Identification of a putative cell receptor for human cytomegalovirus

Differential Expression of PDGF Receptor-α in Human Placental Trophoblasts Leads to Different Entry Pathways by Human Cytomegalovirus Strains

Human cytomegalovirus (CMV) is the leading non-genetic cause of fetal malformation in developed countries. CMV placental infection is a pre-requisite for materno-fetal transmission of virus, and fetal infection. We investigated the roles of the viral pentameric complex gH/gL/pUL128-pUL131A, and cellular platelet-derived growth factor receptor-α (PDGFRα) for CMV infection in first trimester extravillous-derived (SGHPL-4) and villous-derived (HTR-8/SVneo) trophoblast cells. Infection with four CMV clinical and laboratory strains (Merlin, TB40E, Towne, AD169), and Merlin deletion mutants of UL128-, UL130-, and UL131A-genes, showed a cell type-dependent requirement of the viral pentameric complex for infection of trophoblast cells. The viral pentameric complex was essential for infection of villous trophoblasts, but non-essential for extravillous trophoblasts. Blocking of PDGFRα in extravillous trophoblasts, which naturally express PDGFRα, inhibited entry of pentameric complex-deficient CMV strains, but not the entry of pentameric positive CMV strains. Transient expression of PDGFRα in villous trophoblasts, which are naturally deficient in PDGFRα, promoted the entry of CMV strains lacking gH/gL/pUL128-pUL131A, but had no effect on entry of pentameric positive CMV strains. These results suggest PDGFRα is an important cell receptor for entry of CMV mutant strains lacking gH/gL/pUL128-pUL131A complexes in some placental cells, suggesting these entry pathways could be potential antiviral targets.

Role of annexin A2 in cellular entry of rabbit vesivirus

The mechanisms of calicivirus attachment and internalization are not well understood, mainly due to the lack of a reliable cell-culture system for most of its members. In this study, rabbit vesivirus (RaV) virions were shown to bind annexin A2 (ANXA2) in a membrane protein fraction from HEK293T cells, using a virus overlay protein-binding assay and matrix-assisted laser desorption/ionization time-of-flight analysis. A monoclonal anti-ANXA2 antibody and small interfering RNA-mediated knockdown of ANXA2 expression in HEK293T cells reduced virus infection significantly, further supporting the role of ANXA2 in RaV attachment and/or internalization.

Virus entry and innate immune activation

Human cytomegalovirus (HCMV) exhibits an exceptionally broad cellular tropism as it is capable of infecting most major organ systems and cell types. Definitive proof of an essential role for a cellular molecule that serves as an entry receptor has proven very challenging. It is widely hypothesized that receptor utilization, envelope glycoprotein requirements and entry pathways may all vary according to cell type, which is partially supported by the data. What has clearly emerged in recent years is that virus entry is not going undetected by the host. Robust and rapid induction of innate immune response is intimately associated with entry-related events. Here we review the state of knowledge on HCMV cellular entry mediators confronting the scientific challenges by accruing a definitive data set. We also review the roles of pattern recognition receptors such as Toll-like receptors in activation of specific innate immune response and discuss how entry events are tightly coordinated with innate immune initiation steps.

Annexinopathies

Annexins comprise a conserved family of proteins characterised by their ability to bind and order charged phospholipids in membranes, often in response to elevated intracellular calcium. The family members (there are at least 12 in humans) have become specialised over evolutionary time and are involved in a diverse range of cellular functions both inside the cell and extracellularly Although a mutation in an annexin has never been categorically proven to be the cause of a disease state, they have been implicated in pathologies as diverse as autoimmunity, infection, heart disease, diabetes and cancer. 'Annexinopathies' were first described by Jacob H. Rand to describe the pathological sequelae in two disease states, the overexpression of annexin 2 in a patients with a haemorrhagic form of acute promyelocytic leukaemia, and the under-expression of annexin 5 on placental trophoblasts in the antiphospholipid syndrome. In this chapter we will outline some of the more recent observations in regard to these conditions, and describe the involvement of annexins in some other major causes of human morbidity.

Antiviral targets of a chromene derivative from Sargassum micracanthum in the replication of human cytomegalovirus

A chromene derivative (1) obtained from a brown alga, Sargassum micracanthum, has been proved to be a potent inhibitor of human cytomegalovirus (HCMV). In the present study, we evaluated its mode of action by various experimental assays. Time-of-addition experiments revealed that 1 was active if applied to cells before viral DNA synthesis, indicating that it inhibited early events of virus replication including virus adsorption and penetration, and a step immediately after viral internalization. Virus attachment and penetration studies suggested that one of the targets for anti-HCMV action of 1 was virus adsorption to cells and to a lesser extent, virus internalization was delayed in the presence of the compound. Pretreatment of virus particles with 1 showed that the compound exerted dose-dependent virucidal action. The chromene derivative and ganciclovir (GCV), an anti-HCMV drug, were synergistic inhibitors when used in combination. The synergistic effect could be explained by inhibition of different steps in HCMV replication cycle produced by 1 and GCV.

Immunohistochemical and in situ detection of cytomegalovirus in lung autopsies of children immunocompromised by secondary interstitial pneumonia

Secondary interstitial pneumonia (SIP), a disease affecting patients immunocompromised by primary underlying diseases during their treatment in hospital, is frequently associated with cytomegalovirus (CMV) infection, a potentially treatable condition. However, in many cases, no infectious agent can be determined, and this clinical disease rapidly progresses to death. Theoretically, SIP could be caused by CMV, which may be present in such small amounts or such configuration that routine histopathological analysis or viral culture techniques cannot detect the virus. To test the hypothesis that immunohistochemistry (IH) and in situ detection by hybridization (ISH) provides more accurate results than the mere histological demonstration of CMV inclusions, these methods were applied to 37 autopsied lung sections obtained from children immunocompromised by primary underlying diseases and who died of SIP. As a result, the cases were subdivided into three groups: (1) children with SIP CMV inclusions (Diffuse alveolar damage-DAD-related) (n = 7); (2) children with SIP without classical viral inclusions (CMV-DAD-related) (n = 3); (3) children with SIP exhibiting nuclear cytopathic effect (not CMV-NSIP-related) (n = 27). In the first group, all three techniques yielded clearly positive results, whereas IH and ISH indicated that three of the children of the second group had CMV-related DAD without histological demonstration of CMV inclusions. In the third group, there were no positive CMV signals. These data indicate that DAD-related CMV infection is an important cause of SIP and of death in children immunosuppressed by primary underlying diseases, and that IH and in situ detection were more sensitive than the histological demonstration of CMV inclusions. A direct involvement of CMV in SIP exhibiting DAD is likely, but not in the non-specific interstitial pneumonia (NSIP) pattern. We conclude that all children with primary underlying diseases should be investigated for CMV SIP using sensitive IH and in situ tests in conjunction with histological routine procedures.

Characterization of a 105-kDa plasma membrane associated glycoprotein that is involved in West Nile virus binding and infection

This study attempts to isolate and characterize West Nile virus-binding molecules on the plasma membrane of Vero and murine neuroblastoma cells that is responsible for virus entry. Pretreatment of Vero cells with proteases, glycosidases (endoglycosidase H, alpha-mannosidase), and sodium periodate strongly inhibited West Nile virus infection, whereas treatments with phospholipases and heparinases had no effect. The virus overlay protein blot detected a 105-kDa molecule on the plasma membrane extract of Vero and murine neuroblastoma cells that bind to WN virus. Treatment of the 105-kDa molecules with beta-mercaptoethanol resulted in the virus binding to a series of lower molecular weight bands ranging from 30 to 40 kDa. The disruption of disulfide-linked subunits did not affect virus binding. N-linked sugars with mannose residues on the 105-kDa membrane proteins were found to be important in virus binding. Specific antibodies against the 105-kDa glycoprotein were highly effective in blocking virus entry. These results strongly supported the possibility that the 105-kDa protease-sensitive glycoprotein with complex N-linked sugars could be the putative receptor for WN virus.

Differentiation-dependent redistribution of heparan sulfate in epithelial intestinal Caco-2 cells leads to basolateral entry of cytomegalovirus

Human cytomegalovirus (HCMV) causes a broad spectrum of clinical manifestations in immunocompromised patients, including infection of the gastrointestinal tract. To investigate the role of epithelial cells in the gastrointestinal HCMV disease, we used the intestinal epithelial cell line Caco-2, which is permissive for HCMV replication. In differentiated Caco-2 cells, we showed previously that HCMV infection proceeds preferentially from the basolateral membrane, suggesting that receptors for HCMV may be contained predominantly in the basolateral membrane (A. Esclatine et al., 2000, J. Virol. 74, 513-517). Therefore, we examined expression and localization in Caco-2 cells of heparan sulfate (HS) proteoglycan and annexin II, previously implicated in initial events of HCMV infection. We observed that annexin II is expressed in Caco-2 cells, but is not essential for entry of HCMV. We showed that, during the differentiation process, HS, initially present on the entire surface of the membrane of undifferentiated cells, ultimately became sequestered at the basolateral cell surface of fully differentiated cells. We established by biochemical assays that membrane-associated HS proteoglycan mediates both viral attachment to, and subsequent infection of, Caco-2 cells, regardless of the cell differentiation state. Thus, the redistribution of HS is implicated in the basolateral entry of HCMV into differentiated Caco-2 cells.

Interaction of recombinant norwalk virus particles with the 105-kilodalton cellular binding protein, a candidate receptor molecule for virus attachment

Norwalk virus (NV), responsible for outbreaks of acute gastroenteritis, comprises the species of the genus Norwalk-like viruses in the family Caliciviridae. Although the study of the molecular biology of NV has been hampered by a lack of culture systems or small experimental animal models, virus-like particles (VLPs) generated with recombinant baculoviruses harboring the capsid protein gene of NV provide a useful tool for investigating NV-cell interactions. In this study, the attachment of the recombinant VLPs derived from the Ueno virus (UEV), a strain belonging to the genogroup II NVs, to mammalian and insect cells was examined. Kinetic analyses of the binding of the recombinant VLPs of the UEV (rUEVs) to Caco-2 cells demonstrated that the binding was specific and occurred in a dose-dependent manner. Approximately 7.5% of the prebound rUEVs were internalized into the Caco-2 cells. Enzymatic and chemical modification of Caco-2 cell surface molecules suggested that the binding was directly mediated by a protein-protein interaction. A virus overlay protein-binding assay (VOPBA) indicated that rUEVs appeared to bind to a 105-kDa molecule, designated as the NV attachment (NORVA) protein. Furthermore, the assay indicated that its native conformational structure was indispensable for the binding activity. In Caco-2 cells, the NORVA protein was detected when VOPBA was carried out with the VLPs from Seto and Funabashi viruses, which are serologically different NVs from UEV, used as probes. The binding of rUEVs to NORVA protein was also observed in six mammalian cell lines other than Caco-2. These data suggest that the attachment of NV to mammalian cells is mediated by NORVA protein, which is ubiquitously expressed in the mammalian cells. The present study is the first report on the role of the cellular molecule in the binding of recombinant VLPs of NV.

Cloning and epitope mapping of a functional partial fusion receptor for human cytomegalovirus gH

A cDNA clone encoding a partial putative human cytomegalovirus (HCMV) gH fusion receptor (CMVFR) was previously identified. In this report, the cDNA sequence of CMVFR was determined and the role of this CMVFR in HCMV/cell fusion was confirmed by rendering fusion-incompetent MOLT-4 cells susceptible to fusion following transfection with receptor cDNA. Blocking experiments using recombinant gH or either of two MAbs (against recombinant gH or purified viral gH:gL) provided additional evidence for the role of gH binding to this protein in virus fusion. An HCMV-binding domain of 12 aa in the middle hydrophilic region of CMVFR was identified by fusion blocking studies using synthetic receptor peptides. The 1368 bp cDNA of CMVFR contained a predicted ORF of 345 aa with two potential membrane-spanning domains and several possible nuclear localization signals. A search of sequence databases indicated that CMVFR is a novel protein. Further characterization of this cell membrane protein that confers susceptibility to fusion with the viral envelope should provide important information about the mechanism by which HCMV infects cells.

The pathogenicity of cytomegalovirus

Human cytomegalovirus is ubiquitous, yet causes little illness in immunocompetent individuals. Disease is evident in immunodeficient groups such as neonates, transplant recipients and AIDS patients either following a primary infection or reactivation of a latent infection. Little is known of the mechanisms underlying the pathogenicity of the virus. The recent determination of the nucleotide sequence of both human cytomegalovirus (strain AD169) and murine cytomegalovirus (murine cytomegalovirus strain Smith) has allowed an analysis of the biological importance of several virus genes. Studies with human cytomegalovirus have indicated that many viral genes are non-essential for replication in vitro which are thus assumed to be important in the pathogenesis of the virus. This is being examined in the murine model where the role of the gene and its product in disease can be directly examined in vivo using viral mutants in which the relevant gene has been interrupted or deleted. Current information on the role of cytomegalovirus genes in tissue tropism, immune evasion, latency, reactivation from latency and damage is described.

Selective recognition of the membrane-bound CX3C chemokine, fractalkine, by the human cytomegalovirus-encoded broad-spectrum receptor US28

The 7TM receptor, US28, encoded by human cytomegalovirus binds a broad spectrum of endogenous CC chemokines with sub-nanomolar affinity as determined in homologous competition binding assays. We here find that US28 also recognizes the membrane-associated CX3C chemokine, fractalkine, with sub-nanomolar affinity (IC50=0.42+/-0.09 nM). Importantly, although fractalkine could compete with high affinity against the binding of CC chemokines, the secreted CC chemokines were only able to compete for binding against radioactive fractalkine with very low affinity. It is concluded that US28, which is known to enhance cell-cell fusion processes through interaction with an as yet unidentified, human cell-specific factor, has been optimized by cytomegalovirus to selectively recognize the membrane-associated fractalkine. It is suggested that US28 expressed on the surface of infected cells and possibly on the envelope of the virion is involved in transfer of the virus from cell to cell.

Receptor-binding properties of a soluble form of human cytomegalovirus glycoprotein B

The human cytomegalovirus (HCMV) glycoprotein B (gB) (also known as gpUL55) homolog is an important mediator of virus entry and cell-to-cell dissemination of infection. To examine the potential ligand-binding properties of gB, a soluble form of gB (gB-S) was radiolabeled, purified, and tested in cell-binding experiments. Binding of gB-S to human fibroblast cells was found to occur in a dose-dependent, saturable, and specific manner. Scatchard analysis demonstrated a biphasic plot with the following estimated dissociation constants (Kd): Kd1, 4.96 x 10(-6) M; Kd2, 3.07 x 10(-7) M. Cell surface heparan sulfate proteoglycans (HSPGs) were determined to serve as one class of receptors able to facilitate gB-S binding. Both HSPG-deficient Chinese hamster ovary (CHO) cells and fibroblast cells with enzymatically removed HSPGs had 40% reductions in gB-S binding, whereas removal of chondroitin sulfate had no effect. However, a significant proportion of gB-S was able to associate with the cell surface in the absence of HSPGs via an undefined nonheparin component. Binding affinity analysis of gB-S binding to wild-type CHO-K1 cells demonstrated biphasic binding kinetics (Kd1, 9.85 x 10(-6) M; Kd2, 4.03 x 10(-8) M), whereas gB-S binding to HSPG-deficient CHO-677 cells exhibited single-component binding kinetics (Kd, 7.46 x 10(-6) M). Together, these data suggest that gB-S associates with two classes of cellular receptors. The interaction of gB with its receptors is physiologically relevant, as evidenced by an inhibitory effect on HCMV entry when cells were pretreated with purified gB-S. This inhibition was determined to be manifested at the level of virus attachment. We conclude that gB is a ligand for HCMV that mediates an interaction with a cellular receptor(s) during HCMV infection.

Direct interaction between human cytomegalovirus glycoprotein B and cellular annexin II

Cellular annexin II has been shown to specifically bind human cytomegalovirus (HCMV) and be a component of highly purified virions. In this report, we characterize the interaction of annexin II with HCMV. We found that the binding of annexin II to the HCMV envelope occurs partially through the calcium-dependent phospholipid-binding ability of annexin II since some annexin II was dissociated from virions with chelating agents. However, a substantial proportion of virion-associated annexin II was resistant to chelation, which suggested a calcium-independent interaction between annexin II and an HCMV envelope component. The search for a nonphospholipid component to account for this binding led to the discovery that HCMV glycoprotein B (gpUL55) (gB) can physically interact with annexin II. We present three lines of evidence to support the conclusion that HCMV gB can bind host cell annexin II.

Infection of blood and bone marrow cells with the human cytomegalovirus in vivo

The human cytomegalovirus (HCMV) is a major pathogen in immunocompromised patients. Both, primary infection and reactivation of latent virus can cause disease. Peripheral blood leukocytes (PBL) most likely play an important role in viral persistence and dissemination of infection. However, an open question has been whether HCMV actively replicates in PBL in vivo and whether the progenitor cells in the bone marrow are also infected. Previous studies on this issue are controversial. Here we summarize data on the tropism of HCMV for mature leukocyte populations as well as bone marrow progenitor cells during HCMV viremia. All cell populations were highly purified by a fluorescence activated cell sorter (FACS) and analyzed by PCR for the presence of viral genomic DNA. Moreover, mature leukocyte populations were investigated for mRNA expression of regulatory and viral structural proteins. We could show, that HCMV DNA was detected most frequently in granulocytes and monocytes as well as in CD34+ progenitor cells of immunosuppressed patients. Viral mRNA expression was found in granulocytes, monocytes, and lymphocyte fractions. In contrast, no HCMV DNA was found in healthy, seropositive individuals.

Canine parvovirus binds to multiple cellular membrane proteins from both permissive and nonpermissive cell lines

For identification of canine parvovirus (CPV) binding protein, the SDS-solubilized cell membrane fraction from a permissive cell line. CRPK, was subjected to the virus overlay protein blot assay (VOPBA). Competitive inhibition experiments showed the presence of multiple CPV-binding proteins with molecular masses of 36, 35, 33, 31, 29, 27, 25, and 23 kDa. CPV-binding proteins of same molecular masses were also detected in membrane fractions from nonpermissive, as well as other permissive, cell lines. We confirm that the mechanism of nonpermissiveness to CPV is not operative at the cellular attachment level.

Evidence that neomycin inhibits human cytomegalovirus infection of fibroblasts

The effect of phosphoinositide-binding aminoglycosides, such as neomycin, gentamicin and streptomycin, on human cytomegalovirus (HCMV) infection of human fibroblasts MRC-5 was studied. The inhibition of HCMV infection was obtained with all of these molecules but neomycin was more effective than the others. We showed that the inoculation of the cells with cell-free viral suspension in presence of neomycin concentrations above 5 mM at 37 degrees C, inhibited more than 98% the HCMV infection. However, the preincubation of the fibroblasts with neomycin at 4 degrees C, before the removal of the drug and the inoculation of the cells, induced only a 30% decrease in the number of infected cells. Addition of neomycin after the HCMV-binding at 4 degrees C or the infection of the cells was less efficient to inhibit HCMV infection than the standard incubation of neomycin during inoculation of the fibroblasts. Indeed, 1 hour after the inoculation of the cells at 37 degrees C, neomycin still inhibited HCMV infection, but 4 hours after the inoculation, this drug had no effect on HCMV infection. Our findings demonstrated that neomycin must be present at the time of infection in order to exert a full inhibiting effect. The effect of neomycin on the HCMV infection was almost immediate upon the addition of the drug (binding and/or internalization) and after the virus internalization (inhibition of immediate-early events). We suggest that neomycin and other aminoglycoside antibiotics may interact with HCMV glycoproteins for binding to similar structural features of cell surface heparan sulfate proteoglycans and may inhibit HCMV infection in fibroblasts by disrupting phosphoinositide-mediated events in the cells.

A rapid and sensitive radioimmunoassay for the detection of human cytomegalovirus binding and infection of human fibroblasts

A rapid and sensitive radioimmunoassay for the quantitation of HCMV binding and infection of human fibroblasts (HFF) was developed. The protocol involves the use of a monoclonal antibody (27-156) reactive with HCMV gB (alpha-gB), followed by an 125I-labeled second antibody to mouse IgG. Antibody to gB bound specifically to HFF inoculated with HCMV when compared to sham inoculated cells or cells inoculated with HSV (strain KOS). Antibody to gB also bound to HFF infected with HCMV 48 h prior to assay. The binding of antibody to HFF inoculated with HCMV was found to be dependent on antibody concentration and to demonstrate saturable kinetics. Moreover, antibody binding was directly dependent on the concentration of the virus inoculum, using either conventional viral preparations or gradient purified HCMV. The binding of antibody to HFF inoculated with HCMV at 4 degrees C was found to be dependent on antibody concentration and to demonstrate saturable kinetics. Displacement of HCMV binding to HFF with the proteoglycan heparin sulfate could be detected, thus allowing for competitive binding studies. This binding assay allows for the relative quantitation of HCMV binding to cells and will be useful for examining the early events of cell-viral interactions.

Cell types involved in replication and distribution of human cytomegalovirus

As the number of patients suffering from severe HCMV infections has steadily increased, there is a growing need to understand the molecular mechanisms by which the virus causes disease. The factors that control infection at one time and the events leading to virus multiplication at another time are only beginning to be understood. The interaction of HCMV with different host cells is one key for elucidating these processes. Through modern techniques, much has been learned about the biology of HCMV infections in culture systems. In addition to endothelial cells, epithelial cells, and smooth muscle cells, fibroblasts are one cell population preferentially infected in solid tissues in vivo. From these sites of multiplication, the virus may be carried by peripheral monocytes and circulating endothelial cells to reach distant sites of the body. This would explain the multiorgan involvement in acute HCMV infection and the modes of viral transmission. From what has been learned mainly from human fibroblast culture systems, future studies will focus on how HCMV regulates the expression of its putative 200 genes in different host cells at different stages of cell differentiation and activation to result in viral latency and pathogenesis.

Human adenovirus serotype 3 (Ad3) and the Ad3 fiber protein bind to a 130-kDa membrane protein on HeLa cells

The fiber protein of adenovirus mediates the interaction of adenovirus with cell membrane receptors. We have produced the Ad3 fiber protein in the baculovirus expression system. Biochemical, morphological and functional analyses showed that the recombinant fiber was properly folded and functionally competent. The specific binding of Ad3 virus to two HeLa membrane proteins of 130 and 100 kDa was demonstrated with an overlay protein binding assay. In the same assay, Ad3 fiber only recognized the 130-kDa protein. Divalent cations seemed to be important for the interaction of both virus and fiber with these proteins.

Cell membrane bound N-acetylneuraminic acid is involved in the infection of fibroblasts and phorbol-ester differentiated monocyte-like cells with human cytomegalovirus (HCMV)

We focused on the role of membrane bound sugar residues in the infection of fibroblasts and monocyte-like cells with human cytomegalovirus (HCMV). Treatment of phorbol 12-myristate 13-acetate (PMA) differentiated monocyte-like cells THP-1 or human fibroblasts MRC-5 with lectins specific for N-acetylneuraminic acid (NeuAc) blocked infection with HCMV. HCMV failed to infect sialidase-treated differentiated THP-1 cells or MRC-5 cells. By using NeuAc, N-glycolylneuraminic acid (NeuGl) and alpha 2-3, but not alpha 2-6, sialyl-oligosaccharide, the infection of cells was less efficient. NeuAc was more potent inhibitor than NeuGl. These observations suggest that the sialic acid specificity and the nature of the interglycosidic linkage at the end of the complex carbohydrates may play an important role. Analogous experiments indicated that HCMV binds to N-acetylglucosamine (GlcNAc) in addition to NeuAc. Human cytomegalovirus infection in differentiated THP-1 cells and in human fibroblasts was inhibited by incubation of the virus with 20 micrograms/ml of heparin before and during the adsorption period. Treatment of the cells with heparinase or heparitinase inhibited infection with HCMV. We emphasized the role of NeuAc and GlcNAc and heparan sulfate proteoglycans at the surface of the cells, in the early steps of infection of both human fibroblasts and PMA differentiated monocyte-like cells with HCMV.

MHC proteins and heparan sulphate proteoglycans regulate murine cytomegalovirus infection

Factors influencing MCMV infection mediated by MHC class 1 molecules were analysed further as previous studies showed that the effects of the MHC genotype on sensitivity to this virus are important in vivo. Here we show that H-2d, H-2b, H-2r and H-2v macrophages are highly sensitive to MCMV. Moreover, transfection of H-2k L-cells with Kb or Dd conferred sensitivity to MCMV. This was not affected by amino acid substitutions in Kb alpha 1 or alpha 2, although previous studies demonstrated that exchange of the alpha 1 domain of Dd with Ld alpha 1 compromised sensitivity. Here replacement of Kb alpha 3 with Ld alpha 3 reduced susceptibility to low doses of MCMV. In addition, extracellular beta 2-microglobulin (beta 2m) promoted infection of beta 2m-negative RIE/TL8X.1 cells transfected with Db with or without a beta 2m gene. Hence MCMV infection can involve beta 2m and the alpha 1 and alpha 3 domains of MHC heavy chains. MCMV infection of L-cells expressing Dd or Kb was also inhibited by heparin, but infection of the parental L-cell line was not reproducibly affected. A role for heparan sulphate proteoglycan in MHC-mediated MCMV infection was confirmed using cells pre-treated with heparinase I or III, or propagated in chlorate to inhibit the sulphation of the glycosaminoglycan chains.

Host cellular annexin II is associated with cytomegalovirus particles isolated from cultured human fibroblasts

A significant amount of host cellular annexin II was found to be associated with human cytomegalovirus isolated from cultured human fibroblasts (approximately 1,160 molecules per virion). This composition was established by four different analytical approaches that included (i) Western blot (immunoblot) analysis of gradient-purified virions with a monoclonal antibody specific for annexin II, (ii) peptide mapping and sequence analysis of virus-associated proteins and proteins dissociated from virus following EDTA treatment, (iii) electron microscopic immunocytochemistry of gradient-purified virions, and (iv) labeling of virus-associated proteins by lactoperoxidase-catalyzed radioiodination. These results indicated that annexin II was primarily localized to the viral surface, where it bound in a divalent cation-dependent manner. In functional experiments, a rabbit antiserum raised against annexin II inhibited cytomegalovirus plaque formation in human foreskin fibroblast monolayers in a concentration-dependent manner. Cumulatively, these studies demonstrate an association of host annexin II with cytomegalovirus particles and provide evidence for the involvement of this cellular protein in virus infectivity.

Identification of annexin 33 kDa in cultured cells as a binding protein of influenza viruses

The binding of three influenza A and one influenza B virus strains to proteins of three continuously cultured cell lines was studied using protein overlay and immunostaining methods. The results obtained indicated the presence of both sialic acid-dependent and -independent binding of the virus strains; virus binding to proteins in the molecular mass range from about 40 to 103 kDa was dependent on sialic acid, whereas binding to the 33 kDa protein was independent of sialic acid. The 33 kDa binding protein was identified as annexin, a widely distributed non-glycosylated calcium-dependent phospholipid-binding protein.

Identification and characterization of the cell surface 70-kilodalton sialoglycoprotein(s) as a candidate receptor for encephalomyocarditis virus on human nucleated cells

The attachment of encephalomyocarditis (EMC) virus to human nucleated cells susceptible to virus infection was examined with HeLa and K562 cell lines. Both cell types showed specific virus binding competitively blocked by unlabeled virions. The number of binding sites for EMC virus on HeLa and K562 cells were approximately 1.6 x 10(5) and 3.5 x 10(5) per cell, respectively, and dissociation binding constants were 1.1 and 2.7 nM, respectively. Treatment of cells with cycloheximide after pretreatment with trypsin eliminated EMC virus attachment, suggesting that the virus-binding moiety is proteinaceous in nature. Digestion of cells, cell membranes, and sodium deoxycholate-solubilized cell membranes with proteases or neuraminidases or treatment of cells with lectins demonstrated that the EMC virus-cell interaction is mediated by a sialoglycoprotein. Proteins with a molecular mass of 70 kDa were isolated from detergent-solubilized cell membranes of both HeLa and K562 cells by EMC virus affinity chromatography. The purified proteins, as well as their 70-kDa-molecular-mass equivalents detected in intact surface membranes of HeLa and K562 cells, specifically bound EMC virus in a virus overlay protein blot assay, whereas membranes from nonpermissive K562 D clone cells did not. Western immunoblot analysis with glycophorin A-specific antibody confirmed that the identified 70-kDa binding site on K562 cells is not glycophorin A, which is the EMC virus receptor molecule on virus-nonpermissive human erythrocytes (HeLa cells do not express glycophorin A). These results indicate that EMC virus attachment to permissive human cells is mediated by a cell surface sialoglycoprotein(s) with a molecular mass of 70 kDa.

Immunohistochemical study on the infection of herpes simplex virus, human cytomegalovirus, and Epstein-Barr virus in secondary diffuse interstitial pneumonia

We examined the infection of herpes simplex virus (HSV), human cytomegalovirus (HCMV), and Epstein-Barr virus (EBV) in 61 autopsy cases with secondary diffuse interstitial pneumonia (SDIP) by immunohistochemistry and compared our findings with those in 46 individuals without lung complications. There was no significant difference in positivity of HSV infection between SDIP cases (28 of 61; 45.9%) and the controls (24 of 46; 52.2%). However, HSV was more extensively distributed in the lungs of seven SDIP cases than in those of controls and proliferated to form inclusion bodies in host cells of 11 SDIP cases. Twenty-two (36.1%) and 19 (31.1%) SDIP cases were positive for HCMV and EBV, respectively, whereas all the 46 controls were negative for both viruses. Eighteen of 22 HCMV-positive cases contained classical inclusion bodies in host cells. Epstein-Barr virus was detected extensively in the lungs of seven SDIP patients, but no viral inclusion bodies were observed in host cells. These findings indicate that the herpes viruses replicate excessively in a considerable number of SDIP cases, but classical inclusion bodies are not always associated with viral infection in the lungs. Major infected cells of these viruses were alveolar lining pneumocytes and intra-alveolar cells. Herpes simplex virus and EBV were detected in leukocytes as well as in pneumocytes. In addition, compared with HCMV and HSV, EBV frequently multiplied in bronchial or bronchiolar epithelial cells. Preferential host cells for these types of herpes virus were somewhat different from one another.

CD13 (human aminopeptidase N) mediates human cytomegalovirus infection

Human cytomegalovirus (HCMV) infects cells by a series of processes including attachment, penetration via fusion of the envelope with the plasma membrane, and transport of the viral DNA to the nucleus. The details of the early events of HCMV infection are poorly understood. We have recently reported that CD13, human aminopeptidase N, a metalloprotease, is present on blood cells susceptible in vitro to HCMV infection (C. Söderberg, S. Larsson, S. Bergstedt-Lindqvist, and E. Möller, J. Virol. 67:3166-3175, 1993). Here we report that human CD13 is involved in HCMV infection. Antibodies directed against human CD13 not only inhibit infection but also block binding of HCMV virions to susceptible cells. Compounds known to inhibit aminopeptidase activity block HCMV infection. HCMV-resistant murine fibroblasts have heightened susceptibility to HCMV infection after transfection with complementary DNA encoding human CD13. A significant increase in binding of HCMV was observed in the CD13-expressing transfectants compared with neomycin-resistant control mouse cells. However, murine fibroblasts transfected with mutant CD13, lacking a portion of the aminopeptidase active site, remained susceptible to HCMV infection. Thus, human CD13 appears to mediate HCMV infection by a process that increases binding, but its enzymatic domain is not necessary for infection.

Murine cytomegalovirus interacts with major histocompatibility complex class I molecules to establish cellular infection

The expression of stable, correctly folded major histocompatibility complex class I molecules conferred susceptibility to murine cytomegalovirus (MCMV) in cells which were previously resistant to infection, demonstrating that these molecules interact critically with MCMV to initiate infection. All class I molecules could potentiate MCMV infection but H-2Dd and Kb molecules were most efficient. Monoclonal antibodies specific for the alpha 1 and/or alpha 2 domains of Dd and Kb inhibited infection. Infection of L cells transfected with hybrid major histocompatibility complex class I molecules demonstrated that allelic control of susceptibility to MCMV mapped to the alpha 1 domain of Dd when in correct configuration with the alpha 2 and alpha 3 domains. In MCMV-resistant RMA-S cells, an improvement in the conformation of class I molecules introduced susceptibility to infection.

Definition of a subset of human peripheral blood mononuclear cells that are permissive to human cytomegalovirus infection

The identity of cells responsible for transmission of human cytomegalovirus (HCMV) in blood products or bone marrow transplants is unknown. We have tested the capacity of HCMV to in vitro infect human peripheral blood mononuclear cells (PBMC) from healthy donors and found that certain PBMC are permissive to HCMV infection. In vitro-infected viable cells were double stained for surface expression of different HMCV proteins and for cell-type-specific antigens to allow the identification of sensitive cells. All analysis were performed on viable cells, using HCMV-specific monoclonal antibodies and automated flow cytofluorimetry. PBMC were infected either with the laboratory-adapted HCMV strain AD169 or with a virus isolate obtained from a viremic patient. Up to 25% of all PBMC could express the major immediate-early antigen as well as the pp65 antigen, known at the lower matrix protein. Infected cells were mainly CD14+ monocytes, but also a small population of large CD8+ cells were susceptible to HCMV infection. CD19+ B lymphocytes were resistant to HCMV infection. Different populations of infected cells were enriched by using Dynabeads coated with cell-type-specific antibodies, and the presence of infectious virus was demonstrated by incubating the selected and sonicated cell material on human fibroblasts. Only material from infected monocytes and from CD3+ CD8+ cells gave rise to HCMV-specific plaques. The presence of HCMV mRNA as a sign of active viral transcription of the major immediate-early and late pp150 genes in infected cells was demonstrated by using nested reversed polymerase chain reaction. A common denominator was found for all cells that could be infected with HCMV. The CD13 antigen, a 130- to 150-kDa integral membrane protein identical to the enzyme aminopeptidase N, was expressed on all HCMV-permissive cells.

Characterization of lymphocytic choriomeningitis virus-binding protein(s): a candidate cellular receptor for the virus

The attachment of lymphocytic choriomeningitis virus (LCMV) to murine and primate cell lines was quantitated by a fluorescence-activated cell sorter assay in which binding of biotinylated virus was detected with streptavidin-fluorescein isothiocyanate. Cell lines that were readily infected by LCMV (e.g., MC57, Rin, BHK, Vero, and HeLa) bound virus in a dose-dependent manner, whereas no significant binding was observed to lymphocytic cell lines (e.g., RMA and WIL 2) that were not readily infected. Binding was specific and competitively blocked by nonbiotinylated LCMV. It was also blocked by LCMV-specific antiserum and a neutralizing monoclonal antibody to the virus glycoprotein GP-1 but not by antibodies specific for GP-2, indicating that attachment was likely mediated by GP-1. Treatment of cells with any of several proteases abolished LCMV binding, whereas phospholipases including phosphatidylinositol-specific phospholipase C had no effect, indicating that one or more membrane proteins were involved in virus attachment. These proteins were characterized with a virus overlay protein blot assay. Virus bound to protein(s) with a molecular mass of 120 to 140 kDa in membranes from cell lines permissive for LCMV but not from nonpermissive cell lines. Binding was specific, since unlabeled LCMV, but not the unrelated enveloped virus herpes simplex virus type 1, competed with 125I-labeled LCMV for binding to the 120- to 140-kDa band. The proteinaceous nature of the LCMV-binding substance was confirmed by the lack of virus binding to proteinase K-treated membrane components. By contrast, glycosidase treatment of membranes did not abolish virus binding. However, in membranes treated with endoglycosidase F/N-glycosidase F, and/or neuraminidase and in membranes from cells grown in tunicamycin, the molecular mass of the LCMV-binding entity was reduced. Hence, LCMV attachment to rodent fibroblastic cell lines is mediated by a glycoprotein(s) with a molecular mass of 120 to 140 kDa, with complex N-linked sugars that are not involved in virus binding.

Human cytomegalovirus penetrates host cells by pH-independent fusion at the cell surface

Biochemical, genetic, and morphological criteria were used to demonstrate that human cytomegalovirus penetrates permissive fibroblasts and nonpermissive Chinese hamster ovary (CHO) cells by pH-independent fusion between the virus envelope and the host cell plasma membrane and not by low pH-induced fusion within endosomes. Viral immediate early (IE) gene expression and infectivity were unaffected by conditions which block various stages of endocytosis or agents that alter the acidic pH of the endosome. IE gene expression was also evident in a mutant CHO cell line which is defective in endosomal acidification. Morphological analysis of the entry process at the electron microscopic level revealed viral particles in various stages of virion-plasma membrane fusion. In contrast, intact enveloped virions were not observed sequestered within coated pits or vesicular structures. Collectively, the data indicate that the entry pathway by which HCMV gains access to the cytoplasm of fibroblasts and CHO cells in order to initiate infection is via pH independent, virion envelope-plasma membrane fusion.

Construction and characterization of a human cytomegalovirus mutant with the UL18 (class I homolog) gene deleted

The UL18 open reading frame of human cytomegalovirus (HCMV) (which encodes a product homologous to major histocompatibility complex class I heavy chains) has been disrupted by insertion of the beta-galactosidase gene under control of the major HCMV early promoter. The recombinant virus delta UL18 showed no phenotypic differences from wild-type HCMV in terms of single-step growth curves or particle/infectivity ratios, indicating that the UL18 gene product is dispensable for the growth of HCMV in human fibroblasts in vitro. The synthesis of the mature cellular class I heterodimer is shut down in cells infected at a high multiplicity with wild-type HCMV, and a similar effect was seen in delta UL18-infected fibroblasts, suggesting that although the UL18 gene product can associate with beta 2 microglobulin, it is not directly involved in the disruption of class I assembly.

The human fibroblast receptor for gp86 of human cytomegalovirus is a phosphorylated glycoprotein

A human embryonic lung (HEL) cell receptor for gp86 of human cytomegalovirus that functions in virus-cell fusion was further characterized. Anti-idiotype antibodies that mimic gp86 were used to immunoprecipitate the 92.5-kDa fibroblast membrane receptor for gp86, which was preincubated with various endoglycosidases. The receptor, which has a pI ranging from 5.3 to 5.6, appears to be a glycoprotein with primarily N-linked sugar residues, some of which have high concentrations of mannose and some of which are complex oligosaccharides. Western blots (immunoblots) of electrophoretically transferred receptor incubated with various biotinylated lectins confirmed the presence of sugar moieties, including N-acetylglucosamine, glucose or mannose, and galactose, but not fucose or N-acetylgalactosamine. This gp86 receptor from uninfected HEL cells also incorporated radiolabeled phosphate from orthophosphoric acid, indicating that it is a constitutively phosphorylated receptor.

Sulfated polymers inhibit the interaction of human cytomegalovirus with cell surface heparan sulfate

Several sulfated polysaccharides (dextran sulfate, pentosan polysulfate, heparin) and copolymers of acrylic acid with vinylalcohol sulfate have proved to be potent inhibitors of human cytomegalovirus (CMV) infectivity in vitro. Sulfated alpha-cyclodextrins are only weak inhibitors of CMV. A close correlation was found between the 50% inhibitory concentrations of the sulfated polymers for CMV cytopathogenicity, virus-cell binding, and expression of immediate early antigens (IEA) in human embryonic lung (HEL) cells. CMV particles bound specifically to heparin-Sepharose. Sulfated polymers specifically eluted the virus particles from this matrix. Enzymatic digestion of cell surface heparan sulfate, but not of chondroitin sulfate, prevented the cells from being infected with CMV. Moreover, radiolabeled CMV bound efficiently to, and were infective for wild-type Chinese hamster ovary (CHO) cells, whereas virus binding to, and infection of, mutant CHO cell lines that were deficient in either all glycosaminoglycans or heparan sulfate only was significantly impaired. The mechanism of action of the sulfated polymers can be attributed to an inhibitory effect on the binding of CMV particles to the host cells. Presumably, the sulfated polymers interact with the viral envelope site(s) involved in the attachment of the CMV virions to cell surface heparan sulfate.

Receptor-initiated activation of cells and their oncogenes by herpes-family viruses

The interaction of human cytomegalovirus (HCMV) with the cell membrane has been shown to initiate a cascade of physiologic and biochemical responses that result in the transcriptional activation of specific cellular proto-oncogenes. The cell-activation responses initiated by the virus membrane interaction appear to be important for efficient HCMV replication, as pharmacologic inhibition of cell activation responses significantly reduces the expression of immediate early viral genes and the production of infectious progeny virus. Cellular receptor proteins for other viruses have been shown to be molecules with physiologic activities. Binding of virus to these receptors may trigger the cell to initiate changes that are important for efficient viral replication. These viruses may also trigger inappropriate physiologic responses in the absence of viral replication, thereby causing more covert manifestations of viral pathology.

The effects of beta-2-microglobulin on the infectivity of murine cytomegalovirus

The role of beta-2-microglobulin (beta 2m) in murine cytomegalovirus (MCMV) infection of susceptible (H-2d) and resistant (H-2k) murine embryo fibroblasts (MEF) and peritoneal macrophages was evaluated using serum-free virus (SF-MCMV). The infectivity of SF-MCMV was significantly lower than virus propagated in serum, although the concentrations of virions were similar. Infection of cells with SF-MCMV was assessed by measuring the proportion of cells expressing viral antigens, the sizes of plaques formed in fibroblast monolayers and TCID50 titers. Infection of susceptible fibroblasts was significantly increased 1.6-4.7 fold by the addition of whole FCS, a less than 20 kDa FCS fraction, or purified human beta 2m. These supplements also significantly enhanced infection of susceptible macrophages and increased TCID50 titers by 3.5-10 fold in susceptible MEF. In relatively resistant H-2k cells, the TCID50 titer and the proportion of cells expressing viral antigens after infection with SF-MCMV were not affected by beta 2m or FCS, but plaque sizes were increased 2.5-3 fold in resistant BALB.K MEF. When human or murine beta 2m was added to infected cultures, immunogold electron microscopy revealed these proteins to be always associated extracellularly with the tegument material of disrupted multicapsid virions, but rarely with the envelope of intact virions. However, no murine beta 2m was found in association with the envelope or tegument of SF-MCMV. These relatively modest effects of beta 2m which were restricted to genetically susceptible cells, may be due to tegument-bound beta 2m facilitating infection by capsids, or the stabilisation of the conformation of Class 1 molecules by exogenous beta 2m, promoting MCMV binding to the target cell.

Predominant binding of Theiler's viruses to a 34-kilodalton receptor protein on susceptible cell lines

Western immunoblots of BHK-21 cell lysates probed with the highly virulent GDVII and the less virulent BeAn strains of Theiler's murine encephalomyelitis virus (TMEV) revealed predominant binding to a 34-kDa membrane protein and much lower levels of binding to 100- and 18-kDa membrane proteins. Complete inhibition of virus binding to both the 34- and 18-kDa membrane species by excess unlabeled TMEV demonstrated specificity of binding. Virus binding was also blocked by wheat germ agglutinin, which specifically binds to sialic acid residues and blocks TMEV binding to whole BHK-21 cells. Radiolabeled TMEV also bound to 100-, 34-, and 18-kDa membrane proteins expressed on other TMEV permissive cell lines but not on the nonpermissive cell lines tested. These data suggest that a 34-kDa cellular protein may be the primary determinant of susceptibility to TMEV infection by mediating the binding of GDVII and BeAn viruses to susceptible cells.

Cellular oncogene activation by human cytomegalovirus. Lack of correlation with virus infectivity and immediate early gene expression

The contribution of expression of human cytomegalovirus (HCMV) immediate early (IE) genes to the rapid and transient increase in cellular (c)-oncogene (fos, jun, myc) transcription following HCMV infection was investigated. A partial temporal overlap was observed between the increases in c-oncogene RNA levels and the increase in either transcripts from HCMV IE genes or the number of cells in which HCMV IE proteins were detected. The increases in c-oncogene RNA levels, however, slightly preceded the increase in the detection of HCMV IE transcripts or proteins. To distinguish between the temporal coincidence and a direct relationship between expression of HCMV IE genes and the increased transcription of c-oncogenes, the number of cells synthesizing HCMV IE proteins was reduced by infecting with virus stock enriched in defective particles. Alternatively, the synthesis of HCMV IE proteins was essentially eliminated by ultra-violet (UV) irradiation of virus stock or by inhibitors of protein synthesis. Virus stocks enriched in defective particles demonstrated a substantially reduced capacity to direct the synthesis of HCMV IE proteins, but were more efficient in activating c-oncogene expression than infectious virus stocks. Elimination of expression of HCMV IE genes by UV-irradiation of virus stock or by inhibiting de novo viral and/or cellular protein synthesis with cycloheximide (100 micrograms/ml) or anisomycin (100 micrograms/ml) did not eliminate the HCMV-induced increase in RNA levels of c-oncogenes. These data indicate that activation of these early response cellular genes is independent from de novo expression of HCMV IE proteins, and possibly involves biologically active virion proteins that are related to the induction of a cascade of cellular events associated with the binding of HCMV to its cellular receptor.

Anti-idiotype antibodies that mimic gp86 of human cytomegalovirus inhibit viral fusion but not attachment

Human cytomegalovirus (CMV) infects cells by sequential processes involving attachment, fusion with the cell membrane, and penetration of the capsid. We used two monoclonal anti-idiotype that mimic one of the CMV envelope glycoproteins, gp86, to study its role in the early phases of CMV infection. Neither of two such antibodies inhibited virus binding to human embryonic lung (HEL) fibroblasts; however, both antibodies inhibited the fusion of CMV with HEL cells, as measured by an assay in which viral envelope is labeled with a fluorescent amphiphile (octadecyl rhodamine B chloride, or R18), resulting in increased fluorescence during fusion of virus with the cell membrane. Because these anti-idiotype antibodies were shown previously to bind to specific receptors on HEL cell membranes, these findings suggest that both gp86 and its cell membrane receptor may function in the fusion of human CMV with HEL cells.

Identification and partial characterization of a rhesus rotavirus binding glycoprotein on murine enterocytes

In order to assess the possibility that rotavirus binds to a specific cellular receptor on enterocytes, we have used a viral overlay protein blot assay to study viral binding to murine intestinal brush border membranes (BBM). Infectious double-shelled particles of rhesus rotavirus bound specifically to two approximately 300- and 330-kDa glycoproteins from BBM prepared from suckling mice. Significantly less rotavirus binding was observed when adult BBM were examined. Rats have never been shown to harbor natural group A rotavirus infection and correspondingly, rat BBM showed no rotavirus binding activity. In suckling mice, rotavirus was found to bind to villus tip membranes to a much greater extent than to crypt preparations. Rotavirus binding activity was abolished by treatment of membrane preparations with protease. Analysis by glycolytic digestion of BBM with N- and O-glyconases revealed evidence for both N- and O-linked glycosylation of the rotavirus binding protein. Also neuraminidase digestion showed that O-linked sialic acid residues were required for virus binding. Monoclonal antibodies which immunoprecipitate the 300-kDa viral binding glycoprotein react with the apical surface of suckling but not adult enterocytes by Western blot. Baculovirus-expressed vp4, the rotavirus outer capsid spike protein, bound to the 300- and 330-kDa proteins and competed with rotavirus particles for binding sites. The ability of rotavirus to bind via vp4 to large BBM glycoproteins correlates with in vivo rotavirus cell tropism and host range restriction. Specific host cell receptor expression may be important in rotavirus pathogenesis.

Expression of a human cytomegalovirus receptor correlates with infectibility of cells

Previous studies have demonstrated that human cytomegalovirus (HCMV) specifically binds to a fibroblast membrane glycoprotein(s) with a molecular mass from 30 to 34 kDa. In this study, the distribution of the putative receptor proteins was analyzed in a variety of cell types, including cell types representative of those that are infected in vivo. Using a sensitive microbinding assay (to score virus attachment) and an indirect detection method (to score HCMV-binding proteins), we found that the 34- and 32-kDa HCMV binding proteins are ubiquitous molecules, broadly distributed among diverse cell types. In addition, the level of virus attachment was found to correlate with the abundance of the 34- and 32-kDa cellular proteins, while the ability of the virus to penetrate cells and initiate infection did not. The results support the hypothesis that the 34- and 32-kDa cellular proteins represent the HCMV (attachment) receptor. The data also support the notion that additional cellular components are required for virus entry and fusion.

Transcriptional activation of cellular oncogenes fos, jun, and myc by human cytomegalovirus

The mechanisms responsible for the human cytomegalovirus (HCMV)-induced increase in cellular oncogene RNAs for c-jun, c-fos, and c-myc in human embryo lung cells (I. Boldogh, S. AbuBakar, and T. Albrecht, Science 247:561-564, 1990) were investigated. Results of transcription assays indicated that the rapid increase in RNA levels for the above-noted oncogenes was controlled at the transcriptional level and was related to enhanced transcription. The maximum rates of transcription for c-jun and c-fos genes occurred at 40 min postinfection, while for the c-myc gene the maximum rate occurred at about 60 min. The magnitude of HCMV-induced activation of these cellular genes was similar to the activation induced by serum. The half-lives of the cellular oncogenes showed similar decay rates after either serum or HCMV activation when measured by dactinomycin chase. The half-life for c-fos or c-jun was about 20 min, and that for c-myc was about 40 min. Furthermore, inhibition of the RNA increase by dactinomycin or by alpha-amanitin suggested that the increase in RNA levels was due to an increase in the transcriptional activity of oncogenes triggered by HCMV.