CELL-TYPE AND CELL STATE DETERMINE DIFFERENTIAL IN-VITRO GROWTH OF NON-NEUROVIRULENT ICP34.5-NEGATIVE HERPES-SIMPLEX VIRUS TYPE-1 AND TYPE-2

The herpes simplex virus (HSV) gene RL1 encodes the protein ICP34.5, w hich is a specific neurovirulence factor. Null mutants in RL1 fail to replicate in the central nervous system of mice and are therefore tota lly non-neurovirulent. Additionally, they fail to replicate in neurons of the peripheral nervous system, although they are capable of establ ishing and reactivating from a latent infection. As the precise functi on of ICP34.5 in HSV-neuronal interactions is unknown, we have studied the role of ICP34.5 in vitro by examining in detail the phenotypes of RL1-negative viruses in two defined tissue culture systems. The first was mouse embryo fibroblast 3T6 cells, in which RL1-negative mutants are impaired and the in vivo phenotype is mimicked. This impairment is amplified when the cells are in the stationary state. The second was mouse embryo testicular carcinoma F9 cells which, in the undifferentia ted state, provide a reversal of phenotype; wild-type virus fails to g row but RL1-negative virus replicates efficiently. Differentiation res ults in the ability to support wild-type virus growth. The stage at wh ich the replication cycle is blocked plus the role of cellular factors is addressed in both tissue culture systems. Evidence is provided tha t cell type and cell state are crucial to ICP34.5-cellular interaction and hence, based on these parameters, ICP34.5 can be defined as a hos t-range determinant. Identification of cellular proteins that specific ally interact with or are homologues of ICP34.5 may lead to the identi fication of neuron-specific proteins that have a similar role.