Role of cis-acting sequences of the ICPO promoter of herpes simplex virus type 1 in viral pathogenesis, latency and reactivation.

A mutant herpes simplex virus type 1, termed delta Tfi, with a 350 bp deletion of the Sp1, NF-kappaB, TAATGARATs, C/EBP and F2 DNA-binding elements from -420 to -70 relative to the transcriptional start site of ICPO (Vmw 110), was generated and characterized. The efficiency of plating of delta Tfi was reduced on Vero cells and it expressed correctly initiated ICPO RNA in the presence of cycloheximide, although RNA levels were 2.5-fold lower than with wild-type (KOS) and marker-rescued (delta TfiR) viruses. This was consistent with a demonstrated reduction in ICPO protein expression for delta Tfi at early times post-infection and a 3-fold reduction in ICPO-dependent transactivation of the ICP6 promoter. KOS, delta Tfi and delta TfiR replication in murine corneas and trigeminal ganglia were comparable when measured on a complementing cell line, but delta Tfi titres appeared 15- to 50-fold lower when measured on Vero cells. delta Tfi was correspondingly less virulent than wild-type or marker-rescued viruses in both immunocompetent and SCID mice. delta Tfi, however, established and reactivated from latency with efficiencies comparable to wild-type and marker-rescued viruses. These results demonstrate that although this deletion of the ICPO promoter results in lower levels of ICPO in vitro and decreased virulence in vivo, the establishment of and reactivation from latency are unaffected. This indicates that elements which regulate ICPO expression and virulence during acute infection may be distinct from those involved in reactivation.