Role of herpes simplex virus thymidine kinase expression in viral pathogenesis and latency

Herpes simplex virus (HSV) thymidine kinase (TK) expression and the HSV TK gene have been evaluated in studies of gene control, as well as in animal and human studies of viral pathogenesis, including HSV latency. In investigations of the biological role of HSV TK, enzyme expression was noted to be important for HSV infection of nonreplicating cells in culture; and, in experimental animal studies, HSV TK was shown to be important for in vivo latent infection of sensory ganglion neurons. Latency in these studies was determined by the ability of HSV to reactivate from sensory ganglion explants. In recent studies, investigators sought to determine whether the role HSV TK expression plays in latency is primarily in the establishment and maintenance of latency or in the reactivation process. Following infection of experimental animals with HSV TK-deficient mutants, the presence of HSV in ganglia was detected in complementation, rescue, and molecular biological studies. Results suggest that HSV TK expression may be important for HSV reactivation from latency. This was supported by in situ hybridization investigations. In the latter studies, HSV latency associated transcript (LAT) was present in ganglion neurons, although reactivation of HSV from such ganglia was defective. LAT-expressing, reactivation-defective infections established by TK mutants of HSV are considered examples of incomplete latency. From the present review, it appears that HSV TK expression, particularly TK expression of HSV-1, is important for the reactivation of latent HSV infection of sensory ganglion neurons, probably because of limited neuronal TK expression and absent replication capacity of these cells.