Antibody-induced and cytoskeleton-mediated redistribution and shedding of viral glycoproteins, expressed on pseudorabies virus-infected cells

Deletion of gE in Herpes Simplex Virus 1 Leads to Increased Extracellular Virus Production and Augmented Interferon Alpha Production by Peripheral Blood Mononuclear Cells

Herpes simplex virus (HSV) in humans and pseudorabies virus (PRV) in pigs are both alphaherpesviruses. Plasmacytoid dendritic cells (pDCs) make part of the peripheral blood mononuclear cells (PBMCs) and are specialized in producing large amounts of antiviral type I interferon (IFN-I). IFN-I production by PBMCs in response to both HSV-1 and PRV can be virtually exclusively attributed to pDCs. Recently, we discovered that cells infected with gEnull PRV trigger increased production of IFNalpha by porcine PBMCs/pDCs compared with cells infected with wild-type (WT) PRV. This increased IFNalpha response correlates with increased extracellular virus production triggered by gEnull PRV compared with WT PRV. The gE protein and some of its currently described functions are conserved in different alphaherpesviruses, including PRV and HSV-1. In the current study, we report that cells infected with gEnull HSV-1 trigger increased IFNalpha production by human PBMCs and increased extracellular virus production compared with WT HSV-1. Hence, these recently described functions of PRV gE are conserved in HSV-1 gE. Since the increased extracellular virus production and IFNalpha response have also been reported for successful (gEnull) PRV vaccines, the current findings may have important consequences for the rational design of HSV vaccines.

Pseudorabies Virus Infection Causes Downregulation of Ligands for the Activating NK Cell Receptor NKG2D

Herpesviruses display a complex and carefully balanced interaction with important players in the antiviral immune response of immunocompetent natural hosts, including natural killer (NK) cells. With regard to NK cells, this delicate balance is illustrated on the one hand by severe herpesvirus disease reported in individuals with NK cell deficiencies and on the other hand by several NK cell evasion strategies described for herpesviruses. In the current study, we report that porcine cells infected with the porcine alphaherpesvirus pseudorabies virus (PRV) display a rapid and progressive downregulation of ligands for the major activating NK cell receptor NKG2D. This downregulation consists both of a downregulation of NKG2D ligands that are already expressed on the cell surface of an infected cell and an inhibition of cell surface expression of newly expressed NKG2D ligands. Flow cytometry and RT-qPCR assays showed that PRV infection results in downregulation of the porcine NKG2D ligand pULBP1 from the cell surface and a very substantial suppression of mRNA expression of pULBP1 and of another potential NKG2D ligand, pMIC2. Furthermore, PRV-induced NKG2D ligand downregulation was found to be independent of late viral gene expression. In conclusion, we report that PRV infection of host cells results in a very pronounced downregulation of ligands for the activating NK cell receptor NKG2D, representing an additional NK evasion strategy of PRV.

Equine Herpesvirus 1 Bridles T Lymphocytes To Reach Its Target Organs

Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Equine herpesvirus 1 (EHV1) replicates in the respiratory epithelium and disseminates through the body via a cell-associated viremia in leukocytes, despite the presence of neutralizing antibodies. “Hijacked” leukocytes, previously identified as monocytic cells and T lymphocytes, transmit EHV1 to endothelial cells of the endometrium or central nervous system, causing reproductive (abortigenic variants) or neurological (neurological variants) disorders. In the present study, we questioned the potential route of EHV1 infection of T lymphocytes and how EHV1 misuses T lymphocytes as a vehicle to reach the endothelium of the target organs in the absence or presence of immune surveillance. Viral replication was evaluated in activated and quiescent primary T lymphocytes, and the results demonstrated increased infection of activated versus quiescent, CD4⁺ versus CD8⁺, and blood- versus lymph node-derived T cells. Moreover, primarily infected respiratory epithelial cells and circulating monocytic cells efficiently transferred virions to T lymphocytes in the presence of neutralizing antibodies. Albeit T-lymphocytes express all classes of viral proteins early in infection, the expression of viral glycoproteins on their cell surface was restricted. In addition, the release of viral progeny was hampered, resulting in the accumulation of viral nucleocapsids in the T cell nucleus. During contact of infected T lymphocytes with endothelial cells, a late viral protein(s) orchestrates T cell polarization and synapse formation, followed by anterograde dynein-mediated transport and transfer of viral progeny to the engaged cell. This represents a sophisticated but efficient immune evasion strategy to allow transfer of progeny virus from T lymphocytes to adjacent target cells. These results demonstrate that T lymphocytes are susceptible to EHV1 infection and that cell-cell contact transmits infectious virus to and from T lymphocytes.

IMPORTANCE Equine herpesvirus 1 (EHV1) is an ancestral alphaherpesvirus that is related to herpes simplex virus 1 and causes respiratory, reproductive, and neurological disorders in Equidae. EHV1 is indisputably a master at exploiting leukocytes to reach its target organs, accordingly evading the host immunity. However, the role of T lymphocytes in cell-associated viremia remains poorly understood. Here we show that activated T lymphocytes efficiently become infected and support viral replication despite the presence of protective immunity. We demonstrate a restricted expression of viral proteins on the surfaces of infected T cells, which prevents immune recognition. In addition, we indicate a hampered release of progeny, which results in the accumulation of nucleocapsids in the T cell nucleus. Upon engagement with the target endothelium, late viral proteins orchestrate viral synapse formation and viral transfer to the contact cell. Our findings have significant implications for the understanding of EHV1 pathogenesis, which is essential for developing innovative therapies to prevent the devastating clinical symptoms of infection.

Antibody Responses to Bovine Alphaherpesvirus 1 (BoHV-1) in Passively Immunized Calves

To date, in countries where infectious bovine rhinotracheitis (IBR) is widespread, its control is associated with deleted marker vaccines. These products lack one or more genes responsible for the synthesis of glycoproteins or enzymes. In Europe, the most widely used marker vaccine is one in which glycoprotein E (gE-) is deleted, and it is marketed in a killed or modified-live form. Using this type of immunization, it is possible to differentiate vaccinated animals (gE-) from those infected or injected with non-deleted (gE+) products using diagnostic tests specific for gE. The disadvantage of using modified-live gE-products is that they may remain latent in immunized animals and be reactivated or excreted following an immunosuppressive stimulus. For this reason, in the last few years, a new marker vaccine became commercially available containing a double deletion related to genes coding for gE and the synthesis of the thymidine-kinase (tk) enzyme, the latter being associated with the reduction of the neurotropism, latency, and reactivation of the vaccine virus. Intramuscularly and intranasally administered marker products induce a humoral immune response; however, the mother-to-calf antibody kinetics after vaccination with marker vaccines is poorly understood. This review discusses several published articles on this topic.

Anti-KL-6/MUC1 monoclonal antibody reverses resistance to trastuzumab-mediated antibody-dependent cell-mediated cytotoxicity by capping MUC1

Polymorphic epithelial mucin (MUC1) is generally overexpressed on the surface of most adenocarcinomas including breast cancer. MUC1 is associated with chemotherapeutic resistance and immune evasion of cancer cells; however, the association between MUC1 and trastuzumab-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) remains unclear. In this study, using six breast cancer cell lines with differing expression levels and MUC1 distribution, the present results show that cells with MUC1 overexpression and uniform surface distribution were resistant to trastuzumab-mediated ADCC. Importantly, trastuzumab resistance was reversed upon siRNA-mediated MUC1 knockdown and by using anti-KL-6/MUC1 monoclonal antibody (mAb). Additionally, we visually confirmed that anti-KL-6/MUC1 mAb induced capping of MUC1 molecules on the cell surface, resulting the in death of these cells. These results suggest that not only the quantity but also the cell-surface distribution of MUC1 affects the sensitivity of breast cancer cells to trastuzumab-mediated ADCC.

Herpesvirus Evasion of Natural Killer Cells

Natural killer (NK) cells play an important role in the host response against viral infections and cancer development. They are able to kill virus-infected and tumor cells, and they produce different important cytokines that stimulate the antiviral and antitumor adaptive immune response, particularly interferon gamma. NK cells are of particular importance in herpesvirus infections, which is illustrated by systemic and life-threatening herpesvirus disease symptoms in patients with deficiencies in NK cell activity and by the myriad of reports describing herpesvirus NK cell evasion strategies. The latter is particularly obvious for cytomegaloviruses, but increasing evidence indicates that most, if not all, members of the herpesvirus family suppress NK cell activity to some extent. This review discusses the different NK cell evasion strategies described for herpesviruses and how this knowledge may translate to clinical applications.

The Pseudorabies Virus Glycoprotein gE/gI Complex Suppresses Type I Interferon Production by Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDC) play a central role in the antiviral immune response, both in the innate response and in shaping the adaptive response, mainly because of their ability to produce massive amounts of type I interferon (TI-IFN). Here, we report that cells infected with the live attenuated Bartha vaccine strain of porcine alphaherpesvirus pseudorabies virus (PRV) trigger a dramatically increased TI-IFN response by porcine primary pDC compared to cells infected with wild-type PRV strains (Becker and Kaplan). Since Bartha is one of the relatively few examples of a highly successful alphaherpesvirus vaccine, identification of factors that may contribute to its efficacy may provide insights for the rational design of other alphaherpesvirus vaccines. The Bartha vaccine genome displays several mutations compared to the genome of wild-type PRV strains, including a large deletion in the unique short (US) region, encompassing the glycoprotein E (gE), gI, US9, and US2 genes. Using recombinant PRV Becker strains harboring the entire Bartha US deletion or single mutations in the four affected US genes, we demonstrate that the absence of the viral gE/gI complex contributes to the observed increased IFN-α response. Furthermore, we show that the absence of gE leads to an enhanced extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in pDC, which correlates with a higher TI-IFN production by pDC. In conclusion, the PRV Bartha vaccine strain triggers strongly increased TI-IFN production by porcine pDC. Our data further indicate that the gE/gI glycoprotein complex suppresses TI-IFN production by pDC, which represents the first alphaherpesvirus factor that suppresses pDC activity.IMPORTANCE Several alphaherpesviruses, including herpes simpex virus, still lack effective vaccines. However, the highly successful Bartha vaccine has contributed substantially to eradication of the porcine alphaherpesvirus pseudorabies virus (PRV) in several countries. The impact of Bartha on the immune response is still poorly understood. Type I interferon (TI-IFN)-producing plasmacytoid dendritic cells (pDC) may play an important role in vaccine development. Here, we show that Bartha elicits a dramatically increased type I interferon (TI-IFN) response in primary porcine pDC compared to wild-type strains. In addition, we found that the gE/gI complex, which is absent in Bartha, inhibits the pDC TI-IFN response. This is the first description of an immune cell type that is differentially affected by Bartha versus wild-type PRV and is the first report describing an alphaherpesvirus protein that inhibits the TI-IFN response by pDC. These data may therefore contribute to the rational design of other alphaherpesvirus vaccines.

Pseudorabies Virus US3 Protein Kinase Protects Infected Cells from NK Cell-Mediated Lysis via Increased Binding of the Inhibitory NK Cell Receptor CD300a

Several reports have indicated that natural killer (NK) cells are of particular importance in the innate response against herpesvirus infections. As a consequence, herpesviruses have developed diverse mechanisms for evading NK cells, although few such mechanisms have been identified for the largest herpesvirus subfamily, the alphaherpesviruses. The antiviral activity of NK cells is regulated by a complex array of interactions between activating/inhibitory receptors on the NK cell surface and the corresponding ligands on the surfaces of virus-infected cells. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus (PRV) displays previously uncharacterized immune evasion properties: it triggers the binding of the inhibitory NK cell receptor CD300a to the surface of the infected cell, thereby providing increased CD300a-mediated protection of infected cells against NK cell-mediated lysis. US3-mediated CD300a binding was found to depend on aminophospholipid ligands of CD300a and on group I p21-activated kinases. These data identify a novel alphaherpesvirus strategy for evading NK cells and demonstrate, for the first time, a role for CD300a in regulating NK cell activity upon contact with virus-infected target cells.

IMPORTANCE

Herpesviruses have developed fascinating mechanisms to evade elimination by key elements of the host immune system, contributing to their ability to cause lifelong infections with recurrent reactivation events. Natural killer (NK) cells are central in the innate antiviral response. Here we report that the US3 protein kinase of the alphaherpesvirus pseudorabies virus displays a previously uncharacterized capacity for evasion of NK cells. Expression of US3 protects infected cells from NK cell-mediated lysis via increased binding of the inhibitory NK cell receptor CD300a. We show that this US3-mediated increase in CD300a binding depends on aminophospholipids and on cellular p21-activated kinases (PAKs). The identification of this novel NK cell evasion strategy may contribute to the design of improved herpesvirus vaccines and may also have significance for other PAK- and CD300a-modulating viruses and cancer cells.

Pseudorabies virus triggers glycoprotein gE-mediated ERK1/2 activation and ERK1/2-dependent migratory behavior in T cells

The interaction between viruses and immune cells of the host may lead to modulation of intracellular signaling pathways and to subsequent changes in cellular behavior that are of benefit for either virus or host. ERK1/2 (extracellular signal regulated kinase 1/2) signaling represents one of the key cellular signaling axes. Here, using wild-type and gE null virus, recombinant gE, and gE-transfected cells, we show that the gE glycoprotein of the porcine Varicellovirus pseudorabies virus (PRV) triggers ERK1/2 phosphorylation in Jurkat T cells and primary porcine T lymphocytes. PRV-induced ERK1/2 signaling resulted in homotypic T cell aggregation and increased motility of T lymphocytes. Our study reveals a new function of the gE glycoprotein of PRV and suggests that PRV, through activation of ERK1/2 signaling, has a substantial impact on T cell behavior.

IMPORTANCE

Herpesviruses are known to be highly successful in evading the immune system of their hosts, subverting signaling pathways of the host to their own advantage. The ERK1/2 signaling pathway, being involved in many cellular processes, represents a particularly attractive target for viral manipulation. Glycoprotein E (gE) is an important virulence factor of alphaherpesviruses, involved in viral spread. In this study, we show that gE has the previously uncharacterized ability to trigger ERK1/2 phosphorylation in T lymphocytes. We also show that virus-induced ERK1/2 signaling leads to increased migratory behavior of T cells and that migratory T cells can spread the infection to susceptible cells. In conclusion, our results point to a novel function for gE and suggest that virus-induced ERK1/2 activation may trigger PRV-carrying T lymphocytes to migrate and infect other cells susceptible to PRV replication.

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.

Replication of herpes simplex virus: egress of progeny virus at specialized cell membrane sites

In the final stages of the herpes simplex virus 1 (HSV-1) life cycle, a viral nucleocapsid buds into a vesicle of trans-Golgi network (TGN)/endosome origin, acquiring an envelope and an outer vesicular membrane. The virus-containing vesicle then traffics to the plasma membrane where it fuses, exposing a mature virion. Although the process of directed egress has been studied in polarized epithelial cell lines, less work has been done in nonpolarized cell types. In this report, we describe a study of HSV-1 egress as it occurs in nonpolarized cells. The examination of infected Vero cells by electron, confocal, and total internal reflection fluorescence (TIRF) microscopy revealed that HSV-1 was released at specific pocket-like areas of the plasma membrane that were found along the substrate-adherent surface and cell-cell-adherent contacts. Both the membrane composition and cytoskeletal structure of egress sites were found to be modified by infection. The plasma membrane at virion release sites was heavily enriched in viral glycoproteins. Small glycoprotein patches formed early in infection, and virus became associated with these areas as they expanded. Glycoprotein-rich areas formed independently from virion trafficking as confirmed by the use of a UL25 mutant with a defect in capsid nuclear egress. The depolymerization of the cytoskeleton indicated that microtubules were important for the trafficking of virions and glycoproteins to release sites. In addition, the actin cytoskeleton was found to be necessary for maintaining the integrity of egress sites. When actin was depolymerized, the glycoprotein concentrations dispersed across the membrane, as did the surface-associated virus. Lastly, viral glycoprotein E appeared to function in a different manner in nonpolarized cells compared to previous studies of egress in polarized epithelial cells; the total amount of virus released at egress sites was slightly increased in infected Vero cells when gE was absent. However, gE was important for egress site formation, as Vero cells infected with gE deletion mutants formed glycoprotein patches that were significantly reduced in size. The results of this study are interpreted to indicate that the egress of HSV-1 in Vero cells is directed to virally induced, specialized egress sites that form along specific areas of the cell membrane.

The herpes simplex virus 1 IgG fc receptor blocks antibody-mediated complement activation and antibody-dependent cellular cytotoxicity in vivo

Herpes simplex virus 1 (HSV-1) glycoprotein E (gE) mediates cell-to-cell spread and functions as an IgG Fc receptor (FcγR) that blocks the Fc domain of antibody targeting the virus or infected cell. Efforts to assess the functions of the HSV-1 FcγR in vivo have been hampered by difficulties in preparing an FcγR-negative strain that is relatively intact for spread. Here we report the FcγR and spread phenotypes of NS-gE264, which is a mutant strain that has four amino acids inserted after gE residue 264. The virus is defective in IgG Fc binding yet causes zosteriform disease in the mouse flank model that is only minimally reduced compared with wild-type and the rescue strains. The presence of zosteriform disease suggests that NS-gE264 spread functions are well maintained. The HSV-1 FcγR binds the Fc domain of human, but not murine IgG; therefore, to assess FcγR functions in vivo, mice were passively immunized with human IgG antibody to HSV. When antibody was inoculated intraperitoneally 20 h prior to infection or shortly after virus reached the dorsal root ganglia, disease severity was significantly reduced in mice infected with NS-gE264, but not in mice infected with wild-type or rescue virus. Studies of C3 knockout mice and natural killer cell-depleted mice demonstrated that the HSV-1 FcγR blocked both IgG Fc-mediated complement activation and antibody-dependent cellular cytotoxicity. Therefore, the HSV-1 FcγR promotes immune evasion from IgG Fc-mediated activities and likely contributes to virulence at times when antibody is present, such as during recurrent infections.

Functional impairment of PRRSV-specific peripheral CD3+CD8high cells

The replication of porcine reproductive and respiratory syndrome virus (PRRSV) in lungs and lymphoid tissues of PRRSV-infected pigs is already strongly reduced before the appearance of neutralizing antibodies, indicating that other immune mechanisms are involved in eliminating PRRSV at those sites. This study aimed to determine whether PRRSV Lelystad virus (LV)-specific cytotoxic T-lymphocytes (CTL) can efficiently eliminate PRRSV-infected alveolar macrophages. Therefore, CTL assays were performed with PRRSV-infected alveolar macrophages as target cells and autologous peripheral blood mononuclear cells (PBMC) from PRRSV-infected pigs as a source of PRRSV-specific CTL. PBMC of 3 PRRSV-infected pigs were used either directly in CTL assays, or following restimulation in vitro. CTL assays with pseudorabies virus (PRV) Begonia-infected alveolar macrophages and autologous PBMC, from 2 PRV Begonia-inoculated pigs, were performed for validation of the assays. In freshly isolated PBMC, derived from PRRSV-infected pigs, CTL activity towards PRRSV-infected macrophages was not detected until the end of the experiment (56 days post infection-dpi). Restimulating the PBMC with PRRSV in vitro resulted in proliferation of CD3+CD8high cells starting from 14 dpi. Although CD+CD8high cells are generally considered to be CTL, CTL activity was not detected in PRRSV-restimulated PBMC of the 3 pigs until 49 dpi. A weak PRRSV-specific CTL activity was observed only at 56 dpi in PRRSV-restimulated PBMC of one pig. In contrast, a clear CTL activity was observed in PRV Begonia-restimulated PBMC, derived from PRV Begonia-infected pigs, starting from 21 dpi. This study indicates that PBMC of PRRSV-infected pigs contain proliferating CD3+CD8high cells upon restimulation in vitro, but these PBMC fail to exert CTL activity towards PRRSV-infected alveolar macrophages.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

A soluble nonglycosylated recombinant infectious hematopoietic necrosis virus (IHNV) G-protein induces IFNs in rainbow trout (Oncorhynchus mykiss)

Viral glycoproteins interact with cell-surface receptors to mediate virus entry and innate immune system activation. We found that a soluble recombinant infectious hematopoietic necrosis virus G-protein (rIHNV-G) stimulated an early innate immune response mediated by proinflammatory cytokines, IFN1 and IFN-gamma in rainbow trout (Oncorhynchus mykiss) fry. Expression of both IFN1 and IFN-gamma mRNA transcripts was an early event and was rIHNV-G dose-dependent. In addition, preliminary evidence revealed that the innate immune response induced by rIHNV-G protein could protect rainbow trout fry from a subsequent IHNV virus challenge. Finally, the binding and distribution of FITC-rIHNV-G protein on rainbow trout spleen and head kidney leukocytes resemble morphological changes which occur on the cell membrane during antigen-receptor interaction including membrane reorganization, patching, polarization and capping. Thus a soluble nonglycosylated rIHNV-G protein could mediate the activation of rainbow trout leukocytes, with concomitant production of proinflammatory cytokines and IFNs.

The human cytomegalovirus Fc receptor gp68 binds the Fc CH2-CH3 interface of immunoglobulin G

Recognition of immunoglobulin G (IgG) by surface receptors for the Fc domain of immunoglobulin G (Fcgamma), FcgammaRs, can trigger both humoral and cellular immune responses. Two human cytomegalovirus (HCMV)-encoded type I transmembrane receptors with Fcgamma-binding properties (vFcgammaRs), gp34 and gp68, have been identified on the surface of HCMV-infected cells and are assumed to confer protection against IgG-mediated immunity. Here we show that Fcgamma recognition by both vFcgammaRs occurs independently of N-linked glycosylation of Fcgamma, in contrast with the properties of host FcgammaRs. To gain further insight into the interaction with Fcgamma, truncation mutants of the vFcgammaR gp68 ectodomain were probed for Fcgamma binding, resulting in localization of the Fcgamma binding site on gp68 to residues 71 to 289, a region including an immunoglobulin-like domain. Gel filtration and biosensor binding experiments revealed that, unlike host FcgammaRs but similar to the herpes simplex virus type 1 (HSV-1) Fc receptor gE-gI, gp68 binds to the C(H)2-C(H)3 interdomain interface of the Fcgamma dimer with a nanomolar affinity and a 2:1 stoichiometry. Unlike gE-gI, which binds Fcgamma at the slightly basic pH of the extracellular milieu but not at the acidic pH of endosomes, the gp68/Fcgamma complex is stable at pH values from 5.6 to pH 8.1. These data indicate that the mechanistic details of Fc binding by HCMV gp68 differ from those of host FcgammaRs and from that of HSV-1 gE-gI, suggesting distinct functional and recognition properties.

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.

Tyrosine phosphorylation and lipid raft association of pseudorabies virus glycoprotein E during antibody-mediated capping

In specific cell types infected with the alphaherpesviruses herpes simplex virus and pseudorabies virus (PRV), addition of virus-specific antibodies results in redistribution of cell-surface-anchored viral proteins. This redistribution is triggered by the viral protein gE and consists of the directional movement of the antibody-antigen complexes to one pole of the cell. This viral capping process has been associated with increased antibody-resistant virus spread and strongly resembles immunoreceptor capping, a process that is crucial in activation of different immune cells (e.g. capping of Fcgamma-receptors, B and T cell receptors). Here, we report that the PRV gE-mediated viral capping process results in increased Src kinase-mediated tyrosine phosphorylation of the cytoplasmic domain of gE and that a fraction of gE associates with lipid rafts, all very reminiscent of immunoreceptor capping. These results provide evidence that gE-mediated capping is a viral mimicry of immunoreceptor capping.

Inflammation et virus: déclenchement,contournements et détournements de la réponse inflammatoire au cours des infections virales

The inflammatory process aims at opposing an early responseto the viral infections. Inflammation is supposed to delay or limit viral multiplication and dissemination until a specific immune response can be raised. This review introduces the basis of virus-induced inflammation and presents various strategies that are used by viruses to circumvent or exploit inflammation for their own benefit.

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.

Immune escape of equine herpesvirus 1 and other herpesviruses of veterinary importance

Equine herpesvirus (EHV)-1 is a pathogen of horses, well known for its ability to induce abortion and nervous system disorders. Clinical signs may occur despite the presence of a virus-specific immune response in the horse. The current review will summarize the research, on how, EHV-1-infected cells can hide from recognition by the immune system. Research findings on immune evasion of EHV-1 will be compared with those of other herpesviruses of veterinary importance.

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.

In the absence of glycoprotein I (gI), gE determines bovine herpesvirus type 5 neuroinvasiveness and neurovirulence

Bovine herpesvirus type 5 (BHV-5) is an alphaherpesvirus that causes fatal encephalitis in calves. Envelope glycoproteins E (gE) and gI of alphaherpesviruses are important for the pathogenesis in vivo. Previously the authors determined that BHV-5 gE is important for BHV-5 neurovirulence. To determine the role of gI in BHV-5 neurovirulence, the authors have constructed gI-deleted and gI-revertant BHV-5 and analyzed their neuropathogenic properties in a rabbit seizure model. Following intranasal infection, 40% of the rabbits infected with the gI-deleted virus showed severe neurological signs. gI-deleted BHV-5 invaded all the central nervous system (CNS) structures invaded by the gI-revertant BHV-5; however, the number of neurons infected by the gI-deleted virus was similar or slightly reduced (two to four fold). Thus, the gI-deleted virus retained significant neurovirulence and/or neuroinvasive properties when compared with the gE-deleted BHV-5. Pulse-chase analysis revealed that the gE of gI-deleted virus was processed to a larger and a diffused 94- to 100-kDa protein (instead of 94 kDa). The 94- to 100-kDa protein was processed in the Golgi with delayed kinetics but it was endoglycosidase H (EndoH) resistant. In cells infected with gI-deleted virus, there was a reduction in cell-surface gE expression compared to wild-type, which correlated to reduced amount of gE processed in the Golgi. The authors believe that in the absence of gI, BHV-5 gE is sufficient for BHV-5 neurovirulence.

Viral Subversion of the Immune System

The continuous interactions between host and viruses during their co-evolution have shaped not only the immune system but also the countermeasures used by viruses. Studies in the last decade have described the diverse arrays of pathways and molecular targets that are used by viruses to elude immune detection or destruction, or both. These include targeting of pathways for major histocompatibility complex class I and class II antigen presentation, natural killer cell recognition, apoptosis, cytokine signalling, and complement activation. This paper provides an overview of the viral immune-evasion mechanisms described to date. It highlights the contribution of this field to our understanding of the immune system, and the importance of understanding this aspect of the biology of viral infection to develop efficacious and safe vaccines.

Copatching and lipid raft association of different viral glycoproteins expressed on the surfaces of pseudorabies virus-infected cells

Pseudorabies virus (PRV) is a swine alphaherpesvirus that is closely related to human herpes simplex virus (HSV). Both PRV and HSV express a variety of viral envelope glycoproteins in the plasma membranes of infected cells. Here we show that at least four major PRV glycoproteins (gB, gC, gD, and gE) in the plasma membrane of infected swine kidney cells and monocytes seem to be linked, since monospecific antibody-induced patching of any one of these proteins results in copatching of the others. Further, for all four PRV glycoproteins, monospecific antibody-induced patches were enriched in GM1, a typical marker of lipid raft microdomains, but were excluded for transferrin receptor, a nonraft marker, suggesting that these viral proteins may associate with lipid rafts. However, only gB and, to a lesser extent, gE were found in lipid raft fractions by using detergent floatation assays, indicating that gC and gD do not show strong lipid raft association. Addition of methyl-beta-cyclodextrin (MCD), a cholesterol-depleting agent that is commonly used to disrupt lipid rafts, only slightly reduced copatching efficiency between the different viral proteins, indicating that other factors, perhaps tegument-glycoprotein interactions, may be important for the observed copatching events. On the other hand, MCD strongly reduced polarization of the antibody-induced viral glycoprotein patches to a cap structure, a gE-dependent process that has been described for specific PRV- and HSV-infected cells. Therefore, we hypothesize that efficient gE-mediated capping of antibody-antigen patches may require the lipid raft-associated signal transduction machinery.

Immunization strategies to block the herpes simplex virus type 1 immunoglobulin G Fc receptor

Herpes simplex virus type 1 (HSV-1) glycoprotein gE functions as an immunoglobulin G (IgG) Fc receptor (FcgammaR) that promotes immune evasion. When an IgG antibody binds by the F(ab')(2) domain to an HSV antigen, the Fc domain of some of the same antibody molecules binds to the FcgammaR, which blocks Fc-mediated functions. gE is a type 1 membrane glycoprotein with a large ectodomain that is expressed on the virion envelope and infected-cell surface. Our goal was to determine if immunizing with gE protein fragments could produce antibodies that bind by the F(ab')(2) domain to gE and block the FcgammaR, as measured by competitively inhibiting nonimmune human IgG binding to the FcgammaR. Three gE peptides were constructed in baculovirus spanning almost the entire ectodomain and used to immunize mice and rabbits. Two fragments were highly effective at producing antibodies that bind by the F(ab')(2) domain and block the FcgammaR. The most potent of these two antibodies was far more effective at blocking the FcgammaR than antibodies that are only capable of binding by the Fc domains to the FcgammaR, including anti-gC, anti-gD, and nonimmune IgG. These results suggest that immunizing with gE fragments has potential for preventing immune evasion by blocking activities mediated by the HSV-1 FcgammaR.

pH dependence and stoichiometry of binding to the Fc region of IgG by the herpes simplex virus Fc receptor gE-gI

Herpes simplex virus type 1 encodes two glycoproteins, gE and gI, that form a heterodimer on the surface of virions and infected cells. The gE-gI heterodimer has been implicated in cell-to-cell spread of virus and is a receptor for the Fc fragment of IgG. Previous studies localized the gE-gI-binding site on human IgG to a region near the interface between the C(H)2 and C(H)3 domains of Fc, which also serves as the binding site for bacterial and mammalian Fc receptors. Although there are two potential gE-gI-binding sites per Fc homodimer, only one gE-gI heterodimer binds per IgG in gel filtration experiments. Here we report production of recombinant human Fc molecules that contain zero, one, or two potential gE-gI-binding sites and use them in analytical ultracentrifugation experiments to show that two gE-gI heterodimers can bind to each Fc. Further characterization of the gE-gI interaction with Fc reveals a sharp pH dependence of binding, with K(D) values of approximately 340 and approximately 930 nm for the first and second binding events, respectively, at the slightly basic pH of the cell surface (pH 7.4), but undetectable binding at pH 6.0. This strongly pH-dependent interaction suggests a physiological role for gE-gI dissociation from IgG within acidic intracellular compartments, consistent with a mechanism whereby herpes simplex virus promotes intracellular degradation of anti-viral antibodies.

Antibody-induced internalization of viral glycoproteins and gE-gI Fc receptor activity protect pseudorabies virus-infected monocytes from efficient complement-mediated lysis

Pseudorabies virus (PRV)-infected blood monocytes are able to transport virus throughout the body of vaccination-immune pigs. PRV-infected monocytes express viral glycoproteins in their plasma membrane that can be recognized by virus-specific antibodies. Recently, it has been shown that addition of PRV-specific polyclonal immunoglobulins to PRV-infected monocytes at 37 degrees C induces internalization of the majority of plasma membrane-expressed viral glycoproteins. This study investigated whether this process may interfere with efficient antibody-dependent complement-mediated lysis (ADCML) of infected monocytes. Therefore, an ADCML assay was set up in vitro. A significant decrease in the percentage of cells lysed by ADCML was observed when antibody-induced internalization of PRV glycoproteins occurred (P<0.005). Furthermore, it is shown (i) that the PRV gE-gI complex, which, like certain other alpha herpesvirus orthologues, possesses IgG-binding capacity, aids in avoiding efficient ADCML of PRV-infected monocytes and (ii) that the efficiency of PRV gE-gI-mediated evasion of ADCML can be decreased by the presence of gE-gI-specific antibodies.

Responses of herpes simplex virus type 1-infected cells to the presence of extracellular antibodies: gE-dependent glycoprotein capping and enhancement in cell-to-cell spread

Binding of anti-herpes simplex virus (HSV) immunoglobulin G (IgG) to HSV type 1 (HSV-1)-infected HEL and HEp-2 cells causes changes in surface viral glycoprotein distribution, resulting in a capping of all viral glycoproteins towards one pole of the cell. This occurs in a gE-dependent manner. In HEL cells, low concentrations of anti-HSV IgG also enhance cell-to-cell spread of wild-type HSV-1 but not of gE deletion mutant HSV-1. These observations raised the possibility that gE-dependent mechanisms exist that allow some HSV-1-infected cells to respond to the presence of extracellular antibodies by enhancing the antibody-resistant mode of virus transmission.

Absence of viral antigens on the surface of equine herpesvirus-1-infected peripheral blood mononuclear cells: a strategy to avoid complement-mediated lysis

Equine herpesvirus-1 (EHV-1) may cause abortion in vaccination- and infection-immune horses. EHV-1-infected peripheral blood mononuclear cells (PBMCs) play an important role in virus immune evasion. The mechanisms by which infected PBMCs can avoid destruction by EHV-1-specific antibody and equine complement were examined. The majority of EHV-1-infected PBMCs (68.6 %) lacked surface expression of viral antigens and these cells were not susceptible to complement-mediated lysis. In infected PBMCs with surface expression of viral antigens, 63 % showed focal surface expression, whereas 37 % showed general surface expression. General surface expression rendered infected PBMCs susceptible to lysis by antibody and complement (from 5.4 to 31.2 % lysed cells depending on the concentration of antibody and complement). Infected PBMCs with focal surface expression showed significant lysis only in the presence of high concentrations of antibody and complement. Thus, the absence of surface expression protects infected PBMCs against complement-mediated lysis.

Virus complement evasion strategies

The immune system has a variety of tools at its disposal to combat virus infections. These can be subdivided roughly into two categories: 'first line defence', consisting of the non-specific, innate immune system, and 'adaptive immune response', acquired over time following virus infection or vaccination. During evolution, viruses have developed numerous, and often very ingenious, strategies to counteract efficient recognition of virions or virus-infected cells by both innate and adaptive immunity. This review will focus on the different strategies that viruses use to avoid recognition by one of the components of the immune system: the complement system. Complement evasion is of particular importance for viruses, since complement activation is a crucial component of innate immunity (alternative and mannan-binding lectin activation pathway) as well as of adaptive immunity (classical, antibody-dependent complement activation).

Herpes simplex virus type 1 evades the effects of antibody and complement in vivo

Herpes simplex virus type 1 (HSV-1) encodes a complement-interacting glycoprotein, gC, and an immunoglobulin G (IgG) Fc binding glycoprotein, gE, that mediate immune evasion by affecting multiple aspects of innate and acquired immunity, including interfering with complement components C1q, C3, C5, and properdin and blocking antibody-dependent cellular cytotoxicity. Previous studies evaluated the individual contributions of gC and gE to immune evasion. Experiments in a murine model that examines the combined effects of gC and gE immune evasion on pathogenesis are now reported. Virulence of wild-type HSV-1 is compared with mutant viruses defective in gC-mediated C3 binding, gE-mediated IgG Fc binding, or both immune evasion activities. Eliminating both activities greatly increased susceptibility of HSV-1 to antibody and complement neutralization in vitro and markedly reduced virulence in vivo as measured by disease scores, virus titers, and mortality. Studies with C3 knockout mice indicated that other activities attributed to these glycoproteins, such as gC-mediated virus attachment to heparan sulfate or gE-mediated cell-to-cell spread, do not account for the reduced virulence of mutant viruses. The results support the importance of gC and gE immune evasion in vivo and suggest potential new targets for prevention and treatment of HSV disease.

A tyrosine-based motif in the cytoplasmic tail of pseudorabies virus glycoprotein B is important for both antibody-induced internalization of viral glycoproteins and efficient cell-to-cell spread

Pseudorabies virus (PRV), a swine alphaherpesvirus, is capable of causing viremia in vaccinated animals. Two mechanisms that may help PRV avoid recognition by the host immune system during this viremia are direct cell-to-cell spread in tissue and antibody-induced internalization of viral cell surface glycoproteins in PRV-infected blood monocytes, the carrier cells of the virus in the blood. PRV glycoprotein B (gB) is crucial during both processes. Here we show that mutating a tyrosine residue located in a YXXPhi motif in the gB cytoplasmic tail results in decreased efficiency of cell-to-cell spread and a strong reduction in antibody-induced internalization of viral cell surface glycoproteins. Mutating the dileucine motif in the gB tail led to an increased cell-to-cell spread of the virus and the formation of large syncytia.

Involvement of cellular cytoskeleton components in antibody-induced internalization of viral glycoproteins in pseudorabies virus-infected monocytes

Addition of pseudorabies virus (PrV)-specific polyclonal immunoglobulins to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and this may interfere with antibody-dependent cell lysis. We investigated the role of actin, microtubules, clathrin, and dynein, the major cellular components involved in physiological endocytosis during this virological internalization. Porcine monocytes were infected in vitro for 13 h and afterward treated with different concentrations of colchicine, cytochalasin D, latrunculin B, and amantadine-HCl, which inhibit polymerization of microtubules, actin/clathrin, actin, and clathrin, respectively. This resulted in a significant reduction of internalization compared to the nontreated control, indicating that these components are involved in the process. A double labeling was performed during the internalization process and a clear colocalization of actin, microtubules, clathrin, and dynein with the viral glycoproteins was observed at different stages during the internalization process. We conclude that these cellular components are used by PrV to generate the antibody-induced internalization of viral glycoproteins.

Viral subversion of the immune system

This review describes the diverse array of pathways and molecular targets that are used by viruses to elude immune detection and destruction. These include targeting of pathways for major histocompatibility complex-restricted antigen presentation, apoptosis, cytokine-mediated signaling, and humoral immune responses. The continuous interactions between host and pathogens during their coevolution have shaped the immune system, but also the counter measures used by pathogens. Further study of their interactions should improve our ability to manipulate and exploit the various pathogens.

Role of the pseudorabies virus gI cytoplasmic domain in neuroinvasion, virulence, and posttranslational N-linked glycosylation

The glycoproteins I and E of pseudorabies virus are important mediators of cell-to-cell spread and virulence in all animal models tested. Although these two proteins form a complex with one another, ascribing any function to the individual proteins has been difficult. We have shown previously, using nonsense mutations, that the N-terminal ectodomain of the gE protein is sufficient for gE-mediated transsynaptic spread whereas the cytoplasmic domain of the protein is required for full expression of virulence. These same studies demonstrated that the cytoplasmic domain of gE is also required for endocytosis of the protein. In this report, we describe the construction of viruses with nonsense mutations in gI that allowed us to determine the contributions of the gI cytoplasmic domain to protein expression as well as virus neuroinvasion and virulence after infection of the rat eye. We also constructed double mutants with nonsense mutations in both gE and gI so that the contributions of both the gE and gI cytoplasmic domains could be determined. We observed that the gI cytoplasmic domain is required for efficient posttranslational modification of the gI protein. The gE cytoplasmic domain has no effect on gE posttranslational glycosylation. In addition, we found that infection of all gE-gI-dependent anterograde circuits projecting from the rat retina requires both ectodomains and at least one of the cytoplasmic domains of the proteins. The gI cytoplasmic domain promotes transsynaptic spread of virus better than the gE cytoplasmic domain. Interestingly, both gE and gI cytoplasmic tails are required for virulence; lack of either one or both results in an attenuated infection. These data suggest that gE and gI play differential roles in mediating directional neuroinvasion of the rat; however, the gE and gI cytoplasmic domains most likely function together to promote virulence.

Bovine herpesvirus 5 glycoprotein E is important for neuroinvasiveness and neurovirulence in the olfactory pathway of the rabbit

Glycoprotein E (gE) is important for full virulence potential of the alphaherpesviruses in both natural and laboratory hosts. The gE sequence of the neurovirulent bovine herpesvirus 5 (BHV-5) was determined and compared with that of the nonneurovirulent BHV-1. Alignment of the predicted amino acid sequences of BHV-1 and BHV-5 gE open reading frames showed that they had 72% identity and 77% similarity. To determine the role of gE in the differential neuropathogenesis of BHV-1 and BHV-5, we have constructed BHV-1 and BHV-5 recombinants: gE-deleted BHV-5 (BHV-5gEDelta), BHV-5 expressing BHV-1 gE (BHV-5gE1), and BHV-1 expressing BHV-5 gE (BHV-1gE5). Neurovirulence properties of these recombinant viruses were analyzed using a rabbit seizure model (S. I. Chowdhury et al., J. Comp. Pathol. 117:295-310, 1997) that distinguished wild-type BHV-1 and -5 based on their differential neuropathogenesis. Intranasal inoculation of BHV-5 gEDelta and BHV-5gE1 produced significantly reduced neurological signs that affected only 10% of the infected rabbits. The recombinant BHV-1gE5 did not invade the central nervous system (CNS). Virus isolation and immunohistochemistry data suggest that these recombinants replicate and spread significantly less efficiently in the brain than BHV-5 gE revertant or wild-type BHV-5, which produced severe neurological signs in 70 to 80% rabbits. Taken together, the results of neurological signs, brain lesions, virus isolation, and immunohistochemistry indicate that BHV-5 gE is important for efficient neural spread and neurovirulence within the CNS and could not be replaced by BHV-1 gE. However, BHV-5 gE is not required for initial viral entry into olfactory pathway.

Role of anti-gB and -gD antibodies in antibody-induced endocytosis of viral and cellular cell surface glycoproteins expressed on pseudorabies virus-infected monocytes

The addition of porcine pseudorabies virus (PrV)-specific polyclonal IgG antibodies to PrV-infected monocytes induces internalization of plasma membrane-anchored viral glycoproteins and major histocompatibility complex (MHC) class I. Using PrV deletion strains, it was shown that gB and gD are essential for the process to occur. The purpose of the current study was to evaluate whether antibodies directed against single viral glycoproteins are able to induce endocytosis. It was shown that monoclonal antibodies directed against viral glycoprotein gB and gD, but not against gC and gE, are able to induce internalization of their respective ligand. Adding a combination of monoclonal antibodies against gB and gD resulted in endocytosis levels, comparable to the endocytosis levels observed when adding porcine PrV-specific polyclonal antibodies. The addition of genistein and tyrphostin 25, two inhibitors of tyrosine kinase activity, abolished endocytosis induced by monoclonal anti-gB and -gD antibodies in a concentration-dependent manner. The addition of similar concentrations of tyrphostin 1, an inactive tyrphostin, had no effect on endocytosis. It was also shown that a mixture of polyclonal, but not monoclonal, antibodies against gB and gD is able to induce cointernalization of MHC class I. This indicates that MHC class I cointernalization results from a passive catching of the molecules rather than from a specific interaction of the MHC class I molecules with one or more viral glycoproteins. In conclusion, it can be stated that antibody-induced crosslinking of gB and gD induces the activation of a tyrosine phosphorylation-dependent signal transduction pathway, leading to their endocytosis. Cointernalization of other viral glycoproteins and MHC class I is most likely caused by a passive catching of these molecules in the gB and gD aggregates.

Life beyond eradication: veterinary viruses in basic science

To some, the focus of research in virology entails the search for solutions of practical problems. By definition then, attention is limited to those viruses that cause disease or to exploitation of some aspect of virology to a practical end (e.g., antiviral drugs or vaccines). Once a disease is cured, or the agent eradicated, it is time to move on to something else. To others, virology offers the opportunity to study fundamental problems in biology. Work on these problems may offer no obvious practical justification; it is an affliction of the terminally curious, perhaps with the outside hope that something "useful" will come of it. To do this so-called "basic science", one must find the most tractable system to solve the problem, not the system that has "relevance" to disease. I have found that veterinary viruses offer a variety of opportunities to study relevant problems at the fundamental level. To illustrate this point, I describe some recent experiments in my laboratory using pseudorabies virus (PRV), a swine herpesvirus.

Role of the cytoplasmic tail of gE in antibody-induced redistribution of viral glycoproteins expressed on pseudorabies-virus-infected cells

Pseudorabies virus (PrV) glycoprotein gE is a nonessential glycoprotein involved in virulence and spread of the virus. It also has an important, yet unknown, function during antibody-induced capping of viral glycoproteins on the plasma membrane of PrV-infected swine kidney cells. In the present study, it was shown, by the use of a PrV strain expressing a truncated gE glycoprotein, that the cytoplasmic tail of gE is of significant importance for viral glycoprotein capping to occur. In addition, using PrV strains carrying point mutations in the cytoplasmic tail of gE, it was demonstrated that two tyrosine-based motifs are very important for correct functioning of gE during viral glycoprotein capping. Furthermore it was shown that genistein and tyrphostin, two tyrosine kinase activity inhibitors, inhibit viral glycoprotein capping in a concentration-dependent manner. In conclusion, it can be stated that efficient antibody-induced viral glycoprotein capping requires the presence of two YxxL sequences in the cytoplasmic tail of glycoprotein gE, as well as the activation of a tyrosine phosphorylation signal transduction pathway.

Characterization of the interaction between the herpes simplex virus type I Fc receptor and immunoglobulin G

Herpes simplex virus type I (HSV-1) virions and HSV-1-infected cells bind to human immunoglobulin G (hIgG) via its Fc region. A complex of two surface glycoproteins encoded by HSV-1, gE and gI, is responsible for Fc binding. We have co-expressed soluble truncated forms of gE and gI in Chinese hamster ovary cells. Soluble gE-gI complexes can be purified from transfected cell supernatants using a purification scheme that is based upon the Fc receptor function of gE-gI. Using gel filtration and analytical ultracentrifugation, we determined that soluble gE-gI is a heterodimer composed of one molecule of gE and one molecule of gI and that gE-gI heterodimers bind hIgG with a 1:1 stoichiometry. Biosensor-based studies of the binding of wild type or mutant IgG proteins to soluble gE-gI indicate that histidine 435 at the CH2-CH3 domain interface of IgG is a critical residue for IgG binding to gE-gI. We observe many similarities between the characteristics of IgG binding by gE-gI and by rheumatoid factors and bacterial Fc receptors such as Staphylococcus aureus protein A. These observations support a model for the origin of some rheumatoid factors, in which they represent anti-idiotypic antibodies directed against antibodies to bacterial and viral Fc receptors.

Actin polymerization induces shedding of FcgammaRIIIb (CD16) from human neutrophils

FcgammaRIIIb (CD16) is a glycosyl phosphatidylinositol (GPI)-anchored low-affinity IgG receptor, exclusively expressed on human neutrophils. FcgammaRIIIb associates with complement receptor 3 (CR3, Mac-1, CD11b/CD18), which may indirectly link FcgammaRIIIb to the actin cytoskeleton. Upon neutrophil activation, apoptosis, or chemotaxis, FcgammaRIIIb is shed from the cell surface. In all of these events, actin rearrangements play an important role. To establish a role for the actin cytoskeleton in the control of FcgammaRIIIb shedding, we treated human neutrophils with jasplakinolide, an actin-polymerizing peptide. We show that enhanced actin polymerization induces time- and dose-dependent shedding of FcgammaRIIIb. This effect was not restricted to FcgammaRIIIb, because the cell surface expression of CD43, CD44, and L-selectin was also downregulated after induction of actin polymerization. This actin-dependent pathway is staurosporine sensitive but does not appear to involve activation of PKC or CR3. These data show that the actin cytoskeleton can regulate protein ectodomain shedding from human neutrophils.

The disulfide-bonded structure of feline herpesvirus glycoprotein I

Alphaherpesvirus glycoproteins E and I (gE and gI, respectively) assemble into a hetero-oligomeric complex which promotes cell-to-cell transmission, a determining factor of virulence. Focusing on gI of feline herpesvirus (FHV), we examined the role of disulfide bonds during its biosynthesis, its interaction with gE, and gE-gI-mediated spread of the infection in vitro. The protein's disulfide linkage pattern was determined by single and pairwise substitutions for the four conserved cysteine residues in the ectodomain. The resulting mutants were coexpressed with gE in the vaccinia virus-based vTF7-3 system, and the formation and endoplasmic reticulum (ER)-to-Golgi transport of the hetero-oligomeric complex were monitored. The results were corroborated biochemically by performing an endoproteinase Lys-C digestion of a [35S]Cys-labeled secretory recombinant form of gI followed by tricine-sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of the peptides under reducing and nonreducing conditions. We found that (i) gI derivatives lacking Cys79 (C1) and/or Cys223 (C4) still assemble with gE into transport-competent complexes, (ii) mutant proteins lacking Cys91 (C2) and/or Cys102 (C3) bind to gE but are retained in the ER, (iii) radiolabeled endoproteinase Lys-C-generated peptide species containing C1 and C4 are linked through disulfide bonds, and (iv) peptides containing both C2 and C3 are not disulfide linked to any other peptide. From these findings emerges a model in which C1 and C4 as well as C2 and C3 form intramolecular disulfide bridges. Since the cysteines in the ectodomain have been conserved during alphaherpesvirus divergence, we postulate that the model applies for all gI proteins. Analysis of an FHV recombinant with a C1-->S substitution confirmed that the C1-C4 disulfide bond is not essential for the formation of a transport-competent gE-gI complex. The mutation affected the posttranslational modification of gI and caused a slight cold-sensitivity defect in the assembly or the intracellular transport of the gE-gI complex but did not affect plaque size. Thus, C1 and the C1-C4 bond are not essential for gE-gI-mediated cell-to-cell spread, at least not in vitro.

Role of envelope protein gE endocytosis in the pseudorabies virus life cycle

Several groups have reported that certain herpesvirus envelope proteins do not remain on the surface of cells that express them but rather are internalized by endocytosis in a recycling process. The biological function of membrane protein endocytosis in the virus life cycle remains a matter of speculation and debate. In this report, we demonstrate that some, but not all, membrane proteins encoded by the alphaherpesvirus pseudorabies virus (PRV) are internalized after reaching the plasma membrane. Glycoproteins gE and gB are internalized from the plasma membrane of cells, while gI and gC are not internalized efficiently. We show for gE that the cytoplasmic domain of the protein is required for endocytosis. While the gI protein is incapable of endocytosis on its own, it can be internalized when complexed with gE. We demonstrate that endocytosis of the gE-gI complex and gB occurs early after infection of tissue culture cells but that this process stops completely after 6 h of infection, a time that correlates with significant shutoff of host protein synthesis. We also show that gE protein internalized at 4 h postinfection is not present in virions formed at a later time. We discuss the differences in PRV gE and gI endocytosis compared to that of the varicella-zoster virus homologs and the possible roles of glycoprotein endocytosis in the virus life cycle.