Mixed infection with herpes simplex virus type 1 generates recombinants with increased ocular and neurovirulence

The authors used the method of mixed ocular infection and subsequent in vivo selection to isolate Herpes simplex virus type 1 intratypic recombinants with increased ocular virulence and neurovirulence. Four recombinants were studied in some detail (DRG1A3, DRG2A2, DRG3A3, and DRG4A1). The recombinants had lethal doses in 50% of animals tested (LD50) at least 2-3 log units lower than either parent virus (OD4 and CJ394) and caused significantly more severe stromal keratitis, vascularization of the cornea, and blepharitis than either parent. Studies on the ability of DRG1A3 and DRG4A1 to replicate in the eye, trigeminal ganglia, and brain showed that these recombinants replicated to higher titers (1-3.5 log units) than the parents in all three tissues. One of the parents, OD4, spread to the central nervous system with the same kinetics as CJ394, DRG1A3, and DRG4A1 but had a restricted ability to replicate in all tissues, which may account for its lack of virulence. The other parent, CJ394, was nonneurovirulent but replicated to titers which were only 1-1.5 log units lower than the neurovirulent recombinants. These recombinants should be useful in studying virulence determinants in herpetic ocular infections.