RECOMBINANTS ARE ISOLATED AT HIGH-FREQUENCY FOLLOWING IN-VIVO MIXED OCULAR INFECTION WITH 2 AVIRULENT HERPES-SIMPLEX VIRUS TYPE-1 STRAINS

Mixed infections with different strains of herpes simplex virus type 1 (HSV-1) may result in more severe disease than infection with either strain alone. This phenomenon is important because it may facilitate t he identification of virulence genes through the transfer of virulence determinants between complementing strains, and it may pose a problem in the use of attenuated HSV strains for vaccines and gene delivery v ectors. In this study, we have compared the percentage of recombinants present after mixed infection with HSV-1 strains OD4 and 994 in vitro and in vivo. After corneal inoculation, we found that 74% of randomly picked isolates from the trigeminal ganglia were recombinants, compar ed with 59% from the cornea. Twenty-six percent of randomly picked iso lates were recombinant following mixed infection of Vero cells in vitr o. Seventeen recombinant strains isolated from the in vivo mixed infec tions were assayed for ocular virulence, and they were found to exhibi t a wide range of virulence phenotypes. The presence of virulent recom binants suggests that recombination plays a role in the increased dise ase observed in this mixed infection, and the broad range of virulence indicates that there may be multiple genetic factors involved in the increased virulence observed after mixed infection with these two stra ins. The recombinants were also tested for their ability to grow in NI H 3T3 fibroblasts, and though some correlation was observed between gr owth in vitro and ability to cause ocular disease, improved growth in murine cells does not sufficiently explain the increased virulence obs erved in some recombinants.