Proteolytic cleavage of the fusion protein but not membrane fusion is required for measles virus-induced immunosuppression in vitro

Measles immunity and immunosuppression

Effects of measles on the immune system are only partially understood. Lymphoid tissue is a primary site of measles virus (MeV) replication where CD150 is the receptor for infection of both B and T cells. Lymphocyte depletion occurs during the acute phase of infection, but initiation of the adaptive immune response leads to extensive lymphocyte proliferation, production of MeV-specific antibody and T cells, the rash and clearance of infectious virus. Viral RNA persists in lymphoid tissue accompanied by ongoing germinal center proliferation, production of antibody-secreting cells, functionally distinct populations of T cells and antibody avidity maturation to establish life-long immunity. However, at the same time diversity of pre-existing antibodies and numbers of memory and naive B cells are reduced and susceptibility to other infections is increased.

Mutations in the Fusion Protein of Measles Virus That Confer Resistance to the Membrane Fusion Inhibitors Carbobenzoxy-d-Phe-l-Phe-Gly and 4-Nitro-2-Phenylacetyl Amino-Benzamide

The inhibitors carbobenzoxy (Z)-d-Phe-l-Phe-Gly (fusion inhibitor peptide [FIP]) and 4-nitro-2-phenylacetyl amino-benzamide (AS-48) have similar efficacies in blocking membrane fusion and syncytium formation mediated by measles virus (MeV). Other homologues, such as Z-d-Phe, are less effective but may act through the same mechanism. In an attempt to map the site of action of these inhibitors, we generated mutant viruses that were resistant to the inhibitory effects of Z-d-Phe-l-Phe-Gly. These 10 mutations were localized to the heptad repeat B (HRB) region of the fusion protein, and no changes were observed in the viral hemagglutinin, which is the receptor attachment protein. Mutations were validated in a luciferase-based membrane fusion assay, using transfected fusion and hemagglutinin expression plasmids or with syncytium-based assays in Vero, Vero-SLAM, and Vero-Nectin 4 cell lines. The changes I452T, D458N, D458G/V459A, N462K, N462H, G464E, and I483R conferred resistance to both FIP and AS-48 without compromising membrane fusion. The inhibitors did not block hemagglutinin protein-mediated binding to the target cell. Edmonston vaccine/laboratory and IC323 wild-type strains were equally affected by the inhibitors. Escape mutations were mapped upon a three-dimensional (3D) structure modeled from the published crystal structure of parainfluenzavirus 5 fusion protein. The most effective mutations were situated in a region located near the base of the globular head and its junction with the alpha-helical stalk of the prefusion protein. We hypothesize that the fusion inhibitors could interfere with the structural changes that occur between the prefusion and postfusion conformations of the fusion protein.IMPORTANCE Due to lapses in vaccination worldwide that have caused localized outbreaks, measles virus (MeV) has regained importance as a pathogen. Antiviral agents against measles virus are not commercially available but could be useful in conjunction with MeV eradication vaccine programs and as a safeguard in oncolytic viral therapy. Three decades ago, the small hydrophobic peptide Z-d-Phe-l-Phe-Gly (FIP) was shown to block MeV infections and syncytium formation in monkey kidney cell lines. The exact mechanism of its action has yet to be determined, but it does appear to have properties similar to those of another chemical inhibitor, AS-48, which appears to interfere with the conformational change in the viral F protein that is required to elicit membrane fusion. Escape mutations were used to map the site of action for FIP. Knowledge gained from these studies could help in the design of new inhibitors against morbilliviruses and provide additional knowledge concerning the mechanism of virus-mediated membrane fusion.

Use of quantitative real-time RT-PCR to investigate the correlation between viremia and viral shedding of canine distemper virus, and infection outcomes in experimentally infected dogs

We used real-time RT-PCR and virus titration to examine canine distemper virus (CDV) kinetics in peripheral blood and rectal and nasal secretions from 12 experimentally infected dogs. Real-time RT-PCR proved extremely sensitive, and the correlation between the two methods for rectal and nasal (r=0.78, 0.80) samples on the peak day of viral RNA was good. Although the dogs showed diverse symptoms, viral RNA kinetics were similar; the peak of viral RNA in the symptomatic dogs was consistent with the onset of symptoms. These results indicate that real-time RT-PCR is sufficiently sensitive to monitor CDV replication in experimentally infected dogs regardless of the degree of clinical manifestation and suggest that the peak of viral RNA reflects active CDV replication.

Cytoskeletal dynamics: concepts in measles virus replication and immunomodulation

In common with most viruses, measles virus (MV) relies on the integrity of the cytoskeleton of its host cells both with regard to efficient replication in these cells, but also retention of their motility which favors viral dissemination. It is, however, the surface interaction of the viral glycoprotein (gp) complex with receptors present on lymphocytes and dendritic cells (DCs), that signals effective initiation of host cell cytoskeletal dynamics. For DCs, these may act to regulate processes as diverse as viral uptake and sorting, but also the ability of these cells to successfully establish and maintain functional immune synapses (IS) with T cells. In T cells, MV signaling causes actin cytoskeletal paralysis associated with a loss of polarization, adhesion and motility, which has been linked to activation of sphingomyelinases and subsequent accumulation of membrane ceramides. MV modulation of both DC and T cell cytoskeletal dynamics may be important for the understanding of MV immunosuppression at the cellular level.

Measles virus-induced suppression of immune responses

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.

Canine distemper viruses expressing a hemagglutinin without N-glycans lose virulence but retain immunosuppression

Paramyxovirus glycoproteins are posttranslationally modified by the addition of N-linked glycans, which are often necessary for correct folding, processing, and cell surface expression. To establish the contribution of N glycosylation to morbillivirus attachment (H) protein function and overall virulence, we first determined the use of the potential N-glycosylation sites in the canine distemper virus (CDV) H proteins. Biochemical characterization revealed that the three sites conserved in all strains were N glycosylated, whereas only two of the up to five additional sites present in wild-type strains are used. A wild-type virus with an H protein reproducing the vaccine strain N-glycosylation pattern remained lethal in ferrets but with a prolonged course of disease. In contrast, introduction of the vaccine H protein in the wild-type context resulted in complete attenuation. To further characterize the role of N glycosylation in CDV pathogenesis, the N-glycosylation sites of wild-type H proteins were successively deleted, including a nonstandard site, to ultimately generate a nonglycosylated H protein. Despite reduced expression levels, this protein remained fully functional. Recombinant viruses expressing N-glycan-deficient H proteins no longer caused disease, even though their immunosuppressive capacities were retained, indicating that reduced N glycosylation contributes to attenuation without affecting immunosuppression.

Measles virus-induced immunosuppression

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.

Measles virus inhibits human immunodeficiency virus type 1 reverse transcription and replication by blocking cell-cycle progression of CD4+ T lymphocytes

Acute measles virus (MV) infection results in a decrease in plasma human immunodeficiency virus type 1 (HIV-1) RNA levels in co-infected children. An in vitro peripheral blood mononuclear cell (PBMC) culture system was used to assess the mechanisms by which MV blocks HIV-1 replication. MV inhibited proliferation of CD4(+) T lymphocytes, the target cell for HIV-1 replication. In the presence of MV, cells did not progress to G(1b) and S phases, steps critical for the completion of HIV-1 reverse transcription and productive replication. This block in cell-cycle progression was characterized by an increased proportion of CD4(+) and HIV-1-infected cells retained in the parental generation in PBMCs co-cultured with MV and HIV-1, and decreased levels of cyclins and RNA synthesis. Early HIV-1 replication was also inhibited in the presence of MV, as measured by reduced expression of a luciferase reporter gene and lower levels of both early (LTR) and late (LTR-gag) DNA intermediates of HIV-1 reverse transcription in the presence of CCR5-tropic HIV-1. The effects of MV on lymphoproliferation and p24 antigen production were reproduced by n-butyrate and hydroxyurea, drugs that block the cell cycle in G(1a) and G(1)/S, respectively. It was concluded that MV inhibits HIV-1 productive replication in part by blocking the proliferation of CD4(+) T lymphocytes.

Glycoprotein targeting signals influence the distribution of measles virus envelope proteins and virus spread in lymphocytes

We previously demonstrated the presence of tyrosine-dependent motifs for specific sorting of two measles virus (MV) glycoproteins, H and F, to the basolateral surface in polarized epithelial cells. Targeted expression of the glycoproteins was found to be required for virus spread in epithelia via cell-to-cell fusion in vitro and in vivo. In the present study, recombinant MVs (rMVs) with substitutions of the critical tyrosines in the H and F cytoplasmic domains were used to determine whether the sorting signals also play a crucial role for MV replication and spread within lymphocytes, the main target cells of acute MV infection. Immunolocalization revealed that only standard glycoproteins are targeted specifically to the uropod of polarized lymphocytes and cluster on the surface of non-polarized lymphocytes. H and F proteins with tyrosine mutations did not accumulate in uropods, but were distributed homogeneously on the surface and did not colocalize markedly with the matrix (M) protein. Due to the defective interaction with the M protein, all mutant rMVs showed an enhanced fusion capacity, but only rMVs harbouring two mutated glycoproteins showed a marked decrease in virus release from infected lymphocytes. These results demonstrate clearly that the tyrosine-based targeting motifs in the MV glycoproteins are not only important in polarized epithelial cells, but are also active in lymphocytes, thus playing an important role in virus propagation in different key target cells during acute MV infection.

Measles virus nucleocapsid transport to the plasma membrane requires stable expression and surface accumulation of the viral matrix protein

In measles virus (MV)-infected cells the matrix (M) protein plays a key role in virus assembly and budding processes at the plasma membrane because it mediates the contact between the viral surface glycoproteins and the nucleocapsids. By exchanging valine 101, a highly conserved residue among all paramyxoviral M proteins, we generated a recombinant MV (rMV) from cloned cDNA encoding for a M protein with an increased intracellular turnover. The mutant rMV was barely released from the infected cells. This assembly defect was not due to a defective M binding to other matrix- or nucleoproteins, but could rather be assigned to a reduced ability to associate with cellular membranes, and more importantly, to a defective accumulation at the plasma membrane which was accompanied by the deficient transport of nucleocapsids to the cell surface. Thus, we show for the first time that M stability and accumulation at intracellular membranes is a prerequisite for M and nucleocapsid co-transport to the plasma membrane and for subsequent virus assembly and budding processes.

Measles virus contact with T cells impedes cytoskeletal remodeling associated with spreading, polarization, and CD3 clustering

CD3/CD28-induced activation of the PI3/Akt kinase pathway and proliferation is impaired in T cells after contact with the measles virus (MV) glycoprotein (gp) complex. We now show that this signal also impairs actin cytoskeletal remodeling in T cells, which loose their ability to adhere and to promote microvilli formation. MV exposure results in an almost complete collapse of membrane protrusions associated with reduced phosphorylation levels of cofilin and ezrin/radixin/moesin (ERM) proteins. Consistent with their inability to activate Cdc42 and Rac1 in response to the ligation of CD3/CD28, T cells exposed to MV fail to acquire a morphology consistent with spreading and lamellopodia formation. In spite of these impairments of cytoskeleton-driven morphological alterations, these cells are recruited into conjugates with dendritic cells as efficiently as control T cells. The signal elicited by MV, however, prevents T cells to polarize as documented by a failure to redistribute the microtubule organizing center toward the synapse. Moreover, CD3 cannot be efficiently clustered and redistributed to the central region of the immunological synapse. Thus, by inducing microvillar collapse and interfering with cytoskeletal remodeling, MV signaling disturbs the ability of T cells to adhere, spread, and cluster receptors essential for sustained T-cell activation.

Silencing T cells or T-cell silencing: concepts in virus-induced immunosuppression

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.

Neonatal Immunization with a Sindbis Virus-DNA Measles Vaccine Induces Adult-Like Neutralizing Antibodies and Cell-Mediated Immunity in the Presence of Maternal Antibodies

Infants younger than age 9 mo do not respond reliably to the live attenuated measles vaccine due the immaturity of their immune system and the presence of maternal Abs that interfere with successful immunization. We evaluated the immune responses elicited by Sindbis virus replicon-based DNA vaccines encoding measles virus (MV) hemagglutinin (H, pMSIN-H) or both hemagglutinin and fusion (F, pMSINH-FdU) glycoproteins in neonatal mice born to naive and measles-immune mothers. Despite the presence of high levels of maternal Abs, neonatal immunization with pMSIN-H induced long-lasting, high-avidity MV plaque reduction neutralization (PRN) Abs, mainly IgG2a, that also inhibited syncytium formation in CD150(+) B95-8 cells. IgG secreting plasma cells were detected in spleen and bone marrow. Newborns vaccinated with pMSINH-FdU elicited PRN titers that surpassed the protective level (200 mIU/ml) but were short-lived, had low syncytium inhibition capacity, and lacked avidity maturation. This vaccine failed to induce significant PRN titers in the presence of placentally transferred Abs. Both pMSIN-H and pMSINH-FdU elicited strong Th1 type cell-mediated immunity, measured by T cell proliferation and IFN-gamma production, that was unaffected by maternal Abs. Newborns responded to measles DNA vaccines with similar or even higher PRN titers and cell-mediated immunity than adult mice. This study is the first demonstration that a Sindbis virus-based measles DNA vaccine can elicit robust MV immunity in neonates bypassing maternal Abs. Such a vaccine could be followed by the current live attenuated MV vaccine in a heterologous prime-boost to protect against measles early in life.

Immunosuppression caused by measles virus: role of viral proteins

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.

Characterization of immune responses induced by intramuscular vaccination with DNA vaccines encoding measles virus hemagglutinin and/or fusion proteins

Measles virus (MV) hemagglutinin (MV-H) and fusion (MV-F) proteins induce plaque reduction neutralizing (PRN) antibodies and cell-mediated immune responses that protect against clinical measles. DNA vaccines that encode MV-H and MV-F are being investigated as a new generation of measles vaccine to protect infants too young to receive currently licensed attenuated measles vaccines. However, it is unclear whether DNA vaccines encoding both MV-H and MV-F act synergistically to induce stronger immunity than immunization with plasmids encoding MV-H or MV-F alone. To address this question, we generated Sindbis virus-based pSINCP DNA vaccines that encode either MV-H or MV-F alone or bicistronic or fusion system vectors that encode both MV-H and MV-F (to mimic MV infection where both MV-H and MV-F proteins are expressed by the same mammalian cell). Mice immunized with DNA vaccine encoding MV-H alone developed significantly greater PRN titers than mice immunized with bicistronic constructs. Interestingly, the presence of MV-F in the bicistronic constructs stimulated serum MV-specific immunoglobulin G of reduced avidity. By contrast, mice immunized with bicistronic constructs induced equivalent or higher levels of MV-specific gamma interferon responses than mice immunized with DNA vaccine encoding MV-H alone. These data will help guide the design of DNA-based MV vaccines to be used early in life in a heterologous prime-boost strategy.

In vitro suppression of human immunodeficiency virus type 1 replication by measles virus

During the acute phase of measles, human immunodeficiency virus type 1 (HIV-1)-infected children have a transient, but dramatic, decrease in plasma HIV-1 RNA levels (W. J. Moss, J. J. Ryon, M. Monze, F. Cutts, T. C. Quinn, and D. E. Griffin, J. Infect. Dis. 185:1035-1042, 2002). To determine the mechanism(s) by which coinfection with measles virus (MV) decreases HIV-1 replication, we established an in vitro culture system that reproduces this effect. The addition of MV to CCR5- or CXCR4-tropic HIV-1-infected human peripheral blood mononuclear cells (PBMCs) decreased HIV-1 p24 antigen production in a dose-dependent manner. This decrease occurred with the addition of MV before or after HIV-1. The inhibition of HIV-1 p24 antigen production was decreased when UV-inactivated MV or virus-free supernatant fluid from MV-infected PBMCs was used. Inhibition was not due to increased production of chemokines known to block coreceptor usage by HIV-1, a decrease in the percentage of CD4+ T cells, or a decrease in chemokine receptor expression by CD4+ T cells. Viability of PBMCs was decreased only 10 to 20% by MV coinfection; however, lymphocyte proliferation was decreased by 60 to 90% and correlated with decreased production of p24 antigen. These studies showed that an in vitro system of coinfected PBMCs could be used to dissect the mechanism(s) by which MV suppresses HIV-1 replication in coinfected children and suggest that inhibition of lymphocyte proliferation by MV may play a role in the suppression of HIV-1 p24 antigen production.

Impact of measles virus dendritic-cell infection on Th-cell polarization in vitro

Interference of measles virus (MV) with dendritic-cell (DC) functions and deregulation of T-cell differentiation have been proposed to be central to the profound suppression of immune responses to secondary infections up to several weeks after the acute disease. To address the impact of MV infection on the ability of DCs to promote Th-cell differentiation, an in vitro system was used where uninfected, tumour necrosis factor alpha/interleukin (IL) 1 beta-primed DCs were co-cultured with CD45RO(-) T cells in the presence of conditioned media from MV-infected DCs primed under neutral or DC-polarizing conditions. It was found that supernatants of DCs infected with an MV vaccine strain strongly promoted Th1 differentation, whereas those obtained from wild-type MV-infected DCs generated a mixed Th1/Th0 response, irrespective of the conditions used for DC priming. Th-cell commitment in this system did not correlate with the production of IL12 p70, IL18 or IL23. Thus, a combination of these or other, as yet undefined, soluble factors is produced upon MV infection of DCs that strongly promotes Th1/Th0 differentiation.

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: immune suppression and immune responses

Measles is a highly contagious viral disease that remains the leading vaccine-preventable cause of child mortality worldwide. Deaths from measles are due largely to an increased susceptibility to secondary bacterial and viral infections, attributed to a prolonged state of immune suppression. Several abnormalities of the immune system have been described, including changes in lymphocyte number and function, shifts in cytokine responses, immunomodulatory effects of interleukin-10, down regulation of interleukin-12, impaired antigen presentation, and altered interferon alpha/beta signaling pathways. Although the current vaccine is very effective, knowledge of the molecular basis of the immune responses to measles virus could contribute to the development of a safer, more immunogenic measles vaccine. However, the safety of new measles vaccines must be carefully investigated, as two measles vaccines have resulted in unintended immunologic consequences: atypical measles following administration of the formalin-inactivated measles vaccine and increased mortality in girls following administration of high-titer measles vaccines.

Polarized glycoprotein targeting affects the spread of measles virus in vitro and in vivo

We have shown previously that basolateral targeting of plasmid-encoded measles virus (MV) F and H protein is dependent on single tyrosine residues in the cytoplasmic tails of the glycoproteins and is essential for fusion activity in polarized epithelial cells. Here, we present data on the functional importance of polarized glycoprotein expression for the cytopathic properties of infectious MV in culture and for pathogenesis in vivo. By the introduction of single point mutations, we generated recombinant viruses in which the basolateral targeting signal of either one or both glycoproteins was destroyed (tyrosine mutants). As a consequence, the mutated glycoproteins were predominantly expressed on the apical membrane of polarized Madin-Darby canine kidney cells. In contrast to parental MV, none of these virus mutants was able to spread by syncytia formation in polarized cells showing that the presence of both MV glycoproteins at the basolateral cell surface is required for cell-to-cell fusion in vitro. Using cotton rats as an animal model that allows MV replication in the respiratory tract, we showed that basolateral glycoprotein targeting is also of importance for the spread of infection in vivo. Whereas parental MV was able to spread laterally within the respiratory epithelium and from there to cells in the underlying tissue, tyrosine mutants infected only single epithelial and very few subepithelial cells. These data strongly suggest that basolateral targeting of MV glycoproteins helps to overcome the epithelial barrier and thereby facilitates the systemic spread of MV infection in vivo.

Measles Virus 1998–2002: Progress and Controversy

Despite the extensive media exposure that viruses such as West Nile, Norwalk, and Ebola have received lately, and the emerging threat that old pathogens may reappear as new agents of terrorism, measles virus (MV) persists as one of the leading causes of death by infectious agents worldwide, approaching the annual mortality rate of human immunodeficiency virus (HIV)-1. For most MV victims, fatality is indirect: Virus-induced transient immunosuppression predisposes the individual to opportunistic infections that, left untreated, can result in mortality. In rare cases, MV may also cause progressive neurodegenerative disease. During the past five years (1998-2002), development of animal models and the application of reverse genetics and immunological assays have collectively contributed to major progress in our understanding of MV biology and pathogenesis. Nevertheless, questions and controversies remain that are the basis for future research. In this review, major advances and current debates are discussed, including MV receptor usage, the cellular basis of immunosuppression, the suspected role of MV in "nonviral" diseases such as multiple sclerosis and Paget's disease, and the controversy surrounding MV vaccine safety.

Dendritic cells and measles virus infection

Measles is a major cause of childhood mortality in developing countries which is mainly attributed to the ability of measles virus (MV) to suppress general immune responses. Paradoxically, virus-specific immunity is efficiently induced, which leads to viral clearance from the host and confers long-lasting protection against reinfection. As sensitisers of pathogen encounter and instructors of the adaptive immune response, dendritic cells (DCs) may play a decisive role in the induction and quality of the MV-specific immune activation. The ability of MV wild-type strains in particular to infect DCs in vitro is dearly established, and the receptor binding haemagglutinin protein of these viruses essentially determines this particular tropism. DC maturation as induced early after MV infection is likely to be of crucial importance for the induction of MV-specific immunity. DCs may, however, be instrumental in MV-induced immunosuppression. (1) T cell depletion could be brought about by DC-T cell fusion or TRAIL-mediated induction of apoptosis. (2) Inhibition of stimulated IL-12 production from MV-infected DCs might affect T cell responses in qualitative terms in favouring Th2 and suppressing Th1 responses. (3) The viral glycoprotein complex expressed at high levels on infected DCs late in infection is able to directly inhibit T cell proliferation by surface contact-dependent negative signalling. This most likely accounts for the failure of infected DC cultures to stimulate allogeneic and inhibit mitogen-stimulated T cell proliferation in vitro and the pronounced proliferative unresponsiveness of T cell ex vivo to polyclonal and antigen-specific stimulation which is a central finding of MV-induced immunosuppression.

Measles virus infects and suppresses proliferation of T lymphocytes from transgenic mice bearing human signaling lymphocytic activation molecule

Humans are the only natural reservoir of measles virus (MV), one of the most contagious viruses known. MV infection and the profound immunosuppression it causes are currently responsible for nearly one million deaths annually. Human signaling lymphocytic activation molecule (hSLAM) was identified as a receptor for wild-type MV as well as for MV strains prepared as vaccines. To better evaluate the role of hSLAM in MV pathogenesis and MV-induced immunosuppression, we created transgenic (tg) mice that expressed the hSLAM molecule under the control of the lck proximal promoter. hSLAM was expressed on CD4(+) and CD8(+) T cells in the blood and spleen and also on CD4(+), CD8(+), CD4(+) CD8(+), and CD4(-) CD8(-) thymocytes. Wild-type MV, after limited passage on B95-8 marmoset B cells, and the Edmonston laboratory strain of MV infected hSLAM-expressing cells. There was a direct correlation between the amount of hSLAM expressed on the cells' surface and the degree of viral infection. Additionally, MV infection induced downregulation of receptor hSLAM and inhibited cell division and proliferation of hSLAM(+) but not hSLAM(-) T cells. Therefore, these tg mice provide the opportunity for analyzing and comparing MV-T cell interactions and MV pathogenesis in cells expressing only the hSLAM MV receptor with those of tg mice whose T cells selectively express another MV receptor, CD46.

Cleavage at the furin consensus sequence RAR/KR(109) and presence of the intervening peptide of the respiratory syncytial virus fusion protein are dispensable for virus replication in cell culture

Proteolytic processing of the respiratory syncytial virus F (fusion) protein results in the generation of the disulfide-linked subunits F1 and F2 and in the release of pep27, a glycopeptide originally located between the two furin cleavage sites FCS-1 (RKRR(136)) and FCS-2 (RAR/KR(109)). We made use of reverse genetics to study the importance of FCS-2 and of pep27 for BRSV replication in cell culture. Replacement of FCS-2 in the F protein of recombinant viruses by either of the sequences NANR(109), RANN(109) or SANN(109), respectively, abolished proteolytic processing at this position, whereas the cleavage of FCS-1 was not affected. All mutants replicated in calf kidney and Vero cells in the absence of exogenous trypsin, although somewhat higher titers of BRSV containing the NANR(109) or the RANN(109) motif were achieved in the presence of trypsin. The virus mutants showed a reduced cytopathic effect which was lowest in the case of the SANN(109) mutant. These findings demonstrate that cleavage at FCS-2 is dispensable for replication of respiratory syncytial virus in cell culture. A deletion mutant containing FCS-1 but lacking FCS-2 and most of pep27 replicated in cell culture as efficiently as the parental virus, indicating that this domain of the F protein is not essential for virus maturation and infectivity.

Immune responses to measles and measles vaccine: challenges for measles control

Most strategies for reducing global measles morbidity and mortality and eliminating measles are based on the ability to enhance immune responses to measles virus. Challenges to measles elimination and eradication are based in part on the need to sustain high levels of population immunity to interrupt transmission of measles virus. We review aspects of the immunology of measles and measles vaccination with the aim of demonstrating how knowledge of the immune responses is essential to furthering the goals of reducing measles morbidity and mortality and the elimination of measles. Better understanding of the mechanisms of immune suppression after measles, the potential for alternative vaccination strategies to induce immunity in young infants, and the immunologic basis of atypical measles, increased mortality after high-titer measles vaccine, and waning immunity will lead to improved strategies for measles control and elimination.

Importance of the cytoplasmic tails of the measles virus glycoproteins for fusogenic activity and the generation of recombinant measles viruses

The generation of replication-competent measles virus (MV) depends on the incorporation of biologically active, fusogenic glycoprotein complexes, which are required for attachment and penetration into susceptible host cells and for direct virus spread by cell-to-cell fusion. Whereas multiple studies have analyzed the importance of the ectodomains of the MV glycoproteins hemagglutinin (H) and fusion protein (F), we have investigated the role of the cytoplasmic tails of the F and H proteins for the formation of fusogenic complexes. Deletions in the cytoplasmic tails of transiently expressed MV glycoproteins were found to have varying effects on receptor binding, fusion, or fusion promotion activity. F tail truncation to only three amino acids did not affect fusion capacity. In contrast, truncation of the H cytoplasmic tail was limited. H protein mutants with cytoplasmic tails of <14 residues no longer supported F-mediated cell fusion, predominantly due to a decrease in surface expression and receptor binding. This indicates that a minimal length of the H protein tail of 14 amino acids is required to ensure a threshold local density to have sufficient accumulation of fusogenic H-F complexes. By using reverse genetics, a recombinant MV with an F tail of three amino acids (rMV-FcDelta30), as well as an MV with an H tail of 14 residues (rMV-HcDelta20), could be rescued, whereas generation of viruses with shorter H tails failed. Thus, glycoprotein truncation does not interfere with the successful generation of recombinant MV if fusion competence is maintained.

Enhancing the proteolytic maturation of human immunodeficiency virus type 1 envelope glycoproteins

In virus-infected cells, the envelope glycoprotein (Env) precursor, gp160, of human immunodeficiency virus type 1 is cleaved by cellular proteases into a fusion-competent gp120-gp41 heterodimer in which the two subunits are noncovalently associated. However, cleavage can be inefficient when recombinant Env is expressed at high levels, either as a full-length gp160 or as a soluble gp140 truncated immediately N-terminal to the transmembrane domain. We have explored several methods for obtaining fully cleaved Env for use as a vaccine antigen. We tested whether purified Env could be enzymatically digested with purified protease in vitro. Plasmin efficiently cleaved the Env precursor but also cut at a second site in gp120, most probably the V3 loop. In contrast, a soluble form of furin was specific for the gp120-gp41 cleavage site but cleaved inefficiently. Coexpression of Env with the full-length or soluble form of furin enhanced Env cleavage but also reduced Env expression. When the Env cleavage site (REKR) was mutated in order to see if its use by cellular proteases could be enhanced, several mutants were found to be processed more efficiently than the wild-type protein. The optimal cleavage site sequences were RRRRRR, RRRRKR, and RRRKKR. These mutations did not significantly alter the capacity of the Env protein to mediate fusion, so they have not radically perturbed Env structure. Furthermore, unlike that of wild-type Env, expression of the cleavage site mutants was not significantly reduced by furin coexpression. Coexpression of Env cleavage site mutants and furin is therefore a useful method for obtaining high-level expression of processed Env.

Measles virus interactions with cellular receptors: consequences for viral pathogenesis

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.

Disruption of Akt kinase activation is important for immunosuppression induced by measles virus

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.

CD150 (SLAM) is a receptor for measles virus but is not involved in viral contact-mediated proliferation inhibition

Measles virus (MV) interacts with cellular receptors on the surface of peripheral blood lymphocytes (PBL) which mediate virus binding and uptake. Simultaneously, the direct contact of the viral glycoproteins with the cell surface induces a negative signal blocking progression to the S phase of the cell cycle, resulting in a pronounced proliferation inhibition. We selected a monoclonal antibody (MAb 5C6) directed to the surface of highly MV-susceptible B cells (B95a), which inhibits binding to and infection of cells with MV wild-type and vaccine strains. By screening a retroviral cDNA library from human splenocytes (ViraPort; Stratagene) with this antibody, we cloned and identified the recognized molecule as signaling lymphocytic activation molecule (SLAM; CD150), which is identical to the MV receptor recently found by H. Tatsuo et al. (Nature 406:893-897, 2000). After infection of cells, and after surface contact with MV envelope proteins, SLAM is downregulated from the cell surface of activated PBL and cell lines. Although anti-SLAM and/or anti-CD46 antibodies block virus binding, they do not interfere with the contact-mediated proliferation inhibition. In addition, the cell-type-specific expression of SLAM does not correlate with the sensitivity of cells for proliferation inhibition. The data indicate that proliferation inhibition induced by MV contact is independent of the presence or absence of the virus-binding receptors SLAM and CD46.

Paramyxovirus fusion (F) protein: a conformational change on cleavage activation

The fusion (F) protein of the paramyxovirus SV5 promotes both virus-cell and cell-cell fusion. Recently, the atomic structure at 1.4 A of an extremely thermostable six-helix bundle core complex consisting of two heptad repeat regions of the F protein has been described (K. A. Baker, R. E. Dutch, R. A. Lamb, and T. S Jardetsky, Mol. Cell 3, 309-319, 1999). To analyze the conformations of the F protein at various stages of the membrane fusion process and to understand further the role of formation of the six-helix bundle core complex in promotion of membrane fusion, antibodies to peptides corresponding to regions of the F protein were obtained. Major changes in F protein antibody recognition were found after cleavage of the precursor protein F(0) to the fusogenically active disulfide-linked heterodimer, F(1) + F(2), and antibodies directed against the heptad repeat regions recognized only the uncleaved form. A monoclonal antibody directed against the F protein showed increased recognition at the cell surface of the cleaved form of the F protein as compared to uncleaved F protein, again indicating changes in conformation between the uncleaved and cleaved forms of the F protein. Anti-peptide antibodies specific for the heptad repeat regions were unable to precipitate a synthetic protein that consisted of the heptad repeat regions separated only by a small spacer, suggesting that the antibodies are unable to recognize their target regions when the heptad repeats are present in the six-helix bundle core complex. Taken together, these data indicate that the six-helix bundle core complex is not present in the precursor molecule F(0) and that significant conformational changes occur subsequent to cleavage of the F protein.

Maturation of HIV envelope glycoprotein precursors by cellular endoproteases

The entry of enveloped viruses into its host cells is a crucial step for the propagation of viral infection. The envelope glycoprotein complex controls viral tropism and promotes the membrane fusion process. The surface glycoproteins of enveloped viruses are synthesized as inactive precursors and sorted through the constitutive secretory pathway of the infected cells. To be infectious, most of the viruses require viral envelope glycoprotein maturation by host cell endoproteases. In spite of the strong variability of primary sequences observed within different viral envelope glycoproteins, the endoproteolytical cleavage occurs mainly in a highly conserved domain at the carboxy terminus of the basic consensus sequence (Arg-X-Lys/Arg-Arg downward arrow). The same consensus sequence is recognized by the kexin/subtilisin-like serine proteinases (so called convertases) in many cellular substrates such as prohormones, proprotein of receptors, plasma proteins, growth factors and bacterial toxins. Therefore, several groups of investigators have evaluated the implication of convertases in viral envelope glycoprotein cleavage. Using the vaccinia virus overexpression system, furin was first shown to mediate the proteolytic maturation of both human immunodeficiency virus (HIV-1) and influenza virus envelope glycoproteins. In vitro studies demonstrated that purified convertases directly and specifically cleave viral envelope glycoproteins. Although these studies suggested the participation of several enzymes belonging to the convertases family, recent data suggest that other protease families may also participate in the HIV envelope glycoprotein processing. Their role in the physiological maturation process is still hypothetical and the molecular mechanism of the cleavage is not well documented. Crystallization of the hemagglutinin precursor (HA0) of influenza virus allowed further understanding of the molecular interaction between viral precursors and the cellular endoproteases. Furthermore, relationships between differential pathogenicity of influenza strains and their susceptibility to cleavage are molecularly funded. Here we review the most recent data and recent insights demonstrating the crucial role played by this activation step in virus infectivity. We discuss the cellular endoproteases that are implicated in HIV gp160 endoproteolytical maturation into gp120 and gp41.

Measles virus-induced promotion of dendritic cell maturation by soluble mediators does not overcome the immunosuppressive activity of viral glycoproteins on the cell surface

Measles virus (MV) infection promotes maturation of dendritic cells (DC), but also interferes with DC functions, and MV renders the DC inhibitory for T cell proliferation. We now describe that MV infection triggers the release of type I IFN from monocyte-derived DC (Mo-DC) which contributes to DC maturation. There is no evidence that soluble mediators are released interfering with the stimulatory activity of uninfected DC. Since inhibition of allogeneic T cell proliferation was unaffected by a fusion inhibitory peptide (Z-fFG), MV infection of T cells did not contribute to inhibition. Allogeneic T cell proliferation depended on the percentage of DC expressing MV F/H glycoproteins within the DC population and their surface expression levels, was induced upon addition of UV-inactivated MV to a mixed lymphocyte reaction stimulated by lipopolysaccharide-matured DC, and was not induced by DC infected with a recombinant MV encoding the ectodomain of vesicular stomatitis virus G protein (MG/FV) instead of the MV glycoproteins. Similarly, DC infected with MV, but not with MG/FV inhibited mitogen-induced proliferation of T cells. Thus, a dominant inhibitory signal is delivered to T cells by the MV glycoproteins on the surface of DC overcoming positive signals by co-stimulatory molecules promoted by maturation factors released from infected DC.

Measles virus-induced immunosuppression in vitro is independent of complex glycosylation of viral glycoproteins and of hemifusion

Expression of the measles virus (MV) F/H complex on the surface of viral particles, infected cells, or cells transfected to express these proteins (presenter cells [PC]) is necessary and sufficient to induce proliferative arrest in both human and rodent lymphoid cells (responder cells [RC]). This inhibition was found to occur independent of apoptosis and soluble mediators excluded by a pore size filter of 200 nm released from either PC or RC. We now show that reactive oxygen intermediates which might be released by RC or PC also do not contribute to MV-induced immunosuppression in vitro. Using an inhibitor of Golgi-resident mannosidases (deoxymannojirimycin), we found that complex glycosylation of the F and H proteins is not required for the induction of proliferative arrest of RC. As revealed by our previous studies, proteolytic cleavage of the MV F protein precursor into its F1 and F2 subunits, but not of F/H-mediated cellular fusion, was found to be required, since fusion-inhibitory peptides such as Z-D-Phe-L-Phe-Gly (Z-fFG) did not interfere with the induction of proliferative inhibition. We now show that Z-fFG inhibits cellular fusion at the stage of hemifusion by preventing lipid mixing of the outer membrane layer. These results provide strong evidence for a receptor-mediated signal elicited by the MV F/H complex which can be uncoupled from its fusogenic activity is required for the induction of proliferative arrest of human lymphocytes.