Endocytosis and recycling of varicella-zoster virus Fc receptor glycoprotein gE: internalization mediated by a YXXL motif in the cytoplasmic tail

Herpes simplex virus type-1 cVAC formation in neuronal cells is mediated by dynein motor function and glycoprotein retrieval from the plasma membrane

Herpesvirus assembly requires the cytoplasmic association of large macromolecular and membrane structures that derive from both the nucleus and cytoplasmic membrane systems. Results from the study of human cytomegalovirus (HCMV) in cells where it organizes a perinuclear cytoplasmic virus assembly compartment (cVAC) show a clear requirement for the minus-end-directed microtubule motor, dynein, for virus assembly. In contrast, the assembly of herpes simplex virus -1 (HSV-1) in epithelial cells where it forms multiple dispersed, peripheral assembly sites is only mildly inhibited by the microtubule-depolymerizing agent, nocodazole. Here, we make use of a neuronal cell line system in which HSV-1 forms a single cVAC and show that dynein and its co-factor dynactin localize to the cVAC, and dynactin is associated with membranes that contain the virion tegument protein pUL11. We also show that the virus membrane-associated structural proteins pUL51 and the viral envelope glycoprotein gE arrive at the cVAC by different routes. Specifically, gE arrives at the cVAC after retrieval from the plasma membrane, suggesting the need for an intact retrograde transport system. Finally, we demonstrate that inhibition of dynactin function profoundly inhibits cVAC formation and virus production during the cytoplasmic assembly phase of infection.IMPORTANCEMany viruses reorganize cytoplasmic membrane systems and macromolecular transport systems to promote the production of progeny virions. Clarifying the mechanisms by which they accomplish this may reveal novel therapeutic strategies and illustrate mechanisms that are critical for normal cellular organization. Here, we explore the mechanism by which HSV-1 moves macromolecular and membrane cargo to generate a virus assembly compartment in the infected cell. We find that the virus makes use of a well-characterized, microtubule-based transport system that is stabilized against drugs that disrupt microtubules.

Evidence of varicella zoster virus (VZV) reactivation in children with arterial ischemic stroke: Results of the VIPS II Study

Background

Varicella zoster virus (VZV) has been associated with focal cerebral arteriopathy (FCA) and arterial ischemic stroke (AIS) in childhood. The Vascular effects of Infection in Pediatric Stroke (VIPS) II study aimed to examine this relationship in the modern era when most children in North America and Australia receive VZV vaccination with live, attenuated virus.

Methods

This 22-center prospective cohort study enrolled 205 children (28 days-18 years) with AIS (2017-2022), collected baseline [hyperacute (≤72 hours; n=194) and acute (4-6 days; n=181)] and convalescent (1-6 weeks; n=74) serum samples. Sites enrolled 95 stroke-free controls with single serum samples. A virology research laboratory measured VZV IgM and IgG titers by an in-house enzyme-linked immunosorbent assay (ELISA). Baseline IgG seropositivity indicated prior exposure (vaccination/infection) and elevated IgM titers indicated recent reactivation.

Results

Median (IQR) age was 11.6 (5.5-15.6) years for cases and 11.8 (6.8-15.3) years for controls. Baseline serologies indicated prior VZV exposure in 198 cases (97%) and all controls. Parents of cases reported VZV vaccination in 160 (78%) and remote chicken pox in three (1.4%). Twenty cases (9.8%) and three controls (3.1%) had serologic evidence of recent VZV reactivation (p=0.06); all had remote VZV exposure (vaccination in 19 cases and all controls) and all were asymptomatic. Recent VZV reactivation was seen in similar proportions in arteriopathic, cardioembolic, and idiopathic stroke. Of 32 cases of FCA, 4 (12.5%) had recent VZV reactivation, versus no cases of arterial dissection (n=10) or moyamoya (n=16).

Conclusions

Serologic evidence of recent VZV reactivation (≈1-6 weeks prior to stroke) was present in one in 10 cases of childhood AIS, including those without arteriopathy. Clinically silent VZV reactivation may be a childhood stroke trigger despite widespread vaccination. These cases could represent waning immunity with reactivation of either vaccine virus or wild-type virus after an unrecognized secondary VZV infection.

Evaluation of the Immunological Efficacy of an LNP-mRNA Vaccine Prepared from Varicella Zoster Virus Glycoprotein gE with a Double-Mutated Carboxyl Terminus in Different Untranslated Regions in Mice

Cell-mediated immunity (CMI) plays a key role in the effectiveness of varicella zoster virus (VZV) vaccines, and mRNA vaccines have an innate advantage in inducing CMI. Glycoprotein E (gE) has been used widely as an antigen for VZV vaccines, and carboxyl-terminal mutations of gE are associated with VZV titer and infectivity. In addition, the untranslated regions (UTRs) of mRNA affect the stability and half-life of mRNA in the cell and are crucial for protein expression and antigenic translational efficiency. In this study, three UTRs were designed and connected to the nucleic acid sequence of gE-M, which is double mutated in the extracellular region of gE. Then, mRNA with different nucleic acids was encapsulated in lipid nanoparticles (LNPs), forming three LNP-mRNA VZV vaccines, named gE-M-Z, gE-M-M, and gE-M-P. The immune response elicited by these vaccines in mice was evaluated at intervals of 4 weeks, and the mice were sacrificed 2 weeks after the final immunization. In the results, the gE-M-P group, which retains the nucleic acid sequence of gE-M and is connected to Pfizer/BioNTech's BNT162b2 UTRs, induced the strongest humoral immune response and CMI. Because CMI is crucial for protection against VZV and for the design of VZV vaccines, this study provides a feasible strategy for improving the effectiveness and economy of future VZV vaccines.

Coronavirus accessory protein ORF3 biology and its contribution to viral behavior and pathogenesis

Coronavirus porcine epidemic diarrhea virus (PEDV) is classified in the genus Alphacoronavirus, family Coronaviridae that encodes the only accessory protein, ORF3 protein. However, how ORF3 contributes to viral pathogenicity, adaptability, and replication is obscure. In this review, we summarize current knowledge and identify gaps in many aspects of ORF3 protein in PEDV, with emphasis on its unique biological features, including membrane topology, Golgi retention mechanism, potential intrinsic disordered property, functional motifs, protein glycosylation, and codon usage phenotypes related to genetic evolution and gene expression. In addition, we propose intriguing questions related to ORF3 protein that we hope to stimulate further studies and encourage collaboration among virologists worldwide to provide constructive knowledge about the unique characteristics and biological functions of ORF3 protein, by which their potential role in clarifying viral behavior and pathogenesis can be possible.

Human immunoglobulins are transported to HCMV viral envelope by viral Fc gamma receptors-dependent and independent mechanisms

Human cytomegaloviruses (HCMVs) employ many different mechanisms to escape and subvert the host immune system, including expression of the viral IgG Fcγ receptors (vFcγRs) RL11 (gp34), RL12 (gp95), RL13 (gpRL13), and UL119 (gp68) gene products. The role of vFcγRs in HCMV pathogenesis has been reported to operate in infected cells by interfering with IgG-mediated effector functions. We found that gp34 and gp68 are envelope proteins that bind and internalize human IgGs on the surface of infected cells. Internalized IgGs are then transported on the envelope of viral particles in a vFcR-dependent mechanism. This mechanism is also responsible for the incorporation on the virions of the anti-gH neutralizing antibody MSL-109. Intriguingly, we show that gp68 is responsible for MSL-109 incorporation, but it is dispensable for other anti-HCMV antibodies that do not need this function to be transported on mature virions. HCMV-infected cells grown in presence of anti-HCMV monoclonal antibodies generate a viral progeny still infective and possible to be neutralized. This is the first example of a virus carrying neutralizing IgGs on its surface and their possible role is discussed.

Relationships between cerebral small vessel diseases markers and cognitive performance in stroke-free patients with atrial fibrillation

Background

Atrial fibrillation (AF) is related to an increased risk of cognitive dysfunction. Besides clinically overt stroke, AF can damage the brain via several pathophysiological mechanisms. We aimed to assess the potential mediating role of cerebral small vessel disease (SVD) and cognitive performance in individuals with AF.

Methods

Stroke-free individuals with AF from the cardiological outpatient clinic at West China Hospital of Sichuan University were recruited. Extensive neuropsychological testing tools were assessed including global function, domains of attention, executive functions, learning, and memory. 3 T magnetic resonance imaging (MRI) was used for SVD markers assessment of white matter hyperintensities (WMH), lacunes, cerebral microbleeds (CMBs), and enlarged perivascular spaces (EPVS). The correlation between SVD markers and cognitive measures was analyzed by multivariate linear regression models.

Results

We finally enrolled 158 participants, of whom 95 (60.1%) were males. In multivariate models, the presence of lacunes independently associated with Montreal Cognitive Assessment (Model 1: ß = 0.52, Model 2: ß = 0.55), Rey Auditory Verbal Learning Test-immediate and delayed recall (Model 1: ß = 0.49; ß = 0.69; Model 2: ß = 0.53; ß = 0.73) as well as Stroop-A_correct (Model 1: ß = 0.12; Model 2: ß = 0.13), while total WMH severity independently associated with Stroop_time-A (Model 1: ß = 0.24; Model 3: ß = 0.27), Stroop_time-B (Model 1: ß = 0.17; Model 3: ß = 0.17), Stroop_time-C (Model 1: ß = 0.22; Model 3: ß = 0.21) and Shape Trail Test-A (Model 1: ß = 0.17; Model 3: ß = 0.16).

Conclusion

In our cohort of stroke-free individuals with AF, lacunes, and WMHs were independently associated with cognitive decline while EPVS and CMBs did not show significance. Assessment of SVD MRI markers might be valuable for cognition risk stratification and facilitate optimal management of patients with AF.

Identification of Cellular Genes Involved in Baculovirus GP64 Trafficking to the Plasma Membrane

The baculovirus envelope protein GP64 is an essential component of the budded virus and is necessary for efficient virion assembly. Little is known regarding intracellular trafficking of GP64 to the plasma membrane, where it is incorporated into budding virions during egress. To identify host proteins and potential cellular trafficking pathways that are involved in delivery of GP64 to the plasma membrane, we developed and characterized a stable Drosophila cell line that inducibly expresses the AcMNPV GP64 protein and used that cell line in combination with a targeted RNA interference (RNAi) screen of vesicular protein trafficking pathway genes. Of the 37 initial hits from the screen, we validated and examined six host genes that were important for trafficking of GP64 to the cell surface. Validated hits included Rab GTPases Rab1 and Rab4, Clathrin heavy chain, clathrin adaptor protein genes AP-1-2β and AP-2μ, and Snap29. Two gene knockdowns (Rab5 and Exo84) caused substantial increases (up to 2.5-fold) of GP64 on the plasma membrane. We found that a small amount of GP64 is released from cells in exosomes and that some portion of cell surface GP64 is endocytosed, suggesting that recycling helps to maintain GP64 at the cell surface. IMPORTANCE While much is known regarding trafficking of viral envelope proteins in mammalian cells, little is known about this process in insect cells. To begin to understand which factors and pathways are needed for trafficking of insect virus envelope proteins, we engineered a Drosophila melanogaster cell line and implemented an RNAi screen to identify cellular proteins that aid transport of the model baculovirus envelope protein (GP64) to the cell surface. For this we developed an experimental system that leverages the large array of tools available for Drosophila and performed a targeted RNAi screen to identify cellular proteins involved in GP64 trafficking to the cell surface. Since viral envelope proteins are often critical for production of infectious progeny virions, these studies lay the foundation for understanding how either pathogenic insect viruses (baculoviruses) or insect-vectored viruses (e.g., flaviviruses, alphaviruses) egress from cells in tissues such as the midgut to enable systemic virus infection.

Effects of Varicella-Zoster Virus Glycoprotein E Carboxyl-Terminal Mutation on mRNA Vaccine Efficacy

Glycoprotein E (gE) is one of the most abundant glycoproteins in varicella-zoster virus and plays pivotal roles in virus replication and transmission between ganglia cells. Its extracellular domain has been successfully used as an antigen in subunit zoster vaccines. The intracellular C-terminal domain was reported to be decisive for gE trafficking between the endoplasmic reticulum, trans-Golgi network and endosomes and could influence virus spread and virus titers. Considering that the trafficking and distribution of mRNA vaccine-translated gE may be different from those of gE translated against the background of the viral genome (e.g., most gE in virus-infected cells exists as heterodimers with another glycoprotein, gI,), which may influence the immunogenicity of gE-based mRNA vaccines, we compared the humoral and cellular immunity induced by LNP-encapsulated mRNA sequences encoding the whole length of gE, the extracellular domain of gE and a C-terminal double mutant of gE (mutant Y569A with original motif AYRV, which targets gE to TGN, and mutants S593A, S595A, T596A and T598A with the original motif SSTT) that were reported to enhance virus spread and elevate virus titers. The results showed that while the humoral and cellular immunity induced by all of the mRNA vaccines was comparable to or better than that induced by the AS01B-adjuvanted subunit vaccines, the C-terminal double mutant of gE showed stable advantages in all of the indicators tested, including gE-specific IgG titers and T cell responses, and could be adopted as a candidate for both safer varicella vaccines and effective zoster vaccines.

The Structures and Functions of VZV Glycoproteins

The virions of all enveloped viruses, including those of the Herpesviridae, must bind to the cell surface then undergo a process of membrane fusion between the cell plasma membrane and the virus particle envelope. As for all herpesviruses, glycoproteins in the virion envelope are the modus operandi of these events.

Viral Interactions with Adaptor-Protein Complexes: A Ubiquitous Trait among Viral Species

Numerous viruses hijack cellular protein trafficking pathways to mediate cell entry or to rearrange membrane structures thereby promoting viral replication and antagonizing the immune response. Adaptor protein complexes (AP), which mediate protein sorting in endocytic and secretory transport pathways, are one of the conserved viral targets with many viruses possessing AP-interacting motifs. We present here different mechanisms of viral interference with AP complexes and the functional consequences that allow for efficient viral propagation and evasion of host immune defense. The ubiquity of this phenomenon is evidenced by the fact that there are representatives for AP interference in all major viral families, covered in this review. The best described examples are interactions of human immunodeficiency virus and human herpesviruses with AP complexes. Several other viruses, like Ebola, Nipah, and SARS-CoV-2, are pointed out as high priority disease-causative agents supporting the need for deeper understanding of virus-AP interplay which can be exploited in the design of novel antiviral therapies.

Bovine herpesvirus 1 (BHV-1) envelope protein gE subcellular trafficking is contributed by two separate YXXL/Φ motifs within the cytoplasmic tail which together promote efficient virus cell-to-cell spread

Bovine herpesvirus envelope glycoprotein E (gE) and, in particular, the gE cytoplasmic tail (CT) is a virulence determinant in cattle. Also, the gE CT contributes to virus cell-to-cell spread and anterograde neuronal transport. In this study, our goal was to map the gE CT sub-domains that contribute to virus cell-to-cell spread property. A panel of gE-CT specific mutant viruses was constructed and characterized, in vitro, with respect to their plaque phenotypes, gE recycling and gE basolateral membrane targeting. The results revealed that disruption of the tyrosine-based motifs, ₄₆₇YTSL₄₇₀ and ₅₆₃YTVV₅₆₆, individually produced smaller plaque phenotypes than the wild type. However, they were slightly larger than the gE CT-null virus plaques. The Y467A mutation affected the gE endocytosis, gE trans-Golgi network (TGN) recycling, and gE virion incorporation properties. However, the Y563A mutation affected only the gE basolateral cell-surface redistribution function. Notably, the simultaneous Y467A/Y563A mutations produced gE CT-null virus-like plaque phenotypes.

The C-terminus of varicella-zoster virus glycoprotein M contains trafficking motifs that mediate skin virulence in the SCID-human model of VZV pathogenesis

Knowledge about the function of varicella-zoster virus glycoprotein M is limited; the requirement of gM for skin and neural tropism are unknown. VZV gM contains two predicted YXXΦ trafficking motifs and a dileucine motif in the carboxyl-terminus. We constructed a recombinant VZV with gM truncated from the first YXXΦ and five additional viruses with YXXΦ tyrosine substitutions, alone and in combination with dileucine substitution. All recombinant viruses grew to high titer but mutation of the membrane-proximal YXXΦ motif reduced plaque size in cultured cells and altered gM localization. C-terminus truncation had a pronounced effect on virion morphogenesis and plaque size, but not on overall replication kinetics in vitro. Mutation of gM trafficking motifs and truncation attenuated replication in human skin xenografts in vivo; gM truncation did not alter neurotropism. Our results demonstrate that the gM C-terminus is dispensable for virus replication in cultured cells but is important for skin pathogenesis.

Varicella-Zoster Virus ORF9p Binding to Cellular Adaptor Protein Complex 1 Is Important for Viral Infectivity

ORF9p (homologous to herpes simplex virus 1 [HSV-1] VP22) is a varicella-zoster virus (VZV) tegument protein essential for viral replication. Even though its precise functions are far from being fully described, a role in the secondary envelopment of the virus has long been suggested. We performed a yeast two-hybrid screen to identify cellular proteins interacting with ORF9p that might be important for this function. We found 31 ORF9p interaction partners, among which was AP1M1, the μ subunit of the adaptor protein complex 1 (AP-1). AP-1 is a heterotetramer involved in intracellular vesicle-mediated transport and regulates the shuttling of cargo proteins between endosomes and the trans-Golgi network via clathrin-coated vesicles. We confirmed that AP-1 interacts with ORF9p in infected cells and mapped potential interaction motifs within ORF9p. We generated VZV mutants in which each of these motifs was individually impaired and identified leucine 231 in ORF9p to be critical for the interaction with AP-1. Disrupting ORF9p binding to AP-1 by mutating leucine 231 to alanine in ORF9p strongly impaired viral growth, most likely by preventing efficient secondary envelopment of the virus. Leucine 231 is part of a dileucine motif conserved among alphaherpesviruses, and we showed that VP22 of Marek's disease virus and HSV-2 also interacts with AP-1. This indicates that the function of this interaction in secondary envelopment might be conserved as well.IMPORTANCE Herpesviruses are responsible for infections that, especially in immunocompromised patients, can lead to severe complications, including neurological symptoms and strokes. The constant emergence of viral strains resistant to classical antivirals (mainly acyclovir and its derivatives) pleads for the identification of new targets for future antiviral treatments. Cellular adaptor protein (AP) complexes have been implicated in the correct addressing of herpesvirus glycoproteins in infected cells, and the discovery that a major constituent of the varicella-zoster virus tegument interacts with AP-1 reveals a previously unsuspected role of this tegument protein. Unraveling the complex mechanisms leading to virion production will certainly be an important step in the discovery of future therapeutic targets.

A Tyrosine-Based Trafficking Motif of the Tegument Protein pUL71 Is Crucial for Human Cytomegalovirus Secondary Envelopment

The human cytomegalovirus (HCMV) tegument protein pUL71 is required for efficient secondary envelopment and accumulates at the Golgi compartment-derived viral assembly complex (vAC) during infection. Analysis of various C-terminally truncated pUL71 proteins fused to enhanced green fluorescent protein (eGFP) identified amino acids 23 to 34 as important determinants for its Golgi complex localization. Sequence analysis and mutational verification revealed the presence of an N-terminal tyrosine-based trafficking motif (YXXΦ) in pUL71. This led us to hypothesize a requirement of the YXXΦ motif for the function of pUL71 in infection. Mutation of both the tyrosine residue and the entire YXXΦ motif resulted in an altered distribution of mutant pUL71 at the plasma membrane and in the cytoplasm during infection. Both YXXΦ mutant viruses exhibited similarly decreased focal growth and reduced virus yields in supernatants. Ultrastructurally, mutant-virus-infected cells exhibited impaired secondary envelopment manifested by accumulations of capsids undergoing an envelopment process. Additionally, clusters of capsid accumulations surrounding the vAC were observed, similar to the ultrastructural phenotype of a UL71-deficient mutant. The importance of endocytosis and thus the YXXΦ motif for targeting pUL71 to the Golgi complex was further demonstrated when clathrin-mediated endocytosis was inhibited either by coexpression of the C-terminal part of cellular AP180 (AP180-C) or by treatment with methyl-β-cyclodextrin. Both conditions resulted in a plasma membrane accumulation of pUL71. Altogether, these data reveal the presence of a functional N-terminal endocytosis motif that is an important determinant for intracellular localization of pUL71 and that is furthermore required for the function of pUL71 during secondary envelopment of HCMV capsids at the vAC.IMPORTANCE Human cytomegalovirus (HCMV) is the leading cause of birth defects among congenital virus infections and can lead to life-threatening infections in immunocompromised hosts. Current antiviral treatments target viral genome replication and are increasingly overcome by viral mutations. Therefore, identifying new targets for antiviral therapy is important for future development of novel treatment options. A detailed molecular understanding of the complex virus morphogenesis will identify potential viral as well as cellular targets for antiviral intervention. Secondary envelopment is an important viral process through which infectious virus particles are generated and which involves the action of several viral proteins, such as tegument protein pUL71. Targeting of pUL71 to the site of secondary envelopment appears to be crucial for its function during this process and is regulated by utilizing host trafficking mechanisms that are commonly exploited by viral glycoproteins. Thus, intracellular trafficking, if targeted, might present a novel target for antiviral therapy.

Betaherpesvirus Virion Assembly and Egress

Virions are the vehicle for cell-to-cell and host-to-host transmission of viruses. Virions need to be assembled reliably and efficiently, be released from infected cells, survive in the extracellular environment during transmission, recognize and then trigger entry of appropriate target cells, and disassemble in an orderly manner during initiation of a new infection. The betaherpesvirus subfamily includes four human herpesviruses (human cytomegalovirus and human herpesviruses 6A, 6B, and 7), as well as viruses that are the basis of important animal models of infection and immunity. Similar to other herpesviruses, betaherpesvirus virions consist of four main parts (in order from the inside): the genome, capsid, tegument, and envelope. Betaherpesvirus genomes are dsDNA and range in length from ~145 to 240 kb. Virion capsids (or nucleocapsids) are geometrically well-defined vessels that contain one copy of the dsDNA viral genome. The tegument is a collection of several thousand protein and RNA molecules packed into the space between the envelope and the capsid for delivery and immediate activity upon cellular entry at the initiation of an infection. Betaherpesvirus envelopes consist of lipid bilayers studded with virus-encoded glycoproteins; they protect the virion during transmission and mediate virion entry during initiation of new infections. Here, we summarize the mechanisms of betaherpesvirus virion assembly, including how infection modifies, reprograms, hijacks, and otherwise manipulates cellular processes and pathways to produce virion components, assemble the parts into infectious virions, and then transport the nascent virions to the extracellular environment for transmission.

Exocytosis of Varicella-Zoster Virus Virions Involves a Convergence of Endosomal and Autophagy Pathways

Varicella-zoster virus (VZV) is an extremely cell-associated herpesvirus with limited egress of viral particles. The induction of autophagy in VZV-infected monolayers is easily detectable; inhibition of autophagy leads to decreased VZV glycoprotein biosynthesis and diminished viral titers. To explain how autophagic flux could exert a proviral effect on the VZV infectious cycle, we postulated that the VZV exocytosis pathway following secondary envelopment may converge with the autophagy pathway. This hypothesis depended on known similarities between VZV gE and autophagy-related (Atg) Atg9/Atg16L1 trafficking pathways. Investigations were carried out with highly purified fractions of VZV virions. When the virion fraction was tested for the presence of autophagy and endosomal proteins, microtubule-associated protein 1 light chain (MAP1LC3B) and Ras-like GTPase 11 (Rab11) were detected. By two-dimensional (2D) and 3D imaging after immunolabeling, both proteins also colocalized with VZV gE in a proportion of cytoplasmic vesicles. When purified VZV virions were enumerated after immunoelectron microscopy, gold beads were detected on viruses following incubation with antibodies to VZV gE (∼100%), Rab11 (50%), and LC3B (30%). Examination of numerous electron micrographs demonstrated that enveloped virions were housed in single-membraned vesicles; viral particles were not observed in autophagosomes. Taken together, our data suggested that some viral particles after secondary envelopment accumulated in a heterogeneous population of single-membraned vesicular compartments, which were decorated with components from both the endocytic pathway (Rab11) and the autophagy pathway (LC3B). The latter cytoplasmic viral vesicles resembled an amphisome.

IMPORTANCE

VZV infection leads to increased autophagic flux, while inhibition of autophagy leads to a marked reduction in virus spread. In this investigation of the proviral role of autophagy, we found evidence for an intersection of viral exocytosis and autophagy pathways. Specifically, both LC3-II and Rab11 proteins copurified with some infectious VZV particles. The results suggested that a subpopulation of VZV particles were carried to the cell surface in single-walled vesicles with attributes of an amphisome, an organelle formed from the fusion of an endosome and an autophagosome. Our results also addressed the interpretation of autophagy/xenophagy results with mutated herpes simplex virus lacking its ICP34.5 neurovirulence gene (HSVΔ34.5). The VZV genome lacks an ICP34.5 ortholog, yet we found no evidence of VZV particles housed in a double-membraned autophagosome. In other words, xenophagy, a degradative process documented after infection with HSVΔ34.5, was not observed in VZV-infected cells.

Varicella-Zoster Virus Glycoproteins: Entry, Replication, and Pathogenesis

Varicella-zoster virus (VZV), an alphaherpesvirus that causes chicken pox (varicella) and shingles (herpes zoster), is a medically important pathogen that causes considerable morbidity and, on occasion, mortality in immunocompromised patients. Herpes zoster can afflict the elderly with a debilitating condition, postherpetic neuralgia, triggering severe, untreatable pain for months or years. The lipid envelope of VZV, similar to all herpesviruses, contains numerous glycoproteins required for replication and pathogenesis.

PURPOSE OF REVIEW

To summarize the current knowledge about VZV glycoproteins and their roles in cell entry, replication and pathogenesis.

RECENT FINDINGS

The functions for some VZV glycoproteins are known, such as gB, gH and gL in membrane fusion, cell-cell fusion regulation, and receptor binding properties. However, the molecular mechanisms that trigger or mediate VZV glycoproteins remains poorly understood.

SUMMARY

VZV glycoproteins are central to successful replication but their modus operandi during replication and pathogenesis remain elusive requiring further mechanistic based studies.

Preventing Cleavage of the Respiratory Syncytial Virus Attachment Protein in Vero Cells Rescues the Infectivity of Progeny Virus for Primary Human Airway Cultures

All live attenuated respiratory syncytial virus (RSV) vaccines that have advanced to clinical trials have been produced in Vero cells. The attachment (G) glycoprotein in virions produced in these cells is smaller than that produced in other immortalized cells due to cleavage. These virions are 5-fold less infectious for primary well-differentiated human airway epithelial (HAE) cell cultures. Because HAE cells are isolated directly from human airways, Vero cell-grown vaccine virus would very likely be similarly inefficient at initiating infection of the nasal epithelium following vaccination, and therefore, a larger inoculum would be required for effective vaccination. We hypothesized that Vero cell-derived virus containing an intact G protein would be more infectious for HAE cell cultures. Using protease inhibitors with increasing specificity, we identified cathepsin L to be the protease responsible for cleavage. Our evidence suggests that cleavage occurs in the late endosome or lysosome during endocytic recycling. Cathepsin L activity was 100-fold greater in Vero cells than in HeLa cells. In addition, cathepsin L was able to cleave the G protein in Vero cell-grown virions but not in HeLa cell-grown virions, suggesting a difference in G-protein posttranslational modification in the two cell lines. We identified by mutagenesis amino acids important for cleavage, and these amino acids included a likely cathepsin L cleavage site. Virus containing a modified, noncleavable G protein produced in Vero cells was 5-fold more infectious for HAE cells in culture, confirming our hypothesis and indicating the value of including such a mutation in future live attenuated RSV vaccines.

IMPORTANCE

Worldwide, RSV is the second leading infectious cause of infant death, but no vaccine is available. Experimental live attenuated RSV vaccines are grown in Vero cells, but during production the virion attachment (G) glycoprotein is cleaved. Virions containing a cleaved G protein are less infectious for primary airway epithelial cells, the natural RSV target. In the study described here we identified the protease responsible, located the cleavage site, and demonstrated that cleavage likely occurs during endocytic recycling. Moreover, we showed that the infectivity of Vero cell-derived virus for primary airway epithelial cells is increased 5-fold if the virus contains a mutation in the G protein that prevents cleavage. The blocking of cleavage should improve RSV vaccine yield, consequently reducing production costs. Posttranslational cleavage of the fusion glycoprotein of many viruses plays an essential role in activation; however, cleavage of the RSV G protein is a novel example of a detrimental effect of cleavage on virus infectivity.

The herpes virus Fc receptor gE-gI mediates antibody bipolar bridging to clear viral antigens from the cell surface

The Herpes Simplex Virus 1 (HSV-1) glycoprotein gE-gI is a transmembrane Fc receptor found on the surface of infected cells and virions that binds human immunoglobulin G (hIgG). gE-gI can also participate in antibody bipolar bridging (ABB), a process by which the antigen-binding fragments (Fabs) of the IgG bind a viral antigen while the Fc binds to gE-gI. IgG Fc binds gE-gI at basic, but not acidic, pH, suggesting that IgG bound at extracellular pH by cell surface gE-gI would dissociate and be degraded in acidic endosomes/lysosomes if endocytosed. The fate of viral antigens associated with gE-gI-bound IgG had been unknown: they could remain at the cell surface or be endocytosed with IgG. Here, we developed an in vitro model system for ABB and investigated the trafficking of ABB complexes using 4-D confocal fluorescence imaging of ABB complexes with transferrin or epidermal growth factor, well-characterized intracellular trafficking markers. Our data showed that cells expressing gE-gI and the viral antigen HSV-1 gD endocytosed anti-gD IgG and gD in a gE-gI-dependent process, resulting in lysosomal localization. These results suggest that gE-gI can mediate clearance of infected cell surfaces of anti-viral host IgG and viral antigens to evade IgG-mediated responses, representing a general mechanism for viral Fc receptors in immune evasion and viral pathogenesis.

Herpesviruses exploit several host compartments for envelopment

Enveloped viruses acquire their host-derived membrane at a variety of intracellular locations. Herpesviruses are complex entities that undergo several budding and fusion events during an infection. All members of this large family are believed to share a similar life cycle. However, they seemingly differ in terms of acquisition of their mature envelope. Herpes simplex virus is often believed to bud into an existing intracellular compartment, while the related cytomegalovirus may acquire its final envelope from a novel virus-induced assembly compartment. This review focuses on recent advances in the characterization of cellular compartment(s) potentially contributing to herpes virion final envelopment. It also examines the common points between seemingly distinct envelopment pathways and highlights the dynamic nature of intracellular compartments in the context of herpesvirus infections.

The high affinity IgE receptor (FcεRI) expression and function in airway smooth muscle

The airway smooth muscle (ASM) is no longer considered as merely a contractile apparatus and passive recipient of growth factors, neurotransmitters and inflammatory mediators signal but a critical player in the perpetuation and modulation of airway inflammation and remodeling. In recent years, a molecular link between ASM and IgE has been established through Fc epsilon receptors (FcεRs) in modulating the phenotype and function of these cells. Particularly, the expression of high affinity IgE receptor (FcεRI) has been noted in primary human ASM cells in vitro and in vivo within bronchial biopsies of allergic asthmatic subjects. The activation of FcεRI on ASM cells suggests a critical yet almost completely ignored network which may modulate ASM cell function in allergic asthma. This review is intended to provide a historical perspective of IgE effects on ASM and highlights the recent updates in the expression and function of FcεRI, and to present future perspectives of activation of this pathway in ASM cells.

Analysis of the functions of glycoproteins E and I and their promoters during VZV replication in vitro and in skin and T-cell xenografts in the SCID mouse model of VZV pathogenesis

The two VZV glycoproteins, gE and gI, are encoded by genes that are designated open reading frames, ORF67 and ORF68, located in the short unique region of the VZV genome. These proteins have homologs in the other alphaherpesviruses. Like their homologues, VZV gE and gI exhibit prominent co-localization in infected cells and form heterodimers. However, VZV gE is much larger than its homologues because it has a unique N-terminal domain, consisting of 188 amino acids that are not present in these other gene products. VZV gE also differs from the related gE proteins, in that it is essential for viral replication. Targeted mutations of gE that are compatible with VZV replication in cultured cells have varying phenotypes in skin and T-cell xenografts in the SCID mouse model of VZV pathogenesis in vivo. While gI is dispensable for growth in cultured cells in vitro, this glycoprotein is essential for VZV infection of differentiated human skin and T cells in vivo. The promoter regions of gE and gI are regulated by the cellular transactivator, specificity protein factor 1 (Sp1) in combination with the major VZV transactivator in reporter construct experiments and some Sp1 promoter elements are important for VZV virulence in vivo. Further analysis of VZV gE and gI functions and their interactions with other viral and host cell proteins are important areas for studies of VZV replication and pathogenesis.

Optimal replication of human cytomegalovirus correlates with endocytosis of glycoprotein gpUL132

Envelopment of a herpesvirus particle is a complex process of which much is still to be learned. We previously identified the glycoprotein gpUL132 of human cytomegalovirus (HCMV) as an envelope component of the virion. In its carboxy-terminal portion, gpUL132 contains at least four motifs for sorting of transmembrane proteins to endosomes; among them are one dileucine-based signal and three tyrosine-based signals of the YXXØ and NPXY (where X stands for any amino acid, and Ø stands for any bulky hydrophobic amino acid) types. To investigate the role of each of these trafficking signals in intracellular localization and viral replication, we constructed a panel of expression plasmids and recombinant viruses in which the signals were rendered nonfunctional by mutagenesis. In transfected cells wild-type gpUL132 was mainly associated with the trans-Golgi network. Consecutive mutation of the trafficking signals resulted in increasing fractions of the protein localized at the cell surface, with gpUL132 mutated in all four trafficking motifs predominantly associated with the plasma membrane. Concomitant with increased surface expression, endocytosis of mutant gpUL132 was reduced, with a gpUL132 expressing all four motifs in mutated form being almost completely impaired in endocytosis. The replication of recombinant viruses harboring mutations in single trafficking motifs was comparable to replication of wild-type virus. In contrast, viruses containing mutations in three or four of the trafficking signals showed pronounced deficits in replication with a reduction of approximately 100-fold. Moreover, recombinant viruses expressing gpUL132 with three or four trafficking motifs mutated failed to incorporate the mutant protein into the virus particle. These results demonstrate a role of endocytosis of an HCMV envelope glycoprotein for incorporation into the virion and optimal virus replication.

VZV ORF47 serine protein kinase and its viral substrates

ORF47, a serine protein kinase of varicella-zoster virus (VZV) and homolog of herpes simplex virus UL13, is an interesting modulator of VZV pathogenesis. This chapter summarizes research showing that ORF47 protein kinase activity, by virtue of phosphorylation of or binding to various viral substrates, regulates VZV proteins during all phases of viral infection and has a pronounced effect on the trafficking of gE, the predominant VZV glycoprotein, which in turn is critical for cell-to-cell spread of the virus. Casein kinase II, an ubiquitous cellular protein kinase, recognizes a similar but less stringent phosphorylation consensus sequence and can partially compensate for lack of ORF47 activity in VZV-infected cells. Differences between the phosphorylation consensus sites of the viral and cellular kinases are outlined in detail.

Functions of the unique N-terminal region of glycoprotein E in the pathogenesis of varicella-zoster virus infection

Varicella-zoster virus (VZV) is an alphaherpesvirus that infects skin, lymphocytes, and sensory ganglia. VZV glycoprotein E (gE) has a unique N-terminal region (aa1-188), which is required for replication and includes domains involved in secondary envelopment, efficient cell-cell spread, and skin infection in vivo. The nonconserved N-terminal region also mediates binding to the insulin-degrading enzyme (IDE), which is proposed to be a VZV receptor. Using viral mutagenesis to make the recombinant rOka-DeltaP27-G90, we showed that amino acids in this region are required for gE/IDE binding in infected cells; this deletion reduced cell-cell spread in vitro and skin infection in vivo. However, a gE point mutation, linker insertions, and partial deletions in the aa27-90 region, and deletion of a large portion of the unique N-terminal region, aa52-187, had similar or more severe effects on VZV replication in vitro and in vivo without disrupting the gE/IDE interaction. VZV replication in T cells in vivo was not impaired by deletion of gE aa27-90, suggesting that these gE residues are not essential for VZV T cell tropism. However, the rOka-DeltaY51-P187 mutant failed to replicate in T cell xenografts as well as skin in vivo. VZV tropism for T cells and skin, which is necessary for its life cycle in the human host, requires this nonconserved region of the N-terminal region of VZV gE.

Insulin-degrading enzyme binds to the nonglycosylated precursor of varicella-zoster virus gE protein found in the endoplasmic reticulum

Insulin degradation enzyme (IDE) is a 110-kDa zinc metalloprotease found in the cytosol of all cells. IDE degrades insulin and a variety of small proteins including amyloid-beta. Recently, IDE has been proposed as the receptor for varicella-zoster virus (VZV) attachment. During our reassessment, some of the original studies were repeated and expanded in scope. We first confirmed that IDE antibody reduced VZV spread. For additional controls, we repeated the same experiments with herpes simplex virus (HSV)-infected cells as well as uninfected cells. There was a visible reduction in HSV spread but less than seen in the VZV system. Of greater importance, IDE antibody also inhibited the growth of uninfected cells. Second, we repeated the coprecipitation assays. We confirmed that antibodies to VZV gE (open reading frame 68) coprecipitated IDE and that anti-IDE antibody coprecipitated gE. However, the detected gE protein was not the mature 98-kDa form; rather, it was a precursor 73-kDa gE form found in the endoplasmic reticulum. Additional control experiments included VZV-infected cell cultures treated with tunicamycin to block gE glycosylation in the endoplasmic reticulum; again, the anti-IDE antibody coprecipitated a 73-kDa gE product. Finally, Orbitrap mass spectrometry analysis of a chromatographically purified gE sample revealed four cellular proteins associated with the unfolded protein response: BiP (HSPA5), HSPA8, HSPD1, and PPIA (peptidyl-propyl cis-trans isomerase). We conclude that IDE protease binds to the 73-kDa gE precursor and that this event occurs in the cytosol but not as a receptor/ligand interaction.

Development of recombinant varicella-zoster viruses expressing luciferase fusion proteins for live in vivo imaging in human skin and dorsal root ganglia xenografts

Varicella-zoster virus (VZV) is a host specific human pathogen that has been studied using human xenografts in SCID mice. Live whole-animal imaging is an emerging technique to measure protein expression in vivo using luminescence. Currently, it has only been possible to determine VZV protein expression in xenografts postmortem. Therefore, to measure immediate early (IE63) and late (glycoprotein E [gE]) protein expression in vivo viruses expressing IE63 or gE as luciferase fusion proteins were generated. Viable recombinant viruses pOka-63-luciferase and pOka-63/70-luciferase, which had luciferase genes fused to ORF63 and its duplicate ORF70, or pOka-gE-CBR were recovered that expressed IE63 or gE as fusion proteins and generated luminescent plaques. In contrast to pOka-63/70-luciferase viruses, the luciferase gene was rapidly lost in vitro when fused to a single copy of ORF63 or ORF68. IE63 expression was successfully measured in human skin and dorsal root ganglia xenografts infected with the genomically stable pOka-63/70-luciferase viruses. The progress of VZV infection in dorsal root ganglia xenografts was delayed in valacyclovir treated mice but followed a similar trend in untreated mice when the antiviral was withdrawn 28 days post-inoculation. Thus, IE63-luciferase fusion proteins were effective for investigating VZV infection and antiviral activity in human xenografts.

Deletion of the first cysteine-rich region of the varicella-zoster virus glycoprotein E ectodomain abolishes the gE and gI interaction and differentially affects cell-cell spread and viral entry

Varicella-zoster virus (VZV) glycoprotein E (gE) is the most abundant glycoprotein in infected cells and, in contrast to those of other alphaherpesviruses, is essential for viral replication. The gE ectodomain contains a unique N-terminal region required for viral replication, cell-cell spread, and secondary envelopment; this region also binds to the insulin-degrading enzyme (IDE), a proposed VZV receptor. To identify new functional domains of the gE ectodomain, the effect of mutagenesis of the first cysteine-rich region of the gE ectodomain (amino acids 208 to 236) was assessed using VZV cosmids. Deletion of this region was compatible with VZV replication in vitro, but cell-cell spread of the rOka-DeltaCys mutant was reduced significantly. Deletion of the cysteine-rich region abolished the binding of the mutant gE to gI but not to IDE. Preventing gE binding to gI altered the pattern of gE expression at the plasma membrane of infected cells and the posttranslational maturation of gI and its incorporation into viral particles. In contrast, deletion of the first cysteine-rich region did not affect viral entry into human tonsil T cells in vitro or into melanoma cells infected with cell-free VZV. These experiments demonstrate that gE/gI heterodimer formation is essential for efficient cell-cell spread and incorporation of gI into viral particles but that it is dispensable for infectious varicella-zoster virion formation and entry into target cells. Blocking gE binding to gI resulted in severe impairment of VZV infection of human skin xenografts in SCIDhu mice in vivo, documenting the importance of cell fusion mediated by this complex for VZV virulence in skin.

Roles of conserved intracellular sequences regions for the proper expression of dopamine D(2) and D(3) receptors

The dopamine D(2) receptor and D(3) receptor (D(2)R, D(3)R) have high homology in both their amino acid composition and signaling pathways. Virtually all signaling pathways reported thus far overlap between the two receptors with the exception that the D(3)R signals are 2 approximately 5 times less efficient than D(2)R. Previous studies have suggested that conformational constraints of D(3)R might be responsible for the poor coupling with the G protein. To this hypothesis, point mutations were introduced into some of the conserved regions between D(2)R and D(3)R, and their effects on receptor expression were investigated. Among the four conserved intracellular receptor regions examined (TTT motif in the 1(st) intracellular loop, SS motif in the 2(nd) intracellular loop, YxxL and TxxS/xS motifs in the 3(rd) intracellular loop), a mutation of the Thr-Thr-Thr (TTT) motif in the first intracellular loop or the LxxY motif in the 3(rd) intracellular loop markedly decreased the level of D(3)R expression compared with D(2)R. The TTT motif was further mutated individually or in combination to test which residue plays a critical role on the expression of the receptor proteins. Different amino acids between D(2)R and D(3)R in the 1(st) intracellular loop were exchanged to determine if the adjacent amino acid residues are responsible for the differences between D(2)R and D(3)R. The first two threonine residues become more important when the individual threonine residue is mutated. However, all three intact threonine residues are essential for proper expression of the receptor proteins. The neighboring sequences around the triplet threonine residues in the 1(st) loop of D(3)R are not important for proper positioning of the receptor proteins on the plasma membrane. It was concluded that D(2)R has a more flexible overall conformation that can accept mutated residues in the intracellular region than D(3)R, which might be partly responsible for the quantitative differences in the signaling efficiency between D(2)R and D(3)R.

A bovine herpesvirus type 1 mutant virus specifying a carboxyl-terminal truncation of glycoprotein E is defective in anterograde neuronal transport in rabbits and calves

Bovine herpesvirus type 1 (BHV-1) is an important component of the bovine respiratory disease complex (BRDC) in cattle. The ability of BHV-1 to transport anterogradely from neuronal cell bodies in trigeminal ganglia (TG) to nerve ending in the noses and corneas of infected cattle following reactivation from latency plays a significant role in the pathogenesis of BRDC and maintenance of BHV-1 in the cattle population. We have constructed a BHV-1 bacterial artificial chromosome (BAC) clone by inserting an excisable BAC plasmid sequence in the long intergenic region between the glycoprotein B (gB) and UL26 genes. A BAC-excised, reconstituted BHV-1 containing only the 34-bp loxP sequence within the gB-UL26 intergenic region was highly infectious in calves, retained wild-type virulence properties, and reactivated from latency following treatment with dexamethasone. Using a two-step Red-mediated mutagenesis system in Escherichia coli, we constructed a gE cytoplasmic tail-truncated BHV-1 and a gE-rescued BHV-1. Following primary infection, the gE cytoplasmic tail-truncated virus was efficiently transported retrogradely from the nerve endings in the nose and eye to cell bodies in the TG of calves and rabbits. However, following dexamethasone-induced reactivation from latency, the gE mutant virus was not isolated from nasal and ocular sheddings. Reverse transcriptase PCR assays detected VP5 transcription in the TG of rabbits infected with gE-rescued and gE cytoplasmic tail-truncated viruses during primary infection and after dexamethasone treatment but not during latency. Therefore, the BHV-1gE cytoplasmic tail-truncated virus reactivated in the TG; however, it had defective anterograde transport from TG to nose and eye in calves and rabbits.

Egress of light particles among filopodia on the surface of Varicella-Zoster virus-infected cells

Varicella-zoster virus (VZV) is renowned for its very low titer when grown in cultured cells. There remains no single explanation for the low infectivity. In this study, viral particles on the surfaces of infected cells were examined by several imaging technologies. Few surface particles were detected at 48 h postinfection (hpi), but numerous particles were observed at 72 and 96 hpi. At 72 hpi, 75% of the particles resembled light (L) particles, i.e., envelopes without capsids. By 96 hpi, 85% of all particles resembled L particles. Subsequently, the envelopes of complete virions and L particles were investigated to determine their glycoprotein constituents. Glycoproteins gE, gI, and gB were detected in the envelopes of both types of particles in similar numbers; i.e., there appeared to be no difference in the glycoprotein content of the L particles. The viral particles emerged onto the cell surface amid actin-based filopodia, which were present in abundance within viral highways. Viral particles were easily detected at the base of and along the exterior surfaces of the filopodia. VZV particles were not detected within filopodia. In short, these results demonstrate that VZV infection of cultured cells produces a larger proportion of aberrant coreless particles than has been seen with any other previously examined alphaherpesvirus. Further, these results suggested a major disassociation between capsid formation and envelopment as an explanation for the invariably low VZV titer in cultured cells.

Contribution of endocytic motifs in the cytoplasmic tail of herpes simplex virus type 1 glycoprotein B to virus replication and cell-cell fusion

The use of endocytic pathways by viral glycoproteins is thought to play various functions during viral infection. We previously showed in transfection assays that herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is transported from the cell surface back to the trans-Golgi network (TGN) and that two motifs of gB cytoplasmic tail, YTQV and LL, function distinctly in this process. To investigate the role of each of these gB trafficking signals in HSV-1 infection, we constructed recombinant viruses in which each motif was rendered nonfunctional by alanine mutagenesis. In infected cells, wild-type gB was internalized from the cell surface and concentrated in the TGN. Disruption of YTQV abolished internalization of gB during infection, whereas disruption of LL induced accumulation of internalized gB in early recycling endosomes and impaired its return to the TGN. The growth of both recombinants was moderately diminished. Moreover, the fusion phenotype of cells infected with the gB recombinants differed from that of cells infected with the wild-type virus. Cells infected with the YTQV-mutated virus displayed reduced cell-cell fusion, whereas giant syncytia were observed in cells infected with the LL-mutated virus. Furthermore, blocking gB internalization or impairing gB recycling to the cell surface, using drugs or a transdominant negative form of Rab11, significantly reduced cell-cell fusion. These results favor a role for endocytosis in virus replication and suggest that gB intracellular trafficking is involved in the regulation of cell-cell fusion.

Roles of Protein Kinase C and Actin-Binding Protein 280 in the Regulation of Intracellular Trafficking of Dopamine D3 Receptor

D3 dopamine receptor (D3R) is expressed mainly in parts of the brain that control the emotional behaviors. It is believed that the improper regulation of D3R is involved in the etiology of schizophrenia. Desensitization of D3R is weakly associated with G protein-coupled receptor kinase (GRK)/β-arrestin-directed internalization. This suggests that there might be an alternative pathway that regulates D3R signaling. This report shows that D3R undergoes robust protein kinase C (PKC)-dependent sequestration that is accompanied by receptor phosphorylation and the desensitization of signaling. PKC-dependent D3R sequestration, which was enhanced by PKC-β or -δ, was dynamin dependent but independent of GRK, β-arrestin, or caveolin 1. Site-directed mutagenesis of all possible phosphorylation sites within the intracellular loops of D3R identified serine residues at positions 229 and 257 as the critical amino acids responsible for phorbol-12-myristate-13-acetate (PMA)-induced D3R phosphorylation, sequestration, and desensitization. In addition, the LxxY endocytosis motif, which is located between residues 252 and 255, was found to play accommodating roles for PMA-induced D3R sequestration. A continuous interaction with the actin-binding protein 280 (filamin A), which was previously known to interact with D3R, is required for PMA-induced D3R sequestration. In conclusion, the PKC-dependent but GRK-/β-arrestin-independent phosphorylation of D3R is the main pathway responsible for the sequestration and desensitization of D3R. Filamin A is essential for both the efficient signaling and sequestration of D3R.

Kinetics of antibody-induced modulation of respiratory syncytial virus antigens in a human epithelial cell line

Background

The binding of viral-specific antibodies to cell-surface antigens usually results in down modulation of the antigen through redistribution of antigens into patches that subsequently may be internalized by endocytosis or may form caps that can be expelled to the extracellular space. Here, by use of confocal-laser-scanning microscopy we investigated the kinetics of the modulation of respiratory syncytial virus (RSV) antigen by RSV-specific IgG. RSV-infected human epithelial cells (HEp-2) were incubated with anti-RSV polyclonal IgG and, at various incubation times, the RSV-cell-surface-antigen-antibody complexes (RSV Ag-Abs) and intracellular viral proteins were detected by indirect immunoflourescence.

Results

Interaction of anti-RSV polyclonal IgG with RSV HEp-2 infected cells induced relocalization and aggregation of viral glycoproteins in the plasma membrane formed patches that subsequently produced caps or were internalized through clathrin-mediated endocytosis participation. Moreover, the concentration of cell surface RSV Ag-Abs and intracellular viral proteins showed a time dependent cyclic variation and that anti-RSV IgG protected HEp-2 cells from viral-induced death.

Conclusion

The results from this study indicate that interaction between RSV cell surface proteins and specific viral antibodies alter the expression of viral antigens expressed on the cells surface and intracellular viral proteins; furthermore, interfere with viral induced destruction of the cell.

Glycoprotein E (gE) specified by bovine herpesvirus type 5 (BHV-5) enables trans-neuronal virus spread and neurovirulence without being a structural component of enveloped virions

Bovine herpesvirus 5 (BHV-5) is a neurovirulent alpha-herpesvirus that causes fatal encephalitis in calves. We previously demonstrated that deletion of a glycine-rich epitope in the gE ectodomain dramatically reduced BHV-5 neurovirulence. To investigate the role of gE cytoplasmic tail sequences in the neuropathogenesis of BHV-5 in rabbits, we constructed a BHV-5gE recombinant virus with a short residual cytoplasmic domain lacking the YXXL motifs and the acidic (BHV-5gEAm480). In vitro, BHV-5gEAm480 produced on the average smaller plaques, compared with wild-type BHV-5, but it produced on the average substantially larger plaques than the gE ORF-deleted BHV-5. The truncated gE was not phosphorylated, and was not endocytosed from the cell surface. Importantly, the truncated gE was not incorporated into enveloped infectious virions, but its glycosylation and interaction with gI were not affected. In a rabbit model of infection, the BHV-5gEAm480 remained highly virulent, while the gE-null virus was avirulent. The gEAm480 mutant virus invaded most of the central nervous system (CNS) structures that are invaded by the wild-type BHV-5. The number of neurons infected by BHV-5gEAm480 was very similar to the number infected by BHV-5 wild-type and gEAm480-rescued viruses. Collectively, the results suggest that gE functions in transsynaptic transmission of BHV-5 and neurovirulence without being a structural component of the virion particle.

Pathogen recognition and development of particulate vaccines: does size matter?

The use of particulate carriers holds great promise for the development of effective and affordable recombinant vaccines. Rational development requires a detailed understanding of particle up-take and processing mechanisms to target cellular pathways capable of stimulating the required immune responses safely. These mechanisms are in turn based on how the host has evolved to recognize and process pathogens. Pathogens, as well as particulate vaccines, come in a wide range of sizes and biochemical compositions. Some of these also provide 'danger signals' so that antigen 'senting cells (APC), usually dendritic cells (DC), acquire specific stimulatory activity. Herein, we provide an overview of the types of particles currently under investigation for the formulation of vaccines, discuss cellular uptake mechanisms (endocytosis, macropinocytosis, phagocytosis, clathrin-dependent and/or caveloae-mediated) for pathogens and particles of different sizes, as well as antigen possessing and presentation by APC in general, and DC in particular. Since particle size and composition can influence the immune response, inducing humoral and/or cellular immunity, activating CD8 T cells and/or CD4 T cells of T helper 1 and/or T helper 2 type, particle characteristics have a major impact on vaccine efficacy. Recently developed methods for the formulation of particulate vaccines are presented in this issue of Methods, showcasing a range of "cutting edge" particulate vaccines that employ particles ranging from nano to micro-sized. This special issue of Methods further addresses practical issues of production, affordability, reproducibility and stability of formulation, and also includes a discussion of the economic and regulatory challenges encountered in developing vaccines for veterinary use and for common Third World infectious diseases.

Essential functions of the unique N-terminal region of the varicella-zoster virus glycoprotein E ectodomain in viral replication and in the pathogenesis of skin infection

Varicella-zoster virus (VZV) glycoprotein E (gE) is a multifunctional protein important for cell-cell spread, envelopment, and possibly entry. In contrast to other alphaherpesviruses, gE is essential for VZV replication. Interestingly, the N-terminal region of gE, comprised of amino acids 1 to 188, was shown not to be conserved in the other alphaherpesviruses by bioinformatics analysis. Mutational analysis was performed to investigate the functions associated with this unique gE N-terminal region. Linker insertions, serine-to-alanine mutations, and deletions were introduced in the gE N-terminal region in the VZV genome, and the effects of these mutations on virus replication and cell-cell spread, gE trafficking and localization, virion formation, and replication in vivo in the skin were analyzed. In summary, mutagenesis of the gE N-terminal region identified a new functional region in the VZV gE ectodomain essential for cell-cell spread and the pathogenesis of VZV skin tropism and demonstrated that different subdomains of the unique N-terminal region had specific roles in viral replication, cell-cell spread, and secondary envelopment.

Intracellular transport and stability of varicella-zoster virus glycoprotein K

VZV gK, an essential glycoprotein that is conserved among the alphaherpesviruses, is believed to participate in membrane fusion and cytoplasmic virion morphogenesis based on analogy to its HSV-1 homolog. However, the production of VZV gK-specific antibodies has proven difficult presumably due to its highly hydrophobic nature and, therefore, VZV gK has received limited study. To overcome this obstacle, we inserted a FLAG epitope into gK near its amino terminus and produced VZV recombinants expressing epitope-tagged gK (VZV gK-F). These recombinants grew indistinguishably from native VZV, and FLAG-tagged gK could be readily detected in VZV gK-F-infected cells. FACS analysis established that gK is transported to the plasma membrane of infected cells, while indirect immunofluorescence demonstrated that gK accumulates predominately in the Golgi. Using VZV gK-F-infected cells we demonstrated that VZV gK, like several other herpesvirus glycoproteins, is efficiently endocytosed from the plasma membrane. However, pulse-labeling experiments revealed that the half-life of gK is considerably shorter than that of other VZV glycoproteins including gB, gE and gH. This finding suggests that gK may be required in lower abundance than other viral glycoproteins during virion morphogenesis or viral entry.

Porcine reproductive and respiratory syndrome virus-infected alveolar macrophages contain no detectable levels of viral proteins in their plasma membrane and are protected against antibody-dependent, complement-mediated cell lysis

Porcine reproductive and respiratory syndrome virus (PRRSV) can evade the host immune system, which results in prolonged virus replication for several weeks to several months. To date, the mechanisms of PRRSV immune evasion have not been investigated in detail. One possible immune-evasion strategy is to avoid incorporation of viral proteins into the plasma membrane of infected cells, as this prevents recognition by virus-specific antibodies and consequent cell lysis either by the classical complement pathway or by antibody-dependent, cell-mediated cytotoxicity. In this study, viral proteins were not observed in the plasma membrane of in vitro-infected macrophages by using confocal microscopy or flow cytometry. Subsequently, the sensitivity of PRRSV-infected macrophages towards antibody-dependent, complement-mediated cell lysis (ADCML) was determined by using an ADCML assay. A non-significant percentage of PRRSV-infected cells were killed in the assay, showing that in vitro PRRSV-infected macrophages are protected against ADCML. PRRSV proteins were not detected in the plasma membrane of in vivo-infected alveolar macrophages and ADCML was also not observed. Together, these data indicate that viral proteins are not incorporated into the plasma membrane of PRRSV-infected macrophages, which makes infected cells invisible to PRRSV-specific antibodies. This absence of viral proteins on the cell surface could explain the protection against ADCML observed for in vitro and in vivo PRRSV-infected macrophages, and may play a role in virus persistence.

Herpes simplex virus gE/gI must accumulate in the trans-Golgi network at early times and then redistribute to cell junctions to promote cell-cell spread

Herpes simplex virus (HSV) glycoprotein heterodimer gE/gI is necessary for virus spread in epithelial and neuronal tissues. Deletion of the relatively large gE cytoplasmic (CT) domain abrogates the ability of gE/gI to mediate HSV spread. The gE CT domain is required for the sorting of gE/gI to the trans-Golgi network (TGN) in early stages of virus infection, and there are several recognizable TGN sorting motifs grouped near the center of this domain. Late in HSV infection, gE/gI, other viral glycoproteins, and enveloped virions redistribute from the TGN to epithelial cell junctions, and the gE CT domain is also required for this process. Without the gE CT domain, newly enveloped virions are directed to apical surfaces instead of to cell junctions. We hypothesized that the gE CT domain promotes virus envelopment into TGN subdomains from which nascent enveloped virions are sorted to cell junctions, a process that enhances cell-to-cell spread. To characterize elements of the gE CT domain involved in intracellular trafficking and cell-to-cell spread, we constructed a panel of truncation mutants. Specifically, these mutants were used to address whether sorting to the TGN and redistribution to cell junctions are necessary, and sufficient, for gE/gI to promote cell-to-cell spread. gE-519, lacking 32 C-terminal residues, localized normally to the TGN early in infection and then trafficked to cell junctions at late times and mediated virus spread. By contrast, mutants gE-495 (lacking 56 C-terminal residues) and gE-470 (lacking 81 residues) accumulated in the TGN but did not traffic to cell junctions and did not mediate cell-to-cell spread. A fourth mutant, gE-448 (lacking most of the CT domain), did not localize to cell junctions and did not mediate virus spread. Therefore, the capacity of gE/gI to promote cell-cell spread requires early localization to the TGN, but this is not sufficient for virus spread. Additionally, gE CT sequences between residues 495 and 519, which contain no obvious cell sorting motifs, are required to promote gE/gI traffic to cell junctions and cell-to-cell spread.

The why's of Y-based motifs in alphaherpesvirus envelope proteins

The Alphaherpesvirinae are large DNA viruses and represent the largest subfamily of the Herpesviridae with closely related members of man and animal, including herpes simplex virus, varicella-zoster virus, pseudorabies virus, bovine herpesvirus 1, and many others. The viral envelope proteins of alphaherpesviruses are remarkably diverse and are incorporated in the ER, Golgi, and plasma membrane of infected cells. The cytoplasmic domain of many of these envelope proteins contain specific tyrosine-based amino acids. During recent years, accumulating evidence indicates that these tyrosine-based motifs serve different important functions during the virus life cycle, and are implicated in endocytosis processes, intracellular trafficking, basolateral and axonal sorting, and signal transduction events. The current minireview will discuss the functions associated with these tyrosine-based motifs in alphaherpesvirus envelope proteins.

Herpesvirus interference with virus-specific antibodies: bridging antibodies, internalizing antibodies, and hiding from antibodies

Herpesviruses have developed different tools to thwart efficient antibody-dependent neutralisation and lysis of virions and elimination of infected cells. This overview will briefly summarize different of these tools, including (i) viral Fc receptors and the resulting process of antibody bridging, (ii) internalization of individual viral proteins and clustered antibody-antigen complexes from the plasma membrane of infected cells, and (iii) directed egress of virus particles to sites of intimate cell-cell contact that are difficult to access for antibodies.

Trafficking of viral membrane proteins

Many viruses express membrane proteins. For enveloped viruses in particular, membrane proteins are frequently structural components of the virus that mediate the essential tasks of receptor recognition and membrane fusion. The functional activities of these proteins require that they are sorted correctly in infected cells. These sorting events often depend on the ability of the virus to mimic cellular protein trafficking signals and to interact with the cellular trafficking machinery. Importantly, loss or modification of these signals can influence virus infectivity and pathogenesis.

Deletion of gpUL132, a structural component of human cytomegalovirus, results in impaired virus replication in fibroblasts

The coding capacity of human cytomegalovirus (HCMV) for glycoproteins by far exceeds that of other herpesviruses. Few of these proteins have been characterized so far. We have investigated the gene product of reading frame UL132. The putative protein product of UL132 is a glycoprotein with a theoretical mass of 29.8 kDa. Transcription analysis revealed that the gene is transcribed with a true late kinetics from the laboratory-adapted strain AD169 and the low-passage isolate TB40E. Two proteins of 22 to 28 kDa and 45 to 60 kDa were detected in virus-infected cells as well as in extracellular virions. The larger protein carried N-linked carbohydrates. Both protein forms were present in laboratory-adapted strains as well as in low-passage isolates of HCMV. Recombinant viruses with the UL132 gene deleted were constructed in the low-passage HCMV isolate PAN as well as the high-passage isolate AD169. Deletion of UL132 from either genome resulted in a pronounced replication deficit with a reduction of approximately 100-fold for HCMV strain AD169. Thus, the protein product of the UL132 reading frame represents a structural viral glycoprotein of HCMV that has an important function for viral replication in tissue culture.

Herpes simplex virus type 1 capsids transit by the trans-Golgi network, where viral glycoproteins accumulate independently of capsid egress

Egress of herpes capsids from the nucleus to the plasma membrane is a complex multistep transport event that is poorly understood. The current model proposes an initial envelopment at the inner nuclear membrane of capsids newly assembled in the nucleus. The capsids are then released in cytosol by fusion with the outer nuclear membrane. They are finally reenveloped at a downstream organelle before traveling to the plasma membrane for their extracellular release. Although the trans-Golgi network (TGN) is often cited as a potential site of reenvelopment, other organelles have also been proposed, including the Golgi, endoplasmic reticulum-Golgi intermediate compartment, aggresomes, tegusomes, and early or late endosomes. To clarify this important issue, we followed herpes simplex virus type 1 egress by immunofluorescence under conditions that slowed intracellular transport and promoted the accumulation of the otherwise transient reenvelopment intermediate. The data show that the capsids transit by the TGN and point to this compartment as the main reenvelopment site, although a contribution by endosomes cannot formally be excluded. Given that viral glycoproteins are expected to accumulate where capsids acquire their envelope, we examined this prediction and found that all tested could indeed be detected at the TGN. Moreover, this accumulation occurred independently of capsid egress. Surprisingly, capsids were often found immediately adjacent to the viral glycoproteins at the TGN.

Pseudorabies virus glycoprotein gD contains a functional endocytosis motif that acts in concert with an endocytosis motif in gB to drive internalization of antibody-antigen complexes from the surface of infected monocytes

Viral glycoproteins gB and gD of the swine alphaherpesvirus pseudorabies virus (PRV), which is closely related to human herpes simplex virus and varicella-zoster virus, are able to drive internalization of antibody-antigen complexes that may form at the cell surface of infected monocytes, thereby protecting these cells from efficient antibody-mediated lysis. We found earlier that gB relies on an endocytosis motif in its cytoplasmic domain for its function during this internalization process. Here, we report that the PRV gD protein also contains a functional endocytosis motif (YRLL) in its cytoplasmic domain that drives spontaneous endocytosis of gD from the cell surface early in infection and that acts in concert with the endocytosis motif in gB to contribute to efficient internalization of antibody-antigen complexes in PRV-infected monocytes.