Mouse type I IFN-producing cells are immature APCs with plasmacytoid morphology

Flt3 Ligand-Generated Murine Plasmacytoid and Conventional Dendritic Cells Differ in Their Capacity to Prime Naive CD8 T Cells and to Generate Memory Cells In Vivo

Mature dendritic cells (DCs) have the capacity to induce efficient primary T cell response and effector cell differentiation. Thus, these cells are a major tool in the design of various immunotherapeutic protocols. We have tested the capacity of different subsets of matured DCs pulsed with a peptide to induce the differentiation of naive CD8 T cells into memory cells in vivo. Flt3 ligand (FL) induces the differentiation of conventional DCs (cDCs) and plasmacytoid DCs (PDCs) from murine bone marrow precursors in vitro. After maturation, both subsets become strong stimulators of Ag-specific T cell responses in vitro. However, the in vivo T cell stimulatory capacity of these DC subsets has not been studied in detail. In the present study, we demonstrate that mature FL-generated DCs induce efficient peptide-specific CD8 T cell response and memory cell differentiation in vivo. This is mainly due to the cDC subset because the PDC subset induced only a negligible primary CD8 response without detectable levels of memory CD8 T cell differentiation. Thus, in vitro FL-generated mature cDCs, but not PDCs, are potent stimulators of peptide-specific CD8 T cell responses and memory generation in vivo.

A minor population of splenic dendritic cells expressing CD19 mediates IDO-dependent T cell suppression via type I IFN signaling following B7 ligation

By ligating CD80/CD86 (B7) molecules, the synthetic immunomodulatory reagent CTLA4-Ig (soluble synthetic CTLA4 fusion protein) induces expression of the enzyme indoleamine 2,3-dioxygenase (IDO) in some dendritic cells (DCs), which acquire potent T cell regulatory functions as a consequence. Here we show that this response occurred exclusively in a population of splenic DCs co-expressing the marker CD19. B7 ligation induced activation of the transcription factor signal transducer and activator of transcription (STAT1) in sorted CD19+, but not CD19(NEG), DCs. STAT1 activation occurred even when DCs lacked receptors for type II IFN (IFNgamma); however, STAT1 activation and IDO up-regulation were not observed when DCs lacked receptors for type I IFN (IFNalphabeta). Thus, IFNalpha, but not IFNgamma, signaling was essential for STAT1 activation and IDO up-regulation in CD19+ DCs following B7 ligation. Consistent with these findings, B7 ligation also induced sorted CD19+, but not CD19(NEG), DCs to express IFNalpha. Moreover, recombinant IFNalpha induced CD19+, but not CD19(NEG), DCs to mediate IDO-dependent T cell suppression, showing that IFNalpha signaling could substitute for upstream signals from B7. These data reveal that a minor population of splenic DCs expressing the CD19 marker is uniquely responsive to B7 ligation, and that IFNalpha-mediated STAT1 activation is an essential intermediary signaling pathway that promotes IDO induction in these DCs. Thus, CD19+ DCs may be a target for regulatory T cells expressing surface CTLA4, and may suppress T cell responses via induction of IDO.

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 host type I interferon response to viral and bacterial infections

Type I interferons (IFN) are well studied cytokines with anti-viral and immune-modulating functions. Type I IFNs are produced following viral infections, but until recently, the mechanisms of viral recognition leading to IFN production were largely unknown. Toll like receptors (TLRs) have emerged as key transducers of type I IFN during viral infections by recognizing various viral components. Furthermore, much progress has been made in defining the signaling pathways downstream of TLRs for type I IFN production. TLR7 and TLR9 have become apparent as universally important in inducing type I IFN during infection with most viruses, particularly by plasmacytoid dendritic cells. New intracellular viral pattern recognition receptors leading to type I IFN production have been identified. Many bacteria can also induce the up-regulation of these cytokines. Interestingly, recent studies have found a detrimental effect on host cells if type I IFN is produced during infection with the intracellular gram-positive bacterial pathogen, Listeria monocytogenes. This review will discuss the recent advances made in defining the signaling pathways leading to type I IFN production.

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.

Interleukin-12: an update on its immunological activities, signaling and regulation of gene expression

Interleukin-12 (IL-12) is a heterodimeric cytokine composed of the p35 and p40 subunits. It is produced by antigen-presenting cells and plays a critical role in host defense against intracellular microbial infection and control of malignancy via its ability to stimulate both innate and adaptive immune effector cells. The potency of IL-12 renders itself to stringent regulation of the timing, locality and magnitude of its production during an immune response. Subversion of the delicate control and balance frequently leads to immunologic disorders. In this article, we provide an update, since our last review of the subject four years ago, on recent advances in: (1) uncovering of novel activities of IL-12 and related molecules in various immunological settings and models; and (2) dissection of the physiological pathways involved in the modulation of IL-12 production by pathogens and immune regulators. The increased understanding of IL-12 immunobiology and expression will likely benefit the development of therapeutic modalities to correct immune dysfunctions.

Inhibitory NK receptor Ly49Q is expressed on subsets of dendritic cells in a cellular maturation- and cytokine stimulation-dependent manner

Ly49Q is a member of the Ly49 family that is expressed on Gr-1+ cells but not on NK and NKT cells. Ly49Q appears to be involved in regulating cytoskeletal architectures through ITIM-mediated signaling. We provide evidence that dendritic cells (DCs) of certain maturational states expressed Ly49Q, and that IFN-alpha plays an important role in its regulation. Freshly prepared murine plasmacytoid pre-DCs as well as Flt3L-induced plasmacytoid pre-DCs expressed Ly49Q, whereas freshly prepared myeloid DCs did not. However, GM-CSF-induced myeloid DCs showed low levels of Ly49Q expression, and this was significantly enhanced by IFN-alpha. In contrast, other cytokines and ligands for TLRs such as TNF-alpha, IL-6, LPS, and CpG-ODN had little or no effect on Ly49Q expression. Plasmacytoid pre-DCs in all mouse strains examined expressed Ly49Q. Constitutive expression of Ly49Q on myeloid DCs was observed in three restricted mouse strains including 129, NZB, and NZW. As can be seen in other Ly49 family members, Ly49Q expression was affected by MHC class I expression. At the same time, Ly49Q possessed polymorphisms, including at least three alleles. The polymorphic residues lay within the stalk and carbohydrate recognition domain, and two of them, in loop 3 and loop 6 of the carbohydrate recognition domain, are located in the region implicated in the interaction of Ly49A with H-2D(d). Therefore, depending on IFN-alpha, our results imply that Ly49Q serves a role for the biological functions of certain DC subsets through recognition of MHC class I or related molecules.

Plasmacytoid precursor dendritic cells facilitate allogeneic hematopoietic stem cell engraftment

Bone marrow transplantation offers great promise for treating a number of disease states. However, the widespread application of this approach is dependent upon the development of less toxic methods to establish chimerism and avoid graft-versus-host disease (GVHD). CD8⁺/TCR⁻ facilitating cells (FCs) have been shown to enhance engraftment of hematopoietic stem cells (HSCs) in allogeneic recipients without causing GVHD. In the present studies, we have identified the main subpopulation of FCs as plasmacytoid precursor dendritic cells (p-preDCs). FCs and p-preDCs share many phenotypic, morphological, and functional features: both produce IFN-α and TNF-α, both are activated by toll-like receptor (TLR)-9 ligand (CpG ODN) stimulation, and both expand and mature after Flt3 ligand (FL) treatment. FL-mobilized FCs, most of which express a preDC phenotype, significantly enhance engraftment of HSCs and induce donor-specific tolerance to skin allografts. However, p-preDCs alone or p-preDCs from the FC population facilitate HSC engraftment less efficiently than total FCs. Moreover, FCs depleted of preDCs completely fail to facilitate HSC engraftment. These results are the first to define a direct functional role for p-preDCs in HSC engraftment, and also suggest that p-preDCs need to be in a certain state of maturation/activation to be fully functional.

CD4- plasmacytoid dendritic cells (pDCs) migrate in lymph nodes by CpG inoculation and represent a potent functional subset of pDCs

We have recently identified two groups of plasmacytoid dendritic cells (pDCs) isolated from murine liver based on the expression of CD4 and other cell surface markers uniquely expressed by pDCs. Herein, we describe the identification of both CD4+ and CD4- pDCs that clearly exist in lymph nodes (LNs), spleen, liver, thymus, bone marrow, and lung. Normally, CD4+ pDCs are enriched in LNs. However, after in vivo systemic injection with bacterial CpG, a larger number of CD4- pDCs are recruited to the LNs and local inoculation by CpG drives CD4- pDCs migrating into local sentinel LNs, suggesting that CD4- pDCs are the main subpopulation migrating to the peripheral LNs. Furthermore, although both freshly isolated CD4+ pDCs and CD4- pDCs appear as an immature plasmacytoid cell and develop into a DC morphology following activation, the two subsets have strikingly different immune features, including differences in the production pattern of cytokines stimulated with CpG and in T cell activation.

A type I interferon autocrine-paracrine loop is involved in Toll-like receptor-induced interleukin-12p70 secretion by dendritic cells

Dendritic cells (DC) produce interleukin-12 (IL-12) in response to Toll-like receptor (TLR) activation. Two major TLR signaling pathways participate in the response to pathogens: the nuclear factor-kappaB (NF-kappaB)-dependent pathway leading to inflammatory cytokine secretion including IL-12 and the interferon (IFN)-dependent pathway inducing type I IFN and IFN-regulated genes. Here we show that the two pathways cooperate and are likely both necessary for inducing an optimal response to pathogens. R-848/Resiquimod (TLR7 ligand in the mouse and TLR7/8 ligand in human) synergized with poly(I:C) (TLR3 ligand) or lipopolysaccharide (LPS; TLR4 ligand) in inducing high levels of bioactive IL-12p70 secretion and IFN-beta mRNA accumulation by mouse bone marrow-derived DC (BM-DC). Strikingly, IL-12p70 but not IL-12p40 secretion was strongly reduced in BM-DC from STAT1(-/-) and IFNAR(-/-) mice. STAT1 tyrosine-phosphorylation, IL-12p35, and IFN-beta mRNA accumulation were strongly inhibited in IFNAR(-/-) BM-DC activated with the TLR ligand combinations. Similar observation were obtained in human TLR8-expressing monocyte-derived DC (moDC) using neutralizing anti-IFNAR2 antibodies, although results also pointed to a possible involvement of IFN-lambda1 (also known as IL-29). This suggests that TLR engagement on DC induces endogenous IFNs that further synergize with the NF-kappaB pathway for optimal IL-12p70 secretion. Moreover, analysis of interferon regulatory factors (IRF) regulation in moDC suggests a role for IRF7/8 in mediating IRF3-independent type I IFN and possibly IL-12p35 synthesis in response to TLR7/8.

IFN regulatory factor-4 and -8 govern dendritic cell subset development and their functional diversity

Dendritic cells (DCs) are bone marrow (BM)-derived APCs central to both innate and adaptive immunity. DCs are a heterogeneous cell population composed of multiple subsets with diverse functions. The mechanism governing the generation of multiple DC subsets is, however, poorly understood. In this study we investigated the roles of closely related transcription factors, IFN regulatory factor (IRF)-4 and IRF-8, in DC development by analyzing IRF-4(-/-), IRF-8(-/-), and IRF-4(-/-)IRF-8(-/-) (double-knockout) mice. We found that IRF-4 is required for the generation of CD4(+) DCs, whereas IRF-8 is, as reported previously, essential for CD8alpha(+) DCs. Both IRFs support the development of CD4(-)CD8alpha(-) DCs. IRF-8 and, to a lesser degree, IRF-4 contribute to plasmacytoid DC (PDC) development. Thus, the two IRFs together regulate the development of all conventional DCs as well as PDCs. Consistent with these findings, IRF-4, but not IRF-8, was expressed in CD4(+) DCs, whereas only IRF-8 was expressed in CD8alpha(+) DCs. CD4(-)CD8alpha(-) DCs and PDCs expressed both IRFs. We also demonstrate in vitro that GM-CSF-mediated DC differentiation depends on IRF-4, whereas Fms-like tyrosine kinase 3 ligand-mediated differentiation depends mainly on IRF-8. Gene transfer experiments with double-knockout BM cells showed that both IRFs have an overlapping activity and stimulate a common process of DC development. Nonetheless, each IRF also possesses a distinct activity to stimulate subset-specific gene expression, leading to the generation of functionally divergent DCs. Together, IRF-4 and IRF-8 serve as a backbone of the molecular program regulating DC subset development and their functional diversity.

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.

Maturation requirements for dendritic cells in T cell stimulation leading to tolerance versus immunity

The model that dendritic cell (DC) "maturation" describes the change from an immature, antigen-capturing cell to a mature, antigen-presenting cell is well-established. Classification of DCs in terms of function has been problematic previously. It is therefore proposed that mature and not immature DCs are responsible for antigen presentation and stimulation of T cells. Furthermore, DC antigen presentation to T cells can have two outcomes: tolerance or immunity. The particular outcomes appear to be determined by the activation state of the mature DC. DCs can be activated by a range of environmental stimuli or "danger signals". Here, the hypothesis is advanced that activated, mature DCs induce T cell immunity, and resting, nonactivated but fully differentiated mature antigen-presenting DCs can induce tolerance. This proposal extends to conventional DCs and plasmacytoid DCs. The paper also concentrates on the spleen as a site for DC maturation, in light of evidence from this laboratory for differentiation of DCs from splenic precursors in long-term, stroma-dependent cultures. The hypothesis advanced here serves to simplify many current issues regarding DC maturation and function.

Ly49Q defines 2 pDC subsets in mice

Plasmacytoid dendritic cells (pDCs) play an important primary role for antiviral innate immunity by rapidly producing large amounts of type 1 interferon (IFN) upon viral infection. To study pDC biology, we generated a monoclonal antibody, termed 2E6, that recognizes pDCs. Molecular cloning of a cDNA encoding the 2E6 antigen revealed that it is a type II C-type lectin, Ly49Q, that consists of 247 amino acids with high homology to the natural killer (NK) receptor family Ly49, with an immunoreceptor tyrosine-based inhibitory motif in the cytoplasmic domain. Ly49Q is expressed on pDCs but not on NK cells or myeloid dendritic cells. B220+, CD11c+, CD11b– pDCs in bone marrow were divided into Ly49Q+ and Ly49Q– subsets. While both subsets produced IFN-α upon cytosine-phosphate-guanosine (CpG) and herpes simplex virus stimulation, Ly49Q– pDCs responded poorly to influenza virus. In addition, Ly49Q– pDCs produced inflammatory cytokines such as interleukin 6 (IL-6), IL-12, and tumor necrosis factor α (TNF-α) upon stimulation at lower levels than those produced by Ly49Q+ pDCs. In contrast to bone marrow, Ly49Q+ pDCs were only found in peripheral blood, lymph nodes, and spleen. These results indicate that Ly49Q is a specific marker for peripheral pDCs and that expression of Ly49Q defines 2 subsets of pDCs in bone marrow.

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.

Type I interferon dependence of plasmacytoid dendritic cell activation and migration

Differential expression of Toll-like receptor (TLR) by conventional dendritic cells (cDCs) and plasmacytoid DC (pDCs) has been suggested to influence the type of immune response induced by microbial pathogens. In this study we show that, in vivo, cDCs and pDCs are equally activated by TLR4, -7, and -9 ligands. Type I interferon (IFN) was important for pDC activation in vivo in response to all three TLR ligands, whereas cDCs required type I IFN signaling only for TLR9- and partially for TLR7-mediated activation. Although TLR ligands induced in situ migration of spleen cDC into the T cell area, spleen pDCs formed clusters in the marginal zone and in the outer T cell area 6 h after injection of TLR9 and TLR7 ligands, respectively. In vivo treatment with TLR9 ligands decreased pDC ability to migrate ex vivo in response to IFN-induced CXCR3 ligands and increased their response to CCR7 ligands. Unlike cDCs, the migration pattern of pDCs required type I IFN for induction of CXCR3 ligands and responsiveness to CCR7 ligands. These data demonstrate that mouse pDCs differ from cDCs in the in vivo response to TLR ligands, in terms of pattern and type I IFN requirement for activation and migration.

An innately interesting decade of research in immunology

"Nature has provided, in the white corpuscles as you call them-in the phagocytes as we call them-a natural means of devouring and destroying all disease germs. There is at bottom only one genuinely scientific treatment for all diseases, and that is to stimulate the phagocytes." So opined B.B. in G.B. Shaw's The Doctor's Dilemma in a dramatic restatement of a key portion of Ilya Metchnikoff's Nobel Prize address: "Whenever the organism enjoys immunity, the introduction of infectious microbes is followed by the accumulation of mobile cells, of white corpuscles of the blood in particular which absorb the microbes and destroy them. The white corpuscles and the other cells capable of doing this have been designated 'phagocytes,' (i.e., devouring cells) and the whole function that ensures immunity has been given the name of 'phagocytosis'". Based on these insights into the foundation of resistance to infectious disease, Metchnikoff was awarded the 1908 Nobel Prize in Physiology or Medicine together with Paul Ehrlich (Fig. 1). Although both were cited for discoveries in immunity, the contributions of the two men seem worlds apart. Ehrlich's studies did not deal with generic responses to infection, but rather with the highly specific nature of antibodies and their relationship to the cells producing them: "As the cell receptor is obviously preformed, and the artificially produced antitoxin only the consequence, i.e. secondary, one can hardly fail to assume that the antitoxin is nothing else but discharged components of the cell, namely receptors discharged in excess". But biological systems are just that-systems-and the parts need to work together. And so we arrive, a century later, at an appreciation for just how intimately related these two seemingly disparate aspects of host defense really are.

Vaccines that facilitate antigen entry into dendritic cells

Although vaccines have been highly successful in preventing and treating many infectious diseases (including smallpox, polio and diphtheria) diseases prevalent in the developing world such as malaria and HIV, that suppress the host immune system, require new, multiple strategies that will be defined by our growing understanding of specific immune activation. The definition of adjuvants, previously thought of as any substance that enhanced the immunogenicity of antigen, could now include soluble mediators and antigenic carriers that interact with surface molecules present on DC (e.g. LPS, Flt3L, heat shock protein) particulate antigens which are taken up by mechanisms available to APC but not other cell types (e.g. immunostimulatory complexes, latex, polystyrene particles) and viral/bacterial vectors that infect antigen presenting cells (e.g. vaccinia, lentivirus, adenovirus). These approaches, summarized herein, have shown potential in vaccinating against disease in animal models, and in some cases in humans. Of these, particle-antigen conjugates provide rapid formulation of the vaccine, easy storage and wide application, with both carrier and adjuvant functions that activate DC. Combined vaccines of the future could use adjuvants such as virus-like particles and particles targeted towards a predominant cellular type or immune response, with target cell activation enhanced by growth factors or maturation signals prior to, or during immunization. Collectively, these new additions to adjuvant technology provide opportunities for more specific immune regulation than previously available.

Accurate and simple discrimination of mouse pulmonary dendritic cell and macrophage populations by flow cytometry: methodology and new insights

BACKGROUND

The need to accurately discriminate dendritic cells (DCs) and macrophages (Mphs) in mouse lungs is critical given important biological differences. However, a validated flow cytometry-based method is still lacking, resulting in much confusion between both cell types.

METHODS

Single-cell suspensions freshly obtained from collagenase-digested lung tissue were stained with a CD11c-specific monoclonal antibody, detected using a PE-Cy5 or APC-conjugated secondary reagent. Cellular immunophenotype was simultaneously explored using a panel of PE-conjugated markers. The FL1 or FITC-detection channel was reserved for the assessment of autofluorescence.

RESULTS

CD11c-bright cells were heterogeneous and displayed a bimodal distribution with regard to autofluorescence (AF). CD11c+/low-AF cells were lineage-negative and showed features compatible with myeloid DCs. This was confirmed by morphology, potent T-cell stimulatory function in a mixed-leukocyte reaction, surface expression of MHCII and costimulatory molecules, and further immunophenotypical criteria, including the expression of Mac-1 and absence of CD8alpha. In contrast, CD11c+/high-AF cells displayed the features of pulmonary Mphs, including typical Mph morphology, very weak induction of T-cell proliferation, low to absent expression of MHCII and costimulatory molecules, and very low levels of Mac-1 as well as F4/80. We also show that only CD11c+/high-AF cells strongly expressed the macrophage marker MOMA-2, while interestingly Mac-3 was expressed at high levels by CD11c+/high-AF and low-AF alike.

CONCLUSIONS

This study shows that the combination of CD11c-expression and autofluorescence is necessary and sufficient to accurately separate DCs from macrophage subpopulations in mouse lungs.

Close encounters of different kinds: dendritic cells and NK cells take centre stage

Immune responses are generally divided into innate and adaptive responses, and the efficacy of one is thought to be independent of the other. The regulation of immune responses, however, is complex, and accumulating evidence indicates that multiple interactions between immune effector cells are common and are crucial for the initiation, as well as the outcome, of these responses. Dendritic cells, long recognized as key initiators of primary adaptive immunity, are now also seen as crucial regulators of aspects of innate immunity, in particular natural-killer-cell function. Reciprocally, natural killer cells can influence the activity of dendritic cells. Here, we review recent exciting progress in this field, and we highlight the impact of this cellular crosstalk on the design of immune-based therapies for control of infection and cancer.

Interleukin-1 receptor-associated kinase-1 plays an essential role for Toll-like receptor (TLR)7- and TLR9-mediated interferon-{alpha} induction

Toll-like receptors (TLRs) recognize microbial pathogens and trigger innate immune responses. Among TLR family members, TLR7, TLR8, and TLR9 induce interferon (IFN)-α in plasmacytoid dendritic cells (pDCs). This induction requires the formation of a complex consisting of the adaptor MyD88, tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) and IFN regulatory factor (IRF) 7. Here we show an essential role of IL-1 receptor-associated kinase (IRAK)-1 in TLR7- and TLR9-mediated IRF7 signaling pathway. IRAK-1 directly bound and phosphorylated IRF7 in vitro. The kinase activity of IRAK-1 was necessary for transcriptional activation of IRF7. TLR7- and TLR9-mediated IFN-α production was abolished in Irak-1–deficient mice, whereas inflammatory cytokine production was not impaired. Despite normal activation of NF-κB and mitogen-activated protein kinases, IRF7 was not activated by a TLR9 ligand in Irak-1–deficient pDCs. These results indicated that IRAK-1 is a specific regulator for TLR7- and TLR9-mediated IFN-α induction in pDCs.

Tissue-specific positive feedback requirements for production of type I interferon following virus infection

Type I interferon (IFN) is synthesized by most nucleated cells following viral infection. Robust IFN production in cell culture requires positive feedback expression of inducible signaling components, such as the transcription factor IRF7. However, the role of positive feedback and IRF7 in vivo may be more complex. We found that IFN produced locally in the respiratory tract of influenza virus-infected mice displayed characteristics of positive feedback, including Stat1-dependent induction of IRF7 and IFN gene expression. IRF7 expression was similarly stimulus-dependent in most tissues. However, lymphoid tissue constitutively expressed high levels of IRF7 in the absence of induction or positive feedback, and this expression was largely confined to plasmacytoid dendritic cells (DC). These cells rapidly produced large quantities of multiple IFN alpha species following viral infection without positive feedback, whereas other hematopoietic cells, including other DC subtypes, expressed little IRF7 and were poor IFN producers in the absence of positive feedback. These data reveal a dual mechanism for the regulation of IFN production by differential expression of IRF7, involving positive feedback at local sites of infection combined with robust systemic production by IRF7-expressing plasmacytoid DC.

Identification of antigen-capturing cells as basophils

Binding of intact Ag is a hallmark of Ag-specific B cells. Apart from B cells, a small number of non-B cells can bind Ag with comparable efficacy as B cells and are found in the peripheral blood, spleen, and bone marrow of mice. This population has been observed for a long time and recently named "Ag-capturing cells." Their identity remained enigmatic. In this study, we show that these cells are basophilic granulocytes. Their ability to capture Ags is dependent on surface IgE receptors and on Ag-specific plasma IgE molecules appearing after immunization. Several surface markers including surface bound IgE, IL-3R, CD45, CD16/32, and the chemokine receptor CCR2 were used to clearly identify these cells. Cross-linkage of surface Igs results in the release of large amounts of IL-4 and IL-6. The data identify basophils as Ag-capturing cells and support the concept of basophils as important regulators of humoral immune responses.

Dendritic cells and the regulation of the allergic immune response

Studies in mouse models of asthma have revealed a critical role for airway dendritic cells in the induction of Th2 sensitization to inhaled allergens. Under some conditions, subsets of dendritic cells can also induce tolerance or Th1 responses to the same allergens, depending on the context in which the antigen is seen. This article discusses various aspects of DC biology as it relates to allergic sensitization and also provides a summary of the recent evidence that dendritic cells function beyond sensitization.

Characterization of virus-responsive plasmacytoid dendritic cells in the rhesus macaque

Plasmacytoid dendritic cells (PDC) are potent producers of alpha interferon (IFN-alpha) in response to enveloped viruses and provide a critical link between the innate and adaptive immune responses. Although the loss of peripheral blood PDC function and numbers has been linked to human immunodeficiency virus (HIV) progression in humans, a suitable animal model is needed to study the effects of immunodeficiency virus infection on PDC function. The rhesus macaque SIV model closely mimics human HIV infection, and recent studies have identified macaque PDC, potentially making the macaque a good model to study PDC regulation. In this study, we demonstrate that peripheral blood PDC from healthy macaques are both phenotypically and functionally similar to human PDC and that reagents used for human studies can be used to study macaque PDC. Both human and macaque PBMC expressed IFN-alpha in response to herpes simplex virus (HSV), the prototypical activator of PDC, as measured by using an IFN bioassay and IFN-alpha-specific enzyme-linked immunospot assays. Similar to human PDC, macaque PDC were identified by using flow cytometry as CD123+ HLA-DR+ lineage- cells. In addition, like human PDC, macaque PDC expressed intracellular IFN-alpha, tumor necrosis factor alpha, macrophage inflammatory protein 1beta/CCL4, and IFN-inducible protein 10/CXCL10 upon stimulation with HSV, all as determined by intracellular flow cytometry. We found that IFN regulatory factor 7, which is required for the expression of IFN-alpha genes, was, similar to human PDC, expressed at high levels in macaque PDC compared to monocytes and CD8+ T cells. These findings establish the phenotypic and functional similarity of human and macaque PDC and confirm the utility of tools developed for studying human PDC in this animal model.

Progressive and Controlled Development of Mouse Dendritic Cells from Flt3+CD11b+Progenitors In Vitro

Dendritic cells (DC) represent key regulators of the immune system, yet their development from hemopoietic precursors is poorly defined. In this study, we describe an in vitro system for amplification of a Flt3(+)CD11b(+) progenitor from mouse bone marrow with specific cytokines. Such progenitor cells develop into both CD11b(+) and CD11b(-) DC, and CD8alpha(+) and CD8alpha(-) DC in vivo. Furthermore, with GM-CSF, these progenitors synchronously differentiated into fully functional DC in vitro. This two-step culture system yields homogeneous populations of Flt3(+)CD11b(+) progenitor cells in high numbers and allows monitoring the consecutive steps of DC development in vitro under well-defined conditions. We used phenotypic and functional markers and transcriptional profiling by DNA microarrays to study the Flt3(+)CD11b(+) progenitor and differentiated DC. We report here on an extensive analysis of the surface Ag expression of Flt3(+)CD11b(+) progenitor cells and relate that to surface Ag expression of hemopoietic stem cells. Flt3(+)CD11b(+) progenitors studied exhibit a broad overlap of surface Ags with stem cells and express several stem cell Ags such as Flt3, IL-6R, c-kit/SCF receptor, and CD93/AA4.1, CD133/AC133, and CD49f/integrin alpha(6). Thus, Flt3(+)CD11b(+) progenitors express several stem cell surface Ags and develop into both CD11b(+) and CD11b(-) DC, and CD8alpha(+) and CD8alpha(-) DC in vivo, and thus into both of the main conventional DC subtypes.

Derivation of 2 categories of plasmacytoid dendritic cells in murine bone marrow

Plasmacytoid dendritic cells (pDCs) competent to make type I interferon were rigorously defined as a Ly-6C(+) and CD11c(Lo) subset of the B220(+)CD19(-) CD43(+)CD24(Lo) bone marrow (BM) Fraction A. Otherwise similar Ly6C(-) cells expressed the natural killer (NK) markers DX5 and NK1.1. pDCs represented a stable, discrete, and long-lived population. Stem cells and early lymphoid progenitors (ELPs), but not prolymphocytes, were effective precursors of pDCs, and their differentiation was blocked by ligation of Notch receptors. Furthermore, pDCs were present in the BM of RAG1(-/-), CD127/IL-7Ra(-/-), and Pax5(-/-) mice. pDCs in RAG1/GFP knock-in mice could be subdivided, and immunoglobulin D(H)-J(H) rearrangements, as well as transcripts for the B-lineage-related genes Pax5, mb1/CD79a, ebf, and Bcl11a, were identified only in the green fluorescent protein-positive (GFP(+)) pDC1 subset. All pDCs expressed terminal deoxynucleotidyl transferase (TdT), the ETS transcription factor Spi-B, the nuclear factor-kappaB transcription factor RelB, toll-like receptor 9 (TLR9), and interferon consensus sequence binding protein (ICSBP)/interferon regulatory factor 8 (IRF-8) transcripts; lacked CD16 and granulocyte colony-stimulating factor receptor (G-CSFR); and were uniformly interleukin-7 receptor alpha (IL-7Ralpha(-)) AA4.1(Lo), CD27(-), Flk-2(Lo), c-Kit(-), DX-5(-), and CD11b(-), while CD4 and CD8alpha were variable. GFP(+) pDC1 subset was less potent than GFP(-) pDC2s in T allostimulation and production of tumor necrosis factor alpha (TNFalpha), interferon alpha (IFNalpha), and interleukin-6 (IL-6), while only pDC2s made IFNgamma and IL-12 p70. Thus, 2 functionally specialized subsets of pDCs arise in bone marrow from progenitors that diverge from B, T, and NK lineages at an early stage.

Variegation of the Immune Response with Dendritic Cells and Pathogen Recognition Receptors

One of the most fundamental questions in biology is: "How do cells differentiate in the right place, at the right time, into the right kinds?" Understanding the phenomenon of cell differentiation in its spatial and temporal framework is a prelude to understanding the development and physiology of all multicellular systems, including the immune system. Insights over the past 2300 years, since Aristotle, suggest that biological differentiation is guided by the interplay between genetic programs and specific environmental signals. This is exemplified by the mammalian immune response to pathogens, where qualitatively different types can emerge. Although it is appreciated that this type immunity is critical for optimal defense against different pathogens, the early "decision-making mechanisms" are largely obscure. Recent developments in innate immunity and genomics, especially in the biology of dendritic cells (DCs) and pathogen recognition receptors, have stimulated intense research in understanding the mechanisms guiding the differentiation of Th1, Th2, and T regulatory responses. In this study, I summarize recent findings which suggest that activation of DCs via distinct pathogen recognition receptors stimulate different gene expression programs and signaling networks in DCs that guide the variegation of immune responses.

Rapid Activation of Spleen Dendritic Cell Subsets following Lymphocytic Choriomeningitis Virus Infection of Mice: Analysis of the Involvement of Type 1 IFN

In this study, we report the dynamic changes in activation and functions that occur in spleen dendritic cell (sDC) subsets following infection of mice with a natural murine pathogen, lymphocytic choriomeningitis virus (LCMV). Within 24 h postinfection (pi), sDCs acquired the ability to stimulate naive LCMV-specific CD8+ T cells ex vivo. Conventional (CD11chigh CD8+ and CD4+) sDC subsets rapidly up-regulated expression of costimulatory molecules and began to produce proinflammatory cytokines. Their tendency to undergo apoptosis ex vivo simultaneously increased, and in vivo the number of conventional DCs in the spleen decreased markedly, dropping approximately 2-fold by day 3 pi. Conversely, the number of plasmacytoid (CD11clowB220+) DCs in the spleen increased, so that they constituted almost 40% of sDCs by day 3 pi. Type 1 IFN production was up-regulated in plasmacytoid DCs by 24 h pi. Analysis of DC activation and maturation in mice unable to respond to type 1 IFNs implicated these cytokines in driving infection-associated phenotypic activation of conventional DCs and their enhanced tendency to undergo apoptosis, but also indicated the existence of type 1 IFN-independent pathways for the functional maturation of DCs during LCMV infection.

Induction and regulation of IFNs during viral infections

Interferons (IFN)s are involved in numerous immune interactions during viral infections and contribute to both induction and regulation of innate and adaptive antiviral mechanisms. IFNs play a pivotal rule in the outcome of a viral infection, as demonstrated by the impaired resistance against different viruses in mice deficient for the receptors IFNAR-2 and IFNGR. During viral infections, IFNs are involved in numerous immune interactions as inducers, regulators, and effectors of both innate and adaptive antiviral mechanisms. IFN-alpha/beta is produced rapidly when viral factors, such as envelope glycoproteins, CpG DNA, or dsRNA, interact with cellular pattern-recognition receptors (PRRs), such as mannose receptors, toll-like receptors (TLRs), and cytosolic receptors. These host-virus interactions signal downstream to activate transcription factors needed to achieve expression from IFN-alpha/beta genes. These include IFN regulatory factor-3 (IRF-3), IRF-5, IRF-7, c-Jun/ATF-2, and NF-kappaB. In contrast, IFN-gamma is induced by receptor-mediated stimulation or in response to early produced cytokines, including interleukin-2 (IL-12), IL-18, and IFN-alpha/beta, or by stimulation through T cell receptors (TCRs) or natural killer (NK) cell receptors. IFNs signal through transmembrane receptors, activating mainly Jak-Stat pathways but also other signal transduction pathways. Cytokine and TCR-induced IFN-gamma expression uses distinct signal transduction pathways involving such transcription factors as NFAT, Stats and NF-kappaB. This results in induction and activation of numerous intrinsic antiviral factors, such as RNA-activated protein kinase (PKR), the 2-5A system, Mx proteins, and several apoptotic pathways. In addition, IFNs modulate distinct aspects of both innate and adaptive immunity. Thus, IFN-alpha/beta and IFN-gamma affect activities of macrophages, NK cells, dendritic cells (DC), and T cells by enhancing antigen presentation, cell trafficking, and cell differentiation and expression profiles, ultimately resulting in enhanced antiviral effector functions. This review focuses on the latest findings regarding induction and regulation of IFNs, primarily during the early phase of an antiviral immune response. Both cellular and molecular aspects are discussed from the perspective of host-virus interactions.

Cross-talk between dendritic cells and natural killer cells in viral infection

Dendritic cells (DC), first characterized in 1973 by Steinman and Cohn, have been defined as the professional antigen presenting cells (APC), capable of activating naïve T cells much more efficiently than either B cells or macrophages. DC also capture and process antigen more efficiently than other APC, and offer MHC-antigen complexes to T cells at higher densities, and in the context of larger amounts of co-stimulatory molecules (i.e. CD40, CD80 and CD86) at the T cell-DC synapse. Although historically, the principal function of DC is the priming of naïve T cells, more recently they have also been shown to affect the functions of natural killer (NK) cells. Interactions between DC and NK cells may be critical in situations where immune surveillance requires efficient early activation of NK cells, as is the case during infections. This review aims to summarise the interactions that occur between DC and NK cells during viral infection.

Developmental origin of interferon-α–producing dendritic cells from hematopoietic precursors

OBJECTIVE

The aim of this study was to determine the lineage origin of interferon-alpha-producing cells (IPCs), also called plasmacytoid dendritic cells, in mice by evaluating the ability of common lymphoid (CLP) and myeloid (CMP) progenitors to give rise to IPCs.

MATERIALS AND METHODS

Sublethally irradiated C57Bl/6 mice were intravenously transplanted with rigorously purified lymphoid and myeloid progenitors from a congenic mouse strain. At various time points posttransplantation mice were analyzed for donor-derived cells by flow cytometry. The developmental potential of all progenitor populations was also tested in in vitro cultures. In addition, in vitro and in vivo derived IPCs were functionally assessed for their interferon-alpha production after virus challenge.

RESULTS

Transplantation of 1 x 10(4) common myeloid progenitors, 1 x 10(4) common lymphoid progenitors or 2.5 x 10(4) granulocyte/macrophage progenitors all led to the generation of IPCs within 2 to 3 weeks. In general, IPC reconstitution in spleen and liver by CMPs was more efficient than by CLP. Adding Flt3L alone to in vitro cultures was sufficient to support the development of IPCs from myeloid progenitors whereas CLPs required additional survival factors provided either by stroma cells or by introduction of transgenic Bcl-2. Both myeloid- and lymphoid-derived IPC were indistinguishable by function, gene expression, and morphology.

CONCLUSION

Surprisingly, our results clearly show that murine IPCs differentiate from both lineages but are mainly of myeloid origin. These results extend to IPCs the observation made originally in classical dendritic cells that cellular expression of so called lineage markers does not correlate with lineal origin.

Enhanced Production of IL-10 by Dendritic Cells Deficient in CIITA

Dendritic cells (DC) are professional APCs that play a critical role in regulating immunity. In DC, maturation-induced changes in MHC class II expression and Ag presentation require transcriptional regulation by CIITA. To study the role of CIITA in DC, we evaluated key cell functions in DC from CIITA-deficient (CIITA(-/-)) mice. The ability to take up Ag, measured by fluid phase endocytosis, was comparable between CIITA(-/-) and control DC. Although CIITA(-/-) DC lack MHC class II, they maintained normal expression of costimulatory molecules CD80, CD86, and CD40. In contrast, CIITA(-/-) DC activated with LPS or CpG expressed increased IL-10 levels, but normal levels of TNF-alpha and IL-12 relative to control. Enhanced IL-10 was due to greater IL-10 mRNA in CIITA(-/-) DC. Abeta(-/-) DC, which lack MHC class II but express CIITA normally, had exhibited no difference in IL-10 compared with control. When CIITA was cotransfected with an IL-10 promoter-reporter into a mouse monocyte cell line, RAW 264.7, IL-10 promoter activity was decreased. In addition, reintroducing CIITA into CIITA(-/-) DC reduced production of IL-10. In all, these data suggest that CIITA negatively regulates expression of IL-10, and that CIITA may direct DC function in ways that extend beyond control of MHC class II.

Toll-like receptor engagement converts T-cell autoreactivity into overt autoimmune disease

Autoimmune diabetes mellitus in humans is characterized by immunological destruction of pancreatic beta islet cells. We investigated the circumstances under which CD8(+) T cells specific for pancreatic beta-islet antigens induce disease in mice expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein (GP) as a transgene under the control of the rat insulin promoter. In contrast to infection with LCMV, immunization with LCMV-GP derived peptide did not induce autoimmune diabetes despite large numbers of autoreactive cytotoxic T cells. Only subsequent treatment with Toll-like receptor ligands elicited overt autoimmune disease. This difference was critically regulated by the peripheral target organ itself, which upregulated class I major histocompatibility complex (MHC) in response to systemic Toll-like receptor-triggered interferon-alpha production. These data identify the 'inflammatory status' of the target organ as a separate and limiting factor determining the development of autoimmune disease.

Viral activation of macrophages through TLR-dependent and -independent pathways

Induction of cytokine production is important for activation of an efficient host defense response. Macrophages constitute an important source of cytokines. In this study we have investigated the virus-cell interactions triggering induction of cytokine expression in macrophages during viral infections. We found that viral entry and viral gene products produced inside the cell are responsible for activation of induction pathways leading to IFN-alphabeta expression, indicating that virus-cell interactions on the cell surface are not enough. Moreover, by the use of cell lines expressing dominant negative versions of TLR-associated adaptor proteins we demonstrate that Toll/IL-1 receptor domain-containing adaptor inducing IFN-beta is dispensable for all virus-induced cytokine expression examined. However, a cell line expressing dominant negative MyD88 revealed the existence of distinct induction pathways because virus-induced expression of RANTES and TNF-alpha was totally blocked in this cell line whereas IFN-alphabeta expression was much less affected in the absence of signaling via MyD88. In support of this, we also found that inhibitory CpG motifs, which block TLR9 signaling inhibited early HSV-2-induced TNF-alpha and RANTES expression dramatically whereas IFN-alphabeta induction was only slightly affected. This suggests that virus activates macrophages through distinct pathways, of which some are dependent on TLRs signaling through MyD88, whereas others seem to be independent of TLR signaling. Finally we demonstrate that IFN-alphabeta induction in HSV-2-infected macrophages requires a functional dsRNA-activated protein kinase molecule because cells expressing a dsRNA-dependent protein kinase version unable to bind dsRNA do not express IFN-alphabeta on infection.

Reading the viral signature by Toll-like receptors and other pattern recognition receptors

Successful host defense against viral infections relies on early production of type I interferon (IFN) and subsequent activation of a cellular cytotoxic response. The acute IFN and inflammatory response against virus infections is mediated by cellular pattern-recognition receptors (PRRs) that recognize specific molecular structures on viral particles or products of viral replication. Toll-like receptors (TLRs) constitute a class of membrane-bound PRRs capable of detecting microbial infections. While TLR2 and TLR4, which were first identified to recognize Gram-positive and Gram-negative bacteria, respectively, sense specific viral proteins on the cell surface, TLRs 3, 7, 8, and 9 serve as receptors for viral nucleic acids in endosomic compartments. In addition to TLRs, cells express cytoplasmic PRRs such as the RNA helicase retinoic acid inducible gene I and the kinase double-stranded RNA-activated protein kinase R, both of which sense dsRNA, a characteristic signature of viral replication, and initiate a protective cellular response. Here we review the recent progress in our understanding of PRRs and viral infections and discuss the molecular and cellular responses evoked by virus-activated PRRs. Finally, we look into what is currently known about the role of PRRs in viral infections in vivo.

Dendritic cells respond to influenza virus through TLR7- and PKR-independent pathways

Natural interferon-producing cells (IPC) secrete type I IFN (IFN-alpha and -beta) in response to influenza virus. This process is independent of viral replication and is mediated by Toll-like receptor 7 (TLR7), which recognizes single-stranded RNA (ssRNA). DC also express TLR7 but its function in DC response to influenza virus is unknown. To address this, we compared the DC and IPC responses to influenza virus and ssRNA oligoribonucleotides (ORN) that activate TLR7. When stimulated by ORN in vitro and in vivo, DC matured and produced inflammatory cytokines but not IFN-alpha. DC did secrete IFN-alpha in response to influenza virus. However, this response was independent of TLR7 signaling and required viral replication but not dsRNA-activated protein kinase (PKR). We conclude that DC and IPC are hard-wired to secrete IFN-alpha via different pathways, reflecting their complementary but distinct roles in anti-viral immunity.

Dendritic Cell Biology

Dendritic cells (DCs) are a special type of leukocytes able to alert the immune system to the presence of infections. They play a central role in the initiation of both innate and adaptive immune responses. This particular DC feature is regulated by the activation of specific receptors at the cell surface called Toll-like receptors (TLRs) that bind a number of microbial products collectively referred to as microbial-associated molecular patterns (MAMP). TLRs initiate a cascade of events, which together define the process of DC maturation. This phenomenon allows DCs to progressively acquire varying specific functions. DC maturation depends on the nature of the perturbation and permits unique and efficient immune responses for each pathogen. In this review the discussion is focused on DCs in the context of interactions with pathogens and DC-specific functions are highlighted.

Requirement of type I interferon signaling for arthritis triggered by double-stranded RNA

OBJECTIVE

Arthralgias and overt arthritides are often associated with viral infections. Viral infections expose the infected host to proinflammatory double-stranded RNA (dsRNA), which can cause joint inflammation and is a potent activator of interferon-alpha (IFNalpha). The aim of this study was to determine the role of IFNalpha and dsRNA-related signaling molecules in the onset of joint inflammation induced by viral dsRNA.

METHODS

IFNalpha and different forms of RNA were injected into the knee joints of wild-type mice, mice lacking the type I interferon receptor (IFNAR(-/-)), and mice deficient in dsRNA-dependent protein kinase (PKR(-/-)). Histologic evidence of joint damage and the ability of splenocytes to produce cytokines in response to dsRNA or IFNalpha were assessed.

RESULTS

Viral dsRNA, but not short single-stranded RNA, induced arthritis. The arthritis was aggravated by intracellular delivery of dsRNA. The expression of PKR was not mandatory for dsRNA-induced joint inflammation. In contrast, IFNalpha/beta signaling was important for dsRNA-induced joint inflammation because IFNAR(-/-) mice did not develop arthritis. Furthermore, intraarticular deposition of IFNalpha induced arthritis in PKR(-/-) and control mice, whereas IFNAR(-/-) mice were protected. The arthritogenic effect of IFNalpha was attenuated by in vivo depletion of monocyte/macrophages.

CONCLUSION

Arthritis triggered by dsRNA is not dependent on the expression of the dsRNA-signaling molecule PKR (or Toll-like receptor 3, as previously shown), but is associated with the ability to produce type I IFN and is critically dependent on type I IFN receptor signaling. The intrinsic arthritogenic properties of IFNalpha implicate a role of this cytokine in joint manifestations triggered by various interferogenic stimuli.

Regulation of Antigen Presentation and Cross-Presentation in the Dendritic Cell Network: Facts, Hypothesis, and Immunological Implications

Dendritic cells (DCs) are central to the maintenance of immunological tolerance and the initiation and control of immunity. The antigen-presenting properties of DCs enable them to present a sample of self and foreign proteins, contained within an organism at any given time, to the T-cell repertoire. DCs achieve this communication with T cells by displaying antigenic peptides bound to MHC I and MHC II molecules. Here we review the studies carried out over the past 15 years to characterize these antigen presentation mechanisms, emphasizing their significance in relation to DC function in vivo. The life cycles of different DC populations found in vivo are described. Furthermore, we provide a critical assessment of the studies that examine the mechanisms controlling DC MHC class II antigen presentation, which have often reached contradictory conclusions. Finally, we review findings pertaining to the biological mechanisms that enable DCs to present exogenous antigens on their MHC class I molecules, a process known as cross-presentation. Throughout, we highlight what we consider to be major knowledge gaps in the field and speculate on possible directions for future research.

Absence of cross-reactivity between murine Ly-6C and Ly-6G

BACKGROUND

The Ly-6 family has many members, including Ly-6C and Ly-6G. A previous study suggested that the anti-Ly-6G antibody, RB6-8C5, may react with Ly-6Chi murine bone marrow (BM) cells. This finding has been interpreted as cross-reactivity of RB6-8C5 with the Ly-6C antigen, and has been generalized to many hematopoietic cell types, using the terminology Ly-6G/C. The present study was undertaken to determine whether anti-Ly-6G antibodies truly cross-react with the Ly-6C antigen on multiple hematopoietic cell types.

METHODS

Splenocytes, thymocytes, and BM cells obtained from Ly-6.1 and Ly-6.2 strains of mice were stained with a variety of antibodies to Ly-6C and Ly-6G. Flow cytometric analysis was performed on these populations.

RESULTS

Evaluation of anti-Ly-6C and anti-Ly-6G staining showed only Ly-6C expression and no Ly-6G expression on subsets of splenic T and B cells and thymocytes from Ly-6.1 and Ly-6.2 mice. Bone marrow cells were identified that express both Ly-6G and Ly-6C; no Ly-6G+Ly-6C- populations were seen.

CONCLUSIONS

Multiple Ly-6C+ hematopoietic cell populations were identified that do not stain with anti-Ly-6G antibodies. This calls into question the use of the Ly-6G/C nomenclature and suggests that epitopes recognized by anti-Ly-6G antibodies should simply be designated Ly-6G.

Replication-dependent potent IFN-alpha induction in human plasmacytoid dendritic cells by a single-stranded RNA virus

Plasmacytoid dendritic cells sense viral ssRNA or its degradation products via TLR7/8 and CpG motifs within viral DNA via TLR9. Although these two endosomal pathways operate independently of viral replication, little is known about the detection of actively replicating viruses in plasmacytoid dendritic cell (PDC). Replication and transcription of the viral genome of ssRNA viruses as well as many DNA viruses lead to the formation of cytosolic dsRNA absent in noninfected cells. In this study, we used human respiratory syncytial virus (HRSV) encoding a fusion (F) protein for direct cytosolic entry. Both HRSV infection and cytosolic delivery of a 65-nt dsRNA led to potent IFN-alpha induction in PDC, but not in myeloid dendritic cells. Inactivation of HRSV by UV irradiation abrogated IFN-alpha induction in PDC. The comparison of two respiratory syncytial virus (RSV) constructs carrying either the HRSV or the bovine RSV F protein revealed that F-mediated cytosolic entry of RSV was absolutely required for IFN-alpha induction in PDC. HRSV-induced IFN-alpha production was independent of endosomal acidification and of protein kinase R (PKR) kinase activity, as demonstrated with chloroquine and the PKR inhibitor 2-aminopurine, respectively. In contrast, the induction of IFN-alpha by the TLR7/8 ligand R848, by the TLR9 ligand CpG-A ODN 2216, and by inactivated influenza virus (TLR7/8 dependent) was completely blocked by 2-aminopurine. IFN-alpha induction by mouse pathogenic Sendai virus was not affected in PKR- and MyD88-deficient mice, confirming that a ssRNA virus, which is able to directly enter host cells via fusion at the plasma membrane, can be detected by PDC independently of PKR, TLR7/8, and TLR9.

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.

Cross-presentation, dendritic cell subsets, and the generation of immunity to cellular antigens

Cross-presentation involves the uptake and processing of exogenous antigens within the major histocompatibility complex (MHC) class I pathway. This process is primarily performed by dendritic cells (DCs), which are not a single cell type but may be divided into several distinct subsets. Those expressing CD8alpha together with CD205, found primarily in the T-cell areas of the spleen and lymph nodes, are the major subset responsible for cross-presenting cellular antigens. This ability is likely to be important for the generation of cytotoxic T-cell immunity to a variety of antigens, particularly those associated with viral infection, tumorigenesis, and DNA vaccination. At present, it is unclear whether the CD8alpha-expressing DC subset captures antigen directly from target cells or obtains it indirectly from intermediary DCs that traffic from peripheral sites. In this review, we examine the molecular basis for cross-presentation, discuss the role of DC subsets, and examine the contribution of this process to immunity, with some emphasis on DNA vaccination.

Bone marrow plasmacytoid dendritic cells can differentiate into myeloid dendritic cells upon virus infection

Two subsets of dendritic cell (DCs), plasmacytoid (p) and myeloid (m) DCs, have been described in humans and mice. These subsets are known to have divergent roles during an immune response, but their developmental course is unclear. Here we report that virus infection induces bone marrow pDCs to differentiate into mDCs, thereby undergoing profound phenotypic and functional changes including the acquisition of enhanced antigen-presenting capacity and the ability to recognize different microbial structures through Toll-like receptor 4. The conversion of pDCs into mDCs is also induced by the injection of double-stranded RNA and requires type I interferons. Our results establish a precursor-product developmental relationship between these two DC subsets and highlight unexpected plasticity of bone marrow pDCs.

Characterization of colonic and mesenteric lymph node dendritic cell subpopulations in a murine adoptive transfer model of inflammatory bowel disease

Ulcerative colitis and Crohn's disease, collectively termed inflammatory bowel diseases (IBD), are chronic inflammatory diseases of the intestine that afflict more than 4 million people worldwide. Intestinal inflammation is characterized by an abnormal mucosal immune response to normally harmless antigens in the gut flora. In Crohn's disease, the pathogenic mucosal immune response is a typical T helper (TH1) type cell response, whereas ulcerative colitis is predominantly associated with a TH2 response. We are interested in the role of dendritic cells in early immunologic events leading to T cell activation and chronic intestinal inflammation. Using a murine adoptive transfer model of IBD, we found an accumulation of dendritic cells in colon and mesenteric lymph nodes during the early stage of IBD before the appearance of epithelial lesions and tissue degradation. In situ immunostaining and flow-cytometric analysis revealed that approximately 50% of colonic dendritic cells were CD11b B220 myeloid dendritic cells and 50% expressed the CD11b B220 plasmacytoid phenotype. In corresponding mesenteric lymph nodes, approximately 16% were plasmacytoid dendritic cells. Colonic myeloid dendritic cells were shown to express the co-stimulatory molecule CD40. Both, colonic myeloid and plasmacytoid dendritic cells released interferon-alpha in situ and stimulated T cell proliferation ex vivo. Our results show that dendritic cells can mature in the intestine without migrating to mesenteric lymph nodes. Mature intestinal dendritic cells may form a nucleation site for a local T cell response and play an important role in the pathogenesis of IBD.