The role of the microenvironment on the regulation of Epstein-Barr virus latent gene expression

Abstract: Epstein-Barr virus (EBV) is a human-specific gamma-herpes virus of the Lymphocryptovirus genus that has succeeded to colonize more than 90% of the adult population. EBV s coevolution with humans has established a largely harmless co-existence that depends on the variability of viral gene expression and on the immunological host response. Though infection with EBV is generally harmless, the virus is associated with multiple human tumors, such as Burkitt lymphoma, classical Hodgkin lymphoma, nasopharyngeal carcinoma, post-transplant lymphoproliferative disorders, and AIDS lymphomas. The role of EBV in the malignant transformation is still enigmatic even after more than 40 years of research on this virus. EBV readily infects B-lymphocytes in vitro. After infection the virus establishes a latent infection and expresses 9 viral proteins. The concerted effect of these EBV proteins will be the activation and proliferation of the infected B cells. This viral gene expression pattern was named type III latency. As the EBV-infected B cells with type III latency are highly immunogenic they are readily detected and killed by the specific cytotoxic T cells. In contrast to the EBV-infected normal B cells, the majority of the EBV-carrying tumors do not express all the nine proteins; rather they express only EBNA-1 or EBNA-1 together with the latent membrane proteins (LMP-1 and LMP-2). The factors that determine what viral genes EBV will express in the different normal and malignant cells are only partially known. Motivated by the lack of in vitro models in which to study the interaction of EBV with the malignant Hodgkin/Reed-Sternberg (HRS) cells, we infected with EBV one of the Hodgkin lymphoma-derived cell lines and studied the viral gene expression in this EBV-converted subline. In this system we identified two cytokines, IL-4 and IL-13, that could modulate the viral gene expression (specifically, induce the expression of LMP-1) and with their help we could for the first time reconstitute in vitro the EBV gene expression seen in the classical Hodgkin lymphomas in vivo. We have also studied the molecular mechanisms that are responsible for the induction of LMP-1 by IL-4 and IL-13. Through these studies we identified STAT6 as an important inducer of LMP-1 expression. As STAT6 is constitutively activated in the majority of Hodgkin lymphomas, our results not only provides an explanation how LMP-1 is expressed in the EBV-carrying HRS cells, but might also have future therapeutic implications. Further work identified two additional cytokines, IL-10 and IL-21, which could induce the expression of LMP-1 in EBV-positive B cell and NK cell lymphoma-derived cell lines. The effect of IL-21 was pleiotropic: it could induce LMP-1 in cells that did not express it, and it induced the plasma cell differentiation and down-regulation of expression of the EBV nuclear antigens (EBNA-1 to -6) in type III lymphoblastoid and Burkitt lymphoma cell lines. Furthermore, when isolated human CD4+ T cells were co-cultured with different EBVcarrying lymphoma cell lines, we found that upon activation they were capable of inducing the expression of LMP-1, just as the recombinant cytokines did. Altogether our results provide evidence for an important role for the cytokines secreted by CD4+ T or other inflammatory cells in the modulation of EBV latent gene expression.