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

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.

Human metapneumovirus SH and G glycoproteins inhibit macropinocytosis-mediated entry into human dendritic cells and reduce CD4+ T cell activation

Human metapneumovirus (HMPV) is a major etiologic agent of respiratory disease worldwide. HMPV reinfections are common in healthy adults and children, suggesting that the protective immune response to HMPV is incomplete and short-lived. We used gene-deletion viruses to evaluate the role of the attachment G and small hydrophobic SH glycoproteins on virus uptake by primary human monocyte-derived dendritic cells (MDDC) in vitro and on subsequent MDDC maturation and activation of autologous T cells. HMPV with deletion of G and SH (ΔSHG) exhibited increased infectivity but had little effect on MDDC maturation. However, MDDC stimulated with ΔSHG induced increased proliferation of autologous Th1-polarized CD4(+) T cells. This effect was independent of virus replication. Increased T cell proliferation was strictly dependent on contact between virus-stimulated MDDC and CD4(+) T cells. Confocal microscopy revealed that deletion of SH and G was associated with an increased number of immunological synapses between memory CD4(+) T cells and virus-stimulated MDDC. Uptake of HMPV by MDDC was found to be primarily by macropinocytosis. Uptake of wild-type (WT) virus was reduced compared to that of ΔSHG, indicative of inhibition by the SH and G glycoproteins. In addition, DC-SIGN-mediated endocytosis provided a minor alternative pathway that depended on SH and/or G and thus operated only for WT. Altogether, our results show that SH and G glycoproteins reduce the ability of HMPV to be internalized by MDDC, resulting in a reduced ability of the HMPV-stimulated MDDC to activate CD4(+) T cells. This study describes a previously unknown mechanism of virus immune evasion.

IMPORTANCE

Human metapneumovirus (HMPV) is a major etiologic agent of respiratory disease worldwide. HMPV reinfections are common in healthy adults and children, suggesting that the protective immune response to HMPV is incomplete and short-lived. We found that HMPV attachment G and small hydrophobic SH glycoproteins reduce the ability of HMPV to be internalized by macropinocytosis into human dendritic cells (DC). This results in a reduced ability of the HMPV-stimulated DC to activate Th1-polarized CD4(+) T cells. These results contribute to a better understanding of the nature of incomplete protection against this important human respiratory virus, provide new information on the entry of HMPV into human cells, and describe a new mechanism of virus immune evasion.

Human Langerhans cells capture measles virus through Langerin and present viral antigens to CD4⁺ T cells but are incapable of cross-presentation

Langerhans cells (LCs) are a subset of DCs that reside in the upper respiratory tract and are ideally suited to sense respiratory virus infections. Measles virus (MV) is a highly infectious lymphotropic and myelotropic virus that enters the host via the respiratory tract. Here, we show that human primary LCs are capable of capturing MV through the C-type lectin Langerin. Both immature and mature LCs presented MV-derived antigens in the context of HLA class II to MV-specific CD4(+) T cells. Immature LCs were not susceptible to productive infection by MV and did not present endogenous viral antigens in the context of HLA class I. In contrast, mature LCs could be infected by MV and presented de novo synthesized viral antigens to MV-specific CD8(+) T cells. Notably, neither immature nor mature LCs were able to cross-present exogenous UV-inactivated MV or MV-infected apoptotic cells. The lack of direct infection of immature LCs, and the inability of both immature and mature LCs to cross-present MV antigens, suggest that human LCs may not be directly involved in priming MV-specific CD8(+) T cells. Immune activation of LCs seems a prerequisite for MV infection of LCs and subsequent CD8(+) T-cell priming via the endogenous antigen presentation pathway.

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.

Sorting signals in the measles virus wild-type glycoproteins differently influence virus spread in polarized epithelia and lymphocytes

The spread of virus infection within an organism is partially dictated by the receptor usage of the virus and can be influenced by sorting signals present in the viral glycoproteins expressed in infected cells. In previous studies, we have shown that the haemagglutinin (H) and fusion protein (F) of the measles virus (MV) vaccine strain MV(Edm) harbour tyrosine-dependent sorting signals which influence virus spread in both lymphocytes and epithelial cells to a similar degree. In contrast with the vaccine strain, MV wild-type virus does not use CD46 but CD150/SLAM and a not clearly identified molecule on epithelial cells as receptors. To determine differences in viral spread between vaccine and wild-type virus, we generated recombinant MV expressing glycoproteins of both the wild-type strain WTFb and the corresponding tyrosine mutants. In contrast with observations based on vaccine virus glycoproteins, mutations in wild-type virus H and F differently influenced cell-to-cell fusion and replication in polarized epithelia and lymphocytes. For wild-type H, our data suggest a key role of the cytoplasmic tyrosine signal for virus dissemination in vivo. It seems to be important for efficient virus spread between lymphocytes, while the tyrosine signal in the F protein gains importance in epithelial cells as both signals have to be intact to allow efficient spread of infection within epithelia.

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 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.

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.

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.

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.

Hemagglutinin protein of wild-type measles virus activates toll-like receptor 2 signaling

Pattern recognition via Toll-like receptors (TLR) by antigen-presenting cells is an important element of innate immunity. We report that wild-type measles virus but not vaccine strains activate cells via both human and murine TLR2, and this is a property of the hemagglutinin (H) protein. The ability to activate cells via TLR2 by wild-type MV H protein is abolished by mutation of a single amino acid, asparagine at position 481 to tyrosine, as is found in attenuated strains, which is important for interaction with CD46, the receptor for these strains. TLR2 activation by MV wild-type H protein stimulates induction of proinflammatory cytokines such as interleukin-6 (IL-6) in human monocytic cells and surface expression of CD150, the receptor for all MV strains. Confirming the specificity of this interaction, wild-type H protein did not induce IL-6 release in macrophages from TLR2-/- mice. Thus, the unique property of MV wild-type strains to activate TLR2-dependent signals might essentially contribute not only to immune activation but also to viral spread and pathogenicity by upregulating the MV receptor on monocytes.

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.

Respiratory syncytial virus fusion protein mediates inhibition of mitogen-induced T-cell proliferation by contact

Human respiratory syncytial virus (HRSV) and bovine respiratory syncytial virus (BRSV) are major pathogens in infants and calves, respectively. Experimental BRSV infection of calves and lambs is associated with lymphopenia and a reduction in responsiveness of peripheral blood lymphocytes (PBLs) to mitogens ex vivo. In this report, we show that in vitro mitogen-induced proliferation of PBLs is inhibited after contact with RSV-infected and UV-inactivated cells or with cells expressing RSV envelope proteins on the cell surface. The protein responsible was identified as the RSV fusion protein (F), as cells infected with a recombinant RSV expressing F as the single envelope protein or cells transfected with a plasmid encoding F were able to induce this effect. Thus, direct contact with RSV F is necessary and sufficient to inhibit proliferation of PBLs. Interestingly, F derived from HRSV was more efficient in inhibiting human PBL proliferation, while F from BRSV was more efficient in inhibiting bovine PBLs. Since various T-cell activation markers were upregulated after presenter cell contact, T lymphocytes are viable and may still be activated by mitogen. However, a significant fraction of PBLs were delayed or defective in G0/G1 to S-phase transit.

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.