Human Cytomegalovirus. In: Barbanti-brodano G, Bendinelli M, Friedman H, editors. DNA Tumor Viruses. Boston: Springer US; 1995. pp. 295-312. [DOI: 10.1007/978-1-4899-1100-1_16]

Analysis of genomic homology of murine gammaherpesvirus (MHV)-72 to MHV-68 and impact of MHV-72 on the survival and tumorigenesis in the MHV-72-infected CB17 scid/scid and CB17+/+ mice

Murine gammaherpesvirus (MHV)-68-infected mice are well-known as models for Epstein-Barr virus (EBV)-related lymphoproliferative diseases. MHV-72 may be a relative of MHV-68, but any genetic comparison between the two (except for the M7 gene) has never been reported. The genetic compositions of MHV-72 and MHV-68 were compared and the pathology of MHV-72 infection studied in CB-17 severe combined immunodeficiency (scid/scid; SCID) and CB17 wild-type (CB17+/+) mice. The MHV-72 DNA sequence was almost identical to MHV-68 except for approximately 7000 bp corresponding to the MHV-68 M1-M3 genes. Twenty-seven of 30 MHV-72-infected SCID mice (90%) died from generalized infection with intranuclear viral inclusions for approximately 1 month, while MHV-72-infected CB17+/+ mice recovered from acute infection. Long observation and pathological study of 68 MHV-72-infected mice for up to 24 months revealed that the survival rate (29.4%) and survival time (21.3 months) of MHV-72-infected CB17+/+ mice were significantly lower (P = 0.0127) and shorter (P = 0.0065) than those of the controls (61.1% and 22.9 months), respectively. The malignancy development rate (60.3%) of the infected CB17+/+ mice was also significantly higher (P = 0.004) than those of the controls (22.2%). However, no MHV-72 DNA was detected in the tumors of infected mice. MHV-72 may have some tumor-promoting effects but the tumorigenesis in infected CB17+/+ mice is different from EBV-associated tumors.

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.

Human cytomegalovirus virions differentially incorporate viral and host cell RNA during the assembly process

While analyzing human cytomegalovirus (HCMV) gene expression in infected cells by RNA-specific nucleic acid sequence-based amplification (NASBA), positive results were observed for HCMV RNA encoded by several viral genes immediately after the addition of the virus. UV-inactivated virus also gave a positive NASBA result without establishing active infection, suggesting that RNA was associated with the inoculum. Highly purified virions devoid of cellular contamination proved to be positive for viral RNA encoding both immediate-early (UL123) and late (UL65) gene products. Virion-associated RNA might be incorporated specifically or without selection during the virion assembly. In the latter case, cellular RNA would also be present in the virion. A high-abundant cellular RNA encoded by GAPDH and even U1A RNA, which is expressed at low levels, were detected in the virion fraction, whereas cellular DNA was absent. Virion fractionation revealed that cellular RNA was absent in purified de-enveloped capsids. In conclusion, cellular and viral RNA was present between the capsid and envelope of the virion, whereas in the capsid only viral RNA could be detected. The results suggest that virion-associated viral and cellular RNA is incorporated nonspecifically during virion assembly.

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.