[Herpes simplex virus latency, reactivation, and a new antiviral therapy for herpetic keratitis]

Abstract

Although many factors that trigger the herpes simplex virus (HSV) reactivation from latency have been reported, how HSV resides in a latent state in the normal human cornea still needs to be defined. We therefore conducted a series of studies regarding various aspects of HSV infections. To understand how patients subjectively perceived changes in their daily life that could have induced HSV reactivation, we first performed a comprehensive survey on the subjective factors in patients who had experienced recurrent herpetic keratitis. The result of our survey revealed that stress, lack of sleep, shoulder stiffness, and physical fatigue were the key factors. There were various causes for stress, and stress associated with reactivation often occurred between spring and summer. Regarding HSV latency in the normal cornea, we used real-time polymerase chain reaction (PCR) to determine the presence of HSV in the donor and host corneas. The findings showed that on average, those host corneas with a history of HSV keratitis had 1.6 x 10(4) copies/mg of HSV DNA, while the host corneas without a history and the donor corneas had 8.7 and 4.9 x 10(2) copies/mg of HSV DNA, respectively. Based on these observations, it is reasonable to infer that latent viruses could have resided in a normal cornea without a history and were transmitted to a host cornea through corneal transplantation. We also quantified the virus load in tears before and after ocular surgery (one week after corneal transplantation or the next day after vitreous surgery). Our results indicated that both the detection rate and the average copy number of HSV DNA had a tendency to increase postperatively. Moreover, we tried to differentiate the HSV strains that were involved in the recurrent lesions. In only one of the studied cases, could we find a single different nucleotide between two HSV strains. It seemed possible that two different strains of HSV had set off the same episode of reactivation. In recent years, chemokines have become known for their action in mediating inflammatory diseases. We suspected that chemokines might also play a role in the antiviral mechanism and examined the chemokine-derived antiviral activity. We used eight chemokines, including RANTES/CCL5, MIP-lalpha/ CCL3, and MIP-1beta/CCL4, in a murine HSK model with Vero cells. These chemokines directly bound to HSV and the chemokine-bound HSV was later resisted by the neutralizing antibody of envelope protein gB. Furthermore, by electron microscope analysis, it became clear that these chemokines had cut an opening in the HSV envelope. Consequently, these chemokines had significantly inhibited the HSV infection on Vero cells. In addition, the virus load in tears was decreased and the corneal opacity was less severe. We concluded in that study that during early infection, chemokines accumulated in the corneal stroma have the ability to protect cells and tissues from HSV infection. As for antiviral therapy, acyclovir (ACV) eye ointment has been effective for patients with herpetic keratitis. However, patients often find it difficult to successfully follow the treatment due to the required frequent application and the blurred vision after application. On the other hand, valaciclovir (VCV), which is the oral prodrug of ACV, has become commercially available in recent years for treating nonocular herpetic diseases. We therefore examined and compared the efficacies of oral VCV, oral ACV, ACV eye ointment, and ACV eye drops in a murine keratitis model; the group treated with oral VCV did show a significantly good antiviral effect. We have proved that oral VCV can be a beneficial alternative antiviral therapy for patients with difficulty in complying with the ACV eye ointment treatment.