on September 30, 3 days before the Nobel announcement. Unaware of his death at the time of their announcement, the Nobel Committee made the unprecedented decision that his award would stand. Ralph was the consummate physician-scientist to the end. After his diagnosis, he actively participated in his 4.5 years of treatments, creating experimental therapies using his own DCs in conjunction with the therapies devised by his physicians, all the while traveling, lecturing, and most of all pursuing new investigations in his laboratory. For 38 years—from his discovery of DCs to his Nobel Prize—Ralph pioneered the criteria and methods used to identify, isolate, grow, and study DCs. He and his colleagues demonstrated that DCs are initiators of immunity and regulators of tolerance. In his most recent studies, Ralph was harnessing the specialized features of DCs to design improved vaccines. The following synopsis describes some of his seminal discoveries.

Decisions about dendritic cells: past, present, and future

A properly functioning adaptive immune system signifies the best features of life. It is diverse beyond compare, tolerant without fail, and capable of behaving appropriately with a myriad of infections and other challenges. Dendritic cells are required to explain how this remarkable system is energized and directed. I frame this article in terms of the major decisions that my colleagues and I have made in dendritic cell science and some of the guiding themes at the time the decisions were made. As a result of progress worldwide, there is now evidence of a central role for dendritic cells in initiating antigen-specific immunity and tolerance. The in vivo distribution and development of a previously unrecognized white cell lineage is better understood, as is the importance of dendritic cell maturation to link innate and adaptive immunity in response to many stimuli. Our current focus is on antigen uptake receptors on dendritic cells. These receptors enable experiments involving selective targeting of antigens in situ and new approaches to vaccine design in preclinical and clinical systems.

Psoriasis

Psoriasis is a common relapsing and remitting immune-mediated inflammatory disease that affects the skin and joints. This review focuses on current immunogenetic concepts, key cellular players, and axes of cytokines that are thought to contribute to disease pathogenesis. We highlight potential therapeutic targets and give an overview of the currently used immune-targeted therapies.

Immune functions of the skin

The skin, the body's largest organ, helps to secure the integrity of the host and, at the same time, allows the individual to communicate with the outside world. This finely tuned balance between protection from harmful pathogens (mostly microorganisms) and bidirectional signal exchange is provided by a network of structural, cellular, and molecular elements that are collectively referred to as the skin barrier. This "gateway" has a physical, chemical, and immunologic component. The role of the latter is to elicit a powerful defense reaction in the case of danger and, at the same time, to prevent such a reaction against innocuous substances. Immune responses originating in the skin are mounted and executed by cells and molecules of the innate or the adaptive immune system. Innate reactions are typically rapid, poorly discriminating, and do not exhibit memory. Adaptive responses, in contrast, show a high degree of specificity as well as memory but need a protracted time for their development. As a consequence, innate and adaptive responses are consecutive events influencing each other. In fact, we now know that the type and magnitude of the innate reactions govern and often determine the quality and quantity of adaptive responses.

Innate immunity in allergic disease

The innate immune system consists of multiple cell types that express germline-encoded pattern recognition receptors that recognize pathogen-associated molecular patterns (PAMPs) or danger-associated molecular patterns (DAMPs). Allergens are frequently found in forms and mixtures that contain PAMPs and DAMPs. The innate immune system is interposed between the external environment and the internal acquired immune system. It is also an integral part of the airways, gut, and skin. These tissues face continuous exposure to allergens, PAMPs, and DAMPs. Interaction of allergens with the innate immune system normally results in immune tolerance but, in the case of allergic disease, this interaction induces recurring and/or chronic inflammation as well as the loss of immunologic tolerance. Upon activation by allergens, the innate immune response commits the acquired immune response to a variety of outcomes mediated by distinct T-cell subsets, such as T-helper 2, regulatory T, or T-helper 17 cells. New studies highlighted in this review underscore the close relationship between allergens, the innate immune system, and the acquired immune system that promotes homeostasis versus allergic disease.

Pulsing with blast cell lysate or blast-derived total RNA reverses the dendritic cell-mediated cytotoxic activity of cytokine-induced killer cells against allogeneic acute myelogenous leukemia cells

Immunotherapeutic strategies may be a treatment option in patients with refractory acute myelogenous leukemia (AML) or, in cases of complete remission after conventional therapy regimens, may help to reduce disease recurrence or delay time to progression. Evidence suggests a key role of dendritic cells (DCs) in cancer immunotherapy due to their capacity to present tumour antigens to effector cells. We generated cytokine-induced killer (CIK) cells from healthy donors and examined their responses in vitro in an LDH release assay against three cell lines and allogeneic HLA non-matched blasts from three patients with de novo AML after coincubation with autologous peripheral blood monocyte-derived DCs. Although DCs were unable to enhance CIK cell effects against all three cell lines tested, the cytotoxic activity against the patients’ AML cells increased after coculture with mature DCs, which was significant in two of three patients. However, neither prior pulsing of the DCs with blast cell lysates nor with leukemic cell-derived total RNA further enhanced the lytic capacity of the CIK cells. On the contrary, pulsing reduced or even reversed the cytotoxic activity of the effector cells. This decrease of allogeneic cytotoxicity led us to conclude that monocyte-derived DCs may be useful in autologous or allogeneic vaccine strategies for the treatment of AML or in priming donor lymphocytes in vitro, but unfractionated antigens as pulsing agents may have inhibitory effects on T cell efficiency and their employment in immunotherapeutic strategies for AML seems questionable.

Epicutaneous/transcutaneous allergen-specific immunotherapy: rationale and clinical trials

PURPOSE OF REVIEW

IgE-mediated allergies, such as allergic rhinoconjunctivitis and asthma, have become highly prevalent, today affecting up to 35% of the population in industrialized countries. Allergen immunotherapy (also called hyposensitization therapy, desensitization or allergen-specific immunotherapy), the administration of gradually increasing amounts of an allergen, either subcutaneously or via the sublingual or oral route is effective. However, only few allergy patients (<5%) choose immunotherapy, as treatment duration is over years and because allergen administrations are associated with local and in some cases even systemic allergic side effects due to allergen accidentally reaching the circulation. Therefore, ideally the allergen should be administered to a site that contains high numbers of potent antigen-presenting cells in order to enhance efficacy and shorten treatment duration, and ideally that site should also be nonvascularized in order to prevent both systemic distribution of the allergen and systemic allergic side effects. The epidermis, a nonvascularized multilayer epithelium that contains high numbers of potent antigen-presenting Langerhans cells, could therefore be an interesting administration route.

RECENT FINDINGS

We have recently reintroduced transcutaneous or epicutaneous allergen-specific immunotherapy (EPIT) as treatment option for IgE-mediated allergies. This method was found efficacious and safe. Few applications of allergens using skin patches with a treatment duration of a few weeks were sufficient to achieve lasting relief.

SUMMARY

This review gives an overview on the history, the rationale, and the mechanisms of transcutaneous/epicutaneous immunotherapy.

Viral interleukin-10 expressed by human cytomegalovirus during the latent phase of infection modulates latently infected myeloid cell differentiation

The human cytomegalovirus UL111A gene is expressed during latent and productive infections, and it codes for homologs of interleukin-10 (IL-10). We examined whether viral IL-10 expressed during latency altered differentiation of latently infected myeloid progenitors. In comparison to infection with parental virus or mock infection, latent infection with a virus in which the gene encoding viral IL-10 has been deleted upregulated cytokines associated with dendritic cell (DC) formation and increased the proportion of myeloid DCs. These data demonstrate that viral IL-10 restricts the ability of latently infected myeloid progenitors to differentiate into DCs and identifies an immunomodulatory role for viral IL-10 which may limit the host's ability to clear latent virus.

The cellular orchestra in skin allergy; are differences to lung and nose relevant?

PURPOSE OF REVIEW

It has been a long lasting question that although a similar peripheral allergen-specific immune response has been observed, why some patients show only atopic dermatitis, rhinitis and asthma alone or their combinations. The answer resides in the propensity of resident tissue cells and local antigen-presenting cells and T cells for developing an allergic inflammatory immune response. Antigen-presenting cells introduce processed allergens to T helper lymphocytes, where a decision of developing different types of T cell immunity is given under the influence of several cytokines, chemokines, costimulatory signals and regulatory T cells.

RECENT FINDINGS

We focused in this review article on effector T cell subsets, which have been recently described such as Th9, Th17 cells and Th22 cells, which are characterized by their IL-9 and IL-10, IL-17 (or IL-17A) and IL-22 expression, respectively together with other proinflammatory cytokines, which coordinate local tissue inflammation. Both naturally occurring CD4+CD25+ regulatory T (Treg) cells and inducible populations of allergen-specific, IL-10-secreting Treg type 1 cells inhibit allergen-specific effector cells and have been shown to play a central role in the maintenance of peripheral homeostasis and the establishment of controlled immune responses in allergic inflammatory tissues.

SUMMARY

Better understanding and characterization of newly described effector cell subsets and their interaction between antigen presenting cells and resident tissue cells will enlighten our knowledge on the mechanisms of allergic diseases.

Harnessing dendritic cells in inflammatory skin diseases

The skin immune system harbors a complex network of dendritic cells (DCs). Recent studies highlight a diverse functional specialization of skin DC subsets. In addition to generating cellular and humoral immunity against pathogens, skin DCs are involved in tolerogenic mechanisms to ensure the maintenance of immune homeostasis, as well as in pathogenesis of chronic inflammation in the skin when excessive immune responses are initiated and unrestrained. Harnessing DCs by directly targeting DC-derived molecules or selectively modulate DC subsets is a convincing strategy to tackle inflammatory skin diseases. In this review we discuss recent advances underlining the functional specialization of skin DCs and discuss the potential implication for future DC-based therapeutic strategies.

Langerhans cells are not required for graft-versus-host disease

Graft-versus-host disease (GVHD) is initiated and maintained by antigen-presenting cells (APCs) that prime alloreactive donor T cells. APCs are therefore attractive targets for GVHD prevention and treatment. APCs are diverse in phenotype and function, making understanding how APC subsets contribute to GVHD necessary for the development of APC-targeted therapies. Langerhans cells (LCs) have been shown to be sufficient to initiate skin GVHD in a major histocompatibility complex-mismatched model; however, their role when other host APC subsets are intact is unknown. To address this question, we used mice genetically engineered to be deficient in LCs by virtue of expression of diphtheria toxin A under the control of a BAC (bacterial artificial chromosome) transgenic hu-man Langerin locus. Neither CD8- nor CD4-mediated GVHD was diminished in recipients lacking LCs. Similarly, CD8- and CD4-mediated GVHD, including that in the skin, was unaffected if bone marrow came from donors that could not generate LCs, even though donor LCs engrafted in control mice. Engraftment of donor LCs after irradiation in wild-type hosts required donor T cells, with immunofluorescence revealing patches of donor and residual host LCs. Surprisingly, donor LC engraftment in Langerin-diphtheria toxin A (DTA) transgenic hosts was independent of donor T cells, suggesting that a Langerin(+) cell regulates repopulation of the LC compartment.

UV exposure boosts transcutaneous immunization and improves tumor immunity: cytotoxic T-cell priming through the skin

Immunologic approaches to combat cancer aim at the induction of tumor-reactive immune responses to achieve long-term protection. In this context, we recently developed a transcutaneous immunization (TCI) method using the Toll-like receptor (TLR) 7 agonist imiquimod and a peptide epitope. Application onto intact skin induces potent cytotoxic T lymphocyte (CTL) responses and protection against transplanted tumors. The purpose of this study was to explore the effects of UV irradiation on imiquimod-based TCI. Here we show that skin exposure to low-dose UV light before TCI with imiquimod strongly boosts specific CTL responses leading to memory formation and enhanced tumor protection. Toward the mechanisms, we show that the activation of bone-marrow-derived dermal dendritic cells (DCs), but not Langerin-expressing DCs, is responsible for enhanced CTL activation. We describe an optimized TCI method that mediates enhanced CTL and antitumor responses by a DC- and TLR-dependent mechanism. These data may provide the basis for the future development of advanced vaccination protocols against tumors and persistent virus infections.

Dendritic cells continue to capture and present antigens after maturation in vivo

Dendritic cell (DC) maturation is critical for the regulation of T cell responses. The downregulation of endocytosis on maturation is considered a key adaptation that dissociates prior Ag capture by DCs from subsequent T cell engagement. To study the dynamics of Ag capture and presentation in situ, we studied the capacity for Ag uptake by DCs matured in their natural tissue environment. We found that after maturation in vivo, mouse DCs retained a robust capacity to capture soluble Ags. Furthermore, Ags internalized by mature DCs were efficiently presented on MHC class II and cross-presented on MHC class I. These results suggest that under inflammatory conditions, mature DCs may contribute to T cell stimulation without exclusively relying on prior exposure to Ags as immature DC precursors.

Herpes simplex virus type 2 enhances HIV-1 susceptibility by affecting Langerhans cell function

Genital herpes is the most prevalent viral sexually transmitted infection worldwide and is mainly caused by HSV type 2 (HSV-2). HSV-2 infection enhances HIV-1 susceptibility, even in the absence of clinical symptoms. In this study, we investigated the effect of HSV-2 on HIV-1 transmission by mucosal Langerhans cells (LCs). LCs are important in heterosexual transmission because they form a barrier against HIV-1 infection; LCs efficiently capture and degrade HIV-1 through the C-type lectin langerin, thereby preventing HIV-1 transmission. Notably, our data showed that HSV-2 enhanced HIV-1 infection of LCs and subsequent HIV-1 transmission to T cells. HSV-2 interfered with HIV-1 capture by langerin, which allowed efficient HIV-1 infection of LCs. HSV-2 inhibited the antiviral function of langerin at two levels; HSV-2 decreased langerin expression and competed with HIV-1 for langerin binding. HSV-2 replication was not required, because both UV-inactivated HSV-2 and TLR-3 agonist polyinosinic:polycytidylic acid similarly increased HIV-1 transmission by LCs. Therefore, we identified a mechanism by which HSV-2 enhances HIV-1 susceptibility, even in the absence of clinical symptoms. Our data demonstrated that viral coinfections, such as HSV-2, breach the protective function of LCs by abrogating langerin function, which increases HIV-1 susceptibility. These data reinforce the importance of preventing sexually transmitted infections, such as HSV-2, to reduce the transmission of HIV-1.