Pathogenesis of neurotropic herpesviruses: role of viral glycoproteins in neuroinvasion and transneuronal spread

Interferon-gamma response of PBMC indicates productive pseudorabies virus (PRV) infection in swine

In Chinese Meishan/German Landrace cross-bred swine F₂ generation interferon gamma (IFN-γ) production by peripheral blood mononuclear cells (PBMC) was determined directly ex vivo at different time points after survival of a virulent pseudorabies virus (PRV) infection. This reactivity was compared with the reactivity of naïve PBMC. Significant IFN-γ production was determined in ELISA and ELISPOT only after in vitro PBMC re-stimulation with PRV and not with the closely related bovine herpesvirus BHV-1.

The PRV-specific IFN-γ secretion from re-stimulated PBMC showed high levels 6 days after infection, before the presence of serum antibodies, and it persisted at a high level over a 3 months period. The response of a group of eight piglets infected intranasally with PRV varied. Only two animals showed the expected typical fever response. PRV specific IFN-γ production by PBMC clearly indicated that infection had occurred. Early significant IFN-γ production by primed PBMC turned out to be a reliable and specific ex vivo marker for cellular response against productive PRV infection in swine before antibody formation.

Alphaherpesvirus glycoprotein M causes the relocalization of plasma membrane proteins

Herpesvirus glycoprotein M (gM) is a multiple-spanning integral membrane protein found within the envelope of mature herpesviruses and is conserved throughout the Herpesviridae. gM is defined as a non-essential glycoprotein in alphaherpesviruses and has been proposed as playing a role in controlling final envelopment in a late secretory-pathway compartment such as the trans-Golgi network (TGN). Additionally, gM proteins have been shown to inhibit cell-cell fusion in transfection-based assays by an as yet unclear mechanism. Here, the effect of pseudorabies virus (PRV) gM and the herpes simplex virus type 1 (HSV-1) gM/UL49A complex on the fusion events caused by the HSV-1 glycoproteins gB, gD, gH and gL was investigated. Fusion of cells expressing HSV-1 gB, gD, gH and gL was efficiently inhibited by both PRV gM and HSV-1 gM/UL49A. Furthermore, expression of PRV gM or HSV-1 gM/UL49A, which are themselves localized to the TGN, caused both gD and gH/L to be relocalized from the plasma membrane to a juxtanuclear compartment, suggesting that fusion inhibition is caused by the removal of 'fusion' proteins from the cell surface. The ability of gM to cause the relocalization of plasma membrane proteins was not restricted to HSV-1 glycoproteins, as other viral and non-viral proteins were also affected. These data suggest that herpesvirus gM (gM/N) can alter the membrane trafficking itineraries of a broad range of proteins and this may have multiple functions.

Influence of tegument proteins of pseudorabies virus on neuroinvasion and transneuronal spread in the nervous system of adult mice after intranasal inoculation

Pseudorabies virus (PrV) is a neurotropic alphaherpesvirus that, after intranasal infection of adult mice, enters peripheral neurons and propagates to the central nervous system. In recent years we have analyzed the contribution of virus-encoded glycoproteins to neuroinvasion and transneuronal spread (reviewed in T. C. Mettenleiter, Virus Res. 92:197-206, 2003). We now extend our studies to analyze the role of tegument proteins in these processes. To this end, PrV mutants unable to express the UL11, UL37, UL46, UL47, and UL48 tegument proteins, as well as the corresponding rescued viruses, were intranasally instilled into 6- to 8-week-old CD1 strain mice. First, mean survival times were determined which showed that mice infected with the UL46 deletion mutant succumbed to the disease as early as wild-type PrV-infected animals. Survival times increased in the order: PrV-DeltaUL47-, PrV-DeltaUL11-, and PrV-DeltaUL48-infected animals, a finding which parallels the growth phenotype of these viruses in cell culture. In contrast, none of the PrV-DeltaUL37-infected animals died. Upon closer histological examination, all viruses except PrV-DeltaUL37 were able to infect the nasal cavity and propagate to first- and second-order neurons as shown by two-color immunofluorescence. However, neuroinvasion was delayed in PrV-DeltaUL47, PrV-DeltaUL11, and PrV-DeltaUL48, a finding that correlated with the extended survival times. Surprisingly, whereas PrV-DeltaUL48 and PrV-DeltaUL37 replicated to similar titers in cell culture which were approximately 500-fold lower than those of wild-type virus, after intranasal infection of mice PrV-DeltaUL48 was able to infect areas of the brain like wild-type PrV, although only after a considerably longer time period. In contrast, PrV-DeltaUL37 was not able to enter neurons and was restricted to the infection of single cells in the nasal respiratory epithelium. Thus, our data demonstrate the importance of herpesviral tegument proteins in neuronal infection and show a different contribution of tegument proteins to the neuroinvasion phenotype of a neurotropic alphaherpesvirus.

Effects of mutations in the cytoplasmic domain of herpes simplex virus type 1 glycoprotein B on intracellular transport and infectivity

Herpes simplex virus type 1 (HSV-1) is a human pathogen of the alphaherpesvirus family which infects and spreads in the nervous system. Glycoproteins play a key role in the process of assembly and maturation of herpesviruses, which is essential for neuroinvasion and transneuronal spread. Glycoprotein B (gB) is a main component of the HSV-1 envelope and is necessary for the production of infectious particles. The cytoplasmic domain of gB, the longest one among HSV-1 glycoproteins, contains several highly conserved peptide sequences homologous to motifs involved in intracellular sorting. To determine the specific roles of these motifs in processing, subcellular localization, and the capacity of HSV-1 gB to complement a gB-null virus, we generated truncated or point mutated forms of a green fluorescent protein (GFP)-tagged gB. GFP-gB with a deletion in the acidic cluster DGDADEDDL (amino acids [aa] 896 to 904) behaved the same as the parental form. Deletion or disruption of the YTQV motif (aa 889 to 892) abolished internalization and reduced complementation by 60%. Disruption of the LL motif (aa 871 to 872) impaired the return of the protein to the trans-Golgi network (TGN) while enhancing its recycling to the plasma membrane. Truncations from residue E 857 abolished transport and processing of the truncated proteins, which had null complementation activity, through the Golgi complex. Altogether, our results favor a model in which HSV-1 gets its final envelope in the TGN, and they suggest that endocytosis, albeit not necessary, might play a role in infectivity.

Infectious agents and age-related neurodegenerative disorders

chlamdAs with other organ systems, the vulnerability of the nervous system to infectious agents increases with aging. Several different infectious agents can cause neurodegenerative conditions, with prominent examples being human immunodeficiency virus (HIV-1) dementia and prion disorders. Such infections of the central nervous system (CNS) typically have a relatively long incubation period and a chronic progressive course, and are therefore increasing in frequency as more people live longer. Infectious agents may enter the central nervous system in infected migratory macrophages, by transcytosis across blood-brain barrier cells or by intraneuronal transfer from peripheral nerves. Synapses and lipid rafts are important sites at which infectious agents may enter neurons and/or exert their cytotoxic effects. Recent findings suggest the possibility that infectious agents may increase the risk of common age-related neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and stroke. While scenarios can be envisioned whereby viruses such as Chlamydia pneumoniae, herpes simplex and influenza promote damage to neurons during aging, there is no conclusive evidence for a major role of these pathogens in neurodegenerative disorders. In the case of stroke, blood vessels may be adversely affected by bacteria or viruses resulting in atherosclerosis.