Dendritic cells and measles virus infection

The Interplay between Bluetongue Virus Infections and Adaptive Immunity

Viral infections have long provided a platform to understand the workings of immunity. For instance, great strides towards defining basic immunology concepts, such as MHC restriction of antigen presentation or T-cell memory development and maintenance, have been achieved thanks to the study of lymphocytic choriomeningitis virus (LCMV) infections. These studies have also shaped our understanding of antiviral immunity, and in particular T-cell responses. In the present review, we discuss how bluetongue virus (BTV), an economically important arbovirus from the Reoviridae family that affects ruminants, affects adaptive immunity in the natural hosts. During the initial stages of infection, BTV triggers leucopenia in the hosts. The host then mounts an adaptive immune response that controls the disease. In this work, we discuss how BTV triggers CD8+ T-cell expansion and neutralizing antibody responses, yet in some individuals viremia remains detectable after these adaptive immune mechanisms are active. We present some unpublished data showing that BTV infection also affects other T cell populations such as CD4+ T-cells or γδ T-cells, as well as B-cell numbers in the periphery. This review also discusses how BTV evades these adaptive immune mechanisms so that it can be transmitted back to the arthropod host. Understanding the interaction of BTV with immunity could ultimately define the correlates of protection with immune mechanisms that would improve our knowledge of ruminant immunology.

Measles Virus Infection Fosters Dendritic Cell Motility in a 3D Environment to Enhance Transmission to Target Cells in the Respiratory Epithelium

Transmission of measles virus (MV) from dendritic to airway epithelial cells is considered as crucial to viral spread late in infection. Therefore, pathways and effectors governing this process are promising targets for intervention. To identify these, we established a 3D respiratory tract model where MV transmission by infected dendritic cells (DCs) relied on the presence of nectin-4 on H358 lung epithelial cells. Access to recipient cells is an important prerequisite for transmission, and we therefore analyzed migration of MV-exposed DC cultures within the model. Surprisingly, enhanced motility toward the epithelial layer was observed for MV-infected DCs as compared to their uninfected siblings. This occurred independently of factors released from H358 cells indicating that MV infection triggered cytoskeletal remodeling associated with DC polarization enforced velocity. Accordingly, the latter was also observed for MV-infected DCs in collagen matrices and was particularly sensitive to ROCK inhibition indicating infected DCs preferentially employed the amoeboid migration mode. This was also implicated by loss of podosomes and reduced filopodial activity both of which were retained in MV-exposed uninfected DCs. Evidently, sphingosine kinase (SphK) and sphingosine-1-phosphate (S1P) as produced in response to virus-infection in DCs contributed to enhanced velocity because this was abrogated upon inhibition of sphingosine kinase activity. These findings indicate that MV infection promotes a push-and-squeeze fast amoeboid migration mode via the SphK/S1P system characterized by loss of filopodia and podosome dissolution. Consequently, this enables rapid trafficking of virus toward epithelial cells during viral exit.

Differential response of porcine immature monocyte-derived dendritic cells to virulent and inactivated transmissible gastroenteritis virus

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Virulent TGEV down-regulates cell surface markers of immature Mo-DCs.

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Virulent TGEV limits the immature Mo-DCs to secrete IL-12, IFN-γ and IL-10.

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Virulent TGEV impairs the ability of immature Mo-DCs to stimulate T cell proliferation.

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Virulent TGEV inhibits the activation of nuclear factor kappa B (NF-κB) in immature Mo-DCs.

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UV-inactivated TGEV (UV-SHXB) has the opposite effects in immature Mo-DCs.

Exposure of piglets less than 2 weeks of age to virulent transmissible gastroenteritis virus (TGEV) gives rise to mortality as high as 100%, and adult pigs recovering from its infection often become TGEV carriers. These facts suggest an evasion of the immune system by virulent TGEV. In this study, we showed that a virulent TGEV SHXB strain could infect porcine immature monocyte-derived dendritic cells (Mo-DCs), and down-regulate cell surface markers (SLA-II-DR, CD1a and CD80/86). Moreover, SHXB-infected immature Mo-DCs showed low expression of IL-12 and IFN-γ, and also lost the ability to stimulate T cell proliferation. Finally, SHXB inhibited the activation of nuclear factor kappa B (NF-κB) in these cells. Instead, UV-inactivated SHXB (UV-SHXB) had the opposite effects in immature Mo-DCs. In conclusion, the virulent SHXB could severely impair immature Mo-DCs, which might be involved in the pathogenesis of virulent TGEV in vivo.

Histopathological alterations in immune organs of chickens and ducks after experimental infection with virulent 9a5b newcastle disease virus

The chicken and duck are important hosts of Newcastle disease virus (NDV) with distinctive responses to infection. NDV infection in ducks is often subclinical and chronic, while in chickens the infection is clinically apparent and transient. These differences may be due to in part to the host response to NDV infection. This study compares the histopathological changes in the spleen, thymus and bursa of Fabricius following infection with NDV in chickens and ducks. The 9a5b isolate of NDV was inoculated intranasally into specific pathogen-free chickens and Japanese commercial ducks. NDV nucleoprotein (NDV-NP) and interferon (IFN)-β were detected in tissues by immunohistochemistry (IHC), apoptosis was detected by haematoxylin and eosin staining, caspase-3 IHC and the TUNEL assay. Labelling of NDV-NP and lymphoid depletion were most marked in chicken tissues. The pattern of apoptosis in the spleen differed between chickens and ducks. In chickens there were numerous apoptotic cells in the peri-ellipsoidal white pulp and the peri-ellipsoidal, peri-arteriolar and peri-venous lymphoid sheaths, while apoptosis in duck spleens was mainly within the germinal centres. Lymphoid depletion was the main feature in the bursal and thymic tissues of chickens, but apoptosis was marked in these organs in ducks. Expression of IFN-β appeared earlier and was more intense in the tissues from ducks compared with those from chickens. The differences in IFN-β and NDV-NP expression may reflect the relative clinical severity of the infection in the two avian species.

Membrane dynamics and interactions in measles virus dendritic cell infections

Viral entry, compartmentalization and transmission depend on the formation of membrane lipid/protein microdomains concentrating receptors and signalosomes. Dendritic cells (DCs) are prime targets for measles virus (MV) infection, and this interaction promotes immune activation and generalized immunosuppression, yet also MV transport to secondary lymphatics where transmission to T cells occurs. In addition to MV trapping, DC-SIGN interaction can enhance MV uptake by activating cellular sphingomyelinases and, thereby, vertical surface transport of its entry receptor CD150. To exploit DCs as Trojan horses for transport, MV promotes DC maturation accompanied by mobilization, and restrictions of viral replication in these cells may support this process. MV-infected DCs are unable to support formation of functional immune synapses with conjugating T cells and signalling via viral glycoproteins or repulsive ligands (such as semaphorins) plays a key role in the induction of T-cell paralysis. In the absence of antigen recognition, MV transmission from infected DCs to T cells most likely involves formation of polyconjugates which concentrate viral structural proteins, viral receptors and with components enhancing either viral uptake or conjugate stability. Because DCs barely support production of infectious MV particles, these organized interfaces are likely to represent virological synapses essential for MV transmission.

DC-SIGN mediated sphingomyelinase-activation and ceramide generation is essential for enhancement of viral uptake in dendritic cells

As pattern recognition receptor on dendritic cells (DCs), DC-SIGN binds carbohydrate structures on its pathogen ligands and essentially determines host pathogen interactions because it both skews T cell responses and enhances pathogen uptake for cis infection and/or T cell trans-infection. How these processes are initiated at the plasma membrane level is poorly understood. We now show that DC-SIGN ligation on DCs by antibodies, mannan or measles virus (MV) causes rapid activation of neutral and acid sphingomyelinases followed by accumulation of ceramides in the outer membrane leaflet. SMase activation is important in promoting DC-SIGN signaling, but also for enhancement of MV uptake into DCs. DC-SIGN-dependent SMase activation induces efficient, transient recruitment of CD150, which functions both as MV uptake receptor and microbial sensor, from an intracellular Lamp-1+ storage compartment shared with acid sphingomyelinase (ASM) within a few minutes. Subsequently, CD150 is displayed at the cell surface and co-clusters with DC-SIGN. Thus, DC-SIGN ligation initiates SMase-dependent formation of ceramide-enriched membrane microdomains which promote vertical segregation of CD150 from intracellular storage compartments along with ASM. Given the ability to promote receptor and signalosome co-segration into (or exclusion from) ceramide enriched microdomains which provide a favorable environment for membrane fusion, DC-SIGN-dependent SMase activation may be of general importance for modes and efficiency of pathogen uptake into DCs, and their routing to specific compartments, but also for modulating T cell responses.

Dendritic cells (DCs) bear receptors specialized on recognition of patterns specific to pathogens (such as carbohydrates), which can either promote functional activation of these cells (such as TLRs), which renders them capable of efficiently presenting antigens to T cells, or, as DC-SIGN, endocytic uptake as essential for loading MHC molecules. Viruses such as HIV and measles virus (MV) exploit DC-SIGN for both their uptake into DCs and modulation of TLR signaling, yet how this is mechanistically exerted is poorly understood. We now show that DC-SIGN activates sphingomyelinases (SMases) which convert their sphingomyelin substrate into ceramides, thereby catalysing the formation of membrane platforms able to recruit and concentrate receptors and associated signaling components. We found DC-SIGN-dependent SMase activation as essential for DC-SIGN and thereby modulation of TLR signaling, but also for enhancement of MV uptake. This is mediated by a fast, transient recruitment of its entry receptor, CD150, from an intracellular storage compartment to the cell surface where it co-clusters in ceramide enriched platforms with DC-SIGN. The ability to segregate viral receptors into (or exclude them from) membrane microdomains, which, based on their biophysical properties, facilitate membrane fusion, proposes DC-SIGN-mediated SMAse activation as a central regulator of pathogen uptake into DCs.

Sequencing of DC-SIGN promoter indicates an association between promoter variation and risk of nasopharyngeal carcinoma in cantonese

Background

The dendritic cell-specific intercellular adhesion molecule 3 grabbing non-integrin (DC-SIGN) is an important pathogen recognition receptor of the innate immune system. DC-SIGN promoter variants play important role in the susceptibility to various infectious diseases. Nasopharyngeal carcinoma (NPC) is a malignancy that is common in southern China and whether DC-SIGN promoter variants have effects on susceptibility to NPC is still unknown. The aim of this study is to ascertain the potential involvement of DC-SIGN promoter single nucleotide polymorphisms (SNPs) in NPC susceptibility.

Methods

We conducted a case control study based on Cantonese population including 444 NPC patients and 464 controls matched on age and sex. The 1041 bp of DC-SIGN promoter region was directly sequenced for all samples. Sequence alignment and SNP search were inspected using DNAStar analysis programs and haplotype frequencies were estimated in Haploview V 4.0. The associations between the SNPs and the risk of NPC were analyzed using chi-square test and non-conditional logistic regression analysis with SPSS 13.0 software.

Results

A total of six variants were observed in the DC-SIGN promoter region and DC-SIGN -139 GG and -939 AA were significantly associated with NPC risk with adjusted Odds Ratios (ORs) of 2.10 (95% confidence interval [CI] = 1.23-3.59; P = 0.006) and 2.52 (1.29-4.93; P = 0.007) respectively and subjects carrying the risk allele DC-SIGN -871 G had 1.47-fold (95% CI = 1.14-1.90) increased risks of developing NPC (P = 0.003). Haplotype analysis revealed that h1 'AAAG' was significantly associated with protection against NPC (OR = 0.69; P = 0.0002) and the association was still significant when using 1000 permutation test runs (P = 0.001).

Conclusions

Our study indicated that DC-SIGN promoter variants appear to be involved in the susceptibility to NPC and the detailed mechanism of this effect need further studies.

The Biology of TRAIL and the Role of TRAIL-Based Therapeutics in Infectious Diseases

TNF-related apoptosis inducing ligand (TRAIL) is a key mediator of the innate immune response to infection. While TRAIL-mediated apoptosis plays an essential role in the clearance of virus-infected cells, its physiologic role also includes immunosurveilance for cancer cells. Therapeutics that induce TRAIL-mediated apoptosis in cancer cells remain a focus of ongoing investigation in clinical trials, and much has been learned from these studies regarding the efficacy and toxicity of these interventions. These data, combined with data from numerous preclinical studies that detail the important and multifaceted role of TRAIL during infection with human immunodeficiency virus and other viruses, suggest that therapeutic exploitation of TRAIL signaling offers a novel and efficacious strategy for the management of infectious diseases.

Measles studies in the macaque model

Much of our current understanding of measles has come from experiments in non-human primates. In 1911, Goldberger and Anderson showed that macaques inoculated with filtered secretions from measles patients developed measles, thus demonstrating that the causative agent of this disease was a virus. Since then, different monkey species have been used for experimental measles virus infections. Moreover, infection studies in macaques demonstrated that serial passage of the virus in vivo and in vitro resulted in virus attenuation, providing the basis for all current live-attenuated measles vaccines. This chapter will review the macaque model for measles, with a focus on vaccination and immunopathogenesis studies conducted over the last 15 years. In addition, recent data are highlighted demonstrating that the application of a recombinant measles virus strain expressing enhanced green fluorescent protein dramatically increased the sensitivity of virus detection, both in living and sacrificed animals, allowing new approaches to old questions on measles vaccination and pathogenesis.

Measles virus V protein is a decoy substrate for IkappaB kinase alpha and prevents Toll-like receptor 7/9-mediated interferon induction

The central role of plasmacytoid dendritic cells (pDC) in activating host immune responses stems from their high capacity to express alpha interferon (IFN-alpha) after stimulation of Toll-like receptors 7 and 9 (TLR7 and -9). This involves the adapter MyD88 and the kinases interleukin-1 receptor-associated kinase 1 (IRAK1), IRAK4, and IkappaB kinase alpha (IKKalpha), which activates IFN regulatory factor 7 (IRF7) and is independent of the canonical kinases TBK1 and IKKepsilon. We have recently shown that the immunosuppressive measles virus (MV) abolishes TLR7/9/MyD88-dependent IFN induction in human pDC (Schlender et al., J. Virol. 79:5507-5515, 2005), but the molecular mechanisms remained elusive. Here, we have reconstituted the pathway in cell lines and identified IKKalpha and IRF7 as specific targets of the MV V protein (MV-V). Binding of MV-V to IKKalpha resulted in phosphorylation of V on the expense of IRF7 phosphorylation by IKKalpha in vitro and in living cells. This corroborates the role of IKKalpha as the kinase phosphorylating IRF7. MV-V in addition bound to IRF7 and to phosphomimetic IRF7 and inhibited IRF7 transcriptional activity. Binding to both IKKalpha and IRF7 required the 68-amino-acid unique C-terminal domain of V. Inhibition of TLR/MyD88-dependent IFN induction by MV-V is unique among paramyxovirus V proteins and should contribute to the unique immunosuppressive phenotype of measles. The mechanisms employed by MV-V inspire strategies to interfere with immunopathological TLR/MyD88 signaling.

Induction of neutralising antibodies and cellular immune responses against SARS coronavirus by recombinant measles viruses

Live attenuated recombinant measles viruses (rMV) expressing a codon-optimised spike glycoprotein (S) or nucleocapsid protein (N) of severe acute respiratory syndrome-associated coronavirus (SARS-CoV) were generated (rMV-S and rMV-N). Both recombinant viruses stably expressed the corresponding SARS-CoV proteins, grew to similar end titres as the parental strain and induced high antibody titres against MV and the vectored SARS-CoV antigens (S and N) in transgenic mice susceptible to measles infection. The antibodies induced by rMV-S had a high neutralising effect on SARS-CoV as well as on MV. Moreover, significant N-specific cellular immune responses were measured by IFN-gamma ELISPOT assays. The pre-existence of anti-MV antibodies induced by the initial immunisation dose did not inhibit boost of anti-S and anti-N antibodies. Immunisations comprising a mixture of rMV-S and rMV-N induced immune responses similar in magnitude to that of vaccine components administered separately. These data support the suitability of MV as a bivalent candidate vaccine vector against MV and emerging viruses such as SARS-CoV.

Predominant infection of CD150+ lymphocytes and dendritic cells during measles virus infection of macaques

Measles virus (MV) is hypothesized to enter the host by infecting epithelial cells of the respiratory tract, followed by viremia mediated by infected monocytes. However, neither of these cell types express signaling lymphocyte activation molecule (CD150), which has been identified as the receptor for wild-type MV. We have infected rhesus and cynomolgus macaques with a recombinant MV strain expressing enhanced green fluorescent protein (EGFP); thus bringing together the optimal animal model for measles and a virus that can be detected with unprecedented sensitivity. Blood samples and broncho-alveolar lavages were collected every 3 d, and necropsies were performed upon euthanasia 9 or 15 d after infection. EGFP production by MV-infected cells was visualized macroscopically, in both living and sacrificed animals, and microscopically by confocal microscopy and FACS analysis. At the peak of viremia, EGFP fluorescence was detected in skin, respiratory and digestive tract, but most intensely in all lymphoid tissues. B- and T-lymphocytes expressing CD150 were the major target cells for MV infection. Highest percentages (up to 30%) of infected lymphocytes were detected in lymphoid tissues, and the virus preferentially targeted cells with a memory phenotype. Unexpectedly, circulating monocytes did not sustain productive MV infection. In peripheral tissues, large numbers of MV-infected CD11c+ MHC class-II+ myeloid dendritic cells were detected in conjunction with infected T-lymphocytes, suggesting transmission of MV between these cell types. Fluorescent imaging of MV infection in non-human primates demonstrated a crucial role for lymphocytes and dendritic cells in the pathogenesis of measles and measles-associated immunosuppression.

Measles virus P protein suppresses Toll-like receptor signal through up-regulation of ubiquitin-modifying enzyme A20

We recently reported that the activation of NF-kappaB and AP-1 was suppressed in monocytes infected with measles virus, but not in infected epithelial cells. This cell-type-specific suppression of the inflammatory response represents a potential for measles virus to evade host immune system. In the current study, we examined the suppression mechanism of lipopolysaccharide (LPS)-induced, namely Toll-like receptor 4 (TLR4)-mediated, activation of NF-kappaB and AP-1 in measles virus-infected monocytic cells. In the infected cells, LPS treatment failed to induce the formation of active protein kinase complex containing TAK1, TAB2 and tumor necrosis factor receptor-associated factor 6 (TRAF6), dissociate from TLR complexes containing Interleukin-1 receptor-associated kinase 1 (IRAK1). Ubiquitin-modifying enzyme A20, which is a host negative feedback regulator of NF-kappaB, was dramatically up-regulated in infected monocytic cells, but not in infected epithelial cells. Suppression of A20 expression by siRNA restored LPS-induced signaling in infected cells. Measles virus phosphoprotein (P protein) expression was necessary and sufficient for the induction of A20. P protein interacted indirectly with a negative regulatory motif in the A20 gene promoter, and released the suppression of A20 transcription, independent of the activation of NF-kappaB.

Viruses and dendritic cells: enemy mine

Dendritic cells (DCs) act not only as sentinels for detection of, but also as target cells for viruses, and this can be important for viral transport and spread. All subsets of DCs are equipped with a battery of receptors recognizing virus‐associated molecular signatures, and recognition of those launches a maturation programme that results in substantial alterations of morphology, motility and the DCs' interactive properties with the extracellular matrix and scanning T cells. In addition to being sensed, viruses are internalized into DCs and, for the major proportion, processed into peptides that are subsequently presented by major histocompatibility complex (MHC) molecules. Transmission of virus to T cells can occur after completion of their replication cycle if the intracellular milieu of the DC permits that. Alternatively, viruses can remain protected from degradation following entrapment by pattern recognition receptors in intracellular compartments, also referred to as virosomes, which translocate towards the DC/T cell interface. Most likely, transfer of virus to T cells occurs in these junctions, referred to as infectious synapses. In addition to promoting DC maturation, many viruses are able to downmodulate DC development and functions in order to evade immune recognition or to induce a generalized immunosuppression.

Virus recognition by specific natural antibodies and complement results in MHC I cross-presentation

Natural antibodies (NAb) and complement (C’) are important regulators of immune system activation. We have shown previously that the galactosyl‐α1,3‐galactosyl (Galα1,3Gal) xenoantigen and the similar ABO histo‐blood group antigens are transferred onto virus from the producer cell, resulting in sensitisation of the virus to the respective NAb in a C’‐dependent manner. Here we show that measles virus (Mv) that expresses Galα1,3Gal termini can drive the proliferation of human T cells in the presence of serum and autologous DC, whereas without such targets, measles, as expected, suppress T cell reactivity. The use of affinity‐purified NAb to Galα1,3Gal and rabbit C’ demonstrated the components in human serum responsible for this effect. Proteasome inhibition and blocking of antigen presentation showed that the increased T cell proliferation was mediated by MHC class I cross‐presentation of immune complexes. These results lend further support to the idea that polymorphic carbohydrates of the Galα1,3Gal/ABO type serve as important targets for NAb and C’ and that their expression on virus has influenced their evolution by contributing to protection against viral transmission within as well as between species. The adjuvance effect of this recognition, acting as a bridge between the natural innate and adaptive immune systems, also has important implications for vaccine development.

Woodchuck dendritic cells generated from peripheral blood mononuclear cells and transduced with recombinant human adenovirus serotype 5 induce antigen-specific cellular immune responses

Woodchucks infected with the woodchuck hepatitis virus (WHV) is the best available animal model for testing the immunotherapeutic effects of dendritic cells (DCs) in the setting of a chronic infection, as woodchucks develop a persistent infection resembling that seen in humans infected with the hepatitis B virus. In the present study, DCs were generated from woodchuck peripheral blood mononuclear cells (wDCs) in the presence of human granulocyte macrophage colony-stimulating factor (hGM-CSF) and human interleukin 4 (hIL-4). After 7 days of culture, cells with morphology similar to DCs were stained positively with a cross-reactive anti-human CD86 antibody. Functional analysis showed that uptake of FITC-dextran by wDCs was very efficient and was partially inhibited after LPS-induced maturation. Furthermore, wDCs stimulated allogenic lymphocytes and induced proliferation. Moreover, wDCs were transduced efficiently with a human adenovirus serotype 5 for the expression of beta-galactosidase. Following transduction and in vivo administration of such DCs into woodchucks, an antigen-specific cellular immune response was induced. These results demonstrate that wDCs can be generated from the peripheral blood. Following transfection with a recombinant adenovirus wDCs can be used as a feasible and effective tool for eliciting WHV-specific T-cell responses indicating their potential to serve as prophylactic and therapeutic vaccines.

TLR-4 and -6 agonists reverse apoptosis and promote maturation of simian virus 5-infected human dendritic cells through NFkB-dependent pathways

Infection of primary cultures of human immature monocyte-derived dendritic cells (moDC) with the paramyxovirus Simian Virus 5 (SV5) results in extensive cytopathic effect (CPE) and induction of apoptosis, but DC maturation pathways are not activated. In this study, we investigated the relationship between SV5-induced apoptosis and the lack of DC maturation. Reducing CPE and apoptosis in SV5-infected immature DC by the addition of a pancaspase inhibitor resulted in only low level expression of maturation markers CD40, CD80 and CD86, suggesting that SV5 infection either actively blocked maturation pathways or failed to provide sufficient signals to activate maturation. To distinguish between these hypotheses, SV5-infected immature DC were challenged with agonists that stimulate toll-like receptors (TLRs). Treatment with the TLR-4 agonist LPS or TLR-6 agonist FSL1 enhanced cell surface expression of CD40, CD80 and CD86 on SV5-infected cells to levels approaching that of mock-infected TLR-treated moDC, but treatment with agonists for TLR-2, -3, -5 or -8 had little effect. Addition of TLR-4 or -6 agonists to SV5-infected DC also dramatically reduced CPE and apoptosis, but the levels of viral protein and virus yield were not affected. Similarly, SV5-infected immature moDC were matured by treatment with IL-1beta, and these mature infected cells also showed reduced CPE and apoptosis. In the presence of NFkB inhibitors, TLR-4 and -6 agonists did not promote maturation or reduce apoptosis of SV5-infected DC, indicating that maturation and cell survival were both dependent on signaling through NFkB-dependent pathways. Our results suggest a model whereby SV5 replication induces apoptosis in immature DC but fails to provide strong maturation signals, while activation of NFkB-dependent pathways by exogenous ligands can lead to moDC maturation and override SV5-induced cell death.

Immune synapses formed with measles virus-infected dendritic cells are unstable and fail to sustain T cell activation

Interaction with dendritic cells (DCs) is considered as central to immunosuppression induced by viruses, including measles virus (MV). Commonly, viral infection of DCs abrogates their ability to promote T cell expansion, yet underlying mechanisms at a cellular level are undefined. We found that MV-infected DCs only subtly differed from LPS-matured with regard to integrin activation, acquisition of a migratory phenotype and motility. Similarly, the organization of MV-DC/T cell interfaces was consistent with that of functional immune synapses with regard to CD3 clustering and MHC class II surface recruitment. The majority of MV-DC/T cell conjugates was, however, unstable and only promoted abortive T cell activation. Thus, MV-infected DCs retain activities required for initiating, but not sustaining T cell conjugation and activation. This is partially rescued if surface expression of the MV glycoproteins on DCs is abolished by infection with a recombinant MV encoding VSV G protein instead, indicating that these contribute directly to synapse destabilization and thereby act as effectors of T cell inhibition.

Receptor (SLAM [CD150]) recognition and the V protein sustain swift lymphocyte-based invasion of mucosal tissue and lymphatic organs by a morbillivirus

Experimental infections of ferrets with canine distemper virus (CDV) recapitulate many hallmarks of measles: rash, high fever, viremia, depression of delayed-type hypersensitivity responses, lowered leukocyte counts, and reduced lymphocyte proliferation activity. To understand how a morbillivirus invades the host and causes immunosuppression, we generated CDV either unable to recognize one of the receptors or incapable of expressing either one or both of the candidate interferon antagonist proteins V and C. Variants of these viruses expressing green fluorescent protein were also generated. Striking similarities between CDV infection of ferrets and human immunodeficiency virus host invasion were documented: first, massive early replication in the gut-associated lymphatic tissue, including intestinal Peyer's patches, followed by extensive infection of lymphatic organs, including thymus and circulating lymphocytes. Moreover, T cells were selectively depleted. Thus, CDV takes advantage of mucosal surfaces for host invasion and lymphocytes for swift dissemination. A CDV unable to recognize the signaling lymphocytic activation molecule (SLAM [CD150]) that is expressed in lymphocytes and other immune cells did not spread. A V-defective CDV multiplied with reduced efficiency in lymphocytes and did not inhibit the interferon and cytokine responses. Protein C affected the severity of rash and digestive symptoms elicited by V-defective CDV, but it was dispensable for the invasion of the lymphatic organs. These findings prove formally that SLAM recognition is necessary for morbillivirus virulence. They also reveal how two viral proteins affect pathogenesis: V sustains the swift lymphocyte-based invasion of mucosal tissue and lymphatic organs, whereas C sustains subsequent infection phases.

Measles virus induces expression of SIP110, a constitutively membrane clustered lipid phosphatase, which inhibits T cell proliferation

Interference of measles virus (MV) with phosphatidyl-inositol-3-kinase (PI3K) activation in response to T cell receptor ligation was identified as important for the induction of T cell paralysis. We now show that MV exposure of unstimulated T cells induces expression of SIP110, an isoform of the lipid phosphatase SHIP145, which is translated from an intron-derived sequences containing mRNA. We found that MV contact can regulate stimulated exon inclusion into pre-mRNAs by targeting PI3K or MAPK-dependent nuclear translocation and activation of splicing regulatory serine-arginine rich (SR) and Sam68 proteins. Induction of SIP110 in resting T cells relied on MV-dependent interference with basal activity of the PI3K. SIP110 was cloned from MV-exposed T cells, and, when transiently expressed in primary or Jurkat T cells, localized into membrane clusters independently of T cell activation. Confirming that SIP110 is a catalytically active lipid phosphatase, its transgenic expression abolished basal and impaired PMA/ionomycin-stimulated phosphorylation of the Akt kinase which is important for T cell proliferation. Thus MV causes induction of SIP110 expression, which constitutively depletes the cellular phosphoinositol-3,4,5-phosphate pool suggesting that thereby the threshold for activation signals necessary for the induction of T cell proliferation is raised.

A simian virus 5 (SV5) P/V mutant is less cytopathic than wild-type SV5 in human dendritic cells and is a more effective activator of dendritic cell maturation and function

Human epithelial cells infected with the parainfluenza virus simian virus 5 (SV5) show minimal activation of host cell interferon (IFN), cytokine, and cell death pathways. In contrast, a recombinant SV5 P/V gene mutant (rSV5-P/V-CPI-) overexpresses viral gene products and is a potent inducer of IFN, proinflammatory cytokines, and apoptosis in these cells. In this study, we have compared the outcomes of wild-type (WT) SV5 and rSV5-P/V-CPI- infections of primary human dendritic cells (DC), important antigen-presenting cells for initiating adaptive immune responses. We have tested the hypothesis that a P/V mutant which activates host antiviral responses will be a more potent inducer of DC maturation and function than WT rSV5, which suppresses host cell responses. Infection of peripheral blood mononuclear cell-derived immature DC with WT rSV5 resulted in high levels of viral protein and progeny virus but very little increase in cell surface costimulatory molecules or secretion of IFN and proinflammatory cytokines. In contrast, immature DC infected with the rSV5-P/V-CPI- mutant produced only low levels of viral protein and progeny virus, but these infected cells were induced to secrete IFN-alpha and other cytokines and showed elevated levels of maturation markers. Unexpectedly, DC infected with WT rSV5 showed extensive cytopathic effects and increased levels of active caspase-3, while infection of DC with the P/V mutant was largely noncytopathic. In mixed-culture assays, WT rSV5-infected DC were impaired in the ability to stimulate proliferation of autologous CD4+ T cells, whereas DC infected with the P/V mutant were very effective at activating T-cell proliferation. The addition of a pancaspase inhibitor to DC infected with WT rSV5 reduced cytopathic effects and resulted in higher surface expression levels of maturation markers. Our finding that the SV5 P/V mutant has both a reduced cytopathic effect in human DC compared to WT SV5 and an enhanced ability to induce DC function has implications for the rational design of novel recombinant paramyxovirus vectors based on engineered mutations in the viral P/V gene.

Measles virus targets DC-SIGN to enhance dendritic cell infection

Dendritic cells (DCs) are involved in the pathogenesis of measles virus (MV) infection by inducing immune suppression and possibly spreading the virus from the respiratory tract to lymphatic tissues. It is becoming evident that DC function can be modulated by the involvement of different receptors in pathogen interaction. Therefore, we have investigated the relative contributions of different MV-specific receptors on DCs to MV uptake into and infection of these cells. DCs express the MV receptors CD46 and CD150, and we demonstrate that the C-type lectin DC-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN) is a novel receptor for laboratory-adapted and wild-type MV strains. The ligands for DC-SIGN are both MV glycoproteins F and H. In contrast to CD46 and CD150, DC-SIGN does not support MV entry, since DC-SIGN does not confer susceptibility when stably expressed in CHO cells. However, DC-SIGN is important for the infection of immature DCs with MV, since both attachment and infection of immature DCs with MV are blocked in the presence of DC-SIGN inhibitors. Our data demonstrate that DC-SIGN is crucial as an attachment receptor to enhance CD46/CD150-mediated infection of DCs in cis. Moreover, MV might not only target DC-SIGN to infect DCs but may also use DC-SIGN for viral transmission and immune suppression.

Preferential infection of mature dendritic cells by mouse hepatitis virus strain JHM

Mouse hepatitis virus strain JHM (MHV-JHM) causes acute encephalitis and acute and chronic demyelinating diseases in mice. Dendritic cells (DCs) are key cells in the initiation of innate and adaptive immune responses, and infection of these cells could potentially contribute to a dysregulated immune response; consistent with this, recent results suggest that DCs are readily infected by another strain of mouse hepatitis virus, the A59 strain (MHV-A59). Herein, we show that the JHM strain also productively infected DCs. Moreover, mature DCs were at least 10 times more susceptible than immature DCs to infection with MHV-JHM. DC function was impaired after MHV-JHM infection, resulting in decreased stimulation of CD8 T cells in vitro. Preferential infection of mature DCs was not due to differential expression of the MHV-JHM receptor CEACAM-1a on mature or immature cells or to differences in apoptosis. Although we could not detect infected DCs in vivo, both CD8(+) and CD11b(+) splenic DCs were susceptible to infection with MHV-JHM directly ex vivo. This preferential infection of mature DCs may inhibit the development of an efficient immune response to the virus.

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.

Chemokine up-regulation in SARS-coronavirus-infected, monocyte-derived human dendritic cells

Lymphopenia and increasing viral load in the first 10 days of severe acute respiratory syndrome (SARS) suggested immune evasion by SARS-coronavirus (CoV). In this study, we focused on dendritic cells (DCs) which play important roles in linking the innate and adaptive immunity. SARS-CoV was shown to infect both immature and mature human monocyte-derived DCs by electron microscopy and immunofluorescence. The detection of negative strands of SARS-CoV RNA in DCs suggested viral replication. However, no increase in viral RNA was observed. Using cytopathic assays, no increase in virus titer was detected in infected DCs and cell-culture supernatant, confirming that virus replication was incomplete. No induction of apoptosis or maturation was detected in SARS-CoV–infected DCs. The SARS-CoV–infected DCs showed low expression of antiviral cytokines (interferon α [IFN-α], IFN-β, IFN-γ, and interleukin 12p40 [IL-12p40]), moderate up-regulation of proinflammatory cytokines (tumor necrosis factor α [TNF-α] and IL-6) but significant up-regulation of inflammatory chemokines (macrophage inflammatory protein 1α [MIP-1α], regulated on activation normal T cell expressed and secreted [RANTES]), interferon-inducible protein of 10 kDa [IP-10], and monocyte chemoattractant protein 1 [MCP-1]). The lack of antiviral cytokine response against a background of intense chemokine up-regulation could represent a mechanism of immune evasion by SARS-CoV.

Analysis of human peripheral blood samples from fatal and nonfatal cases of Ebola (Sudan) hemorrhagic fever: cellular responses, virus load, and nitric oxide levels

Peripheral blood samples obtained from patients during an outbreak of Ebola virus (Sudan species) disease in Uganda in 2000 were used to phenotype peripheral blood mononuclear cells (PBMC), quantitate gene expression, measure antigenemia, and determine nitric oxide levels. It was determined that as the severity of disease increased in infected patients, there was a corresponding increase in antigenemia and leukopenia. Blood smears revealed thrombocytopenia, a left shift in neutrophils (in some cases degenerating), and atypical lymphocytes. Infected patients who died had reduced numbers of T cells, CD8(+) T cells, and activated (HLA-DR(+)) CD8(+) T cells, while the opposite was noted for patients who survived the disease. Expression levels of cytokines, Fas antigen, and Fas ligand (TaqMan quantitation) in PBMC from infected patients were not significantly different from those in uninfected patients (treated in the same isolation wards), nor was there a significant increase in expression compared to healthy volunteers (United States). This unresponsive state of PBMC from infected patients despite high levels of circulating antigen and virus replication suggests that some form of immunosuppression had developed. Ebola virus RNA levels (virus load) in PBMC specimens were found to be much higher in infected patients who died than patients who survived the disease. Similarly, blood levels of nitric oxide were much higher in fatal cases (increasing with disease severity), and extremely elevated levels (>/=150 microM) would have negatively affected vascular tone and contributed to virus-induced shock.

Defining T-cell-mediated immune responses in rotavirus-infected juvenile rhesus macaques

The appearance of virus-specific CD4(+) and/or CD8(+) T lymphocytes in peripheral blood of captive juvenile rhesus macaques (Macaca mulatta) was observed following rotavirus infection. These cell-mediated immune responses were measured following experimental or natural infection after rotavirus was isolated from stool specimens of asymptomatic animals. The virus isolated was a new strain of simian rotavirus that we named TUCH (for Tulane University and Cincinnati Children's Hospital). Restimulation of peripheral T lymphocytes by inactivated double- or triple-layered TUCH rotavirus particles containing either VP6 or VP4 and VP7 on their respective surfaces resulted in increased quantities of interleukin-6 (IL-6) and IL-12 in cell culture supernatants. Recall responses to rotavirus by CD4(+) and CD8(+) T lymphocytes were associated with accumulation of intracellular IL-6 and gamma interferon. Antigen presentation of TUCH rotavirus to lymphocytes was mediated via differentiated cultures of monocyte-derived dendritic (HLA-DR(+)) cells. This is the first report demonstrating cell-mediated immune responses to rotavirus in nonhuman primates. Further exploration of rhesus macaques in vaccine trials with human rotavirus vaccine candidates is the major objective of future studies.

Measles virus interacts with and alters signal transduction in T-cell lipid rafts

By a contact-dependent surface interaction, the measles virus (MV) glycoprotein complex induces a pronounced inhibition of T-cell proliferation. We now show that MV directly interacts with glycosphingolipid-enriched membrane microdomains on human primary T cells and alters recruitment and segregation of membrane proximal signaling components. Contact-dependent interference with T-cell receptor-stimulated tyrosine phosphorylation and Ca mobilization is a late event seen 24 h after MV treatment. In contrast, stimulated recruitment of pleckstrin homology domain-containing proteins such as Akt and Vav is inhibited early after MV contact, as is segregation of the activated Akt kinase from rafts. Tyrosine phosphorylation of the regulatory subunit of the phosphatidylinositol 3-kinase (PI3K), p85, is apparently normal then, yet this protein fails to partition to the lipid raft fraction, and this is associated with stable expression of its negative regulator Cbl-b. Thus, by interaction with lipid rafts, MV contact initially targets recruitment of PI3K by preventing stimulated Cbl-b degradation and activation of PI3K-dependent signaling components.

Measles virus interacts with human SLAM receptor on dendritic cells to cause immunosuppression

Measles virus (MV) infects dendritic cells (DCs) resulting in immunosuppression. Human DCs express two MV receptors: CD46 and human signaling lymphocyte activation molecule (hSLAM); thus, the role played by either alone is unclear. Because wild-type (wt) MV uses hSLAM receptor preferentially, we dissected the molecular basis of MV-DC interaction and resultant immunosuppression through the hSLAM receptor by creating transgenic (tg) mice expressing hSLAM on DCs. After infection with wt MV, murine splenic DCs expressing hSLAM receptor had less B7-1, B7-2, CD40, MHC class I, and MHC class II molecules on their surfaces and displayed an increased rate of apoptosis when compared to uninfected DCs. Further, MV-infected DCs failed to stimulate allogeneic T cells and inhibited mitogen-dependent T-cell proliferation. Individual expression of human SLAM, interferon alpha/beta receptor, tumor necrosis factor-alpha, and lymphotoxin-alpha or beta from T cells was not required for MV-infected DCs to inhibit the proliferation of T cells.