Nonviral microbodies with viral antigenicity produced in cytomegalovirus-infected cells

Human cytomegalovirus induces neuronal gene expression for viral maturation

Viral invasion of the host cell causes some of the most dramatic changes in biology. Human cytomegalovirus (HCMV) extensively remodels host cells, altering nuclear shape and generating a cytoplasmic viral-induced assembly compartment (vIAC). How these striking morphology changes take place in the context of host gene regulation is still emerging. Here, we discovered that histone variant macroH2A1 is essential for producing infectious progeny. Because virion maturation and cellular remodeling are closely linked processes, we investigated structural changes in the host cell upon HCMV infection. We discovered that macroH2A1 is necessary for HCMV-induced reorganization of the host nucleus, cytoskeleton, and endoplasmic reticulum. Furthermore, using RNA-seq we found that while all viral genes were highly expressed in the absence of macroH2A1, many HCMV-induced host genes were not. Remarkably, hundreds of these HCMV-induced macroH2A1-dependent host genes are associated with neuronal synapse formation and vesicle trafficking. Knock-down of these HCMV-induced neuronal genes during infection resulted in malformed vIACs and smaller plaques, establishing their importance to HCMV infection. Together, our findings demonstrate that HCMV manipulates host gene expression by hijacking a dormant neuronal secretory pathway for efficient virion maturation.

Proteome changes of fibroblasts and endothelial cells upon incubation with human cytomegalovirus subviral Dense Bodies

Human cytomegalovirus (HCMV) is a pathogen of high medical relevance. Subviral Dense Bodies (DB) were developed as a vaccine candidate to ameliorate the severe consequences of HCMV infection. Development of such a candidate vaccine for human application requires detailed knowledge of its interaction with the host. A comprehensive mass spectrometry (MS)- based analysis was performed regarding the changes in the proteome of cell culture cells, exposed to DB.

Subviral Dense Bodies of Human Cytomegalovirus Enhance Interferon-Beta Responses in Infected Cells and Impair Progeny Production

(1) Background: Infection with human cytomegalovirus (HCMV) leads to the production and release of subviral particles, termed Dense Bodies (DB). They are enclosed by a membrane resembling the viral envelope. This membrane mediates the entrance of DBs into cells in a way that is comparable to virus infection. HCMV attachment and entry trigger the induction of interferon synthesis and secretion, and the subsequent expression of interferon-regulated genes (IRGs) that might inhibit replication of the virus. Recently, we demonstrated that DBs induce a robust interferon response in the absence of infection. Little is known thus far, including how DBs influence HCMV infection and virus-host interaction. (2) Methods: Purified DBs were used to study the impact on virus replication and on the innate defense mechanisms of the cell. (3) Results: The incubation of cells with DBs at the time of infection had little effect on viral genome replication. Preincubation of DBs, however, led to a marked reduction in viral release from infected cells. These cells showed an enhancement of the cytopathic effect, associated with a moderate increase in early apoptosis. Despite virus-induced mechanisms to limit the interferon response, the induction of interferon-regulated genes (IRGs) was upregulated by DB treatment. (4) Conclusions: DBs sensitize cells against viral infection, comparable to the effects of interferons. The activities of these particles need to be considered when studying viral-host interaction.

Subviral Dense Bodies of Human Cytomegalovirus Induce an Antiviral Type I Interferon Response

(1) Background: Cells infected with the human cytomegalovirus (HCMV) produce subviral particles, termed dense bodies (DBs), both in-vitro and in-vivo. They are released from cells, comparable to infectious virions, and are enclosed by a membrane that resembles the viral envelope and mediates the entry into cells. To date, little is known about how the DB uptake influences the gene expression in target cells. The purpose of this study was to investigate the impact of DBs on cells, in the absence of a viral infection. (2) Methods: Mass spectrometry, immunoblot analyses, siRNA knockdown, and a CRISPR-CAS9 knockout, were used to investigate the changes in cellular gene expression following a DB exposure; (3) Results: A number of interferon-regulated genes (IRGs) were upregulated after the fibroblasts and endothelial cells were exposed to DBs. This upregulation was dependent on the DB entry and mediated by the type I interferon signaling through the JAK-STAT pathway. The induction of IRGs was mediated by the sensing of the DB-introduced DNA by the pattern recognition receptor cGAS. (4) Conclusions: The induction of a strong type I IFN response by DBs is a unique feature of the HCMV infection. The release of DBs may serve as a danger signal and concomitantly contribute to the induction of a strong, antiviral immune response.

An Attenuated Strain of Human Cytomegalovirus for the Establishment of a Subviral Particle Vaccine

Human cytomegalovirus (HCMV) infection is associated with severe disease conditions either following congenital transmission of the virus or viral reactivation in immunosuppressed individuals. Consequently, the establishment of a protective vaccine is of high medical need. Several candidates have been tested in preclinical and clinical studies, yet no vaccine has been licensed. Subviral dense bodies (DB) are a promising vaccine candidate. We have recently provided a GMP-compliant protocol for the production of DB, based on a genetically modified version of the HCMV laboratory strain Towne, expressing the pentameric complex of envelope protein gH-gL-pUL128-131 (Towne-UL130rep). In this work, we genetically attenuated Towne-UL130rep by abrogating the expression of the tegument protein pUL25 and by fusing the destabilizing domain ddFKBP to the N-terminus of the IE1- and IE2-proteins of HCMV. The resulting strain, termed TR-VAC, produced high amounts of DB under IE1/IE2 repressive conditions and concomitant supplementation of the viral terminase inhibitor letermovir to the producer cell culture. TR-VAC DB retained the capacity to induce neutralizing antibodies. A complex pattern of host protein induction was observed by mass spectrometry following exposure of primary human monocytes with TR-VAC DB. Human monocyte-derived dendritic cells (DC) moderately increased the expression of activation markers and MHC molecules upon stimulation with TR-VAC DB. In a co-culture with autologous T cells, the TR-VAC DB-stimulated DC induced a robust HCMV-specific T cell-activation and –proliferation. Exposure of donor-derived monocytic cells to DB led to the activation of a rapid innate immune response. This comprehensive data set thus shows that TR-VAC is an optimal attenuated seed virus strain for the production of a DB vaccine to be tested in clinical studies.

Identification and Characterization of Epithelial Cell-Derived Dense Bodies Produced upon Cytomegalovirus Infection

Dense bodies (DB) are complex, noninfectious particles produced during CMVinfection containing envelope and tegument proteins that may be ideal candidates as vaccines. Although DB were previously described in fibroblasts, no evidence of DB formation has been shown after propagating CMV in epithelial cells. In the present study, both fibroblast MRC-5 and epithelial ARPE-19 cells were used to study DB production during CMV infection. We demonstrate the formation of epithelial cell-derived DB, mostly located as cytoplasmic inclusions in the perinuclear area of the infected cell. DB were gradient-purified, and the nature of the viral particles was confirmed using CMV-specific immunelabeling. Epithelial cell-derived DB had higher density and more homogeneous size (200-300 nm) compared to fibroblast-derived DB (100-600 nm).In agreement with previous results characterizing DB from CMV-infected fibroblasts, the pp65 tegument protein was predominant in the epithelial cell-derived DB. Our results also suggest that epithelial cells had more CMV capsids in the cytoplasm and had spherical bodies compatible with nucleus condensation (pyknosis) in cells undergoing apoptosis that were not detected in MRC-5 infected cells at the tested time post-infection. Our results demonstrate the formation of DB in CMV-infected ARPE-19 epithelial cells that may be suitable candidate to develop a multiprotein vaccine with antigenic properties similar to that of the virions while not including the viral genome.

Production- and Purification-Relevant Properties of Human and Murine Cytomegalovirus

There is a large unmet need for a prophylactic vaccine against human cytomegalovirus (HCMV) to combat the ubiquitous infection that is ongoing with this pathogen. A vaccination against HCMV could protect immunocompromised patients and prevent birth defects caused by congenital HCMV infections. Moreover, cytomegalovirus (CMV) has a number of features that make it a very interesting vector platform for gene therapy. In both cases, preparation of a highly purified virus is a prerequisite for safe and effective application. Murine CMV (MCMV) is by far the most studied model for HCMV infections with regard to the principles that govern the immune surveillance of CMVs. Knowledge transfer from MCMV and mice to HCMV and humans could be facilitated by better understanding and characterization of the biological and biophysical properties of both viruses. We carried out a detailed investigation of HCMV and MCMV growth kinetics as well as stability under the influence of clarification and different storage conditions. Further, we investigated the possibilities to concentrate and purify both viruses by ultracentrifugation and ion-exchange chromatography. Defective enveloped particles were not separately analyzed; however, the behavior of exosomes was examined during all experiments. The effectiveness of procedures was monitored using CCID₅₀ assay, Nanoparticle tracking analysis, ELISA for host cell proteins, and quantitative PCR for host cell DNA. MCMV generally proved to be more robust in handling. Despite its greater sensitivity, HCMV was efficiently (100% recovery) purified and concentrated by anion-exchange chromatography using QA monolithic support. The majority of the host genomic DNA as well as most of the host cell proteins were removed by this procedure.

SAMHD1 phosphorylation and cytoplasmic relocalization after human cytomegalovirus infection limits its antiviral activity

SAMHD1 is a host restriction factor that functions to restrict both retroviruses and DNA viruses, based on its nuclear deoxynucleotide triphosphate (dNTP) hydrolase activity that limits availability of intracellular dNTP pools. In the present study, we demonstrate that SAMHD1 expression was increased following human cytomegalovirus (HCMV) infection, with only a modest effect on infectious virus production. SAMHD1 was rapidly phosphorylated at residue T592 after infection by cellular cyclin-dependent kinases, especially Cdk2, and by the viral kinase pUL97, resulting in a significant fraction of phosho-SAMHD1 being relocalized to the cytoplasm of infected fibroblasts, in association with viral particles and dense bodies. Thus, our findings indicate that HCMV-dependent SAMHD1 cytoplasmic delocalization and inactivation may represent a potential novel mechanism of HCMV evasion from host antiviral restriction activities.

Production Strategies for Pentamer-Positive Subviral Dense Bodies as a Safe Human Cytomegalovirus Vaccine

Infections with the human cytomegalovirus (HCMV) are associated with severe clinical manifestations in children following prenatal transmission and after viral reactivation in immunosuppressed individuals. The development of an HCMV vaccine has long been requested but there is still no licensed product available. Subviral dense bodies (DB) are immunogenic in pre-clinical models and are thus a promising HCMV vaccine candidate. Recently, we established a virus based on the laboratory strain Towne that synthesizes large numbers of DB containing the pentameric protein complex gH/gL/UL128-131 (Towne-UL130repΔGFP). The work presented here focuses on providing strategies for the production of a safe vaccine based on that strain. A GMP-compliant protocol for DB production was established. Furthermore, the DB producer strain Towne-UL130rep was attenuated by deleting the UL25 open reading frame. Additional genetic modifications aim to abrogate its capacity to replicate in vivo by conditionally expressing pUL51 using the Shield-1/FKBP destabilization system. We further show that the terminase inhibitor letermovir can be used to reduce infectious virus contamination of a DB vaccine by more than two orders of magnitude. Taken together, strategies are provided here that allow for the production of a safe and immunogenic DB vaccine for clinical testing.

Dense Bodies of a gH/gL/UL128/UL130/UL131 Pentamer-Repaired Towne Strain of Human Cytomegalovirus Induce an Enhanced Neutralizing Antibody Response

The development of a vaccine against human cytomegalovirus infection (HCMV) is a high-priority medical goal. The viral pentameric protein complex consisting of glycoprotein H (gH)/gL/UL128-131A (PC) is considered to be an important vaccine component. Its relevance to the induction of a protective antibody response is, however, still a matter of debate. We addressed this issue by using subviral dense bodies (DBs) of HCMV. DBs are exceptionally immunogenic. Laboratory HCMV strain DBs harbor important neutralizing antibody targets, like the glycoproteins B, H, L, M, and N, but they are devoid of the PC. To be able to directly compare the impact of the PC on the levels of neutralizing antibody (NT-abs) responses, a PC-positive variant of the HCMV laboratory strain Towne was established by bacterial artificial chromosome (BAC) mutagenesis (Towne-UL130rep). This strain synthesized PC-positive DBs upon infection of fibroblasts. These DBs were used in side-by-side immunizations with PC-negative Towne DBs. Mouse and rabbit sera were tested to address the impact of the PC on DB immunogenicity. The neutralizing antibody response to PC-positive DBs was superior to that of PC-negative DBs, as tested on fibroblasts, epithelial cells, and endothelial cells and for both animal species used. The experiments revealed the potential of the PC to enhance the antibody response against HCMV. Of particular interest was the finding that PC-positive DBs induced an antibody response that blocked the infection of fibroblasts by a PC-positive viral strain more efficiently than sera following immunizations with PC-negative particles.IMPORTANCE Infections with the human cytomegalovirus (HCMV) may cause severe and even life-threatening disease manifestations in newborns and immunosuppressed individuals. Several strategies for the development of a vaccine against this virus are currently pursued. A critical question in this respect refers to the antigenic composition of a successful vaccine. Using a subviral particle vaccine candidate, we show here that one protein complex of HCMV, termed the pentameric complex (PC), enhances the neutralizing antibody response against viral infection of different cell types. We further show for the first time that this not only relates to the infection of epithelial or endothelial cells; the presence of the PC in the particles also enhanced the neutralizing antibody response against the infection of fibroblasts by HCMV. Together, these findings argue in favor of including the PC in strategies for HCMV vaccine development.

Virus-Like Vesicles of Kaposi's Sarcoma-Associated Herpesvirus Activate Lytic Replication by Triggering Differentiation Signaling

Virus-like vesicles (VLVs) are membrane-enclosed vesicles that resemble native enveloped viruses in organization but lack the viral capsid and genome. During the productive infection of tumor-associated gammaherpesviruses, both virions and VLVs are produced and are released into the extracellular space. However, studies of gammaherpesvirus-associated VLVs have been largely restricted by the technical difficulty of separating VLVs from mature virions. Here we report a strategy of selectively isolating VLVs by using a Kaposi's sarcoma-associated herpesvirus (KSHV) mutant that is defective in small capsid protein and is unable to produce mature virions. Using mass spectrometry analysis, we found that VLVs contained viral glycoproteins required for cellular entry, as well as tegument proteins involved in regulating lytic replication, but lacked capsid proteins. Functional analysis showed that VLVs induced the expression of the viral lytic activator RTA, initiating KSHV lytic gene expression. Furthermore, employing RNA sequencing, we performed a genomewide analysis of cellular responses triggered by VLVs and found that PRDM1, a master regulator in cell differentiation, was significantly upregulated. In the context of KSHV replication, we demonstrated that VLV-induced upregulation of PRDM1 was necessary and sufficient to reactivate KSHV by activating its RTA promoter. In sum, our study systematically examined the composition of VLVs and demonstrated their biological roles in manipulating host cell responses and facilitating KSHV lytic replication.IMPORTANCE Cells lytically infected with tumor-associated herpesviruses produce a high proportion of virus-like vesicles (VLVs). The composition and function of VLVs have not been well defined, largely due to the inability to efficiently isolate VLVs that are free of virions. Using a cell system capable of establishing latent KSHV infection and robust reactivation, we successfully isolated VLVs from a KSHV mutant defective in the small capsid protein. We quantitatively analyzed proteins and microRNAs in VLVs and characterized the roles of VLVs in manipulating host cells and facilitating viral infection. More importantly, we demonstrated that by upregulating PRDM1 expression, VLVs triggered differentiation signaling in targeted cells and facilitated viral lytic infection via activation of the RTA promoter. Our study not only demonstrates a new strategy for isolating VLVs but also shows the important roles of KSHV-associated VLVs in intercellular communication and the viral life cycle.

Human cytomegalovirus microRNAs are carried by virions and dense bodies and are delivered to target cells

Human cytomegalovirus (HCMV) infection results in the production of virions, dense bodies (DBs) and non-infectious enveloped particles, all of which incorporate proteins and RNAs that can be transferred to host cells. Here, we investigated whether virions and DBs also carry microRNAs (miRNAs) and assessed their delivery and functionality in cells. Human lung fibroblasts (MRC-5) were infected with the HCMV strain AD169, and conditioned cell culture medium was collected and centrifuged. The pellets were treated with RNase-ONE, and the virions and DBs were purified with a potassium tartrate–glycerol gradient and dialysed. The virions and DBs were incubated with micrococcal nuclease, DNA and RNA were extracted and then analysed with TaqMan PCR assays, while the proteins were examined with Western blots. To assess the delivery of miRNAs to cells and their functionality, virions and DBs were irradiated with UV light. The purity of the virions and DBs was confirmed by typical morphology, the presence of the structural protein pp65 and the HCMV genome, the ability to infect MRC-5 cells and the absence of the host genome. RNA analysis revealed the presence of 14 HCMV-encoded miRNAs (UL22A-5p, US25-1-5p, UL22A-3p, US5-2-3p, UL112-3p, US25-2-3p, US25-2-5p, US33-3p, US5-1, UL36-5p, US4-5p, UL36-3p, UL70-5p and US25-1-3p), HCMV immediate-early mRNA and long non-coding RNA2.7, moreover, two host-encoded miRNAs (hsa-miR-218-5p and hsa-miR-21-5p) and beta-2-microglobulin RNA. UV-irradiated virions and DBs delivered viral miRNAs (US25-1-5p and UL112-3p) to the host cells, and miR-US25-1-5p was functional in a luciferase reporter assay. We conclude that virions and DBs carry miRNAs that are biologically functional and can be delivered to cells, which may affect cellular processes.

Basic Biological Sciences

Cytomegaloviruses undergo the classical morphologic stages of replication in hedgehog submandibular gland duct cells. A profound cytomegaly is associated with the infection. These features indicate that the hedgehog may be an excellent model in which to study the biology of cytomegalovirus-salivary gland interactions.

Use of flow cytometry for characterization of human cytomegalovirus vaccine particles

Despite a 40-year effort, an effective vaccine against human cytomegalovirus (HCMV) remains an unmet medical need. The discovery of potent neutralizing epitopes on the pentameric gH complex (gH/gL/UL128/130/131) has reenergized HCMV vaccine development. Our whole-virus vaccine candidate, currently in a Phase I clinical trial, is based on the attenuated AD169 strain with restored expression of the pentameric gH complex. Given the complexity of a whole-virus vaccine, improved analytical methods have been developed to better characterize heterogeneous viral particles released from infected cells during vaccine production. Here we show the utility of a commercial flow cytometer for the detection of individual HCMV particles, either via light scattering or using fluorescence after labeling of specific antigens. Rapid measurements requiring minimal material provide near real-time information on particle concentration, distributions of different particle types, and product purity. Additionally, utilizing immunoreagents has allowed us to characterize the distribution of key antigens across individual particles and particle types.

Morphologic differentiation of viruses beyond the family level

Electron microscopy has been instrumental in the identification of viruses by being able to characterize a virus to the family level. There are a few cases where morphologic or morphogenesis factors can be used to differentiate further, to the genus level. These include viruses in the families Poxviridae, Reoviridae, Retroviridae, Herpesviridae, Filoviridae, and Bunyaviridae.

Role of human cytomegalovirus tegument proteins in virion assembly

Like other herpesviruses, human cytomegalovirus (HCMV) contains a unique proteinaceous layer between the virion envelope and capsid, termed the tegument. Upon infection, the contents of the tegument layer are delivered to the host cell, along with the capsid and the viral genome, where they facilitate the initial stages of virus replication. The tegument proteins also play important roles in virion assembly and this dual nature makes them attractive potential targets for antiviral therapies. While our knowledge regarding tegument protein function during the initiation of infection has been the subject of intense study, their roles in assembly are much less well understood. In this review, we will focus on recent studies that highlight the functions of HCMV tegument proteins during assembly, and pose key questions for further investigation.

Development of a high-throughput assay to measure the neutralization capability of anti-cytomegalovirus antibodies

Infection by human cytomegalovirus (CMV) elicits a strong humoral immune response and robust anti-CMV antibody production. Diagnosis of virus infection can be carried out by using a variety of serological assays; however, quantification of serum antibodies against CMV may not present an accurate measure of a patient's ability to control a virus infection. CMV strains that express green fluorescent protein (GFP) fusion proteins can be used as screening tools for evaluating characteristics of CMV infection in vitro. In this study, we employed a CMV virus strain, AD169, that ectopically expresses a yellow fluorescent protein (YFP) fused to the immediate-early 2 (IE2) protein product (AD169IE2-YFP) to quantify a CMV infection in human cells. We created a high-throughput cell-based assay that requires minimal amounts of material and provides a platform for rapid analysis of the initial phase of virus infection, including virus attachment, fusion, and immediate-early viral gene expression. The AD169IE2-YFP cell infection system was utilized to develop a neutralization assay with a monoclonal antibody against the viral surface glycoprotein gH. The high-throughput assay was extended to measure the neutralization capacity of serum from CMV-positive subjects. These findings describe a sensitive and specific assay for the quantification of a key immunological response that plays a role in limiting CMV dissemination and transmission. Collectively, we have demonstrated that a robust high-throughput infection assay can analyze the early steps of the CMV life cycle and quantify the potency of biological reagents to attenuate a virus infection.

Cytomegalovirus UL103 controls virion and dense body egress

Human cytomegalovirus UL103 encodes a tegument protein that is conserved across herpesvirus subgroups. Mutant viruses lacking this gene product exhibit dramatically reduced accumulation of cell-free virus progeny and poor cell-to-cell spread. Given that viral proteins and viral DNA accumulate with normal kinetics in cells infected with mutant virus, UL103 appears to function during the late phase of replication, playing a critical role in egress of capsidless dense bodies and virions. Few dense bodies were observed in the extracellular space in mutant virus-infected cells in the presence or absence of the DNA encapsidation inhibitor 2-bromo-5,6-dichloro-1-(β-d-ribofuranosyl)benzimidazole. Upon reversal of encapsidation inhibition, UL103 had a striking impact on accumulation of cell-free virus, but not on accumulation of cell-associated virus. Thus, UL103 plays a novel and important role during maturation, regulating virus particle and dense body egress from infected cells.

Identification of putative functional motifs in viral proteins essential for human cytomegalovirus DNA replication

Six of the eleven genes essential for Human cytomegalovirus (HCMV) DNA synthesis have been analyzed for putative structural motifs that may have a significant functional role in DNA replication. The genes studied encode for the DNA polymerase accessory protein (UL44), single-stranded DNA binding protein (UL57), primase-helicase complex (UL70, UL102, and UL105), and the putative initiator protein (UL84). The full-length open reading frames of these genes were highly conserved between ten isolates with amino acid sequence identity of >97% for all genes. Using ScanProsite software from the Expert Protein Analysis System (ExPASy) proteomics server, we have mapped putative motifs throughout these HCMV replication genes. Interesting motifs identified include casein kinase-2 (CKII) phosphorylation sites, a microbodies signal motif in UL57, and an ATP binding site in the putative UL105 helicase. Our investigations have also elucidated motif-rich regions of the UL44 DNA polymerase accessory protein and identified cysteine motifs that have potential implications for UL57 and UL70 primase. Taken together, these findings provide insights to regions of these HCMV replication proteins that are important for post-translation modification, activation, and overall function, and this information can be utilized to target further research into these proteins and advance the development of novel antiviral agents that target these processes.

Exogenous introduction of an immunodominant peptide from the non-structural IE1 protein of human cytomegalovirus into the MHC class I presentation pathway by recombinant dense bodies

Exogenous introduction of particle-associated proteins of human cytomegalovirus (HCMV) into the major histocompatibility complex (MHC) class I presentation pathway by subviral dense bodies (DB) is an effective way to sensitize cells against CD8 T-cell (CTL) recognition and killing. Consequently, these particles have been proposed as a platform for vaccine development. We have developed a strategy to refine the antigenic composition of DB. For proof of principle, an HCMV recombinant (RV-VM3) was generated that encoded the immunodominant CTL determinant IE1TMY from the IE1 protein in fusion with the major constituent of DB, the tegument protein pp65. To generate RV-VM3, a bacterial artificial chromosome containing the HCMV genome was modified by applying positive/negative selection based on the expression of the bacterial galactokinase in conjunction with lambda Red-mediated homologous recombination. This method allowed the efficient and seamless insertion of the DNA sequence encoding IE1TMY in frame into the pp65 open reading frame (UL83) of the viral genome. RV-VM3 expressed its fusion protein to high levels. The fusion protein was packaged into DB and into virions. Its delivery into fibroblasts by these viral particles led to the loading of the MHC class I presentation pathway with IE1TMY and to efficient killing by specific CTLs. This demonstrated that a heterologous peptide, not naturally present in HCMV particles, can be processed from a recombinant, DB-derived protein to be subsequently presented by MHC class I. The results presented here provide a rationale for the optimization of a vaccine based on recombinant DB.

A guide to viral inclusions, membrane rearrangements, factories, and viroplasm produced during virus replication

Virus replication can cause extensive rearrangement of host cell cytoskeletal and membrane compartments leading to the “cytopathic effect” that has been the hallmark of virus infection in tissue culture for many years. Recent studies are beginning to redefine these signs of viral infection in terms of specific effects of viruses on cellular processes. In this chapter, these concepts have been illustrated by describing the replication sites produced by many different viruses. In many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated platforms in the cell that concentrate replicase proteins, virus genomes, and host proteins required for replication, and thereby increase the efficiency of replication. Interestingly, these same structures, called virus factories, virus inclusions, or virosomes, can recruit host components that are associated with cellular defences against infection and cell stress. It is possible that cellular defence pathways can be subverted by viruses to generate sites of replication. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release.

L-particle production during primary replication of pseudorabies virus in the nasal mucosa of swine

Different tissue culture cell lines infected with a number of alphaherpesviruses produce, in addition to virions, light particles (L particles). L particles are composed of the envelope and tegument components of the virion but totally lack the proteins of the capsid and the virus genome; therefore, they are noninfectious. In this electron microscopy report, we show that L particles are produced during primary replication of the alphaherpesvirus pseudorabies virus (PRV) in the nasal mucosa of experimentally infected swine, its natural host. Although PRV infected different types of cells of the respiratory and olfactory mucosae, PRV L particles were found to be produced exclusively by epithelial cells and fibroblasts. We observed that formation of noninfectious particles occurred by budding of condensed tegument at the inner nuclear membrane and at membranes of cytoplasmic vesicles, resulting in intracisternal and intravesicular L particles, respectively. Both forms of capsidless particles were clearly distinguishable by the presence of prominent surface projections on the envelope and the higher electron density of the tegument, morphological features which were only observed in intravesicular L particles. Moreover, intravesicular but not intracisternal L particles were found to be released by exocytosis and were also identified extracellularly. Comparative analysis between PRV virion and L-particle morphogenesis indicates that both types of virus particles share a common intracellular pathway of assembly and egress but that they show different production patterns during the replication cycle of PRV.

Experimental preemptive immunotherapy of murine cytomegalovirus disease with CD8 T-cell lines specific for ppM83 and pM84, the two homologs of human cytomegalovirus tegument protein ppUL83 (pp65)

CD8 T cells are the principal antiviral effectors controlling cytomegalovirus (CMV) infection. For human CMV, the virion tegument protein ppUL83 (pp65) has been identified as a source of immunodominant peptides and is regarded as a candidate for cytoimmunotherapy and vaccination. Two sequence homologs of ppUL83 are known for murine CMV, namely the virion protein ppM83 (pp105) expressed late in the viral replication cycle and the nonstructural protein pM84 (p65) expressed in the early phase. Here we show that ppM83, unlike ppUL83, is not delivered into the antigen presentation pathway after virus penetration before or in absence of viral gene expression, while other virion proteins of murine CMV are processed along this route. In cytokine secretion-based assays, ppM83 and pM84 appeared to barely contribute to the acute immune response and to immunological memory. Specifically, the frequencies of M83 and M84 peptide-specific CD8 T cells were low and undetectable, respectively. Nonetheless, in a murine model of cytoimmunotherapy of lethal CMV disease, M83 and M84 peptide-specific cytolytic T-cell lines proved to be highly efficient in resolving productive infection in multiple organs of cell transfer recipients. These findings demonstrate that proteins which fail to prime a quantitatively dominant immune response can nevertheless represent relevant antigens in the effector phase. We conclude that quantitative and qualitative immunodominance are not necessarily correlated. As a consequence of these findings, there is no longer a rationale for considering T-cell abundance as the key criterion for choosing specificities to be included in immunotherapy and immunoprophylaxis of CMV disease and of viral infections in general.

Dense bodies of human cytomegalovirus induce both humoral and cellular immune responses in the absence of viral gene expression

Infection of fibroblast cell cultures with human cytomegalovirus (HCMV) leads to the production of significant amounts of defective enveloped particles, termed dense bodies (DB). These noninfectious structures contain major antigenic determinants which are responsible for induction of both the humoral and the cellular immune response against HCMV. We tested the hypothesis that, by virtue of their unique antigenic and structural properties, DB could induce a significant immune response in the absence of infectious virus. Mice were immunized with gradient-purified DB, which were either left untreated or subjected to sequential rounds of sonication and freeze-thawing to prevent cellular entry. Titers of neutralizing antibodies induced by DB were in a range comparable to levels present in convalescent human sera. The virus-neutralizing antibody response was surprisingly durable, with neutralizing antibodies detected 12 months following primary immunization. The HCMV-specific major histocompatibility complex class I-restricted cytolytic T-cell (CTL) response was assayed using mice transgenic for the human HLA-A2 molecule. Immunization with DB led to high levels of HCMV-specific CTL in the absence of de novo viral protein synthesis. Maximal total cytolytic activity in mice immunized with DB was nearly as efficient as the cytolytic activity induced by a standard immunization with murine cytomegalovirus. Furthermore, DB induced a typical T-helper 1 (Th1)-dominated immune response in mice, as determined by cytokine and immunoglobulin G isotype analysis. Induction of humoral and cellular immune responses was achieved without the concomitant use of adjuvant. We thus propose that DB can serve as a basis for the future development of a recombinant nonreplicating vaccine against HCMV. Finally, such particles could be engineered for efficient delivery of antigens from other pathogens to the immune system.

Human cytomegalovirus nuclear and cytoplasmic dense bodies

One of the characteristic features of cytomegalovirus (CMV) replication is the formation of cytoplasmic dense bodies. Recent findings revealed similar structures also in the nuclei of CMV-infected cells. By transmission electron microscopy, immuno electronmicroscopy, and cytochemistry, we have studied the morphogenetic steps and macromolecular composition of both structures. Our results show that both structures contain DNA, RNA and viral antigenic proteins. Nuclear dense bodies are probably an expression of a stimulated cellular metabolism, while cytoplasmic dense bodies may represent the site where surplus cellular and viral molecules are stored before being eliminated.

Successful treatment of disseminated cutaneous cytomegalic inclusion disease associated with Hodgkin's disease

A case of disseminated cutaneous cytomegalic inclusion disease associated with Hodgkin's disease is described. The patient had a diffuse eruption of pruritic, erosive erythematous nodules. Histologically, many inclusion bodies were observed in perivascular areas of the skin lesions. Immunohistologically, the inclusion bodies positively stained with both anticytomegalovirus antibody and anti-factor VIII-related antibody. On electron microscopy many virus particles and dense bodies were found in the area where inclusion bodies were observed. Treatment with high-dose intravenous acyclovir and a large amount of immunoglobulin resulted in prompt healing of the skin lesions. Subsequently, the patient's Hodgkin's disease was well controlled on chemotherapy. The patient steadily improved without relapse of skin lesions 1 year after antiviral therapy was administered.

Identification and characterization of a human cytomegalovirus gene coding for a membrane protein that is conserved among human herpesviruses

A rabbit antiserum was raised against envelope material from purified human cytomegalovirus strain AD169. The serum recognized polypeptides 200, 170, 160, 75, 58, and 45 kilodaltons in size. It was used to screen a cDNA library constructed from poly(A)+ RNA from human cytomegalovirus-infected cells in the expression vector lambda gt11. A recombinant bacteriophage expressing cytomegalovirus-specific sequences was identified, and the corresponding gene was mapped to the HindIII R fragment. The gene is transcribed into a late 1.5-kilobase RNA. The nucleotide sequence of the coding region was determined. Computer analysis of the gene product revealed a polypeptide containing multiple potential membrane-spanning domains, representing a type of protein not identified in the envelope of herpesviruses before. The protein shows homology on the amino acid level to hypothetical proteins from reading frames BBRF3 of Epstein-Barr virus, UL10 of herpes simplex virus type 1, and ORF50 of varicella-zoster virus. By using an antiserum raised against procaryote-expressed parts of the cytomegalovirus membrane protein, a 45-kilodalton structural component of the virus was identified as the gene product.

Staining qualities of cytomegalovirus inclusions in the lungs of patients with the acquired immunodeficiency syndrome: a potential source of diagnostic misinterpretation

Gomori's methenamine silver and the periodic acid--Schiff reaction stain cytoplasmic but not intranuclear cytomegalovirus inclusions. The biochemical basis of these staining qualities is discussed, and the staining characteristics are correlated with supporting ultrastructural observations. Failure to recognize this phenomenon may lead to erroneous diagnostic conclusions at the light microscopic level.

Anti-complement immunofluorescence establishes nuclear localization of human cytomegalovirus matrix protein

A monospecific, polyclonal antiserum to the 69-kDa matrix protein of human cytomegalovirus (HCMV) was prepared in a guinea pig and used to determine the intracellular distribution of this viral antigen. The resulting antiserum was specific for infected cells as tested by immunofluorescence, and specific for the HCMV matrix protein as determined by "nitrocellulose immunoassay" of electrophoretically separated, infected-cell proteins. Antibodies were reacted with fixed, infected human fibroblasts, and visualized by the anti-complement immunofluorescence procedure to avoid complications arising from the strong IgG Fc binding activity of the infected-cell-specific cytoplasmic inclusion. Results establish that the matrix protein is located in the nucleus, and indicate that it is concentrated in the nucleoplasm rather than within the intranuclear inclusions.

Epidermal hyperplasias of northern pike (Esox lucius) associated with herpesvirus and C-type particles

Northern pike from several locations in central Canada were observed to have two different types of hyperplastic epidermal lesions on the body and fins. One type of epidermal hyperplasia consisted of flat lesions, bluish-white in color with a granular or "gritty" appearance. Histological examination of this granular lesion showed the presence of grossly hypertrophied cells surrounded by normal sized epidermal cells. The enlarged nuclei in thin section contained many typical herpesvirus capsids measuring 100 nm in diameter. A large proportion of the cytoplasmic mass consisted of many dark staining inclusions in which were embedded numerous herpes-like virions. For this herpesvirus of pike the name esocid herpesvirus 1 is proposed in keeping with the herpesvirus nomenclature of Roizman et al. (19). The second type of epidermal hyperplasia was a smooth convex whitish translucent tissue mass consisting largely of a population of randomly oriented normal sized undifferentiated cells. Electron microscopic examination showed them to be associated with clusters of C-type retrovirus measuring 150 nm in diameter. The formation of these virus particles was by budding from the cytoplasmic membrane into the inter-cellular spaces. Neither the herpesvirus nor the C-type particles could be isolated in fish cell cultures.

Isolation and characterization of a noninfectious virion-like particle released from cells infected with human strains of cytomegalovirus

Three types of virus particles have been recovered from the culture medium of human foreskin fibroblasts infected with human strains of cytomegalovirus (HCMV). Two of these, virions and dense bodies, are routinely observed and have been described by others. The third, produced in lesser amounts, has not been previously characterized. This particle, separable from virions by rate-velocity sedimentation, is morphologically distinguished from them only by core structure. Radiolabeling and biological assays have established that these particles, like dense bodies, lack DNA and are not infectious. Based on these properties, we have designated this virion-like structure as a noninfectious enveloped particle (NIEP). Comparisons of the protein constituents of these three particles has shown that dense bodies have the simplest composition. Approximately 95% of their protein mass is represented by a 69,000 Da (69K) matrix-like protein. While dense bodies appear to have a normal complement of virion glycoproteins, they completely lack other predominant virion species. The protein compositions of virions and NIEPs are more complex than that of dense bodies, and are distinguished from one another by the presence in NIEPs of a 35,000 Da (35K) protein absent from the two other particles. Biosynthetic radiolabeling and cell fractionation experiments have demonstrated that this 35K protein is produced only in infected cells, is phosphorylated and partitions with the nuclear fraction. These and other results suggest that this protein is the HCMV counterpart of the previously described B-capsid proteins VP22a of herpes simplex and 37K of CMV (strain Colburn). NIEPs are produced by all HCMV strains examined and have not been observed in preparations of herpes simplex virus- or Old World monkey CMV-infected cells. Although this particle is generally present in much lower amounts than virions, strain AD169 overproduces NIEPs by approximately 10-fold. We have also found that the additional NIEP protein of AD169 has an apparently larger size (i.e., 36K) than the corresponding protein of other strains. The correlation between AD169 NIEP overproduction and its altered protein suggests that the two may be causally related.

Chorioretinal biopsy in a patient with leukaemia

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Comparison of human cytomegalovirus growth in MRC-5 human fibroblasts, brain, and choroid plexus cells in vitro

Cell cultures derived from human brain, choroid plexus, and human lung fibroblasts (MRC-5) were infected with the Towne strain of human cytomegalovirus (CMV). The cytopathic effect, beginning 24-48 hours after infection, was characterized by foci of enlarged rounded cells that spread slowly and eventually coalesced to destroy the entire monolayer within one week. Cowdry type A inclusion bodies and herpes virus nucleocapsids were seen in infected cells. CMV-specific antigen was demonstrated by immunofluorescence in fibroblasts and astrocytic cells of brain cultures and in cells of choroid plexus cultures as well as in MRC-5 fibroblasts. Despite these morphologic and immunochemical similarities the growth of CMV differed in cells of brain and choroid plexus origin as compared with MRC-5 cells. In brain and choroid plexus cell cultures most of the virus remained cell-associated throughout the observation period of one week, whereas in MRC-5 cells the CMV was found in both cell-associated and cell-free fractions of harvested material.

Ultrastructural development and persistence of guinea pig cytomegalovirus in duet cells of guinea pig submaxillary gland

Salivary glands from Hartley guinea pigs were experimentally infected with guinea pig cytomegalovirus (GPCMV) and examined by light and electron microscopy at different time intervals. Characteristic intranuclear and intracytoplasmic viral inclusions were observed in duct cells of infected animals. Viral inclusion counts and infectivity titers in the salivary gland reached maximum levels by 3 to 4 weeks after infection; infectivity persisted, though at reduced levels, for at least 30 weeks. Electron microscopic examination of viral inclusions revealed several developmental events including nucleocapsid assembly, envelopment of nucleocapsids at the inner nuclear membrane and their enclosure by a thin vacuolar membrane. While contained within cytoplasmic vacuoles, enveloped virions acquired surface spikes. Cytoplasmic vacuoles containing virions subsequently coalesced and discharged mature virions at the cell surface into the lumen of the salivary gland duct. The data indicate that the ultrastructural development of GPCMV in the guinea pig salivary gland shows many similarities to that of human cytomegalovirus in humans. The salivary gland may provide a primary locus for virus shedding and horizontal transmission of cytomegalovirus.

Humoral immune response to virions and dense bodies of human cytomegalovirus determined by enzyme immunofluorescence assay

An enzyme immunoassay utilizing a fluorogenic substrate (EIFA) was employed to compare humoral antibody responses to purified virion and dense body antigens in human cytomegalovirus (CMV) infections. Results of these antibody assays were also compared to those obtained by plaque reduction neutralization and complement fixation (CF). In adults with CMV infections diagnosed by a significant rise in CF antibody titer, there was also a vigorous antibody response to the virion and dense body antigens. There was better correlation between antibody levels to the virion and dense body antigens than between antibody levels detected by other combinations of tests. Antibody responses to the virion and dense body antigens were similar, but not identical, suggesting that although the two structures share major antigens, they may also possess unique antigens to which the host can mount independent antibody responses. All individuals with CMV neutralizing antibody also had virion and dense body antibodies, but there was poor correlation between neutralization titers and those detected by EIFA, perhaps reflecting greater strain specificity of the neutralization test. Some individuals with serological diagnoses of Mycoplasma pneumoniae or influenza A infection showed fourfold or greater antibody titer rises to CMV in two or more assays, suggesting reactivation of latent CMV in these infections.

Persistence of the cytomegalovirus genome in human cells

A small percentage of human fibroblast cells survived high-multiplicity infection by cytomegalovirus and were isolated as persistently infected cultures. Approximately 30% of the cells were in the productive phase of infection, since virus-specific structural antigens and virions were associated with these cells. The remaining cells contained neither viral structural antigens nor particles. Nuclear DNA from these nonproductive cells contained approximately 120 genome equivalents of viral DNA per cell as determined by reassociation kinetics. In situ hybridization confirmed that nuclei from nonproductive cells contained a significant amount of viral DNA that was distributed in most of these cells. Early virus-induced proteins and antigens were also detected. Nonproductive cells continued to grow, and there was a slow, spontaneous transition of some of these cells to productive viral replication. The majority of the viral DNA in nonproductive cells persisted with restricted gene expression. When infectious virus production was eliminated by growing the persistently infected cultures in the presence of anticytomegalovirus serum, approximately 45 genome equivalents of the viral DNA persisted per cell. The reassociation reaction approached completion. After removal of the antiserum and subculturing, infectious virus production resumed. Therefore, it was assumed that all sequences of the viral genome remained associated with these cells. Restriction of cytomegalovirus gene expression in persistently infected cell cultures is discussed.

Stimulation of human lymphocytes by cytomegalovirions and dense bodies

Lymphocytes from healthy adult individuals were examined for their ability to incorporate thymidine in the presence of cytomegalovirus (CMV) and dense bodies. It was found that lymphocytes from donors with antibodies to CMV were stimulated to incorporate thymidine-14C both by preparation of CMV and CMV dense bodies. Lymphocytes from CMV seronegative donors did not respond. Dilution experiments and the dose-response curve indicate that the stimulation induced by the dense body preparation was not caused by the small amounts of contaminating CMV particles. These results indicate that in healthy human adults there is a correlation between CMV seropositivity and in vitro lymphocyte transformation, induced by either CMV or by dense bodies.

Biological autoxidation. I. Decontrolled iron: an ultimate carcinogen and toxicant: an hypothesis

Ionic iron at physiological pH hydrolyzes into insoluble aggregates, which disperse on slight acidification. Uncontrolled ionic iron promotes autoxidation, which crosslinks biomolecules and produces destructive activated oxygen. Defenses against autoxidative crosslinking include: 1. ferritin, the macromolecular scavenger of iron; 2. metabolic turnover, which prevents irreversible crosslinking through early catabolic degradation and replacement; and 3. enzymatic deactivation of oxygen. I am proposing that the anticrosslinking defenses are defeated by transient actions of metabolic perturbations, toxicants, oxidants and "foreign bodies", which cause oxidative crosslinking of proteins and lipids into irreversible tissue imprint: indigestible bodies containing porous limited-access spaces (LASs). The pores exclude the macromolecular ferritin and the digestive and antiautoxidation enzymes but admit ionic iron which, sheltered from ferritin, accumulates into decontrolled-iron pathogen (DIP). DIP utilizes the energy of ambient pH fluctuations to erupt from the LAS, swamp the available ferritin, poison the surroundings, catalyze autoxidation and crosslink cell components into additional LAS carriers. With time and sufficient promotion by pH fluctuations or metal-complexing agents, DIP and LAS expand. DIP injures through heavy-metal inhibition of life processes and catalysis of autoxidation. Typically, carcinogenic initiators are protein denaturants, cell poisons, "foreign bodies" and autoxidation catalysts. These are DIP-initiating properties, and DIP may be a preneoplastic stage of carcinogenesis. A DIP-model interpretation is given for the growth of asbestos bodies. DIP is an inorganic parasite. It may envelope and attack phagocytized particles.

Syncytium formation caused by human cytomegalovirus in human embryonic lung fibroblasts

Large syncytia were regularly produced after prolonged infection in vitro of human embryonic fibroblasts by AD169 and other strains of human CMV. The syncytia showed typical CMV intranuclear inclusion bodies and herpesvirus particles in the nuclei and cytoplasm. It is proposed that syncytium formation follows abortive infection of the fibroblasts with defective virus.

Herpes simplex virus and human cytomegalovirus replication in WI-38 cells. III. Cytochemical localization of lysosomal enzymes in infected cells

Cytochemical localization of the lysosomal enzymes acid phosphatase and arylsulfatase in cells infected by herpes simplex virus (HSV) or human cytomegalovirus (CMV) showed the following interactions between viruses and host cell lysosomes: (i) many enveloped progeny viruses were located within cytoplasmic vacuoles containing lysosomal enzyme activity; (ii) naked cytoplasmic capsids appeared to acquire an envelope by budding directly into lysosomes; and (iii) many of the cytoplasmic dense bodies that are characteristic of CMV-infected cells and are thought to represent noninfectious aggregates of CMV structural proteins (I. Sarov and I. Abady, Virology 66:464-473, 1975) also acquired a limiting membrane by budding into lysosomes. Autophagy of other cytoplasmic elements was not observed, suggesting that there is some specificity involved in the association of viral particles and CMV dense bodies with lysosomes. Despite the presence of potentially destructive hydrolases, there was little evidence of significant morphological damage to intralysosomal viruses, and high titers of infectious particles were released into the medium. It would therefore appear that significant levels of HSV and CMV infectivity normally persist even though many progeny particles are directly exposed to lysosomal enzymes.

Cytomegalovirus proteins: II. Polypeptide composition of cytomegalovirus complement-fixing antigen

Complement-fixing (CF) antigen, prepared from cytomegalovirus (CMV)-infected cells and solubilized by urea and Tween-20, was fractionated on agarose gel columns. Its molecular weight was 1.5--5 X 10(7) daltons, and it contained 5 major peptides with molecular weights from 60,000--90,000 daltons. The peptide with a molecular weight of 70,000 daltons which was present in the solubilized CF antigen was absent in a soluble fraction from infected cells without CF activity and may be, therefore, important in this respect. The antigens present in the CF antigen did not elicit neutralizing antibody.

Replication of cytomegalovirus in human epitheloid diploid cell line

Human diploid BAMB cells with epitheloid morphology, which had been derived from amniotic fluid cells, were capable of supporting the replication of human cytomegalovirus (CMV), without prior treatment of the cells with halogenated pyrimidines. The growth of this virus in BAMB cells and in human diploid fibroblastoid (LEP) cells was compared in parallel tests. Virus replication was slower and less efficient in the former than in the latter system. The most characteristic morphological feature of the CMV-infected BAMB cells was the formation of multinucleated giant cells which frequently contained more than a hundred nuclei; such cells were not seen in LEP cultures. The development of ultrastructural changes was slower in BAMB cells than in LEP cells. The additional most marked differences concerned the place of viral envelopment and the production of cytoplasmic dense bodies. While in LEP cells most nucleocapsids were enveloped from the inner leaflet of the nuclear membrane, in the other system a great majority of the particles acquired their envelopes by budding into vacuoles. Cytoplasmic dense bodies were rare in infected LEP cells but very frequent in BAMB cells. Budding of these structures into vacuoles was also observed.

Antisera to human cytomegalovirus produced in hamsters: reactivity in radioimmunoassay and other antibody assay systems

Hamsters immunized with human cytomegalovirus (CMV) concentrated and purified by polyethylene glycol precipitation and density gradient centrifugation produced antisera with high titers of specific viral antibody, and which showed no significant reactivity with human host cell components. The antisera had high titers of CMV antibody in complement fixation, indirect fluorescent-antibody (FA), and neutralization tests, but titers obtained by indirect radioimmunoassay (RIA) were markedly higher. The antisera were used to follow the development of CMV antigen in infected host cells by indirect RIA and indirect FA staining. Virus-specific antigen was first detectable by RIA at 8 h after infection, and by FA staining at 16 h; cells contained optimal amounts of antigen for RIA and FA assays at 72 to 100 h postinfection. Immune globulins from the antisera were labeled with 125I for use in direct RIA. The labeled globulins gave highly specific reactions with CMV-infected cells, including those infected with low-passage isolates, and showed no reactivity with cells infected with other human herpesviruses or certain other human viruses.

Human cytomegalovirus: glycoproteins associated with virions and dense bodies

The glycoproteins associated with the membranes of cytomegalovirions and dense bodies were characterized by their relative mobility, percentage of glucosamine incorporation, and molecular weight. Eight glycopolypeptides were repeatedly detectable. Three glycopolypeptides of higher molecular weight with low levels of glucosamine incorporation were occasionally detectable. These latter glycopolypeptides may be precursors or aggregates of the glycopolypeptides with lower molecular weights. The glycoproteins associated with the membranes were on the surface, as determined by iodination with 125I of virions and dense bodies partially purified in gradients of D-sorbitol. Velocity centrifugation in linear gradients of D-sorbitol was used to obtain concentrated and partially purified preparations of infectious cytomegalovirus. Viral infectivity and the membranes of cytomegalovirions and dense bodies were stable in gradients of sorbitol, but cellular contaminants were not completely removed. Additional centrifugation in CsCl separated both cellular contaminants and viral nucleocapsids from virions and dense bodies. Many dense bodies, which are considered to be aberrant forms of cytomegalovirus, had the same size, sedimentation properties, and density as virions. Consequently, they were not separable from virions by various centrifugation techniques. Electron microscopy demonstrated that purified virions and dense bodies were qualitatively free of extraneous material and that each dense body was bounded by a membrane, as evidenced by its double-tract appearance. Antisera to a preparation of purified virions and dense bodies, or to their glycoproteins, contained antibodies that neutralized viral infectivity and reacted with antigens in cells infected with cytomegalovirus. However, these same antisera did not contain antibodies that reacted with uninfected cells. The glycoproteins associated with the membranes of cytomegalovirions and dense bodies are considered to be specified by the cytomegalovirus genome.

Cytomegalovirus proteins. I. Polypeptides of virions and dense bodies

Cytomegalovirus virions and dense bodies were purified by sucrose velocity and equilibrium centrifugation from the medium of fibroblasts infected with the strain AD169. The final virus preparations were purified more than 228-fold with respect to cellular proteins as determined by double-isotopic labeling and at least 1,600-fold on the basis of changes in the ratio of total protein to virus particles. The protein content of purified particles approximated that found for purified preparations of other herpesviruses. Twenty polypeptides ranging from 22,000 to greater than 230,000 molecular weight were detected in purified virus preparations by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Polypeptides of virions and dense bodies were allocated on the basis of analyses of preparations containing differing percentages of virions and dense bodies. Six polypeptides were represented predominantly or exclusively in virions, and four polypeptides were represented predominantly or exclusively in dense bodies, whereas the remainder appeared to be shared by both types of particles. Four polypeptides were glycosylated, and at least three of these appeared to be shared by both particles. Four polypeptides were glycosylated, and at least three of these appeared to be shared by both particle types. The protein composition of cytomegalovirus differs profoundly from that of herpes simplex virus.

Envelopment and the envelopes of infectious bovine rhinotracheitis virus in ultrathin sections

An electron microsopic study of cell cultures and bovine foetal tracheal organ cultures infected with infectious bovine rhinotracheitis (IBR) virus showed the following. a) The difference in the site at which the outer envelope is acquired in the infected cell is responsible for some morphological differences between the virions. Where envelopment of the capsids occurs by budding into cytoplasmic tubules, dense material adjacent to the tubules is often incorporated between the capsid and the outer envelope, giving a pleomorphic appearance to the latter. No dense material is seen in the same position if envelopment occurs in the nucleus. b) Dense material adjacent to the cytoplasmic tubules is not only incorporated in the virions, but may also bud into the cytoplasmic tubules without nucleocapsids, thus giving rise to dense bodies. c) Dense material in the virions is closely adjacent to the inner side of the outer envelope, but is separated from the capsid by a lucent zone. A similar zone can be seen around some nucleocapsids in the nucleus. d) The outer envelope of numerous extracellular virions and of those located in the cytoplasmic tubules shows the unit membrane structure with projections. The outer envelope of intranuclear particles and of those located in the perinuclear cisterna had the appearance of a dense membrane.

Structural proteins of human cytomegalovirus

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