A thymidine kinase deficient HSV-2 strain causes acute keratitis and establishes trigeminal ganglionic latency, but poorly reactivates in vivo

A Comparison of Pseudorabies Virus Latency to Other A-Herpesvirinae Subfamily Members

Pseudorabies virus (PRV), the causative agent of Aujeszky’s disease, is one of the most important infectious pathogens threatening the global pig industry. Like other members of alphaherpesviruses, PRV establishes a lifelong latent infection and occasionally reactivates from latency after stress stimulus in infected pigs. Latent infected pigs can then serve as the source of recurrent infection, which is one of the difficulties for PRV eradication. Virus latency refers to the retention of viral complete genomes without production of infectious progeny virus; however, following stress stimulus, the virus can be reactivated into lytic infection, which is known as the latency-reactivation cycle. Recently, several research have indicated that alphaherpesvirus latency and reactivation is regulated by a complex interplay between virus, neurons, and the immune system. However, with those limited reports, the relevant advances in PRV latency are lagging behind. Therefore, in this review we focus on the regulatory mechanisms in PRV latency via summarizing the progress of PRV itself and that of other alphaherpesviruses, which will improve our understanding in the underlying mechanism of PRV latency and help design novel therapeutic strategies to control PRV latency.

Virulence of herpes simplex virus 1 harbouring a UAG stop codon between the first and second initiation codon in the thymidine kinase gene

Herpes simplex virus 1 (HSV-1)-TK(8UAG) expresses a truncated thymidine kinase (TK) translated from the second initiation codon due to a stop UAG codon at the 8th position (counted from the first initiation codon). Here, we showed that the sensitivity of HSV-1-TK(8UAG) to acyclovir (ACV) is similar to that of control HSV-1 wild-type (WT), which expresses an intact TK protein. However, HSV-1-TK(44UAG), which expresses a truncated TK due to a UAG codon at position 44, showed lower sensitivity to ACV. A mouse infection model was used to compare the virulence of HSV-1-TK(8UAG) and HSV-1-TK(44UAG) with that of HSV-1 wild-type (WT). The 50% lethal dose (LD₅₀) value of HSV-1-TK(44UAG) was 7.8-fold higher than that of HSV-1-TK(8UAG), whereas the LD₅₀ value of HSV-1-TK(8UAG) was the same as that of the parental HSV-1 WT. There were no statistically significant differences between HSV-1-TK(44UAG), HSV-1-TK(8UAG), and HSV-1 WT with respect to replication capacity and viral TK mRNA expression in mouse brain. Thus, the virulence of HSV-1 expressing a truncated viral TK translated from the second initiation codon might depend on the position of the UAG stop codon.

"Non-Essential" Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

Viruses encode for structural proteins that participate in virion formation and include capsid and envelope proteins. In addition, viruses encode for an array of non-structural accessory proteins important for replication, spread, and immune evasion in the host and are often linked to virus pathogenesis. Most virus accessory proteins are non-essential for growth in cell culture because of the simplicity of the infection barriers or because they have roles only during a state of the infection that does not exist in cell cultures (i.e., tissue-specific functions), or finally because host factors in cell culture can complement their absence. For these reasons, the study of most nonessential viral factors is more complex and requires development of suitable cell culture systems and in vivo models. Approximately half of the proteins encoded by the herpes simplex virus 1 (HSV-1) genome have been classified as non-essential. These proteins have essential roles in vivo in counteracting antiviral responses, facilitating the spread of the virus from the sites of initial infection to the peripheral nervous system, where it establishes lifelong reservoirs, virus pathogenesis, and other regulatory roles during infection. Understanding the functions of the non-essential proteins of herpesviruses is important to understand mechanisms of viral pathogenesis but also to harness properties of these viruses for therapeutic purposes. Here, we have provided a comprehensive summary of the functions of HSV-1 non-essential proteins.

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

Resistance of herpes simplex viruses to nucleoside analogues: mechanisms, prevalence, and management

Herpes simplex viruses (HSV) type 1 and type 2 are responsible for recurrent orolabial and genital infections. The standard therapy for the management of HSV infections includes acyclovir (ACV) and penciclovir (PCV) with their respective prodrugs valacyclovir and famciclovir. These compounds are phosphorylated by the viral thymidine kinase (TK) and then by cellular kinases. The triphosphate forms selectively inhibit the viral DNA polymerase (DNA pol) activity. Drug-resistant HSV isolates are frequently recovered from immunocompromised patients but rarely found in immunocompetent subjects. The gold standard phenotypic method for evaluating the susceptibility of HSV isolates to antiviral drugs is the plaque reduction assay. Plaque autoradiography allows the associated phenotype to be distinguished (TK-wild-type, TK-negative, TK-low-producer, or TK-altered viruses or mixtures of wild-type and mutant viruses). Genotypic characterization of drug-resistant isolates can reveal mutations located in the viral TK and/or in the DNA pol genes. Recombinant HSV mutants can be generated to analyze the contribution of each specific mutation with regard to the drug resistance phenotype. Most ACV-resistant mutants exhibit some reduction in their capacity to establish latency and to reactivate, as well as in their degree of neurovirulence in animal models of HSV infection. For instance, TK-negative HSV mutants establish latency with a lower efficiency than wild-type strains and reactivate poorly. DNA pol HSV mutants exhibit different degrees of attenuation of neurovirulence. The management of ACV- or PCV-resistant HSV infections includes the use of the pyrophosphate analogue foscarnet and the nucleotide analogue cidofovir. There is a need to develop new antiherpetic compounds with different mechanisms of action.

Mucosal delivery of CpG oligodeoxynucleotides expands functional dendritic cells and macrophages in the vagina

Antigen-presenting cells (APC) are specialized sentinel cells that sense pathogens within tissues and then activate appropriate immune effector cells in lymphoid organs. Recent evidence, however, suggests that APC can also induce effector cells in non-lymphoid organs. The purpose of this study was to determine the effect of intravaginal (IVAG) delivery of CpG-oligodeoxynucleotide (ODN) on expansion of resident genital APC. Our results show that delivery of CpG-ODN to the murine genital tract induced a rapid and significant, but transient expansion of genital APC in situ. As early as 12 hr post CpG-ODN delivery, we observed an enhanced level of F4/80+ major histocompatibility complex (MHC) class II-negative macrophages in the genital tissue. This was followed by increased levels of F4/80/MHC class II double-positive cells, as well as MHC class II, CD11c and CD86 triple-positive dendritic cells (DC) at 48 hr. Expanded APC levels at 48 hr post CpG-ODN resulted in increased ability of genital cells to induce an allogenic mixed leucocyte reaction. By 72 hr after CpG-ODN treatment, APC levels were not distinguishable from naive levels. Therefore, these results clearly show that administration of CpG-ODN to the genital tract induced a marked but transient enhancement of APC within the genital tissue, and that these APC appear to possess functional capacity. Furthermore, these results indicate that IVAG-CpG-ODN may be an important factor for the enhancement of local antigen presentation in the genital tract through increased DC numbers.

Intravaginal administration of herpes simplex virus type 2 to mice leads to infection of several neural and extraneural sites

Female mice have been used extensively to study mucosal immunity against herpes simplex virus type 2 (HSV-2) infection of the vagina, but comparatively little is known about the spread of this virus to other tissues. Here the authors have used immunolabeling to demonstrate that HSV-2 infected the vaginal epithelium; the epithelium covering the vulva, perineum, and anal canal; and perineal hair follicles and sebaceous glands. The kinetics and basal localization of the immunolabeling indicated that the virus spread horizontally within the epithelial layer, starting in the vagina and then proceeding to the distal epithelial sites. HSV-2 also spread from the vagina to multiple neuronal sites including the paracervical ganglia (PCG), which are the major autonomic ganglia of the pelvis. The authors demonstrated both sympathetic and parasympathetic neurons in the PCG by labeling of acetylcholinesterase and tryosine hydroxlyase, and noted that infection was limited mainly or entirely to parasympathetic neurons. Infection of the PCG was correlated with the presence of virus in the autonomic ganglia in the walls of the rectum and urinary bladder, which in turn correlated with distention of these organs and retention of urine and feces. HSV-2 infection was also detected in cell bodies and axons in the lumbosacral sympathetic chain, in lumbosacral dorsal root ganglia, and in the dorsal portions of the lumbar spinal cord. Collectively, the data show that vaginal HSV-2 infection in mice leads to subsequent infection of multiple neural and epithelial sites. This information should be useful for development of a mouse model that can be used to study HSV-2 latency and for development of therapeutic vaccines to prevent recurrent infections.

Biological properties of herpes simplex virus 2 replication-defective mutant strains in a murine nasal infection model

We used a mouse nasal model of herpes simplex virus 2 (HSV-2) infection to examine the biological properties of HSV-2 wild-type (wt), TK-negative, and replication-defective strains in vivo. Nasal septa tissue is the major site of wt viral replication post intranasal (i.n.) inoculation. The HSV-2 strain 186 syn(+)-1 wt virus caused lethal encephalitis at doses of 10(4) PFU and above per nostril, and at lower doses no neurons in the trigeminal ganglia were positive for the latency-associated transcript, indicating a lack of latent infection. The 186DeltaKpn TK-negative mutant virus replicated in nasal septa tissue but showed low-level replication in trigeminal ganglia at only one timepoint. In situ hybridization of trigeminal ganglia showed that the number of LAT-positive neurons was proportional to the inoculum dose from 10(3) to 10(6) PFU per nare. The replication-defective mutant virus 5BlacZ showed no replication in nasal septa tissue and no persistence of viral DNA at the inoculation site or the trigeminal ganglia. Nevertheless, inoculation of 5BlacZ or the double-mutant dl5-29 at distal sites reduced acute replication and latent infection of 186DeltaKpn following intranasal challenge. This infection model provides a biological system to test the properties of HSV-2 strains and shows that replication-defective mutant strains do not persist at sites of inoculation or in sensory ganglia but can induce immune protection that reduces the latent viral load of a challenge virus.

Intravenous acyclovir treatment for extensive herpetic keratitis in a liver transplant patient

Herpetic infection is a common complication among immune suppressed patients following heart, kidney and bone marrow transplantations, in leukemia patients, in AIDS patients, and during treatment with cytotoxic drugs. In the cases described in the literature, oral acyclovir was recommended as a treatment for the acute infection, as well as for prophylaxis. Intravenous acyclovir is not a routine treatment for herpetic keratitis, but is recommended for cases of insufficient clinical response to oral treatment, and defective absorption of acyclovir by the gastrointestinal tract. We present a patient who underwent 4 liver transplantations, was treated regularly with immunosuppressive drugs, and who developed extensive herpetic keratitis. The keratitis was resistant to both topical ointment and oral acyclovir treatment. Recovery was only achieved following the intravenous administration of acyclovir. We recommend intravenous acyclovir treatment at a very early stage for immune suppressed patients with extensive herpes simplex keratitis.

Cloning and restriction endonuclease mapping of herpes simplex virus type-1 strains H129 and +GC

EcoRI fragments of herpes simplex virus I (HSV-1) strains H129 and +GC were cloned and the EcoRI and BglII restriction enzyme sites were mapped. Comparison of these enzyme sites with the sequence of HSV-1 strain 17syn+ demonstrated that all EcoRI sites were identical. For H129, the BglII sites were also found to match strain 17syn+ BglII sites. With one exception, the BglII sites in strain +GC also aligned with the strain 17syn+ sequence. The one exception was a missing BglII site from strain +GC located between bases 25,149 and 25,154 in the EcoRI D fragment within the viral deoxyribonuclease gene (UL12). The BglII site represents the first difference to be mapped within HSV-1 strains H129 and +GC which have unique pathobiological properties in animal models of acute and reactivated infections.