The comparative biology of latent herpes simplex virus type 1 and type 2 infections: latency-associated transcript promoter activity and expression in vitro and in infected mice

Neuroinflammation in Alzheimer's Disease: A Potential Role of Nose-Picking in Pathogen Entry via the Olfactory System?

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by progressive cognitive decline and memory impairment. Many possible factors might contribute to the development of AD, including amyloid peptide and tau deposition, but more recent evidence suggests that neuroinflammation may also play an-at least partial-role in its pathogenesis. In recent years, emerging research has explored the possible involvement of external, invading pathogens in starting or accelerating the neuroinflammatory processes in AD. In this narrative review, we advance the hypothesis that neuroinflammation in AD might be partially caused by viral, bacterial, and fungal pathogens entering the brain through the nose and the olfactory system. The olfactory system represents a plausible route for pathogen entry, given its direct anatomical connection to the brain and its involvement in the early stages of AD. We discuss the potential mechanisms through which pathogens may exploit the olfactory pathway to initiate neuroinflammation, one of them being accidental exposure of the olfactory mucosa to hands contaminated with soil and feces when picking one's nose.

Immunization with HSV-1 glycoprotein C prevents immune evasion from complement and enhances the efficacy of an HSV-1 glycoprotein D subunit vaccine

Herpes simplex virus type 1 (HSV-1) glycoprotein C (gC-1) binds complement component C3b and inhibits complement-mediated immunity. HSV-1 glycoprotein D (gD-1) is a potent immunogen and a candidate antigen for a subunit vaccine. We evaluated whether combined immunization with gD-1 and gC-1 provides better protection against challenge than gD-1 alone based on antibodies to gC-1 preventing HSV-1-mediated immune evasion. IgG purified from mice immunized with gC-1 blocked C3b binding to gC-1 and greatly increased neutralization by gD-1 IgG in the presence of complement. Passive transfer of gC-1 IgG protected complement intact mice against HSV-1 challenge but not C3 knockout mice, indicating that gC-1 antibody activity in vivo is complement-dependent. Immunizing mice with gD-1 and gC-1 provided better protection than gD-1 alone in preventing zosteriform disease and infection of dorsal root ganglia. Therefore, gC-1 immunization prevents HSV-1 evasion from complement and enhances the protection provided by gD-1 immunization.

Investigation of the mechanism by which herpes simplex virus type 1 LAT sequences modulate preferential establishment of latent infection in mouse trigeminal ganglia

We previously demonstrated that herpes simplex virus type 1 (HSV-1) preferentially establishes latent infection in monoclonal antibody (MAb) A5-positive ganglionic neurons and that a 2.8-kb portion of the HSV-1 genome, corresponding to the 5' end of the LAT (latency-associated transcript) coding region, is responsible for this phenotype (38, 65). In the current study we carried out further genetic mapping of this latency phenotype and investigated some of the mechanisms that might be responsible. Studies with the chimeric virus HSV-1 17syn+/LAT2, an HSV-1 virus engineered to express HSV-2 LAT, demonstrated that this virus exhibited an HSV-2 latency phenotype, preferentially establishing latency in MAb KH10-positive neurons. This result is complementary to that previously described for the chimeric virus HSV-2 333/LAT1 and indicate that the HSV-1 latency phenotype can be changed to that of HSV-2 by substitution of a 2.8-kb piece of complementary viral DNA. Sequential studies in which we evaluated the pattern of HSV-1 latent infection of the mouse trigeminal ganglion following ocular inoculation with viruses with deletions of functional thymidine kinase, glycoprotein E, ICP0, and US9 protein demonstrate that preferential establishment of HSV-1 latent infection in A5-positive neurons is not a consequence of (i) differential access of HSV-1 to A5-positive neurons,(ii) differential cell-to-cell spread of HSV-1 to A5-positive neurons, (iii) differential "round-trip" spread of HSV-1 to A5-positive neurons, or (iv) expression of ICP0. Additional mapping studies with the HSV-1 LAT deletion viruses dLAT371, 17DeltaSty, and 17Delta348 indicate that most of the LAT 5' exon is not required for HSV-1 to preferentially establish latent infection in A5-positive neurons.

Cell type specific accumulation of the major latency-associated transcript (LAT) of herpes simplex virus type 2 in LAT transgenic mice

We performed in situ hybridization to determine the cell type specific accumulation of the intron of the latency-associated transcript (LAT) in tissues in HSV-2 LAT transgenic mice in which LAT expression is driven by its native promoter. We identified LAT in multiple cell types in most tissues analyzed from HSV-2 LAT transgenic mice. While weak to moderate signals were seen in brain and spinal cord neurons, epithelial cells, and muscle cells, the strongest signals were detected in neurons from dorsal root and trigeminal ganglia. About 70-86% of neurons in these ganglia were LAT-positive with varying signal intensities, while cells surrounding the neurons were LAT-negative. The frequency of A5 or KH10-positive neurons was similar in LAT-positive and total neurons. These data indicate that HSV-2 LAT promoter activity is not restricted to neurons and that LAT accumulation in ganglionic neurons is likely regulated by cell-specific factors.

The 2.2-kilobase latency-associated transcript of herpes simplex virus type 2 does not modulate viral replication, reactivation, or establishment of latency in transgenic mice

To better understand the mechanisms responsible for the observed effects of deletions in the promoter region of the latency-associated transcript (LAT) gene in impairing herpes simplex virus (HSV) reactivation, we generated mice transgenic for a 5.5-kb HSV type 2 (HSV-2) genomic fragment spanning the major LAT, along with the LAT promoter and flanking regions, in the C57BL/6 background. The mice expressed abundant 2.2-kb major LATs in trigeminal ganglia (TG) and other tissues. The effects of the transgene on HSV-2 infection, latency, and reactivation were assessed. When infected with wild-type (WT) HSV-2 or its LAT promoter deletion (LAT(-)) mutant, primary lung fibroblast lines established from normal C57BL/6 and transgenic mice supported virus growth equally well. The replication of these viruses in the mouse eye and their spread to TG and brains were similar. The quantities of latent viral DNA in TG of transgenic and normal mice, as determined by real-time PCR, were comparable. UV light-induced reactivation of the LAT(-) mutant from transgenic mice (0 to 7%) was no more frequent than that from normal mice (0 to 14%), while WT virus was reactivated from 13 to 54% of normal mice and 22 to 54% of transgenic mice. The cumulative data indicate that, when expressed transgenically, the HSV-2 major LAT cannot influence HSV-2 infection or latency and cannot complement the defect in reactivation of the LAT(-) mutant. These results imply that the phenotype of reduced reactivation associated with the LAT(-) mutant is related to a function encoded in the LAT promoter but not to the major LAT itself.

Analysis of individual human trigeminal ganglia for latent herpes simplex virus type 1 and varicella-zoster virus nucleic acids using real-time PCR

Herpes simplex virus type 1 (HSV-1) and varicella-zoster virus (VZV) establish latent infections in the peripheral nervous system following primary infection. During latency both virus genomes exhibit limited transcription, with the HSV-1 LATs and at least four VZV transcripts consistently detected in latently infected human ganglia. In this study we used real-time PCR quantitation to determine the viral DNA copy number in individual trigeminal ganglia (TG) from 17 subjects. The number of HSV-1 genomes was not significantly different between the left and right TG from the same individual and varied per subject from 42.9 to 677.9 copies per 100 ng of DNA. The number of VZV genomes was also not significantly different between left and right TG from the same individual and varied per subject from 37.0 to 3,560.5 copies per 100 ng of DNA. HSV-1 LAT transcripts were consistently detected in ganglia containing latent HSV-1 and varied in relative expression by >500-fold. Of the three VZV transcripts analyzed, only transcripts mapping to gene 63 were consistently detected in latently infected ganglia and varied in relative expression by >2,000-fold. Thus, it appears that, similar to LAT transcription in HSV-1 latently infected ganglia, VZV gene 63 transcription is a hallmark of VZV latency.