Mutants of herpes simplex virus type 1 (HSV-1) lacking glycoproteins gG, gE, gI or the putative gJ were constructed by inserting a expression cassette within the US4, US8, US7 and US5 genes respectively. Revertant viruses were then constructed by rescue with a wild-type DNA fragment. Each of these mutant viruses, by comparison with the parental virus HSV-1 SC16, exhibited normal particle to infectivity ratios, and had no discernible phenotypic abnormalities in baby hamster kidney-21 cells following high or low multiplicity infections. Infection of mice by scarification of the ear with these mutant viruses showed the following. (i) Interruption of the US5 (gJ) gene has no effect on the ability of HSV-1 to multiply at the inoculation site or its ability to enter or multiply in the peripheral or central nervous system (CNS). This shows that the US5 gene provides a convenient site for the insertion of foreign genes for both and studies. (ii) Disruption of the US4 (gG) gene results in marginal attenuation in the mouse ear model. (iii) Disruption of the US7 (gI) or US8 (gE) genes results in pronounced attenuation; virus was rapidly cleared from the inoculation site and was barely detectable in sensory ganglia or in the CNS. The failure of gI-negative or gE-negative viruses to replicate efficiently at the inoculation site led to the investigation of virus behaviour in epithelial cells . Viruses lacking gE or gI adsorbed to and entered these cells at normal rates compared with the parental virus, but formed minute plaques. This is consistent with a failure of cell-to-cell spread by the cell contact route. This was confirmed by measurement of the rate of increase in infectious centre numbers following low multiplicity infections. The view that gE and gI influence interactions between cells at the plasma membrane was reinforced by showing that the introduction of disrupted gE or gI genes into a syncytial, but otherwise syngeneic, background resulted in a non-syncytial phenotype. We conclude that the gE-gI complex plays a part, at least in some cell types, in the interactions at the cell surface that allow transmission of the virus from infected to uninfected cells by cell contact. In syncytial strains this leads to uncontrolled membrane fusion. The observation that virions lacking gE or gI enter cells at apparently normal rates reinforces the view that cell-cell fusion is not analogous to the fusion of the virion envelope with the plasma membrane for nucleocapsid entry. It is also apparent that the phenotypes of HSV-1 mutants lacking gI or gE are similar in many respects to those reported for mutants of pseudorabies virus lacking the gE homologue.


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