Suppression of dendritic cell activation by anthrax lethal toxin and edema toxin depends on multiple factors including cell source, stimulus used, and function tested

Bacillus anthracis produces lethal toxin (LT) and edema toxin (ET), and they suppress the function of LPS-stimulated dendritic cells (DCs). Because DCs respond differently to various microbial stimuli, we compared toxin effects in bone marrow DCs stimulated with either LPS or Legionella pneumophila (Lp). LT, not ET, was more toxic for cells from BALB/c than from C57BL/6 (B6) as measured by 7-AAD uptake; however, ET suppressed CD11c expression. LT suppressed IL-12, IL-6, and TNF-alpha in cells from BALB/c and B6 mice but increased IL-1beta in LPS-stimulated cultures. ET also suppressed IL-12 and TNF-alpha, but increased IL-6 and IL-1beta in Lp-stimulated cells from B6. Regarding maturation marker expression, LT increased MHCII and CD86 while suppressing CD40 and CD80; ET generally decreased marker expression across all groups. We conclude that the suppression of cytokine production by anthrax toxins is dependent on variables, including the source of the DCs, the type of stimulus and cytokine measured, and the individual toxin tested. However, LT and ET enhancement or suppression of maturation marker expression is more related to the marker studied than the stimuli or cell source. Anthrax toxins are not uniformly suppressive of DC function but instead can increase function under defined conditions.     

A polysaccharide extracted from Grifola frondosa enhances the anti-tumor activity of bone marrow-derived dendritic cell-based immunotherapy against murine colon cancer

We previously isolated the novel heteropolysaccharide maitake Z-fraction (MZF) from the maitake mushroom (Grifola frondosa), and demonstrated that MZF significantly inhibited tumor growth by inducing cell-mediated immunity. In this study, we demonstrated that MZF upregulated the expression of CD80, CD86, CD83, and MHC II on bone marrow-derived dendritic cells (DCs) and significantly increased interleukin-12 (IL-12) and tumor necrosis factor-alpha production by DCs in a dose-dependent manner. MZF-treated DCs significantly stimulated both allogeneic and antigen-specific syngenic T cell responses and enhanced antigen-specific interferon-gamma (IFN-γ) production by syngenic CD4⁺ T cells; however, MZF-treated DCs did not affect IL-4 production. Furthermore, the enhancement of IFN-γ production in CD4⁺ T cells, which was induced by MZF-treated DCs, was completely inhibited by the addition of an anti-IL-12 antibody. These results indicate that MZF induced DC maturation and antigen-specific Th1 response by enhancing DC-produced IL-12. We also demonstrated that DCs pulsed with colon-26 tumor lysate in the presence of MZF induced both therapeutic and preventive effects on colon-26 tumor development in BALB/c mice. These results suggest that MZF could be a potential effective adjuvant to enhance immunotherapy using DC-based vaccination.     

IL-12 or IL-4 prime human NK cells to mediate functionally divergent interactions with dendritic cells or tumors

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

TLR9-signaling pathways are involved in Kilham rat virus-induced autoimmune diabetes in the biobreeding diabetes-resistant rat

Viral infections are associated epidemiologically with the expression of type 1 diabetes in humans, but the mechanisms underlying this putative association are unknown. To investigate the role of viruses in diabetes, we used a model of viral induction of autoimmune diabetes in genetically susceptible biobreeding diabetes-resistant (BBDR) rats. BBDR rats do not develop diabetes in viral-Ab-free environments, but approximately 25% of animals infected with the parvovirus Kilham rat virus (KRV) develop autoimmune diabetes via a mechanism that does not involve beta cell infection. Using this model, we recently documented that TLR agonists synergize with KRV infection and increase disease penetrance. We now report that KRV itself activates innate immunity through TLR ligation. We show that KRV infection strongly stimulates BBDR splenocytes to produce the proinflammatory cytokines IL-6 and IL-12p40 but not TNF-alpha. KRV infection induces high levels of IL-12p40 by splenic B cells and Flt-3-ligand-induced bone marrow-derived dendritic cells (DCs) but only low levels of IL-12p40 production by thioglycolate-elicited peritoneal macrophages or GM-CSF plus IL-4-induced bone marrow-derived DCs. KRV-induced cytokine production is blocked by pharmacological inhibitors of protein kinase R and NF-kappaB. Genomic KRV DNA also induces BBDR splenocytes and Flt-3L-induced DCs from wild-type but not TLR9-deficient mice to produce IL-12p40; KRV-induced up-regulation of B lymphocytes can be blocked by TLR9 antagonists including inhibitory CpG and chloroquine. Administration of chloroquine to virus-infected BBDR rats decreases the incidence of diabetes and decreases blood levels of IL-12p40. Our data implicate the TLR9-signaling pathway in KRV-induced innate immune activation and autoimmune diabetes in the BBDR rat.     

Dendritic cells trigger tumor cell death by a nitric oxide-dependent mechanism

Dendritic cells (DCs) are well known for their capacity to induce adaptive antitumor immune response through Ag presentation and tumor-specific T cell activation. Recent findings reveal that besides this role, DCs may display additional antitumor effects. In this study, we provide evidence that LPS- or IFN-gamma-activated rat bone marrow-derived dendritic cells (BMDCs) display killing properties against tumor cells. These cytotoxic BMDCs exhibit a mature DC phenotype, produce high amounts of IL-12, IL-6, and TNF-alpha, and retain their phagocytic properties. BMDC-mediated tumor cell killing requires cell-cell contact and depends on NO production, but not on perforin/granzyme or on death receptors. Furthermore, dead tumor cells do not exhibit characteristics of apoptosis. Thus, intratumoral LPS injections induce an increase of inducible NO synthase expression in tumor-infiltrating DCs associated with a significant arrest of tumor growth. Altogether, these results suggest that LPS-activated BMDCs represent powerful tumoricidal cells which enforce their potential as anticancer cellular vaccines.     

Human dendritic cells are activated by dengue virus infection: enhancement by gamma interferon and implications for disease pathogenesis

The ability of dendritic cells (DCs) to shape the adaptive immune response to viral infection is mediated largely by their maturation and activation state as determined by the surface expression of HLA molecules, costimulatory molecules, and cytokine production. Dengue is an emerging arboviral disease where the severity of illness is influenced by the adaptive immune response to the virus. In this report, we have demonstrated that dengue virus infects and replicates in immature human myeloid DCs. Exposure to live dengue virus led to maturation and activation of both the infected and surrounding, uninfected DCs and stimulated production of tumor necrosis factor alpha (TNF-alpha) and alpha interferon (IFN-alpha). Activation of the dengue virus-infected DCs was blunted compared to the surrounding, uninfected DCs, and dengue virus infection induced low-level release of interleukin-12 p70 (IL-12 p70), a key cytokine in the development of cell-mediated immunity (CMI). Upon the addition of IFN-gamma, there was enhanced activation of dengue virus-infected DCs and enhanced dengue virus-induced IL-12 p70 release. The data suggest a model whereby DCs are the early, primary target of dengue virus in natural infection and the vigor of CMI is modulated by the relative presence or absence of IFN-gamma in the microenvironment surrounding the virus-infected DCs. These findings are relevant to understanding the pathogenesis of dengue hemorrhagic fever and the design of new vaccination and therapeutic strategies.     

Interplay between CD8α+ dendritic cells and monocytes in response to Listeria monocytogenes infection attenuates T cell responses

During the course of a microbial infection, different antigen presenting cells (APCs) are exposed and contribute to the ensuing immune response. CD8α(+) dendritic cells (DCs) are an important coordinator of early immune responses to the intracellular bacteria Listeria monocytogenes (Lm) and are crucial for CD8(+) T cell immunity. In this study, we examine the contribution of different primary APCs to inducing immune responses against Lm. We find that CD8α(+) DCs are the most susceptible to infection while plasmacytoid DCs are not infected. Moreover, CD8α(+) DCs are the only DC subset capable of priming an immune response to Lm in vitro and are also the only APC studied that do so when transferred into β2 microglobulin deficient mice which lack endogenous cross-presentation. Upon infection, CD11b(+) DCs primarily secrete low levels of TNFα while CD8α(+) DCs secrete IL-12 p70. Infected monocytes secrete high levels of TNFα and IL-12p70, cytokines associated with activated inflammatory macrophages. Furthermore, co-culture of infected CD8α(+) DCs and CD11b+ DCs with monocytes enhances production of IL-12 p70 and TNFα. However, the presence of monocytes in DC/T cell co-cultures attenuates T cell priming against Lm-derived antigens in vitro and in vivo. This suppressive activity of spleen-derived monocytes is mediated in part by both TNFα and inducible nitric oxide synthase (iNOS). Thus these monocytes enhance IL-12 production to Lm infection, but concurrently abrogate DC-mediated T cell priming.     

Hepatitis C virus core and nonstructural protein 3 proteins induce pro- and anti-inflammatory cytokines and inhibit dendritic cell differentiation

Antiviral immunity requires recognition of viral pathogens and activation of cytotoxic and Th cells by innate immune cells. In this study, we demonstrate that hepatitis C virus (HCV) core and nonstructural protein 3 (NS3), but not envelope 2 proteins (E2), activate monocytes and myeloid dendritic cells (DCs) and partially reproduce abnormalities found in chronic HCV infection. HCV core or NS3 (not E2) triggered inflammatory cytokine mRNA and TNF-alpha production in monocytes. Degradation of I-kappa B alpha suggested involvement of NF-kappa B activation. HCV core and NS3 induced production of the anti-inflammatory cytokine, IL-10. Both monocyte TNF-alpha and IL-10 levels were higher upon HCV core and NS3 protein stimulation in HCV-infected patients than in normals. HCV core and NS3 (not E2) inhibited differentiation and allostimulatory capacity of immature DCs similar to defects in HCV infection. This was associated with elevated IL-10 and decreased IL-2 levels during T cell proliferation. Increased IL-10 was produced by HCV patients' DCs and by core- or NS3-treated normal DCs, while IL-12 was decreased only in HCV DCs. Addition of anti-IL-10 Ab, not IL-12, ameliorated T cell proliferation with HCV core- or NS3-treated DCs. Reduced allostimulatory capacity in HCV core- and NS3-treated immature DCs, but not in DCs of HCV patients, was reversed by LPS maturation, suggesting more complex DC defects in vivo than those mediated by core or NS3 proteins. Our results reveal that HCV core and NS3 proteins activate monocytes and inhibit DC differentiation in the absence of the intact virus and mediate some of the immunoinhibitory effects of HCV via IL-10 induction.     

NKp46 and NKG2D recognition of infected dendritic cells is necessary for NK cell activation in the human response to influenza infection

At an early phase of viral infection, contact and cooperation between dendritic cells (DCs) and NK cells activates innate immunity, and also influences recruitment, when needed, of adaptive immunity. Influenza, an adaptable fast-evolving virus, annually causes acute, widespread infections that challenge the innate and adaptive immunity of humanity. In this study, we dissect and define the molecular mechanisms by which influenza-infected, human DCs activate resting, autologous NK cells. Three events in NK cell activation showed different requirements for soluble mediators made by infected DCs and for signals arising from contact with infected DCs. IFN-alpha was mainly responsible for enhanced NK cytolysis and also important for CD69 up-regulation, whereas IL-12 was necessary for enhancing IFN-gamma production. Increased CD69 expression and IFN-gamma production, but not increased cytolysis, required recognition of influenza-infected DCs by two NK cell receptors: NKG2D and NKp46. Abs specific for these receptors or their known ligands (UL16-binding proteins 1-3 class I-like molecules for NKG2D and influenza hemagglutinin for NKp46) inhibited CD69 expression and IFN-gamma production. Activation of NK cells by influenza-infected DCs and polyinosinic:polycytidylic acid (poly(I:C))-treated DCs was distinguished. Poly(I:C)-treated DCs did not express the UL16-binding protein 3 ligand for NKG2D, and in the absence of the influenza hemagglutinin there was no involvement of NKp46.     

Functional dichotomy of dendritic cells following interaction with Leishmania braziliensis: infected cells produce high levels of TNF-alpha, whereas bystander dendritic cells are activated to promote T cell responses

Leishmania braziliensis infections are often associated with exaggerated immune responses that can sometimes lead to severe disease associated with high levels of IFN-gamma and TNF-alpha. To explore the role played by dendritic cells (DCs) in these responses, we characterized DCs that were exposed to L. braziliensis. We found that DCs cultured with L. braziliensis parasites up-regulated DC activation markers and produced IL-12 and TNF-alpha. However, not all DCs in the culture became infected, and an analysis of infected and uninfected DCs demonstrated that the up-regulation of activation markers and IL-12 production was primarily confined to the uninfected (bystander) DCs. Further studies with Transwell chambers and parasite fractions indicated that the activation of bystander DCs was mediated by a soluble parasite product, in a type 1 IFN- and MyD88-independent, but TNF-alpha-dependent fashion, and that the activated DCs were more efficient at presenting Ag than control DCs. In contrast, L. braziliensis-infected DCs failed to up-regulate activation markers, but exhibited a dramatic enhancement in their ability to produce TNF-alpha in response to LPS as compared with uninfected DCs. These findings uncover a dual role for DCs in L. braziliensis infection: T cell activation by bystander DCs due to enhanced Ag-presenting capacity following exposure to soluble parasite products, and increased production of TNF-alpha by infected cells that may contribute to the local control of the parasites, but concomitantly induce immunopathology.     

Dendritic cells in innate immune responses against HIV

Dendritic cells (DCs) were recently found to be innate immunity effectors against tumoral cells and viruses. (i) In response to most viruses, including HIV, plasmacytoid DCs are responsible for most of the type I IFN secretion, which is strongly anti-viral and induces TH1 type responses. Myeloid DCs secrete IL-12, which is also important for TH1-type and cytotoxic responses. In HIV patient blood, both DC population numbers decrease as early as the primary stage. Plasmacytoid DC numbers correlate with type I IFN secretion, which is a prognosis predictor, particularly under treatment. IL-12 secretion is also defective. Immunotherapies to replace the defective cytokines or to restore a potentially defective DC-T lymphocyte feed-back might help patients restore their immune responses under antiviral therapy. (ii) After measles and other viral infections, or incubation with dsRNA, DCs become cytotoxic and consequently exhibit natural killer function, through upregulation of type I IFN secretion which enhances TRAIL expression. In HIV infection, this mechanism was not demonstrated yet, but it might a) be responsible for the massive apoptosis of uninfected lymphocytes, and b) increase specific immunity through cross-presentation of antigens from infected cells killed by DCs. (iii) DCs direct expansion and effector functions of NK cells in the absence of adaptive-type cytokines and modulate NKT cell IFN-gamma production. Reciprocally, NK activation triggers DC maturation. HIV-1 Tat inhibits NK cell cytotoxicity directly and probably through inhibition of IL-12 secretion by DC. Therefore, understanding the functions of DCs in innate immune responses and in pathogenesis will help obtain better HIV replication control.     

Fever range temperature promotes TLR4 expression and signaling in dendritic cells

Fever improves survival and shortens disease duration in microbial infections. However, the mechanisms of these beneficial responses still remain elusive. Toll-like receptors (TLRs) play important roles in sensing microbes invading and therefore we hypothesized that fever range temperature may enhance responsiveness of dendritic cells (DCs) to lipopolysaccharide (LPS) by promoting TLR4 expression and signaling. In this study, we found that pretreatment of DCs with 39.5 degrees C temperature can up-regulate TLR4 expression in DCs and enhances LPS-induced DC production of interleukins (IL) IL-6, IL-10 and IL-12 but not tumor necrosis factor alpha (TNF-alpha). Blockade of the autocrine action of IL-10 could increase LPS-induced TNF-alpha and IL-12 production in DCs. Further experiments confirmed that TLR4 ligation activates extracellular signal-regulated kinase (ERK), p38, and nuclear factor-kappaB pathways more potently in DCs pretreated with 39.5 degrees C. We conclude that fever range temperature can promote TLR4 expression and signaling in DCs, leading to enhancement of immune responses to inflammatory stimuli. These results might reveal a possible mechanistic explanation for the significance of fever in activating innate immune responses.     

Bovine dendritic cells are more permissive for Mycobacterium bovis replication than macrophages, but release more IL-12 and induce better immune T-cell proliferation

Bovine dendritic cells (DCs) were obtained by incubating blood monocytes with interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). The ability of DCs to phagocytose and allow the replication of virulent Mycobacterium bovis in vitro was studied, and compared with bovine blood monocyte-derived macrophages. In addition, the release of cytokines by M. bovis-infected DCs was assessed. DCs were shown to phagocytose M. bovis efficiently, and allowed a more substantial replication of M. bovis when compared to macrophages, as assessed by the metabolic activity of intracellular bacteria. During the course of M. bovis infection, it was found that macrophages released substantial amounts of pro-inflammatory factors such as tumour necrosis factor-alpha (TNF-alpha), nitric oxide (NO) and interleukin-1 beta (IL-1 beta). M. bovis-infected DCs released much smaller quantities of NO, IL-1 beta and TNF-alpha (5- to 10-fold lower amounts), when compared to macrophages. Treating cells with interferon-gamma (IFN-gamma) before and during the in vitro infection process was shown to increase the release of NO, TNF-alpha and IL-1 beta by M. bovis-infected macrophages, but not by M. bovis-infected DCs. M. bovis-infected macrophages released more interleukin-10 (IL-10) than infected DCs. Treating cells with IFN-gamma/LPS was shown to reduce M. bovis metabolic activity in infected macrophages, but had no such impact on M. bovis metabolic activity in infected DCs. A variety of T-cell-derived cytokines (IFN-gamma, GM-CSF, IL-4) had no impact on the replication of M. bovis in infected DCs. On the other hand, DCs infected with M. bovis sustained a more efficient replication of autologous sensitized T lymphocytes compared to M. bovis-infected macrophages. M. bovis-infected DCs released more substantial amounts of interleukin-12 (IL-12) than similarly infected macrophages. These data suggest a complementary role for DCs and macrophages with regard to bacteriostatic activity and induction of an efficient immune response against M. bovis.     

Polymorphonuclear neutrophil-derived ectosomes interfere with the maturation of monocyte-derived dendritic cells

Polymorphonuclear neutrophils (PMNs) are a key component of the innate immune system. Their activation leads to the release of potent antimicrobial agents through degranulation. Simultaneously, PMNs release cell surface-derived microvesicles, so-called ectosomes (PMN-Ect). PMN-Ect are rightside-out vesicles with a diameter of 50-200 nm. They expose phosphatidylserine in the outer leaflet of their membrane and down-modulate monocyte/macrophage-activation in vitro. In this study, we analyzed the effects of PMN-Ect on maturation of human monocyte-derived dendritic cells (MoDCs). Intriguingly, exposing immature MoDCs to PMN-Ect modified their morphology, reduced their phagocytic activity, and increased the release of TGF-beta1. When immature MoDCs were incubated with PMN-Ect and stimulated with the TLR4 ligand LPS, the maturation process was partially inhibited as evidenced by reduced expression of cell surface markers (CD40, CD80, CD83, CD86, and HLA-DP DQ DR), inhibition of cytokine-release (IL-8, IL-10, IL-12, and TNF-alpha), and a reduced capacity to induce T cell proliferation. Together these data provide evidence that PMN-Ect have the ability to modify MoDC maturation and function. PMN-Ect may thus represent an as yet unidentified host-factor influencing MoDC maturation at the site of injury, thereby possibly impacting on downstream MoDC-dependent immunity.     

Activation and suppression of renin-angiotensin system in human dendritic cells

We previously identified the gene expression of renin-angiotensin system in human monocyte-derived dendritic cells (DCs). This study was conducted to examine the mechanisms by which angiotensin II and captopril, the inhibitor of the angiotensin-converting enzyme (ACE), affect human DCs. In DCs, lipopolysaccharide (LPS)-induced production of tumor necrosis factor-alpha (TNF-alpha), interleukin-(IL)-1alpha, IL-10, IL-12, and IL-18 was significantly inhibited by captopril. In contrast, angiotensin II treatment resulted in a significant increase in TNF-alpha and IL-6 protein biosynthesis by DCs. In addition, we have studied the global expression of 2400 genes in DCs from two donors. Here, we demonstrated the specific down-regulation of the ACE gene expression in captopril-treated DCs. Our finding indicates the possible activation of NF-kappaB through the up-regulation of expressions of MEFV gene (encoding PYRIN protein) and heterogeneous nuclear ribonucleoprotein R in DCs. This is the first study on the modulation of cytokine and gene expression by angiotensin II and captopril in DCs.     

Reovirus activates human dendritic cells to promote innate antitumor immunity

Oncolytic viruses can exert their antitumor activity via direct oncolysis or activation of antitumor immunity. Although reovirus is currently under clinical investigation for the treatment of localized or disseminated cancer, any potential immune contribution to its efficacy has not been addressed. This is the first study to investigate the ability of reovirus to activate human dendritic cells (DC), key regulators of both innate and adaptive immune responses. Reovirus induced DC maturation and stimulated the production of the proinflammatory cytokines IFN-alpha, TNF-alpha, IL-12p70, and IL-6. Activation of DC by reovirus was not dependent on viral replication, while cytokine production (but not phenotypic maturation) was inhibited by blockade of PKR and NF-kappaB signaling. Upon coculture with autologous NK cells, reovirus-activated DC up-regulated IFN-gamma production and increased NK cytolytic activity. Moreover, short-term coculture of reovirus-activated DC with autologous T cells also enhanced T cell cytokine secretion (IL-2 and IFN-gamma) and induced non-Ag restricted tumor cell killing. These data demonstrate for the first time that reovirus directly activates human DC and that reovirus-activated DC stimulate innate killing by not only NK cells, but also T cells, suggesting a novel potential role for T cells in oncolytic virus-induced local tumor cell death. Hence reovirus recognition by DC may trigger innate effector mechanisms to complement the virus's direct cytotoxicity, potentially enhancing the efficacy of reovirus as a therapeutic agent.     

Innate immunity in tuberculosis: myths and truth

Tuberculosis is the most important bacterial infection world wide. The causative agent, Mycobacterium tuberculosis survives and proliferates within macrophages. Immune mediators such as interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha) activate macrophages and promote bacterial killing. IFN-gamma is predominantly secreted by innate cells (mainly natural killer (NK) cells) and by T cells upon instruction by interleukin 12 (IL-12) and IL-18. These cytokines are primarily produced by dendritic cells and macrophages in response to Toll-like receptor (TLR) signalling interaction with tubercle bacilli. These signals also induce pro-inflammatory cytokines (including IL-1beta and TNF-alpha), chemokines and defensins. The inflammatory environment further recruits innate effector cells such as macrophages, polymorphonuclear neutrophils (PMN) and NK cells to the infectious foci. This eventually leads to the downstream establishment of acquired T cell immunity which appears to be protective in more than 90% of infected individuals. Robust innate immune activation is considered an essential prerequisite for protective immunity and vaccine efficacy. However, data published so far provide a muddled view of the functional importance of innate immunity in tuberculosis. Here we critically discuss certain aspects of innate immunity, namely PMN, TLRs and NK cells, as characterised in tuberculosis to date, and their contribution to protection and pathology.     

Influenza virus evades innate and adaptive immunity via the NS1 protein

Both antibodies and T cells contribute to immunity against influenza virus infection. However, the generation of strong Th1 immunity is crucial for viral clearance. Interestingly, we found that human dendritic cells (DCs) infected with influenza A virus have lower allospecific Th1-cell stimulatory abilities than DCs activated by other stimuli, such as lipopolysaccharide and Newcastle disease virus infection. This weak stimulatory activity correlates with a suboptimal maturation of the DCs following infection with influenza A virus. We next investigated whether the influenza A virus NS1 protein could be responsible for the low levels of DC maturation after influenza virus infection. The NS1 protein is an important virulence factor associated with the suppression of innate immunity via the inhibition of type I interferon (IFN) production in infected cells. Using recombinant influenza and Newcastle disease viruses, with or without the NS1 gene from influenza virus, we found that the induction of a genetic program underlying DC maturation, migration, and T-cell stimulatory activity is specifically suppressed by the expression of the NS1 protein. Among the genes affected by NS1 are those coding for macrophage inflammatory protein 1beta, interleukin-12 p35 (IL-12 p35), IL-23 p19, RANTES, IL-8, IFN-alpha/beta, and CCR7. These results indicate that the influenza A virus NS1 protein is a bifunctional viral immunosuppressor which inhibits innate immunity by preventing type I IFN release and inhibits adaptive immunity by attenuating human DC maturation and the capacity of DCs to induce T-cell responses. Our observations also support the potential use of NS1 mutant influenza viruses as live attenuated influenza virus vaccines.     

Cutting edge: the acquisition of TLR tolerance during malaria infection impacts T cell activation

An effective immune response to infection requires control of pathogen growth while minimizing inflammation-associated pathology. During malaria infection, this balance is particularly important. Murine malaria is characterized by early production of proinflammatory cytokines, which declines in the face of continuing parasitemia. The mechanism by which this occurs remains poorly understood. In this study, we investigated the role of dendritic cells (DCs) in regulating pro- and anti-inflammatory cytokine responses. As malaria infection progresses, DCs become refractory to TLR-mediated IL-12 and TNF-alpha production, while increasing their ability to produce IL-10 and retaining the capacity for activation of naive T cells. IL-12-secreting DCs from early infection stimulate an IFN-gamma-dominated T cell response, whereas IL-10-secreting DCs from later stages induce an IL-10-dominated T cell response. We suggest that phenotypic changes in DCs during Plasmodium yoelii infection represent a mechanism of controlling host inflammation while maintaining effective adaptive immunity.     

Systemic tumor necrosis factor generated during lethal Plasmodium infections impairs dendritic cell function

Dendritic cells (DCs) initiate innate and adaptive immune responses including those against malaria. Although several studies have shown that DC function is normal during malaria, other studies have shown compromised function. To establish why these studies had different findings, we examined DCs from mice infected with two lethal species of parasite, Plasmodium berghei or P. vinckei, and compared them to DCs from nonlethal P. yoelii 17XNL or P. chabaudi infections. These studies found that DCs from only the lethal infections became uniformly mature 7 days after infection and were functionally impaired as they were unable to endocytose latex particles, secrete IL-12, or present OVA to transgenic OTII T cells. These changes coincided with a peak in levels of systemic TNF-alpha. Because TNF-alpha is known to mature DCs, we used TNF-KO mice to determine the role of this cytokine in the loss of DC function. In the TNF-KO mice, phenotype, Ag presentation, and IL-12 secretion by DCs were restored to normal following both lethal infections. This study shows that the systemic production of TNF-alpha contributes to poor DC function during lethal infections. These studies may explain, at least in part, immunosuppression that is associated with malaria.