Replication at body temperature selects a neurovirulent herpes simplex virus type 2

Analysis of herpes simplex virus ICP0 promoter function in sensory neurons during acute infection, establishment of latency, and reactivation in vivo

We have begun an analysis of the functional architecture of the ICP0 promoter in neurons in vivo with the ultimate goal of determining how this gene is regulated during reactivation in vivo. Promoter/reporter mutants in which the Escherichia coli beta-galactosidase (beta-Gal) gene was driven by various permutations of the ICP0 promoter were employed to permit the analysis of promoter function without the added complications that would arise due to inappropriate regulation of ICP0 protein levels. A whole-ganglion immunohistochemical staining procedure (N. M. Sawtell, J. Virol. 77:4127-4138, 2003) was used for direct comparisons of the expression of the promoter/reporter gene to expression of the native protein in the same cell. In this way, the expression of the putative wild-type promoter could be validated and results for mutant promoters could be compared to expression of the native gene. We found that a DNA fragment from bp -562 through the methionine start codon of the ICP0 gene contained all sequences required for properly regulated ICP0 expression in diverse cell types (including sensory neurons of the trigeminal ganglia [TG]) in vitro and in vivo, as indicated by colocalization of ICP0 and beta-Gal. Truncation of the ICP0 promoter to bp -145 or -129 resulted in the loss of immediate-early (alpha) kinetics. The truncated promoters expressed high levels of the reporter gene with leaky late (gamma1) kinetics in vitro and in some cell types in vivo. Unexpectedly, the truncated promoters did not express in TG neurons. Thus, TAATGARAT or other sequences upstream of bp -145 in the ICP0 promoter are required for basal expression of ICP0 in neurons but are not required for basal expression in other cells in vivo. There was a >95% concordance between reporter and native protein expression detected with the 562-bp promoter in neurons during the acute stage. However, this was not the case during reactivation from latency in vivo, as nearly twice as many neurons contained detectable beta-Gal as contained detectable ICP0. This same 562-bp promoter/reporter cassette, when placed in the context of a latency-associated transcript (LAT) null mutant, resulted in >95% concordance of expression of beta-Gal and ICP0 during reactivation in vivo. These last results strongly suggest that there is a posttranscriptional constraint on the expression of ICP0 protein during reactivation from latency and that this constraint is mediated by LAT.

Herpes simplex virus type 1 latency-associated transcript gene promotes neuronal survival

A complex interaction has evolved between the host's peripheral nervous system (PNS) and herpes simplex virus type 1 (HSV-1). Sensory neurons are permissive for viral replication, yet the virus can also enter a latent state in these cells. The interplay of viral and neuronal signals that regulate the switch between the viral lytic and latent states is not understood. The latency-associated transcript (LAT) regulates the establishment of the latent state and is required for >65% of the latent infections established by HSV-1 (R. L. Thompson and N. M. Sawtell, J. Virol. 71:5432-5440, 1997). To further investigate how LAT functions, a 1.9-kb deletion that includes the entire LAT promoter and 827 bp of the 5' end of the primary LAT mRNA was introduced into strain 17syn+. The wild-type parent, three independently derived deletion mutants, and two independently derived genomically rescued variants of the mutants were analyzed in a mouse ocular model. The number of latent sites established in trigeminal ganglion (TG) neurons was determined using a single-cell quantitative PCR assay for the viral genome on purified TG neurons. It was found that the LAT null mutants established ~75% fewer latent infections than the number established by the parental strain or rescued variant. The reduced establishment phenotype of LAT null mutants was due at least in part to a dramatic increase in the loss of TG neurons in animals infected with the LAT mutants. Over half of the neurons in the TG were destroyed following infection with the LAT mutants, and this was significantly more than were lost following infection with wild type. This is the first demonstration that the HSV LAT locus prevents the destruction of sensory neurons. The death of these neurons did not appear to be the result of increased apoptosis as measured by a terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling assay. Animals latently infected with the LAT null mutants reactivated less frequently in vivo and this was consistent with the reduction in the number of neurons in which latency was established. Thus, one function of the LAT gene is to protect sensory neurons and enhance the establishment of latency in the PNS.

A herpes simplex virus DNA polymerase mutation that specifically attenuates neurovirulence in mice

Herpes simplex virus can infect the mammalian brain causing lethal encephalitis (neurovirulence). Previously, herpes simplex virus mutants that are attenuated for neurovirulence have exhibited defects in replication in brain and/or blocks to replication in neuronal cells. We investigated the attenuation of neurovirulence of mutant PAAr5, which exhibits resistance to antiviral drugs due to altered viral DNA polymerase. Following intracerebral inoculation of 7-week-old CD1 mice, PAAr5 was 30-fold attenuated for neurovirulence compared to its wild-type parent. A drug-sensitive virus derived by marker rescue with DNA polymerase gene sequences exhibited neurovirulence that was essentially indistinguishable from that of wild-type virus, demonstrating that attenuation was due to a polymerase mutation. PAAr5 replicated in brain similarly to wild-type virus unlike another polymerase mutant, 615.8, that exhibited a similar degree of attenuation. The attenuation of PAAr5 was not associated with altered particle to PFU ratios nor with any obvious reductions in viral antigen expression in neurons, spread, histopathology, or TUNEL staining suggestive of apoptotic cells. Thus PAAr5 differs from other mutants that are attenuated for neurovirulence. Understanding how a polymerase mutation specifically attenuates neurovirulence may shed light on how herpes simplex virus can cause lethal encephalitis.

A herpes simplex virus ribonucleotide reductase deletion mutant is defective for productive acute and reactivatable latent infections of mice and for replication in mouse cells

Herpes simplex virus encodes a ribonucleotide reductase that is not essential for virus growth in dividing cells at 37 degrees. This enzyme has been proposed as a target for antiviral drugs; its utility in this regard could depend upon its importance in vivo. To test the requirement of viral ribonucleotide reductase in a mammalian host, we tested a mutant virus, lacking most of the gene encoding the ribonucleotide reductase large subunit, in a mouse eye model of pathogenesis and latency where the wild-type virus establishes reactivatable latent infections in trigeminal ganglia following corneal inoculation. The deletion mutant was severely impaired in its ability to replicate acutely in the eye and in the trigeminal ganglion and failed to establish reactivatable latent infections. In contrast, a recombinant virus in which the deleted sequences were restored was competent for both acute and latent infections. The defects of the deletion mutant in the mouse may be related to its severely impaired growth at 38 degrees in mouse cells relative to its growth in Vero cells. These results indicate that ribonucleotide reductase is critical for productive acute and reactivatable latent infections in mice and replication in mouse cells at 38 degrees and suggest that caution be exercised in extrapolating from studies conducted in mice to human infections when judging the utility of this enzyme as a target for antiviral chemotherapy.

Neurovirulence of individual plaque stocks of Herpes simplex virus type 2 strain HG 52

The virulence of ten independent plaque stocks of HSV-2 strain HG 52 was determined by intracranial inoculation of 3 week old BALB/c mice. The ten stocks exhibited a range of LD50 which could be grouped into three classes; high (less than 10(3) pfu/mouse), intermediate (10(3)-10(4) pfu/mouse) and low (greater than 10(5) pfu/mouse). Analysis of the ten stocks using six different restriction endonucleases showed no significant variation in the size and distribution of fragments. Isolates from the brains of dead mice had restriction enzyme profiles indistinguishable from the initial infecting virus. The pfu:particle ratios of the virus stocks were comparable. There were no significant differences between the single growth cycle characteristics in vitro of the parental HG 52 and the various plaque purified stocks. In vivo after intracranial inoculation the low virulence stocks grew poorly while those of high virulence grew well. Plaques were picked from the high and low virulence stocks after passage five times in vitro and re-assayed for virulence. In some cases there were differences in the LD50s of passaged plaque isolates from that of parental stock. The restriction enzyme profiles of the passaged plaque isolates remained unchanged. The analysis has clearly demonstrated virulence heterogeneity within a single HSV-2 virus stock.

Autonomic nervous system involvement in experimental genital infection by herpes simplex virus type 2

Peroxidase-antiperoxidase technique and histology were employed to elucidate the peripheral routes involved in HSV-2 progression from vagina towards the central nervous system in mice. 12 week-old female Balb/c mice were intravaginally infected with 5 X 10(5)LD50 of HSV-2. Sixty per cent of animals developed vulvovaginitis, perigenital alopecia and hind-limb paresia. Death occurred at 9-11 days post-infection. Colon dilatation and urinary bladder distention were observed in all cases. Complete transversal sections from vulva to kidneys were obtained of each animal, including the spinal cord in situ. Herpes antigen were regularly detected in vulvovaginal epithelium, intramural, perigenital and perivesical small nerves. Besides, their invariable presence in Auerbach's plexus and sympathetic ganglia, strongly suggests preferential autonomic nervous system involvement in the progression of HSV-2 intravaginal infection towards the spinal cord.