THE LATENT HERPES-SIMPLEX VIRUS TYPE-1 GENOME COPY NUMBER IN INDIVIDUAL NEURONS IS VIRUS-STRAIN SPECIFIC AND CORRELATES WITH REACTIVATION

The viral genetic elements that determine the in vivo reactivation eff iciencies of fully replication competent wild-type herpes simplex viru s (HSV) strains have not been identified. Among the common laboratory strains, KOS reactivates in vivo at a lower efficiency than either str ain 17syn+ or strain McKrae., An important first step in understanding the molecular basis for this observation is to distinguish between vi ral genetic factors that regulate the establishment of latency from th ose that directly regulate reactivation. Reported here are experiments performed to determine whether the reduced reactivation of KOS was as sociated with a reduced ability to establish or maintain latent infect ions, For comparative purposes, latent infections were quantified by ( i) quantitative PCR on DNA extracted from whole ganglia, (ii) the numb er of latency-associated transcript (LAT) promoter-positive neurons, u sing KOS and 17syn+ LAT promoter-beta-galactosidase reporter mutants, and (iii) contextual analysis of DNA, Mice latently infected with 17sy n+-based strains contained more HSV type 1 (HSV-1) DNA in their gangli a than those infected with KOS strains, but this difference was not st atistically significant. The number of latently infected neurons also did not differ significantly between ganglia latently infected with ei ther the low- or high-reactivator strains. In addition to the number o f latent sites, the number of viral genome copies within the individua l latently infected neurons has recently been demonstrated to be varia ble. Interestingly, neurons latently infected with KOS contained signi ficantly fewer viral genome copies than those infected with either 17s yn+ or McKrae. Thus, the HSV-1 genome copy number profile is viral str ain specific and positively correlates with the ability to reactivate in vivo. This is the first demonstration that the number of HSV genome copies within individual latently infected neurons is regulated by vi ral genetic factors, These findings suggest that the latent genome cop y number may be an important parameter for subsequent induced reactiva tion in vivo.