Immune activation suppresses initiation of lytic Epstein-Barr virus infection

A metabolic dependency of EBV can be targeted to hinder B cell transformation

After infection of B cells, Epstein-Barr virus (EBV) engages host pathways that mediate cell proliferation and transformation, contributing to the propensity of the virus to drive immune dysregulation and lymphomagenesis. We found that the EBV protein EBNA2 initiates nicotinamide adenine dinucleotide (NAD) de novo biosynthesis by driving expression of the metabolic enzyme indoleamine 2,3-dioxygenase 1 (IDO1) in infected B cells. Virus-enforced NAD production sustained mitochondrial complex I activity, to match adenosine triphosphate (ATP) production with bioenergetic requirements of proliferation and transformation. In transplant patients, IDO1 expression in EBV-infected B cells, and a serum signature of increased IDO1 activity, preceded development of lymphoma. In humanized mice infected with EBV, IDO1 inhibition reduced both viremia and lymphomagenesis. Virus-orchestrated NAD biosynthesis is therefore a druggable metabolic vulnerability of EBV-driven B cell transformation, opening therapeutic possibilities for EBV-related diseases.

EBV Reactivation from Latency Is a Degrading Experience for the Host

During reactivation from latency, gammaherpesviruses radically restructure their host cell to produce virion particles. To achieve this and thwart cellular defenses, they induce rapid degradation of cytoplasmic mRNAs, suppressing host gene expression. In this article, we review mechanisms of shutoff by Epstein-Barr virus (EBV) and other gammaherpesviruses. In EBV, canonical host shutoff is accomplished through the action of the versatile BGLF5 nuclease expressed during lytic reactivation. We explore how BGLF5 induces mRNA degradation, the mechanisms by which specificity is achieved, and the consequences for host gene expression. We also consider non-canonical mechanisms of EBV-induced host shutoff. Finally, we summarize the limitations and barriers to accurate measurements of the EBV host shutoff phenomenon.

Anti-viral and pro-inflammatory functions of Toll-like receptors during gamma-herpesvirus infections

Toll-like receptors (TLRs) control anti-viral responses both directly in infected cells and in responding cells of the immune systems. Therefore, they are crucial for responses against the oncogenic γ-herpesviruses Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus and the related murine virus MHV68, which directly infect immune system cells. However, since these viruses also cause lifelong persistent infections, TLRs may also be involved in modulation of inflammation during latent infection and contribute to virus-driven tumorigenesis. This review summarizes work on both of these aspects of TLR/γ-herpesvirus interactions, as well as results showing that TLR activity can drive these viruses' re-entry into the replicative lytic cycle.

Toll-like receptor-mediated innate immunity against herpesviridae infection: a current perspective on viral infection signaling pathways

Background

In the past decades, researchers have demonstrated the critical role of Toll-like receptors (TLRs) in the innate immune system. They recognize viral components and trigger immune signal cascades to subsequently promote the activation of the immune system.

Main body

Herpesviridae family members trigger TLRs to elicit cytokines in the process of infection to activate antiviral innate immune responses in host cells. This review aims to clarify the role of TLRs in the innate immunity defense against herpesviridae, and systematically describes the processes of TLR actions and herpesviridae recognition as well as the signal transduction pathways involved.

Conclusions

Future studies of the interactions between TLRs and herpesviridae infections, especially the subsequent signaling pathways, will not only contribute to the planning of effective antiviral therapies but also provide new molecular targets for the development of antiviral drugs.

Epstein-Barr Virus and Innate Immunity: Friends or Foes?

Epstein–Barr virus (EBV) successfully persists in the vast majority of adults but causes lymphoid and epithelial malignancies in a small fraction of latently infected individuals. Innate immunity is the first-line antiviral defense, which EBV has to evade in favor of its own replication and infection. EBV uses multiple strategies to perturb innate immune signaling pathways activated by Toll-like, RIG-I-like, NOD-like, and AIM2-like receptors as well as cyclic GMP-AMP synthase. EBV also counteracts interferon production and signaling, including TBK1-IRF3 and JAK-STAT pathways. However, activation of innate immunity also triggers pro-inflammatory response and proteolytic cleavage of caspases, both of which exhibit proviral activity under some circumstances. Pathogenic inflammation also contributes to EBV oncogenesis. EBV activates NFκB signaling and induces pro-inflammatory cytokines. Through differential modulation of the proviral and antiviral roles of caspases and other host factors at different stages of infection, EBV usurps cellular programs for death and inflammation to its own benefits. The outcome of EBV infection is governed by a delicate interplay between innate immunity and EBV. A better understanding of this interplay will instruct prevention and intervention of EBV-associated cancers.

Cyclin-dependent kinase 1 and survivin as potential therapeutic targets against nasal natural killer/T-cell lymphoma

Nasal natural killer/T-cell lymphoma (NNKTL) is closely associated with Epstein-Barr virus (EBV) and is characterized by poor prognosis, resulting from rapid progression of lesions in the affected organs. Recent data have shown that NNKTL is associated with the aberrant expression of cyclin-dependent kinase 1 (CDK1) and its downstream target survivin, but little is known about the functional roles of CDK1 and survivin in NNKTL. In the current study, we show that knockdown of the EBV-encoded oncoprotein latent membrane protein 1 (LMP1) induces downregulation of CDK1 and survivin in NNKTL cells. Immunohistochemistry detected CDK1 and survivin expression in LMP1-positive cells of NNKTL biopsy specimens. Inhibition of CDK1 and survivin in NNKTL cells with several inhibitors led to a dose-dependent decrease in cell proliferation. In addition, the Sp1 inhibitor mithramycin, which can downregulate both CDK1 and survivin, significantly suppressed the growth of established NNKTL in a murine xenograft model. Our results suggest that LMP1 upregulation of CDK1 and survivin may be essential for NNKTL progression. Furthermore, targeting CDK1 and survivin with Sp1 inhibitors such as mithramycin may be an effective approach to treat NNKTL, which is considered to be a treatment-refractory lymphoma.

Gene Analysis of Epstein-Barr Virus-Associated Lymphomas in Hu-PBL/SCID Chimeras

Aims and background

The mechanisms of Epstein-Barr virus (EBV)-associated tumor development are incompletely understood. The aim of this study was to investigate the gene expression of EBV-associated lymphomas in hu-PBL/SCID mice.

Methods

Human peripheral blood lymphocytes (hu-PBL) from EBV-seropositive donors were transplanted into severe combined immunodeficiency (SCID) mice. In situ hybridization was used to detect EBV-encoded small RNA-1 (EBER1) in tumor tissues. Mutation of TP53 exons 5–8 in EBV-induced lymphomas was analyzed by PCR-SSCP. Immunohistochemical staining was used to examine EBV gene products and cellular oncoproteins.

Results

Twenty-one of 29 mice developed tumors. EBER1 was positive in the nuclei of almost all tumor cells. Immunohistochemistry showed positive staining of LMP1, EBNA2 and ZEBRA in a small number of tumor cells. Immunohistochemically detectable p53 protein expression was common (85.7%), but TP53 gene mutations were identified in only four cases (19.1%) of EBV-associated lymphomas. Positivity rates of C-myc, Bcl-2 and Bax expression were 100%, 95.2%, and 90.5%, respectively, in the 21 cases of EBV-associated lymphomas.

Conclusions

Our preliminary findings suggest that EBV-associated lymphomas in hu-PBL/SCID chimeras show EBV infection, expression of oncogenic viral genes, and overexpression of cellular oncogenes. TP53 gene mutations are rare but p53 protein is commonly expressed in EBV-associated lymphomas.

IRAK4 is essential for TLR9-induced suppression of Epstein-Barr virus BZLF1 transcription in Akata Burkitt's lymphoma cells

Burkitt’s lymphoma (BL) is the most common childhood cancer in equatorial Africa, and is endemic to areas where people are chronically co-infected with Epstein-Barr virus (EBV) and the malaria pathogen Plasmodium falciparum. The contribution of these pathogens in the oncogenic process remains poorly understood. We showed earlier that the activation of Toll-like receptor (TLR) 9 by hemozoin, a disposal product formed from the digestion of blood by P. falciparum, suppresses the lytic reactivation of EBV in BL cells. EBV lytic reactivation is regulated by the expression of transcription factor Zta (ZEBRA), encoded by the EBV gene BZLF1. Here, we explore in the BL cell line Akata, the mechanism involved in repression by TLR9 of expression of BZLF1. We show that BZLF1 repression is mediated upon TLR9 engagement by a mechanism that is largely independent of de novo protein synthesis. By CRISPR/Cas9-induced inactivation of TLR9, MyD88, IRAK4 and IRAK1 we confirm that BZLF1 repression is dependent on functional TLR9 and MyD88 signaling, and identify IRAK4 as an essential element for TLR9-induced repression of BZLF1 expression upon BCR cross-linking. Our results unprecedentedly show that TLR9-mediated inhibition of lytic EBV is largely independent of new protein synthesis and demonstrate the central roles of MyD88 and IRAK4 in this process contributing to EBV’s persistence in the host’s B-cell pool.

Epstein-Barr virus lytic infection promotes activation of Toll-like receptor 8 innate immune response in systemic sclerosis monocytes

Background

Monocytes/macrophages are activated in several autoimmune diseases, including systemic sclerosis (scleroderma; SSc), with increased expression of interferon (IFN)-regulatory genes and inflammatory cytokines, suggesting dysregulation of the innate immune response in autoimmunity. In this study, we investigated whether the lytic form of Epstein-Barr virus (EBV) infection (infectious EBV) is present in scleroderma monocytes and contributes to their activation in SSc.

Methods

Monocytes were isolated from peripheral blood mononuclear cells (PBMCs) depleted of the CD19+ cell fraction, using CD14/CD16 negative-depletion. Circulating monocytes from SSc and healthy donors (HDs) were infected with EBV. Gene expression of innate immune mediators were evaluated in EBV-infected monocytes from SSc and HDs. Involvement of Toll-like receptor (TLR)8 in viral-mediated TLR8 response was investigated by comparing the TLR8 expression induced by infectious EBV to the expression stimulated by CL075/TLR8/agonist-ligand in the presence of TLR8 inhibitor in THP-1 cells.

Results

Infectious EBV strongly induced TLR8 expression in infected SSc and HD monocytes in vitro. Markers of activated monocytes, such as IFN-regulated genes and chemokines, were upregulated in SSc- and HD-EBV-infected monocytes. Inhibiting TLR8 expression reduced virally induced TLR8 in THP-1 infected cells, demonstrating that innate immune activation by infectious EBV is partially dependent on TLR8. Viral mRNA and proteins were detected in freshly isolated SSc monocytes. Microarray analysis substantiated the evidence of an increased IFN signature and altered level of TLR8 expression in SSc monocytes carrying infectious EBV compared to HD monocytes.

Conclusion

This study provides the first evidence of infectious EBV in monocytes from patients with SSc and links EBV to the activation of TLR8 and IFN innate immune response in freshly isolated SSc monocytes. This study provides the first evidence of EBV replication activating the TLR8 molecular pathway in primary monocytes. Immunogenicity of infectious EBV suggests a novel mechanism mediating monocyte inflammation in SSc, by which EBV triggers the innate immune response in infected cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s13075-017-1237-9) contains supplementary material, which is available to authorized users.

Post-transplant lymphoproliferative disorders: from epidemiology to pathogenesis-driven treatment

Post-transplant lymphoproliferative disorders (PTLDs) represent the most severe complication of both solid organ and hematopoietic stem cell transplantation. The Epstein-Barr Virus (EBV) is the main driver of PTLD, particularly those occurring early after transplantation. EBV-driven malignancies are associated with selective expression of latent viral proteins, but uncontrolled lytic replication may favor early phases of cell transformation. Besides immunodepression, persistent immune activation and chronic inflammation play an important role in both virus reactivation and expansion of EBV-infected B cells. EBV-induced immortalization requires the expression of telomerase. TERT, the rate-limiting component of the telomerase complex, is central in the switch from the lytic to the latent viral program, and TERT inhibition induces the EBV lytic cycle and cell death. Immunotherapy and combination of EBV lytic cycle inducers with antiviral drugs are promising strategies to improve the treatment of PTLD patients. This review is aimed at providing an update on the intriguing association between EBV and PTLD, mainly focusing on cases arising after kidney and liver transplantation, which account for the vast majority of transplants.

Telomerase activity impacts on Epstein-Barr virus infection of AGS cells

The Epstein-Barr virus (EBV) is transmitted from host-to-host via saliva and is associated with epithelial malignancies including nasopharyngeal carcinoma (NPC) and some forms of gastric carcinoma (GC). Nevertheless, EBV does not transform epithelial cells in vitro where it is rapidly lost from infected primary epithelial cells or epithelial tumor cells. Long-term infection by EBV, however, can be established in hTERT-immortalized nasopharyngeal epithelial cells. Here, we hypothesized that increased telomerase activity in epithelial cells enhances their susceptibility to infection by EBV. Using HONE-1, AGS and HEK293 cells we generated epithelial model cell lines with increased or suppressed telomerase activity by stable ectopic expression of hTERT or of a catalytically inactive, dominant negative hTERT mutant. Infection experiments with recombinant prototypic EBV (rB95.8), recombinant NPC EBV (rM81) with increased epithelial cell tropism compared to B95.8, or recombinant B95.8 EBV with BZLF1-knockout that is not able to undergo lytic replication, revealed that infection frequencies positively correlate with telomerase activity in AGS cells but also partly depend on the cellular background. AGS cells with increased telomerase activity showed increased expression mainly of latent EBV genes, suggesting that increased telomerase activity directly acts on the EBV infection of epithelial cells by facilitating latent EBV gene expression early upon virus inoculation. Thus, our results indicate that infection of epithelial cells by EBV is a very selective process involving, among others, telomerase activity and cellular background to allow for optimized host-to-host transmission via saliva.

Chromatin Structure of Epstein-Barr Virus Latent Episomes

EBV latent infection is characterized by a highly restricted pattern of viral gene expression. EBV can establish latent infections in multiple different tissue types with remarkable variation and plasticity in viral transcription and replication. During latency, the viral genome persists as a multi-copy episome, a non-integrated-closed circular DNA with nucleosome structure similar to cellular chromosomes. Chromatin assembly and histone modifications contribute to the regulation of viral gene expression, DNA replication, and episome persistence during latency. This review focuses on how EBV latency is regulated by chromatin and its associated processes.

Activation of NF-κB via endosomal Toll-like receptor 7 (TLR7) or TLR9 suppresses murine herpesvirus 68 reactivation

In order to understand and possibly treat B-cell malignancies associated with latent gammaherpesvirus infection, it is vital to understand the factors that control the balance between the two transcriptional states of gammaherpesviruses: latency and lytic replication. We used murine gammaherpesvirus 68 (MHV 68) as a model system to investigate how engagement of endosomal Toll-like receptors (TLRs) impacts reactivation from latency in vitro and establishment of latent infection in vivo. We found that treatment with TLR7 ligand R848 or TLR9 ligand CpG oligodeoxynucleotide (ODN) suppresses reactivation of MHV 68 in vitro. These suppressive effects correlated with the ability to activate cellular transcription factor NF-κB. Downregulation of TLR9 by RNA interference in vitro led to a reduction of nuclear levels of NF-κB p65 and consequently to an increase of spontaneous reactivation in cells latently infected with MHV 68, indicating that the TLR9 pathway suppresses spontaneous reactivation events. In vivo, sustained stimulation of TLR7 by repeated R848 treatment led to an increased frequency of infected splenocytes compared to mock-treated control results. Frequencies of infected splenic B cells in tlr7-/- or tlr9-/- mice after establishment of latency did not differ from those seen with their wild-type counterparts. Nevertheless, MHV 68-infected B cells from tlr9-/- mice showed a higher frequency of reactivation than B cells from wild-type or tlr7-/- mice in ex vivo reactivation assays. Thus, we show a suppressive effect of TLR7 or TLR9 triggering on MHV 68 reactivation that correlates with NF-κB activation and that the mere presence of a functional TLR9 signaling pathway contributes to dampen lytic gammaherpesvirus reactivation in infected cells.

IMPORTANCE

A hallmark of gammaherpesviruses is their establishment of latency in B cells that is reversible through lytic reactivation. Latency can result in B-cell malignancies. Activation of the innate immune system is thought to contribute to controlling the switch between the transcriptional states of latency and reactivation. Nevertheless, the mechanisms involved are not clear. Here, we show that engagement of Toll-like receptor 7 (TLR7) and TLR9 suppresses reactivation of murine gammaherpesvirus MHV 68 in vitro and that stimulation of TLR7 in vivo increases the frequency of infected cells. TLR7 and TLR9 are innate immunity sensors of nucleic acids localized in endosomes. Additionally, we demonstrate that impairment of TLR9 signaling in latently infected B cells leads to increased reactivation. Thus, activated endosomal TLR7 and TLR9 pathways play an important role in promoting establishment of latent gammaherpesvirus infection. Counteracting signaling of these pathways allows reactivation and could represent treatment targets in gammaherpesvirus-associated malignancies.

Epstein-Barr virus-driven lymphomagenesis in the context of human immunodeficiency virus type 1 infection

Epstein-Barr virus (EBV) is a ubiquitous human γ-herpes virus which establishes a life-long asymptomatic infection in immunocompetent hosts. In human immunodeficiency virus type 1 (HIV-1) infected patients, the impaired immunosurveillance against EBV may favor the development of EBV-related diseases, ranging from lymphoproliferative disorders to B cell non-Hodgkin's lymphomas (NHL). Antiretroviral therapy (ART) has significantly modified the natural course of HIV-1 infection, resulting in decreased HIV-1 plasmaviremia, increased CD4 lymphocytes, and decreased opportunistic infections, indicating a restoration of immune functions. However, the impact of ART appears to be less favorable on EBV-related malignancies than on other AIDS-defining tumors, such as Kaposi's sarcoma, and NHL remains the most common cancer during the ART era. EBV-driven tumors are associated with selective expression of latent oncogenic proteins, but uncontrolled lytic cycle with virus replication and/or reactivation may favor cell transformation, at least in the early phases. Several host's factors may promote EBV reactivation and replication; besides immunodepression, inflammation/chronic immune stimulation may play an important role. Microbial pathogen-associated molecular patterns and endogenous damage-associated molecular patterns, through Toll-like receptors, activate the immune system and may promote EBV reactivation and/or polyclonal expansion of EBV-infected cells. A body of evidence suggests that chronic immune stimulation is a hallmark of HIV-1 pathogenesis and may persist even in ART-treated patients. This review focuses on lymphomagenesis driven by EBV both in the context of the natural history of HIV-1 infection and in ART-treated patients. Understanding the mechanisms involved in the expansion of EBV-infected cells is a premise for the identification of prognostic markers of EBV-associated malignancies.

Oropharyngeal group A streptococcal colonization disrupts latent Epstein-Barr virus infection

Epstein-Barr virus (EBV) infects >90% of the human population within the first 2 decades of life and establishes reversible latent infection in B cells. The stimuli that lead to switching from latent to lytic EBV infection in vivo are still elusive. Group A streptococci (GAS) are a common cause of bacterial pharyngotonsillitis in children and adolescents and colonize the tonsils and pharynx of up to 20% of healthy children. Thus, concomitant presence of EBV and GAS in the same individual is frequent. Here, we show that EBV carriers who are colonized with GAS shed EBV particles in higher numbers in their saliva, compared with EBV carriers not colonized with GAS. Messenger RNA levels of the master lytic regulatory EBV gene BZLF1 were more frequently detected in tonsils from EBV carriers colonized with GAS than from EBV carriers not colonized. Heat-killed GAS, potentially mimicking GAS colonization, elicited lytic EBV in latently infected lymphoblastoid cell lines (LCLs) partially via Toll-like receptor 2 triggering, as did purified GAS peptidoglycan. Thus, colonization by GAS might benefit EBV by increasing the EBV load in saliva and thereby enhancing the likelihood of EBV spread to other hosts.

Early gene expression changes by Epstein-Barr virus infection of B-cells indicate CDKs and survivin as therapeutic targets for post-transplant lymphoproliferative diseases

Lymphoproliferative diseases (LPDs) associated with Epstein-Barr virus (EBV) infection cause significant morbidity and mortality in bone marrow and solid organ transplant recipients. To gain insight into LPD pathogenesis and to identify potential effective therapeutic approaches, we investigated early molecular events leading to B-cell transformation by gene expression profiling of EBV-infected B-cells from tonsils by Affymetrix microarray 72 hr postinfection when the B-cells hyperproliferation phase starts. Cell cycle and apoptosis were the most significantly affected pathways and enriched gene sets. In particular, we found significantly increased expression of cyclin-dependent kinase (CDK)1 and CCNB1 (cyclin B1) and of one of their downstream targets BIRC5 (survivin). Importantly, the strong upregulation of the antiapoptotic protein survivin was confirmed in lymphoblastoid cell lines (LCLs) and 71% of EBV-positive post-transplant EBV-LPD lesions scored positive for survivin. The validity of early transforming events for the identification of therapeutic targets for EBV-LPD was confirmed by the marked antiproliferative effect of the CDK inhibitor flavopiridol on LCLs and by the strong induction of apoptosis by survivin inhibition with YM155 or terameprocol. Our results suggest that targeting of CDKs and/or survivin in post-transplant EBV-LPD by specific inhibitors might be an important approach to control and eliminate EBV-transformed B-cells that should be further considered.

Assessment of the effect of TLR7/8, TLR9 agonists and CD40 ligand on the transformation efficiency of Epstein-Barr virus in human B lymphocytes by limiting dilution assay

Infection of human B cells with Epstein-Barr virus (EBV) induces polyclonal activation in almost all infected cells, but a small proportion of infected cells are transformed to immortalized lymphoblastoid cell lines. Since B cells are activated also by CD40 ligand (CD40L) and Toll-like receptor (TLR) agonists via a similar signaling pathway, it is likely that costimulation through these molecules could result in synergistic enhancement of the transformation efficiency of EBV. In this study, the stimulatory effect of TLR7/8 (R848), TLR9 (CpG) agonists and/or CD40L on transformation efficiency of EBV in normal human B cells was assessed using the limiting dilution assay. Costimulation of peripheral blood mononuclear cells (PBMCs) with CpG and R848, but not CD40L, increased significantly the frequency of EBV transformed B cells (p < 0.001). Neither synergistic nor additive effects were observed between TLR agonists and CD40L and also TLR7/8 and TLR9 agonists. Costimulation with R848, CpG and CD40L enhanced the proliferative response of B cells infected with EBV. This effect was more evident when enriched B cells were employed, compared to PBMCs. The promoting effect of TLR agonists stimulation, implies that EBV may take advantage of the genes induced by the TLR stimulation pathway for viral latency and oncogenesis.

TLR9 agonists induced cell death in Burkitt's lymphoma cells is variable and influenced by TLR9 polymorphism

Toll-like receptor 9 (TLR9) triggering is a promising novel strategy to combat cancer as it induces innate and adaptive immunity responses. B-cell lymphoma is unique in this context as tumor cells express TLR9 and may harbor latent Epstein-Barr virus (EBV), a gamma-herpesvirus with remarkable oncogenic potential when latent. Latent EBV may be promoted by TLR9 triggering via suppression of lytic EBV. Here, we elaborated an initial assessment of the impact of TLR9 triggering on EBV-positive and EBV-negative B-cell lymphoma using Burkitt's lymphoma (BL) cell lines as an in vitro model. We show that, independent of the presence of EBV, the TLR9 ligand oligodeoxynucleotide (ODN) CpG-2006 may or may not induce caspase-dependent cell death in BL cells. Moreover, ODN CpG-2006-induced cell death responses of BL cells were associated with TLR9 single-nucleotide polymorphisms (SNPs) rs5743836 or rs352140, which we detected in primary BL tumors and in peripheral blood from healthy individuals at similar frequencies. Thus, our findings suggest that the effect of TLR9 agonists on BL cells should be tested in vitro before installment of therapy and TLR9 SNPs in BL patients should be determined as potential biological markers for the therapeutic response to treatment targeting innate immunity.

EBV lytic-phase protein BGLF5 contributes to TLR9 downregulation during productive infection

Viruses use a wide range of strategies to modulate the host immune response. The human gammaherpesvirus EBV, causative agent of infectious mononucleosis and several malignant tumors, encodes proteins that subvert immune responses, notably those mediated by T cells. Less is known about EBV interference with innate immunity, more specifically at the level of TLR-mediated pathogen recognition. The viral dsDNA sensor TLR9 is expressed on B cells, a natural target of EBV infection. Here, we show that EBV particles trigger innate immune signaling pathways through TLR9. Furthermore, using an in vitro system for productive EBV infection, it has now been possible to compare the expression of TLRs by EBV(-) and EBV(+) human B cells during the latent and lytic phases of infection. Several TLRs were found to be differentially expressed either in latently EBV-infected cells or after induction of the lytic cycle. In particular, TLR9 expression was profoundly decreased at both the RNA and protein levels during productive EBV infection. We identified the EBV lytic-phase protein BGLF5 as a protein that contributes to downregulating TLR9 levels through RNA degradation. Reducing the levels of a pattern-recognition receptor capable of sensing the presence of EBV provides a mechanism by which the virus could obstruct host innate antiviral responses.

B cells from HIV-infected patients with primary central nervous system lymphoma display an activated phenotype and have a blunted TNF-α response to TLR9 triggering

Each cell in HIV-associated primary central nervous system lymphoma (PCNSL) harbors latent EBV. Notably, the triggering of TLR9, a key event in HIV pathogenesis, also promotes EBV latency and transformation. We hypothesized that because only a minority of HIV-infected patients develops PCNSL, their B cells exhibit aberrant signaling responses to TLR9 triggering. We found higher levels of IL-6, CD80, and CD86 expression at baseline in B cells of those patients than in B cells of matched controls, whereas TNF-a expression was lower. Notably, on TLR9 triggering with CpG 2006, CD80 and TNF-α were up-regulated to a lesser extent in B cells of the former than in those of matched controls. The reduced up-regulation of CD80 might be explained by its higher baseline expression resulting in a more blunted response rather than a specific deficit of the signaling response to TLR9 triggering. However, this cannot explain the blunted TNF-α response, which warrants further investigation. Finally, since increased IL-6 expression is linked to EBV-associated Hodgkin’s lymphoma, the enhanced baseline expression of IL-6 might be important in the pathogenesis of PCNSL in HIV-infected patients.

TLR9 triggering in Burkitt's lymphoma cell lines suppresses the EBV BZLF1 transcription via histone modification

Endemic Burkitt's lymphoma (BL) is considered to preferentially develop in equatorial Africa because of chronic co-infection with Epstein-Barr virus (EBV) and the malaria pathogen Plasmodium falciparum. The interaction and contribution of both pathogens in the oncogenic process are poorly understood. Earlier, we showed that immune activation with a synthetic Toll-like receptor 9 (TLR9) ligand suppresses the initiation of EBV lytic replication in primary human B cells. In this study we investigate the mechanism involved in the suppression of EBV lytic gene expression in BL cell lines. We show that this suppression is dependent on functional TLR9 and MyD88 signaling but independent of downstream signaling elements, including phosphatidylinositol-3 kinase, mitogen-activated protein kinases and nuclear factor-kappaB. We identified TLR9 triggering resulting in histone modifications to negatively affect the activation of the promoter of EBV's master regulatory lytic gene BZLF1. Finally, we show that P. falciparum hemozoin, a natural TLR9 ligand, suppresses induction of EBV lytic gene expression in a dose-dependent manner. Thus, we provide evidence for a possible interaction between P. falciparum and EBV at the B-cell level and the mechanism involved in suppressing lytic and thereby reinforcing latent EBV that has unique oncogenic potential.

A novel method for making human monoclonal antibodies

We have developed a B cell immortalization method for low B cell numbers per well using simultaneous B cell stimulation by CpG2006 and B cell infection by Epstein-Barr virus (EBV), followed by an additional CpG2006 and interleukin-2 (IL-2) stimulus. Using this method, immunoglobulin G (IgG)-producing immortalized B cell lines were generated from peripheral blood IgG⁺CD22⁺ B cells with an efficiency of up to 83%. Antibody can already be obtained from the culture supernatant after 3–4 weeks. Moreover, clonality analysis demonstrated monoclonality in 87% of the resulting immortalized B cell lines. Given the high immortalization efficiency and monoclonality rate, evidence is provided that no further subcloning is necessary. An important application of this B cell immortalization method is the characterization of (autoreactive) antibodies from patients with autoimmune disease. This could eventually lead to the identification of new autoantigens, disease markers or targets for therapy.

Plasma cell toll-like receptor (TLR) expression differs from that of B cells, and plasma cell TLR triggering enhances immunoglobulin production

Toll-like receptors (TLRs) are key receptors of the innate immune system and show cell subset-specific expression. We investigated the messenger RNA (mRNA) expression of TLR genes in human haematopoietic stem cells (HSC), in naïve B cells, in memory B cells, in plasma cells from palatine tonsils and in plasma cells from peripheral blood. HSC and plasma cells showed unrestricted expression of TLR1-TLR9, in contrast to B cells which lacked TLR3, TLR4 and TLR8 but expressed mRNA of all other TLRs. We demonstrated, for the first time, that TLR triggering of terminally differentiated plasma cells augments immunoglobulin production. Thus, boosting the immediate antibody response by plasma cells upon pathogen recognition may point to a novel role of TLRs.

Cytokine induction after Epstein-Barr virus infection of peripheral T-lymphocytes in vitro

Background: Although the presence of Epstein-Barr virus (EBV) in different T-cell malignancies has been widely reported, there is very few data available for EBV infection of normal T cells. This leads to the lack of knowledge on the early events after T cell infection. Objective: Investigate the early events occurring after normal human peripheral T-cells are infected with EBV in vitro. Methods: T-cells were treated with EBV in vitro. The expression of tumor necrosis factor- α (TNF-α) mRNA were determined using reverse-transcription (RT)-PCR, and the level of TNF-α and interferon- γ (IFN-γ) in the culture supernatant were measured using ELISA. The effect of virus inactivation on cytokine induction from T-cells was also determined. Results: At the beginning of T cell infection by EBV, the expression of several lytic EBV transcripts (BALF5, BcLF1, and BLLF1) were observed using RT-PCR. This indicated the susceptibility of in vitro EBV infection and the entering lytic cycle of EBV-infected T-cells. The interactions of EBV with T-cells lead to induction of inflammatory cytokines, tumour necrosis factor- α (TNF-α) and interferon- γ (IFN-γ), production from the T-cells. Inactivation of the virus by UV irradiation eliminated the TNF-α and IFN-γ induction by EBV, suggesting the involvement in the expression of viral gene(s). Conclusion: This in vitro analysis demonstrated the cytokine induction by EBV after primary infection of T-cells.

Toll-like receptor agonists synergistically increase proliferation and activation of B cells by epstein-barr virus

Epstein-Barr virus (EBV) efficiently drives proliferation of human primary B cells in vitro, a process relevant for human diseases such as infectious mononucleosis and posttransplant lymphoproliferative disease. Human B-cell proliferation is also driven by ligands of Toll-like receptors (TLRs), notably viral or bacterial DNA containing unmethylated CpG dinucleotides, which triggers TLR9. Here we quantitatively investigated how TLR stimuli influence EBV-driven B-cell proliferation and expression of effector molecules. CpG DNA synergistically increased EBV-driven proliferation and transformation, T-cell costimulatory molecules, and early production of interleukin-6. CpG DNA alone activated only memory B cells, but CpG DNA enhanced EBV-mediated transformation of both memory and naive B cells. Ligands for TLR2 or TLR7/8 or whole bacteria had a weaker but still superadditive effect on B-cell transformation. Additionally, CpG DNA facilitated the release of transforming virus by established EBV-infected lymphoblastoid cell lines. These results suggest that the proliferation of EBV-infected B cells and their capability to interact with immune effector cells may be directly influenced by components of bacteria or other microbes present at the site of infection.

Generation of potent neutralizing human monoclonal antibodies against cytomegalovirus infection from immune B cells

Background

Human monoclonal antibodies (mAbs) generated as a result of the immune response are likely to be the most effective therapeutic antibodies, particularly in the case of infectious diseases against which the immune response is protective.

Human cytomegalovirus (HCMV) is an ubiquitous opportunistic virus that is the most serious pathogenic agent in transplant patients. The available therapeutic armamentarium (e.g. HCMV hyperimmune globulins or antivirals) is associated with severe side effects and the emergence of drug-resistant strains; therefore, neutralizing human mAb may be a decisive alternative in the prevention of primary and re-activated HCMV infections in these patients.

Results

The purpose of this study was to generate neutralizing mAb against HCMV from the immunological repertoire of immune donors. To this aim, we designed an efficient technology relying on two discrete and sequential steps: first, human B-lymphocytes are stimulated with TLR9-agonists and IL-2; second, after both additives are removed, the cells are infected with EBV. Using this strategy we obtained 29 clones secreting IgG neutralizing the HCMV infectivity; four among these were further characterized. All of the mAbs neutralize the infection in different combinations of HCMV strains and target cells, with a potency ~20 fold higher than that of the HCMV hyperimmune globulins, currently used in transplant recipients. Recombinant human monoclonal IgG1 suitable as a prophylactic or therapeutic tool in clinical applications has been generated.

Conclusion

The technology described has proven to be more reproducible, efficient and rapid than previously reported techniques, and can be adopted at low overall costs by any cell biology laboratory for the development of fully human mAbs for immunotherapeutic uses.

Distinct ex vivo susceptibility of B-cell subsets to epstein-barr virus infection according to differentiation status and tissue origin

Epstein-Barr virus (EBV) uses tonsils as the portal of entry to establish persistent infection. EBV is found in various B-cell subsets in tonsils but exclusively in memory B cells in peripheral blood. The in vitro susceptibilities of B-cell subsets to EBV infection have been studied solely qualitatively. In this work, we examined quantitatively the in vitro susceptibilities of various B-cell subsets from different tissue origins to EBV infection. First, we established a centrifugation-based inoculation protocol (spinoculation) that resulted in a significantly increased proportion of infected cells compared to that obtained by conventional inoculation, enabling a detailed susceptibility analysis. Importantly, B-cell infection occurred via the known EBV receptors and infected cells showed EBV mRNA expression patterns similar to those observed after conventional inoculation, validating our approach. Tonsillar naïve and memory B cells were infected ex vivo at similar frequencies. In contrast, memory B cells from blood, which represent B cells from various lymphoid tissues, were infected at lower frequencies than their naïve counterparts. Immunoglobulin A (IgA)-positive or IgG-positive tonsillar memory B cells were significantly more susceptible to EBV infection than IgM-positive counterparts. Memory B cells were transformed with lower efficiency than naïve B cells. This result was paralleled by lower proliferation rates. In summary, these data suggest that EBV exploits the B-cell differentiation status and tissue origin to establish persistent infection.

Latent membrane protein 2B regulates susceptibility to induction of lytic Epstein-Barr virus infection

The B-lymphotropic Epstein-Barr virus (EBV) encodes two isoforms of latent membrane protein 2 (LMP2), LMP2A and LMP2B, which are expressed during latency in B cells. The function of LMP2B is largely unknown, whereas LMP2A blocks B-cell receptor (BCR) signaling transduction and induction of lytic EBV infection, thereby promoting B-cell survival. Transfection experiments on LMP2B in EBV-negative B cells and the silencing of LMP2B in EBV-harboring Burkitt's lymphoma-derived Akata cells suggest that LMP2B interferes with the function of LMP2A, but the role of LMP2B in the presence of functional EBV has not been established. Here, LMP2B, LMP2A, or both were overexpressed in EBV-harboring Akata cells to study the function of LMP2B. The overexpression of LMP2B increased the magnitude of EBV switching from its latent to its lytic form upon BCR cross-linking, as indicated by a more-enhanced upregulation and expression of EBV lytic genes and significantly increased production of transforming EBV compared to Akata vector control cells or LMP2A-overexpressing cells. Moreover, LMP2B lowered the degree of BCR cross-linking required to induce lytic EBV infection. Finally, LMP2B colocalized with LMP2A as demonstrated by immunoprecipitation and immunofluorescence and restored calcium mobilization upon BCR cross-linking, a signaling process inhibited by LMP2A. Thus, our findings suggest that LMP2B negatively regulates the function of LMP2A in preventing the switch from latent to lytic EBV replication.