Egress of varicella-zoster virus from the melanoma cell: a tropism for the melanocyte

Abstract

The pathway of envelopment and egress of the varicella-zoster virus (VZV) and the primary site of viral production within the epidermal layer of the skin are not fully understood. There are several hypotheses to explain how the virus may receive an envelope as it travels to the surface of the monolayer. In this study, we expand earlier reports and provide a more detailed explanation of the growth of VZV in human melanoma cells. Human melanoma cells were selected because they are a malignant derivative of the melanocyte, the melaninproducing cell which originates in the neural crest. We were able to observe the cytopathic effects of syncytial formation and the pattern of egress of virions at the surfaces of infected monolayers by scanning electron microscopy and laser-scanning confocal microscopy. The egressed virions did not appear uniformly over the syncytial surface, rather they were present in elongated patterns which were designated viral highways. In order to document the pathway by which VZV travels from the host cell nucleus to the outer cell membrane, melanoma cells were infected and then processed for examination by transmission electron microscopy (TEM) at increasing intervals postinfection. At the early time points, within minutes to hours postinfection, it was not possible to localize the input virus by TEM. Thus, viral particles first observed at 24 h postinfection were considered progeny virus. On the basis of the TEM observations, the following sequence of events was considered most likely. Nucleocapsids passed through the inner nuclear membrane and acquired an envelope, after which they were seen in the endoplasmic reticulum. Enveloped virions within vacuoles derived from the endoplasmic reticulum passed into the cytoplasm. Thereafter, vacuoles containing nascent enveloped particles acquired viral glycoproteins by fusion with vesicles derived from the Golgi. The vacuoles containing virions fused with the outer plasma membrane and the particles appeared on the surface of the infected cell. Late in infection, enveloped virions were also present within the nuclei of infected cells; the most likely mechanism was retrograde flow from the perinuclear space back into the nucleus. Thus, this study suggests a role for the melanocyte in the pathogenesis of VZV infection, because all steps in viral egress can be accounted for if VZV subsumes the cellular pathways required for melanogenesis.

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

“…Whether these different ways are mutually exclusive or are indicative of multiple pathways for herpesvirus egress is unclear at present. A cell-type dependence in this respect was demonstrated for VZV (18). However, we were unable to detect differences using cells from either pigs, cattle, or monkeys.…”

“…EM and immunolabeling of PrV. For routine electron microscopy (EM), noninfected and infected cell cultures in petri dishes (4,5,8,10,12,14,16,18,20, and 24 h postinfection [p.i.]) were fixed at different times after infection for 60 min with 2.5% glutaraldehyde buffered in 0.1 M Na-cacodylate (pH 7.2; 300 mOsmol) (Merck, Darmstadt, Germany).…”

Autophagy and the Effects of Its Inhibition on Varicella-Zoster Virus Glycoprotein Biosynthesis and Infectivity

“…Because of the low titer of the inoculum, several replication cycles are required before all cells in a monolayer are infected (50). Another confounder is that VZV is highly fusogenic in certain cells; thus, fusion of one infected cell with a second newly infected cell obscures the end of a replication cycle and the beginning of another (67).…”

Deletion of the R78 G Protein-Coupled Receptor Gene from Rat Cytomegalovirus Results in an Attenuated, Syncytium-Inducing Mutant Strain

“…Previously, syncytium formation has been observed in many different cell types infected with a variety of herpesvirus species: HCMV-infected human amnion cells (28), varicella-zoster virus-infected human melanoma cells (34), Epstein-Barr virus-superinfected Raji cells (8), HHV-6-infected human primary fetal astrocytes (35), HHV-7-infected T lymphocytes (56), and RCMV-infected Rat2 cells (11). In these cases, syncytium formation appears to be associated with the cell type or MOI, since the same virus strains fail to produce these syncytium (syn) phenotypes upon infection of other permissive cell lines (8,28,34,35) or after infection at lower titers (8,56). Well-defined syn loci were found within genomes of a limited number of herpesvirus species, in particular within the genomes of herpes simplex type 1 (HSV-1) strains: UL20 (6), UL24 (40), UL27-gB (29), and UL53-gK (37).…”