A thin-section study of the morphogenesis of Aujeszky's disease virus in synchronously infected cell cultures

Egress of alphaherpesviruses: comparative ultrastructural study

Egress of four important alphaherpesviruses, equine herpesvirus 1 (EHV-1), herpes simplex virus type 1 (HSV-1), infectious laryngotracheitis virus (ILTV), and pseudorabies virus (PrV), was investigated by electron microscopy of infected cell lines of different origins. In all virus-cell systems analyzed, similar observations were made concerning the different stages of virion morphogenesis. After intranuclear assembly, nucleocapsids bud at the inner leaflet of the nuclear membrane, resulting in enveloped particles in the perinuclear space that contain a sharply bordered rim of tegument and a smooth envelope surface. Egress from the perinuclear cisterna primarily occurs by fusion of the primary envelope with the outer leaflet of the nuclear membrane, which has been visualized for HSV-1 and EHV-1 for the first time. The resulting intracytoplasmic naked nucleocapsids are enveloped at membranes of the trans-Golgi network (TGN), as shown by immunogold labeling with a TGN-specific antiserum. Virions containing their final envelope differ in morphology from particles within the perinuclear cisterna by visible surface projections and a diffuse tegument. Particularly striking was the addition of a large amount of tegument material to ILTV capsids in the cytoplasm. Extracellular virions were morphologically identical to virions within Golgi-derived vesicles, but distinct from virions in the perinuclear space. Studies with gB- and gH-deleted PrV mutants indicated that these two glycoproteins, which are essential for virus entry and direct cell-to-cell spread, are dispensable for egress. Taken together, our studies indicate that the deenvelopment-reenvelopment process of herpesvirus maturation also occurs in EHV-1, HSV-1, and ILTV and that membrane fusion processes occurring during egress are substantially different from those during entry and direct viral cell-to-cell spread.

Ultrastructural analysis of the replication cycle of pseudorabies virus in cell culture: a reassessment

We reinvestigated major steps in the replicative cycle of pseudorabies virus (PrV) by electron microscopy of infected cultured cells. Virions attached to the cell surface were found in two distinct stages, with a distance of 12 to 14 nm or 6 to 8 nm between virion envelope and cell surface, respectively. After fusion of virion envelope and cell membrane, immunogold labeling using a monoclonal antibody against the envelope glycoprotein gE demonstrated a rapid drift of gE from the fusion site, indicating significant lateral movement of viral glycoproteins during or immediately after the fusion event. Naked nucleocapsids in the cytoplasm frequently appeared close to microtubules prior to transport to nuclear pores. At the nuclear pore, nucleocapsids invariably were oriented with one vertex pointing to the central granulum at a distance of about 40 nm and viral DNA appeared to be released via the vertex region into the nucleoplasm. Intranuclear maturation followed the typical herpesvirus nucleocapsid morphogenesis pathway. Regarding egress, our observations indicate that primary envelopment of nucleocapsids occurred at the inner leaflet of the nuclear membrane by budding into the perinuclear cisterna. This nuclear membrane-derived envelope exhibited a smooth surface which contrasts the envelope obtained by putative reenvelopment at tubular vesicles in the Golgi area which is characterized by distinct surface projections. Loss of the primary envelope and release of the nucleocapsid into the cytoplasm appeared to occur by fusion of envelope and outer leaflet of the nuclear membrane. Nucleocapsids were also found engulfed by both lamella of the nuclear membrane. This vesiculation process released nucleocapsids surrounded by two membranes into the cytoplasm. Our data also indicate that fusion between the two membranes then leads to release of naked nucleocapsids in the Golgi area. Egress of virions appeared to occur via transport vesicles containing one or more virus particles by fusion of vesicle and cell membrane. Our data thus support biochemical data and mutant virus studies of (i) two steps of attachment, (ii) the involvement of microtubules in the transport of nucleocapsids to the nuclear pore, and (iii) secondary envelopment in the trans-Golgi area in PrV infection.

Pathogenesis of Aujeszky's disease virus infection in swine tracheal organ culture

Two different strains of Aujeszky's disease virus (ADV) were inoculated into swine tracheal organ culture. Both viruses replicated in and destroyed the tracheal epithelium and epithelial cells. ADV antigen was first localized in ciliated epithelial cells by fluorescent antibody and immunoperoxidase examinations. Corresponding to the distribution of ADV antigen, many ADV particles were observed in ciliated epithelial cells. Results demonstrated that the tracheal epithelium infected with ADV is reduced in its ciliary activity.

Effect of brefeldin A on alphaherpesvirus membrane protein glycosylation and virus egress

In this work we used brefeldin A (BFA), a specific inhibitor of export to the Golgi apparatus, to study pseudorabies virus viral glycoprotein processing and virus egress. BFA had little effect on initial synthesis and cotranslational modification of viral glycoproteins in the endoplasmic reticulum (ER), but it disrupted subsequent glycoprotein maturation and export. Additionally, single-step growth experiments demonstrated that after the addition of BFA, accumulation of infectious virus stopped abruptly. BFA interruption of virus egress was reversible. Electron microscopic analysis of infected cells demonstrated BFA-induced disappearance of the Golgi apparatus accompanied by a dramatic accumulation of enveloped virions between the inner and outer nuclear membranes and also in the ER. Large numbers of envelope-free capsids were also present in the cytoplasm of all samples. In control samples, these capsids were preferentially associated with the forming face of Golgi bodies and acquired a membrane envelope derived from the trans-cisternae. Our results are consistent with a multistep pathway for envelopment of pseudorabies virus that involves initial acquisition of a membrane by budding of capsids through the inner leaf of the nuclear envelope followed by deenvelopment and release of these capsids from the ER into the cytoplasm in proximity to the trans-Golgi. The released capsids then acquire a bilaminar double envelope containing mature viral glycoproteins at the trans-Golgi. The resulting double-membraned virus is transported to the plasma membrane, where membrane fusion releases a mature, enveloped virus particle from the cell.

Neuronal and transneuronal tracing in the trigeminal system of the rat using the herpes virus suis

The herpes virus suis has been used as a tracer for the pathways in the central nervous system of the rat. The viruses have been inoculated in various peripheral structures innervated by the trigeminal nerve, namely in the cornea by instillation or scarification, in the anterior chamber of the eye by injection and also by subconjunctival injection, nasal instillation and injection in the masseter muscle. The herpes virus suis is easy to detect by immunofluorescence or electron microscopy, the tracing is precise because it does not diffuse, as some other tracers. The virus is replicated at the site of inoculation and at each neuronal relay, thus 'fresh' tracer is continuously brought into the system. The herpes virus suis is transported by retrograde axonal flow. It has been observed in the motor sensory, sympathetic and parasympathetic pathways up to the central nuclei, which demonstrates transneuronal transport. The selectivity of this tracer, applied to the trigeminal pathways, has allowed us to understand the function of the 3 types of neurons present in the trigeminal ganglion, namely to confirm their somatotopy and establish their central projections in the trigeminal system.

Studies on Herpesvirus scophthalmi infection of turbot Scophthalmus maximus (L.) ultrastructural observations

Abstract. Recent heavy mortalities amongst O+ group turbot at a fish farm were found to be associated with a herpes‐type viral infection of the epithelia of the skin and gills. The morphology of the virus is described with ultrastructural observations on its morphogenesis and release from infected cells.