Impaired IFN-alpha secretion by plasmacytoid dendritic cells induced by TLR9 activation in chronic idiopathic urticaria

P>Background The evolution and therapeutic outcome of American tegumentary leishmaniasis (ATL) depend upon many factors, including the balance between Th1 and Th2 cytokines to control parasite multiplication and lesion extension. Other cytokines known for their role in inflammatory processes such as interleukin IL-17 or IL-18 as well as factors controlling keratinocyte differentiation and the inflammatory process in the skin, like the Notch system, could also be involved in the disease outcome. Notch receptors are a group of transmembrane proteins that regulate cell fate decisions during development and adulthood in many tissues, including keratinocyte differentiation and T-cell lineage commitment, depending on their activation by specific groups of ligands (Delta-like or Jagged). Objectives To compare the in situ expression of Notch system proteins (receptors, ligands and transcriptional factors) and cytokines possibly involved in the disease outcome (IL-17, IL-18, IL-23 and transforming growth factor-beta) in ATL cutaneous and mucosal lesions, according to the response to therapy with N-methyl glucamine. Methods Cutaneous and mucosal biopsies obtained from patients prior to therapy with N-methyl glucamine were analysed by immunohistochemistry and real-time polymerase chain reaction. Results Notch receptors and Delta-like ligands were found increased in patients with ATL, particularly those with poor response to therapy or with mucosal lesions. Conclusions The increase of Notch receptors and Delta-like ligands in patients with a poor response to treatment suggests that these patients would require a more aggressive therapeutic approach or at least a more thorough and rigorous follow-up.

Plasmacytoid dendritic cells: one-trick ponies or workhorses of the immune system?

Plasmacytoid dendritic cells (pDCs) were first described as interferon-producing cells and, for many years, their overlapping characteristics with both lymphocytes and classical dendritic cells (cDCs) created confusion over their exact ontogeny. In this Viewpoint article, Nature Reviews Immunology asks five leaders in the field to discuss their thoughts on the development and functions of pDCs--do these cells serve mainly as a major source of type I interferons or do they also make other important contributions to immune responses?

Plasmacytoid dendritic cells in melanoma: can we revert bad into good?

Tumor-infiltrating plasmacytoid dendritic cells (pDCs) promote an immunosuppressive milieu that drives tumor growth in melanoma. This phenomenon typically results from the lack of appropriate pDC activation signals in the tumor microenvironment, but it is also actively controlled by tumor cells, which have evolved strategies to inhibit type I IFN production by pDCs. In this issue, Camisaschi et al. identify a new mechanism in which tumors avoid type I IFN production by triggering LAG-3-dependent activation of pDCs. Combination therapies that restore pDC functionality and trigger innate activation to produce type I IFN should be envisaged to induce effective antitumor immunity.

Plasmacytoid dendritic cells and regulatory T cells in the tumor microenvironment: A dangerous liaison.

Tumor-infiltrating plasmacytoid dendritic cells (pDCs) have been associated with poor patient prognosis. We have recently uncovered the ability of pDCs to activate and expand a subset of tumor-infiltrating FOXP3(+) regulatory T cells that express inducible costimulator (ICOS), providing new insights into the mechanisms that govern the escape of cancer from immunosurveillance.

Mouse CD11c(+) B220(+) Gr1(+) plasmacytoid dendritic cells develop independently of the T-cell lineage.

The developmental origin of dendritic cells (DCs) is controversial. In the mouse CD8alpha(+) and CD8alpha(-) DC subsets are often considered to be of lymphoid and myeloid origin respectively, although evidence on this point is conflicting. Very recently a novel CD11c(+) B220(+) DC subset has been identified that appears to be the murine counterpart to interferon alpha (IFNalpha)-producing human plasmacytoid DCs (PDCs). We show here that CD11c(+) B220(+) mouse PDCs, like human PDCs, are present in the thymus and express T lineage markers such as CD8alpha and CD4. However, the intrathymic development of PDCs can be completely dissociated from immature T lineage cells in mixed chimeras established with bone marrow cells from mice deficient for either Notch-1 or T-cell factor 1, two independent mutations that severely block early T-cell development. Our data indicate that thymic PDCs do not arise from a bipotential T/DC precursor.

Plasmacytoid dendritic cells in the skin: to sense or not to sense nucleic acids.

Plasmacytoid dendritic cells (pDCs) are specialized sensors of viral nucleic acids that initiate protective immunity through the production of type I interferons (IFNs). Normally, pDCs fail to sense host-derived self-nucleic acids but do so when self-nucleic acids form complexes with endogenous antimicrobial peptides produced in damaged skin. Whereas regulated expression of antimicrobial peptides may lead to pDC activation and protective immune responses to skin injury, overexpression of antimicrobial peptides in psoriasis drives excessive sensing of self-nucleic acids by pDCs resulting in IFN-driven autoimmunity. In skin tumors, pDCs are unable to sense self-nucleic acids; however, therapeutic activation of pDCs by synthetic nucleic acids or analogues can be exploited to generate antitumor immunity.

Plasmacytoid dendritic cells sense skin injury and promote wound healing through type I interferons.

Plasmacytoid dendritic cells (pDCs) are specialized type I interferon (IFN-α/β)-producing cells that express intracellular toll-like receptor (TLR) 7 and TLR9 and recognize viral nucleic acids in the context of infections. We show that pDCs also have the ability to sense host-derived nucleic acids released in common skin wounds. pDCs were found to rapidly infiltrate both murine and human skin wounds and to transiently produce type I IFNs via TLR7- and TLR9-dependent recognition of nucleic acids. This process was critical for the induction of early inflammatory responses and reepithelization of injured skin. Cathelicidin peptides, which facilitate immune recognition of released nucleic acids by promoting their access to intracellular TLR compartments, were rapidly induced in skin wounds and were sufficient but not necessary to stimulate pDC activation and type I IFN production. These data uncover a new role of pDCs in sensing tissue damage and promoting wound repair at skin surfaces.

Neutrophils activate plasmacytoid dendritic cells by releasing self-DNA-peptide complexes in systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is a severe and incurable autoimmune disease characterized by chronic activation of plasmacytoid dendritic cells (pDCs) and production of autoantibodies against nuclear self-antigens by hyperreactive B cells. Neutrophils are also implicated in disease pathogenesis; however, the mechanisms involved are unknown. Here, we identified in the sera of SLE patients immunogenic complexes composed of neutrophil-derived antimicrobial peptides and self-DNA. These complexes were produced by activated neutrophils in the form of web-like structures known as neutrophil extracellular traps (NETs) and efficiently triggered innate pDC activation via Toll-like receptor 9 (TLR9). SLE patients were found to develop autoantibodies to both the self-DNA and antimicrobial peptides in NETs, indicating that these complexes could also serve as autoantigens to trigger B cell activation. Circulating neutrophils from SLE patients released more NETs than those from healthy donors; this was further stimulated by the antimicrobial autoantibodies, suggesting a mechanism for the chronic release of immunogenic complexes in SLE. Our data establish a link between neutrophils, pDC activation, and autoimmunity in SLE, providing new potential targets for the treatment of this devastating disease.

Plasmacytoid dendritic cells promote immunosuppression in ovarian cancer via ICOS costimulation of Foxp3(+) T-regulatory cells.

Epithelial ovarian cancer (EOC) is the fifth most common cause of cancer death among women. Despite its immunogenicity, effective antitumor responses are limited, due, in part, to the presence of forkhead box protein 3-positive (Foxp3(+)) T regulatory (Treg) cells in the tumor microenvironment. However, the mechanisms that regulate the accumulation and the suppressive function of these Foxp3(+) Treg cells are poorly understood. Here, we found that the majority of Foxp3(+) Treg cells accumulating in the tumor microenvironment of EOCs belong to the subset of Foxp3(+) Treg cells expressing inducible costimulator (ICOS). The expansion and the suppressive function of these cells were strictly dependent on ICOS-L costimulation provided by tumor plasmacytoid dendritic cells (pDC). Accordingly, ICOS(+) Foxp3(+) Treg cells were found to localize in close vicinity of tumor pDCs, and their number directly correlated with the numbers of pDCs in the tumors. Furthermore, pDCs and ICOS(+) Foxp3(+) Treg cells were found to be strong predictors for disease progression in patients with ovarian cancer, with ICOS(+) Treg cell subset being a stronger predictor than total Foxp3(+) Treg cells. These findings suggest an essential role for pDCs and ICOS-L in immunosuppression mediated by ICOS(+) Foxp3(+) Treg cells, leading to tumor progression in ovarian cancer.

Plasmacytoid Dendritic Cells Are Productively Infected and Activated through TLR-7 Early after Arenavirus Infection.

The antiviral response is largely mediated by dendritic cells (DCs), including conventional (c) DCs that function as antigen-presenting cells, and plasmacytoid (p) DCs that produce type I interferons, making them an attractive target for viruses. We find that the Old World arenaviruses lymphocytic choriomeningitis virus clone 13 (LCMV Cl13) and Lassa virus bind pDCs to a greater extent than cDCs. Consistently, LCMV Cl13 targets pDCs early after in vivo infection of its natural murine host and establishes a productive and robust replication cycle. pDCs coproduce type I interferons and proinflammatory cytokines, with the former being induced in both infected and uninfected pDCs, demonstrating a dissociation from intrinsic virus replication. TLR7 globally mediates pDC responses, limits pDC viral load, and promotes rapid innate and adaptive immune cell activation. These early events likely help dictate the outcome of infections with arenaviruses and other DC-replicating viruses and shed light on potential therapeutic targets.

Immunodetection of myeloid and plasmacytoid dendritic cells in mammary carcinomas of female dogs

ABSTRACT: Dendritic cells have attracted great interest from researchers as they may be used as targets of tumor immune evasion mechanisms. The main objective of this study was to evaluate the relationship between the dendritic cells (DCs) subpopulation in simple type mammary carcinomas in female dogs. Two groups of samples were used: the control group consisted of 18 samples of mammary tissue without changes and the tumor group with 26 simple type mammary carcinomas. In these groups, we evaluated the immunodetection of immature and mature myeloid DCs, plasmacytoid DCs and MHC-II. In mammary tumor, mature myeloid DCs predominated in the peritumoral region, while immature myeloid DCs and plasmacytoid DCs were evident in the intratumoral region. Immunostaining of MHC-II was visualized in mammary acini (control group), in tumor cells and inflammatory infiltration associated with tumors. The comparison between the control and tumor groups showed a statistically significant difference between immature myeloid DCs, mature myeloid DCs and plasmacytoid DCs. The immunodetection of MHC-II was not significant when comparing the groups. The predominance of immature DCs in the tumor group is possibly related to an inefficient immune response, promoting the development and survival of tumor cells. The presence of plasmacytoid DCs in the same group suggests a worse prognosis for female dogs with mammary tumors. Therefore, the ability of differentiation of canine dendritic cells could be influenced by neoplastic cells and by the tumor microenvironment.

The role of myeloid receptors on murine plasmacytoid dendritic cells in induction of type I interferon

This study tested the hypothesis that a set of predominantly myeloid restricted receptors (F4/80, CD36, Dectin-1, CD200 receptor and mannan binding lectins) and the broadly expressed CD200 played a role in a key function of plasmacytoid DC (pDC), virally induced type I interferon (IFN) production. The Dectin-1 ligands zymosan, glucan phosphate and the anti-Dectin-1 monoclonal antibody (mAb) 2A11 had no effect on influenza virus induced IFNα/β production by murine splenic pDC. However, mannan, a broad blocking reagent against mannose specific receptors, inhibited IFNα/β production by pDC in response to inactivated influenza virus. Moreover, viral glycoproteins (influenza virus haemagglutinin and HIV-1 gp120) stimulated IFNα/β production by splenocytes in a mannan-inhibitable manner, implicating the function of a lectin in glycoprotein induced IFN production. Lastly, the effect of CD200 on IFN induction was investigated. CD200 knock-out macrophages produced more IFNα than wild-type macrophages in response to polyI:C, a MyD88-independent stimulus, consistent with CD200's known inhibitory effect on myeloid cells. In contrast, blocking CD200 with an anti-CD200 mAb resulted in reduced IFNα production by pDC-containing splenocytes in response to CpG and influenza virus (MyD88-dependent stimuli). This suggests there could be a differential effect of CD200 on MyD88 dependent and independent IFN induction pathways in pDC and macrophages. This study supports the hypothesis that a mannan-inhibitable lectin and CD200 are involved in virally induced type I IFN induction.

Neonatal plasmacytoid dendritic cells at the interface of innate and adaptive immunity = Neonatale plasmazytoide dendritische Zellen - an der Schnittstelle zwischen angeborener und adaptiver Immunität

Human cord blood plasmacytoid dendritic cells (PDC) react to stimulation with CPG A and CPG B with an increase in cell surface activation and maturation markers and cytokine production, similar to adult PDC. Intracellular phosphorylation in neonatal PDC did not benefit from CPG stimulation, in contrast to adult PDC. Cord blood PDC primed with CPG A, CPG B and CD40L do not promote division of autologous T cells contrary to adult PDC. Priming of neonatal PDC with CPG A or CPG B does not induce a clear bias in T helper cell response towards Th1 or Th2 while adult PDC trend towards a Th2 response.

PLASMACYTOID DENDRITIC CELLS SENSE SKIN INJURY AND PROMOTE WOUND HEALING THROUGH TYPE I INTERFERONS

Plasmacytoid dendritic cells (pDCs) are a rare population of circulating cells, which selectively express intracellular Toll-like receptors (TLR)-7 and TLR-9 and have the capacity to produce large amounts of type I IFNs (IFN-a/b) in response to viruses or host derived nucleic acid containing complexes. pDCs are normally absent in skin but accumulate in the skin of psoriasis patients where their chronic activation to produce IFN-a/b drives the disease formation. Whether pDCs and their activation to produce IFN-a/b play a functional role in healthy skin is unknown. Here we show that pDCs are rapidly and transiently recruited into healthy human and mouse skin upon epidermal injury. Infiltrating pDCs were found to sense nucleic acids in wounded skin via TLRs, leading to the production of IFN-a/b. The production of IFN-a/b was paralleled by a short lived expression of cathelicidins, which form complexes with extracellular nucleic acids and activated pDCs to produce IFN-a/b in vitro. In vivo, cathelicidins were sufficient but not necessary for the induction of IFN-a/b in wounded skin, suggesting redundancy of this pathway. Depletion of pDCs or inhibition of IFN-a/bR signaling significantly impaired the inflammatory response and delayed re-epithelialization of skin wounds. Thus we uncover a novel role of pDCs in sensing skin injury via TLR mediated recognition of nucleic acids and demonstrate their involvement in the early inflammatory process and wound healing response through the production of IFN-a/b.

Porcine cathelicidins efficiently complex and deliver nucleic acids to plasmacytoid dendritic cells and can thereby mediate bacteria-induced IFN-α responses.

Cathelicidins constitute potent antimicrobial peptides characterized by a high cationic charge that enables strong interactions with nucleic acids. In fact, the only human cathelicidin LL-37 triggers rapid sensing of nucleic acids by plasmacytoid dendritic cells (pDC). Among the porcine cathelicidins, phylogenetic analysis of the C-terminal mature peptide showed that porcine myeloid antimicrobial peptide (PMAP)-36 was the most closely related of the 11 porcine cathelicidins to human LL-37. Despite several investigations evaluating potent antimicrobial functions of porcine cathelicidins, nothing is known about their ability to promote pDC activation. We therefore investigated the capacity of the proline-arginine-rich 39-aa peptide, PMAP-23, PMAP-36, and protegrin-1 to complex with bacterial DNA or synthetic RNA molecules and facilitate pDC activation. We demonstrate that these peptides mediate a rapid and efficient uptake of nucleic acids within minutes, followed by robust IFN-α responses. The highest positively charged cathelicidin, PMAP-36, was found to be the most potent peptide tested for this effect. The peptide-DNA complexes were internalized and also found to associate with the cell membranes of pDC. The amphipathic conformation typical of PMAP-36 was not required for IFN-α induction in pDC. We also demonstrate that PMAP-36 can mediate IFN-α induction in pDC stimulated by Escherichia coli, which alone fail to activate pDC. This response was weaker with a scrambled PMAP-36, relating to its lower antimicrobial activity. Collectively, our data suggest that the antimicrobial and nucleic acid-complexing properties of cathelicidins can mediate pDC activation-promoting adaptive immune responses against microbial infections.