Phagocytosis of antigens by Langerhans cells in vitro

Mannosylated Polycations Target CD206+ Antigen-Presenting Cells and Mediate T-Cell-Specific Activation in Cancer Vaccination

Immunotherapy is deemed one of the most powerful therapeutic approaches to treat cancer. However, limited response and tumor specificity are still major challenges to address. Herein, mannosylated polycations targeting mannose receptor- are developed as vectors for plasmid DNA (pDNA)-based vaccines to improve selective delivery of genetic material to antigen-presenting cells and enhance immune cell activation. Three diblock glycopolycations (M₁₅A₁₂, M₂₉A₂₅, and M₅₈A₄₅) and two triblock copolymers (M₂₉A₂₉B₉ and M₆₂A₅₂B₃₂) are generated by using mannose (M), agmatine (A), and butyl (B) derivatives to target CD206, complex nucleic acids, and favor the endosomal escape, respectively. All glycopolycations efficiently complex pDNA at N/P ratios <5, protecting the pDNA from degradation in a physiological milieu. M₅₈A₄₅ and M₆₂A₅₂B₃₂ complexed with plasmid encoding for antigenic ovalbumin (pOVA) trigger the immune activation of cultured dendritic cells, which present the SIINFEKL antigenic peptide via specific major histocompatibility complex-I. Importantly, administration of M₅₈A₄₅/pOVA elicits SIINFEKL-specific T-cell response in C56BL/6 mice bearing the melanoma tumor model B16-OVA, well in line with a reduction in tumor growth. These results qualify mannosylation as an efficient strategy to target immune cells in cancer vaccination and emphasize the potential of these glycopolycations as effective delivery vehicles for nucleic acids.

How cell migration helps immune sentinels

The immune system relies on the migratory capacity of its cellular components, which must be mobile in order to defend the host from invading micro-organisms or malignant cells. This applies in particular to immune sentinels from the myeloid lineage, i.e. macrophages and dendritic cells. Cell migration is already at work during mammalian early development, when myeloid cell precursors migrate from the yolk sac, an extra embryonic structure, to colonize tissues and form the pool of tissue-resident macrophages. Later, this is accompanied by a migration wave of precursors and monocytes from the bone marrow to secondary lymphoid organs and the peripheral tissues. They differentiate into DCs and monocyte-derived macrophages. During adult life, cell migration endows immune cells with the ability to patrol their environment as well as to circulate between peripheral tissues and lymphoid organs. Hence migration of immune cells is key to building an efficient defense system for an organism. In this review, we will describe how cell migratory capacity regulates the various stages in the life of myeloid cells from development to tissue patrolling, and migration to lymph nodes. We will focus on the role of the actin cytoskeletal machinery and its regulators, and how it contributes to the establishment and function of the immune system.

Evaluation of blood cell viability rate, gene expression, and O-GlcNAcylation profiles as indicative signatures for fungal stimulation of salmonid cell models

Fungal diseases of fish are a significant economic problem in aquaculture. Using high-throughput expression analysis, we identified potential transcript markers in primary head kidney and secondary embryonic cells from salmonid fish after stimulation with the inactivated fungi Mucor hiemalis and Fusarium aveneacium and with purified fungal molecular patterns. The transcript levels of most of the 45 selected genes were altered in head-kidney cells after 24 h of stimulation with fungal antigens. Stimulation with the inactivated fungus M. hiemalis induced the most pronounced transcriptional changes, including the pathogen receptor-encoding genes CLEC18A and TLR22, the cytokine-encoding genes IL6 and TNF, and the gene encoding the antimicrobial peptide LEAP2. In parallel, we analyzed the total GlcNAcylation status of embryonic salmonid cells with or without stimulation with inactivated fungi. O-GlcNAcylation modulates gene expression, intracellular protein, and signal activity, but we detected no significant differences after a 3-h stimulation. A pathway analysis tool identified the "apoptosis of leukocytes" based on the expression profile 24 h after fungal stimulation. Fluorescence microscopy combined with flow cytometry revealed apoptosis in 50 % of head-kidney leukocytes after 3 h stimulation with M. hiemalis, but this level decreased by > 5% after 24 h of stimulation. The number of apoptotic cells significantly increased in all blood cells after a 3-h stimulation with fungal molecular patterns compared to unstimulated controls. This in vitro approach identified transcript-based parameters that were strongly modulated by fungal infections of salmonid fish.

Immune control of Mycobacterium tuberculosis is dependent on both soluble TNFRp55 and soluble TNFRp75

Tuberculosis presents a global health challenge, and tumour necrosis factor (TNF) signalling is required for host immunity against Mycobacterium tuberculosis (Mtb). TNF receptor shedding, however, compromises effective immunity by reducing bioactive TNF through the formation of inactive complexes. In this study, we first compared the effect of total soluble TNF receptors using a transgenic p55^ΔNS /p75^-/- murine strain on host protection during a low-dose aerosol Mtb H37Rv challenge. We report that the presence of membrane-bound TNFRp55 alone in the absence of TNFRp75 results in superior control of a primary Mtb infection where p55^ΔNS /p75^-/- hyperactive dendritic cells displayed an increased capacity to induce a hyperactive Mtb-specific CD4⁺ T-cell response. p55^ΔNS /p75^-/- dendritic cells expressed a higher frequency of MHCII and increased MFIs for both CD86 and MHCII, while CD4⁺ T cells had higher expression of CD44 and IFN-γ. Next, the relative contributions of soluble TNFRp55 and soluble TNFRp75 to host protection against either primary Mtb infection or during reactivation of latent tuberculosis were delineated by comparing the experimental outcomes of control C57BL/6 mice to transgenic p55^ΔNS /p75^-/- , p55^ΔNS and p75^-/- mouse strains. We found that soluble TNFRp55 is redundant for immune regulation during the chronic stages of a primary Mtb infection. However, TNFRp55 together with soluble TNFRp75 has a crucial role in immune regulation of reactivation of latent tuberculosis.

An IRF1-IRF4 Toggle-Switch Controls Tolerogenic and Immunogenic Transcriptional Programming in Human Langerhans Cells

Langerhans cells (LCs) reside in the epidermis as a dense network of immune system sentinels, coordinating both immunogenic and tolerogenic immune responses. To determine molecular switches directing induction of LC immune activation, we performed mathematical modelling of gene regulatory networks identified by single cell RNA sequencing of LCs exposed to TNF-alpha, a key pro-inflammatory signal produced by the skin. Our approach delineated three programmes of LC phenotypic activation (immunogenic, tolerogenic or ambivalent), and confirmed that TNF-alpha enhanced LC immunogenic programming. Through regulon analysis followed by mutual information modelling, we identified IRF1 as the key transcription factor for the regulation of immunogenicity in LCs. Application of a mathematical toggle switch model, coupling IRF1 with tolerance-inducing transcription factors, determined the key set of transcription factors regulating the switch between tolerance and immunogenicity, and correctly predicted LC behaviour in LCs derived from different body sites. Our findings provide a mechanistic explanation of how combinatorial interactions between different transcription factors can coordinate specific transcriptional programmes in human LCs, interpreting the microenvironmental context of the local tissue microenvironments.

Hepatic Dendritic Cells in the Development and Progression of Metabolic Steatohepatitis

Metabolic Associated Fatty liver disease (MAFLD) is a global health problem and represents the most common cause of chronic liver disease in the world. MAFLD spectrum goes from simple steatosis to cirrhosis, in between metabolic steatohepatitis with progressive fibrosis, which pathogenesis is not completely understood. Hence, the role of the immune system has become an important fact in the trigger of inflammatory cascades in metabolic steatohepatitis and in the activation of hepatic stellate cells (HSCs). Among, the more studied immune cells in the pathogenesis of MAFLD are macrophages, T cells, natural killer and dendritic cells. In particular, hepatic dendritic cells had recently attracted a special attention, with a dual role in the pathogenesis of MAFLD. These cells have the capacity to switch from a tolerant state to active state inducing an inflammatory cascade. Furthermore, these cells play a role in the lipid storage within the liver, having, thus providing a crucial nexus between inflammation and lipid metabolism. In this review, we will discuss the current knowledge on the dual role of dendritic cells in lipid accumulation, as wells as in the triggering of hepatic inflammation and hepatocytes cell death in metabolic steatohepatitis.

Molecular mechanisms of dendritic cell migration in immunity and cancer

Dendritic cells (DCs) are a heterogeneous population of antigen-presenting cells that act to bridge innate and adaptive immunity. DCs are critical in mounting effective immune responses to tissue damage, pathogens and cancer. Immature DCs continuously sample tissues and engulf antigens via endocytic pathways such as phagocytosis or macropinocytosis, which result in DC activation. Activated DCs undergo a maturation process by downregulating endocytosis and upregulating surface proteins controlling migration to lymphoid tissues where DC-mediated antigen presentation initiates adaptive immune responses. To traffic to lymphoid tissues, DCs must adapt their motility mechanisms to migrate within a wide variety of tissue types and cross barriers to enter lymphatics. All steps of DC migration involve cell-cell or cell-substrate interactions. This review discusses DC migration mechanisms in immunity and cancer with a focus on the role of cytoskeletal processes and cell surface proteins, including integrins, lectins and tetraspanins. Understanding the adapting molecular mechanisms controlling DC migration in immunity provides the basis for therapeutic interventions to dampen immune activation in autoimmunity, or to improve anti-tumour immune responses.

Antigen presentation by dendritic cells and their instruction of CD4+ T helper cell responses

Dendritic cells are powerful antigen-presenting cells that are essential for the priming of T cell responses. In addition to providing T-cell-receptor ligands and co-stimulatory molecules for naive T cell activation and expansion, dendritic cells are thought to also provide signals for the differentiation of CD4+ T cells into effector T cell populations. The mechanisms by which dendritic cells are able to adapt and respond to the great variety of infectious stimuli they are confronted with, and prime an appropriate CD4+ T cell response, are only partly understood. It is known that in the steady-state dendritic cells are highly heterogenous both in phenotype and transcriptional profile, and that this variability is dependent on developmental lineage, maturation stage, and the tissue environment in which dendritic cells are located. Exposure to infectious agents interfaces with this pre-existing heterogeneity by providing ligands for pattern-recognition and toll-like receptors that are variably expressed on different dendritic cell subsets, and elicit production of cytokines and chemokines to support innate cell activation and drive T cell differentiation. Here we review current information on dendritic cell biology, their heterogeneity, and the properties of different dendritic cell subsets. We then consider the signals required for the development of different types of Th immune responses, and the cellular and molecular evidence implicating different subsets of dendritic cells in providing such signals. We outline how dendritic cell subsets tailor their response according to the infectious agent, and how such transcriptional plasticity enables them to drive different types of immune responses.

Dendritic Cells in Anticancer Vaccination: Rationale for Ex Vivo Loading or In Vivo Targeting

Dendritic cells (DCs) have shown great potential as a component or target in the landscape of cancer immunotherapy. Different in vivo and ex vivo strategies of DC vaccine generation with different outcomes have been proposed. Numerous clinical trials have demonstrated their efficacy and safety in cancer patients. However, there is no consensus regarding which DC-based vaccine generation method is preferable. A problem of result comparison between trials in which different DC-loading or -targeting approaches have been applied remains. The employment of different DC generation and maturation methods, antigens and administration routes from trial to trial also limits the objective comparison of DC vaccines. In the present review, we discuss different methods of DC vaccine generation. We conclude that standardized trial designs, treatment settings and outcome assessment criteria will help to determine which DC vaccine generation approach should be applied in certain cancer cases. This will result in a reduction in alternatives in the selection of preferable DC-based vaccine tactics in patient. Moreover, it has become clear that the application of a DC vaccine alone is not sufficient and combination immunotherapy with recent advances, such as immune checkpoint inhibitors, should be employed to achieve a better clinical response and outcome.

Zymosan by-passes the requirement for pulmonary antigen encounter in lung tissue-resident memory CD8+ T cell development

Tissue-resident memory T cells (Trm) in the lung provide a frontline defence against respiratory pathogens. Vaccination models that lodge CD8⁺ Trm populations in the lung have been developed, all of which incorporate the local delivery of antigen plus adjuvant into the airways; a necessary approach as local cognate antigen recognition is required for optimal lung Trm development. Although pulmonary delivery of antigen is important for lung Trm development, the impact the co-administered adjuvant has on Trm differentiation is unclear. We show that while altering the adjuvant co-administered with the pulmonary delivered antigen does not impact the size of the lung Trm population, a particular adjuvant, zymosan, when administered into the airways without antigen can drive effector CD8⁺ T cells to differentiate into lung Trm. Zymosan signalling via dectin-1 receptor was sufficient to promote antigen-independent lung Trm development. When combined with an injectable influenza vaccination regime, intranasal zymosan delivery significantly boosted the size of the influenza virus-specific lung Trm population. Our results highlight that eliciting the appropriate local inflammatory milieu can by-pass the requirement for local antigen recognition in lung Trm development and emphasises that the appropriate selection of adjuvant can greatly improve vaccines that aim to elicit pulmonary Trm.

Evaluation of a Particulate Breast Cancer Vaccine Delivered via Skin

Breast cancer impacts female population globally and is the second most common cancer for females. With various limitations and adverse effects of current therapies, several immunotherapies are being explored. Development of an effective breast cancer vaccine can be a groundbreaking immunotherapeutic approach. Such approaches are being evaluated by several clinical trials currently. On similar lines, our research study aims to evaluate a particulate breast cancer vaccine delivered via skin. This particulate breast cancer vaccine was prepared by spray drying technique and utilized murine breast cancer whole cell lysate as a source of tumor-associated antigens. The average size of the particulate vaccine was 1.5 μm, which resembled the pathogenic species, thereby assisting in phagocytosis and antigen presentation leading to further activation of the immune response. The particulate vaccine was delivered via skin using commercially available metal microneedles. Methylene blue staining and confocal microscopy were used to visualize the microchannels. The results showed that microneedles created aqueous conduits of 50 ± 10 μm to deliver the microparticulate vaccine to the skin layers. Further, an in vivo comparison of immune response depicted significantly higher concentration of serum IgG, IgG2a, and B and T cell (CD4+ and CD8+) populations in the vaccinated animals than the control animals (p < 0.001). Upon challenge with live murine breast cancer cells, the vaccinated animals showed five times more tumor suppression than the control animals confirming the immune response activation and protection (p < 0.001). This research paves a way for individualized immunotherapy following surgical tumor removal to prolong relapse episodes.

Dendritic Cell-Based Cancer Vaccines

Dendritic cells (DC) are specialized immune cells that play a critical role in promoting an immune response against Ags, which can include foreign pathogenic Ags and self-tumor Ags. DC are capable of boosting a memory T cell response but most importantly they are effective initiators of naive T cell responses. Many years of studies have focused on the use of DC vaccines against cancer to initiate and shape an antitumor-specific immune response and/or boost existing spontaneous antitumor T cell responses. In this study we give a brief overview of DC biology, function, and cellular subsets, and review the current status of the field of DC as cancer vaccines.

Distribution of mannose receptor in blunt snout bream (Megalobrama amblycephala) during the embryonic development and its immune response to the challenge of Aeromonas hydrophila

The mannose receptor (MR) is a type I transmembrane protein. Its ectodomain has eight C-type lectin-like domains, which are able to recognize and mediate the phagocytosis of a wide range of pathogens. Comprehensive studies have revealed that mammalian MR is widely distributed in the mononuclear phagocyte system (MPS, previously known as the reticuloendothelial system) and play a key role both in the physiological clearance and cell activation. Hitherto, neither the MR distribution, nor the function of clearance and cell activation has been investigated in fish. In the previous study, we have reported the full-length cDNA of blunt snout bream MR, analyzed its structure and relative mRNA expression during embryogenesis and in the liver, head kidney, spleen and intestine of fish after stimulation with killed Aeromonas hydrophila. In the present study, we developed a rabbit polyclonal antibody against MR and undertook a systematic survey of the expression of MR at the protein level by immunohistochemistry. To get more information about MR function, the mRNA expression of MR, pro-inflammatory factor TNF-α and anti-inflammatory factor ARG2 genes was measured by qRT-PCR in the liver, head kidney, and spleen after A. hydrophila challenge. We first observed MR expression in the yolk sac at the fertilized egg stage and possibly MR was expressed by early macrophages. We also showed the MR distribution in head kidney, body kidney, spleen, liver, intestine, muscle, brain, heart, and gills. Following A. hydrophila challenge the MR immunoreactive cells became more widespread in head kidney and spleen, which are the major reticuloendothelial systems of fish. The quantitative studies at mRNA levels showed that there exists a high correlation between MR expression and immune cytokine expressions after bacteria challenge.

Skin Immune Landscape: Inside and Outside the Organism

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

Dendritic cell targeted vaccines: Recent progresses and challenges

Dendritic cells (DCs) are known to be a set of morphology, structure and function of heterogeneous professional antigen presenting cells (APCs), as well as the strongest functional antigen presenting cells, which can absorb, process and present antigens. As the key regulators of innate and adaptive immune responses, DCs are at the center of the immune system and capable of interacting with both B cells and T cells, thereby manipulating the humoral and cellular immune responses. DCs provide an essential link between the innate and adaptive immunity, and the strong immune activation function of DCs and their properties of natural adjuvants, make them a valuable target for antigen delivery. Targeting antigens to DC-specific endocytic receptors in combination with the relevant antibodies or ligands along with immunostimulatory adjuvants has been recently recognized as a promising strategy for designing an effective vaccine that elicits a strong and durable T cell response against intracellular pathogens and cancer. This opinion article provides a brief summary of the rationales, superiorities and challenges of existing DC-targeting approaches.

Langerhans cell: exciting developments in health and disease

Langerhans cells (LCs) have been the subject of much research since their discovery in 1868. LCs belong to the subset of leucocytes called dendritic cells. They are present in the epidermis and the pilosebaceous apparatus and monitor the cutaneous environment for changes in homeostasis. During embryogenesis, a wave of yolk sac macrophages seed the fetal skin. Then, fetal liver monocytes largely replace the yolk sac macrophages and comprise the majority of adult LCs. In the presence of skin irritation, LCs process antigen and travel to regional lymph nodes to present antigen to reactive T lymphocytes. Changes in LCs' surface markers during the journey occur under the influence of cytokines. The difference in expression of surface markers and the ability to resist radiation have allowed researchers to differentiate LCs from the murine Langerin-positive dermal dendritic cells. Exciting discoveries have been made recently regarding their role in inflammatory skin diseases, cancer and HIV. New research has shown that antibodies blocking CD1a appear to mitigate inflammation in contact hypersensitivity reactions and psoriasis. While it has been established that LCs have the potential to induce effector cells of the adaptive immune system to counter oncogenesis, recent studies have demonstrated that LCs coordinate with natural killer cells to impair development of squamous cell carcinoma caused by chemical carcinogens. However, LCs may also physiologically suppress T cells and permit keratinocyte transformation and tumorigenesis. Although long known to play a primary role in the progression of HIV infection, it is now understood that LCs also possess the ability to restrict the progression of the disease. There is a pressing need to discover more about how these cells affect various aspects of health and disease; new information gathered thus far seems promising and exciting.

Dendritic cells in the host response to implanted materials

The role of dendritic cells (DCs) and their targeted manipulation in the body's response to implanted materials is an important and developing area of investigation, and a large component of the emerging field of biomaterials-based immune engineering. The key position of DCs in the immune system, serving to bridge innate and adaptive immunity, is facilitated by rich diversity in type and function and places DCs as a critical mediator to biomaterials of both synthetic and natural origins. This review presents current views regarding DC biology and summarizes recent findings in DC responses to implanted biomaterials. Based on these findings, there is promise that the directed programming of application-specific DC responses to biomaterials can become a reality, enabling and enhancing applications almost as diverse as the larger field of biomaterials itself.

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

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

Rotavirus Recombinant VP6 Nanotubes Act as an Immunomodulator and Delivery Vehicle for Norovirus Virus-Like Particles

We have recently shown that tubular form of rotavirus (RV) recombinant VP6 protein has an in vivo adjuvant effect on the immunogenicity of norovirus (NoV) virus-like particle (VLP) vaccine candidate. In here, we investigated in vitro effect of VP6 on antigen presenting cell (APC) activation and maturation and whether VP6 facilitates NoV VLP uptake by these APCs. Mouse macrophage cell line RAW 264.7 and dendritic cell line JAWSII were used as model APCs. Internalization of VP6, cell surface expression of CD40, CD80, CD86, and major histocompatibility class II molecules, and cytokine and chemokine production were analyzed. VP6 nanotubes were efficiently internalized by APCs. VP6 upregulated the expression of cell surface activation and maturation molecules and induced secretion of several proinflammatory cytokines and chemokines. The mechanism of VP6 action was shown to be partially dependent on lipid raft-mediated endocytic pathway as shown by methyl-β-cyclodextrin inhibition on tumor necrosis factor α secretion. These findings add to the understanding of mechanism by which VP6 exerts its immunostimulatory and immunomodulatory actions and further support its use as a part of nonlive RV-NoV combination vaccine.

Effect of Biophysical Properties of Phosphatidylserine Particle on Immune Tolerance Induction Toward Factor VIII in a Hemophilia A Mouse Model

A major complication in the replacement therapy of Factor VIII (FVIII) for Hemophilia A is the development of unwanted immune responses. Previous studies from our laboratory have shown that pretreatment of FVIII in the presence of phosphatidylserine (PS) resulted in hyporesponsiveness to subsequent administration of FVIII alone, due to the ability of PS to convert an immunogen to a tolerogen. We investigated the importance of biophysical properties of PS liposomes on its ability to convert an immunogen to a tolerogen. PS particles were prepared differing in size, protein-lipid topology, lamellarity, and % association to FVIII keeping the composition of the particle same. PS particles were prepared in 2 different sizes with differing biophysical properties: smaller particles in the nanometer range (200 nm) and larger size particles in the micron range (2 μm). Hemophilia A animals treated with both the nanometer and micron size PS particles showed a significant reduction in anti-FVIII antibody titers when compared to animals receiving free FVIII alone. Upon rechallenge with free FVIII animals that received FVIII along with the nanometer size particle continued to show reduced antibody responses. Animals receiving the micron size particle showed a slight increase in titers although they remained significantly lower than the free FVIII treated group. Upon culture with bone marrow derived dendritic cells, the nanometer size particle showed a reduction in CD40 expression and an increase in transforming growth factor-β cytokine production, which was not observed with the micron size particle. These results show that biophysical properties of PS play an important role in tolerance.

Dendritic Cells Enhance HIV Infection of Memory CD4(+) T Cells in Human Lymphoid Tissues

Dendritic cells (DCs) play a key role in controlling infections by coordinating innate and adaptive immune responses to invading pathogens. Paradoxically, DCs can increase HIV-1 dissemination in vitro by binding and transferring infectious virions to CD4(+) T cells, a process called transinfection. Transinfection has been well characterized in cultured cell lines and circulating primary T cells, but it is unknown whether DCs enhance infection of CD4(+) T cells in vivo. In untreated HIV infection, massive CD4(+) T-cell infection and depletion occur in secondary lymphoid tissues long before decline is evident in the peripheral circulation. To study the role of DCs in HIV infection of lymphoid tissues, we utilized human tonsil tissues, cultured either as tissue blocks or as aggregate suspension cultures, in single-round infection experiments. In these experiments, addition of monocyte-derived DCs (MDDCs) to the cultures increased T-cell infection, particularly in CD4(+) T cells expressing lower levels of HLA-DR. Subset analysis demonstrated that MDDCs increased HIV-1 infection of central and effector memory T-cell populations. Depletion of endogenous myeloid DCs (myDCs) from the cultures decreased memory T-cell infection, and readdition of MDDCs restored infection to predepletion levels. Using an HIV-1 fusion assay, we found that MDDCs equally increased HIV delivery into naïve, central, and effector memory T cells in the cultures, whereas predepletion of myDCs reduced fusion into memory T cells. Together, these data suggest that resident myDCs facilitate memory T-cell infection in lymphoid tissues, implicating DC-mediated transinfection in driving HIV dissemination within these tissues in untreated HIV/AIDS.

Sphingosine 1-phospate differentially modulates maturation and function of human Langerhans-like cells

BACKGROUND

As mediators between innate and adaptive immune responses, Langerhans cells (LCs) are in the focus of recent investigations to determine their role in allergic inflammatory diseases like allergic contact dermatitis and atopic dermatitis. Sphingosine 1-phosphate (S1P) is a crucial lipid mediator in the skin and potentially interferes with LC homeostasis but also functional properties, such as cytokine release, migration and antigen-uptake which are considered to be key events in the initiation and maintenance of pathological disorders.

OBJECTIVE

Here, we used human Langerhans-like cells to study the influence of S1P-mediated signalling on LC maturation, cytokine release, migration and endocytosis.

METHODS

Immature Langerhans-like cells were generated from the human acute myeloid leukaemia cell line MUTZ-3 (MUTZ-LCs) and human primary monocytes (MoLCs). S1P receptor expression was determined by quantitative RT-PCR and western blotting. Expression of maturation markers were investigated by flow cytometry. The influence of S1P signalling on cytokine release was quantified by ELISA. Migration assays and FITC-dextran uptake in the presence of S1P, specific S1 P receptor agonists and antagonists as well as fingolimod (FTY720) were analysed through fluorescence microscopy and flow cytometry.

RESULTS

S1P receptor protein expression was confirmed for S1P1, S1P2 and S1P4 in MUTZ-LCs and S1P1 and S1P2 in MoLCs. In mature cells S1P receptors were downregulated. S1P did not induce maturation in MUTZ-LCs, whereas in MoLCs CD83 and CD86 were slightly upregulated. IL-8 release of MUTZ-LCs matured in the presence of S1P was not altered, however, reduced IL-6 and IL-12p70 levels were observed in mature MoLCs. Interestingly, immature MUTZ-LCs revealed a significantly increased S1P-dependent migratory capacity, whereas CCL20 induced migration was significantly decreased in the presence of S1P. Furthermore, migratory capacity towards CCL21 in mature MUTZ-LCs but not MoLCs was significantly lower when cells were stimulated with S1P. S1P, FTY720 and specific S1P receptor agonists did not modulate the endocytotic capacity of immature MUTZ-LCs and MoLCs. These findings were further supported by testing specific antagonists of S1P1-4 in the absence or presence of S1P.

CONCLUSION

Our data demonstrate that S1P regulates key events of human LC maturation including cytokine release and migration. These findings are of particular importance when considering the potential use of S1P in inflammatory skin disorders.

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.

The Role of Dendritic Cells in Fibrosis Progression in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is the most frequent cause of chronic liver disease. NAFLD encompasses a wide range of pathologies, from simple steatosis to steatosis with inflammation to fibrosis. The pathogenesis of NAFLD progression has not been completely elucidated, and different liver cells could be implicated. This review focuses on the current evidence of the role of liver dendritic cells (DCs) in the progression from NAFLD to fibrosis. Liver DCs are a heterogeneous population of hepatic antigen-presenting cells; their main function is to induce T-cell mediated immunity by antigen processing and presentation to T cells. During the steady state liver DCs are immature and tolerogenic. However, in an environment of chronic inflammation, DCs are transformed to potent inducers of immune responses. There is evidence about the role of DC in liver fibrosis, but it is not clearly understood. Interestingly, there might be a link between lipid metabolism and DC function, suggesting that immunogenic DCs are associated with liver lipid storage, representing a possible pathophysiological mechanism in NAFLD development. A better understanding of the interaction between inflammatory pathways and the different cell types and the effect on the progression of NAFLD is of great relevance.

The ins and outs of MHC class II-mediated antigen processing and presentation

Antigenic peptide-loaded MHC class II molecules (peptide-MHC class II) are constitutively expressed on the surface of professional antigen-presenting cells (APCs), including dendritic cells, B cells, macrophages and thymic epithelial cells, and are presented to antigen-specific CD4(+) T cells. The mechanisms of antigen uptake, the nature of the antigen processing compartments and the lifetime of cell surface peptide-MHC class II complexes can vary depending on the type of APC. It is likely that these differences are important for the function of each distinct APC subset in the generation of effective adaptive immune responses. In this Review, we describe our current knowledge of the mechanisms of uptake and processing of antigens, the intracellular formation of peptide-MHC class II complexes, the intracellular trafficking of peptide-MHC class II complexes to the APC plasma membrane and their ultimate degradation.

The inability of wild-type rabies virus to activate dendritic cells is dependent on the glycoprotein and correlates with its low level of the de novo-synthesized leader RNA

Dendritic cells (DCs) are the most efficient antigen-presenting cells, playing a key role in the adaptive immune responses to viral infections. Our studies demonstrate that wild-type (wt) rabies virus (RABV) does not activate DCs. Adoptive transfer of DCs primed with wt RABV did not activate DCs, stimulate virus neutralizing antibodies (VNA), or protect recipients against challenge. However, adoptive transfer of DCs primed with laboratory-attenuated RABV resulted in DC activation, production of VNA, and protection against challenge. In vitro studies with recombinant RABV (laboratory-attenuated RABV expressing the glycoprotein or the phosphoprotein from wt RABV) demonstrate that DC activation is dependent on the glycoprotein and involves the IPS-1 pathway. Furthermore, binding to and entry into DCs by wt RABV is severely blocked, and the copy number of de novo-synthesized leader RNA was two logs lower in DCs infected with the wt than in DCs treated with laboratory-attenuated RABV. However, transient transfection of DCs with synthesized leader RNA from either wt or attenuated RABV is capable of activating DCs in a dose-dependent manner. Thus, the inability of wt RABV to activate DCs correlates with its low level of the de novo-synthesized leader RNA.

IMPORTANCE

Rabies remains a public health threat, with more than 55,000 fatalities each year around the world. Since DCs play a key role in the adaptive immune responses to viral infections, we investigated the ability of rabies virus (RABV) to activate DCs. It was found that the adoptive transfer of DCs primed with wt RABV did not activate DCs, stimulate VNA, or protect mice against lethal challenge. However, laboratory-attenuated RABV mediates the activation of DCs via the IPS-1 pathway and is glycoprotein dependent. We further show that wt RABV evades DC-mediated immune activation by inefficient binding/entry into DCs and as a result of a reduced level of de novo-synthesized leader RNA. These findings may have important implications in the development of efficient rabies vaccines.

Soluble TNFRp75 regulates host protective immunity against Mycobacterium tuberculosis

Development of host protective immunity against Mycobacterium tuberculosis infection is critically dependent on the inflammatory cytokine TNF. TNF signals through 2 receptors, TNFRp55 and TNFRp75; however, the role of TNFRp75-dependent signaling in immune regulation is poorly defined. Here we found that mice lacking TNFRp75 exhibit greater control of M. tuberculosis infection compared with WT mice. TNFRp75-/- mice developed effective bactericidal granulomas and demonstrated increased pulmonary recruitment of activated DCs. Moreover, IL-12p40-dependent migration of DCs to lung draining LNs of infected TNFRp75-/- mice was substantially higher than that observed in WT M. tuberculosis-infected animals and was associated with enhanced frequencies of activated M. tuberculosis-specific IFN-γ-expressing CD4+ T cells. In WT mice, TNFRp75 shedding correlated with markedly reduced bioactive TNF levels and IL-12p40 expression. Neutralization of TNFRp75 in M. tuberculosis-infected WT BM-derived DCs (BMDCs) increased production of bioactive TNF and IL-12p40 to a level equivalent to that produced by TNFRp75-/- BMDCs. Addition of exogenous TNFRp75 to TNFRp75-/- BMDCs infected with M. tuberculosis decreased IL-12p40 synthesis, demonstrating that TNFRp75 shedding regulates DC activation. These data indicate that TNFRp75 shedding downmodulates protective immune function and reduces host resistance and survival; therefore, targeting TNFRp75 may be beneficial for improving disease outcome.

Vaccine strategies for glioblastoma: progress and future directions

Recent advances in glioblastoma therapy have led to optimism that more effective therapies will improve outcomes. Immunotherapy is a promising approach that has demonstrated the potential to eradicate cancer cells with cellular-level accuracy while minimizing damage to surrounding healthy tissue. Several vaccination strategies have been evaluated for activity against glioblastoma in clinical trials. These include peptide vaccines, polyvalent dendritic cell vaccines, heat shock protein vaccines and adoptive immunotherapy. In this review, we highlight clinical trials representative of each of these approaches and discuss strategies for integrating these therapies into routine patient care.

Cryptococcus gattii is killed by dendritic cells, but evades adaptive immunity by failing to induce dendritic cell maturation

During adaptive immunity to pathogens, dendritic cells (DCs) capture, kill, process, and present microbial Ags to T cells. Ag presentation is accompanied by DC maturation driven by appropriate costimulatory signals. However, current understanding of the intricate regulation of these processes remains limited. Cryptococcus gattii, an emerging fungal pathogen in the Pacific Northwest of Canada and the United States, fails to stimulate an effective immune response in otherwise healthy hosts leading to morbidity or death. Because immunity to fungal pathogens requires intact cell-mediated immunity initiated by DCs, we asked whether C. gattii causes dysregulation of DC functions. C. gattii was efficiently bound and internalized by human monocyte-derived DCs, trafficked to late phagolysosomes, and killed. Yet, even with this degree of DC activation, the organism evaded pathways leading to DC maturation. Despite the ability to recognize and kill C. gattii, immature DCs failed to mature; there was no increased expression of MHC class II, CD86, CD83, CD80, and CCR7, or decrease of CD11c and CD32, which resulted in suboptimal T cell responses. Remarkably, no increase in TNF-α was observed in the presence of C. gattii. However, addition of recombinant TNF-α or stimulation that led to TNF-α production restored DC maturation and restored T cell responses. Thus, despite early killing, C. gattii evades DC maturation, providing a potential explanation for its ability to infect immunocompetent individuals. We have also established that DCs retain the ability to recognize and kill C. gattii without triggering TNF-α, suggesting independent or divergent activation pathways among essential DC functions.

Low dose zymosan ameliorates both chronic and relapsing experimental autoimmune encephalomyelitis

Zymosan has previously been reported to have both pro-inflammatory and anti-inflammatory effects. Here we demonstrate that low dose zymosan prevented or reversed chronic and relapsing paralysis in EAE. In suppressing CNS autoimmune inflammation, zymosan not only regulated APC costimulator and MHC class II expression, but also promoted differentiation of regulatory T cells. Following adoptive transfer of zymosan-primed CD4(+) T cells, recipient mice were protected from EAE. In contrast, a MAPK inhibitor and a blocker of β-glucan, reversed the effects of zymosan. These results demonstrate that zymosan may be a promising beneficial agent for Multiple Sclerosis (MS).

Subunit vaccines of the future: the need for safe, customized and optimized particulate delivery systems

A major challenge for current vaccine development is the fact that many new subunit vaccines based on highly purified recombinant proteins are poorly immunogenic and mobilize insufficient immune responses for protective immunity. Adjuvants are therefore needed in vaccine formulations to enhance, direct and maintain the immune response to vaccine antigens. Few adjuvants are currently approved for human use that mainly induce humoral immunity, and there is therefore an unmet medical need for development of effective and safe adjuvants that in addition can stimulate cellular or mucosal immunity, or combinations thereof, depending on the requirements for protection against the specific disease. Vaccine delivery systems are important components of adjuvants that allow proper delivery of antigens to antigen-presenting cells. Moreover, they often possess intrinsic immunopotentiating activity and/or can be customized towards a given immunological profile by the appropriate combination with immunopotentiating compounds. This article reviews the current status of human-tailored vaccine delivery with special focus on how to design safe particulate vaccine delivery systems with respect to composition, physicochemical properties, antigen association and choice of administration route, in order to better customize vaccine formulations towards specific diseases in the future.

Post-transcriptional CD59 gene silencing by siRNAs induces enhanced human T lymphocyte response to tumor cell lysate-loaded DCs

CD59 is a complement regulatory protein known to prevent the membrane attack complex (MAC) from assembling. To investigate the role of CD59 molecules in human T cell activation in response to exogenous antigens, gene silencing via small interfering RNAs (siRNAs) was carried out. Subsequent T cell activation in response to both autologous dendritic cells (DCs) loaded with tumor lysate and beads coated with anti-CD3, anti-CD28 and anti-CD59 antibodies was investigated. The findings demonstrated that decreased CD59 expression on T cells significantly enhanced activation and proliferation of CD4(+) T cells and CD8(+) T cells while the expansion of CD4(+) CD25(+) regulatory T cells (Tregs) was not affected, and CD59 mediated inhibition of T cell activation requires the binding of CD59 with its ligand on antigen-presenting cells (APCs). The data support that CD59 down-regulates antigen-specific activation of human T lymphocytes in a ligand-dependent manner.

A TLR4 agonist synergizes with dendritic cell-directed lentiviral vectors for inducing antigen-specific immune responses

TLR4 agonists can be used as adjuvants to trigger innate immune responses of antigen-presenting cells (APCs) such as dendritic cells (DCs) to enhance vaccine-specific immunity. Adjuvant effects of TLR4 agonists are mediated by downstream signaling controlled by both MyD88 and TRIF adapter proteins. In this study, we investigated the adjuvanting capacity of glucopyranosyl lipid A (GLA), a chemically synthesized TLR4 agonist, to boost antigen-specific immunity elicited by DC-directed lentiviral vectors (DC-LV). We found that stimulation by this agonist in vitro can activate DCs in a TLR4-dependent manner. The agonist can significantly boost DC-LV-induced humoral and cellular immune responses, resulting in better antitumor reactions in response to tumor challenges. We observed that the adjuvant-mediated enhancement of cytotoxic CD8(+) T cell responses is CD4(+) T cell-dependent and determined that in vitro the agonist stimulation involves the participation of both MyD88 and TRIF pathways to activate DCs. In vivo immunization study however revealed that adjuvant effects depend more on the MyD88 signaling as TRIF(-/-) mice but not MyD88(-/-) mice were able to maintain the enhanced CD8(+) T cell responses upon DC-LV immunization. Thus, our study supports the use of this TLR4 agonist as a potent adjuvant candidate for boosting DC-LV immunization.

Dendritic cell vaccines

Despite progress in brain tumor therapy, the prognosis of malignant glioma patients remains dismal. Among the new treatments currently being investigated, immunotherapy is theoretically very attractive since it offers the potential for high tumor-specific cytotoxicity. Increasing numbers of reports demonstrate that systemic immunotherapy using dendritic cells is capable of inducing an antiglioma response. Therefore, dendritic cell-based immunotherapy could be a new treatment modality for patients with glioma. In this chapter, we will discuss the implications of these findings for glioma therapy, reviewing current literature on dendritic cell-based glioma immunotherapy. We will overview the role of dendritic cells in immunobiology, the central nervous system and tumor immunology, before outlining dendritic cell therapy results in clinical trials and future directions. Dendritic cell-based immunotherapy strategies appear promising as an approach to successfully induce an antitumor immune response in patients with glioma, where it seems to be safe and without major side effects. The development of methods for manipulating dendritic cells for the purpose of vaccination will enhance the clinical usefulness of these cells for biotherapy. Its efficacy should be further determined in randomized, controlled clinical trials.

TLR and nucleotide-binding oligomerization domain-like receptor signals differentially regulate exogenous antigen presentation

The effect of dendritic cell (DC) maturation on MHC class II-restricted Ag presentation is well studied, but less is known about the effects of DC maturation on MHC class I-restricted cross-presentation. We investigated the ability of mature DCs to present Ags from cells infected with HSV-1. Pretreatment with pure LPS increased cross-presentation in a manner dependent on both MyD88 and Toll/IL-1R domain-containing adaptor inducing IFN-β, whereas a similar dose of a less pure LPS preparation inhibited cross-presentation. The difference could not be attributed to differences in uptake or phenotypic maturation. The likely contaminant responsible for shutting down cross-presentation is peptidoglycan (PGN). Addition of PGN to pure LPS abrogated its ability to enhance cross-presentation. Direct activation of DCs with PGN inhibited cross-presentation through nucleotide-binding oligomerization domain-like receptor signaling. These results demonstrate that different maturation stimuli can have opposite impacts on the ability of DCs to cross-present viral Ags.

Real-time visualization of macromolecule uptake by epidermal Langerhans cells in living animals

As a skin-resident member of the dendritic cell family, Langerhans cells (LCs) are generally regarded to function as professional antigen-presenting cells. Here we report a simple method to visualize the endocytotic activity of LCs in living animals. BALB/c mice received subcutaneous injection of FITC-conjugated dextran (DX) probes into the ear skin and were then examined under confocal microscopy. Large numbers of FITC(+) epidermal cells became detectable 12-24 hours after injection as background fluorescence signals began to disappear. Most (>90%) of the FITC(+) epidermal cells expressed Langerin, and >95% of Langerin(+) epidermal cells exhibited significant FITC signals. To assess intracellular localization, Alexa Fluor 546-conjugated DX probes were locally injected into IAβ-enhanced green fluorescent protein (EGFP) knock-in mice and Langerin-EGFP-diphtheria toxin receptor mice--three dimensional rotation images showed close association of most of the internalized DX probes with major histocompatibility complex (MHC) class II molecules, but not with Langerin molecules. These observations support the current view that LCs constantly sample surrounding materials, including harmful and innocuous antigens, at the environmental interface. Our data also validate the potential utility of the newly developed imaging approach to monitor LC function in wild-type animals.

The glucocorticoid dexamethasone programs human dendritic cells for enhanced phagocytosis of apoptotic neutrophils and inflammatory response

GCs are powerful anti-inflammatory compounds inhibiting inflammatory cell recruitment and production of proinflammatory cytokines. We have recently found that DCs, the key players of T cell priming and polarization, respond to allogeneic apoptotic neutrophils with proinflammatory cytokine release and Th1 cell activation. Here, we show that monocyte-derived human DCs develop their capacity to engulf apoptotic cells by up-regulating a set of apoptophagocytic genes. This gene expression pattern was reprogrammed when differentiation took place in the presence of the synthetic GC Dex, which increased the expression of phagocytosis receptors MERTK and CD14, the bridging molecule C1QA, DNASE2, and ADORA3. The increased phagocytosis was attenuated by the addition of ADORA3 antagonist and could not be observed when bone marrow-derived DCs of ADORA3 KO mice were treated with Dex. The GC-treated human DCs loaded with allogeneic apoptotic neutrophils secreted, in response to LPS and IFN-γ, the inflammatory cytokine TNF-α. Furthermore, the Dex-treated DCs could activate autologous T lymphocytes toward Th1 effector cells, and this was enhanced by their exposure to allogeneic apoptotic neutrophils.

Virus aggregating peptide enhances the cell-mediated response to influenza virus vaccine

Given the poor immunogenicity of current H5N1 influenza vaccines, additives and adjuvants remain a viable solution for increasing efficacy. Here, we demonstrate that a 20-amino acid peptide (EB) possessing influenza antiviral activity also enhances the immune response to H5N1 vaccination in mice. The addition of EB to formalin-inactivated whole-virus vaccine induced virion aggregation and these aggregates were readily engulfed by phagocytic cells in vitro. In vivo, mice vaccinated with a suboptimal dose of inactivated vaccine containing EB peptide had reduced morbidity, improved viral clearance, and faster recovery than mice receiving vaccine alone. This phenomenon was not accompanied by an increase in virus-specific antibodies. Instead, cell-mediated immunity was enhanced as demonstrated by increased interferon-γ production from splenocytes. This data demonstrates that the EB peptide may a useful adjuvant for boosting the efficacy of poorly immunogenic influenza vaccines.

License to kill: Formulation requirements for optimal priming of CD8(+) CTL responses with particulate vaccine delivery systems

Induction of CD8(+) T-cell responses is critical for the immunological control of a variety of diseases upon vaccination. Modern subunit vaccines are based on highly purified recombinant proteins. The high purity represents a major advancement in terms of vaccine safety compared to previous vaccination strategies with live attenuated or whole killed pathogens, but typically renders vaccine antigens poorly immunogenic and insufficient in mobilizing protective immunity. Adjuvants are therefore needed in vaccine formulations to enhance, direct and maintain the immune response to vaccine antigens. However, a weakness of many adjuvants is the lack of induction of CD8(+) T-cell responses against protein antigens, which are required for protection against challenging and difficult infectious diseases such as AIDS and for therapeutic cancer vaccination. Within the last decade, adjuvant systems that can induce CD8(+) T-cell responses have been developed and the first clinical trials demonstrating the clinical relevance of such formulations have been performed. This paper reviews the current status of lipid- and polymer-based particulate antigen delivery systems capable of stimulating CD8(+) T-cell immunity with special focus on mechanisms of priming and pharmaceutical requirements for optimal activation of cytotoxic T-lymphocytes that can kill virus-infected or abnormal (cancer) cells.

Rare lung diseases III: pulmonary Langerhans' cell histiocytosis

Pulmonary Langerhans' cell histiocytosis (PLCH) is an unusual cystic lung disease that is also characterized by extrapulmonary manifestations. The current review discusses the presenting features and relevant diagnostic testing and treatment options for PLCH in the context of a clinical case. While the focus of the present article is adult PLCH and its pulmonary manifestations, it is important for clinicians to distinguish the adult and pediatric forms of the disease, as well as to be alert for possible extrapulmonary complications. A major theme of the current series of articles on rare lung diseases has been the translation of insights gained from fundamental research to the clinic. Accordingly, the understanding of dendritic cell biology in this disease has led to important advances in the care of patients with PLCH.

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.

Langerhans cells and dermal dendritic cells capture protein antigens in the skin: possible targets for vaccination through the skin

Dendritic cells capture and process antigen and present it to T lymphocytes in the lymphoid organs. Dendritic cells of the skin, including epidermal Langerhans cells, langerin(+) and langerin(negative) dermal dendritic cells are ideally positioned to take up pathogens that enter the body through the skin or vaccines that are administered into (intradermal) or onto (epicutaneous) the skin. The antigen uptake properties of skin dendritic cells have not thoroughly been studied yet. We therefore investigated the uptake of the fluorochrome-conjugated model antigen ovalbumin (OVA) by skin dendritic cells in the mouse. OVA was readily taken up by immature Langerhans cells both in situ and in cell suspensions. When offered to Langerhans cells in situ either by "bathing" skin explants in OVA-containing culture medium or by intradermal injection they retained the captured OVA for at least 2-3 days when migrating into the culture medium and, importantly, into the draining lymph nodes. Also langerin(+) and - to a larger extent - langerin(negative) skin dendritic cells took up and transported OVA to the lymph nodes. Interestingly, mature Langerhans cells were still capable of ingesting substantial amounts of OVA, indicating that predominantly receptor-mediated endocytosis is operative in these cells. Unlike macropinocytosis, this pathway of endocytosis is not shut down upon dendritic cell maturation. These observations indicate that in intradermal vaccination schemes, Langerhans cells from the epidermis are prominently involved. They were recently shown to possess the capacity to induce functional cytotoxic T lymphocytes. Furthermore, the potential to markedly enhance antigen uptake and processing by targeting antigen to c-type lectin receptors on Langerhans cells was also recently demonstrated. Our data provide a rationale and an incentive to explore in more detail antigen targeting to Langerhans cells with the aim of harnessing it for immunotherapy.

Langerhans cells: critical regulators of skin immunity?

Langerhans cells (LC) are members of the heterogenous family of professional antigen presenting dendritic cells (DC). They are identified by the C-type lectin receptor Langerin and form a contiguous network in the epidermis. Consequently, LC are an integral part of the skin barrier to the environment and were considered to be critical inducers of skin immunity, whereas dermal DC were largely overlooked. However, with the identification of a distinct subset of Langerin expressing dermal DC, the situation in the skin has become more complex and the relative contribution of the different cutaneous DC populations in balancing immunity and tolerance has become a matter of active debate. Here, we briefly review the classical paradigm and recent challenges of LC function, before focusing on advances concerning their role in contact hypersensitivity and ultraviolet radiation-induced immunosuppression obtained with in vivo LC ablation models. We then discuss novel LC/DC-specific gene targeting approaches currently used to dissect the role of the regulatory cytokines transforming growth factor-beta and interleukin-10 to govern LC and DC function in vivo. This second generation of LC-specific genetically engineered mice will considerably extend our understanding of the molecular control of LC function in regulating skin immunity and tolerance in the near future.

Mature dendritic cells use endocytic receptors to capture and present antigens

In response to inflammatory stimuli, dendritic cells (DCs) trigger the process of maturation, a terminal differentiation program required to initiate T-lymphocyte responses. A hallmark of maturation is down-regulation of endocytosis, which is widely assumed to restrict the ability of mature DCs to capture and present antigens encountered after the initial stimulus. We found that mature DCs continue to accumulate antigens, especially by receptor-mediated endocytosis and phagocytosis. Internalized antigens are transported normally to late endosomes and lysosomes, loaded onto MHC class II molecules (MHCII), and then presented efficiently to T cells. This occurs despite the fact that maturation results in the general depletion of MHCII from late endocytic compartments, with MHCII enrichment being typically thought to be a required feature of antigen processing and peptide loading compartments. Internalized antigens can also be cross-presented on MHC class I molecules, without any reduction in efficiency relative to immature DCs. Thus, although mature DCs markedly down-regulate their capacity for macropinocytosis, they continue to capture, process, and present antigens internalized via endocytic receptors, suggesting that they may continuously initiate responses to newly encountered antigens during the course of an infection.

Pidotimod promotes functional maturation of dendritic cells and displays adjuvant properties at the nasal mucosa level

Mucosal dendritic cells (DCs) are very important in the process of antigen presentation to T cells, playing a key role in the induction of primary and secondary immune responses. Pidotimod is a synthetic substance capable of modulating immune cell functions, but the effect of pidotimod on human DCs has not been investigated yet. Here we demonstrate the ability of pidotimod to induce DC maturation and up-regulate the expression of HLA-DR and co-stimulatory molecules CD83 and CD86, which are fundamental for communication with adaptative immunity cells. Pidotimod also stimulated DCs to release high amounts of pro-inflammatory molecules such as MCP-1 and TNF-alpha cytokines and to drive T cell proliferation and differentiation towards a Th1 phenotype. Moreover, we demonstrate that pidotimod in vivo promotes strong and specific humoral and cellular immune response when co-administered intranasally with a model antigen. Taken together our data suggest the possibility to use pidotimod as adjuvant molecule to facilitate the activation of the innate immune system as well as to promote an effective mucosal and systemic immune response.