Molecular cloning of membrane cofactor protein (MCP; CD46) on B95a cell, an Epstein-Barr virus-transformed marmoset B cell line: B95a-MCP is susceptible to infection by the CAM, but not the Nagahata strain of the measles virus

Infection of Different Cell Lines of Neural Origin with Subacute Sclerosing Panencephalitis (SSPE) Virus

Measles virus is the causative agent of subacute sclerosing panencephalitis (SSPE). The viruses isolated from brain cells of patients with SSPE (called SSPE viruses) are defective in cell-free virus production in vitro. To investigate the cell tropism of three strains of SSPE virus (Osaka-1, Osaka-2, Osaka-3), SSPE virus-infected cell cultures were treated with cytochalasin D to prepare virus-like particles (CD-VLPs). All CD-VLPs formed syncytia after infection in CHO cells expressing CD150 but not in those expressing CD46. In addition, an antibody to CD46 did not block the infection of Vero cells by SSPE CDVLPs. The results were consistent with our previous suggestion that one or more unidentified receptors might be involved in the entry process. Infection with the CD-VLPs from three SSPE strains was further examined in different human cell lines, including those of neural origin, and was found to induce syncytia in epithelial cells (HeLa and 293T) as well as neuroblastoma cells (IMR-32 and SK-N-SH) with varying efficiency. SSPE CD-VLPs also infected glioblastoma cells (A172) and astrocytoma cells (U-251) but syncytial formation was rarely induced. These epithelial and neural cell lines were not permissive for the replication of wild-type MV. Together with our previous observations, these results suggest that the cell entry receptor is the major factor determining the cell tropism of SSPE viruses. Further studies are necessary to identify other viral and/or cellular factors that might be involved in the replication of SSPE virus in specific neural cells and in the brain.

Novel marmoset (Callithrix jacchus) model of human Herpesvirus 6A and 6B infections: immunologic, virologic and radiologic characterization

Human Herpesvirus 6 (HHV-6) is a ubiquitous virus with an estimated seroprevalence of 95% in the adult population. HHV-6 is associated with several neurologic disorders, including multiple sclerosis, an inflammatory demyelinating disease affecting the CNS. Animal models of HHV-6 infection would help clarify its role in human disease but have been slow to develop because rodents lack CD46, the receptor for cellular entry. Therefore, we investigated the effects of HHV-6 infections in a non-human primate, the common marmoset Callithrix jacchus. We inoculated a total of 12 marmosets with HHV-6A and HHV-6B intravenously and HHV-6A intranasally. Animals were monitored for 25 weeks post-inoculation clinically, immunologically and by MRI. Marmosets inoculated with HHV-6A intravenously exhibited neurologic symptoms and generated virus-specific antibody responses, while those inoculated intravenously with HHV-6B were asymptomatic and generated comparatively lower antibody responses. Viral DNA was detected at a low frequency in paraffin-embedded CNS tissue of a subset of marmosets inoculated with HHV-6A and HHV-6B intravenously. When different routes of HHV-6A inoculation were compared, intravenous inoculation resulted in virus-specific antibody responses and infrequent detection of viral DNA in the periphery, while intranasal inoculation resulted in negligible virus-specific antibody responses and frequent detection of viral DNA in the periphery. Moreover, marmosets inoculated with HHV-6A intravenously exhibited neurologic symptoms, while marmosets inoculated with HHV-6A intranasally were asymptomatic. We demonstrate that a marmoset model of HHV-6 infection can serve to further define the contribution of this ubiquitous virus to human neurologic disorders.

Reduced ability of hemagglutinin of the CAM-70 measles virus vaccine strain to use receptors CD46 and SLAM

The CAM-70 measles virus (MV) vaccine strain is currently used for vaccination against measles. We examined the fusion-inducing ability of the CAM-70 hemagglutinin (H) protein and found that it was impaired in both CD46- and signaling lymphocyte activation molecule (SLAM)-expressing cells. We also generated recombinant MVs possessing H genes derived from the CAM-70 strain. The CAM-70 H protein impaired viral growth in both CD46- and SLAM-expressing cells. In peripheral blood lymphocytes (PBL) and monocyte-derived dendritic cells (Mo-DC), the CAM-70 strain did not grow efficiently. Infection with recombinant MVs revealed that impaired growth of the CAM-70 strain was attributed to the H gene only partly in PBL and largely in Mo-DC. Thus, impaired fusion-inducing ability of the H protein may be one of the underlying molecular mechanisms resulting in the attenuation of the CAM-70 strain.

Cell surface activation of the alternative complement pathway by the fusion protein of measles virus

Measles virus (MV)-infected cells are activators of the alternative human complement pathway, resulting in high deposition of C3b on the cell surface. Activation was observed independent of whether CD46 was used as a cellular receptor and did not correlate with CD46 down-regulation. The virus itself was an activator of the alternative pathway and was covered by C3b/C3bi, resulting in some loss in infectivity without loss of virus binding to target cells. The cell surface expression of MV fusion (F), but not haemagglutinin, envelope protein resulted in complement activation of the Factor B-dependent alternative pathway in a dose-dependent manner and F-C3b complexes were formed. The underlying activation mechanism was not related to any decrease in cell surface expression of the complement regulators CD46 and CD55. The C3b/C3bi coating of MV-infected cells and virus should ensure enhanced targeting of MV antigens to the immune system, through binding to complement receptors.

Receptor use by vesicular stomatitis virus pseudotypes with glycoproteins of defective variants of measles virus isolated from brains of patients with subacute sclerosing panencephalitis

The vaccine or Vero cell-adapted strains of measles virus (MV) have been reported to use CD46 as a cell entry receptor, while lymphotropic MVs preferentially use the signalling lymphocyte activation molecule (SLAM or CD150). In contrast to the virus obtained from patients with acute measles, little is known about the receptor that is used by defective variants of MV isolated from patients with subacute sclerosing panencephalitis (SSPE). The receptor-binding properties of SSPE strains of MV were analysed using vesicular stomatitis virus pseudotypes expressing the envelope glycoproteins of SSPE strains of MV. Such pseudotype viruses could use SLAM but not CD46 for entry. The pseudotype viruses with SSPE envelope glycoproteins could enter Vero cells, which do not express SLAM. In addition, their entry was not blocked by the monoclonal antibody to CD46, pointing to another entry receptor for SSPE strains on Vero cells. Furthermore, the unknown receptor(s), distinct from SLAM and CD46, may be present on cell lines derived from lymphoid and neural cells. Biochemical characterization of the receptor present on Vero cells and SK-N-SH neuroblastoma cells was consistent with a glycoprotein. Identification of additional entry receptors for MV will provide new insights into the mechanism of spread of MV in the central nervous system and possible reasons for differences between MVs isolated from patients with acute measles and SSPE.

Susceptibility of human dendritic cells (DCs) to measles virus (MV) depends on their activation stages in conjunction with the level of CDw150: role of Toll stimulators in DC maturation and MV amplification

Human CD46 (membrane cofactor protein, or MCP) and CDw150 (signaling lymphocyte activation molecule, or SLAM) serve as receptors for measles virus (MV), which induces marked host immune suppression. Although monocytes express CD46, they are considerably resistant to MV. Once monocytes differentiate into immature myeloid dendritic cells (iDCs) (GM-CSF + IL-4-treated), the cells become susceptible to MV. Therefore, we have identified CD46-adapted and CDw150-adapted strains of MV, and the dynamics of CD46 and CDw150 during monocyte-iDC conversion were examined in conjunction with MV susceptibility. Strikingly, CDw150 was not detected in monocytes and moderately induced in iDCs, while CD46 was constantly expressed in monocyte-to-iDC differentiation. Thus, iDCs were found to become highly permissive to CDw150-adapted MV strains via expression of CDw150. In fact, polyclonal and monoclonal antibodies that specifically blocked the MV receptor function of CD46 or CDw150 cancelled MV replication in iDCs according to the preferential usage of either CD46 or CDw150 in each strain of MV. Next, we showed that DCs that matured via stimulation of their Toll-like receptors (TLRs) 2 and/or 4 exhibited an approximately fivefold increase in CDw150 at the protein level, and concomitantly, higher levels of MV amplification were observed in mixed culture of lymphocytes than in iDCs without TLR2/4 stimuli. Hence, amplification of CDw150-dependent MV strains was augmented in DCs parallel with the levels of CDw150 in the presence of lymphocytes possessing CDw150. TLR-mediated functional potential of DCs may affect the degree of MV amplification through distinct MV strain-specific receptor usage of CDw150 or CD46.

Human herpesvirus 6 variant A but not variant B induces fusion from without in a variety of human cells through a human herpesvirus 6 entry receptor, CD46

Human herpesvirus 6 (HHV-6) is a lymphotropic betaherpesvirus that productively infects T cells and monocytes. HHV-6 isolates can be differentiated into two groups, variants A and B (HHV-6A and HHV-6B). Here, we show a functional difference between HHV-6A and -6B in that HHV-6A induced syncytium formation of diverse human cells but HHV-6B did not. The syncytium formation induced by HHV-6A was observed 2 h after infection; moreover, it was found in the presence of cycloheximide, indicating that HHV-6A induced fusion from without (FFWO) in the target cells. Furthermore, the fusion event was dependent on the expression of the HHV-6 entry receptor, CD46, on the target cell membrane. In addition, we determined that short consensus repeat 2 (SCR2), -3, and -4 of the CD46 ectodomain were essential for the formation of the virus-induced syncytia. Monoclonal antibodies against glycoproteins B and H of HHV-6A inhibited the fusion event, indicating that the syncytium formation induced by HHV-6A required glycoproteins H and B. These findings suggest that FFWO, which HHV-6A induced in a variety of cell lines, may play an important role in the pathogenesis of HHV-6A, not only in lymphocytes but also in various tissues, because CD46 is expressed ubiquitously in human tissues.

A CD46CD[55-46] chimeric receptor, eight short consensus repeats long, acts as an inhibitor of both CD46 (MCP)- and CD150 (SLAM)-mediated cell-cell fusion induced by CD46-using measles virus

According to their cellular receptor use, measles virus (MV) strains can be separated into two phenotypes, CD46-using and CD46-non-using. A long chimeric receptor, CD46CD[55-46], was generated from the CD46 backbone, encompassing the four short consensus repeat (SCR) domains of CD46 linked via a flexible glycine hinge to SCR1 and SCR2 of CD55, SCR3 and SCR4 of CD46 and the STP, transmembrane and cytoplasmic tail of CD46. This chimeric receptor was proficient for MV binding but deficient in mediating MV-induced cell-to-cell fusion and virus replication, possibly due to the extended distance between the MV haemagglutinin (H) binding site (CD46 SCR1-SCR2) and the cell membrane. When coexpressed with either wild-type CD46 or CD150, this fusion-incompetent receptor exerted a dominant negative effect and inhibited both cell-to-cell fusion and entry of MV with CD46-using, but not CD46-non-using, phenotype. A soluble octameric CD46-C4bpalpha exhibited similar CD46- and CD150-mediated fusion inhibition properties only against CD46-using MV. This suggests that the long CD46CD[55-46] receptor acts by sequestering incoming MV prior to its binding to the shorter functional CD46 or CD150 receptor.

Molecular assembly of CD46 with CD9, alpha3-beta1 integrin and protein tyrosine phosphatase SHP-1 in human macrophages through differentiation by GM-CSF

Human CD46, formerly membrane cofactor protein (MCP), binds and inactivates complement C3b and serves as a receptor for measles virus (MV), thereby protecting cells from homologous complement and sustaining systemic viral infection. CD46 on activated macrophages (Mphi) but not intact monocytes is presumed to be the factor responsible for virus-mediated immune modulation including down-regulation of IL-12 production. As CD46 is expressed on both Mphi and monocytes, the molecular mechanisms responsible for these distinct immune responses remain largely unknown. Here, we found that peripheral blood monocytes treated for 5--8 days with GM-CSF (i.e. mature Mphi) acquired the capacity to assemble CD9, alpha3-beta1 integrin and the tyrosine phosphatase SHP-1 with their CD46. Prior to this maturation stage, Mphi expressed sufficient amounts of CD9 and CD46 but showed no such complex formation, and as in intact monocytes MV replication was markedly suppressed. By flow cytometry and confocal microscopy, the complex was found to assemble on the surface in cells treated with approximately 6 days with GM-CSF but not for approximately 2 days. Notably, an alternative MV receptor SLAM CDw150 was neither expressed nor recruited to this complex throughout GM-CSF-mediated Mphi differentiation. These responses and molecular links were not reproduced in the hamster cell line CHO expressing human CD46 although these cells acquired high susceptibility to MV. Based on these observations, MV susceptibility in human myeloid lineages appears not to be as simple as that observed in human CD46-transfected non-myeloid cells. The molecular complex involving CD46 may confer high MV permissiveness leading to immune modulation in Mphi.

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.

Functional modulation of human macrophages through CD46 (measles virus receptor): production of IL-12 p40 and nitric oxide in association with recruitment of protein-tyrosine phosphatase SHP-1 to CD46

Human CD46, formerly membrane cofactor protein, binds and inactivates complement C3b and serves as a receptor for measles virus (MV), thereby protecting cells from homologous complement and sustaining systemic measles infection. Suppression of cell-mediated immunity, including down-regulation of IL-12 production, has been reported on macrophages (Mphi) by cross-linking their CD46. The intracellular events responsible for these immune responses, however, remain unknown. In this study, we found that 6- to 8-day GM-CSF-treated peripheral blood monocytes acquired the capacity to recruit protein-tyrosine phosphatase SHP-1 to their CD46 and concomitantly were able to produce IL-12 p40 and NO. These responses were induced by stimulation with mAbs F(ab')(2) against CD46 that block MV binding or by a wild-type MV strain Kohno MV strain (KO; UV treated or untreated) that was reported to induce early phase CD46 down-regulation. Direct ligation of CD46 by these reagents, but not intracellular MV replication, was required for these cellular responses. Interestingly, the KO strain failed to replicate in the 6- to 8-day GM-CSF-cultured Mphi, while other MV strains replicated to form syncytia under the same conditions. When stimulated with the KO strain, rapid and transient dissociation of SHP-1 from CD46 was observed. These and previous results provide strong evidence that CD46 serves as a signal modulatory molecule and that the properties of ligands determine suppression or activation of an innate immune system at a specific maturation stage of human Mphi.