Functional profiling of a human cytomegalovirus genome

Role of tumor cell immune escape mechanisms in cytomegalovirus-mediated oncomodulation

It has been known for a long time that cytomegalovirus (CMV) has evolved mechanisms that allow the escape from the host immune surveillance. In the past, many efforts have been done to elucidate the molecular mechanisms underlying this virus-mediated immune escape and thus virus persistence. However, it is unknown, whether CMV may also impair immune responses directed against tumor cells. This might have severe consequences on tumor progression and may explain the growing evidence for CMV-mediated oncomodulation. This review summarizes recent work on CMV-mediated immune escape mechanisms of tumor cells and oncomodulation and proposes novel aspects that may be important for understanding the CMV-associated tumor progression.

Double-stranded RNA binding by human cytomegalovirus pTRS1

The human cytomegalovirus (HCMV) TRS1 and IRS1 genes rescue replication of vaccinia virus (VV) that has a deletion of the double-stranded RNA binding protein gene E3L (VVDeltaE3L). Like E3L, these HCMV genes block the activation of key interferon-induced, double-stranded RNA (dsRNA)-activated antiviral pathways. We investigated the hypothesis that the products of these HCMV genes act by binding to dsRNA. pTRS1 expressed by cell-free translation or by infection of mammalian cells with HCMV or recombinant VV bound to dsRNA. Competition experiments revealed that pTRS1 preferentially bound to dsRNA compared to double-stranded DNA or single-stranded RNA. 5'- and 3'-end deletion analyses mapped the TRS1 dsRNA-binding domain to amino acids 74 through 248, a region of identity to pIRS1 that contains no homology to known dsRNA-binding proteins. Deletion of the majority of this region (Delta86-246) completely abrogated dsRNA binding. To determine the role of the dsRNA-binding domain in the rescue of VVDeltaE3L replication, wild-type or deletion mutants of TRS1 were transfected into HeLa cells, which were then infected with VVDeltaE3L. While full-length TRS1 rescued VVDeltaE3L replication, deletion mutants affecting a carboxy-terminal region of TRS1 that is not required for dsRNA binding failed to rescue VVDeltaE3L. Analyses of stable cell lines revealed that the carboxy-terminal domain is necessary to prevent the shutoff of protein synthesis and the phosphorylation of eIF2alpha after VVDeltaE3L infection. Thus, pTRS1 contains an unconventional dsRNA-binding domain at its amino terminus, but a second function involving the carboxy terminus is also required for countering host cell antiviral responses.

Two gamma interferon-activated site-like elements in the human cytomegalovirus major immediate-early promoter/enhancer are important for viral replication

Human cytomegalovirus (HCMV) infection directly initiates a signal transduction pathway that leads to activation of a large number of cellular interferon-stimulated genes (ISGs). Our previous studies demonstrated that two interferon response elements, the interferon-stimulated response element and gamma interferon-activated site (GAS), in the ISG promoters serve as HCMV response sites (VRS). Interestingly, two GAS-like VRS elements (VRS1) were also present in the HCMV major immediate-early promoter-enhancer (MIEP/E). In this study, the importance of these VRS elements in viral replication was investigated. We demonstrate that the expression of the major IE genes, IE1 and IE2, is interferon inducible. To understand the biological significance of this signal transduction pathway in HCMV major IE expression, the two VRS1 in the MIEP/E were mutated. Mutant HCMVs in which the VRS elements were deleted or that contained point mutations grew dramatically more slowly than wild-type virus at a low multiplicity of infection (MOI). Insertion of wild-type VRS1 into the mutant viral genome rescued the slow growth phenotype. Furthermore, the expression levels of major IE RNAs and proteins were greatly reduced during infection with the VRS mutants at a low MOI. HCMV microarray analysis indicated that infection of host cells with the VRS mutant virus resulted in a global reduction in the expression of viral genes. Collectively, these data demonstrate that the two VRS elements in the MIEP/E are necessary for efficient viral gene expression and replication. This study suggests that although the HCMV-initiated signal transduction pathway results in induction of cellular antiviral genes, it also functions to stimulate viral major IE gene expression. This might be a new viral strategy in which the pathway is used to regulate gene expression and play a role in reactivation.

Coupling generation of cytomegalovirus deletion mutants and amplification of viral BAC clones

Human cytomegalovirus (HCMV) genome manipulation has always been difficult. Recently, the introduction of full-length HCMV DNA into Escherichia coli as an artificial bacterial chromosome (BAC) clone has allowed reliable targeted mutagenesis. Here, we show the next generation of improvement in designing recombinant HCMV, which will also be applicable to other viral BAC clones. An inducible origin of replication linked with an antibiotic resistance marker was used as a cassette for targeted replacement of sequences within a HCMV BAC clone, TowneBAC. The origin of replication allowed for the induction of increased amounts BAC DNA that improved recovery, ease of use and transfections for mutant viruses. By specific deletion of UL147 and the recombinant GFP gene, we have shown that targeted deletion of a gene and selection for a recombinant genome are coupled with the ability to amplify the BAC clone DNA. These HCMV BAC clones were amplified approximately 10-fold. In the case of the removal of GFP from the clone TowneBAC shown in this study, the resulting BAC DNA preparation following amplification was used for successful primary cell transfection. Both parental and deletion BAC clone transfections gave similar levels of recombinant HCMV, and the GFP deletion virus replicated the same as the TowneBAC in a multi-step growth curve analysis.

Human cytomegalovirus tegument protein ppUL35 is important for viral replication and particle formation

The tegument proteins ppUL35 and ppUL82 (pp71) of human cytomegalovirus (HCMV) physically interact and cooperatively activate the major immediate-early transcription. While an HCMV mutant lacking UL82 displayed a multiplicity of infection (MOI)-dependent growth, the biological significance of ppUL35 has not been addressed so far. We generated a mutant virus with a deletion of the UL35 gene. Using an MOI of 0.1, the progeny virus yield of this mutant was reduced by a factor of 1,000; however, when infected at a low MOI (0.01), the gene was essential. Characterization of the replication cycle showed that the mutant virus had two defects: when virus inoculum was standardized by the amount of viral DNA, a reduced immediate-early gene expression was observed, leading to a strongly delayed expression of lytic genes. A second defect was apparent in the virus assembly, as fewer enveloped particles and no dense bodies were present in cells infected with the mutant virus. However, the particles produced by wild-type and mutant viruses did not show significant ultrastructural differences. These results suggest an important role for ppUL35 in immediate-early gene expression and virus assembly.

Dendritic-cell infection by human cytomegalovirus is restricted to strains carrying functional UL131-128 genes and mediates efficient viral antigen presentation to CD8+ T cells

Human cytomegalovirus (HCMV) genetic determinants of endothelial-cell tropism and virus transfer to leukocytes (both polymorphonuclear and monocyte) have been recently identified in the UL131-128 genes. Here it is documented that the same genetic determinants of HCMV are responsible for monocyte-derived dendritic-cell (DC) tropism, i.e. all endotheliotropic and leukotropic strains of HCMV are also DC-tropic (or dendrotropic). In fact, all recent clinical HCMV isolates and deletion mutants sparing the UL131-128 locus as well as the endotheliotropic revertants AD169 and Towne were able to productively infect DC following co-culture with infected endothelial cells. On the contrary, the same clinical isolates extensively propagated in human fibroblasts, the UL131-128 deletion mutants and the reference laboratory strains were not. Peak extracellular virus titres in DC were reached 4-7 days post-infection (p.i.). Viral proteins pp65 and p72 were detected 1-3 h p.i., involving the great majority of DC 24 h p.i., while gB was abundantly detected 96 h p.i., when a cytopathic effect first appeared. Infection of DC with cell-free virus released into the medium could only be achieved with HCMV strains extensively adapted to growth in endothelial cells, reaching the peak titres 10 days p.i. DC infected for 24 h with cell-free virus and incubated for 16 h with autologous peripheral blood mononuclear cells were found to act as a potent stimulator of both HCMV-specific CD4+- and CD8+-mediated immune responses, as determined by cytokine flow cytometry. DC incubated with inactivated crude whole viral antigen preparations were only capable of eliciting a significant CD4+-mediated immune response.

Analysis of human cytomegalovirus oriLyt sequence requirements in the context of the viral genome

During the lytic phase of infection, replication of herpesvirus genomes initiates at the lytic origin of replication, oriLyt. Many herpesviruses harbor more than one lytic origin, but so far, only one oriLyt has been identified for human cytomegalovirus (HCMV). Evidence for the existence of additional lytic origins of HCMV has remained elusive. On the basis of transient replication assays with cloned viral fragments, HCMV oriLyt was described as a core region of 1.5 kbp (minimal oriLyt) flanked by auxiliary sequences required for maximal replication activity (complete oriLyt). It remained unclear whether minimal oriLyt alone can drive the replication of HCMV in the absence of its accessory regions. To investigate the sequence requirements of oriLyt in the context of the viral genome, mutant genomes were constructed lacking either minimal or complete oriLyt. These genomes were not infectious, suggesting that HCMV contains only one lytic origin of replication. Either minimal or complete oriLyt was then ectopically reinserted into the oriLyt-depleted genomes. Only the mutant genomes carrying complete oriLyt led to infectious progeny. Remarkably, inversion of the 1.5-kbp core origin relative to its flanking regions resulted in a replication-defective genome. Mutant genomes carrying minimal oriLyt plus the left flanking region gave rise to minifoci, but genomes harboring minimal oriLyt together with the right flanking region were noninfectious. We conclude that the previously defined minimal lytic origin is not sufficient to drive replication of the HCMV genome. Rather, our results underline the importance of the accessory regions and their correct arrangement for the function of HCMV oriLyt.

Cell death suppression by cytomegaloviruses

Cytomegaloviruses (CMVs), a subset of betaherpesviruses, employ multiple strategies to suppress apoptosis in infected cells and thus to delay their death. Human cytomegalovirus (HCMV) encodes at least two proteins that directly interfere with the apoptotic signaling pathways, viral inhibitor of caspase-8-induced apoptosis vICA (pUL36), and mitochondria-localized inhibitor of apoptosis vMIA (pUL37 x 1). vICA associates with pro-caspase-8 and appears to block its recruitment to the death-inducing signaling complex (DISC), a step preceding caspase-8 activation. vMIA binds and sequesters Bax at mitochondria, and interferes with BH3-only-death-factor/Bax-complex-mediated permeabilization of mitochondria. vMIA does not seem to either interact with Bak, a close structural and functional homologue of Bax, or to suppress Bak-mediated permeabilization of mitochondria and Bak-mediated apoptosis. All sequenced betaherpesviruses, including CMVs, encode close homologues of vICA, and those vICA homologues that have been tested, were found to be functional cell death suppressors. Overt sequence homologues of vMIA were found only in the genomes of primate CMVs, but recent observations made with murine CMV (MCMV) indicate that non-primate CMVs may also encode a cell death suppressor functionally resembling vMIA. The exact physiological roles and relative contributions of vMIA and vICA in suppressing death of CMV-infected cells in vivo have not been elucidated. There is strong evidence that the cell death suppressing function of vMIA is indispensable, and that vICA is dispensable for replication of HCMV. In addition to suppressed caspase-8 activation and sequestered Bax, CMV-infected cells display several other phenomena, less well characterized, that may diminish, directly or indirectly the extent of cell death.

Identification of viral genes essential for replication of murine gamma-herpesvirus 68 using signature-tagged mutagenesis

Gamma-herpesviruses, Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus are important human pathogens, because they are involved in tumor development. Murine gamma-herpesvirus-68 (MHV-68 or gammaHV-68) has emerged as a small animal model system for the study of gamma-herpesvirus pathogenesis and host-virus interactions. To identify the genes required for viral replication in vitro and in vivo, we generated 1,152 mutants using signature-tagged transposon mutagenesis on an infectious bacterial artificial chromosome of MHV-68. Almost every ORF was mutated by random insertion. For each ORF, a mutant with an insertion proximal to the N terminus of each ORF was examined for the ability to grow in fibroblasts. Our results indicate that 41 genes are essential for in vitro growth, whereas 26 are nonessential and 6 attenuated. Replication-competent mutants were pooled to infect mice, which led to the discovery of ORF 54 being important for MHV-68 to replicate in the lung. This genetic analysis of a tumor-associated herpesvirus at the whole genome level validates signature-tagged transposon mutagenesis screening as an effective genetic system to identify important virulent genes in vivo and define interactions with the host immune system.

Characterization of Host Immunity to cytomegalovirus pp150 (UL32)

The basic phosphoprotein 150 (pp150), the product of UL32 (unique long domain 32) gene of human cytomegalovirus (CMV), is an abundant component of the viral tegument and a target of human leukocyte antigen (HLA)-restricted cytotoxic T cells (CTLs) after infection. Identification of minimal cytotoxic epitopes (MCEs) from this CMV protein is of importance for peptide-based vaccines and immunotherapeutic approaches. Several pp150-specific CTL clones were derived from peripheral blood mononuclear cells of healthy CMV-positive donors with autologous fibroblasts infected either with CMV AD169 or with a recombinant vaccinia virus expressing full-length pp150 protein. HLA A*0301- and HLA A*6801-restricted CD8+ pp150 T-cell clones derived from different donors were found to efficiently kill autologous CMV-infected fibroblasts. Fine mapping of each MCE first used a T-cell epitope prediction algorithm. Overlapping peptides within the recognized regions were screened. The analysis identified pp150(792-802) and pp150(945-955) as MCEs for the HLA A*6801 and the HLA A*0301 pp150 clones, respectively. In vitro stimulation by recombinant modified vaccinia Ankara virus expressing full-length pp150 elicited high frequencies of CMV-CTL and interferon gamma production specific for the MCE identified in all subjects. The consistent presence of pp150 T cells in CMV-exposed individuals supports a role for this antigen in shaping the antiviral CTL response and indicates that pp150 could be a pivotal constituent of prophylactic and therapeutic CMV vaccines.

Genetic polymorphisms among human cytomegalovirus (HCMV) wild-type strains

Human cytomegalovirus (HCMV) clinical isolates display genetic polymorphisms in multiple genes. Some authors have suggested that those polymorphisms may be implicated in HCMV-induced immunopathogenesis, as well as in strain-specific behaviours, such as tissue-tropism and ability to establish persistent or latent infections. This review summarises the features of the main clustered HCMV polymorphic open reading frames and also briefly cites other variable loci within the viral genome. The implications of gene polymorphisms are discussed in terms of potentially advantageous higher fitness obtained by the strain, but also taking into account that the published data are often speculative. The last section of this review summarises and critically analyses the main literature reports about the linkage of strain specific genotypes with clinical manifestations of HCMV disease in different patient populations affected by severe cytomegalovirus infections, namely immunocompromised subjects and congenitally infected newborns.

Downregulation of natural killer cell-activating ligand CD155 by human cytomegalovirus UL141

Natural killer (NK) cells are crucial in the control of cytomegalovirus infections in mice and humans. Here we show that the viral UL141 gene product has an immunomodulatory function that is associated with low-passage strains of human cytomegalovirus. UL141 mediated efficient protection of cells against killing by a wide range of human NK cell populations, including interferon-alpha-stimulated bulk cultures, polyclonal NK cell lines and most NK cell clones tested. Evasion of NK cell killing was mediated by UL141 blocking surface expression of CD155, which was previously identified as a ligand for NK cell-activating receptors CD226 (DNAM-1) and CD96 (TACTILE). The breadth of the UL141-mediated effect indicates that CD155 has a key role in regulating NK cell function.

Human cytomegalovirus proteins encoded by UL37 exon 1 protect infected fibroblasts against virus-induced apoptosis and are required for efficient virus replication

Human cytomegalovirus (HCMV) strain AD169 mutants carrying transposon insertions or large deletions in UL37 exon 1 (UL37x1) were recovered from modified bacterial artificial chromosomes by reconstitution in human fibroblasts expressing the adenovirus anti-apoptotic protein E1B19K. UL37x1 mutant growth was severely compromised in normal fibroblasts, with minimal release of infectious progeny. Growth in E1B19K-expressing cells was restored, but did not reach wild-type levels. Normal fibroblasts infected by UL37x1 mutants underwent apoptosis spontaneously between 48 and 96 h after infection. Apoptosis was inhibited by treatment of cells with the broad-spectrum caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone, resulting in substantially increased release of virus. Inhibition of viral DNA replication by phosphonoformate or ganciclovir also inhibited apoptosis, implying that death was triggered by late viral functions or by replication and packaging of the viral genome. Immunofluorescent staining showed that although viral proteins accumulated normally during delayed-early phase and viral DNA replication compartments formed, viral late proteins were detected only rarely, suggesting that spontaneous apoptosis occurs early in late phase. These results demonstrate that anti-apoptotic proteins encoded by HCMV UL37x1 [pUL37x1 (vMIA), gpUL37 and gpUL37(M)] prevent apoptosis that would otherwise be initiated by the replication programme of the virus and are required for efficient and sustainable virus replication.

Role of the proximal enhancer of the major immediate-early promoter in human cytomegalovirus replication

The human cytomegalovirus (CMV) enhancer has a distal component (positions -550 to -300) and a proximal component (-300 to -39) relative to the transcription start site (+1) of the major immediate-early (MIE) promoter. Without the distal enhancer, human CMV replicates slower and has a small-plaque phenotype. We determined the sequence requirements of the proximal enhancer by making 5'-end deletions to positions -223, -173, -116, -67, and -39. Even though recombinant virus with the proximal enhancer deleted to -39 has the minimal TATA box-containing MIE promoter element, it cannot replicate independently in human fibroblast cells. Recombinant virus with a deletion to -67 has an Sp-1 transcription factor binding site which may represent a minimal enhancer element for recombinant virus replication in human fibroblast cells. Although recombinant virus with a deletion to -223 replicates to titers at least 100-fold less than that of the wild-type virus, it replicates to titers 8-fold higher than that of recombinant virus with a deletion to -173 and 20-fold higher than that of virus with a deletion to -67. Recombinant virus with a deletion to -173 replicates more efficiently than that with a deletion to -116. There was a direct correlation between the level of infectious virus replication and time after infection, amount of MIE gene transcription, MIE and early viral protein synthesis, and viral DNA synthesis. The extent of the proximal enhancer determines the efficiency of viral replication.

Identification of a mouse cytomegalovirus gene selectively targeting CD86 expression on antigen-presenting cells

We and others have shown that infection of dendritic cells with murine cytomegalovirus (MCMV) leads to severe functional impairment of these antigen-presenting cells (D. M. Andrews, C. E. Andoniou, F. Granucci, P. Ricciardi-Castagnoli, and M. A. Degli-Esposti, Nat. Immunol. 2:1077-1084, 2001; S. Mathys, T. Schroeder, J. Ellwart, U. H. Koszinowski, M. Messerle, and U. Just, J. Infect. Dis. 187:988-999, 2003). Phenotypically, reduced surface expression of costimulatory molecules and major histocompatibility complex molecules was detected. In order to identify the molecular basis for the observed effects, we generated MCMV mutants with large deletions of nonessential genes. The study was facilitated by the finding that a monocyte-macrophage cell line displayed similar phenotypic alterations after MCMV infection. By analyzing the expression of cell surface molecules on infected cells, we identified a mutant virus which is no longer able to downmodulate the expression of the costimulatory molecule CD86. Additional mutants with smaller deletions allowed us to pin down the responsible gene to a certain genomic region. RNA analysis led to the identification of the spliced gene m147.5, encoding a protein with 145 amino acids. Experiments with an m147.5 mutant revealed that the protein affects CD86 expression only, suggesting that additional MCMV genes are responsible for downmodulation of the other surface molecules. Identification of viral gene products interfering with functionally important proteins of antigen-presenting cells will provide the basis to dissect the complex interaction of CMV with these important cells and to evaluate the biological importance of these viral genes in vivo.

Alteration of cellular RNA splicing and polyadenylation machineries during productive human cytomegalovirus infection

Alternative processing of human cytomegalovirus (HCMV) UL37 pre-mRNA predominantly produces the unspliced UL37 exon 1 (UL37x1) RNA and multiple, lower abundance, alternatively spliced UL37 RNAs. The relative abundance of UL37x1 unspliced RNA is surprising because it requires the favoured use of a polyadenylation signal within UL37 intron 1, just upstream of the UL37 exon 2 (UL37x2) acceptor. Here, it was shown that a downstream element (DSE) in UL37x2 strongly enhanced processing at the UL37x1 polyadenylation site, but did not influence UL37x1-x2 splicing. There was a potential binding site (UCUU) for polypyrimidine tract-binding protein (PTB) at the UL37x1 polyadenylation/cleavage site and its mutation to UGGG reduced both polyadenylation and splicing of UL37x1-x2 minigene pre-mRNA, suggesting a role in both RNA processing events. To determine whether lytic HCMV infection altered the balance of RNA processing factors, which bind to UL37 pre-mRNA cis elements, these were investigated in permissively infected primary and immortalized human diploid fibroblasts (HFFs) and epithelial cells. Induction of polyadenylation factors in HCMV-infected, serum-starved (G(0)) HFFs was also investigated. Permissive HCMV infection consistently increased, albeit with different kinetics, the abundance of cleavage stimulation factor 64 (CstF-64) and PTB, and altered hypo-phosphorylated SF2 in different cell types. Moreover, the preponderance of UL37x1 RNA increased during infection and correlated with CstF-64 induction, whereas the complexity of the lower abundance UL37 spliced RNAs transiently increased following reduction of hypo-phosphorylated SF2. Collectively, multiple UL37 RNA polyadenylation cis elements and induced cellular factors in HCMV-infected cells strongly favoured the production of UL37x1 unspliced RNA.

Identification of proteins in human cytomegalovirus (HCMV) particles: the HCMV proteome

Human cytomegalovirus (HCMV), a member of the herpesvirus family, is a large complex enveloped virus composed of both viral and cellular gene products. While the sequence of the HCMV genome has been known for over a decade, the full set of viral and cellular proteins that compose the HCMV virion are unknown. To approach this problem we have utilized gel-free two-dimensional capillary liquid chromatography-tandem mass spectrometry (MS/MS) and Fourier transform ion cyclotron resonance MS to identify and determine the relative abundances of viral and cellular proteins in purified HCMV AD169 virions and dense bodies. Analysis of the proteins from purified HCMV virion preparations has indicated that the particle contains significantly more viral proteins than previously known. In this study, we identified 71 HCMV-encoded proteins that included 12 proteins encoded by known viral open reading frames (ORFs) previously not associated with virions and 12 proteins from novel viral ORFs. Analysis of the relative abundance of HCMV proteins indicated that the predominant virion protein was the pp65 tegument protein and that gM rather than gB was the most abundant glycoprotein. We have also identified over 70 host cellular proteins in HCMV virions, which include cellular structural proteins, enzymes, and chaperones. In addition, analysis of HCMV dense bodies indicated that these viral particles are composed of 29 viral proteins with a reduced quantity of cellular proteins in comparison to HCMV virions. This study provides the first comprehensive quantitative analysis of the viral and cellular proteins that compose infectious particles of a large complex virus.

Human cytomegalovirus UL84 insertion mutant defective for viral DNA synthesis and growth

Human cytomegalovirus (HCMV) UL84 is required for oriLyt-dependent DNA replication, and evidence from transient transfection assays suggests that UL84 directly participates in DNA synthesis. In addition, because of its apparent interaction with IE2, UL84 is implicated as a possible regulatory protein. To address the role of UL84 in the context of the viral genome, we generated a recombinant HCMV bacterial artificial chromosome (BAC) construct that did not express the UL84 gene product. This construct, BAC-IN84/Ep, displayed a null phenotype in that it failed to produce infectious virus after transfection into human fibroblast cells, whereas a revertant virus readily produced viral plaques and, subsequently, infectious virus. Real-time quantitative PCR showed that BAC-IN84/Ep was defective for DNA synthesis in that no increase in the accumulation of viral DNA was observed in transfected cells. We were unable to complement BAC-IN84/Ep in trans; however, oriLyt-dependent DNA replication was observed by the cotransfection of UL84 and BAC-IN84/Ep. An analysis of viral mRNA by real-time PCR indicated that, even in the absence of DNA synthesis, all representative kinetic classes of genes were expressed in cells transfected with BAC-IN84/Ep. The detection of UL44 and IE2 by immunofluorescence in BAC-IN84/Ep-transfected cells showed that these proteins failed to partition into replication compartments, indicating that UL84 expression is essential for the formation of these proteins into replication centers within the context of the viral genome. These results show that UL84 provides an essential DNA replication function and influences the subcellular localization of other viral proteins.

Immune escape and exploitation strategies of cytomegaloviruses: impact on and imitation of the major histocompatibility system

Cytomegalovirus (CMV) has yielded many insights into immune escape mechanisms. Both human and mouse CMV encode a diverse array of gene products, many of which appear to modulate the immune response in the host. Some deflect the host response to infection and contribute to lifelong viral persistence while others exploit immune cells that respond to infection. Here, the viral functions that modulate and mimic host major histocompatibility complex (MHC) function will be reviewed. Viral gene products related to both classical and non-classical components of the MHC system assure the virus will persist in immunocompetent individuals. Examples of host countermeasures that neutralize viral immunomodulatory functions have emerged in the characterization of viral functions that contribute to this stand-off in CMVs that infect humans, other primates and rodents. CMV-induced disease occurs when the immune system is not yet developed, such as in the developing fetus, or when it is compromised, such as in allograft transplant recipients, suggesting that the balance between virus escape and host control is central to pathogenesis. Although evidence supports the dominant role of immune escape in CMV pathogenesis and persistence, MHC-related immunomodulatory functions have been ascribed only subtle impact on pathogenesis and the immune response during natural infection. Viral gene products that interface with the MHC system may impact natural killer cell function, antigen presentation, and T lymphocyte immune surveillance. Many also interact with other cells, particularly those in the myeloid lineage, with consequences that have not been explored. Overall, the virus-encoded modulatory functions that have been acquired by CMV likely ensure survival and adaptation to the wide range of mammalian host species in which they are found.

Human cytomegalovirus UL76 encodes a novel virion-associated protein that is able to inhibit viral replication

The human cytomegalovirus (HCMV) UL76 gene encodes a highly conserved herpesvirus protein, pUL76, which is able to modulate gene expression in either activation or repression. In this study, two specific transcripts were found to contain the reading frame of UL76, one a 4.5-kb and the other a 5.5-kb tricistronic mRNA encoding the UL76, UL77, and UL78 open reading frames. Both transcripts were expressed with true late kinetics, as revealed by data showing inhibition of production in the presence of phosphonoformic acid. Immediately after viral infection, pUL76 was found in the nuclear fraction and was detected in cells in the presence of the protein synthesis inhibitor cycloheximide. Subsequent virus particle purification and Western blot analysis revealed that two forms of pUL76 are associated within mature virions. The high-molecular-mass protein (H-pUL76) was verified as originating from a free form of pUL76 by cross-linking with an unknown protein(s). By performing a biochemical fractionation experiment with purified virions, we provide evidence that pUL76 and H-pUL76 are associated with the detergent-soluble (envelope) and -insoluble (tegument/capsid) fractions, respectively. Both results were consistent with the images exhibited by immunoelectron microscopy, which showed that the distribution of gold particles labeled by the anti-pUL76 antibody juxtaposed the compartments of the envelope and the tegument/capsid of the virion. Evidence indicated that expression of pUL76 at the immediate-early phase of the viral replication cycle leads to the inhibition of HCMV production. The viral constituent pUL76, with a dominant-negative effect on replication, may provide a novel mechanism for HCMV's resumption of latency.

Characterization of the human cytomegalovirus UL34 gene

UL34 encodes the transcriptional repressor of the human cytomegalovirus immune evasion gene, US3, and is essential for viral replication in tissue culture. Two different monocistronic transcripts originate from UL34 at early and late times postinfection and encode two predominant proteins and a third, minor protein. The UL34 proteins are differentially expressed throughout the viral replication cycle, with both proteins localizing to the nucleus and repressing expression of the US3 gene.

Evidence that the human cytomegalovirus 46-kDa UL72 protein is not an active dUTPase but a late protein dispensable for replication in fibroblasts

The Human Cytomegalovirus (HCMV) UL72 gene is considered to be the equivalent of the dUTPase gene of the Alpha- and Gamma-herpesviruses. To characterize its function, the expression profiles of UL72 at both the RNA and the protein level were determined. The gene is expressed with a late kinetics and the corresponding UL72 46-kDa protein accumulates late during infection in the cytoplasm of infected cells. The pUL72 was expressed in E. coli and the purified recombinant protein did not display a detectable dUTPase activity. The viral yields of reconstituted HCMV RVDeltaUL72 viruses carrying a deletion within the UL72 ORF demonstrated a moderate growth defect following low MOI infections, whereas their DNA synthesis profiles were not significantly different from those of the parental HCMV RVAD169. These results demonstrate that the UL72 gene product is not a dUTPase and is not essential for replication in human fibroblasts.

Internal cleavage of the human cytomegalovirus UL37 immediate-early glycoprotein and divergent trafficking of its proteolytic fragments

The human cytomegalovirus UL37 gene encodes at least three isoforms, which share N-terminal UL37 exon 1 (UL37x1) sequences. UL37 proteins traffic dually into the endoplasmic reticulum (ER) and to mitochondria. Trafficking of the UL37 glycoprotein (gpUL37) in relation to its post-translational processing was investigated. gpUL37 is internally cleaved in the ER and its products traffic differentially. Its C-terminal fragment (UL37(COOH)) is ER-localized and N-glycosylated. Unlike conventional ER signal sequences, its N-terminal fragment is stable and traffics to mitochondria. Inhibition of N-glycosylation did not block pUL37 cleavage and dramatically decreased the levels of but not of UL37(COOH). pUL37(M), which differs from gpUL37 by the lack of residues 178-262 and hence the UL37x3 consensus signal peptidase cleavage site, traffics into the ER and mitochondria, but is neither cleaved nor N-glycosylated. This finding of a relationship between ER processing and mitochondrial importation of UL37 proteins is unique for herpesvirus proteins.

Genetic content of wild-type human cytomegalovirus

The genetic content of wild-type human cytomegalovirus was investigated by sequencing the 235 645 bp genome of a low passage strain (Merlin). Substantial regions of the genome (genes RL1-UL11, UL105-UL112 and UL120-UL150) were also sequenced in several other strains, including two that had not been passaged in cell culture. Comparative analyses, which employed the published genome sequence of a high passage strain (AD169), indicated that Merlin accurately reflects the wild-type complement of 165 genes, containing no obvious mutations other than a single nucleotide substitution that truncates gene UL128. A sizeable subset of genes exhibits unusually high variation between strains, and comprises many, but not all, of those that encode proteins known or predicted to be secreted or membrane-associated. In contrast to unpassaged strains, all of the passaged strains analysed have visibly disabling mutations in one or both of two groups of genes that may influence cell tropism. One comprises UL128, UL130 and UL131A, which putatively encode secreted proteins, and the other contains RL5A, RL13 and UL9, which are members of the RL11 glycoprotein gene family. The case in support of a lack of protein-coding potential in the region between UL105 and UL111A was also strengthened.

Cellular repressor inhibits human cytomegalovirus transcription from the UL127 promoter

The region of the human cytomegalovirus (HCMV) genome between the UL127 promoter and the major immediate-early (MIE) enhancer is referred to as the unique region. The role of this region during a viral infection is not known. In wild-type HCMV-infected permissive fibroblasts, there is no transcription from the UL127 promoter at any time during productive infection. Our investigators previously reported that the region upstream of the UL127 TATA box repressed expression from the UL127 promoter (C. A. Lundquist et al., J. Virol. 73:9039-9052, 1999). The region was reported to contain functional NF1 DNA binding sites (L. Hennighausen and B. Fleckenstein, EMBO J. 5:1367-1371, 1986). Sequence analysis of this region detected additional consensus binding sites for three transcriptional regulatory proteins, FoxA (HNF-3), suppressor of Hairy wing, and CAAT displacement protein. The cis-acting elements in the unique region prevented activation of the early UL127 promoter by the HCMV MIE proteins. In contrast, deletion of the region permitted very high activation of the UL127 promoter by the viral MIE proteins. Mutation of the NF1 sites had no effect on the basal activity of the promoter. To determine the role of the other sites in the context of the viral genome, recombinant viruses were generated in which each putative repressor site was mutated and the effect on the UL127 promoter was analyzed. Mutation of the putative Fox-like site resulted in a significant increase in expression from the viral early UL127 promoter. Insertion of wild-type Fox-like sites between the HCMV immediate-early (IE) US3 TATA box and the upstream NF-kappaB-responsive enhancer (R2) also significantly decreased gene expression, but mutated Fox-like sites did not. The wild-type Fox-like site inhibits activation of a viral IE enhancer-containing promoter. Cellular protein, which is present in uninfected or infected permissive cell nuclear extracts, binds to the wild-type Fox-like site but not to mutated sites. Reasons for repression of UL127 gene transcription during productive infection are discussed.

Quantitative analysis of human herpesvirus 6 cell tropism

Although HHV-6A and -6B are known to replicate preferably in human T-lymphocytes, in vitro infection of several other cell types has been described. Also, the finding that both variants use the ubiquitous molecule CD46 as a membrane receptor fully supports the possibility of a broad cellular tropism. However, productive infection, which requires complete progression through the viral replication cycle, depends on multiple cellular processes. Our studies were aimed at determining the differences in replication efficiency according to the cell type infected and at relating these differences to the sequential transcriptional events preceding DNA replication. A strong expression of immediate-early, early, and late genes was only seen in the T-lymphoblastoma lines. In the other cell lines, there was no clear correlation between the level of transcription and the final outcome of replication. Finally, we investigated the cytopathic effects of HHV-6 on different cell types of neural origin (oligodendrocytes, astrocytes, and neurons) in greater detail, and found that although all three sustained HHV-6 replication, HHV-6A was more neurovirulent than HHV-6B. This was confirmed in primary human oligodendrocyte cultures.

[In vivo synaptic transmission in the smooth muscle cells of the guinea-pig vas deferens]

BACKGROUND: Brain metastases (BMs) develop by largely unknown mechanisms and cause major morbidity and mortality in patients with solid tumors. Human cytomegalovirus (HCMV) is frequently detected in tumor tissue from patients with different cancers. Here, we aimed to determine the prevalence and potential prognostic role of HCMV in BMs. METHODS: We obtained archived samples of BMs from 41 patients with breast cancer and 37 with colorectal cancer and paired primary tumor tissues from 13 and 12 patients in each respective group. In addition, primary breast cancer tissues from 15 patients were included. HCMV proteins were detected with an immunohistochemical technique and Western blot. HCMV nucleic acids were detected with TaqMan polymerase chain reaction (PCR) assay. RESULTS: HCMV proteins were abundantly expressed in 99% of BM specimens, and in 12 of 13 (92%) paired primary breast cancer specimens. All 12 paired colon cancer samples were positive for HCMV proteins. Protein staining was mainly confined to neoplastic cells. Western blot analysis detected an HCMV-IE reactive protein in 53% of breast cancer specimens, and PCR detected the presence of HCMV DNA and transcripts in 92% and 80% of samples, respectively. Patients with high-level expression of HCMV-IE proteins in their tumors had a shorter time to tumor progression and shorter overall survival. CONCLUSIONS: The prevalence of HCMV proteins and nucleic acids is very high in primary and metastatic tumors and may drive the development of metastatic brain tumors; therefore, this virus may represent a potential therapeutic target in metastatic cancer.