Measles virus, a paramyxovirus of the Morbillivirus genus, is responsible for an acute childhood illness that infects over 40 million people and leads to the deaths of more than 1 million people annually (C. J. Murray and A. D. Lopez, Lancet 349:1269-1276, 1997). Measles virus infection is characterized by virus-induced immune suppression that creates susceptibility to opportunistic infections. Here we demonstrate that measles virus can inhibit cytokine responses by direct interference with host STAT protein-dependent signaling systems. Expression of the measles V protein prevents alpha, beta, and gamma interferon-induced transcriptional responses. Furthermore, it can interfere with signaling by interleukin-6 and the non-receptor tyrosine kinase, v-Src. Affinity purification demonstrates that the measles V protein associates with cellular STAT1, STAT2, STAT3, and IRF9, as well as several unidentified partners. Mechanistic studies indicate that while the measles V protein does not interfere with STAT1 or STAT2 tyrosine phosphorylation, it causes a defect in IFN-induced STAT nuclear accumulation. The defective STAT nuclear redistribution is also observed in measles virus-infected cells, where some of the STAT protein is detected in cytoplasmic bodies that contain viral nucleocapsid protein and nucleic acids. Interference with STAT-inducible transcription may provide a novel intracellular mechanism for measles virus-induced cytokine inhibition that links innate immune evasion to adaptive immune suppression.

Measles virus (MV), a member of the genus Morbillivirus in the family Paramyxoviridae, is an enveloped virus with a non-segmented, negative-strand RNA genome. It has two envelope glycoproteins, the haemagglutinin (H) and fusion proteins, which are responsible for attachment and membrane fusion, respectively. Human signalling lymphocyte activation molecule (SLAM; also called CD150), a membrane glycoprotein of the immunoglobulin superfamily, acts as a cellular receptor for MV. SLAM is expressed on immature thymocytes, activated lymphocytes, macrophages and dendritic cells and regulates production of interleukin (IL)-4 and IL-13 by CD4+ T cells, as well as production of IL-12, tumour necrosis factor alpha and nitric oxide by macrophages. The distribution of SLAM is in accord with the lymphotropism and immunosuppressive nature of MV. Canine distemper virus and Rinderpest virus, other members of the genus Morbillivirus, also use canine and bovine SLAM as receptors, respectively. Laboratory-adapted MV strains may use the ubiquitously expressed CD46, a complement-regulatory molecule, as an alternative receptor through amino acid substitutions in the H protein. Furthermore, MV can infect SLAM- cells, albeit inefficiently, via the SLAM- and CD46-independent pathway, which may account for MV infection of epithelial, endothelial and neuronal cells in vivo. MV infection, however, is not determined entirely by the H protein-receptor interaction, and other MV proteins can also contribute to its efficient growth by facilitating virus replication at post-entry steps. Identification of SLAM as the principal receptor for MV has provided us with an important clue for better understanding of MV tropism and pathogenesis.

Measles virus (MV), a member of the family Paramyxoviridae, encodes C and V non-structural proteins. To clarify the functions of MV C and V proteins, HeLa cell lines constitutively expressing C or V protein were established. We found that expression of V protein inhibited interferon (IFN)-alpha/beta signaling but not IFN-gamma signaling. C protein had no inhibitory effect on IFN signaling in our experimental condition. Degradation of selective signal transducers and activators of transcription (STAT) proteins was not observed in HeLa cells expressing V protein. In contrast, tyrosine phosphorylation of both STAT1 and STAT2 was inhibited in these cells after IFN-beta stimulation.

Type I interferons (IFNalpha/beta) are an important part of innate immunity to viral infections because they induce an antiviral response and limit viral replication until the adaptive response clears the infection. Since the nonstructural proteins of several paramyxoviruses inhibit the IFNalpha/beta response, we chose to explore the role of the C protein of measles virus (MV) in such inhibition. Previous studies have suggested that the MV C protein may serve as a virulence factor, but its role in the pathogenesis of MV remains undefined. In the present study, a recombinant MV strain that does not express the C protein (MV C-) and its parental strain (Ed Tag) were used. Growth of MV C- was restricted in human peripheral blood mononuclear cells and HeLa cells, but in the presence of neutralizing antibodies to IFNalpha/beta, MV C- produced titers that were equivalent to those of Ed Tag. In addition, expression of the MV C protein from plasmid DNA inhibited the production of an IFNalpha/beta responsive reporter gene and, to a lesser extent, inhibited an IFNgamma responsive reporter gene. The ability of the MV C protein to suppress the IFNalpha/beta response was confirmed using a biologic assay. After IFNbeta stimulation, HeLa cells infected with Ed Tag produced five-fold less IFNalpha/beta than cells infected with MV C-. While the mechanism of inhibition remains unclear, these data suggest that the MV C protein plays an important role in the pathogenesis of MV by inhibiting IFNalpha/beta signaling.

Two genera, the Respirovirus (Sendai virus (SeV) and human parainfluenza virus (hPIV3) and the Rubulavirus (simian virus (SV) 5, SV41, mumps virus and hPIV2), of the three in the subfamily Paramyxovirinae inhibit interferon (IFN) signalling to circumvent the IFN response. The viral protein responsible for the inhibition is the C protein for respirovirus SeV and the V protein for the rubulaviruses, both of which are multifunctional accessory proteins expressed from the P gene. SeV suppresses IFN-stimulated tyrosine phosphorylation of signal transducers and activators of transcription (STATs) at an early phase of infection and further inhibits the downstream signalling without degrading any of the signalling components in most cell lines. On the contrary, the Rubulavirus V protein targets Stat1 or Stat2 for degradation. Proteasome-mediated degradation appears to be involved in most cases. Studies on the molecular mechanisms by which paramyxoviruses evade the IFN response will offer important information for modulating the JAK-STAT pathway, designing novel antiviral drugs and recombinant live vaccines, and improving paramyxovirus expression vectors for gene therapy.

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.

To establish infections, viruses use various strategies to suppress the host defense mechanism, such as interferon (IFN)-induced antiviral state. We found that cells infected with a wild strain of measles virus (MeV) displayed nearly complete suppression of IFN-alpha-induced antiviral state, but not IFN-gamma-induced state. This phenomenon is due to the suppression of IFN-alpha-inducible gene expression at a transcriptional level. In the IFN-alpha signal transduction pathway, Jak1 phosphorylation induced by IFN-alpha is dramatically suppressed in MeV-infected cells; however, phosphorylation induced by IFN-gamma is not. We performed immunoprecipitation experiments using antibodies against type 1 IFN receptor chain 1 (INFAR1) and antibody against RACK1, which is reported to be a scaffold protein interacting with type I IFN receptor chain 2 and STAT1. These experiments indicated that IFNAR1 forms a complex containing the MeV-accessory proteins C and V, RACK1, and STAT1 in MeV-infected cells but not in uninfected cells. Composition of this complex in the infected cells altered little by IFN-alpha treatment. These results indicate that MeV suppresses the IFN-alpha, but not IFN-gamma, signaling pathway by inhibition of Jak1 phosphorylation. Our data suggest that functional disorder of the type I IFN receptor complex is due to "freezing" of the receptor through its association with the C and/or V proteins of MeV.

Measles is an important cause of child mortality that has a seemingly paradoxical interaction with the immune system. In most individuals, the immune response is successful in eventually clearing measles virus (MV) infection and in establishing life-long immunity. However, infection is also associated with persistence of viral RNA and several weeks of immune suppression, including loss of delayed type hypersensitivity responses and increased susceptibility to secondary infections. The initial T-cell response includes CD8+ and T-helper 1 CD4+ T cells important for control of infectious virus. As viral RNA persists, there is a shift to a T-helper 2 CD4+ T-cell response that likely promotes B-cell maturation and durable antibody responses but may suppress macrophage activation and T-helper 1 responses to new infections. Suppression of mitogen-induced lymphocyte proliferation can be induced by lymphocyte infection with MV or by lymphocyte exposure to a complex of the hemagglutinin and fusion surface glycoproteins without infection. Dendritic cells (DCs) are susceptible to infection and can transmit infection to lymphocytes. MV-infected DCs are unable to stimulate a mixed lymphocyte reaction and can induce lymphocyte unresponsiveness through expression of MV glycoproteins. Thus, multiple factors may contribute both to measles-induced immune suppression and to the establishment of durable protective immunity.

Surface-contact-mediated signaling induced by the measles virus (MV) fusion and hemagglutinin glycoproteins is necessary and sufficient to induce T-cell unresponsiveness in vitro and in vivo. To define the intracellular pathways involved, we analyzed interleukin (IL)-2R signaling in primary human T cells and in Kit-225 cells. Unlike IL-2-dependent activation of JAK/STAT pathways, activation of Akt kinase was impaired after MV contact both in vitro and in vivo. MV interference with Akt activation was important for immunosuppression, as expression of a catalytically active Akt prevented negative signaling by the MV glycoproteins. Thus, we show here that MV exploits a novel strategy to interfere with T-cell activation during immunosuppression.

Measles virus (MV) enters cells by membrane fusion at the cell surface at neutral pH. Two glycoproteins mediate this process: the hemagglutinin (H) and fusion (F) proteins. The H-protein binds to receptors, while the F-protein mediates fusion of the viral and cellular membranes. H naturally interacts with at least three different receptors. The wild-type virus primarily uses the signaling lymphocyte activation molecule (SLAM, CD150) expressed on certain lymphatic cells, while the vaccine strain has gained the ability to also use the ubiquitous membrane cofactor protein (MCP, CD46), a regulator of complement activation. Additionally, MV infects polarized epithelial cells through an unidentified receptor (EpR). The footprints of the three receptors on H have been characterized, and the focus of research is shifting to the characterization of receptor-specific conformational changes that occur in the H-protein dimer and how these are transmitted to the F-protein trimer. It was also shown that MV attachment and cell entry can be readily targeted to designated receptors by adding specificity determinants to the H-protein. These studies have contributed to our understanding of membrane fusion by the glycoprotein complex of paramyxoviruses in general.

Immunosuppression is the major cause of infant death associated with acute measles and therefore of substantial clinical importance. Major hallmarks of this generalized modulation of immune functions are (1) lymphopenia, (2) a prolonged cytokine imbalance consistent with suppression of cellular immunity to secondary infections, and (3) silencing of peripheral blood lymphocytes, which cannot expand in response to ex vivo stimulation. Lymphopenia results from depletion, which can occur basically at any stage of lymphocyte development, and evidently, expression of the major MV receptor CD150 plays an important role in targeting these cells. Virus transfer to T cells is thought to be mediated by dendritic cells (DCs), which are considered central to the induction of T cell silencing and functional skewing. As a consequence of MV interaction, viability and functional differentiation of DCs and thereby their expression pattern of co-stimulatory molecules and soluble mediators are modulated. Moreover, MV proteins expressed by these cells actively silence T cells by interfering with signaling pathways essential for T cell activation.

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.

Measles virus (MV) causes transient but profound immunosuppression resulting in increased susceptibility to secondary bacterial and viral infections. Due to the development of these opportunistic infections, measles remains the leading vaccine-preventable cause of child death worldwide. Different immune abnormalities have been associated with measles, including disappearance of delayed-type hypersensitivity reactions, impaired lymphocyte and antigen-presenting cell functions, down-regulation of pro-inflammatory interleukin 12 production and altered interferon alpha/beta signalling pathways. Several MV proteins have been suggested to hinder immune functions: hemagglutinin, fusion protein, nucleoprotein and the non-structural V and C proteins. This review will focus on the novel functions attributed to MV proteins in the immunosuppression associated with measles. Here, we highlight new advances in the field, emphasising the interaction between MV proteins and their cellular targets, in particular the cell membrane receptors, CD46, CD150, TLR2 and FcgammaRII in the induction of immunological abnormalities associated with measles.

Measles is a highly contagious disease, which was responsible for high infant mortality before the advent of an effective vaccine in 1963. In immunocompetent individuals, measles virus (MV) infection triggers an effective immune response that starts with innate responses and then leads to successful adaptive immunity, including cell-mediated immunity and humoral immunity. The virus is cleared and lifelong protection is acquired. However, changing epidemiology of measles due to vaccination as well as severe immunodeficiency has created new pockets of individuals vulnerable to measles. This chapter reviews the knowledge on effective measles-specific immune responses induced by natural infection and vaccination and explores problems arising in specific cases of immunodeficiency, infant immunity, and ineffective vaccination against measles.

Because viruses are obligate parasites, numerous partnerships between measles virus and cellular molecules can be expected. At the entry level, measles virus uses at least two cellular receptors, CD150 and a yet to be identified epithelial receptor to which the virus H protein binds. This dual receptor strategy illuminates the natural infection and inter-human propagation of this lymphotropic virus. The attenuated vaccine strains use CD46 as an additional receptor, which results in a tropism alteration. Surprisingly, the intracellular viral and cellular protein partnership leading to optimal virus life cycle remains mostly a black box, while the interactions between viral proteins that sustain the RNA-dependant RNA polymerase activity (i.e., transcription and replication), the particle assembly and the polarised virus budding are documented. Hsp72 is the only cellular protein that is known to regulate the virus transcription and replication through its interaction with the viral N protein. The viral P protein is phosphorylated by the casein kinase II with undetermined functional consequences. The cellular partnership that controls the intracellular trafficking of viral components, the assembly and/or the budding of measles virus, remains unknown. The virus to cell innate immunity war is better documented. The 5' triphosphate-ended virus leader transcript is recognised by RIG-I, a cellular helicase, and induces the interferon response. Measles virus V protein binds to the MDAS helicase and prevents the MDA5-mediated activation of interferon. By interacting with STAT1 and Jak1, the viral P and V proteins prevent the type I interferon receptor (IFNAR) signalling. The virus N protein interacts with eIF3-p40 to inhibit the translation of cellular mRNA. The H protein binds to TLR2, which then transduces an activation signal and CD150 expression in monocytes. The P protein activates the expression of the ubiquitin modifier A20, thus blocking the TLR4-mediated signalling. Few other partnerships between measles virus components and cellular proteins have been postulated or demonstrated, and they need further investigations to understand their physiopathological outcome.

The ability to evade or suppress the host's immune response is a property of many viruses, indicating that this provides an advantage for the pathogen to spread efficiently or even to establish a persistent infection. The type and complexity of its genome and cell tropism but also its preferred type of host interaction are important parameters which define the strategy of a given virus to modulate the immune system in an optimal manner. Because they take a central position in any antiviral defence, the activation and function of T cells are the predominant target of many viral immunosuppressive regimens. In this review, two different strategies whereby this could be achieved are summarized. Retroviruses can infect professional antigen-presenting cells and impair their maturation and functional properties. This coincides with differentiation and expansion of silencing T cells referred to as regulatory T cells with suppressive activity, mainly to CD8+ effector T cells. The second concept, outlined for measles virus, is a direct, contact-mediated silencing of T cells which acquire a transient paralytic state.

Following measles virus (MV) infection, host innate immune responses promptly operate to purge the virus. Detection of alerting measles viral components or replication intermediates by pattern-recognizing host machinery of Toll-like receptors and RNA helicases triggers signaling to synthesize array of anti-viral and immunoregulatory molecules, including type I interferon (IFN). Diverse subtypes of dendritic cells (DCs) play pivotal roles in both host innate immunity on the primary MV-infected site and initiating adaptive immune responses on secondary lymphoid tissues. Responding to the predictable host immune responses, MV appears to have devised multiple strategies to evade, suppress, or even utilize host innate immunity and DC responses. This review focuses on versatile actions of MV-induced type I IFNs causing beneficial or deleterious influence on host immunity and the interplay between MV and heterogeneous DCs at distinct locations.

Immunosuppression is the major cause of infant death associated with acute measles. Hallmarks of this generalized modulation of immune functions include: (1) lymphopenia, (2) a prolonged cytokine imbalance consistent with suppression of cellular immunity to secondary infections and (3) silencing of peripheral blood lymphocytes that fail to expand in response to ex vivo stimulation. Lymphopenia results from depletion of T cells by mechanisms also involving MV infection, and expression of the major MV receptor CD150 plays an important role for targeting these cells. Virus transfer to T cells is thought to be mediated by dendritic cells (DCs), which are considered as central to the induction of T-cell silencing and functional skewing. MV interaction modulates functional differentiation of DCs, and thereby expression pattern of costimulatory molecules and soluble mediators. Moreover, MV proteins expressed by these cells actively silence T cells by interfering with signaling pathways essential for T-cell activation. As an essential component of this interference, the MV glycoprotein complex activates cellular sphingomyelinases in a contact-dependent manner, and these are effective at preventing stimulated rearrangements of the actin cytoskeleton and thereby morphological and functional polarization and motility of T cells.

Although CNS complications occurring early and late after acute measles are a serious problem and often fatal, the transient immunosuppression lasting for several weeks after the rash is the major cause of measles-related morbidity and mortality worldwide. This review is focused on the interactions of measles virus (MV) with cellular receptors on neural and lymphoid cells which are important elements in viral pathogenesis. First, the cognate MV receptors, CD46 and CD150, are important components of viral tropism by mediating binding and entry. Second, however, additional unknown cellular surface molecules may (independently of viral uptake) after interaction with the MV glycoprotein complex act as signaling molecules and thereby modulate cellular survival, proliferation, and specific functions.