Plasmacytoid Dendritic Cells Regulate Th Cell Responses through OX40 Ligand and Type I IFNs

Requirements for the functional expression of OX40 ligand on human activated CD4+ and CD8+ T cells

Interaction between OX40 expressed on activated T cells and its ligand (OX40L) on antigen presenting cells (APC) provides a co-stimulatory signal for T cells to promote acquired immunity. In the present study, we have examined various culture conditions for optimum OX40L expression on T cells stimulated with immobilized anti-CD3/CD28 monoclonal antibodies (mAbs). Although the day 3 primed T cells expressed minimal OX40L, after repeated stimulations both the CD4+ and CD8+ T cells became OX40L positive as determined by flow cytometry. Interleukin (IL)-12 interfered with the OX40L expression. Among activated T cells, a higher frequency of CD8+ T cells expressed OX40L than CD4+ T cells. By blocking OX40L-OX40 interaction by an anti-OX40 mAb, the number of OX40L+ T cells significantly increased. Screening of various cytokines showed that transforming growth factor (TGF)-beta1 was capable of induction of OX40L on the activated T cells within 3 days. The OX40L expressed on T cells was functional, as they bound soluble OX40 and stimulated human immunodeficiency virus-1 (HIV-1) production from cell lines chronically infected with HIV-1 and expressing OX40. Altogether the present study findings indicate that functional OX40L is inducible on human activated CD4+ and CD8+ T cells, and that the expression is enhanced by TGF-beta1.

Plasmacytoid dendritic cells--virus experts of innate immunity

Plasmacytoid dendritic cells (pDC) have emerged as a principal subset of dendritic cells in both human and mouse. PDC morphology, surface markers, their migration in vivo and the ability to rapidly produce large amounts of type I interferons (IFN-alpha/beta) in response to toll like receptor (TLR) triggering sets them apart from other dendritic cell subsets. This review highlights the features that make pDC uniquely able to sense and respond to viral infection.

Inactivated Sendai virus particles eradicate tumors by inducing immune responses through blocking regulatory T cells

UV-inactivated, replication-defective Sendai virus particles [hemagglutinating virus of Japan envelope (HVJ-E)] injected into murine colon carcinoma (CT26) tumors growing in syngeneic BALB/c mice eradicated 60% to 80% of the tumors and obviously inhibited the growth of the remainder. Induced adaptive antitumor immune responses were dominant in the tumor eradication process because the effect was abrogated in severe combined immunodeficient mice. Murine and human dendritic cells underwent dose-dependent maturation by HVJ-E in vitro. Profiles of cytokines secreted by dendritic cells after HVJ-E stimulation showed that the amount of interleukin-6 (IL-6) released was comparable to that elicited by live HVJ. Real-time reverse transcription-PCR and immunohistochemistry revealed that HVJ-E induced a remarkable infiltration of dendritic cells and CD4+ and CD8+ T cells into tumors. In addition, CT26-specific CTLs were induced with the evidence of enhanced CD8+ T-cell activation in a CD4+CD25- T cell-dependent manner. On the other hand, conditioned medium from dendritic cells stimulated by HVJ-E rescued CD4+CD25- effector T-cell proliferation from Foxp3+CD4+CD25+ regulatory T cell (Treg)-mediated suppression and IL-6 was presumably dominant for this phenomenon. We also confirmed such rescue in mice treated with HVJ-E in vivo. Moreover, antitumor effect of HVJ-E was significantly reduced by an in vivo blockade of IL-6 signaling. This is the first report to show that HVJ-E alone can eradicate tumors and the mechanism through which it induces antitumor immune responses. Because it can enhance antitumor immunity and simultaneously remove Treg-mediated suppression, HVJ-E shows promise as a novel therapeutic for cancer immunotherapy.

Characterization of myeloid and plasmacytoid dendritic cells in human lung

Dendritic cells (DCs) are bone marrow-derived mononuclear cells that play a central role in the initiation of immune responses. Because human lung DCs have been incompletely characterized, we enumerated and phenotyped mononuclear cell populations from excess lung tissue obtained at surgery. Myeloid DCs (MDCs) were identified as CD1c(+)CD11c(+)CD14(-)HLA-DR(+) cells and comprised approximately 2% of low autofluorescent (LAF) mononuclear cells. Plasmacytoid DCs (PDCs) were characterized as CD123(+)CD11c(-)CD14(-)HLA-DR(+) cells and comprised approximately 1.0% of the LAF mononuclear cells. Cells enriched in MDCs expressed CD86, moderate CD80, and little CD40, but cells enriched in PDCs had little to no expression of these three costimulatory molecules. CD11c(+)CD14(-) lineage-negative (MDC-enriched) LAF cells were isolated and shown to be much more potent in stimulating an alloreaction than CD11c(+)CD14(+) lineage-negative (monocyte-enriched) LAF cells. PDC-enriched cells were more capable of responding to a TLR-7 agonist by secreting IFN-alpha than MDC-enriched cells. MDC-enriched cells were either CD123(+) or CD123(-), but both subsets secreted cytokines and chemokines typical of MDC upon stimulation with a TLR-4 agonist and both subsets failed to secrete IFN-alpha upon stimulation with a TLR-7 agonist. By immunohistochemistry, we identified MDCs throughout different anatomical locations of the lung. However, our method did not allow the localization of PDCs with certainty. In conclusion, in the human lung MDCs were twice as numerous and expressed higher levels of costimulatory molecules than PDCs. Our data suggest that both lung DC subsets exert distinct immune modulatory functions.

High mobility group B1 protein suppresses the human plasmacytoid dendritic cell response to TLR9 agonists

Plasmacytoid dendritic cells (PDC) are innate immune effector cells that are recruited to sites of chronic inflammation, where they modify the quality and nature of the adaptive immune response. PDCs modulate adaptive immunity in response to signals delivered within the local inflammatory milieu by pathogen- or damage-associated molecular pattern, molecules, and activated immune cells (including NK, T, and myeloid dendritic cells). High mobility group B1 (HMGB1) is a recently identified damage-associated molecular pattern that is released during necrotic cell death and also secreted from activated macrophages, NK cells, and mature myeloid dendritic cells. We have investigated the effect of HMGB1 on the function of PDCs. In this study, we demonstrate that HMGB1 suppresses PDC cytokine secretion and maturation in response to TLR9 agonists including the hypomethylated oligodeoxynucleotide CpG- and DNA-containing viruses. HMGB1-inhibited secretion of several proinflammatory cytokines including IFN-alpha, IL-6, TNF-alpha, inducible protein-10, and IL-12. In addition, HMGB1 prevented the CpG induced up-regulation of costimulatory molecules on the surface of PDC and potently suppressed their ability to drive generation of IFN-gamma-secreting T cells. Our observations suggest that HMGB1 may play a critical role in regulating the immune response during chronic inflammation and tissue damage through modulation of PDC function.

Plasmacytoid dendritic cells prime IL-10-producing T regulatory cells by inducible costimulator ligand

Although there is evidence for distinct roles of myeloid dendritic cells (DCs [mDCs]) and plasmacytoid pre-DCs (pDCs) in regulating T cell–mediated adaptive immunity, the concept of functional DC subsets has been questioned because of the lack of a molecular mechanism to explain these differences. In this study, we provide direct evidence that maturing mDCs and pDCs express different sets of molecules for T cell priming. Although both maturing mDCs and pDCs upregulate the expression of CD80 and CD86, only pDCs upregulate the expression of inducible costimulator ligand (ICOS-L) and maintain high expression levels upon differentiation into mature DCs. High ICOS-L expression endows maturing pDCs with the ability to induce the differentiation of naive CD4 T cells to produce interleukin-10 (IL-10) but not the T helper (Th)2 cytokines IL-4, -5, and -13. These IL-10–producing T cells are T regulatory cells, and their generation by ICOS-L is independent of pDC-driven Th1 and Th2 differentiation, although, in the later condition, some contribution from endogenous IL-4 cannot be completely ruled out. Thus, in contrast to mDCs, pDCs are poised to express ICOS-L upon maturation, which leads to the generation of IL-10–producing T regulatory cells. Our findings demonstrate that mDC and pDCs are intrinsically different in the expression of costimulatory molecules that drive distinct types of T cell responses.

Needleless intranasal administration of HVJ-E containing allergen attenuates experimental allergic rhinitis

Allergic rhinitis (AR) is one of the most common chronic diseases. Although current medications are highly effective in controlling its symptoms, they do not reverse the allergen-specific hypersensitivities that underlie the disease. Immunoglobulin E is a key mediator of AR, and preventing its production is clinically important. In this study, we developed an efficient needleless intranasal protein delivery system using the hemagglutinating virus of Japan envelope vector (HVJ-E). Intranasal delivery of ovalbumin (OVA) once a week for 3 weeks using this system enhanced OVA-induced interferon-gamma production by murine splenocytes. This treatment also attenuated the OVA-induced release interleukin-4 (IL-4) and IL-5 from splenocytes and the production of plasma OVA-specific immunoglobulin E in OVA-sensitive AR model mice. Thus, allergen-containing HVJ-E may be useful for noninvasive treatment of AR.

Approaches to studying costimulation of human antiviral T cell responses: prospects for immunotherapeutic vaccines

The generation of strong and specific CD8 T cell responses is important in the control of viral infections. Costimulatory molecules provide signals necessary for the development or maintenance of these responses. A major focus of our laboratory is to investigate the role of costimulatory molecules of the TNFR and CD28 families in antiviral responses. Our aim is to translate information obtained using murine models to the study of these molecules using human cells. We have devised an in vitro system using recombinant replication- deficient adenovirus to deliver costimulatory molecules to antigen-presenting cells that are then used to stimulate autologous T cells from both healthy and HIV-infected individuals. Here we describe our findings and discuss the implications of incorporating costimulatory molecules into viral vector vaccine strategies.

Fundamental mechanisms of host immune responses to infection

Chronic inflammatory periodontal disease is caused by host immune responses to periodontal microorganisms. Although periodontal disease seems to progress locally in the oral cavity, the actual immune responses proceed at two sites, one being the local area peripheral to the periodontium, and the other being in secondary lymphoid tissues, such as lymph nodes and the spleen. To investigate the pathogenesis of periodontal diseases, it is essential to understand exactly how the immune system works against microbial infections. The past decade has produced remarkable advances in our understanding of host immune responses. This review highlights two of these developments: cross-talk between innate and adaptive immunity mediated by dendritic cells via toll-like receptors; and antigen-specific immune regulation by dendritic cells and T cells.

Activation of plasmacytoid dendritic cells with TLR9 agonists initiates invariant NKT cell-mediated cross-talk with myeloid dendritic cells

CD1d-restricted invariant NK T (iNKT) cells and dendritic cells (DCs) have been shown to play crucial roles in various types of immune responses, including TLR9-dependent antiviral responses initiated by plasmacytoid DCs (pDCs). However, the mechanism by which this occurs is enigmatic because TLRs are absent in iNKT cells and human pDCs do not express CD1d. To explore this process, pDCs were activated with CpG oligodeoxyribonucleotides, which stimulated the secretion of several cytokines such as type I and TNF-alpha. These cytokines and other soluble factors potently induced the expression of activation markers on iNKT cells, selectively enhanced double-negative iNKT cell survival, but did not induce their expansion or production of cytokines. Notably, pDC-derived factors licensed iNKT cells to respond to myeloid DCs: an important downstream cellular target of iNKT cell effector function and a critical contributor to the initiation of adaptive immune responses. This interaction supports the notion that iNKT cells can mediate cross-talk between DC subsets known to express mutually exclusive TLR and cytokine profiles.

Ikaros is required for plasmacytoid dendritic cell differentiation

Plasmacytoid dendritic cells (pDCs) are specialized DCs that produce high levels of type I IFN upon viral infection. Despite their key immunoregulatory role, little is known about pDC ontogeny or how developmental events regulate their function. We show that mice expressing low levels of the transcription factor Ikaros (Ik(L/L)) lack peripheral pDCs, but not other DC subsets. Loss of pDCs is associated with an inability to produce type I IFN after challenge with Toll-like receptor-7 and -9 ligands, or murine cytomegalovirus (MCMV) infection. In contrast, conventional DCs are present in normal numbers and exhibit normal responses in vivo after challenge with MCMV or inactivated toxoplasma antigen. Interestingly, Ik(L/L) bone marrow (BM) cells contain a pDC population that appears blocked at the Ly-49Q- stage of differentiation and fails to terminally differentiate in response to Flt-3L, a cytokine required for pDC differentiation. This differentiation block is strictly dependent on a cell-intrinsic requirement for Ikaros in pDC-committed precursors. Global gene expression profiling of Ik(L/L) BM pDCs reveals an up-regulation of genes not normally expressed, or expressed at low levels, in WT pDCs. These studies suggest that Ikaros controls pDC differentiation by silencing a large array of genes.

Plasmacytoid dendritic cells: in search of their niche in immune responses

Plasmacytoid dendritic cells (pDCs) have been associated with several names and functions over time, reflecting the limited availability of specific markers and their ability to produce large amounts of type I interferons, present antigens, as well as prime disparate T-cell helper responses. Yet, there is increasing evidence that pDCs are a distinct cell type in the innate immune system. This review highlights aspects in which pDCs are unique in comparison to other antigen-presenting cells in regulating innate and adaptive immune responses.

Eminent role of ICOS costimulation for T cells interacting with plasmacytoid dendritic cells

CD4+ CD45RO+ T cells could mature freshly isolated human plasmacytoid dendritic cells (PDC) in a superantigen-driven culture in a similar way to recombinant interleukin-3 (IL-3). Mature PDC expressed significantly higher levels of inducible costimulator ligand (ICOS-L), but similar levels of CD80 and CD86, when compared to mature monocyte-derived DC (moDC). We therefore directly compared the capacities of mature PDC and moDC to activate T cells. A similar T helper type 1 (Th1)/Th2 pattern of cytokines was generated in both systems, but significantly higher levels of IL-3, IL-4 and IL-10 were induced by PDC. In T cells interacting with PDC, the ICOS/ICOS-L costimulatory pathway played a pre-eminent role in the generation of IL-3 and IL-10, CD28 was central to the induction of IL-2, and both pathways were equally important for the generation of other cytokines. In cocultures with moDC, the CD28 pathway was dominant over ICOS under all circumstances, except for the ICOS-mediated release of IL-10. In general, our data demonstrate an eminent role of ICOS in the interaction of T cells with PDC, and thus modify the current paradigm of CD28 dominance for the costimulation of T cells interacting with professional antigen-presenting cells. In particular, our data highlight the role of ICOS in the generation of IL-3, a factor central to the biology of human PDC.

A network-based analysis of allergen-challenged CD4+ T cells from patients with allergic rhinitis

We performed a network-based analysis of DNA microarray data from allergen-challenged CD4(+) T cells from patients with seasonal allergic rhinitis. Differentially expressed genes were organized into a functionally annotated network using the Ingenuity Knowledge Database, which is based on manual review of more than 200,000 publications. The main function of this network is the regulation of lymphocyte apoptosis, a role associated with several genes of the tuber necrosis factor superfamily. The expression of TNFRSF4, one of the genes in this family, was found to be 48 times higher in allergen-challenged cells than in diluent-challenged cells. TNFRSF4 is known to inhibit apoptosis and to enhance Th2 proliferation. Examination of a different material of allergen-stimulated peripheral blood mononuclear cells showed a higher number of interleukin-4(+) type 2 CD4(+) T (Th2) cells in patients than in controls (P<0.01), as well as a higher number of non-apoptotic Th2 cells in patients (P<0.01). The number of Th2 cells expressing TNFRSF4, TNFSF7 and TNFRSF1B was also significantly higher in patients. Treatment with anti-TNFSF4 resulted in a significantly decreased number of Th2 cells (P<0.05). A logical inference from all this is that the proliferation of allergen-challenged Th2 cells is associated with a decreased apoptosis of Th2 cells and an increase in TNFRSF4 signalling.

Roles of OX40 in the pathogenesis and the control of diseases

OX40 belongs to the tumor necrosis factor receptor superfamily, and its expression is restricted to activated T-cells. Ligation of OX40 during T-cell-dendritic cell interaction is crucial for clonal expansion of antigen-specific T-cells and generation of T-cell memory. The ligand of OX40 (OX40L) is expressed not only on dendritic cells but also on other cell types, such as B-cells, vascular endothelial cells, natural killer cells, and mast cells. The pathophysiological relevance of this broad distribution needs further investigation. In particular, OX40L on vascular endothelial cells may play a role in inflammatory vasculitis as well as in atherosclerotic change. Recent studies with animal models have indicated the critical involvement of OX40 in the pathogenesis of a variety of immunologic abnormalities of inflammatory, autoimmune, infectious, allergic, and allotransplantation-related diseases. Blockade of OX40-OX40L interaction has been shown to prevent, cure, or ameliorate these diseases. In contrast, activation of OX40 is known to break an existing state of tolerance in malignancies, leading to a reactivation of antitumor immunity. These findings clearly suggest that the OX40/OX40L system is one of the most promising targets of immune intervention for treatment of these diseases.

Aberrant composition of the dendritic cell population in hepatic lymph nodes of patients with hepatocellular carcinoma

Patients with hepatocellular carcinoma (HCC) are characterized by a weak T-cell response to their tumor, and chronic carriers of hepatitis B virus or hepatitis C virus have a poor T-cell response against the virus. These inadequate T-cell responses may be due to insufficient activation of the T cells by dendritic cells (DCs). Because lymph nodes (LNs) are the primary site of antigen-specific T-cell activation, we hypothesized that hepatic LNs of patients with HCC and/or chronic viral hepatitis might have aberrant compositions of their DC populations. To address this hypothesis, we enumerated mature myeloid DCs (MDCs) and plasmacytoid DCs (PDCs) in hepatic LNs by quantitative immunohistochemistry. Patients with HCC and chronic viral hepatitis and patients with chronic viral hepatitis without HCC were compared with patients with liver inflammation of nonviral etiology and with organ donors with healthy livers. The numbers of PDCs and mature MDCs in hepatic LNs of patients with chronic viral hepatitis did not differ from those of patients with liver inflammation of nonviral etiology nor from individuals with healthy livers. However, hepatic LNs of patients with HBV or HCV infection complicated by HCC showed a 1.5-fold reduction in numbers of mature MDCs and a 4-fold increase in numbers of PDCs in their T-cell areas compared with those of patients with viral hepatitis only (P <.01). In conclusion, patients with HCC have an aberrant composition of the DC population in their hepatic LNs. This may be one of the causes of the inadequate T-cell response against HCC in these patients.

Targeted chemotherapy against intraperitoneally disseminated colon carcinoma using a cationized gelatin–conjugated HVJ envelope vector

The hemagglutinating virus of Japan envelope (HVJ-E; Sendai virus) vector derived from inactivated HVJ particles can be used to deliver DNA, proteins, and drugs into cells both in vitro and in vivo. HVJ-E is capable of delivering bleomycin, an anticancer drug, to various cancer cell lines, thereby producing 300-fold greater cytotoxicity than administration of bleomycin alone. In a mouse model of peritoneally disseminated colon cancer, we injected HVJ-E containing the luciferase gene into the peritoneum. Unexpectedly, luciferase gene expression was not observed within the tumor deposits or any organs. However, when combined with cationized gelatin (CG), CG-HVJ-E produced a high level of luciferase gene expression primarily within the tumor deposits. Forty-eight hours after introducing colon cancer cells into the peritoneum of experimental mice, CG-HVJ-E with or without bleomycin was injected into the abdominal cavity. Following six injections of bleomycin-incorporated CG-HVJ-E, complete responses were observed in 40% of the mice examined. All of the mice that received either empty CG-HVJ-E or bleomycin alone died within 40 days of having cancer cells introduced into the peritoneum. When the mice with complete responses were rechallenged with colon cancer cells from the same cell line, no tumors developed. Thus, CG-HVJ-E may suppress peritoneal dissemination of cancer.

In vitro-Generated Viral Double-Stranded RNA in Contrast to Polyinosinic : Polycytidylic Acid Induces Interferon-alpha in Human Plasmacytoid Dendritic Cells

Double-stranded RNA (dsRNA) arises in the cytoplasm during viral replication and was shown to participate in the interferon (IFN)-alpha induction process. Besides the intracellular recognition, released dsRNA from dying, infected cells can function as a pathogen-associated molecular pattern (PAMP) for the innate immune system. In the present study, in vitro-generated dsRNA fragments of genomic sequences of Newcastle disease virus were used to induce IFN-alpha release in human peripheral blood mononuclear cells (PBMC), in immature myeloid dendritic cells (mDC) and in immature plasmacytoid DC (pDC). The extracellular administration of dsRNA fragments but not the application of the corresponding single-stranded RNA (ssRNA) strands led to an IFN-alpha production in PBMC. The synthetic dsRNA analogue polyinosinic acid : polycytidylic acid [Poly(I : C)] could only stimulate IFN-alpha production in enriched mDC but not in pDC. In contrast, dsRNA fragments induced IFN-alpha only in pDC. Complexation of dsRNA fragments with transfection reagents increased the efficiency of IFN-alpha induction and commuted ssRNA molecules into IFN-alpha inducers. However, stimulation of in vitro-generated murine Toll-like receptor 7 (TLR7) knockout DC and human TLR-transfected HEK293 cells with dsRNA fragments gave no evidences for the involvement of pDC-specific TLR7 or TLR9 in the observed IFN-alpha induction.

Renal transplantation reverses functional deficiencies in circulating dendritic cell subsets in chronic renal failure patients

BACKGROUND

Dendritic cell (DC) subsets play critical roles in regulation of innate and adaptive immune responses. These important antigen-presenting cells have not been extensively analyzed in chronic renal failure (CRF), during dialysis, or before and after renal transplantation.

METHODS

The incidence of circulating precursor (pre)-DC subsets relative to total peripheral blood mononuclear cells was analyzed in healthy controls, haemodialysis patients, peritoneal dialysis patients, CRF patients, and renal transplant (RT) recipients. DC subsets were identified and characterized phenotypically by multicolour flow cytometric analysis and purified by immunomagnetic bead isolation respectively. Cytokine production and circulating DC mobilizing cytokines were determined by ELISA.

RESULTS

The incidence of circulating prePDC was reduced in all patients, but the incidence of circulating preMDC was comparable in RT and dialysis patients compared to healthy controls. CRF patients exhibited the lowest incidence of circulating preMDC and prePDC. Immunomagnetic bead-isolated preMDC and prePDC from haemodialysis patients were functionally impaired (reduced expression of surface costimulatory molecules and interleukin-12p70 production following bacterial lipopolysaccharide stimulation, and reduced interferon-alpha production following herpes simplex virus stimulation respectively, compared to healthy controls and RT recipients. Glomerular filtration rate correlated significantly with the incidence of circulating preMDC, but not prePDC.

CONCLUSIONS

Deficiencies in the incidence and function of precursor DC can be reversed with successful renal transplantation achieving normal renal function. However, the finding of reduced incidence of circulating prePDC in the peripheral blood in RT recipients may be of significance in the pathogenesis of infections and malignancies.

The glucocorticoid-induced tumor necrosis factor receptor-related gene modulates the response to Candida albicans infection

The glucocorticoid-induced tumor necrosis factor (TNF) receptor-related gene (GITR; TNFRSF18) modulates immune response activating coaccessory signals in T cells and is expressed at high levels in CD4+CD25+ cells. Its ligand (GITRL) is expressed in antigen-presenting cells, where it is capable of promoting signaling. We investigated the role of GITR/GITRL interaction during disseminated candidiasis in GITR knockout (GITR-/-) mice. GITR-/- mice survived longer and had a significantly decreased yeast load in kidneys and brain compared to GITR+/+ mice. Since protective immunity to the fungus is mediated by antigen-specific T helper (Th) 1 cells, we studied in vitro cytokine production following infection. CD4+ T cells of GITR-/- mice demonstrated a more efficient Th1 polarization as suggested by a two- to threefold decreased production of interleukin- (IL-)4 and IL-10 and a four- to fivefold increased production of gamma interferon compared to GITR+/+ mice. This effect was not due to differences in lymphocyte and dendritic cell (DC) subpopulations in infected mice as demonstrated by flow cytometric studies. To verify whether DC activity was differently modulated, DCs were cocultured with CD4+ T cells in the presence of heat-inactivated Candida albicans. DCs, cocultured with GITR+/+ CD4+CD25+ cells produced a lower amount of IL-12 than DCs cocultured with GITR-/- CD4+CD25+ T cells. These results suggest that GITR regulates susceptibility to systemic candidiasis by negatively modulating IL-12 production and promoting polarization of CD4+ T cells towards Th2 by analogy with OX40, another TNF receptor superfamily member.

CpG ODN enhance antigen-specific NKT cell activation via plasmacytoid dendritic cells

Human Valpha24+ Vbeta11+ natural killer T cells (NKT cells) are "natural memory" T cells that detect glycolipid antigens such as alpha-galactosylceramide (alpha-GalCer) presented on CD1d. In the present study we found that highly purified Valpha24+ NKT cells lack TLR9 mRNA, and thus are not sensitive towards stimulation with CpG oligodeoxynucleotides (ODN). Within PBMC, however, CpG ODN synergistically activated NKT cells stimulated with their cognate antigen alpha-GalCer. Depletion of plasmacytoid dendritic cells (PDC) or myeloid dendritic cells (MDC) revealed that both DC subsets were necessary for the synergistic activation of NKT cells by alpha-GalCer and CpG ODN. While PDC were responsible for the stimulation of NKT cells with CpG ODN, MDC but not PDC presented alpha-GalCer via CD1d. Partial activation of NKT cells was mediated by PDC-derived IFN-alpha, whereas full activation of NKT cells as indicated by IFN-gamma production required cell-to-cell contact of PDC and NKT cells in addition to IFN-alpha; OX40 was involved in this interaction. We conclude that CpG-activated PDC enhance alpha-GalCer-specific NKT cell activation, and bias activated NKT cells towards a Th1 phenotype. Our results lead to a novel concept of PDC function: to regulate effector activity of antigen-stimulated T cells in a cell contact-dependent manner without the need of simultaneous presentation of the cognate T cell antigen.

Nitric oxide inhibits IFN-alpha production of human plasmacytoid dendritic cells partly via a guanosine 3',5'-cyclic monophosphate-dependent pathway

NO, a free radical gas, is known to be critically involved not only in vascular relaxation but also in host defense. Besides direct bactericidal effects, NO has been shown to inhibit Th1 responses and modulate immune responses in vivo, although the precise mechanism is unclear. In this study, we examined the effect of NO on human plasmacytoid dendritic cells (pDCs) to explore the possibility that NO might affect innate as well as adaptive immunity through pDCs. We found that NO suppressed IFN-alpha production of pDCs partly via a cGMP-dependent mechanism, which was accompanied by down-regulation of IFN regulatory factor 7 expression. Furthermore, treatment of pDCs with NO decreased production of IL-6 and TNF-alpha and up-regulated OX40 ligand expression. In accordance with these changes, pDCs treated with NO plus CpG-oligodeoxynucleotide AAC-30 promoted differentiation of naive CD4(+) T cells into a Th2 phenotype. Moreover, pDCs did not express inducible NO synthase even after treatment with AAC-30, LPS, and several cytokines. These results suggest that exogenous NO and its second messenger, cGMP, alter innate as well as adaptive immune response through modulating the functions of pDCs and may be involved in the pathogenesis of certain Th2-dominant allergic diseases.

Virus overrides the propensity of human CD40L-activated plasmacytoid dendritic cells to produce Th2 mediators through synergistic induction of IFN-{gamma} and Th1 chemokine production

Depending on the activation status, plasmacytoid dendritic cells (PDC) and myeloid DC have the ability to induce CD4 T cell development toward T helper cell type 1 (Th1) or Th2 pathways. Thus, we tested whether different activation signals could also have an impact on the profile of chemokines produced by human PDC. Signals that induce human PDC to promote a type 1 response (i.e., viruses) and a type 2 response [i.e., CD40 ligand (CD40L)] also induced PDC isolated from tonsils to secrete chemokines preferentially attracting Th1 cells [such as interferon-gamma (IFN-gamma)-inducible protein (IP)-10/CXC chemokine ligand 10 (CXCL10) and macrophage inflammatory protein-1beta/CC chemokine ligand 4 (CCL4)] or Th2 cells (such as thymus and activation-regulated chemokine/CCL17 and monocyte-derived chemokine/CCL22), respectively. Activated natural killer cells were preferentially recruited by supernatants of virus-activated PDC, and supernatants of CD40L-activated PDC attracted memory CD4(+) T cells, particularly the CD4(+)CD45RO(+)CD25(+) T cells described for their regulatory activities. It is striking that CD40L and virus synergized to trigger the production of IFN-gamma by PDC, which induces another Th1-attracting chemokine monokine-induced by IFN-gamma/CXCL9 and cooperates with endogenous type I IFN for IP-10/CXCL10 production. In conclusion, our studies reveal that PDC participate in the selective recruitment of effector cells of innate and adaptive immune responses and that virus converts the CD40L-induced Th2 chemokine patterns of PDC into a potent Th1 mediator profile through an autocrine loop of IFN-gamma.

IPC: professional type 1 interferon-producing cells and plasmacytoid dendritic cell precursors

Type 1 interferon-(alpha, beta, omega)-producing cells (IPCs), also known as plasmacytoid dendritic cell precursors (pDCs), represent 0.2%-0.8% of peripheral blood mononuclear cells in both humans and mice. IPCs display plasma cell morphology, selectively express Toll-like receptor (TLR)-7 and TLR9, and are specialized in rapidly secreting massive amounts of type 1 interferon following viral stimulation. IPCs can promote the function of natural killer cells, B cells, T cells, and myeloid DCs through type 1 interferons during an antiviral immune response. At a later stage of viral infection, IPCs differentiate into a unique type of mature dendritic cell, which directly regulates the function of T cells and thus links innate and adaptive immune responses. After more than two decades of effort by researchers, IPCs finally claim their place in the hematopoietic chart as the most important cell type in antiviral innate immunity. Understanding IPC biology holds future promise for developing cures for infectious diseases, cancer, and autoimmune diseases.

TNF/TNFR family members in costimulation of T cell responses

Several members of the tumor necrosis factor receptor (TNFR) family function after initial T cell activation to sustain T cell responses. This review focuses on CD27, 4-1BB (CD137), OX40 (CD134), HVEM, CD30, and GITR, all of which can have costimulatory effects on T cells. The effects of these costimulatory TNFR family members can often be functionally, temporally, or spatially segregated from those of CD28 and from each other. The sequential and transient regulation of T cell activation/survival signals by different costimulators may function to allow longevity of the response while maintaining tight control of T cell survival. Depending on the disease condition, stimulation via costimulatory TNF family members can exacerbate or ameliorate disease. Despite these complexities, stimulation or blockade of TNFR family costimulators shows promise for several therapeutic applications, including cancer, infectious disease, transplantation, and autoimmunity.

Inhibition of toll-like receptor 7- and 9-mediated alpha/beta interferon production in human plasmacytoid dendritic cells by respiratory syncytial virus and measles virus

Human plasmacytoid dendritic cells (PDC) are key sentinels alerting both innate and adaptive immune responses through production of huge amounts of alpha/beta interferon (IFN). IFN induction in PDC is triggered by outside-in signal transduction pathways through Toll-like receptor 7 (TLR7) and TLR9 as well as by recognition of cytosolic virus-specific patterns. TLR7 and TLR9 ligands include single-stranded RNA and CpG-rich DNA, respectively, as well as synthetic derivatives thereof which are being evaluated as therapeutic immune modulators promoting Th1 immune responses. Here, we identify the first viruses able to block IFN production by PDC. Both TLR-dependent and -independent IFN responses are abolished in human PDC infected with clinical isolates of respiratory syncytial virus (RSV), RSV strain A2, and measles virus Schwarz, in contrast to RSV strain Long, which we previously identified as a potent IFN inducer in human PDC (Hornung et al., J. Immunol. 173:5935-5943, 2004). Notably, IFN synthesis of PDC activated by the TLR7 and TLR9 agonists resiquimod (R848) and CpG oligodeoxynucleotide 2216 is switched off by subsequent infection by RSV A2 and measles virus. The capacity of RSV and measles virus of human PDC to shut down IFN production should contribute to the characteristic features of these viruses, such as Th2-biased immune pathology, immune suppression, and superinfection.

The importance of being a pDC in antiviral immunity: the IFN mission versus Ag presentation?

Plasmacytoid pre-dendritic cells (pDCs) comprise a pivotal element of antiviral immune responses. They recognize viral components, leading to type I interferon (IFN) production, and affect adaptive defense strategies designed to eliminate viral pathogens. These strategies include the ability of pDCs to modulate virus-specific CD8(+) T-cell responses. Although a great deal has been learned recently about pDCs, our knowledge of whether, how and why pDCs might function as antigen-presenting cells is extremely limited, and now is a prime time for exploring the unknowns of this area. This Opinion will focus on the IFN production and T-cell priming capacity of pDCs, and will argue that IFN production (and not T-cell priming) is the main function of pDCs during viral infection.

Pathogen-induced private conversations between natural killer and dendritic cells

Natural killer (NK) cells and dendritic cells (DCs) are recruited to inflammatory tissues in response to infection. Following priming by pathogen-derived products, their reciprocal interactions result in a potent activating crosstalk that regulates both the quality and the intensity of innate immune responses. Thus, pathogen-primed NK cells, in the presence of cytokines released by DCs, become activated. At this stage they favor DC maturation and also select the most suitable DCs for subsequent migration to lymph nodes and priming of T cells. In addition, a specialized subset of NK cells might directly participate in the process of T-cell priming via the release of interferon (IFN)gamma. Thus, the reciprocal crosstalk between NK cells and DCs that is induced by microbial products not only promotes rapid innate responses against pathogens but also favor the generation of appropriate downstream adaptive responses.

The role of type I interferons in non-viral infections

For a long time, the family of type I interferons (IFN-alpha/beta) has received little attention outside the fields of virology and tumor immunology. In recent years, IFN-alpha/beta regained the interest of immunologists, due to the phenotypic and functional characterization of IFN-alpha/beta-producing cells, the definition of novel immunomodulatory functions and signaling pathways of IFN-alpha/beta, and the observation that IFN-alpha/beta not only exerts antiviral effects but is also relevant for the pathogenesis or control of certain bacterial and protozoan infections. This review summarizes the current knowledge on the production and function of IFN-alpha/beta during non-viral infections in vitro and in vivo.

Functional Diversity and Plasticity of Human Dendritic Cell Subsets

The induction of different types of innate and adaptive immune responses, depending on the nature of the antigens and the environmental context, is crucial to cope with a variety of pathogens and concurrently to avoid pathologic reaction to self antigens. Recent studies have elucidated that the diversity of immune responses is critically controlled by dendritic cells (DCs). Two DC subsets, myeloid DCs and plasmacytoid DCs, have been identified in humans. The DC subsets recognize different microbial pathogens by expressing distinct repertoires of Toll-like receptors and induce different types of innate and adaptive immune responses, depending on the environmental factors. In particular, plasmacytoid DC precursors produce vast amounts of type I interferons in response to viruses and thus play an important role in antiviral immunity. Elucidating the cellular and molecular mechanisms that modulate the functions of the 2 DC subsets will lead to an understanding of the pathogenesis of various immune-related diseases and to the development of novel immunologic therapies.

IL-12 or IL-4 Prime Human NK Cells to Mediate Functionally Divergent Interactions with Dendritic Cells or Tumors

In the course of inflammatory responses in peripheral tissues, NK cells may be exposed to cytokines such as IL-12 and IL-4 released by other cell types that may influence their functional activities. In the present study we comparatively analyzed purified human peripheral blood NK cells that had been exposed to either IL-12 or IL-4 during short (overnight) incubation. We show that although IL-12-cultured NK cells produced abundant IFN-gamma, TNF-alpha, and GM-CSF in response to stimuli acting on the NKp46-activating receptor, IL-4-cultured NK cells did not release detectable levels of these cytokines. In contrast, IL-4-cultured NK cells produced significant levels of TNF-alpha and GM-CSF only when stimulated with PMA and ionomycin. In no instance could the production of IL-5 and IL-13 be detected. Importantly, IL-12-cultured, but not IL-4-cultured, NK cells displayed strong cytolytic activity against various tumor cells or immature dendritic cells (DCs). Moreover, only NK cells that had been cultured in IL-12 were able to induce substantial DC maturation. Our data suggest that NK cells exposed to IL-12 for a time interval compatible with in vivo responses may favor the selection of appropriate mature DCs for subsequent Th1 cell priming in secondary lymphoid organs. On the contrary, NK cells exposed to IL-4 do not exert DC selection, may impair efficient Th1 priming, and favor either tolerogenic or Th2-type responses.

Plasmacytoid dendritic cells in immunity

Human and mouse plasmacytoid dendritic cells have been shown to correspond to a specialized cell population that produces large amounts of type I interferons in response to viruses, the so-called natural interferon-producing cells. As a result, intensive investigation is now focused on the potential functions of plasmacytoid dendritic cells in both innate and adaptive immunity. Here we review recent progress on the characterization of plasmacytoid dendritic cell origin, development, migration and function in immunity and tolerance, as well as their effect on human diseases.

Plasmacytoid dendritic cell precursors/type I interferon-producing cells sense viral infection by Toll-like receptor (TLR) 7 and TLR9

Plasmacytoid dendritic cell (pDC) precursors, also called type I IFN (alpha/beta/omega)-producing cells (IPCs), are the key effectors in the innate immune system because of their extraordinary capacity to produce type I IFNs against microbial infection, particularly viral infection. In contrast to myeloid DCs, human pDC/IPCs selectively express Toll-like receptor (TLR) 7 and TLR9 within the endosomal compartment. These receptors are specifically designed to recognize the nucleoside-based products derived from RNA viruses and DNA viruses. Therefore, this expression profile potentially enables pDC/IPCs to sense a variety of viruses. Stimulation of TLR7 or TLR9 leads to type I IFN responses through the MyD88 pathway. Thus, pDC/IPCs may play a central role in host defense against viral infection through the TLR7 and TLR9 system.