Electron microscopic observations on the development of herpes simplex virus

Autophagy and antiviral defense

Targeting intracellular components for lysosomal degradation by autophagy not only maintains cellular homeostasis but also counteracts the effects of external stimuli, including invading pathogens. Among various kinds of pathogens, viruses have been extensively shown to induce autophagy to benefit viral growth in infected cells and to modulate host defense responses, such as innate antiviral immunity. Recently, numerous lines of evidence have implied that virus-induced autophagy triggers multilayer mechanisms to regulate the innate antiviral response of host cells, thus promoting a balance in virus-host cell interactions. In this review, the detailed mechanisms underlying autophagy and the innate antiviral immune response are first described. Then, I summarize the current information regarding the diverse functional role(s) of autophagy in the control of antiviral defenses against different types of viral infections. Moreover, the physiological significance of autophagy-regulated antiviral responses on the viral life cycle and the potential autophagy alterations induced by virus-associated antiviral signaling is further discussed.

Herpes Simplex Virus 1 Infection of Neuronal and Non-Neuronal Cells Elicits Specific Innate Immune Responses and Immune Evasion Mechanisms

Alphaherpesviruses (α-HV) are a large family of double-stranded DNA viruses which cause many human and animal diseases. There are three human α-HVs: Herpes Simplex Viruses (HSV-1 and HSV-2) and Varicella Zoster Virus (VZV). All α-HV have evolved multiple strategies to suppress or exploit host cell innate immune signaling pathways to aid in their infections. All α-HVs initially infect epithelial cells (primary site of infection), and later spread to infect innervating sensory neurons. As with all herpesviruses, α-HVs have both a lytic (productive) and latent (dormant) stage of infection. During the lytic stage, the virus rapidly replicates in epithelial cells before it is cleared by the immune system. In contrast, latent infection in host neurons is a life-long infection. Upon infection of mucosal epithelial cells, herpesviruses immediately employ a variety of cellular mechanisms to evade host detection during active replication. Next, infectious viral progeny bud from infected cells and fuse to neuronal axonal terminals. Here, the nucleocapsid is transported via sensory neuron axons to the ganglion cell body, where latency is established until viral reactivation. This review will primarily focus on how HSV-1 induces various innate immune responses, including host cell recognition of viral constituents by pattern-recognition receptors (PRRs), induction of IFN-mediated immune responses involving toll-like receptor (TLR) signaling pathways, and cyclic GMP-AMP synthase stimulator of interferon genes (cGAS-STING). This review focuses on these pathways along with other mechanisms including autophagy and the complement system. We will summarize and discuss recent evidence which has revealed how HSV-1 is able to manipulate and evade host antiviral innate immune responses both in neuronal (sensory neurons of the trigeminal ganglia) and non-neuronal (epithelial) cells. Understanding the innate immune response mechanisms triggered by HSV-1 infection, and the mechanisms of innate immune evasion, will impact the development of future therapeutic treatments.

Similarities and Differences in the Development of Laboratory Strains and Freshly Isolated Strains of Herpes Simplex Virus in HEp-2 Cells: Electron Microscopy

HEp-2 cells infected with two laboratory strains (mP and MP) and two freshly isolated strains (F and G) of herpes simplex virus were fixed at intervals between 4 and 50 hr postinfection and sectioned, and were then examined with the electron microscope. These studies revealed the following. (i) All four strains caused identical segregation of nucleoli and aggregation of host chromosomes at the nuclear membrane. (ii) The development of MP virus could not be differentiated from that of its parent mP strain. (iii) There were quantitative differences between laboratory (mP) and freshly isolated (F) type 1 strains. Thus, cells infected with F contained numerous nuclear crystals of nucleocapsids and relatively few cytoplasmic structures containing enveloped nucleocapsids. Conversely, cells infected with mP or with MP virus contained numerous cytoplasmic structures with enveloped nucleocapsids and relatively few nuclear crystals of nucleocapsids. (iv) There were qualitative differences between type 2 strain (G) isolated from genital lesions and type 1 strains. Thus, cells infected with the G strain contain numerous filaments in nuclei and unenveloped and partially enveloped nucleocapsids in the cytoplasm. Of particular interest is the finding that cytoplasmic membranes in apposition to nucleocapsids were thickened and bent as if they were enveloping the particle. The significance of the qualitative differences in the development of the four strains is discussed.

CYTOCHEMICAL STUDIES OF THE NUCLEOPROTEINS OF HELA CELLS INFECTED WITH HERPES VIRUS

The morphological and cytochemical changes in HeLa cells infected with herpes virus have been studied at frequent intervals during infection and related to the growth of virus and the multiplicity of the virus inoculum. Infection with a high multiplicity inoculum produced enlargement and extrusion of small ribonucleoprotein (RNP) bodies in the nucleoli (nucleolini) to form RNP bodies in the nucleoplasm (B bodies) beginning (1/2) hour after infection. 3 hours after infection, RNP of the pars amorpha appeared to diffuse into the adjacent nucleoplasm, where, (1/2) hour later, the classical type A inclusion or A body first appeared. The A bodies displaced the B bodies and the nucleoli and eventually filled the nucleus. 6 hours after infection, minute granules containing RNA, DNA, and non-histone protein appeared inside the A bodies (A granules) and increased in number until the late stages of infection, when they disappeared. 18 hours after infection, at the time when the A bodies came to fill the nucleus completely, extrusion of RNP from the nucleus produced cytoplasmic masses which have been termed C bodies. B bodies were formed in the majority of cells before the maturation of infectious virus, but the number of B bodies could not be correlated with the amount of virus in the cell or with the multiplicity of the inoculum. It is suggested that the formation of B bodies may be the result of inhibition of the onset of mitotic division by a mechanism which does not inhibit the formation of RNA in the nucleolini. The nature of the A bodies, the A granules, and the C bodies is discussed and it is concluded that the A granules may represent aggregations of maturing virus in the nucleus. The progression of some C mitotic metaphases to the formation of post-C mitotic multinucleated giant cells is described. These are distinct from syncytia formed by cell fusion.

Digesting Oneself and Digesting Microbes

Although research in this area is still in a stage of infancy, it seems likely that the lysosomal degradation pathway of autophagy plays an evolutionarily conserved role in antiviral immunity. The interferon-inducible, antiviral PKR signaling pathway positively regulates autophagy, and both mammalian and plant autophagy genes restrict viral replication and protect against virus-induced cell death. Given this role of autophagy in innate immunity, it is not surprising that viruses have evolved numerous strategies to inhibit host autophagy. Different viral gene products can either modulate autophagy regulatory signals or directly interact with components of the autophagy execution machinery. Moreover, certain RNA viruses have managed to “co-apt” the autophagy pathway, selectively utilizing certain components of the dynamic membrane rearrangement system to promote their own replication inside the host cytoplasm.

In addition to this newly emerging role of autophagy in innate immunity, autophagy plays an important role in many other fundamental biological processes, including tissue homeostasis, differentiation and development, cell growth control, and the prevention of aging. Accordingly, the inhibition of host autophagy by viral gene products has important implications not only for understanding mechanisms of immune evasion, but also for understanding novel mechanisms of viral pathogenesis. It will be interesting to dissect the role of viral inhibition of autophagy in acute, persistent, and latent viral replication, as well as in the pathogenesis of cancer and other medical diseases.

Envelopment of human cytomegalovirus occurs by budding into Golgi-derived vacuole compartments positive for gB, Rab 3, trans-golgi network 46, and mannosidase II

Although considerable progress has been made towards characterizing virus assembly processes, assignment of the site of tegumentation and envelopment for human cytomegalovirus (HCMV) is still not clear. In this study, we examined the envelopment of HCMV particles in human lung fibroblasts (HF) HL 411 and HL 19, human umbilical vein endothelial cells, human pulmonary arterial endothelial cells, and arterial smooth muscle cells at different time points after infection by electron microscopy (EM), immunohistochemistry, and confocal microscopy analysis. Double-immunofluorescence labeling experiments demonstrated colocalization of the HCMV glycoprotein B (gB) with the Golgi resident enzyme mannosidase II, the Golgi marker TGN (trans-Golgi network) 46, and the secretory vacuole marker Rab 3 in all cell types investigated. Final envelopment of tegumented capsids was observed at 5 days postinfection by EM, when tegumented capsids budded into subcellular compartments located in the cytoplasm, in close proximity to the Golgi apparatus. Immunogold labeling and EM analysis confirmed staining of the budding compartment with HCMV gB, Rab 3, and mannosidase II in HL 411 cells. However, the markers Rab 1, Rab 2, Rab 7, Lamp 1 (late endosomes and lysosomes), and Lamp 2 (lysosomes) neither showed specific staining of the budding compartment in the immunogold labeling experiments nor colocalized with gB in the immunofluorescent colocalization experiments in any cell type studied. Together, these results suggest that the final envelopment of HCMV particles takes place mainly into a Golgi-derived secretory vacuole destined for the plasma membrane, which may release new infectious virus particles by fusion with the plasma membrane.

Cytoplasmic domain signal sequences that mediate transport of varicella-zoster virus gB from the endoplasmic reticulum to the Golgi

Normal herpesvirus assembly and egress depend on the correct intracellular localization of viral glycoproteins. While several post-Golgi transport motifs have been characterized within the cytoplasmic domains of various viral glycoproteins, few specific endoplasmic reticulum (ER)-to-Golgi transport signals have been described. We report the identification of two regions within the 125-amino-acid cytoplasmic domain of Varicella-Zoster virus gB that are required for its ER-to-Golgi transport. Native gB or gB containing deletions and specific point mutations in its cytoplasmic domain was expressed in mammalian cells. ER-to-Golgi transport of gB was assessed by indirect immunofluorescence and by the acquisition of Golgi-dependent posttranslational modifications. These studies revealed that the ER-to-Golgi transport of gB requires a nine-amino-acid region (YMTLVSAAE) within its cytoplasmic domain. Mutations of individual amino acids within this region markedly impaired the transport of gB from the ER to the Golgi, indicating that this domain functions by a sequence-dependent mechanism. Deletion of the C-terminal 17 amino acids of the gB cytoplasmic domain was also shown to impair the transport of gB from the ER to the Golgi. However, internal mutations within this region did not disrupt the transport of gB, indicating that its function during gB transport is not sequence dependent. Native gB is also transported to the nuclear membrane of transfected cells. gB lacking as many as 67 amino acids from the C terminus of its cytoplasmic domain continued to be transported to the nuclear membrane at apparently normal levels, indicating that the cytoplasmic domain of gB is not required for nuclear membrane localization.

Genetic analysis of the role of herpes simplex virus type 1 glycoprotein K in infectious virus production and egress

Herpes simplex virus type 1 (KOS)DeltagK is a mutant virus which lacks glycoprotein K (gK) and exhibits defects in virion egress (S. Jayachandra, A. Baghian, and K. G. Kousoulas, J. Virol. 69:5401-5413, 1997). To further understand the role of gK in virus egress, we constructed recombinant viruses, DeltagKhpd-1, -2, -3, and -4, that specified gK amino-terminal portions of 139, 239, 268, and 326 amino acids, respectively, corresponding to truncations immediately after each of the four putative membrane-spanning domains of gK. DeltagKhpd-1 and DeltagKhpd-2 viruses produced lower yields and smaller plaques than DeltagK. Numerous DeltagKhpd-1 capsids accumulated predominately within large double-membrane vesicles of which the inner membrane appeared to be derived from viral envelopes while the outer membrane appeared to originate from the outer nuclear membrane. The mutant virus DeltagKhpd-3 produced higher yields and larger plaques than the DeltagK virus. The mutant virus DeltagKhpd-4 produced yields and plaques similar to those of the wild-type virus strain KOS, indicating that deletion of the carboxy-terminal 12 amino acids did not adversely affect virus replication and egress. Comparisons of the gK primary sequences specified by alphaherpesviruses revealed the presence of a cysteine-rich motif (CXXCC), located within domain III in the lumen side of gK, and a tyrosine-based motif, YTKPhi (where Phi is any bulky hydrophobic amino acid), located between the second and third hydrophobic domains (domain II) in the cytoplasmic side of gK. The mutant virus gK/Y183S, which was constructed to specify gK with a single-amino-acid change (Y to S) within the YTKPhi motif, replicated less efficiently than the DeltagK virus. The mutant virus gK/C304S-C307S, which was constructed to specify two serine instead of cysteine residues within the cysteine-rich motif (CXXCC changed to SXXSC) of gK domain III, replicated more efficiently than the DeltagK virus. Our data suggests that gK contains domains in its amino-terminal portion that promote aberrant nucleocapsid envelopment and/or membrane fusion between different virion envelopes and contains domains within its domains II and III that function in virus replication and egress.

Role of the membrane anchoring and cytoplasmic domains in intracellular transport and localization of viral glycoproteins

Nuclear factor-κB (NF-κB) binds to nucleotide sequences between -80 and -70 bp upstream of the transcriptional start site in the interleukin-8 (IL-8) promoter and is crucial for transcription of the IL-8 gene. We showed that exogenous nitric oxide in the form of a nitric oxide donor significantly reduced IL-8 mRNA in cytokine-activated ECV304. Similarly, nitric oxide significantly reduced migration of polymorphonuclear neutrophils through cytokine-activated ECV304 monolayers, an IL-8-dependent process. Using a luciferase reporter construct containing the NF-κB site of the IL-8 gene, we showed that exposing cytokine-activated ECV304 to exogenous nitric oxide resulted in significant reduction of NF-κB binding. Follow-up studies using a luciferase reporter construct possessing a mutated NF-κB site confirmed that the luciferase activity observed in the NF-κB reporter resulted from NF-κB binding. These studies demonstrate that nitric oxide, supplied exogenously into reactions containing activated endothelium, down-regulates pro-inflammatory activity, such as the secretion of chemokines, and functional activity, such as transendothelial migration of neutrophils. Key words: interleukin-8, nuclear factor κ B, transendothelial migration, nitric oxide.

Electrorotation studies of baby hamster kidney fibroblasts infected with herpes simplex virus type 1

The dielectric properties of baby hamster kidney fibroblast (BHK(C-13)) cells have been measured using electrorotation before and after infection with herpes simplex virus type 1 (HSV-1). The dielectric properties and morphology of the cells were investigated as a function of time after infection. The mean specific capacitance of the uninfected cells was 2.0 microF/cm2, reducing to a value of 1. 5 microF/cm2 at 12 h after infection. This change was interpreted as arising from changes in the cell membrane morphology coupled with alterations in the composition of the cell membrane as infection progressed. The measured changes in the cell capacitance were correlated with alterations in cellular morphology determined from scanning electron microscope (SEM) images. Between 9 and 12 h after infection the internal permittivity of the cell exhibited a rapid change, reducing in value from 75epsilono to 58epsilono, which can be correlated with the generation of large numbers of Golgi-derived membrane vesicles and enveloped viral capsids. The data are discussed in relation to the known life cycle of HSV-1 and indicate that electrorotation can be used to observe dynamic changes in both the dielectric and morphological properties of virus-infected cells. Calculations of the dielectrophoretic spectrum of uninfected and infected cells have been performed, and the results show that cells in the two states could be separated using appropriate frequencies and electrode arrays.

Live-cell analysis of a green fluorescent protein-tagged herpes simplex virus infection

Many stages of the herpes simplex virus maturation pathway have not yet been defined. In particular, little is known about the assembly of the virion tegument compartment and its subsequent incorporation into maturing virus particles. Here we describe the construction of a herpes simplex virus type 1 (HSV-1) recombinant in which we have replaced the gene encoding a major tegument protein, VP22, with a gene expressing a green fluorescent protein (GFP)-VP22 fusion protein (GFP-22). We show that this virus has growth properties identical to those of the parental virus and that newly synthesized GFP-22 is detectable in live cells as early as 3 h postinfection. Moreover, we show that GFP-22 is incorporated into the HSV-1 virion as efficiently as VP22, resulting in particles which are visible by fluorescence microscopy. Consequently, we have used time lapse confocal microscopy to monitor GFP-22 in live-cell infection, and we present time lapse animations of GFP-22 localization throughout the virus life cycle. These animations demonstrate that GFP-22 is present in a diffuse cytoplasmic location when it is initially expressed but evolves into particulate material which travels through an exclusively cytoplasmic pathway to the cell periphery. In this way, we have for the first time visualized the trafficking of a herpesvirus structural component within live, infected cells.

Effect of p-fluorophenylalanine of psittacosis virus in tissue cultures

Tanami, Yoh (University of Texas Medical Branch, Galveston) and Morris Pollard. Effect of p-fluorophenylalanine on psittacosis virus in tissue cultures. J. Bacteriol. 83:437-442. 1962.-The inhibitory effect of p-fluorophenylalanine (FPA) on maturation of psittacosis virus was investigated, with attention to the time sequence of viral protein synthesis. Extracellular virus particles were not inactivated by FPA at a concentration of 100 mug per ml, at which level it interfered with maturation of intracellular virus. When FPA was added to infected tissue cultures earlier than 15 hr after infection, intracellular virus maturation was suppressed. However, when FPA was added after 15 hr, infective virus was produced, which indicates that the synthesis of a FPA-sensitive virus precursor (presumably viral protein) had already occurred. A latent ("dormant") infection of psittacosis virus, established in a medium deficient in phenylalanine and tyrosine, was also investigated.

A correlative study by electron and light microscopy of the development of type 5 adenovirus. I. Electron microscopy

Stages in the nuclear changes consequent to infection with type 5 adenovirus are shown and described. Viral development seems to be confined to the nucleus where characteristic particles are found. The shape of the intracellular virus depends upon the method of preservation employed, appearing spherical after osmium tetroxide or freezing-substitution, occasionally exhibiting angulated faces after formalin and often assuming an hexagonal profile after potassium permanganate. The non-viral crystals are encountered in zones of low density, and it is suggested that crystallization results from the accumulation of protein in these areas. An hypothesis is presented to explain why these crystals, in contrast to the insect polyhedra, contain few viral particles.

Electron microscopic study of the cytopathology of ECHO virus infection in cultivated cells

The cultivated monkey kidney cell is subject to changes when infected with ECHO viruses 6, 9, and 19. The electron microscope reveals three stages of infection: (a) initial stage. The nucleus appears granular with chromatin condensation on the nuclear envelope. The cytoplasm contains electron transparent vesicles and vacuoles forming nests. (b) Intermediate stage. The nucleus seems to diminish, appearing more pycnotic and displaced toward the periphery. The cytoplasm is filled with electron transparent vacuoles and vesicles, and dense masses as well as some spiral bodies are seen. The mitochondria retain their shape. Dense particles are seen, which are possibly of viral nature. (c) Final stage. The nucleus is contracted to a narrow strip close to the cellular membrane or is completely destroyed. The cytoplasm shows no apparent changes. Crystals are frequently observed in cells infected with ECHO viruses 6 and 19, consisting of dense particles with an average diameter of 14.4 mmicro ranging from approximately 13.2 to 15.6 mmicro for ECHO virus 6, and 14.5 mmicro ranging from approximately 12.5 to 16.5 mmicro for ECHO virus 19. These particles are clustered in hexagonal packages forming angles of 75 degrees and 105 degrees . The particles in most crystals are arranged in rows separated by a constant distance, the latter varying from one crystal to another and being approximately 1.5 and 2.5 times the distance between particles. Other particles were observed which, however, are not considered to be of viral nature.

The development of vaccinia virus in Earle's L strain cells as examined by electron microscopy

A favorable system which is amenable to frequent and reproducible sampling, consisting of suspension cultures of strain L cells and vaccinia virus, was employed to study the animal virus-mammalian host cell relationship. The three principal aspects investigated concerned the adsorption and penetration of vaccinia into the host, the relationship between the sequence of virus development and the production of infectious particles, and the changes in the fine structure of the host cells. Experiments in which a very high multiplicity of infection was used revealed that vaccinia is phagocytized by L cells in less than 1 hour after being added to the culture, without any apparent loss of its outer limiting membranes. Regions of dense fibrous material, thought to be foci of presumptive virus multiplication, appear in the cytoplasm 2 hours after infection. A correlation between electron microscope studies and formation of infectious particles shows that although immature forms of the virus appear 4 hours after infection, infectious particles are produced 6 hours after infection of the culture, at the time when mature forms of vaccinia appear for the first time in thinly sectioned cells. Spread of the infection is gradual until eventually, after 24 hours, virus is being elaborated throughout the cytoplasm. Addition of vaccinia to monolayer cultures induced fusion of L cells and rapid formation of multinucleate giant forms. In both suspension and stationary cultures infected cells elaborate a variety of membranous structures not present in normal L cells. These take the form of tube-like lamellar and vesicular formations, or appear as complex reticular networks or as multi-laminar membranes within degenerating mitochondria.

Some unusual features of fine structure observed in HeLa cells

HeLa cells from conventional culture media have been studied in thin sections with the electron microscope; in many cases cells were examined in sets of sections cut in series. The fine structure of the cells is described including three unusual features not hitherto reported. It has been found that numerous cells contained rows of parallel smooth surfaced cisternae spaced about 150 mµ apart and communicating with rough surfaced elements of the endoplasmic reticulum. These cisternae resembled "annulate lamellae" but did not contain regular arrays of pores. In many cells an area of juxtanuclear cytoplasm was occupied by a membranous structure composed of closely applied pairs of narrow cisternae either arranged in concentric rings or else extending in several directions in a haphazard manner. Sparse particles were present on the outer membranes of each pair of cisternae. Communications between the double cisternae and other membrane-bounded structures were not observed. A small number of cells contained areas of cytoplasm devoid of organelles and filled with amorphous fuzzy material. The observations recorded are discussed.

Structure and development of viruses as observed in the electron microscope. V. Western equine encephalomyelitis virus

Stages in the development and release of Western equine encephalomyelitis virus are illustrated and described. It is suggested that precursor particles 22 mmicro in diameter differentiate at template sites close to membranes bordering cytoplasmic vacuoles and that these particles either pass into the lumen of the vacuole, acquiring in the process a coat and peripheral membrane, or are dispersed in the cytoplasm and extruded through the cellular wall, emerging as viral particles on the surface. Although necrosis and dissolution of the cell with release of contents, including virus, may intervene at any stage of infection, ejection of virus from the vacuoles presumably can occur without rupture of the cell. The virus consists of a 30 mmicro core separated by a zone of lesser density from a sharply defined peripheral membrane 45 to 48 mmicro in diameter. Precursor particles, as well as viral particles, occasionally crystallize, the former in the cytoplasm, the latter in vacuoles and probably on the cellular surface.

Observations on the fine structure of mature herpes simplex virus and on the composition of its nucleoid

The fine structure and composition of mature herpes virus have been investigated in thin sections by electron microscopy. The virus was grown in cultured HeLa cells and was collected with them. Tests for the biological activity of the infected cultures were included in the first half of the work. Preparations were fixed with both osmium and permanganate, and were embedded either in methacrylate or in aquon, a water-miscible fraction of a commercially available epoxy resin. In further experiments material fixed with permanganate was subjected to the action of specific nucleases or control medium before embedding. All the preparations showed numerous uniform particles around and between the cells and this was paralleled by considerable biological activity where tests were made on samples of the culture fluids. Mature herpes virus has been found to be round and to measure, when dehydration shrinkage is avoided, about 165 mmicro in diameter. The particle contained an eccentric round-ended rod-shaped, electron-opaque nucleoid lying in an inner zone of low density. A dense outer zone or viroplasm surrounded this, no membrane being present between the two zones. After permanganate fixation the particle was found to have an outer limiting membrane showing a triple-layered structure morphologically indistinguishable from that of the plasma membrane of the HeLa cells. The results of the digestion experiments show that herpes virus contains nucleic acid of deoxyribose type and that this is localized in the dense nucleoid. Both the findings and the methods used are discussed.

An electron microscope study of polyoma virus in hamster kidney

Electron microscope studies were made of hamster kidneys taken at daily intervals after injection of a variant of polyoma virus into newborn animals. Particular attention was paid to the period 5 to 6 days after injection at which time the necrotizing response was at its peak and virus particles were seen in greatest numbers. The most numerous particles were about 28 mµ in diameter. They were observed mainly within nuclei of stromal cells and are similar to the particles seen in large numbers in polyoma-infected mouse cells growing in vitro. They were not observed in cells of fully developed tumors. Filamentous or tubular structures closely associated with the 28 mµ particles and probably concerned in their formation are described. Considerable quantities of viral material were contained within cytoplasmic inclusions. In some of the inclusions larger particles of diameter 60 mµ were observed. The origin of these particles and their relation to the 28 mµ particles is discussed.

Structure and development of viruses as observed in the electron microscope. VI. ECHO virus, type 9

Sequential stages in the development and release of ECHO 9 virus have been illustrated and described. It is suggested that viral particles differentiate and become oriented in columns upon a fine filamentous lattice at cytoplasmic template sites which are distinct from the endoplasmic reticulum. Subsequently, virus is dispersed in the peripheral cytoplasm and gains egress from the cell through rents in the plasma membrane. Complete cellular disruption with viral release may supervene. The virus consists of a 13 to 15 mµ dense core and a poorly defined outer membrane, 22 to 24 mµ in diameter. Incomplete forms, lacking the core, are observed in the cytoplasm but have not been seen in the extracellular space.

Observations on the mode of release of herpes virus from infected HeLa cells

The development of a well adapted strain of herpes virus has been studied in HeLa cells using thin sectioning techniques for electron microscopy. Particular attention was directed to events in the cytoplasm and certain new features were observed. Profuse immature particles with a nucleoid and single limiting membrane were present in the nuclei of infected cells, often in crystalline array; morphologically indistinguishable immature particles were also found very frequently in the cytoplasm. Cells with such particles were intact and well preserved, and contained smooth vacuoles apparently derived from the Golgi component of the endoplasmic reticulum. The cytoplasmic particles escaped from the cells by bulging out as buds through the cell membrane or through that of the cytoplasmic vacuoles until they were attached only by a pedicle and then became free. During this process the particles were gradually enclosed by the membrane through which they passed and carried a coat of it with them as they matured. After permanganate fixation the triple-layered structure of the cell membrane and vacuolar membranes was evident and was identical with that of the outer coat of the mature virus. These findings are discussed both in relation to different types of virus structure and to function in the endoplasmic reticulum and cell membrane.

A correlative study by electron and light microscopy of the development of type 5 adenovirus. II. Light microscopy

The evolution of the intranuclear lesion produced by type 5 adenovirus in HEp-2 and HeLa cells is described as seen in the light microscope and the bodies formed in the course of the infection characterized histochemically. Some 12 hours after infection acidophilic protein bodies, without appreciable nucleic acid, first appear in the nucleus and coalesce into a network. Within or in association with this material, DNA-containing masses (viral aggregates) are formed which rapidly increase in amount and then coalesce. At the same time, a protein is produced, histochemically different from that of the acidophilic or basophilic structures mentioned, within the infected nucleus, which constitutes a matrix within which regular cytstals of a protein, (presumably non-viral) materialize. These structural and histochemical features are correlated with details which have been observed in parallel studies with the electron microscope.

Herpes simplex virus type 1 glycoprotein K is not essential for infectious virus production in actively replicating cells but is required for efficient envelopment and translocation of infectious virions from the cytoplasm to the extracellular space

We characterized the glycoprotein K (gK)-null herpes simplex virus type 1 [HSV-1] (KOS) delta gK and compared it to the gK-null virus HSV-1 F-gKbeta (L. Hutchinson et al., J. Virol. 69:5401-5413, 1995). delta gK and F-gKbeta mutant viruses produced small plaques on Vero cell monolayers at 48 h postinfection. F-gKbeta caused extensive fusion of 143TK cells that was sensitive to melittin, a specific inhibitor of gK-induced cell fusion, while delta gK virus did not fuse 143TK cells. A recombinant plasmid containing the truncated gK gene specified by F-gKbeta failed to rescue the ICP27-null virus KOS (d27-1), while a plasmid with the delta gK deletion rescued the d27-1 virus efficiently. delta gK virus yield was approximately 100,000-fold lower in stationary cells than in actively replicating Vero cells. The plaquing efficiencies of delta gK and F-gKbeta virus stocks on VK302 cells were similar, while the plaquing efficiency of F-gKbeta virus stocks on Vero cells was reduced nearly 10,000-fold in comparison to that of delta gK virus. Mutant delta gK and F-gKbeta infectious virions accumulated within Vero and HEp-2 cells but failed to translocate to extracellular spaces. delta gK capsids accumulated in the nuclei of Vero but not HEp-2 cells. Enveloped delta gK virions were visualized in the cytoplasms of both Vero and HEp-2 cells, and viral capsids were found in the cytoplasm of HEp-2 cells within vesicles. Glycoproteins B, C, D, and H were expressed on the surface of delta gK-infected Vero cells in amounts similar to those for KOS-infected Vero cells. These results indicate that gK is involved in nucleocapsid envelopment, and more importantly in the translocation of infectious virions from the cytoplasm to the extracellular spaces, and that actively replicating cells can partially compensate for the envelopment but not for the cellular egress deficiency of the delta gK virus. Comparison of delta gK and F-gKbeta viruses suggests that the inefficient viral replication and plaquing efficiency of F-gKbeta virus in Vero cells and its syncytial phenotype in 143TK- cells are most likely due to expression of a truncated gK.

MORPHOLOGICAL AND BIOLOGICAL STUDIES ON A VIRUS IN CULTURED LYMPHOBLASTS FROM BURKITT'S LYMPHOMA

Lymphoblasts of two tissue culture strains (EB1 and EB2) from different biopsy specimens of Burkitt's lymphoma have been examined in thin sections by electron microscopy, and have each been found to carry a morphologically identical virus. The virus was observed in samples taken over many months, being present in about 1 to 2 per cent of the cells in two forms: Immature particles about 75 mmicro in diameter which were seen in both the nucleus and cytoplasm; and larger mature particles with a diameter of 110 to 115 mmicro, which were either within membrane-bounded cytoplasmic spaces or at the cell surface. There was some indication that the particles matured by budding through the cytoplasmic membranes. Both types of particle occurred in dead degenerating cells or, less frequently, in intact altered cells. The characteristic alterations of the latter included margination of the chromatin, fragmentation of the nuclear envelope, beaded opaque material in the mitochondria, and, with one of the cell strains (EB1), sheaves of altered spindle tubules. All attempts to isolate and identify the virus carried by the two strains of lymphoblasts failed. No pathological effects were caused in 8-day chick embryos inoculated either with whole lymphoblasts or extracts of disrupted lymphoblasts, using the intraallantoic, amniotic, and chorioallantoic routes, and the extraembryonic fluids of such chicks were without haemagglutinating activity for human, chicken, guinea pig, or monkey erythrocytes. Whole lymphoblasts or lymphoblast extracts were likewise without effect when inoculated intraperitoneally into newborn hamsters or two strains of newborn mice. Similar lymphoblast inocula did not cause detectable changes in 9 different test tissue culture systems even after 8 blind passages. The nature of the unknown, unidentified virus in the cultured lymphoblasts from Burkitt's lymphomas is considered and its possible relationship to the cells discussed.

ELECTRON MICROSCOPE STUDIES OF RABIES VIRUS IN MOUSE BRAIN

The cells of brains of 2- and 3-day old mice infected with street rabies virus were examined in the electron microscope. It was observed that characteristic rod-like or elongated particles were found within a "matrix" in the cytoplasm of nerve cells and of astrocytes. These rod-like particles can be separated into two types, on the basis of slightly different morphological features. One particle is 110 to 120 mµ wide and has double-membraned coats; the other is 120 to 130 mµ wide and is covered by a single limiting membrane. The former is closely associated with the endoplasmic reticulum. The biological relationship between the two types is unknown, but both types of particles are considered to be street rabies viruses because of their structural features. It is believed that segmentation and branching of elongated particles may play a role in virus multiplication. Negri bodies appear as dense round bodies containing various coarse structures but no virus particles.

THE ENTRY AND DISTRIBUTION OF HERPES VIRUS AND COLLOIDAL GOLD IN HELA CELLS AFTER CONTACT IN SUSPENSION

The way in which herpes virus of a well adapted strain penetrates susceptible HeLa cells has been investigated using thin sectioning techniques for electron microscopy. Mature virus particles and cells were mixed together in suspension cultures for 15, 30, 60, or 120 minutes so that the stages in virus uptake could be followed in sequence. The ingestion of particles of colloidal gold by HeLa cells under similar conditions was studied for comparison in parallel experiments. After 15 minutes' contact, the mature virus was found adsorbed on the surface of the cells but separated from them by a narrow gap in which phosphotungstic acid staining was sometimes able to reveal an extraneous coat which appeared as an amorphous layer on the outer aspect of the plasma membrane. When mixing continued for longer the particles were present in deep invaginations or actual cytoplasmic vacuoles, with their outer layers in various stages of stripping and digestion. The stripped, naked, central portion of the virus was occasionally found in these vacuoles but was more commonly free in the cytoplasmic matrix; the mode of transition between these sites could not be determined. Where contact continued for 2 hours these phenomena were much less frequently observed. The larger particles of colloidal gold were ingested in the same way as the virus, but smaller ones were taken up in micropinocytosis vesicles. The gold passed through membrane-bounded cytoplasmic spaces to accumulate in vacuoles from which, in contrast to herpes particles, it did not escape. These findings are discussed, and considered with particular reference to their bearing on the initiation of infection, the uptake and disposal of particles by cells, and the influence on the latter of virus morphology.

AN ELECTRON MICROSCOPE STUDY OF THE DEVELOPMENT OF A MOUSE HEPATITIS VIRUS IN TISSUE CULTURE CELLS

Samples taken at different intervals of time from suspension cultures of the NCTC 1469 line of mouse liver—derived (ML) cells infected with a mouse hepatitis virus have been studied with the electron microscope. The experiments revealed that the viruses are incorporated into the cells by viropexis within 1 hour after being added to the culture. An increasing number of particles are found later inside dense cytoplasmic corpuscles similar to lysosomes. In the cytoplasm of the cells from the samples taken 7 hours after inoculation, two organized structures generally associated and never seen in the controls are observed: one consists of dense material arranged in a reticular disposition (reticular inclusion); the other is formed by small tubules organized in a complex pattern (tubular body). No evidence has been found concerning their origin. Their significance is discussed. With the progression of the infection a system of membrane-bounded tubules and cisternae is differentiated in the cytoplasm of the ML cells. In the lumen of these tubules or cisternae, which are occupied by a dense material, numerous virus particles are observed. The virus particles which originate in association with the limiting membranes of tubules and cisternae are released into their lumen by a "budding" process. The virus particles are 75 mµ in diameter and possess a nucleoid constituted of dense particles or rods limiting an electron transparent core. The virus limiting membrane is sometimes covered by an outer layer of a dense material. In the cells from the samples taken 14 to 20 hours after inoculation, larger zones of the cell cytoplasm are occupied by inclusion bodies formed by channels or cisternae with their lumens containing numerous virus particles. In the samples taken 20 hours or more after the inoculation numerous cells show evident signs of degeneration.

ELECTRON MICROSCOPE OBSERVATIONS ON THE SURFACE ADENOSINE TRIPHOSPHATASE-LIKE ENZYMES OF HELA CELLS INFECTED WITH HERPES VIRUS

HeLa cells infected with herpes simplex virus have been examined in thin sections by electron microscopy after cytochemical staining for the presence of surface enzymes splitting adenosine triphosphate. As with uninfected HeLa cultures (18), the opaque enzyme reaction product was localized at the plasma membranes of about half the cells, tending to be present where there were microvilli and absent on smooth surfaces. Where mature extracellular herpes particles were found in association with cell membranes showing the enzyme activity, they were invariably likewise stained, and conversely, those mature particles which lay close against cells without reaction product at the surface were themselves free of it. Particles found budding into cytoplasmic vacuoles were also always without opaque deposit since this was never seen at vacuolar membranes, even in cells having the activity at the surface. The enzyme reaction product thus provided a marker indicating the manner in which the particles escape from cells and mature by budding out through cellular membranes, carrying, in the process, a portion of the latter on to themselves to form the outer viral limiting membrane. In some instances, virus particles were observed with more opaque material covering them than was present at the cell membrane with which they were associated. This finding has been taken as evidence for a physiological waxing and waning of surface enzyme activity of adenosine triphosphatase type. The fine structure of the mature extracellular virus as prepared here, using glutaraldehyde fixation, is also recorded. The observations and interpretations are discussed in full.

Targeting of glycoprotein I (gE) of varicella-zoster virus to the trans-Golgi network by an AYRV sequence and an acidic amino acid-rich patch in the cytosolic domain of the molecule

Previous studies suggested that varicella-zoster virus (VZV) envelope glycoproteins (gps) are selectively transported to the trans-Golgi network (TGN) and that the cytosolic domain of gpI (gE) targets it to the TGN. To identify targeting signals in the gpI cytosolic domain, intracellular protein trafficking was studied in transfected cells expressing chimeric proteins in which a full-length or mutated gpI cytosolic domain was fused to the gpI transmembrane domain and interleukin-2 receptor (tac) ectodomain. Expressed protein was visualized with antibodies to tac. A targeting sequence (AYRV) and a second, acidic amino acid-rich region of the gpI cytosolic domain (putative signal patch) were each sufficient to cause expressed protein to colocalize with TGN markers. This targeting was lost when the tyrosine of the AYRV sequence was replaced with glycine or lysine, when arginine was replaced with glutamic acid, or when valine was substituted with lysine. In contrast, tyrosine could be replaced by phenylalanine and valine could be substituted with leucine. Mutation of alanine to aspartic acid or deletion of alanine abolished TGN targeting. Exposure of transfected cells to antibodies to the tac ectodomain revealed that the TCN targeting of expressed tac-gpI chimeric proteins occurred as a result of selective retrieval from the plasmalemma. These data suggest that the AYRV sequence and a second signaling patch in the cytosolic domain of gpI are responsible for its targeting to the TGN. The observations also support the hypothesis that the TGN plays a critical role in the envelopment of VZV.

Construction and characterization of secreted and chimeric transmembrane forms of Drosophila acetylcholinesterase: a large truncation of the C-terminal signal peptide does not eliminate glycoinositol phospholipid anchoring

Despite advances in understanding the cell biology of glycoinositol phospholipid (GPI)-anchored proteins in cultured cells, the in vivo functions of GPI anchors have remained elusive. We have focused on Drosophila acetylcholinesterase (AChE) as a model GPI-anchored protein that can be manipulated in vivo with sophisticated genetic techniques. In Drosophila, AChE is found only as a GPI-anchored G2 form encoded by the Ace locus on the third chromosome. To pursue our goal of replacing wild-type GPI-anchored AChE with forms that have alternative anchor structures in transgenic files, we report the construction of two secreted forms of Drosophila AChE (SEC1 and SEC2) and a chimeric form (TM-AChE) anchored by the transmembrane and cytoplasmic domains of herpes simplex virus type 1 glycoprotein C. To confirm that the biochemical properties of these AChEs were unchanged from GPI-AChE except as predicted, we made stably transfected Drosophila Schneider Line 2(S2) cells expressing each of the four forms. TM-AChE, SEC1, and SEC2 had the same catalytic activity and quaternary structure as wild type. TM-AChE was expressed as an amphiphilic membrane-bound protein resistant to an enzyme that cleaves GPI-AChE (phosphatidylinositol-specific phospholipase C), and the same percentage of TM-AChE and GPI-AChE was on the cell surface according to immunofluorescence and pharmacological data. SEC1 and SEC2 were constructed by truncating the C-terminal signal peptide initially present in GPI-AChE: in SEC1 the last 25 residues of this 34-residue peptide were deleted while in SEC2 the last 29 were deleted. Both SEC1 and SEC2 were efficiently secreted and are very stable in culture medium; with one cloned SEC1-expressing line, AChE accumulated to as high as 100 mg/liter. Surprisingly, 5-10% of SEC1 was attached to a GPI anchor, but SEC2 showed no GPI anchoring. Since no differences in catalytic activity were observed among the four AChEs, and since the same percentage of GPI-AChE and TM-AChE were on the cell surface, we contend that in vivo experiments in which GPI-AChE is replaced can be interpreted solely on the basis of the altered anchoring domain.

Envelopment of varicella-zoster virus: targeting of viral glycoproteins to the trans-Golgi network

Previous studies suggested that varicella-zoster virus derives its final envelope from the trans-Golgi network (TGN) and that envelope glycoproteins (gps) are transported to the TGN independently of nucleocapsids. We tested the hypothesis that gpI is targeted to the TGN as a result of a signal sequence or patch encoded in its cytosolic domain. cDNAs encoding gpI wild type (wt) and a truncated mutant gpI(trc) lacking transmembrane and cytosolic domains were cloned by using the PCR. Cells transfected with cDNA encoding gpI(wt) or gpI(trc) synthesized and N glycosylated the proteins. gpI(wt) accumulated in the TGN, some reached the plasmalemma, but none was secreted. In contrast, gpI(trc) was retained and probably degraded in the endoplasmic reticulum; none was found on cell surfaces, but some was secreted. The distribution of gpI(trc) was not affected by deletion of potential glycosylation sites. To locate a potential gpI-targeting sequence, cells were transfected with cDNA encoding chimeric proteins in which the ectodomain of a plasmalemmal marker, the interleukin-2 receptor (tac), was fused to different domains of gpI. A chimeric protein in which tac was fused with the transmembrane and cytoplasmic domains of gpI was targeted to the TGN. In contrast, a chimeric protein in which tac was fused only with the gpI transmembrane domain passed through the TGN and concentrated in endosomes. We conclude that gpI is targeted to the TGN as a result of a targeting sequence or patch in its cytosolic domain.

Herpes simplex virus glycoprotein K promotes egress of virus particles

Herpes simplex virus (HSV) glycoprotein K (gK) is thought to be intimately involved in the process by which infected cells fuse because HSV syncytial mutations frequently alter the gK (UL53) gene. Previously, we characterized gK produced in cells infected with wild-type HSV or syncytial HSV mutants and found that the glycoprotein was localized to nuclear and endoplasmic reticulum membranes and did not reach the cell surface (L. Hutchinson, C. Roop, and D. C. Johnson, J. Virol. 69:4556-4563, 1995). In this study, we have characterized a mutant HSV type 1, denoted F-gK beta, in which a lacZ gene cassette was inserted into the gK coding sequences. Since gK was found to be essential for virus replication, F-gK beta was propagated on complementing cells which can express gK. F-gK beta produced normal plaques bounded by nonfused cells when plated on complementing cells, although syncytia were observed when the cells produced smaller amounts of gK. In contrast, F-gK beta produced only microscopic plaques on Vero cells and normal human fibroblasts (which do not express gK) and these plaques were reduced by 10(2) to 10(6) in number. Further, large numbers of nonenveloped capsids accumulated in the cytoplasm of F-gK beta-infected Vero cells, virus particles did not reach the cell surface, and the few enveloped particles that were produced exhibited a reduced capacity to enter cells and initiate an infection of complementing cells. Overexpression of gK in HSV-infected cells also caused defects in virus egress, although particles accumulated in the perinuclear space and large multilamellar membranous structures juxtaposed with the nuclear envelope were observed. Together, these results demonstrate that gK regulates or facilitates egress of HSV from cells. How this property is connected to cell fusion is not clear. In this regard, gK may alter cell surface transport of viral particles or other viral components directly involved in the fusion process.

Membrane anchoring domain of herpes simplex virus glycoprotein gB is sufficient for nuclear envelope localization

We have used the glycoprotein gB of herpes simplex virus type 1 (gB-1), which buds from the inner nuclear membrane, as a model protein to study localization of membrane proteins in the nuclear envelope. To determine whether specific domains of gB-1 glycoprotein are involved in localization in the nuclear envelope, we have used deletion mutants of gB-1 protein as well as chimeric proteins constructed by replacing the domains of the cell surface glycoprotein G of vesicular stomatitis virus with the corresponding domains of gB. Mutant and chimeric proteins expressed in COS cells were localized by immunoelectron microscopy. A chimeric protein (gB-G) containing the ectodomain of gB and the transmembrane and cytoplasmic domains of G did not localize in the nuclear envelope. When the ectodomain of G was fused to the transmembrane and cytoplasmic domains of gB, however, the resulting chimeric protein (G-gB) was localized in the nuclear envelope. Substitution of the transmembrane domain of G with the 69 hydrophobic amino acids containing the membrane anchoring domain of gB allowed the hybrid protein (G-tmgB) to be localized in the nuclear envelope, suggesting that residues 721 to 795 of gB can promote retention of proteins in the nuclear envelope. Deletion mutations in the hydrophobic region further showed that a transmembrane segment of 21 hydrophobic amino acids, residues 774 to 795 of gB, was sufficient for localization in the nuclear envelope. Since wild-type gB and the mutant and chimeric proteins that were localized in the nuclear envelope were also retained in the endoplasmic reticulum, the membrane spanning segment of gB could also influence retention in the endoplasmic reticulum.

Immunoelectron microscopic localization of herpes simplex virus glycoprotein gB in the nuclear envelope of infected cells

Herpesvirus, such as herpes simplex type 1 (HSV-1) acquire their envelope by budding through a modified inner membrane of the nuclear envelope which forms thick and dense patches at the site of budding. This suggests that some of the viral envelope glycoproteins must be transported to the nuclear envelope in order to be incorporated into the virus. In an effort to establish the localization of the HSV-1 glycoprotein gB-1 in the nuclear envelope of HSV-1 infected cells directly, we have studied the distribution of the glycoprotein gB-1 by immunoelectron microscopy using a polyclonal anti gB-1 antibody. A specific accumulation of gB-1 in the nuclear envelope, which was five times more labeled than the plasma membrane was observed. The glycoprotein gB-1 was localized in both the outer and the inner membrane of the nuclear envelope. The labeling over the nuclear envelope was distributed evenly and no preferential concentration of gB-1 around or within the patches where the virus buds was detected. The nucleocapsids were found to be labeled only when they become associated with the nuclear envelope indicating that gB-1 is incorporated into the virus at this site.

Temperature sensitivity of herpes simplex virus type 1 is a tissue-dependent phenomenon

The temperature sensitivity of herpes simplex virus type 1 (HSV-1) was assessed in primary cultures of mouse central nervous system (MNS) cells and mouse embryo cells (MEC). Infectious yields were determined and the ultrastructural morphogenesis of HSV-1 particles was compared following incubation at 37 or 40.5 degrees C. Yields of infectious virus were significantly reduced for both types of cell cultures following incubation at 40.5 degrees C. However, the effect of supraoptimal temperature (40.5 degrees C) on HSV-1 replication in MEC was significantly greater than the effect of supraoptimal temperature on virus replication in MNS cells. With respect to viral morphogenesis, no significant differences were found in either the quantity or the appearance (empty versus electron opaque core) of intranuclear particles present per infected nucleus, regardless of cell type or incubation temperature. However, complete virus particles (enveloped capsids with dense cores) were never observed in MEC at 40.5 degrees C, either intracytoplasmically or extracellularly. In contrast, complete virus particles were observed in MNS cell cultures at 40.5 degrees C, albeit in reduced numbers. At the permissive temperature (37 degrees C), complete intracytoplasmic and/or extracellular virus particles were associated with every infected cell in the MNS cell or MEC cultures. Thus an interactional effect on HSV-1 replication was found between cell culture type and incubation temperature.

Herpes simplex virus envelopment and maturation studied by fracture label

Herpes simplex virus envelopment and maturation were investigated by thin-section fracture label. The distribution of glycoproteins B and D was analyzed by labeling with antibodies; the precursor and mature forms of the glycoproteins were differentiated by labeling with the lectins concanavalin A (ConA) and wheat germ agglutinin (WGA), respectively. We report that the two glycoproteins were readily detected in the intracellular virion, whether located between the inner and outer nuclear membranes or within cytoplasmic membrane-bound vesicles and in the inner and outer nuclear membranes themselves. The enveloped virion between the inner and outer nuclear membranes labeled with ConA but not with WGA. During the transit to the extracellular space the reactivity of the virion membranes with ConA decreased and that with WGA ensued. The results document that herpes simplex viruses acquire at the inner nuclear membrane an envelope carrying the immature forms of the glycoproteins and that during the transit to the extracellular space the envelope glycoproteins become of the fully processed type.