Cutting edge: progesterone regulates IFN-alpha production by plasmacytoid dendritic cells

Use of the progesterone (Pg) birth control depot medroxyprogesterone acetate (DMPA) increases a woman's risk for sexually transmitted infection with HIV or HSV-2 via unknown mechanisms. Plasmacytoid dendritic cells (pDCs) are circulating and tissue-resident sentinels capable of making large quantities of IFN-alpha upon recognizing viruses through TLRs 7 and 9. In this study, we show that Pg inhibits TLR9-induced IFN-alpha production by human and mouse pDCs and that DMPA impairs TLR9- and virus-induced IFN-alpha production by pDCs in mice, providing a potential explanation for how DMPA impairs innate antiviral immunity in women. Pg failed to inhibit the Mda-5 pathway of IFN-alpha induction in dendritic cells, suggesting that Pg regulates select antiviral DC programs. This may occur through selective blockade of IFN regulatory factor-7 activation, a novel steroid action. Thus, through inhibition of TLR-mediated IFN-alpha production by pDCs, Pg may regulate antiviral immunity.     

Cutting edge: Expression of functional CD137 receptor by dendritic cells

Interaction between dendritic cells (DCs) and T cells is a prerequisite for the initiation of a T cell response. The molecular nature of this interaction remains to be fully characterized. We report in this work that freshly isolated mouse splenic DCs and bone marrow-derived DCs express CD137 on the cell surface and in soluble form. Triggering CD137 increased the secretion of IL-6 and IL-12 from DCs. More importantly, infusion of an agonistic mAb to CD137 into naive mice enhanced the ability of DCs to stimulate T cell proliferation in response to both alloantigens and a nominal Ag in vitro. This enhancement of DC function is not mediated through activation of T cells, because the effect was also observed in RAG-1 knockout mice that lack T cells. Our findings implicate CD137 as an important receptor involved in the modulation of DC function.     

Cutting edge: selective usage of chemokine receptors by plasmacytoid dendritic cells

The existence of dendritic cell (DC) subsets is firmly established, but their trafficking properties are virtually unknown. In this study, we show that myeloid (M-DCs) and plasmacytoid (P-DCs) DCs isolated from human blood differ widely in the capacity to migrate to chemotactic stimuli. The pattern of chemokine receptors expressed by blood M-DCs and P-DCs, with the exception of CCR7, is similar. However, most chemokine receptors of P-DCs, in particular those specific for inflammatory chemokines and classical chemotactic agonists, are not functional in circulating cells. Following maturation induced by CD40 ligation, the receptors for inflammatory chemokines are down-regulated, and CCR7 on P-DCs becomes coupled to migration. The drastically impaired capacity of blood P-DCs to migrate in response to inflammatory chemotactic signals contrasts with the response to lymph node-homing chemokines, indicating a propensity to migrate to secondary lymphoid organs rather than to sites of inflammation.     

Cutting edge: differential chemokine production by myeloid and plasmacytoid dendritic cells

To examine the different roles of myeloid dendritic cells (M-DCs) and plasmacytoid dendritic cells (P-DCs) in the induction and regulation of immune response, we have studied chemokine secretion by freshly isolated DC subsets in response to bacterial, viral, and T cell-derived stimuli. M-DCs selectively produced very high levels of the homeostatic chemokines CC chemokine ligand (CCL)17 and CCL22, while P-DCs produced very little if any. In contrast, the proinflammatory chemokine CCL3 was secreted mostly by P-DCs, whereas CCL4 and CXC chemokine ligand 8 were produced by both subsets. The selective production of CCL17 and CCL22 by M-DCs but not P-DCs was confirmed in vivo by immunohistology on human reactive lymph node sections. The high production of CCR4 ligands by M-DCs suggests their capacity to selectively recruit at sites of inflammation T cells with regulatory properties or with a Th2 phenotype, whereas P-DCs, by preferentially secreting CCR1/CCR5 ligands, would mostly recruit effector T cells and, in particular, Th1-type cells.     

Cutting edge: Autocrine TGF-beta sustains default tolerogenesis by IDO-competent dendritic cells

CD8(-) and CD8(+) dendritic cells (DCs) are distinct subsets of mouse splenic accessory cells with opposite but flexible programs of Ag presentation, leading to immunogenic and tolerogenic responses, respectively. In this study, we show that the default tolerogenic function of CD8(+) DCs relies on autocrine TGF-beta, which sustains the activation of IDO in response to environmental stimuli. CD8(-) DCs do not produce TGF-beta, yet externally added TGF-beta induces IDO and turns those cells from immunogenic into tolerogenic cells. The acquisition of a suppressive phenotype by CD8(-) DCs correlates with activation of the PI3K/Akt and noncanonical NF-kappaB pathways. These data are the first to link TGF-beta signaling with IDO in controlling spontaneous tolerogenesis by DCs.     

Cutting edge: proangiogenic properties of alternatively activated dendritic cells

Angiogenesis plays an important role in tissue remodeling and repair during the late phase of inflammation. In the present study, we show that human dendritic cells (DC) that matured in the presence of anti-inflammatory molecules such as calcitriol, PGE2, or IL-10 (alternatively activated DC) selectively secrete the potent angiogenic cytokine vascular endothelial growth factor (VEGF) isoforms VEGF165 and VEGF121. No VEGF production was observed in immature or classically activated DC. Also, the capacity to produce VEGF was restricted to the myeloid DC subset. When implanted in the chick embryo chorioallantoic membrane, alternatively activated DC elicit a marked angiogenic response, which is inhibited by neutralizing anti-VEGF Abs and by the VEGFR-2 inhibitor SU5416. Therefore, alternatively activated DC may contribute to the resolution of the inflammatory reaction by promoting VEGF-induced angiogenesis.     

Cutting edge: Clec9A+ dendritic cells mediate the development of experimental cerebral malaria

Plasmodium infections trigger strong innate and acquired immune responses, which can lead to severe complications, including the most feared and often fatal cerebral malaria (CM). To begin to dissect the roles of different dendritic cell (DC) subsets in Plasmodium-induced pathology, we have generated a transgenic strain, Clec9A-diphtheria toxin receptor that allows us to ablate in vivo Clec9A(+) DCs. Specifically, we have analyzed the in vivo contribution of this DC subset in an experimental CM model using Plasmodium berghei, and we provide strong evidence that the absence of this DC subset resulted in complete resistance to experimental CM. This was accompanied with dramatic reduction of brain CD8(+) T cells, and those few cerebral CD8(+) T cells present had a less activated phenotype, unlike their wildtype counterparts that expressed IFN-γ and especially granzyme B. This almost complete absence of local cellular responses was also associated with reduced parasite load in the brain.     

Cutting edge: Th2 response induction by dendritic cells: a role for CD40.

We investigated the influence of dendritic cell (DC) CD40 expression on Th2 and Th1 development by in vivo transfer of Ag-pulsed bone marrow-derived DC generated from wild-type (WT) or CD40(-/-) mice. Contrary to expectation, CD40(-/-) DC primed with Ag that inherently induce a Th2 response (soluble egg Ag from Schistosoma mansoni) failed to induce a Th2 response or any compensatory Th1 response, whereas CD40(-/-)DC primed with Ag that inherently induce a Th1 response (Propionibacterium acnes) generated a competent Th1 response. Thus, DC expression of CD40 is a prerequisite for initiation of Th2, but not Th1, responses by these Ag. Consistent with this, CD154(-/-) mice, unlike WT mice, failed to mount a Th2 response when directly injected with schistosome eggs but mounted a normal Th1 response after challenge with P. acnes. CD40-CD154 interaction can therefore play a major role in Th2 response induction.     

Cutting edge: central nervous system plasmacytoid dendritic cells regulate the severity of relapsing experimental autoimmune encephalomyelitis

Plasmacytoid dendritic cells (pDCs) have both stimulatory and regulatory effects on T cells. pDCs are a major CNS-infiltrating dendritic cell population during experimental autoimmune encephalomyelitis but, unlike myeloid dendritic cells, have a minor role in T cell activation and epitope spreading. We show that depletion of pDCs during either the acute or relapse phases of experimental autoimmune encephalomyelitis resulted in exacerbation of disease severity. pDC depletion significantly enhanced CNS but not peripheral CD4(+) T cell activation, as well as IL-17 and IFN-gamma production. Moreover, CNS pDCs suppressed CNS myeloid dendritic cell-driven production of IL-17, IFN-gamma, and IL-10 in an IDO-independent manner. The data demonstrate that pDCs play a critical regulatory role in negatively regulating pathogenic CNS CD4(+) T cell responses, highlighting a new role for pDCs in inflammatory autoimmune disease.     

Cutting edge: histamine inhibits IFN-alpha release from plasmacytoid dendritic cells

Plasmacytoid dendritic cells (DC) are professional APC and a major source of type I IFN following viral infection. We previously showed that histamine alters the cytokine profiles of maturing monocyte-derived DC resulting in a change from Th1 to Th2 in their T cell polarizing function. In this study, we show that human plasmacytoid DC, activated by either CpG oligodeoxynucleotides or viral infection, also respond to histamine through H2 receptors, leading to a marked down-regulation of IFN-alpha and TNF-alpha and a moderate switch in their capacity to polarize naive T cells. Our findings provide an explanation for low levels of type I IFN frequently observed in atopic individuals.     

Cutting edge: conventional dendritic cells are the critical APC required for the induction of experimental cerebral malaria

Cerebral malaria (CM) is a serious complication of Plasmodium falciparum infection, causing significant morbidity and mortality among young children and nonimmune adults in the developing world. Although previous work on experimental CM has identified T cells as key mediators of pathology, the APCs and subsets therein required to initiate immunopathology remain unknown. In this study, we show that conventional dendritic cells but not plasmacytoid dendritic cells are required for the induction of malaria parasite-specific CD4+ T cell responses and subsequent experimental CM. These data have important implications for the development of malaria vaccines and the therapeutic management of CM.     

Cutting edge: impairment of dendritic cells and adaptive immunity by Ebola and Lassa viruses

Acute infection of humans with Ebola and Lassa viruses, two principal etiologic agents of hemorrhagic fevers, often results in a paradoxical pattern of immune responses: early infection, characterized by an outpouring of inflammatory mediators such as TNF-alpha, IL-1 beta, and IL-6, vs late stage infections, which are associated with poor immune responses. The mechanisms underlying these diverse outcomes are poorly understood. In particular, the role played by cells of the innate immune system, such as dendritic cells (DC), is not known. In this study, we show that Ebola and Lassa viruses infect human monocyte-derived DC and impair their function. Monocyte-derived DC exposed to either virus fail to secrete proinflammatory cytokines, do not up-regulate costimulatory molecules, and are poor stimulators of T cells. These data represent the first evidence for a mechanism by which Ebola and Lassa viruses target DC to impair adaptive immunity.     

Cutting edge: CCR4 mediates antigen-primed T cell binding to activated dendritic cells.

The binding of a T cell to an Ag-laden dendritic cell (DC) is a critical step of the acquired immune response. Herein, we address whether a DC-produced chemokine can induce the arrest of T cells on DC under dynamic flow conditions. Ag-primed T cells and a T cell line were observed to rapidly ( approximately 0.5 s) bind to immobilized DC at low shear stress (0.1-0.2 dynes/cm(2)) in a pertussis toxin-sensitive fashion. Quantitatively, Ag-primed T cells displayed 2- to 3-fold enhanced binding to DC compared with unprimed T cells (p < 0.01). In contrast to naive T cells, primed T cell arrest was largely inhibited by pertussis toxin, neutralization of the CC chemokine, macrophage-derived chemokine (CCL22), or by desensitization of the CCL22 receptor, CCR4. Our results demonstrate that DC-derived CCL22 induces rapid binding of activated T cells under dynamic conditions and that Ag-primed and naive T cells fundamentally differ with respect to chemokine-dependent binding to DC.     

Treatment of established colon carcinoma-bearing mice by dendritic cells pulsed with lysates of heat-treated tumor cells

To investigate the therapeutic effect of dendritic cells pulsed with lysates of heat-treated CT26 colon carcinoma cells. Bone marrow-derived DCs were pulsed with lysates of heat-treated tumor cells and were used to immunize BALB/c mice with established colon carcinoma. Cytotoxic T lymphocyte (CTL) response was detected. The therapeutic effect induced by DCs was observed by tumor weight and survival time. DCs pulsed with lysates of heat-treated tumor cells markedly induced specific cytotoxic activity of CTLs...     

Induction of anti-tumor immunity by vaccination with dendritic cells pulsed with anti-CD44 IgG opsonized tumor cells

Due to the pivotal role that dendritic cells (DC) play in eliciting and maintaining functional anti-tumor T cell responses, these APC have been exploited against tumors. DC express several receptors for the Fc portion of IgG (Fcγ receptors) that mediate the internalization of antigen-IgG complexes and promote efficient MHC class I and II restricted antigen presentation. In this study, the efficacy of vaccination with DC pulsed with apoptotic B16 melanoma cells opsonized with an anti-CD44 IgG (B16-CD44) was explored. Immature bone marrow derived DC grown in vitro with IL-4 and GM-CSF were pulsed with B16-CD44. After 48 h of pulsing, maturation of DC was demonstrated by production of IL-12 and upregulation of CD80 and CD40 expression. To test the efficacy of vaccination with DC+B16-CD44, mice were vaccinated subcutaneously Lymphocytes from mice vaccinated with DC+B16-CD44 produced IFN-γ in response to B16 melanoma lysates as well as an MHC class I restricted B16 melanoma-associated peptide, indicating B16 specific CD8 T cell activation. Upon challenge with viable B16 cells, all mice vaccinated with DC alone developed tumor compared to 40% of mice vaccinated with DC+B16-CD44; 60% of the latter mice remained tumor free for at least 8 months. In addition, established lung tumors and distant metastases were significantly reduced in mice treated with DC+B16-CD44. Lastly, delayed growth of established subcutaneous tumors was induced by combination therapy with anti-CD44 antibodies followed by DC injection. This study demonstrates the efficacy of targeting tumor antigens to DC via Fcγ receptors.     

Cutting edge: dissociation between autoimmune response and clinical disease after vaccination with dendritic cells

Autoimmunity represents a caveat to the use of dendritic cells (DCs) as adjuvant for human vaccines. We derived DCs from normal BALB/c mice or from mice prone to autoimmunity (NZB x NZW) F(1). We allowed DCs to phagocytose apoptotic thymocytes and vaccinated syngeneic animals. All mice developed anti-nuclear and anti-dsDNA Abs. Autoantibodies in normal mice were transient, without clinical or histological features of autoimmunity or tissue involvement. In contrast, autoimmunity was maintained in susceptible mice, which underwent renal failure and precociously died. The data suggest that DC vaccination consistently triggers autoimmune responses. However, clinical autoimmunity develops in susceptible subjects only.     

Cutting edge: activation of NK T cells by CD1d and alpha-galactosylceramide directs conventional T cells to the acquisition of a Th2 phenotype.

NK T cells recognize glycolipid Ags such as alpha-galactosylceramide (alpha-GalCer) presented by the MHC class I-like molecule CD1d. In this paper we have studied the in vivo effects of alpha-GalCer on the generation of adaptive immune responses. Treatment of mice with alpha-GalCer resulted in rapid activation of NK T cells and production of the cytokines IL-4 and IFN-gamma. However, after this initial stimulation, NK T cells became polarized for the production of IL-4. Further, as soon as 6 days after alpha-GalCer injection, a marked increase in serum IgE levels was observed. Administration of alpha-GalCer at the time of priming of mice with protein Ag resulted in the generation of Ag-specific Th2 cells and a profound increase in the production of IgE. Collectively, these findings indicate that alpha-GalCer may be useful for modulating immune responses toward a Th2 phenotype during prophylaxis and therapy.     

Cutting edge: peripheral neuropeptides attract immature and arrest mature blood-derived dendritic cells

Dendritic cells (DC) are highly motile and play a key role in mediating immune responses in various tissues and lymphatic organs. We investigated locomotion of mononuclear cell-derived DC at different maturation stages toward gradients of sensory neuropeptides in vitro. Calcitonin gene-related peptide, vasoactive intestinal polypeptide, secretin, and secretoneurin induced immature DC chemotaxis comparable to the potency of RANTES, whereas substance P and macrophage-inflammatory protein-3beta stimulated immature cell migration only slightly. Checkerboard analyses revealed a true chemotactic response induced by neuropeptides. Upon maturation of DC, neuropeptides inhibited spontaneous, macrophage-inflammatory protein-3beta- and 6Ckine-induced cell migration. Maturation-dependent changes in migratory behavior coincided with distinct neuropeptide-induced signal transduction in DC. Peripheral neuropeptides might guide immature DC to peripheral nerve fibers where high concentrations of these peptides can arrest the meanwhile matured cells. It seems that one function of sensory nerves is to fasten DC at sites of inflammation.     

Cutting edge: a novel role for Fas ligand in facilitating antigen acquisition by dendritic cells

Fas ligand (FasL)-expressing tumor cells are found to effectively mediate rejection of the coinoculated FasL negative parental cells while having no effect on the growth of histologically distinct tumor cells. These observations indicate that FasL induces a specific immune response against Ag derived from FasL-bearing tumors and suggest a possible role for FasL in tumor Ag presentation. Indeed, tumor cells expressing FasL can efficiently interact with dendritic cells (DCs) and this interaction requires the expression of membrane-bound FasL on tumors and Fas on DCs. Moreover, DCs cocultured with FasL-expressing tumors are able to elicit a tumor-specific immune response in vivo, suggesting that DCs acquire tumor Ag during the Fas/FasL-mediated DC-tumor contact. These results identify a novel role for FasL in augmenting tumor-DC interactions and subsequent tumor Ag acquisition by DCs, and suggest that FasL-expressing tumor cells could be used to generate tumor-specific DC vaccines.     

Cutting edge: murine dendritic cells require IL-15R alpha to prime NK cells

NK cells protect hosts against viral pathogens and transformed cells, and dendritic cells (DCs) play important roles in activating NK cells. We now find that murine IL-15Ralpha-deficient DCs fail to support NK cell cytolytic activity and elaboration of IFN-gamma, despite the fact that these DCs express normal levels of costimulatory molecules and IL-12. By contrast, IL-15Ralpha expression on NK cells is entirely dispensable for their activation by DCs. In addition, blockade with anti-IL-15Ralpha and anti-IL-2Rbeta but not anti-IL-2Ralpha-specific Abs prevents NK cell activation by wild-type DCs. Finally, presentation of IL-15 by purified IL-15Ralpha/Fc in trans synergizes with IL-12 to support NK cell priming. These findings suggest that murine DCs require IL-15Ralpha to present IL-15 in trans to NK cells during NK cell priming.