Herpes simplex virus type 1 infection of activated cytotoxic T cells: Induction of fratricide as a mechanism of viral immune evasion

Multisystem Inflammatory Syndrome in Children and Long COVID: The SARS-CoV-2 Viral Superantigen Hypothesis

Multisystem inflammatory syndrome in children (MIS-C) is a febrile pediatric inflammatory disease that may develop weeks after initial SARS-CoV-2 infection or exposure. MIS-C involves systemic hyperinflammation and multiorgan involvement, including severe cardiovascular, gastrointestinal (GI) and neurological symptoms. Some clinical attributes of MIS-C-such as persistent fever, rashes, conjunctivitis and oral mucosa changes (red fissured lips and strawberry tongue)-overlap with features of Kawasaki disease (KD). In addition, MIS-C shares striking clinical similarities with toxic shock syndrome (TSS), which is triggered by bacterial superantigens (SAgs). The remarkable similarities between MIS-C and TSS prompted a search for SAg-like structures in the SARS-CoV-2 virus and the discovery of a unique SAg-like motif highly similar to a Staphylococcal enterotoxin B (SEB) fragment in the SARS-CoV-2 spike 1 (S1) glycoprotein. Computational studies suggest that the SAg-like motif has a high affinity for binding T-cell receptors (TCRs) and MHC Class II proteins. Immunosequencing of peripheral blood samples from MIS-C patients revealed a profound expansion of TCR β variable gene 11-2 (TRBV11-2), which correlates with MIS-C severity and serum cytokine levels, consistent with a SAg-triggered immune response. Computational sequence analysis of SARS-CoV-2 spike further identified conserved neurotoxin-like motifs which may alter neuronal cell function and contribute to neurological symptoms in COVID-19 and MIS-C patients. Additionally, autoantibodies are detected during MIS-C, which may indicate development of post-SARS-CoV-2 autoreactive and autoimmune responses. Finally, prolonged persistence of SARS-CoV-2 RNA in the gut, increased gut permeability and elevated levels of circulating S1 have been observed in children with MIS-C. Accordingly, we hypothesize that continuous and prolonged exposure to the viral SAg-like and neurotoxin-like motifs in SARS-CoV-2 spike may promote autoimmunity leading to the development of post-acute COVID-19 syndromes, including MIS-C and long COVID, as well as the neurological complications resulting from SARS-CoV-2 infection.

Entry of Alphaherpesviruses

Alphaherpesviruses are enveloped viruses that enter cells by fusing the viral membrane with a host cell membrane, either within an endocytic vesicle or at the plasma membrane. This entry event is mediated by a set of essential entry glycoproteins, including glycoprotein D (gD), gHgL, and gB. gHgL and gB are conserved among herpesviruses, but gD is unique to the alphaherpesviruses and is not encoded by all alphaherpesviruses. gD is a receptor-binding protein, the heterodimer gHgL serves as a fusion regulator, and gB is a class III viral fusion protein. Sequential interactions among these glycoproteins are thought to trigger the virus to fuse at the right place and time. Structural studies of these glycoproteins from multiple alphaherpesviruses has enabled the design and interpretation of functional studies. The structures of gD in a receptor- bound and in an unliganded form reveal a conformational change in the C terminus of the gD ectodomain upon receptor binding that may serve as a signal for fusion. By mapping neutralizing antibodies to the gHgL structures and constructing interspecies chimeric forms of gHgL, interaction sites for both gD and gB on gHgL have been proposed. A comparison of the post fusion structure of gB and an alternative conformation of gB visualized using cryo- electron tomography suggests that gB undergoes substantial refolding to execute membrane fusion. Although these structures have provided excellent insights into the entry mechanism, many questions remain about how these viruses coordinate the interactions and conformational changes required for entry.

The PD-1/PD-L1 Axis and Virus Infections: A Delicate Balance

Programmed cell death protein (PD-1) and its ligands play a fundamental role in the evasion of tumor cells from antitumor immunity. Less well appreciated is the fact that the PD-1/PD-L1 axis also regulates antiviral immune responses and is therefore modulated by a number of viruses. Upregulation of PD-1 and its ligands PD-L1 and PD-L2 is observed during acute virus infection and after infection with persistent viruses including important human pathogens such as human immunodeficiency virus (HIV), hepatitis C virus (HCV), and hepatitis B virus (HBV). Experimental evidence suggests that insufficient signaling through the PD-1 pathway promotes immunopathology during acute infection by exaggerating primary T cell responses. If chronic infection is established, however, high levels of PD-1 expression can have unfavorable immunological consequences. Exhaustion and suppression of antiviral immune responses can result in viral immune evasion. The role of the PD-1/PD-L1 axis during viral infections is further complicated by evidence that PD-L1 also mediates inflammatory effects in the acute phase of an immune response. In this review, we discuss the intricate interplay between viruses and the PD-1/PD-L1 axis.

Herpes Simplex Virus Evasion of Early Host Antiviral Responses

Herpes simplex viruses type 1 (HSV-1) and type 2 (HSV-2) have co-evolved with humans for thousands of years and are present at a high prevalence in the population worldwide. HSV infections are responsible for several illnesses including skin and mucosal lesions, blindness and even life-threatening encephalitis in both, immunocompetent and immunocompromised individuals of all ages. Therefore, diseases caused by HSVs represent significant public health burdens. Similar to other herpesviruses, HSV-1 and HSV-2 produce lifelong infections in the host by establishing latency in neurons and sporadically reactivating from these cells, eliciting recurrences that are accompanied by viral shedding in both, symptomatic and asymptomatic individuals. The ability of HSVs to persist and recur in otherwise healthy individuals is likely given by the numerous virulence factors that these viruses have evolved to evade host antiviral responses. Here, we review and discuss molecular mechanisms used by HSVs to evade early innate antiviral responses, which are the first lines of defense against these viruses. A comprehensive understanding of how HSVs evade host early antiviral responses could contribute to the development of novel therapies and vaccines to counteract these viruses.

Immuno-metabolic changes in herpes virus infection

Recent evidences indicate that change in cellular metabolic pathways can alter immune response and function of the host; emphasizing the role of metabolome in health and diseases. Human Herpes simplex virus type-1 (HSV-1) and type-2 (HSV-2) causes diseases from asymptomatic to highly prevalent oral and genital herpes, recurrent blisters or neurological complications. Immune responses against HSV are complex with delicate interplay between innate signaling pathways and adaptive immune responses. The innate response involves the induction of protective IFN-1; while Natural Killer (NK) cells and plasmacytoid Dendritic Cells (pDC) confer in vivo adaptive anti-HSV response along with humoral and cellular components in controlling infection and latency. Metabolic changes lead to up-/down-regulation of several cytokines and chemokines like IFN-γ, IL-2, IL-4, IL-10 and MIP1β in HSV infection and recurrences. Recently, the viral protein ICP0 has been identified as an attenuator of TLR signaling, that inhibit innate responses to HSV. This review will summarize the role of metabolome in innate and adaptive effectors in infection, pathogenesis and immune control of HSV, highlighting the delicate interplay between the metabolic changes and immunity.

What Goes Around, Comes Around - HSV-1 Replication in Monocyte-Derived Dendritic Cells

HSV-1 is a very successful human pathogen, known for its high sero-prevalence and the ability to infect a wide range of different cell types, including dendritic cells (DCs). As very potent antigen-presenting cells DCs play an important role in the induction of antiviral immune responses and therefore represent a strategic target for viral-mediated immune escape mechanisms. It is known that HSV-1 completes its gene expression profile in immature as well as in mature DCs, while lytic infection is only found in immature DCs (iDCs). Notably, HSV-1 infected mature DCs (mDCs) fail to release infectious progeny virions into the supernatant. Apart from HSV-1 dissemination via extracellular routes cell-to-cell spread counteracts a yet unknown mechanism by which the virus is trapped in mDCs and not released into the supernatant. The dissemination in a cell-cell contact-dependent manner enables HSV-1 to infect bystander cells without the exposure toward the extracellular environment. This supports the virus to successfully infect the host and establish latency. In this review the mechanism of HSV-1 replication in iDCs and mDCs and its immunological as well as virological implications, will be discussed.

Herpes simplex virus 1 infection of T cells causes VP11/12-dependent phosphorylation and degradation of the cellular protein Dok-2

Previous studies have shown that HSV-1 infection of lymphocytes induces the tyrosine phosphorylation of several proteins that might correspond to viral or host proteins. VP11/12, a viral tegument protein, is the major HSV-induced tyrosine phosphorylated protein identified thus far. In this report, we demonstrated that the cellular adaptor proteins Dok-2 and Dok-1 are tyrosine phosphorylated upon HSV-1 infection. In addition, HSV-1 induced the selective degradation of Dok-2. Finally, we provide evidence that Dok-2 interacts with VP11/12, and that HSV-induced tyrosine phosphorylation and degradation of Dok-2 require VP11/12. Inactivation of either the Src Family Kinases binding motifs or the SHC binding motif of VP11/12 eliminated the interaction of Dok-2 with VP11/12. Elimination of the binding of Dok-2 to VP11/12 prevented Dok-2 phosphorylation and degradation. We propose that HSV-induced Dok phosphorylation and Dok-2 degradation is an immune evasion mechanism to inactivate T cells that might play an important role in HSV pathogenesis.

Replication-Competent Viruses as Cancer Immunotherapeutics: Emerging Clinical Data

Replication-competent (oncolytic) viruses (OV) as cancer immunotherapeutics have gained an increasing level of attention over the last few years while the clinical evidence of virus-mediated antitumor immune responses is still anecdotal. Multiple clinical studies are currently ongoing and more immunomonitoring results are expected within the next five years. All viruses can be recognized by the immune system and are therefore potential candidates for immune therapeutics. However, each virus activates innate immune system by using different combination of recognition receptors/pathways which leads to qualitatively different adaptive immune responses. This review summarizes immunological findings in cancer patients following treatment with replication-competent viruses.

Therapeutic Use of Soluble Fas Ligand Ameliorates Acute and Recurrent Herpetic Stromal Keratitis in Mice

PURPOSE

The present study was designed to test the therapeutic value of soluble FasL (sFasL) in an acute model of herpetic stromal keratitis (HSK) and, more importantly, a recurrent model of HSK using BALB/c, BALB-lpr, and National Institutes of Health (NIH) mice.

METHODS

Mice were infected either acutely with the KOS strain of herpes simplex virus 1 (HSV-1) or latently with the McKrae strain of HSV-1. Acutely infected mice as well as ultraviolet-B (UV-B) reactivated mice (recurrent infection) were treated with sFasL, or soluble TNF-related apoptosis inducing ligand (sTRAIL), or BSA daily or 3 times/wk by using either a combination of subconjunctival injection and topical ointment, or with topical ointment alone. These mice then were evaluated for corneal opacity and neovascularization for 6 weeks.

RESULTS

Following acute and recurrent HSV-1 infection, wild-type BALB/c mice treated with sFasL displayed significantly reduced incidence of corneal opacity and neovascularization compared to the control animals. However, BALB-lpr mice, which are deficient in Fas+ inflammatory cells, displayed no such differences in ocular disease, as expected. Latently infected NIH mice treated with sFasL displayed similar results. Flow cytometric analysis revealed that the corneal inflammatory infiltrate in those treated with sFasL was significantly less than in sTRAIL- or BSA-treated mice. Furthermore, corneas from sFasL-treated mice displayed relatively more cells undergoing apoptosis.

CONCLUSIONS

This study provides evidence that sFasL treatment has potential therapeutic benefit in reducing inflammatory infiltrate and neovascularization in primary and recurrent forms of HSK, and that it does so by augmenting the restriction of Fas+ inflammatory cells mediated by membrane FasL.

How do viruses control mitochondria-mediated apoptosis?

There is no doubt that viruses require cells to successfully reproduce and effectively infect the next host. The question is what is the fate of the infected cells? All eukaryotic cells can "sense" viral infections and exhibit defence strategies to oppose viral replication and spread. This often leads to the elimination of the infected cells by programmed cell death or apoptosis. This "sacrifice" of infected cells represents the most primordial response of multicellular organisms to viruses. Subverting host cell apoptosis, at least for some time, is therefore a crucial strategy of viruses to ensure their replication, the production of essential viral proteins, virus assembly and the spreading to new hosts. For that reason many viruses harbor apoptosis inhibitory genes, which once inside infected cells are expressed to circumvent apoptosis induction during the virus reproduction phase. On the other hand, viruses can take advantage of stimulating apoptosis to (i) facilitate shedding and hence dissemination, (ii) to prevent infected cells from presenting viral antigens to the immune system or (iii) to kill non-infected bystander and immune cells which would limit viral propagation. Hence the decision whether an infected host cell undergoes apoptosis or not depends on virus type and pathogenicity, its capacity to oppose antiviral responses of the infected cells and/or to evade any attack from immune cells. Viral genomes have therefore been adapted throughout evolution to satisfy the need of a particular virus to induce or inhibit apoptosis during its life cycle. Here we review the different strategies used by viruses to interfere with the two major apoptosis as well as with the innate immune signaling pathways in mammalian cells. We will focus on the intrinsic mitochondrial pathway and discuss new ideas about how particular viruses could activately engage mitochondria to induce apoptosis of their host.

Distinct APC subtypes drive spatially segregated CD4+ and CD8+ T-cell effector activity during skin infection with HSV-1

Efficient infection control requires potent T-cell responses at sites of pathogen replication. However, the regulation of T-cell effector function in situ remains poorly understood. Here, we show key differences in the regulation of effector activity between CD4⁺ and CD8⁺ T-cells during skin infection with HSV-1. IFN-γ-producing CD4⁺ T cells disseminated widely throughout the skin and draining lymph nodes (LN), clearly exceeding the epithelial distribution of infectious virus. By contrast, IFN-γ-producing CD8⁺ T cells were only found within the infected epidermal layer of the skin and associated hair follicles. Mechanistically, while various subsets of lymphoid- and skin-derived dendritic cells (DC) elicited IFN-γ production by CD4⁺ T cells, CD8⁺ T cells responded exclusively to infected epidermal cells directly presenting viral antigen. Notably, uninfected cross-presenting DCs from both skin and LNs failed to trigger IFN-γ production by CD8⁺ T-cells. Thus, we describe a previously unappreciated complexity in the regulation of CD4⁺ and CD8⁺ T-cell effector activity that is subset-specific, microanatomically distinct and involves largely non-overlapping types of antigen-presenting cells (APC).

HSV-1 is a widely distributed pathogen causing a life-long latent infection associated with periodic bouts of reactivation and severe clinical complications. Adaptive immune responses encompassing CD4⁺ and CD8⁺ T-cell activities are key to both the clearance of infectious virus and the control of latent infection. However, precisely how such T-cell responses are regulated, particularly within acutely infected peripheral tissues, remains poorly understood. Using a mouse model of HSV-1 skin infection, we describe a complex regulation of T-cell responses at the site of acute infection. These responses were subset-specific and anatomically distinct, with CD4⁺ and CD8⁺ T-cell activities being directed to distinct anatomical compartments within the skin. While IFN-γ-producing CD4⁺ T cells were broadly distributed, including skin regions a considerable distance away from infected cells, CD8⁺ T-cell activity was strictly confined to directly infected epithelial compartments. This unexpected spatial segregation was a direct consequence of the involvement of largely non-overlapping types of antigen-presenting cells in driving CD4⁺ and CD8⁺ T-cell effector activity. Our results provide novel insights into the cellular regulation of T-cell immunity within peripheral tissues and have the potential to guide the development of T-cell subset-specific approaches for therapeutic and prophylactic intervention in antimicrobial immunity and autoimmunity.

Structural proteins of Kaposi's sarcoma-associated herpesvirus antagonize p53-mediated apoptosis

The tumor suppressor p53 is a central regulatory molecule of apoptosis and is commonly mutated in tumors. Kaposi's sarcoma-associated herpesvirus (KSHV)-related malignancies express wild-type p53. Accordingly, KSHV encodes proteins that counteract the cell death-inducing effects of p53. Here, the effects of all KSHV genes on the p53 signaling pathway were systematically analyzed using the reversely transfected cell microarray technology. With this approach we detected eight KSHV-encoded genes with potent p53 inhibiting activity in addition to the previously described inhibitory effects of KSHV genes ORF50, K10 and K10.5. Interestingly, the three most potent newly identified inhibitors were KSHV structural proteins, namely ORF22 (glycoprotein H), ORF25 (major capsid protein) and ORF64 (tegument protein). Validation of these results with a classical transfection approach showed that these proteins inhibited p53 signaling in a dose-dependent manner and that this effect could be reversed by small interfering RNA-mediated knockdown of the respective viral gene. All three genes inhibited p53-mediated apoptosis in response to Nutlin-3 treatment in non-infected and KSHV-infected cells. Addressing putative mechanisms, we could show that these proteins could also inhibit the transactivation of the promoters of apoptotic mediators of p53 such as BAX and PIG3. Altogether, we demonstrate for the first time that structural proteins of KSHV can counteract p53-induced apoptosis. These proteins are expressed in the late lytic phase of the viral life cycle and are incorporated into the KSHV virion. Accordingly, these genes may inhibit cell death in the productive and in the early entrance phase of KSHV infection.

Comparison of the host immune response to herpes simplex virus 1 (HSV-1) and HSV-2 at two different mucosal sites

A study was undertaken to compare the host immune responses to herpes simplex virus 1 (HSV-1) and HSV-2 infection by the ocular or genital route in mice. Titers of HSV-2 from tissue samples were elevated regardless of the route of infection. The elevation in titers of HSV-2, including cell infiltration and cytokine/chemokine levels in the central nervous system relative to those found following HSV-1 infection, was correlative with inflammation. These results underscore a dichotomy between the host immune responses to closely related alphaherpesviruses.

The role of MAPK in CD4(+) T cells toll-like receptor 9-mediated signaling following HHV-6 infection

Human herpesvirus-6 (HHV-6) is an important immunosuppressive and immunomodulatory virus that primarily infects immune cells (mainly CD4(+) T cells) and strongly suppresses the proliferation of infected cells. Toll-like receptors are pattern-recognition receptors essential for the development of an appropriate innate immune defense against infection. To understand the role of CD4(+) T cells in the innate response to HHV-6 infection and the involvement of TLRs, we used an in vitro infection model and observed that the infection of CD4(+) T cells resulted in the activation of JNK/SAPK via up-regulation of toll-like receptor 9 (TLR9). Associated with JNK activation, annexin V-PI staining indicated that HHV-6A was a strong inducer of apoptosis. Apoptotic response associated cytokines, IL-6 and TNF-α also induced by HHV-6A infection.

Herpes simplex virus type 1-induced FasL expression in human monocytic cells and its implications for cell death, viral replication, and immune evasion

Herpes simplex virus type 1 (HSV-1) is a ubiquitously occurring pathogen that infects humans early in childhood. The virus persists as a latent infection in dorsal root ganglia, especially of the trigeminal nerve, and frequently becomes reactivated in humans under conditions of stress. Monocytic cells constitute an important component of the innate and adaptive immune responses. We show here for the first time that HSV-1 stimulates human FasL promoter and induces de novo expression of FasL on the surface of human monocytic cells, including monocytes and macrophages. This virus-induced FasL expression causes death of monocytic cells growing in suspension, but not in monolayers (e.g., macrophages). The addition of a broad-spectrum caspase inhibitor, as well as anti-FasL antibodies, reduced cell death but increased viral replication in the virus-infected cell cultures. We also show here for the first time that the virus-induced de novo expression of FasL on the cell surface acts as an immune evasion mechanism by causing the death of interacting human CD4+ T cells, CD8+ T cells, and natural killer (NK) cells. Our study provides novel insights on FasL expression and cell death in HSV-infected human monocytic cells and their impact on interacting immune cells.

Systemic immunological tolerance to ocular antigens is mediated by TRAIL-expressing CD8+ T cells

Systemic immunological tolerance to Ag encountered in the eye restricts the formation of potentially damaging immune responses that would otherwise be initiated at other anatomical locations. We previously demonstrated that tolerance to Ag administered via the anterior chamber (AC) of the eye required Fas ligand-mediated apoptotic death of inflammatory cells that enter the eye in response to the antigenic challenge. Moreover, the systemic tolerance induced after AC injection of Ag was mediated by CD8(+) regulatory T cells. This study examined the mechanism by which these CD8(+) regulatory T cells mediate tolerance after AC injection of Ag. AC injection of Ag did not prime CD4(+) T cells and led to increased TRAIL expression by splenic CD8(+) T cells. Unlike wild-type mice, Trail(-/-) or Dr5(-/-) mice did not develop tolerance to Ag injected into the eye, even though responding lymphocytes underwent apoptosis in the AC of the eyes of these mice. CD8(+) T cells from Trail(-/-) mice that were first injected via the AC with Ag were unable to transfer tolerance to naive recipient wild-type mice, but CD8(+) T cells from AC-injected wild-type or Dr5(-/-) mice could transfer tolerance. Importantly, the transferred wild-type (Trail(+/+)) CD8(+) T cells were also able to decrease the number of infiltrating inflammatory cells into the eye; however, Trail(-/-) CD8(+) T cells were unable to limit the inflammatory cell ingress. Together, our data suggest that "helpless" CD8(+) regulatory T cells generated after AC injection of Ag enforce systemic tolerance in a TRAIL-dependent manner to inhibit inflammation in the eye.

Herpes simplex virus glycoprotein D interferes with binding of herpesvirus entry mediator to its ligands through downregulation and direct competition

To initiate membrane fusion and virus entry, herpes simplex virus (HSV) gD binds to a cellular receptor such as herpesvirus entry mediator (HVEM). HVEM is a tumor necrosis factor (TNF) receptor family member with four natural ligands that either stimulate (LIGHT and LTα) or inhibit (BTLA and CD160) T cell function. We hypothesized that the interaction of gD with HVEM affects the binding of natural ligands, thereby modulating the immune response during infection. Here, we investigated the effect that gD has on the interaction of HVEM with its natural ligands. First, HSV gD on virions or cells downregulates HVEM from the cell surface. Similarly, trans-interaction with BTLA or LIGHT also downregulates HVEM from the cell surface, suggesting that HSV may subvert a natural mechanism for regulating HVEM activity. Second, we showed that wild-type gD had the lowest affinity for HVEM compared with the four natural ligands. Moreover, gD directly competed for binding to HVEM with BTLA but not LTα or LIGHT, indicating the possibility that gD selectively controls HVEM signals. On the other hand, natural ligands influence the use of HVEM by HSV. For instance, soluble BTLA, LTα, and LIGHT inhibited the binding of wild-type gD to HVEM, and soluble BTLA and LTα blocked HSV infection of HVEM-expressing cells. Thus, gD is at the center of the interplay between HVEM and its ligands. It can interfere with HVEM function in two ways, by competing with the natural ligands and by downregulating HVEM from the cell surface.

Viruses and Langerhans cells

Langerhans cells (LCs) are the resident dendritic cells (DCs) of epidermis in human mucosal stratified squamous epithelium and the skin. A phenotypically similar DC has recently been discovered as a minor population in the murine dermis. In epidermis, LCs function as sentinel antigen-presenting cells that can capture invading viruses such as herpes simplex virus (HSV), varicella-zoster virus (VZV) and human immunodeficiency virus (HIV). This interaction between LCs and viruses results in highly variable responses, depending on the virus as discussed in this review. For example, HSV induces apoptosis in LCs but HIV does not. LCs seem to be the first in a complex chain of antigen presentation to T cells in lymph nodes for HSV and possibly VZV, or they transport virus to T cells, as described for HIV and maybe VZV. Together with epidermal keratinocytes they may also have a role in the initial innate immune response at the site of infection in the epidermis, although this is not fully known. The full spectrum of biological responses of LCs even to these viruses has yet to be understood and will require complementary studies in human LCs in vitro and in murine models in vivo.

Two overlapping domains of a lyssavirus matrix protein that acts on different cell death pathways

The lyssavirus matrix (M) protein induces apoptosis. The regions of the M protein that are essential for triggering cell death pathways are not yet clearly defined. We therefore compared the M proteins from two viruses that have contrasting characteristics in terms of cellular apoptosis: a genotype 3 lyssavirus, Mokola virus (MOK), and a genotype 1 rabies virus isolated from a dog from Thailand (THA). We identified a 20-amino-acid fragment (corresponding to positions 67 to 86) that retained the cell death activities of the full-length M protein from MOK via both the tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and inhibition of cytochrome c oxidase (CcO) activity. We found that the amino acids at positions 77 and 81 have an essential role in triggering these two cell death pathways. Directed mutagenesis demonstrated that the amino acid at position 77 affects CcO activity, whereas the amino acid at position 81 affects TRAIL-dependent apoptosis. Mutations in the full-length M protein that compromised induction of either of these two pathways resulted in delayed apoptosis compared with the time to apoptosis for the nonmutated control.

Caspase 9 is essential for herpes simplex virus type 2-induced apoptosis in T cells

Herpes simplex virus type 2 (HSV-2) induces apoptosis in T cells by a caspase-dependent mechanism. Apoptosis can occur via extrinsic (death receptor) and/or intrinsic (mitochondrial) pathways. Here, we show that the initiator caspase for the intrinsic pathway is activated in T cells following HSV-2 exposure. To determine the respective contributions of intrinsic and extrinsic pathways, we assessed apoptosis in Jurkat cells that are deficient in caspase 8 or Fas-associating protein with death domain (FADD) for the extrinsic pathway and in cells deficient in caspase 9 for the intrinsic pathway. Our results indicate HSV-2-induced apoptosis in T cells occurs via the intrinsic pathway.

Herpes simplex virus type 1 (HSV-1)-induced apoptosis in human dendritic cells as a result of downregulation of cellular FLICE-inhibitory protein and reduced expression of HSV-1 antiapoptotic latency-associated transcript sequences

Herpes simplex virus type 1 (HSV-1) is one of the most frequent and successful human pathogens. It targets immature dendritic cells (iDCs) to interfere with the antiviral immune response. The mechanisms underlying apoptosis of HSV-1-infected iDCs are not fully understood. Previously, we have shown that HSV-1-induced apoptosis of iDCs is associated with downregulation of the cellular FLICE-inhibitory protein (c-FLIP), a potent inhibitor of caspase-8-mediated apoptosis. In this study, we prove that HSV-1 induces degradation of c-FLIP in a proteasome-independent manner. In addition, by using c-FLIP-specific small interfering RNA (siRNA) we show for the first time that downregulation of c-FLIP expression is sufficient to drive uninfected iDCs into apoptosis, underlining the importance of this molecule for iDC survival. Surprisingly, we also observed virus-induced c-FLIP downregulation in epithelial cells and many other cell types that do not undergo apoptosis after HSV-1 infection. Microarray analyses revealed that HSV-1-encoded latency-associated transcript (LAT) sequences, which can substitute for c-FLIP as an inhibitor of caspase-8-mediated apoptosis, are much less abundant in iDCs as compared to epithelial cells. Finally, iDCs infected with an HSV-1 LAT knockout mutant showed increased apoptosis when compared to iDCs infected with the corresponding wild-type HSV-1. Taken together, our results demonstrate that apoptosis of HSV-1-infected iDCs requires both c-FLIP downregulation and diminished expression of viral LAT.

Herpes simplex virus requires VP11/12 to induce phosphorylation of the activation loop tyrosine (Y394) of the Src family kinase Lck in T lymphocytes

Herpes simplex virus (HSV) tegument proteins are released into the cytoplasm during viral entry and hence are among the first viral proteins encountered by an infected cell. Despite the implied importance of these proteins in the evasion of host defenses, the function of some, like virion protein 11/12 (VP11/12), have not been clearly defined. Previously, we reported that VP11/12 is strongly tyrosine phosphorylated during the infection of lymphocytes but not in fibroblasts or an epithelial cell line (G. Zahariadis, M. J. Wagner, R. C. Doepker, J. M. Maciejko, C. M. Crider, K. R. Jerome, and J. R. Smiley, J. Virol. 82:6098-6108, 2008). We also showed that tyrosine phosphorylation depends in part on the activity of the lymphocyte-specific Src family kinase (SFK) Lck in Jurkat T cells. These data suggested that VP11/12 is a substrate of Lck and that Lck is activated during HSV infection. Here, we show that HSV infection markedly increases the fraction of Lck phosphorylated on its activation loop tyrosine (Y394), a feature characteristic of activated Lck. A previous report implicated the immediate-early protein ICP0 and the viral serine/threonine kinases US3 and UL13 in the induction of a similar activated phenotype of SFKs other than Lck in fibroblasts and suggested that ICP0 interacts directly with SFKs through their SH3 domain. However, we were unable to detect an interaction between ICP0 and Lck in T lymphocytes, and we show that ICP0, US3, and UL13 are not strictly required for Lck activation. In contrast, VP11/12 interacted with Lck or Lck signaling complexes and was strictly required for Lck activation during HSV infection. Thus, VP11/12 likely modulates host cell signaling pathways for the benefit of the virus.

Fas-mediated apoptotic signaling in the mouse brain following reovirus infection

Type 3 (T3) reovirus strains induce apoptotic neuronal cell death and lethal encephalitis in infected mice. T3 strain Dearing (T3D)-induced apoptosis in primary neuronal cultures occurs by a Fas-mediated mechanism and requires the activation of caspase 8. We now show that Fas mRNA is upregulated in the brains of mice infected with encephalitic reovirus T3D and T3 strain Abney (T3A) but not following infection with nonencephalitic reovirus type 1 strain Lang. Fas is upregulated in regions of the brain that are injured during infection with T3 reovirus strains and colocalizes with virus antigen in individual neurons. In contrast, levels of FasL mRNA induced by encephalitic and nonencephalitic reovirus strains do not differ significantly. Caspase 8, the initiator caspase associated with Fas-mediated apoptosis, is activated in the cortex and hippocampal regions of both T3D- and T3A-infected mice. Furthermore, Bid cleavage and the activation of caspase 9 in the brains of T3D-infected mice suggest that the caspase 8-dependent activation of mitochondrial apoptotic signaling contributes to virus-induced apoptosis. We have previously shown that the inhibition of c-Jun N-terminal kinase (JNK) signaling blocks T3D-induced apoptosis and improves the outcome of virus-induced encephalitis. We now show that the reovirus-induced upregulation of Fas requires JNK signaling, thereby providing a link between reovirus-induced death receptor signaling and mitogen-activated protein kinase pathways and a potential mechanism for the therapeutic action of JNK inhibition.

Langerhans cells and viral immunity

Langerhans cells (LC) are a unique dendritic cell subset that are located in mucosal stratified squamous epithelium and skin epidermis. Their location is ideally suited for their function as antigen presenting cells that capture invading viruses and induce anti-viral immunity. However, it is becoming evident that the interaction between LC and viruses can result in different responses, depending on the virus and the receptors involved. Here we will discuss the recent data on the similarities and differences in roles of LC in viral immunity to and infection with HIV, herpes simplex and varicella-zoster virus. Although all three viruses interact with LC during initial infection, the effects can be quite different, reflecting differences in biology and pathogenesis.

TNF-alpha mediates pseudorabies virus-induced apoptosis via the activation of p38 MAPK and JNK/SAPK signaling

PRV infection causes apoptosis in vitro and in vivo. However, the significance of PRV-induced apoptosis and its signaling pathways is still unknown. This work investigates the role of MAPK pathways in mediating PRV-induced apoptosis. Flow cytometry, apoptosis ELISA and western blotting using antibodies against cleaved caspase-3, -6 and PARP demonstrated that PRV induces apoptosis in a time- and dose-dependent manner. p38 and JNK/SAPK inhibitors significantly protected cells from PRV-induced apoptosis. Inhibitor treatment did not affect Us3a gene transcription and progeny virus production. Western blotting revealed that PRV activates p38 and JNK/SAPK signaling. Inhibition of NF-kappaB had no effect on PRV-mediated apoptosis. Non-replicative PRV failed to activate p38 and JNK/SAPK or induce apoptosis. PRV infection increases TNF-alpha transcription, translation and secretion, as well as TNF-alpha receptor expression. Inhibition of p38 and JNK/SAPK reduced PRV-induced TNF-alpha up-regulation. Neutralization assay confirmed that TNF-alpha is a key mediator involved in PRV-induced apoptosis.

Malignant catarrhal fever induced by alcelaphine herpesvirus 1 is associated with proliferation of CD8+ T cells supporting a latent infection

Alcelaphine herpesvirus 1 (AlHV-1), carried by wildebeest asymptomatically, causes malignant catarrhal fever (WD-MCF) when cross-species transmitted to a variety of susceptible species of the Artiodactyla order. Experimentally, WD-MCF can be induced in rabbits. The lesions observed are very similar to those described in natural host species. Here, we used the rabbit model and in vivo 5-Bromo-2'-Deoxyuridine (BrdU) incorporation to study WD-MCF pathogenesis. The results obtained can be summarized as follows. (i) AlHV-1 infection induces CD8(+) T cell proliferation detectable as early as 15 days post-inoculation. (ii) While the viral load in peripheral blood mononuclear cells remains below the detection level during most of the incubation period, it increases drastically few days before death. At that time, at least 10% of CD8(+ )cells carry the viral genome; while CD11b(+), IgM(+) and CD4(+) cells do not. (iii) RT-PCR analyses of mononuclear cells isolated from the spleen and the popliteal lymph node of infected rabbits revealed no expression of ORF25 and ORF9, low or no expression of ORF50, and high or no expression of ORF73. Based on these data, we propose a new model for the pathogenesis of WD-MCF. This model relies on proliferation of infected CD8(+) cells supporting a predominantly latent infection.

Understanding HSV-1 entry glycoproteins

Herpes Simplex Virus‐1 is a common infectious agent, but the precise detail of entry and infection of cells has only now begun to be clarified. Four viral surface glycoproteins (gB, gD, gH and gL) are required. This review summarises the known structure and function of each of these essential viral envelope glycoproteins, and explores what is known about their close cooperation with each other in mediating cellular membrane fusion. It is suggested that, following gD binding to one of its entry receptors, membrane fusion is mediated by gB and the heterodimer gH/gL. Significantly, these four entry glycoproteins also play a key role in the interaction between HSV and the host immune system. The glycoproteins serve an important role as targets of adaptive immunity. However, recent studies have demonstrated that the same proteins also play a key role in initiating the early innate immune response to HSV. Understanding the complex functions of these HSV proteins may be essential for successful development of vaccines for HSV. Copyright © 2007 John Wiley & Sons, Ltd.

Role of Bcl-2 expression for productive herpes simplex virus 2 replication

Herpes simplex viruses infect a variety of cells in vitro. However, not all infected cells sustain a fully productive replication of these viruses. We have shown that, in U937 monocytoid cells, herpes simplex virus 2 (HSV-2) causes a low-productive infection characterized by apoptosis as cytopathic effect at a late stage of infection. This effect was associated with a down-regulation of the Bcl-2 protein. We therefore asked whether destabilization of Bcl-2 expression could act as a limiting factor for the productive HSV-2 infection. We found that overexpression of Bcl-2 in U937 cells dramatically increased the capability of these cells to sustain a fully productive infection, while protecting against apoptosis induced by HSV-2. Overall, our data indicate that Bcl-2 expression acts as a regulator of HSV-2 replication.

Apoptosis and antigen receptor function in T and B cells following exposure to herpes simplex virus

T cells are an essential component of the immune response against herpes simplex virus (HSV) infection. We previously reported that incubation of T cells with HSV-infected fibroblasts inhibits subsequent T cell antigen receptor signal transduction. In the current study, we found that incubation of T cells with HSV-infected fibroblasts also leads to apoptosis in exposed T cells. Apoptosis was observed in Jurkat cells, a T cell leukemia line, and also in CD4(+) cells isolated from human peripheral blood mononuclear cells. Direct infection of these cells with HSV also resulted in apoptosis. Clinical isolates of both HSV type 1 and 2 induced apoptosis in infected T cells at comparable levels to cells infected with laboratory strains of HSV, suggesting an immune evasion mechanism that may be clinically relevant. Further understanding of these viral immune evasion mechanisms could be exploited for better management of HSV infection.

T helper lymphocytes rescue CTL from activation-induced cell death

T cell activation is characterized by a vast expansion of Ag-specific T cells followed by an equally extensive reduction in T cell numbers. This decline is due, in part, to activation-induced apoptosis of the responding T cells during repeated encounter with Ag. In the current study, we used solid-phase MHC class I/peptide monomers to cause activation-induced cell death (AICD) of previously activated CD8 T cells in an Ag-specific manner. AICD occurred rapidly and was mediated primarily by Fas-FasL interactions. Most interestingly, we observed that Th cells could provide survival signals to CTL significantly reducing the level of AICD. Both Th1 and Th2 subsets were capable of protecting CTL from AICD, and a major role for soluble factors in this protection was ruled out, as cell-to-cell contact was an essential component of this Th-mediated protection. Upon encounter with Ag-expressing tumor cells, CTL underwent significant apoptosis. However, in the presence of Th cells, the CTL not only were protected against death, but also had significantly greater lytic ability. In vivo tumor protection studies using peptide immunization showed that the activation of Ag-specific Th cells was crucial for optimal protection, but did not affect the magnitude of the CTL response in the lymphoid tissues. In this study, we examine the type of help that CD4 T cells may provide and propose a model of Th cell-CTL interaction that reduces CTL death. Our results show a novel role for Th cells in the maintenance of CTL responses.

Inhibition of TCR signaling by herpes simplex virus

T lymphocytes are an essential component of the immune response against HSV infection. We previously reported that T cells became functionally impaired or inactivated after contacting HSV-infected fibroblasts. In our current study, we investigate the mechanisms of inactivation. We report that HSV-infected fibroblasts or HSV alone can inactivate T cells by profoundly inhibiting TCR signal transduction. Inactivation requires HSV penetration into T cells but not de novo transcription or translation. In HSV-inactivated T cells stimulated through the TCR, phosphorylation of Zap70 occurs normally. However, TCR signaling is inhibited at linker for activation of T cells (LAT) and at steps distal to LAT in the TCR signal cascade including inhibition of calcium flux and inhibition of multiple MAPK. Inactivation of T cells by HSV leads to the reduced phosphorylation of LAT at tyrosine residues critical for TCR signal propagation. Treatment of T cells with tyrosine phosphatase inhibitors attenuates inactivation by HSV, and stimulus with a mitogen that bypasses LAT phosphorylation overcomes inactivation. Our findings elucidate a potentially novel method of viral immune evasion that could be exploited to better manage HSV infection, aid in vaccine design, or allow targeted manipulation of T cell function.

E2 of hepatitis C virus inhibits apoptosis

Hepatitis C virus (HCV) is the major causative agent of chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma, and can be involved in very long chronic infections up to 30 years or more. Therefore, it has been speculated that HCV possesses mechanisms capable of modulating host defense systems such as innate and adaptive immunity. To investigate this virus-host interaction, we generated HCV replicons containing various HCV structural proteins and then analyzed the sensitivity of replicon-containing cells to the apoptosis-inducing agent, TRAIL. TRAIL-induced apoptosis was monitored by cleavage of procaspase-3 and procaspase-9 as well as that of their substrate poly(ADP-ribose) polymerase. TRAIL-induced apoptosis was inhibited in cells expressing HCV E2. Moreover, expression of HCV E2 enhanced the colony forming efficiency of replicon-containing cells by 25-fold. Blockage of apoptosis by E2 seems to be related to inhibition of TRAIL-induced cytochrome c release from the mitochondria. Based on these results, we propose that E2 augments persistent HCV infection by blocking host-induced apoptosis of infected cells.

Regulated expression of galectin-1 after in vitro productive infection with herpes simplex virus type 1: implications for T cell apoptosis

Apoptosis of cytotoxic T lymphocytes by herpes simplex virus type-1 (HSV-1) has been reported to be a relevant mechanism of viral immune evasion. Galectin-1 (Gal-1), an endogenous lectin involved in T-cell apoptosis, has recently gained considerable attention as a novel mechanism of tumor-immune evasion. Here we investigated whether infection of cells with HSV-1 can modulate the expression of Gal-1. Results show that pro-apoptotic Gal-1, but not Gal-3, is remarkably up-regulated in cell cultures infected with HSV-1. In addition, this protein is secreted to the extracellular milieu, where it contributes to apoptosis of activated T cells in a carbohydrate-dependent manner. Since many viruses have evolved mechanisms to counteract the antiviral response raised by the infected host, our results suggest that HSV-1 may use galectin-1 as a weapon to kill activated T cells and evade specific immune responses.

A tale of 2 alpha-herpesviruses: lessons for vaccinologists

Of the 8 known herpesviruses that affect human beings, we only have successful vaccines against varicella zoster virus. This brief review compares the pathogenesis of varicella zoster virus with that of the closely related alpha-herpesviruses herpes simplex virus 1 and 2, for which we have no satisfactory vaccines. The main objective of this review is to learn lessons from the success of varicella zoster virus vaccine that could be exploited for the development of successful vaccines against herpes simplex virus and perhaps against other herpes viruses.

Induction of apoptosis by herpes simplex virus-1 in neonatal, but not adult, neutrophils

We report a study on the effect of herpes simplex virus 1 (HSV-1) infection on apoptosis of neutrophils from both adults and neonates and present evidence showing that HSV-1 enhances apoptosis in neonatal, but not adult, neutrophils. HSV-1 enhanced the expression of both Fas and Fas ligand on the surface of neonatal neutrophils. Treatments with anti-Fas antibody and a Fas ligand inhibitor significantly reduced the induction of apoptosis by HSV-1. Using an ELISA assay, it was found that HSV-1 infection also leads to increased release of soluble FasL from HSV-1-infected neonatal neutrophils. Increased neonatal neutrophil apoptosis following HSV-1 infection may represent an important mechanism by which HSV-1 may diminish the antiviral response of neonatal neutrophils and might explain, at least in part, the severity of infections that are caused in newborns by this herpesvirus.

Temporary inhibition of neuronal apoptosis in Aujeszky's disease virus-infected swine

Previous studies have shown that during acute infection of the porcine trigeminal ganglia (TG), Aujeszky's disease virus (ADV)-infected neurons are protected from apoptosis induced by the virus itself and by cells of the immune system. However, TG neurons productively infected by ADV finally die and are phagocytosed by adjacent cells, a fact that leads us to speculate that the inhibition of neuronal apoptosis by ADV may be temporary rather than absolute. To address this issue we used TG and brain stem from pigs during acute infection by ADV. Infected cells were detected by immunohistochemical staining of viral antigens, whereas apoptotic cells were identified with an anti-active caspase-3 antibody, the TUNEL assay and by transmission electron microscopy. The results obtained in this study support the contention that the inhibition of neuronal apoptosis by ADV is temporary, since activation of caspase-3 could be detected in infected neurons at late stages in infection and because foci of advanced neuronophagia contained neurons exhibiting typical ultrastructural features of apoptosis.

Infection of dendritic cells by a gamma2-herpesvirus induces functional modulation

The murine gamma-herpesvirus-68 (gammaHV68) establishes viral latency in dendritic cells (DCs). In the present study, we examined the specific consequences of DC infection by gammaHV68, both in vivo and in vitro. Ex vivo analysis of infected mice showed that the virus colonizes respiratory DCs very early after infection and that all subsets of splenic DCs analyzed are viral targets. We have developed and characterized an in vitro model of gammaHV68 infection of DCs. Using this model, we demonstrated that viral infection neither induces full DC maturation nor interferes with exogenous activation, which is assessed by cell surface phenotypic changes. However, whereas gammaHV68 infection alone failed to elicit cytokine secretion, IL-10 secretion of exogenously activated DCs was enhanced. Furthermore, gammaHV68-infected DCs efficiently stimulated virus-specific T cell hybridomas but failed to induce alloreactive stimulation of normal T cells. These data indicate that viral infection doesn't interfere with Ag processing and presentation but does interfere with the ability of DCs to activate T cells. The inhibition of T cell activation was partially reversed by blocking IL-10. Analysis of infected mice shows elevated levels of IL-10 expression in DCs and that lack of endogenous IL-10 is associated with decreased gammaHV68 long-term latency. Taken together, these observations indicate that gamma2-herpesvirus infection of DCs is a mechanism of viral immune evasion, partially mediated by IL-10.

Viral strategies for evading antiviral cellular immune responses of the host

The host invariably responds to infecting viruses by activating its innate immune system and mounting virus-specific humoral and cellular immune responses. These responses are aimed at controlling viral replication and eliminating the infecting virus from the host. However, viruses have evolved numerous strategies to counter and evade host's antiviral responses. Providing specific examples from the published literature, we discuss in this review article various strategies that viruses have developed to evade antiviral cellular responses of the host. Unraveling these viral strategies allows a better understanding of the host-pathogen interactions and their coevolution. This knowledge is important for identifying novel molecular targets for developing antiviral reagents. Finally, it may also help devise new knowledge-based strategies for developing antiviral vaccines.

Dendritic cells in viral pathogenesis: protective or defective?

Dendritic cells (DC) are potent antigen-presenting cells that are critical in the initiation of immune responses to control and/or eliminate viral infections. Recent studies have investigated the effects of virus infection on the biology of DC. This review summarizes these changes, focusing on both the DC parameters affected and the viral factors involved. In addition, the central role of DC biology in the pathogenesis of several viral families, including herpesviruses, paramyxoviruses and retroviruses, is explored. The field of pathogen recognition by DC is addressed, focusing on its role in protecting the host from viral infection, as well as the ability of viruses to exploit such host receptor ligation and signalling to their replicative advantage. The hypothesis is proposed that virus and host have evolved a symbiotic relationship to ensure both viral transmission and host survival.

Apoptosis in normal and diseased oral tissues

Apoptotic cell death plays an important role in maintenance of the normal physiological state and in the pathogenesis of diseases in the body. Over the last three decades the molecular mechanisms of apoptosis have been unravelled leading to development of novel therapeutic approaches. This paper aims to present current knowledge of the role of apoptosis in normal oral tissues and in the development of oral diseases.

Human CD8+ T cell blasts are more sensitive than CD4+ T cell blasts to regulation by APO2L/TRAIL

The mechanisms responsible for the down-modulation of the activation of separated CD4(+) or CD8(+) human T cell blasts were studied using cells obtained from healthy donors. In the presence of IL-2, human CD8(+) T cell blasts were more sensitive than CD4(+) T cell blasts to regulation by APO2 ligand/TNF-related apoptosis-inducing ligand (APO2L/TRAIL), while both T cell subsets were equally sensitive to Fas/CD95 regulation. This regulation was defined as inhibition of IL-2-dependent T cell growth in the absence of cell death induction, characterized by cell cycle arrest in G(2)/M. The physiological validity of these observations was corroborated by the demonstration of intracellular FasL and APO2L/TRAIL expression in CD4(+) and CD8(+) T cell blasts, which were secreted in their bioactive form into the supernatant upon PHA, CD3 or CD59 reactivation. Additionally, the inhibition of IL-2-dependent CD4(+) or CD8(+) T cell blast growth upon CD3 or CD59 ligation was dependent, at least partially, on FasL and/or APO2L/TRAIL. These data precisely define the role of APO2L/TRAIL in the regulation of human T cell activation.

Herpes simplex virus type 1 latency-associated transcript expression protects trigeminal ganglion neurons from apoptosis

Upon infection of murine trigeminal ganglia with herpes simplex virus type 1 (HSV-1), an immune response is initiated resulting in significant infiltration of CD8+ T cells. Previous investigators have observed a lack of apoptosis in HSV-1 trigeminal ganglia even in the presence of cytotoxic immune cells. To determine the role of the latency-associated transcript (LAT) in inhibiting apoptosis, we examined mice during acute and latent infection with HSV-1 (strain 17 or a LAT-negative deletion mutant strain 17 N/H) by terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling (TUNEL) and fluorescence-activated cell sorting (FACS). FACS analysis revealed CD8+ T cells in the trigeminal ganglia by day 7, with more being present in 17- than 17 N/H-infected trigeminal ganglia (6.22% versus 3.5%) and a decrease in number through day 30 (2.7% to 1.2%). To detect apoptotic CD8+ T cells, sections were assayed by TUNEL and stained for CD8+ T cells. By day 7, approximately 10% of CD8+ T cells in both 17- and 17 N/H-infected trigeminal ganglia had undergone apoptosis. By day 30, 58% and 74% of all T cells had undergone apoptosis in 17- and 17 N/H-infected trigeminal ganglia, respectively. Furthermore, no HSV strain 17-infected trigeminal ganglion neurons were apoptotic, but 0.087% of 17deltaSty and 0.98% of 17 N/H-infected neurons were apoptotic. We conclude that the antiapoptotic effect of LAT appears to require the LAT promoter, with most of the antiapoptotic effect mapping within the StyI (+447) to the HpaI (+1667) region and a minor contribution from the upstream StyI (+76) to StyI (+447) region.

Herpes simplex virus infection of human dendritic cells induces apoptosis and allows cross-presentation via uninfected dendritic cells

HSV efficiently infects dendritic cells (DCs) in their immature state and induces down-regulation of costimulatory and adhesion molecules. As in mice, HSV infection of human DCs also leads to their rapid and progressive apoptosis, and we show that both early and late viral proteins contribute to its induction. Because topical HSV infection is confined to the epidermis, Langerhans cells are expected to be the major APCs in draining lymph nodes. However, recent observations in murine models show T cell activation to be mediated by nonepidermal DC subsets, suggesting cross-presentation of viral Ag. In this study we provide an explanation for this phenomenon, demonstrating that HSV-infected apoptotic DCs are readily phagocytosed by uninfected bystander DCs, which, in turn, stimulate virus-specific CD8+ T cell clones.

High susceptibility of a human oligodendroglial cell line to herpes simplex type 1 infection

More than 20 infectious agents, ranging from retroviruses to mycobacteria, have been associated with multiple sclerosis onset or relapses in which oligodendrocytes, the myelin-forming cells of the central nervous system, are the initial target of the pathogenic status. In this work, the nature of the susceptibility of the human precursor oligodendroglial KG-1C cell line to herpes simplex virus type 1 (HSV-1) was investigated. Infection of KG-1C cells was characterized by a high level of virus production and a notable progression of the cytopathic effect. After infection, there was a significant shut-off of host mRNA translation, which was correlated with evident synthesis of viral proteins. An examination by electron microscopy of the infected cells revealed the presence of large clusters of mitochondria located in the proximity of intracellular HSV-1 particle groups. In addition, transmission electron microscopy and nuclear fluorescence analysis showed neither signs of chromatin condensation nor of apoptotic bodies. Furthermore, procaspase-3 remained uncleaved, suggesting that apoptosis does not take place, at least in this system. Finally, expression and localization of MAL2, a subpopulation of detergent-insoluble lipid raft protein, was studied. Detection of MAL2 significantly increased after infection and it was colocalized with HSV-1 proteins. From these findings the authors conclude that human oligodendrocyte-like cells are highly susceptible to HSV-1 infection. The implications of this for central nervous system viral infection are discussed.

Comparative usage of herpesvirus entry mediator A and nectin-1 by laboratory strains and clinical isolates of herpes simplex virus

The herpesvirus entry mediator A (HVEM/HveA) and nectin-1 (HveC/CD111) are two major receptors for herpes simplex virus (HSV). Although structurally unrelated, both receptors can independently mediate entry of wild-type (wt) HSV-1 and HSV-2 by interacting with the viral envelope glycoprotein D (gD). Laboratory strains with defined mutations in gD (e.g. rid1) do not use HVEM but use nectin-2 (HveB/CD112) for entry. The relative usage of HVEM and nectin-1 during HSV infection in vivo is not known. In the absence of a defined in vivo model, we used in vitro approaches to address this question. First, we screened HSV clinical isolates from various origins for receptor tropism and found that all used both HVEM and nectin-1. Second, we determined the numbers of surface receptors on various susceptible and resistant cell lines as well as on primary fibroblasts derived from an individual with cleft lip/palate ectodermal dysplasia (CLPED1). Although CLPED1 cells can only express a defective form of nectin-1, they allowed entry of wild type and mutant HSV strains by usage of either HVEM or nectin-2. Finally, we compared the ability of HVEM and nectin-1 to mediate entry when expressed at varying cell surface densities. Both receptors showed a direct relationship between the number of receptors and HSV susceptibility. Direct comparison of receptors suggests that nectin-1 is more efficient at promoting entry than HVEM. Overall, our data suggest that both receptors play a role during HSV infection in vivo and that both are highly efficient even at low levels of expression.

Evidence for the transmissibility of atopy: hypothesis

The etiology of atopy is unknown. Its family distribution suggests transmissibility. Populations moving from countries with a low incidence to those with a high incidence increase to the higher rate. African and New Guinea village groups developed asthma with return of individuals who have acquired atopy in the city. Protection (and possibly immunity) develops with early exposure to child care or to affected older siblings. T helper (Th) type 2 clones driving specific allergies remain active even without further allergen exposure. Other IgE responses remain normal. Once boosted to completeness, the patterns of skin test results remain quite stable, possibly by the localization of abnormality maintained by immunity. An example of a virus causing the immortality of Th2 cells is herpes simplex virus type 1. It infects mouse or human Th2 cells and, although it does not multiply, causes immortality by increasing FAS-mediated apoptosis of T cells directed against the infected cells. Human T-cell leukemia virus 1 and probably others use similar ploys. Abnormal levels of FAS receptors and resistance to FAS apoptosis in nasal polyp lymphocytes and abnormal Th2 clones of atopy are interesting in this regard. The localizing role of a staphylococcal superantigen in atopic dermatitis, and possibly in autoimmunity in nonatopic eczema and intrinsic asthma, encourage the consideration of roles for microorganisms in localization and etiology. The epidemiology and characteristics of atopic disease support the plausibility of a viral hypothesis.

CTL Are Inactivated by Herpes Simplex Virus-Infected Cells Expressing a Viral Protein Kinase

Numerous cell-to-cell signals tightly regulate CTL function. Human fibroblasts infected with HSV type 1 or 2 can generate such a signal and inactivate human CTL. Inactivated CTL lose their ability to release cytotoxic granules and synthesize cytokines when triggered through the TCR. Inactivation requires cell-to-cell contact between CTL and HSV-infected cells. However, inactivated CTL are not infected with HSV. The inactivation of CTL is sustainable, as CTL function remains impaired when the CTL are removed from the HSV-infected cells. IL-2 treatment does not alter inactivation, and the inactivated phenotype is not transferable between CTL, distinguishing this phenotype from traditional anergy and T regulatory cell models. CTL inactivated by HSV-infected cells are not apoptotic, and the inactivated state can be overcome by phorbol ester stimulation, suggesting that inactivated CTL are viable and that the signaling block is specific to the TCR. HSV-infected cells require the expression of U(S)3, a viral protein kinase, to transmit the inactivating signal. Elucidation of the molecular nature of this signaling pathway may allow targeted manipulation of CTL function.