ICP34.5 INFLUENCES HERPES-SIMPLEX VIRUS TYPE-1 MATURATION AND EGRESS FROM INFECTED-CELLS IN-VITRO

We have previously demonstrated that efficient replication of mutant h erpes simplex virus which fails to synthesize the polypeptide ICP34.5 is cell type and cell state dependent. ICP34.5 negative viruses do not grow in stationary state mouse embryo fibroblast 3T6 cells whereas th e growth kinetics in BHK cells are indistinguishable from those of wil d-type. We now demonstrate that this defect is not due to an inability of mutant virus to adsorb to 3T6 cells but rather to an inability to spread from the initially infected cells. Electron microscopic studies with wild-type HSV in both BHK and 3T6 cells revealed virus particles equally distributed between nucleus and cytoplasm, and additionally i n the extracellular matrix. In BHK cells infected with the ICP34.5 neg ative mutant 1716, virus is likewise distributed between nucleus and c ytoplasm but in 50 % of the infected cells there is marked delaminatio n and swelling of the nuclear membrane. In addition there is evidence of a significant number of particles trapped between the nuclear lamel lae. When 1716 is used to infect 3T6 cells, over 90 % of the virus par ticles are confined to the nuclei and the number of infected cells rem ains constant between 24 and 48 h with no increase in the proportion o f extracellular virus. Failure to express ICP34.5 appears therefore to result in a defect in virus maturation and egress from the nuclei of infected cells. Egress of HSV from the nuclei to the extracellular spa ce is thought to occur via two pathways. We postulate that lack of exp ression of ICP34.5 results in one of these pathways being blocked. In BHK cells this leads to overloading of the alternative pathway with a buildup of particles in the nuclear lamellae and associated endoplasmi c reticulum. In stationary state 3T6 cells, it appears that there is n o functional alternative pathway. We conclude that ICP34.5 exerts an e ffect on HSV maturation by controlling the passage of virus through in fected cells.