Induction of the lytic viral cycle in Epstein Barr virus carrying Burkitt lymphoma lines is accompanied by increased expression of major histocompatibility complex molecules

Targeting Epstein-Barr Virus in Nasopharyngeal Carcinoma

Nasopharyngeal carcinoma (NPC) is consistently associated with Epstein-Barr virus (EBV) infection in regions in which it is endemic, including Southern China and Southeast Asia. The high mortality rates of NPC patients with advanced and recurrent disease highlight the urgent need for effective treatments. While recent genomic studies have revealed few druggable targets, the unique interaction between the EBV infection and host cells in NPC strongly implies that targeting EBV may be an efficient approach to cure this virus-associated cancer. Key features of EBV-associated NPC are the persistence of an episomal EBV genome and the requirement for multiple viral latent gene products to enable malignant transformation. Many translational studies have been conducted to exploit these unique features to develop pharmaceutical agents and therapeutic strategies that target EBV latent proteins and induce lytic reactivation in NPC. In particular, inhibitors of the EBV latent protein EBNA1 have been intensively explored, because of this protein's essential roles in maintaining EBV latency and viral genome replication in NPC cells. In addition, recent advances in chemical bioengineering are driving the development of therapeutic agents targeting the critical functional regions of EBNA1. Promising therapeutic effects of the resulting EBNA1-specific inhibitors have been shown in EBV-positive NPC tumors. The efficacy of multiple classes of EBV lytic inducers for NPC cytolytic therapy has also been long investigated. However, the lytic-induction efficiency of these compounds varies among different EBV-positive NPC models in a cell-context-dependent manner. In each tumor, NPC cells can evolve and acquire somatic changes to maintain EBV latency during cancer progression. Unfortunately, the poor understanding of the cellular mechanisms regulating EBV latency-to-lytic switching in NPC cells limits the clinical application of EBV cytolytic treatment. In this review, we discuss the potential approaches for improvement of the above-mentioned EBV-targeting strategies.

Role of ATM in the formation of the replication compartment during lytic replication of Epstein-Barr virus in nasopharyngeal epithelial cells

Epstein-Barr virus (EBV), a type of oncogenic herpesvirus, is associated with human malignancies. Previous studies have shown that lytic reactivation of EBV in latently infected cells induces an ATM-dependent DNA damage response (DDR). The involvement of ATM activation has been implicated in inducing viral lytic gene transcription to promote lytic reactivation. Its contribution to the formation of a replication compartment during lytic reactivation of EBV remains poorly defined. In this study, the role of ATM in viral DNA replication was investigated in EBV-infected nasopharyngeal epithelial cells. We observed that induction of lytic infection of EBV triggers ATM activation and localization of DDR proteins at the viral replication compartments. Suppression of ATM activity using a small interfering RNA (siRNA) approach or a specific chemical inhibitor profoundly suppressed replication of EBV DNA and production of infectious virions in EBV-infected cells induced to undergo lytic reactivation. We further showed that phosphorylation of Sp1 at the serine-101 residue is essential in promoting the accretion of EBV replication proteins at the replication compartment, which is crucial for replication of viral DNA. Knockdown of Sp1 expression by siRNA effectively suppressed the replication of viral DNA and localization of EBV replication proteins to the replication compartments. Our study supports an important role of ATM activation in lytic reactivation of EBV in epithelial cells, and phosphorylation of Sp1 is an essential process downstream of ATM activation involved in the formation of viral replication compartments. Our study revealed an essential role of the ATM-dependent DDR pathway in lytic reactivation of EBV, suggesting a potential antiviral replication strategy using specific DDR inhibitors.

IMPORTANCE

Epstein-Barr virus (EBV) is closely associated with human malignancies, including undifferentiated nasopharyngeal carcinoma (NPC), which has a high prevalence in southern China. EBV can establish either latent or lytic infection depending on the cellular context of infected host cells. Recent studies have highlighted the importance of the DNA damage response (DDR), a surveillance mechanism that evolves to maintain genome integrity, in regulating lytic EBV replication. However, the underlying molecular events are largely undefined. ATM is consistently activated in EBV-infected epithelial cells when they are induced to undergo lytic reactivation. Suppression of ATM inhibits replication of viral DNA. Furthermore, we observed that phosphorylation of Sp1 at the serine-101 residue, a downstream event of ATM activation, plays an essential role in the formation of viral replication compartments for replication of virus DNA. Our study provides new insights into the mechanism through which EBV utilizes the host cell machinery to promote replication of viral DNA upon lytic reactivation.

Zidovudine-based lytic-inducing chemotherapy for Epstein-Barr virus-related lymphomas

Treatment of Epstein-Barr virus (EBV)-related lymphomas with lytic-inducing agents is an attractive targeted approach for eliminating virus-infected tumor cells. Zidovudine (AZT) is an excellent substrate for EBV-thymidine kinase: it can induce EBV lytic gene expression and apoptosis in primary EBV+ lymphoma cell lines. We hypothesized that the combination of AZT with lytic-inducing chemotherapy agents would be effective in treating EBV+ lymphomas. We report a retrospective analysis of 19 patients with aggressive EBV+ non-Hodgkin lymphoma, including nine cases of acquired immune deficiency syndrome-associated primary central nervous system lymphoma (AIDS-PCNSL) treated with AZT-based chemotherapy. Our results demonstrate that high-dose AZT-methotrexate is efficacious in treating highly aggressive systemic EBV+ lymphomas in the upfront setting. In primary EBV+ lymphoma cell lines, the combination of AZT with hydroxyurea resulted in synergistic EBV lytic induction and cell death. Further, AZT-hydroxyurea treatment resulted in dramatic responses in patients with AIDS-PCNSL. The combination of AZT with chemotherapy, especially lytic-inducing agents, should be explored further in clinical trials for the treatment of EBV-related lymphomas.

Getting the message direct manipulation of host mRNA accumulation during gammaherpesvirus lytic infection

The Gammaherpesvirinae subfamily of herpesviruses comprises lymphotropic viruses, including the oncogenic human pathogens Epstein-Barr virus and Kaposi's sarcoma-associated herpesvirus. During lytic infection, gammaherpesviruses manipulate host gene expression to optimize the cellular environment for viral replication and to evade the immune response. Additionally, although a lytically infected cell will itself be killed in the process of viral replication, lytic infection can contribute to pathogenesis by inducing the secretion of paracrine factors with functions in cell survival and proliferation, and angiogenesis. The mechanisms by which these viruses manipulate host gene expression are varied and target the accumulation of cellular mRNAs and their translation, signaling pathways, and protein stability. Here, we discuss how gammaherpesviral proteins directly influence host mRNA biogenesis and stability, either selectively or globally, in order to fine-tune the cellular environment to the advantage of the virus. Appreciation of the mechanisms by which these viruses interface with and adapt normal cellular processes continues to inform our understanding of gammaherpesviral biology and the regulation of mRNA accumulation and turnover in our own cells.

Inhibition of p38 MAP kinase pathway induces apoptosis and prevents Epstein Barr virus reactivation in Raji cells exposed to lytic cycle inducing compounds

Background

EBV lytic cycle activators, such as phorbol esters, anti-immunoglobulin, transforming growth factor β (TGFβ), sodium butyrate, induce apoptosis in EBV-negative but not in EBV-positive Burkitt's lymphoma (BL) cells. To investigate the molecular mechanisms allowing EBV-infected cells to be protected, we examined the expression of viral and cellular antiapoptotic proteins as well as the activation of signal transduction pathways in BL-derived Raji cells exposed to lytic cycle inducing agents.

Results

Our data show that, following EBV activation, the latent membrane protein 1 (LMP1) and the cellular anti-apoptotic proteins MCL-1 and BCL-2 were quickly up-regulated and that Raji cells remained viable even when exposed simultaneously to P(BU)₂, sodium butyrate and TGFβ. We report here that inhibition of p38 pathway, during EBV activation, led to a three fold increment of apoptosis and largely prevented lytic gene expression.

Conclusion

These findings indicate that, during the switch from the latent to the lytic phase of EBV infection, p38 MAPK phosphorylation plays a key role both for protecting the host cells from apoptosis as well as for inducing viral reactivation. Because Raji cells are defective for late antigens expression, we hypothesize that the increment of LMP1 gene expression in the early phases of EBV lytic cycle might contribute to the survival of the EBV-positive cells.

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.

Inhibition of Poly(ADP-ribose)polymerase impairs Epstein Barr Virus lytic cycle progression

BACKGROUND

Poly(ADP-ribosylation) is a post-translational modification of nuclear proteins involved in several cellular events as well as in processes that characterize the infective cycle of some viruses. In the present study, we investigated the role of poly(ADP-ribosylation) on Epstein-Barr Virus (EBV) lytic cycle activation.

RESULTS

Inhibition of PARP-1 by 3-aminobenzamide (3-ABA) during EBV induction, diminished cell damage and apoptosis in the non-productive Raji cell line while markedly reducing the release of viral particles in the productive Jijoye cells. Furthermore, incubation with 3-ABA up-regulated the levels of LMP1 and EBNA2 latent viral proteins. At the same time, it slightly affected the expression of the immediate early BZLF1 gene, but largely down-regulated the levels of the early BFRF1 protein. The modulation of the expression of both latent and lytic EBV genes appeared to be post-transcriptionally regulated.

CONCLUSION

Taken together the data indicate that PARP-1 plays a role in the progression of EBV lytic cycle and therefore, PARP inhibitors might represent suitable pharmacological adjuncts to control viral spread in EBV productive infection.

Impaired transporter associated with antigen processing-dependent peptide transport during productive EBV infection

Human herpesviruses, including EBV, persist for life in infected individuals. During the lytic replicative cycle that is required for the production of infectious virus and transmission to another host, many viral Ags are expressed. Especially at this stage, immune evasion strategies are likely to be advantageous to avoid elimination of virus-producing cells. However, little is known about immune escape during productive EBV infection because no fully permissive infection model is available. In this study, we have developed a novel strategy to isolate populations of cells in an EBV lytic cycle based on the expression of a reporter gene under the control of an EBV early lytic cycle promoter. Thus, induction of the viral lytic cycle in transfected EBV(+) B lymphoma cells resulted in concomitant reporter expression, allowing us, for the first time, to isolate highly purified cell populations in lytic cycle for biochemical and functional studies. Compared with latently infected B cells, cells supporting EBV lytic cycle displayed down-regulation of surface HLA class I, class II, and CD20, whereas expression levels of other surface markers remained unaffected. Moreover, during lytic cycle peptide transport into the endoplasmic reticulum, was reduced to <30% of levels found in latent infection. Because steady-state levels of TAP proteins were unaffected, these results point toward EBV-induced interference with TAP function as a specific mechanism contributing to the reduced levels of cell surface HLA class I. Our data implicate that EBV lytic cycle genes encode functions to evade T cell recognition, thereby creating a window for the generation of viral progeny.

In vitro EBV-infected subline of KMH2, derived from Hodgkin lymphoma, expresses only EBNA-1, while CD40 ligand and IL-4 induce LMP-1 but not EBNA-2

In about 50% of classical Hodgkin lymphomas, the Hodgkin/Reed Sternberg (H/RS) cells carry Epstein-Barr virus (EBV). The viral gene expression in these cells is restricted to EBNA-1, EBERs, LMP-1 and LMP-2 (type II latency). The origin of H/RS cells was defined as crippled germinal center B cells that escaped apoptosis. In spite of numerous attempts, only few typical Hodgkin lymphoma (HL) lines have been established. This suggests that the cells require survival factors that they receive in the in vivo microenvironment. If EBV is expected to drive the cells for growth in culture, the absence of EBNA-2 may explain the incapacity of H/RS cells for in vitro proliferation. In EBV carrying B lymphocytes, functional EBNA-2 and LMP-1 proteins are required for in vitro growth. For analysis of the interaction between EBV and the H/RS cells, we infected the CD21-positive HL line KMH2 with the B958 and Akata viral strains. Only EBNA-1 expression was detected in a few cells in spite of the fact that all cells could be infected. Using a neomycin-resistance-tagged recombinant EBV strain (Akata-Neo) we established an EBV-positive subline that was carried on selective medium. In contrast to the type II EBV expression pattern of H/RS cells in vivo, the KMH2 EBV cells did not express LMP-1. The EBV expression pattern could be modified in this type I subline. LMP-1 could be induced by the histone deacetylase inhibitors TSA and n-butyrate, by 5-AzaC, a demethylating agent, and by phorbol ester. None of these treatments induced EBNA-2. Importantly, exposure to CD40 ligand and IL-4 induced LMP-1 without EBNA-2 expression and lytic replication. The KMH2 EBV cells expressed LMP-2A, but not LMP-2B mRNAs. This result is highly relevant for the type II expression pattern of H/RS cells in vivo, since these stimuli can be provided by the surrounding activated T lymphocytes.

Latent membrane protein 1 inhibits Epstein-Barr virus lytic cycle induction and progress via different mechanisms

Epstein-Barr virus (EBV) is a potent growth-transforming agent of human B cells. It has previously been shown that viral latent membrane protein 1 (LMP1) is essential for EBV-induced transformation of normal B cells and contributes to maintenance of latency in vitro. Using the EBV-positive Burkitt's lymphoma line P3HR1-c16, which lacks LMP1 during latency and which can readily be activated into virus-productive lytic cycle, we found that LMP1 inhibits lytic cycle induction via the transcription factor NF-kappa B. In addition, LMP1 inhibits lytic cycle progress via two distinct NF-kappa B-independent mechanisms: one involving the cytosolic C-terminal activating regions and the other involving the transmembrane region of LMP1. These findings indicate that in B cells EBV self-limits its lytic cycle via three distinct LMP1-mediated mechanisms.

The lytic cycle of Epstein-Barr virus is associated with decreased expression of cell surface major histocompatibility complex class I and class II molecules

Human herpesviruses utilize an impressive range of strategies to evade the immune system during their lytic replicative cycle, including reducing the expression of cell surface major histocompatibility complex (MHC) and immunostimulatory molecules required for recognition and lysis by virus-specific cytotoxic T cells. Study of possible immune evasion strategies by Epstein-Barr virus (EBV) in lytically infected cells has been hampered by the lack of an appropriate permissive culture model. Using two-color immunofluorescence staining of cell surface antigens and EBV-encoded lytic cycle antigens, we examined EBV-transformed B-cell lines in which a small subpopulation of cells had spontaneously entered the lytic cycle. Cells in the lytic cycle showed a four- to fivefold decrease in cell surface expression of MHC class I molecules relative to that in latently infected cells. Expression of MHC class II molecules, CD40, and CD54 was reduced by 40 to 50% on cells in the lytic cycle, while no decrease was observed in cell surface expression of CD19, CD80, and CD86. Downregulation of MHC class I expression was found to be an early-lytic-cycle event, since it was observed when progress through late lytic cycle was blocked by treatment with acyclovir. The immediate-early transactivator of the EBV lytic cycle, BZLF1, did not directly affect expression of MHC class I molecules. However, BZLF1 completely inhibited the upregulation of MHC class I expression mediated by the EBV cell-transforming protein, LMP1. This novel function of BZLF1 elucidates the paradox of how MHC class I expression can be downregulated when LMP1, which upregulates MHC class I expression in latent infection, remains expressed in the lytic cycle.

n-butyrate downregulates the stimulatory function of peripheral blood-derived antigen-presenting cells: a potential mechanism for modulating T-cell responses by short-chain fatty acids

Modulation of proliferative T-cell responses by n-butyrate has been suggested to result from direct interference with cell cycle progression. Considering the important role of antigen-presenting cells (APC) in T-cell activation, we were particularly interested in studying the impact of n-butyrate on these cells. We demonstrated that pretreatment of human peripheral blood mononuclear cells (PBMC) or monocytes with this agent resulted in a dose- and time-dependent downregulation of their capability to stimulate T-cell responses with a similar pattern of inhibition when this agent was present throughout the culture period. Pretreatment with n-butyrate was effective in preventing both alloresponses and T-cell proliferation to immobilized anti-CD3 monoclonal antibody (mAb) suggesting alteration of costimulatory function. Flow cytometric analysis revealed that interferon-gamma (IFN-gamma)-induced upregulation of B7-1 expression on monocytes was profoundly inhibited by n-butyrate. Furthermore, this agent significantly suppressed the expression of intercellular adhesion molecule-1 (ICAM-1) or lymphocyte function-associated antigen-3 (LFA-3). In contrast, constitutive as well as cytokine-induced expression of B7-2 was enhanced by n-butyrate. Additionally, in monocytes, but not in T cells, treatment with n-butyrate led to significant alteration of membrane integrity owing to apoptotic cell death. Our findings indicate that modulation of T-cell responses by n-butyrate could also result from altered APC function, possibly as a consequence of downregulating distinct adhesion and/or costimulatory receptors as well as of inducing apoptosis. A potential clinical relevance of short-chain fatty acids for reducing T-cell-mediated immune reactions via modulating APC function is speculated.

Regulation and specificity of MNDA expression in monocytes, macrophages, and leukemia/B lymphoma cell lines

The expression of the human myeloid cell nuclear differentiation antigen (MNDA) was observed specifically in cells of the granulocyte-macrophage lineage in our earlier reports. The specificity of MNDA expression for cells in the granulocyte-macrophage lineage was reexamined in cell lines established from patients with Philadelphia chromosome-positive chronic myeloid leukemia. Cell lines that expressed MNDA exhibited myeloid cell features and granulocyte or monocyte differentiation could be induced in vitro, while cell lines exhibiting properties of very early stage cells or multipotential cells did not express MNDA. Cells originating from cases of Burkitt's lymphoma were negative. By contrast, three lymphoblastoid cell lines (immortalized in vitro with Epstein-Barr virus) were weakly positive and MNDA was up-regulated by interferon-alpha (IFN-alpha) treatment. As we reported previously, MNDA mRNA level in adherent monocytes is elevated by IFN-alpha; in this study, we further assessed MNDA expression in in vitro monocyte-derived macrophages. Three additional agents (endotoxin, phytohemagglutinin, and phorbol ester) and other conditions that affect function, cytokine production, differentiation, and/or growth of monocytes were examined for their ability to alter MNDA expression. The results varied with the agent, cell type, and stage of differentiation. Changes in MNDA expression occurred slowly (hours to days), suggesting that MNDA could mediate changes realized over a long period. The results also reveal a discordance in certain MNDA positive cells between steady-state levels or changes in levels of protein and mRNA indicating that the regulation of MNDA expression occurs at more than one point. Changes in MNDA expression are consistent with a role in opposing macrophage differentiation and activation of monocytes/macrophages.