Are dendritic cells afraid of commitment?

Timing and intensity of exposure to interferon-γ critically determines the function of monocyte-derived dendritic cells

A growing body of evidence suggests that inflammatory cytokines have a dualistic role in immunity. In this study, we sought to determine the direct effects of interferon-γ (IFN-γ) on the differentiation and maturation of human peripheral blood monocyte-derived dendritic cells (moDC). Here, we report that following differentiation of monocytes into moDC with granulocyte-macrophage colony-stimulating factor and interleukin-4, IFN-γ induces moDC maturation and up-regulates the co-stimulatory markers CD80/CD86/CD95 and MHC Class I, enabling moDC to effectively generate antigen-specific CD4(+) and CD8(+) T-cell responses for multiple viral and tumour antigens. Early exposure of monocytes to high concentrations of IFN-γ during differentiation promotes the formation of macrophages. However, under low concentrations of IFN-γ, monocytes continue to differentiate into dendritic cells possessing a unique gene-expression profile, resulting in impairments in subsequent maturation by IFN-γ or lipopolysaccharide and an inability to generate effective antigen-specific CD4(+) and CD8(+) T-cell responses. These findings demonstrate that IFN-γ imparts differential programmes on moDC that shape the antigen-specific T-cell responses they induce. Timing and intensity of exposure to IFN-γ can therefore determine the functional capacity of moDC.

Identification of immune factors regulating antitumor immunity using polymeric vaccines with multiple adjuvants

The innate cellular and molecular components required to mediate effective vaccination against weak tumor-associated antigens remain unclear. In this study, we used polymeric cancer vaccines incorporating different classes of adjuvants to induce tumor protection, to identify dendritic cell (DC) subsets and cytokines critical to this efficacy. Three-dimensional, porous polymer matrices loaded with tumor lysates and presenting distinct combinations of granulocyte macrophage colony-stimulating factor (GM-CSF) and various Toll-like receptor (TLR) agonists affected 70% to 90% prophylactic tumor protection in B16-F10 melanoma models. In aggressive, therapeutic B16 models, the vaccine systems incorporating GM-CSF in combination with P(I:C) or CpG-ODN induced the complete regression of solid tumors (≤40 mm(2)), resulting in 33% long-term survival. Regression analysis revealed that the numbers of vaccine-resident CD8(+) DCs, plasmacytoid DCs (pDC), along with local interleukin (IL)-12, and granulocyte colony-stimulating factor (G-CSF) concentrations correlated strongly to vaccine efficacy regardless of adjuvant type. Furthermore, vaccine studies in Batf3(-/-) mice revealed that CD8(+) DCs are required to affect tumor protection, as vaccines in these mice were deficient in cytotoxic T lymphocytes priming and IL-12 induction in comparison with wild-type. These studies broadly demonstrate that three-dimensional polymeric vaccines provide a potent platform for prophylactic and therapeutic protection, and can be used as a tool to identify critical components of a desired immune response. Specifically, these results suggest that CD8(+) DCs, pDCs, IL-12, and G-CSF play important roles in priming effective antitumor responses with these vaccines.

Generation of hypoallergenic neoglycoconjugates for dendritic cell targeted vaccination: a novel tool for specific immunotherapy

The incidence of allergic disorders and asthma continuously increased over the past decades, consuming a considerable proportion of the health care budget. Allergen-specific subcutaneous immunotherapy represents the only intervention treating the underlying causes of type I allergies, but still suffers from unwanted side effects and low compliance. There is an urgent need for novel approaches improving safety and efficacy of this therapy. In the present study we investigated carbohydrate-mediated targeting of allergens to dermal antigen-presenting cells and its influence on immunogenicity and allergenicity. Mannan, high (40 kDa) and low (6 kDa) molecular weight dextran, and maltodextrin were covalently attached to ovalbumin and papain via mild carbohydrate oxidation resulting in neoglycocomplexes of various sizes. In particular, mannan-conjugates were efficiently taken up by dendritic cells in vivo leading to elevated humoral immune responses against the protein moiety and a shift from IgE to IgG. Beyond providing an adjuvant effect, papain glycocomplexes also proved to mask B-cell epitopes, thus rendering the allergen derivative hypoallergenic.

The present data demonstrate that carbohydrate-modified allergens combine targeting of antigen presenting cells with hypoallergenicity, offering the potential for low dose allergen-specific immunotherapy while concomitantly reducing the risk of side effects.

The type I interferon response during viral infections: a "SWOT" analysis

The type I interferon (IFN) response is a strong and crucial moderator for the control of viral infections. The strength of this system is illustrated by the fact that, despite some temporary discomfort like a common cold or diarrhea, most viral infections will not cause major harm to the healthy immunocompetent host. To achieve this, the immune system is equipped with a wide array of pattern recognition receptors and the subsequent coordinated type I IFN response orchestrated by plasmacytoid dendritic cells (pDCs) and conventional dendritic cells (cDCs). The production of type I IFN subtypes by dendritic cells (DCs), but also other cells is crucial for the execution of many antiviral processes. Despite this coordinated response, morbidity and mortality are still common in viral disease due to the ability of viruses to exploit the weaknesses of the immune system. Viruses successfully evade immunity and infection can result in aberrant immune responses. However, these weaknesses also open opportunities for improvement via clinical interventions as can be seen in current vaccination and antiviral treatment programs. The application of IFNs, Toll-like receptor ligands, DCs, and antiviral proteins is now being investigated to further limit viral infections. Unfortunately, a common threat during stimulation of immunity is the possible initiation or aggravation of autoimmunity. Also the translation from animal models to the human situation remains difficult. With a Strengths-Weaknesses-Opportunities-Threats ("SWOT") analysis, we discuss the interaction between host and virus as well as (future) therapeutic options, related to the type I IFN system.

In situ regulation of DC subsets and T cells mediates tumor regression in mice

Vaccines are largely ineffective for patients with established cancer, as advanced disease requires potent and sustained activation of CD8(+) cytotoxic T lymphocytes (CTLs) to kill tumor cells and clear the disease. Recent studies have found that subsets of dendritic cells (DCs) specialize in antigen cross-presentation and in the production of cytokines, which regulate both CTLs and T regulatory (Treg) cells that shut down effector T cell responses. Here, we addressed the hypothesis that coordinated regulation of a DC network, and plasmacytoid DCs (pDCs) and CD8(+) DCs in particular, could enhance host immunity in mice. We used functionalized biomaterials incorporating various combinations of an inflammatory cytokine, immune danger signal, and tumor lysates to control the activation and localization of host DC populations in situ. The numbers of pDCs and CD8(+) DCs, and the endogenous production of interleukin-12, all correlated strongly with the magnitude of protective antitumor immunity and the generation of potent CD8(+) CTLs. Vaccination by this method maintained local and systemic CTL responses for extended periods while inhibiting FoxP3 Treg activity during antigen clearance, resulting in complete regression of distant and established melanoma tumors. The efficacy of this vaccine as a monotherapy against large invasive tumors may be a result of the local activity of pDCs and CD8(+) DCs induced by persistent danger and antigen signaling at the vaccine site. These results indicate that a critical pattern of DC subsets correlates with the evolution of therapeutic antitumor responses and provide a template for future vaccine design.

Plasmacytoid dendritic cells and type I IFN: 50 years of convergent history

It has been 50 years since the initial descriptions of what are now known as plasmacytoid dendritic cells (pDC) and type I IFN. pDC, which are infrequent cells found in the peripheral blood and lymphoid organs, are the most potent producers of type I and type III IFNs in the body. pDC produce IFN-alpha in response to both DNA and RNA enveloped viruses by virtue of their ribonucleic acids signaling in the endosome through TLR9 and TLR7, respectively. This stimulation, which also occurs with DNA or RNA-containing immune complexes and synthetic TLR7 and -9 agonists, is dependent upon the transcription factor IRF-7, which is expressed at high constitutive levels in pDC. In addition to releasing as much as 3-10pg of IFN-alpha/cell, pDC are also potent modulators of the immune response. In this review, we discuss the signaling pathways in pDC, their roles in linking innate and adaptive immunity, and their roles in infectious disease and autoimmunity.

Aspects of human cytomegalovirus latency and reactivation

Primary infection of healthy individuals with human cytomegalovirus (HCMV) is usually asymptomatic and results in the establishment of a lifelong latent infection of the host. Although no overt HCMV disease is observed in healthy carriers, due to effective immune control, severe clinical symptoms associated with HCMV reactivation are observed in immunocompromised transplant patients and HIV sufferers. Work from a number of laboratories has identified the myeloid lineage as one important site for HCMV latency and reactivation and thus has been the subject of extensive study. Attempts to elucidate the mechanisms controlling viral latency have shown that cellular transcription factors and histone proteins influence HCMV gene expression profoundly and that the type of cellular environment virus encounters upon infection may have a critical role in determining a lytic or latent infection and subsequent reactivation from latency. Furthermore, the identification of a number of viral gene products expressed during latent infection suggests a more active role for HCMV during latency. Defining the role of these viral proteins in latently infected cells will be important for our full understanding of HCMV latency and reactivation in vivo.

Dendritic cells: nature and classification

Dendritic cells (DCs) are antigen (Ag)-presenting cells (APCs) characterized by a unique capacity to stimulate naive T cells and initiate primary immune responses. Recent studies suggest that DCs also play critical roles in the induction of central and peripheral immunological tolerance, regulate the types of T cell immune responses, and function as sentinels in innate immunity against microbes. The diverse functions of DCs in immune regulation depend on the heterogeneity of DC subsets and their functional plasticity. Here we review recent progress in our understanding of the nature and classification of DCs.

The role of type I interferon production by dendritic cells in host defense

Type I interferons (IFN) and dendritic cells (DC) share an overlapping history, with rapidly accumulating evidence for vital roles for both production of type 1 IFN by DC and the interaction of this IFN both with DC and components of the innate and adaptive immune responses. Within the innate immune response, the plasmacytoid DC (pDC) are the "professional" IFN producing cells, expressing specialized toll-like receptors (TLR7 and -9) and high constitutive expression of IRF-7 that allow them to respond to viruses with rapid and extremely robust IFN production; following activation and production of IFN, the pDC subsequently mature into antigen presenting cells that help to shape the adaptive immune response. However, like most cells in the body, the myeloid or conventional DC (mDC or cDC) also produce type I IFNs, albeit typically at a lower level than that observed with pDC, and this IFN is also important in innate and adaptive immunity induced by these classic antigen presenting cells. These two major DC subsets and their IFN products interact both with each other as well as with NK cells, monocytes, T helper cells, T cytotoxic cells, T regulatory cells and B cells to orchestrate the early immune response. This review discusses some of the converging history of DC and IFN as well as mechanisms for IFN induction in DC and the effects of this IFN on the developing immune response.

Dendritic cell subsets and immune regulation in the lung

The lung is continuously exposed to inhaled particles, microbes and harmless antigens to which either immunity or tolerance is induced. Dendritic cells are mainly recognized for their extraordinary capacity to induce a primary immune response in the lung. Recent evidence suggests that particular subsets of DCs are essential in the decision between immunity or tolerance. Moreover, DCs play an essential role during secondary immune responses in the lung, where they control the inflammatory reaction. These novel concepts are of particular interest in understanding the pathogenesis of asthma, a disorder of aberrant immune reactivity to inhaled harmless allergens.

Natural killer dendritic cells are an intermediate of developing dendritic cells

NK dendritic cells (DCs; NKDCs) appear to emerge as a distinct DC subset in humans and rodents, which have the functions of NK cells and DCs. However, the developmental relationship of NKDCs (CD11c(+)NK1.1(+)) to CD11c(+)NK1.1(-) DCs has not been addressed. Herein, we show that NKDCs exist exclusively in the compartment of CD11c(+)MHC II(-) cells in the steady state and express variable levels of DC subset markers, such as the IFN-producing killer DC marker B220, in a tissue-dependent manner. They can differentiate into NK1.1(-) DCs, which is accompanied by the up-regulation of MHC Class II molecules and down-regulation of NK1.1 upon adoptive transfer. However, NK cells (NK(+)CD11c(-)) did not differentiate into NK1.1(+)CD11c(+) cells upon adoptive transfer. Bone marrow-derived Ly6C(+) monocytes can be a potential progenitor of NKDCs, as some of them can differentiate into CD11c(+)NK1.1(+) as well as CD11c(+)NK1.1(-) cells in vivo. The steady-state NKDCs have a great capacity to lyse tumor cells but little capability to present antigens. Our studies suggest that NKDCs are an intermediate of developing DCs. These cells appear to bear the unique surface phenotype of CD11c(+)NK1.1(+)MHC II(-) and possess strong cytotoxic function yet show a poor ability to present antigen in the steady state. These findings suggest that NKDCs may play a critical role in linking innate and adaptive immunity.

Characterisation of a dendritic cell subset in synovial tissue which strongly expresses Jak/STAT transcription factors from patients with rheumatoid arthritis

OBJECTIVES

To characterise the phenotype of the putative dendritic cells strongly expressing Jak3 and STAT4, which have been previously identified in the synovial tissue of patients with active rheumatoid arthritis (RA).

METHODS

Synovial biopsy specimens were obtained at arthroscopy from 30 patients with active RA (42 synovial biopsies). Immunohistological analysis was performed using monoclonal antibodies to detect dendritic cell subsets, including activation markers and cytokines relevant to dendritic cell function. Co-localisation of cell surface markers and cytokines was assessed primarily using sequential sections, with results confirmed by dual immunohistochemistry and immunofluorescence with confocal microscopy.

RESULTS

The dendritic cells identified in RA synovial tissue that strongly express Jak3 also strongly express STAT4 and STAT 6 and are correlated with the presence of serum rheumatoid factor. These cells are not confined to a single dendritic cell subset, with cells having phenotypes consistent with both myeloid- and plasmacytoid-type dendritic cells. The activation status of these dendritic cells suggests that they are maturing or mature dendritic cells. These dendritic cells produce IL12 as well as interferon alpha and gamma.

CONCLUSIONS

The close correlation of these dendritic cells with the presence of serum rheumatoid factor, a prognostic factor for worse disease outcome, and the strong expression by these cells of components of the Jak/STAT transcription factor pathway suggest a potential therapeutic target for the treatment of RA.

STAT3: a potential therapeutic target in dendritic cells for the induction of transplant tolerance

Dendritic cells (DCs) control the segue from innate to adaptive immunity. Moreover, depending upon their milieu, DCs can either induce or inhibit immune responses. Whether DCs are immune stimulatory or tolerogenic apparently rests with whether or not the DCs express activated signal transducer and activator of transcription-3 (STAT3), the transcription factor induced by IL-6-like cytokines and IL-10. DCs expressing activated STAT3 produce less IL-12, which results in less effector T cell development. Moreover, DCs expressing activated STAT3 also express the tryptophan-catabolising enzyme indoleamine 2,3-dioxygenase. The kynurenine products of tryptophan catabolism induce T cell apoptosis; this area is of major interest to researchers working on tolerogenic DCs. In various disease models ranging from tumours to autoimmune diseases, administration of STAT3-activating cytokines resulted in attenuation of immune responses. Other corroborating evidence was obtained using conditional STAT3-deficient mice, or mice defective in cytokine signalling. Thus, persistently activating STAT3 in DCs may be a feasible strategy for controlling allograft rejection.

Differentiation to the CCR2+ inflammatory phenotype in vivo is a constitutive, time-limited property of blood monocytes and is independent of local inflammatory mediators

It is proposed that CCR2+ monocytes are specifically recruited to inflammatory sites, whereas CCR2- monocytes are recruited to normal tissue to become resident macrophages. Whether these subsets represent separate lineages, how differential trafficking is regulated and whether monocytes undergo further differentiation is uncertain. Using a mouse model of autoimmune uveoretinitis we examined monocyte trafficking to the inflamed retina in vivo. We show that bone marrow-derived CD11b+ F4/80- monocytes require 24 to 48 h within the circulation and lymphoid system before acquiring the CCR2+ phenotype and trafficking to the inflamed retina is enabled. This phenotype, and the capacity to traffic were lost by 72 h. Monocyte CCR2 expression followed a similar time course in normal mice indicating that differentiation to an inflammatory phenotype is a constitutive, time-limited property, independent of local inflammatory mediators. Phenotypic analysis of adoptively transferred cells indicated that circulating inflammatory monocytes also differentiate into CD11c+ and B220+ dendritic cells and F4/80+ tissue macrophages in vivo. Our data supports the hypothesis of continuous extravasation and progressive differentiation over time of inflammatory monocytes in the circulation rather than replication within the actively inflamed tissue, and supports the concept of myeloid dendritic cell differentiation from trafficking monocytes under physiological conditions in vivo.

Maturation of function in dendritic cells for tolerance and immunity

The capacity of antigen presenting dendritic cells (DC) to function in both tolerance and immunity is now well documented. The function and characteristics of different DC subsets are reviewed here and their capacity to activate T cells under different conditions of maturation and activation is discussed. The immunogenic potential of exosomes produced by DC is also considered in light of evidence that the capacity of exosomes to activate T cells for tolerance or immunity appears to mirror that of the parent DC. A model is proposed whereby exosomes produced by immature DC can function to maintain peripheral tolerance, while exosomes produced by more mature DC can stimulate effector T cells.

The Colony-Stimulating Factor 1 Receptor Is Expressed on Dendritic Cells during Differentiation and Regulates Their Expansion

The lineage of dendritic cells (DC), and in particular their relationship to monocytes and macrophages, remains obscure. Furthermore, the requirement for the macrophage growth factor CSF-1 during DC homeostasis is unclear. Using a transgenic mouse in which the promoter for the CSF-1R (c-fms) directs the expression of enhanced GFP in cells of the myeloid lineage, we determined that although the c-fms promoter is inactive in DC precursors, it is up-regulated in all DC subsets during differentiation. Furthermore, plasmacytoid DC and all CD11c(high) DC subsets are reduced by 50-70% in CSF-1-deficient osteopetrotic mice, confirming that CSF-1 signaling is required for the optimal differentiation of DC in vivo. These data provide additional evidence that the majority of tissue DC is of myeloid origin during steady state and supports a close relationship between DC and macrophage biology in vivo.

Dendritic cell expansion occurs in mesenteric lymph nodes of B10.BR mice infected with the murine nematode parasite Trichuris muris

Dendritic cells (DCs) are a crucial element in the immune system and bridge innate and adaptive immunity. CD11c+ B220- DCs residing in Peyer's patches (PPs) have the ability to produce interleukin 10 (IL-10) and induce T helper (Th2) development. Evidence suggests that CD11c+ B220- DCs maintain the gut environment by suppressing Th1 responses with IL-10, resulting in a Th2-dominat gut environment. Th2 effectors are required for protection against the murine nematode parasite Trichuris muris, and thus CD11c+ B220- DCs may be involved in the induction of Th2 cells in T. muris infection. In the present study, the kinetics of CD11c+ B220- DCs were analyzed in mesenteric lymph nodes of B10.BR mice infected with the E-J isolate of T. muris, and the cellular expansion of CD11c+ B220- DCs was also observed. As well, the DC expansion was consistent with the occurrence of worm expulsion augmented by IL-4 and IL-13. The evidence here suggests the involvement of CD11c+ B220- DCs in protective Th2 responses to T. muris infection.