Dendritic cells as sensors of environmental perturbations

Dendritic cells were discovered 25 years ago as professional antigen presenting cells bridging together innate and adaptive immunity. Recently additional functions of dendritic cells have been uncovered indicating a relevant role of dendritic cells in immune system regulation. Indeed, they are the professional sensors of the immune system that can detect perturbations caused by non-self infectious as well as self non-infectious signals in most tissues. Dendritic cells discriminate both antigen amounts and antigen persistence through their receptor repertoire via the integration of different signaling pathways. The environment plays an essential role in conditioning the effector functions of dendritic cells leading either to the activation or suppression of adaptive immunity.     

Analysis of proteomic profiles and functional properties of human peripheral blood myeloid dendritic cells,monocyte-derived dendritic cells and the dendritic cell-like KG-1 cells reveals distinct characteristics

Background  

Dendritic cells (DCs) are specialized antigen presenting cells that play a pivotal role in bridging innate and adaptive immune responses. Given the scarcity of peripheral blood myeloid dendritic cells (mDCs) investigators have used different model systems for studying DC biology. Monocyte-derived dendritic cells (moDCs) and KG-1 cells are routinely used as mDC models, but a thorough comparison of these cells has not yet been carried out, particularly in relation to their proteomes. We therefore sought to run a comparative study of the proteomes and functional properties of these cells.     

mRNA-transfected dendritic cells: a promising strategy in immunotherapy

Dendritic cells play a central role in the initiation of the immune response as they are the only antigen-presenting cells able to prime naive T cells. This makes the dendritic cells the vector of choice to use as a cell-based vaccine in immunotherapy. Although there are several strategies to deliver antigen to dendritic cells, the ones transfected with mRNA coding for tumor or viral antigens are able to induce potent antigen specific T-cell responses directed against multiple epitopes. In this review, we report several advances made in the field of anti-tumoral and anti-HIV immunotherapy using mRNA-transfected dendritic cells-based approaches.     

Immunogenicity: role of dendritic cells

In the development of the immune response, the dendritic cell subset of leukocytes plays a key role in enhnacing immunogenicity. Dendritic cells can pick up antigens in the tissues and move to lymphoid organs, through which T cells continually recirculate. It is proposed that dendritic cells at these sites express functions which have been identified in tissue culture models. These involve efficient binding to antigen-specific T lymphocytes, as well as the induction of the lymphokines and growth factor receptors required for immunity. The dendritic cell system, apparently under the control of cytokines, is a sentinel designed to signal T cells that a significant antigen burden is present, and to generate the activated T lymphoblasts that interact with many other cell types to bring about an immune response.     

Dendritic cells and interferon-mediated autoimmunity

Dendritic cells (DCs) are central cells of the immune responses. They can be considered as the most influential antigen-presenting cells in the body because of their unique role in initiating immunity against most types of antigens. Recent studies have clearly established that the state of maturation of DC can be crucial for the ability of these antigen-presenting cells to inhibit or induce T-cell-mediated autoimmune diseases. Type I interferon has been shown to be produced at very high amounts by a specific type of DC (pDC). In recent years, the study of multiple autoimmune diseases has pointed to a central role for type I interferon (IFN-I) in disease pathogenesis, in particular through the IFN-molecular signature deciphered in some of these diseases. One hypothesis would be that IFN directly affects multiple actors of the immune reaction such as T cells and B cells and that it can induce the unabated activation of peripheral dendritic cells. On the other hand, type II IFN has been considered as pathogenic in multiple autoimmune diseases leading to the paradigm of TH-1 type autoimmune diseases. The discovery of the TH-17 type of cells and the protective role IFN-gamma can exert on particular phases of these diseases urge one to re-evaluate this assumption.     

Rapid downregulation of antigen processing enzymes in ex vivo generated human monocyte derived dendritic cells occur endogenously in extended cultures

Dendritic cells, the most potent antigen presenting cells, have been shown in murine models to induce immune responses against many antigens. Their role in the initiation of antitumour immunity has received enormous attention. Their ability to process and present antigen is dependent on their state of maturation. This study examines the activity of human monocyte-derived dendritic cells at two different time points and the corresponding changes in the proteolytic enzyme activity. Dendritic cells were produced from peripheral blood mononuclear cells of normal volunteers. Plastic adherent cells were cultured for 5 or 7 days with recombinant human (rh)GM-CSF and rhIL-4. Flow cytometry showed that day 5 dendritic cells (DC) were less mature than day 7 DC as indicated by the expression of CD1a, CD11c, CD14, CD80, CD83, CD86 and MHC-II. Proteolytic activity of the enzymes cathepsin C and cathepsin G and phagocytosis of particulate antigens also showed significant differences between d5 dendritic cells and d7 dendritic cells. Allogeneic costimulatory activity of d7 dendritic cells was also significantly increased. Induction of immunity requires active presentation of antigens by antigen processing cells on their MHC-I and/or MHC-II molecules. Study of peptide carriers and peptide precursor molecules showed a significant decrease in CLIP levels in the day 7 DC, suggesting their decreased ability to process antigens but no difference in their ability to load MHC-II molecules. These findings indicate that the length of time in culture, in the absence of exogenous maturation - inducing stimuli affects dendritic cell maturation. Intracellular enzymatic activities of dendritic cells also changed rapidly with small changes in phenotype.     

C-type lectin receptors on dendritic cells and Langerhans cells

Dendritic cells and Langerhans cells are specialized for the recognition of pathogens and have a pivotal role in the control of immunity. As guardians of the immune system, they are present in essentially every organ and tissue, where they operate at the interface of innate and acquired immunity. Recently, several C-type lectin and lectin-like receptors have been characterized that are expressed abundantly on the surface of these professional antigen-presenting cells. It is now becoming clear that lectin receptors not only serve as antigen receptors but also regulate the migration of dendritic cells and their interaction with lymphocytes.     

Murine dendritic cell development: difficulties associated with subset analysis

Dendritic cells are bone marrow-derived professional antigen presenting cells that play major roles in both the induction of primary immune responses and tolerance. It has become clear that dendritic cells are a heterogeneous group of cells that vary in cell surface marker expression and function. Multiple dendritic cell subsets have now been defined in mouse lymphoid organs and peripheral tissues. A knowledge of the function and relationship between dendritic cell subsets will be essential for understanding the regulation of immune homeostasis, immune responses and tolerance. While an increasing number of dendritic cell progenitors are being identified, the pathways that connect them remain unclear. In addition, it is unclear whether the functional divisions reflect maturation status, subset specialization or functional plasticity in response to specific pathogen and environmental signals. This review summarizes the current knowledge about the function and lineage relationship of dendritic cell subsets. It also discusses some of the difficulties associated with dendritic cell subset analysis.     

ILT3+/ILT4+ tolerogenic dendritic cells and their influence on allograft survival

Dendritic cells, the most powerful antigen-presenting cells, are important for triggering of the immune responses to allo-antigens. However, they also play a fundamental role in the peripheral tolerance maintenance. Tolerance is enhanced by the presence on the dendritic cell surface of the inhibitor receptors ILT3 and ILT4. They recruit protein tyrosine-phosphatases to their ITIM domains and inhibit antigen-presenting cell activation, leading T cell hypo-responsivensess. Moreover, these receptors favor a bidirectional interaction with T-suppressor and T-regulator cells, generating an antigen-specific immunoregulator cascade, in which the dendritic cell behaves as a tolerogenic cell. In the current review, analysis is centered on the biology and behavior of the tolerogenic dendritic cells that express high levels of ILT3 and ILT4. Some molecular and genetics aspects of these receptors are discussed as well as their importance in the modulation of the allo-specific antigen immune response to transplants.     

Dendritic cells and liver fibrosis

Dendritic cells are a relative rare population of specialized antigen presenting cells that are distributed through most lymphoid and non-lymphoid tissues and play a critical role in linking the innate and adaptive arms of the immune system. The liver contains a heterogeneous population of dendritic cells that may contribute to liver inflammation and fibrosis through a number of mechanisms. This review summarizes current knowledge on the development and characterization of liver dendritic cells and their potential impact on liver fibrosis. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.     

Preparation of peptide-loaded dendritic cells for cancer immunotherapy

Dendritic cell-based vaccines are being evaluated in clinical trials to determine their ability to activate clinically relevant tumor antigen-specific immune responses. Although some groups isolate dendritic cells from peripheral blood, most have found it more efficient to generate large numbers from peripheral blood progenitors, particularly plastic adherent or CD14+ monocytes, in media supplemented with GM-CSF and IL-4. These DC may then be matured, if desired, and loaded with antigen, such as tumor-associated peptides, prior to administration. We describe the scheme that we are currently using to generate peptide-loaded dendritic cells for our clinical trials of cancer immunotherapy.     

Understanding the cell biology of antigen presentation: the dendritic cell contribution

The study of the cell biology of antigen processing and presentation has greatly contributed to our understanding of the immune response. The work of many immunologically inclined cell biologists has also permitted us to gain new insights on cellular mechanisms shared by many cell types. Dendritic cells are master regulators of the immune system and consequently have received a lot of attention in recent years. With the aim of controlling antigen processing and presentation, the solutions used by dendritic cells to respond to environmental changes are numerous and surprising. In the presence of pathogens, dendritic cells regulate strongly their endocytic pathway by interfering with uptake, proteolysis, membrane dynamics and transport in and out of the lysosome to become the most potent antigen-presenting cells known.     

Dendritic cells presenting tumor antigen

 Since the first identification of dendritic cells by Steinman and Cohn in 1973, progress in understanding their biology has included the development of novel methods of cell culture, recognition of critical aspects of migration and maturation, and appreciation of their major role as antigen-presenting cells (APC), and how this activity is regulated by cytokines and expression of accessory molecules. Dendritic cells are the major APC involved in the initiation of the immune response and the development of tolerance. There is considerable evidence that they can acquire antigen in the peripheral tissues and process, transport, and present it to T cells in secondary lymphoid tissue. A number of studies show that, in vitro or in vivo, antigen-pulsed dendritic cells can directly sensitize T cells and stimulate the development of antigen-specific immune responses, including both protective and therapeutic antitumor responses. In this paper, several important aspects of dendritic cell biology are discussed and a number of studies confirming the role of these professional APC in antitumor immunity are reviewed. Received: 6 August 1996 / Accepted: 20 September 1996     

Dendritic cells in intestinal immune regulation

A breakdown in intestinal homeostasis can result in chronic inflammatory diseases of the gut including inflammatory bowel disease, coeliac disease and allergy. Dendritic cells, through their ability to orchestrate protective immunity and immune tolerance in the host, have a key role in shaping the intestinal immune response. The mechanisms through which dendritic cells can respond to environmental cues in the intestine and select appropriate immune responses have until recently been poorly understood. Here, we review recent work that is beginning to identify factors responsible for intestinal conditioning of dendritic-cell function and the subsequent decision between tolerance and immunity in the intestine.     

Dendritic cells and host resistance to infection

Host defence against infection requires an integrated response of both the innate and adaptive arms of the immune system. Emerging data indicate that dendritic cells contribute an essential part to the initiation and regulation of adaptive immunity. Dendritic cells guard the sites of pathogen entry to the host and are uniquely suited to detect and capture invading microbes. Upon recognition of microbial structures and appropriate activation, a maturation programme is triggered and dendritic cells migrate to lymphoid organs to stimulate a primary cell-mediated immune response. Moreover, dendritic cells play a critical role in shaping the emerging response, thereby controlling the course of infection. They can discriminate between various types of microorganisms and are capable of producing different cytokines in response to different microbial stimuli. On the other hand, pathogens developed numerous strategies to evade and subvert dendritic cell functions. Elucidating the interactions of dendritic cells with microbial pathogens may lead to novel strategies for combating infectious diseases by dendritic cell-based vaccination and immunotherapy. This review highlights recent advances in our knowledge of the unique role of dendritic cells in counteracting microbial infections.     

Hepatitis B virus surface antigen can activate dendritic cells and modulate T helper type immune response

Hepatitis B virus surface antigen (HBsAg) is a major antigen of hepatitis B virus (HBV). Dendritic cells (DC) of HBV carriers have been reported to exhibit functional impairment. In this study, the role of HBsAg on mice bone marrow-derived dendritic cells and immune responses in vivo was studied. The immune modulatory function of HBsAg was explored by using mice bone marrow-derived dendritic cells in vitro and also by examining an ovalbumin (OVA) specific immune response in vivo. Treatment of dendritic cells with HBsAg resulted in enhanced cell surface expression of cluster of differentiation (CD) 80, CD83, CD86, and major histocompatibility complex (MHC) class II, and enhanced production of interleukin (IL)-12 p40 and IL-12 p70. Treatment of dendritic cells with HBsAg resulted in decreased T cell secretion of IL-5 by OVA stimulation. In addition, the results showed stronger OVA-specific immunoglobulin (Ig) M and weaker IgG responses in mice sera when they had been immunized with OVA and co-injected with HBsAg. It was also found that the mice exhibited significant enhancement of anti-OVA IgG2a antibody (Ab), as well as marked inhibition of IgG1 Ab production. In cellular immune responses, IL-5 production was significantly decreased and interferon (IFN)-γ increased in the group co-injected with HBsAg. On the other hand, the induction of lymphoproliferative response to OVA stimulation in spleen cells was decreased in the HBsAg co-injected group. These results demonstrate that HBsAg can affect the differentiation of T helper (Th) cells, which might provide a strategy for improving its prophylactic and therapeutic efficacy.     

Professional type I interferon-producing cells--a unique subpopulation of dendritic cells

Dendritic cells (DC) represent a rare but multifunctional population of cells with the capacity to prime and orchestrate antigen-specific immune responses. Both human and mouse DC are classified to myeloid and plasmacytoid DC (pDC) with distinct functional activities. These DC subsets can be found in the peripheral blood and tissues as resting cells and act as sensors of environmental changes. Activation of DC by various stimuli induces morphological and functional changes and transforms these cells to potent antigen presenting and secretory cells. A newly identified precursor subset of human DC has recently been identified as professional type I interferon producing cells (IPC) with multiple functional activities. Interferon-producing cells, also referred as pDC act as a link between innate and adaptive immunity and possess the capacity to instruct and regulate pathogen- and tumor-specific immune responses. The role of IPC/pDC--partly mediated by type I interferons--has also been demonstrated in the pathogenesis of various diseases and could be used as a target for modulating immune responses.     

Transfection of murine dendritic cell line (JAWS II) by a nonviral transfection reagent

Dendritic cells are the most potent antigen-presenting cells that initiate and modulate the host immune system. Based on their immunostimulatory activity, a variety of strategies have been developed to use dendritic cells as vaccines and immunotherapeutic agents against infection and cancer. Genetically modified dendritic cells are useful for immunotherapeutic purposes because of their sustained activity in vivo. However, transfection of dendritic cells with plasmid DNA has been very difficult. While the viral transfection is associated with nonspecific activation of dendritic cells, commonly used nonviral transfection reagents have a low efficiency of transfection. Here we describe an improved, simple, less time-consuming transfection protocol using the nonviral nonliposomal lipid polymer, TransIT-TKO transfection reagent, for transfecting murine dendritic cells (JAWS II) with the gene that encodes Coccidioides immitis antigen 2 (Ag2). The JAWS II cells were cotransfected with pHYG-enhanced green fluorescent protein (EGFP) and pVR1012-C. immitis Ag2 plasmid DNAs using TransIT-TKO reagent. We reproducibly obtained 30%-50% transfection efficiency. The transfected cells maintained their immature phenotype and were functionally active. In addition, the flexibility of this agent for expressing multiple antigens (GFP and C. immitis Ag2) offers an advantage of delivering multiple immunogens.     

miRNA调控树突状细胞的分化、成熟和功能

miRNA是一类高度保守的内源性非编码小RNA,主要作用于靶mRNA的3′-非翻译区,在转录后水平调控基因表达。miRNA可调控造血细胞的增殖、分化及免疫系统的内环境稳定,在固有免疫和适应性免疫中发挥重要的作用。树突状细胞(dendritic cell,DC)是目前发现的抗原递呈能力最强的细胞,是启动、调控并维持免疫应答的中心环节。证据显示,miRNA也参与了树突状细胞的发育、分化和功能的调控,本文将综述miRNA与树突状细胞的关系的最新研究进展。