Steady-state and inflammatory dendritic-cell development

Development of dendritic-cell lineages

Dendritic cells (DCs) are a heterogenous population of bone-marrow-derived immune cells. Although all DCs share a common ability to process and present antigen to naive T cells for the initiation of an immune response, they differ in surface markers, migratory patterns, localization, and cytokine production. DCs were originally considered to be myeloid cells, but recent findings have demonstrated that DCs can develop not only from myeloid- but also from lymphoid-committed progenitors. The common feature of the progenitors capable of developing into DCs is the surface expression of Flt3 receptor. The development of different populations of DCs is differentially regulated by various transcription factors and cytokines. This review summarizes the recent advances made in the field of DC development.

The role of the interferon regulatory factor (IRF) family in dendritic cell development and function

Dendritic cells (DCs) are powerful sensors of foreign pathogens as well as cancer cells and provide the first line of defence against infection. They also serve as a major link between innate and adaptive immunity. Immature DCs respond to incoming danger signals and undergo maturation to produce high levels of proinflammatory cytokines including type I interferons (IFNs) to establish innate immunity. They then present antigens to T lymphocytes to stimulate lasting specific immune responses. Recent studies point to the importance of DCs in the induction of peripheral tolerance. Transcription factors of the IRF family have emerged as crucial controllers of many aspects of DC activity, playing an essential role in the establishment of early innate immunity. Furthermore, eight of the nine members of the IRF family have been shown to control either the differentiation and/or the functional activities of DCs. In this review, we focus on three aspects of DC properties that are under the control of IRFs: (1) the development and differentiation, (2) maturation in response to toll-like receptor (TLR) signalling and the production of anti-microbial cytokines, and (3) activation and expansion of lymphocytes to generate protective or tolerogenic immune responses.

Monocyte and dendritic cell recruitment and activation during oral Salmonella infection

Immunity to bacterial infection involves the joint effort of the innate and adaptive immune systems. The innate immune response is triggered when the body senses bacterial components, such as lipopolysaccharide, that alarm the body of the invader. An array of cell types function in the innate response. These cells are rapidly recruited to the infection site and activated to optimally perform their functions. The adaptive immune response follows the innate response, and one cell type in particular, dendritic cells (DCs), are the critical link between the innate and adaptive responses. This review will summarize recent data concerning the events that occur early during oral infection with the intracellular pathogen Salmonella, with emphasis on the phagocytic cells involved in combating the infection in the gut-associated lymphoid tissues. In particular, recent findings concerning the recruitment and activation of mononuclear phagocyte populations and dendritic cell subsets will be presented after an overview of the Salmonella infection model.

Characterization of human immunodeficiency virus type 1 replication in immature and mature dendritic cells reveals dissociable cis- and trans-infection

Dendritic cells (DCs) transmit human immunodeficiency virus type 1 (HIV-1) to CD4(+) T cells through the trans- and cis-infection pathways; however, little is known about the relative efficiencies of these pathways and whether they are interdependent. Here we compare cis- and trans-infections of HIV-1 mediated by immature DCs (iDCs) and mature DCs (mDCs), using replication-competent and single-cycle HIV-1. Monocyte-derived iDCs were differentiated into various types of mDCs by lipopolysaccharide (LPS), tumor necrosis factor alpha (TNF-alpha), and CD40 ligand (CD40L). iDCs and CD40L-induced mDCs were susceptible to HIV-1 infection and mediated efficient viral transmission to CD4(+) T cells. Although HIV-1 cis-infection was partially restricted in TNF-alpha-induced mDCs and profoundly blocked in LPS-induced mDCs, these cells efficiently promoted HIV-1 trans-infection of CD4(+) T cells. The postentry restriction of HIV-1 infection in LPS-induced mDCs was identified at the levels of reverse transcription and postintegration, using real-time PCR quantification of viral DNA and integration. Furthermore, nucleofection of DCs with HIV-1 proviral DNA confirmed that impaired gene expression of LPS-induced mDCs was responsible for the postentry restriction of HIV-1 infection. Our results suggest that various DC subsets in vivo may differentially contribute to HIV-1 dissemination via dissociable cis- and trans-infections.

Tolerogenic dendritic cells and the quest for transplant tolerance

In recent years, there has been a shift from the perception of dendritic cells (DCs) solely as inducers of immune reactivity to the view that these cells are crucial regulators of immunity, which includes their ability to induce and maintain tolerance. Advances in our understanding of the phenotypical and functional plasticity of DCs, and in our ability to manipulate their development and maturation in vitro and in vivo, has provided a basis for the therapeutic harnessing of their inherent tolerogenicity. In this Review, we integrate the available information on the role of DCs in the induction of tolerance, with a focus on transplantation.

Dendritic cell-regulatory T-cell interactions control self-directed immunity

In addition to their immunostimulatory capacity, dendritic cells (DCs) play a crucial role in central and peripheral tolerance mechanisms. In the absence of an infection, immature DCs constantly take up, process and present self-antigens to specific T cells, which leads to the induction of T-cell anergy or deletion. In recent years, several additional mechanisms have been identified by which DCs constantly downregulate immune responses to maintain immunological tolerance. Among these are the complex interactions between several DC subtypes and different types of regulatory T cells. In this review, we summarize recent key findings and concepts in this field.

Notch-RBP-J signaling controls the homeostasis of CD8- dendritic cells in the spleen

Signaling through Notch receptors and their transcriptional effector RBP-J is essential for lymphocyte development and function, whereas its role in other immune cell types is unclear. We tested the function of the canonical Notch–RBP-J pathway in dendritic cell (DC) development and maintenance in vivo. Genetic inactivation of RBP-J in the bone marrow did not preclude DC lineage commitment but caused the reduction of splenic DC fraction. The inactivation of RBP-J in DCs using a novel DC-specific deleter strain caused selective loss of the splenic CD8⁻ DC subset and reduced the frequency of cytokine-secreting CD8⁻ DCs after challenge with Toll-like receptor ligands. In contrast, other splenic DC subsets and DCs in the lymph nodes and tissues were unaffected. The RBP-J–deficient splenic CD8⁻ DCs were depleted at the postprogenitor stage, exhibited increased apoptosis, and lost the expression of the Notch target gene Deltex1. In the spleen, CD8⁻ DCs were found adjacent to cells expressing the Notch ligand Delta-like 1 in the marginal zone (MZ). Thus, canonical Notch–RBP-J signaling controls the maintenance of CD8⁻ DCs in the splenic MZ, revealing an unexpected role of the Notch pathway in the innate immune system.

The role of dendritic cells in multiple sclerosis

Although multiple sclerosis (MS) is a presumed T-cell- mediated disease, it is unclear what triggers the development of neuroantigen-specific T cells. Autoreactive CD4(+) T cells are activated by antigen presenting cells; dendritic cells (DCs) are the primary antigen presenting cells directing T-cell functions and are, therefore, extremely important in directing the immune pathology characteristic of MS. Three important concepts have emerged regarding DCs in MS. First, DCs are present within the healthy central nervous system (CNS) in association with the cerebrospinal fluid space and microvasculature. Therefore, the potential for sampling of CNS antigens in similar fashion to other tissues and organs exists and likely plays an integral role in CNS immunity. The degree of involvement, as well as the source, of these CNS DCs has been addressed by several studies using the experimental autoimmune encephalomyelitis animal model. Second, DCs are found within MS lesions and have been shown to be functionally abnormal in patients with MS. Lastly, therapeutics directed at DCs could potentially be engineered for treatment in MS and in fact may already be involved in the mechanisms of current immunomodulatory therapies.

Dendritic cells at the osteo-immune interface: implications for inflammation-induced bone loss

Within the past decade, the critical roles of T cells and T cell-mediated immunity in inflammation-induced osteoclastogenesis and subsequent bone loss have been extensively studied, thereby establishing the new paradigm of osteoimmunology. Therefore, dendritic cells (DCs), the most potent antigen-presenting cells, responsible for activation of naïve T cells and orchestration of the immune response, became critically situated at the osteo-immune interface. Today, emerging new evidence suggests that DC may be directly involved in inflammation-induced osteoclastogenesis and bone loss, by acting as osteoclast (OC) precursors that can further develop into DC-derived OCs (DDOC) under inflammatory conditions. These findings have tremendous implications, because in addition to DC's important roles in regulating innate and adaptive immunity, a direct contribution by these cells to inflammation-induced bone loss may provide a promising therapeutic target not only for controlling inflammation but also for modulating bone destruction.

Major differences in inflammatory dendritic cells and their products distinguish atopic dermatitis from psoriasis

BACKGROUND

Atopic dermatitis (AD) and psoriasis represent contrasting poles of the T(H)1 versus T(H)2 paradigm. Both diseases have been associated with increased numbers of dendritic cells (DCs) in the skin, but the similarities and differences in DC populations need to be established.

OBJECTIVE

We aimed to characterize the specific DC subsets, as well as chemokine and cytokine environment in chronic AD compared with psoriasis.

METHODS

Skin biopsies were obtained from patients with acute exacerbation of chronic AD (n = 18), psoriasis (n = 15), and healthy volunteers (n = 15) for microarray analysis, RT-PCR, immunohistochemistry, and double-label immunofluorescence.

RESULTS

Myeloid DCs upregulate CCL17 and CCL18 in AD, as opposed to TNF-alpha and inducible nitric oxide synthase (iNOS) in psoriasis. In our study, we identified cells phenotypically identical to the inflammatory dendritic epidermal cells in the dermis in both diseases, although to a lesser extent in psoriasis. We found substantially higher numbers of dermal CCL22 producing plasmacytoid DCs in AD. The thymic stromal lymphopoietin receptor showed significantly higher expression in AD, whereas the thymic stromal lymphopoietin ligand was upregulated more in psoriasis.

CONCLUSION

There are major differences in myeloid and plasmacytoid subsets of cutaneous DCs and the chemokine/cytokine environment between AD and psoriasis. Distinct subsets within the CD11c(+) population may influence polarization through the production of regulatory mediators, including iNOS, TNF, CCL17, and CCL18. Plasmacytoid DCs may also influence T(H)2 polarization, having a more important role in AD than previously appreciated.

CLINICAL IMPLICATIONS

Dermal inflammatory dendritic cells in AD and TNF and iNOS-producing DCs in psoriasis, and/or their regulatory products, may be potential targets for future therapeutic interventions.

DC research in Australia

Australian researchers have contributed significantly to the understanding of DC biology and clinical application over the past 25 years. Active DC research programs are in place in all major centers, pursuing the key questions of DC phylogeny, physiology and clinical applicability. Pre-clinical and clinical research include the pathophysiology of DC in malignancy, autoimmunity, chronic viral infection, chronic renal failure and transplantation medicine. In addition, Australian laboratories have uncovered some of the subtle complexities of DC subsets, often utilizing novel investigational tools discovered in their laboratories. Above all, Australian DC research has benefited from the existence of a potent culture of active collaboration, which has led to key interactions between cellular immunologists, clinician scientists and clinical researchers. These collaborations have led to the emergence of DC research programs that extend from in vitro and animal models of DC biology through each step of clinical translation and into active clinical trials.

Monocyte-derived dendritic cells formed at the infection site control the induction of protective T helper 1 responses against Leishmania

Infection-induced inflammatory reactions involve a strong increase in dendritic cells (DCs) at the infection site and draining lymph nodes (dLNs). Whether inflammatory DCs are recruited to these locations or differentiate locally, and what their functional relevance is, remain unclear. Here we showed that during Leishmania infection, monocytes were recruited to the dermis and differentiated into "dermal monocyte-derived DCs," which subsequently migrated into the dLNs. In addition, monocyte recruitment to the dLNs resulted in the differentiation into "LN monocyte-derived DCs." Analysis of the kinetics of monocyte differentiation into DCs, susceptibility to infection, IL-12 production, and L. major-specific T cell stimulation potential suggest that dermal monocyte-derived DCs controlled the induction of protective T helper 1 responses against Leishmania. Thus, the demonstration of monocyte differentiation potential into DCs during in vivo infection and of local DC differentiation in inflammatory foci suggests that de novo formed monocyte-derived DCs are essential in T cell immunity against pathogens.

Hold On, the Monocytes Are Coming!

The role that monocyte-derived dendritic cells play in vivo remains enigmatic. León et al. (2007) now show that these cells develop "on demand" at a site of Leishmania infection to promote a life-saving T helper 1 immune response.

Genomics of TGF-β1 signaling in stem cell commitment and dendritic cell development

Dendritic cells are professional antigen presenting cells and central for establishing and maintaining immunity and immunological tolerance. They develop from hematopoietic stem cells through successive steps of lineage commitment and differentiation. Dendritic cell development and function are regulated by specific cytokines, including transforming growth factor type beta1 (TGF-beta1). Our previous work demonstrated the importance of TGF-beta1 signaling for dendritic cell development and subset specification. Here, we used genome-wide gene expression profiling with DNA microarrays to investigate the activity of TGF-beta1 on gene expression in dendritic cell development. This study identified specific gene categories induced by TGF-beta1 with an impact on dendritic cell biology.