A deletion mutant of the latency-associated transcript of herpes simplex virus type 1 reactivates from the latent state with reduced frequency.

We have generated and characterized a deletion mutant of herpes simplex virus type-1, dlLAT1.8, which lacks the putative promoter region, transcriptional start site, and 1,015 base pairs of the DNA sequences specifying the latency-associated transcripts (LATs). When tested in a CD-1 mouse ocular model, dlLAT1.8 was replication competent in the eye and in ganglia during acute infection but reactivated from explant cultures of ganglia with reduced efficiency (49%) relative to those of wild-type and marker-rescued viruses (94 and 85%, respectively) despite the fact that levels of mutant viral DNA in ganglia during latent infection were comparable to wild-type levels. The neurovirulence of KOS was not significantly altered by the removal of sequences specifying the LATs, as judged by numbers of animals dying on or before 30 days postinfection. Examination of ganglia latently infected with dlLAT1.8 by in situ hybridization revealed no LAT expression. The genotype of reactivated virus was identical to that of input dlLAT1.8 virus as judged by Southern blot analysis. These studies suggest that although the LATs are not essential for the establishment and reactivation of latency in our model, they may play a role in determining the frequency of reactivation of virus from the latent state.