The herpes simplex virus type 1 (HSV-1) mutant in1814 possesses an insertion mutation that abolishes trans-activation of immediate early (IE) transcription by the virion protein Vmw65. Interactions between in 1814 and the host cell were examined by use of an in vitro latency system which relies on infection of human foetal lung (HFL) cells at 42 °C to prevent lytic growth of virus. Mutant in 1814 was retained in HFL cells after infection at low m.o.i, and incubation at 42 °C, and was reactivated by superinfection of monolayers with viruses that express the HSV-1 IE protein Vmw110. Moreover, latency was established by in1814 in an analogous manner at 37 °C. The low cytotoxicity of in1814 enabled an investigation of latency after infection at high m.o.i. (five particles per cell) to be undertaken. At 42 °C, or at 37 oC in the presence of an inhibitor of DNA synthesis, in1814 DNA was maintained at low abundance (one to eight copies per infected cell) in a non-linear configuration. The absence of trans-activation by Vmw65 therefore predisposes HSV to latency, as opposed to lytic growth, in HFL cells, resulting in the retention of the genome in a form resembling that found in vivo.

Reactivation of latent herpes simplex virus type 2 (HSV-2) by the immediate-early protein VmwllO was studied by using an in vitro latency system. Adenovirus recombinants that express VmwllO reactivated latent HSV-2. An HSV-1 mutant possessing a deletion in a carboxy-terminal region of VmwllO reactivated latent HSV-2, whereas mutant FXE, which has a deletion in the second exon, did not. Therefore, VmwllO alone is required to reactivate latent HSV-2 in vitro, and the region of VmwllO defined by the deletion in FXE is important for this process.