Human cell receptor CD46 is down regulated through recognition of a membrane-proximal region of the cytoplasmic domain in persistent measles virus infection

A recombinant measles virus vaccine strain rMV-Hu191 has oncolytic effect against human gastric cancer by inducing apoptotic cell death requiring integrity of lipid raft microdomains

Live-attenuated strain of measles virus (MV) has oncolytic effect. In this study, the antitumor effect of rMV-Hu191, a recombinant Chinese Hu191 MV generated in our laboratory by efficient reverse genetics system, was evaluated in gastric cancer (GC). From our data, rMV-Hu191 induced cytopathic effects and inhibited tumor proliferation both in vitro and in vivo by inducing caspase-dependent apoptosis. In mice bearing GC xenografts, tumor size was reduced and survival was prolonged significantly after intratumoral injections of rMV-Hu191. Furthermore, lipid rafts, a type of membrane microdomain with specific lipid compositions, played an important role in facilitating entry of rMV-Hu191. Integrity of lipid rafts was required for successful viral infection as well as subsequent cell apoptosis, but was not required for viral binding and replication. CD46, a MV membrane receptor, was found to be partially localized in lipid rafts microdomains. This is the first study to demonstrate that Chinese Hu191 MV vaccine strain could be used as a potentially effective therapeutic agent in GC treatment. As part of the underlying cellular mechanism, the integrity of lipid rafts is required for viral entry and to exercise the oncolytic effect.

Structure of the extracellular portion of CD46 provides insights into its interactions with complement proteins and pathogens

The human membrane cofactor protein (MCP, CD46) is a central component of the innate immune system. CD46 protects autologous cells from complement attack by binding to complement proteins C3b and C4b and serving as a cofactor for their cleavage. Recent data show that CD46 also plays a role in mediating acquired immune responses, and in triggering autophagy. In addition to these physiologic functions, a significant number of pathogens, including select adenoviruses, measles virus, human herpes virus 6 (HHV-6), Streptococci, and Neisseria, use CD46 as a cell attachment receptor. We have determined the crystal structure of the extracellular region of CD46 in complex with the human adenovirus type 11 fiber knob. Extracellular CD46 comprises four short consensus repeats (SCR1-SCR4) that form an elongated structure resembling a hockey stick, with a long shaft and a short blade. Domains SCR1, SCR2 and SCR3 are arranged in a nearly linear fashion. Unexpectedly, however, the structure reveals a profound bend between domains SCR3 and SCR4, which has implications for the interactions with ligands as well as the orientation of the protein at the cell surface. This bend can be attributed to an insertion of five hydrophobic residues in a SCR3 surface loop. Residues in this loop have been implicated in interactions with complement, indicating that the bend participates in binding to C3b and C4b. The structure provides an accurate framework for mapping all known ligand binding sites onto the surface of CD46, thereby advancing an understanding of how CD46 acts as a receptor for pathogens and physiologic ligands of the immune system.

The human membrane cofactor protein (MCP, CD46) is expressed on all nucleated cells and serves as a marker that prevents host cells from destruction by the immune system. It functions as a cofactor that helps to inactivate the C3b and C4b molecules, which are central components of the complement system. In addition to its role in regulation complement activation, CD46 is also used by a large number of pathogens, including measles virus and adenovirus, as a receptor to allow these pathogens to attach to the cell surface and initiate an infection. We have determined the three-dimensional structure of the bulk of the extracellular region of CD46 using X-ray crystallography. This structure provides detailed information about the location of previously identified residues that play a role in the interactions with C3b, C4b, and several pathogens, advancing an understanding of the function of the CD46 protein as a host and pathogen receptor. Moreover, the structure also reveals an unexpected, bent conformation of the protein that has implications for how the binding sites are presented at the cell surface.

Membrane cofactor protein (MCP, CD46) binding to clinical isolates of Streptococcus pyogenes: binding to M type 18 strains is independent of Emm or Enn proteins

The complement regulatory protein CD46 (MCP, membrane cofactor protein) is used as a cell receptor by a number of bacterial and viral pathogens, including Streptococcus pyogenes (Group A Streptococci). The highly variable M (Emm) proteins are virulence factors of S. pyogenes, and Emm proteins of serotypes 5, 6 or 22 are able of binding to CD46, thus mediating the binding of Streptococci to human cells. In this work, using a soluble construction encompassing the extracellular domain of human CD46, we have analyzed its binding to clinical isolates of S. pyogenes, including isolates of the M types 1, 3 and 18 that are frequently found in invasive infections or rheumatic fever. Our data show a strong binding of CD46 to bacteria of M types 1, 3, 8, 18, 24, 28, 29, 31 and 78; weak binding to M6 and M29 and no binding to M types 11, 12, M27 or M30. Surprisingly, CD46 bound to isogenic mutants of one clinical M18 isolate lacking the Emm protein or Emm and the Emm-related protein Enn, regardless of having capsule or not. In addition, these isogenic mutants bound to keratinocytes in a CD46-dependent manner, confirming the role of CD46 as one of the cell receptors for Group A Streptococci. Furthermore, CD46 did not bind to a recombinant Emm 18 construct, confirming that Emm is not involved in CD46 binding to M18 bacteria. Emm-dependent and -independent CD46 binding of clinical isolates of Streptococci confirms the importance of CD46 as a cell target that might confer pathogens some biological advantages over the host.

The short consensus repeats 1 and 2, not the cytoplasmic domain, of human CD46 are crucial for infection of subgroup B adenovirus serotype 35

Human CD46 (membrane cofactor protein) has recently been identified to be an attachment receptor for subgroup B adenoviruses (Ads); however, the precise interaction between human CD46 and subgroup B Ads are just beginning to be understood. In this study, to characterize the interaction between human CD46 and subgroup B Ads, varieties of mutant CD46 were tested for their ability to act as a receptor for Ad serotype 35 (Ad35), which belongs to subgroup B. In addition, we determined Ad35 vector-mediated transgene expression and cellular uptake of Ad35 vectors in the presence of a set of anti-CD46 antibodies. Our data demonstrated that the short consensus repeats (SCRs) 1 and 2 in human CD46 are important for interaction with Ad35, whereas the cytoplasmic domain of human CD46 was found not to be required for the function as an Ad35 receptor. Rather, a complete deletion of the cytoplasmic domain of human CD46 increased the transduction efficiencies of Ad35 vectors. This information should help in elucidation of the mechanism of subgroup B Ad infection, as well in the improvement of the subgroup B Ad vectors.

Molecular mechanisms of measles virus persistence

As measles virus causes subacute sclerosing panencephalitis and measles inclusion body encephalitis due to its ability to establish human persistent infection, without symptoms for the time between the acute infection and the onset of clinical symptoms, it has been the paradigm for a long term persistent as opposed to chronic infection by an RNA virus. We have reviewed the mechanisms of persistence of the virus and discuss specific mutations associated with CNS infection affecting the matrix and fusion protein genes. These are placed in the context of our current understanding of the viral replication cycle. We also consider the proposed mechanisms of persistence of the virus in replicating cell cultures and conclude that no general mechanistic model can be derived from our current state of knowledge. Finally, we indicate how reverse genetics approaches and the use of mouse models with specific knock-out and knock-in modifications can further our understanding of measles virus persistence.

Measles virus superinfection immunity and receptor redistribution in persistently infected NT2 cells

A recombinant measles virus (MV) expressing red fluorescent protein (MVDsRed1) was used to produce a persistently infected cell line (piNT2-MVDsRed1) from human neural precursor (NT2) cells. A similar cell line (piNT2-MVeGFP) was generated using a virus that expresses enhanced green fluorescent protein. Intracytoplasmic inclusions containing the viral nucleocapsid protein were evident in all cells and viral glycoproteins were present at the cell surface. Nevertheless, the cells did not release infectious virus nor did they fuse to generate syncytia. Uninfected NT2 cells express the MV receptor CD46 uniformly over their surface, whereas CD46 was present in cell surface aggregates in the piNT2 cells. There was no decrease in the overall amount of CD46 in piNT2 compared to NT2 cells. Cell-to-cell fusion was observed when piNT2 cells were overlaid onto confluent monolayers of MV receptor-positive cells, indicating that the viral glycoproteins were correctly folded and processed. Infectious virus was released from the underlying cells, indicating that persistence was not due to gross mutations in the virus genome. Persistently infected cells were superinfected with MV or canine distemper virus and cytopathic effects were not observed. However, mumps virus could readily infect the cells, indicating that superinfection immunity is not caused by general soluble antiviral factors. As MVeGFP and MVDsRed1 are antigenically indistinguishable but phenotypically distinct it was possible to use them to measure the degree of superinfection immunity in the absence of any cytopathic effect. Only small numbers of non-fusing green fluorescent piNT2-MVDsRed1 cells (1 : 300 000) were identified in which superinfecting MVeGFP entered, replicated and expressed its genes.

Mechanism of CD150 (SLAM) down regulation from the host cell surface by measles virus hemagglutinin protein

Measles virus has been reported to enter host cells via either of two cellular receptors, CD46 and CD150 (SLAM). CD46 is found on most cells of higher primates, while SLAM is expressed on activated B, T, and dendritic cells and is an important regulatory molecule of the immune system. Previous reports have shown that measles virus can down regulate expression of its two cellular receptors on the host cell surface during infection. In this study, the process of down regulation of SLAM by measles virus was investigated. We demonstrated that expression of the hemagglutinin (H) protein of measles virus was sufficient for down regulation. Our studies provided evidence that interactions between H and SLAM in the endoplasmic reticulum (ER) can promote the down regulation of SLAM but not CD46. In addition, we demonstrated that interactions between H and SLAM at the host cell surface can also contribute to SLAM down regulation. These results indicate that two mechanisms involving either intracellular interactions between H and SLAM in the ER or receptor-mediated binding to H at the surfaces of host cells can lead to the down regulation of SLAM during measles virus infection.

C3 promotes clearance of Klebsiella pneumoniae by A549 epithelial cells

The airway epithelium represents a primary site for contact between microbes and their hosts. To assess the role of complement in this event, we studied the interaction between the A549 cell line derived from human alveolar epithelial cells and a major nosocomial pathogen, Klebsiella pneumoniae, in the presence of serum. In vitro, we found that C3 opsonization of poorly encapsulated K. pneumoniae clinical isolates and an unencapsulated mutant enhanced dramatically bacterial internalization by A549 epithelial cells compared to highly encapsulated clinical isolates. Local complement components (either present in the human bronchoalveolar lavage or produced by A549 epithelial cells) were sufficient to opsonize K. pneumoniae. CD46 could competitively inhibit the internalization of K. pneumoniae by the epithelial cells, suggesting that CD46 is a receptor for the binding of complement-opsonized K. pneumoniae to these cells. We observed that poorly encapsulated strains appeared into the alveolar epithelial cells in vivo but that (by contrast) they were completely avirulent in a mouse model of pneumonia compared to the highly encapsulated strains. Our results show that bacterial opsonization by complement enhances the internalization of the avirulent microorganisms by nonphagocytic cells such as A549 epithelial cells and allows an efficient innate defense.

Ligand binding determines whether CD46 is internalized by clathrin-coated pits or macropinocytosis

CD46 is a ubiquitous human cell surface receptor for the complement components C3b and C4b and for various pathogens, including the measles virus and human herpes virus 6. Ligand binding to CD46 affects (i) protection of autologous cells from complement attack by breakdown of complement components, (ii) intracellular signals that affect the regulation of immune cell function, (iii) antigen presentation, and (iv) down-regulation of cell surface CD46. Recent evidence indicates that CD46 signaling can link innate and acquired immune function. The molecular mechanisms for these processes and the importance of intracellular trafficking of the receptor have not yet been elucidated. We demonstrate here that, in nonlymphoid cells, CD46 is constitutively internalized via clathrin-coated pits, traffics to multivesicular bodies, and is recycled to the cell surface. However, cross-linking of CD46 at the cell surface, by either multivalent antibody or by measles virus, induces pseudopodia that engulf the ligand in a process similar to macropinocytosis, and leads to the degradation of cell surface CD46. Thus, we have elucidated two pathways for CD46 internalization, which are regulated by the valence of cross-linking of CD46 and which utilize either clathrin-coated pits or pseudopodial extension. This has important implications for CD46 signaling, antigen presentation, CD46 down-regulation, and engulfment of pathogens.

CD46 is phosphorylated at tyrosine 354 upon infection of epithelial cells by Neisseria gonorrhoeae

The Neisseria type IV pilus promotes bacterial adhesion to host cells. The pilus binds CD46, a complement-regulatory glycoprotein present on nucleated human cells (Källström et al., 1997). CD46 mutants with truncated cytoplasmic tails fail to support bacterial adhesion (Källström et al., 2001), suggesting that this region of the molecule also plays an important role in infection. Here, we report that infection of human epithelial cells by piliated Neisseria gonorrhoeae (GC) leads to rapid tyrosine phosphorylation of CD46. Studies with wild-type and mutant tail fusion constructs demonstrate that Src kinase phosphorylates tyrosine 354 in the Cyt2 isoform of the CD46 cytoplasmic tail. Consistent with these findings, infection studies show that PP2, a specific Src family kinase inhibitor, but not PP3, an inactive variant of this drug, reduces the ability of epithelial cells to support bacterial adhesion. Several lines of evidence point to the role of c-Yes, a member of the Src family of nonreceptor tyrosine kinases, in CD46 phosphorylation. GC infection causes c-Yes to aggregate in the host cell cortex beneath adherent bacteria, increases binding of c-Yes to CD46, and stimulates c-Yes kinase activity. Finally, c-Yes immunoprecipitated from epithelial cells is able to phosphorylate the wild-type Cyt2 tail but not the mutant derivative in which tyrosine 354 has been substituted with alanine. We conclude that GC infection leads to rapid tyrosine phosphorylation of the CD46 Cyt2 tail and that the Src kinase c-Yes is involved in this reaction. Together, the findings reported here and elsewhere strongly suggest that pilus binding to CD46 is not a simple static process. Rather, they support a model in which pilus interaction with CD46 promotes signaling cascades important for Neisseria infectivity.

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.

Hemadsorption expressed by cloned H genes from subacute sclerosing panencephalitis (SSPE) viruses and their possible progenitor measles viruses isolated in Osaka, Japan

Most subacute sclerosing panencephalitis (SSPE) viruses, including our Osaka-1, -2, and -3 strains isolated in Osaka, have shown negative hemadsorption (HAD) by African green monkey red blood cells. This property has been thought to be characteristic of SSPE virus as compared to the positive reaction of the standard Edmonston strain of measles virus (MV). However, this assumption has become quite obscure because MV mutates frequently at the genetic level during its multiplication and also because recent field strains isolated by lymphoblastoid cell lines have shown negative HAD. To investigate the above issue, the nucleotide sequences of the hemagglutinin (H) genes from SSPE virus Osaka-1, -2, or -3 strains were compared to those of various MV field strains isolated in Osaka by Vero cells. The H gene sequences of three SSPE strains were relatively conserved without such biased hypermutation as had been observed in the matrix (M) gene of three SSPE strains. However, this analysis of the H gene sequence of the SSPE viruses enabled us to deduce possible progenitor MVs, which are in agreement with the deduction from the M gene analysis we reported previously. The HAD of Vero cells transfected with the cloned H cDNAs from the SSPE strains and their progenitors suggested that negative HAD of the SSPE viruses has been maintained as one of original properties of the progenitor MVs rather than having been acquired as an altered one during long-term persistent infection in the brains of patients with SSPE.

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.

Interaction of CD46 with measles virus: accessory role of CD46 short consensus repeat IV

To define further the accessory role(s) of the CD46 (membrane cofactor protein) short consensus repeat (SCR) III and IV domains in the interaction of CD46 with measles virus (MV), chimeric proteins were generated by substituting domains from the structurally related protein decay accelerating factor (DAF, CD55): x3DAF (exchange of CD46 SCR III) and x4DAF (exchange of SCR IV). Transfected CHO cell lines that stably expressed these chimeric proteins were compared for MV binding and infection. Compared with wild-type CD46 (I-II-III-IV), a significant decrease in MV binding was observed with x4DAF. Despite this limited binding, these cells were still capable of supporting virus entry. In a quantitative fusion assay, no significant differences in fusion were observed as a result of the exchange of either CD46 SCR III or IV. However, the down-regulation of cell surface CD46 typically observed following MV infection was abolished with x4DAF, as was the redistribution of CD46 on the cell surface. Thus, CD46 SCR IV appears to be required for optimal virus binding and receptor down-regulation, although importantly, in spite of these functional limitations, x4DAF can still be used for MV entry.

Measles virus spread by cell-cell contacts: uncoupling of contact-mediated receptor (CD46) downregulation from virus uptake

CD46, which serves as a receptor for measles virus (MV; strain Edmonston), is rapidly downregulated from the cell surface after contact with viral particles or infected cells. We show here that the same two CD46 complement control protein (CCP) domains responsible for primary MV attachment mediate its downregulation. Optimal downregulation efficiency was obtained with CD46 recombinants containing CCP domains 1 and 2, whereas CCP 1, alone and duplicated, induced a slight downregulation. Using persistently infected monocytic/promyelocytic U937 cells which release very small amounts of infectious virus, and uninfected HeLa cells as contact partners, we then showed that during contact the formation of CD46-containing patches and caps precedes CD46 internalization. Nevertheless, neither substances inhibiting capping nor the fusion-inhibiting peptide Z-D-Phe-L-Phe-Gly-OH (FIP) blocked CD46 downregulation. Thus, CD46 downregulation can be uncoupled from fusion and subsequent virus uptake. Interestingly, in that system cell-cell contacts lead to a remarkably efficient infection of the target cells which is only partially inhibited by FIP. The finding that the contact of an infected with uninfected cells results in transfer of infectious viral material without significant (complete) fusion of the donor with the recipient cell suggests that microfusion events and/or FIP-independent mechanisms may mediate the transfer of MV infectivity from cell to cell.

Human CD46 enhances nitric oxide production in mouse macrophages in response to measles virus infection in the presence of gamma interferon: dependence on the CD46 cytoplasmic domains

CD46 is a transmembrane complement regulatory protein widely expressed on nucleated human cells. Laboratory-adapted strains of measles virus (MV) bind to the extracellular domains of CD46 to enter human cells. The cytoplasmic portion of CD46 consists of a common juxtamembrane region and different distal sequences called Cyt1 and Cyt2. The biological functions of these cytoplasmic sequences are unknown. In this study, we show that expression of human CD46 with the Cyt1 cytoplasmic domain in mouse macrophages enhances production of nitric oxide (NO) in response to MV infection in the presence of gamma interferon (IFN-gamma). Human CD46 does not increase the basal levels of NO production in mouse macrophages and does not augment NO production induced by double-stranded polyribonucleotides. Replacing the cytoplasmic domain of human CD46 with Cyt2 reduces MV and IFN-gamma-induced NO production in mouse macrophages. Deleting the entire cytoplasmic domains of human CD46 does not prevent MV infection but markedly attenuates NO production in response to MV and IFN-gamma. Mouse macrophages expressing a tailless human CD46 mutant are more susceptible to MV infection and produce 2 to 3 orders of magnitude more infectious virus than mouse macrophages expressing human CD46 with intact cytoplasmic domains. These results reveal a novel function of CD46 dependent on the cytoplasmic domains (especially Cyt1), which augments NO production in macrophages. These findings may have significant implications for roles of CD46 in innate immunity and MV pathogenesis.

Persistent infection promotes cross-species transmissibility of mouse hepatitis virus

Persistent infection with mouse hepatitis virus (MHV) strain A59 in murine DBT (delayed brain tumor) cells resulted in the emergence of host range variants, designated V51A and V51B, at 210 days postinfection. These host range mutants replicated efficiently in normally nonpermissive Chinese hamster ovary (CHO), in human hepatocarcinoma (HepG2), and to a lesser extent in human breast carcinoma (MCF7) cell lines. Little if any replication was noted in baby hamster kidney (BHK), green African monkey kidney (COS-7), feline kidney (CRFK), and swine testicular (ST) cell lines. By fluorescent antibody (FA) staining, persistent viruses V10B and V30B, isolated at days 38 and 119 days postinfection, also demonstrated very low levels of replication in human HepG2 cells. These data suggest that persistence may rapidly select for host range expansion of animal viruses. Pretreatment of HepG2 cells with a polyclonal antibody directed against human carcinoembryonic antigens (CEA) or with some monoclonal antibodies (Col-1, Col-4, Col-12, and Col-14) that bind human CEA significantly inhibited V51B infection. Under identical conditions, little or no blockade was evident with other monoclonal antibodies (kat4c or Col-6) which also bind the human CEA glycoproteins. In addition, an antibody (EDDA) directed against irrelevant antigens did not block V51B replication. Pretreatment with the Col-4 and Col-14 antibodies did not block Sindbis virus replication in HepG2 cells or MHV infection in DBT cells, suggesting that one or more CEA glycoproteins likely functioned as receptors for V51B entry into human cell lines. To test this hypothesis, the human biliary glycoprotein (Bgp) and CEA genes were cloned and expressed in normally nonpermissive BHK cell lines by using noncytopathic Sindbis virus replicons (pSinRep19). By growth curves and FA staining, human CEA and to a much lesser extent human Bgp functioned as receptors for V51B entry. Furthermore, V51B replication was blocked with polyclonal antiserum directed against human CEA and Bgp. Under identical conditions, the parental MHV strain A59 failed to replicate in BHK cells expressing human Bgp or CEA. These data suggest that MHV persistence may promote virus cross-species transmissibility by selecting for virus variants that recognize phylogenetic homologues of the normal receptor.

Expression of human complement regulatory protein CD46 restricts measles virus replication in mouse macrophages

Measles virus (MV) can infect mouse macrophages to cause a prolonged non-cytopathic infection that produces low levels of infectious virus for days. We have generated RAW264.7 mouse macrophages expressing human CD46, a cell surface complement regulatory protein that serves as a receptor for laboratory-adapted strains of MV. Laboratory-adapted MV strains efficiently enter the CD46-positive mouse macrophages to cause a cytopathic infection with extensive multinucleated cells and pseudopodia-like extensions. However, MV infection of mouse macrophages through CD46 is self-limiting. Both viral protein synthesis and infectious virus production are abruptly terminated after the second day of infection. This novel virus-cell interaction is seen only in mouse macrophages but not in mouse or hamster fibroblasts expressing human CD46. The possible role of CD46 in macrophage antiviral response restricting MV replication is discussed.

Post-translational modification and intracellular localization of a splice product of CD46 cloned from human testis: role of the intracellular domains in O-glycosylation

We obtained a unique CD46 cDNA, STc/CY4, from the human testis, the predicted amino acid sequence of which suggested the presence of a novel isoform of CD46. This message was present predominantly in the testis, and the predicted isoform possessed a short (11 amino acids) transmembrane section (TM) and an unidentified cytoplasmic tail (CY). When expressed in Chinese hamster ovary (CHO) cells, this CD46 isoform underwent no O-glycosylation and was mostly retained in the endoplasmic reticulum. This unusual behaviour of the new isoform was due in part to the short TM and the unusual sequences of the CY. The molecular mass of this isoform was 42,000, approximately 20,000 smaller than conventional CD46. These properties of the STc/CY4 isoform were similar to those of sperm CD46. The only difference between sperm CD46 and the STc/CY4 isoform expressed on CHO cells was that only the latter possessed N-linked sugars of high mannose types. Since the STc/CY4 isoform may behave like sperm CD46 in cellular localization and post-translational modification, studies of sperm-egg interassociation were performed using hamster eggs and CHO cell clones expressing various isoforms including the STc/CY4. Rosette formation was seen most effectively between hamster eggs and STc/CY4-expressing CHO cells. These results infer that O-glycosylation perturbs CD46-mediated sperm-binding to eggs and thus sperm CD46 lacking O-linked sugars can serve as an adhesion molecule. The possible role of CD46 in fertilization and the structural differences between sperm and conventional CD46 are discussed.

A single amino acid change in the hemagglutinin protein of measles virus determines its ability to bind CD46 and reveals another receptor on marmoset B cells

This paper provides evidence for a measles virus receptor other than CD46 on transformed marmoset and human B cells. We first showed that most tissues of marmosets are missing the SCR1 domain of CD46, which is essential for the binding of Edmonston measles virus, a laboratory strain that has been propagated in Vero monkey kidney cells. In spite of this deletion, the common marmoset was shown to be susceptible to infections by wild-type isolates of measles virus, although they did not support Edmonston measles virus production. As one would expect from these results, measles virus could not be propagated in owl monkey or marmoset kidney cell lines, but surprisingly, both a wild-type isolate (Montefiore 89) and the Edmonston laboratory strain of measles virus grew efficiently in B95-8 marmoset B cells. In addition, antibodies directed against CD46 had no effect on wild-type infections of marmoset B cells and only partially inhibited the replication of the Edmonston laboratory strain in the same cells. A direct binding assay with insect cells expressing the hemagglutinin (H) proteins of either the Edmonston or Montefiore 89 measles virus strains was used to probe the receptors on these B cells. Insect cells expressing Edmonston H but not the wild-type H bound to rodent cells with CD46 on their surface. On the other hand, both the Montefiore 89 H and Edmonston H proteins adhered to marmoset and human B cells. Most wild-type H proteins have asparagine residues at position 481 and can be converted to a CD46-binding phenotype by replacement of the residue with tyrosine. Similarly, the Edmonston H protein did not bind CD46 when its Tyr481 was converted to asparagine. However, this mutation did not affect the ability of Edmonston H to bind marmoset and human B cells. The preceding results provide evidence, through the use of a direct binding assay, that a second receptor for measles virus is present on primate B cells.

Moesin is not a receptor for measles virus entry into mouse embryonic stem cells

The involvement of moesin in measles virus (MV) entry was investigated with moesin-positive and -negative mouse embryonic stem (ES) cells. MV infection of these cells was very ineffective and was independent of moesin expression. Furthermore, when these cells were transfected to express human CD46, a 100-fold increase in syncytium formation was observed with these cells and was independent of the expression of moesin. The only obvious difference between moesin-positive and -negative ES cells was the shape of the syncytia formed. Moesin-negative ES cells expressing or not expressing human CD46 formed separate pieces of fragmented syncytia which were torn apart during spreading, whereas ES cells expressing moesin exhibited typical syncytia. In addition, moesin was not detected on the surface of any murine cells or cell lines that we have tested by a flow cytometric assay with moesin-specific antibodies. These findings indicate that murine moesin is neither a receptor nor a CD46 coreceptor for MV entry into mouse ES cells. Moesin is involved in actin filament-plasma membrane interactions as a cross-linker, and it affects only the spreading and shape of MV-mediated syncytia.

The murine coronavirus mouse hepatitis virus strain A59 from persistently infected murine cells exhibits an extended host range

In murine 17 Cl 1 cells persistently infected with murine coronavirus mouse hepatitis virus strain A59 (MHV-A59), expression of the virus receptor glycoprotein MHVR was markedly reduced (S. G. Sawicki, J. H. Lu, and K. V. Holmes, J. Virol. 69:5535-5543, 1995). Virus isolated from passage 600 of the persistently infected cells made smaller plaques on 17 Cl 1 cells than did MHV-A59. Unlike the parental MHV-A59, this variant virus also infected the BHK-21 (BHK) line of hamster cells. Virus plaque purified on BHK cells (MHV/BHK) grew more slowly in murine cells than did MHV-A59, and the rate of viral RNA synthesis was lower and the development of the viral nucleocapsid (N) protein was slower than those of MHV-A59. MHV/BHK was 100-fold more resistant to neutralization with the purified soluble recombinant MHV receptor glycoprotein (sMHVR) than was MHV-A59. Pretreatment of 17 Cl 1 cells with anti-MHVR monoclonal antibody CC1 protected the cells from infection with MHV-A59 but only partially protected them from infection with MHV/BHK. Thus, although MHV/BHK could still utilize MHVR as a receptor, its interactions with the receptor were significantly different from those of MHV-A59. To determine whether a hemagglutinin esterase (HE) glycoprotein that could bind the virions to 9-O-acetylated neuraminic acid moieties on the cell surface was expressed by MHV/BHK, an in situ esterase assay was used. No expression of HE activity was detected in 17 Cl 1 cells infected with MHV/BHK, suggesting that this virus, like MHV-A59, bound to cell membranes via its S glycoprotein. MHV/BHK was able to infect cell lines from many mammalian species, including murine (17 Cl 1), hamster (BHK), feline (Fcwf), bovine (MDBK), rat (RIE), monkey (Vero), and human (L132 and HeLa) cell lines. MHV/BHK could not infect dog kidney (MDCK I) or swine testis (ST) cell lines. Thus, in persistently infected murine cell lines that express very low levels of virus receptor MHVR and which also have and may express alternative virus receptors of lesser efficiency, there is a strong selective advantage for virus with altered interactions with receptor (D. S. Chen, M. Asanaka, F. S. Chen, J. E. Shively, and M. M. C. Lai, J. Virol. 71:1688-1691, 1997; D. S. Chen, M. Asanaka, K. Yokomori, F.-I. Wang, S. B. Hwang, H.-P. Li, and M. M. C. Lai, Proc. Natl. Acad. Sci. USA 92:12095-12099, 1995; P. Nedellec, G. S. Dveksler, E. Daniels, C. Turbide, B. Chow, A. A. Basile, K. V. Holmes, and N. Beauchemin, J. Virol. 68:4525-4537, 1994). Possibly, in coronavirus-infected animals, replication of the virus in tissues that express low levels of receptor might also select viruses with altered receptor recognition and extended host range.

Artificial mutations and natural variations in the CD46 molecules from human and monkey cells define regions important for measles virus binding

CD46 was previously shown to be a primate-specific receptor for the Edmonston strain of measles virus. This receptor consists of four short consensus regions (SCR1 to SCR4) which normally function in complement regulation. Measles virus has recently been shown to interact with SCR1 and SCR2. In this study, receptors on different types of monkey erythrocytes were employed as "natural mutant proteins" to further define the virus binding regions of CD46. Erythrocytes from African green monkeys and rhesus macaques hemagglutinate in the presence of measles virus, while baboon erythrocytes were the least efficient of the Old World monkey cells used in these assays. Subsequent studies demonstrated that the SCR2 domain of baboon CD46 contained an Arg-to-Gln mutation at amino acid position 103 which accounted for reduced hemagglutination activity. Surprisingly, none of the New World monkey erythrocytes hemagglutinated in the presence of virus. Sequencing of cDNAs derived from the lymphocytes of these New World monkeys and analysis of their erythrocytes with SCR1-specific polyclonal antibodies indicated that the SCR1 domain was deleted in these cells. Additional experiments, which used 35 different site-specific mutations inserted into CD46, were performed to complement the preceding studies. The effects of these artificial mutations were documented with a convenient binding assay using insect cells expressing the measles virus hemagglutinin. Mutations which mimicked the change found in baboon CD46 or another which deleted the SCR2 glycosylation site reduced binding substantially. Another mutation which altered GluArg to AlaAla at positions 58 and 59, totally abolished binding. Finally, the epitopes for two monoclonal antibodies which inhibit measles virus attachment were mapped to the same regions implicated by mutagenesis.

Identification of a cytoplasmic Tyr-X-X-Leu motif essential for down regulation of the human cell receptor CD46 in persistent measles virus infection

To investigate the sequence requirements for measles virus (MV)-induced receptor down regulation, we transfected the human CD46 gene into simian cells persistently infected by the Biken strain of MV. Surface expression of CD46 is drastically reduced in these cells. Deletion analysis has shown that the juxtamembrane region of the CD46 cytoplasmic domain is essential for down regulation. Deleting a Tyr-Arg-Tyr-Leu sequence in this region or changing these residues to Ala prevents CD46 down regulation from the infected cell surface. Alanine-scanning mutagenesis has identified two amino acid residues, Tyr and Leu, forming a Tyr-X-X-Leu motif critical for CD46 down regulation. Mutations that prevent CD46 down regulation enhance syncytium formation. These results indicate that CD46 down regulation limits the cytopathic effects in a persistent MV infection and that CD46 down regulation requires a cytoplasmic Tyr-X-X-Leu sequence which resembles known motifs for membrane protein trafficking and receptor signalling.