Reactivation of thymidine kinase-defective herpes simplex virus is enhanced by nucleoside

Competition and complementation between thymidine kinase-negative and wild-type herpes simplex virus during co-infection of mouse trigeminal ganglia

Laboratory strains of herpes simplex virus lacking thymidine kinase (TK) cannot replicate acutely to detectable levels in mouse trigeminal ganglia and do not reactivate from latency. However, many pathogenic clinical isolates that are resistant to the antiviral drug acyclovir are heterogeneous populations of TK-negative (TK(-)) and TK-positive (TK(+)) viruses. To recapitulate this in vivo, mice were infected with mixtures of wild-type virus and a recombinant TK(-) mutant in various ratios. Following co-infection, the replication, number of latent viral genomes and reactivation efficiency of TK(+) virus in trigeminal ganglia were reduced in a manner related to the amount of TK(-) virus in the inoculum. TK(+) virus did not always complement the acute replication or increase the number of latent viral genomes of TK(-) mutant in mouse ganglia. Even so, TK(+) virus could still confer the pathogenic phenotype to a TK(-) mutant, somehow providing sufficient TK activity in trans to permit a TK(-) mutant to reactivate from latently infected ganglia.

Intracellular uptake of thymidine and antiherpetic drugs for thymidine kinase-deficient mutants of herpes simplex virus type 1

The influence of the thymidine (Thd) kinase (TK) of herpes simplex virus type 1 (HSV-1) on the intracellular uptake and anabolism of nucleosides has been investigated. To compare the differences between the TK-positive (TK(+)) and TK-deficient strains, acyclovir (ACV)-resistant strains were cloned from a cell culture and classified into 2 groups, viz. the TK-partial (TK(p)) and TK-negative (TK(-)). The cellular uptake of thymidine was highly dependent on the viral TK (vTK) activity. The TK(+) strain showed the highest level of intracellular thymidine uptake, the TK(p) strain a moderate level, which varied from strain to strain, and the TK(-) and mock strains showed little uptake. The inhibition of viral replication by ACV, ganciclovir (GCV) and penciclovir (PCV) did not decrease the Thd uptake at all. On the contrary, a notable increase found to be induced by ACV. The influence of the vTK on the uptake of GCV or PCV was much greater than that of ACV. The metabolism was generally less dependent on the vTK activity than the influx. The influx and phosphorylation rates of GCV and PCV were dependent on the substrate specificity of the vTK.

Failure of thymidine kinase-negative herpes simplex virus to reactivate from latency following efficient establishment

Thymidine kinase-negative mutants of herpes simplex virus did not reactivate from latency in mouse trigeminal ganglia, even when their latent viral loads were comparable to those that permitted reactivation by wild-type virus. Thus, reduced establishment of latency does not suffice to account for the failure to reactivate.

Translational compensation of a frameshift mutation affecting herpes simplex virus thymidine kinase is sufficient to permit reactivation from latency

Herpes simplex virus thymidine kinase is important for reactivation of virus from its latent state and is a target for the antiviral drug acyclovir. Most acyclovir-resistant isolates have mutations in the thymidine kinase gene; however, how these mutations confer clinically relevant resistance is unclear. Reactivation from explanted mouse ganglia was previously observed with a patient-derived drug-resistant isolate carrying a single guanine insertion within a run of guanines in the thymidine kinase gene. Despite this mutation, low levels of active enzyme were synthesized following an unusual ribosomal frameshift. Here we report that a virus, generated from a pretherapy isolate from the same patient, engineered to lack thymidine kinase activity, was competent for reactivation. This suggested that the clinical isolate contains alleles of other genes that permit reactivation in the absence of thymidine kinase. Therefore, to establish whether thymidine kinase synthesized via a ribosomal frameshift was sufficient for reactivation under conditions where reactivation requires this enzyme, we introduced the mutation into the well-characterized strain KOS. This mutant virus reactivated from latency, albeit less efficiently than KOS. Plaque autoradiography revealed three phenotypes of reactivating viruses: uniformly low thymidine kinase activity, mixed high and low activity, and uniformly high activity. We generated a recombinant thymidine kinase-null virus from a reactivating virus expressing uniformly low activity. This virus did not reactivate, confirming that mutations in other genes that would influence reactivation had not arisen. Therefore, in strains that require thymidine kinase for reactivation from latency, low levels of enzyme synthesized via a ribosomal frameshift can suffice.

Inhibition of herpes simplex virus reactivation by dipyridamole

Herpes simplex virus (HSV) reactivation from latency was investigated. Reactivation of thymidine kinase-negative HSV, which is defective for reactivation, was greatly enhanced by thymidine (TdR). The reactivation-enhancing effect of TdR was blocked by dipyridamole (DPM), a known nucleoside transport inhibitor. DPM also inhibited wild-type HSV reactivation, suggesting potential antiviral use.

Optimized viral dose and transient immunosuppression enable herpes simplex virus ICP0-null mutants To establish wild-type levels of latency in vivo

The reduced efficiency with which herpes simplex virus type 1 (HSV-1) mutants establish latent infections in vivo has been a fundamental obstacle in efforts to determine the roles of individual viral genes in HSV-1 reactivation. For example, in the absence of the "nonessential" viral immediate-early protein, ICP0, HSV-1 is severely impaired in its ability to (i) replicate at the site of inoculation and (ii) establish latency in neurons of the peripheral nervous system. The mouse ocular model of HSV latency was used in the present study to determine if the conditions of infection can be manipulated such that replication-impaired, ICP0-null mutants establish wild-type levels of latency, as measured by viral genome loads in latently infected trigeminal ganglia (TG). To this end, the effects of inoculum size and transient immunosuppression on the levels of acute replication in mouse eyes and of viral DNA in latently infected TG were examined. Following inoculation of mice with 2 x 10(3), 2 x 10(4), 2 x 10(5), or 2 x 10(6) PFU/eye, wild-type virus replicated in mouse eyes and established latency in TG with similar efficiencies at all four doses. In contrast, increasing the inoculum size of the ICP0-null mutants n212 and 7134 from 2 x 10(5) to 2 x 10(6) PFU/eye significantly decreased the levels of infectious virus detected in the tear films of mice from days 4 to 9 postinfection. In an attempt to establish the biological basis for this finding, the effect of viral dose on the induction of the host proinflammatory response was examined. Quantitative reverse transcription-PCR demonstrated that increasing the inoculum of 7134 from 2 x 10(4) to 2 x 10(6) PFU/eye significantly increased the expression of proinflammatory (interleukin 6), cell adhesion (intercellular adhesion molecule 1), and phagocyte-associated (CD11b) genes in mouse eyes 24 h postinfection. Furthermore, transient immunosuppression of mice with cyclophosphamide, but not cyclosporin A, significantly enhanced both the levels of acute n212 and 7134 replication in the eye and the levels of mutant viral genomes present in latently infected TG in a dose-dependent manner. Thus, the results of this study demonstrate that acute replication in the eye and the number of ICP0-null mutant genomes in latently infected TG can be increased to wild-type levels for both n212 and 7134 by (i) optimization of inoculum size and (ii) transient immunosuppression with cyclophosphamide.

Interferons regulate the phenotype of wild-type and mutant herpes simplex viruses in vivo

Mechanisms responsible for neuroattenuation of herpes simplex virus (HSV) have been defined previously by studies of mutant viruses in cultured cells. The hypothesis that null mutations in host genes can override the attenuated phenotype of null mutations in certain viral genes was tested. Mutants such as those in infected cell protein (ICP) 0, thymidine kinase, ribonucleotide reductase, virion host shutoff, and ICP34.5 are reduced in their capacity to replicate in nondividing cells in culture and in vivo. The replication of these viruses was examined in eyes and trigeminal ganglia for 1-7 d after corneal inoculation in mice with null mutations (-/-) in interferon receptors (IFNR) for type I IFNs (IFN-alpha/betaR), type II IFN (IFN-gammaR), and both type I and type II IFNs (IFN-alpha/beta/gammaR). Viral titers in eyes and ganglia of IFN-gammaR-/- mice were not significantly different from congenic controls. However, in IFN-alpha/betaR-/- or IFN-alpha/beta/gammaR-/- mice, growth of all mutants, including those with significantly impaired growth in cell culture, was enhanced by up to 1,000-fold in eyes and trigeminal ganglia. Blepharitis and clinical signs of infection were evident in IFN-alpha/betaR-/- and IFN-alpha/beta/gammaR-/- but not control mice for all viruses. Also, IFNs were shown to significantly reduce productive infection of, and spread from intact, but not scarified, corneas. Particularly striking was restoration of near-normal trigeminal ganglion replication and neurovirulence of an ICP34.5 mutant in IFN-alpha/betaR-/- mice. These data show that IFNs play a major role in limiting mutant and wild-type HSV replication in the cornea and in the nervous system. In addition, the in vivo target of ICP34.5 may be host IFN responses. These experiments demonstrate an unsuspected role for host factors in defining the phenotypes of some HSV mutants in vivo. The phenotypes of mutant viruses therefore cannot be interpreted based solely upon studies in cell culture but must be considered carefully in the context of host factors that may define the in vivo phenotype.

Molecular aspects of herpes simplex virus I latency, reactivation, and recurrence

The application of molecular biology in the study of the pathogenesis of herpes simplex virus type 1 (HSV-1) has led to significant advances in our understanding of mechanisms that regulate virus behavior in sensory neurons and epithelial tissue. Such study has provided insight into the relationship of host and viral factors that regulate latency, reactivation, and recurrent disease. This review attempts to distill decades of information involving human, animal, and cell culture studies of HSV-1 with the goal of correlating molecular events with the clinical and laboratory behavior of the virus during latency, reactivation, and recurrent disease. The purpose of such an attempt is to acquaint the clinician/scientist with the current thinking in the field, and to provide key references upon which current opinions rest.

Human thymidine kinase can functionally replace herpes simplex virus type 1 thymidine kinase for viral replication in mouse sensory ganglia and reactivation from latency upon explant

Herpes simplex virus type 1 thymidine kinase exhibits a strikingly broad substrate specificity. It is capable of phosphorylating deoxythymidine and deoxyuridine as does human thymidine kinase, deoxycytidine as does human deoxycytidine kinase, the cytosolic kinase whose amino acid sequence it most closely resembles, and thymidylate as does human thymidylate kinase. Following peripheral inoculation of mice, viral thymidine kinase is ordinarily required for viral replication in ganglia and for reactivation from latency following ganglionic explant. To determine which activity of the viral kinase is important for replication and reactivation in mouse ganglia, recombinant viruses lacking viral thymidine kinase but expressing individual human kinases were constructed. Each recombinant virus expressed the appropriate kinase activity with early kinetics following infection of cultured cells. The virus expressing human thymidine kinase exhibited thymidine phosphorylation activity equivalent to approximately 5% of that of wild-type virus in a quantitative plaque autoradiography assay. Nevertheless, it was competent for ganglionic replication and reactivation following corneal inoculation of mice. The virus expressing human thymidylate kinase was partially competent for these activities despite failing to express detectable thymidine kinase activity. The virus expressing human deoxycytidine kinase failed to replicate acutely in neurons or to reactivate from latency. Therefore, it appears that low levels of thymidine phosphorylation suffice to fulfill the role of the viral enzyme in ganglia and that this role can be partially fulfilled by thymidylate kinase activity alone.

Inhibition of herpes simplex virus reactivation by dipyridamole in a mouse model

Herpes simplex virus (HSV) thymidine kinase (TK) has been demonstrated to be important for reactivation from latency. Specifically, HSV latency-associated transcripts (LAT) are expressed during latent infection established by TK-negative (TK-) HSV mutants, but reactivation is minimal. TK- HSV, however, readily reactivated in the presence of exogenous thymidine (TdR) in explant medium [Tenser et al. (1996): Journal of Virology 70:1271-1276]. In the present report this was further studied by evaluating the effect of dipyridamole (DPM) on HSV reactivation. DPM is known to interfere with nucleoside transport. Inhibition of TdR-enhanced reactivation of TK- HSV and inhibition of reactivation of wild-type TK+ HSV were evaluated in an experimental mouse model of latency. Without DPM, TK- HSV reactivation was increased from 0% to 88% with TdR in explant medium, demonstrating TdR-enhanced reactivation of TK- HSV (as seen previously), TdR-enhanced reactivation of TK- HSV was decreased when DPM (25 or 50 microM) was also present, to 30%-60% and to 0%, respectively. Secondly, DPM also decreased reactivation of wild-type TK+ HSV. The reactivation frequency of latently infected dorsal root ganglia was 90% in standard medium (no added TdR), and this was decreased by DPM to 9% and 0%, respectively. Reactivation of trigeminal ganglia in standard medium was 100%, and this decreased to 59% and 23%, respectively. The possibility of a direct toxic effect of DPM on ganglion neurons to explain the results was unlikely. DPM had a modest antiviral effect on HSV replication in cell culture, and its efficacy in blocking reactivation may be related to this activity, probably by inhibition of nucleoside transport.