Interleukin-21 regulates expression of the immediate-early lytic cycle genes and proteins in Epstein-Barr Virus infected B cells

EBV dUTPase: A Novel Modulator of Inflammation and the Tumor Microenvironment in EBV-Associated Malignancies

There is increasing evidence that put into question the classical dogma that the Epstein-Barr virus (EBV) exists in cells as either a lytic virus in which new progeny is produced or in a latent state in which no progeny is produced. Notably, a third state has now been described, known as the abortive-lytic phase, which is characterized by the expression of some immediate early (IE) and early (E) genes, but no new virus progeny is produced. While the function of these IE and E gene products is not well understood, several recent studies support the concept they may contribute to tumor promotion by altering the tumor microenvironment (TME). The mechanisms by which these viral gene products may contribute to tumorigenesis remain unclear; however, it has been proposed that some of them promote cellular growth, immune evasion, and/or inhibit apoptosis. One of these EBV early gene products is the deoxyuridine triphosphate nucleotidohydrolase (dUTPase) encoded by BLLF3, which not only contributes to the establishment of latency through the production of activin A and IL-21, but it may also alter the TME, thus promoting oncogenesis.

Interleukin 21 signaling in B cells is required for efficient establishment of murine gammaherpesvirus latency

The human gammaherpesviruses take advantage of normal B cell differentiation pathways to establish life-long infection in memory B cells. Murine gammaherpesvirus 68 (MHV68) infection of laboratory strains of mice also leads to life-long infection in memory B cells. To gain access to the memory B cell population, MHV68 infected B cells pass through the germinal center reaction during the onset of latency and require signals from T follicular helper (T_FH) cells for proliferation. Interleukin 21 (IL-21), one of the secreted factors produced by T_FH cells, plays an important role in both the maintenance of the germinal center response as well as in the generation of long-lived plasma cells. Using IL-21R deficient mice, we show that IL-21 signaling is required for efficient establishment of MHV68 infection. In the absence of IL-21 signaling, fewer infected splenocytes are able to gain access to either the germinal center B cell population or the plasma cell population – the latter being a major site of MHV68 reactivation. Furthermore, the germinal center B cell population in IL-21R^-/- mice is skewed towards the non-proliferating centrocyte phenotype, resulting in reduced expansion of infected B cells. Additionally, the reduced frequency of infected plasma cells results in a significant reduction in the frequency of splenocytes capable of reactivating virus. This defect in establishment of MHV68 infection is intrinsic to B cells, as MHV68 preferentially establishes infection in IL-21R sufficient B cells in mixed bone marrow chimeric mice. Taken together, these data indicate that IL-21 signaling plays multiple roles during establishment of MHV68 infection, and identify IL-21 as a critical T_FH cell-derived factor for efficient establishment of gammaherpesvirus B cell latency.

Gammaherpesviruses establish life-long infection in B cells by taking advantage of the host immune response that is generated during primary infection. During initial infection, the immune system responds by inducing rapid proliferation of responding B cells during the germinal center reaction. This response is highly coordinated and relies on the interplay of multiple cell types. CD4 T helper cells are an important component of the germinal center reaction in that they communicate with B cells by providing both proliferation and survival signals. Gammaherpesviruses infect B cells that receive these signals, resulting in proliferation and survival of infected cells, allowing the virus to establish life-long infection. Here we show that interleukin 21 (IL-21), one of the signaling factors produced by CD4 T cells, is required for efficient establishment of infection in a mouse model of gammaherpesvirus infection. In the absence of IL-21 signaling, the viral load is markedly reduced and the composition of the infected cell population is altered to cell types that are less proliferative and produce less virus. These results demonstrate how gammaherpesviruses are able to take advantage of the immune response being generated against it to establish lifelong infection.

Regulation of Epstein-Barr virus reactivation from latency

The Epstein-Barr virus (EBV) is a human gamma-herpesvirus that is implicated in various types of proliferative diseases. Upon infection, it predominantly establishes latency in B cells and cannot ever be eradicated; it persists for the host's lifetime. Reactivation of the virus from latency depends on expression of the viral immediate-early gene, BamHI Z fragment leftward open reading frame 1 (BZLF1). The BZLF1 promoter normally exhibits only low basal activity but is activated in response to chemical or biological inducers, such as 12-O-tetradecanoylphorbol-13-acetate, calcium ionophore, histone deacetylase inhibitor, or anti-Ig. Transcription from the BZLF1 promoter is activated by myocyte enhancer factor 2, specificity protein 1, b-Zip type transcription factors and mediating epigenetic modifications of the promoter, such as histone acetylation and H3K4me3. In contrast, repression of the promoter is mediated by transcriptional suppressors, such as ZEB, ZIIR-BP, and jun dimerization protein 2, causing suppressive histone modifications like histone H3K27me3, H3K9me2/3 and H4K20me3. Interestingly, there is little CpG DNA methylation of the promoter, indicating that DNA methylation is not crucial for suppression of BZLF1. This review will focus on the molecular mechanisms by which the EBV lytic switch is controlled and discuss the physiological significance of this switching for its survival and oncogenesis.

Interleukin-21 overexpression dominates T cell response to Epstein-Barr virus in a fatal case of X-linked lymphoproliferative syndrome type 1

Interleukin-21 (IL-21) is a cytokine whose actions are closely related to B cell differentiation into plasma cells as well as to CD8(+) cytolytic T cell effector and memory generation, influencing the T lymphocyte response to different viruses. X-linked lymphoproliferative syndrome type 1 (XLP-1) is a primary immunodeficiency syndrome that is characterized by a high susceptibility to Epstein-Barr virus. We observed in a pediatric patient with XLP-1 that IL-21 was expressed in nearly all peripheral blood CD4(+) and CD8(+) T cells. However, IL-21 could not be found in the lymph nodes, suggesting massive mobilization of activated cells toward the infection's target organs, where IL-21-producing cells were detected, resulting in large areas of tissue damage.

Epstein-Barr virus LMP2A increases IL-10 production in mitogen-stimulated primary B-cells and B-cell lymphomas

Epstein-Barr virus (EBV) latently infected B-cells are the precursors of EBV-associated malignancies. EBV-infection induces the production of pro-survival and anti-inflammatory cytokines that may be important in the transition between latency and malignancy. One EBV protein, LMP2A, can be detected in both latently infected resting B-cells and in EBV-associated malignancies. Therefore, we tested the ability of LMP2A to influence cytokine production using both LMP2A-Tg primary B-cells and LMP2A-expressing B-cell lines. Our data demonstrate that LMP2A does not globally alter B-cell-produced cytokine levels, but specifically targets IL-10. Additional studies using ELISA and real-time-RT-PCR confirm that LMP2A utilizes PI3-kinase to increase IL-10 levels. Finally, the data demonstrate that LMP2A-expressing B-cell lines are more dependent on IL-10 for survival in comparison to LMP2A-negative B-cell lines. These data identify a novel function of LMP2A in the alteration of a cytokine that is important for both tumour survival and anti-tumour responses.

Epstein-Barr virus LMP2A imposes sensitivity to apoptosis

In cell lines, the Epstein–Barr virus (EBV)-encoded protein latent membrane protein 2A (LMP2A) protects B-cells from apoptosis by blocking B-cell receptor (BCR) signalling. However, EBV-infected B-cells in vivo are extremely different from cell lines. This study used a murine transgenic model in which B-cells express LMP2A and a BCR specific for hen egg lysozyme to determine whether LMP2A protects resting and antigen-activated B-cells from apoptosis. LMP2A allows BCR signal transduction and induces constitutive activation of NF-κB to increase Bcl-2 levels that afford LMP2A-mediated protection from apoptosis in the absence or presence of antigen. In contrast, low levels of NF-κB inhibitor only affected Bcl-2 and Bcl-xL levels and increased apoptosis in LMP2A-negative B-cells after BCR cross-linking. These data suggest that LMP2A uniquely makes resting B-cells sensitive to NF-κB inhibition and apoptosis and suggest that NF-κB may be a novel target to eradicate latently EBV-infected B-cells.

Transient Epstein-Barr virus reactivation in CD3 monoclonal antibody-treated patients

Here we report a unique situation in which an early and synchronized Epstein-Barr virus (EBV) reactivation was induced by a 6-day course of treatment with a humanized CD3-specific monoclonal antibody in patients with recent onset of type 1 diabetes. The virologic and immunologic analysis demonstrated that this reactivation was transient, self-limited, and isolated, associated with the rapid advent of an EBV-specific T-cell response. The anti-CD3 antibody administration induced short-lasting immunosuppression and minor yet clear-cut signs of T-cell activation that preceded viral reactivation. Early posttransplant monitoring of renal and islet allograft recipients showed that no comparable phenomenon was observed after the administration of full-dose immunosuppressive therapy. This EBV reactivation remains of no apparent clinical concern over the long term and should not preclude further development of therapeutic anti-CD3 antibodies. This phenomenon may also direct new research avenues to understand the still ill-defined nature of stimuli triggering EBV reactivation in vivo.