Murine plasmacytoid pre-dendritic cells generated from Flt3 ligand-supplemented bone marrow cultures are immature APCs

Understanding the multiple functions of Gr-1(+) cell subpopulations during microbial infection

The murine cell surface determinant Gr-1 is expressed at high level on neutrophils. Depletion of polymorphonuclear leukocytes with anti-Gr-1(+) monoclonal antibody results in increased susceptibility and dysregulated immunity to many microbial pathogens, a finding widely interpreted to indicate the importance of neutrophils during infection. Yet, in recent years it has become clear that additional cell types express the Gr-1 determinant, including dendritic cell and monocyte subpopulations. In this review, we evaluate current knowledge on the functional aspects of Gr-1(+) cell populations. We focus on infection with the opportunistic protozoan Toxoplasma gondii, a case where host survival depends on an intact Gr-1(+) cell population.

Dendritic cell subsets and toll-like receptors

Toll-like receptors exist as highly conserved pathogen sensors throughout the animal kingdom and they represent a key family of molecules bridging the ancient innate and adaptive immune systems. The first molecules of adaptive immunity appeared in the cartilaginous fishes and, with these, major histocompatibility proteins and cells expressing these molecules, and thus, by definition, the advent of antigen-presenting cells and the "professional" antigen-presenting cells, the dendritic cells. Dendritic cells themselves are highly specialized subsets of cells with the major roles of antigen presentation and stimulation of lymphocytes. The dendritic cell functions of inducing immunity are regulated by their own activation status, which is governed by their encounter with pathogen-associated molecular patterns that signal through pattern recognition receptors, including Toll-like receptors, expressed at the surface and within the cytoplasm and endosomal membranes of dendritic cells. Thus although dendritic cells play a crucial role in the induction of adaptive immunity, the adaptive response is itself initiated at the level of ancient receptors of the innate immune system. A further degree in the complexity of dendritic cell activation is established by the fact that not all dendritic cells are equal. Dendritic cells exist as multiple subsets that vary in location, function, and phenotype. Distinct dendritic cell subsets display great variation in the type of Toll-like receptors expressed and consequently variation in the type of pathogens sensed and the subsequent type of immune responses initiated.

High-affinity uptake of kynurenine and nitric oxide-mediated inhibition of indoleamine 2,3-dioxygenase in bone marrow-derived myeloid dendritic cells

Indoleamine 2,3-dioxygenase (IDO)-initiated tryptophan metabolism along the kynurenine (Kyn) pathway in some dendritic cells (DC) such as plasmacytoid DC (pDC) regulates T-cell responses. It is unclear whether bone marrow-derived myeloid DC (BMDC) express functional IDO. The IDO expression was examined in CD11c(+)CD11b(+) BMDC differentiated from mouse bone marrow cells using GM-CSF. CpG oligodeoxynucleotides (CpG) induced the expression of IDO protein with the production of nitric oxide (NO) in BMDC in cultures for 24h. In the enzyme assay using cellular extracts of BMDC, the IDO activity of BMDC stimulated with CpG was enhanced by the addition of a NO synthase (NOS) inhibitor, suggesting that IDO activity was suppressed by NO production. On the other hand, the concentration of Kyn in the culture supernatant of BMDC was not increased by stimulation with CpG. Exogenously added Kyn was taken up by BMDC independently of CpG stimulation and NO production, and the uptake of Kyn was inhibited by a transport system L-specific inhibitor or high concentrations of tryptophan. The uptake of tryptophan by BMDC was markedly lower than that of Kyn. In conclusion, IDO activity in BMDC is down-regulated by NO production, whereas BMDC strongly take up exogenous Kyn.

Development of plasmacytoid dendritic cells in bone marrow stromal cell niches requires CXCL12-CXCR4 chemokine signaling

Plasmacytoid dendritic cells (pDCs), also known as type I interferon (IFN)–producingcells, are thought to play central roles in antiviral immunity and the pathogenesis of some autoimmune diseases. pDCs are produced from hematopoietic stem cells in bone marrow. However, the environmental regulation of the development of pDCs is not fully understood. Here, we show that the numbers of pDCs and their earliest progenitors are severely reduced in the absence of CXCR4, the primary physiologic receptor for CXC chemokine ligand 12 (CXCL12), also known as stromal cell–derived factor-1 (SDF-1) in vivo. In vitro, CXCL12 induces a significant increase in pDC numbers generated from primitive hematopoietic cells, and pDCs and their progenitors migrate to CXCL12. In addition, most pDCs are in contact with CXCL12-abundant reticular (CAR) cells in the intersinal space of bone marrow, although many primitive hematopoietic cells adjoin CAR cells surrounding sinusoidal endothelial cells or residing near the bone surface. Thus we identified CXCL12 as a key regulator of pDC development produced by cellular niches, providing new targets for pDC therapeutic control.

Type I interferon regulates pDC maturation and Ly49Q expression

Ly49Q is expressed on peripheral mouse plasmacytoid dendritic cells (pDC). Immature Ly49Q-negative pDC precursors acquire Ly49Q in the bone marrow and then migrate into the periphery. While searching for molecules that regulate pDC maturation, we found that type I interferon (IFN) inhibited Ly49Q acquisition in vitro. Infections that induce type I IFN production by cells other than pDC (a condition mimicked by poly(I:C) injection in vivo) increase the prevalence of Ly49Q(-) pDC in the bone marrow and peripheral lymphoid organs in wild-type but not IFN-alpha/beta receptor knockout BALB/c mice. Moreover, in vivo exposure to type I IFN causes some Ly49Q(-), but not Ly49Q(+), pDC to convert to conventional DC, defined as B220(-) CD11c(+) CD11b(+) cells. These data suggest that type I IFN regulates pDC development and affects their distribution in the body.

Matrix protein mediated shutdown of host cell metabolism limits vesicular stomatitis virus-induced interferon-alpha responses to plasmacytoid dendritic cells

Upon infection with many different viruses, plasmacytoid dendritic cells (pDC) produce large amounts of type I interferon (IFN-alpha/beta). To address why upon vesicular stomatitis virus (VSV) infection pDC, but not conventional myeloid DC (mDC), are induced to produce IFN-alpha, pDC and mDC were differentiated from bone marrow cells (BM-DC). Upon VSV infection BM-pDC produced IFN-alpha, whereas BM-mDC did not. Notably, upon infection with VSV-M2, a VSV variant expressing a M51R mutant matrix (M) protein that showed a reduced sequestration of host cell metabolism, BM-pDC and BM-mDC mounted massive IFN-alpha responses. Both DC subsets showed comparable RNA levels of retinoic acid inducible gene-I (RIG-I) and Toll-like receptor (TLR) 7 and were able to respond upon triggering with double-stranded RNA (dsRNA) or single-stranded RNA (ssRNA) analogs. Moreover, upon VSV-M2 infection IFN-alpha production by both DC subsets was largely dependent on viral replication. Interestingly, upon virus infection BM-pDC, but not BM-mDC, up-regulated mRNA levels of nuclear export factors Nup96/98, probably reflecting cellular mechanisms to circumvent viral escape strategies. Collectively, these results indicated that cell types induced to produce IFN-alpha upon viral infection are not primarily defined by cellular receptor configurations but rather by complex virus/host cell interactions.

Vaccination with plasmacytoid dendritic cells induces protection against infection with Leishmania major in mice

DC-based vaccination against Leishmania major induces a parasite-specific Th1 response and long-lasting protective immunity in susceptible mice. Since distinct DC subsets have been proposed to direct the predominant development of either Th1 or Th2 cells, we analyzed the capability of plasmacytoid DC (pDC) to induce protection and elicit a Th1 response against L. major. Pulsing with L. major lysate induced the activation and maturation of semi-mature murine pDC that had been isolated from the spleen, as indicated by up-regulation of the co-stimulatory molecules CD86 and CD80, but did not enhance the level of IFN-alpha secretion by pDC. Vaccination of susceptible mice with L. major lysate-pulsed pDC induced highly effective T cell-mediated immunity against subsequent infection with L. major parasites. Surprisingly, the protection was not accompanied by a polarized Th1 cytokine profile. Co-activation of pDC with CpG-containing oligodeoxynucleotides, which has been shown to be critical for activating the protective potential of myeloid DC, was not required for the protective effect of L. major antigen-pulsed pDC. These findings demonstrate that antigen-loaded pDC are able to induce T cell-mediated protection against a parasite disease and that experimental leishmaniasis is a suitable model to elucidate the mechanisms underlying DC-based vaccination against infections.

Development and function of murine B220+CD11c+NK1.1+ cells identify them as a subset of NK cells

Lymphoid organs contain a B220⁺CD11c⁺NK1.1⁺ cell population that was recently characterized as a novel dendritic cell (DC) subset that functionally overlaps with natural killer (NK) cells and plasmacytoid DCs (PDCs). Using Siglec-H and NK1.1 markers, we unambiguously dissected B220⁺CD11c⁺ cells and found that PDCs are the only professional interferon (IFN)-α–producing cells within this heterogeneous population. In contrast, B220⁺CD11c⁺NK1.1⁺ cells are a discrete NK cell subset capable of producing higher levels of IFN-γ than conventional NK cells. Unlike DCs, only a minute fraction of B220⁺CD11c⁺NK1.1⁺ cells in the spleen expressed major histocompatibility complex class II ex vivo or after stimulation with CpG. Consistent with being a NK cell subset, B220⁺CD11c⁺NK1.1⁺ cells depended primarily on interleukin 15 and common cytokine receptor γ chain signaling for their development. In terms of function, expression of distinctive cell surface receptors, and location in lymphoid organs, NK1.1⁺B220⁺CD11c⁺ appear to be the murine equivalent of human CD56^bright NK cells.

Development of plasmacytoid and conventional dendritic cell subtypes from single precursor cells derived in vitro and in vivo

The development of functionally specialized subtypes of dendritic cells (DCs) can be modeled through the culture of bone marrow with the ligand for the cytokine receptor Flt3. Such cultures produce DCs resembling spleen plasmacytoid DCs (pDCs), CD8(+) conventional DCs (cDCs) and CD8(-) cDCs. Here we isolated two sequential DC-committed precursor cells from such cultures: dividing 'pro-DCs', which gave rise to transitional 'pre-DCs' en route to differentiating into the three distinct DC subtypes (pDCs, CD8(+) cDCs and CD8(-) cDCs). We also isolated an in vivo equivalent of the DC-committed pro-DC precursor cell, which also gave rise to the three DC subtypes. Clonal analysis of the progeny of individual pro-DC precursors demonstrated that some pro-DC precursors gave rise to all three DC subtypes, some produced cDCs but not pDCs, and some were fully committed to a single DC subtype. Thus, commitment to particular DC subtypes begins mainly at this pro-DC stage.

M-CSF: a novel plasmacytoid and conventional dendritic cell poietin

The critical importance of plasmacytoid dendritic cells (pDCs) in viral infection, autoimmunity, and tolerance has focused major attention on these cells that are rare in blood and immune organs of humans and mice. The recent development of an Flt-3 ligand (FL) culture system of bone marrow cells has led to the simple generation of large numbers of pDCs that resemble their in vivo steady-state counterparts. The FL system has allowed unforeseen insight into the biology of pDCs, and it is assumed that FL is the crucial growth factor for these cells. Surprisingly we have found that a cell type with high capacity for interferon-alpha (IFN-alpha) production in response to CpG-containing oligonucleotides, a feature of pDCs, develop within macrophage-colony-stimulating factor (M-CSF)-generated bone marrow cultures. Analysis of this phenomenon revealed that M-CSF is able to drive pDCs as well as conventional DCs (cDCs) from BM precursor cells in vitro. Furthermore, application of M-CSF to mice was able to drive pDCs and cDCs development in vivo. It is noteworthy that using mice deficient in FL indicated that the M-CSF-driven generation of pDCs and cDCs in vitro and in vivo was independent of endogenous FL.

Dendritic cells: ontogeny

Dendritic cells (DC) play key rolls in various aspects of immunity. The functions of DC depend on the subsets as well as their location or activation status. Understanding developmental lineages, precursors and inducing factors for various DC subsets would help their clinical application, but despite extensive efforts, the precise ontogeny of various DC, remain unclear and complex. Because of their many functional similarities to macrophages, DC were originally thought to be of myeloid-lineage, an idea supported by many in vitro studies where monocytes or GM-CSF (a key myeloid growth factor) has been extensively used for generating DC. However, there has been considerable evidence which suggests the existence of lymphoid-lineage DC. After the confusion of myeloid-/lymphoid-DC concept regarding DC surface markers, we have now reached a consensus that each DC subset can differentiate through both myeloid- and lymphoid-lineages. The identification of committed populations (such as common myeloid- and lymphoid progenitors) as precursors for every DC subsets and findings from various knockout (KO) mice that have selected lymphoid- or myeloid-lineage deficiency appear to indicate flexibility of DC development rather than their lineage restriction. Why is DC development so flexible unlike other hematopoitic cells? It might be because there is developmental redundancy to maintain such important populations in any occasions, or such developmental flexibility would be advantageous for DC to be able to differentiate from any "available" precursors in situ irrespective of their lineages. This review will cover ontogeny of conventional (CD8 +/- DC) DC, plasmacytoid DC and skin Langerhans cells, and recently-identified many Pre-DC (immediate DC precursor) populations, in addition to monocytes and plasmacytoid DC, will also be discussed.

Dendritic cells: nature and classification

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

Dendritic cell vaccination

There has been a surge of interest in the use of dendritic cell (DC) vaccination as cellular immunotherapy for numerous cancers. Despite some encouraging results, this therapeutic modality is far from being considered as a therapy for cancer. This review will first discuss preclinical DC vaccination in murine models of cancer, with an emphasis on comparative studies investigating different methods of antigen priming. We will then comment on the various murine DC subsets and how these relate to human DC preparations used for clinical studies. Finally, the methodology used to generate human DCs and some recent clinical trials in several cancers are reviewed.

Type I interferon inhibition and dendritic cell activation during gammaherpesvirus respiratory infection

The respiratory tract is a major mucosal site for microorganism entry into the body, and type I interferon (IFN) and dendritic cells constitute a first line of defense against viral infections. We have analyzed the interaction between a model DNA virus, plasmacytoid dendritic cells, and type I IFN during lung infection of mice. Our data show that murine gammaherpesvirus 68 (gammaHV68) inhibits type I IFN secretion by dendritic cells and that plasmacytoid dendritic cells are necessary for conventional dendritic cell maturation in response to gammaHV68. Following gammaHV68 intranasal inoculation, the local and systemic IFN-alpha/beta response is below detectable levels, and plasmacytoid dendritic cells are activated and recruited into the lung with a tissue distribution that differs from that of conventional dendritic cells. Our results suggest that plasmacytoid dendritic cells and type I IFN have important but independent roles during the early response to a respiratory gammaHV68 infection. gammaHV68 infection inhibits type I IFN production by dendritic cells and is a poor inducer of IFN-alpha/beta in vivo, which may serve as an immune evasion strategy.

Vesicular stomatitis virus glycoprotein G activates a specific antiviral Toll-like receptor 4-dependent pathway

We have previously shown that mutations of CD14 or TLR4 impair type I interferon (IFN) production and macrophage survival during infection with vesicular stomatitis virus (VSV). We now report that VSV glycoprotein G (gpG) is essential for the induction of a previously unrecognized CD14/TLR4-dependent response pathway in which the adapter TRAM has predominant importance, absent any need for MyD88 or Mal, and with only a partial requirement for TRIF. Downstream of TRAM, IRF7 activation leads to a type I IFN response. The pathway is utilized by myeloid dendritic cells (mDCs) and macrophages rather than plasmacytoid DCs. This new mode of TLR4 signal transduction, which does not stimulate NF-kappaB activation, reveals the importance of viral protein recognition by mDCs and shows that TLR4 can drive qualitatively different events within the cell in response to different ligands.

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.

Generation and maturation of bone marrow-derived DCs under serum-free conditions

Standard protocols for the generation of murine dendritic cells (DCs) employ medium supplemented with heat-inactivated fetal calf serum (FCS). Recently, several attempts have been made to avoid serum exposure during DC culture. The impetus for these efforts has been a desire to generate DCs for clinical use, as preclinical data have demonstrated their efficacy in immune activation and in immune suppression both in vitro and in vivo. However, these protocols have resulted in contradictory outcomes with respect to DC survival in culture and activation status. In this report, we compared several serum-free culture conditions with respect to survival, differentiation, activation, and cytokine profile of murine DC progenitors. DC progenitors can survive only in some serum-free conditions. Surprisingly, DCs grown in serum-free medium display a higher expression of activation markers upon stimulation. They produce increased IL-12 and decreased IL-6 following stimulation. Furthermore, DCs derived under serum-free conditions may express unusual surface markers, B220 and Ly6C/G, implying an increased differentiation to plasmacytoid DCs (pDCs).

Plasmacytoid dendritic cells synergize with myeloid dendritic cells in the induction of antigen-specific antitumor immune responses

Plasmacytoid dendritic cells (pDC) are capable of producing high levels of type I IFNs upon viral stimulation, and play a central role in modulating innate and adaptive immunity against viral infections. Whereas many studies have assessed myeloid dendritic cells (mDC) in the induction of antitumor immune responses, the role of pDC in antitumor immunity has not been addressed. Moreover, the interaction of pDC with other dendritic cell subsets has not been evaluated. In this study, we analyzed the capacity of pDC in stimulating an Ag-specific T cell response. Immunization of mice with Ag-pulsed, activated pDC significantly augmented Ag-specific CD8(+) CTL responses, and protected mice from a subsequent tumor challenge. Immunization with a mixture of activated pDC plus mDC resulted in increased levels of Ag-specific CD8(+) T cells and an enhanced antitumor response compared with immunization with either dendritic cell subset alone. Synergy between pDC and mDC in their ability to activate T cells was dependent on MHC I expression by mDC, but not pDC, suggesting that pDC enhanced the ability of mDC to present Ag to T cells. Our results demonstrate that pDC and mDC can interact synergistically to induce an Ag-specific antitumor immune response in vivo.

Regulation of dendritic cell differentiation and subset distribution by the zinc finger protein CTCF

The molecular mechanisms that regulate DC differentiation and subset distribution are largely unknown. In this study we report the identification of the C(2)H(2) zinc finger transcription factors (TF) CTCF as a regulator of DC differentiation. CTCF was expressed in human and murine DC and its expression was downregulated during the differentiation of human monocyte-derived DC. Enforced expression of CTCF during the differentiation of murine BM-derived DC (BMDC) caused increased apoptosis and reduced proliferation leading to a dramatically reduced number of CTCF transduced DC. The CTCF expressing BMDC that developed had a more immature phenotype than control cells, and showed defects in maturation upon TLR stimulation. Furthermore, in vivo expression of CTCF led to an increase in the percentage of plasmacytoid DC (pDC) within the DC lineage. Our data provide new insight into molecular mechanisms regulating DC differentiation and subset development and identify CTCF as a factor involved in the regulation of these important processes.

CpG-B ODNs potently induce low levels of IFN-alphabeta and induce IFN-alphabeta-dependent MHC-I cross-presentation in DCs as effectively as CpG-A and CpG-C ODNs

Deoxycytidyl-deoxyguanosine [(CpG)3] oligodeoxynucleotides (ODNs) signal through TLR9 to induce type-I IFN (IFN-alphabeta) and IFN-alphabeta-dependent MHC-I cross-presentation of exogenous antigens by dendritic cells (DCs). A puzzle was presented by our observation that three ODN classes, CpG-A, CpG-B, and CpG-C, had similar efficacy for induction of IFN-alphabeta-dependent MHC-I antigen cross-presentation by myeloid DCs despite greatly differing for induction of IFN-alphabeta (CpG-A>CpG-C>>CpG-B). All ODN classes similarly enhanced plasmacytoid DC (pDC) presentation of exogenous MHC-I-restricted peptide, although pDCs did not cross-process protein antigen. MHC-I and the transporter for antigen presentation were induced by all ODN classes or IFN-alpha. CpG-B ODNs were slightly more potent than CpG-A or CpG-C ODNs for induction of low levels of IFN-alphabeta but less efficacious at high concentrations than CpG-A or CpG-C ODNs. Low levels of IFN-alphabeta induced by CpG-B ODNs sufficed for full induction of MHC-I cross-presentation. Thus, CpG-B ODNs are slightly more potent but less efficacious than CpG-A and CpG-C ODNs for induction of IFN-alphabeta. High sensitivity to IFN-alphabeta allows CpG-B ODNs to be equally efficacious for induction of MHC-I cross-presentation. CpG-B ODNs may be effective for inducing therapeutic responses that require low levels of IFN-alphabeta and may avoid unnecessarily high induction of IFN-alphabeta.

Differentiation of dendritic cell subsets from mouse bone marrow

Dendritic cells (DC) are key regulators of the immune system. They are capable of stimulating lymphocytes to generate potent cell-mediated and humoral immune responses against pathogens and tumor cells. DC not only activate lymphocytes, but can also educate T cells to tolerate self-antigens, thereby minimizing autoimmune reactions. Another peculiarity of the DC system is the large variety of subsets described, both in the human and in the mouse, according to surface phenotype and organ distribution. Different protocols have been developed to differentiate DC from total mouse bone marrow in vitro. Here, we describe the isolation of a specific DC progenitor population, referred to as preimmunocytes, and document protocols for their differentiation into various DC subsets.

Steady-state and inflammatory dendritic-cell development

The developmental pathways that lead to the production of antigen-presenting dendritic cells (DCs) are beginning to be understood. These are the last of the pathways of haematopoiesis to be mapped. The existence of many specialized subtypes of DC has complicated this endeavour, as has the need to distinguish the DCs formed in steady state from those produced during an inflammatory response. Here we review studies that lead to the concept that different types of DC develop through different branches of haematopoietic pathways that involve different immediate precursor cells. Furthermore, these studies show that many individual tissues generate their own DCs locally, from a reservoir of immediate DC precursors, rather than depending on a continuous flux of DCs from the bone marrow.

Plasmacytoid dendritic cells limit viral replication, pulmonary inflammation, and airway hyperresponsiveness in respiratory syncytial virus infection

Plasmacytoid dendritic cells (pDC), as major producers of IFN-alpha, are thought not only to be pivotal in antiviral immunity, but also to limit allergic inflammation. In this study, we delineate the role of pDC in a mouse model of respiratory syncytial virus (RSV)-induced airway inflammation. Bone marrow-derived pDC generated high levels of IFN-alpha upon RSV infection, and the percentage of pDC expressing MHC class II and maturation-associated costimulatory molecules was increased. However, their weak Ag-presenting capacity was not enhanced. Furthermore, pDC induced marked levels of IL-10 in T cell cultures irrespective of infection. In vivo, numbers of pDC in the lung increased early after RSV infection and remained elevated throughout the inflammatory phase and the resolution phase of infection. Depletion of pDC resulted in increases in peak RSV titers, pulmonary inflammation, and airway hyperresponsiveness. In contrast, adoptive transfer of activated pDC to the airways reduced RSV copy numbers. In conclusion, RSV infection induces activation of murine pDC with robust IFN-alpha production, limiting replication and accelerating elimination of RSV. In addition to this innate response, pDC also may play an immune regulatory role in reducing pulmonary inflammation and inhibiting the development of airway hyperresponsiveness.

Plasmacytoid dendritic cells do not migrate in intestinal or hepatic lymph

Plasmacytoid dendritic cells (pDCs) recognize pathogen-associated molecules, particularly viral, and represent an important mechanism in innate defense. They may however, also have roles in steady-state tolerogenic responses at mucosal sites. pDCs can be isolated from blood, mucosa, and lymph nodes (LNs). Although pDCs can express peripherally derived Ags in LNs and at mucosal sites, it is not clear whether pDCs actually migrate from the periphery in lymph or whether LN pDCs acquire Ags by other mechanisms. To determine whether pDCs migrate in lymph, intestine or liver-draining LNs were removed and thoracic duct leukocytes (TDLs) were collected. TDLs expressing MHC-II and CD45R, but not TCRalphabeta or CD45RA, were then analyzed. These enriched TDLs neither transcribe type I IFNs nor secrete inflammatory cytokines in response to viral stimuli in vitro or after a TLR7/8 stimulus in vivo. In addition, these TDLs do not express CD5, CD90, CD200, or Siglec-H, but do express Ig, and therefore represent B cells, despite their lack of CD45RA expression. Intestinal and hepatic lymph are hence devoid of bona fide pDCs under both steady-state conditions and after TLR7/8 stimulation. This shows that any role for pDCs in Ag-specific T cell activation or tolerance must differ from the roles of classical dendritic cells, because it cannot result from peripheral Ag capture, followed by migration of pDCs via lymph to the LN.

Prion Protein Expression by Mouse Dendritic Cells Is Restricted to the Nonplasmacytoid Subsets and Correlates with the Maturation State

Expression of the physiological cellular prion protein (PrP(C)) is remarkably regulated during differentiation and activation of cells of the immune system. Among these, dendritic cells (DCs) display particularly high levels of membrane PrP(C), which increase upon maturation, in parallel with that of molecules involved in Ag presentation to T cells. Freshly isolated mouse Langerhans cells, dermal DCs, and DCs from thymus, spleen, and mesenteric lymph nodes expressed low to intermediate levels of PrP(C). Highest levels of both PrP(C) and MHC class II molecules were displayed by lymph node CD8alpha(int) DCs, which represent fully mature cells having migrated from peripheral tissues. Maturation induced by overnight culture resulted in increased levels of surface PrP(C), as did in vivo DC activation by bacterial LPS. Studies on Fms-like tyrosine kinase 3 ligand bone marrow-differentiated B220(-) DCs confirmed that PrP(C) expression followed that of MHC class II and costimulatory molecules, and correlated with IL-12 production in response to TLR-9 engagement by CpG. However, at variance with conventional DCs, B220(+) plasmacytoid DCs isolated from the spleen, or in vitro differentiated, did not significantly express PrP(C), both before and after activation by TLR-9 engagement. PrP knockout mice displayed higher numbers of spleen CD8alpha(+) DCs, but no significant differences in their maturation response to stimulation through TLR-4 and TLR-9 were noticed. Results are discussed in relation to the functional relevance of PrP(C) expression by DCs in the induction of T cell responses, and to the pathophysiology of prion diseases.

Dendritic cells and tumor microenvironment: a dangerous liaison

The fact that the immune response to cancer is compromised has been convincingly demonstrated in murine tumor models as well as in cancer patients. The unresponsiveness of the host immune system is one of the major mechanisms of tumor escape as well as an important factor that limits the success of cancer immunotherapy. Inadequate function of professional antigen presenting cells dendritic cells (DC) in cancer is one of the major elements of compromised anti-tumor immune response. Despite substantial progress in recent years, the mechanism of inadequate DC function in cancer still remains unclear. The tumor microenvironment has emerged as an important component contributing to DC malfunction. In this review we will discuss the potential role of tumor microenvironment in DC dysfunction.

Natural killer cell-dendritic cell crosstalk in the initiation of immune responses

Dendritic cells (DCs) and natural killer (NK) cells play a critical role in early defences against cancer and infections. They specialise in complementary functions, including IL-12 or IFN-alpha/beta secretion and antigen presentation for the former, and IFN-gamma secretion and killing of infected or tumour cells for the latter. Both DCs and NK cells are also sensors of the immune system that have developed different, but partially overlapping, systems to identify pathology associated danger signals. Evidence of NK-DC interaction has accumulated recently. This interaction may lead to NK cell activation, DC activation, or apoptosis depending on the activation status of both cell types. Thus, the outcome of NK-DC crosstalk is likely to influence both innate and adaptive immune responses. This review addresses the molecular mechanisms under-lying the different NK-DC interactions, and their in vivo significance in anti-tumour or antimicrobial immunity. Finally, we discuss the potential clinical implications of this new field.

What is natural in natural killer cells?

Denomination is a not a trivial issue in human activities in general, and in science in particular. Natural killer (NK) cells have been originally characterized as cytolytic effector lymphocytes present in mouse spleen and human peripheral blood. The "NK" name was originally coined after the NK cell's ability to kill targets cells without requirement for a prior exposure to these targets, in contrast to cytolytic T cells (Kiessling, R., Klein, E., Wigzell, H. 1975. "Natural" killer cells in the mouse. I. Cytotoxic cells with specificity for mouse Moloney leukemia cells. Specificity and distribution according to genotype. Eur J Immunol 5:112-117). In this review, an attempt is made to revisit the basis for this vastly admitted definition of NK cells, and to enumerate some of the unsolved questions related to NK cell biology. An emphasis will be given to recent data showing that circulating NK cells are not steady state killers unless they have gone through a process of functional maturation. This process that is referred as to "NK cell education" thereafter involves self MHC class I recognition via inhibitory receptors, but also some still unidentified factors. Therefore, the NK denomination does not reflect a genuine property of these lymphocytes that appear to more and more resemble T and B lymphocytes.

Differential expansion, activation and effector functions of conventional and plasmacytoid dendritic cells in mouse tissues transiently infected with Listeria monocytogenes

Dendritic cells (DC) are crucial in generating immunity to infection. Here we characterize changes in DC in terms of number, activation and effector functions, focusing on conventional DC (cDC) and plasmacytoid DC (pDC), in Listeria-infected mice. Kinetic studies showed a subset- and tissue-specific expansion of cDC and upregulation of CD80 and CD86 on splenic and mesenteric lymph node (MLN) cDC after intragastric infection. Expansion of pDC was more prolonged than cDC, and pDC upregulated CD86 and MHC-II, but not CD80, in both the spleen and MLN. cDC were an important source of IL-12 but not TNF-alpha during infection, while pDC made neither of these cytokines. Instead other CD11c(int) cells produced these cytokines. Using five-colour flow cytometry and double intracellular cytokine staining, we detected phenotypically similar CD11c(int)CD11b(+)Gr1(+) cells with distinct capacities to produce TNF-alpha/IL-12 or TNF-alpha/iNOS (inducible nitric oxide synthase) in Listeria-infected tissues. IL-12p70 was also produced by sorted CD11c(hi) and CD11c(int)CD11b(+)Gr1(+) cells. Furthermore, production of TNF-alpha, iNOS and IL-12 was differentially dependent on cellular localization of the bacteria. Cytosol-restricted bacteria induced TNF-alpha and iNOS-producing cells, albeit at lower frequency than wild-type bacteria. In contrast, IL-12 was induced only with wild-type bacteria. These data provide new insight into the relative abundance and function of distinct CD11c-expressing populations during the early stage of Listeria infection.

Expression of the class A macrophage scavenger receptor on specific subpopulations of murine dendritic cells limits their endotoxin response

Dendritic cells (DC) function at the interface of innate and acquired immunity and are uniquely sensitive to specific stimuli. Pattern recognition receptors (PRR) on these cells are critically important because of their ability to recognise and initiate responses to conserved microbial-associated molecular signatures. With the exception of Toll-like receptors (TLR), we know relatively little about the specific distribution of other PRR amongst populations of DC. Here, we describe the expression of the murine class A macrophage scavenger receptor (SR-A) and show that it is restricted to specific subpopulations of bone marrow-derived and splenic DC. Importantly, we demonstrate that the receptor significantly alters the response of DC to endotoxin. In contrast to the activities of other PRR that have so far been examined, uniquely SR-A limits the maturation response; SR-A-/- cells display enhanced CD40 expression and TNF-alpha production. We discuss the potential contributions of SR-A to DC biology in the context of the known multiple activities of this receptor.

The synthetic analogue of mycoplasmal lipoprotein FSL-1 induces dendritic cell maturation through Toll-like receptor 2

Granulocyte-macrophage colony-stimulating factor-differentiated bone marrow-derived dendritic cells were stimulated with the synthetic lipopeptide S-(2,3-bispalmitoyloxypropyl)-CGDPKHSPKSF (FSL-1) or the Escherichia coli lipopolysaccharide. FSL-1 induced the production of TNF-alpha and IL-12 by C57BL/6-derived bone marrow-derived dendritic cells but not by bone marrow-derived dendritic cells from Toll-like receptor 2-deficient (TLR2(-/-)) mice. Lipopolysaccharide induced the production of TNF-alpha and IL-12 by bone marrow-derived dendritic cells derived from either type of mice. FSL-1 did not induce production of IL-10 by bone marrow-derived dendritic cells from either type of mice, whereas lipopolysaccharide induced small amounts of IL-10 by bone marrow-derived dendritic cells from both types of mice. The upregulation by FSL-1 of the expression of CD80, CD86 and the MHC class II molecule IA(b) was dose- and time-dependent on the surfaces of C57BL/6-derived bone marrow-derived dendritic cells but not on the surface of TLR2(-/-)-derived bone marrow-derived dendritic cells. Lipopolysaccharide upregulated the expression of these molecules on the surfaces of bone marrow-derived dendritic cells from both types of mice. The expression of CD11c on the surfaces of C57BL/6-derived bone marrow-derived dendritic cells was upregulated by stimulation with both FSL-1 and lipopolysaccharide up to 12 h; thereafter, the expression was downregulated. The results suggest that FSL-1 can accelerate maturation of bone marrow-derived dendritic cells and this FSL-1 activity is mediated by TLR2.

Microenvironmental niches in the bone marrow required for B-cell development

B-cell development is known to occur in a complex bone-marrow microenvironment but its functional organization remains unclear. It is thought that bone-marrow stromal cells create distinct microenvironments, known as niches, that provide support for haematopoiesis and B-cell development. Although it has been more than 20 years since the development of a culture system that allows the growth of B-cell progenitors on bone-marrow-derived stromal cells in vitro, it is only recently that studies have provided a novel basis for understanding the nature of the niches for B-cell development in vivo. This article summarizes the recent advances in research on the earliest B-cell precursors, their requisite environmental factors and the cellular niches that supply these factors and maintain B cells during their development.

Matrix protein mutant of vesicular stomatitis virus stimulates maturation of myeloid dendritic cells

Matrix (M) protein mutants of vesicular stomatitis virus have recently been used as oncolytic viruses for tumor therapies and are being developed as vaccine vectors for heterologous antigens. Because dendritic cell (DC) maturation is an important correlate of tumor immunosurveillance and vaccine efficacy, we sought to determine the ability of a recombinant M protein mutant virus (rM51R-M virus) to mature DC in vitro. We have previously shown that rM51R-M virus is defective at inhibiting host gene expression in several cell lines compared to its recombinant wild-type counterpart, rwt virus. Therefore, rM51R-M virus allows the expression of genes involved in antiviral responses, such as the type I interferon (IFN) gene. Our results demonstrate that, in contrast to the rwt virus, rM51R-M virus induced the maturation of myeloid DC (mDC) populations, as indicated by an increase in the surface expression of CD40, CD80, and CD86 as well as the secretion of interleukin-12 (IL-12), IL-6, and type I IFN. In addition, mDC infected with rM51R-M virus effectively activated naïve T cells in vitro, whereas rwt virus-infected mDC were defective in antigen presentation. The inability of rwt virus to induce mDC maturation was correlated with the inhibition of host gene expression in rwt virus-infected cells. Our studies also indicated that the production of costimulatory molecules on mDC by rM51R-M virus was dependent on the type I IFN receptor, while maturation induced by this virus was largely independent of MyD88. These data indicate that rM51R-M virus effectively stimulates the maturation of mDC and has the potential to promote effective T-cell responses to vector-expressed antigens, activate DC at tumor sites during therapy, and aid in tumor immunosurveillance and destruction.

Dexamethasone preferentially suppresses plasmacytoid dendritic cell differentiation and enhances their apoptotic death

Plasmacytoid dendritic cells (pDC) are an important source of type-1 interferon (IFN) following microbial infection and also play key roles in the induction of innate and adaptive immune responses. Here, we show that the glucocorticoid (GC) dexamethasone (Dex) strikingly reduces pDC (and myeloid DC) numbers in secondary lymphoid tissue and liver of normal and hematopoietic growth factor-mobilized mice and suppresses pDC differentiation from bone marrow precursors in vitro. Moreover, the apoptotic death of pDC in vitro was enhanced by exposure to Dex. Notably, however, Toll-like receptor 9 expression and virally induced IFNalpha production by residual pDC from Dex-treated animals were unaffected. Thus, whereas marked reduction in absolute numbers of pDC by GC may predispose to viral infection, often associated with GC-mediated immunosuppression, reductions in pDC and IFNalpha production may contribute to the beneficial effects on GC observed in systemic autoimmune disease, in which that both pDC and IFNalpha have been implicated.

Towards an understanding of the transcription factor network of dendritic cell development

Dendritic cells (DCs) are antigen-presenting cells of the immune system and develop from hematopoietic stem cells through successive steps of lineage commitment and differentiation. The three major DC populations are epidermal Langerhans cells, tissue/interstitial/dermal DCs and plasmacytoid DCs. We review how gene-targeted mutations in mice have contributed to our understanding of how the various DC subpopulations develop. These studies have revealed both overlapping and distinct pathways of DC differentiation and show that there is no obvious correlation between transcription factor knockout phenotypes and a lymphoid or myeloid origin of DCs.

“Endogenous adjuvant” activity of the RNA components of lupus autoantigens Sm/RNP and Ro 60

OBJECTIVE

Most lupus patients produce autoantibodies against small ribonucleoproteins such as Sm/RNP and Ro 60 (containing U1 and Y1-Y5 RNAs, respectively). We undertook this study to investigate whether the RNA components of these antigens, which contain extensive tracts of single- and double-stranded RNA, signatures of viral infection, activate innate immunity.

METHODS

U1 and Y RNAs were affinity purified from K562 cells. Murine bone marrow-derived dendritic cells (DCs), human HEK 293 cells, and murine RAW264.7 cells were stimulated with U1 RNA and other known Toll-like receptor (TLR) ligands. Expression of the interferon (IFN)-inducible gene Mx1 and other genes was quantified using real-time polymerase chain reaction, and cytokine production was measured by enzyme-linked immunosorbent assay. DC maturation was assessed using flow cytometry.

RESULTS

Purified U1 and Y1-Y5 RNAs and synthetic stem-loop II of U1 RNA stimulated type I IFN (IFN-I) production by cell lines and murine bone marrow-derived DCs and promoted DC maturation (CD86 expression). U1 RNA-stimulated, but not TLR-3 ligand-stimulated, IFN-I was blocked by bafilomycin A1, indicating that immunostimulation by U1 RNA requires endosomal acidification. Myeloid differentiation factor 88-deficient cells responded poorly to U1 RNA, suggesting that an endosomal TLR, probably TLR-7, mediates the stimulatory effects of U1 RNA. U1 RNA-induced IFN-I and interleukin-6 production also were protein kinase R (PKR) dependent (abrogated by 2-aminopurine and greatly reduced in PKR-/- cells).

CONCLUSION

We conclude that the RNA components of the Ro 60 (Y1-Y5 RNA) and Sm/RNP (U1 RNA) small ribonucleoproteins act as endogenous adjuvants that could play a role in the pathogenesis of autoimmunity by stimulating DC maturation and IFN-I production.

Enhancement of dendritic cell antigen cross-presentation by CpG DNA involves type I IFN and stabilization of class I MHC mRNA

Dendritic cells (DCs) internalize exogenous Ags and process them for cross-presentation by class I MHC (MHC-I) to CD8+ T cells. This processing can occur by transporter for Ag presentation (TAP)-dependent or TAP-independent mechanisms. We observed that CpG DNA enhanced cross-presentation of Ags by Flt-3L-cultured bone marrow-derived murine DCs by a type I IFN (IFN-alphabeta)-dependent mechanism. Myeloid DCs provided cross-presentation function in this system. Both TAP1 knockout and wild-type DCs showed enhanced cross-presentation when treated with CpG DNA at 26 degrees C, demonstrating that TAP is not essential to this regulatory mechanism, although TAP is an important determinant of MHC-I expression. Enhancement of cross-processing by CpG DNA did not involve increased Ag uptake or proteolysis but did correlate with IFN-alphabeta-dependent increases in expression of MHC-I mRNA and protein. Increased MHC-I mRNA levels resulted in part from stabilization of MHC-I mRNA, a novel posttranscriptional mechanism for regulation of MHC-I expression. Thus, a major mechanism by which CpG oligodeoxynucleotide increase cross presentation by DCs appears to be an IFN-alphabeta-mediated increase in MHC-I synthesis.

The microglial "activation" continuum: from innate to adaptive responses

Microglia are innate immune cells of myeloid origin that take up residence in the central nervous system (CNS) during embryogenesis. While classically regarded as macrophage-like cells, it is becoming increasingly clear that reactive microglia play more diverse roles in the CNS. Microglial "activation" is often used to refer to a single phenotype; however, in this review we consider that a continuum of microglial activation exists, with phagocytic response (innate activation) at one end and antigen presenting cell function (adaptive activation) at the other. Where activated microglia fall in this spectrum seems to be highly dependent on the type of stimulation provided. We begin by addressing the classical roles of peripheral innate immune cells including macrophages and dendritic cells, which seem to define the edges of this continuum. We then discuss various types of microglial stimulation, including Toll-like receptor engagement by pathogen-associated molecular patterns, microglial challenge with myelin epitopes or Alzheimer's β-amyloid in the presence or absence of CD40L co-stimulation, and Alzheimer disease "immunotherapy". Based on the wide spectrum of stimulus-specific microglial responses, we interpret these cells as immune cells that demonstrate remarkable plasticity following activation. This interpretation has relevance for neurodegenerative/neuroinflammatory diseases where reactive microglia play an etiological role; in particular viral/bacterial encephalitis, multiple sclerosis and Alzheimer disease.

Development of murine plasmacytoid dendritic cell subsets

Plasmacytoid predendritic cells (pDC) are a haematopoietic cell population with a characteristic plasma cell-like morphology found in many tissues of the mouse, including blood, thymus, bone marrow, liver, and the T-cell areas of lymphoid organs. Recent studies of pDC have revealed them to be crucial mediators linking the innate and adaptive arms of the immune system. In this review, rather than focus on pDC function, we focus on recent evidence regarding pDC development. We examine the requirements for pDC development from several perspectives, including organ localization, cytokine requirements, development from myeloid- and lymphoid-restricted bone marrow precursors, expression of lineage-restricted markers, transcription factor dependence, and markers that separate pDC into distinct subsets.

Thymic stromal lymphopoietin as a key initiator of allergic airway inflammation in mice

The cytokine thymic stromal lymphopoietin (TSLP) has been linked to human allergic inflammatory diseases. We show here that TSLP expression was increased in the lungs of mice with antigen-induced asthma, whereas TSLP receptor-deficient mice had considerably attenuated disease. Lung-specific expression of a Tslp transgene induced airway inflammation and hyperreactivity characterized by T helper type 2 cytokines and increased immunoglobulin E. The lungs of Tslp-transgenic mice showed massive infiltration of leukocytes, goblet cell hyperplasia and subepithelial fibrosis. TSLP was capable of activating bone marrow-derived dendritic cells to upregulate costimulatory molecules and produce the T helper type 2 cell-attracting chemokine CCL17. These findings suggest that TSLP is an important factor necessary and sufficient for the initiation of allergic airway inflammation.

Cutting edge: generation of splenic CD8+ and CD8- dendritic cell equivalents in Fms-like tyrosine kinase 3 ligand bone marrow cultures

We demonstrate that functional and phenotypic equivalents of mouse splenic CD8(+) and CD8(-) conventional dendritic cell (cDC) subsets can be generated in vitro when bone marrow is cultured with fms-like tyrosine kinase 3 (flt3) ligand. In addition to CD45RA(high) plasmacytoid DC, two distinct CD24(high) and CD11b(high) cDC subsets were present, and these subsets showed equivalent properties to splenic CD8(+) and CD8(-) cDC, respectively, in the following: 1) surface expression of CD11b, CD24, and signal regulatory protein-alpha; 2) developmental dependence on, and mRNA expression of, IFN regulatory factor-8; 3) mRNA expression of TLRs and chemokine receptors; 4) production of IL-12 p40/70, IFN-alpha, MIP-1alpha, and RANTES in response to TLR ligands; 5) expression of cystatin C; and 6) cross-presentation of exogenous Ag to CD8 T cells. Furthermore, despite lacking surface CD8 expression, the CD24(high) subset contained CD8 mRNA and up-regulated surface expression when transferred into mice. This culture system allows access to bona fide counterparts of the splenic DC subsets.

Plasmacytoid dendritic cell precursors induce allogeneic T-cell hyporesponsiveness and prolong heart graft survival

Dendritic cell (DC) precursors were propagated from C57BL/10 (B10; H2b) mouse bone marrow in fms-like tyrosine kinase 3 ligand. Cosignaling molecule (B7-1/B7-2 and B7-H1) expression and stimulatory capacity of precursor (pre)-plasmacytoid (p)DC (CD11c+B220+CD11b-CD19-) and classic myeloid DC (MDC) for allogeneic (C3H; H2k) T cells were compared. Unstimulated pre-pDC exhibited very low levels of surface MHC class II and classic costimulatory molecules (B7-1/B7-2), whereas a minor population expressed B7-H1 at levels higher than on MDC. The pre-pDC were ineffective T-cell stimulators and induced nonspecific hyporesponsiveness to rechallenge with donor alloantigens in vitro and in vivo. Following stimulation with CpG-oligonucleotide (CpG-ODN), B7 molecule expression was upregulated on pre-pDC, however the ratio between coinhibitory (B7-H1) and costimulatory (B7-1/B7-2) signals was much higher (five- to six-fold) on pre-pDC than MDC. Blockade of B7-H1 expression on pDC increased their T-cell allostimulatory capacity significantly. A single preoperative infusion of C3H hosts with pre-pDC prolonged B10 heart graft survival significantly but nonspecifically compared with untreated mice (median survival times 22 vs. 9 days, respectively). Thus, pre-pDC of donor origin have potential to regulate T-cell responses to alloantigens and can prolong organ graft survival.

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.

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.

Dendritic cell function in mice lacking Plexin C1

Semaphorins are secreted or transmembrane proteins that provide essential repulsive guidance cues to growing axons or endothelial cells through their receptors of the Plexin and Neuropilin family. Semaphorins and Plexins are also expressed in the immune system where their function remains elusive. In particular, Plexin C1 is expressed by mouse dendritic cells (DCs) and is the receptor for the poxvirus semaphorin homolog A39R. We previously found that Plexin C1 engagement by A39R inhibits integrin-mediated DC adhesion and chemokine-induced migration. Here, we show that a cellular ligand for Plexin C1 is expressed both by activated T cells and DCs, suggesting that Plexin C1 might be engaged on DCs both in cis and in trans. We used Plexin C1(-/-) mice to explore the role of Plexin C1 in DC function. DC development is unaffected in these mice. In two different in vivo assays, Plexin C1(-/-) DC migration to lymph nodes (LNs) was lower than that of wild-type (WT) DC but this difference was not statistically significant. Plexin C1(-/-) bone marrow-derived DCs induced normal in vitro T cell responses but reduced in vivo T cell responses when injected subcutaneously to WT mice. Finally, in vivo T cell responses to ovalbumin peptide and contact hypersensitivity to dinitrofluorobenzene were slightly decreased in Plexin C1(-/-) mice. These results suggest a role for Plexin C1 in DC migration or mobility within the LNs.

Natural-killer cells and dendritic cells: "l'union fait la force"

Several recent publications have focused on the newly described interactions between natural-killer (NK) cells and dendritic cells (DCs). Activated NK cells induce DC maturation either directly or in synergy with suboptimal levels of microbial signals. Immature DCs appear susceptible to autologous NK-cell-mediated cytolysis while mature DCs are protected. NK-cell-induced DC activation is dependent on both tumor necrosis factor-alpha (TNF-alpha)/interferon-gamma (IFN-gamma) secretion and a cell-cell contact involving NKp30. In vitro, interleukin-12 (IL-12)/IL-18, IL-15, and IFN-alpha/beta production by activated DCs enhance, in turn, NK-cell IFN-gamma production, proliferation, and cytotoxic potential, respectively. In vivo, NK-cell/DC interactions may occur in lymphoid organs as well as in nonlymphoid tissues, and their consequences are multiple. By inducing DC activation, NK-cell activation induced by tumor cells can indirectly promote antitumoral T-cell responses. Reciprocally, DCs activated through Toll-like receptors (TLRs) induce potent NK-cell activation in antiviral responses. Thus, DCs and NK cells are equipped with complementary sets of receptors that allow the recognition of various pathogenic agents, emphasizing the role of NK-cell/DC crosstalk in the coordination of innate and adaptive immune responses.