The herpes simplex virus type 1 ribonucleotide reductase is required for acute retinal disease

We have used a herpes simplex virus type 1 (HSV-1) ribonucleotide reductase (RR) null mutant (ICP6Δ) to determine if the HSV-1 RR is required for acute retinal disease. Injection of the ICP6Δ mutant into the vitreous induced mild transient signs of infection (vitreal infiltrate, retinal inflammation, and changes in retinal cytology). In contrast, the parental KOS and a revertant virus (ICP6Δ + 3.1) in which the RR gene had been restored, caused severe retinitis. Injection of media alone also induced mild transient signs of disease. Two months after infection, ICP6Δ injected eyes could not be distinguished from normal eyes. Repeated injection of ICP6Δ (3 times, 2 weeks apart) resulted in vitreal infiltrate near the site of injection but the retina did not appear damaged. The mutant, ICP6Δ, grew to peak titers 1 x 10³ to 1 x 10⁵-fold lower and cleared faster than KOS or ICP6Δ + 3.1 in the injected eyes suggesting that the reduced virulence was due to reduced ability of the virus to grow. These results show that the viral RR is required for acute retinal disease.